JPH0542183B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control circuit for a photodetection receiver.

Optical heterodyne detection receivers perform signal detection by combining received signal light and local oscillation light. Since detection is possible, it is advantageous for extending the distance of optical fiber communication systems. Generally, in optical receivers,
In order to ensure the quality of the demodulated signal over a wide range of received signal light levels, an automatic gain control circuit is required to compensate for fluctuations in the received signal light level and to always maintain the output signal level of the receiving amplifier circuit at a predetermined value. be. Conventionally, in optical direct detection receivers, there are methods to control the amplification degree of the reception amplifier circuit, methods to use an avalanche photodiode (APD) as a photodetector and control the amplification factor of this APD, and methods to use both of the above methods together. etc. are used. Although the above method can be used in an optical heterodyne detection receiver, it is almost impossible to use the above method. In other words, in an optical heterodyne detection receiver, an APD is installed in the photodetector in order to increase the level of local oscillation light and prevent sensitivity deterioration due to dark current noise in the photodetector, thermal noise in the receiving amplifier circuit, etc. Even when used, the multiplication factor of the APD reaches its maximum value at 1 or a small value close to 1, at which the optical reception sensitivity is the highest. For this reason, the conventional optical heterodyne detection receiver has a drawback that the control range of the automatic gain control circuit, that is, the permissible level (dynamic range) of the received light is narrow.

An object of the present invention is to provide an optical heterodyne detection receiving apparatus in which an automatic gain control circuit has a wide control range.

The optical detection receiver of the present invention includes a local oscillation light source, an optical multiplexer that multiplexes the output of the local oscillation light source and signal light, a photodetector into which light from the optical multiplexer is input, and a photodetector that receives the light from the optical multiplexer. In a photodetection receiving device comprising a receiving amplifying circuit that amplifies the output signal of a photodetector and a processing circuit that processes the output signal of the receiving amplifying circuit, the level of the output signal of the receiving amplifying circuit is always at a predetermined value. The present invention is characterized in that it includes a control device that controls the level of the output light of the local oscillation light source that is input to the photodetector so that the level of the light output from the local oscillation light source is input to the photodetector.

In optical heterodyne detection, the level (amplitude) of the photodetector output is proportional to _√L . However, the multiplication factor of the photodetector is 1, _I3 is the photocurrent of the signal light and the photodetector output, and IL is the photocurrent of the photodetector output due to the locally oscillated light.

In the photodetection receiving device of the present invention, IL is controlled so that IsIL becomes a constant value by utilizing the above relationship that the level of the photodetector output is proportional to _√L .
In other words, as Is becomes smaller, IL is increased so that √ _L becomes a constant value, and when Is becomes larger, _IL is decreased, so the level of the output signal of the receiving amplifier circuit can always be controlled to a predetermined value. . Therefore,
If the signal is a pulse, the level of the pulse input to the discriminator does not change even if the level of the received light increases, so the permissible level range of the received light is wider than in the conventional case where the level of the local oscillation light is not controlled. Can be done widely.

Note that the case where control of the amplification degree of the reception amplifier circuit is also used will be explained in more detail below. The signal-to-noise ratio in optical heterodyne detection is approximately proportional to the S/N ratio of equation (1) below.

S/N=IsI _L /2eI _L B+K _L I ² _L B+it ² B (1) However, K _L is a parameter representing the noise of the local oscillation light, ² is a parameter representing the thermal noise of the receiving amplifier circuit, and e is the electron charge. , B is the noise bandwidth of the receiving amplifier circuit, the numerator of equation (1) is the signal power, the first term on the right side of the denominator of equation (1) is the shot noise power due to the local oscillation light, and the second term is the shot noise power of the local oscillation light. The third term of the noise power represents the thermal noise power of the receiving amplifier circuit. From equation (1), the optimal value of I _L that maximizes the S/N _is found, becomes. When I _L > I _LO , K _L I _L ² is large and the S/N decreases. On the other hand, when I _L <I _LO , IsI _L becomes small and the influence of ² becomes relatively large, resulting in a decrease in S/N. Therefore, in a small range of Is, it is better to fix _IL to the optimum value _ILO without controlling _IL .
In the optical detection receiving device of the present invention, which uses both the level control of the local oscillation light and the control of the amplification degree of the receiving amplifier circuit, I _L is fixed to a value approximately close to I _LO in a small Is range, and By controlling the amplification degree of the receiving amplifier circuit, the level of the output of the receiving amplifier circuit is kept constant by changing Is. In a range where Is becomes large and the output level cannot be controlled by the reception amplifier circuit, the output level of the reception amplifier circuit is kept constant by controlling _IL . Note that in a large Is range, the S/N is sufficiently large, so by controlling I _L , the S/N is
Even if N decreases, it is sufficiently large compared to the required S/N value and no problem will occur.

In the optical detection receiving device of the present invention, fluctuations in the output level of the receiving amplifier circuit due to fluctuations in Is are compensated for by controlling I _L in addition to controlling the amplification degree of the receiving amplifier circuit, so that the permissible level of the signal light is Wide range (dynamic range).

Next, the present invention will be described by way of examples with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, and shows an example in which a control device for controlling the level of local oscillation light is composed of a peak value detection circuit, a comparison circuit, and an optical controller. be. Signal light 1 with a wavelength of 1.3 μm is input as a parallel beam to optical multiplexer 2, where it is multiplexed with local oscillation light 5 from local oscillation light source 3, condensed by lens 6, and then input to photodetector 7. are doing. The signal light 1 and the local oscillation light 5 are adjusted so that their polarization planes and wavefronts match on the light receiving surface of the photodetector 7. Signal light 1 has a bit rate of 32Mb/s
ASK amplitude modulated with a binary pulse signal of
(Amplitude Shift Keying) signal. The local oscillation light source 3 is composed of an InGaAsP semiconductor laser, and the frequency of the local oscillation light 5 is controlled to be 320 MHz higher than that of the signal light 1. The photodetector 7 uses a photodiode (PD) made of InGaAs material. Signal light 1
and the local oscillation light 5 are optically heterodyne detected by the photodetector 7, and are 32Mb/s with an intermediate frequency of 320MHz.
The signal is converted into an ASK signal and guided to the reception amplifier circuit 8. In the reception amplification circuit 8, the output signal of the photodetector 7 is amplified by an intermediate frequency amplifier 21, then envelope detected by an envelope detector 22, and further amplified by a baseband amplifier 23 to generate a 32 Mb/s binary pulse. It is output as a signal. The output signal of the reception amplifier circuit 8 is divided into two parts, one of which is sent to the timing extractor 24.
and a discriminator 25,
Here, the signal is identified and reproduced and a signal is output to the output terminal 10. The other output signal of the reception amplifier circuit 8 is
It is guided to the peak value detection circuit 11, where 32Mb/
The peak value of the s pulse signal is detected and the comparison circuit 12
is guided by. The comparison circuit 12 compares the output signal of the peak value detection circuit 11 with a reference value, and determines that the output signal of the peak value detection circuit 11 has a certain predetermined value, that is, the pulse peak value of the output signal of the reception amplifier circuit 8. A first control signal 13 is output that controls the amplification degree of the reception amplifier circuit 8 so that it becomes a certain constant value. The comparison circuit 12 is equipped with a circuit that sets a threshold for the first control signal 13, and when the first control signal 13 does not exceed the threshold, the first control signal 13 is supplied as is to the receiving amplifier circuit 8. be done. Note that the first control signal 13 is transmitted to the receiving amplifier circuit 8.
intermediate frequency amplifier 21 and baseband amplifier 23
The degree of amplification is controlled. If the first control signal 13 exceeds the threshold, the first control signal 1
3 is fixed at the threshold value, and the comparison circuit 12 outputs the second control signal 14. The threshold value serves to limit the control range of the amplification degree of the reception amplifier circuit 8, and prevents signal distortion in the reception amplifier circuit 8. Note that the second control signal 14 is not output within a range in which the first control signal 13 does not exceed the threshold value. An optical controller 4 is inserted between the local oscillation light source 3 and the optical multiplexer 2, and this optical controller 4 uses a second control signal 14 as an input signal to attenuate the level of the local oscillation light 5. Control with. Light controller 4 is LiNbO ₃
This is a type of optical modulator that utilizes a waveguide-type electro-optic effect made of a material of When this happens, the amount of optical attenuation is increased depending on the magnitude, and the level of the locally oscillated light 5 is lowered. In this embodiment, the minimum optical reception level of the signal light 1 at which the bit error rate of the signal outputted to the output terminal 10 is 10 ^-9 or less was -55 dBm in pulse peak value. In the range where the signal light 1 was -35 dBm or less, the amplification degree of the receiving amplifier circuit 8 was changed, and the level of the output signal was controlled to a constant value with a pulse amplitude of 0.8 V. When the signal light 1 becomes -35dBm or more, the first control signal 1
3 exceeds the threshold, the amplification degree of the reception amplifier circuit 8 is fixed to the value when the signal light 1 is -35 dBm, and the second control signal 14 is output from the comparison circuit 12. The optical controller 4 can attenuate the level of the local oscillation light 5 by a maximum of 15 dB, so if the signal light 1 is -35 dBm or more, -
In the range of 20 dBm or less, the optical controller 4 adjusts the level of the output signal of the receiving amplifier circuit 8 by adjusting the pulse amplitude.
I was able to control it to a constant value of 0.8V. That is, the dynamic range of signal light 1 was 35 dB, assuming a bit error rate of 10 ^-9 . On the other hand, the dynamic range of a conventional optical heterodyne detection receiver that controls only the amplification degree of the receiver amplifier circuit 8 is
Since it is 20 dB, the improvement in dynamic range by the optical heterodyne detection receiver of this example is
It is large at 5dB.

In the above embodiment, the signal light 1 was an ASK signal that was amplitude modulated with a 32 Mb/s binary pulse signal, but it may also be a pulse phase modulated signal or a pulse frequency modulated signal, or the bit rate may also be changed. Other values are also acceptable. In the case of a pulse phase modulated signal, an appropriate detector may be used depending on the type of signal, such as using a differential delay detector instead of the envelope detector 22. Further, in the above embodiment, the input signal of the comparator 12 is obtained from the output signal of the receiving amplifier circuit 8, but the intermediate frequency amplifier circuit 2 of the receiving amplifier circuit 8
The output level of the intermediate frequency amplification circuit 21 may be controlled so that the output level of the intermediate frequency amplifier circuit 21 becomes a predetermined value. Further, in the above embodiment, the intermediate frequency is set to 320 MHz, but the intermediate frequency may have another value as long as it is possible to demodulate the signal. Further, the wavelength of the signal light 1 and the local oscillation light 5 may also be other wavelengths than 1.3 μm, such as 1.55 μm. In addition, in the above embodiment, the optical controller 4 is made of LiNbO ₃
Although a type of optical modulator that utilizes a waveguide-type electro-optic effect is used, other materials such as LiTaO ₃ may be used, or a bulk structure other than a waveguide structure may be used. Furthermore, it may be an optical modulator that utilizes a magneto-optic effect made of a material such as YIG, or a variable optical attenuator or mechanical variable optical attenuator that uses a material such as liquid crystal. Good too.

Further, although the above embodiment uses optical heterodyne detection using an intermediate frequency, it may be optical homodyne detection that does not use an intermediate frequency. In this case, a baseband amplification circuit is used in the reception amplification circuit 8 instead of the intermediate frequency amplifier 21.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. In the figure, 1... Signal light, 2... Optical multiplexer, 3... Local oscillation light source, 4... Optical controller, 5... Local oscillation light, 6
... Lens, 7 ... Photodetector, 8 ... Reception amplifier circuit, 9 ... Processing circuit, 10 ... Output terminal, 11 ...
... Peak value detection circuit, 12 ... Comparison circuit, 13, 1
4...Control signal.

Claims (1)

[Claims] 1. A local oscillation light source, an optical multiplexer that multiplexes the output light of the local oscillation light source and signal light, a photodetector into which light from the optical multiplexer is input, and a photodetector that receives the light from the optical multiplexer. In a photodetection receiving device comprising a receiving amplifying circuit that amplifies a signal output from a detector and a processing circuit that processes an output signal of the receiving amplifying circuit, the level of the output signal of the receiving amplifying circuit is always at a predetermined value. A photodetection receiving device comprising: a control device for controlling a level of output light from the local oscillation light source input to the photodetector so that the output light level is input to the photodetector. 2. The control device comprises a peak value detection circuit, and an optical controller connected to the peak value detection circuit and installed in the optical path of the locally oscillated light. The optical detection receiving device according to item 1. 3. The control device includes an optical controller installed in the optical path of the local oscillation light, a peak value detection circuit, and a signal for controlling the amplification degree of the reception amplifier circuit based on a signal from the peak value detection circuit. 2. The optical detection receiving apparatus according to claim 1, further comprising a circuit for generating a signal for controlling the optical controller.