JPH03251827A - Optical phase detection system - Google Patents

Abstract

PURPOSE:To reduce a demodulation error by multiplexing signal light and local oscillation light in the same polarization state and extracting a Q component signal and an I component signal, and making those signals 180 deg. out of phase with each other and performing optical detection. CONSTITUTION:The signal light S1 which is polarized and compensated into a linear polarization state having a 45 deg. angle to the axis of the polarized wave separating element 10a of an optical 90 deg. hybrid 10 is made incident on the element 10a. The local oscillation light L0 of a linear polarized wave oscillated by a laser light source 2 for local oscillation, on the other hand, is made into local oscillation light L1 in a circular polarization state by a 1/4-wavelength plate 10b and made incident on a polarization light separating element 10c. The P and S polarization components of signal light beams S2 and S3 and local oscillation light beams L2 and L3 which are separated by the elements 10a and 10c are multiplexed by half-mirrors 10e and 10d respectively. The light signals and local oscillation light beams which are obtained by the multiplexing are light-received by balanced receivers 11 and 12 to obtain light- reception outputs Vb1 and Vb2. Thus, the outputs Vb1 and Vb2 having suppressed intensity noises are multiplied by a mixer 6 to obtain an output V3 to bring the light source 2 under feedback control through a control circuit 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical phase detection method, and particularly to an optical phase detection method that performs phase detection using a Q component and an I component.

[Conventional technology] Optical PSK (Phase 5hift Keyin)
g) In homodyne transmission, in order to spread the phase modulated signal light for 1 m, the phase θg of the local light is changed to the phase θS of the signal light.
It is necessary to perform synchronous control. The phase of the local light is determined by Costas type light PLL (Pha
(seLocked Loop) allows for synchronous control.

FIG. 3 is a block diagram of an optical phase detection method using a conventional optical 90° hybrid. In the figure, 1 is an optical 90° hybrid that combines signal light S1 and local light LO to extract output lights with a phase difference of 90°, 2 is a local laser light source that oscillates local light LO, and 3 is A polarization-maintaining optical fiber is used to propagate the local light 10 while preserving the polarization plane; 4 and 5 are optical receivers that convert optical signals into electrical signals; and 6 is an output signal V1. A mixer 7 for mixing V2 is a control circuit for controlling the phase of the local laser beam 'a2, and 8 is a sign determiner for demodulating the modulated signal. In addition, the optical 90° hybrid uses linearly polarized local light L.
A quarter-wave plate 1a that makes O circularly polarized, and a half mirror 1b that combines circularly polarized local light L1 and signal light S1.
and a polarization and light separation element 1C for extracting light of orthogonal polarization planes, respectively. In addition, in the figure, ◎ indicates the S-polarized light component, ↓ indicates the P-polarized light component, 7 indicates the linearly polarized light at 45° with respect to the optical axis of the polarization separation element 1c, ○ indicates the circularly polarized state, and the dotted arrow indicates the traveling direction of the light. show.

An output V1. which is obtained by combining the signal light S1 whose polarization has been compensated in advance using the optical 90° hybrid 1 and the local light 10 and receiving and detecting the light by the optical receiver 4.5. V2 is Q (Quad
In phase) component v1 and the signal light are combined in the same phase in the phase synchronization state1 (Inphase)
It corresponds to either component v2.

Which of (1) and (2) becomes VQ or VI will be described later. The figure shows a case where V1=VQ and V2=V[. Here, generally the outputs of the Q component and I component are VQ, V
When I, VQ and VI are proportional to 5 inches (θS-θρ) and cos(θs-θρ), respectively. Therefore, in order to cancel the phase modulation component included in θS, similar to the Costas loop in wireless communication, vl and ■2 are multiplied by mixer 6 to obtain v3 proportional to 3in2 (θS - θρ), and The phase synchronization state can be maintained by applying feedback control to the local laser light source 2 using the obtained VC as a phase control signal for local light. Also, V2 (=V
According to I), the modulated signal component of θS can be demodulated by the sign determiner 8, and the demodulated signal SO can be extracted.

Next, when using this optical phase detection method, the optical 90' hybrid 1 necessary for extracting the outputs of the Q component and I component will be described below.

The signal light S1, which has been polarization-compensated in advance to be linearly polarized at 45 degrees with respect to the optical axis of the polarization separation element 1C, and the local light L1, which has been made into a circularly polarized state by the quarter-wave plate 18, are connected to the half mirror 1b. The local light L2' and the signal light 82 are multiplexed by
', the signal light $3-, which is the S and P polarized light components, and the local light 3- and the signal light S
It emits local light from 4- and is separated into 4-. Here, local light L3-
The phase relationship between L4- and L4- is that when the circularly polarized local light [1 is right-handed (clockwise) when viewed from the quarter-wave plate 1a, the local light L4-
- is delayed by 90 degrees in phase from the local light L3', and when rotating to the left, it emits local light, and 3- emits local light and is delayed by 90 degrees in phase from 4-.

-The force signal light S1 has the same phase and equal intensity signal light 83-, S4
- separated into Therefore, the signal light S1, the local light S,
When the P-polarized light components are each detected by the optical receiver 4.5, when the rotation is left-handed, the received-detection output of the S-polarized component becomes VQ, and the received-detected output VP of the P-polarized light component becomes Vl, and conversely, when the right-handed rotation occurs, VP becomes VQ.
, VS becomes V I .

Other 90° hybrid configurations shown in FIG. 3 include examples using waveguides and fiber couplers. Also light 90
'Signal light S with 3dB coupler without using hybrid
1. A balanced optical PLL in which local light LO is multiplexed and received and detected by a balanced receiver has already been proposed (I
EEE Photonics Let,, vol, 1. N
O, 11, Nov, 1989 DI) 395-397)
.

[Problem to be solved by the invention] In the optical 90° hybrid 1 shown in Fig. 3, due to the polarization characteristics of the half mirror 1b, the output of the circularly polarized local light L1 combined with the signal light S1 by the half mirror 1b. In this case, it is extremely difficult to maintain local light 2- in a circularly polarized state. Therefore, during phase control, signal light S1 and local light LO
A residual phase offset occurs, increasing the probability of demodulation errors and deteriorating phase detection sensitivity. In addition, with this configuration, it is not possible to suppress the intensity optical noise of the local light source, and the S/
There was a problem in that N deteriorated, the quality of the demodulated signal SO deteriorated, and the phase of the local laser light source 2 was erroneously controlled. Note that an optical 90° hybrid using a waveguide has a problem of large loss, and an optical 90° hybrid using a fiber coupler has problems with accuracy and stability, so it cannot be applied to the optical phase detection method.

On the other hand, in a balanced optical PLL, the spectral linewidth of the signal light and local light is 1/10 of that in the Costas optical PLL.
The problem is that it is difficult to obtain a stable narrow linewidth light source.

As described above, conventional optical phase detection methods have many demodulation errors, and a practical optical phase detection method cannot be realized due to deterioration of demodulated signal quality due to the influence of intense optical noise of local light.

The present invention has been made to solve the problems of the prior art described above, and an object of the present invention is to provide a practical optical phase detection method that has few demodulation errors and can obtain high-quality phase detection.

[Means for Solving the Problems] The above problems can be solved by using a signal light whose polarization state has been compensated in advance to be linear polarization, and a local light emitted from a local oscillation laser. are combined in a multiplexer to extract an I component and a Q component that is delayed by 90' from the phase of the local light of the I component, and using the I and Q components to combine the signal component and the local light. In an optical phase detection method that demodulates the I component while maintaining phase synchronization, the signal light and circularly polarized local light are separated into orthogonal polarization components of a P polarization component and an S polarization component, and the separated A first output light in which the signal light and the local light having the same polarization component are combined to obtain the I component and the Q component, respectively, and each of the obtained I component and Q component has a phase different from each other by 180 degrees. and the second output light, convert the first output light and the second output light into electrical signals, and then calculate the fixed phase offset between the first and second signals converted into the electrical signals. Adjusting the phase and performing differential synthesis, synchronizing the phase of the signal light and the local light using the differentially synthesized electrical signal, and demodulating one of the electrical signals corresponding to the I component to obtain a demodulated signal. This is achieved by employing the above configuration means, which is characterized by the following.

[Function] Since the present invention has taken the above-mentioned measures, after making the polarization states of the signal light and the local light the same in advance, the phase difference of the local light is stabilized at 90' in order to combine them and obtain the respective output lights. Moreover, it becomes possible to control the phase difference between the signal light and the local light to almost zero regardless of the polarization characteristics of the multiplexing means. Therefore, since no residual phase offset occurs between the signal light and the local light, the respective output lights are outputted with a reduced probability of demodulation errors.

Furthermore, the phase of the fixed phase offset between the electric signals converted from the respective output lights is adjusted and differentially synthesized, and the obtained local light intensity noise is suppressed to produce extremely good signal light by the respective electric outputs. Demodulation is performed while maintaining the phase synchronization state of the local light.

The present invention will be described in detail below with reference to the drawings, but the same numbers will be given to the same parts as in the conventional structure to avoid redundant explanation.

[Example] FIG. 1 is a configuration diagram of an optical phase detection method according to the present invention,
The difference from the conventional configuration (Fig. 3) is that the signal light S1 and local light beam 1 are separated into S and P polarized components, and then the light is combined into a 90' hybrid 10, which suppresses the influence of local light intensity noise. The present invention uses balanced receivers 11 and 12 of a polarization diversity optical reception system that demodulates optical signals into electrical signals.

First, the optical 90° hybrid used in the present invention will be explained. FIG. 2(a) is a configuration diagram of the optical 90° hybrid 10 used in the present invention, and 10a is a polarization separation for separating the linearly polarized signal light S1 into orthogonal S polarization component S2 and P polarization component S3. Element 10b is a quarter-wave plate for converting the linearly polarized local light LO into circularly polarized light, and 10c is a quarter wavelength plate for converting the linearly polarized local light LO into circularly polarized local light LO and separating the circularly polarized local light LO into orthogonal S-polarized light component L2 and P-polarized light component L3. Polarization separation element 10d is a half mirror (semi-transparent 1) that combines the S-polarization component signal light S2 and the S-polarization component local light L2, and 10e combines the P-polarization component signal light S3 and the P-polarization component local light L2. Half mirror that combines 3
(semi-transparent mirror).

Next, the operation of the optical 90° hybrid 10 will be explained. The signal light S is polarization-compensated in advance to be in a linearly polarized state having an angle of 45° with respect to the axis of the polarization separation element 10a.
1 is incident on the polarization separation element 10a. On the other hand, local light L of linearly polarized light waves oscillated from local laser light 1112
O is first made into a circularly polarized local light L1 by the quarter-wave plate 10b, and is incident on the polarization separation element 10c. Polarization separation element 10a, signal light 82.83 separated by IOC
and local light 12. P and S of L3, each light editing component S3 and L3,
And S2 and L2 are combined by half mirrors 10e and 10d, respectively. S4°L5 obtained by combining and 85. L4 and 86. L7 and 87.16 each balanced receiver 1
1.12 and received light output Vb1. Obtain Vb2. Second
In Figure (a)r, b1 is the Q component and Vb2 is the I component.

That is, when the circularly polarized wave of the local light L1 is right-handed (II counterclockwise) when viewed from the quarter-wave plate 1ob, the P-polarized light component L3 lags the S-polarized light component L2 by 90'', and the balanced receiver 11.
12+7) Output Vb1. Vb2 is each Q component (Sin(
(proportional to θS - θg)) and ■ component (COS (θS - θg)
g)). Also, the circularly polarized wave of local light is 1/
When viewed from the 4-wavelength plate 10b, when rotating to the left, the situation is reversed, and b1 is 1.
Among the components, Vb2 becomes the Q component.

As mentioned above, the optical 90° hybrid 10 of the present invention has the following characteristics:
The polarization state of signal light S1 and local light S1 is separated by polarization separation element 10.
After making them the same in advance with a and 10c, half mirror-1oc+
, half mirror 10d, 10e for multiplexing with ioe
The local light L2. The phase difference of L3 can be stably maintained at 90".Therefore, no residual phase offset occurs between the signal light and the local light, so the probability of demodulation errors can be reduced.Also, the half mirror 1
0d, 10e and polarization separation elements 10a, 10c and 1/
Since each optical component of the four-wavelength plate 10b can be integrated to form the optical 90° hybrid 10, it is possible to reduce optical coupling loss and downsize.

Next, the configuration of the balanced drag lever 11-12 used in the present invention is shown in FIG. 2(b). In addition, the output light 84 (signal light) of the half mirror 10e will be described below.

15 (local light) is the Q+ component signal, Q4 component signal and 180
The output lights 35 (signal light) and L4 (local light) of the half mirror 10e, which have different degrees of phase, are the Q-component signals, and the output lights S6 (signal light) and L7 (local light) of the half mirror 10d are the bi-component signals. Signal, ■ “Output light S7 (signal light) of half mirror 10d whose phase is 180 degrees different from the component signal
, L6 (local light) is a -component signal.

As shown in the figure, 20a and 20b are light 90° hybrid 1
A Q+ component signal (I+ component signal) and a Q-component signal (I
- an optical fiber for extracting the optical signal (component signal), 218.
21b is an optical receiver that converts an optical signal into an electrical signal, and 22 is an optical receiver 21a.

21b output signal D1. A phase adjuster 23 is for adjusting the fixed phase offset existing between D2, and a differential synthesis circuit 23 is for differentially synthesizing the phase-adjusted electrical signal D3 and the output signal D2 of the optical receiver 21b. .

In the present invention, since the balanced receiver 11 (12) that differentially combines signals of the same polarization component is used, the half mirror 10e (1
0d) operates as a π-hybrid, and the optical receiver 21
In the outputs of a and 21b, the beat signal between the signal light and the local light is output in reverse phase, and the local light intensity noise is output in the same phase. Therefore,
Output signals D1. of the optical receivers 21a, 21b. By adjusting the fixed phase offset that exists between D2 with the phase adjuster 22, and then differentially combining the signals D3 and D2 with the differential combiner 23, an output signal with suppressed local light intensity noise is obtained. V
bl can be obtained. Although the explanation is omitted, the local light intensity noise can be similarly suppressed for the I component signal, which is the other polarization component.

Balanced receiver 1 with intensity noise suppressed as described above
1.12 outputs Vbl and Vb2 are equal to Vbl (-VQ) and Vb2 in order to cancel the phase modulation component, as in the conventional case.
(=VI) is multiplied by the mixer 6 to obtain V3, and the VC obtained through the control circuit 7 is used as the phase control signal for the local oscillator light to perform feedback III control on the local oscillator laser light source 2. Good phase synchronization can be maintained. Further, the modulated signal component of θS can be demodulated by the sign determiner 8 using Vb2 (-VI), and the demodulated signal SO can be extracted.

In this way, the local light intensity noise of vbl and Vb2 obtained by the balanced receiver 11.
By using (12) in combination, sufficient suppression is achieved, making it possible to perform phase detection with a good S/N ratio.

[Effects of the Invention] Thus, the present invention provides the same polarization state before transmitting the signal light S.
1 and the local light 10 are combined to extract the Q component signal and the ■ component signal, and in order to optically detect each signal with a 180 degree phase change, there is a residual phase offset between the signal light and the local light. It is possible to perform optical phase detection in which the Q component signal and the 1-component signal are less likely to be affected by the intensity noise of the local light. Therefore, phase detection with few demodulation errors and high quality can be achieved.

In addition, polarization separation elements 10a, 10c and half mirror 1
By arranging 0d and 10e diagonally and integrating them including the 1/4 wavelength plate 10b, the optical 90° hybrid 1
0 operation can be stabilized.

Therefore, the present invention can control the phase of local light in synchronization with the phase of signal light with high precision, and can be widely applied to optical homodyne transmission and coherent optical communication systems.

[Brief explanation of drawings]

Fig. 1 is a block diagram of the optical phase detection system according to the present invention, Fig. 2 (a) is a block diagram of the optical 90° hybrid used in the present invention, and Fig. 2 (b) is the structure of the parastorage bar used in the present invention. 3 are block diagrams of a conventional optical phase detection system. 81-S7. S2''~S4-...Signal light 10~L7.
L2- to L4-... Local light 1.10... Light 90° hybrid 1 C, 10a, 1 Qc... Polarization separation element 1 b,
10d, 10e...Half mirror-1a, 10b-1/
4-wavelength plate 2...Local laser light source 3...Polarization maintaining optical fiber 4, 5.21 a, 21 b-*Reception B6...Mixer 7...Control circuit 8...Sign determination Devices 11, 12... Balanced receivers 20a, 20b... Optical fibers 22... Phase adjuster 23... Differential synthesis circuit small heating station

Claims (1)

[Claims] 1. The signal light whose polarization state has been compensated in advance to be linearly polarized light and the local light emitted from the local oscillation laser are combined by a multiplexer to combine the I component and the I component. A Q component that is delayed by 90° from the phase of the local light is extracted, and the I component and the Q component are used to synchronize the phase of the signal component and the local light.
In the optical phase detection method for demodulating the components, the signal light and the circularly polarized local light are separated into orthogonal polarization components of a P polarization component and an S polarization component, and the separated signals of the same polarization components are obtained. The light and the local light are combined to obtain the I component and the Q component, respectively, and the obtained I component and Q component each have a first output light and a second output light that have a phase different from each other by 180 degrees. After converting the first output light and the second output light into electrical signals, the fixed phase offset between the first and second signals converted to the electrical signals is adjusted to generate a differential signal. The optical phase is characterized in that the signal light and the local light are phase-synchronized using the differentially combined electrical signals, and one of the electrical signals corresponding to the I component is demodulated to obtain a demodulated signal. Detection method.