JP2633593B2 - Polarization diversity optical receiver for coherent optical communication - Google Patents

Description

Detailed Description of the Invention [Table of Contents] Overview Industrial application field Conventional technology (Figs. 10 to 12) Problems to be solved by the invention Means for solving the problems (Fig. 1) (FIG. 1) Example Description of the first embodiment (FIGS. 3 to 5) Description of the second embodiment (FIGS. 6 to 9) Effects of the Invention [Outline] Polarization Diversity for Coherent Optical Communication Regarding an optical receiver, in particular, a polarization diversity optical receiver using a baseband combining method, in a device using a baseband combining method, a control for weakening a correlation between both detection outputs and stabilizing an intermediate frequency signal. An optical local oscillation circuit that oscillates a local oscillation light, and an optical signal for each of two types of polarization components of the signal light transmitted through the optical fiber. The local oscillation light from the local oscillation circuit A mixing circuit that combines the signals, a detection circuit that detects a signal for each polarization component from the mixing circuit, an addition circuit that adds the detection output of each polarization component from the detection circuit, and an addition circuit that adds the addition circuits. A frequency discriminating circuit for discriminating the frequency of the signal obtained by the adding circuit and outputting a corresponding voltage signal in order to control the optical local oscillation circuit based on the output; and converting the output of the frequency discriminating circuit into a current signal. A control circuit including a driver circuit for inputting to an oscillation frequency control input terminal of the optical local oscillation circuit, and the adding circuit adds one of the detection outputs of the respective polarization components from the detection circuit. A delay circuit is provided to delay the detection before and give each detection output a delay time difference, thereby weakening the correlation between the two detection outputs.

[Industrial applications]

The present invention relates to a polarization diversity optical receiver for coherent heterodyne optical communication, and more particularly to a polarization diversity optical receiver using a baseband combining method.

2. Description of the Related Art In recent years, a coherent optical communication system that uses information such as amplitude, frequency, and phase of an optical carrier with increased coherency to derive ultimate performance of optical communication has been spotlighted.

One of the problems for realizing such a coherent optical communication system is fluctuation of the polarization state. As a countermeasure, a polarization diversity reception system has been proposed.

In this polarization diversity reception system, signals of two polarization components are synthesized. As a method for this, an intermediate frequency synthesis method and a baseband synthesis method are considered.

Here, the intermediate frequency synthesizing method is a method in which the phases of two polarization components are matched in advance and then adding and synthesizing them. In the baseband synthesizing method, two polarization components are detected and then added. It is a method of combining.

By the way, the intermediate frequency synthesizing method is difficult to perform phase matching operation and is difficult to put into practical use, whereas the baseband synthesizing method is a method that requires two detection circuits but is easy to put into practical use.

[Conventional technology]

FIG. 10 is a block diagram showing a coherent optical communication system. In FIG. 10, reference numeral 1 'denotes a transmitting unit, and the transmitting unit 1' comprises a light source 2, an optical modulation circuit 3, and a light source stabilizing device 4. Have.

Here, for example, a semiconductor laser capable of single-mode oscillation is used as the light source 2. If the above-described semiconductor laser is used as the light source stabilizing device 4, a device having a temperature control device for controlling the temperature of the laser to be constant, for example, is used.

Further, the light modulation circuit 3 performs FSK modulation (pulse frequency modulation) on an input signal by, for example, slightly changing the drive current of the semiconductor laser.

Reference numeral 5 'denotes an optical fiber as a transmission medium. As the optical fiber 5', for example, a single mode optical fiber is used.

Reference numeral 6 'denotes a receiving unit. The receiving unit 6' includes an optical local oscillation circuit 7 for oscillating local oscillation light, a signal light transmitted through the optical fiber 5 ', and a local oscillation light from the optical local oscillation circuit 7. , A detection circuit 9 for detecting a mixed output of the mixing circuit 8, and a demodulation / identification circuit 10 'for extracting a signal corresponding to the input signal from the detection output of the detection circuit 9. I have.

Here, a semiconductor laser is also used as the optical local oscillation circuit 7, and the device is stabilized by controlling the temperature of the optical local oscillation circuit 7 to be constant by the temperature control circuit.

With such a configuration, in principle, coherent optical communication can be performed. However, one of the problems for realizing such a coherent optical communication method is the fluctuation of the polarization state as described above. It is preferable to adopt a polarization diversity receiving method.

Then, in order to realize the polarization diversity receiving system, it is preferable to use the baseband combining method as described above.

Next, FIG. 11 shows a block diagram of a polarization diversity optical receiving apparatus which can be considered when the polarization diversity optical receiving system is to be realized using the baseband combining method.

In FIG. 11, reference numeral 6 denotes a polarization diversity optical receiver as a receiving unit. The polarization diversity optical receiver 6 includes an optical local oscillation circuit 7 for oscillating local oscillation light,
A mixing circuit 8 for mixing the local oscillation light from the optical local oscillation circuit 7 for each of two kinds of polarization components of the signal light transmitted through the optical fiber 5, and a signal for each polarization component from the mixing circuit 8 , A demodulation / addition / identification circuit 10 for extracting a signal corresponding to a transmission input signal from a detection output of the detection circuit 9, and a detection output e _s of each polarization component from the detection circuit 9. An adder circuit 11 'for adding e _p and a control circuit 12 for controlling the oscillation frequency of the optical local oscillation circuit 7 based on the added output of the adder circuit 11' are provided.

Here, the control circuit 12 discriminates the frequency of the signal obtained by the adder circuit 11 'and outputs a corresponding voltage signal. The control circuit 12 converts the output of the frequency discrimination circuit into a current signal and outputs the current signal. An automatic frequency control circuit (AF) having a drive circuit input to the oscillation frequency control input terminal of the circuit 7
C).

By feeding back the detection output information in this way, the intermediate frequency is stabilized.

[Problems to be solved by the invention]

However, in such a polarization diversity optical receiver using the baseband combining method shown in FIG.
One of the detection output e _s, the phase of e _p do not match, adding it, the control signal to the optical local oscillating circuit 7 as the case (the intermediate frequency signal for the AFC) possibly becomes zero is there.

That is, now, for example, consider a heterodyne type coherent optical communication system FSK (or ASK), it can be expressed the detection output (intermediate frequency signal) e _s, a e _p as follows.

e _p = (a) ^1/2 S (t) cos (ωt) (1) e _s = (1-a) ^1/2 S (t) cos (ωt + θ) (2) where a Is (signal output light power of p polarization component / all signal output light power), S (t) is S ₀ m (t), [where,
m (t) = 0 (space), 1 (mark)], and θ is a phase difference due to polarization.

When both are synthesized as they are, the output signal e (t)
Is e (t) = e _p (t) + _es (t) = S (t) {(a) ^1/2 cos (ωt) + (1-a) ^1/2 cos (ωt + θ)} 3) Here, considering the worst case (a = 1/2, θ = π), in this case, e (t) = 0, whereby the intermediate frequency signal for AFC becomes I can't take it.

 The situation at this time is shown in FIGS. 12 (a) to (c).

Here, FIGS. 12 (a) and (b) show the detection output, respectively.
FIG. 12C shows an example of the waveform of e _p and e _s , and FIG. 12C shows an example of the composite waveform e.

Considering now the heterodyne coherent optical communication system DPSK (Differential Phase Shift Keying), it can be expressed the detection output (intermediate frequency) e _s, a e _p as follows.

e _p = (a) ^1/2 S ₀ cos (ωt + ψ (t)) (4) e _s = (1-a) ^1/2 S ₀ cos (ωt + ψ (t) + θ)... (5) Where ψ (t) is πm (t) [where m (t) =
0 (space), 1 (mark)].

When both are synthesized as they are, the output signal e (t)
E (t) = e _p (t) + _es (t) = S ₀ ｛(a) ^1/2 cos (ωt + ψ (t)) + (1−a) ^1/2 cos (ωt + ψ (t) + θ )｝ (6) Here, considering the worst case (a = 1/2, θ = π), also in this case, e (t) = 0, so that FSK (or As in the case of the heterodyne type coherent optical communication system (ASK), an intermediate frequency signal for AFC cannot be obtained.

The present invention is intended to solve such a problem. In the case of using the baseband synthesis method, the correlation between the two detection outputs is weakened by giving each detection output a delay time difference, so that the intermediate frequency signal is output. An object of the present invention is to provide a polarization diversity optical receiver for coherent optical communication in which a control signal for stabilization can be obtained in any case.

[Means for solving the problem]

 FIG. 1 is a block diagram showing the principle of the present invention.

In FIG. 1, reference numeral 6 denotes a polarization diversity optical receiver as a receiving unit. The polarization diversity optical receiver 6 includes an optical local oscillation circuit 7, a mixing circuit 8, a detection circuit 9, a demodulation / addition / identification circuit 10 And an adder circuit 11 and a control circuit 12.

Here, the optical local oscillation circuit 7 oscillates local oscillation light, and the mixing circuit 8 performs local oscillation from the optical local oscillation circuit 7 for each of two kinds of polarization components of the signal light transmitted through the optical fiber 5. The detection circuit 9 mixes light, and the detection circuit 9 detects a signal for each polarization component from the mixing circuit 8.
The demodulation / addition / identification circuit 10 extracts a signal corresponding to the transmission input signal from the detection output of the detection circuit 9.

Further, the adder circuit 11 is intended to adding the detection output e _s, e _p of each polarization components from the detection circuit 9, while an adder 13 for this, further the adding circuit 11, the detection circuit detection output e _s of each polarization component from 9, or an addition before in delaying by the detection output e _s of e _p, both detection output by to have a delay time difference e _p e _s, correlation e _p Delay circuit 1 that weakens the relationship
4 and an automatic gain control circuit (A
GC circuit) 15.

Further, the control circuit 12 controls the oscillation frequency of the optical local oscillation circuit 7 based on the addition output of the addition circuit 11, and for this purpose, the control circuit 12 adjusts the frequency of the signal obtained by the addition circuit 11. An automatic frequency control comprising a frequency discriminating circuit for discriminating and outputting a corresponding voltage signal, and a driver circuit for converting an output of the frequency discriminating circuit into a current signal and inputting the current signal to an oscillation frequency control input terminal of the optical local oscillation circuit 7. It is configured as a circuit (AFC).

(Operation)

Thus detection output information e _s, when the e _p is fed back, the detection output information e _s, e each detection output information one is made to delay the _p e _s, the delay time difference e _p is Motasa Are added and synthesized. As a result, the correlation between the two signals is weakened, and the combined signal does not become zero.

In addition, the output gain after addition is adjusted by an automatic gain control circuit (AG
C circuit) is adjusted by 15.

As described above, since an intermediate frequency signal having a required frequency can be reliably obtained and a required gain can be secured, the intermediate frequency can be stabilized.

〔Example〕

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(A) Description of First Embodiment FIG. 2 is a block diagram showing a first embodiment of the present invention. FIG. 2 shows an FSK (or ASK) heterodyne type coherent optical communication system. Things.

In FIG. 2, reference numeral 6 denotes a polarization diversity optical receiver as a receiving unit. The polarization diversity optical receiver 6 includes an optical local oscillation circuit 7, a mixing circuit 8, and a detection circuit.
9, in addition to the demodulation / addition / identification circuit 10, an addition circuit 11 and a control circuit 12 are provided.

Here, the optical local oscillation circuit 7 oscillates local oscillation light Plo, uses a semiconductor laser, and is controlled by a light source stabilizing device 18 configured as a temperature control device so that the laser temperature becomes constant. I have.

The mixing circuit 8 mixes the local oscillation light Plo from the optical local oscillation circuit 7 for each of two kinds of polarization components of the signal light Psi transmitted through the optical fiber 5, and outputs the signal from the optical fiber 5. A demultiplexing optical fiber coupler 19 for demultiplexing the optical Psi and the local oscillation light Plo from the optical local oscillation circuit 7, and a polarization maintaining fiber coupler for maintaining the polarization state
It has 20.

Here, the energy distribution of the signal light Psi to the P polarization component is a
In Psi, the energy distribution to the S polarization of the signal light Psi is (1
-A) Psi, the energy distribution of the local oscillation light Plo to the P polarization component is 1 / 2Plo, and the energy distribution of the local oscillation light Plo to the S polarization component is 1 / 2Plo.

Note that a is (signal output light power of p polarization component / all signal output light power).

The detection circuit 9 detects a signal for each polarization component from the mixing circuit 8, and includes photoelectric conversion circuits 21P and 21S;
Post amplifiers 22P and 22S and bandpass filters 23P and 23S are provided.

Here, the photoelectric conversion circuits 21P and 21S are circuits that receive an optical signal and convert it into an electric signal, and the post amplifiers 22P and 22S are circuits that amplify the electric output from the photoelectric conversion circuits 21P and 21S.
Bandpass filter 23P, 23S baseband signal only required frequency band by filtering (intermediate frequency signal) e _p, is a circuit that outputs e _s.

The demodulation / addition / identification circuit 10 detects the detection output of the detection circuit 9
It has a demodulation function to extract a signal corresponding to the transmission input signal from e _p and e _s , and has a delay detection circuit 24P, 24S, an addition circuit 2
5, a low-pass filter 26 and an identification circuit 27 are provided.

Here, the delay detection circuits 24P and 24S have delay circuits 24P-1 and 24S-1, respectively, for branching a signal and delaying one of them.
The delay time in each of the delay circuits 24P-1 and 24S-1 is set to almost 0 in the case of FSK (ASK).

Further, the addition circuit 25 outputs the output Vp of each of the delay detection circuits 24P and 24S,
This is a circuit that adds and combines Vs.

Further, the low-pass filter 26 is a circuit for removing noise, and the identification circuit 27 is a circuit for identifying 0, 1 information from the signal and extracting a signal corresponding to the transmission input signal. The identification circuit 27 also has a function of extracting a timing signal for identifying 0 and 1 from the input signal.

By the way, the adder circuit 11 includes bandpass filters 23P, 23S
Detection output e _s of the polarized components from, but adding the e _p,
Although an adder 13 for this, further the adding circuit 11, a band-pass filter 23P, the detection output of the respective polarization components from the 23S e _s, for example before adding the one e _s of e _p each detection output by 1 bit delay e _s, a delay time difference e _p (i.e., phase difference) both detected output e _s by to have a provided with a delay circuit 14 to weaken the correlation of e _p, after addition output An automatic gain control circuit (AGC circuit) 15 for adjusting the gain is provided.

Further, the control circuit 12 controls the oscillation frequency of the optical local oscillation circuit 7 based on the addition output of the addition circuit 11, and for this purpose, the control circuit 12 adjusts the frequency of the signal obtained by the addition circuit 11. A frequency discriminating circuit 16 for discriminating and outputting a corresponding voltage signal, and a drive circuit 17 for converting the output of the frequency discriminating circuit 16 into a current signal and inputting the current signal to the oscillation frequency control input terminal of the optical local oscillation circuit 7 are provided. It is configured as an automatic frequency control circuit (AFC).

In FIG. 2, reference numeral 1 denotes a transmitting unit, and the transmitting unit 1 includes a light source 2, a light source stabilizer 4, and an amplifier (amplifier) 28.

Here, as the light source 2, for example, a semiconductor laser capable of single-mode oscillation is used. In addition, the light source stabilizing device 4
If a semiconductor laser is used as described above, for example, a semiconductor laser having a temperature control device for controlling the temperature of the laser to be constant is used.

Further, the amplifier 28 amplifies the input signal (data) and applies the current signal _IAC to the oscillation frequency control terminal of the light source 2.
_{DC is} also supplied.

With this, optical modulation is performed, and ASK is performed on the input signal.
Modulation (pulse amplitude modulation) or FSK modulation (pulse frequency modulation) can be performed.

In FIG. 2, reference numeral 5 denotes an optical fiber as a transmission medium.

The construction described above, the detection output data e _s, when the e _p is fed back, the detection output information e _s, e _p of one e _s the detection output information is allowed to delay e _s, a phase difference e _p is After being given, both are added and synthesized. The output signal e at this time
(T) is as follows.

_{e (t) = e p (} t) + e s (t-T) = (a) 1/2 S (t) cos (ωt) + (1-a) 1/2 S (t-T) cos (ω (T−T) + θ) (7) Here, considering the case where a = 1/2 and θ = π, in this case, e (t) = (1/2) ^1/2 ｛ S (t) + S (t−T)｝ cos
(Ωt) (8) Here, ωt = 2nπ (n is an integer).

 The situation at this time is shown in FIGS. 3 (a) to 3 (c).

Here, FIGS. 3 (a) and 3 (b) show detection outputs, respectively.
FIG. 3C shows an example of a waveform of e _p and e _s , and FIG. 3C shows an example of a composite waveform e.

Thereby, some intermediate frequency signal for AFC can be obtained. This means that both signals e _p (t) and e _s (t−T)
Is weakened.

When a = 1/2 and θ = 0, e (t) = (1/2) ^1/2 {S (t) −S (t−T)} cos
(Ωt) (9) Here, ωs = 2nπ (n is an integer).

FIG. 4A to FIG. 4C show the state at this time.

Here, FIGS. 4 (a) and 4 (b) show detection outputs, respectively.
FIG. 4 (c) shows an example of the waveform of e _p and e _s , and FIG. 4 (c) shows an example of the composite waveform e. In this case as well, it can be seen that some intermediate frequency signal for AFC can be obtained.

In addition, the output gain after addition is adjusted by an automatic gain control circuit (AG
C circuit 15), but the capability of the AGC circuit 15 will be described briefly.

Consider the average power E of the intermediate frequency signal that has been delayed and combined by one bit as described above. Assuming that the mark ratio is m, the probability Pkl (t) is as follows.

That is, P ₀₀ (t) = (1−m) ² , P ₁₀ (t) = m (1−
m), P ₀₁ (t) = m (1-m), and P ₁₁ (t) = m ² .

 Then, the average power E (θ) is as follows.

E (θ) = P ₀₀ (t) <e ₀₀ (t) ² > + P ₁₀ (t) <e ₁₀ (t) ² > + P ₀₁ (t) <e ₀₁ (t) ² > + P ₁₁ (t) < ^{_{e 11 (t) 2> =}} (mS 0 2/2) + {a (1-a)} 1/2 m 2 S 0 2 cosθ ...... (10) ^{_{where, <e 00 (t) 2}} > is m (t) = 0, m (t-T)
= 0, the squared time average of the synthesized intermediate frequency signal,
<E ₁₀ (t) ² > is the squared time average of the synthesized intermediate frequency signal when m (t) = 1 and m (t−T) = 0, <e ₁₀ (t) ² >
₀₁ (t) ² > is the squared time average of the synthesized intermediate frequency signal when m (t) = 0, m (t−T) = 1, <e ₁₁ (t)
² > is the squared time average of the synthesized intermediate frequency signal when m (t) = 1 and m (t−T) = 1.

From the above equation (10), the maximum value and the minimum value of E (θ) are as follows.

Emax = E (0) = ( mS 0 2/2) + {a (1-a)} 1/2 m 2 S 0 2 ...... (11) Emin = E (π) = (mS 0 2/2) − {A (1-a)} ^1/2 m ² S ₀ ² (12)

Further, the variation V of the average power E (θ) with respect to θ is expressed by Em
Expressed by the ratio of ax to Emin, it is as follows.

V = Emax / Emin = [1 + 2 ｛a (1-a)｝ ^1/2 m] / [1-2 ｛a (1-a)｝ ^1/2 m] (13)

As can be seen from the equation (13), V is a function of a and m. For example, the average power E when m = 0.5 is obtained.
FIG. 5 shows (θ) as a function of a.

From FIG. 5 and equation (6), when a = 1/2, V becomes maximum.

 That is, Vmax = (1 + m) / (1-m).

Therefore, the gain adjustment capability of the AGC circuit 15 may be set so as to cover Vmax. However, when the mark ratio is m = 1/2, the average AGC circuit 15 can sufficiently adjust the gain by three times the average power. is there.

In this way, an intermediate frequency signal having a required frequency can be reliably obtained and a required gain can be secured, so that the oscillation frequency (intermediate frequency) of the optical local oscillation circuit 7 can be obtained.
Can be sufficiently stabilized.

(B) Description of the Second Embodiment FIG. 6 is a block diagram showing a second embodiment of the present invention. The one shown in FIG. 2 is for a DPSK heterodyne type coherent optical communication system.

In FIG. 6, reference numeral 6 denotes a polarization diversity optical receiver as a receiving unit. The polarization diversity optical receiver 6 includes an optical local oscillation circuit 7, a mixing circuit 8, and a detection circuit.
9, in addition to the demodulation / addition / identification circuit 10, an addition circuit 11 and a control circuit 12 are provided.

Here, since the optical local oscillation circuit 7, the mixing circuit 8, the detection circuit 9, the demodulation / addition / identification circuit 10, and the control circuit 12 are substantially the same as those in the first embodiment, their detailed description will be given. Is omitted.

By the way, the adder circuit 11 includes bandpass filters 23P, 23S
Detection output e _s of the polarized components from, but adding the e _p,
Although an adder 13 for this, further the adding circuit 11, a band-pass filter 23P, the detection output e _s of the polarized components from the 23S, before adding for example 1 bit e _s of e _p each detection output e _s is delayed, the delay time difference e _p (i.e., phase difference) both detected output e _s by to have a provided with a delay circuit 14 to weaken the correlation of e _p, the output frequency after the addition Doubler 29 for doubling, and automatic gain control circuit (AGC circuit) 15 for adjusting the output gain of doubler 29
With

Here, the difference from the above-described first embodiment is that a doubler 29 is provided. By using the doubler 29, the spectrum based on the phase information of the intermediate frequency signal can be canceled.

Note that the delay detection circuits 24P and 24S have delay circuits 24P-1 and 24S-
The delay time at 1 is set to 1 bit in the case of DPSK.

In FIG. 6, reference numeral 1 denotes a transmission unit for DPSK. This transmission unit 1 includes a light source 2, an optical modulation circuit 3, a light source stabilizing device 4, a waveform shaping circuit 30, an amplifier 31, and differential encoding. The circuit 32 includes a crystal oscillator 33.

Here, as the light source 2, for example, a semiconductor laser capable of single-mode oscillation is used. In addition, the light source stabilizing device 4
If a semiconductor laser is used as described above, for example, a semiconductor laser having a temperature control device for controlling the temperature of the laser to be constant is used.

Further, the differential encoding circuit 32 receives the reference signal and the input signal (data) from the crystal oscillator 33 and differentially encodes and outputs the differential signal. The output of the differential encoding circuit 32 is an amplifier.
The signal is inputted to the optical modulation circuit 3 via the waveform shaping circuit 30 and the signal 31.

With this, optical modulation is performed, and DPSK is performed on the input signal.
Modulation can be performed.

 Reference numeral 5 denotes an optical fiber as a transmission medium.

The construction described above, the detection output data e _s, when the e _p is fed back, the detection output information e _s, e _p of one e _s the detection output information is allowed to delay e _s, a phase difference e _p is After being given, both are added and synthesized. The output signal e at this time
(T) is as follows.

e (t) = e _p (t) + _es (t−T) = S ₀ ｛(a) ^1/2 cos (ωt + ψ (t)) + (1 + a) ^1/2 cos (ωt + ψ (t−T) + θ )｝ (14) Here, considering the case of a = 1/2, θ = 0, in this case, e (t) = (S ₀ (1/2) ^1/2 ｛cos ( ωt + ψ (t)) + cos (ωt + ψ (t−T))｝ (15), where ωt = 2nπ (n is an integer).

 The situation at this time is shown in FIGS. 7 (a) to (c).

Here, FIGS. 7 (a) and 7 (b) show detection outputs, respectively.
FIG. 7C shows an example of a waveform of e _p and e _s , and FIG. 7C shows an example of a composite waveform e.

Thereby, some intermediate frequency signal for AFC can be obtained. This means that both signals e _p (t) and e _s (t−T)
Is weakened.

Considering the case where a = 1/2 and θ = π / 2, in this case, e (t) = (S ₀ (1/2) ^1/2 ｛cos (ωt + ψ (t)) + cos (Ωt + {(t−T) + π / 2)} (16), where ωt = 2nπ (n is an integer).

 The situation at this time is shown in FIGS. 8 (a) to (c).

Here, FIGS. 8 (a) and 8 (b) show detection outputs, respectively.
FIG. 8C shows an example of a waveform of e _p and e _s , and FIG. 8C shows an example of a composite waveform e.

Thus, also in this case, some intermediate frequency signal for AFC can be obtained. This is the two signals e _p (t), e
_This is because the correlation of _s (t−T) becomes weak.

In addition, the output gain after addition is adjusted by an automatic gain control circuit (AG
C circuit 15), but the capability of the AGC circuit 15 will be described briefly.

Consider the average power E of the intermediate frequency signal that has been delayed and combined by one bit as described above. Now, assuming that the mark rate is m, the probability Pkl (t) is as follows as in the case described above.

That is, P ₀₀ (t) = (1−m) ² , P ₁₀ (t) = m (1−
m), P ₀₁ (t) = m (1-m), and P ₁₁ (t) = m ² .

 Then, the average power E (θ) is as follows.

E (θ) = P ₀₀ (t) <e ₀₀ (t) ² > + P ₁₀ (t) <e ₁₀ (t) ² > + P ₀₁ (t) <e ₀₁ (t) ² > + P ₁₁ (t) < e ₁₁ (t) ² > (17) where <e ₀₀ (t) ² > is m (t) = 0, m (t−T)
= 0, the squared time average of the synthesized intermediate frequency signal of F, <e ₁₀ (t) ² > is the synthesized intermediate frequency of m (t) = 1, m (t−T) = 0 Mean squared time of signal, <e ₀₁
(T) ² > is the squared time average of the synthesized intermediate frequency signal when m (t) = 0, m (t−T) = 1, <e ₁₁ (t) ²
> Is the squared time average of the synthesized intermediate frequency signal when m (t) = 1 and m (t−T) = 1.

In this case, since <e ₀₀ (t) ² > = <e ₁₁ (t) ² > and <e ₁₀ (t) ² > = <e ₀₁ (t) ² >, the average power E (θ) is E (θ) = {P ₀₀ (t) + P ₁₁ (t)} <e ₀₀ (t) ² > + P ₁₀ (t) + P ₀₁ (t)｝ <e ₁₀ (t) ² > = S ₀ ² [( 1/2) + (1-2 m) ² {a (1-a)} ^1/2 cos θ] (18)

From the above equation (18), the maximum value and the minimum value of E (θ) are as follows.

Emax = E (0) = S 0 2 ^{[(1/2) + (1-2m) 2} {a (1-a)} 1/2 ]
...... becomes (19) Emin = E (π ) = S 0 2 ^{[{(1/2) + (1-2m)} 2 {a (1 -a)} 1/2 ] ...... (20).

Further, a variation V with respect to the average power E (θ) is represented by Emax.
Expressed as a ratio with Emin, it is as follows.

V = Emax / Emin = [(1/2) + (1-2m) ² {a (1-a)} ^1/2 ]} [(1/2)-(1-2m) ² } a (1- a)｝ ^1/2 ] (21)

Here, considering the case where the mark rate is m = 0.5, in this case, V = 1 regardless of the value of a.

 When a = 1/2, V becomes maximum.

Therefore, Vmax = {1+ (1-2m) ² } / {1- (1-2m) ₂ } (22)

From the above, FIG. 9 shows the relationship between the change amount V of the average power E (θ) with respect to θ and the mark ratio m. In FIG. 9, the characteristic indicated by DPSK is for the DPSK system, and the characteristic indicated by FSK is for the FSK system.

From FIG. 9, in the DSPK system, the mark rate is 1/11 (minimum mark rate) or 10/11 (maximum mark rate).
In this case, it can be seen that the gain margin of the AGC circuit 15 only needs to be about 7 dB.

Therefore, the gain adjustment capability of the AGC circuit 15 is approximately 7
It is sufficient to use dB, but this can be sufficiently handled by the ordinary AGC circuit 15.

Thus, also in the case of the second embodiment, the intermediate frequency signal having the required frequency can be reliably obtained and the required gain can be ensured.
The oscillation frequency (intermediate frequency) can be sufficiently stabilized.

In each embodiment described above, the detection output e _s, it may be the other e _p is delayed before the addition of e _p.

The number of bits to be delayed by the delay circuit 14 of the adder circuit 11 is not limited to 1 bit, but may be 1 bit × N (N is generally an integer value, but need not be an integer value).

〔The invention's effect〕

As described in detail above, according to the polarization diversity optical receiving apparatus for coherent optical communication of the present invention, in the apparatus using the baseband combining method, the addition circuit provided in the intermediate frequency feedback line includes: A simple configuration that has a delay circuit that weakens the correlation between the two detection outputs by delaying any of the detection outputs of the polarization components before addition and giving each detection output a delay time difference. In any case, a control signal for stabilizing the intermediate frequency signal can be obtained by weakening the correlation, which has an advantage that the intermediate frequency can be sufficiently stabilized.

[Brief description of the drawings]

1 is a block diagram showing the principle of the present invention, FIG. 2 is a block diagram showing a first embodiment of the present invention, and FIGS. 3 (a) to (c) and FIGS. 4 (a) to (c) are books. FIG. 5 is a waveform diagram for explaining the operation of the first embodiment of the present invention. FIG. 5 is a graph for explaining the operation of the first embodiment of the present invention. FIG. 6 is a block diagram showing the second embodiment of the present invention. FIGS. 7 (a) to 7 (c) and 8 (a) to 8 (c) are waveform diagrams for explaining the operation of the second embodiment of the present invention, and FIG. 9 is the first embodiment of the present invention. FIG. 10 is a block diagram showing a coherent optical communication system, FIG. 11 is a block diagram showing an example of a polarization diversity optical receiver for coherent optical communication, FIG. (A)-(c) are waveform diagrams for explaining the operation of the device shown in FIG. In the figure, 1 is a transmitting unit, 2 is a light source, 3 is an optical modulation circuit, 4 is a light source stabilizing device, 5 is an optical fiber, 6 is a polarization diversity optical receiving device as a receiving unit, 7 is an optical local oscillation circuit, 8 is a mixing circuit, 9 is a detection circuit, 10 is a demodulation / addition / identification circuit, 11 is an addition circuit, 12 is a control circuit, 13 is an adder, 14 is a delay circuit, 15 is an automatic gain control circuit (AGC circuit), 16 is a frequency discrimination circuit, 17 is a drive circuit, 18 is a light source stabilizing device, 19 is a demultiplexing optical fiber coupler, 20 is a polarization maintaining fiber coupler, 21P and 21S are photoelectric conversion circuits, 22P and 22S are post amplifiers, 23P, 23S is a bandpass filter, 24P, 24S is a delay detection circuit, 24P-1, 24S-1 is a delay circuit, 25 is an addition circuit, 26 is a low-pass filter, 27 is an identification circuit, 28 is an amplifier, 29 is a doubler, 30 is a waveform shaping circuit, 31 is an amplifier, 32 is a differential encoding circuit, 33 is water It is an oscillator.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tetsuya Kiyonaga 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Yoshito Onoda 1015 Kamidadanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited ( 56) References The Institute of Electrical Engineers of Japan Material Communications Society of Japan 17-22 (published by the Institute of Electrical Engineers of Japan on October 29, 1987)

Claims (2)

(57) [Claims] 1. A polarization diversity optical receiver (6) for coherent optical communication, comprising: an optical local oscillation circuit (7) for oscillating local oscillation light; and a signal light transmitted through an optical fiber (5). A mixing circuit (8) for mixing the local oscillation light from the optical local oscillation circuit (7) for each of two types of polarization components, and a signal for each polarization component from the mixing circuit (8) for detection; And a detection output (e _s ) of each polarization component from the detection circuit (9),
An adder (11) for adding (e _p ); and an adder (11) for controlling the oscillation frequency of the optical local oscillator (7) based on the added output of the adder (11).
A frequency discriminating circuit for discriminating the frequency of the signal obtained in step 1 and outputting a corresponding voltage signal, and converting the output of the frequency discriminating circuit into a current signal and inputting it to the oscillation frequency control input terminal of the optical local oscillation circuit (7) And a control circuit (12) including a driver circuit for performing the above operation. The adder circuit (11) includes a control circuit (12) for detecting detection outputs (e _s ) and (e _p ) of the respective polarization components from the detection circuit (9). A delay circuit (14) that weakens the correlation between the two detection outputs (e _s ) and (e _p ) by delaying one of the detection outputs (e _s ) and (e _p ) so as to have a delay time difference between them. A coherent optical communication polarization diversity optical receiver comprising: 2. The polarization for coherent optical communication according to claim 1, wherein said addition circuit (11) has an automatic gain control circuit (15) for adjusting an output gain after addition. Diversity optical receiver.