JPH10232415A - Method and device for generating phase conjugate light and covering wavelength, and system with the device, and manufacture of the device. - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a phase conjugate light generator which is efficient, fast, broadband, and also polarized-wave-independent, by splitting a signal beam of light into planes of polarization orthogonal to each other, and generating each phase conjugate beam of light for synthesizing polarized waves. SOLUTION: A signal beam of light ES is split into polarized waves of a first polarization component ES1 and a second polarization component ES2 by a first polarization beam splitter(PBS) 32. The first polarization component ES1 is supplied to a first DFB laser diode 1 (#1) and the first phase conjugate beam of light EC1 is outputted. Moreover, a second DFB laser diode 1 (#2) is used for the second polarization component ES2 and the second phase conjugate beam of light EC2 is outputted. These polarization components ES1 and ES2 are orthogonal to each other. And, the first and second polarization beams of light EC1 and EC2 are polarization-synthesized for obtaining one phase conjugate beam of light EC.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for generating phase conjugate light and wavelength conversion, and a system having the apparatus.

[0002]

2. Description of the Related Art With the development of low-loss silica optical fibers, many optical fiber communication systems using optical fibers as transmission lines have been put into practical use. The optical fiber itself has a very wide bandwidth. However, the transmission capacity of the optical fiber is actually limited by the system design. The most important limitation is due to waveform distortion due to chromatic dispersion occurring in the optical fiber.
Optical fibers also attenuate optical signals, for example, at a rate of about 0.2 dB / km, and the losses due to this attenuation have been compensated for by the use of optical amplifiers, including erbium-doped fiber amplifiers (EDFAs).

[0003] Chromatic dispersion, often referred to simply as dispersion, is a phenomenon in which the group velocity of an optical signal in an optical fiber changes as a function of the wavelength (frequency) of the optical signal. For example, in a standard single mode fiber, 1.3 μm
For wavelengths shorter than m, optical signals having longer wavelengths propagate faster than optical signals having shorter wavelengths,
The resulting variance is usually referred to as the normal variance.
For wavelengths longer than 1.3 μm, optical signals with shorter wavelengths propagate faster than optical signals with longer wavelengths, and the resulting dispersion is called anomalous dispersion.

In recent years, attention has been paid to non-linearity due to an increase in optical signal power due to the use of EDFA. The most important nonlinearity of an optical fiber that limits the transmission capacity is the optical Kerr effect. The optical Kerr effect is a phenomenon in which the refractive index of an optical fiber changes according to the intensity of an optical signal. The change in refractive index modulates the phase of the optical signal propagating in the optical fiber, resulting in frequency chirping that changes the signal spectrum. This phenomenon is called self-phase modulation (self-phase modulation:
SPM). The spectrum is expanded by SPM, and the waveform distortion due to chromatic dispersion is further increased.

As described above, the chromatic dispersion and the Kerr effect give waveform distortion to an optical signal as the transmission distance increases. Therefore, chromatic dispersion and non-linearity need to be controlled, compensated or suppressed to allow long-distance transmission over optical fibers.

As a technique for controlling chromatic dispersion and nonlinearity, a technique using a regenerative repeater including an electronic circuit for a main signal is known. For example, a plurality of regenerative repeaters are arranged in the middle of the transmission line, and in each regenerative repeater, optical / electrical conversion, regeneration processing, and electrical / optical conversion are performed in this order before the waveform distortion of the optical signal becomes excessive. Will be However, this method has a problem that an expensive and complicated regenerative repeater is required, and an electronic circuit of the regenerative repeater limits the bit rate of the main signal.

Optical soliton is known as a technique for compensating for chromatic dispersion and nonlinearity. An optical signal pulse having precisely defined amplitude, pulse width, and peak power is generated for a given value of anomalous dispersion, whereby the SPM due to the optical Kerr effect and the pulse compression due to the anomalous dispersion and the dispersion The pulse spread balances, and the optical soliton propagates while maintaining its waveform.

Another technique for compensating for chromatic dispersion and nonlinearity is the application of optical phase conjugation. For example, a method for compensating for chromatic dispersion in a transmission line is described in Yari (Yari).
v) et al. (A. Yariv, D. Fekete,
and DM Pepper, “Compensation for channel dispe
rsion by nonlinear optical phase conjugation ”Op
t. Lett., vol. 4, pp. 52-54, 1979). At the midpoint of the transmission line, the optical signal is converted into phase conjugate light, and waveform distortion due to chromatic dispersion received in the first half of the transmission line is compensated for by distortion due to chromatic dispersion in the second half of the transmission line.

In particular, if the factor of the phase change of the electric field at the two points is the same, and the environmental change causing the factor is gradual within the light propagation time between the two points, the intermediate between the two points The phase change is compensated for by arranging a phase conjugator (phase conjugate light generator) in (S. Watanabe, “Co
mpensation of phase fluctuation in a transmission
line by optical conjugation ”Opt. Lett., vol. 17,
pp. 1355-1357, 1992). Therefore, by using the phase conjugator, the waveform distortion caused by the SPM is also compensated. However, if the distribution of the optical power before and after the phase conjugator is asymmetric, the non-linearity compensation is incomplete.

The inventor has previously proposed a technique for overcoming incomplete compensation due to optical power asymmetry when using a phase conjugator (S. Watanabe and M. Shirasak).
i, “Exact compensation for both chromatic dispers
ion and Kerr effect in a transmission fiber using
optical phase conjugation ”J. Lightwave Technol., v
ol. 14, pp. 243-248, 1996). The phase conjugator is arranged near a point in the transmission path where the total value of the dispersion value or the nonlinear effect before and after the phase conjugator becomes equal, and various parameters before and after the phase conjugator are set for each minute section.

Regarding the phase conjugator and its application to optical fiber communication, for example, an application by the present inventor (Japanese Patent Application No.
No. 509844, Japanese Patent Application No. 7-44574, Japanese Patent Application No. 7
-304229, JP-A-7-98464 and JP-A-7-301830).

A method for generating a phase conjugate wave using a traveling wave type semiconductor laser amplifier is described in [1] A. MECOZZI ET AL., IEEE.
JOURNAL OF QUANTUM ELECTRONICS, VOL.31, NO.4, APRI
L 1995, PP.689-699. in this way,
As shown in FIG. 6 of that document, the pump (excitation)
The light and the probe light (also referred to as signal light) are combined by a directional coupler, and the combined pump light and probe light are input to a traveling-wave semiconductor laser amplifier via a lens and optical isolation, and thereby phase conjugate. Waves are extracted from the traveling wave type semiconductor laser amplifier. The pump light is obtained by converting light output from a color center laser (CCL) into an optical isolator (OI) and Babine Soleil (BABINE).
(T-SOLEIL) compensator and lens input to the directional coupler. Further, the probe light converts light output from an external cavity semiconductor laser (ECLD) into optical isolation, a λ / 2 plate and a λ / 4
It is given by input to the directional coupler through the plate.

A method of generating a phase conjugate wave using a semiconductor laser instead of a semiconductor laser amplifier is described in [2] PATR
ICK P. IANNONE ET AL., IEEE JOURNAL OF QUANTUM ELE
CTRONICS, VOL.31, NO.7, JULY 1995, PP.1285-1291. In this method, a device having substantially the same mechanism as that of the document [1] is used except that a semiconductor laser is used. The semiconductor laser oscillates light having the same wavelength as the wavelength of pump light injected from the outside.

These two examples are common in that pump light and probe light are input to one end of a semiconductor laser amplifier or a semiconductor laser, and pump light, probe light, and a phase conjugate wave are extracted from the other end.

On the other hand, a method of inputting probe light from the first end face of a semiconductor laser that oscillates pump light and outputting a phase conjugate wave from the same first end face has been proposed.
[3] S. MURATA EL AL., APPL. PHYS. LETT. 58 (14),
8 APRIL 1991, PP.1458-1460.

In the methods described in the above documents [1] and [2], a light source for generating probe light, a light source for generating pump light, and a semiconductor laser amplifier or semiconductor laser for generating a phase conjugate wave are used. The need for three optical devices complicates the optical system that couples them. In particular, an optical coupler for efficiently coupling the probe light and the pump light is required.

In the method described in the document [3], it is necessary to form a high-reflectance reflective film on the non-output end face of the semiconductor laser that outputs a phase conjugate wave. Perot mode exists. Therefore, the wavelength of the phase conjugate wave is limited to the wavelength that resonates in the Fabry-Perot mode, as described in document [3].

Recently, a method for generating phase conjugate light by non-degenerate four-wave mixing (FWM) in an oscillating DFB-LD has been reported in the following paper. H. Kuwatsuka, H. Shoji, M. Matsuda and H. Ishikaw
a, “THz frequency conversion using nondegenerate
four-wave mixing prosess in a lasing long-cavity
λ / 4- shifted laser ”, ELECTRONICS LETTERS, Vol.3
1, No. 24, pp. 2108-2110, 1995. A method of generating phase conjugate light by this method will be briefly described. A semiconductor high gain medium with high carrier injection is
Due to its large third-order nonlinear sensitivity, it is one of the most suitable materials for performing four-wave mixing. In a state where a semiconductor laser is oscillating, high-intensity oscillating light exists inside the semiconductor laser. Therefore, it is theoretically possible that four-wave mixing occurs when light is input from the outside and phase conjugate light is generated. Although known, actually, when light is externally applied to the oscillating laser, there is a problem that the oscillating light is drawn to the wavelength of the light or the oscillating light becomes unstable. Further, even if phase conjugate light is generated, phase conjugate light can be generated only with light having a wavelength that resonates with the resonator constituting the semiconductor laser, and the wavelength cannot be freely converted.

In the above paper, a quarter-wave phase shift DF
Inside the B semiconductor laser, two diffraction gratings that reflect only light of a wavelength to be oscillated are formed so that they are phase-shifted from each other by お 互 い wavelength. Oscillation light is strongly confined inside the semiconductor laser by these two diffraction gratings, and light having a wavelength different from that of the oscillation light passes through without being reflected inside the laser by coating both end faces of the semiconductor laser with antireflection. . Therefore, it is possible to generate the phase conjugate light of the light input to the semiconductor laser from the outside by using the oscillation light as the pump light, and to achieve high-efficiency, high-speed, wide-band conversion without using the pump light from the outside. Is possible.

[0020]

The conversion efficiency of the phase conjugate light generator depends on the coincidence of the polarization planes of the probe light and the pump light. Does not have the ability to preserve the wavefront. Therefore, in order to configure an optical system using optical phase conjugation, it is necessary to realize a phase conjugate light generator having high efficiency, high speed and wide band, and having no polarization dependency.

The objects of the present invention are as follows. (1) To provide a method and apparatus capable of increasing the conversion efficiency from a signal light beam to a phase conjugate light beam.

(2) To provide a method and apparatus in which the conversion efficiency from a signal light beam to a phase conjugate light beam does not largely depend on the polarization state of the signal light beam. (3) To provide a method and apparatus for widening a conversion band in conversion from a signal light beam to a phase conjugate light beam, and a method for manufacturing the apparatus.

(4) To provide a method and an apparatus for obtaining a phase conjugate light beam having an excellent signal-to-noise ratio. (5) Having the device of (1) to (4), or (1)
To provide a system suitable for optical communication to which the method of (4) is applied.

(6) To provide a system having a novel configuration suitable for constructing a highly flexible optical network system.

[0025]

According to one aspect of the present invention, there is provided a method having the following features (1) to (58):
An apparatus or system is provided.

(1) (a) A signal light beam is divided into a first polarization component having a first polarization plane and a second polarization component having a second polarization plane perpendicular to the first polarization plane. (B) supplying the first and second polarization components to a distributed feedback (DFB) laser diode to provide first and second polarization components corresponding to the first and second polarization components, respectively. Generating a second phase conjugate light beam; and (c) generating the first phase conjugate light beam.
And polarization combining the second phase conjugate light beam.

(2) In the method of the above (1), the step (b) is such that the DFB laser diode generates a pump light having a wavelength different from the wavelength of the signal light beam. Injecting a current into the DFB laser diode to generate the first and second phase conjugate light beams by four-wave mixing based on the pump light.

(3) The method according to (1) above, wherein
The FB laser diode is composed of first and second DFB laser diodes receiving the first and second polarization components, respectively, and the steps (a) and (c) are respectively performed by the first and second polarization beam splitters. The method made by.

(4) The method according to (1) above, wherein
The FB laser diode has first and second excitation ends for receiving the first and second polarization components, respectively, and the first and second phase conjugate light beams respectively correspond to the second and second phase conjugate light beams.
And the output from the first excitation end, wherein steps (a) and (c) are performed by a common polarization beam splitter.

(5) The signal light beam is polarized into a first polarization component having a first polarization plane and a second polarization component having a second polarization plane perpendicular to the first polarization plane. Means for splitting and distributed feedback for supplying the first and second polarization components and generating first and second phase conjugate light beams corresponding to the first and second polarization components, respectively. DFB) device comprising a laser diode.

(6) In the apparatus according to (5), the polarization separating means includes a first port for receiving the signal light beam and a first port for outputting the first and second polarization components, respectively. A first polarization beam splitter having second and third ports, wherein the DFB laser diode comprises a first and second DFB laser diode operatively connected to the second and third ports, respectively. And the first and second phase conjugate light beams are respectively the first and second phase conjugate light beams.
Further comprising a second polarization beam splitter that is output from the DFB laser diode and that polarization-combines the first and second phase conjugate light beams.

(7) In the apparatus of (6), the first DFB laser diode generates a first pump light having a third polarization plane, and the first phase conjugate light beam is In the first DFB laser diode, the first
, And the second DFB laser diode generates a second pump light having a fourth polarization plane, and the second DFB laser diode generates a second pump light having a fourth polarization plane.
Is generated by four-wave mixing based on the second polarization component and the second pump light in the second DFB laser diode, and the first polarization plane and the third polarization Apparatus further comprising means for rotating the polarization plane by 90 ° so that the wavefront coincides and the second polarization plane coincides with the fourth polarization plane.

(8) The apparatus according to (7), wherein the rotating means is a first mirror operatively connected between the first polarization beam splitter and the second DFB laser diode. A device comprising: a half-wave plate; and a second half-wave plate operatively connected between the first DFB laser diode and the second polarization beam splitter.

(9) The apparatus according to (7), wherein the rotating means includes a polarization maintaining fiber. (10) The device according to (6), wherein the first and second devices are
The polarization beam splitter is formed on a common waveguide substrate.

(11) In the apparatus according to (5), the means for polarization separation includes a polarization beam splitter having first to fourth ports, and the first port has the signal beam. A light beam is supplied and the first and third ports and the second and fourth ports are coupled by the first polarization plane, and the first and second ports are coupled to each other and the third and third ports are coupled to each other. The fourth port is coupled by the second polarization plane, the first and second polarization components are output from the third and second ports, respectively, and the DFB laser diode is connected to the first and second ports. It has first and second excitation ends respectively receiving a second polarization component, generates pump light having a third polarization plane, and the first and second phase conjugate light beams respectively have the second and third phase conjugate light beams. And output from the first excitation end respectively It is supplied to the second and third port, the first and second polarization plane of one of said first and second polarization plane to match the third polarization plane 90
° The device further comprising means for rotating.

(12) The apparatus according to (11), wherein the rotating means includes a half-wave plate. (13) The apparatus according to (11), wherein the rotating means includes a polarization maintaining fiber.

(14) The apparatus according to (11), further comprising an optical circulator having fifth to seventh ports, wherein one of the fifth to seventh ports is the polarization beam splitter. And the fourth port of the polarization beam splitter is reflectionless terminated.

(15) The apparatus according to (11), further comprising a first optical circulator having fifth to seventh ports, and a second optical circulator having eighth to tenth ports. One of the fifth to seventh ports is connected to a first port of the polarization beam splitter, and one of the eighth to tenth ports is connected to a first port of the polarization beam splitter. Device connected to port 4.

(16) The device according to (15), wherein the second device is cascaded to the DFB laser diode.
The second DFB laser diode.
A laser diode for generating a second pump light having a polarization plane perpendicular to the third polarization plane;

(17) The apparatus according to (5), further comprising a means for injecting a current into the DFB laser diode so that the DFB laser diode generates pump light, wherein the pump in the DFB laser diode is provided. An apparatus for generating the first and second phase conjugate light beams by light-based four-wave mixing.

(18) The apparatus according to (17), wherein the DFB laser diode has a substantially central portion at 4
An apparatus comprising: a diffraction grating having a one-wavelength phase shift structure; and an electrode for injecting the current, wherein the electrode includes a plurality of portions divided in the direction of the diffraction grating.

(19) (a) injecting a current into the DFB laser diode so that the distributed feedback (DFB) laser diode generates pump light; and (b) supplying a signal light beam to the DFB laser diode. Generating a phase conjugate light beam by four-wave mixing based on the signal light beam and the pump light in the DFB laser diode; and (c) non-linearly converting the signal light beam, the pump light, and the phase conjugate light beam. Feeding the optical medium to increase the power of the phase conjugate light beam by four-wave mixing in the nonlinear optical medium.

(20) A distributed feedback (DFB) laser diode to which a signal light beam is supplied, means for injecting a current into the DFB laser diode so that the DFB laser diode generates pump light, and the DFB laser diode And a non-linear optical medium optically connected to the DFB laser diode, and a phase conjugate light beam is generated by four-wave mixing based on the signal light beam and the pump light in the DFB laser diode. An apparatus for increasing the power of the phase conjugate light beam by mixing.

(21) The device according to (20), wherein the nonlinear optical medium includes a semiconductor optical amplifier. (22) The device according to (20), wherein the nonlinear optical medium includes an optical fiber.

(23) The apparatus according to (22), wherein the optical fiber has a zero dispersion wavelength substantially equal to the wavelength of the pump light. (24) The apparatus according to (22), further comprising means for frequency-modulating or phase-modulating the pump light, whereby stimulated Brillouin scattering in the optical fiber is suppressed.

(25) The apparatus according to (22), wherein the optical fiber has a nonlinear coefficient large enough to shorten the length of the optical fiber to such an extent that the optical fiber has a polarization plane preserving ability. Equipment.

(26) The apparatus according to (25), wherein the optical fiber includes a GeO ₂ -doped core and a fluorine-doped clad. (27) The apparatus according to (25), wherein the optical fiber is a single mode fiber, and the single mode fiber has a mode field diameter smaller than that of a single mode fiber used as a transmission line. Equipment.

(28) The apparatus according to (20), further comprising an optical amplifier operatively connected between the DFB laser diode and the nonlinear optical medium. (29) (a) A signal light beam composed of a first polarization component having a first polarization plane and a second polarization component having a second polarization plane perpendicular to the first polarization plane is converted into the above-described signal light beam. First DF for generating pump light having a polarization plane corresponding to the first polarization plane
The first DFB laser diode has a polarization plane corresponding to the first polarization plane by four-wave mixing based on the first polarization component and the first pump light supplied to the first DFB laser diode. Generating a phase conjugate light beam of (b); and (b) the first phase conjugate light beam output from the first DFB laser diode and the second polarization transmitted through the first DFB laser diode. To the second DFB laser diode that generates a second pump light having a polarization plane corresponding to the second polarization plane, and supplies the second polarization component in the second DFB laser diode. And generating a second phase conjugate light beam having a polarization plane corresponding to the second polarization plane by four-wave mixing based on the second pump light. .

(30) A first DFB laser diode for generating a first pump light having a first polarization plane, and a first DFB laser diode cascaded to the first DFB laser diode and perpendicular to the first polarization plane. A second DFB laser diode for generating a second pump light having two polarization planes, the first and second polarization components having polarization planes respectively corresponding to the first and second polarization planes Is supplied to the first DFB laser diode, the first DFB laser diode performs four-wave mixing based on the first polarization component and the first pump light to perform the first DFB laser diode. Having a polarization plane corresponding to the polarization plane of
And the second polarization component passes through the first DFB laser diode and passes through the second DF.
The B laser diode generates a second phase conjugate light beam having a polarization plane corresponding to the second polarization plane by four-wave mixing based on the second polarization component and the second pump light. The phase conjugate light beam of the second DF
A device that transmits a B laser diode.

(31) The apparatus according to the above (30), further comprising a polarization dependent element for giving different loss or gain to the light having the first and second polarization planes. The polarization dependent element is the first and second D
The first and the second in each of the FB laser diodes
An apparatus provided to suppress a difference in transmittance of light having a polarization plane.

(32) (a) injecting a current into the distributed feedback (DFB) laser diode so that the laser diode generates pump light;
A signal light beam is supplied to the FB laser diode, and the above D
Generating a phase conjugate light beam by four-wave mixing based on the signal light beam and the pump light in the FB laser diode; (c) the signal light beam and the phase conjugate light beam output from the DFB laser diode And supplying the pump light to an optical band stop filter having a stop band including the wavelength of the pump light.

(33) A distributed feedback (DFB) laser diode to which a signal light beam is supplied, and the DFB laser diode generates pump light, and the DFB laser diode is subjected to four-wave mixing based on the pump light and the signal light beam. Means for injecting a current into the DFB laser diode so as to convert the signal light beam into a phase conjugate light beam, and a stop band including a wavelength of the pump light;
An apparatus comprising: an optical band stop filter to which the signal light beam output from the DFB laser diode, the phase conjugate light beam, and the pump light are supplied.

(34) The apparatus according to the above (33), wherein the optical band rejection filter comprises a fiber grating. (35) (a) supplying a signal light beam to an optical band stop filter having a stop band including a predetermined wavelength; and (b) phase conjugating the signal light beam output from the optical band stop filter. Supplying to a light generator to generate a phase conjugate light beam having a wavelength substantially equal to the predetermined wavelength.

(36) An optical band stop filter having a stop band including a predetermined wavelength and supplied with a signal light beam, and the signal light beam output from the optical band stop filter is supplied. A phase conjugate light generator for generating a phase conjugate light beam having a wavelength substantially equal to a predetermined wavelength by four-wave mixing.

(37) In the apparatus according to (36), the phase conjugate light generator includes a distributed feedback (DFB) laser diode to which the signal light beam is supplied, and the DFB laser diode generates pump light. Means for injecting a current into the DFB laser diode so as to perform the operation.

(38) The apparatus according to (36), wherein the phase conjugate light generator comprises: a pump light source for outputting pump light; a non-linear optical medium to which the signal light beam is supplied;
Means for optically connecting the pump light source and the nonlinear optical medium such that the pump light is supplied to the nonlinear optical medium.

(39) The apparatus according to the above (36), wherein the optical band rejection filter comprises a fiber grating. (40) (a) supplying a signal light beam to the first nonlinear optical medium; and (b) forming a phase conjugate light beam in the first nonlinear optical medium based on four-wave mixing using pump light. Generating (c) the first
Supplying the signal light beam, the phase conjugate light beam, and the pump light output from the nonlinear optical medium to a second nonlinear optical medium.

(41) A first nonlinear optical medium that receives a signal light beam and generates a phase conjugate light beam based on four-wave mixing using pump light, and is cascaded to the first nonlinear optical medium. An apparatus comprising: the signal light beam output from the first nonlinear optical medium; the phase conjugate light beam; and a second nonlinear optical medium to which the pump light is supplied.

(42) In the apparatus according to (41), the first nonlinear optical medium is a semiconductor chip for providing a first conversion band, and the second nonlinear optical medium is the first nonlinear optical medium. An apparatus comprising an optical fiber for providing a second conversion band wider than the conversion band.

(43) The apparatus according to (42), wherein said semiconductor chip is provided by a semiconductor optical amplifier, and further comprises a pump light source for supplying said semiconductor optical amplifier with said pump light.

(44) The apparatus according to (42), wherein said semiconductor chip is provided by a distributed feedback (DFB) laser diode, and a current is supplied to said DFB laser diode so that said DFB laser diode generates said pump light. Further comprising a means for injecting.

(45) The apparatus according to (42), wherein said optical fiber has a zero dispersion wavelength substantially equal to the wavelength of said pump light. (46) The apparatus according to (42), further comprising a feedback loop for controlling the wavelength of the pump light so that the power of the phase conjugate light beam becomes higher.

(47) (a) supplying a signal light beam to the semiconductor nonlinear optical medium, and (b) generating a phase conjugate light beam in the semiconductor nonlinear optical medium based on four-wave mixing using pump light. The step of causing
(C) setting the wavelength of the signal light beam to be longer than the wavelength of the pump light.

(48) The semiconductor nonlinear optical medium to which the signal light beam is supplied and the semiconductor nonlinear optical medium are phase-conjugated based on four-wave mixing using pump light having a wavelength shorter than the wavelength of the signal light beam. Means for pumping the semiconductor nonlinear optical medium to generate a light beam.

(49) In the apparatus according to (48), the semiconductor nonlinear optical medium is provided by a semiconductor optical amplifier, and the pumping means includes a pump light source for supplying the pump light to the semiconductor optical amplifier. Including equipment.

(50) The apparatus according to (48), wherein said semiconductor nonlinear optical medium is provided by a distributed feedback (DFB) laser diode, and said pumping means is such that said DFB laser diode generates said pump light. And means for injecting current into said DFB laser diode.

(51) A first optical fiber for transmitting a signal light beam, a phase conjugate light generator for converting the signal light beam into a phase conjugate light beam, and transmitting the phase conjugate light beam A second optical fiber for
The phase conjugate light generator includes the above (5) to (18), (2)
0) to (28), (30), (31), (33), (3)
4), (36) to (39), (41) to (46), (4)
8) The apparatus according to any one of (50) to (50), or the phase conjugate light generator includes (1), (19),
(29) A system to which the method described in any one of (32), (35), (40), and (47) is applied.

(52) The system according to (51), wherein when the first and second optical fibers are virtually divided into the same number of sections, respectively, counting is performed from the phase conjugate light generator. The product of the average value of the chromatic dispersion and the product of the section lengths of the two sections substantially match, and the product of the average value of the optical power, the average value of the nonlinear coefficient, and the section length of the two sections substantially match. system.

(53) The system according to (51), wherein the optical power and the nonlinearity at each of the two points of the first and second optical fibers having the same cumulative chromatic dispersion from the phase conjugate light generator. A system in which the ratio between the product of the coefficients and the chromatic dispersion is substantially equal.

(54) The system according to (51), wherein each of the two points of the first and second optical fibers has the same cumulative value of the product of the optical power and the nonlinear coefficient from the phase conjugate light generator. Wherein the ratio of the product of the optical power and the non-linear coefficient and the chromatic dispersion in the first embodiment substantially coincides.

(55) In the system of (51), the product of the average value and the length of the chromatic dispersion of the first optical fiber is the product of the average value and the length of the chromatic dispersion of the second optical fiber. A system that substantially matches the product.

(56) The system according to (55), wherein the product of the average value of the optical power and the average value of the nonlinear coefficient in the first optical fiber and the length of the first optical fiber is the second optical fiber. A system substantially corresponding to the product of the average value of the optical power and the average value of the non-linear coefficient in the optical fiber and the length of the second optical fiber.

(57) A plurality of optically connected units are provided, each of the plurality of units being (51) to (51)
A system including the system according to any one of (56). (58) A plurality of units optically connected, and at least one optical signal insertion / branching device provided at a connection point of the plurality of units, wherein each of the plurality of units transmits signal light. A first optical fiber for converting the signal light into a phase conjugate light; and a second optical fiber for transmitting the phase conjugate light. Wherein the chromatic dispersion and the optical Kerr effect are compensated by the chromatic dispersion and the optical Kerr effect in the second optical fiber.

According to another aspect of the present invention, there is provided an apparatus or system having the following features (1 ') to (24'). (1 ′) The signal light is separated into two polarization components orthogonal to each other, and the first signal light component is subjected to the first phase conjugate by four-wave mixing in the first tertiary nonlinear medium using the first pump light. After the light is converted into light, the polarization direction is rotated by 90 °, the second signal light component is rotated by 90 ° in the polarization plane, and then the second signal light is subjected to the second excitation light using the second excitation light having substantially the same wavelength as the first excitation light. The optical phase conjugation that converts the two phase conjugate lights into the second phase conjugate light with almost the same generation efficiency as the first phase conjugate light by the four-wave mixing in the third-order nonlinear medium, and combines the two phase conjugate lights with each other to make a polarization synthesis. vessel.

(2 ') In the above (1'), a DFB-LD or a quarter-wave (λ / 4) phase shift-DFB-LD is used as a third-order nonlinear medium, and this is used as an excitation light for generating four-wave mixing. DFB-LD or 1/4 wavelength (λ /
4) Phase shift—Uses oscillation light of DFB-LD.

(3 ') In the above (2'), the signal light is passed through an optical circulator and then polarization-separated by a polarization separator. Of the two orthogonal polarization components, the signal polarized parallel to the active layer surface of the DFB-LD is obtained. 1 is incident on this first end face,
The oscillation light emitted from the second end face is converted into the first phase conjugate light by four-wave mixing using the excitation light as the excitation light.
The second polarization component perpendicular to the active layer surface of D has a polarization direction of 90
After rotating, it is incident on the second end face of the DFB-LD,
The oscillating light emitted from the first end face is converted into the second phase conjugate light by four-wave mixing using the excitation light as the excitation light, and the first phase conjugate light is rotated by 90 ° in the polarization direction. 2
The second phase conjugate light component is incident on the port of the polarization splitter from which the first signal light component is emitted, and the two phase conjugate light components are polarization-combined. After that, the signal light in the optical circulator is input to the port from which the signal light is emitted.

(4 ') In the above (1') to (3 '), each optical device is coupled by using a polarization maintaining fiber, and the 90 ° rotation of the polarization plane is caused by two principal axes of polarization of the polarization maintaining fiber. Are orthogonally combined.

(5 ') In the above (2'), the polarization separation and the polarization synthesis are performed by using the branch synthesis by the LiNbO ₃ waveguide. (6 ′) The signal light is input to the first end face of the DFB-LD, and the phase conjugate light is generated by four-wave mixing using the oscillation light output from the second end face of the DFB-LD as excitation light;
The signal light, the oscillation light, and the phase conjugate light output from the second end face are input to a third-order nonlinear optical medium disposed outside the DFB-LD, and four-wave mixing in which the oscillation light is used as excitation light is performed. Generate.

(7 ') In (6'), a semiconductor optical amplifier is used as the third-order nonlinear optical medium. (8 ') In the above (6'), an optical fiber is used as the third-order nonlinear optical medium.

(9 ') In the above (8'), the wavelength of the pump light is made substantially coincident with the zero dispersion wavelength of the optical fiber. (10 ') In (8'), the excitation light is subjected to frequency or phase modulation to suppress stimulated Brillouin scattering (SBS) in the optical fiber. (11 ') First optical fiber for transmitting signal light And a phase conjugate light generator that receives the signal light supplied from the first optical fiber and generates a phase conjugate light corresponding to the signal light; and the phase conjugate light supplied from the phase conjugate light generator. A second optical fiber for receiving the light and transmitting the phase conjugate light; and when the first and second optical fibers are each divided into the same number, the phase conjugate light generator outputs The average value of the chromatic dispersion of the corresponding section when counted in order is the same sign and a value substantially inversely proportional to the length of each divided section, and the optical frequency in each divided section,
In an optical fiber communication system in which the average value of the product of the optical power and the third-order nonlinear coefficient is set so as to be substantially inversely proportional to the length of each divided section, the phase conjugate light generator (1 ')
To (10 ').

(12 ') A first optical fiber for transmitting a signal light, and a phase conjugate light for receiving the signal light supplied from the first optical fiber and generating a phase conjugate light corresponding to the signal light A second optical fiber that receives the phase conjugate light supplied from the phase conjugate light generator and transmits the phase conjugate light; and a total dispersion value of the first and second optical fibers. In the optical fiber communication system in which is set to be substantially equal, the configurations described in the above (1 ′) to (10 ′) are used as the phase conjugate light generator.

(13 ') In (12') above, the product of the nonlinear coefficient, the average optical power, and the length of the optical fiber in the first and second optical fibers is set to be substantially equal.

(14 ′) In the above (11 ′), the dispersion of the value of the sign opposite to the dispersion of these optical fibers is determined during, before or after either or both of the first and second optical fibers. One or more dispersion compensators to be provided are inserted.

(15 ') In the above (11') to (14 '), the loss of the first or second optical fiber is compensated by the relay optical amplifier. (16 ') In the above (11') to (15 '), the first
First and second terminal stations are respectively disposed on the upstream side of the optical fiber and on the downstream side of the second optical fiber, the first terminal station having a first transmitter and a second receiver, and Terminal 2 has the first
And a second transmitter. The first signal light output from the first transmitter is transmitted through a first optical fiber to a first signal fiber.
After being transmitted through the second optical fiber,
The second signal light received by the receiver and output from the second transmitter is converted into the second phase conjugate light after transmission through the second optical fiber, and the second signal light is transmitted through the first optical fiber. Receive at the receiver.

(17 ') In the above (1') to (16 '), the signal light is a wavelength multiplexed signal of a signal having optical carriers of different wavelengths. (18 ') In the above (11') to (17 '), the first optical fiber and the phase conjugator are arranged at the transmitting terminal, and the second optical fiber has an optical amplifier at an interval shorter than the nonlinear length. Relay transmission.

(19 ') In the above (11') to (18 '), the second optical fiber and the phase conjugator are arranged at the receiving terminal. Relay transmission at short intervals.

(20 ') In the above (17'), a plurality of signal lights having different wavelengths are used as the signal lights, and the wavelength-division multiplexed signal of the plurality of signal lights has a different dispersion value for each channel. 11 '), the signal light transmitted through the plurality of first optical fibers and converted into phase conjugate light for each channel and then combined, or the signal light combined and converted into phase conjugate light and converted into the second In the system for transmitting the optical fiber, a dispersion compensator is inserted into one or both of the plurality of first optical fibers and the second optical fibers.

(21 ') In the above (20'), the wavelength-division multiplexed signal light is transmitted through a plurality of first optical fibers, multiplexed, branched, and multiplexed by a plurality of phase conjugators and optical filters. One or more channels of the phase conjugate light are extracted, and the extracted signal is transmitted through a second optical fiber attached to each of these phase conjugate devices and the optical filter.

(22 ') In the above (21'), a first optical fiber common to a plurality of channels is transmitted and then branched, and a plurality of phase conjugators and optical filters are used to multiplex these wavelength-multiplexed phase conjugate lights. The signal of one or more of the channels is extracted, and the extracted signal is transmitted through a second optical fiber associated with each of the phase conjugator and the optical filter.

(23 ') In (22'), separate optical fibers are transmitted for each channel before inputting to the common first optical fiber. (24 ') In the above (11') to (23 '), the first
Alternatively, the signal power or the wavelength input to the second optical fiber is adjusted so that the reception state is optimized.

[0091]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Now, consider a case where an optical pulse propagates through a dispersion medium. When an unchirped pulse passes through the dispersion medium, the normal dispersion medium (∂ ² β / ∂
When ω ² > 0), the pulse shifts to the low frequency side at the rise of the pulse, and shifts to the high frequency side at the fall. In the case of an anomalous dispersion medium (∂ ² β / ∂ω ² <0), it shifts to the high frequency side at the rising of the pulse,
It shifts to the lower frequency side at the fall. Where β
Represents a propagation constant, and ω represents an angular frequency of light. In a normal dispersion medium, the group velocity increases as the wavelength increases, and in the abnormal dispersion medium, the group velocity increases as the wavelength decreases, so that the pulse width increases in any case.

On the other hand, when the light intensity is high, the refractive index changes by Δn (t) = n ₂ | E (t) | ² due to the optical Kerr effect. Here, n ₂ is an amount called a nonlinear refractive index. In the case of a normal silica fiber, the value is, for example, about 3.2 × 10 ⁻²⁰ m ² / W. When an optical pulse undergoes the optical Kerr effect in a nonlinear medium, Δω (t) = − ∂Δφ (t) / ∂t = − (2πn ₂ / λ) (∂ | E (t) | ² / ∂t) Δz Only the spectrum spreads (chirps). Here, Δz is the interaction length.

This phenomenon is generally caused by self-phase modulation (Self-p modulation).
ise modulation (SPM). Due to this SPM, the light pulse shifts to the low frequency side at the rise, and shifts to the high frequency side at the fall. Due to the chirping due to this SPM, the effect of dispersion becomes more pronounced and consequently the pulse distortion becomes more pronounced.
Therefore, when the optical pulse is subjected to the optical Kerr effect in the dispersion medium, the pulse is further diffused in the case of the normal dispersion medium than in the case of the dispersion alone, but pulse compression occurs in the case of the abnormal dispersion medium.

Therefore, considering the effect of chromatic dispersion described above, a large pulse spread occurs in the case of a normal dispersion medium, and in the case of an abnormal dispersion medium, pulse spread due to chromatic dispersion and pulse compression due to SPM are large. The effect of one appears. An optical soliton balances these two effects.

In general, it is considered that it is more convenient to apply pulse compression by SPM in an anomalous dispersion medium to maintain a higher signal-to-noise ratio (S / N). With the ability to transmit at the level of optical power and the development of dispersion-shifted fibers that have enabled relatively small chromatic dispersion values, it has become impossible to say that pulse compression should generally be added. Was.

That is, the pulse compression effect becomes too large, and a large waveform distortion occurs. In particular, in the case of an NRZ pulse, since pulse compression occurs intensively at the rising and falling portions of the pulse, a sharp waveform change or, in extreme cases, the falling portion overtakes the rising portion and the pulse becomes 3 pulses. Sometimes things break apart. Further, in the case of long-distance optical amplification multi-relay transmission, there is a problem that four-wave mixing occurs between the signal light as excitation light and the spontaneous emission light of the optical amplifier, and the S / N is significantly reduced (modulation instability). Qualitative; modulation instability).

The distortion of the optical pulse caused by the chromatic dispersion and the non-linearity as described above can be compensated by applying the phase conjugate optics. For example, the signal light beam transmitted by the first optical fiber transmission line is converted into a phase conjugate light beam by the phase conjugate light generator, and the phase conjugate light beam is transmitted by the second optical fiber transmission line. First
By setting parameters relating to chromatic dispersion and nonlinearity in the second optical fiber appropriately, it is possible to obtain an optical pulse having substantially no distortion at the output end of the second optical fiber.

However, the conversion efficiency of a signal light beam into a phase conjugate light beam in a phase conjugate light generator generally depends on the polarization state of the signal light beam. There is no need for a phase conjugate light generator.

To construct a phase conjugate light generator whose conversion efficiency has no polarization dependence, a polarization scrambling method, a polarization diversity method, or a polarization active control method can be applied. Furthermore, polarization maintaining fiber (Polarization Maintai
The use of an optical fiber transmission line made of ning fiber (PMF) can also eliminate the polarization dependence of the conversion efficiency in the phase conjugate light generator. In the present invention, a polarization diversity method is employed to eliminate the polarization dependence of the conversion efficiency.

FIG. 1 is a diagram showing a first method according to the present invention. First, in the polarization separation process, the signal light beam E _S is separated into two polarization components E _S1 and E _S2 .
The polarization components E _S1 and E _S2 have polarization planes orthogonal to each other.

Next, in the conversion process, the polarization components E _S1 and E _S2 are converted into the corresponding phase conjugate light beams E _C1 and E _C2 , respectively. The polarization planes of the phase conjugate light beams E _C1 and E _C2 coincide with the polarization planes of the polarization components E _S1 and E _S2 , respectively.

In the polarization combining process, the phase conjugate light beams E _C1 and E _C2 are polarized and combined to obtain a phase conjugate light beam E _C. According to the invention, in the process of conversion one or two distributed feedback (DFB)
A laser diode is used.

When one DFB laser diode is used in the conversion process, the polarization components E _S1 and E _S2 are supplied to the first excitation end and the second excitation end of the DFB laser diode, respectively. The phase conjugate light beams E _C1 and E _C2 are output from the end and the first excitation end, respectively. In this case, the processes of polarization separation and polarization synthesis can be performed by a common polarization beam splitter.
Here, the term "excitation end" is used to mean the end face of the active layer of the DFB laser diode.

When two DFB laser diodes are used in the conversion process, these DFB laser diodes convert the polarization component E _S1 to the phase conjugate light beam E _C1 and the polarization component E _S2 , respectively. It is used for conversion into a phase conjugate light beam E _C2 . In this case, the processes of polarization separation and polarization synthesis can be performed by different polarization beam splitters.

Preferably, a current is injected into the DFB laser diode so that the DFB laser diode generates a pump light having a wavelength different from the wavelength of the signal light beam E _S , whereby four-wave mixing in the DFB laser diode is performed. Phase conjugate light beams E _C1 and E _C2 are generated.

According to the method of FIG. 1, the polarization components E _S1 and E _S1
Because the phase conjugate light beam E _c based on both the _S2 is obtained, the conversion efficiency is less likely to depend on the polarization state of the signal light beam E _s. That is, the polarization dependence is reduced or eliminated.

FIG. 2 is a diagram showing a phase conjugate light generator by nondegenerate four-wave mixing applicable to the present invention. Distributed feedback (DFB)
An optical fiber 2 is optically connected to a first excitation end of a laser diode 1 via a lens 3, and an optical filter is connected to a second excitation end via a lens 6 and an optical fiber 4. 10 is optically connected. DF
A drive current is supplied from the drive circuit 7 to the B laser diode 1.

The DFB laser diode 1 has a structure as shown in FIGS. 3 and 4, for example. 3, the upper surface of the n-InP substrate 11 has n-InGaAsP
A guide layer 12 is formed, and a diffraction grating 13 having a waveform whose film thickness changes periodically in the light traveling direction is formed on a joint surface between them. As shown in FIG. 4, the diffraction grating 13 has a period λ / 4 (λ:
It has a phase shift structure shifted by the wavelength of light in the waveguide structure).

On the guide layer 12, an undoped multiple quantum well (MQW) active layer 14 is formed.
4 has a p-InGaAsP buffer layer 15 and p-
The InP layer 16 is formed in this order.

The MQW active layer 14 is a 7 nm-thick In _{x ′}
Ga _{1-x ′} As (x ′ = 0.532) well layer and thickness 10 n
m of _{Ga x In 1-x As y} P 1-y (x = 0.283, y
= 0.611) The barrier layer and the barrier layer are alternately laminated by five layers.

From the p-InP layer 16 to the n-InP substrate 11
Is patterned in a convex shape up to the upper portion, and its planar shape is a stripe shape extending in the light traveling direction. A p-InP layer 17 and an n-InP layer 18 are formed in this order on both sides of the stripe-shaped protrusions of the n-InP substrate 11, and the uppermost p-InP layer 16 and n-
A p-InGaAsP layer 19 is formed on the InP layer 18.

The n-side electrode 2 is provided on the lower surface of the n-InP substrate 11.
0 is formed, and three p-side electrodes 21a, 21b, and 21c are formed on the p-InGaAsP layer 19.

Both ends of the DFB laser diode 1 (first
And the second excitation end) are coated with an anti-reflection film 22 for transmitting at least phase conjugate light.
The cavity length of the DFB laser diode 1 is, for example, 900 μm.
m, the length of the central p-side electrode 21b is, for example, about 580 μm.
m and the lengths of the p-side electrodes 21a and 21c near both ends are, for example, about 160 μm, respectively.

The operation of the phase conjugate light generator will be described. First, the p-side electrode 21 of the DFB laser diode 1
a, 21b and 21c through the MQW active layer 14
By supplying a drive current to the side electrode 20, light having a wavelength of 1549 nm is continuously oscillated at an output of 40 mW in the MQW active layer 14. In this case, the electrodes 21a, 21b and 21
A current of, for example, 400 mA is passed through c.

The light oscillating in the DFB laser diode 1 has a narrow and stable spectrum due to a single laser mode and a narrow gain bandwidth. Therefore, light oscillated by the DFB laser diode 1 is used as pump light for four-wave mixing.

In FIG. 2, the optical fiber 2 and the lens 3
When the probe light is supplied to the first excitation end of the DFB laser diode 1 through the lens, light having some spectral peaks is emitted from the second excitation end to the lens 6 and the optical fiber 4.
Is output via. When the spectrum of the output light was examined by an optical spectrum analyzer (not shown), the result shown in FIG. 5 was obtained.

In FIG. 5, the wavelength of the pump light is 1549.
In addition to a spectral peak at a wavelength of 1569 nm and a wavelength of the probe light of 1569 nm, a spectral peak also exists at a wavelength of 1529 nm, and this spectral peak corresponds to phase conjugate light. When the angular frequency of the probe light is ω _s , the angular frequency of the pump light is ω _P , and the angular frequency of the phase conjugate light is ω _C , the following equation holds.

Ω _C = 2ω _P −ω _S As described above, by generating the phase conjugate light by the non-degenerate four-wave mixing, the optical frequency conversion from the probe light (signal light) to the phase conjugate light, that is, the wavelength conversion is performed. It turns out that it becomes possible. In this wavelength conversion process, when the probe light is modulated by the main signal, the modulation is also preserved in the phase conjugate light, so this kind of wavelength conversion function constructs a network as described later. Above is very useful.

As described above, when the pump light is generated inside the DFB laser diode 1, a mechanism for coupling the probe light and the pump light becomes unnecessary, and the structure of the phase conjugate light generator is simplified. Therefore, the size of the optical communication device in which the generator is incorporated can be reduced.

Further, since the pump light is generated in the DFB laser diode 1, it is not necessary to consider the attenuation of the pump light intensity caused by passing through the optical fiber when the pump light is to be input from the outside. The conversion efficiency from the probe light to the phase conjugate light can be increased by the strong pump light. Note that the intensity of the obtained phase conjugate light is proportional to the square of the intensity of the pump light.

Furthermore, although the DFB laser diode 1 has a single oscillation mode, the wavelength can be freely changed. As a method of changing the wavelength, there is a method of changing the distribution of the current supplied to the active layer 14. This will be described specifically.

The three p-side electrodes 21a, 21b and 21c
It is known that a single oscillation mode of the DFB laser diode 1 shifts when the magnitude of the current flowing through the DFB laser diode 1 is shifted (Y. KOTAKI et al., OFC'90, THURSDAY MORNING,
159).

For example, if the current injected to the p-side electrodes 21a and 21c near both ends of the DFB laser diode 1 is kept constant and the current injected to the central p-side electrode 21b is increased, the oscillation wavelength becomes longer. Shift to the wavelength side. The adjustment of the current flowing through each of the p-side electrodes 21a, 21b, 21c is performed by the drive circuit 7.

Therefore, when a DFB laser diode 1 having a plurality of p-side electrodes 21a, 21b and 21c as shown in FIG. 4 and having antireflection films 22 formed on both end surfaces is used, the wavelength of the pump light is reduced. Since the wavelength can be freely changed, the wavelength of the phase conjugate light can be freely changed accordingly. Thus, the wavelength conversion of the optical signal of each channel can be performed in the wavelength division multiplexing optical communication by using the above-described phase conjugate light generator.

In the above-described example, the DFB laser diode 1 is constituted by the InP / InGaAsP layer structure. However, an InP / InAlGaAs layer structure or the like may be employed. Further, a material based on a GaAs substrate may be used.

Since the phase conjugate light generated by the DFB laser diode 1 is output together with the probe light and the pump light, if it is desired to extract only the phase conjugate light, an optical filter is provided outside the output end of the DFB laser diode 1. 10 may be arranged. The optical filter 10 is a DF in FIG.
Between the B laser diode 1 and the lens 6 or the lens 6
And the optical fiber 4.

Next, an experiment of generating phase conjugate light using the above-mentioned DFB laser diode will be described. Λ / 4 phase shift DF with AR coating (anti-reflection coating) on both ends
A wavelength conversion experiment was performed using a B laser diode (resonator length 900 μm) and a module having a single mode fiber (SMF) coupled before and after the laser diode. This module was oscillated with an element output of 40 mW (wavelength λ _{p of} pump light = 1550 nm), a signal light having a wavelength λ _S was input from the front end face, and the spectrum of light output from the rear end face was observed.

FIG. 6 shows how the conversion efficiency changes with respect to the detuning frequency Δf between the pump light and the signal light. Δf = 1
-8.7d at 25 GHz (wavelength difference 1.0 nm)
B, -2 even when Δf = 2.5 THz (20 nm)
A conversion efficiency of 3 dB was obtained. Conversion with high efficiency up to the THz region is possible, and application to wavelength conversion of wavelength division multiplexed signal light or the like can be expected. Further, almost no band limitation due to the Fabry-Perot mode was observed by the AR coating.

Next, in order to confirm that the converted light is phase conjugate light, a dispersion compensation experiment in short pulse transmission was attempted. RZ signal pulse of about 23 ps width generated using a two-stage LiNbO ₃ modulator (λ _S = 1552 nm)
To a first 50 km long SMF (dispersion: +18.1 ps)
/ Nm / km), the wavelength is converted by a DFB laser diode into light of λ _C = 1548 nm, and the converted light is converted into a second SMF (+17.8 ps / 51 km) having a length of 51 km.
(nm / km).

FIGS. 7A and 7B show pulse shapes after transmission and transmission of 101 km, respectively. The pulse shape of the converted light is reproduced with respect to the transmitted signal light (FIG. 7B), and it can be seen that the converted light satisfies the phase conjugate relationship with the signal light. For comparison, FIG. 7C shows a pulse shape in the case of transmitting 101 km without using the phase conjugate light generator. The pulse shape is significantly distorted due to chromatic dispersion and optical Kerr effect.

From the above experimental results, it was found that the phase conjugate light generator using the DFB laser diode provided 50 Gb / s
It can be seen that the waveform distortion of a considerable high-speed optical signal (pulse) can be compensated.

As described above, when the pump light is generated in the DFB laser diode, a mechanism for coupling the probe light and the pump light becomes unnecessary, and the structure of the phase conjugate light generator is simplified. Therefore, the size of the optical communication device in which the phase conjugate light generator is incorporated can be reduced.

FIG. 8 shows a first embodiment of the phase conjugate light generator according to FIG.
It is a figure showing an embodiment. A first polarization beam splitter (PBS) 32 is used to split the signal light beam E _S into a first polarization component E _S1 and a second polarization component E _S2 . The first polarization component E _S1 is supplied to a first DFB laser diode 1 (# 1) driven to generate a pump light E _P1, and a first phase conjugate light beam E _S1 is supplied.
_C1 is output from the DFB laser diode (# 1).
The polarization component E _S1 , the pump light E _P1, and the phase conjugate light beam E _C1
Have the same polarization plane.

For the second polarization component E _S2 , the second DFB
The laser diode 1 (# 2) is used. The DFB laser diode 1 (# 2) is driven to generate the second pump light E _P2 . Here, the driving circuits and the like for each of the DFB laser diodes 1 (# 1 and # 2) and the like are omitted (the same applies hereinafter), and the pump lights E _P1 and E _P1.
It is assumed that the polarization plane of _P2 is parallel.

Polarization component E_S1And E_S2Polarization planes are
And the second polarization component E _S2Polarization plane of the second
Pump light E_P2And the polarization component E_S2Is the second
To be supplied to DFB laser diode 1 (# 2)
す る wavelength plate (λ / 2) 34 is used to
I have.

The half-wave plate 34 is operatively connected between the polarization beam splitter 32 and the DFB laser diode 1 (# 2). In this application, operative connection between one element and another element includes the case where these elements are directly connected, and furthermore, the transfer of an electric signal or an optical signal between these elements. This includes cases where these elements are provided with as much relevance as possible.

A second phase conjugate light beam E _C2 is output from the DFB laser diode 1 (# 2). A second polarization beam splitter (PBS) 38 is used to polarization combine the first and second phase conjugate light beams E _C1 and E _C2 to obtain one phase conjugate light beam E _C. . Here, since the polarization beam splitter 38 is provided corresponding to the polarization beam splitter 32, the first phase conjugate light beam E _C1 from the DFB laser diode 1 (# 1) is used for the half-wave plate 36. After the polarization plane is rotated by 90 °, it is supplied to the polarization beam splitter 38.

From the signal light beam E _S to the phase conjugate light beam E
_In order to completely eliminate the polarization dependence of the conversion efficiency into _C , the DFB laser diodes 1 (# 1 and # 2) having the same characteristics are used, and the polarization beam splitter 32 and the polarization beam splitter 38 are used. DF up to
The lengths of the optical paths including the B laser diodes 1 (# 1 and # 2) may be made equal, but the present invention is not limited to this.

DFB laser diode 1 (# 1 and # 1)
The identity of the characteristic of 2) is, for example, that the pump lights _EP1 and _EP2
And the driving conditions such that the power and the wavelength of the light are substantially equal. For this purpose, for example, the position of the λ / 4 phase shift in the active layer 14 shown in FIG. 4 is appropriately set, or the distribution of the current supplied to the active layer 14 is adjusted.

As described above, when the position of the λ / 4 phase shift is substantially at the midpoint of the active layer 14 and a symmetrical structure is obtained, the driving current I _C (injection to the electrode 21b) is obtained. ) Is set equal to the drive current I _S (the current injected into the electrodes 21a and 21c), it is possible to equalize the conversion efficiency in generating bidirectional phase conjugate light as described later.

The power or wavelength of the pump light E _P1 and E _P2 may be adjusted by adjusting the temperature of the DFB laser diode 1 (# 1 and # 2). In this embodiment,
Although the half-wave plates 34 and 36 are used to rotate the polarization plane by 90 °, the polarization plane may be rotated by 90 ° by another structure.

For example, one polarization preserving fiber (PMF) is used in place of the half-wave plate 34, and the PMF at one end is rotated by 90 ° with respect to the main axis at the other end.
Can be twisted. Alternatively, two PMFs connected in series may be used instead of the half-wave plate 34, and these may be connected such that their main axes are orthogonal at these connection points.

In order to reduce the polarization dispersion in the case of using the PMF, the former connection mode is more preferable. This method of rotating the plane of polarization using the PMF is applicable to all embodiments of the present invention.

The first embodiment shown in FIG. 8 has a symmetrical structure with respect to the input port of the signal light beam E _{S and} the output port of the phase conjugate light beam E _C. When this phase conjugate light generator is applied to an optical communication system, a phase conjugate light beam can be generated for both the upstream channel and the downstream channel, and the conversion efficiency does not depend on the polarization state.

FIG. 9 shows a second embodiment of the phase conjugate light generator according to FIG.
It is a figure showing an embodiment. In this embodiment, a waveguide substrate 4 is used instead of the polarization beam splitters 32 and 38 in FIG.
Polarization beam splitters 32 'and 3 formed on
8 'is used.

The polarization beam splitters 32 'and 38'
Is provided, for example, by a waveguide structure formed on a LiNbO ₃ substrate. In this case, the elements functioning as the half-wave plates 34 and 36 are LiNbO ₃ optical waveguide and Si
This can be realized by a combination of an O ₂ film and the like.

DFB laser diode 1 (# 1 and # 1)
2) is accommodated in a groove formed on the waveguide substrate 40, for example. In this case, by using the half-wave plates 34 and 36, the DFB laser diode 1 (# 1 and # 2)
Since the active layers can be set in parallel with each other, manufacturing is easy.

By the way, due to the aforementioned symmetry or bidirectionality of the DFB laser diode, one DF
Polarization diversity can be implemented using a B laser diode. Specifically, it is as follows.

FIG. 10 is a diagram showing a third embodiment of the phase conjugate light generator according to FIG. Here, one DFB laser diode 1 having excitation ends 1A and 1B and one polarization beam splitter 4 for polarization separation and polarization synthesis are provided.
2 is formed.

The polarization beam splitter 42 has four ports 42A, 42B, 42C and 42D. Ports 42A and 42C and ports 42B and 42D are connected by a TE polarization plane, and ports 42A and 42B and ports 42C and 42D are connected by a TM polarization plane.

Here, the expressions “TE polarization plane” and “TM polarization plane” are used for convenience to express two polarization states that are relatively orthogonal. In the figure, T
The E polarization plane is parallel to the active layer of the DFB laser diode 1 and the plane of the paper, and the TM polarization plane is perpendicular to the plane of the paper.

The port 42C is a DFB laser diode 1.
Optically connected to the excitation end 1A of the
It is optically connected to the excitation end 1B of the DFB laser diode 1 via a two-wave plate 44. Also, port 42D
Are optically non-reflection terminated.

The optical circulator 46 is used to separate (drop) the obtained phase conjugate light beam E _C from the signal light beam E _S. The optical circulator 46 has three ports 46A, 46B and 46C.
The optical circulator 46 functions to output light input from the port 46A from the port 46B and output light input from the port 46B from the port 46C.

The port 46A is connected to the input port 48 to which the signal light beam E _S is supplied, the port 46B is connected to the port 42A of the polarization beam splitter 42, and the port 46C is connected to the phase conjugate light beam E _C. Are connected to the output port 50.

[0155] Port 48,46A and 46B the signal light beam supplied to the port 42A through E _S has a first polarization component E _S1 and TM polarization plane having a TE polarization plane by the polarization beam splitter 42 The polarization is separated into a second polarization component _ES2 .

The first polarization component E _S1 is supplied from port 42C to D
The FB laser diode 1 is supplied to the excitation end 1A,
Polarization component E _S2 in is supplied to the excitation end 1B of the DFB laser diode 1 through the half-wave plate 44 from the port 42B.

When the second polarization component E _S2 passes through the half-wave plate 44, its polarization plane is shifted from the TM polarization plane by T.
Converted to E polarization plane. Therefore, the first and second polarization components E _S1 and E _S2 supplied to the DFB laser diode 1
Have both TE polarization planes.

The pump light generated in the DFB laser diode 1 mainly has a TE polarization plane. The pump light includes a first pump light component E _P1 in the direction from the excitation end 1A to the excitation end 1B and the excitation end. The second pump light component E _P2 extends in the direction from 1B to the excitation end 1A.

The first polarization component E supplied to the excitation end 1A
Fourth-wave mixing based on _S1 and the first pump light component E _P1 generates a first phase conjugate light beam E _C1 having a TE polarization plane in the DFB laser diode 1, and the phase conjugate light beam E _C1 is excited. The beam is supplied from the end 1B to the port 42B of the polarization beam splitter 42 through the half-wave plate 44. Therefore, the phase conjugate light beam E _C1 has a TM polarization plane at the port 42B.

At the excitation end 1B of the DFB laser diode 1,
The supplied second polarization component E_S2And the second pump light component E
_P2DFB laser diode by four-wave mixing based on
In the mode 1, the second phase conjugate light beam E_C2Occurs
And the phase conjugate light beam E _C2Has a TE polarization plane
From the excitation end 1A to the port 4 of the polarization beam splitter 42
2C.

The phase conjugate light beams E _C1 and E _C2 supplied to the polarization beam splitter 42 are polarization-combined to form a phase conjugate light beam E _C , and this phase conjugate light beam E _C is supplied to the ports 42A, 46B, 46C and The data is output through the 50 in this order.

In this embodiment, since one DFB laser diode 1 having the above-described characteristics is used, it is easy to make the conversion efficiencies for the polarization components E _S1 and E _S2 coincide. Match conversion efficiency, for example, it can be easily performed by setting the DFB operating conditions of the laser diode 1 as mentioned above, thereby, the signal light beam phase conjugate light having a constant intensity regardless of the polarization state of E _S A beam E _C can be obtained.

Further, in this embodiment, since the clockwise optical path length and the counterclockwise optical path length in the optical loop are the same, the polarization combination of the phase conjugate light beams E _C1 and E _C2 is performed with the timing. Can be done with This allows
Accurate operation of the phase conjugate light generator is ensured.

As the polarization beam splitter 42, one using a polarization separation film such as a dielectric multilayer film, a bulk type using a crystal such as calcite, a fiber type or the like can be used.

An optical filter may be connected to the output port 50 in order to extract only the phase conjugate light beam E _C. The optical loop in this embodiment can be provided by spatial coupling using a lens system or coupling by an optical fiber or optical waveguide. In particular, when an optical fiber is used, the use of a polarization maintaining fiber (PMF) or the additional use of a polarization controller is employed to maintain the polarization state. When using PMF, as described above, 1
This is convenient because the / 2 wavelength plate can be omitted.

FIG. 11 is a diagram showing an arrangement for a demonstration experiment of the embodiment of FIG. The signal light beam E _S given as linearly polarized supplied to the optical circulator 46, the polarization plane to rotate in the range between 0 ° to 180 °, rotatable polarizer 52 was used. The polarization beam splitter 42 and the excitation end 1A of the DFB laser diode 1 were connected by a polarization plane preserving fiber (PMF) 54, and the polarization beam splitter 42 and the excitation end 1B of the DFB laser diode 1 were connected by a PMF 56.

In order for the first polarization component E _S1 having the TE polarization plane to be supplied to the excitation end 1A of the DFB laser diode 1 as it is in the polarized state, the directions of the main axes at both ends of the PMF 54 must be one. I do. In contrast, P
In the MF 56, main axes at both ends of the PMF 56 are orthogonal to each other in order to achieve the function without using the half-wave plate 44. As a result, the second polarization component E _S2 having the TM polarization plane output from the polarization beam splitter 42 is transmitted from the excitation ends 1B to D with the TE polarization plane.
Input to FB laser diode 1.

Referring to FIG. 12, there is shown data obtained in the experiment of FIG. 12, the vertical axis represents the conversion efficiency η _C (dB), and the horizontal axis represents the polarization angle θ (deg).
Is represented. The conversion efficiency η _C is given by η _C = P _C / P _S when the power of the signal light beam E _S is P _S and the power of the phase conjugate light beam E _C is P _C. The polarization angle θ is defined by the angle between the polarization plane of the signal light beam E _S input as linear polarization and the TE polarization plane.

In the case of the prior art in which signal light is incident on one DFB laser diode in only one direction, as indicated by reference numeral 58, the conversion efficiency η _C largely fluctuates according to the polarization angle θ (proportional to cos ² θ). ), Θ = 90 ° did not convert at all. On the other hand, according to the third embodiment of FIG. 10, the conversion efficiency η _C is suppressed to a change smaller than 0.4 dB with respect to the change of the polarization angle θ, and a sufficient characteristic in the system design is obtained. It was confirmed that it was obtained.

For additional details regarding the experiments of FIGS. 11 and 12, see Electronics Letters, Vol. 33, No. 4, pp. 3
16-317, 1997. FIG. 13 is a diagram showing a fourth embodiment of the phase conjugate light generator according to FIG. FIG.
0 in the third embodiment, the one-channel signal light beam E _S
Against whereas is generating a phase conjugate light beam E _C of one channel, in this embodiment, two-channel signal light beam E _S10 and the phase conjugate light beam E of 2 channels for E _S20 of _C10 And E _C20 are generated.

Although the functions of the third embodiment shown in FIG. 10 are included in this embodiment, the beams E _S , E _S1 , and E _S1 shown in FIG.
The notations E _S2 , E _C1 , E _C2 and E _C are respectively
_{E S10, E S11, E S12} , has been changed to E _C11, E _C12 and E _C10.

The port 42D of the polarization beam splitter 42
Are connected to the optical circulator 62 without non-reflection termination. The optical circulator 62 has ports 62A and 62B.
And 62C.

The optical circulator 62 outputs the light input from the port 62A from the port 62B, and outputs the light input from the port 62B from the port 62C. Port 62
A is the input port 6 of the signal light beam E _S20 of the second channel.
4, the port 62B is connected to the port 42D of the polarization beam splitter 42, and the port 62B is connected to the port 62D.
C is connected to the output port 66 of the phase conjugate light beam E _C20 of the second channel.

Note that the signal light beam E in the second channel
Regarding the conversion from _S20 to the phase conjugate light beam E _C20 ,
Since it can be easily understood according to the conversion from the signal light beam E _S10 to the phase conjugate light beam E _C10 in the first channel, the description is omitted. Here, it is assumed that the DFB laser diode 1 also generates pump light having a TM polarization plane.

In a general DFB laser diode, high generation efficiency may not necessarily be obtained for both the pump light having the TE polarization plane and the pump light having the TM polarization plane. In such a case, two DFs
B laser diodes are preferably used in cascade connection. Specifically, it is as follows.

Referring to FIG. 33, there is shown a modification of the phase conjugate light generator of FIG. Here, another DFB laser diode 1 'is provided between the DFB laser diode 1 and the polarization beam splitter 42, and the DFB laser diodes 1 and 1' are cascaded. DFB laser diode 1 is mainly T
Pump light having an E polarization plane is generated, and the DFB laser diode 1 'mainly generates pump light having a TM polarization plane.

According to this embodiment, the signal light beam E
_The DFB laser diode 1 mainly contributes to the conversion from _S10 to the phase conjugate light beam E _C10 , and the signal light beam E _C10 .
_The DFB laser diode 1 'mainly contributes to the conversion from _S20 to the phase conjugate light beam E _C20 .

The principle of generation and wavelength conversion of the phase conjugate light in the DFB laser diodes 1 and 1 'can be easily understood according to the embodiments described above, and the description thereof will be omitted.

In the configuration of FIG. 33, the DFB laser diodes 1 and 1 'are included in the optical loop including the polarization beam splitter 42 and the half-wave plate 44.
A phase conjugate light generator may be constructed by taking out only the cascade-connected DFB laser diodes 1 and 1 '. That is, in this case, the DFB laser diode 1
Generates pump light having a TE polarization plane, and the DFB laser diode 1 'generates pump light having a TM polarization plane. Therefore, one of the cascade-connected DFB laser diodes 1 and 1' is used. , A converted phase conjugate light beam is output from the other, and the conversion efficiency in that case does not depend on the polarization state of the input signal light beam. Also, cascade-connected DFB laser diodes 1 and 1 '
Has bidirectionality, so that when the phase conjugate light generator is applied to a bidirectional transmission system, it is possible to eliminate the polarization dependence of the conversion efficiency for both bidirectional channels.

FIGS. 14A and 14B are views for explaining the second method according to the present invention. (A) of FIG.
As shown in FIG. 7, when phase conjugate light is generated by non-degenerate four-wave mixing using a third-order nonlinear optical medium (χ ⁽³⁾ ) 68 such as an optical fiber and a semiconductor optical amplifier, the angular frequency ω _S The signal light and the pump light having the angular frequency ω _P (ω _P ≠ ω _S ) are input to the nonlinear optical medium 68 via the optical coupler 70 on the same optical path. The reason why the optical coupler 70 is used is that when the signal light and the pump light are output from different light sources, they are supplied to the nonlinear optical medium 68 along the same optical path to enable their interaction.

Based on the four-wave mixing of the signal light and the pump light in the nonlinear optical medium 68, the angular frequency 2ω _P −ω _S
Is generated from the nonlinear optical medium 68 together with the signal light and the pump light.

The term "non-degenerate" is used to mean that the wavelength (angular frequency) of the signal light is different from the wavelength (angular frequency) of the pump light. Since the wavelength of the signal light, the wavelength of the pump light, and the wavelength of the phase conjugate light satisfy the above-described relationship, the wavelength conversion is performed simultaneously with the generation of the phase conjugate light. Therefore, in this application, except for the title of the invention and the technical field to which the invention pertains, the term “generation of phase conjugate light” means the meanings of phase conjugate conversion and wavelength conversion from probe light (signal light) to phase conjugate light. It should be understood as a concept including.

As shown in FIG. 14B, the DFB
When the laser diode 1 is used as a non-linear optical medium, pump light is generated in the DFB laser diode 1 by injecting a current into the DFB laser diode 1. Therefore, by supplying only the signal light from the outside to the DFB laser diode 1, phase conjugate light is generated, and the signal light, the pump light, and the phase conjugate light are output from the DFB laser diode 1. The effect of using such a DFB laser diode 1 as a nonlinear optical medium is as described above.

It should be noted here that the DFB laser diode 1 does not have a Fabry-Perot mode, so that not only can external signal light be input, but also signal light,
The point is that pump light and phase conjugate light can be extracted. That is, by optically cascading the DFB laser diode 1 and the nonlinear optical medium 68, the power of the phase conjugate light can be increased.

In the second method according to the present invention, first, a current is injected into the DFB laser diode 1 so that the DFB laser diode 1 generates pump light. Next, signal light is supplied to the DFB laser diode 1,
Phase conjugate light is generated by four-wave mixing based on the signal light and the pump in the DFB laser diode 1.

Then, the signal light, pump light and phase conjugate light output from the DFB laser diode 1 are supplied to the nonlinear optical medium 68, and the power of the phase conjugate light is increased by four-wave mixing in the nonlinear optical medium 68. .

When the second method according to the present invention is carried out, signal light, pump light and phase conjugate light are output from the DFB laser diode 1 on the same optical path. The optical coupler 70 as shown in FIG. As a result, DFB
It is easy to maintain high-power pump light in the laser diode 1 and the nonlinear optical medium 68, and the conversion efficiency from signal light to phase conjugate light can be increased.

The second method according to the present invention can be implemented in combination with the first method according to the present invention. For example, in the arrangement for the demonstration experiment shown in FIG. 11, polarization maintaining fibers (PMFs) 54 and 56 are connected to the two excitation ends 1A and 1B of the DFB laser diode 1, respectively.

In general, since the optical fiber has the property of a third-order nonlinear optical medium, the fiber 54 shown in FIG.
And 56, the phase conjugate light beams E _C2 and E _C1 are amplified in the fibers 54 and 56, respectively, and as a result, the phase conjugate light beam E _C obtained by polarization combining is obtained. Power can be increased. Specifically, it is as follows.

The third-order nonlinear effect (specifically, the value of γ) is increased.
To obtain a non-linear refractive index n_TwoMode to increase
What is necessary is just to reduce a feed diameter (MFD). Nonlinear refraction
Rate n _TwoIs to increase the cladding
Add GeO to the core_TwoThere is a method to add
You. By such a method, the nonlinear refractive index n_TwoAnd the value of
5 × 10^-20m^Two/ W or larger value
You.

On the other hand, it is possible to reduce the MFD by designing the non-refractive index difference between the core and the clad or designing the shape of the core (similar to DCF). These techniques, a large value exceeding 15W ^-1 miles ^-1 as gamma value is obtained (in the usual ^{DSF γ ≒ 2.6W -1 km -1)} . It is also possible to use a fiber having such a large γ value as a zero dispersion fiber.

Since the conversion efficiency is proportional to the square of γPL,
In the fiber specially designed as described above (special fiber), the length is 2.6 / 15 (≒ 1) in order to generate a third-order nonlinear effect similar to that of a normal DSF.
/5.8). For example, normal DSF
If a length of about 20 km is required to generate a third-order nonlinear effect using
A similar effect can be obtained with a length of about 4 km. Actually, since the loss is reduced by the shortening, the length of the special fiber can be further reduced. Also,
In such a special fiber, an extremely wide conversion band is expected to be realized because the precision of the control of the zero dispersion wavelength is improved. Furthermore, if the fiber length is several km (for example, 6 km), the polarization plane preserving (polarization preserving) ability is ensured. Therefore, application of such a special fiber to the present invention has high conversion efficiency and polarization. It is extremely useful in obtaining independent conversion efficiency.

In the present invention, in order to make the conversion efficiencies in the two directions within the DFB laser diode equal, the conversion in each of the DFB laser diode and the optical fiber provided as a third-order nonlinear optical medium provided therewith is required. Efficiencies may be equal, or the total amount of conversion by the DFB laser diode and each fiber may be equal.

As described above, the first and second embodiments of the present invention
By combining the methods described above, it is possible to generate phase conjugate light having high conversion efficiency without polarization dependence of conversion efficiency.

FIG. 15 is a diagram showing a first embodiment of the phase conjugate light generator according to (a) and (b) of FIG. Here, as the nonlinear optical medium 68, a semiconductor optical amplifier (SO
A) 70 is used.

The DFB laser diode 1 has a pump light E _P
And the signal light E _S is
It is supplied to the laser diode 1. Phase conjugate light E _C is generated by four-wave mixing based on the signal light E _S and the pump light E _P in the DFB laser diode 1.

The signal light E is output from the DFB laser diode 1.
_S, Pump light E_PAnd phase conjugate light E _CIs output and this
Are supplied to the SOA 70. Four in SOA70
Phase conjugate light E by light wave mixing_CPower is increased,
The phase conjugate light E_CIs output from the SOA 70.

Referring to FIG. 16, there is shown a second preferred embodiment of the phase conjugate light generator according to (a) and (b) of FIG. Here, an optical fiber 72 is used as the nonlinear optical medium 68. As the optical fiber 72, a single mode fiber is desirable, and in order to increase the conversion efficiency, it is effective to make the zero dispersion wavelength of the optical fiber 72 substantially coincide with the wavelength of the pump light. For example, when the wavelength of the pump light is in the 1.5 μm band, the zero-dispersion wavelength of the optical fiber 72 and the wavelength of the pump light can be matched by using a dispersion-shifted fiber (DSF) as the optical fiber 72. it can.

The signal light E _S supplied to the optical fiber 72,
The power of the pump light E _P or the phase conjugate light E _C is changed by stimulated Brillouin scattering (Stimulated) of the optical fiber 72.
(Brillouin Scattering: SBS)
When the threshold value is exceeded, the conversion efficiency decreases and the power of the obtained phase conjugate light decreases.

[0200] In order to suppress the effect of SBS may be performed frequency modulation or phase modulation on pumping beam E _P or the signal light E _S. Therefore, in this embodiment, the DF
The modulation circuit 74 is connected to the B laser diode 1. A modulation speed of several hundred KHz or less is sufficient, and when the signal speed of the signal light is Gb / s or more, deterioration of transmission quality due to the modulation does not matter.

The modulation circuit 74 superimposes a low-frequency signal corresponding to the modulation speed on the current supplied to one of the electrodes 21a, 21b and 21c shown in FIG. 4, for example. FIG.
DFB laser diode 1 has a high frequency modulation efficiency, so that the influence of SBS can be easily suppressed (S. Ogita, Y. Kotaki, M. Matsuda, Y. Kuwa).
hara, H. Onaka, H. Miyata, and H. Ishikawa, “FM r
esponse of narrow-linewidth, multielectrode λ / 4 s
hift DFB laser, ”IEEE Photon. Technol. Lett., Vo
l.2, pp. 165-166, 1990.).

Hereinafter, an application example of the present invention to an optical communication system will be described. The application example shown in FIG. 17 is a system that enables compensation for waveform distortion due to chromatic dispersion of a transmission optical fiber and nonlinear optical Kerr effect. This application is described in the above-mentioned applications by the present inventor (Japanese Patent Application Nos. 6-509844, 7-44574, 7-304229, 7-98464 and 7-301830). )
, But will be described below.

Output signal light E from the transmitter (TX)_SThe
1 optical fiber F1 (length L₁, Dispersion D₁, Nonlinear coefficient
γ₁), And enters the phase conjugate light generator (PC).
Power. Phase conjugate light E on PC_CInto a second optical fiber.
Iva F2 (Length L_Two, Dispersion D _Two, The nonlinear coefficient γ_Two)
Optical transmission to a transmitter (RX). At the receiver, this phase
The signal light is received by the photodetector to detect the signal. The transmission signal
Modulation methods include optical amplitude (intensity), frequency, phase, etc.
The phase conjugate light is used for signal detection.
Optical detection and optical heterodyne after extraction with a bandpass filter
Detection can be considered.

The optical fiber used here is a single mode silica fiber (SMF) in many cases, and a 1.3 μm zero-dispersion optical fiber or a 1.55 μm dispersion shift fiber generally used in optical communication. (DSF) is a typical example. Further, the signal light may be a wavelength multiplexed signal composed of a plurality of light sources having different wavelengths.

In order to compensate for chromatic dispersion in an optical fiber and waveform distortion due to self-phase modulation (Self Phase Modulation) in the system shown in FIG. 17, the dispersion of the corresponding portion across the PC and the magnitude of the non-linear effect Should be the same.
Here, the corresponding portions refer to two portions where the dispersion or the cumulative value of the optical Kerr effect measured from the PC becomes equal. In other words, when the transmission path is divided, P
Regarding C, the variance and the magnitude of the nonlinear effect may be the same in each of the divided sections at the target positions in the above sense.
This means that the variance value in each divided section should be the same, and that D ₁ / γ ₁ P ₁ = D ₂ / γ ₂ P ₂ (1a) holds in each section. Is shown. Here, P ₁ and P ₂ are the optical power in each part, and γ _j = ωn _2j / cA _effj (2a) represents the nonlinear coefficient of the optical Kerr effect in the optical fiber. Here, ω represents the optical angular frequency, c represents the speed of light in a vacuum, and n _2j and A _effj represent the nonlinear refractive index and the effective core area of the optical fiber j (j = 1, 2), respectively.

In order to compensate for the decrease due to the loss of the nonlinear effect along the transmission line, the dispersion may be reduced or the optical Kerr effect may be increased. Varying the value of dispersion is possible and promising depending on the design of the optical fiber. For example, Dispersion Shifted Fiber
Currently, it is actively performed by changing the zero-dispersion wavelength of the optical fiber, and by changing the relative refractive index difference and the core diameter between the core and the clad of the optical fiber. On the other hand, it is possible to change the optical Kerr effect by changing the nonlinear refractive index or the light intensity.

A dispersion compensating fiber (Dispersion Compensati)
on Fiber), the dispersion is gradually decreased in proportion to the change of the optical Kerr effect in the longitudinal direction.
A high-speed and long-distance transmission is made possible by configuring the system with a transmission path using DCF) and DSF with normal dispersion.

In long-distance transmission using an optical amplifier,
It has been found that the use of a normal dispersion fiber is good for reducing nonlinear distortion (modulation instability) due to noise light of an optical amplifier, and this configuration is promising.

In addition to the above strict compensation method, when the change of the optical Kerr effect is not so large (for example, when the relay interval of the optical amplifier is sufficiently short compared to the nonlinear length), the following average power is used. An approximation holds.

D ₁ 'L ₁ = D ₂ ' L ₂ (3a) γ ₁ P ₁ 'L ₁ = γ ₂ P ₂ ' L ₂ (4a) Here, P ₁ 'and P ₂ ' are respectively Optical fiber Fj
(J = 1, 2), and D ₁ ′,
D ₂ ′ is the average dispersion of the optical fiber Fj.

Further, the ideal waveform compensation condition expression (1a)
Is not satisfied, but it is also possible to appropriately arrange a dispersion compensator by arranging the dispersion of the opposite sign in the transmission path. This method is particularly effective in long-distance transmission such as undersea transmission. The reason is as follows.

That is, in the compensation using the PC, it is necessary to make the waveform distortion in the optical fiber before and after the PC the same.
Therefore, the waveform is most distorted before and after the PC. Therefore, at the position of the PC, the spectrum of the light pulse is in the widest state. on the other hand,
Noise is added from the PC and the optical amplifier on the transmission line,
The S / N degradation due to this noise increases as the spectrum increases. Therefore, designing the system so that the spectrum spread before and after the PC is small is very effective in extending the transmission distance.

In this sense, it is effective to reduce the total dispersion value of the transmission line by dispersion compensation in the middle of the transmission line. Next, FIG. 18 shows an example in which the present invention is applied to a wavelength division multiplexing (WDM) transmission system.

N-channel wavelength multiplexed signal light E _S1 ,
E _S2 , ..., E _SN (frequency: ω _S1 , ω _S2 , ..., ω _SN )
After transmitted by the optical fiber F1, the wavelength-multiplexed phase conjugate light E _C1, E _C2 of N-channel by PC, ..., E _{CN (Frequency: ω C1, ω C2, ...} , ω CN) is converted into optical fiber F2
And then receive it.

In the dispersion compensation by the PC, since the sign of the dispersion needs to be the same before and after the PC, the arrangement is as shown in FIG. 19 with respect to the zero dispersion. The zero-dispersion wavelengths of the optical fibers F1 and F2 are ω ₁₀ and ω ₂₀ , respectively. In the case of the figure, the normal variance is converted to the normal variance. In this case, since the secondary dispersion (dispersion gradient) exists in the ordinary transmission line, the first channel (ch.
The absolute value of the dispersion for the optical fiber F2 is the smallest while the absolute value of the dispersion for the 1) is the smallest. Therefore, it is impossible to perform complete dispersion compensation on all channels at the same time.

In order to compensate all channels equally ideally, as shown in FIG. 20, signal light is transmitted for each channel through separate optical fibers F11, F12,..., Optical fiber F1N. when the power _{(P 11, P 12, ...} , P 1N) commensurate with different dispersion transmit at. .., PC-N for all channels by one PC.
To convert them into phase conjugate light, and convert them into a common optical fiber F2.
To transmit and receive. At this time, the dispersion and the nonlinear effect of each channel are compensated by the method described above.

FIG. 21 is a diagram showing a third applied example of the present invention. , TX-N output signal lights E _S1 , E _S2 ,..., E _SN having different wavelengths (optical frequencies) from each other. The angular frequencies of these signal lights are ω _S1 , ω _{S2 ,.}
It is.

These signal lights are transmitted by a plurality of first optical fibers F11,..., F1N, added and branched by an optical multi / demultiplexer comprising a star coupler or the like.

The branched signal lights are supplied to phase conjugate light generators PC-1,..., PC-M, respectively. The phase conjugate light generators PC-1,..., PC-M generate phase conjugate light corresponding to at least one of the supplied signal lights.
The generated phase conjugate lights are respectively applied to optical filters OF1,.
After passing through the OFM, a plurality of second optical fibers F21,
,..., F2M, the optical receivers RX-1,.
X-M.

Transmitted by a plurality of second optical fibers
Phase conjugate light is E ′_C1, E '_C2, ..., E '_CN)
Have been. First optical fiber F1j (j = 1,..., N)
Are of length L_1JAnd the variance is D_1j, The nonlinear coefficient is γ
_1jAnd the power of each signal light is P_1jAnd Ma
In addition, that of the second optical fiber F2k (k = 1,..., M)
The length of each is L_2k, Dispersion is D _2k, The nonlinear coefficient is γ_2kIn
And the power of each phase conjugate light is P_2kAnd

At this time, each parameter is set so that the following two conditions are satisfied. D _1j L _1j = D _2k L _2k = (constant) γ _1j P _1j / D _1j = γ _2k P _2k / D _2k = (constant) The term “constant” here means an arbitrary section in each fiber. Is constant.

Here, the compensation for the waveform distortion by each second optical fiber F2k is set so as to be optimized for the phase conjugate light passing through the band of the optical filter OFk. Further, the phase conjugate generator PC-k and the optical filter OFk
The channel E ′ _Ck extracted by the combination is a phase conjugate light of a plurality of channels included in the band of the optical filter near one arbitrary channel of the signal light or the vicinity thereof.

For example, when the transmitters TX-1,..., TX-N and the fibers F11,..., F1N are provided in the transmitting terminal, the dispersion or nonlinear effect in each fiber F1j is set equal. In this case, each receiver RX-
k, the phase conjugate light generator PC- with respect to the fiber F2k so that k can select a desired channel.
k and the combination of the functions of the optical filter OFk are controlled. Such control is performed by, for example, controlling the wavelength of the pump light in each phase conjugate light generator and / or controlling the center wavelength passing through each optical filter. for that purpose,
It is desirable to apply a tunable optical filter.

This system functions as, for example, a distribution system when the second optical fiber is used as a transmission line, and performs channel switching when the second optical fiber is in a receiving station or a repeater. (Cross Connect) Functions as a system.

FIG. 22 is a diagram showing a fourth applied example of the present invention. This system is characterized in that a common first optical fiber F1 is used for a plurality of optical transmitters TX-1,..., TX-N as compared with FIG.

With this change, the input end of the first optical fiber F1 is connected to each optical transmitter TX-j via an optical multiplexer.
And the output terminal is connected to each phase conjugate light generator PC-k via an optical demultiplexer.

The dispersion in the common first optical fiber F1 is made substantially constant for all channels. For example, as the first optical fiber F1, the above-described DD-DCF, dispersion-shifted fiber having large dispersion, and 1.
The above condition can be satisfied by using a 1.3 μm band zero dispersion fiber for a 55 μm band signal light and a 1.55 μm band zero dispersion fiber for a 1.3 μm band signal light.

With respect to such a common first optical fiber F1, when each second optical fiber F2k satisfies the condition of the present invention, an optimum reception state can be obtained for each channel.

FIG. 23 is a diagram showing a fifth applied example of the present invention. Here, as the first optical fiber, a combination of N optical fibers F11 ',..., F1N' having a relatively large dispersion and a common optical fiber F1 'having a relatively small dispersion is used.

The optical fibers F11 ',..., F1N' and the optical fiber F1 'are connected by an optical multiplexer, and the optical fiber F1' and each phase conjugate light generator PC-k are connected.
Are connected by an optical demultiplexer.

Also in this system, by satisfying predetermined conditions for the first optical fiber and the second optical fiber, it is possible to satisfactorily compensate waveform distortion for each channel and obtain an optimum reception state. Can be.

FIG. 24 shows a configuration example of a wavelength division multiplexing transmission system integrating these functions. After transmitting the wavelength-division multiplexed signal through the first optical fiber, the wavelength division multiplexed signal is split and converted into phase conjugate light having an optimum wavelength for each channel. These wavelength multiplexed phase conjugate signal lights are combined and transmitted to the receiver via the second optical fiber. With this configuration, it is possible to completely compensate for the waveform distortion of all the channels even when the transmission path has second-order dispersion.

Example of application of the present invention to a bidirectional optical transmission system
Is shown in FIG. Wavelength λ from TX-1 in the first terminal
_S1Signal light E_S1After transmission through the optical fiber F1, the DFB-L
Oscillation light (pump light) E in the same direction within D_P1Using
Wavelength λ _C1Phase conjugate light E_C1Into an optical fiber
2 and then received by RX-1 in the second terminal station.
You. On the other hand, the wavelength λ from TX-2 in the second terminal station_S2No faith
Light E_S2After transmission through the optical fiber F2, the DFB-LD
Oscillation light (pump light) E in the same direction_P2Using
Wavelength λ_C2Phase conjugate light E_C2Into an optical fiber F1
After transmission, the signal is received by RX-2 in the first terminal station.

At this time, the optical fiber F1 and the optical fiber
The wavelength of the signal light transmitted by F2 is used for each transmission path.
It is desirable to be within the transmission band of the bandpass filter. Immediately
C, λ _S1And λ_C2, And λ_C1And λ_S2Are in the same transmission band
It will be set as follows. Of course, each signal light at this time
May be a wavelength multiplexed signal light.

Next, an example of realizing a lightwave network using a phase conjugate light generator will be described. FIG. 26 is a diagram for explaining the principle of the lightwave network. An optical transmitter (OS) 202 outputs a signal beam.

The first optical fiber 204 has a first end 204A and a second end 204B corresponding to an input end and an output end of a signal beam, respectively. A first phase conjugate light generator (1st PC) 206 is operatively connected to the second end 204B.

The first phase conjugate light generator 206 converts the signal beam supplied from the first optical fiber 204 into a first phase conjugate beam and outputs it. Second optical fiber 2
08 has a third end 208A and a fourth end 208B corresponding to the input end and the output end of the first phase conjugate beam, respectively. A second phase conjugate light generator (2nd PC) 210 is operatively connected to the fourth end 208B.

The second phase conjugate light generator 210 converts the first phase conjugate beam supplied from the second optical fiber 208 into a second phase conjugate beam and outputs it. The third optical fiber 212 has a fifth end 212A and a sixth end 2 corresponding to an input end and an output end of the second phase conjugate beam, respectively.
12B.

In order to receive the second phase conjugate beam transmitted by the third optical fiber 212, an optical receiver (O
R) 214 are provided. Second optical fiber 208
Is set in the middle of the process. The system waypoint 216 is defined later.

The second optical fiber 208 is connected to the third end 208
A first part 281 between A and system waypoint 216
And a second portion 282 between the system midpoint 216 and the fourth end 208B.

In the present invention, each parameter in the optical fibers 204, 208 and 212 is set as follows. First, the first optical fiber 204 is virtually divided into N (N is an integer greater than 1) sections 204 (# 1,..., #N), and the first portion 2 of the second optical fiber 208 is
81 is virtually divided into the same number of sections 281 (# 1,..., #N). At this time, the first phase conjugate light generator 20
The product of the average value and the section length of the chromatic dispersion of two corresponding sections counted from 6 is made to substantially match. That is,
I-th (1 ≦ i ≦ N) -th section 204 of the first optical fiber 204 counted from the first phase conjugate light generator 206
The average value and the section length of the chromatic dispersion (or dispersion parameter) of (#i) are D _1i and L _1i , respectively, and are transmitted from the first phase conjugate light generator 206 in the first portion 281 of the second optical fiber 208. I-th section 281 (#i)
When the average value and the section length of the chromatic dispersion (or dispersion parameter) of D are defined as D _2i and L _2i , respectively, D _1i L _1i = D _2i L _2i (1) is satisfied.

Further, the average value of the optical power and the average value of the nonlinear coefficient in the section 204 (#i) are P _1i and γ _1i , respectively, and the average value of the optical power and the average value of the nonlinear coefficient in the section 281 (#i) _{Are defined} as P _2i and γ _2i , respectively, P _1i γ _1i L _1i = P _2i γ _2i L _2i (2) is satisfied.

On the other hand, the second portion 282 of the second optical fiber 208 has M (M is an integer greater than 1) sections 282.
(# 1,..., #M), and the third optical fiber 212 is virtually divided into the same number of sections 212 (# 1,..., #M).

At this time, the second optical fiber 208
In the part 282, the average value and the section length of the chromatic dispersion of the j (1 ≦ j ≦ M) -th section 282 (#j) counted from the second phase conjugate light generator 210 are D _3j and L _3j , respectively. When the average value and the section length of the chromatic dispersion of the j-th section 212 (#j) counted from the second phase conjugate light generator 210 in the third optical fiber 212 are D _4j and L _4j , respectively: _{3j L 3j = D 4j L 4j} ... (3) are satisfied. Further, the average value of the optical power and the average value of the nonlinear coefficient in the section 282 (#j) are P _3j and γ _3j , respectively, and the average value of the optical power and the average value of the nonlinear coefficient in the section 212 (#j) are P _{When 4j} and γ _4j are satisfied, P _3j γ _3j L _3j = P _4j γ _4j L _4j (4) is satisfied.

In FIG. 26, although the waveform distortion once increases before and after the first phase conjugate generator 206, the chromatic dispersion and nonlinearity at the system midpoint 216 depend on the conditions of equations (1) and (2). Is compensated, and the waveform returns to the original state once. The recovered waveform is again distorted before and after the second phase conjugate generator 210, but the chromatic dispersion and the nonlinearity are compensated in the optical receiver 214 by the conditions of the equations (3) and (4). , The waveform is restored again.

The configuration shown in FIG. 26 is tolerant of errors in setting parameters such as the length of the second optical fiber 208 which may be laid on the sea floor or the like. That is,
Even if the waveform does not completely return to the original state at the system midpoint 216, this imperfection is eliminated by the second part 28.
2. By reproducing the signal with the second phase conjugate light generator 210 and the third optical fiber 212, it is possible to completely restore the waveform in the optical receiver 214.

Referring to FIG. 27, the principle of compensation for chromatic dispersion and nonlinearity is shown. This compensation principle is shown in FIG.
The same applies to other cases. Here, the optical transmitter 20
The principle of compensation from 2 to the system midpoint 216 will be described. First, prior to the description of FIG. 27, general items of the phase conjugate wave will be described.

An optical signal E (x,
y, z, t) = F (x, y) φ (z, t) exp [i
(Ωt−kz)] can be generally described by the following nonlinear wave equation. Here, F (x, y) represents a lateral mode distribution, φ (z, t) represents a complex envelope of light, and φ (z, t) changes sufficiently slowly as compared with the frequency ω of light. Suppose that.

[0249]

(Equation 1)

Here, T = t−β ₁ z (β ₁ is a propagation constant), α is the loss of the fiber, β ₂ is the chromatic dispersion of the fiber,

[0251]

(Equation 2)

Represents a third-order nonlinear coefficient (coefficient of optical Kerr effect). Here, n ₂ and A _eff represent the nonlinear refractive index and effective core area of the fiber, respectively. c is the speed of light in a vacuum. Here, the first order dispersion is considered, and higher order dispersions are omitted. Α, β, and γ are functions of z, and are represented as α (z), β (z), and γ (z), respectively. Further, the position of the phase conjugate light generator is set to the origin (z = 0). Here, the following normalization function is introduced.

[0253]

(Equation 3)

Here,

[0255]

(Equation 4)

Represents the amplitude. When α (z)> 0, the transmission path has a loss, and when α (z) <0, it has a gain. A (z) ≡A (0) represents the case without loss. A (z) ² = P (z) corresponds to the optical power. By substituting equations (7) and (8) into equation (5), the following evolution equation is obtained.

[0257]

(Equation 5)

Here, the following conversion is performed.

[0259]

(Equation 6)

As a result, equation (9) can be converted as follows.

[0261]

(Equation 7)

Here, sgn [β ₂ ] ≡ ± 1 is β ₂ >
0, that is, +1 in the case of normal variance, and -1 in the case of β ₂ <0, that is, variance of ≧ ₂ . If the equation (11) holds, the complex conjugate holds, and the following equation is obtained.

[0263]

(Equation 8)

The complex conjugate light u ^* obeys the same evolution equation as that for u. However, the propagation direction at that time is reversed. This operation is a correct operation of the phase conjugator. Particularly in a transmission type phase conjugator, the above is equivalent to inverting the phase shift due to chromatic dispersion and SPM.

Here, in FIG. 27, the length of the first optical fiber 204 is L ₁ and the length of the second optical fiber 204 is L _1.
It is assumed that the length of the first portion 281 of L 8 is L ₂ . In addition, the phase conjugate light generator 206 is arranged at the origin z = 0 (ζ = 0) of the z-axis coordinate and the ζ coordinate. System midpoint 21
The z coordinate and ζ coordinate of 6 are L ₂ and ζ ₀ , respectively.

In the first optical fiber 204, the signal beam u (Es) propagates according to the development equation (11). The signal beam u is converted into a phase conjugate beam u ^* (Ec) by the phase conjugate light generator 206. The phase conjugate beam u ^* propagates in the first portion 281 of the second optical fiber 208 according to the equation (12).

At this time, the phase conjugate light generator 206 on the ζ axis
(11) within a normalized distance dζ at any two points − に, あ る symmetrical with respect to the position (の = 0)
If the values of the respective parameters are set so that the coefficients of the first and second terms on the right side of the equation become equal, u ^* in な る becomes a phase conjugate wave of u in −ζ. That is, the following two equations are conditions.

[0268]

(Equation 9)

Expression (13) indicates that the dispersion signs of the first optical fiber 204 and the first portion 281 must be equal. Considering that γ> 0 and A (z) ² > 0 in the fiber, the above conditions can be summarized as follows.

[0270]

(Equation 10)

The sign of the phase shift due to chromatic dispersion and SPM in (−ζ) in the first optical fiber 204 is inverted by the phase conjugate light generator 206. Therefore, the waveform distortion due to the phase shift is compensated by the distortion due to the phase shift in (ζ) in the first portion 281. By repeating the above-described compensation for each section, compensation can be performed over the entire length.

Next, the above compensation conditions will be described by z coordinates. From equation (15),

[0273]

[Equation 11]

Is obtained. That is, the condition is that the ratio of the chromatic dispersion to the product of the nonlinear coefficient and the optical power in each section is made equal. Here, −z ₁ and z ₂ are two points that satisfy the following equation.

[0275]

(Equation 12)

From equations (16) and (17), (18) and (1)
9) is obtained.

[0277]

(Equation 13)

Dz ₁ and dz ₂ are the lengths of the small sections at −z ₁ and z ₂ , respectively. Each section length is inversely proportional to the variance in the section or the product of the nonlinear coefficient and the optical power. Here, considering the relationship between the variance β ₂ and the dispersion parameter D, D = − (2πc / λ ² ) β ₂ , (1
The following relationships are obtained from equations 8) and (19). D is a function of z and is also represented as D (z).

[0279]

[Equation 14]

Regarding the dispersion and the non-linearity, it is understood that the condition for compensation is that the increment in one of the two positions symmetric with respect to the phase conjugate light generator 206 is equal to the decrease in the other.

Equations (20) and (21) are necessary conditions for compensation, and show that the total amount of dispersion and the total amount of the optical Kerr effect are equal in two corresponding sections. That is,
The validity of the conditions of the expressions (1) to (4) was confirmed.

In particular, when α, D, and γ are constant and the fluctuation of the power is small, by integrating the equations (20) and (21), D ₁ L ₁ = D ₂ L ₂ (22) γ ₁ P ₁ L ₁ = γ ₂ P ₂ L ₂ (23) is obtained. Here, P ₁ and P ₂ are average powers in the first optical fiber 204 and the first portion 281 respectively. D ₁ and γ ₁ are the first optical fibers 20 respectively.
4, the average value of the dispersion parameter and the average value of the nonlinear coefficient.
D ₂ and γ ₂ are the average value of the dispersion parameter and the average value of the nonlinear coefficient of the first portion 281 respectively. (22),
Equation (23) matches the condition in the SPM compensation method by dispersion compensation and average value approximation.

Practically, the present invention can be implemented only by satisfying the condition of the expression (22). That is, according to the present invention, a first optical fiber having a first end and a second end respectively corresponding to an input end and an output end of a signal beam;
A phase conjugate light generator operatively connected to the second end for converting the signal beam into a phase conjugate beam and outputting the converted signal; a third end and a fourth end corresponding to an input end and an output end of the phase conjugate beam, respectively. A second optical fiber having an end,
An optical fiber communication system is provided, wherein the product of the average value and the length of the chromatic dispersion of the first optical fiber substantially matches the product of the average value and the length of the chromatic dispersion of the second optical fiber.

Preferably, in order to satisfy the condition of the expression (23), the average value of the optical power and the average value of the nonlinear coefficient in the first optical fiber and the product of the length of the first optical fiber are The average value of the optical power and the average value of the nonlinear coefficient in the second optical fiber and the product of the length of the second optical fiber substantially coincide with each other.

In the case where a plurality of optical amplifiers are provided on the optical path including the first and second optical fibers, the interval between two adjacent optical amplifiers is determined by the nonlinear length of the optical path (optical fiber). It is desirable to set shorter.

In FIG. 27, the system intermediate point 216
The principle of compensation upstream of is shown. The principle of compensation on the downstream side of the system intermediate point 216 can be understood in the same manner, and the description thereof is omitted.

In the description with reference to FIG. 27, the normalized coordinates are defined by the cumulative value of the chromatic dispersion from the phase conjugate light generator 206, as shown in equation (10). As a result, the required condition is, as shown by the equation (15), on the first optical fiber 204 and the first portion 281 having the same cumulative value of the chromatic dispersion from the phase conjugate light generator 206. The ratio between the product of the optical power and the nonlinear coefficient and the chromatic dispersion at each of the two points substantially coincides.

In FIG. 27, the phase conjugate light generator 20
The normalized coordinates may be defined by the cumulative value of the nonlinear effect from Step 6 (ie, the cumulative value of the product of the optical power and the nonlinear coefficient). In this case, the ratio between the chromatic dispersion and the product of the optical power and the nonlinear coefficient at each of the two points on the first optical fiber 204 and the first portion 281 having the same cumulative value from the phase conjugate light generator 206 is obtained. Are substantially the same.

As described above, the total amount of the dispersion of the optical fiber and the total amount of the optical Kerr (Kerr) effect between the first optical fiber and the second optical fiber connected to the phase conjugate light generator. Is set to be equal to
It can be seen that the optical pulse waveform input to the optical fiber and the optical pulse waveform output from the second optical fiber are compensated by the phase conjugate light generator so that they have substantially the same shape. That is, on the transmitting side of the optical pulse (the input end of the first optical fiber) and the receiving side of the optical pulse (the output end of the second optical fiber), optical pulse waveforms having substantially the same shape can be obtained. Optical ADM (Add
By providing a Drop Multiplexer (optical signal add / drop device), in the optical ADM, it is possible to receive a received optical pulse in almost the same state as a transmitted optical pulse. For this reason, in each ADM, it is possible to eliminate the need for the process of reproducing (waveform shaping / timing) of the received optical pulse without deteriorating the SNR of the received optical pulse, and to construct a highly flexible system. Become. A so-called lightwave network to which this principle is applied will be described below.

FIG. 28 is a diagram showing a ring lightwave network using a phase conjugate light generator. In the figure, each of nodes (Nodes) 1, 2, and 3 is an optical ADM, and includes an outer optical fiber ring (single mode optical fiber transmission line) and an inner optical fiber ring (similarly single mode optical fiber transmission line). It is connected to the.

A phase conjugate light generator (PC12, PC21, PC23, PC32, PC) is provided between the outer optical fiber ring and the inner optical fiber ring between the nodes 1, 2, and 3.
13, PC 31). Each PC or each node is provided at a position where the total amount of dispersion and the total amount of the optical Kerr effect of the input side optical fiber ring and the output side optical fiber ring are equal.

A case will be described in which the node 1 transmits a signal to the node 2 using a lightwave having a wavelength λ12, and the node 2 transmits a signal to the node 1 using a lightwave having a wavelength λ21.
The node 1 transmits the light wave of λ12 to the outer optical fiber ring 10
Send to 1.

The PC 12 generates a phase conjugate light λ′12 of the lightwave of λ12 received from the optical fiber ring 101. As the PC 12, it is preferable to use the above-mentioned DFB-LD.

The PC 12 outputs the phase conjugate light λ′12 of λ12.
Is input to the optical fiber ring 102 and transmitted to the node 2. At node 2, λ '
Twelve lightwaves are received and treated as an optical signal from node 1.

By providing the PC 12 at a position where the total amount of dispersion of the optical fiber ring 101 and the optical fiber ring 102 is equal to the total amount of the optical Kerr effect, the optical signal λ12 inserted into the outer optical fiber ring at the node 1 is provided. The phase conjugate light λ′12 having substantially the same waveform as the above can be branched from the outer optical fiber ring at the node 2. Therefore, the node 2 does not need to perform complicated waveform shaping and timing regeneration of the received optical signal.

When transmitting a signal from node 2 to node 1, an inner optical fiber ring is used. That is, node 2
When transmitting a signal from to the node 1 using a lightwave having a wavelength λ21, the lightwave having the wavelength λ21 is transmitted to the optical fiber ring 103.

The PC 21 generates a phase conjugate light λ′21 of the light wave λ21 received from the optical fiber ring 103,
The signal is transmitted to the optical fiber ring 104. The node 1 receives the lightwave λ'21 from the optical fiber ring 104, and receives the lightwave λ21 'as a transmission signal from the node 2.

By providing the PC 21 at a position where the total amount of dispersion of the optical fiber ring 103 and the optical fiber ring 104 is equal to the total amount of the optical Kerr effect, the optical signal λ 21 inserted into the inner optical fiber ring at the node 2 is provided. The phase conjugate light λ'21 having substantially the same waveform as the above can be branched from the inner optical fiber ring at the node 1.

Therefore, the node 1 does not need to perform complicated waveform shaping and timing reproduction of the received optical signal. The communication from the node 1 to the node 3 is performed using the wavelength λ13 via the inner optical fiber ring 106, and the communication from the node 3 to the node 1 is performed using the outer optical fiber ring 10.
5 and using a lightwave of wavelength λ31.

The PC 13 outputs the phase conjugate light λ′13 of λ13.
Is generated and input to the optical fiber ring 108. The node 3 receives the lightwave of λ'13 as an optical signal from the node 1.

The communication from the node 3 to the node 1 is as follows.
This is performed via the outer optical fiber ring 107. PC31
Generates a phase conjugate light λ′31 of λ31 and inputs it to the optical fiber ring 105. The node 1 receives the lightwave of λ'31 as an optical signal from the node 3.

Similarly, the communication from the node 2 to the node 3 is transmitted via the outer optical fiber ring 112 to the wavelength λ23.
The communication from the node 3 to the node 2 is performed using the lightwave of the wavelength λ32 via the inner optical fiber ring 109.

The PC 23 receives the phase conjugate light λ'23 of λ 23
Is generated and input to the optical fiber ring 110. The node 3 receives the lightwave of λ′23 as an optical signal from the node 2.

The PC 32 generates a phase conjugate light λ′32 of λ32 and inputs the conjugate light to the optical fiber ring 111. The node 2 receives the lightwave of λ'32 as an optical signal from the node 3.

In the ring lightwave network shown in FIG. 28, communication can be continued even when the optical fiber ring is disconnected. That is, when the optical fiber ring 101 is disconnected, communication from the node 1 to the node 2 can be continued by using a bypass composed of the inner optical fiber rings 106, 108, 109, and 111.

Here, "the optical fiber ring is broken"
This includes the case where the optical fiber ring is physically damaged and transmission becomes impossible, and the case where the optical fiber ring overflows the transmission capacity and becomes difficult to transmit.

[0307] In this case, the node 1 transmits a lightwave having the wavelength λ12 to the optical fiber ring 106. A PC 12 ′ is provided at the position of the PC 13, and the PC 12 ′ generates a phase conjugate light of λ ″ 12 from the light wave of λ 12,
The signal is transmitted to the optical fiber ring 108.

The node 3 transmits the light wave of λ ″ 12,
The signal is transmitted to the optical fiber ring 109. The PC 12 ″ is provided at the position of the PC 32, and the optical fiber ring 111 is provided.
The phase conjugate light of λ′12 is generated from the lightwave of λ ″ 12 received from the optical fiber ring 111 and transmitted to the optical fiber ring 111.

At the node 2, the lightwave of λ′12 received from the optical fiber ring 111 is received as an optical signal from the node 1. In the above case, the PC provided at the position of the PC 13
12 ′ and the PC 12 ″ provided at the position of the PC 32,
Since two-stage phase conjugate light generation is performed, two-stage wavelength conversion is performed.

Therefore, by appropriately selecting the pump light wavelength used in the PCs 12 ′ and 12 ″, the lightwave λ12 received from the node 1 and the lightwave λ′12 transmitted to the node 2 become P
It is preferable to make the wavelength of the input light wave λ12 and the wavelength of the phase conjugate light λ'12 at C12 equal.

By setting as described above, the node 1
In this case, the same light source can be used even in the event of a failure, and the same optical receiving system can be used in the node 2. Therefore, in the node 1, only one light source for the node 1 is prepared, and the connection to the optical fiber ring 101 or the connection to the optical fiber ring 106 is selected by an optical switch according to the failure situation. Just need.

In this case, at the node 2, the optical fiber ring 102 and the optical fiber ring 111 are simply connected to the same receiving system, respectively, and the lightwave λ '
12 can be received. Conversely, the node 1 always transmits the lightwave λ12 to the optical fiber ring 101 and the optical fiber ring 106, and the node 2 receives the lightwave λ'12 from the optical fiber ring 102 depending on the failure state. It is sufficient to select whether to receive the lightwave λ′12 from the optical fiber ring 111.

In the above, PC12 'and PC12 "
Can be provided at the same position as the PC 13 and the PC 32 because the PC 13 is provided at a position where the total amount of dispersion of the optical fiber ring 106 and the optical fiber ring 109 is equal to the total amount of the optical Kerr effect. Is provided at a position where the total amount of dispersion of the optical fiber ring 109 and the optical fiber ring 111 is equal to the total amount of the optical Kerr effect.

Incidentally, as a detour from the node 2 to the node 1, the optical fiber rings 112, 110, 107, 10
5 can be used, and as a detour from the node 2 to the node 3, optical fiber rings 103, 104,
The paths of 106 and 108 can be used, and the paths of the optical fiber rings 107, 105, 101 and 102 can be used as a detour from the node 3 to the node 2. The operation and the like are performed from node 1 to node 2 as described above.
It is the same as the case of the detour of communication to.

In the above example, the case where different optical paths are used as the inner optical fiber ring and the outer optical fiber ring has been described. However, the same optical path is used by using different wavelengths for the light waves λ12 and λ21. Can be used for bidirectional optical communication. In this case, since the outer optical fiber ring and the inner optical fiber ring are physically the same, the total amount of dispersion and the total amount of the optical Kerr effect are naturally equal. Can be provided.

FIG. 29 shows a specific configuration of node 1. In the figure, DMUX is an optical wavelength separation device,
It separates the optical waves into optically different wavelengths. The MUX is an optical wavelength multiplexing device for multiplexing light having optically different wavelengths and coupling the multiplexed light to one optical fiber. Nodes 2 and 3 can be similarly configured. When bidirectional optical transmission is performed using one optical fiber ring, the optical fiber ring 101 may be connected to the DMUX, and the optical fiber ring 105 may be connected to the MUX, as shown by the dotted line in the figure.

FIG. 30 shows a phase conjugate light generator PC12, P
3 shows a specific configuration of C21. PC12, PC
13 ′ and PC 32 ′, as described above, DFB-
It is preferable to use LD.

By using the DFB-LD, the phase conjugate light generator can be greatly reduced in size and simplified. Therefore, in optical wavelength division multiplexing communication, a phase conjugate light generator is provided for each wavelength as shown in FIG. , Wavelength conversion can be performed individually.

Therefore, it is not necessary to perform control for expanding the required band of the phase conjugate light generator. In FIG. 30, a band-pass optical filter that transmits only the wavelength of the phase conjugate light is provided to input only the phase conjugate light to the optical fiber ring (to remove the probe light and the pump light).

FIG. 31 shows still another configuration example of the lightwave network of FIG. In the figure, ○ indicates the same node as in FIG. 28 and has a function of inserting / branching a lightwave of a specific wavelength. In this case, the difference from FIG. 28 is that the phase conjugate light generator PC has an optical branching / switching function. FIG. 32 shows a specific configuration of the PC 121.
This will be described with reference to FIG.

Now, the PC 121 is a node (Node).
11 sub network (Sub-Network)
k) Consider a case where an optical signal is received from 1 and the optical signal is transmitted to the sub-network on the node 12 side. Node 11
Sub-network 1 is an optical wavelength demultiplexer DMUX
And separated into light waves λ11 to λ1j for each wavelength. When the lightwaves of λ11 to λ1i are used for optical communication of the sub-network 1, the respective lightwaves λ11 to λ1
i, and extracts only this phase conjugate light by an optical filter, and multiplexes them with an optical wavelength multiplexing device MU.
X is multiplexed and input to the node 12 side of the sub-network 1. When the lightwaves λ1m to λ1j are used for communication with a main network (Main-Network), the respective lightwaves λ1m to λ1j are used.
, And only the phase conjugate light is extracted by an optical filter, and these are multiplexed into an optical wavelength multiplexing device MUX.
, Multiplexed, and input to the optical fiber 130 of the main network. In this case, node 11 and PC1
, The optical fiber 131 between the PC 21 and the node 1
2, the total amount of dispersion and the total amount of the optical Kerr effect are set equal to each other, and further, the optical fiber 131 between the node 11 and the PC 121, and the optical fiber 131 between the PC 121 and the node 10 The total amount of dispersion and the total amount of the optical Kerr effect are set equal to the optical fiber 130.

In the node 10, the path of the light wave can be switched using, for example, an optical matrix switch described in Japanese Patent Publication No. 6-66982. By using this optical matrix switch, PC12
4, lightwave signals can be transmitted to any of PC125 and PC126.

In the embodiments shown in FIGS. 28 to 32, a phase conjugate light generator not using a DFB laser diode can be adopted. This type of phase conjugate light generator includes, for example, a nonlinear optical medium (for example, an optical fiber or a semiconductor optical amplifier) to which a signal light beam is supplied, a pump light source that outputs pump light, and a pump light that supplies pump light to the nonlinear optical medium. And an optical coupler for performing the operation. In the nonlinear optical medium, a phase conjugate light beam is generated by four-wave mixing based on the signal light beam and the pump light, and the phase conjugate light beam is output from the nonlinear optical medium.

FIGS. 34 (A), (B) and (C) show FIG.
FIG. 14 is a cross-sectional view showing a modification of the DFB laser diode of FIG. FIG. 34A shows a first end surface (cleavage surface) inclined with respect to a plane perpendicular to a bonding surface between the layers 12, 14 and 15, and a second end surface substantially perpendicular to the bonding surface. (Cleavage plane).
The first end face is supplied with a signal light beam, and the second end face outputs a signal light beam, pump light, and a phase conjugate light beam. In the configuration shown in FIG. 34A, light reflected from the first end face toward the inside of the DFB laser diode is in a leaky mode.
It is prevented from being guided into. As a result, a phase conjugate light beam can be generated stably. Therefore, the configuration shown in FIG.
It is suitable for a one-way type phase conjugate light generator as shown in 15 and 16.

FIGS. 34 (B) and 34 (C) show FIG.
9 shows a DFB laser diode suitable for a bidirectional phase conjugate light generator as shown at 0, 11, 13 and 33. Each of the DFB laser diodes shown in FIGS. 34B and 34C has first and second end faces (cleavage planes) inclined with respect to a plane perpendicular to the bonding plane.
The first and second end faces in FIG. 34B are substantially parallel to each other, and the first and second end faces in FIG. 34C are not parallel to each other. According to the configuration shown in FIG. 34 (B) or (C), the light reflected at each of the first and second end faces is in a leaky mode, so that the reflected light is guided into the waveguide layer 12. Is prevented. As a result, it is possible to stably generate phase conjugate light beams that propagate in opposite directions in the DFB laser diode.

To further suppress the influence of reflected light, FIG.
In the modification shown in FIG. 4, an antireflection film (reference numeral 2 in FIG. 4) is provided on one or both of the first and second end faces.
2 (see FIG. 2). With proper design of the anti-reflection coating, a reflectance of less than 0.1% can be obtained.

FIG. 35 is a diagram showing a first modification of the phase conjugate light generator shown in FIG. Here, an optical band stop filter 202 is additionally provided. Filter 2
02 is optically connected between the port 46C of the optical circulator 46 and the output port 50,
2 removes the pump light components E _P1 and E _P2 generated in the DFB laser diode 1.

FIG. 36 shows the wavelength characteristic of the transmittance of the optical band stop filter 202 shown in FIG. The filter 202 has a narrow stop band including the wavelengths λ _P of the pump light components E _P1 and E _P2 . That is, the transmittance in a region near the wavelength λ _P is substantially 0%, and the transmittance in other regions is substantially 100%. The wavelength characteristic as shown in FIG.
2 can be obtained by using a fiber grating.

When the refractive index of an optical medium (eg, glass) changes permanently due to light irradiation, the medium is said to be photosensitive. By using this property, a fiber grating can be manufactured in the core of the optical fiber. The characteristics of such a fiber grating are
Bragg reflection of light in a narrow band near the resonance wavelength determined by the grating pitch and the effective refractive index of the fiber mode. The fiber grating can be manufactured by, for example, irradiating an excimer laser oscillating at a wavelength of 248 nm or 193 nm using a phase mask (KO Hill, B. Malo,
F. Bilodeau, DC Johnson, and J. Albert, “Brag
g gratings fabricated in monomode photosensitive o
ptical fiber by UV exposure through a phase mas
k ”, Applied Physics Letters, Vol.62, No.10, pp.10
35-1037, March 8, 1993). Therefore, by optimizing the resonance wavelength of the fiber grating, the wavelength λ
A narrow element band of the optical band rejection filter including _P can be obtained.

In particular, when the stop band of the optical band stop filter 202 shown in FIG. 35 has a center wavelength substantially equal to the wavelength λ _P , the pump light components E _P1 and E _P2 generated in the DFB laser diode 1 are obtained. Is the filter 202
Effectively removed. Therefore, the pump light component E _P1
And E _P2 are not output from the output port 50. As described above, the influence of the pump light on the receiving station or the optical device disposed downstream of the optical transmission line is reduced, and the processing (extraction, amplification, etc.) of the phase conjugate light beam is facilitated.

FIG. 37 is a diagram showing a second modification of the phase conjugate light generator shown in FIG. Filter 2 in FIG.
02, two optical band stop filters 202 (# 1 and # 2) are provided. The filter 202 (# 1) is optically connected between the half-wave plate 44 and the excitation end 1B of the DFB laser diode 1, and the filter 202 (# 1)
(# 2) is optically connected between the excitation end 1A of the DFB laser diode 1 and the port 42C of the polarization beam splitter 42. Each of the filters 202 (# 1 and # 2) has a wavelength characteristic equivalent to that of the filter 202 shown in FIG. 35, that is, a wavelength characteristic as shown in FIG. Therefore, according to the embodiment shown in FIG.
Pump light components E _P1 and E _P2 generated in the B laser diode 1 are respectively filtered by filters 202 (# 1 and # 1).
2), and is not output from the port 50.

When the phase conjugate light generator includes a DFB laser diode, the effect of using the above-described optical band stop filter is enormous. Because, the pump light generated and output from the DFB laser diode is:
Since the power of the residual component of the pump light introduced from the outside into the nonlinear optical medium to generate the phase conjugate light is large, the influence of such high power pump light is This is because it easily occurs on the downstream side of the generator.

FIG. 38 shows a third example of the phase conjugate light generator of FIG.
It is a figure which shows the modification of. Here, an optical band stop filter 204 that is optically connected between the input port 48 and the port 46A of the optical circulator 46 is additionally provided. The filter 204 is made of, for example, a fiber grating. Filter 204 has a narrow stopband containing a predetermined wavelength. The predetermined wavelength is the wavelength λ of the phase conjugate light beams E _C1 and E _C2 generated by four-wave mixing in the DFB laser diode 1.
It is set to substantially match _c .

FIG. 39A shows the wavelength characteristic of the transmittance of the optical band stop filter 204 shown in FIG.
The transmittance in a region near the wavelength λ _c is substantially 0%, and the transmittance in other regions is substantially 100%.

FIG. 39B shows a power spectrum of light transmitted through the optical band rejection filter 204 shown in FIG. The input light beam supplied to the input port 48 has ASE (spontaneous emission light) noise and ASE at wavelength λ _s .
And a signal component (E _s ) superimposed on the noise. By the input light beam passes through the optical band stop filter 204, a portion of the ASE noise is eliminated in the vicinity of the wavelength lambda _c.

FIG. 39C shows a power spectrum of light output from the phase conjugate light generator shown in FIG. The signal light beam E _s (polarization components E _s1 and E _s2 ) and the pump light E in the DFB laser diode 1
_p a result of four-wave mixing based on (pump light components E _p1 and E _p2), a phase conjugate light beam E _c (E _c1 and E _c2) the wavelength λ
Occurs at _c . The resulting phase conjugate light beam provides a good signal-to-noise ratio (SNR) because the ASE noise has been previously removed near wavelength λ _c .

FIG. 40 is a diagram showing a modification of the phase conjugate light generator shown in FIG. DFB laser diode 1
An optical amplifier 206 optically connected between the optical amplifier and the nonlinear optical medium 68 is additionally provided. DFB laser diode 1 is driven so as to generate pump light E _p, the signal light beam E _s is supplied to the DFB laser diode 1. Phase conjugate light beam E _c is generated by four-wave mixing based on the signal light beam E _s and the pump light E _p in the DFB laser diode 1. The signal light beam E _s , the pump light _Ep and the phase conjugate light beam E _c output from the DFB laser diode 1 are amplified in the optical amplifier 206 and then supplied to the nonlinear optical medium 68.
In the nonlinear optical medium 68, the phase conjugate light beam E
_c is enhanced by four-wave mixing, and the nonlinear optical medium 68
Output from In particular, in this embodiment, the pump light E _p
Before being supplied to the nonlinear optical medium 68.
Because it is amplified by the non-linear effect is enhanced in the nonlinear optical medium 68, the power of the resulting phase conjugate light beam E _c is effectively increased.

For amplification in the wavelength band of 1.5 μm, an erbium-doped fiber amplifier (EDFA) is suitable as the optical amplifier 206. The nonlinear optical medium 68 is, for example, an optical fiber such as a semiconductor optical amplifier (SOA) or a dispersion shift fiber (DSF). Optical fibers have been used as the nonlinear optical medium 68, if the zero-dispersion wavelength is substantially matched to the wavelength of the pump light E _p,
Because it is easy to satisfy the phase matching condition, it is possible to increase the power of the resulting phase conjugate light beam E _c.

FIG. 41 is a diagram showing an embodiment of a polarization independent phase conjugate light generator according to the present invention. This phase conjugate light generator has cascaded DFB laser diodes 1 and 1 ', for example as taken out of the optical loop of FIG. DFB laser diode 1 is driven so as to generate pump light E _p having a TE polarization plane,
The DFB laser diode 1 'is driven to generate a pump light E _p ' having a TM polarization plane. The has a polarization plane DFB laser diode signal light beam E _s supplied to one corresponding to the TE and TM polarization planes, respectively 1
And the second signal component. The first signal component is DF
In B the laser diode 1 is converted through four-wave mixing process based on the first signal component and the pump light E _p in the first phase conjugate light component, the second signal component is transmitted through the DFB laser diode 1. The second signal component is then converted into a second phase conjugate light component through a four-wave mixing process based on the second signal component and the pump light E _p ′ in the DFB laser diode 1 ′, Is transmitted through the DFB laser diode 1 '. Phase conjugate light component in the first and second is output as the phase conjugate light beam E _c from the DFB laser diode 1 '. According to this embodiment, since both the first and second signal components are converted into a phase conjugate light beam, the polarization dependence of the conversion efficiency is reduced.

FIG. 42 is a diagram showing a modification of the phase conjugate light generator shown in FIG. DFB laser diode 1
And 1 'each have a different transmission for the TE and TM polarization modes, the degree of polarization dependence reduction may be worse. To address this possibility,
Here, a polarization dependent element 208 is provided between the DFB laser diodes 1 and 1 '. Element 208 has different losses or gains for TE and TM polarization modes, and element 208 is set or adjusted so that the polarization dependence of the conversion efficiency in this phase conjugate light generator is minimized. ing. As the element 208, for example, an optical amplifier or a polarizer can be used.

By the way, the first and second nonlinear optical media (for example, the DFB laser diode 1 and the optical fiber 7 shown in FIG.
When performed in a phase conjugate light generator including 2),
The conversion efficiency and the conversion band are determined by the sum of the nonlinear effects in the first and second nonlinear optical media. here,
The conversion band is defined by the maximum detuning wavelength (detuning frequency) of the pump light and the signal light under the condition that a phase conjugate light beam of a certain power is obtained. Generally, the optical fiber as the second nonlinear optical medium has a wider conversion band than the DFB laser diode or the semiconductor optical amplifier as the first nonlinear optical medium. Therefore, a combination of a DFB laser diode or a semiconductor optical amplifier as the first nonlinear optical medium and an optical fiber as the second nonlinear optical medium provides a phase conjugate light generator having high conversion efficiency and a wide conversion band.

A dispersion-shifted fiber (DSF) used for general purposes has a nonlinear coefficient γ of about 2.6 W ^−1.
It has a value of only about km ⁻¹ , and therefore, in order to obtain sufficient conversion efficiency, it is required that the fiber length be 10 km or more. Therefore, there is a demand for providing a DSF having a nonlinear coefficient γ large enough to shorten the fiber length. If the length of the DSF used as the second nonlinear optical medium can be reduced, the management of the zero-dispersion wavelength becomes easy, and therefore, the wavelength of the pump light is substantially reduced to the zero-dispersion wavelength of the DSF. Therefore, it is easy to achieve the same, and as a result, a wide conversion band can be obtained.

The non-linear coefficient γ is given by γ = ωn ₂ / cA _eff . Where ω is the optical frequency, n ₂ and A _eff
Is the nonlinear refractive index and effective core area of the optical fiber, respectively, and c is the speed of light. Therefore, in order to obtain a large nonlinear coefficient γ, it is effective to increase the nonlinear refractive index n ₂ or reduce the mode field diameter (MFD) of the DSF corresponding to the effective core area A _eff . The non-linear refractive index n ₂ can be increased by, for example, doping the cladding with fluorine or the like or doping the core with high concentration GeO ₂ . 25 to 3 GeO ₂ in the core
By doping 0 mol%, the nonlinear refractive index n ₂
As a result, a large value of 5 × 10 ⁻²⁰ m ² / W or more is obtained. The MFD can be reduced by designing the relative refractive index difference Δ or the shape of the core. The design of such a DSF is similar to that of a DCF (dispersion compensation fiber). For example, an MFD value smaller than 4 μm is obtained by doping the core with GeO ₂ at 25 to 30 mol% and setting the relative refractive index difference Δ to 2.5 to 3.0%. As a total effect of these, 15W ^-1 k
A large nonlinear coefficient γ value of m ⁻¹ or more is obtained.

Another important point is that the DSF that provides such a large value of the nonlinear coefficient γ should have a zero dispersion wavelength included in the pump band. Such agreement between the zero-dispersion wavelength and the pump band is possible by setting the fiber parameters (for example, the relative refractive index difference Δ and the MFD) as follows. In a normal optical fiber, when the relative refractive index difference Δ is increased under the condition that the MFD is kept constant, the dispersion value increases in the normal dispersion region. The above-described DD-DCF used for pre-compensation or post-compensation by the phase conjugate light generator is realized by such a principle. On the other hand, as the core diameter increases, the dispersion decreases, and conversely, as the core diameter decreases, the dispersion increases. Therefore, after setting the MFD to a certain value suitable for the pump band, the core diameter may be adjusted so that the zero dispersion wavelength matches a predetermined value of the pump light.

The conversion efficiency is γPL (P is optical power, L is D
Therefore, a DSF having a large non-linear coefficient γ is 2.6 / 1 as compared with a normal DSF because it is proportional to the square of the SF length).
The same conversion efficiency can be achieved with a length of about 5 ≒ 1 / 5.7. As described above, in order to obtain a sufficiently large conversion efficiency, a normal DSF requires a length of about 10 km, whereas an optical fiber having such a large nonlinear coefficient γ requires: Similar conversion efficiency can be obtained with a length of about 1 to 2 km. Actually, since the loss can be reduced by reducing the fiber length, the fiber length can be further reduced to obtain the same conversion efficiency. In such a short-length DSF, the controllability of the zero-dispersion wavelength is improved, so that the wavelength of the pump light can be easily controlled to substantially coincide with the zero-dispersion wavelength, and a wide conversion band can be obtained. Can be provided. Furthermore,
If the fiber length is several km, the polarization preserving ability is ensured. Therefore, such an application of the DSF to the present invention is effective in achieving high conversion efficiency and a wide conversion band and eliminating polarization dependence. Extremely effective.

In an apparatus having an optical fiber as a nonlinear optical medium for generating phase conjugate light, the conversion band is limited by the dispersion of the optical fiber. Therefore, the dispersion in the longitudinal direction of the optical fiber is completely controlled. For example, when there is only one zero-dispersion wavelength over the entire length (nonlinear length), the wavelength of the pump light is adjusted to the zero-dispersion wavelength, so that it is practically infinite. A large (unlimited within a range where the dispersion slope is linear) conversion band is obtained. Actually, the zero dispersion wavelength varies in the longitudinal direction of the optical fiber due to the manufacturing technology of the optical fiber, so that the phase matching condition deviates from the ideal state, thereby limiting the conversion band.

However, even in such a case, the optical fiber is cut and divided into a plurality of small sections, and sections having similar zero-dispersion wavelengths are connected to each other by a splice or the like (the initial state). An optical fiber suitable for providing a phase conjugate light generator having a wide conversion band can be obtained despite the same average dispersion over the entire length and in the order different from the order counted from the fiber end). .

Alternatively, a large number of fibers having a length (for example, several hundred meters or less) capable of controlling the dispersion with high precision necessary to obtain a sufficiently wide conversion band are prepared in advance.
A wide conversion band can be obtained by combining fibers having a required zero-dispersion wavelength to obtain a fiber having a length necessary for obtaining a required conversion efficiency and manufacturing a phase conjugate light generator using the fiber. .

When the conversion band is expanded in this manner, a portion having a small zero dispersion wavelength or a portion having a small dispersion of the zero dispersion wavelength is collected near the pump light input end of a fiber having a high pump light power. It is effective. If necessary, the conversion band can be further increased by appropriately increasing the number of divisions or by arranging the dispersion values positively and negatively alternately at locations far from the pump light input end where dispersion values are relatively large. Can be expanded.

As described above, according to one aspect of the present invention,
A method for manufacturing a device having an optical fiber as a nonlinear optical medium for generating phase conjugate light,
(A) cutting an optical fiber to divide it into a plurality of sections; and (b) rearranging and joining the plurality of sections so that a conversion band by non-degenerate four-wave mixing is expanded. Is provided.

Desirably, the dispersion value (eg, the dispersion value for the pump light) of each of the plurality of sections is measured, and the section having a relatively small dispersion value near the input end when the pump light is input to the optical fiber. Are arranged in such a manner that are arranged. Thereby, the phase matching condition can be effectively obtained in a portion where the power of the pump light is high, so that the conversion band is effectively expanded.

Desirably, at least a part of the plurality of sections is connected so that the variance values are alternately positive and negative. As a result, the average dispersion of each portion of the optical fiber can be suppressed to a small value, so that the conversion band can be effectively expanded.

According to another aspect of the present invention, there is provided a method for manufacturing a device having an optical fiber as a nonlinear optical medium for generating phase conjugate light, comprising the steps of: Dividing into sections, (b)
Measuring a variance value (for example, a variance value for pump light) of each of the plurality of sections; and (c) selecting only sections having a variance value sufficiently small to obtain a required conversion band by non-degenerate four-wave mixing. And stitching together.

Next, a method of expanding the conversion band using the nonlinear effect will be described. Assuming an optical fiber having a large nonlinear effect (including a case where the power P _{0 of the} pump light is large), the frequency difference between the signal light and the pump light is represented by Ω (≡ | ω _p −ω _c
| = | Ω _s −ω _p |), the amount of phase mismatch Δk in four-wave mixing is given by the following equation.

Δk = β ₂ Ω ² + 2γP ₀ where β ₂ is the dispersion value at the wavelength of the pump light.
In a normal fiber, since 2γP ₀ is sufficiently small, the condition for phase matching (Δk = 0) is given by β ₂ = 0. On the other hand, in the case of a fiber having a large nonlinear effect, the value of 2γP ₀ cannot be neglected, and the condition for phase matching changes. Since 2γP ₀ is always a positive value, the condition for phase matching is that β ₂ be a certain negative value. That is, the wavelength of the pump light may be arranged in the anomalous dispersion region. At this time, the detuning frequency Ω _{1 at which} the best phase matching is obtained is given by the following equation.

Ω ₁ = (2γP ₀ / | β ₂ |) ^1/2 Therefore, by appropriately adjusting the pump light with respect to γ and P ₀ , the conversion band can be extended to the vicinity of Ω ₁ .

FIGS. 43 and 44 show a phase conjugate light generator having a high conversion efficiency and a wide conversion band. Each of these phase conjugate light generators has a combination of a cascade-connected first nonlinear optical medium 68 (# 1) and second nonlinear optical medium 68 (# 2). FIG.
Shows a case where the first nonlinear optical medium 68 (# 1) includes a semiconductor optical amplifier (SOA) 70, and FIG.
Shows a case where the nonlinear optical medium 68 (# 1) includes the DFB laser diode 1. 43 and 44, the second nonlinear optical medium 68 (# 2) includes the DSF 72.

In the embodiment shown in FIG. 43, the supplied signal light beam and the pump light output from the pump light source 210 are added by the optical coupler 209, and the SOA 70
Is input to A phase conjugate light beam is generated by four-wave mixing based on the signal light beam and the pump light in the SOA 70. The signal light beam, pump light and phase conjugate light beam output from the SOA 70 are
Supplied to In the DSF 72, the power of the phase conjugate light beam is increased by four-wave mixing, and the phase conjugate light beam is then output from the DSF 72. In the process using the combination of the SOA 70 and the DSF 72, the conversion efficiency is improved.

In order to widen the conversion band and further increase the conversion efficiency, a feedback loop including an optical bandpass filter 216, a photodetector 218 and a control unit 220 is provided here. The pump light source 210 is driven by a drive circuit 212. Drive circuit 21
2 adjusts the wavelength of the pump light according to the supplied control signal. The light output from the DSF 72 is applied to the optical coupler 214.
, And one of these beams is supplied to an optical bandpass filter 216. The filter 216 has a narrow pass band including the wavelength λ _c of the phase conjugate light beam. The beam component that has passed through the filter 216 is converted by the photodetector 218 into an electric signal having a level (for example, a voltage level) corresponding to the optical power. The control unit 220 receives the signal output from the photodetector 218, and generates the above-described control signal so that the light power detected by the photodetector 218 becomes higher. As a result of such feedback control, the wavelength of the pump light supplied to the SOA 70 and the DSF 72 is controlled to be equal to a desired value (for example, the zero dispersion wavelength of the DSF 72). Thereby,
A wide conversion band and high conversion efficiency can be obtained. This is because the wavelength of the pump light is equal to the optimum value when the power of the obtained phase conjugate light beam is maximized. When a laser diode is used as the pump light source 210, the wavelength of the pump light can be changed by the injection current or temperature of the laser diode.

In the embodiment shown in FIG. 44, the control signal output from the control unit 220 is supplied to the drive circuit 7 of the DFB laser diode 1. Drive circuit 7
Controls the wavelength of the pump light generated in the DFB laser diode 1 based on the supplied control signal (see the description of FIGS. 2 to 5). As a result, the wavelength of the pump light is controlled to match a desired value (for example, the zero-dispersion wavelength of the DSF 72), thereby obtaining a wide conversion band and high conversion efficiency. Here, the drive current of the DFB laser diode 1 is controlled, but the temperature of the DFB laser diode may be controlled.

Although not shown, the first nonlinear optical medium 6
8 (# 1) and the second nonlinear optical medium (# 2), an optical amplifier is provided in advance, and the second nonlinear optical medium 68 (#
The power of the pump light supplied to 2) may be increased. This further increases the conversion efficiency.

By the way, in the non-degenerate four-wave mixing process in a semiconductor nonlinear optical medium such as a DFB laser diode and a semiconductor optical amplifier, when the wavelength of the signal light beam is longer than the wavelength of the pump light (λ _p <λ _s ). The conversion efficiency is higher than the conversion efficiency when the wavelength of the signal light is shorter than the wavelength of the pump light (λ _s <λ _p ). This is thought to be due to the following reasons. In a conversion process using a semiconductor nonlinear optical medium, four-wave mixing occurs based on the total effect of the following third-order nonlinear effects.

(1) Modulation effect of carrier density (band 0.
(1 nm or less) (2) Carrier heating effect (about 10 nm band) (3) Spectral hole burning effect (band 50 nm)
As a result, the light generated through the four-wave mixing process
The phase relationship is λ_p<Λ _sIn the case of
And λ_s<Λ_pIn the case of cancel each other out
Because Therefore, using a semiconductor nonlinear optical medium,
When generating a phase conjugate light beam, the signal light beam
By setting the wavelength longer than the wavelength of the pump light
Thus, the conversion efficiency can be increased.

The limitation of the wavelength relationship in the semiconductor nonlinear optical medium can be avoided by cascading an external nonlinear optical medium having no wavelength limitation in the semiconductor nonlinear optical medium. That is, as described above, the combination of the DFB laser diode or the semiconductor optical amplifier as the first nonlinear optical medium and the optical fiber as the second nonlinear optical medium avoids the above-described limitation of the wavelength relationship. This is effective in expanding the conversion band.

When the semiconductor optical amplifier and the optical fiber are connected in cascade, their order is not limited. In addition, even when the first and second optical fibers that provide different conversion bands are cascaded to provide a phase conjugate light generator, the band limitation caused by the narrower conversion band can be eliminated. Such a combination of nonlinear optical media is also effective in expanding the conversion band.

[0366]

According to the present invention, it is possible to ideally compensate for chromatic dispersion in an optical fiber and waveform distortion due to the optical Kerr effect, thereby enabling long-distance optical fiber transmission at a light speed and a large capacity. Become.

According to the configuration or procedure of the present invention described above, the following effects can be obtained. (1) It is possible to provide a method and apparatus capable of increasing the conversion efficiency from a signal light beam to a phase conjugate light beam.

(2) It is possible to provide a method and apparatus capable of widening a conversion band in conversion from a signal light beam to a phase conjugate light beam, and a method of manufacturing the apparatus. (3) It is possible to provide a method and an apparatus in which the conversion efficiency from a signal light beam to a phase conjugate light beam hardly depends on the polarization state of the signal light beam.

(4) It is possible to provide a method and apparatus capable of obtaining a phase conjugate light beam having a good signal-to-noise ratio. (5) Having the device of (1) to (4), or (1)
It is possible to provide a system suitable for optical communication to which the method of (4) is applied.

(6) It is possible to provide a novel system having a phase conjugate light generator suitable for providing an optical network system with high flexibility.

[Brief description of the drawings]

FIG. 1 shows a first method according to the invention.

FIG. 2 is a diagram showing a phase conjugate light generator applicable to the present invention.

FIG. 3 is a partially cutaway perspective view of the DFB laser diode of FIG. 2;

FIG. 4 is a sectional view taken along the line II in FIG. 3;

FIG. 5 is a diagram illustrating an example of a spectrum of light output from the DFB laser diode of FIG. 2;

FIG. 6 is a graph showing signal light frequency dependence of conversion efficiency.

FIG. 7 shows experimental results, in which (a) shows a transmission pulse waveform,
(B) is a pulse waveform after transmission of a 101 km SMF (single mode optical fiber) when a phase conjugate light generator (PC) is used, and (c) is a pulse waveform when no PC is used.
It is a pulse waveform after SMF transmission of 01 km.

FIG. 8 is a diagram showing a first embodiment of the phase conjugate light generator according to FIG. 1;

FIG. 9 is a diagram showing a second embodiment of the phase conjugate light generator according to FIG. 1;

FIG. 10 is a diagram showing a third embodiment of the phase conjugate light generator according to FIG. 1;

11 is a diagram showing an arrangement for a demonstration experiment of the embodiment of FIG. 10;

FIG. 12 is a graph showing data obtained in the experiment of FIG.

FIG. 13 is a diagram showing a fourth embodiment of the phase conjugate light generator according to FIG. 1;

FIG. 14 is a diagram for explaining a second method according to the present invention.

FIG. 15 is a diagram showing a first embodiment of the phase conjugate light generator according to FIG. 14;

FIG. 16 is a diagram showing a second embodiment of the phase conjugate light generator according to FIG. 14;

FIG. 17 is a block diagram of an optical communication system showing an application example of the present invention.

FIG. 18 is a diagram illustrating a first application example of the present invention to a wavelength division multiplexing transmission system.

FIG. 19 is a diagram for explaining a frequency arrangement when the present invention is applied to a wavelength division multiplexing transmission system.

FIG. 20 is a diagram illustrating a second application example of the present invention to a wavelength division multiplexing transmission system.

FIG. 21 is a diagram illustrating a third application example of the present invention to a wavelength division multiplexing transmission system.

FIG. 22 is a diagram illustrating a fourth application example of the present invention to a wavelength division multiplexing transmission system.

FIG. 23 is a diagram showing a fifth application example of the present invention to a wavelength division multiplexing transmission system.

FIG. 24 is a diagram illustrating a sixth application example of the present invention to a wavelength division multiplexing transmission system.

FIG. 25 is a diagram illustrating an application example of the present invention to a bidirectional transmission system.

FIG. 26 is a diagram illustrating the basic principle of a lightwave network.

FIG. 27 is an explanatory diagram of a compensation principle in FIG. 26;

FIG. 28 is a diagram illustrating a system configuration example of a ring network using a phase conjugate light generator (PC).

FIG. 29 is a configuration diagram of a node (Node) 1 in FIG. 28;

30 is a configuration diagram of a PC (phase conjugate light generator) in FIG. 28.

31 is a diagram illustrating an example of a WDM network (optical wavelength division multiplexing network) obtained by further developing the ring network of FIG. 28.

32 is a phase conjugate light generator (PC) 1 in FIG.
FIG. 21 is a diagram illustrating an example of the configuration of a first embodiment.

FIG. 33 is a diagram showing a modification of the phase conjugate light generator of FIG. 13;

FIG. 34 is a diagram showing a modification of the DFB laser diode of FIG. 4;

FIG. 35 is a diagram showing a first modification of the phase conjugate light generator of FIG. 10;

36 is a diagram illustrating a wavelength characteristic of transmittance of the optical band rejection filter of FIG. 35;

FIG. 37 is a diagram showing a second modification of the phase conjugate light generator of FIG. 10;

FIG. 38 is a diagram showing a third modification of the phase conjugate light generator of FIG. 10;

FIG. 39 is a diagram for explaining the operation of the optical band rejection filter in FIG. 38;

FIG. 40 is a diagram showing a modification of the phase conjugate light generator of FIG.

FIG. 41 is a diagram showing an embodiment of a polarization independent phase conjugate light generator according to the present invention.

FIG. 42 is a diagram showing a modification of the phase conjugate light generator of FIG. 41.

FIG. 43 is a diagram illustrating an embodiment of a phase conjugate light generator having high conversion efficiency and a wide conversion band.

FIG. 44 is a diagram showing another embodiment of the phase conjugate light generator having a high conversion efficiency and a wide conversion band.

[Explanation of symbols]

 1 DFB laser diode 32, 38, 42 Polarization beam splitter 34, 36, 44 1/2 wavelength plate 46 Optical circulator

Claims (63)

[Claims] (A) A signal light beam is converted into a first polarization component having a first polarization plane and a second polarization component having a second polarization plane perpendicular to the first polarization plane. (B) distributed feedback (DF) of the first and second polarization components.
B) supplying to a laser diode to generate first and second phase conjugate light beams corresponding to the first and second polarization components, respectively; and (c) the first and second phase conjugates. Polarization combining the light beam. 2. The method according to claim 1, wherein said step (b) comprises: said DFB laser diode such that said DFB laser diode generates pump light having a wavelength different from the wavelength of said signal light beam. Injecting a current into the DFB laser diode to generate the first and second phase conjugate light beams by four-wave mixing based on the pump light. 3. The method according to claim 1, wherein said DFB laser diode comprises first and second DFB laser diodes receiving said first and second polarization components, respectively. ) And (c) are the first and second
The method performed by the polarization beam splitter. 4. The method of claim 1, wherein said DFB laser diode has first and second excitation ends for receiving said first and second polarization components, respectively. A method wherein the first and second phase conjugate light beams are output from the second and first excitation ends, respectively, and wherein steps (a) and (c) are performed by a common polarization beam splitter. 5. The polarization separation of a signal light beam into a first polarization component having a first polarization plane and a second polarization component having a second polarization plane perpendicular to the first polarization plane. Means for supplying the first and second polarization components and receiving the first and second polarization components, respectively.
And a distributed feedback (DFB) laser diode for generating first and second phase conjugate light beams corresponding to the second polarization component. 6. The apparatus according to claim 5, wherein said polarization separating means outputs a first port for receiving said signal light beam and said first and second polarization components, respectively. A first polarization beam splitter having second and third ports, wherein the first and second DFB laser diodes are operatively connected to the second and third ports, respectively.
A laser diode, wherein the first and second phase conjugate light beams are output from the first and second DFB laser diodes, respectively, and a second polarization multiplexing of the first and second phase conjugate light beams is performed. An apparatus further comprising a polarization beam splitter according to (1). 7. The apparatus according to claim 6, wherein the first DFB laser diode generates a first pump light having a third plane of polarization, and wherein the first phase conjugate light beam is The first polarization component in the first DFB laser diode and the first polarization component;
The second DFB laser diode generates a second pump light having a fourth plane of polarization, and the second phase conjugate light beam generates the second DFB laser beam. The second polarization component in the laser diode and the second polarization component;
The first polarization plane and the third polarization plane are generated by four-wave mixing based on the pump light, and the polarization directions are set so that the second polarization plane and the fourth polarization plane coincide with each other. An apparatus further comprising means for rotating the wavefront by 90 °. 8. The apparatus according to claim 7, wherein said means for rotating comprises a first one operatively connected between said first polarization beam splitter and said second DFB laser diode. A device comprising: a half-wave plate; and a second half-wave plate operatively connected between the first DFB laser diode and the second polarization beam splitter. 9. The apparatus according to claim 7, wherein said rotating means includes a polarization maintaining fiber. 10. The apparatus according to claim 6, wherein said first and second polarization beam splitters are formed on a common waveguide substrate. 11. The apparatus according to claim 5, wherein said means for polarization separation comprises a polarization beam splitter having first to fourth ports, said first port having said signal. A light beam is supplied and the first and third ports and the second and fourth ports are coupled by the first polarization plane, and the first and second ports are coupled to each other and the third and third ports are coupled to each other. The fourth port is coupled by the second polarization plane, the first and second polarization components are output from the third and second ports, respectively, and the DFB laser diode is connected to the first and second ports. A first excitation end receiving a second polarization component and a second excitation end receiving the second polarization component;
Generating pump light having a third polarization plane, wherein the first and second phase conjugate light beams are output from the second and first excitation ends, respectively, and supplied to the second and third ports, respectively. Then, one of the first and second polarization planes is set to 9 so that the first and second polarization planes coincide with the third polarization plane.
An apparatus further comprising means for rotating 0 °. 12. The apparatus according to claim 11, wherein said rotating means includes a half-wave plate. 13. The apparatus according to claim 11, wherein said rotating means comprises a polarization maintaining fiber. 14. The apparatus according to claim 11, further comprising an optical circulator having fifth to seventh ports, wherein one of the fifth to seventh ports is the polarization beam splitter. And a fourth port of the polarization beam splitter is non-reflectively terminated. 15. The apparatus according to claim 11, further comprising: a first optical circulator having fifth to seventh ports; and a second optical circulator having eighth to tenth ports. One of the fifth to seventh ports is connected to a first port of the polarization beam splitter, and one of the eighth to tenth ports is connected to a first port of the polarization beam splitter. Device connected to port 4. 16. The apparatus of claim 15, further comprising a second DFB laser diode cascaded with said DFB laser diode, said second DFB laser diode being cascaded to said DFB laser diode.
The B laser diode is a device for generating a second pump light having a polarization plane perpendicular to the third polarization plane. 17. The apparatus according to claim 5, further comprising means for injecting current into said DFB laser diode so that said DFB laser diode generates pump light, wherein said pump in said DFB laser diode is provided. An apparatus for generating the first and second phase conjugate light beams by light-based four-wave mixing. 18. The device according to claim 17, wherein the DFB laser diode has a diffraction grating having a quarter-wave phase shift structure at a substantially central portion thereof, and the DFB laser diode is configured to inject the current. And an electrode including a plurality of portions divided in the direction of the diffraction grating. 19. Injecting a current into the DFB laser diode so that the distributed feedback (DFB) laser diode generates pump light; and (b) supplying a signal light beam to the DFB laser diode. Generating a phase conjugate light beam by four-wave mixing based on the signal light beam and the pump light in the DFB laser diode; and (c) nonlinearly optics the signal light beam, the pump light, and the phase conjugate light beam. Feeding the medium to increase the power of the phase conjugate light beam by four-wave mixing in the nonlinear optical medium. 20. A distributed feedback (DFB) laser diode to which a signal light beam is supplied; means for injecting a current into the DFB laser diode so that the DFB laser diode generates pump light; A non-linear optical medium optically connected, a four-wave mixing based on the signal light beam and the pump light in the DFB laser diode generates a phase conjugate light beam, and a four-wave mixing in the non-linear optical medium. A device for increasing the power of the phase conjugate light beam. 21. The apparatus according to claim 20, wherein said nonlinear optical medium includes a semiconductor optical amplifier. 22. The apparatus according to claim 20, wherein said nonlinear optical medium includes an optical fiber. 23. The apparatus according to claim 22, wherein the optical fiber has a zero dispersion wavelength substantially equal to a wavelength of the pump light. 24. The apparatus according to claim 22, further comprising means for frequency-modulating or phase-modulating the pump light, whereby stimulated Brillouin scattering in the optical fiber is suppressed. 25. The apparatus according to claim 22, wherein the optical fiber has a nonlinear coefficient large enough to shorten the length of the optical fiber to such an extent that the optical fiber has a polarization plane preserving ability. Equipment. 26. The apparatus according to claim 25, wherein the optical fiber includes a GeO ₂ -doped core and a fluorine-doped cladding. 27. The apparatus according to claim 25, wherein the optical fiber is a single mode fiber, and the single mode fiber has a mode field diameter smaller than that of a single mode fiber used as a transmission line. An apparatus having a. 28. The apparatus according to claim 20, further comprising an optical amplifier operatively connected between said DFB laser diode and said nonlinear optical medium. 29. (a) A signal light beam comprising a first polarization component having a first polarization plane and a second polarization component having a second polarization plane perpendicular to the first polarization plane. Is supplied to a first distributed feedback (DFB) laser diode that generates pump light having a polarization plane corresponding to the first polarization plane, and the first polarization component in the first DFB laser diode and the first Generating a first phase conjugate light beam having a polarization plane corresponding to the first polarization plane by four-wave mixing based on the first pump light; and (b) outputting from the first DFB laser diode. The first phase conjugate light beam and the second polarization component transmitted through the first DFB laser diode are used to generate a second pump light having a polarization plane corresponding to the second polarization plane. The second distributed feedback (D B) having a polarization plane corresponding to the second polarization plane by four-wave mixing based on the second polarization component and the second pump light supplied to the laser diode in the second DFB laser diode; Generating a second phase conjugate light beam. 30. A first distributed feedback (DFB) laser diode for generating a first pump light having a first plane of polarization, and cascaded to the first DFB laser diode and connected to the first plane of polarization. A second distributed feedback (DFB) laser diode for generating a second pump light having a second perpendicular polarization plane, the first having a polarization plane respectively corresponding to the first and second polarization planes. When the signal light beam including the second polarization component and the second polarization component is supplied to the first DFB laser diode, the first DFB laser diode is based on the first polarization component and the first pump light. Fourth wave mixing generates a first phase conjugate light beam having a polarization plane corresponding to the first polarization plane, and the second polarization component is converted to the first polarization conjugate light beam.
And the second DFB laser diode has a polarization plane corresponding to the second polarization plane by four-wave mixing based on the second polarization component and the second pump light. An apparatus for generating a phase conjugate light beam, wherein the first phase conjugate light beam passes through the second DFB laser diode. 31. The apparatus of claim 30, further comprising a polarization dependent element for providing different losses or gains to the light having the first and second polarization planes. Dependent elements are the first and second DFB
An apparatus provided to suppress a difference in transmittance of light having the first and second polarization planes in each of the laser diodes. 32. (a) injecting current into the distributed feedback (DFB) laser diode so that the laser diode generates pump light; and (b) supplying a signal light beam to the DFB laser diode. Generating a phase conjugate light beam by four-wave mixing based on the signal light beam and the pump light in the DFB laser diode; and (c) the signal light beam output from the DFB laser diode and the phase conjugate light. Supplying the beam and the pump light to an optical band stop filter having a stop band including the wavelength of the pump light. 33. A distributed feedback (DFB) laser diode to which a signal light beam is supplied, wherein said DFB laser diode generates pump light, and said DFB laser diode is formed by four-wave mixing based on said pump light and said signal light beam. Means for injecting a current into the DFB laser diode so as to convert the signal light beam into a phase conjugate light beam, and a stop band including a wavelength of the pump light;
An apparatus comprising: an optical band stop filter to which the signal light beam output from the laser diode, the phase conjugate light beam, and the pump light are supplied. 34. The apparatus according to claim 33, wherein the optical band stop filter comprises a fiber grating. 35. (a) supplying a signal light beam to an optical band stop filter having a stop band including a predetermined wavelength; and (b) applying the signal light beam output from the optical band stop filter. Providing a phase conjugate light generator and generating a phase conjugate light beam having a wavelength substantially equal to said predetermined wavelength. 36. An optical band stop filter having a stop band including a predetermined wavelength, to which a signal light beam is supplied, and the signal light beam output from the optical band stop filter is supplied, and A phase conjugate light generator for generating a phase conjugate light beam having a wavelength substantially equal to the determined wavelength by four-wave mixing. 37. The apparatus according to claim 36, wherein the phase conjugate light generator includes a distributed feedback (DFB) laser diode to which the signal light beam is supplied, and the DFB laser diode generates pump light. Means for injecting a current into the DFB laser diode so as to perform the operation. 38. The apparatus according to claim 36, wherein the phase conjugate light generator includes a pump light source that outputs a pump light, a nonlinear optical medium to which the signal light beam is supplied, and the pump light. Means for optically connecting the pump light source and the nonlinear optical medium so as to be supplied to the nonlinear optical medium. 39. The apparatus according to claim 36, wherein the optical band stop filter comprises a fiber grating. 40. (a) a step of supplying a signal light beam to a first nonlinear optical medium; and (b) the first light beam based on four-wave mixing using pump light.
Generating a phase conjugate light beam in the nonlinear optical medium of (c), and (c) converting the signal light beam, the phase conjugate light beam, and the pump light output from the first nonlinear optical medium to a second nonlinear optical medium. Supplying the medium. 41. A first nonlinear optical medium which is supplied with a signal light beam and generates a phase conjugate light beam based on four-wave mixing using pump light, and a cascade-connected to the first nonlinear optical medium, First
The signal light beam, the phase conjugate light beam, and the pump light output from the nonlinear optical medium
A device comprising: a non-linear optical medium; 42. The apparatus according to claim 41, wherein said first nonlinear optical medium comprises a semiconductor chip for providing a first conversion band, and said second nonlinear optical medium comprises said first nonlinear optical medium. An apparatus comprising an optical fiber for providing a second conversion band wider than the conversion band. 43. The apparatus according to claim 42, wherein the semiconductor chip is provided by a semiconductor optical amplifier,
An apparatus further comprising a pump light source for supplying the pump light to the semiconductor optical amplifier. 44. The apparatus according to claim 42, wherein the semiconductor chip is provided by a distributed feedback (DFB) laser diode, and a current is applied to the DFB laser diode such that the DFB laser diode generates the pump light. Further comprising a means for injecting. 45. The apparatus according to claim 42, wherein the optical fiber has a zero dispersion wavelength substantially equal to a wavelength of the pump light. 46. The apparatus according to claim 42, further comprising a feedback loop for controlling the wavelength of the pump light so that the power of the phase conjugate light beam is higher. 47. (a) supplying a signal light beam to a semiconductor nonlinear optical medium; and (b) generating a phase conjugate light beam in the semiconductor nonlinear optical medium based on four-wave mixing using pump light. And (c) setting the wavelength of the signal light beam to be longer than the wavelength of the pump light. 48. A semiconductor nonlinear optical medium to which a signal light beam is supplied and a phase conjugate light based on four-wave mixing using pump light having a wavelength shorter than the wavelength of the signal light beam. Means for pumping the semiconductor nonlinear optical medium to generate a beam. 49. The apparatus according to claim 48, wherein said semiconductor nonlinear optical medium is provided by a semiconductor optical amplifier, and said pumping means comprises a pump light source for supplying said pump light to said semiconductor optical amplifier. Including equipment. 50. The apparatus according to claim 48, wherein said semiconductor nonlinear optical medium is provided by a distributed feedback (DFB) laser diode, and said pumping means is such that said DFB laser diode generates said pump light. And means for injecting current into said DFB laser diode. 51. A first optical fiber for transmitting a signal light beam, a phase conjugate light generator for converting the signal light beam into a phase conjugate light beam, and for transmitting the phase conjugate light beam The phase conjugate light generator according to any one of claims 5 to 18, 20 to 28, 30, 31, 33, 34, 36 to 39, 41 to 46, and 48 to 50. A system comprising the device described in. 52. The system according to claim 51, wherein when the first and second optical fibers are virtually divided into the same number of sections, respectively, counting from the phase conjugate light generator. The product of the average value of the chromatic dispersion and the product of the section lengths of the two sections substantially match, and the product of the average value of the optical power, the average value of the nonlinear coefficient, and the section length of the two sections substantially match system. 53. The system according to claim 51, wherein the optical power and the nonlinearity at each of the two points of the first and second optical fibers having the same cumulative chromatic dispersion from the phase conjugate light generator. A system in which the ratio between the product of the coefficients and the chromatic dispersion is substantially equal. 54. The system according to claim 51, wherein the accumulated value of the product of the optical power and the nonlinear coefficient from the phase conjugate light generator is equal to each other.
A system wherein the ratio of the product of the optical power and the non-linear coefficient at each of the points to the chromatic dispersion is substantially identical. 55. The system according to claim 51, wherein the product of the average value and the length of the chromatic dispersion of the first optical fiber is the product of the average value and the length of the chromatic dispersion of the second optical fiber. A system that substantially matches the product. 56. The system according to claim 55, wherein the product of the average value of the optical power and the average value of the nonlinear coefficient in the first optical fiber and the length of the first optical fiber is the second optical fiber. A system substantially corresponding to the product of the average value of the optical power and the average value of the non-linear coefficient in the optical fiber and the length of the second optical fiber. 57. A system comprising a plurality of optically connected units, each of said plurality of units including the system of any of claims 51-56. 58. A system comprising: a plurality of optically connected units; and at least one optical signal insertion / branching device provided at a connection point between the plurality of units, wherein each of the plurality of units transmits a signal light. A first optical fiber for transmitting, a means for converting the signal light into a phase conjugate light, and a second optical fiber for transmitting the phase conjugate light; A system wherein chromatic dispersion and optical Kerr effect in an optical fiber are compensated by chromatic dispersion and optical Kerr effect in the second optical fiber. 59. The apparatus according to claim 41, wherein said first nonlinear optical medium comprises a semiconductor chip, and said second nonlinear optical medium comprises an optical fiber. 60. A method for manufacturing a device having an optical fiber as a non-linear optical medium for generating phase conjugate light, comprising: (a) cutting the optical fiber and dividing the optical fiber into a plurality of sections; (B) rearranging and joining the plurality of sections so that a conversion band by non-degenerate four-wave mixing is expanded. 61. The method according to claim 60, wherein the step (b) includes a step of measuring a dispersion value of each of the plurality of sections, and an input when pump light is input to the optical fiber. A method in which the plurality of sections are rearranged such that a section having a relatively small variance value is arranged on a side closer to an end. 62. The method according to claim 61, wherein at least some of the plurality of sections are joined such that the variance values are alternately positive and negative. 63. A method for manufacturing a device having an optical fiber as a non-linear optical medium for generating phase conjugate light, comprising: (a) cutting the optical fiber to divide it into a plurality of sections; (B) measuring a variance value of each of the plurality of sections; and (c) selecting and joining only sections having a variance value small enough to obtain a required conversion band by non-degenerate four-wave mixing. A method that includes