JP4040583B2 - Optical transmission system - Google Patents

Description

  The present invention relates to an optical transmission system.

  Due to 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 by optical fiber is practically 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, but the loss due to this attenuation has been compensated by the adoption of optical amplifiers such as erbium doped fiber amplifiers (EDFAs).

  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, for wavelengths shorter than 1.3 μm, an optical signal with a longer wavelength propagates faster than an optical signal with a shorter wavelength, and the resulting dispersion is Usually called normal dispersion. For wavelengths longer than 1.3 μm, an optical signal having a shorter wavelength propagates faster than an optical signal having a longer wavelength, and the resulting dispersion is referred to as anomalous dispersion.

  In recent years, non-linearity has attracted attention due to the increase in optical signal power due to the adoption 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 with the intensity of an optical signal. The change in refractive index modulates the phase of the optical signal propagating through the optical fiber, resulting in frequency chirping that alters the signal spectrum. This phenomenon is known as self-phase modulation (SPM). The spectrum is expanded by SPM, and waveform distortion due to wavelength dispersion is further increased.

  As described above, the chromatic dispersion and the Kerr effect give waveform distortion to the optical signal as the transmission distance increases. Therefore, in order to enable long-distance transmission over an optical fiber, chromatic dispersion and nonlinearity need to be controlled, compensated or suppressed.

  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. 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. Is called. However, this method has a problem that an expensive and complicated regenerative repeater is required and the electronic circuit of the regenerative repeater limits the bit rate of the main signal.

  Optical solitons are known as a technique for compensating for chromatic dispersion and nonlinearity. An optical signal pulse having an amplitude, a pulse width and a peak power accurately defined for a given value of anomalous dispersion is generated, whereby pulse compression by SPM and anomalous dispersion due to the optical Kerr effect, and by 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 has been proposed by Yariv et al. (A. Yariv, D. Fekete, and DM Pepper, “Compensation for channel dispersion by nonlinear optical phase conjugation” Opt. Lett., Vol. 4, pp. 52-54, 1979). The optical signal is converted into phase conjugate light at the intermediate point of the transmission line, and the waveform distortion caused by the chromatic dispersion received in the first half of the transmission line is compensated by the distortion caused by the chromatic dispersion in the second half of the transmission line.

  In particular, if the factors of the phase change of the electric field at the two points are the same, and the environmental change that causes the factors is gentle within the light propagation time between the two points, the phase conjugate is intermediate between the two points. Phase change is compensated by arranging a phase conjugate light generator (S. Watanabe, “Compensation of phase fluctuation in a transmission line by optical conjugation” Opt. Lett., Vol. 17, pp. 1355 -1357, 1992). Therefore, the waveform distortion caused by the SPM is also compensated by adopting the phase conjugator. However, if the optical power distribution is asymmetric before and after the phase conjugator, the nonlinearity compensation is incomplete.

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

Using a third-order nonlinear optical medium such as an optical fiber and a semiconductor optical amplifier, phase conjugate light can be generated by non-degenerate four-wave mixing. When the pump light angular frequency omega _S of the signal light and the angular frequency _{ω P (ω P ≠ ω S} ) is supplied to the nonlinear optical medium, based on four-wave mixing of the signal light and the pump light in the nonlinear optical medium Then, phase conjugate light (converted signal light) having an angular frequency of 2ω _P −ω _S is generated, and this phase conjugate light is output from the nonlinear optical medium together with the signal light and the pump light.

  “Non-degenerate” is used in the sense that the wavelength of the signal light and the wavelength of the pump light are different. Since the wavelength of the signal light, the wavelength of the pump light, and the wavelength (angular frequency) of the phase conjugate light satisfy the relationship described above, wavelength conversion is performed simultaneously with the generation of the phase conjugate light.

  The conversion efficiency in the conversion from signal light to phase conjugate light depends on the coincidence of the polarization planes of the signal light and the pump light. However, since a general optical fiber transmission line does not have the ability to maintain the plane of polarization, the polarization state of the signal light to be converted varies with time. Therefore, the device for phase conjugate conversion and wavelength conversion is required to have no polarization dependency. Here, “no polarization dependence” means that the conversion efficiency is substantially constant regardless of the polarization state of the signal light to be converted.

  On the other hand, when a device for phase conjugate conversion and wavelength conversion is applied to WDM (wavelength division multiplexing), it is required that the conversion band is sufficiently wide to increase the number of channels that can be converted at one time. The The conversion band is defined by the maximum detuning wavelength (or detuning frequency) of the pump light and the signal light under conditions where phase conjugate light with a certain power is obtained.

  Accordingly, an object of the present invention is to provide a device for phase conjugate conversion and wavelength conversion without polarization dependency and a system including the device.

  Another object of the present invention is to provide an apparatus for phase conjugate conversion and wavelength conversion with a wide conversion band and a system including the apparatus.

According to the present invention, a first wavelength multiplexing network that performs wavelength multiplexing transmission in a first wavelength band, a second wavelength multiplexing network that performs wavelength multiplexing transmission in a second wavelength band, and a first wavelength multiplexing network A four-wave mixer disposed between the second wavelength multiplexing networks, wherein the four-wave mixer receives light of the first wavelength band from the first wavelength multiplexing network and overlaps the second wavelength band. To the third wavelength band to be output to the second wavelength division multiplexing network, the four-wave mixer has first, second and third ports, the first port supplied to the first port The light of the first wavelength band from one wavelength division multiplexing network is output from the second port, the light supplied to the second port is output from the third port, and the polarization plane is applied to the first port. Are first and second orthogonal to each other Signal light including the polarization component and the pump light are supplied, and have an optical element output from the second port, and first, second, and third ports, and the first port is connected to the optical element. Optically connected to a second port, the first and second ports are coupled by a first polarization plane which is a polarization plane of the first polarization component, and the first and third ports are coupled to each other by the first port. A polarization beam splitter coupled by a second polarization plane that is a polarization plane of two polarization components, and a polarization mode to be maintained between the first end and the second end, wherein the first end is Optically connected to the second port of the polarization beam splitter so that the first polarization plane matches the polarization mode, and the second polarization plane matches the polarization mode at the second end. Polarization maintaining optically connected to the third port of the polarization beam splitter And the four-wave mixing caused by the four-wave mixing generated in the polarization maintaining fiber by the signal light and the pump light output from the second port of the polarization beam splitter to the first end. The first phase conjugate light included in the third wavelength band is supplied from the second end to the third port of the polarization beam splitter, and has a second polarization plane, and the third phase conjugate light of the polarization beam splitter. The second phase conjugate light included in the third wavelength band by the four-wave mixing generated in the polarization maintaining fiber by the signal light and the pump light output from the port to the second end is transmitted from the first end. Supplied to the second port of the polarization beam splitter, and the first and second phase conjugate lights are supplied from the first port of the polarization beam splitter to the second port of the optical element, and at least the optical element An optical transmission system is provided that outputs to the second wavelength division multiplexing network through the third port .

  According to the present invention, it is possible to provide an apparatus for phase conjugate conversion and wavelength conversion having a wide conversion band. In addition, according to the present invention, it is possible to provide an apparatus for phase conjugate conversion and wavelength conversion in which conversion efficiency in conversion from signal light to converted light does not depend on the polarization state of the signal light to be converted. Occurs. Furthermore, the apparatus according to the present invention has an effect that it is possible to provide a new system that is suitable for large-capacity transmission and that is rich in flexibility.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Consider the case where an optical pulse propagates through a dispersion medium. When a non-chirped pulse passes through the dispersion medium, if it is a normal dispersion medium (∂ ² β / ∂ω ² > 0), it shifts to the low frequency side at the rising edge of the pulse and shifts to the high frequency side at the falling edge To do. In the case of an anomalous dispersion medium (∂ ² β / ∂ω ² <0), it shifts to the high frequency side at the rising edge of the pulse and shifts to the low frequency side at the falling edge. Here, β represents a propagation constant, and ω represents the angular frequency of light. In the normal dispersion medium, the longer the wavelength, the faster the group velocity, and in the anomalous dispersion medium, the shorter the wavelength, the faster the group velocity. Therefore, in any case, the pulse width increases.

On the other hand, when the light intensity is high, the refractive index is Δn (t) = n ₂ | E (t) | ^{2 due to the} optical Kerr effect.
Only changes. Here, n2 is a quantity called a nonlinear refractive index, and 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 is subjected to the optical Kerr effect in a nonlinear medium,
Δω (t) = − ∂Δφ (t) / ∂t
= − (2πn ₂ / λ) (∂ | E (t) | ² / ∂t) Δz
Only the spectrum diffuses (chirps). Here, Δz is the interaction length.

  This phenomenon is generally referred to as self-phase modulation (SPM). This SPM shifts to the low frequency side at the rise of the optical pulse, and shifts to the high frequency side at the fall. Due to this chirping by SPM, the influence of dispersion becomes more conspicuous, and as a result, pulse distortion becomes more significant. Therefore, when the optical pulse is subjected to the optical Kerr effect in the dispersion medium, in the case of the normal dispersion medium, the pulse is further diffused than in the case of only the dispersion, but in the case of the anomalous dispersion medium, pulse compression occurs.

  Therefore, considering the effects of chromatic dispersion described above, large pulse diffusion occurs in the case of a normal dispersion medium, and in the case of an anomalous dispersion medium, the larger effect of pulse diffusion by chromatic dispersion and pulse compression by SPM. Appears. Optical solitons are a balance between these two effects.

  In general, it is apt to think that it is convenient to apply a pulse compression by SPM in an anomalous dispersion medium so that a high signal-to-noise ratio (S / N) can be maintained. It has become impossible to say that it is better to add pulse compression in general because it is possible to transmit with power and to develop a relatively small chromatic dispersion value by developing a dispersion shifted fiber.

  That is, the pulse compression effect becomes too great and a large waveform distortion occurs. In particular, in the case of an NRZ pulse, pulse compression occurs intensively at the rising and falling portions of the pulse. Therefore, in an extreme waveform change or in an extreme case, the falling portion overtakes the rising portion, and the pulse is 3 Some things break up. In addition, in the case of long-distance optical amplification multi-relay transmission, there is a problem that four-wave mixing occurs between signal light as excitation light and spontaneous emission light from an optical amplifier, resulting in a significant decrease in S / N (modulation instability). Qualitative (modulation instability).

  Optical pulse distortion due to chromatic dispersion and nonlinearity as described above can be compensated by applying phase conjugate optics. For example, a signal light beam transmitted by a first optical fiber transmission line is converted into a phase conjugate light beam by a phase conjugate light generator, and the phase conjugate light beam is transmitted by a second optical fiber transmission line. By appropriately setting parameters related to chromatic dispersion and nonlinearity in the first and second optical fibers, it is possible to obtain an optical pulse substantially free from distortion at the output end of the second optical fiber. .

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

  In order to configure a phase conjugate light generator whose conversion efficiency does not depend on polarization, a polarization scrambling method, a polarization diversity method, or a polarization active control method can be applied. Furthermore, the polarization dependence of the conversion efficiency in the phase conjugate light generator can also be eliminated by using an optical fiber transmission line composed of a polarization maintaining fiber (PMF). In the present invention, the polarization diversity method is adopted in order to eliminate the polarization dependency of the conversion efficiency.

  FIG. 1 shows a first embodiment of the device according to the invention. In the figure, reference numeral 2 is an input port, 4 is a pump light source, 6 is an optical coupler, 8 is an optical circulator, 10 is a polarization beam splitter, 12 is a polarization maintaining fiber (PMF), 14 is an optical bandpass filter, 16 Indicates an output port. Details of the configuration and operation of this apparatus will be described later.

FIG. 2 is an explanatory diagram of the operating principle of the apparatus according to the present invention. First, in the polarization separation process, the signal light ES is separated into two polarization components E _S (# 1) and E _S (# 2). The polarization components E _S (# 1) and E _S (# 2) have orthogonal polarization planes. Next, in the conversion process, the polarization components E _S (# 1) and E _S (# 2) are converted into corresponding phase conjugate lights E _C (# 1) and E _C (# 2), respectively. The polarization planes of the phase conjugate light E _C (# 1) and E _C (# 2) coincide with the polarization planes of the polarization components E _S (# 1) and E _S (# 2), respectively. Then, in the polarization combining process, the phase conjugate light E _C (# 1) and E _C (# 2) are subjected to polarization combining, and the phase conjugate light (converted signal light) E _C (# 1) and E _C ( # 2) is obtained.

  In the apparatus shown in FIG. 1, the polarization separation and polarization synthesis processes are executed in the polarization beam splitter 10, and the conversion process is executed in the PMF 12 as a nonlinear optical medium.

  FIG. 3 is an explanatory diagram of the polarization mode of the PMF 12. The PMF 12 includes a core 18 having a relatively high refractive index, a clad 20 having a relatively low refractive index covering the core 18, and a pair of stress applying portions 22 provided on both sides of the core 18 in the clad 20. . In the cross section of the PMF 12, an axis passing through the centers of the core 18 and the stress applying portion 22 is defined as an X axis, and an axis passing through the center of the core 18 and perpendicular to the X axis is defined as a Y axis. Each of the X axis and the Y axis is referred to as a main axis. Since the propagation constant of the polarization component having a polarization plane parallel to the X axis is significantly different from the propagation constant of the polarization component having a polarization plane parallel to the Y axis, each polarization component maintains its polarization state while maintaining its polarization state. Can propagate inside.

  According to the inventor's measurement, it has been clarified that the zero dispersion wavelength for the polarization component having the polarization plane parallel to the X axis is different from the zero dispersion wavelength for the polarization component having the polarization plane parallel to the Y axis.

  Therefore, when performing polarization diversity for one channel using both of the two main axes (X axis and Y axis) of the PMF 12, the conversion band cannot be widened due to the difference in the zero dispersion wavelength.

  In the present invention, since polarization diversity is performed using only one of the two main axes as the polarization mode, the conversion band can be expanded. Specifically, it is as follows.

In FIG. 1, the input port 2 is supplied with signal light E _S (wavelength λ _S ) to be converted. The pump light source 4 outputs pump light E _P (wavelength λ _P ). The pump light source 4 can be provided by a laser diode, for example. In this case, the pump light E _P is substantially linearly polarized and its plane of polarization is set as described later. The polarization state of the signal light E _S is arbitrary.

  The signal light and the pump light are combined or added by the optical coupler 6 and supplied to the port 8A of the optical circulator 8. The optical circulator 8 functions to output the light supplied to the port 8A from the port 8B, output the light supplied to the port 8B from the port 8C, and output the light supplied to the port 8C from the port 8A. . Note that the third function is not used in this embodiment.

  The polarization beam splitter 10 has ports 10A, 10B, and 10C. The port 10A is optically connected to the port 8B of the optical circulator 8. The ports 10A and 10B are coupled by a TM component having a first polarization plane perpendicular to the paper surface (first polarization component), and the ports 10A and 10C have a second polarization surface parallel to the paper surface. They are coupled by the TE component (second polarization component).

  Both ends of the PMF 12 are optically connected to the ports 10B and 10C of the polarization beam splitter 10, respectively. For convenience of explanation, it is assumed that the polarization mode to be maintained by the PMF 12 is given by the Y axis shown in FIG. In the port 10B, the first polarization plane perpendicular to the paper surface conforms to the polarization mode of the PMF 12, and in the port 10C, the second polarization plane horizontal to the paper surface conforms to the polarization mode. That is, in the port 10B, the Y axis of the PMF 12 is perpendicular to the paper surface, and in the port 10C, the Y axis is parallel to the paper surface. The PMF 12 is spatially twisted by 90 ° between the ports 10B and 10C.

  The signal light and pump light supplied to the port 8A of the optical circulator 8 are output from the port 8B. The output light is divided into a TM component and a TE component by the polarization beam splitter 10. The TM component propagates in the PMF 12 from the port 10B toward the port 10C, whereas the TE component propagates in the PMF 12 from the port 10C toward the port 10B. When the TM component is output from the port 10B, it has a plane of polarization perpendicular to the plane of the paper, but the plane of polarization is rotated 90 ° after passing through the PFM 12 and is parallel to the plane of the paper. The TM component that has propagated around passes from the port 10C to the port 10A. On the other hand, when the TE component is output from the port 10C, it has a plane of polarization that is horizontal to the plane of the paper, but the plane of polarization is rotated by 90 ° in the PMF 12 and becomes perpendicular to the plane of the paper at the port 10B. Accordingly, the TE component propagating counterclockwise in the PMF 12 passes from the port 10B to the port 10A.

  The light combined in this way is supplied from the port 10A of the polarization beam splitter 10 to the port 8B of the optical circulator 8, and is output from the port 8C. The output light passes through the optical bandpass filter 14 and is output from the output port 16.

The light output from the port 10A of the polarization beam splitter 10 as a result of the polarization synthesis includes signal light and pump light, and also newly generated phase conjugate light (converted signal light) E _C (wavelength) in the PMF 12 λ _C ). That is, part of the signal light is converted into phase conjugate light by four-wave mixing based on the signal light and pump light in the PMF 12. The wavelengths λ _S , λ _P, and λ _C satisfy the above-described relationship and are different from each other. Therefore, the phase conjugate light is extracted by the optical bandpass filter 14. That is, the optical bandpass filter 14 has a pass band that includes the wavelength λ _C and does not include λ _S and λ _P.

  As described above, since the polarization mode corresponding to one main axis (Y axis) of the PMF 12 is used, the wavelength of the pump light can be accurately matched with the zero dispersion wavelength related to the Y axis of the PMF 12, and the conversion band is increased. Enlarged. According to the present invention, the conversion band that has been about 40 nm can be expanded to about 100 nm.

  When a laser diode is used as the pump light source 4, the pump light is substantially given by linearly polarized light. In this case, the polarization plane of the pump light is set so that the conversion efficiency in the conversion from the signal light to the phase conjugate light does not depend on the polarization state of the signal light. Specifically, the polarization plane of the pump light is set to be inclined by 45 ° with respect to the paper surface at the port 10A of the polarization beam splitter 10 so that the distribution ratio of the pump light to the TM component and the TE component becomes equal. I will.

  As described above, since the diversity according to the present invention is applied, it is possible to provide a converter having no polarization dependency.

  FIG. 4 shows a second embodiment of the device according to the invention. Here, a modulation circuit 24 is connected to the pump light source 4 in order to modulate or dithering the phase or frequency of the pump light output from the pump light source 4. In order to increase the generation efficiency of four-wave mixing in the PMF 12, it is effective to increase the power of the pump light. However, if the power of the pump light is increased too much, the pump light is reflected in the PMF 12 by stimulated Brillouin scattering (SBS). End up. In the present embodiment, since the phase or frequency of the pump light is modulated or disturbed, the generation of SBS is suppressed and a converter with high conversion efficiency can be provided. The modulation frequency may be several hundred KHz (for example, 150 KHz). In this embodiment, for example, direct modulation is applied to the pump light source 4 provided by the LD, but indirect modulation using an external modulator may be applied.

  FIG. 5 is a diagram showing a first embodiment of the system according to the present invention. Here, the waveform distortion due to the chromatic dispersion of the optical fiber transmission line and the nonlinear optical Kerr effect is compensated. For this purpose, the apparatus shown in FIG. 1 or FIG. 4 is used as a converter (or phase conjugate light generator: PC) at approximately the midpoint of the optical fiber transmission line.

The output signal light ES from the transmitter (TX) is optically transmitted through the first optical fiber F1 (length L ₁ , dispersion D ₁ , nonlinear coefficient γ ₁ ), and then input to the phase conjugate light generator (PC). . The light is converted into phase conjugate light E _C by the PC, and optically transmitted to the receiver (RX) through the second optical fiber F2 (length L ₂ , dispersion D ₂ , nonlinear coefficient γ ₂ ). In the receiver, the phase conjugate light is received by a light receiver to detect a signal. Any method such as optical amplitude (intensity), frequency, phase, etc. can be applied to the modulation method of the transmission signal. For signal detection, optical direct detection or optical heterodyne detection after phase conjugate light is extracted by a bandpass filter, etc. Can be considered.

  In many cases, the optical fiber used here is a single-mode quartz fiber (SMF), and a 1.3 μm zero-dispersion optical fiber or a 1.55 μm dispersion-shifted fiber (DSF) generally used in optical communication. Etc. are typical examples.

  Further, the signal light may be wavelength division multiplexed (WDM) signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths. In this case, by arranging the wavelengths of a plurality of optical signals at unequal intervals on the wavelength axis, generation of undesired FWM in the optical fiber transmission line can be suppressed and crosstalk can be prevented (described later). The same applies to the description of FIG.

In order to compensate for the chromatic dispersion in the optical fiber and the waveform distortion due to self phase modulation with the system shown in FIG. 5, the dispersion of the corresponding portion and the magnitude of the nonlinear effect are made the same across the PC. do it. Here, the corresponding parts are two parts where the accumulated values of dispersion or optical Kerr effect measured from the PC are equal. That is, when the transmission path is divided, the dispersion and the nonlinear effect may be made the same in each divided section at the target position in the above sense in order from the side closer to the PC. This also makes the variance values in each segmented section the same, and in each section,
_{D 1 / γ 1 P 1 =} D 2 / γ 2 P 2 ... (1a)
It is shown that it should be made to hold. Here, P ₁ and P ₂ are the optical power in each part,
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 vacuum, and n _2j and A _effj represent the nonlinear refractive index and effective core area of the optical fiber Fj (j = 1, 2), respectively.

  In order to compensate for the decrease due to the loss of the non-linear effect along the transmission line, the dispersion should be reduced or the optical Kerr effect should be increased. Changing the value of dispersion is possible and promising depending on the design of the optical fiber. For example, it is actively performed by changing the zero dispersion wavelength of a dispersion shifted fiber, or changing the relative refractive index difference and the core diameter of the core and 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 changing the light intensity.

  A system is composed of a transmission line using a dispersion-decreasing DCF (DD-DCF) having a structure in which the dispersion value of the dispersion-compensating fiber is gradually reduced in the longitudinal direction so as to be proportional to the change of the optical Kerr effect, and a DSF having a normal dispersion. By doing so, high-speed and long-distance transmission becomes possible.

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

  In addition to the exact compensation method described above, when the change in the optical Kerr effect is not so large (such as when the optical amplifier relay interval is sufficiently shorter than the nonlinear length), the following approximation by average power 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 the average power in the optical fiber Fj (j = 1, 2), respectively, and D ₁ ′ and D ₂ ′ are the average dispersion of the optical fiber Fj.

  Furthermore, although the ideal waveform compensation conditional expression (1a) is not satisfied, it is also possible to appropriately dispose a dispersion compensator by disposing dispersion with an opposite sign on the transmission line.

  This method is particularly effective in long-distance transmission such as submarine 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 that the same. For this reason, the waveform is most distorted before and after the PC. Therefore, at the position of the PC, the spectrum of the light pulse is most widened. On the other hand, noise is added from the PC and the optical amplifier of the transmission path, and the S / N degradation due to this noise is larger as the spectrum is wider. 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.

Since the nonlinear coefficient γ of a normal DSF (dispersion shifted fiber) is as small as about 2.6 W ⁻¹ km ⁻¹ , sufficient conversion efficiency is obtained when a normal DSF is used as a nonlinear optical medium for generating phase conjugate light. In order to obtain it, the fiber length is required to be 10 km or more. Therefore, it is desired to provide a DSF having a nonlinear coefficient γ that is sufficiently large to shorten the fiber length. If the length of the DSF used as the nonlinear optical medium for generating the phase conjugate light can be shortened, the zero dispersion wavelength can be managed with high accuracy, and therefore the wavelength of the pump light is changed to the DSF. Therefore, it is easy to accurately match the zero-dispersion wavelength, and as a result, a wide conversion band can be obtained. Here, the conversion band is defined as the maximum detuning wavelength (detuning frequency) of the pump light and the signal light under the condition that phase conjugate light with a certain power is obtained.

In order to increase the nonlinear coefficient γ, it is effective to increase the nonlinear refractive index n ₂ or decrease the mode field diameter (MFD) corresponding to the effective core area A _eff . To increase the nonlinear refractive index n _2, for example, fluorine, etc. may be doped or heavily doped with GeO ₂ in the core to the cladding. The GeO ₂ by 25 to 30 mol% doping the core, the nonlinear refractive index n2 as 5 × 10 ^-20 m in ² / large value above W is obtained (usually silica fiber to about 3.2 × 10 ^{- 20} m ² / W). The MFD can be reduced by designing the relative refractive index difference Δ or the shape of the core. The DSF design is the same as that of DCF (dispersion compensating fiber). For example, MFD values smaller than 4 μm can be 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 these comprehensive effects, a large nonlinear coefficient γ value of 15 W ⁻¹ km ⁻¹ or more is obtained.

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

The conversion efficiency η _C in an optical fiber of length L and loss α is
η _C = exp (−αL) (γP _P L) ² (5a)
Can be approximated by Here, P _P is the average pump light power. Therefore, a fiber having a nonlinear coefficient γ of 15 W ⁻¹ km ⁻¹ can achieve the same conversion efficiency with a length of about 2.6 / 15≈1 / 5.7 compared to a normal DSF. In a normal DSF, in order to obtain a sufficiently large conversion efficiency, a length of about 10 km is necessary as described above, whereas in a fiber having such a large nonlinear coefficient γ, Similar conversion efficiency can be obtained with a length of about 1 to 2 km. Actually, the loss is reduced as the fiber length is shortened, so that the fiber length can be further shortened in order to obtain the same conversion efficiency. In such a short DSF, the controllability of the zero dispersion wavelength is improved, so that the wavelength of the pump light can be exactly matched to the zero dispersion wavelength, and a wide conversion band can be obtained. Furthermore, if the fiber length is several kilometers, the polarization plane preserving ability is ensured. Therefore, the use of such DSF is extremely effective in achieving high conversion efficiency and a wide conversion band and eliminating polarization dependence. It is valid.

  In order to effectively generate four-wave mixing using an optical fiber, it is important to match the phases of signal light, pump light, and phase conjugate light. The phase mismatch amount Δk is approximated as follows.

Δk = δω ² β ₂ (ω _P ) + 2γP _P (6a)
Here, β ₂ (ω _P ) is the chromatic dispersion at the pump light frequency ω _P , and δω is the frequency difference between the signal light and the pump light. Unless the pump light with an extraordinarily large power (for example, 100 mW or more) is used, the second term of the equation (6a) is sufficiently smaller than the first term, and can be ignored. Accordingly, phase matching (making Δk as close to 0 as possible) can be obtained by matching the wavelength of the pump light with the zero dispersion wavelength of the fiber. However, in an actual fiber, the zero dispersion wavelength varies in the longitudinal direction, so it is not easy to maintain the phase matching condition over the entire length of the fiber.

  Thus, 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, if the dispersion in the longitudinal direction of the optical fiber is completely controlled, for example, an optical fiber having a single zero dispersion wavelength over the entire length (exactly nonlinear length) is produced, the pump light wavelength is set to the zero dispersion wavelength. Therefore, a virtually infinite conversion band (infinitely wide within a range in which the dispersion slope is linear) can be obtained. However, in reality, the zero dispersion wavelength varies in the longitudinal direction due to a problem in the manufacturing technique 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 by splicing or the like (counted from the original fiber end). By doing so, an optical fiber suitable for providing a phase conjugator having a wide conversion band can be obtained even though the average dispersion in the entire length is the same.

  Alternatively, a large number of fibers with a length (for example, several hundreds of meters or less) that can be controlled with high accuracy to the extent necessary to obtain a sufficiently wide conversion band are prepared in advance, and those having the required zero dispersion wavelength are combined. A wide conversion band can be obtained by obtaining a fiber having a length necessary for splicing and obtaining a required conversion efficiency and using this to provide a phase conjugator.

  When the conversion band is expanded in this way, the power of the pump light is high near the pump light input end of the nonlinear optical medium. Therefore, a portion with a small zero dispersion wavelength or a zero dispersion wavelength near the pump light input end. It is effective to collect portions with small variations. In addition, if necessary, the number of divisions is increased sequentially, or in places where the dispersion value is relatively large at a position away from the pump light input end, further conversion is performed by appropriately combining the dispersion values such as by alternately arranging positive and negative values. Bandwidth can be expanded.

For example, a nonlinear length may be used as a standard for determining how short each section is to be sufficient when dividing an optical fiber. As in the compensation of nonlinear effects, in FWM (four wave mixing) in a fiber that is sufficiently shorter than the nonlinear length, it can be considered that phase matching depends on the average dispersion value of the fiber. As an example, in an FWM using a pump light power of about 30 mW with a fiber having a nonlinear coefficient γ of 2.6 W ⁻¹ m ⁻¹ , the nonlinear length is about 12.8 km, so about 1/10, that is, 1 km. The degree is one standard. As another example, in an FWM using a pump light power of about 30 mW with a fiber having a nonlinear coefficient γ of 15 W ⁻¹ km ⁻¹ , the nonlinear length is about 2.2 km, that is, about 1/10 of that. 200m will be one standard. In any case, if the average zero-dispersion wavelength of a fiber that is sufficiently shorter than the nonlinear length is measured and a nonlinear optical medium having a required conversion efficiency is provided by combining the same values, a phase conjugate with a wide conversion band can be obtained. Can be obtained.

  Thus, according to the present invention, a first method for manufacturing a device having a nonlinear optical medium for generating phase conjugate light is provided. In this method, the optical fiber is first cut and divided into a plurality of sections, and then the plurality of sections are rearranged and connected so that the conversion band in non-degenerate four-wave mixing using a nonlinear optical medium is maximized. Together, a nonlinear optical medium is provided. By supplying pump light and signal light to this nonlinear optical medium, phase conjugate light is generated. Since the conversion band from signal light to phase conjugate light is sufficiently wide, for example, when WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths is used as signal light, The plurality of optical signals are collectively converted into phase conjugate light (a plurality of phase conjugate light signals).

  Desirably, the dispersion value of each of the plurality of sections (for example, the dispersion value for the pump light) is measured, and a section having a relatively small dispersion value is arranged on the side close to the input end when the pump light is input to the nonlinear optical medium. A plurality of sections are rearranged as shown. As a result, the phase matching condition can be effectively obtained at 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 are connected so that the positive and negative of the dispersion value are alternated. Thereby, since the average dispersion | distribution of each part of an optical fiber can be restrained small, the conversion band can be expanded effectively.

  The present invention also provides a second method for manufacturing a device having a nonlinear optical medium for generating phase conjugate light. In this method, first, the optical fiber is cut and divided into a plurality of sections, and then dispersion values (for example, dispersion values for the pump light) of the plurality of sections are measured, and thereafter, a non-linear optical medium is used. A non-linear optical medium can be obtained by selecting and connecting only sections having a dispersion value sufficiently small to obtain a required conversion band by degenerate four-wave mixing. Even when a phase conjugator is configured using the nonlinear optical medium obtained by the second method, a wide conversion band is obtained, so that WDM signal light can be converted all at once.

  In each of the first and second methods according to the present invention, the optical fiber is first cut and divided into a plurality of sections, but the present invention is not limited to this. For example, the optical fiber can be cut as necessary as follows.

  That is, according to the present invention, a third method for manufacturing a device having a nonlinear optical medium for generating phase conjugate light is provided. In this method, first, the deviation of the zero dispersion wavelength of the optical fiber is measured, and then the zero dispersion wavelength of each fiber cut and cut when the measured deviation exceeds a predetermined range. Of the optical fiber having a zero dispersion wavelength substantially equal to the wavelength of the pump light, or a cut fiber is selected, and the selected fibers are spliced together. As a result, a nonlinear optical medium is obtained.

  The measurement of the deviation of the zero dispersion wavelength can be performed, for example, using the fact that the generation efficiency of the four-wave mixing differs according to the zero dispersion wavelength. In general, chromatic dispersion can be obtained by measuring the wavelength dependence of the group velocity, but as described above, the phase matching of four-wave mixing is best when the pump light wavelength and the zero-dispersion wavelength match. Therefore, the zero-dispersion wavelength is generated by four-wave mixing (phase conjugate light) with respect to the pump light wavelength in a state where the wavelength difference between the pump light and the signal light is set to a relatively large constant value of, for example, about 10 to 20 nm. The efficiency can be measured and determined as the pump light wavelength that gives the maximum generation efficiency. The generation efficiency of four-wave mixing is proportional to the square of the intensity of the pump light. Therefore, when the zero-dispersion wavelength changes in the longitudinal direction of the optical fiber, in general, the signal light and the pumping light differ depending on whether they are input from one end face of the optical fiber or the other end face. The zero dispersion wavelength is measured. Therefore, the deviation of the zero dispersion wavelength of the optical fiber can be obtained based on the measured values of these two zero dispersion wavelengths. Specifically, it is as follows.

Referring to FIG. 6, a manufacturing process 120 of a nonlinear optical medium having a small deviation of zero dispersion wavelength is shown. In step 122, an allowable range Δλ ₀ of the zero dispersion wavelength is determined. The range Δλ ₀ can be determined as a required characteristic of the system from a required conversion band, and a specific value thereof is, for example, 2 nm. Next, at step 124, the zero dispersion wavelength deviation δλ is measured. For example, given the optical fiber F1, the zero dispersion wavelength λ ₀₁ obtained when the signal light and the pumping light are input from the first end of the optical fiber F1 and the optical fiber F1 due to the generation efficiency of the four-wave mixing described above. The zero-dispersion wavelength λ ₀₂ obtained when the signal light and the pump light are input from the second end is measured. In this case, | λ ₀₁ −λ ₀₂ | can be used as an alternative value for the deviation δλ of the zero dispersion wavelength.

Subsequently in step 126, whether the deviation δλ is smaller than the range [Delta] [lambda] ₀ is determined. Here, to describe the previous flow as a [delta] [lambda] ≧ [Delta] [lambda] _0, in step 128, the optical fiber F1 is divided into two optical fibers F1A and F1B cleavage. Returning to step 124 after step 128, the deviation δλ is measured for each of the optical fibers F1A and F1B, and a determination is made at step 126 for each measured value. Here, assuming that each deviation δλ is smaller than Δλ0, this flow ends. Note that the cut point of the optical fiber F1 in step 128 is arbitrary, and therefore the lengths of the optical fibers F1A and F1B may be equal or different.

In the above description, steps 124 and 126 are repeated, but steps 124 and 126 may or may not be repeated. For example, when the optical fiber F2 having a small deviation of the zero dispersion wavelength is given, the condition is satisfied by the first determination in step 126, and in this case, the optical fiber F2 is not cut. On the other hand, when an optical fiber F3 having a long dispersion of zero dispersion wavelength is given, the optical fiber F3 is divided into optical fibers F3A and F3B in the first step 128, and the optical fiber F3A is divided in the second determination step 126. If the condition is satisfied but the optical fiber F3B does not satisfy the condition, the optical fiber F3B may be divided into the optical fibers F3B1 and F3B2 in the second step 128, and this flow may end. In this case, three optical fibers F3A, F3B1, and F3B2 are obtained from the original optical fiber F3, and the deviation of the zero dispersion wavelength of each fiber is smaller than the allowable range Δλ ₀ .

  A plurality of optical fiber pieces (optical fibers F1A, F1B,...) Obtained in this way are arranged for each value of zero dispersion wavelength, and zero substantially equal to the wavelength of pump light for four-wave mixing. By selecting and splicing optical fiber pieces having a dispersion wavelength so as to obtain a required conversion efficiency, it is possible to obtain a nonlinear optical medium in which variation of the zero dispersion wavelength in the longitudinal direction is extremely small. A wide conversion band can be obtained by configuring a phase conjugator using this nonlinear optical medium.

Even if the values of the zero-dispersion wavelengths λ ₀₁ and λ ₀₂ are substantially the same, an optical fiber having a large variation in the longitudinal direction of the zero-dispersion wavelength is also assumed. For example, this is the case where the longitudinal distribution of the zero dispersion wavelength is symmetric with respect to the longitudinal center of the optical fiber. In such a case, prior to the process 120, the optical fiber may be divided into at least two optical fiber pieces, and the process 120 may be applied to each optical fiber piece. Alternatively, the process 120 may be repeated multiple times.

  The nonlinear optical medium manufacturing method described above can also be applied to PMF. The effect is as follows.

  When ideal dispersion management is realized, the polarization state of each light wave becomes a limiting condition for phase matching. Even if the polarization states of the signal light, the pump light, and the phase conjugate light are matched at the fiber input end, the polarization states of the light waves having different wavelengths gradually shift due to the influence of polarization dispersion during propagation through the fiber. Since this deviation increases as the wavelength deviation increases, the conversion band is limited. The most effective way to circumvent this limitation is to fix the polarization state in the fiber. Specifically, as described in the apparatus of FIG. 1 or FIG. 4, PMF may be used as the nonlinear optical medium. Therefore, the nonlinear constant is large, the management of the zero dispersion wavelength is ideal, and the DSF converted into the PMF is ideal as a fiber four-wave mixer, and the wavelength of the pump light is set to the zero dispersion wavelength of this fiber. By matching, a four-wave mixer having a very wide conversion band can be provided.

  FIG. 7 shows a third embodiment of the device according to the invention. Here, an apparatus that can be used for bidirectional transmission is shown due to the symmetry of the operations of the polarization beam splitter 10 and the PMF 12.

The phase conjugate light E _C1 (wavelength λ _C1 ) is generated by four-wave mixing in the PMF 12 based on the signal light E _S (wavelength λ _S ) and the pump light E _P1 (wavelength λ _P1 ). It is the same. Here, the polarization beam splitter 10 has another port 10D, and is provided in the PFM 12 based on the second signal light E _S2 (wavelength λ _S2 ) and the second pump light E _P2 (wavelength λ _P2 ). In order to generate the second phase conjugate light E _C2 (λ _C2 ) by four-wave mixing, it corresponds to the input port 2, the pump light source 4, the optical coupler 6, the optical circulator 8, the optical bandpass filter 14 and the output port 16, respectively. An input port 2 ', a pump light source 4', an optical coupler 6 ', an optical circulator 8', an optical bandpass filter 14 ', and an output port 16' are provided. The ports 10B and 10D are coupled by a polarization component horizontal to the paper surface, and the ports 10C and 10D are coupled by a polarization component having a polarization surface perpendicular to the paper surface. Accordingly, the second signal light E _S2 is divided into polarization components TM2 and TE2 in the same manner as the signal light E _S1 is divided into the polarization components TM1 and TE1 in the polarization beam splitter 10.

According to this configuration, the generation of the phase conjugate light E _C1 by the signal light E _S1 and the pump light E _P1 is performed in relation to one main axis (Y axis) of the PMF 12, whereas the second signal light E _S2 is generated. The phase conjugate light E _C2 based on the second pump light E _P2 is generated in relation to the other principal axis (X axis) of the PMF 12, and the two phase conjugate lights E _C1 and E _C2 can be generated.

  Accordingly, in a system having a two-channel optical transmission line, by applying the input port 2 and the output port 16 to one channel and applying the input port 2 'and the output port 16' to the other channel, Conversion can be performed on a two-channel transmission line in the apparatus. Although the effects described in the embodiment of FIG. 1 can be obtained for both channels, the details are as described above, and the description thereof is omitted.

  Next, an implementation example of a lightwave network using a phase conjugate light generator will be described. FIG. 8 is a diagram for explaining the principle of the lightwave network.

  An optical transmitter (OS) 202 outputs a signal beam.

  The first optical fiber (fiber span) 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.

  The second optical fiber (fiber span) 208 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 212B corresponding to the input end and the output end of the second phase conjugate beam, respectively.

  An optical receiver (OR) 214 is provided to receive the second phase conjugate beam transmitted by the third optical fiber 212.

  A system intermediate point 216 is set in the middle of the second optical fiber 208. System midpoint 216 will be defined later.

  The second optical fiber 208 includes a first portion 281 between the third end 208A and the system midpoint 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 sections (N is an integer greater than 1) 204 (# 1,..., #N), and the first portion 281 of the second optical fiber 208 is obtained. Are virtually divided into the same number of sections 281 (# 1,..., #N). At this time, the product of the average value of the chromatic dispersion and the section length of the two corresponding sections counted from the first phase conjugate light generator 206 are substantially matched. That is, in the first optical fiber 204, the average value and interval of the chromatic dispersion (or dispersion parameter) of the i (1 ≦ i ≦ N) -th interval 204 (#i) counted from the first phase conjugate light generator 206. The lengths are D _1i and L _1i , respectively, and the chromatic dispersion (or dispersion) of the i-th section 281 (#i) counted from the first phase conjugate light generator 206 in the first portion 281 of the second optical fiber 208. Parameter) average value and interval length are 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 respectively P _2i and γ _2i ,
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 is virtually divided into M (M is an integer greater than 1) sections 282 (# 1,..., #M), and the third optical fiber 212 is obtained. Are virtually divided into the same number of sections 212 (# 1,..., #M).

At this time, the average value of the chromatic dispersion of the j (1 ≦ j ≦ M) -th section 282 (#j) counted from the second phase conjugate light generator 210 in the second portion 282 of the second optical fiber 208. And the section lengths are D _3j and L _3j , respectively, and the average value and section length of the chromatic dispersion of the jth section 212 (#j) counted from the second phase conjugate light generator 210 in the third optical fiber 212 are When D _4j and L _4j respectively,
D _3j L _3j = D _4j L _4j (3)
Is 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 respectively P _4j and γ _4j ,
P _3j γ _3j L _3j = P _4j γ _4j L _4j (4)
Is satisfied.

  In FIG. 8, the waveform distortion once increases before and after the first phase conjugation generator 206, but the chromatic dispersion and nonlinearity are compensated at the system intermediate point 216 by the conditions of the equations (1) and (2). The waveform once returns to the original state. The recovered waveform is again distorted before and after the second phase conjugate generator 210. However, the optical receiver 214 compensates for chromatic dispersion and nonlinearity under the conditions of the equations (3) and (4). The waveform is restored again.

  Further, the configuration of FIG. 8 is tolerant to setting errors of parameters such as the length of the second optical fiber 208 that may be laid on the seabed or the like. That is, even if the waveform does not completely return to the original state at the system intermediate point 216, this imperfection is reproduced by the second portion 282, the second phase conjugate light generator 210, and the third optical fiber 212. By doing so, the waveform can be completely restored in the optical receiver 214.

  Referring to FIG. 9, the principle of chromatic dispersion and nonlinearity compensation is shown. This compensation principle is the same in other embodiments. Here, the principle of compensation from the optical transmitter 202 to the system intermediate point 216 will be described. First, prior to the description of FIG. 9, general items of the phase conjugate wave will be described.

  The propagation of an optical signal E (x, y, z, t) = F (x, y) φ (z, t) exp [i (ωt−kz)] in optical fiber transmission is generally described by the following nonlinear wave equation. Is possible. Here, F (x, y) represents a transverse mode distribution, φ (z, t) represents a light complex envelope, and φ (z, t) changes sufficiently slowly compared to the light frequency ω. Assume that.

ここに、Ｔ＝ｔ−β₁ ｚ（β₁ は伝搬定数）、αはファイバの損失、β₂ はファイバの波長分散を表し、

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

は、３次の非線形係数（光カー効果の係数）を表す。ここに、ｎ₂とＡ_eff はそれぞれファイバの非線形屈折率と有効コア断面積を表す。ｃは真空中の光速である。ここでは１次分散までを考慮し、それより高次の分散は省略した。また、α，β，γはｚの関数であるとし、それぞれα（ｚ），β（ｚ），γ（ｚ）と表されるものとする。更に、位相共役光発生器の位置を原点（ｚ＝０）とする。ここで、以下の規格化関数を導入する。

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 vacuum. Here, up to the first order dispersion was considered, and higher order dispersion was omitted. Further, α, β, and γ are functions of z, and are expressed 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.

ここに、

here,

は、振幅を表し、α（ｚ）＞０の場合は伝送路が損失を持ち、α（ｚ）＜０の場合は利得を持つことをそれぞれ表す。Ａ（ｚ）≡Ａ（０）は損失無しの場合を表す。また、Ａ（ｚ）² ＝Ｐ（ｚ）は光パワーに相当する。（７），（８）式を（５）式に代入すると、次の発展方程式が得られる。

Represents an amplitude. When α (z)> 0, the transmission path has a loss, and when α (z) <0, it represents a gain. A (z) ≡A (0) represents the case of no loss. A (z) ² = P (z) corresponds to the optical power. Substituting equations (7) and (8) into equation (5) yields the following evolution equation:

ここで以下の変換を行う。

Here, the following conversion is performed.

その結果、（９）式は以下のように変換できる。

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

ここで、ｓｇｎ［β₂ ］≡±１は、β₂ ＞０，即ち正常分散の場合には＋１を、β₂ ＜０，即ち以上分散の場合には−１をそれぞれとる。（１１）式が成り立てばその複素共役も成り立ち、次の式が得られる。

Here, sgn [β ₂ ] ≡ ± 1 takes β ₂ > 0, that is, +1 when normal dispersion, and −1 when β ₂ <0, that is, above dispersion. If equation (11) holds, the complex conjugate also holds, and the following equation is obtained.

複素共役光ｕ^* はｕに対する発展方程式と同じ発展方程式に従う。但し、その際の伝搬方向は反転する。この動作は正しく位相共役器の動作である。特に透過型の位相共役器においては上記のことは波長分散とＳＰＭとによる位相シフトを反転させることと等価である。

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

Here, in FIG. 9, it is assumed that the length of the first optical fiber 204 is L ₁ and the length of the first portion 281 of the second optical fiber 208 is L ₂ . The phase conjugate light generator 206 is disposed at the origin z = 0 (ζ = 0) of the z-axis coordinate and the ζ coordinate. The z coordinate and ζ coordinate of the system intermediate point 216 are L ₂ and ζ ₀ , respectively.

In the first optical fiber 204, the signal beam u (Es) propagates according to the evolution 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, in the normalized distance dζ at any two points −ζ and ζ that are symmetrical with respect to the position (ζ = 0) of the phase conjugate light generator 206 on the ζ axis, If the values of the parameters are set so that the coefficients of the two terms are equal, u ^{* at} ζ becomes the phase conjugate wave of u at −ζ. That is, the following two expressions are the conditions.

（１３）式は第１の光ファイバ２０４及び第１の部分２８１の分散の符号が等しい必要性を示している。ファイバ内では、γ＞０，Ａ（ｚ）² ＞０であることを考慮すると、上記条件は次のようにまとめることができる。

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

第１の光ファイバ２０４内の（−ζ）における波長分散とＳＰＭとによる位相シフトは位相共役光発生器２０６により符号が反転する。従って、この位相シフトによる波形歪みは第１の部分２８１内の（ζ）における位相シフトによる歪みにより補償される。このように区間毎に上記のような設定による補償を繰り返していけば、全長に渡る補償が可能となる。

The phase shift due to chromatic dispersion and SPM at (−ζ) in the first optical fiber 204 is inverted in sign by the phase conjugate light generator 206. Therefore, the waveform distortion due to this phase shift is compensated by the distortion due to the phase shift at (ζ) in the first portion 281. Thus, if compensation by the above settings is repeated for each section, compensation over the entire length becomes possible.

  Next, the compensation condition is described in terms of the z coordinate. From equation (15)

を得る。即ち、各区間内での非線形係数と光パワーの積に対する波長分散の比を等しくすることが条件となる。ここで、−ｚ₁ ，ｚ₂ は次の式を満足させる２点である。

Get. 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 equal. Here, −z ₁ and z ₂ are two points that satisfy the following expression.

（１６），（１７）式より（１８），（１９）式が得られる。

Equations (18) and (19) are obtained from equations (16) and (17).

ｄｚ₁ ，ｄｚ₂ はそれぞれ−ｚ₁ ，ｚ₂ における小区間の長さであり、各区間長は当該区間内の分散に反比例するか或いは非線形係数と光パワーの積に反比例する。ここで、分散β₂ と分散パラメータＤの関係、Ｄ＝−（２πｃ／λ² ）β₂を考慮すれば、（１８），（１９）式より以下の関係が得られる。Ｄはｚの関数であり、Ｄ（ｚ）とも表される。

dz ₁ and dz ₂ are the lengths of the small sections at −z ₁ and z ₂ , respectively, and each section length is inversely proportional to the dispersion in the section or inversely proportional to the product of the nonlinear coefficient and the optical power. Here, considering the relationship between the dispersion β ₂ and the dispersion parameter D, D = − (2πc / λ ² ) β ₂ , the following relationship is obtained from the equations (18) and (19). D is a function of z and is also expressed as D (z).

分散及び非線形性についていずれも位相共役光発生器２０６に関して対称な２つの位置の一方における増加分と他方の減少分とが等しいことが補償の条件であることがわかる。

It can be seen that the condition for compensation is that the increase in one of the two positions symmetric with respect to the phase conjugate light generator 206 is equal to the decrease in the other in both dispersion and nonlinearity.

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

In particular, when α, D, and γ are constant and the power fluctuation is small, if the equations (20) and (21) are integrated,
D ₁ L ₁ = D ₂ L ₂ (22)
γ ₁ P ₁ L ₁ = γ ₂ P ₂ L ₂ (23)
Get. Here, P ₁ and P ₂ are average powers in the first optical fiber 204 and the first portion 281, respectively. In addition, D ₁ and γ ₁ are the average value of the dispersion parameter and the average value of the nonlinear coefficient of the first optical fiber 204, respectively. D ₂ and γ ₂ are the average value of the dispersion parameter and the nonlinear coefficient of the first portion 281, respectively. Average value. Equations (22) and (23) agree with the conditions in the compensation method of SPM 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 corresponding to an input end and an output end of a signal beam, respectively, and a phase of the signal beam operatively connected to the second end. A phase conjugate light generator for converting and outputting a conjugate beam; and a second optical fiber having a third end and a fourth end corresponding to an input end and an output end of the phase conjugate beam, respectively. An optical fiber communication system is provided in which the product of the average value and the length of the chromatic dispersion of the 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 Equation (23), 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 value. It substantially coincides with the product of the average value of the optical power and the average value of the nonlinear coefficient in the optical fiber and the length of the second optical fiber.

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

  In FIG. 9, the principle of compensation upstream of the system midpoint 216 is shown. Since the principle of compensation at the downstream side of the system midpoint 216 can be understood in the same way, the description thereof is omitted.

  In the description with reference to FIG. 9, the normalized coordinates are defined by the accumulated value of chromatic dispersion from the phase conjugate light generator 206, as shown in the equation (10). As a result, the required condition is that on the first optical fiber 204 and the first portion 281 where the accumulated value of the chromatic dispersion from the phase conjugate light generator 206 is equal, as shown by the equation (15). The ratio between the product of the optical power and nonlinear coefficient at each of the two points and the chromatic dispersion is substantially the same.

  In FIG. 9, the normalized coordinates may be defined by the accumulated value of the nonlinear effect from the phase conjugate light generator 206 (that is, the accumulated value of the product of the optical power and the nonlinear coefficient). In this case, the ratio of the product of the optical power and nonlinear coefficient and the chromatic dispersion 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 substantially the same.

  As described above, the total dispersion amount of the optical fiber and the total amount of the optical Kerr effect are equal between the first optical fiber and the second optical fiber connected to the phase conjugate light generator. By setting so that the optical pulse waveform input to the first optical fiber and the optical pulse waveform output from the second optical fiber have substantially the same shape, compensation is performed by the phase conjugate light generator. You can see that That is, an optical pulse waveform having substantially the same shape is obtained on the optical pulse transmission side (input end of the first optical fiber) and the optical pulse reception side (output end of the second optical fiber). By providing an optical ADM (Add Drop Multiplexer: optical signal inserting / branching device) at the input end and the output end, the optical ADM can receive the received optical pulse in almost the same state as the transmitted optical pulse. Therefore, in each ADM, it is possible to eliminate the process of regenerating the received optical pulse (waveform shaping / timing recovery) without degrading the SNR of the received optical pulse, and it is possible to construct a highly flexible system. Become. A so-called lightwave network applying this principle will be described below.

  Since the apparatus according to the present invention has a wide conversion band, it can be effectively applied to wavelength division multiplexing. Specifically, it is as follows.

FIG. 10 is a diagram for explaining batch conversion of WDM signal light by a phase conjugator having a wide conversion band. WDM signal light, different wavelengths lambda _1, lambda _2, ..., obtained by wavelength division multiplexing (WDM) optical signals of N channels with lambda _N. Here, it is assumed that λ ₁ is the shortest wavelength and λ _N is the longest wavelength. The wavelength λP of the pump light is set shorter than λ _1, for example. The WDM signal light is converted into converted light by non-degenerate four-wave mixing using pump light. The converted light comprises N-channel converted optical signals having different wavelengths λ ₁ ′, λ ₂ ′,..., Λ _N ′. The arrangement of the optical signal of each channel in the WDM signal light and the arrangement of the converted optical signals in the converted light are symmetric with respect to the wavelength λ _{P of the} pump light.

  In the four-wave mixing using an optical fiber or PMF as a nonlinear optical medium, the conversion band is substantially flat, so that wavelength conversion and phase conjugate conversion can be performed on the optical signal of each channel with substantially the same conversion efficiency. . Therefore, it is possible to compensate for the waveform distortion due to the chromatic dispersion of the transmission path and the nonlinear effect for each channel, and long distance and large capacity transmission is possible. In FIG. 10, the conversion from the long wavelength band to the short wavelength band is shown, but since the conversion band by the optical fiber is symmetric with respect to the zero dispersion wavelength, the conversion from the short wavelength band to the long wavelength band is similarly performed. It goes without saying that it is possible.

FIG. 11 is a diagram showing a second embodiment of the system according to the present invention. A plurality of optical fiber networks NW1, NW2, and NW3 to which each WDM is applied are connected by an optical fiber transmission line 140 and a node 142. A phase conjugator PC11 is provided in the middle of the optical fiber transmission line 140 in order to perform conversion between the networks NW1 and NW2, and in the middle of the optical fiber transmission line 140 in order to perform conversion between the networks NW2 and NW3. A phase conjugator PC23 is provided. In the networks NW1, NW2, and NW3, it is assumed that WDM transmissions in different wavelength bands λ _1j , λ _2j , and λ _3j are performed, respectively. The phase conjugater PC11 performs wavelength conversion and phase conjugate conversion between the wavelength bands _λ1j and _λ2j , and the phase conjugater PC23 performs wavelength conversion and phase conjugate conversion between the wavelength bands _λ2j and _λ3j . In the middle of the optical fiber transmission line 140, there are some positions where the waveform distortion due to the chromatic dispersion and the nonlinear effect is most improved according to the present invention. Therefore, each node 142 is provided at such a position. Each node 142 includes an optical add / drop device for adding and extracting optical signals. The optical add / drop device functions for all channels or a part of channels in the WDM signal light or converted light. For example, if the wavelength band λ _1j of the optical fiber network NW1 is given by the WDM signal light shown in FIG. 10, and the wavelength of the pump light in the phase conjugator PC11 is λp, the wavelength band λ of the optical fiber network NW2 _2j is given by the band of the converted light.

According to such a system configuration, since the waveform distortion compensation and the wavelength conversion function by the phase conjugator are effectively utilized, it is possible to construct a long-distance and large-capacity system rich in flexibility. In addition, such applications for inter-network transmission are particularly important recently:
(1) Broadband optical amplifier;
(2) Variety of dispersion of optical fibers used as transmission lines.

  Of these, (1) relates to the recent widening of the bandwidth of EDFA (erbium-doped fiber amplifier), and (2) relates to the speeding up of transmission signals and dispersion control for performing WDM transmission. Recently, an EDFA having a wide bandwidth exceeding 50 nm and excellent in gain flatness directed to WDM has been developed. In the future, the band will be further expanded, and a broadband EDFA of about 60 to 80 nm will be developed. Such widening of the EDFA is of course useful for increasing the number of WDM channels (transmission capacity), but it is possible to introduce a new concept in inter-network transmission as shown in FIG. And

  For example, as shown in FIG. 12, when the wavelength bands of the optical fiber networks NW1 and NW2 in FIG. 11 are set, effective transmission according to the present invention is possible between the optical fiber networks NW1 and NW2. In FIG. 12, reference numeral 144 denotes a relatively flat gain band of an optical amplifier (for example, EDFA).

  One of the reasons that the wavelength band used for each network is different is that the optical fiber as a transmission path used for each network is different. Optical fibers that have already been put into practical use include a 1.3 μm zero dispersion single mode fiber (so-called standard SMF) and a 1.55 μm dispersion shifted fiber (DSF). On the other hand, with the recent development of EDFA, the center of high-speed and long-distance transmission has become 1.55 μm band. Standard SMF exhibits a large anomalous dispersion value of about +16 to +20 ps / nm / km, whereas DSF can suppress the dispersion value to a small dispersion value of about ± 1 to 2 ps / nm / km. DSF is more advantageous for high-speed and long-distance transmission. However, many standard SMFs have already been laid, and there are many networks that must use these as transmission lines. In connection from such a network to a network using a DSF, wavelength conversion to a wavelength band that provides an optimum dispersion value of the DSF is required. Therefore, the present invention is effective in such a case.

  On the other hand, the present invention is also effective in connection between networks using DSFs. The reason is that a smaller dispersion is not always advantageous in WDM. In a relatively high-speed WDM, the power level of each channel needs to be set to be quite high in order to ensure a required signal-to-noise ratio (SNR). In this case, if the dispersion of the optical fiber used as the transmission path is small, crosstalk between adjacent channels occurs due to four-wave mixing, and transmission characteristics deteriorate. In order to avoid this influence, recently, a relatively large dispersion fiber (Nonzero dispersion-shifted fiber) in which the zero dispersion wavelength is greatly shifted from the signal band may be used. Since the variety of optical fibers used as transmission paths is thus rich, network configurations in various wavelength bands are possible. When connecting between such networks, wideband wavelength conversion and Phase conjugate conversion is effective.

  Recently, the variety of EDFAs is increasing along with optical fibers. However, the general EDFA is a type having gain peaks in 1.53 μm band and 1.55 μm band. Of these, the former is called the blue band and the latter is called the red band.

  FIG. 13 is a diagram illustrating another setting example of the wavelength band in FIG. Here, the wavelength band of the optical fiber network NW1 is included in the red band of the EDFA indicated by reference numeral 146, and the wavelength band of the optical fiber network NW2 is included in the blue band of the EDFA indicated by reference numeral 148. According to such a setting, when the optical fiber transmission line 140 or each network includes an inline EDFA, phase conjugate conversion between the red band and the blue band can be easily performed.

FIG. 14 is a diagram showing an example of the distributed arrangement in FIG. D ₁ and D ₂ (each unit is ps / nm / km) represent dispersion in the optical fiber networks NW1 and NW2, respectively. In the figure, an example of performing WDM using normal dispersion fiber in each network is shown. As shown in FIG. 10, since the channel arrangement is inverted by wavelength conversion, the influence of dispersion before and after conversion on each channel is expected to be different. However, the influence of dispersion on the channel near the center is almost the same. This problem can be solved by performing dispersion compensation in each network. The distribution within each network may be normal distribution or abnormal distribution.

As described above, according to the present invention, a plurality of optical fiber networks for WDM signal light obtained by wavelength division multiplexing (WDM) of a plurality of optical signals having different wavelengths, and at least one conversion for connecting them. An optical fiber communication system is provided. When the converter performs wavelength conversion and phase conjugate conversion of a plurality of optical signals at once, it is easy to construct a flexible long-distance and large-capacity system.

  Recently, an EDFA having a flat gain over the entire red band and blue band has been developed. An optical amplifier having a band of about 40 nm (generally 1525 to 1565 nm) is realized by co-doping mainly aluminum with erbium to a high concentration and combining a gain equalizer. This band may be referred to as the C band.

  On the other hand, an optical amplifier having a flat gain in the 1570-1610 nm band longer than the C band is also being developed. This band may be referred to as the L band. This optical amplifier is called a gain-shifted EDFA, and the length of an EDFA co-doped with aluminum is made longer than that of a C-band to ensure a gain in the L-band that originally has a small gain per unit length. It is.

  An optical amplifier having a band near 80 nm has been developed by combining a C-band optical amplifier and an L-band optical amplifier in parallel. Furthermore, an optical amplifier having a bandwidth of nearly 80 nm is being developed using telluride-based EDFA.

  FIG. 15 is a diagram showing another setting example of the wavelength band in FIG. Here, the wavelength band of the optical fiber network NW1 is included in the L band as indicated by reference numeral 152, and the wavelength band of the optical fiber network NW2 is included in the C band as indicated by reference numeral 154.

  Thus, according to the present invention, since a wide conversion band can be obtained, the combination of the device according to the present invention and a broadband optical amplifier is extremely useful in constructing a flexible long-distance and large-capacity system. is there.

  As shown in FIG. 16, generally, when WDM signal light is input to the four-wave mixer FWM, WDM signal light and converted light are respectively output. A four-wave mixer FWM can be provided by the device according to the invention. In a four-wave mixer having a sufficiently high efficiency, the signal light is amplified by the parametric effect. In this case, if the parametric gain for the signal light is G, the converted light has a gain of (G-1). As described above, the four-wave mixer has a great feature that it can pass through the traffic light and output the converted light, and can be applied to the WDM network as described below by utilizing this feature.

FIG. 17 is a diagram showing a third embodiment of the system according to the present invention. Here, the optical fiber networks NW1 and NW2 as described with reference to FIG. 11 are connected by a four-wave mixer FWM to enable conversion between the wavelength band λ _1j and the wavelength band λ _2j . As the four-wave mixer FWM, for example, an apparatus shown in FIG. 7 capable of bidirectional transmission can be used.

  FIG. 18 is a diagram showing a fourth embodiment of the system according to the present invention. Each of the optical fiber networks NW1 and NW2 shown in FIG. 17 should be interpreted as including a function as a normal transmission path. That is, when bidirectional transmission as shown in FIG. 18 is performed, the downlink and uplink should be understood as optical fiber networks NW1 and NW2, respectively.

In the system shown in FIG. 18, in order to perform loopback from the downlink to the uplink, the wavelength band λ _1j is converted into the wavelength band λ _2j by the four-wave mixer FWM, and the loopback from the uplink to the downlink is performed. For this purpose, the wavelength band λ _2j is converted to the wavelength λ _1j by another four-wave mixer FWM. As each four-wave mixer FWM, the apparatus shown in FIG. 1 or FIG. 4 can be used. Alternatively, these two four-wave mixers FWM may be provided by one device as shown in FIG.

  So far, the application of exchanging all of the wavelength-converted WDM signal light has been described so far, but by combining with the above-mentioned optical ADM, any channel of the WDM signal light can be exchanged. It becomes possible to construct a more flexible system.

FIG. 19 is a diagram showing a fifth embodiment of the system according to the present invention. Here, the WDM signal light in the wavelength band λ _1j in the optical fiber network NW1 is converted into converted light by the four-wave mixer FWM, and the optical add multiplexer ADM extracts and extracts an arbitrary wavelength channel based on the converted light. A signal is sent to the optical fiber network NW2. Also, the WDM signal light in the wavelength band λ _2j in the optical fiber network NW2 is converted into converted light by another four-wave mixer FWM, and based on this converted light, the optical add multiplexer ADM extracts a channel of an arbitrary wavelength, The extracted signal is sent to the optical fiber network NW1.

  FIG. 20 is a diagram showing a sixth embodiment of the system according to the present invention. In contrast to the system shown in FIG. 19, this system is characterized in that each four-wave mixer FWM is arranged inside each of the optical fiber networks NW1 and NW2. Accordingly, in each of the optical fiber networks NW1 and NW2, the WDM signal light in each wavelength band can pass through the four-wave mixer FWM. This configuration can also be applied to bidirectional transmission and the like, similar to the system shown in FIG.

  FIG. 21 is a diagram showing a seventh embodiment of the system according to the present invention. Wavelength channel signals appropriately extracted by an optical add multiplexer ADM sequentially arranged in the transmission path are converted into wavelengths of the optical fiber networks NW1 and NW2 by the four-wave mixer FWM and then sent to the networks. . On the other hand, the WDM signal light of each network is also wavelength-converted to the band of the transmission path by the four-wave mixer FWM, and a signal of a necessary wavelength channel is sent to the transmission path.

  FIG. 22 is a diagram showing an eighth embodiment of the system according to the present invention. In contrast to the system shown in FIG. 11, this system is characterized in that an optical add multiplexer ADM is provided in place of each node 142. That is, each optical add multiplexer ADM is provided at a point where the waveform distortion is most compensated by each phase conjugate light generator PC, and a signal of an arbitrary wavelength channel is input / output here.

FIG. 23 is a diagram showing a ninth embodiment of the system according to the present invention. Here, a common wavelength band λ _S is set in the optical fiber transmission line 140, and each phase conjugate light generator PC is provided in each of the optical fiber networks NW1, NW2, and NW3. In each phase conjugate light generator PC, wavelength conversion and waveform distortion compensation are performed, and a signal of an arbitrary wavelength is sent to the optical fiber transmission line 140 by the optical add multiplexer ADM, and the optical add from the optical fiber transmission line 140 A signal of an arbitrary wavelength channel extracted by the multiplexer ADM is converted into a wavelength band of each network by using the phase conjugate light generator PC and captured.

FIG. 1 is a diagram showing a first embodiment of the apparatus according to the present invention. FIG. 2 is an explanatory diagram of the operating principle of the apparatus according to the present invention. FIG. 3 is an explanatory diagram of the polarization mode of the PMF (polarization maintaining fiber). FIG. 4 shows a second embodiment of the device according to the invention. FIG. 5 is a diagram showing a first embodiment of the system according to the present invention. FIG. 6 is a diagram illustrating an example of a manufacturing process of a nonlinear optical medium. FIG. 7 shows a third embodiment of the device according to the invention. FIG. 8 is a diagram for explaining the basic principle of a lightwave network. FIG. 9 is an explanatory diagram of the compensation principle in FIG. FIG. 10 is a diagram for explaining batch conversion of WDM signal light. FIG. 11 is a diagram showing a second embodiment of the system according to the present invention. FIG. 12 is a diagram showing a setting example of the wavelength band in the system according to the present invention. FIG. 13 is a diagram showing another setting example of the wavelength band in the system according to the present invention. FIG. 14 is a diagram showing an example of distributed arrangement in the system according to the present invention. FIG. 15 is a diagram showing still another example of setting the wavelength band in the system according to the present invention. FIG. 16 is an explanatory diagram of a four-wave mixer. FIG. 17 is a diagram showing a third embodiment of the system according to the present invention. FIG. 18 is a diagram showing a fourth embodiment of the system according to the present invention. FIG. 19 is a diagram showing a fifth embodiment of the system according to the present invention. FIG. 20 is a diagram showing a sixth embodiment of the system according to the present invention. FIG. 21 is a diagram showing a seventh embodiment of the system according to the present invention. FIG. 22 is a diagram showing an eighth embodiment of the system according to the present invention. FIG. 23 is a diagram showing a ninth embodiment of the system according to the present invention.

Explanation of symbols

NW1 First wavelength division multiplexing network NW2 Second wavelength division multiplexing network FWM Four-wave mixer

Claims (1)

A first wavelength division multiplexing network for performing wavelength division multiplexing transmission in a first wavelength band;
A second wavelength division multiplexing network for performing wavelength division multiplexing transmission in the second wavelength band;
A four-wave mixer disposed between the first WDM network and the second WDM network;
The four-wave mixer receives light in the first wavelength band from the first wavelength multiplexing network, converts the light into a third wavelength band that does not overlap the second wavelength band, and outputs the third wavelength band to the second wavelength multiplexing network. And
The four-wave mixer has first, second, and third ports, and the light of the first wavelength band from the first wavelength division multiplexing network supplied to the first port is supplied to the second port. And the light supplied to the second port is output from the third port. The first port includes signal light and pump light including first and second polarization components whose polarization planes are orthogonal to each other. And an optical element output from the second port;
The first port is optically connected to the second port of the optical element, and the plane of polarization of the first polarization component is between the first port and the second port. A polarization beam splitter coupled by a first polarization plane, and the first and third ports are coupled by a second polarization plane which is a polarization plane of the second polarization component;
A polarization mode to be maintained between the first end and the second end, wherein the first end is a second of the polarization beam splitter such that the first polarization plane is adapted to the polarization mode. A polarization maintaining fiber optically connected to the port, the second end optically connected to the third port of the polarization beam splitter such that the second polarization plane is adapted to the polarization mode; With
The third wavelength band by four-wave mixing generated in the polarization maintaining fiber by signal light and pump light having a first polarization plane and output from the second port of the polarization beam splitter to the first end Is supplied to the third port of the polarization beam splitter from the second end,
The third wavelength band by four-wave mixing generated in the polarization maintaining fiber by signal light and pump light having a second polarization plane and output from the third port of the polarization beam splitter to the second end Is supplied from the first end to the second port of the polarization beam splitter,
The first and second phase conjugate lights are supplied from the first port of the polarization beam splitter to the second port of the optical element, and output to the second wavelength multiplexing network through at least the third port of the optical element. optical transmission system characterized in that it is.

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