JP5042054B2 - All-optical regenerative repeater - Google Patents

Description

  The present invention relates to an all-optical regenerative repeater.

  In recent years, the progress of optical fiber communication technology has been remarkable, and in addition to the intensity modulation system that sends the digital signal “1” and “0” corresponding to the blinking of the optical signal that has been used in the past, it supports the digital signal. The phase modulation system that changes the phase of light has entered the stage of practical application.

  Various methods have been studied for demodulating light modulated by the optical phase modulation method. Among them, the one using an optical homodyne receiver has the highest reception sensitivity, and this Various studies have been conducted. The present application mainly relates to an optical homodyne receiver using an injection-locked laser.

  A conventional optical homodyne receiver using an injection locked laser is disclosed in Non-Patent Document 1, for example. 1 is a block diagram showing an example of an optical homodyne receiver using an injection-locked laser, and corresponds to FIG. 1 (FIG. 1) of Non-Patent Document 1.

  An optical signal that has been optically phase-modulated and transmitted through an optical fiber transmission line (not shown) is input to the input terminal 1. The input optical signal is branched into two by the optical branching device 2 at a predetermined ratio. One of the input optical signals branched by the optical splitter 2 is input to one input terminal A of the two input terminals of the optical 180-degree hybrid 5. The other of the input optical signals branched by the optical splitter 2 is input to the injection locked laser 3. The injection-locked laser 3 is constituted by a semiconductor laser or an optical fiber laser, and generally outputs light that is synchronized with the frequency and phase of the input light. In addition, by appropriately setting the characteristics of the injection-locked laser 3, it is possible to remove almost all the modulation component of the input light and output only the carrier component of the input light. The output light of the injection-locked laser 3 is appropriately adjusted in optical phase by the optical phase adjuster 4 and input to the other input terminal B of the optical 180-degree hybrid 5.

Here, the electrolysis E _i of the optical signal subjected to the optical phase modulation input to the input terminal 1 is expressed as follows.

E _i = E ₀ cos (ωt + θ _m + θ _i ) (1)
However, E ₀ is the electric field amplitude of the optical signal, ω is the angular frequency of the optical signal, θ _m is the modulation component, and takes a value of 0 or π in the binary phase modulation system. Furthermore, θ _i is a constant phase.

  An optical signal input to one input terminal A of the optical 180-degree hybrid 5 is expressed as follows.

E _U = E ₁ cos (ωt + θ _m + θ ₁ ) (2)
However, E ₁ is the electric field amplitude of the optical signal, theta ₁ is a constant phase.
Further, the optical signal input to the other input terminal B of the light 180 degree hybrid 5, if a modulation component theta _m by injection-locked laser 3 is completely removed, as follows.

E _L = E ₂ cos (ωt + θ ₂ ) (3)
However, E ₂ is the electric field amplitude of the optical signal, and θ ₂ is a constant phase.

Therefore, the output current _{i 1} of the light receiver 6-1 is as shown in equation (4).

i ₁ = R cos (θ _m + θ ₁ −θ ₂ −π / 2)
= Rsin (θ _m + θ ₁ −θ ₂ ) (4)
Further, the output current _{i 2} of the light receiver 6-2 is as shown in equation (5).

i ₂ = R cos (θ _m + θ ₁ −θ ₂ + π / 2)
= −Rsin (θ _m + θ ₁ −θ ₂ ) (5)
Here, R is a constant.

Therefore, the output voltage v ₀ of the differential amplifier 8 is expressed by the expression (6) by the action of the load resistors 7-1 and 7-2 and the differential amplifier 8.

v ₀ ∝sin (θ _m + θ ₁ −θ ₂ ) (6)
Since θ _m takes a value of 0 or π, by making θ ₁ −θ ₂ = π / 2, the value of sin (θ _m + θ ₁ −θ ₂ ) is ± 1, and demodulation is performed correctly. You can see that it is done.

As described above, the conventional optical homodyne receiver shown in FIG. 1 can demodulate the phase-modulated optical signal by optical homodyne detection to obtain a demodulated electric signal.
"Analysis of a homodyne receiver using an injection-locked semiconductor laser", published by Oliver Lidoyne et al., IEEE Journal of Lightwave Technology, Vol. 9, No.5, 659-665, May 1991 “All-Optical 2R Regeneration of 40-Gb / s SignalImpaired by Intrachannel Four-Wave Mixing” Su et al., IEEE Photonics Technology Letters, Vol. 15, No.2, 2003, pages 350-352, published in February 2003 “All-Optical Wavelength Conversion by Semiconductor Optical Amplifiers”, T.K. Durhuus et al., IEEE Journal of Lightwave Technology, Vol. 14, No. 6, 1996, pages 942 to 954, published in June 1996. “All Optical ASK to DPSK Format Conversion Using Cross-Phase Modulation in a Nonlinear Photonic Crystal Fiber”, S.C. H. Lee et al., CLEOPacific Rim 2005, paper CFJ2-5, pages 1579 to 1580, published in 2005

  The optical regenerative repeater removes noise from the light received from the transmission line, amplifies it, and then sends it to the transmission line. FIG. 2 is a block diagram showing a configuration of a conventional optical regenerative repeater. The optical regenerative repeater shown in FIG. 2 is obtained by adding a function of outputting a regenerated phase-modulated optical signal to the optical homodyne receiver of FIG.

  In FIG. 2, the same members as those of the optical homodyne receiver shown in FIG. The demodulated data signal that is the output of the differential amplifier 8 is input to the regenerative repeater circuit 10. The regenerative repeater circuit 10 suppresses noise, distortion, and jitter of the demodulated data signal. Therefore, the output electrical signal of the regenerative repeater circuit 10 is a data signal with a fixed amplitude and phase.

  The electrical signal regenerated by the regenerative repeater circuit 10 is amplified to a predetermined voltage by the amplifier 12 and then applied to the terminal of the optical phase modulator 13. The optical phase modulator 13 applies phase modulation to the input optical signal according to the input voltage, and the output optical signal of the laser oscillator 11 is used as the input optical signal. Therefore, an optical signal phase-modulated according to the output voltage of the amplifier 12 is output to the output terminal 14.

  As can be understood from the above description, in the conventional optical regenerative repeater shown in FIG. 2, the regenerated phase modulated optical signal corresponding to the phase modulation information of the phase modulated optical signal input to the input terminal 1 is output. Output from the terminal 14. In general, the phase-modulated optical signal input from the input terminal 1 is transmitted through an optical transmission line composed of an optical fiber and an optical amplifier, and therefore generally involves noise and distortion. If a regenerative repeater is used, it is possible to remove those influences.

  However, in the conventional optical regenerative repeater, as described above, the optical signal received in the demodulation process is once converted into an electrical signal, regenerated by the regenerative repeater circuit 10, and then converted again into the phase modulated optical signal. . In general, in signal processing using an electric signal, there is an upper limit to the frequency that can be handled, so the operation transmission rate of the optical regenerative repeater as shown in FIG. 2 is limited to about 50 Gbit / s. However, research and development related to Ethernet (registered trademark) operating at 100 Gbit / s is progressing due to recent technological innovation, and it is expected to be put to practical use in the near future.

  In order to transmit an Ethernet signal operating at 100 Gbit / s, modulation having a transmission rate of 100 Gbit / s or higher is generally required. However, the optical regenerative repeater of FIG. 2 has a problem that it cannot be expected to operate at such a high transmission rate as described above.

  An object of the present invention is to provide an all-optical regenerative repeater capable of high-speed operation without conversion to an electrical signal.

An object of the present invention is an all-optical regenerative repeater that is arranged in a transmission line, accepts a phase-modulated optical signal from the transmission line, removes a noise component of the phase-modulated optical signal, and sends it again to the transmission line. There,
An optical branching device for branching the received phase-modulated optical signal at a predetermined ratio;
An injection-locked laser that receives the other phase-modulated optical signal branched from the optical splitter, removes a modulation component of the phase-modulated optical signal, and outputs an optical signal including a carrier wave component;
An optical phase adjuster for adjusting the phase of the optical signal output from the injection-locked laser;
One phase-modulated optical signal branched from the optical splitter is received at a first input terminal, and an optical signal output from the optical phase adjuster is received at a second input terminal, and the first input terminal receives the optical signal. The phase of the received optical signal is delayed by π / 2, and the optical signal received at the second input terminal is output as it is from the first output terminal and received at the first input terminal. An optical 180-degree hybrid that outputs the signal from the second output terminal as it is and with the phase of the optical signal received at the second input terminal delayed by π / 2;
An all-optical regenerative repeater for intensity-modulated light that removes noise and distortion of the intensity-modulated optical signal output from the first output terminal of the optical 180-degree hybrid;
An all-optical wavelength converter that receives an optical signal output from the all-optical regenerative repeater for intensity-modulated light, converts the wavelength of the received optical signal, and outputs the optical signal as a different wavelength;
An all-optical phase modulator that receives the intensity-modulated optical signal output from the all-optical wavelength converter and generates a phase-modulated optical signal corresponding to the intensity of the intensity-modulated optical signal. This is achieved by an optical regenerative repeater.

In a preferred embodiment, the all-optical phase modulator comprises:
A signal laser for outputting an optical signal having a wavelength different from that of the received intensity-modulated optical signal;
An optical multiplexer that combines the intensity-modulated optical signal and the optical signal output from the signal laser;
A nonlinear optical device for subjecting the optical signal output from the signal laser to optical phase modulation by the intensity-modulated optical signal;
An optical bandpass filter that outputs only the optical signal subjected to the optical phase modulation.

In another preferred embodiment, a second optical branching device for receiving the optical signal output from the optical phase adjuster and branching the received optical signal is provided.
The all-optical phase modulator comprises:
An optical multiplexer for combining the intensity-modulated optical signal and the optical signal received from the second optical splitter;
A non-linear optical device for subjecting the optical signal received from the second optical splitter to optical phase modulation by the intensity-modulated optical signal;
An optical bandpass filter that outputs only the optical signal subjected to the optical phase modulation.

In another preferred embodiment, an optical signal received from the second output terminal of the optical 180-degree hybrid is input to the first input terminal of the optical 180-degree hybrid; An optical phase stabilization circuit for generating a correction signal for correcting the phase of the optical signal input to the second input terminal so as to eliminate the phase difference of the optical signal input to the second input terminal;
The optical phase adjuster receives the output from the optical phase stabilization circuit, and changes the phase of the optical signal input to the second input terminal of the optical 180-degree hybrid.

In a more preferred embodiment, the optical phase stabilization circuit is
An optical receiver for receiving an optical signal from the second output terminal of the optical 180-degree hybrid and converting it into an electrical signal;
An oscillator that generates an electrical signal having a sufficiently low frequency compared to the transmission speed of the phase-modulated optical signal;
A multiplier for multiplying the output signal from the optical receiver by the output signal from the oscillator;
An amplifier for amplifying the output signal from the multiplier;
A low-pass filter that passes only a predetermined low-frequency component from the output signal from the amplifier; and
An adder for adding the output signal from the low-pass filter and the output signal from the oscillator;

  According to the present invention, it is possible to provide an all-optical regenerative repeater capable of high-speed operation without conversion to an electric signal.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 3 is a block diagram showing the configuration of the all-optical regenerative repeater according to the first embodiment of the present invention. As shown in FIG. 3, the all-optical regenerative repeater 15 according to the present embodiment includes an optical splitter 2, an injection-locked laser 3, an optical phase adjuster 4, an optical 180-degree hybrid 5, and an all-optical light for intensity-modulated light. A regenerative repeater 20, an optical terminator 22, an all-optical wavelength converter 25, and an all-optical phase modulator 21 are provided.

  As shown in FIG. 3, an optical signal that has been optically phase-modulated and transmitted through an optical fiber transmission line (not shown) is input to the input terminal 1. The input optical signal is branched into two by the optical branching device 2 at a predetermined ratio. One of the input optical signals branched by the optical splitter 2 is input to one input terminal A of the two input terminals of the optical 180-degree hybrid 5. The other of the input optical signals branched by the optical splitter 2 is input to the injection locked laser 3.

  As in the case shown in FIG. 1, the output light of the injection-locked laser 3 is output in synchronization with the frequency and phase of the input light. In addition, by appropriately setting the characteristics of the injection-locked laser 3, it is possible to remove almost all of the modulation component of the input light and output only the carrier line segment of the input light. The output light of the injection-locked laser 3 is appropriately adjusted in optical phase by the optical phase adjuster 4 and input to the other input terminal B of the optical 180-degree hybrid 5.

The electrolysis E _i of the optical signal subjected to the optical phase modulation input to the input terminal is expressed by the equation (1) as in the description of the prior art.

E _i = E ₀ cos (ωt + θ _m + θ _i ) (1)
The optical 180-degree hybrid 5 has input terminals A and B and output terminals C and D, and the phase of light when passing through the optical 180-degree hybrid 5 undergoes the following phase shift.
The optical signal input to the input terminal A is output from the output terminal C with a phase delay of π / 2.
The optical signal input to the input terminal A is output from the output terminal D with the same phase.
The optical signal input to the input terminal B is output from the output terminal C as it is.
The optical signal input to the input terminal B is output from the output terminal D with a phase delay of π / 2.

  For example, in this embodiment, an optical fiber coupler, a transflective mirror, or the like can be used as the light 180-degree hybrid 5. The light input to the input terminal A of the 180-degree hybrid 5 is output to the output terminals C and D at an appropriate branch ratio, and similarly, the light input to the input terminal B is output at the appropriate branch ratio. Output to output terminals C and D, respectively. In the present embodiment, the branching ratio is 50 percent.

  The optical signal input to one input terminal A of the optical 180-degree hybrid 5 and the optical signal input to the other input terminal B of the optical 180-degree hybrid 5 are respectively expressed by equations (2) and (3). It will be shown in

E _U = E ₁ cos (ωt + θ _m + θ ₁ ) (2)
E _L = E ₂ cos (ωt + θ ₂ ) (3)
In the present embodiment, an optical terminator 22 is connected to the other output terminal D of the light 180-degree hybrid 5, and the light output from the other output terminal D is terminated without being reflected. The output light from one output terminal C of the light 180-degree hybrid is expressed by the following equation (7).

E ₃ = E ₁ cos (ωt + θ _m + θ ₁ −π / 2) + E ₂ cos (ωt + θ ₂ ) (7)
If E ₁ = E ₂ ≡E, the equation (7) can be rewritten as follows.

E ₃ = Ecos (ωt + θ _m + θ ₁ −π / 2) + Ecos (ωt + θ ₂ )
= Esin (ωt + θ _m + θ ₁ ) + Ecos (ωt + θ ₂ ) (8)
Here, θ ₂ = θ ₁ + π / 2 (9)
By adjusting the value of θ ₂ by the optical phase adjuster 4 so that the electric field amplitude E ₃ of the output light becomes as shown in equation (10).

E ₃ = Esin (ωt + θ _m + θ ₁ ) + Ecos (ωt + θ ₁ + π / 2)
= Esin (ωt + θ _m + θ ₁ ) −Esin (ωt + θ ₁ ) (10)
Since θ _m takes a value of 0 or π, the above equation (10) is expressed as equations (11a) and (11b).

When θ _m = 0, E ₃ = 0 (11a)
When θ _m = π, E ₃ = −2Esin (ωt + θ ₁ ) (11b)
Therefore, according to the value of theta _m, the presence or absence of the optical signal is determined. This indicates that the phase-modulated optical signal has been converted into a light intensity-modulated signal. This is nothing but the phase-modulated optical signal is demodulated as it is without being converted into an electrical signal.

  The signal demodulated at the optical level is input to the all-optical regenerative repeater 20 for intensity-modulated light. The all-optical regenerative repeater 20 for intensity-modulated light removes noise and distortion of the input intensity-modulated light. FIG. 4 is a block diagram illustrating a configuration example of the all-optical regenerative repeater 20 for intensity-modulated light.

  Note that the all-optical regenerative repeater for intensity-modulated light shown in FIG. 4 is a known one, and is proposed in Non-Patent Document 2, for example. The intensity-modulated optical signal with noise and distortion input to the input terminal 40 is amplified to a specified power by the high-power optical amplifier 41 and input to the nonlinear optical device 42. In the nonlinear optical device 42, the spectrum width of the input optical signal is widened by the nonlinear optical effect. It is known that noise and distortion of an input optical signal are reduced by passing the optical signal with the widened spectral width through the optical bandpass filter 43.

  The optical signal whose noise and distortion are reduced by the all-optical regenerative repeater 20 for intensity-modulated light is input to the all-optical wavelength converter 25 and output as output light having a different wavelength. FIG. 5 is a block diagram illustrating a configuration example of the all-optical wavelength converter 25. The configuration shown in FIG. 5 is known, and is proposed in Non-Patent Document 3, for example.

  The input optical signal is input to the input terminal 50. The optical signal input from the input terminal 50 is combined by the optical combiner 52 with an optical signal having a wavelength different from that of the input optical signal output from the optical transmitter 51. The combined optical signal is input to the semiconductor optical amplifier 53. Due to the gain saturation phenomenon of the semiconductor optical amplifier 53, the output optical signal of the optical transmitter 51 is converted into the intensity-modulated optical signal input to the input terminal 50. , Subjected to logic-inverted intensity modulation.

  The optical filter 54 is set so as to pass only the same wavelength component as the output light wavelength of the optical transmitter 51, and the optical signal input to the input terminal 50 cannot pass. In other words, wavelength-converted intensity-modulated light having modulation information obtained by logically inverting the optical signal input to the input terminal 50 is output to the output terminal 55. As described herein, in the all-optical wavelength converter 25, logical inversion occurs simultaneously with wavelength conversion. Generally, however, logical inversion often occurs in data communication. There is no problem in data demodulation itself. In general, data by an optical signal sent to a transmission path includes a header, and the header includes a determined data string. Therefore, on the receiving side, the presence or absence of logic inversion can be known by referring to the determined data string.

  The optical signal wavelength-converted by the all-optical wavelength converter 25 is input to the all-optical phase modulator 21. The all-optical phase modulator 21 generates phase-modulated light corresponding to the intensity of input intensity-modulated light, and does not have an electric signal processing portion. FIG. 6 is a block diagram illustrating a configuration example of the all-optical phase modulator 21. The all-optical phase modulator shown in FIG. 6 is a known one, and is proposed in Non-Patent Document 4, for example.

  In FIG. 6, the intensity modulated optical signal is input to the input terminal 60. The intensity-modulated light input to the input terminal 60 is amplified by the high-power optical amplifier 61, the output optical signal of the signal laser 62 having a wavelength different from that of the intensity-modulated light input to the input terminal, and the optical multiplexer 63. Are combined. The two combined optical signals are input to the nonlinear optical device 64.

  In the nonlinear optical device 64, the optical signal output from the signal laser 62 is subjected to optical phase modulation by the modulated data signal of the intensity-modulated light input from the input terminal 60 due to the nonlinear optical effect (cross-phase modulation phenomenon). . The optical bandpass filter 65 is set so as to pass only the wavelength of the optical signal that has undergone phase modulation and not to pass the wavelength of the input intensity-modulated light. Therefore, only the optical signal subjected to optical phase modulation is output to the output terminal 66. Thus, by using the all-optical phase modulator shown in FIG. 6, an optical phase modulation signal corresponding to the modulation data of the input intensity modulated light can be output.

  According to the all-optical regenerative repeater 15 according to the present embodiment shown in FIG. 3, the phase-modulated optical signal input from the input terminal 1 is demodulated at the optical level and then all-optical regenerative repeater for intensity-modulated light. The noise and distortion are removed by the optical device 20, and the wavelength is converted by the all-optical wavelength converter 25. Then, the light is optically phase-modulated again by the all-optical phase modulator 21 and guided to the output terminal 23.

  Here, in order to make the wavelength of the phase-modulated optical signal input to the input terminal 1 the same as the wavelength of the phase-modulated optical signal output from the output terminal 23, the optical signal is once converted by the all-optical wavelength converter 25. Needs to be converted. The reason is that in the all-optical phase modulator 21, as shown in FIG. 6, the light that is finally optically phase-modulated and output is the light that is output from the signal laser 62, and this is the input terminal 60. This is because it must be different from the wavelength of the light input from. That is, in FIG. 3, in order to make the wavelength of the phase modulation optical signal input to the input terminal 1 and the wavelength of the phase modulation optical signal output from the output terminal 23 the same, the wavelength of the signal laser in FIG. In FIG. 3, the wavelength of the phase-modulated optical signal input to the input terminal 1 may be the same.

  As can be understood from the above description, in FIG. 3, when the wavelength of the phase modulated optical signal input to the input terminal 1 and the wavelength of the phase modulated optical signal output from the output terminal 23 do not have to be the same. The all-optical wavelength converter 25 is not always necessary, and the output of the all-optical regenerative repeater 20 for intensity-modulated light may be directly connected to the input of the all-optical phase modulator 21.

  According to this embodiment, the noise and distortion of the optical signal transmitted through the transmission line and input to the all-optical regenerative repeater are removed, amplified, and transmitted again without any change to an electrical signal. It can be sent to the road.

  Next, a second embodiment of the present invention will be described. FIG. 7 is a block diagram showing the configuration of the all-optical regenerative repeater according to the second embodiment of the present invention. 7, the same components as those of the all-optical regenerative repeater according to the first embodiment shown in FIG.

  As shown in FIG. 7, the all-optical regenerative repeater 70 according to the second embodiment includes an optical splitter 2, an injection-locked laser 3, an optical phase adjuster 4, an optical splitter 26, and an optical 180-degree hybrid 5. And an all-optical regenerative repeater 20 for intensity-modulated light, an optical terminator 22, an all-optical wavelength converter 25, and an all-optical phase modulator 27.

  The all-optical regenerative repeater 70 according to the second embodiment differs from the all-optical regenerative repeater 15 according to the first embodiment in the following points. In the first embodiment, as shown in FIG. 6, a signal laser 62 is used to generate a signal that is a source of the optical phase-modulated optical signal output from the all-optical phase modulator 21. However, in the second embodiment, the output light of the injection-locked laser 3 is branched into two by the optical branching unit 26 without newly providing the signal laser 62, and one of the optical branching units 26. Is output to the all-optical phase modulator 27, thereby playing the same role as the light generated by the signal laser 62 according to the first embodiment.

  FIG. 8 is a block diagram showing a configuration of the optical phase modulator 27 according to the second embodiment. In FIG. 8, the same components as those of the optical phase modulator 21 according to the first embodiment shown in FIG. In the all-optical phase modulator 27 according to the second embodiment, light that is to be optically phase-modulated is input from the input terminal 80. An intensity-modulated optical signal having modulation information is input from the input terminal 60.

  The optical signal input from the input terminal 60 is amplified by the high-power optical amplifier 61 and is combined with the optical signal input from the input terminal 80 by the optical multiplexer 63, and is nonlinear as in the first embodiment. Optical phase modulation is performed by the optical device 64.

  Next, a third embodiment of the present invention will be described. In the first embodiment and the second embodiment, it is considered that the phases of the optical signals combined in the optical 180-degree hybrid 5 do not change each other. However, the phase of the two optical signals combined by the optical 180-degree hybrid 5 may constantly change due to changes in the external environment such as ambient temperature changes and vibrations. Therefore, in the third embodiment, these variations are corrected.

  FIG. 9 is a block diagram showing the configuration of the all-optical regenerative repeater according to the third embodiment of the present invention. The all-optical regenerative repeater 90 according to the third embodiment includes an optical splitter 2, an injection-locked laser 3, an optical phase modulator 39, an optical 180-degree hybrid 5, an all-optical regenerative repeater 20 for intensity-modulated light, An all-optical wavelength converter 25, an all-optical phase modulator 21, and an optical phase stabilizing circuit 30 are provided.

  The all-optical regenerative repeater 90 according to the third embodiment is different from the all-optical regenerative repeater 15 according to the first embodiment in the following points. In the first embodiment, as shown in FIG. 3, the output from the other output terminal D of the optical 180-degree hybrid 5 is given to the optical terminator 22 and terminated, but the third embodiment In, the optical phase stabilization circuit 30 corrects the phase of the optical signal in the optical phase modulator 39 using the output light from the other output terminal D of the optical 180-degree hybrid 5.

  In the first embodiment and the second embodiment, an optical phase adjuster 4 that receives the output light from the injection-locked laser 3 is provided, whereas in the third embodiment, An optical phase modulator 39 that receives the light of the injection-locked laser 3 is provided. The former is a fixed phase shifter whose phase is fixed. On the other hand, the latter can change the phase by the control of the optical phase stabilization circuit 30 as described later. However, actually, the optical phase adjuster 4 and the optical phase modulator 39 are also realized by the same device (for example, an optical phase modulator using lithium niobate), and the difference is that the phase is fixed. It is only variable.

In FIG. 12, the horizontal axis δ ₀ represents the first optical path from the optical splitter 2 in FIG. 9 to one input terminal A of the optical 180-degree hybrid 5, the injection-locked laser 3, and the optical phase modulator 39. Then, the phase difference from the second path reaching the other input terminal B of the light 180-degree hybrid. In FIG. 12, the vertical axis I represents the intensity of the optical signal output from the output terminal C of the optical 180-degree hybrid 5 in FIG. 9 and input to the all-optical regenerative repeater 20 for intensity-modulated light. In the following description, for the sake of simplicity, it is assumed that the phase shift of π / 2 already described does not occur in the optical 180-degree hybrid 5. Actually, the phase shift of π / 2 occurs in the optical 180-degree hybrid 5, and therefore, in the following description, it is necessary to correctly consider the phase shift value π / 2. For this purpose, in the following description, the phase of the first optical path may be shifted by −π / 2.

As shown in FIG. 12, if δ ₀ = 0, the phase difference between the first optical path and the second optical path is 0. The light passing through the optical path and the light passing through the second optical path are strengthened, and the intensity I is maximized (point P in FIG. 12). On the other hand, when a phase difference of ± π occurs between the two, I = 0. Generally, the phase difference between the first optical path and the second optical path may change from time to time, and the output light intensity is located at the point P + or the point P- in FIG. Therefore, it is desirable to follow this constantly and return to the point P in FIG.

  For this purpose, minute low-frequency modulation is applied to the optical phase of the second optical path by the output of an oscillator 31 (see FIG. 10) in the optical phase stabilization circuit 30 described later.

  Considering the same as described in the first embodiment, the optical electric field output from the other output terminal D of the optical 180-degree hybrid 5 is expressed as follows.

E ₄ = Ecos (ωt + θ _m + θ ₁ ) + Ecos (ωt + θ ₂ −π / 2)
= Ecos (ωt + θ _m + θ ₁ ) + Esin (ωt + θ ₂ ) (12)
Here, from equation (9)
θ ₂ = θ ₁ + π / 2 (13)
Θ ₂ is set so that Applying equation (13) to equation (12) yields equation (14).

E ₄ = Ecos (ωt + θ _m + θ ₁ ) + Esin (ωt + θ ₁ + π / 2)
= Ecos (ωt + θ _m + θ ₁ ) + Ecos (ωt + θ ₁ ) (14)
The optical phase stabilization circuit 30 in FIG. 9 will be described in more detail. FIG. 10 is a block diagram showing a configuration of an optical phase stabilization circuit 30 according to the third exemplary embodiment of the present invention. As shown in FIG. 10, an oscillator 31, a multiplier 32, an optical receiver 33, an amplifier 34, a low-pass filter 35, and an adder 36 are provided.

  The oscillator 31 generates an electric signal having a frequency sufficiently lower than the transmission speed of the phase-modulated optical signal. As a generated frequency of the oscillator 31, for example, a value of about several tens kHz to several hundreds kHz is adopted. The output of the oscillator 31 is input to the optical phase modulator 39 via the adder 36. As a result, of the two optical signals branched by the optical splitter 9 in FIG. 9, the optical signal that has passed through the path (second optical path) passing through the injection-locked laser 3 and the optical phase modulator 39 is: In addition to optical phase modulation performed at the transmission speed, minute optical phase modulation is performed by a low-frequency electrical signal that is an output of the oscillator 31. In consideration of this minute phase modulation, the equation (14) can be rewritten as the equation (15).

However, δ ₀ represents the phase difference between the first optical path (path reaching the one input terminal A of the optical 180-degree hybrid 5 from the optical splitter 2) and the second optical path caused by a change in the external environment. Δ and ω _m collected in the optical phase modulator 39 are the amplitude and angular frequency of the low-frequency electric signal, respectively.

The field _{E 4} is received by the optical receiver 33 shown in FIG. 10. It is assumed that the frequency band of the optical receiver 33 is sufficiently smaller than the frequency corresponding to the transmission speed and sufficiently larger than the frequency of the low frequency electric signal. Then, the output current i of the optical receiver 33 is expressed as shown in Equation (16).

In consideration of the frequency band of the optical receiver 33, it represents a time average over a time sufficiently longer than one bit time corresponding to the transmission speed and sufficiently shorter than the period of the low-frequency electric signal. That is, in equation (16), the right side is the average value of the square of the absolute value of the electric field E _4. The time average is applied to the modulation term by the data signal, and the following equation (17) can be obtained from the equations (15) and (16).

Here, when θ _m = 0, the equation (18) is obtained from the equation (17).

Here, the phase shift due to environmental fluctuation is sufficiently small, and the modulation by the low-frequency electrical signal is also sufficiently small. That is, if δ ₀ is sufficiently smaller than 1 (δ ₀ << 1) and δ is sufficiently smaller than 1 (δ << 1), the equation (18) becomes the equation (19).

i∝−δ ₀ δ sin ω _m t (19)
In the equation (17), when θ _m = π, the equation (20) is obtained.

i∝cos ² ((δ ₀ + δ sin ω _m t) / 2) (20)
Eventually, since it is the same as the case of θ _m = 0, equation (19) is finally obtained.

In FIG. 10, the multiplier 32 outputs an electric signal proportional to the product of the output electric signal of the oscillator 31 and the output electric signal of the optical receiver 33. Therefore, the output voltage v _out is as shown in equation (21).

v _out ∝− (δ ₀ δsin ω _m t) sin ω _m t
=-([Delta] _{0 [} delta] / 2). (1-cos2 [omega] _mt ) (21)
The output of the multiplier 32 is appropriately amplified by an amplifier 34 and then passes through a low-pass filter 35 that passes only a frequency component sufficiently lower than the angular frequency ω _m . The signal v _{out2 that} has passed through the low-pass filter 35 is as shown in equation (22) from equation (21).

v _out2 = −G · (δ ₀ δ / 2) (22)
Here, G is the gain of the amplifier 34.

As can be understood from the equation (22), the output signal of the low-pass filter 35 has a value proportional to δ ₀ and the sign is inverted. Therefore, by applying the signal v _out2 represented by the equation (22) to the optical phase modulator 28 via the adder 36, feedback control is performed so that δ ₀ becomes “0”. That is, even if the phase difference between the first optical path and the second optical path in FIG. 9 fluctuates due to external environmental fluctuations, it can be seen that the fluctuations can be removed by the operation of the optical phase stabilization circuit 30. .

  As described above, according to the third embodiment of the present invention, even when the phase difference between the first optical path and the second optical path due to the external environment fluctuation is changed, the fluctuation is continuously removed. Thus, it is possible to provide an all-optical regenerative repeater capable of stable operation.

  Next, a fourth embodiment of the present invention will be described. FIG. 11 is a block diagram showing the configuration of the all-optical regenerative repeater according to the fourth embodiment of the present invention. As shown in FIG. 11, the all-optical regenerative repeater 100 according to the fourth embodiment includes an optical splitter 2, an injection-locked laser 3, an optical phase modulator 39, an optical splitter 26, and an optical 180-degree hybrid 5. , An all-optical regenerative repeater 20 for intensity-modulated light, an all-optical wavelength converter 25, an all-optical phase modulator 27, and an optical phase stabilization circuit 30. The optical phase stabilization circuit 30 is the same as that shown in the third embodiment.

  In the third embodiment, the all-optical phase modulator 21 includes a signal laser 62 as shown in FIG. On the other hand, in the fourth embodiment, as shown in FIG. 8, the all-optical phase modulator 27 uses the optical splitter 26 to output the output light of the injection-locked laser 3 instead of using the signal laser. The branched optical signal is received and this optical signal is used.

  In the all-optical regenerative repeater 100 according to the fourth embodiment, similarly to the second embodiment, the output light of the injection-locked laser 3 is branched into two by the optical branching device 26, and the optical branching is performed. One output of the detector 26 is input to the all-optical phase modulator 27. In the optical phase modulator 27, as shown in FIG. 8, the optical signal input from the input terminal 60 is amplified by the high output optical amplifier 61, and the optical signal input from the input terminal 80 and the optical multiplexer 63. Combined and optical phase modulated by the nonlinear optical device 64.

  Further, in the all-optical regenerative repeater 100 according to the fourth embodiment, similarly to the third embodiment, the optical phase is obtained using the output light from the other output terminal D of the optical 180-degree hybrid 5. The stabilization circuit 30 corrects the phase of the optical signal in the optical phase modulator 39.

  The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the invention described in the claims, and these are also included in the scope of the present invention. Needless to say.

FIG. 1 is a block diagram showing an example of an optical homodyne receiver using an injection-locked laser. FIG. 2 is a block diagram showing a configuration of a conventional optical regenerative repeater. FIG. 3 is a block diagram showing the configuration of the all-optical regenerative repeater according to the first embodiment of the present invention. FIG. 4 is a block diagram illustrating a configuration example of an all-optical regenerative repeater for intensity-modulated light. FIG. 5 is a block diagram illustrating a configuration example of the all-optical wavelength converter. FIG. 6 is a block diagram illustrating a configuration example of the all-optical phase modulator. FIG. 7 is a block diagram showing the configuration of the all-optical regenerative repeater according to the second embodiment of the present invention. FIG. 8 is a block diagram illustrating a configuration of an optical phase modulator according to the second embodiment. FIG. 9 is a block diagram showing the configuration of the all-optical regenerative repeater according to the third embodiment of the present invention. FIG. 10 is a block diagram showing a configuration of an optical phase stabilization circuit according to the third embodiment of the present invention. FIG. 11 is a block diagram showing the configuration of the all-optical regenerative repeater according to the fourth embodiment of the present invention. FIG. 12 is a graph illustrating the phase difference between the first optical path and the second optical path and the intensity of the combined light.

Explanation of symbols

2 optical splitter 3 injection-locked laser 4 optical phase adjuster 5 optical 180-degree hybrid 15 all-optical regenerative repeater 20 all-optical regenerative repeater 21 for intensity-modulated light 21 all-optical phase modulator 22 optical terminator 25 all-optical wavelength conversion 41 High-power optical amplifier 42 Non-linear optical device 43 Optical band-pass filter 51 Optical transmitter 52 Optical combiner 53 Semiconductor optical amplifier 54 Optical filter 61 High-power optical amplifier 62 Signal laser 63 Optical multiplexer 64 Non-linear optical device 65 Optical band-pass filter

Claims (3)

An all-optical regenerative repeater that is arranged in a transmission line, accepts a phase-modulated optical signal from the transmission line, removes a noise component of the phase-modulated optical signal, and sends it again to the transmission line,
An optical branching device for branching the received phase-modulated optical signal at a predetermined ratio;
An injection-locked laser that receives the other phase-modulated optical signal branched from the optical splitter, removes a modulation component of the phase-modulated optical signal, and outputs an optical signal including a carrier wave component;
An optical phase adjuster for adjusting the phase of the optical signal output from the injection-locked laser;
One phase-modulated optical signal branched from the optical splitter is received at a first input terminal, and an optical signal output from the optical phase adjuster is received at a second input terminal, and the first input terminal receives the optical signal. The phase of the received optical signal is delayed by π / 2 and output from the first output terminal, and the received optical signal at the second input terminal is output as it is from the first output terminal. The signal received at the input terminal is output as it is from the second output terminal, and the light output from the second output terminal is delayed by π / 2 by the phase of the optical signal received at the second input terminal. 180 degree hybrid,
An all-optical regenerative repeater for intensity-modulated light that removes noise and distortion of the intensity-modulated optical signal output from the first output terminal of the optical 180-degree hybrid;
An all-optical wavelength converter that receives an optical signal output from the all-optical regenerative repeater for intensity-modulated light, converts the wavelength of the received optical signal, and outputs the optical signal as a different wavelength;
An all-optical phase modulator that receives the intensity-modulated optical signal output from the all-optical wavelength converter and generates a phase-modulated optical signal according to the intensity of the intensity-modulated optical signal;
A second optical branching unit that receives the optical signal output from the optical phase adjuster and branches the received optical signal;
The all-optical phase modulator comprises:
An optical multiplexer for combining the intensity-modulated optical signal and the optical signal received from the second optical splitter;
A non-linear optical device for subjecting the optical signal received from the second optical splitter to optical phase modulation by the intensity-modulated optical signal;
An all-optical regenerative repeater comprising: an optical bandpass filter that outputs only the optical signal subjected to the optical phase modulation . An optical signal received from the second output terminal of the optical 180-degree hybrid, an optical signal input to the first input terminal of the optical 180-degree hybrid, and light input to the second input terminal An optical phase stabilization circuit for generating a correction signal for correcting the phase of the optical signal input to the second input terminal so as to eliminate the phase difference of the signal;
The optical phase adjuster receives an output from the optical phase stabilization circuit and changes the phase of an optical signal input to the second input terminal of the optical 180-degree hybrid. All optical regeneration repeater according to 1. The optical phase stabilization circuit is
An optical receiver for receiving an optical signal from the second output terminal of the optical 180-degree hybrid and converting it into an electrical signal;
An oscillator that generates an electrical signal having a sufficiently low frequency compared to the transmission speed of the phase-modulated optical signal;
A multiplier for multiplying the output signal from the optical receiver by the output signal from the oscillator;
An amplifier for amplifying the output signal from the multiplier;
A low-pass filter that passes only a predetermined low-frequency component from the output signal from the amplifier; and
The all-optical regenerative repeater according to claim 2 , further comprising an adder that adds an output signal from the low-pass filter and an output signal from the oscillator.