JPS6123121A - Optical heterodyne receiving method - Google Patents

Abstract

PURPOSE:To eliminate deterioration in S/N ratio by separating the multiplexed light of input signal light and local oscillation light into two light beams which have mutually orthogonal planes of polarization, and controlling the polarization state of the input signal light so that one separated light beam is minimum or maximum. CONSTITUTION:The input signal light 1 is made incident on the polarization control element consisting of a phase compensating element 2 and a polarization rotary element 3, and its output signal light 4 is multiplexed by an optical multiplexer 7 with local oscillation light 6 from a local oscillator 5. This multiplexed output light 8 is separated by a polarization splitting element 9 into light beams 10 and 11 which have mutually orthogonal planes of linear polarization, and they are supplied to the 1st and the 2nd photodetectors 12 and 13 for heterodyne detection. The 2nd intermediate frequency signal 15 is shifted in phase by pi/2 through a phase shifter 16 and a driving circuit 20 generates a control signal 21 corresponding to the value of the output 19 of a mixer 18; and a phase compensating element 2 is controlled to shift the phase of the input signal light 1, thereby making the signal line 4 into a linear polarized wave. The polarization direction of the signal light is changed to a desired direction through the polarization rotary element 3 and the output of a photodetector 12 is detected by a detecting circuit 28 to obtain a demodulated signal output 29 with high efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical heterodyne reception method used in optical communication systems, optical information processing systems, and the like.

(Prior art and its problems) In general, the optical heterogain detection method has the great advantage of increasing reception sensitivity by 10 to 100 times or more compared to the conventional optical direct detection method, so it can be used for long-distance optical communication trunk lines. It is an effective optical detection method for various types of high-sensitivity optical sensors.

In order to achieve high reception sensitivity in this optical detection method, efficient multiplexing of the signal light and local oscillation light is required.
Polarization state, beam diameter, etc. must be matched. However, in optical communications, the polarization state of signal light propagated through a long-distance optical fiber is affected by various disturbances applied to the optical fiber and changes over time. The state of polarization varies depending on the state of the object to be measured. Therefore, stable and efficient multiplexing cannot be performed if the current state remains as it is.

One way to solve this problem is to monitor the polarization state of the signal light during reception, and then control the polarization state of the multi-signal light or local oscillation light to match the polarization states of the signal light and the local oscillation light. For example, the method described in the following literature is considered as a polarization control method. Ohara et al.
“Optical circuit for output polarization compensation of single-mode optical fiber” 1977
Proceedings of the Annual National Conference of the Institute of Electronics and Communication Engineers 874.1980
Year. However, in the general method <i, a part of the signal light must be separated in order to monitor the signal light, and there is a problem that a loss occurs in the signal light at that time.

Further, the combined light obtained by combining the signal light and the local oscillation light is input to the polarization separation element, and the first linearly polarized light having mutually orthogonal polarization planes is input. Polarization diversity optical reception is also being considered, in which the second light beam is divided into two, each incident on a separate light receiving element, converted into an electrical signal, and the electrical signals are processed and combined. When combining electrical signals obtained from two light-receiving elements with equal gain using this method, the S/N ratio of the demodulated signal changes depending on the polarization state of the signal light, and the S/N ratio of the demodulated signal changes by up to 3 dB.
/N deterioration is a problem. Also, if you multiply two electrical signals by a coefficient proportional to the signal strength and add them together, S
/N deterioration is eliminated, but there is a problem that the circuit configuration becomes complicated. (Okoshi et al. “Polarization Diperity Optical Receiver for Heterogain/Coherent Optical Fiber Communication” Institute of Electronics and Communication Engineers Communication Systems Study Group Material C383-22, 1
983) (Object of the Invention) The object of the present invention is to eliminate the above-mentioned problems, simultaneously realize control of the polarization state of the signal light and reduction of the signal light component for monitoring, and achieve constant sensitivity without S/N deterioration. An object of the present invention is to provide an optical heterodyne reception method that can perform optical heterodyne reception.

(Structure of the Invention) The optical heterogain receiving method of the present invention includes a process of multiplexing input signal light and local oscillation light, a process of separating the combined light into two light beams whose polarization planes are orthogonal to each other, and a process of multiplexing input signal light and local oscillation light. a process of converting each beam into an electrical signal;
and a process of obtaining a demodulated signal output from the electrical signal, and further, the phase difference between the two electrical signals is n.
= (n=0, 1, 2, -...) and includes a process for controlling the polarization state of the input signal light so that one of the electrical signals becomes minimum or maximum. ing.

(Detailed Description of Configuration) In the present invention, first, an input signal light and a local oscillation light are multiplexed in an optical multiplexer, and the multiplexed light is sent to a first polarization splitter whose polarization planes are orthogonal to each other. split into a second light beam. Here, the polarization state of the input signal light is controlled so that most of the input signal light travels to the first light beam side through the polarization separation element.

In this case, the polarization states of the signal light of the first light beam and the local oscillation light are the same, and since most of the #1 signal light is included in this first light beam, this is detected by optical heterodyne detection. From this output, a demodulated signal output can be stably obtained with almost no S/N deterioration. In the present invention, the polarization state of the signal light is detected and controlled in the following manner so that most of the signal light travels to the first light beam. Here, for simplicity, consider a case where linearly polarized light is used as the locally oscillated light, and its polarization direction forms an angle of 450 with the eigenaxis of the polarization separation element. In this case, the locally oscillated light is separated by the polarization separation element into two linearly polarized lights having the same phase or a phase difference of π and having the same amplitude. Here, for simplicity, we will consider the case of the same phase. Since the polarization separation element separates the local oscillation light and the signal light into two orthogonal linearly polarized waves, it is possible to perform heterodyne detection on each of the two light beams separated by the polarization separation element, and it is possible to perform heterodyne detection on each of the two light beams separated by the polarization separation element, so that the signal light has the same intermediate frequency. Two intermediate frequency signals are obtained. In this case, as mentioned above, the amplitude and phase of the local oscillation light components of the two light beams are equal, so 2
The amplitude ratio and phase difference between the two intermediate frequency signals correspond to the polarization state of the signal light. Therefore, if the polarization state of the signal light is controlled so that the phase difference between the two intermediate frequency signals becomes zero, the signal light becomes linearly polarized.

Furthermore, by controlling the polarization direction of the signal light so that one of the two intermediate frequency signals becomes zero, the multi-signal light can be separated into only one light beam by the polarization separation element. can. Detection of phase difference.

The control may be performed by applying a phase shift of π/2 to one of the two intermediate frequency signals, then multiplying them together, and controlling the phase of the signal light so that the output becomes zero. In addition, the amplitude can be controlled so that the intermediate frequency signal from the required optical beam is at its maximum or the other intermediate frequency signal is at its minimum.One way to control this is to minutely modulate the polarization direction of the signal light. A possible method is to detect the change in the polarization, create a control signal from two periods of the detected signal, and control the polarization direction of the signal light. Furthermore, even if the local oscillation light is a non-linearly polarized wave, if the polarization state is known, the first oscillation light separated by the polarization separation element can be used. Since the phase difference of the locally emitted light component of the second light beam is known, control can be performed taking this phase difference into consideration.

(Embodiment) FIG. 1 is a block diagram of a first embodiment of the present invention, and FIG. 2 is a diagram showing the output of a second photodetector. Input signal light 1 is incident on a polarization control element composed of a phase compensation element 2 and a polarization rotation element 3. The signal light 4 output from this polarization control element is multiplexed with the locally oscillated light 6 emitted from the local oscillator 5 by an optical multiplexer 7. This combined output light 8 is passed through a polarization splitting element 9 into a first wave having two linearly polarized waves orthogonal to each other. are split into second light beams 10, 11, respectively. Heterodyne detection is performed by the second photodetector 12.13. Here, as the local oscillation light 6, a linearly polarized wave is used, and its polarization direction is the eigenaxis of the polarization separation element and 4
I adjusted it so that it had an angle of 50, so the first. Second
of the outputs of the photodetectors 1.2 and 13. The phase difference between the second intermediate frequency signals 14 and 15 takes a value corresponding to the polarization state of the signal light 4. That is, when the signal light 4 has linear polarization, the first. The phase difference between the second intermediate frequency signals 14 and 15 is zero or π, and when the signal light 4 has circular polarization, the phase difference between the first
The phase difference between the second intermediate frequency signals 14 and 15 is π/2 or uπ. When the signal light 4 is elliptically polarized, it takes an intermediate value between linearly polarized and circularly polarized waves. Here, a phase shifter 16 gives a phase shift of π/2 to the second intermediate frequency signal 15,
The output signal 17 and the first intermediate frequency signal 14 are combined by the mixer 8. The average output 19 from the mixer 18 is zero when the signal light 4 is linearly polarized, and takes a positive or negative value in other cases depending on the polarization state of the signal light 4. Therefore, the control signal 2 corresponding to this positive or negative value
1 is generated by the drive circuit 20, and this controls the phase compensation element 2 to give an appropriate phase shift to the input signal light 1, thereby making the polarization state of the signal light 4 linearly polarized. be able to. Further, the direction of polarization is controlled by K as follows. Here, the polarization rotation element 3 is driven by the sine wave drive circuit 22, and the polarization direction of the signal light 4 is modulated by a sine wave as shown in FIG. Vessel 13
The monitor output 23 changes according to the modulation signal, and the phase and period at that time change depending on the polarization direction as shown in FIGS. 2(b), (e), and (d). Figure 2 (b): Shift in the polarization direction to the + side; Figure 2 (
c): Desired polarization; Figure 2 (d)? The shift to one side of the polarization direction is detected by the detection circuit 25 by comparing the nine periods of the phase with the monitor output 24 of the sine wave drive circuit,
By controlling the polarization rotation element 3 using the detection output 26, the polarization direction of the signal light can be directed to a desired direction. Therefore, if the output signal 27 of the first photodetector 12 is detected by the detection circuit 28, a demodulated signal output 29 can be obtained with high efficiency.

In this embodiment, a Babinet-Soleiluke phase compensation plate was used as the phase compensation element 2. Further, a λ/2 plate was used as the polarization rotation element 3, and the polarization direction was rotated by rotating its eigenaxis. As the local oscillator 5, a semiconductor laser,
The optical multiplexer 7 is a mirror with a transmittance of 70% and the reflectance is 30%, and the polarization separation element 9 is a Rochon prism.
A photodiode was used as the second photodetector 12.13. A double-balanced mixer was used as the mixer 18, and a control signal 21 was generated from the output of the mixer 18 to control the phase compensator plate to control the polarization state.

In addition, the eigenaxis of the λ/2 plate is set to p I by the sine wave drive circuit 22.
It was changed minutely at a frequency of KHz. Here, the phase relationship of the monitor output 23 obtained from the second photodetector 13 is adjusted to λ/ so that the monitor output 23 changes at a cycle of 2 KHz.
By adjusting the direction of the characteristic axes of the two plates, the intermediate frequency signal 15 obtained by the second photodetector 13 gradually becomes smaller, and the ratio with the first intermediate frequency signal 14 is reduced to 1/100 or less. I was able to do it. As a result, most of the multi-input signal light 1 travels toward the first light beam 10, and at this time, its polarization state matches that of the local oscillation light, so that the output of the first photodetector 12 is always transmitted to the first light beam 10. We were able to obtain demodulated signals with high efficiency.

FIG. 3 is a block diagram of a second embodiment of the present invention, and FIG. 4 is a diagram showing the sign of the mixer average output with respect to the polarization direction. This embodiment differs from the first embodiment in the method of detecting the polarization direction of the signal light 4.

In the case of this embodiment, the first. Second photodetector 12°13
The outputs 30.31 are mixed together by mixer 32. At this time, depending on which direction the polarization direction of the signal light deviates from the eigenaxis, the average output takes a positive or negative value, as shown in Figure 4. Control is performed by determining the direction in which the direction should be corrected.

The other configurations are similar to the first embodiment. In this embodiment as well, the cylinder 1. The ratio of the second intermediate frequency signal 14.15 can always be 100 or more,
Highly efficient demodulation was achieved.

In addition to the above-described embodiments, various modifications can be made to the present invention. For example, the phase compensation element 2 and the polarization rotation element 3 are each a λ4 plate.

Any device that functions as a λ/2 plate will suffice, and in order to reduce the loss of the signal light, use a device that applies pressure to the optical fiber with a piezoelectric element, or a fiber-type polarization compensator in which the optical fiber is wound in a ring shape. There is a way. Furthermore, in the first embodiment,
The polarization direction was detected by monitoring the output of the second photodetector 13 and minimizing it, but by monitoring the output of the first photodetector 12 and controlling it so that it was maximized. It is also possible.

(Effects of the Invention) As described above, in the optical heterodyne reception method of the present invention, as the polarization state of the signal light is controlled, the signal light component for monitoring also decreases. Signal light and local oscillation light are always combined with high efficiency. Therefore, according to the present invention, even if the polarization state of the signal light changes, the S/N
An optical heterogain reception method without deterioration can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the first embodiment of the present invention, and FIG. 2 (
a) to (d) are diagrams showing the output of the second photodetector, FIG. 3 is a block diagram of the second embodiment of the present invention, and FIG. 4 is a diagram showing the sign of the mixer average output with respect to the polarization direction. FIG. In the figure, 1... input signal light 2...
・Phase compensation element 3...Polarization rotation element 5...Local oscillator 7...Optical multiplexer 9...Polarization separation element 12.13-・Photodetector 16.-Phase shifter 18.
32...Mixer 20--Drive circuit 22...Sine wave drive circuit 25...Detection circuit 28...Detection circuit 29--Demodulated signal output.

Claims (1)

[Claims] a process of combining the input signal light and the local oscillation light; a process of separating the combined light into two light beams whose polarization planes are orthogonal to each other; a process of converting each of the light beams into electrical signals; and a process of obtaining a demodulated signal output from either one of the signals, wherein the phase difference between the two electrical signals is nπ (n=0, 1, 2, . . . ), and , an optical heterodyne reception method comprising a process of controlling the polarization state of the input signal light so that one of the electrical signals becomes a minimum or a maximum.