JPS6047524A - Optical receiver - Google Patents

Abstract

PURPOSE:To prevent deterioration in S/N ratio and improve transmission performance by making signal light and local oscillation light coincident in electric field direction through polarization, demultiplexing, and multiplexing, performing photodetection and summing up voltages, and changing local oscillation in polarization state. CONSTITUTION:A polarizing demultiplexer and multiplexer 7 separates signal light 11 and local oscillation light 12 into linear polarized light beams 11p, 12s and 11s, 12p which cross each other at right angles, and they are multiplexed into projection light beams 13 and 14. Further, polarizers 9 and 9' make two kinds of linear polarized light beams of the projection light beams 13 and 14 coincident in electric field direction, and a couple of photodetectors 5 and 5' detect individually the projection light beams 13 and 14 which are coincident in electric field direction. Then, a signal processing circuit 10 sums up the voltages of the detectors 5 and 5', and a polarizing modulator 8 is interposed between a local oscillation power source 3 and the multiplexer 7 to change the polarization state of the local oscillation light 12 so that the output of the circuit 10 is maximum.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical receiving device, and is useful when applied to an optical heterodyne type device.

FIG. 1 is a block diagram showing a conventional optical receiver. In the figure, 1 is an optical fiber used as an optical transmission line, 2 is a polarization compensation circuit, 3 is a local light source, and 4 is a multiplexer that combines the signal light input from the optical fiber 1 and the local light from the local light source 3. vessel, 5
6 is a photodetector, and 6 is a signal processing circuit. In general, the polarization state of the signal light in the optical fiber 1 varies depending on the temperature of the optical fiber, external force, etc., and therefore, when optically detected using direct local light, the detection output varies. Therefore, in the prior art, the polarization state of the signal light is detected and the polarization compensation circuit 2 matches the polarization state of the local light.

However, in this conventional technique, since a part of the signal light is used to detect the polarization state of the signal light, the amount of light incident on the optical detector 5 decreases, and the S/N ratio deteriorates. Furthermore, since the signal light passes through the polarization compensation circuit 2, attenuation of the signal light occurs. Furthermore, the longer the length of the optical fiber 1, the more the polarization state of the signal light fluctuates, and the fluctuation period becomes shorter. therefore,
The polarization compensation circuit 2 is required to have high speed and a large compensation range. However, it has not been possible to simultaneously satisfy high speed and a large compensation range.

In other words, a high-speed polarization compensation circuit uses electro-optic effects,
There are products that use the magneto-optical effect, but the compensation range is small. On the other hand, it is also possible to perform polarization compensation mechanically, but in this case, high speed performance is lost. Furthermore, the output signal S
For the N ratio, when a transparent mirror, a waveguide type multiplexer, etc. are used for the multiplexer 4, the loss in multiplexing is 3 dB, so the signal optical power is ``PS'', the local optical power is PL, and the local optical power is PL. If the shot noise limit is sufficiently large, S in this conventional configuration is
If N is SNo, 5No==-! -Ps-! -PL/'Pt, Nst =
It becomes Ps/2N2 2 2 . Here, Nst is a shot noise component. Therefore, if the signal light component Ps is attenuated by the polarization compensation circuit 2 or the like, it causes 8N deterioration.

As described above, the prior art has disadvantages in that 8N deterioration occurs due to polarization compensation, and at the same time, the polarization compensation circuit 2 has a high speed and a narrow compensation range.

An object of the present invention is to provide an optical receiver that performs optical detection with a high signal-to-noise ratio without depending on the polarization state of signal light input to the optical receiver. A first configuration of the present invention to achieve such an object is an optical receiving device that obtains a signal by optically detecting a signal light input from an optical transmission line using the local light of a local light source. The linearly polarized vertical component of the signal light is combined with the linearly polarized horizontal component of the local light to form the first output light, and the horizontal component of the signal light is linearly polarized. a polarization separation/synthesizer that combines the polarized light and the linearly polarized vertical component of the local light to produce a second output light; and a first unit that matches the electric field directions of the two types of linearly polarized light included in the first output light a second polarizer that matches the electric field directions of the two types of linearly polarized light included in the second output light, and individually optically detects the first and second output lights with the same electric field directions. A pair of optical detectors are inserted between the local light source and the polarization splitter/combiner, and the output of the signal processing circuit is inserted between the local light source and the polarization splitter/combiner. The present invention is characterized by comprising a polarization modulator that changes the polarization state of the local light so as to maximize the polarization state of the local light. Further, the second configuration of the present invention is an optical receiving device that obtains a signal by optically detecting the signal light input from the optical transmission line using the local light of the local light source, in which the signal light and the local light are separated respectively. One of the separated signal lights and one of the separated local lights are combined and sent out as a first emitted light, and the other separated signal light and the other separated local light are combined and sent out as a second outgoing light. A splitter that receives the first emitted light and passes only the linearly polarized vertical component of the signal light and the linearly polarized horizontal component of the local light with the electric field directions of both linearly polarized lights being the same. A first polarizer receives a second output light and converts only the linearly polarized horizontal component of the signal light and the linearly polarized vertical component of the local light into a state in which the electric field directions of both linearly polarized lights match. a second polarizer that passes through the polarizer, a pair of photodetectors that individually photodetect the linearly polarized light that has passed through the first and second polarizers, and a voltage summation of the outputs from the pair of photodetectors. a polarization modulator that is inserted between the local light source and the polarization splitter/combiner and changes the polarization state of the local light so that the output of the signal processing circuit is maximized; It is characterized by:

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 2 shows an embodiment of the invention. In the figure, 1 is an optical fiber, 3 is a local light source, 5 and 5' are optical detectors, 7 is a polarization splitter/combiner, 8 is a polarization modulator, 9.9' is a polarizer, and 10
is a signal processing circuit.

FIG. 3 is an explanatory diagram for explaining the operation of the polarization splitter/combiner 7 used in this embodiment. In the figure, 11 is the signal light, 12 is the local light, 13 and 1.4 are the emitted lights, respectively, solid lines 11p and 12p are the linearly polarized vertical components of the signal light and the local light that pass directly through the polarization splitter/combiner 7, and the broken line 113 ,128
is the linearly polarized horizontal component of the signal light and local light reflected by the polarization splitter/combiner 7.

Next, the operation of this embodiment will be explained. The signal light 11 from the optical fiber 1 to the optical receiving device is polarized by the polarization splitter/combiner 7 into linearly polarized light LIP and IIs according to the polarization state of the incident light, resulting in output light 13°14. On the other hand, the local light is similarly polarized and separated into linearly polarized light 12p12s and output light 13.1
It becomes 4. Therefore, the output light 13 includes the linearly polarized light lip of the signal light and the linearly polarized light 12g of the local light, and the output light 14
The linearly polarized light IIs of the signal light and the linearly polarized light 12p of the local light are
It is included. In this case, the output light 13 of the polarization splitter/combiner 7 is linearly polarized light 11p.

12s are orthogonal to each other, so linearly polarized light 11p.

The polarizers 9 are arranged at an angle of 45° with respect to the electric field direction of 12s, so that the electric field directions coincide with each other. On the other hand, a polarizer 9' is similarly provided for the output light 14, so that the linearly polarized light 11s, 1
The electric field direction of 2p is made to match. The emitted light 13° 14 having the same electric field direction is optically detected by a photodetector 5 to obtain an electric signal.

The signals of the respective photodetectors 5 and 5' are processed by a signal processing circuit 10.
and add the voltages to obtain the output signal.

Here, the signal optical power incident on the photodetector is Ps.

When the local light power is PL and the photodetector conversion efficiency is K, the photodetector output power S becomes S=@PsIIPL. Generally, after long-distance transmission through an optical fiber, the polarization state of the output light (light incident on the optical receiver) changes, so the signal light component separated by the polarization splitter/combiner 7 also changes, so the optical detector output S also changes. Here, if the power of the output light 13.14 of the polarization splitter/combiner 7 normalized by the incident light power is A and (1-A), and similarly, each power of the local light is B and (1-B), The condition for maximizing the output voltage calculated by voltage 710 in the signal processing circuit 1° is A=B, and the output voltage becomes constant. Therefore, a part of the output voltage is fed back to the polarization modulator 8 inserted between the local light source 3 and the polarization splitter/combiner 7 to change the polarization state of the local light so that the output signal voltage is always the maximum. By controlling the distribution ratio B, it is possible to always keep the output voltage constant and maximum regardless of the polarization state of the incident light. Here, the fluctuation range of the polarization state of the incident light is very wide, but the range in which the local light is distributed to B and (1-B) is ok and El<1, so the polarization state of the local light is linearly polarized light and is 0. It is good within the range of ~90 degrees. A polarization modulator using an electro-optic effect can be used for this purpose, making it possible to reduce the size, speed, and loss.

In this example, the loss of the signal light is 3 dB(
1) This is the loss and recirculation caused by the conventional multiplexer 4. Furthermore, the transmission loss of the polarization splitter/combiner 7 and the polarizers 9 and 9' is small.

Therefore, when the 8N ratio is calculated using the distribution ratio A for the signal light of the polarization splitter/combiner 7, the SN ratio of the combined output is as follows.

Here A-=1. -1: or when A=O, that is, when the signal light is separated by the polarization splitter/combiner 7 and the total amount of light is distributed to one side, S/N = Ps/2Nst = SN.

This corresponds to the S/N ratio when there is no loss of signal light in a conventional optical receiver. Also, when the distribution ratio A is 100, 87N =
28No., 3dB improvement was made, polarizer 9,9
It is possible to compensate for the loss of the signal light received at '.

It should be noted that the polarization splitter/combiner 7, the polarization modulator 8, and the polarizer 9, which are the components of the optical receiver according to this embodiment, can be made into optical ICs using optical waveguides, achieving small size and high reliability. can.

FIG. 4 shows another embodiment of the present invention, in which the polarization splitter/combiner 7 of FIG. 2 is replaced by a demultiplexer 15 that equally divides both the signal light and the local light.

In operation in this configuration example, the signal light 11 and the local light 12 are divided into equal parts by the demultiplexer 15 regardless of their polarization states. Therefore, the polarizer 9'g/// is installed so that components orthogonal to each other with respect to the signal light 11 and the local light 12 pass through.The linearly polarized light 11 with the same electric field direction is emitted from the polarizer 9'.
P, 12s are outputted, and linearly polarized light 11s*12p with the same electric field direction is outputted from polarizer 9#. In this way, the polarizers g //, g I
The output light of I/ will be the same as the output light of polarizer 9.9' in FIG. Therefore, if the same operation as shown in FIG. 2 is carried out from now on, a constant (maximum) output signal will be obtained that is independent of the incident polarization. Also, the S/N ratio will be the same value.

Note that if the incident polarization with a distribution ratio A of 1 or close to 0 persists, the SN ratio deteriorates by 3 dB. In such a case, an infinitely rotating half-wave plate or the like is installed between the optical fiber 1 and the polarization splitter/combiner 7 in the embodiment shown in FIG. 2, and between the optical fiber and the demultiplexer 15 in the embodiment shown in FIG. Insert between and the distribution ratio A is. −
If the control is carried out so that Such control only needs to be performed regularly when the distribution ratio A is close to 1 or O (high speed is not required). Without performing any processing on the signal light, it is possible to respond quickly to infinite fluctuations in the polarization state of the signal light and keep the output signal constant and at a maximum value.

Furthermore, since the loss of signal light is smaller than that of the conventional type, the SN ratio of the output signal is improved. Because of these effects, when the received output signal is dependent on the polarization state of the signal light in optical heterodyne transmission systems, etc., by using the optical receiver according to the present invention, a high-performance transmission system can be realized, and the transmission path can be This has the advantage that the requirements for the polarization percentage of the optical fiber used as an optical fiber are relaxed.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram showing a conventional optical receiver, and Figure 2 is a diagram showing the configuration of a conventional optical receiver.
FIG. 3 is an explanatory diagram for explaining the polarization state of the polarization splitter/combiner used in this embodiment, and FIG. 4 is an embodiment of the second invention. FIG. In the drawing, 1 is the optical fiber, 2 is the polarization compensation circuit, 3 is the local light source, and 4 is the optical fiber.
is a multiplexer, 5.5′ is a photodetector, 6 is a signal processing circuit, and 7
is a polarization separation/synthesizer, 8 is a polarization modulator, 9.9', 9'',
9'' is a polarizer, 10 is a signal processing circuit, 11 is a signal light, l
ip, 118 is linearly polarized signal light, 12 local light, 12
T), 12s is the linearly polarized local light, 13.14 is the output light of the polarization splitter/combiner, and 15 is the demultiplexer. Patent applicant Nippon Telegraph and Telephone Public Corporation agent Patent attorney Shibu Mitsuishi (and one other person) Procedural amendment January 24, 1980 Dear Commissioner of the Japan Patent Office 1 Indication of the case 1982 Patent No. 155089 1972 referee
Judgment No. 2, Name of the invention Optical receiving device 3 Relationship with the case of the person making the amendment Patent applicant 1-6 Uchisaiwai-cho-chome, Chiyoda-ku, Tokyo (422) Nippon Telegraph and Telephone Public Corporation Postal code 107 6, increase due to amendment Number of inventions: 1 7. “Claims” and “Detailed description of the invention” of the specification to be amended;
Each column of "Brief explanation of drawings" and drawings. 8. Contents of the amendment (s-1) The description in the "Claims" column of the specification will be amended to the "Amended Claims" section of the attached appendix. (8-2) The statement in the "Detailed Description of the Invention" column of the specification is amended as follows. 0) "Polarization state" written on the last line of page 3
and correct it. (b) "Characterized by receiving the first emitted light..." written on page 7, line 3 to line 17 of the same page is corrected as follows. ``A first polarizer that receives a first emitted light and allows only a linearly polarized light component having a plane of polarization in one direction of the emitted light to pass; A second polarizer that allows only the linearly polarized light component in a direction orthogonal to the linearly polarized light that has passed through the first polarizer to pass through, and the linearly polarized light that has passed through the first and second polarizers is individually optically detected. A pair of optical detectors, a signal processing circuit that adds voltages of outputs from the pair of optical detectors, and a signal processing circuit that is inserted between the local light source and the polarization splitter/combiner, so that the output of the signal processing circuit is maximized. A third configuration of the present invention is characterized in that it includes a polarization modulator that changes the polarization state of the local light as in In an optical receiver that obtains a signal by optically detecting the local light from the source, there is a multiplexer that combines the signal light and the local light, and a polarization that separates the combined light into two linearly polarized lights that are orthogonal to each other. a separator, a pair of photodetectors that optically detect each of the two separated linearly polarized lights individually, a signal processing circuit that adds voltages to the outputs from the pair of photodetectors, and the local light source and polarization separation. It is characterized by comprising a polarization modulator that is inserted between the combiners and changes the polarization state of the local light so that the output of the signal processing circuit is maximized. (→ Correct “photodetector 5” written on page 9, line 9 to “photodetector 5.5”J. 5 shows still another embodiment of the present invention. In this embodiment, in the configuration of FIG. 4, the demultiplexer 15 is replaced by a multiplexer 16, and the polarizers 9-91- is configured by the analyzer separator 17.The operation in this configuration example is performed by the multiplexer 16.
The signal light 11 and the local light 12 are multiplexed to obtain a multiplexed light 18. When this combined light 18'jk is separated into mutually orthogonal linearly polarized light by the polarization splitter 17, it becomes the same as the output light from the polarizers 9 and 91 in FIG. Therefore, if the same operation as shown in FIG. 2 is carried out thereafter, a constant maximum output can be obtained regardless of the incident polarization. Note that when the amount of light of the local light 12 is sufficiently large compared to the shot noise limit, the transmission loss in the multiplexer 6 is made large for the local light and small for the signal light. Since signal light loss is small, the S/N ratio can be improved. ”
. (f) After "between the demultiplexer 15" written on the second line of page 13, add ", in the embodiment of FIG. 5, between the optical fiber 1 and the multiplexer 16". (8-3) The statement in the "Brief Description of Drawings" column of the specification is amended as follows. 0 Insert "Fig. 5 is a block diagram showing an embodiment of the third invention" between the 5th line of page 14 and the 6th line of the same page. ←) After "15 is a demultiplexer" written on page 14, line 14, "16 is a multiplexer, 17 is a polarization correction patent claim. 1. Going to kindergarten (Figure 5) l. Reverse correction patent Claims (1) In an optical receiver that obtains a signal by optically detecting a signal light input from an optical transmission line using local light from a local light source, the signal light and the local light are separated into mutually orthogonal linearly polarized lights. At the same time, the linearly polarized vertical component of the signal light and the linearly polarized horizontal component of the local light are combined to form the first radiation light, and the linearly polarized horizontal component of the signal light and the vertical component of the local light are combined. a polarization separation/synthesizer that combines the linearly polarized light of a second polarizer that matches the electric field directions of two types of linearly polarized light included in the emitted light; a pair of optical detectors that individually optically detect the first and second emitted lights whose electric field directions match; A signal processing circuit that adds voltages to the outputs from a pair of optical detectors is inserted between the local light source and the polarization splitter/combiner. An optical receiver comprising: a polarization modulator that changes the state of polarization. In the optical receiving device, the signal light and the local light are each separated, and the signal light of the other is combined with the separated local light, and the signal light of the other is combined and sent out as the first output light, and the separated signal light of the other side is combined. a pair of optical detectors that individually optically detect the linearly polarized light that has passed through the demultiplexer polarizer, which combines the local light from the other separated source light and sends it out as second output light; and this pair of optical detectors. a signal processing circuit that adds voltages to outputs from the local light source; and a polarized light that is inserted between the local light source and the polarization separation/combiner and changes the polarization state of the local light so that the output of the signal processing circuit is maximized. An optical receiving device comprising a modulator.

Claims (2)

[Claims] (1) In an optical receiving device that obtains a signal by optically detecting a signal light input from an optical transmission line using the local light of a local light source, the signal light and the local light are separated into mutually orthogonal linearly polarized light, and The linearly polarized vertical component of the signal light and the linearly polarized horizontal component of the local light are combined to form a first output light, and the linearly polarized horizontal component of the signal light and the linearly polarized vertical component of the local light are combined. a polarization separation/synthesizer that combines the two types of linearly polarized light to produce a second output light; a first polarizer that matches the electric field directions of two types of linearly polarized light included in the first output light; A second method that matches the electric field directions of the two types of linearly polarized light included.
a pair of photodetectors that individually optically detect the first and second emitted lights having the same electric field direction, and a signal processing circuit that adds voltages of outputs from the pair of photodetectors; A polarization modulator inserted between the local light source and the polarization separation/combiner for changing the polarization state of the local light so that the output of the signal processing circuit is maximized. Optical receiver. (2) In an optical receiving device that obtains a signal by optically detecting the signal light input from the optical transmission line using the local light of a local light source, the signal light and the local light are separated, and one of the separated signal lights and the other are separated. a splitter that combines one of the separated local light beams and sends it out as a first output light, and combines the other separated signal light and the other separated local light beam and sends it out as a second output light; a first polarizer that receives the emitted light of the signal light and passes only the linearly polarized vertical component of the signal light and the linearly polarized horizontal component of the local light with the electric field directions of both linearly polarized lights being the same; , which receives the second emitted light and passes only the linearly polarized horizontal component of the signal light and the linearly polarized vertical component of the local light with the electric field directions of the two and one lights being the same. a pair of photodetectors that individually optically detect the linearly polarized light that has passed through the first and second polarizers, and a signal processing circuit that adds voltages to outputs from the pair of photodetectors. , a polarization modulator that is inserted between the local light source and the polarization splitter/combiner and changes the polarization state of the local light so that the output of the signal processing circuit is maximized;
An optical receiving device characterized by comprising: