JPH0671233B2 - Double balanced receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] When a received light signal and a locally oscillated laser light are added to an optical coupler and mixed, the locally oscillated laser light is modulated, and as a result, a modulation signal component to be included in a reception output signal. Extract
The input phase to the light receiving element, the gain of the light receiving element, the phase of the output signal of the light receiving element, etc. are controlled so that the modulation signal component is minimized, the difference in the frequency characteristics of the light receiving element is compensated, and the intensity noise suppression effect Is to improve.

[Industrial application field]

The present invention relates to a double-balanced light receiving device in which the intensity noise suppression effect of locally oscillated laser light is enhanced.

The received light signal and the locally oscillated laser light are mixed by an optical coupler, the two output lights from this optical coupler are respectively received by two light receiving elements, and the output signals of these light receiving elements are combined to obtain a light receiving output signal. The double-balanced light receiving device is capable of effectively using the light receiving signal and the local oscillation laser light. Since it has an intensity noise suppression effect, it is regarded as a promising light receiving device for heterodyne coherent optical communication.

In such a double-balanced light receiving device, the intensity noise suppression effect is reduced due to the difference in the characteristics of the two light receiving elements, the variation in mounting, the difference in the gain of the amplifier in the subsequent stage, etc. Is required.

[Prior Art] A conventional double-balanced light receiving device has, for example, a configuration shown in FIG. 6, in which a received light signal and a locally oscillated laser light are added to an optical coupler 61 to be mixed and mixed output light Are added to the light receiving elements 62 and 63, respectively. The output signals of the light receiving elements 62, 63 are amplified by the amplifiers 64, 65 and are added to the subtractor 66, the difference between the output signals of the light receiving elements 62, 63 is output, and the output signal becomes the reception output signal and the latter stage. Circuit (not shown)
Added to.

The output signals i ₁ and i ₂ of the light receiving elements 62 and 63 are Becomes

Note that P _S is the received light signal, P _L is the locally oscillated laser light, ε is the branching ratio of the two outputs of the optical coupler, f _IF is the intermediate frequency, φ is the received signal and the position of the locally oscillated laser light at the time of the optical coupler output. It is a phase difference.

When ε in the equations (1) and (2) is set to ε = 0.5, the difference (i ₁ −i ₂ ) between the output signals of the light receiving elements 62 and 63 is obtained by the subtractor 66, and the intermediate frequency component e is extracted. Becomes

The noise component is mainly the intensity noise of the local oscillation laser light, shot noise, thermal noise of the circuit, etc.For the intensity noise of the local oscillation laser light, the difference between the output signals of the light receiving elements 62, 63 should be obtained. Can be removed by.

FIG. 7 shows a circuit diagram of a conventional example using the 3 dB optical coupler 71. The received light signal and the local oscillation laser light are added to the 3 dB optical coupler 71, and the output light thereof is added to the light receiving elements 72 and 73.
These light receiving elements 72 and 73 are connected in series, and the connection point is connected to the gate of a field effect transistor 76 which constitutes an amplifier. Also, 74 and 75 are resistors, and 77 is a capacitor.

By adding an output signal to the gate of the field effect transistor 76 from the connection point where the light receiving elements 72 and 73 are connected in series,
The difference between the output signals of the light receiving elements 72 and 73 is obtained and amplified by the field effect transistor 76.
Similar to the subtractor 66 in the figure, a reception output signal as an intermediate frequency component can be obtained.

[Problems to be solved by the invention]

As described above, the received light signal and the locally generated disadvantager light are mixed by the 3 dB optical coupler 71, and the difference between the output signals of the light receiving elements 72 and 73 which receive the two output lights of the 3 dB optical coupler 71 is calculated. The double-balanced light-receiving device using the received output signal can suppress the intensity noise of the locally oscillated laser light, but this intensity noise suppression characteristic has a drawback that it is deteriorated in a high frequency region.

For example, FIG. 8 shows a case where one output light of the 3 dB optical coupler 71 is received by one light receiving element in the double balanced light receiving device having the configuration shown in FIG. On the other hand, in the case of the double balanced light receiving device in which the two output lights of the 3 dB optical coupler 71 are respectively received by the light receiving elements 72 and 73, the one shown by the dotted curve b is obtained. That is, by using the double balanced light receiving device, the intensity noise of the locally oscillated laser light can be significantly suppressed.

In actual coherent optical communication, the intermediate frequency is a few
It is selected in the order of GHz, and a signal exists near the intermediate frequency, and it is necessary to sufficiently suppress intensity noise near the intermediate frequency. However, the dotted curve b
As can be seen from the above, in the conventional double balanced photodetector,
There was a drawback that the intensity noise suppression effect gradually decreased from about 1 GHz.

An object of the present invention is to improve the intensity noise suppression effect of the locally oscillated laser light of the double balanced light receiving device.

[Means for solving problems]

The double-balanced light receiving device of the present invention will be described with reference to FIG. 1. An optical coupler 1 to which a received light signal and a locally oscillated laser light are added, and an optical signal power obtained by combining these is split into two.
The light receiving elements 2 and 3 which respectively receive two output lights output from the optical coupler 1 are provided, and these light receiving elements 2 and 3 are provided.
In the double-balanced light receiving device that synthesizes the output signal of 1 by the synthesizing unit 6 into the received output signal, the amplitude of the locally oscillated laser beam applied to the optical coupler 1 is a predetermined modulation signal different from the received output signal. The modulator 4 that modulates with, and the modulation signal component by the modulator 4 is extracted from a part of the received output signal, and the amplitude and phase of the input to the light receiving elements 2 and 3 are adjusted so that this modulation signal component is minimized. A control circuit 5 for controlling or controlling the gain of the light receiving elements 2 and 3 and the phase control of the output signal is provided.

[Action]

When the local oscillation laser light is modulated by the modulator 4, if the frequency characteristics of the light receiving elements 2 and 3 and the characteristics of the amplifier and the like are uniform, the modulation signal component by the modulator 4 is suppressed. Therefore, by extracting this modulated signal component from the received output signal and controlling it to be the minimum,
The intensity noise suppression characteristic can be improved by automatically compensating for the difference in the frequency characteristics of the light receiving elements 2 and 3, or the gain difference and phase difference of the amplifier. That is, the reason why the effect of suppressing the intensity noise of the locally oscillated laser light decreases in the high frequency region is mainly the sixth reason.
This is caused by the difference in the frequency characteristics of the two light receiving elements 62, 63 or 72, 73 in FIG. 7 or FIG. 7 or the gain difference or phase difference between the amplifiers 64, 65 in FIG. It is also caused by the phase difference due to the parasitic factor of. Therefore, by compensating for such a difference in frequency characteristic, a gain difference, and a phase difference, it is possible to improve the noise intensity suppression characteristic.

〔Example〕

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

FIG. 2 is a block diagram of the first embodiment of the present invention.
Is an optical coupler, 12 and 13 are light emitting elements such as photodiodes, phototransistors, and avalanche photodiodes, 14 is a modulator that modulates light intensity, 15 is a control circuit including a lock-in amplifier, and 16 is a local oscillation laser. A semiconductor laser that outputs light, 17 is an oscillator, 18 and 19 are amplifiers, and 20 is a subtractor.

The locally oscillated laser light from the semiconductor laser 16 is added to the optical coupler 11 after being finely modulated by the output signal of the oscillator 17 in the modulator 14, that is, a predetermined modulation signal different from the reception output signal described later. In the optical coupler 1, the received light signal is mixed and the two output lights from the optical coupler 11 are incident on the light receiving elements 12 and 13, respectively. The output signals of the light receiving elements 12 and 13 are amplified by the amplifiers 18 and 19, the amplified output signal is added to the subtractor 20, and the subtraction output signal is output as the reception output signal.

A part of this reception output signal is applied to the control circuit 15 together with the output signal of the oscillator 17. The control circuit 15 includes a lock-in amplifier, which extracts a modulation signal component by the modulator 14 and controls the gain of the light receiving element 12 so that the modulation signal component becomes small. Therefore, the intensity noise of the locally oscillated laser light can be suppressed even in the high frequency region.

When the frequency characteristics of the light receiving elements 12 and 13 and the gain and phase characteristics of the amplifiers 18 and 19 are the same, the modulated signal components included in the output signals of the amplifiers 18 and 19 are equal, and are canceled by the subtractor 20. That is, the received output signal applied to the control circuit 15 does not include the modulation signal component. However, if there is a difference in the characteristics of each part, the received output signal will contain a modulation signal component, so that each part is controlled so that this modulation signal component is zero. In this embodiment, The gain of the light receiving element 12 is controlled.

FIG. 3 is a block diagram of the second embodiment of the present invention.
Is an optical coupler, 22 and 23 are light receiving elements, 24 is a modulator, 25 is a control circuit, 26 is a semiconductor laser, 27 is an oscillator, 28 and 29 are amplifiers, 30 is a subtractor, and 31 is a phase shifter. Also in this embodiment, the locally oscillated laser light from the semiconductor laser 26 is modulated by the output signal of the oscillator 27 at the modulator 24, and the modulated locally oscillated laser light and the received light signal are optically coupled.
In addition to 21, they are mixed, converted into an electric signal by the light receiving elements 22 and 23, amplified by the amplifiers 28 and 29, and the difference between the output signals of the light receiving elements 22 and 23 is obtained by the subtractor 30 to obtain the reception output signal. It is a thing.

The modulation signal component contained in the received output signal is controlled by the control circuit 25.
, So that the modulation signal component becomes smaller,
The phase shifter 31 is controlled to compensate for the difference in the phase characteristics in the paths of the output signals of the light receiving elements 22 and 23, whereby the phase of the modulation signal component and the intensity noise component in the signal applied to the subtractor 30 is compensated. To cancel the intensity noise of the locally oscillated laser light.

FIG. 4 is a block diagram of the third embodiment of the present invention.
Is an optical coupler, 42 and 43 are light receiving elements, 44 is a modulator, 45 is a control circuit, 46 is a semiconductor laser, 47 is an oscillator, 48 is an amplifier, 49
Is a bias circuit. This embodiment shows a case where the light receiving elements 2 and 43 such as avalanche photodiodes are connected in series, and the bias circuit 49 is controlled by the control circuit 45 to control the gain and phase of the light receiving element 42. 45
The modulated signal component contained in the received output signal is extracted by
The bias circuit 49 is controlled so that this modulation signal component becomes small.

FIG. 5 is a block diagram of the fourth embodiment of the present invention.
Is an optical coupler, 52 and 53 are light receiving elements, 54 is a modulator, 55 is a control circuit, 56 is a semiconductor laser, 57 is an oscillator, 58 is an amplifier, 59
Is an optical control element such as an electro-optical effect element capable of controlling the phase or intensity of light. In this embodiment, the modulation signal component contained in the received output signal is extracted in the control circuit 55, the optical control element 59 is controlled so that the modulation signal component becomes small, and the optical coupler 51 enters the light receiving element 53. The phase or intensity of the light incident on the light receiving element 52 is controlled with respect to the phase or intensity of the generated light. Thereby, the light receiving element 5
The modulation signal components output from 2, 53 become equal, the modulation signal component is not included in the signal output from the connection point of the light receiving elements 52, 53, and the intensity noise component is also suppressed. .

It is also possible to combine the control means in each of the above-described embodiments, and it is of course possible to use another solid-state laser or the like instead of the semiconductor laser that generates the local oscillation laser light.

〔The invention's effect〕

As described above, according to the present invention, the locally oscillated laser light applied to the optical coupler 1 is modulated by the modulator 4 with a predetermined modulation signal different from the reception output signal, and the modulation signal component included in the reception output signal is generated. Then, the control circuit 5 controls the light receiving element 2, so that the modulation signal component is minimized.
The input to 3 is controlled by an optical control element or the like, or the output is controlled for the gain control of the light receiving elements 2 and 3 and the gain or phase of the amplifier. The optical coupler 1 is connected to the two light receiving elements 2 and 3. It is possible to automatically suppress intensity noise due to the phase difference of the incident light, the difference in the frequency characteristics of the light receiving elements 2 and 3, the difference in the frequency characteristics such as the gain of the amplifier, etc. There is an advantage that it is possible to perform stable coherent optical communication by compensating for characteristic fluctuations in each part.

[Brief description of drawings]

FIG. 1 is a block diagram of the principle of the present invention, FIGS. 2 to 5 are block diagrams of first to fourth embodiments of the present invention, FIG. 6 is a block diagram of a conventional example, and FIG. 7 is a conventional example. FIG. 8 is a diagram of intensity noise suppression characteristic curve. 1,11,21,31,41,51 are optical couplers, 2,3,12,13,22,23,42,43,
52,53 are light receiving elements, 4,14,24,44,54 are modulators, 5,15,25,4
5,55 is a control circuit including a lock-in amplifier, 6 is a synthesizing unit, 16,26,46,56 are semiconductor lasers, 17,27,47,57 are oscillators for outputting modulation signals, 18,19,28, 29, 48, 58 are amplifiers, 20,
30 is a subtractor, 31 is a phase shifter, 49 is a bias circuit, and 59 is an optical control element.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuya Kiyonaga 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor Chiaki Osawa 1015, Kamedotachu, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited ( 56) References JP-A-60-172842 (JP, A)

Claims (1)

[Claims] 1. An optical coupler (1) to which a received light signal and a locally oscillated laser light are added, and which splits an optical signal power obtained by combining the two, and two outputs output from the optical coupler (1). Light receiving elements (2) and (3) that receive light respectively
In the double-balanced light-receiving device, which comprises the output signals of the light-receiving elements (2) and (3) to form a reception output signal, the amplitude of the locally oscillated laser light applied to the optical coupler (1). And a modulator (4) that modulates with a predetermined modulation signal different from the reception output signal, and a modulation signal component by the modulator (4) is extracted from a part of the reception output signal, and the modulation signal component is minimized. And a control circuit (5) for controlling the input to the light receiving elements (2), (3) or the output signal of the light receiving elements (2), (3). Double balanced photo detector.