JP6432628B2 - Coherent detector - Google Patents

Description

  The present invention relates to a coherent detector used on the receiving side of coherent optical communication.

  With the recent increase in capacity of optical communication, attention has been focused on coherent communication using phase modulation and the like, which makes it easier to improve bandwidth utilization efficiency by multi-leveling compared to conventional intensity modulation. In communication using phase modulation, information is superimposed on the phase and transmitted.

  As a reception method in coherent communication, there are a reception method using homodyne detection and a reception method using heterodyne detection. In homodyne detection, local light having the same frequency and phase as the carrier wave of the phase-modulated optical phase modulation signal is generated at the receiving end, and demodulation is performed by causing interference between the carrier wave of the optical phase modulation signal and the local light. In heterodyne detection, a carrier wave of an optical phase modulation signal subjected to phase modulation and local light having a slightly different frequency are caused to interfere with each other, and down-converted to perform demodulation. Both homodyne detection and heterodyne detection can be realized by using the phase synchronization of the optical phase modulation signal and the local light.

  With reference to FIG. 6, a typical configuration of a coherent detector that performs phase synchronization between an optical phase modulation signal and local light will be described. FIG. 6 is a schematic diagram for explaining a typical configuration of a coherent detector.

  The 90 ° optical hybrid 112 generates a first interference signal by combining the optical phase modulation signal and the local light. Further, the 90 ° optical hybrid 112 generates a second interference signal by combining the optical phase modulation signal and a signal obtained by shifting the local light by π / 2 (90 °) (for example, Non-Patent Document 1). reference).

  The first balance detector 114-1 generates a first modulated electric signal by performing balance detection on the first interference signal. The second balance detector 114-2 generates a second modulated electric signal by performing balance detection on the second interference signal.

  The first amplifier 116-1 amplifies the first modulated electric signal. The output of the first amplifier 116-1 is branched into two, one being sent to the modulation component removing means 120 and the other being outputted as a demodulated signal. The second amplifier 116-2 amplifies the second modulated signal. The output of the second amplifier 116-2 is branched into two, one being sent to the modulation component removing means 120 and the other being outputted as a demodulated signal.

  The modulation component removing unit 120 removes the modulation component from the first and second modulated electrical signals and extracts the phase error component to generate a phase error signal. The loop filter 150 smoothes the phase error signal and sends it to the optical voltage controlled oscillator (optical VCO) 160.

  The optical VCO 160 generates local light corresponding to the phase error component and sends it to the 90 ° optical hybrid 120.

  A typical configuration of the optical VCO 160 will be described with reference to FIGS. FIG. 7 is a schematic diagram for explaining a typical configuration of the optical VCO 160. FIG. 8 is a schematic diagram showing an optical spectrum.

  The optical VCO 160 includes a voltage controlled oscillator (VCO) 162, an amplifier 174, a Mach-Zehnder type intensity modulator (hereinafter also simply referred to as an intensity modulator) 176, and an optical bandpass filter 178. The VCO 162 changes its own oscillation frequency according to the phase error component Δf. The oscillation signal generated by the VCO 162 is amplified to an amplitude necessary for driving the intensity modulator 176 by the amplifier 174 and sent to the intensity modulator 176.

Intensity modulator 176, the continuous light source 144 (also referred to as continuous light frequency.) The resulting frequency _{f 0} of the continuous light (FIG. 8 (A)) to modulate the (FIG. 8 (B)). At the output of the intensity modulator 176, frequency components f ₀ ± Δf shifted by the same frequency Δf as the output of the VCO 162 appear on both sides of the continuous optical frequency f ₀ .

The optical bandpass filter 178 transmits one of the two frequency components (here, f ₀ + Δf) of the output of the intensity modulator 176 as local light.

  The coherent detector operates so that the optical phase modulation signal and the local light are synchronized (FIG. 8C).

Takahide Sakamoto, Akito Chiba, Atsushi Kanno, Isao Morohashi, Tetsuya Kawanishi, "Real-Time Homodyne Reception of 40-Gb / s BPSK Signal by Digital Optical Phase-Locked Loop," ECOC 2010, 19-23 September, 2010, Torino, Italy. L.G.Kazonovsky, "Performance Analysis and Laser Linewidth Requirements for Optical PSK Heterodyne Communications Systems", J. Lightwave Technology, LT-4,415-425 (1986). S. Norimatsu, "Linewidth Requirements for Optical Synchronous Detection Systems with Nonnegligible Loop Delay Time", J. Lightwave Technology, Vol. 10, No. 3, MARCH 1992.

  A major problem with coherent detectors is that the phase noise of phase-synchronized signals deteriorates as the loop propagation delay time required for feedback control increases.

  This phase noise is caused by the fact that the continuous light source used to generate local light and the light source of the optical phase modulation signal carrier on the transmission side have a finite line width. Since the phase changes that occur according to the finite line width are uncorrelated with each other, if the loop propagation delay time required for feedback control is long, phase fluctuation fluctuations increase during this propagation delay time (for example, non-delayed). (See Patent Document 2 or 3).

  As a result, fluctuations in the phase difference between the local light and the optical phase modulation signal in the synchronized state become large, and the demodulation performance deteriorates.

  In order to suppress the deterioration of the phase noise, it is necessary to shorten the loop propagation time required for feedback control in the coherent detector, or to reduce the line width of the local light and the optical phase modulation signal. It has become a big constraint on.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a coherent detector that cancels phase noise derived from a light source.

In order to achieve the above-described object, the coherent detector of the present invention includes a continuous light source, an optical phase comparator, an optical VCO, and a variable optical delay device. Continuous light source generates continuous light having a frequency f _0. Optical phase comparing unit, and the optical phase modulation signal of the carrier frequency f _{0 +} Delta] f, first phase representing the phase error between the first local light having a frequency f _{0 +} 2.DELTA.f shifted by a predetermined frequency from the carrier frequency f _{0 +} Delta] f A second phase error signal indicating a phase error between the error signal and the optical phase modulation signal and the second local light having the same frequency as the carrier frequency f ₀ + Δf is generated. The optical VCO generates first local light and second local light from a signal obtained by mixing the first phase error signal with a high frequency signal and continuous light. The variable optical delay device gives a delay corresponding to the intensity of the first phase error signal to the second local light.

  The coherent detector outputs an optical phase modulation signal and a second local light interference signal as a demodulated signal.

  Note that the variable optical delay device may be configured to give a delay corresponding to the intensity of the first phase error signal and the second phase error signal to the second local light.

  According to the coherent detector of the present invention, a delay corresponding to the intensity of the first phase error signal by the heterodyne detection with phase synchronization established is given to the second local light. Thereby, it is possible to cancel the phase noise existing between the second local light and the optical phase modulation signal.

It is a schematic diagram for demonstrating embodiment of the coherent detector of this invention. It is a schematic diagram of the optical phase comparison part with which a coherent detector is provided. It is a schematic diagram of the modulation component removal means with which a coherent detector is provided. It is a schematic diagram of optical VCO with which a coherent detector is provided. It is a schematic diagram (1) which shows an optical spectrum. It is a schematic diagram for demonstrating the typical structure of a coherent detector. It is a schematic diagram for demonstrating the typical structure of optical VCO. It is a schematic diagram (2) which shows an optical spectrum.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiments are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described, but it is merely a preferred example. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

  With reference to FIGS. 1 to 5, an embodiment of a coherent detector according to the present invention that performs phase synchronization between an optical phase modulation signal and local light will be described. FIG. 1 is a schematic diagram for explaining an embodiment of a coherent detector according to the present invention. FIG. 2 is a schematic diagram of an optical phase comparison unit provided in the coherent detector. FIG. 3 is a schematic diagram of the modulation component removal means provided in the coherent detector, and shows the modulation component removal means in the case of a QPSK signal. FIG. 4 is a schematic diagram of an optical VCO included in the coherent detector. FIG. 5 is a schematic diagram showing an optical spectrum.

  The coherent detector includes an optical phase comparison unit 10, a multiplier 40, a high frequency oscillator 42, a continuous light source 44, a first loop filter 50, an optical VCO 60, a demultiplexer 80, a variable optical delay 82, a multiplexer 84, a first A two-loop filter 90, a delay device 92, an amplifier 94, and an adder 96 are provided.

An optical phase modulation signal and local light are input to the optical phase comparison unit 10. The local light includes the first local light X and the second local light Y. The first local light X has a frequency f ₀ + 2Δf that is shifted from the carrier frequency f ₀ + Δf of the optical phase modulation signal by a fixed frequency. The second local light Y has the same frequency f ₀ + Δf as the carrier frequency f ₀ + Δf of the optical phase modulation signal.

  As shown in FIG. 2, the optical phase comparison unit 10 includes, for example, a 90 ° optical hybrid 12, first and second balance detectors 14-1 and 14-2, first and second amplifiers 16-1, and 16-2, first and second bandpass filters 18-1 and 18-2, first and second lowpass filters 22-1 and 22-2, and first and second modulation component removing means 20 And 24.

  The 90 ° optical hybrid 12 generates the first interference signal by combining the optical phase modulation signal and the local light. The 90 ° optical hybrid 12 generates a second interference signal by combining the optical phase modulation signal and a signal obtained by shifting the local light by π / 2 (90 °).

  The first balance detector 14-1 generates the first modulated electric signal by performing balance detection on the first interference signal. The second balance detector 14-2 generates a second modulated electric signal by performing balance detection on the second interference signal.

  The first amplifier 16-1 amplifies the first modulated electric signal. The output of the first amplifier 16-1 is branched into two, one being sent to the first band pass filter 18-1 and the other being sent to the first low pass filter 22-1.

  The second amplifier 16-2 amplifies the second modulated electric signal. The output of the second amplifier 16-2 is branched into two, one being sent to the second bandpass filter 18-2 and the other being sent to the second lowpass filter 22-2.

  The first and second band pass filters 18-1 and 18-2 transmit and transmit a beat component that appears in the vicinity of the frequency Δf, which is obtained from the optical phase modulation signal and the first local light X. Is sent to the first modulation component removing means 20.

  The first modulation component removing means 20 removes the modulation component from the first and second modulated electric signals and generates a first phase error signal. For example, when the optical phase modulation signal is QPSK, the first phase error signal includes a phase error component that is four times the phase difference between the first local light X and the optical phase modulation signal.

  As shown in FIG. 3, the first modulation component removing unit 20 includes, for example, an addition processing unit 26, a subtraction processing unit 28, and first to third multiplication processing units 30, 32, and 34. The first modulated electric signal (indicated by arrow S130-1 in the figure) input to the first modulation component removing means 20 is branched into three (indicated by arrows S131, S132 and S133 in the figure), respectively. The data is sent to the addition processing unit 26, the subtraction processing unit 28, and the first multiplication processing unit 30. Similarly, the second modulated electric signal (indicated by an arrow S130-2 in the figure) input to the first modulation component removing means 20 is branched into three (in the figure, by arrows S134, S135, and S136). Are sent to the addition processing unit 26, the subtraction processing unit 28, and the first multiplication processing unit 30, respectively.

  The addition processing unit 26 adds the first modulated electric signal S131 and the second modulated electric signal S134 to generate an added signal S162. The first modulated electrical signal S131 is an I-axis signal (sin (θ + d)), and the second modulated electrical signal S134 is a Q-axis signal (−cos (θ + d)). Therefore, the addition signal S162 is sin (θ + d) −cos (θ + d).

  The subtraction processing unit 28 performs subtraction between the first modulated electrical signal S132 and the second modulated electrical signal S135 to generate a subtracted signal S164. At this time, the subtraction signal S164 is sin (θ + d) + cos (θ + d).

  The first multiplication processor 30 multiplies the first modulated electrical signal S133 and the second modulated electrical signal S136 to generate a multiplied signal S166. At this time, the multiplication signal S166 is −cos (θ + d) · sin (θ + d) = − 1/2 sin (2θ + 2d).

The second multiplication processing unit 32 multiplies the addition signal S162 generated by the addition processing unit 26 and the subtraction signal S164 generated by the subtraction processing unit 28. At this time, the multiplication signal S168 in the second multiplication processing unit 32 is {sin (θ + d) −cos (θ + d)} · {sin (θ + d) + cos (θ + d)} = sin ² (θ + d) −cos ² (θ + d). ) = − Cos (2θ + 2d).

  The third multiplication processing unit 34 multiplies the multiplication signal S166 generated by the first multiplication processing unit 30 and the multiplication signal S168 generated by the second multiplication processing unit 32 to obtain a phase error signal S100. Generate. At this time, the phase error signal S100 is {−1/2 sin (2θ + 2d)} · {−cos (2θ + 2d)} = − 1/4 sin (4θ + 4d).

  In the case of a QPSK signal, the data string d is d = kπ / 2 (k = 0, 1, 2, 3). Therefore, the phase error signal S100 becomes sin 4θ, and four times the phase error is extracted.

  The first and second low-pass filters 22-1 and 22-2 transmit a beat component that appears in the vicinity of direct current, which is obtained from the optical phase modulation signal and the second local light Y. The components that pass through the first and second low-pass filters 22-1 and 2 are branched into two, one is sent to the second modulation component removing means 24, and the other is output from the coherent detector as a demodulated signal. .

  The second modulation component removing unit 24 removes the modulation component from the first and second modulated electric signals and generates a second phase error signal. For example, when the optical phase modulation signal is QPSK, the second phase error signal includes a phase error component that is four times the phase difference between the second local light Y and the optical phase modulation signal.

  Since the second modulation component removing unit 24 can be configured in the same manner as the first modulation component removing unit 20, the description thereof is omitted.

  The first phase error signal generated by the first modulation component removing unit 20 is sent to the multiplier 40.

The multiplier 40 mixes the first phase error signal and the high-frequency signal that is the output of the high-frequency oscillator 42. The high-frequency oscillator 42 has an oscillation frequency 4Δf that is four times the difference Δf between the oscillation frequency f ₀ of the continuous light source 44 and the carrier frequency of the optical phase modulation signal. Therefore, when the first phase error signal and the high frequency signal are mixed, a signal in which the frequency of the first phase error signal is shifted by 4Δf can be obtained. The output of the multiplier 40 is branched into two and sent to the first loop filter 50 and the delay unit 92.

  The first loop filter 50 smoothes the output of the multiplier 40 and sends it to the optical VCO 60.

  As shown in FIG. 4, the optical VCO 60 includes a divider 64 in addition to a conventional configuration including a voltage controlled oscillator (VCO) 62, an amplifier 74, a Mach-Zehnder type intensity modulator 76, and an optical bandpass filter 78. , A multiplier 68, a delay unit 66, a combiner 72, and a 1/4 frequency divider 70.

  The VCO 62 changes its own oscillation frequency according to the phase error component Δf caused by the first phase error signal included in the signal received from the first loop filter 50. The oscillation signal generated by the VCO 62 is sent to the divider 64. The divider 64 branches the oscillation signal into two. One of the two branches is sent to the combiner 72 via the delay unit 66. The other of the two branches is sent to the combiner 72 via the multiplier 68.

  A high frequency signal having a frequency 4Δf generated by the high frequency oscillator 42 is input to the ¼ frequency divider 70. The 1/4 divider 70 generates a signal having a frequency Δf that is 1/4 of the frequency of the high-frequency signal by dividing the high-frequency signal by 1/4.

  The multiplier 68 mixes the signal of the frequency Δf sent from the quarter frequency divider 70 and the signal of Δf sent from the VCO 62 via the divider 64 to generate a signal having a frequency of 2Δf.

  The combiner 72 adds the 2Δf signal sent through the multiplier 68 and the Δf signal whose phase timing is adjusted by the delay unit 66. Thereby, the output of the combiner 72 becomes a signal having two frequency components Δf and 2Δf. The output of the combiner 72 is amplified to a predetermined intensity by the amplifier 74 and then input to the intensity modulator 76. At this time, the signal having the frequency 2Δf generated by the multiplier 68 has the phase θ of the VCO 62. Further, a component in the vicinity of a direct current (DC) generated as a result of mixing by the multiplier 68 is removed when the amplifier 74 amplifies.

The intensity modulator 76 modulates the intensity of the continuous light having the frequency f ₀ generated by the continuous light source 44 with the output of the amplifier 74. As a result, the output of the intensity modulator 76, on both sides of the continuous optical frequency f _0, the frequency component f ₀ ± Delta] f shifted by the same frequency Delta] f as the output of the VCO, frequency component f ₀ ± 2.DELTA.f shifted by frequency 2.DELTA.f Appears.

The optical bandpass filter 78 transmits one component (f ₀ + Δf, f ₀ + 2Δf) of the continuous optical frequency f ₀ out of the frequency components of the output of the intensity modulator 76 as local light, and the demultiplexer 80. Send to.

The local lights X and Y output from the optical VCO 60 are respectively output to the first route and the second route by the duplexer 80. Here, the frequency of the first local light X is f ₀ + 2Δf, and the frequency of the second local light Y is f ₀ + Δf. The first local light X output to the first route is sent to the multiplexer 84. The second local light Y output to the second route is sent to the multiplexer 84 via the variable optical delay device 82. The variable optical delay device 82 has a function of changing the phase of light conducted by an external signal according to the external signal, such as an optical fiber stretcher.

  The multiplexer 84 combines the local lights X and Y from the two paths and sends them to the optical phase comparator 10.

  The first loop configured to include the optical phase comparison unit 10, the multiplier 40, the first loop filter 50, and the optical VCO 60 enables heterodyne detection using the first local light X of the optical phase modulation signal.

  However, in this state, the phase error component is proportional to the product of the line width of the continuous light source 44 and the line width of the light source of the optical phase modulation signal on the transmission side and the loop propagation delay time in the first loop. Phase noise is added.

  On the other hand, when phase synchronization is established in the first loop, the second local light Y and the optical phase modulation signal are also phase-synchronized, and homodyne detection is realized. In this state, only the components near DC are extracted by the low-pass filters 22-1 and 2 in the optical phase comparator 10, and demodulation is performed in a state including phase noise.

  Further, since the first route through which the first local light X is conducted and the second route through which the second local light Y is conducted are different, the first local light X and the second local light are emitted in this portion. During Y, phase fluctuations caused by fluctuations in the optical path length due to temperature changes or the like occur. Therefore, this phase fluctuation is included.

  Here, the phase noise existing between the first local light X and the optical phase modulation signal is the same as the phase noise between the second local light Y and the optical phase modulation signal. Therefore, the second loop is used to cancel out the phase noise and the phase fluctuation described above. The second loop includes an optical phase comparison unit 10, a second loop filter 90, and a variable optical delay device 82.

  The second phase error signal is the sum of the phase error caused by the first local light X and the second local light Y and the phase noise between the first local light X and the optical phase modulation signal.

  Therefore, first, in order to cancel the phase noise, a signal component proportional to the phase noise is separated from the first loop in which phase synchronization has already been established, the timing is appropriately adjusted by the delay unit 92, and the signal intensity is adjusted by the amplifier 94. After appropriate adjustment, the adder 96 adds to the second loop. Thereby, the phase noise existing between the second local light Y and the optical phase modulation signal can be canceled out.

  That is, if the delay amount of the variable optical delay device 82 is changed so as to cancel the phase noise existing between the optical phase modulation signal and the first local light X, the optical phase modulation signal and the second local light Y The phase noise during is also canceled out.

  Further, the phase fluctuation caused by the fluctuation of the optical path difference between the first local light X and the second local light Y can be canceled by controlling the variable optical delay device 82 by feedback control in the second loop. It is. As described above, the optical phase-modulated signal can be demodulated with phase noise much smaller than that of a normal optical phase-locked loop.

  If the variation in the optical path length difference between the first path where the first local light X is conducted and the second path where the second local light Y is conducted is negligible, the second loop filter 90 and The output of the amplifier 94 may be input to the variable optical delay device 82 without providing the adder 96.

Although the case where the optical phase modulation signal is a QPSK signal has been described here, the present invention is not limited to this and can be applied to a BPSK signal or the like. When the optical phase modulation signal is a BPSK signal, the high frequency oscillator 42 has an oscillation frequency 2Δf that is twice the difference Δf between the oscillation frequency f ₀ of the continuous light source 44 and the carrier frequency of the optical phase modulation signal, Instead of the 1/4 frequency divider 70, a 1/2 frequency divider may be used.

DESCRIPTION OF SYMBOLS 10 Optical phase comparison part 12, 112 90 degree optical hybrid 14, 114 Balance detector 16, 74, 94, 116, 174 Amplifier 18 Band pass filter 20, 24, 120 Modulation component removal means 22 Low pass filter 26 Addition processing part 28 Subtraction Processing unit 30, 32, 34 Multiply processing unit 40, 68 Multiplier 42 High frequency oscillator 44, 144 Continuous light source 50 First loop filter 60, 160 Optical VCO
62, 162 VCO
64 Divider 66, 92 Delay device 70 1/4 frequency divider 72 Combiner 76, 176 Intensity modulator 78, 178 Optical band pass filter 80 Demultiplexer 82 Variable optical delay device 84 Multiplexer 90 Second loop filter 94 Amplifier 96 Adder 150 Loop filter

Claims (2)

A continuous light source that generates continuous light of frequency f ₀ ;
An optical phase modulation signal of the carrier frequency f _{0 +} Delta] f, the first phase error signal indicating a phase error between the first local light having the frequency f _{0 +} 2.DELTA.f shifted by a predetermined frequency from the carrier frequency f _{0 +} Delta] f and, An optical phase comparison unit that generates a second phase error signal indicating a phase error between the optical phase modulation signal and a second local light having the same frequency as the carrier frequency f ₀ + Δf;
An optical voltage controlled oscillator that generates the first local light and the second local light from the signal obtained by mixing the first phase error signal with a high frequency signal and the continuous light;
A variable optical delay device that provides the second local light with a delay according to the intensity of the first phase error signal;
A coherent detector that outputs the optical phase modulation signal and the second local light interference signal as a demodulated signal. 2. The coherent detector according to claim 1, wherein the variable optical delay device gives a delay corresponding to the intensity of the first phase error signal and the second phase error signal to the second local light. 3.

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