JP3004268B1 - 2 optical signal generator - Google Patents

Abstract

A two-optical signal generator capable of easily changing the optical frequencies of two optical signals by the same amount as compared with a conventional example while maintaining the optical frequency difference. SOLUTION: Light sources 21 and 40 generate first and second optical signals having polarization directions orthogonal to each other, and an optical fiber cable 33 retains polarization and has saturable absorption characteristics.
The polarizing beam splitter 32 enters the first optical signal into the optical fiber cable 33, enters the second optical signal into the optical fiber cable 33, and reflects the first optical signal output from the other end by the reflecting mirror 35. Optical fiber cable 3
3, a standing wave distribution of the first optical signal is generated in the optical fiber cable 33, and a grating portion is optically written on the optical fiber cable 33 to be formed.
The formed grating section generates a second optical signal whose optical frequency is selected so as to have an optical frequency difference corresponding to the refractive index difference between the two optical signals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to an optical fiber link system and the like, and has a predetermined optical frequency difference (optical wavelength difference).
And a two-optical signal generator for generating two optical signals capable of changing the optical frequency (optical wavelength) of the two optical signals by the same amount.

[0002]

2. Description of the Related Art An optical fiber link system modulates a digital data signal into an optical signal and transmits it to a radio base station. What to send.

FIG. 6 is a block diagram showing a configuration of a first conventional optical fiber link system. In FIG. 6, a light source 1 such as a semiconductor laser converts an optical signal modulated by an input digital data signal into a first optical signal (optical frequency f1) via an optical multiplexer 3 and an optical splitter 4. Output to the amplifier 5. On the other hand, the light source 2 such as a semiconductor laser has an optical frequency
And outputs the generated optical signal to the optical amplifier 5 via the optical multiplexer 3 and the optical splitter 4 as a second optical signal (optical frequency f2). Here, the difference in optical frequency | f1-f
2 | is, for example, several tens to several hundreds GHz as shown in FIG.
Is set to the millimeter-wave band radio frequency. The optical amplifier 5 power-amplifies the input optical signal, and then,
The signal is transmitted to the optical receiver 200 via the optical fiber cable 300 connecting the optical receiver 200 in the wireless base station.

On the other hand, a mixed optical signal obtained by mixing the first and second optical signals branched from the optical splitter 4 is photoelectrically converted by a photoelectric converter 6 such as a high-speed photodiode having nonlinear photoelectric conversion characteristics. After that, the signal is converted into a lower-frequency high-frequency signal by a frequency converter including the millimeter-wave signal oscillator 7 and the mixer 8. Next, from the components of the converted high-frequency signal, a high-frequency signal proportional to the difference | f1−f2 | Output to the controller 10. With the optical frequency loop circuit configured as described above, the optical frequency controller 10
On the basis of the input high-frequency signal, the optical frequency f2 of the second optical signal generated by the light source 2 is calculated by the difference |
is controlled so that f1-f2 | is constant. That is,
The photoelectric converter 6 extracts interference components of the two optical signals so that the difference between the oscillation frequencies of the two light sources 1 and 2 corresponds to the millimeter wave frequency, and compares the interference components with the millimeter wave signal generator 7. The optical frequency of one light source 2 is controlled by the error signal. Regarding the optical transmitter 101, for example, the related art document 1 “RP Braun, et al.,” “Optical millimetre”
-wave generation and transmission experiments for
mobile 60GHz band Communications ”, Electronics Let
ters, Vol. 32 pp. 626-627, 1996 ".

In the optical receiver 200, the optical amplifier 11 receives an optical signal via the optical fiber cable 300 and outputs the signal to the photoelectric converter 12. The photoelectric converter 12 includes a high-speed photodiode having non-linear photoelectric conversion characteristics. The photoelectric converter 12 photoelectrically converts an input optical signal and outputs the optical signal to the band-pass filter 13. As shown in FIG. 8, the band-pass filter 13 converts the optical frequency difference f0 = |
After extracting a millimeter-wave band radio signal corresponding to f1−f2 |, the radio signal is output to the radio transmitter 14. The wireless transmitter 14
A power amplifier is provided, which amplifies power of an input radio signal and transmits the amplified radio signal via the antenna 15.

FIG. 9 shows a two-optical signal generator 2 according to a second conventional example.
It is a block diagram which shows the structure of 0e. In the second conventional example, a rare-earth-doped polarization-maintaining optical fiber cable (hereinafter, referred to as a polarization-maintaining optical fiber cable) used for an optical fiber amplifier or the like and doped with a rare earth material such as Er and maintaining the polarization. The two fiber grating portions 23 and 24 are formed by writing a refractive index grating by irradiating ultraviolet rays to two portions 33), and a resonance structure is formed between the two fiber grating portions 23 and 24. Here, the polarization maintaining optical fiber cable 22 is, as shown in FIG.
A clad 22c is formed around the core 22a,
The tension member 22b is inserted into the clad 22c.

The light source 2 which is, for example, a semiconductor laser
The optical signal having a predetermined linear polarization generated by the optical fiber 1 is incident on the polarization-maintaining optical fiber cable 22 having two fiber grating sections 23 and 24 via one end thereof. As a result, as shown in FIGS. 11 and 12, between the two fiber grating portions 23 and 24, the effective refractive indices n ₁ and n ₂ are different between the orthogonal polarizations. The laser oscillates at optical frequencies different from each other due to the optical frequency difference of the band, and two optical signals of, for example, the millimeter wave band that are laser-oscillated are output from the other end of the polarization maintaining optical fiber cable 22. Regarding this second conventional example, see, for example, prior art document 2 “WH Loh, et al.,” 40
GHz optical-millimetre wave generation with a dual
polarization distributed feedback fiber laser ”, E
electronics Letters 1997 vol. 33 pp.594-595,1997
Year ".

In the second conventional example, reflection at the fiber grating portions 23 and 24 occurs according to the following equation.

2n _i Λ = mλ _i , where i = 1,2, where m is a predetermined natural number, Λ is the period of the fiber grating sections 23 and 24, and λ _i is an optical wavelength. is there. Effective refractive index difference | n ₁ of fiber grating portions 23 and 24 in polarization maintaining optical fiber cable 22
−n ₂ | can be set between 1/1000 and 1/100000, so that the frequency difference corresponding to the millimeter wave frequency is 2
One optical signal can be generated.

[0009]

However, in the first conventional example, in the optical transmitter 101 of the optical fiber link system, a high-frequency signal such as the photoelectric converter 6 and the millimeter-wave signal oscillator 7 having broadband photoelectric conversion characteristics is provided. In addition to the necessity of a device, there is also a limitation in the control system pull-in range with respect to the oscillation frequency difference. There was a problem that it was difficult to shift.

In the second conventional example, it is difficult to make the absolute optical frequencies of two optical signals variable. Further, there is a gain competition between the orthogonal polarizations inside the resonator between the two fiber grating sections 23 and 24, so that the oscillation intensity difference between the two optical signals cannot be controlled separately, and the generation of a millimeter-wave band radio signal occurs. There was a problem that efficiency was affected.
When the length of the optical fiber cable 300 as the transmission path is increased, the introduced optical fiber cable 30
It is necessary to consider the dispersion characteristic of 0, and it is necessary to oscillate two optical signals in a frequency range where the chromatic dispersion value is small.
Therefore, there is a problem that it is necessary to appropriately select the grating period of the fiber grating sections 23 and 24 in the polarization maintaining optical fiber cable 22 according to the introduced optical fiber cable 300.

An object of the present invention is to solve the above problems and solve the above problem.
An object of the present invention is to provide a two-optical signal generator that can easily change the optical frequency (optical wavelength) of two optical signals by the same amount as compared with the conventional example while maintaining the optical frequency difference.

[0012]

According to a first aspect of the present invention, there is provided a two-optical signal generator for generating a first optical signal having a first polarization direction and a first optical frequency. And the first
A second light source for generating a second optical signal having a second optical frequency having a second polarization direction substantially orthogonal to the polarization direction of the second light source; an optical fiber cable having polarization maintaining and saturable absorption characteristics; A polarizing means inserted between the first light source and the second light source and one end of the optical fiber cable, and connected to the other end of the optical fiber cable to reflect the first optical signal And a light reflecting means for inputting a first optical signal from the first light source to the optical fiber cable through one end thereof, and a second light signal from the second light source. An optical signal is incident on the optical fiber cable via one end thereof, and a first optical signal output from the other end of the optical fiber cable is reflected by the light reflecting means via the other end to the optical fiber cable. By entering Generating a standing wave distribution of the first optical signal in the optical fiber cable to optically write a grating portion in the optical fiber cable to form the grating portion; and forming the grating portion in the optical fiber cable by the formed grating portion. An optical frequency shifter configured to generate a second optical signal whose optical frequency is selected so as to have an optical frequency difference corresponding to a refractive index difference between the first and second optical signals, and a second optical signal from the first light source; By changing the optical frequency of one optical signal, the optical frequency of the second optical signal is changed while maintaining the optical frequency difference between the first and second optical signals.

Further, the two-optical signal generator according to claim 2 is
2. The two-optical signal generator according to claim 1, wherein said first and second light sources are semiconductor lasers.

Further, the two-light signal generator according to the third aspect is the two-light signal generator according to the first aspect, wherein the first light source is a semiconductor laser, and the second light source is a feedback type optical oscillator. There is a feature.

Further, the two-optical signal generator according to claim 4 is
2. The two-optical signal generator according to claim 1, wherein said first and second light sources are semiconductor lasers, and said light reflecting means comprises:
A diffraction grating, and a rotation mechanism for changing a reflection angle of the first optical signal by changing an inclination angle of the diffraction grating, and changing the inclination angle of the diffraction grating by the rotation mechanism. The optical frequency of one optical signal is changed.

Further, a two-optical signal generator according to a fifth aspect of the present invention is the two-optical signal generator according to the first aspect, wherein the two-optical signal generator is formed at the other end of the optical fiber cable instead of the light reflecting means. A fiber grating for reflecting the first optical signal; and a changing means for changing the temperature or the tension of the fiber grating. The changing means changes the temperature or the tension of the fiber grating, thereby changing the temperature or the tension. The optical frequency of the first optical signal is changed.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment> FIG. 1 is a block diagram showing the configuration of an optical fiber link system according to a first embodiment of the present invention. FIG. 2 is a block diagram showing the two-optical signal generator shown in FIG. It is a block diagram which shows the structure of 20a. The optical fiber link system of the first embodiment is characterized by including two optical signal generators 20 as compared with the first conventional example of FIG. 6, and other configurations are the same as those of the first conventional example. The same is true.

In FIG. 2, a light source 21 is, for example, a semiconductor laser and has a first polarization direction Df1 (optical isolator 31) generated according to an input digital data signal.
2 is perpendicular to the optical axis of, for example, the plane of FIG. In other words, the polarization direction is a direction perpendicular to the longitudinal direction of the rare-earth-doped polarization-maintaining optical fiber cable 33. ), And modulates the intensity of the linearly polarized single-mode optical signal (first optical signal (optical frequency f
1) is output to the optical distributor 30. The optical splitter 30 splits the input first optical signal into two, outputs the split first optical signal to an external device as a first optical signal, and outputs a rare earth-doped polarization maintaining optical fiber cable via an optical isolator 31 and a polarizing beam splitter 32. (Hereinafter, it is referred to as a polarization maintaining optical fiber cable). Here, the polarization beam splitter 32 is provided with the optical isolator 31.
The first optical signal having the first polarization direction Df1 is passed through the polarization maintaining optical fiber cable 33 as it is.

The polarization maintaining optical fiber cable 33 is made of Er.
As shown in FIG. 10, a rare earth material such as 500 to 1000 ppm is added, and the optical fiber cable is polarization-maintained. Polarization maintaining optical fiber cable 33
Is connected to a reflecting mirror 35 and an optical terminator 36 via a polarizing beam splitter 34. Here, while the reflecting mirror 35 is connected to the first polarization direction Df1 side of the polarization beam splitter 34, the second polarization direction Df2 (the longitudinal direction of the polarization-maintaining optical fiber cable 33) is orthogonal to the first polarization direction Df1. The optical terminator 36 is connected to the polarization direction parallel to the direction.).

A semiconductor laser device 40 is connected to the polarization beam splitter 32 on the second polarization direction Df2 side. The semiconductor laser device 40 has a known structure. For example, a semiconductor laser medium 42 made of, for example, InP GaAs is provided on one end surface of a semiconductor multilayer film having a relatively high reflectance with respect to a predetermined wavelength. While the reflection film 43 is formed, an antireflection film 41 which is, for example, an AR coat layer (here, AR is an abbreviation of Anti Reflection) is formed on the other end surface of the semiconductor laser medium 42. Second polarization direction Df generated by semiconductor laser device 40
2 is transmitted to the polarization maintaining optical fiber cable 3 through the polarization beam splitter 32.
3 via one end thereof, and output to an external device as a second optical signal (optical frequency f2).

In the optical fiber cable 33 to which a rare earth material such as Er is added, a grating is formed optically by the following method. That is, a rare earth-doped optical fiber cable 3 used in an optical amplifier or the like.
No. 3 has a high absorption coefficient of infrared light and behaves as a laser medium when the light intensity of the input light exceeds a threshold. However, the input light having an intensity less than the threshold can be treated as a saturable absorption medium. The absorption coefficient can be changed depending on the intensity of the input light. For strong input light, it becomes a transparent transparent body, and for weak input light, it becomes an absorber with a large absorptance. That is, a standing wave distribution that changes at the optical frequency f1 occurs. Therefore, as shown in FIG. 2, a first optical signal of linearly polarized light single mode is incident from one end of the polarization maintaining optical fiber cable 33 doped with the rare earth material,
The incident first optical signal is specularly reflected by the reflecting mirror 35 and input from the other end of the polarization maintaining optical fiber cable 33, that is, the first optical signal is incident from both ends thereof, and the rare earth-doped polarization maintaining optical fiber By generating a standing wave of light in the cable 33, the absorption coefficient in the longitudinal direction of the optical fiber cable 33 changes at a half cycle of the light wavelength, and the refractive index also changes periodically. Thereby, the fiber grating portion can be optically written and formed.

At this time, an optical signal having a polarization direction Df2 orthogonal to the first optical signal is transmitted to the semiconductor laser device 4.
When the light enters the polarization-maintaining optical fiber cable 33 from 0 through the polarization beam splitter 32, the optical grating fiber portion written optically causes the effective refractive index n ₁ orthogonal to the optical frequency f2 in the polarization-maintaining optical fiber cable 33 to be orthogonal to each other. , N ₂ , only the optical signal having the optical frequency difference corresponding to the difference | n ₁ −n ₂ | is reflected and selected by the fiber grating unit. Here, each design of the semiconductor laser device 40 is designed such that the semiconductor laser device 40 oscillates at an optical frequency f2 shifted from the optical frequency f1 by an optical frequency difference corresponding to the difference | n ₁ −n ₂ | of the effective refractive index. Set parameters.

The above equation 1 is rewritten as follows.

2n _i Λ = mλ _i , where i = 1,2, where m is a predetermined natural number, Λ is the period of the fiber grating section, and λ _i is the optical wavelength.

In the above equation (2), n ₁ and n ₂ are optical frequencies f
2, the period ２ of the fiber grating portion can be changed by changing the optical frequency f1, and as is apparent from Equation 2, by changing the optical frequency f1, the optical wavelength λ is changed. _i can be varied. Here, the difference | n ₁ −n ₂ |
Is constant, the optical frequency difference | f1-f2 | is constant.

Therefore, the polarization maintaining optical fiber cable 33
Light source 21 when writing fiber grating section to
For example, if the oscillation wavelength of the laser is changed by changing the bias current of the semiconductor laser, the pitch of the fiber grating also changes according to the oscillation wavelength. Absolute optical frequency f1, f
2 can be varied. That is, the fiber grating portion written in the optical fiber cable 33 to which the rare earth material is added functions as an optical frequency shifter.

As described above, according to the first embodiment, even if the optical frequency of the first light source 21 is changed, the second
Frequency f2 of the semiconductor laser device 40 as the light source
Can be changed so as to maintain the oscillation frequency difference from the optical frequency f1 of the first optical signal. In addition, a configuration in which the intensities of the two light sources are equalized is also possible. Further, since an optical frequency shifter serving as a frequency reference is formed outside the optical amplification region of the polarization maintaining optical fiber cable 33, there is no competition for gain in the optical amplification section which is a problem in the second conventional example.
Further, the two-optical signal generator 20a having a very simple configuration and high reliability as compared with the first conventional example can be realized.

<Second Embodiment> FIG. 3 is a block diagram showing a configuration of a two-optical signal generator 20b according to a second embodiment of the present invention. The two-optical signal generator 20b of the second embodiment is different from the two-optical signal generator 20 of the first embodiment in FIG.
a, in place of the semiconductor laser device 40,
A second light source comprising the loop optical circuit 50 is provided. Hereinafter, the difference will be described in detail.

In FIG. 3, the loop optical circuit 50 includes an optical circulator 51, an optical amplifier 52, and an optical splitter 53.
The optical band-pass filter 54 and the optical isolator 55 are connected in a loop shape to a relatively wide pass band covering the band of the oscillation wavelength, thereby forming a feedback optical oscillator. Second polarization direction Df2 of polarization beam splitter 32
The side terminal is connected to a first terminal of an optical circulator 51, and an optical signal input to the first terminal is passed through the second terminal to an optical amplifier 52, an optical splitter 53, an optical bandpass filter 54, The signal is input to the third terminal of the optical circulator 51 via the optical isolator 55. The optical signal input to the third terminal of the optical circulator 51 is output from the first terminal to the deflection beam splitter 32. Here, the oscillation light frequency of the feedback optical oscillator constituted by the loop optical circuit 50 is determined by the distance in the loop, and is set to be the light frequency f2 as in the first embodiment. Then, the optical splitter 53 splits and outputs the second optical signal oscillated by the feedback type optical oscillator to an external device.

The two-optical signal generator 20b of the second embodiment configured as described above operates and has the same effect as the first embodiment except for the loop optical circuit 50.

<Third Embodiment> FIG. 4 is a block diagram showing a configuration of a two-optical signal generator 20c according to a third embodiment of the present invention. The two-optical signal generator 20c of the first embodiment shown in FIG.
(1) Instead of the light source 21, a semiconductor laser device 60
Is provided. (2) A semiconductor laser device 70 is provided instead of the semiconductor laser device 40. (3) Instead of the reflecting mirror 35, an optical lens 37, a diffraction grating 38 and a rotation mechanism 38a thereof are provided. Here, the third
The two-optical signal generator 20c of the embodiment has the polarization maintaining optical fiber cable 33 in the resonator of the external resonator type laser device including the semiconductor laser device 60 and the diffraction grating 38, and the wavelength is selected by the diffraction grating 38. It is characterized in that the standing wave distribution according to the optical frequency of the light wave is written in the polarization-maintaining optical fiber cable 33 doped with Er.
Hereinafter, the difference will be described in detail.

In FIG. 4, a semiconductor laser device 60
Is a semiconductor laser medium 61 such as InPGaAs, for example.
An AR coat layer 62 for preventing reflection is formed on one end surface of the optical fiber, and a linearly polarized single-mode optical signal having a first polarization direction Df1 is converted into a polarization beam splitter 32, a polarization maintaining optical fiber cable 33, and a polarization maintaining optical fiber cable 33. Beam splitter 3
4. The laser is confined and excited in the resonator of the external resonator type laser device by the optical lens 37 and the diffraction grating 38. At this time, the polarization maintaining optical fiber cable 33 includes:
The standing wave distribution is written, forms a grating, and the linearly polarized single-mode optical signal having the first polarization direction Df1 modulates the intensity of the input optical signal according to the input digital data signal. For example, the light is output to an external device as a first optical signal via a known external optical modulator 65 such as a Mach-Zehnder optical modulator. Here, the oscillation light frequency of the first optical signal is the diffraction grating 38.
, Ie, the reflection angle, can be changed by changing the rotation mechanism 38a.

In the semiconductor laser device 70, an AR coat layer 72 for preventing reflection is formed on one end surface of a semiconductor laser medium 71 such as InGaAs, for example, and the second optical signal is formed by the first optical signal. By reflecting the written grating in the polarization maintaining optical fiber cable 33 as a reflector, resonance occurs between the semiconductor laser device 70 and the grating. Here, a second polarization direction D orthogonal to the first polarization direction Df1
A linearly-polarized single-mode optical signal having a frequency f2 is generated as a second optical signal (optical frequency f2) and is incident on one end of a polarization-maintaining optical fiber cable 33 via a polarization beam splitter 32 and output to an external device. I do.

The two-optical signal generator 20c of the third embodiment configured as described above operates in the same manner as in the first embodiment and has the same effects. Further, the optical frequency of the first optical signal is changed by changing the inclination angle of the diffraction grating 38. At this time, the variable range of the optical frequency of the first optical signal is compared with that of the first embodiment. Can be widened. Thereby, the optical frequency f of the first and second optical signals
The variable range of 1, f2 can be made wider than in the first embodiment.

<Fourth Embodiment> FIG. 5 is a block diagram showing a configuration of a two-optical signal generator 20d according to a fourth embodiment of the present invention. The two-optical signal generator 20d according to the fourth embodiment is different from the two-optical signal generator 20d according to the third embodiment in FIG.
(1) The device 23, 36, 37, 38, 38a at the other end of the polarization-maintaining optical fiber cable 33 is not provided, and the other end of the polarization-maintaining optical fiber cable 33 is irradiated with ultraviolet light. And a temperature controller 39 for controlling the temperature of the fiber grating portion 33a. Hereinafter, the difference will be described in detail.

In FIG. 5, the other end of the polarization-maintaining optical fiber cable 33 emits ultraviolet rays to form a fiber grating portion 33a having a periodic pitch at which the first optical signal of the optical frequency f1 is reflected.
As is well known, the periodic pitch at which the first optical signal is reflected decreases the optical frequency f1 by increasing the temperature of the fiber grating portion 33a, and increases the optical frequency f1 by decreasing the temperature. be able to. That is, by changing the temperature of the fiber grating section 33a by the temperature controller 39, the first
And the optical frequency f2 of the semiconductor laser device 40 as the second light source is changed so as to maintain the oscillation frequency difference from the optical frequency f1 of the first optical signal. it can.

The two-optical signal generator 20d of the fourth embodiment configured as described above operates in the same manner as the first embodiment and has the same effects. Further, the two-optical signal generator 20d of the fourth embodiment has a simpler configuration and can reduce the manufacturing cost as compared with the first embodiment.

In the above-described fourth embodiment, the temperature of the fiber grating section 33a is controlled. However, the present invention is not limited to this.
May be wound around a cylindrical drum (not shown) to apply tension so as to pull the optical fiber cable of the fiber grating portion 33a. In this case, the first in the fiber grating portion 33a
As is known, the optical frequency f1 can be lowered by increasing the tension of the fiber grating portion 33a, while increasing the optical frequency f1 by decreasing the tension. . That is, the fiber grating unit 3 is controlled by the tension controller.
By changing the tension of 3a, the optical frequency of the first light source 21 is changed, and the optical frequency f2 of the semiconductor laser device 40, which is the second light source, is compared with the optical frequency f1 of the first optical signal. It can be changed so as to keep the oscillation frequency difference.

<Modification> In the above embodiment, the first
Is orthogonal to the second optical signal, but the present invention is not limited to this, and it is sufficient that the optical signal is substantially orthogonal so that it can be polarized by the polarization beam splitter 32.

[0040]

As described above in detail, according to the two-optical signal generator of the present invention, a first light source for generating a first optical signal having a first polarization direction and a first optical frequency is provided. A second polarization direction substantially orthogonal to the first polarization direction.
A second light source for generating a second optical signal having an optical frequency of, a polarization maintaining optical fiber cable having a saturable absorption characteristic, the first light source and the second light source, and A polarizing means inserted between the first light source and the other end of the optical fiber cable, and a light reflecting means for reflecting the first optical signal; A first optical signal from the optical fiber cable is incident on the optical fiber cable via one end thereof, and a second optical signal from the second light source is incident on the optical fiber cable via one end thereof, and The first optical signal output from the other end of the fiber cable is reflected by the light reflecting means and is incident on the optical fiber cable via the other end, so that the first optical signal is output from the first end of the optical fiber cable.
Generating a standing wave distribution of the optical signal of the optical signal and optically writing and forming the grating portion on the optical fiber cable;
A light for generating a second optical signal whose optical frequency is selected by the formed grating section so as to have an optical frequency difference corresponding to a refractive index difference between the first and second optical signals in the optical fiber cable. A frequency shifter is configured to change the optical frequency of the first optical signal from the first light source, thereby maintaining the optical frequency difference between the first and second optical signals while maintaining the second optical signal. Change the optical frequency of the optical signal.

Therefore, according to the present invention, even if the optical frequency of the first light source is changed, the optical frequency f2 of the second light source is
It can be changed so as to keep the oscillation frequency difference from the optical frequency f1 of the first optical signal. In addition, a configuration in which the intensities of the two light sources are equalized is also possible. Further, since the optical frequency shifter serving as a frequency reference is formed outside the optical amplification region of the optical fiber cable, there is no competition for gain in the optical amplification section which is a problem in the second conventional example. In addition,
As compared with the first conventional example, a two-optical signal generator having a very simple configuration and high reliability can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical fiber link system according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a two-optical signal generator 20a of FIG.

FIG. 3 is a block diagram showing a configuration of a two-optical signal generator 20b according to a second embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a two-optical signal generator 20c according to a third embodiment of the present invention.

FIG. 5 is a block diagram illustrating a configuration of a two-optical signal generator 20d according to a fourth embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a first conventional example of an optical fiber link system.

7 is a spectrum diagram showing optical frequency spectra of two optical signals generated by the optical transmitter 101 in FIG.

8 is a spectrum diagram showing an electric frequency spectrum of an electric signal after photoelectric conversion by the optical transmitter 200 of FIG.

FIG. 9 is a block diagram showing a configuration of a second optical signal generator 20e according to a second conventional example.

10 is a cross-sectional view showing a cross section of the rare earth-doped polarization maintaining optical fiber cable 22 of FIG.

11 is a polarization maintaining optical fiber cable 22 shown in FIG.
FIG. 4 is a diagram showing that the effective refractive index differs depending on the polarization direction in FIG.

FIG. 12 is a spectrum diagram showing an optical frequency spectrum of two optical signals generated by the two optical signal generator 20e of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 3 ... Optical multiplexer, 5 ... Optical amplifier, 11 ... Optical amplifier, 12 ... Photoelectric converter, 13 ... Bandpass filter, 14 ... Wireless transmitter, 15 ... Antenna, 20 ... 2 optical signal generator, 21 ... Light source, DESCRIPTION OF SYMBOLS 30 ... Optical distributor, 31 ... Optical isolator, 32 ... Polarization beam splitter, 33 ... Rare earth doped polarization maintaining optical fiber cable, 33a ... Fiber grating part, 34 ... Polarization beam splitter, 35 ... Reflection mirror, 36 ... Optical terminator 37: Optical lens, 38: Diffraction grating, 38a: Rotating mechanism, 39: Temperature controller, 40: Semiconductor laser device, 41: Antireflection film, 42: Semiconductor laser medium, 43: High reflection film, 50: Loop optical circuit 51, an optical circulator, 52, an optical amplifier, 53, an optical splitter, 54, an optical band-pass filter, 55, an optical isolator, 60, half Conductive laser device, 61: semiconductor laser medium, 62: AR coat layer, 65: external light modulator, 70: semiconductor laser device, 71: semiconductor laser medium, 72: AR coat layer, 100: optical transmitter, 200: light Receiver, 300: Optical fiber cable.

──────────────────────────────────────────────────続 き Continued on the front page (56) References Electronics Letters Vol. 33, No. 7 p. pp. 594-595 Optics Letters Vol. 22, No. 23, p. p. 1757-1759 (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 2/02 G02B 6/10 H01S 3/10 H04B 10/04 H04B 10/06 H04B 10/14 JICST file (JOIS) WPI ( DIALOG)

Claims (5)

(57) [Claims] A first light source for generating a first optical signal at a first optical frequency having a first polarization orientation; and a second light source having a second polarization orientation substantially orthogonal to the first polarization orientation. A second light source for generating a second optical signal of a second optical frequency, an optical fiber cable having polarization maintaining and saturable absorption characteristics, the first light source and the second light source, and the optical fiber A polarizing means inserted between one end of the cable, and a light reflecting means connected to the other end of the optical fiber cable and reflecting the first optical signal; A first optical signal from the light source is incident on the optical fiber cable via one end thereof, and a second optical signal from the second light source is incident on the optical fiber cable via one end thereof, The first output from the other end of the optical fiber cable Is reflected by the light reflecting means and is incident on the optical fiber cable through the other end, thereby generating a standing wave distribution of the first optical signal in the optical fiber cable and optically grating the optical signal. The optical frequency is written so as to have an optical frequency difference corresponding to a refractive index difference for the first and second optical signals in the optical fiber cable by the formed grating portion. Forming an optical frequency shifter for generating a selected second optical signal; and changing an optical frequency of the first optical signal from the first light source to thereby provide an optical frequency shifter between the first and second optical signals. 2. The two-optical signal generator, wherein the optical frequency of the second optical signal is changed while maintaining the optical frequency difference. 2. The two-optical signal generator according to claim 1, wherein said first and second light sources are semiconductor lasers. 3. The first light source is a semiconductor laser.
2. The two-optical signal generator according to claim 1, wherein said second light source is a feedback type optical oscillator. 4. The first and second light sources are semiconductor lasers, and the light reflecting means changes a reflection angle of a first optical signal by changing a diffraction grating and an inclination angle of the diffraction grating. 2. The two-optical signal generator according to claim 1, further comprising: a rotation mechanism configured to change an inclination angle of the diffraction grating by the rotation mechanism, thereby changing an optical frequency of the first optical signal. 3. . 5. A fiber grating portion, which is formed at the other end of the optical fiber cable and reflects the first optical signal, instead of the light reflecting means, and changes the temperature or tension of the fiber grating portion. 2. The optical frequency of the first optical signal is changed by changing a temperature or a tension of the fiber grating section by the changing means.
Optical signal generator.