JP3576546B2 - Application of phase conjugate optics to optical systems - Google Patents

Description

  The present invention generally relates to the application of phase conjugate optics to optical communication, optical measurement, optical information processing and other fields, and more particularly, to a first phase conjugate light generator and a relay light provided with the same. The present invention relates to a transmission system, a bidirectional repeater optical transmission system, an optical distribution system, an optical switching system, an optical selection system, and an optical AND circuit system, secondly, to an optical communication system using phase conjugate light, and thirdly to these systems. It relates to a possible light modulator.

  First, a first field of application of the present invention will be described. The following embodiments of the present invention relate to a phase conjugate light generating apparatus and a relay optical transmission system, a bidirectional relay optical transmission system, an optical distribution system, an optical switching system, an optical selection system, and an optical AND circuit system including the apparatus.

  By using nonlinear optics, it is possible to achieve new functions and to improve the characteristics of optical systems, which cannot be obtained by conventional optical technology. In particular, if phase conjugate light, which has been actively studied recently, is used, compensation for phase fluctuations and chromatic dispersion in optical transmission lines, reading of sensitive information, amplification of low noise, and quantum states such as squeezed states are possible. A wide range of applications of optical technology to the generation of light is possible.

  Conventionally, for example, in order to realize functions such as optical amplification, branching, and addition of information, a system is combined with an optical device having only each function (specifically, an optical amplifier, an optical splitter, an optical coupler, etc.). Was composed. Further, for example, a method of compensating for phase fluctuations, chromatic dispersion, and the like received in an optical transmission line has been mainly used on the signal transmission / reception side, particularly on the reception side.

  An object of the first application field of the present invention is to provide an optical modulator having a novel configuration applicable to various optical systems.

  Another object in the first field of application of the invention is to provide a useful optical system with this optical modulator.

  According to the present invention, a probe light source, an excitation light source, a nonlinear optical medium, and a probe light / excitation light supply unit that supplies the probe light from the probe light source to the nonlinear optical medium together with the excitation light from the excitation light source. And a modulating means operatively connected to the pumping light source to modulate the pumping light with an information signal, wherein the optical modulator outputs modulated phase conjugate light from the nonlinear optical medium.

  Preferably, the nonlinear optical medium has a third-order nonlinear optical effect, and the phase conjugate light is generated in the nonlinear optical medium by four-wave mixing.

  More preferably, the modulating means modulates the amplitude or intensity of the excitation light.

  By applying phase conjugate optics to an optical system according to the present invention, it is possible to achieve new functions and to improve the characteristics of the optical system, which cannot be obtained by the conventional optical technology.

  Hereinafter, an embodiment of the present invention in a first application field will be described in detail with reference to the drawings.

  FIG. 1 is an explanatory diagram of the principle of generating phase conjugate light by the nonlinear optical effect. When generating phase conjugate light using a nonlinear optical medium, it is desirable to use parametric light amplification (optical parametric process) or four-wave mixing. Parametric optical amplifiers use a second-order nonlinear optical effect, whereas four-wave mixing uses a third-order nonlinear optical effect, but are physically the same phenomenon.

Now, as shown in FIG. 1, a high intensity of the excitation light E _P (frequency omega _P) in a state in which is incident to the nonlinear optical medium NOM, idler light of the signal light E _S and the frequency omega _I of the frequency omega _S When E and _I is incident on the nonlinear optical medium NOM, the second or third-order nonlinear optical process, the frequency omega _S 'and the frequency omega idler light E _I of the _I' signal light E _S of the output from the nonlinear optical medium NOM Is done. At this time, the following relationship is established according to the law of conservation of energy.

特に、４光波混合における信号光、励起光及びアイドラ光の周波数配置を図２に示す。Ωは信号光と励起光の離調周波数である。周波数軸上において、信号光及びアイドラ光は励起光を中心として対称の位置にあることがわかる。

In particular, FIG. 2 shows the frequency arrangement of signal light, pump light and idler light in four-wave mixing. Ω is the detuning frequency between the signal light and the pump light. It can be seen that on the frequency axis, the signal light and the idler light are located symmetrically about the pump light.

  Assuming that the interaction length of the nonlinear optical effect in the principle of FIG. 1 is L, the generation equation is given as follows.

となる。ｎ及びχ⁽³⁾はそれぞれ非線形媒質ＮＯＭの屈折率及び３次の非線形光学定数を表す。また、＜χ⁽³⁾＞は非線形光学定数χ⁽³⁾の非線形媒質ＮＯＭにおける偏波状態についての平均を表す。(3), (4)式より、信号光及びアイドラ光に対する利得Ｇ_S及びＧ_Iは、

It becomes. n and χ ⁽³⁾ represent the refractive index and the third-order nonlinear optical constant of the nonlinear medium NOM, respectively. <Χ ⁽³⁾ > represents the average of the state of polarization of the nonlinear optical constant 非 線形^{(3) in} the nonlinear medium NOM. From equations (3) and (4), the gains G _S and G _I for the signal light and the idler light are

で与えられることがわかる。通常、ω_I／ω_S≒１であるから、非線形光学効果による位相共役光の発生においては、信号の増幅が実現されることがわかる。また、この場合における利得は、励起光の強度に依存するので、本発明の位相共役光発生装置において光変調を行って、位相共役光発生装置から出力される光に新たな情報を載せることができる。

Is given by Usually, since ω _I / ω _S ≒ 1, it is understood that signal amplification is realized in the generation of phase conjugate light by the nonlinear optical effect. In addition, since the gain in this case depends on the intensity of the pump light, it is necessary to perform light modulation in the phase conjugate light generation device of the present invention and to add new information to light output from the phase conjugate light generation device. it can.

The output idler light E _I ′ generated based on the principle of FIG. 1 is a phase conjugate light with respect to the input signal light E _S. This is apparent from the case where there is no input idler light (E _I = 0) in the equations (3) and (4) (E _I ′ corresponds to the complex conjugate of E _S ).

  Next, the property of time reversal, which is a major feature of this phase conjugate light, will be described. Now, it is assumed that input probe light traveling in the + z direction (corresponding to unmodulated input signal light) is represented by the following equation as a plane wave.

ここで、Ａ_S（ｒ）は電場の複素振幅、ｒは空間座標ベクトル、ω_Sはプローブ光の周波数、ｔは時間、ｋ_Sは波数ベクトルを表し、ｃ．ｃ．はその直前の項の複素共役をとることを意味する。但し、波数ベクトルの大きさｋ_Sは、光路の屈折率をｎ、真空中の光速をｃとすると、ｋ_S＝ω_Sｎ／ｃで与えられる。このとき、(9) 式で表される光の位相共役光は、次の(10), (11)式で表される。

Here, A _S (r) is the complex amplitude of the electric field, r is the spatial coordinate vector, ω _S is the frequency of the probe light, t is time, k _S is a wave vector, c. c. Means that the complex conjugate of the term immediately before is taken. However, the magnitude k _S of the wave number vector is given by k _S = ω _S n / c, where n is the refractive index of the optical path and c is the speed of light in vacuum. At this time, the phase conjugate light of the light represented by the expression (9) is represented by the following expressions (10) and (11).

ここで、(10)式は＋ｚ方向の進行波である透過型位相共役光を表し、(11)式は−ｚ方向の進行波である反射型位相共役光を表す。(9),(11)式より
明らかなように、反射型位相共役光については、

Here, equation (10) represents transmission phase conjugate light that is a traveling wave in the + z direction, and equation (11) represents reflection phase conjugate light that is a traveling wave in the −z direction. As is clear from equations (9) and (11), for the reflection-type phase conjugate light,

が成り立ち、位相共役光が時間反転の性質を持つことがわかる。また、(9),(10)式から、透過型位相共役光については、横方向の空間座標成分について時間反転の性質を持つことがわかる。この時間反転の性質を用いることによって、光伝送路で受ける線形の位相歪み（例えば波長分散の影響）や偏波変動等の位相揺らぎを補償可能である。

Holds that the phase conjugate light has the property of time reversal. In addition, from the expressions (9) and (10), it can be seen that the transmissive phase conjugate light has the property of time reversal with respect to the spatial coordinate component in the horizontal direction. By using the nature of the time reversal, it is possible to compensate for phase fluctuations such as linear phase distortion (for example, the influence of chromatic dispersion) and polarization fluctuation received on the optical transmission line.

  FIG. 3 is a block diagram showing a basic configuration of the phase conjugate light generating device of the present invention. 1 is a nonlinear optical medium, 2 is a pump light source, 3 is a signal light input port, 4 is a signal light / pump light supply means, 5 is a signal light output port, 6 is a phase conjugate light output port, and 7 is a signal light / phase conjugate. The light extraction means are respectively shown.

  The excitation light source 2 outputs excitation light. The signal light / pumping light supply unit 4 supplies the input signal light supplied to the signal light input port 3 to the nonlinear optical medium 1 together with the pumping light from the pumping light source 2. The signal light / phase conjugate light extracting means 7 extracts the output signal light and the phase conjugate light generated by the interaction between the input signal light supplied to the nonlinear optical medium 1 and the pump light, and extracts the signal light output port 5 and the signal light output port 5 respectively. Output from the phase conjugate light output port 6.

  According to this configuration, the signal light and the pump light can be made incident on the nonlinear optical medium 1 to generate phase conjugate light. Further, since the generated phase conjugate light can be extracted from the phase conjugate light output port 6, the apparatus can be applied to various optical systems.

  FIG. 4 is a block diagram of a phase conjugate light generator according to the first embodiment of the present invention. In this embodiment, the signal light / pumping light supply means 4 of FIG. 3 includes an optical coupler 11 having at least three ports. The optical coupler 11 has ports 11A, 11B, and 11C, and functions to output light supplied to the ports 11A and 11B from the port 11C. The port 11A of the optical coupler 11 is connected to the signal light input port 3, the port 11B is connected to the pump light source 2, and the port 11C is connected to the first end of the nonlinear optical medium 1. In the specification of the present application, the term “connection” means an operative connection, including a case where optical connection is directly made, and further, connection through an optical device such as an optical filter, an optical isolator and an optical amplifier. Including As the optical coupler 11, a fiber fusion type, a half mirror, a polarization beam splitter, an optical multiplexer, or the like can be used. Further, as the nonlinear optical medium 1, an optical fiber, a semiconductor laser, a semiconductor optical amplifier, an optical crystal exhibiting a secondary or tertiary nonlinear optical effect, or the like can be used.

  When the signal light and the excitation light are supplied to the nonlinear optical medium 1 using the optical coupler 11 as in this embodiment, the signal light and the excitation light enter the nonlinear optical medium 1 from the first end of the nonlinear optical medium 1 through the same optical path. Therefore, in the nonlinear optical medium 1, the signal light and the pump light propagate in the same direction. For example, when the nonlinear optical medium 1 exhibits a third-order nonlinear optical effect, one-way pump-type four-wave mixing occurs. Thus, the phase conjugate light can be extracted from the second end of the nonlinear optical medium 1.

  When the intensity of the excitation light supplied from the excitation light source 2 is sufficiently high, a gain is generated in the nonlinear optical medium 1 as described above, and the amplified signal light and phase conjugate light are output from the nonlinear optical medium 1. . In order to separate and extract the signal light and the phase conjugate light, in this embodiment, the signal light / phase conjugate light extraction means 7 in FIG. 3 includes an optical divider 12 and optical filters 13 and 14. The optical divider 12 has ports 12A, 12B, and 12C, and functions to split the light supplied to the port 12A into two and output from the ports 12B and 12C, respectively. The port 12A of the optical divider 12 is connected to the second end of the nonlinear optical medium 1. As the optical divider 12, for example, a fiber fusion type, a half mirror, a polarization beam splitter, an optical demultiplexer, or the like is used. The optical filter 13 is inserted in the optical path between the port 12B of the optical divider 12 and the signal light output port 5, and its pass band includes the frequency of the output signal light. The optical filter 14 is inserted in the optical path between the port 12C of the optical divider 12 and the phase conjugate light output port 6, and its pass band includes the frequency of the phase conjugate light.

  When non-degenerate four-wave mixing is caused in the nonlinear optical medium 1 by slightly changing the frequency of the pump light and the frequency of the signal light, the output signal light and the phase conjugate light are optically separated in this manner. can do. For example, when a laser diode is used as the excitation light source 2, the gain in the nonlinear optical medium 1 is modulated by superimposing an information signal on the drive current and performing amplitude modulation or intensity modulation of the excitation light. Modulated output signal light and phase conjugate light can be obtained.

  FIG. 5 is a block diagram of a main part of a phase conjugate light generator for explaining the second and third embodiments of the present invention. These embodiments are characterized in that two pumping lights having the same frequency are respectively incident on the nonlinear optical medium 1 in opposite directions to each other to generate bidirectional pumping four-wave mixing. Two excitation light sources 21 and 22 are provided corresponding to the excitation light source 2 in FIG. The signal light / pumping light supply unit 4 in FIG. 3 includes two optical couplers 23 and 24 in order to cause the pumping light from the pumping light sources 21 and 22 to enter the nonlinear optical medium 1 in opposite directions. The optical coupler 23 has ports 23A, 23B and 23C, and functions to output the light supplied to the ports 23A and 23B from the port 23C and output the light supplied to the port 23C from the port 23A. The port 23B of the optical coupler 23 is connected to the pump light source 21, and the port 23C is connected to the first end of the nonlinear optical medium 1. The optical coupler 24 has ports 24A, 24B, and 24C, and functions to output the light supplied to the port 24A from the port 24C and output the light supplied to the port 24B from the port 24A. The port 24A of the optical coupler 24 is connected to the second end of the nonlinear optical medium 1, and the port 24B is connected to the pump light source 22. In the illustrated example, two independent pumping light sources 21 and 22 are used. However, the pumping light from one pumping light source having a high output intensity is branched into two, and the port 23B of the optical coupler 23 and the You may make it supply to the port 24B. In this case, control for matching the frequencies of the two pump lights becomes unnecessary.

  FIG. 6 is a block diagram of a phase conjugate light generator according to a second embodiment of the present invention. This embodiment is suitable for extracting the phase conjugate light generated in the backward direction by bidirectional pumping four-wave mixing in the nonlinear optical medium 1 in the main part of FIG. In order to extract the phase conjugate light generated in the opposite direction to the signal light propagating in the nonlinear optical medium 1, in this embodiment, the signal light / phase conjugate light extraction means 7 of FIG. And 33. The optical divider 31 has ports 31A, 31B, and 31C, and functions to output the light supplied to the port 31A from the port 31C and output the light supplied to the port 31C from the port 31B. The optical filter 32 is inserted in the optical path between the port 24C of the optical coupler 24 and the signal light output port 5, and its pass band includes the frequency of the output signal light. The optical filter 33 is inserted in the optical path between the port 31B of the optical divider 31 and the phase conjugate light output port 6, and its pass band includes the frequency of the phase conjugate light.

  FIG. 7 is a block diagram of a phase conjugate light generator according to a third embodiment of the present invention. This embodiment is suitable for extracting phase conjugate light generated in the forward direction and the backward direction by the bidirectional pumping four-wave mixing in the nonlinear optical medium 1 in the main part of FIG. In order to extract the phase conjugate light generated in the forward direction and the backward direction, in this embodiment, phase conjugate light output ports 6A and 6B are provided corresponding to the phase conjugate light output port 6 of FIG. The light / phase conjugate light extraction means 7 includes optical dividers 31 and 41 and optical filters 33, 42 and 43. The optical divider 31 and the optical filter 33 are for extracting the phase conjugate light generated in the backward direction as in the second embodiment of FIG. 6, and the optical filter 33 has a phase conjugate light output port 6B. Connected. The optical divider 41 and the optical filters 42 and 43 are for extracting the phase conjugate light generated in the forward direction. The optical divider 41 has ports 41A, 41B and 41C, branches the light supplied to the port 41A into two, and outputs the light from the ports 41B and 41C, respectively. The port 41A of the optical divider 41 is connected to the port 24C of the optical coupler 24. The optical filter 42 is inserted in the optical path between the port 41B of the optical divider 41 and the signal light output port 5, and its pass band includes the frequency of the output signal light. The optical filter 43 is inserted in the optical path between the port 41C of the optical divider 41 and the phase conjugate light output port 6A, and its pass band includes the frequency of the phase conjugate light.

  FIG. 8 is a block diagram of a phase conjugate light generator according to a fourth embodiment of the present invention. This embodiment is characterized in that a modulator 51 for modulating the pump light based on the input data is further provided as compared with the basic configuration of FIG. When the modulating means 51 modulates the intensity or amplitude of the pump light, the gain in the nonlinear optical medium 1 changes accordingly, so that the signal light output from the signal light output port 5 and the signal light output from the phase conjugate light output port 6 are output. The modulation can be performed on both of the phase conjugate light. Further, when the modulating means 51 modulates the frequency of the pump light, the phase conjugate light is modulated because the difference between the signal light frequency and the pump light frequency is equal to the difference between the pump light frequency and the phase conjugate light frequency. Can be frequency modulated. When the modulating means 51 modulates the intensity of the excitation light, the modulation degree may be reduced to superimpose the modulation component on the output signal light and / or the phase conjugate light, or the modulation degree may be increased. Thus, the phase conjugate light may be modulated on / off. When the modulation means 51 is applied to the second embodiment of FIG. 6 or the third embodiment of FIG. 7 to perform intensity modulation of the excitation light source, the modulation means 51 outputs the light from one of the excitation light sources 21 and 22. The excitation light may be modulated. In the above-described embodiment of the phase conjugate light generating device, optical amplification may be performed on the pump light, the output signal light, and the phase conjugate light as needed.

  9A, 9B and 9C are block diagrams of a relay optical transmission system according to a fifth embodiment of the present invention. These systems respectively include a transmitting station 61, a receiving station 62, an optical transmission line 63 laid between the transmitting station 61 and the receiving station 62, and a relay station 64 inserted in the optical transmission line 63. Have.

  In the example shown in FIG. 9A, the relay station 64 includes a phase conjugate light generator 65, and the phase conjugate light generator 65 is supplied from the transmitting station 61 to the signal light input port 3 via the optical transmission line 63. The phase conjugate light generated based on the input signal light is transmitted from the phase conjugate light output port 6 to the receiving station 62 via the optical transmission line 63. The phase conjugate light generator 65 has, for example, the basic configuration shown in FIG. According to this configuration, since the phase conjugate light generator 65 is provided in the middle of the optical transmission line 63, chromatic dispersion and the like generated in the optical transmission line 63 can be compensated, and long-distance optical transmission becomes possible. . In addition, by setting the gain for the phase conjugate light in the phase conjugate light generation device 65, the signal light intensity attenuated in the optical transmission line 63 can be compensated.

  In the example shown in FIG. 9B, the relay station 64 further includes, in addition to the phase conjugate light generator 65, a modulation unit 66 that modulates the pump light based on the input data. The input data includes, for example, monitoring data of the relay station 64. According to this configuration, information unique to the relay station 64, such as monitoring data, can be transmitted to the receiving station 62. The output signal light from the signal light output port 5 of the phase conjugate light generator 65 is transmitted to the receiving station 62, and the modulated phase conjugate light output from the phase conjugate light output port 6 is transmitted to the transmitting station 61. The configuration may be changed so as to be transmitted to

  In the example shown in FIG. 9C, the light transmitted by the transmitting station 61 is modulated by transmission data, and the relay station 64 includes a demodulation unit 67 in addition to the phase conjugate light generating device 65. The demodulation means 67 receives the output signal light output from the signal light output port 5 of the phase conjugate light generator 65 and reproduces demodulated data corresponding to the transmission data in the transmitting station 61. The modulation scheme in the transmitting station 61 is, for example, intensity modulation for coherent light or non-coherent light, or amplitude modulation or angle modulation for coherent light. As the detection method in the receiving station 62, when the modulation method in the transmitting station 61 is intensity modulation, direct detection using a photodetector such as a photodiode is suitable, and the modulation method in the transmitting station 61 is suitable for coherent light. In the case of amplitude modulation or angle modulation, heterodyne detection or homodyne detection in which the received light and the local light are made incident on the light receiving surface of a light receiver such as a photodiode on the same optical path is suitable. According to this configuration, the transmission data transmitted from the transmitting station 61 to the receiving station 62 can be monitored by the relay station 64.

  9A, 9B and 9C are also applicable to frequency division multiplex transmission. In this case, it is desirable that the relay station 64 has an excitation light source for each frequency, but when the frequency division multiplexing interval is close, the phase conjugate light of all channels is converted using one excitation light source. It can also be generated.

  FIG. 10 is a block diagram of a relay optical transmission system according to a sixth embodiment of the present invention. This embodiment is different from the system shown in FIG. 9A, FIG. 9B or FIG. 9C in that N (N is a natural number greater than 1) relay stations 64 (# 1) between the transmitting station 61 and the receiving station 62. ~ # N) is inserted. The signal light input port 3 of the phase conjugate light generator 65 included in the first relay station 64 (# 1) counted from the transmitting station 61 side is connected to the transmitting station 61. Also, the phase conjugate light generator of the n-th (n is a natural number larger than 1 and smaller than (N + 1)) counting from the transmitting station 61 side among the relay stations 64 (# 1 to #N). The 65 signal light input port 3 is connected to the phase conjugate light output port 6 of the phase conjugate light generator 65 of the (n-1) th relay station. Further, the phase conjugate light output port 6 of the phase conjugate light generator 65 of the first relay station 64 (#N) counted from the receiving station 62 is connected to the receiving station 62. According to this embodiment, optical transmission over a longer distance becomes possible as compared with any of the embodiments shown in FIGS. 9A, 9B and 9C.

  FIG. 11 is a block diagram of a bidirectional repeater optical transmission system according to a seventh embodiment of the present invention. This system includes a transmission / reception station 73 having an optical transmitter 71 and an optical receiver 72, another transmission / reception station 76 having an optical transmitter 74 and an optical receiver 75, and an uplink installed between the transmission / reception stations 73 and 76. An optical transmission line 77 and a downstream optical transmission line 78 are provided, and a relay station 79 inserted between the upstream and downstream optical transmission lines 77 and 78 is provided. Relay station 79 includes a phase conjugate light generator 65 'modified to apply to bidirectional transmission. The phase conjugate light generator 65 'has first and second excitation light sources (not shown) for outputting the first and second excitation lights, respectively. The generated phase conjugate light is output to the phase conjugate light output port 6A and 6B.

FIG. 12 is an explanatory diagram of the frequency arrangement of each light in the system of FIG. ω _P1 is the first pump light, ω _S1 is the first signal light supplied from the optical transmitter 71 to the signal light input port 3 of the phase conjugate light generator 65 ′ via the upstream optical transmission line 77, ω _I1 Is the first phase conjugate light generated in the phase conjugate light generator 65 ′ based on the first pump light and the first signal light, ω _P2 is the second pump light, and ω _S2 is the downstream light from the optical transmitter 74. The second signal light ω _I2 supplied to the signal light output port 5 of the phase conjugate light generator 65 ′ via the transmission line 78 is a phase conjugate light generator based on the second pump light and the second signal light. 65 represents the frequency of the second phase conjugate light generated at 65 '. The first pump light and the second pump light have different frequencies, for example, and the frequencies of the first and second signal lights are slightly different from the frequencies of the first and second pump lights, respectively. Is set. The first and second phase conjugate lights appear at positions on the frequency axis symmetric with respect to the first and second signal lights with the first and second pump lights as centers. If the first and second signal lights have slightly different frequencies so that they do not interfere with each other, a single frequency (such as the first and second signal lights entering the gain band) is used. Band) may be used.

  The phase conjugate light generation device 65 'generates a first phase conjugate light based on the first signal light and the first pump light supplied to the signal light input port 3, and the first phase conjugate light is The light is transmitted from the phase conjugate light output port 6A to the optical receiver 75 via the upstream optical transmission line 77. Further, the phase conjugate light generation device 65 ′ generates a second phase conjugate light based on the second signal light and the second pump light supplied from the optical transmitter 74 to the signal light output port 5. The second phase conjugate light is transmitted from the phase conjugate light output port 6B to the optical receiver 72 via the downstream optical transmission line 78. Incidentally, the phase conjugate light generator 65 'can be configured according to the third embodiment of FIG. In the third embodiment of FIG. 7, the signal light output port 5 is for sending out the output signal light. In the seventh embodiment of FIG. 11, the signal light output port 5 is connected to the input port for the down direction. Used as a port. 9A to 9C, the frequency division multiplexing can be applied to the seventh embodiment of FIG. 11 as well.

FIG. 13 is a block diagram of a bidirectional repeater optical transmission system showing an eighth embodiment of the present invention. This embodiment is different from the seventh embodiment in FIG. 11 in that N (N is a natural number greater than 1) relay stations 79 (# 1 to #N) are provided between the transmitting and receiving stations 73 and 76. It is characterized by. Each of the relay stations 79 (# 1 to #N) has a phase conjugate light generator 65 'in FIG. The signal light input port 3 and the phase conjugate light output port 6B of the phase conjugate light generation device 65 'of the first relay station 79 (# 1) counted from the transmitting / receiving station 73 have the optical transmitter 71 and the light receiving port, respectively. Device 72. Also, the phase conjugate light generator 65 'of the n-th (n is a natural number larger than 1 and smaller than (N + 1)) relay station 79 counted from the transmitting / receiving station 73 among the relay stations 79 (# 1 to #N). The signal light input port 3 and the phase conjugate light output port 6B are connected to the phase conjugate light output port 6A and the signal light output port 5, respectively, of the phase conjugate light generator 65 'of the (n-1) th relay station. You. Further, the signal light output port 5 and the phase conjugate light output port 6A of the phase conjugate light generator 65 'of the first relay station 79 (#N) counted from the transmitting / receiving station 76 are connected to the optical transmitter 74 and the optical transmitter 74, respectively. Connected to receiver 75. In this embodiment, in order to set the first pump light in each phase conjugate light generator 65 'to the same frequency and to set the second pump light in each phase conjugate light generator 65' to the same frequency, the signal light and the What is necessary is just to reverse the frequency arrangement relationship of the phase conjugate light alternately. For example, when the frequency arrangement in the phase conjugate light generator 65 'of the relay station 79 (# 1) is set as shown in FIG. 2, the phase conjugate light generator 65 of the relay station 79 (# 2) is set. 'Should be set so that ω _S = ω _P −Ω and ω _I = ω _P + Ω.

FIG. 14 is a block diagram of an optical distribution system showing a ninth embodiment of the present invention. This system is provided with a plurality of phase conjugate light generators 65, and the signal light output port 5 and the phase conjugate light output port 6 of the upper one of the plurality of phase conjugate light generators 65 are respectively the signal light output ports of the immediately lower one. Connected to input port 3. In this embodiment, the degradation of the S / N ratio every time one light beam passes through one phase conjugate light generator 65 can be reduced to 3 dB (addition of quantum noise) at a minimum. On the other hand, in a conventional optical distribution system, one optical distribution unit has at least one 1: 1 optical coupler and an optical amplifier. At the distribution unit, the SN ratio was degraded by at least 6 dB. Therefore, according to this embodiment, light distribution with extremely low noise and no loss due to distribution can be realized. The gains for the signal light and the phase conjugate light output from the ports 5 and 6 of the phase conjugate light generator 65 are different from each other as shown in the equations (7) and (8). By operating the phase conjugate light generator 65 under the condition of ω _I / ω _S ≒ 1, this gain difference can be ignored.

  FIG. 15 is a block diagram of an optical switching system showing a tenth embodiment of the present invention. This embodiment is characterized in that a switching means 81 for turning on and off the pumping light source 2 is further provided as compared with the basic configuration of FIG. When the excitation light source 2 is on, the intensity of the excitation light is controlled such that phase conjugate light is generated in the nonlinear optical medium 1, and when the excitation light source 2 is off, no phase conjugate light is generated in the nonlinear optical medium 1. In this way, the intensity of the excitation light is controlled. According to this embodiment, it is possible to selectively switch between a state where the phase conjugate light is generated and a state where the phase conjugate light is not generated by the operation of the switching means 81.

  FIG. 16 is a block diagram of an optical selection system showing an eleventh embodiment of the present invention. This embodiment is different from the basic configuration shown in FIG. 3 in that a sweep unit 91 for sweeping the frequency of the pump light output from the pump light source 2 is further provided. And a plurality of signal lights are supplied. In this embodiment, a plurality of light sources 92 (# 1, # 2,...) That output signal lights having different frequencies are used in order to supply a plurality of frequency-division multiplexed signal lights. The signal light from the light sources 92 (# 1, # 2,...) Is multiplexed by multiplexing means 93 such as a multiplexer and supplied to the signal light input port 3. In this embodiment, when the frequency of the pump light is changed by the sweep means 91, the gain band for generating the phase conjugate light is also swept on the frequency axis. Therefore, the signal light in the gain band can be alternatively selected from the plurality of signal lights, and the phase conjugate light corresponding to the signal light can be generated, so that the channel selection in the frequency division multiplexing system can be easily performed. It can be carried out.

FIG. 17 is a block diagram of an optical AND circuit system showing a twelfth embodiment of the present invention. In this embodiment, the pump light sources 21 and 22, the optical couplers 23 and 24, the optical divider 31, and the optical filter 33 in the second embodiment of FIG. 6 are used, and the pump light control means 101 is further provided. Excitation light control unit 101 includes a driving circuit 102 for varying the intensity of the excitation light supplied from the pumping light source 21 to the port 23B of the optical coupler 23 in accordance with the high and low input logic data Q _1, the optical coupler 24 from the pumping light source 22 the intensity of the pumping light supplied to the port 24B and a driving circuit 103 vary according to the high and low input logic data Q _2. The signal light input port 3 is supplied with, for example, unmodulated signal light from the light source 104. Then, the intensities of the signal light and the pump light are adjusted so that the phase conjugate light is output from the phase conjugate light output port 6 only when the input logical data Q ₁ and Q ₂ are both high. The relationship between the input logic data Q ₁ and Q ₂ and the logic level X ₁ of the phase conjugate light for these at this time is shown in the table.

表から明らかなように、この実施例によると、２つの入力論理データに対する位相共役光の強度レベルがアンド回路の出力として得られていることがわかる。尚、２つの論理データは電気信号であっても良いし光信号であっても良い。また、この実施例においては、非線形光学媒質１における非線形光学効果自体の応答時間はピコ秒程度であるので、極めて高速な演算の実現が可能になる。

As is apparent from the table, according to this embodiment, the intensity level of the phase conjugate light with respect to the two input logic data is obtained as the output of the AND circuit. The two logical data may be electrical signals or optical signals. Further, in this embodiment, since the response time of the nonlinear optical effect itself in the nonlinear optical medium 1 is on the order of picoseconds, an extremely high-speed operation can be realized.

  By the way, when light used for optical measurement or optical communication utilizing the wave nature of light is considered as an oscillating electric field, the amplitude and phase of the oscillating electric field fluctuate for various reasons. In particular, quantum fluctuation of the field is inevitable. Therefore, such fluctuations ultimately determine the accuracy in optical measurement and the receiving sensitivity in optical communication. Fluctuations in one quantity of a field are governed by the uncertainty relationship between that quantity and another quantity in a particular relationship. That is, the product of the magnitudes of the fluctuations of these two amounts does not fall below the predetermined value. However, if the fluctuation of one amount is allowed to increase, there is a possibility that the fluctuation of the other amount can be reduced. Based on this idea, attempts have been made to create a place with less fluctuation. The field thus created is called a squeezed state, and the light in this state is called a squeezed light.

  A squeezed state can be generated by using the phase conjugate light generation device of the present invention. That is, when the intensity of the pump light is high, the quantum handling of the output signal light and the phase conjugate light becomes possible, and the output signal light and the phase conjugate light have a parametric quantum correlation with each other, and the squeezed state is obtained. Generation becomes possible, and measurement accuracy in optical measurement and sensitivity in optical communication can be increased.

  As described above, according to the first aspect of the present invention, there is an effect that it is possible to provide a phase conjugate light generation device having a novel configuration applicable to various optical systems. In addition, there is an effect that various useful optical systems including the phase conjugate light generator can be provided.

  Next, a second field of application of the present invention will be described. The following embodiments of the present invention relate to an optical communication system using phase conjugate light.

  An object of the second application field of the present invention is to use a multifunctional phase conjugate light generating means and apply it to an optical communication system.

  Specifically, an object of the following embodiments of the present invention is to provide an optical communication system in which phase fluctuation in an optical transmission line is compensated.

  According to the present invention, there is provided an optical communication system in which an optical transmission line is provided between a master station and a slave station, wherein the master station has a phase conjugate light generating means and a first modulation means, and the slave station has a probe light. Generating means and first demodulating means, wherein the probe light generating means supplies the probe light to a first end of the optical transmission path, and the phase conjugate light generating means outputs a signal from a second end of the optical transmission path. Generating a phase conjugate light with respect to the probe light and supplying the phase conjugate light to a second end of the optical transmission line, wherein the first modulating means modulates the phase conjugate light according to first input data; A system is provided in which the first demodulation means demodulates the first input data based on the phase conjugate light output from a first end of the optical transmission line.

  In this system, a phase conjugate light for the probe light transmitted from the slave station via the optical transmission line is generated in the master station, and the phase conjugate light is modulated by, for example, a data signal and transmitted to the slave station via the optical transmission line. Therefore, phase fluctuations in the optical transmission line are compensated by the nature of the time inversion in the phase conjugate light.

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

  FIG. 18 is a block diagram showing a basic configuration of the optical communication system of the present invention. This system is configured by connecting a master station 201 and a slave station 202 via an optical transmission line 203. The main station 201 has a phase conjugate light generation means 204 and a (first) modulation means 205. The slave station 202 has a probe light generator 206 and a (first) demodulator 207. The probe light generator 206 supplies the modulated or unmodulated probe light to the first end of the optical transmission line 203. The phase conjugate light generating means 204 receives the probe light output from the second end of the optical transmission line 203, generates a phase conjugate light for the probe light, and outputs the phase conjugate light to the second end of the optical transmission line 203. Supply. The modulating means 205 modulates the phase conjugate light according to the input data. The demodulation unit 207 demodulates input data based on the phase conjugate light output from the first end of the optical transmission path 203. The modulating means 205 may be operatively connected to the phase conjugate light generating means 204 to internally modulate the phase conjugate light, or may generate the phase conjugate light independently of the phase conjugate light generating means 204. The phase conjugate light supplied from the means 204 to the second end of the optical transmission path 203 may be externally modulated.

  FIG. 19 is a block diagram showing a specific configuration example of the main station 201 in FIG. The phase conjugate light generation means 204 includes a nonlinear optical medium 211, excitation light generation means 212 for generating excitation light, and probe light / excitation light supply means 213. The probe light / excitation light supply unit 213 supplies the excitation light supplied from the excitation light generation unit 212 to the nonlinear optical medium 211 together with the probe light supplied from the slave station 202 in FIG. The phase conjugate light generated in the nonlinear optical medium 211 is supplied to the optical transmission path 203 in FIG. 18 via the same path as the probe light supply path in the opposite direction or via a path different from the probe light supply path.

  When the phase conjugate light generation means 204 configured as shown in FIG. 19 is used, the modulation means 205 for modulating the phase conjugate light can include the excitation light modulation means 214 for modulating the excitation light according to the input data. . The pumping light modulator 214 directly modulates the light source of the pumping light generator 212 or indirectly modulates the pumping light supplied from the pumping light generator 212 to the nonlinear optical medium 211 via the probe light / pumping light supplier 213. I do. When generating phase conjugate light using the nonlinear optical medium 211, it is desirable to use parametric amplification (optical parametric process) or four-wave mixing. Parametric optical amplification uses a second-order nonlinear optical effect, while four-wave mixing uses a third-order nonlinear optical effect. When the phase conjugate light is generated by four-wave mixing, first and second pump lights having the same frequency are used as the pump light, and the first and second pump lights are supplied to the nonlinear optical medium in opposite directions. Is desirable. In the case of such two-way pumping type four-wave mixing, phase conjugate light can be efficiently generated. Of course, four-wave mixing may be generated using one excitation light source.

FIG. 20 shows the principle of generation of phase conjugate light when four-wave mixing is caused by supplying first and second pump lights of the same frequency to a nonlinear optical medium exhibiting a third-order nonlinear optical effect in opposite directions. FIG. In a state where the first and second excitation lights _EP1 and _EP2 are incident on the nonlinear optical medium NOM exhibiting the third-order nonlinear optical effect in directions opposite to each other, the input signal light (probe light) is applied to the nonlinear optical medium NOM. Is supplied, a third-order nonlinear optical process (specifically, the first and second pumping by the spatial diffraction grating formed by one of the first and second pumping light and the input signal light). the other one of the diffraction of light), the frequency omega _S, from the input signal light E _S wave number k _S, the frequency omega _S, wavenumber k _S output signal light E _S 'and the frequency omega _I of the output idler wave number k _I Light E _I ′ is generated. The fact that the output idler light E _I 'corresponds to the phase conjugate light of the input signal light E _S will be described later. In particular, when the first and second pump lights supplied in opposite directions have the same frequency (ω _P ), the wave number becomes k _I = −k _S, and thus the idler light becomes the input signal light. The light is output in the direction opposite to the incident direction, thereby realizing a reflection-type phase conjugate light generator (phase conjugate mirror). At this time, the following relationship holds according to the law of conservation of energy.

図２１は、４光波混合における信号光、励起光及びアイドラ光の周波数配置を説明するための図である。Ωは信号光と励起光の離調周波数を表す。周波数軸上において、信号光及びアイドラ光は励起光を中心として対称の位置にあることが分かる。

FIG. 21 is a diagram for explaining the frequency arrangement of signal light, pump light, and idler light in four-wave mixing. Ω represents the detuning frequency of the signal light and the pump light. It can be seen that, on the frequency axis, the signal light and the idler light are located symmetrically about the pump light.

  Assuming that the interaction length of the nonlinear optical effect in the principle of FIG. 20 is L, the generation equation is given as follows.

となる。ｎ及びχ⁽³⁾はそれぞれ非線形媒質ＮＯＭの屈折率及び３次の非線形光学定数を表す。また、＜χ⁽³⁾＞は非線形光学定数χ⁽³⁾の全ての偏波状態についての平均を表す。

It becomes. n and χ ⁽³⁾ represent the refractive index and the third-order nonlinear optical constant of the nonlinear medium NOM, respectively. <Χ ⁽³⁾ > represents the average of all the polarization states of the nonlinear optical constant 全 て⁽³⁾ .

From the equations (14) and (15), the gains G _S and G _I for the signal light and the idler light are respectively:

で与えられることがわかる。通常、ω_I／ω_S≒１であるから、非線形光学効果による位相共役光の発生においては、信号の増幅が実現されることがわかる。また、この場合における利得は、励起光の強度に依存するので、励起光の強度を変調することで、アイドラ光について強度変調を行うことができる。図２０の原理に基づいて発生する出力アイドラ光Ｅ_I′は入力信号光Ｅ_Sに対する位相共役光になっている。このことは、(14),(15) 式において、入力アイドラ光がない場合（Ｅ_I＝０）を考えてみれば明らかである（Ｅ_I′はＥ_Sの複素共役に相当）。

Is given by Usually, since ω _I / ω _S ≒ 1, it is understood that signal amplification is realized in the generation of phase conjugate light by the nonlinear optical effect. Further, since the gain in this case depends on the intensity of the excitation light, the intensity of the idler light can be modulated by modulating the intensity of the excitation light. The output idler light E _I ′ generated based on the principle of FIG. 20 is a phase conjugate light with respect to the input signal light E _S. This is apparent from the case where there is no input idler light (E _I = 0) in the equations (14) and (15) (E _I ′ corresponds to the complex conjugate of E _S ).

  Next, the property of time reversal, which is a major feature of this phase conjugate light, will be described. Now, it is assumed that an input signal light traveling in the + z direction (corresponding to an unmodulated input probe light) is represented by the following equation as a plane wave.

ここで、Ａ_S（ｒ）は電場の複素振幅、ｒは空間座標ベクトル、ω_Sはプローブ光の周波数、ｔは時間、ｋ_Sは波数ベクトルを表し、ｃ．ｃ．はその直前の項の複素共役をとることを意味する。但し、波数ベクトルの大きさｋ_Sは、光路の屈折率をｎ、真空中の光速をｃとすると、ｋ_S＝ω_Sｎ／ｃで与えられる。このとき、(20)式で表される光の位相共役光は、次の(21)式で表される。

Here, A _S (r) is the complex amplitude of the electric field, r is the spatial coordinate vector, ω _S is the frequency of the probe light, t is time, k _S is a wave vector, c. c. Means that the complex conjugate of the term immediately before is taken. However, the magnitude k _S of the wave number vector is given by k _S = ω _S n / c, where n is the refractive index of the optical path and c is the speed of light in vacuum. At this time, the phase conjugate light of the light represented by the expression (20) is represented by the following expression (21).

(21)式は−ｚ方向の進行波である反射型の位相共役光を表している。(21)式より明らかなように、反射型の位相共役光については、

Equation (21) represents reflection-type phase conjugate light that is a traveling wave in the −z direction. As is clear from equation (21), for the reflection-type phase conjugate light,

が成り立ち、位相共役光が時間反転の性質を持つことがわかる。この時間反転の性質を用いることによって、光伝送路で受ける定常的な位相歪み（例えば波長分散の影響）や偏波変動等の位相揺らぎを補償可能である。

Holds that the phase conjugate light has the property of time reversal. By using the nature of the time reversal, it is possible to compensate for phase fluctuations such as steady phase distortion (for example, influence of chromatic dispersion) and polarization fluctuation received in the optical transmission line.

FIG. 22 is a block diagram showing an embodiment of the master station 204 shown in FIGS. In this embodiment, an optical fiber 221 is used as the nonlinear optical medium 211 in FIG. The optical fiber 221 is preferably a silica-based single mode fiber. Further, two laser diodes 222 and 223 are used corresponding to the excitation light generating means 212 in FIG. Laser diodes 222 and 223 output first and second pump lights, respectively, and their frequencies are equal. As the nonlinear optical medium 211, in addition to the optical fiber 221, a light-induced refractive index (photorefractive) effect medium such as BaTiO ₃ , various organic compounds, various semiconductors, particularly a traveling-wave semiconductor optical amplifier and a Fabry-Perot semiconductor light An amplifier can also be used. Further, an optical waveguide structure made of LiNbO ₃ or the like can be adopted. In any case, it is possible to generate phase conjugate light by mainly using FWM of these media, and to shorten the optical path length by using a nonlinear optical medium having high generation efficiency of phase conjugate light is required to reduce the phase length. This is effective in facilitating the matching and widening the band. In order to make the first and second pump lights from the laser diodes 222 and 223 enter the optical fiber 221 as a nonlinear optical medium in opposite directions, respectively, the probe light / pump light supply means 213 in FIG. Optical couplers 224 and 225 are included. The optical coupler 224 has ports 224A, 224B, and 224C, and functions to output at least light supplied to the port 224A from the port 224B and output light supplied to the port 224B from the port 224C. The port 224A of the optical coupler 224 is connected to the laser diode 222, the port 224B is connected to the first end of the optical fiber 221, and the port 224C is connected to the signal light output port 226. The optical coupler 225 has ports 225A, 225B, and 225C, and outputs at least the light supplied to the port 225A from the port 225B, outputs the light supplied to the port 225B from the port 225C, and outputs the light supplied to the port 225C. It functions to output from the port 225B. Port 225A of optical coupler 225 is connected to laser diode 223, port 225B is connected to the second end of optical fiber 221, and port 225C is connected to port 227 for probe light input and phase conjugate light output. As the optical couplers 224 and 225, for example, a fiber fusion type, a half mirror, an optical multiplexer, a polarization beam splitter, or the like is used.

  In this embodiment, the excitation light modulation means 214 in FIG. 19 includes a drive control circuit 228 for the laser diodes 222 and 223. The drive control circuit 228 supplies a drive current to the laser diodes 222 and 223 so that the first and second excitation lights supplied to the optical fiber 221 generate phase conjugate light with respect to the input probe light. The drive current supplied to 222 and / or 223 is changed according to the input data. When performing the intensity modulation or the amplitude modulation on the phase conjugate light, the drive control circuit 228 controls the laser diode so that the intensity or the amplitude of at least one of the first and second excitation lights is modulated according to the input data. The drive current supplied to 222 and 223 is controlled. When frequency modulation is performed on the phase conjugate light, the drive control circuit 228 supplies a drive current to be supplied to the laser diodes 222 and 223 so that the frequencies of the first and second excitation lights are modulated according to the input data. Control. When the first and second pump lights supplied from the laser diodes 222 and 223 to the optical fiber 221 are frequency-modulated, the difference between the frequency of the signal light and the frequency of the pump light becomes the difference between the frequency of the pump light and the phase conjugate light. The phase conjugate light is frequency-modulated from the relationship that it is equal to the frequency difference (see FIG. 21).

  According to this embodiment, the first and second pumping lights from the laser diodes 222 and 223 can be supplied to the optical fiber 221 as the nonlinear optical medium in opposite directions, respectively. By mixing, phase conjugate light with respect to the input probe light can be efficiently generated. Also, since the phase conjugate light can be extracted from the port 227 along the same path as the supply path of the input probe light, the system of FIG. 18 can be easily realized by connecting the port 227 to the optical transmission path 203 of FIG. can do. Note that the port 226 in FIG. 22 can be used for demodulation when the probe light supplied to the port 227 is modulated as described later.

FIGS. 23A and 23B show the phase conjugate light and the second conjugate light in the case where the drive control circuit 228 in FIG. 22 changes the drive current supplied to the laser diode 23 to intensity-modulate the phase conjugate light to intensity-modulate the second excitation light. FIG. 4 is a diagram for explaining a waveform of excitation light No. 2; In FIG. 23A, the lower part shows the waveform of the second excitation light E _P2 , and the upper part shows the waveform of the phase conjugate light E _I ′. The second pumping light E _P2 is intensity-modulated by an analog signal (in this example, a sine wave signal), and the second pumping light _EP 2 is higher than the value _{EP 2} ⁽⁰⁾ satisfying the signal gain G = 1. The phase conjugate light is output only when the value is “1”. Therefore, the waveform of the phase conjugate light E _I ′ is equivalent to a half-wave rectified waveform of the sine wave signal. FIG. 23B is a waveform diagram of the phase conjugate light and the second excitation light in which the intensity of the second excitation light is modulated by a digital signal according to FIG. 23A. Also in this example, the phase conjugate light is output only when the second pumping light _EP2 is higher than the threshold value _EP2 ⁽⁰⁾ . The present invention can also be implemented when the intensity or amplitude of the second excitation light is constantly changed in a region higher than the threshold value _EP2 ⁽⁰⁾ .

  FIG. 24 is a block diagram showing another basic configuration of the optical communication system of the present invention. This basic configuration is different from the basic configuration in FIG. 18 in that the first modulating means 205 of the main station 201 modulates the phase conjugate light according to the first input data, and the slave station 202 Second modulation means 231 for modulating the probe light according to the data, and second demodulation in which main station 201 demodulates the second input data based on the output light of phase conjugate light generation means 204 And means 32. According to this configuration, the transmission data can be loaded on the probe light in the direction from the slave station 202 to the master station 201, and the transmission data can be loaded on the phase conjugate light in the direction from the master station 201 to the slave station 202. It is possible to perform directional transmission and to compensate for phase fluctuations in the optical transmission path in the direction from the master station 201 to the slave station 202.

  When the second modulating means 231 modulates the frequency or phase of the probe light, the first modulating means 205 can employ intensity modulation or amplitude modulation as a modulation method for the phase conjugate light. By doing so, the first and second demodulation means 207 and 232 can easily demodulate the first and second input data, respectively. Further, the second modulating means 231 modulates the intensity or amplitude of the probe light with a relatively low modulation degree, and the first modulating means 205 modulates the intensity or amplitude of the phase conjugate light with a relatively high modulation degree. However, demodulation can be performed well even if the reverse is true. The second demodulation means 232 in FIG. 24 is connected to, for example, the port 226 in FIG. 22, and is based on the signal light (corresponding to the amplified modulation probe light) amplified in the optical fiber 221 in FIG. Demodulate the second input data.

  FIG. 25 is a block diagram showing a first embodiment of a slave station applicable to the system of FIG. 18 or FIG. In this embodiment, when the phase conjugate light is intensity-modulated, the transmission data is demodulated by direct detection. The optical coupler 241 has ports 241A, 241B, and 241C, and functions to output the light supplied to the port 241A from the port 241B and output the light supplied to the port 241C from the port 241A. As the optical coupler 241, for example, an optical circulator can be used. The port 241A of the optical coupler 241 is connected to the optical transmission line 203 in FIG. 18 or FIG. 24, the port 241B is connected to a light receiver 242 such as a photodiode, and the port 241C is connected to a laser diode 244 as a probe light source. You.

  When the intensity-modulated phase conjugate light is supplied to the light receiver 242 via the ports 241A and 241B of the optical coupler 241 in this order, the output electric signal of the light receiver 242 corresponds to the intensity change of the transmitted phase conjugate light. Change. Therefore, the transmission data can be reproduced by processing the output electric signal of the light receiver 242 by the demodulation circuit 243 configured as usual using the discriminator or the like. The laser diode 244 as a light source of the probe light is driven by the drive circuit 245. When the slave station of this embodiment is applied to the system of FIG. 18, the drive circuit 245 drives the laser diode 244 in a steady state, and a constant probe light is output from the laser diode 244. The light passes through the ports 241C and 241A in this order and is transmitted to the optical transmission line 203 in FIG. On the other hand, when the slave station of this embodiment is applied to the system of FIG. 24, the driving circuit 245 drives the laser diode 244 so that the modulated probe light is output from the laser diode 244. When amplitude modulation, phase modulation, or frequency modulation is performed on the coherent phase conjugate light, the slave station can reproduce transmission data by heterodyne detection using local light. In this case, heterodyne detection can be performed by causing the phase conjugate light transmitted to the slave station to enter the light receiving surface of the light receiver together with the local light.

  FIG. 26 is a block diagram showing a second embodiment of the slave station applicable to the system of FIG. 18 or FIG. This embodiment is characterized in that, when heterodyne detection is performed in a slave station, the probe light from one light source is split into two and one of them is used as local light. The phase conjugate light transmitted by the optical transmission line 203 in FIG. 18 or 24 passes through the ports 241A and 241B of the optical coupler 241 in this order, further passes through the half mirror 251 and enters the light receiver 252 such as a photodiode. The probe light output from the laser diode 253 is split by the half mirror 254, and one of the split probe lights is sent to the optical transmission path through the ports 241C and 241A of the optical coupler 241 in this order. The other one of the branched probe lights is reflected by the half mirror 254, is further reflected as local light by the half mirror 251 and is incident on the light receiving surface of the light receiver 252 along the same optical path as the phase conjugate light.

  In this case, the local light source and the signal light source can be shared, and since the local light source is installed near the receiver, it is possible to receive the local light at a high optical power level, and therefore, it is possible to secure a high receiving sensitivity. Easy. When the phase conjugate light and the local light enter the light receiving surface of the light receiver 252 in the same optical path, an intermediate frequency signal having a frequency corresponding to the difference between the frequency of the phase conjugate light and the frequency of the local light is obtained as the output of the light receiver 252. Can be Therefore, the transmission data can be demodulated based on the intermediate frequency signal supplied from the photodetector 252 by using the demodulation circuit 256 configured as usual by the filter detection method or the synchronous detection method. In FIG. 26, reference numeral 255 denotes a drive circuit of the laser diode 253, and this drive circuit 255 corresponds to the drive circuit 245 of FIG.

  FIG. 27 is an explanatory diagram of the spectrum of the intermediate frequency signal output from the light receiver 252 of FIG. As described with reference to FIG. 21, when phase conjugate light is generated by four-wave mixing using a nonlinear optical medium, the detuning frequency between the probe light and the excitation light is Ω, and the detuning frequency between the excitation light and the phase conjugate light is Also becomes Ω, and as a result, the difference between the frequency of the probe light and the frequency of the phase conjugate light becomes 2Ω. Therefore, when the branch probe light is used as the local light in the embodiment of FIG. 26, the center frequency of the intermediate frequency signal is 2Ω.

  By the way, when heterodyne detection is performed as in the embodiment of FIG. 26, it is necessary to make the polarization planes of the phase conjugate light and the local light incident on the light receiver 252 coincide with each other to stabilize the output level of the intermediate frequency signal. Required for Since the output light of the laser diode 253 usually has a polarization state close to linearly polarized light, the local light supplied from the laser diode 253 to the light receiver 252 via the half mirrors 254 and 251 is a linear light having a constant polarization plane. It can be considered as polarized light. On the other hand, in the propagation mode of a single mode fiber often used as an optical transmission line, there are two polarization modes whose polarization planes are orthogonal to each other, and these two polarization modes are coupled due to various disturbances, and as a result, The polarization state of the light supplied to the first end of the single mode fiber does not match the polarization state of the light output from the second end of the fiber. Therefore, when a single mode fiber is used as the optical transmission path connected to the port 241A of the optical coupler 241 in FIG. 26, the optical fiber is supplied from the port 241B of the optical coupler 241 to the light receiver 252 via the half mirror 251. The polarization state of the phase conjugate light varies with time due to environmental changes and the like. If the polarization state fluctuates, the level of the intermediate frequency signal output from the light receiver 252 fluctuates, and in the worst case, the phase conjugate light and the local light do not interfere with each other, and the signal cannot be received.

  In order to cope with such a problem, in the conventional coherent optical communication system, the polarization state of the signal light (corresponding to the phase conjugate light in the embodiment of FIG. 26) and / or the local light supplied to the optical receiver is changed. The polarization was controlled so that Instead of polarization control, use of a polarization maintaining fiber as an optical transmission line or application of a polarization diversity system can also address the above-mentioned problem, but in any case, the system configuration becomes complicated. There are drawbacks.

  FIG. 28 is a block diagram of an optical communication system showing an embodiment capable of maintaining a high level of the intermediate frequency signal without taking polarization control or other measures. As the slave station 202, for example, the one shown in FIG. 26 is used. The optical transmission line 203 connecting the master station 201 and the slave station 202 is a single mode fiber 261 in this embodiment. The main station 201 has two phase conjugate light generators 262 and 263 and a polarization beam splitter 264. The phase conjugate light generators 262 and 263 generate linearly-polarized phase conjugate light having the same polarization plane with respect to the supplied linearly-polarized probe light, and transmit the phase conjugate light in the opposite direction to the probe light supply path. I do. The polarizing beam splitter 264 has ports 264A, 264B, and 264C, separates the light supplied to the port 264A into two orthogonal polarization components, outputs the light from the ports 264B and 264C, and outputs the orthogonal light from the ports 264B and 264C, respectively. It functions to output the polarization component from port 264A. The port 264A of the polarization beam splitter 264 is connected to the single mode fiber 261, and the ports 264B and 264C are connected to the phase conjugate light generators 262 and 263 via a transmission line having a polarization state maintaining function such as a polarization maintaining fiber. Is done.

  The probe light transmitted from the slave station 202 to the master station 201 via the single mode fiber 261 is polarized and separated by the polarization beam splitter 264 into two polarization components whose polarization planes are orthogonal to each other. These two polarized components of the probe light are supplied to the phase conjugate light generators 262 and 263, respectively, while maintaining their polarization state, where the phase conjugate light having the corresponding polarization planes is generated. The phase conjugate lights output from the phase conjugate light generators 262 and 263 are polarization-synthesized by the polarization beam splitter 264 and transmitted to the slave station 202 by the single mode fiber 261. Since the two polarization components of the phase conjugate light transmitted from the phase conjugate light generators 262 and 263 are transmitted to the slave station 202 exactly following the change in the polarization state of the probe light, the change in the polarization state in the single mode fiber 261 is performed. Is longer than the round trip time of the light between the master station 201 and the slave station 202, the polarization state of the phase conjugate light at the output end of the single mode fiber 261 is supplied to the single mode fiber 261 as probe light. In this way, it is possible to always perform heterodyne detection in the optimum polarization state. As described above, according to the present embodiment, there is no need to apply the polarization control or the polarization diversity system, and thus it is possible to provide a simple and high-performance system. Note that the difference between the optical path length between the phase conjugate light generator 262 and the polarization beam splitter 264 and the optical path length between the phase conjugate light generator 263 and the polarization beam splitter 264 is desirably set shorter than the coherent length of the light source. .

  The main station 201 may be configured to have only one phase conjugate light generator 262. In this case, the phase conjugate light generator 263 and the polarization beam splitter 264 among the components of the main station 201 in FIG. 28 are omitted, and the phase conjugate light generator 262 is connected to the single mode optical fiber 261 via the polarization active controller. Connected. Then, the polarization state of the phase conjugate light (usually linear polarization) in which light having an arbitrary polarization state input from the single mode fiber 261 to the main station 201 is generated by the phase conjugate light generator 262 by the active polarization controller. And input to the phase conjugate light generator 262. In this way, the generated phase conjugate light is converted by the polarization active controller into a polarization state when output from the single mode fiber 261 again. Therefore, when this phase conjugate light propagates through the single mode fiber 261 in the reverse direction and returns to the slave station 202, the phase becomes the same as the first polarization state.

  In the system shown in FIG. 18 or FIG. 24, when the probe light supplied from the probe light generating means 206 to the optical transmission line 203 is a plurality of frequency division multiplexed probe lights, the same number of phase conjugates as the number of frequency division multiplexing are used. Frequency division multiplexing optical transmission is possible by using the light generating means 204 or by using the broadband phase conjugate light generating means 204 which can respectively generate phase conjugate light for a plurality of frequency division multiplexed probe lights. become.

  FIG. 29 is a block diagram of an optical communication system showing an embodiment in which the present invention is applied to an optical distribution system. In the first embodiment in this system, a plurality of subscribers 271 each corresponding to the master station 201 and a distribution station 272 corresponding to the slave station 202 are connected via an optical multi / demultiplexer 273. It is. The single frequency or frequency division multiplexed probe light from the distribution station 272 is distributed by the optical multi / demultiplexer 273 and supplied to each subscriber 271. In each subscriber 271, transmission data is reproduced based on each distribution probe light, while transmission data such as a request signal from each subscriber 271 is transmitted to the distribution station 72 by phase conjugate light. In this embodiment, since each subscriber 271 can amplify the output signal light and the phase conjugate light, the distribution loss in the optical multi / demultiplexer 273 can be compensated, and the optical addition with a simple configuration can be performed. User system can be realized. Each subscriber 271 has, for example, a specific configuration example of the master station shown in FIG. 19, and in this case, the frequency of each pump light generated by each pump light generating means 212 is made different, so that the distribution station 272 is provided. Side, for example, it is easy to detect which subscriber is the request signal.

  The second embodiment in this system is a case where a distribution station 272 corresponding to a master station and a plurality of subscribers 271 corresponding to a slave station are connected via an optical multi / demultiplexer 273. The probe light of the frequency assigned to each of the subscribers 271 or the probe light distributed from the common light source is combined by the optical multi / demultiplexer 273 and supplied to the distribution station 272. The distribution station 272 converts the probe light into phase conjugate light, places transmission data on the phase conjugate light, and transmits the data to each subscriber 271 again. In generating phase conjugate light, when probe light of a different frequency is assigned to each of the subscribers, the same number of phase conjugate light generating means as the number of subscribers are used, or a plurality of probes are used. A wide-band phase conjugate light generating means capable of generating phase conjugate light with respect to light is used. It is also possible to carry a request signal from each subscriber on the probe light from each subscriber.

  As described above, according to the second aspect of the present invention, since the phase conjugate light generating means having multi-functionality can be applied to the optical communication system, the predetermined function can be simplified. This has the effect of being able to be realized by Further, according to the present invention, it is possible to provide an optical communication system that compensates for phase fluctuations in an optical transmission line.

  Subsequently, a third application field of the present invention will be described. The following embodiments of the present invention relate to an optical modulator applicable to various optical systems.

Conventionally, as an optical modulator, an optical modulator that modulates an optical device with an electric signal is known. For example, amplitude modulation and frequency (phase) modulation by modulating an injection current of a laser diode, intensity modulation and phase modulation by modulating a bias voltage of a LiNbO ₃ optical waveguide, and the like are actively performed. As optical systems speed up, the performance of conventional optical modulators is approaching its limits. On the other hand, there are also expectations for higher speed optical systems. In addition, as optical amplification repeaters have become popular, optical repeaters that do not depend on the transmission speed have been required. In order to meet such demands, the development of an optical modulator using light without electrical processing (a so-called all-optical modulator) is required.

  An object in a third field of application of the invention is to provide such an optical modulator.

  FIG. 30 is a block diagram showing a basic configuration of the optical modulator according to the present invention. This optical modulator includes a probe light source 301, an excitation light source 302, a nonlinear optical medium 303, and a probe light / excitation light for supplying the probe light from the probe light source 301 to the nonlinear optical medium 303 together with the excitation light from the excitation light source 302. It comprises a supply means 304 and a modulation means 305 operatively connected to the excitation light source 302 for modulating the excitation light with an information signal. Then, the modulated phase conjugate light is output from the nonlinear optical medium 303.

The generation and the properties of the phase conjugate light by the four-wave mixing (FWM) using the nonlinear optical medium have been described by, for example, Expressions (13) to (22) in the second application field. How (phase conjugate light) is modulated will be described by taking amplitude (intensity) modulation, frequency modulation, and phase modulation as examples. Now, Table as follows input signal light in FWM (probe _{light) E S (ω S, t} ), the pumping light E _P (ω _P, t) and an output idler light _{E I '(ω P, t} ) , respectively Shall be A _S (ω _S , t), A _P (ω _P , t) and A _I (ω _I , t) are the amplitudes of the probe light, excitation light and output idler light, respectively, and ω _S , ω _P and ω _I are respectively The frequencies of the probe light, the excitation light, and the output idler light, t represents time, and φ _S (t), φ _P (t), and φ _I (t) represent the phases of the probe light, the excitation light, and the output idler light, respectively. .

ここで、入力アイドラ光がない場合（Ｅ_I＝０）を考えると、(13)式乃至(16)式より以下の関係が得られる。

Here, considering the case where there is no input idler light (E _I = 0), the following relationship is obtained from the expressions (13) to (16).

（１）振幅（強度）変調
(26)式より、励起光の振幅Ａ_P（ω_P，ｔ）を変化させると、その絶対値の二乗に比例して出力アイドラ光（位相共役光）の振幅Ａ_I（ω_I，ｔ）が変化することがわかる。即ち、(29)式及び(30)式で表されるように励起光に振幅（強度）変調をかけると、(31)式で与えられるように出力アイドラ光が振幅（強度）変調されるものである。

(1) Amplitude (intensity) modulation
From equation (26), when the amplitude A _P (ω _P , t) of the pump light is changed, the amplitude A _I (ω _I , t) of the output idler light (phase conjugate light) is proportional to the square of the absolute value. Change. That is, when amplitude (intensity) modulation is applied to the pump light as expressed by the equations (29) and (30), the output idler light is amplitude (intensity) modulated as given by the equation (31) It is.

このとき、励起光Ｅ_P（ω_P，ｔ）の消光比をγとすると、得られるアイドラ光Ｅ_I′（ω_P，ｔ）の消光比はγ²となる。例えば、γ＝−１５ｄＢとすると、γ²＝−３０ｄＢとなるから、大幅な消光比の改善が可能になる。

In this case, the pumping light E _P (ω _P, t) when the extinction ratio of the gamma, the extinction ratio of the obtained idler light _{E I '(ω P, t} ) is the gamma ^2. For example, if γ = −15 dB, γ ² = −30 dB, so that the extinction ratio can be greatly improved.

The idler light E _I ′ (ω _P , t) is output only when the pump light E _P (ω _P , t) has a value higher than the value E _P ⁽⁰⁾ satisfying the signal gain G = 1. This is shown in FIG. 31A. Here, the excitation light is modulated by a sine wave. When the excitation light is modulated on-off by a digital signal, the waveform becomes as shown in FIG. 31B. By the way, in particular, in the case of digital modulation, the pulse shape of the excitation light is actually a rectangular shape as shown in FIG. 31C, and considering that the amplitude of the idler light is proportional to the square of the amplitude of the excitation light, It can be seen that the pulse width of the idler light is narrower than the pulse width of the excitation light. This is shown in FIG. 31C. Therefore, it is possible to generate an RZ pulse having an arbitrary duty ratio and shape by using such a property.
(2) Assuming that the frequency modulation pump light is subjected to frequency modulation as expressed by the equation (32), the output idler light is frequency-modulated as given by the equation (33).

この場合、出力アイドラ光に対する変調度が励起光に対するそれの二倍になるため、出力アイドラ光に変調度ｍの周波数変調を与えるために必要な励起光の変調度はその半分のｍ／２でよいことになる。
（３）位相変調
励起光に(34)式で示されるように位相変調をかけるとすると、出力アイドラ光は(35)式で与えられるように位相変調される。

In this case, the modulation factor for the output idler light is twice that of the pump light, so that the modulation factor of the pump light required to give the output idler light the frequency modulation of the modulation factor m is half that of m / 2. It will be good.
(3) Phase Modulation Assuming that the pump light is subjected to phase modulation as shown by the equation (34), the output idler light is phase-modulated as given by the equation (35).

この場合にも、周波数変調の場合と同様に、アイドラ光に対する変調度は励起光に対するそれの二倍になるため、アイドラ光に変調度ｍの周波数変調を与えるための励起光の変調度はその半分のｍ／２でよいことになる。(33)式及び(34)式より、出力アイドラ光の位相雑音は励起光の位相雑音の二倍と信号光の位相雑音との和になる。従って、光周波数変調或いは光位相変調を行う場合には、用いる光源の位相雑音が極力小さいことが望ましい。即ち、プローブ光源及び励起光源としてはコヒーレント光源を用いるのが望ましい。

Also in this case, similarly to the case of the frequency modulation, the modulation factor for the idler light is twice as large as that for the excitation light. A half m / 2 is sufficient. From equations (33) and (34), the phase noise of the output idler light is the sum of twice the phase noise of the pump light and the phase noise of the signal light. Therefore, when performing optical frequency modulation or optical phase modulation, it is desirable that the phase noise of the light source used is as small as possible. That is, it is desirable to use a coherent light source as the probe light source and the excitation light source.

In FIG. 30, specific examples of the nonlinear medium 303 for generating idler light by the FWM include a crystal medium such as TiBaO ₃ and LiNbO ₃ , various organic media, various semiconductors (for example, a semiconductor optical amplifier), and furthermore, Fiber and the like. In either case, a phase conjugation mirror (PCM) type optical modulator can be configured by using the counter excitation configuration, and a transmission type phase conjugator type optical modulator can be configured by using the front excitation configuration. it can. When the present invention is applied to optical fiber communication, an optical fiber is suitable as a nonlinear optical medium in view of matching with an optical transmission line. Hereinafter, some examples using an optical fiber as a nonlinear optical medium will be described.

  FIG. 32 is a block diagram showing a first embodiment of the optical modulator according to the present invention. An optical fiber 321 corresponding to the nonlinear optical medium 303, the excitation light source 302, the probe light / excitation light supply means 304, the probe light source 301 and the modulation means 305, an excitation LD (laser diode) 322, an optical coupler 323, and a probe LD 324 in FIG. And a modulation circuit 325. The optical fiber 321 as a nonlinear optical medium is preferably a single mode fiber. In this case, when a non-degenerate FWM is generated by making the wavelength of the probe light slightly different from the wavelength of the pump light, the wavelength that gives the zero dispersion of the optical fiber 321 is the wavelength of the pump light (the oscillation wavelength of the pump LD 322). To match. The optical coupler 323 has four ports 323A, 323B, 323C and 323D. The probe LD 324 is connected to the port 323A, the pumping LD 322 is connected to the port 323B, the first end of the optical fiber 321 is connected to the port 323C, and the port 323D is dead-ended. The optical coupler 323 functions to output at least the light supplied to the ports 323A and 323B from the port 323C. As the optical coupler 323, for example, a fiber fusion type, a half mirror, an optical multiplexer, A polarizing beam splitter or the like is used.

  According to this configuration, both the probe light supplied to the port 323A of the optical coupler 323 and the pump light supplied to the port 323B can be guided to the optical fiber 321 which is a nonlinear optical medium. Phase conjugate light (idler light) can be generated. Since the pump light is modulated by the modulation circuit 325 connected to the pump LD 322, the modulated idler light can be output from the second end of the optical fiber 321 according to the above-described principle. The modulated idler light is transmitted to, for example, an optical transmission line including an optical fiber (not shown).

  When an optical fiber is used as the non-linear optical medium, it is desirable to match the phases of the probe light and the idler light in order to efficiently generate FWM in the optical fiber. Effective methods for this include a method using a degenerate FWM in which the wavelengths of the pump light and the probe light are matched, and a method of matching the wavelength of the pump light with the zero dispersion wavelength of the optical fiber. In particular, when the latter method is adopted, the phase velocities of the probe light and the idler light can be equalized while maintaining a complex conjugate relationship with each other (in the approximation up to the second-order dispersion). Phase matching becomes possible.

  By the way, when optical modulation is performed by the FWM in the optical fiber, if the intensity of the excitation light is increased to increase the modulation efficiency, the excitation light is reflected in the optical fiber by stimulated Brillouin scattering (SBS). , The conversion efficiency is saturated. In particular, when amplitude (intensity) modulation or no modulation is applied (CW), SBS occurs at an input power of about +7 to 8 dBm in a single mode fiber. An embodiment excluding such an influence of SBS will be described below.

  FIG. 33 is a block diagram showing a second embodiment of the optical modulator according to the present invention. In this embodiment, the pumping light from the pumping LD 322 frequency-modulated by the oscillator 327 having a relatively low frequency ω ′ is input to the port 323B of the optical coupler 323 via the optical modulator 326. The optical modulator 326 is driven by the modulation circuit 325 to which the information signal is input, and modulates the amplitude (intensity) of the excitation light passing therethrough. This makes it possible to reduce the power density of the pumping light per unit frequency in the optical fiber 321 and suppress the SBS. In the case where the pump light is directly modulated instead of indirectly modulated by the optical modulator 326, a low-frequency signal having a frequency ω 'may be superimposed on the information signal and supplied to the pump LD 322. It is desirable that the frequency ω ′ of the oscillator 326 be sufficiently low so as not to affect the information signal supplied to the modulation circuit 325. In this embodiment, when the modulated phase conjugate light is obtained, the excitation light is subjected to low-speed frequency modulation. However, even when the unmodulated phase conjugate light is obtained as in the above-described embodiment, the excitation light is not modulated. SBS can be suppressed by frequency modulation.

  FIG. 34 is a block diagram showing a third embodiment of the optical modulator according to the present invention. In this embodiment, in contrast to the first embodiment of FIG. 32, the phase conjugate light (output idler light) generated in the optical fiber 321 is not shown through the optical bandpass filter 331 and the optical amplifier 332 in this order. It is characterized in that it is transmitted to an optical transmission line. The optical amplifier 332 is, for example, a linear optical amplifier. One configuration example of the optical amplifier 332 includes a doped fiber doped with a rare earth element such as Er (erbium), a pump light source that outputs pump light, and light to amplify the pump light (here, an output idler light). Means for supplying to the doped fiber. The optical band-pass filter 331 is for removing undesired light such as light from the probe LD 324 and the pump LD 322 and noise light, and outputs only the output idler light modulated by the light modulator from the optical modulator. be able to. Further, by removing such unnecessary light, for example, the operation of the optical amplifier 332 is prevented from being saturated by the pumping light from the pumping LD 322, and the output idler light generated in the optical fiber 321 is sufficiently amplified. can do. In general, since the intensity of the excitation light from the excitation LD 322 is extremely higher than the intensity of the probe light and the output idler light, unnecessary light such as the excitation light is removed by using the optical bandpass filter 331 to obtain high intensity. There is no fear that the exciting pump light will cause a further nonlinear optical effect in the optical transmission line at the subsequent stage. Further, by removing unnecessary light with the optical bandpass filter 331, it is possible to eliminate the difficulty of demodulation due to the presence of the pump light when reproducing the demodulated signal on the receiving side based on the modulated output idler light. When the intensity of the excitation light supplied from the excitation LD 322 is sufficiently high, the intensity of the output idler light generated in the optical fiber 321 may be higher than the intensity of the probe light supplied to the optical fiber 321. Therefore, when such an amplifying action occurs, the optical amplifier 322 may not be used.

  FIG. 35 is a block diagram showing a fourth embodiment of the optical modulator according to the present invention. This embodiment is characterized in that a polarization scrambler 341 is provided between the pump LD 322 and the port 323B of the optical coupler 323, as compared with the first embodiment of FIG. Generally, in the propagation mode of a single mode fiber, there are two polarization modes whose polarization planes are orthogonal to each other, and these two polarization modes are coupled under the influence of various disturbances. The polarization state of the supplied light does not match the polarization state of the light output from the second end of the single mode fiber. Therefore, when the probe light from the probe LD 324 is supplied to the optical coupler 323 via a relatively long single mode fiber, such as when the optical modulator of the present invention is incorporated in an optical repeater, or the like. The polarization state of the probe light supplied to the optical fiber 321 as a nonlinear optical medium fluctuates with time due to environmental changes and the like. On the other hand, as is apparent from the principle of generating the output idler light (phase conjugate light), the conversion efficiency from the probe light to the phase conjugate light depends on the polarization state of the probe light supplied to the nonlinear optical medium and the polarization state of the excitation light. Depends on state and relationship. According to the fourth embodiment shown in FIG. 35, the excitation light from the excitation LD 322 is combined with the probe light via the polarization scrambler 341. Therefore, even when the polarization state of the supplied probe light changes with time. In addition, the operation of the optical modulator can be stabilized by keeping the conversion efficiency from the probe light to the phase conjugate light constant.

  The polarization scrambler 341 is configured as usual using a 波長 wavelength plate, a 板 wavelength plate, or the like. For example, when the excitation light supplied from the excitation LD 322 is substantially linearly polarized, the polarization plane is rotated. Works as follows. Since the fluctuation of the polarization state of the supplied probe light due to a change in environmental conditions and the like is relatively slow, the operating frequency of the polarization scrambler 341 (eg, the reciprocal of the rotation period of the polarization plane) is set to about 1 to 100 KHz. By doing so, the polarization dependency can be sufficiently eliminated. In this example, the polarization scrambler 341 acts on the excitation light supplied from the excitation LD 322, but the polarization scrambler may act on the probe light.

  FIG. 36 is a block diagram showing a fifth embodiment of the optical modulator according to the present invention. In this embodiment, a polarization beam splitter 351 for separating the probe light supplied from the probe LD 324 into first and second polarization components whose polarization planes are orthogonal to each other, and first and second polarization components from the polarization beam splitter 351 are formed. Phase conjugate light generators 352 and 353 that respectively generate phase conjugate light (output idler light) based on the above, and a polarization combiner 354 that combines the phase conjugate lights from the phase conjugate light generators 352 and 353 are used. The polarization combiner 354 is, for example, a polarization beam splitter, and the phase conjugate light combined by the polarization combiner 354 is transmitted to an optical transmission path (not shown). As the phase conjugate light generators 352 and 353, for example, those obtained by removing the probe LD 324 and the modulation circuit 325 from the configuration of the first embodiment in FIG. 32 can be used. A modulation circuit 325 'is provided for modulating the excitation light in the phase conjugate light generators 352 and 353.

  According to this embodiment, since the first and second polarization components of the probe light supplied to the phase conjugate light generators 352 and 353 are both linearly polarized light, the probe light is supplied to the phase conjugate light generators 352 and 353. It is easy to make the polarization state of the probe light (first or second polarization component) coincide with the polarization state of the excitation light, and the polarization dependence can be eliminated. That is, the generation efficiency of the phase conjugate light can be kept constant regardless of the change in the polarization state of the probe light supplied from the probe LD 324. In this embodiment, the optical path length of the optical path from the polarization beam splitter 351 to the polarization combiner 354 via the phase conjugate light generator 352 and the optical path length from the polarization beam splitter 351 to the polarization combiner 354 via the phase conjugate light generator 353. It is desirable to make the difference from the optical path length sufficiently smaller than the traveling distance of light in one time slot T of the signal. For example, when the information signal supplied to the modulation circuit 325 'is an NRZ signal of 10 Gb / s, the traveling distance of the light in one time slot T is about 2 mm, and the optical path length difference is 1/10 of that. Is desirably suppressed to about 0.2 mm or less, which corresponds to

  FIG. 37 is a diagram showing a sixth embodiment of the optical modulator according to the present invention. An optical coupler 323 is used as a probe light / excitation light supply unit, and a polarization maintaining fiber 355 is used as a nonlinear optical medium. The probe light from the probe LD 324 and the pump light from the pump LD 322 are combined by the optical coupler 323 and input to the polarization maintaining fiber 355. The excitation light is substantially linearly polarized light having a predetermined polarization plane, and the arrangement of the excitation LD 322 and the like are set such that the predetermined polarization plane is inclined by approximately 45 ° with respect to the main axis of the polarization maintaining fiber 355. Is set. This makes it possible to keep the two orthogonal polarization components of the pump light power within the polarization maintaining fiber constant, so that the generation efficiency of the output idler light is stably maintained with respect to the probe light having an arbitrary polarization state. be able to.

  In the embodiment of FIG. 37, as the length of the polarization maintaining fiber 355 used as the nonlinear optical medium increases, the phase due to the slight difference in the refractive index of the polarization maintaining fiber 355 with respect to the polarization in the two main axis directions is increased. Since a shift may occur, it is desirable to increase the pump light power or increase the nonlinear constant of the polarization maintaining fiber 355 so that a short polarization maintaining fiber 355 is sufficient. The degree of the phase shift between the two orthogonal polarization components is determined by the material and structure of the polarization maintaining fiber 355. In a standard fiber, a shift of 17 ps occurs for a length of 10 m. Therefore, a signal having a bit rate of about 60 Gb / s appears as 1-bit polarization dispersion. In this case, the transmission speed of a signal that can be actually transmitted is about 10 Gb / s. As the length of the polarization maintaining fiber 355 increases, the transmission speed of a transmittable signal further decreases. Next, a description will be given of an embodiment capable of coping with a signal having a high bit rate without shortening the length of the polarization maintaining fiber as the nonlinear optical medium.

  FIG. 38 is a block diagram showing a seventh embodiment of the optical modulator according to the present invention. This embodiment is characterized in that the non-linear optical medium consists of two polarization maintaining fibers 355A and 355B of substantially the same length as compared to the embodiment of FIG. The polarization maintaining fibers 355A and 355B are connected such that their main axes are orthogonal to each other. The excitation light is substantially linearly polarized light having a predetermined polarization plane. The probe light and the excitation light that have been combined by the optical coupler 323 are supplied to the first end of the polarization maintaining fiber 355A. Here, the arrangement and the like of the pump LD 322 are set such that the polarization plane of the pump light is inclined at approximately 45 ° with respect to the main axis of the polarization maintaining fiber 355A. A second end of the polarization maintaining fiber 355A is connected to a first end of the polarization maintaining fiber 355B. From the second end of the polarization maintaining fiber 355B, modulated phase conjugate light generated in the polarization maintaining fibers 355A and 355B is output. In this embodiment, since the lengths of the polarization maintaining fibers 355A and 355B having substantially the same characteristics are set to be equal, the phase shift between the two orthogonal polarization components generated in the polarization maintaining fiber 355A is reduced. Even if the total length of the polarization maintaining fibers 355A and 355B is long, the transmission speed of the signal is not limited even if the phase shift between the two orthogonal polarization components generated at 355B is canceled.

  FIG. 39 is a block diagram showing an eighth embodiment of the optical modulator according to the present invention. The pump LD 322 is used as the pump light source, and the optical fiber 321 is used as the nonlinear optical medium, which is the same as the embodiments shown in FIGS. In this embodiment, the probe light / excitation light supply means includes an optical coupler 361 and a polarization beam splitter 362 in order to guide the probe light and the excitation light to the optical fiber 321 in both directions. The optical coupler 361 has ports 361A, 361B and 361C, and outputs the light supplied to the ports 361A and 361B from the port 361C. The probe LD324 is connected to the port 361A, and the excitation LD322 is connected to the port 361B. The polarizing beam splitter 362 has ports 362A, 362B, 362C and 362D, separates the light supplied to the ports 362A and 362B into two orthogonal polarization components, and outputs these two polarization components from the ports 362C and 362D, respectively. . The polarization beam splitter 362 separates the light supplied to the ports 362C and 362D into two orthogonal polarization components, and outputs these two polarization components from the ports 362A and 362B, respectively. The port 362A is connected to the port 361C of the optical coupler 361, the port 362B is connected to an optical transmission line (not shown), and the port 362C and the optical fiber 321 are connected between 362D. In the middle of the optical fiber 321, there is provided a polarization controller 363, which is configured normally using a 波長 wavelength plate, a 波長 wavelength plate, or the like. The polarization controller 363 is supplied to the optical fiber 321. The control is performed so that the polarization state of the reflected light and the polarization state of the light output from the optical fiber 321 match.

  The probe light supplied to the optical coupler 361 is combined with the excitation light from the excitation LD 322, and the probe light and the excitation light are polarized by the polarization beam splitter 362 into a first polarization component and a polarization orthogonal to the polarization plane of the first polarization component. And a second polarized light component having a surface. The first and second polarization components respectively propagate through the optical fiber 321 in opposite directions, and are polarized and combined when passing through the polarization beam splitter 362 again, and output from the port 362B. The polarization plane of the excitation light output from the excitation LD 322 is set such that the distribution ratio of the excitation light from the excitation LD 322 to the first and second polarization components separated by the polarization beam splitter 362 is 1: 1. Is done. That is, the excitation LD 322 is set such that the polarization plane of the excitation light supplied to the port 362A of the polarization beam splitter 362 is inclined by approximately 45 ° with respect to the polarization planes of the first and second polarization components. By doing so, the orthogonal two-polarized light components of the probe light guided in opposite directions to the optical fiber 321 act on the polarization planes in which the orthogonal two-polarized light components of the excitation light coincide with each other. When the phase conjugate lights generated in the opposite directions are combined by the polarization beam splitter 362 and output from the port 362B, the phase conjugate light is obtained with a constant conversion efficiency regardless of the change in the polarization state of the supplied probe light. be able to.

  FIG. 40 is a block diagram showing a ninth embodiment of the optical modulator according to the present invention. This embodiment is characterized in that a polarization maintaining fiber 321 'is used as a nonlinear optical medium as compared with the embodiment of FIG. The polarization maintaining fiber 321 'is connected to the polarization beam splitter 362 such that the polarization state of the light supplied to the polarization maintaining fiber 321' matches the polarization state of the light output from the polarization maintaining fiber 321 '. You. In this case, the main axis of the polarization maintaining fiber 321 ′ is parallel to the plane of polarization of linearly polarized light separated by the polarization beam splitter 362. According to this embodiment, since the polarization controller 363 of FIG. 39 is not required, the configuration of the device can be simplified.

  FIG. 41 is a block diagram showing a tenth embodiment of the optical modulator according to the present invention. An optical fiber 321 as a nonlinear optical medium, an excitation LD 322, a probe LD 324, a polarization beam splitter 362, and a polarization controller 363 are the same as in the embodiment of FIG. In this embodiment, the probe light / excitation light supply means includes an optical coupler 371 and a half-wave plate 373 in order to separate the excitation light remaining without being consumed when the phase conjugate light is generated from the generated phase conjugate light. And a polarizing beam splitter 362. The probe light / excitation light supply unit further includes an optical circulator 372 to separate the port for supplying the excitation light from the port for extracting the phase conjugate light. The optical circulator 372 has three ports 372A, 372B and 372C, outputs the light supplied to the port 372A from the port 372B, outputs the light supplied to the port 372B from the port 372C, and supplies the light to the port 372C. It functions to output light from port 372A. The probe LD324 is connected to the port 372A, and the port 372C is connected to an optical transmission line (not shown). The optical coupler 371 has four ports 371A, 371B, 371C and 371D, distributes the light supplied to the ports 371A and 371B equally, outputs the light from the ports 371C and 371D, and outputs the light supplied to the ports 371C and 371D. Evenly distributed and output from ports 371A and 371B. As the optical coupler 371, for example, a half mirror or a fiber fusion type is used. The pump LD 322 is connected to the port 371A, the port 372B of the circulator 372 is connected to the port 371B, and the port 371D is connected to the port 362B of the polarization beam splitter 362. The half-wave plate 373 is inserted into the optical path between the port 371C of the optical coupler 371 and the port 362A of the polarization beam splitter 362, and the half-wave plate 373 rotates the polarization plane of the supplied light by 90 °. In this embodiment, the polarization state of the excitation light supplied to the port 371A of the optical coupler 371 matches the polarization state of the probe light supplied to the port 371B of the optical coupler 371 from the probe LD 324 via the optical circulator 372. It has been like that.

  Now, the operation in this embodiment will be described assuming that these excitation light and probe light are linearly polarized light having a polarization plane perpendicular to the paper surface. The excitation light and the probe light supplied to the ports 371A and 371B of the optical coupler 371 are equally distributed and output from the ports 371C and 371D. The probe light and the excitation light output from the port 371C are rotated by 90 ° in the polarization plane by the half-wave plate 373, and supplied to the port 362A of the polarization beam splitter 362 as linearly polarized light having a polarization plane parallel to the paper. You. The probe light and the excitation light supplied to the port 362A are supplied from the port 362D to the optical fiber 321. When the probe light and the excitation light propagate in the optical fiber 321 counterclockwise in the drawing, phase conjugate light is generated in the same direction. The phase conjugate light and the remaining pump light are supplied from the port 362C to the polarization beam splitter 362 and output from the port 362B. On the other hand, since the probe light and the excitation light supplied from the port 371D of the optical coupler 371 to the port 362B of the polarization beam splitter 362 have a polarization plane perpendicular to the paper, the probe light and the excitation light are supplied to the port 362D. From the optical fiber 321, and propagates in the optical fiber 321 in the counterclockwise direction in the figure to generate phase conjugate light. The phase conjugate light and the remaining pump light are supplied from the port 362C to the polarization beam splitter 362, and output from the port 362A. The phase conjugate light and the excitation light supplied from the port 362A to the 波長 wavelength plate 373 are supplied to the port 371C of the optical coupler 371 as linearly polarized light parallel to the paper surface after the polarization plane is rotated by 90 °. The excitation light and the phase conjugate light supplied from the half-wave plate 373 to the port 371C and the excitation light and the phase conjugate light supplied from the port 362B of the polarizing beam splitter 362 to the port 371D of the optical coupler 371 are both printed on the paper. They have parallel planes of polarization, and the lengths of the optical paths through which they pass are exactly the same. Therefore, of the pump light and the phase conjugate light supplied to the ports 371C and 371D in the optical coupler 371, the pump light is mainly output from the port 371A, and the phase conjugate light is mainly output from the port 371B. The light output from the port 371B of the optical coupler 371 is transmitted to an optical transmission line (not shown) via the optical circulator 372.

  According to this embodiment, the excitation light remaining when the phase conjugate light is generated in the optical fiber 321 as the nonlinear optical medium and the generated phase conjugate light are passed through an optical filter (for example, the optical bandpass filter 331 in FIG. 34). Can be separated without use. Since the intensity of the excitation light used for generating the phase conjugate light is extremely higher than the intensity of the probe light and the generated phase conjugate light, the separation of the high intensity excitation light from the phase conjugate light requires the steps shown in FIG. Is useful.

  In the embodiment described above, the number of the probe light is one. However, the present invention can be applied to a plurality of frequency division multiplexed probe lights.

  FIG. 42 is a block diagram showing an eleventh embodiment of the optical modulator according to the present invention. The probe light from the single mode fiber SMF-1 enters the first port of the optical coupler 382. The second port of the optical fiber 382 is supplied with excitation light from an excitation light source 386 modulated by an information signal source 384 via a polarization controller 388. When the probe light and the excitation light added by the optical coupler 382 enter the nonlinear optical medium 390, phase conjugate light is generated here, and the phase conjugate light is branched into two by the optical coupler 392. One of the split phase conjugate lights is sent out to the single mode fiber SMF-2, and the other split phase conjugate light passes through the optical filter 394 and is converted into an electric signal by the optical receiver 396. The comparator 398 controls the polarization state of the pump light and the oscillation wavelength of the pump light source 386 so that the output level of the light receiver 396 becomes maximum. The wavelength of the excitation light is controlled by the temperature and bias current of the laser diode used as the excitation light source 386.

  According to this embodiment, since the polarization state of the excitation light is actively controlled in accordance with the polarization state of the probe light, phase conjugate light is generated with stable conversion efficiency regardless of the polarization state of the probe light. be able to.

  In the embodiment of the optical modulator according to the present invention described above, the probe light source is installed relatively close to the nonlinear optical medium, and the optical modulator is applied to the transmitting station. Is also applicable. That is, by inputting the probe light sent from the transmitting station to the nonlinear optical medium together with the excitation light (modulated) at the relay station, it is possible to write information on the phase conjugate light generated in the nonlinear optical medium. It is.

  As described above, by applying the phase conjugate optics to the optical system according to the present invention, it is possible to achieve new functions and improve the characteristics of the optical system, which cannot be obtained by the conventional optical technology. Other effects obtained by the present invention are as described above, and the description thereof will be omitted.

FIG. 1 is a diagram illustrating the principle of generation of phase conjugate light. FIG. 2 is an explanatory diagram of the frequency arrangement of the signal light, the pump light, and the idler light. FIG. 3 is a block diagram of a phase conjugate light generator showing a basic configuration of the present invention. FIG. 4 is a block diagram of a phase conjugate light generator according to the first embodiment of the present invention. FIG. 5 is a block diagram of a main part of a phase conjugate light generator for explaining the second and third embodiments of the present invention. FIG. 6 is a block diagram of a phase conjugate light generator according to a second embodiment of the present invention. FIG. 7 is a block diagram of a phase conjugate light generator according to a third embodiment of the present invention. FIG. 8 is a block diagram of a phase conjugate light generator according to a fourth embodiment of the present invention. 9A, 9B and 9C are block diagrams of a relay optical transmission system according to a fifth embodiment of the present invention. FIG. 10 is a block diagram of a relay optical transmission system according to a sixth embodiment of the present invention. FIG. 11 is a block diagram of a bidirectional repeater optical transmission system according to a seventh embodiment of the present invention. FIG. 12 is an explanatory diagram of the frequency allocation in the system of FIG. FIG. 13 is a block diagram of a bidirectional repeater optical transmission system showing an eighth embodiment of the present invention. FIG. 14 is a block diagram of an optical distribution system showing a ninth embodiment of the present invention. FIG. 15 is a block diagram of an optical switching system showing a tenth embodiment of the present invention. FIG. 16 is a block diagram of an optical selection system showing an eleventh embodiment of the present invention. FIG. 17 is a block diagram of an optical AND circuit system showing a twelfth embodiment of the present invention. FIG. 18 is a block diagram showing a basic configuration of the optical communication system of the present invention. FIG. 19 is a block diagram showing a specific configuration of the master station in the present invention. FIG. 20 is a diagram illustrating the principle of generation of phase conjugate light. FIG. 21 is an explanatory diagram of the frequency arrangement of the signal light, the excitation light, and the idler light (phase conjugate light). FIG. 22 is a block diagram showing an embodiment of the master station in the present invention. FIGS. 23A and 23B are explanatory diagrams of intensity modulation of phase conjugate light. FIG. 24 is a block diagram showing another basic configuration of the optical communication system of the present invention. FIG. 25 is a block diagram showing a first embodiment of a slave station according to the present invention. FIG. 26 is a block diagram showing a second embodiment of the slave station according to the present invention. FIG. 27 is an explanatory diagram of the spectrum of the intermediate frequency signal in the embodiment of FIG. FIG. 28 is a block diagram of an optical communication system showing an embodiment in which the polarization control is unnecessary. FIG. 29 is a block diagram of an optical communication system showing an embodiment in which the present invention is applied to an optical distribution system. FIG. 30 is a block diagram showing a basic configuration of the optical modulator according to the present invention. 31A, 31B, and 31C are explanatory diagrams of the modulation of the output idler light by the modulation of the pump light. FIG. 32 is a block diagram showing a first embodiment of the optical modulator according to the present invention. FIG. 33 is a block diagram showing a second embodiment of the optical modulator according to the present invention. FIG. 34 is a block diagram showing a third embodiment of the optical modulator according to the present invention. FIG. 35 is a block diagram showing a fourth embodiment of the optical modulator according to the present invention. FIG. 36 is a block diagram showing a fifth embodiment of the optical modulator according to the present invention. FIG. 37 is a block diagram showing a sixth embodiment of the optical modulator according to the present invention. FIG. 38 is a block diagram showing a seventh embodiment of the optical modulator according to the present invention. FIG. 39 is a block diagram showing an eighth embodiment of the optical modulator according to the present invention. FIG. 40 is a block diagram showing a ninth embodiment of the optical modulator according to the present invention. FIG. 41 is a block diagram showing a tenth embodiment of the optical modulator according to the present invention. FIG. 42 is a block diagram showing an eleventh embodiment of the optical modulator according to the present invention.

Explanation of reference numerals

301 Probe light source 302 Excitation light source 303 Nonlinear optical medium 304 Probe light / excitation light supply means 305 Modulation means

Claims (21)

A probe light source,
An excitation light source;
A nonlinear optical medium,
Probe light / excitation light supply means for supplying the probe light from the probe light source to the nonlinear optical medium together with the excitation light from the excitation light source;
Modulating means operatively connected to the excitation light source and modulating the excitation light with an information signal,
An optical modulator that outputs a phase conjugate light modulated from the nonlinear optical medium. The nonlinear optical medium exhibits a third-order nonlinear optical effect,
The optical modulator according to claim 1, wherein the phase conjugate light is generated by four-wave mixing in the nonlinear optical medium.   The optical modulator according to claim 1, wherein the modulation means modulates the amplitude or the intensity of the excitation light.   2. The optical modulator according to claim 1, wherein said modulating means modulates a frequency of said excitation light.   2. The optical modulator according to claim 1, wherein said modulating means modulates a phase of said excitation light.   The optical modulator of claim 1, further comprising an optical bandpass filter operatively connected to the non-linear optical medium, the passband of which includes the wavelength of the phase conjugate light.   The optical modulator according to claim 6, further comprising an optical amplifier operatively connected to the optical bandpass filter and amplifying the phase conjugate light.   The optical modulator according to claim 1, wherein the nonlinear optical medium is an optical fiber.   9. The optical modulator according to claim 8, wherein a wavelength of the probe light is slightly different from a wavelength of the pump light, and a wavelength that gives the optical fiber zero dispersion substantially coincides with a wavelength of the pump light.   The optical modulator according to claim 1, wherein the pump light is frequency-modulated by a signal that is sufficiently slower than the information signal. A polarization separation unit that separates the probe light into first and second polarization components whose polarization planes are orthogonal to each other; and a polarization combination unit.
The nonlinear optical medium receives the first and second polarization components of the probe light and outputs phase conjugate light for the first and second polarization components, respectively, and the phase conjugate lights are combined by the polarization combining means. The optical modulator according to claim 1, wherein: The nonlinear optical medium is a polarization maintaining fiber;
The excitation light is substantially linearly polarized light having a predetermined polarization plane,
2. The optical modulator according to claim 1, wherein the excitation light source is set such that the predetermined polarization plane is inclined at approximately 45 degrees with respect to a main axis of the polarization maintaining fiber. The nonlinear optical medium is first and second polarization maintaining fibers of substantially the same length,
The first and second polarization maintaining fibers are connected such that their main axes are orthogonal to each other,
The excitation light is substantially linearly polarized light having a predetermined polarization plane,
The pump light is supplied to the first polarization maintaining fiber;
2. The optical modulator according to claim 1, wherein the excitation light source is set such that the predetermined polarization plane is inclined at approximately 45 degrees with respect to a main axis of the first polarization maintaining fiber. The probe light / excitation light supply means has first to third ports,
The probe light source and the excitation light source are connected to the first and second ports, respectively.
An optical coupler for outputting the light supplied to the first and second ports from the third port;
It has fourth to seventh ports,
The fourth port is connected to the third port;
The modulated phase conjugate light is output from the fifth port,
The sixth and seventh ports are respectively connected to first and second ends of the optical fiber,
Outputting orthogonal two-polarized light components of the light supplied to the fourth and fifth ports from the sixth and seventh ports, respectively;
A polarizing beam splitter that outputs orthogonal two-polarized light components of the light supplied to the sixth and seventh ports from the fourth and fifth ports, respectively.
9. The optical modulator according to claim 8, wherein the excitation light source is set such that a polarization plane of the excitation light supplied to the fourth port is inclined by approximately 45 degrees with respect to a polarization plane of the two orthogonal polarization components. .   15. The optical modulator according to claim 14, further comprising a polarization controller that controls the polarization state of light supplied to the optical fiber and the polarization state of light output from the optical fiber so as to match.   The optical modulator according to claim 14, wherein the optical fiber is a polarization maintaining fiber. The probe light / excitation light supply means has first to fourth ports,
The first port is supplied with the excitation light,
The probe light is supplied to the second port,
The light supplied to the first and second ports is equally distributed and output from the third and fourth ports, and the light supplied to the third and fourth ports is equally distributed and the first and second lights are distributed. An optical coupler that outputs from two ports,
Having fifth to eighth ports,
The sixth port is connected to the fourth port;
The seventh and eighth ports are connected to first and second ends of the optical fiber, respectively.
The orthogonal bi-polarization components of the light supplied to the fifth and sixth ports are output from the seventh and eighth ports, and the orthogonal bi-polarization components of the light supplied to the seventh and eighth ports are output to the fifth and sixth ports. And a polarization beam splitter output from the sixth port;
A half-wave plate inserted into an optical path between the third port and the fifth port,
9. The optical modulator according to claim 8, wherein the excitation light and the probe light supplied to the first and second ports respectively have the same polarization state. A ninth port connected to the probe light source, a tenth port connected to the second port, and an eleventh port for outputting the modulated phase conjugate light,
18. The optical modulator according to claim 17, further comprising: an optical circulator that outputs light supplied to the ninth port from the first O port and outputs light supplied to the first O port from the eleventh port. The probe light / excitation light supply means, a first port to which the probe light is supplied,
A second port to which the pumping light is supplied, and a third port connected to a first end of the optical fiber;
An optical coupler that outputs the probe light and the excitation light respectively supplied to the first and second ports from the third port,
The optical modulator according to claim 8, wherein the modulated phase conjugate light is output from a second end of the optical fiber.   20. The optical modulator according to claim 19, further comprising a polarization scrambler for disturbing a polarization state of the probe light inserted in an optical path between the probe light source and the first port.   20. The optical modulator according to claim 19, further comprising a polarization scrambler inserted into an optical path between the excitation light source and the second port to disturb the polarization state of the excitation light.

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