JPS612438A - Photodetecting circuit - Google Patents

Abstract

PURPOSE:To constitute a photodetecting circuit low in distortion by detecting a distortion component generated from an avalanche photodiode (APD) and controlling an APD bias so that the distortion component is minimized. CONSTITUTION:An optical signal from an optical fiber F is converted into an electric signal by the APD1. The electric signal is amplified by an amplifier 2 and the amplified signal is inputted to an LPF5 passing a video signal up to fv, a BPF6 for fp and a BPF for 2fp. A pilot signal passing through the BPF6 is converted into a DC signal corresponding to the pilot signal level and a variable gain amplifier 8 is controlled by the DC signal to perform AGC. On the other hand, a secondary harmonic distortion component passed through a band filter 7 is converted into a DC signal corresponding to the distortion level by a detection smoothing part 10 and an APD bias voltage is controlled by a bias control part 3' so that the DC signal is minimized. The variation of a signal output due to the change of the APD bias is compensated by said AGC.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a receiving circuit for optical communications, and particularly to a low distortion optical receiving circuit suitable for signal transmission.

[Background of the invention]

2. Description of the Related Art Conventionally, an avalanche photodiode (hereinafter abbreviated as APD) has often been used as an optical-to-electrical conversion element in a receiver of an optical communication device. FIG. 1 shows the configuration of an optical-to-electrical converter of an optical receiver using an APD. Figure 1(a)
Here, the optical signal input from the optical fiber F is converted into an electrical signal by the APDl. This electrical signal is amplified by an amplifier 2 and sent to a demodulator or the like. In such a receiver, the signal-to-noise ratio of the received signal is closely related to the multiplication factor of the APD, and there is an optimal multiplication factor that provides the best signal-to-noise ratio for a constant received light power. In Figure 1(a), AP
The APD bias is controlled by the D bias control unit 3 so that the APD multiplication factor becomes the optimum multiplication factor.

On the other hand, in the configuration shown in FIG. 1(b), a part of the output of the amplifier 2 is taken out and converted into a DC signal corresponding to the signal level by the detection smoothing section 4. A bias controller 3 controls the bias of APDl so as to keep this DC signal constant. That is, A
Automatic gain adjustment (
Automatic Galn Control (hereinafter abbreviated as AGC) is performed.

However, in these configurations, no consideration is given to receiver distortion. This is because the distortion characteristics of conventional optical communication devices are dominated by the distortion characteristics of the light emitting element. However, in recent years, the distortion of light emitting elements has been reduced, and the distortion of optical transmitters has been significantly reduced compared to conventional ones by methods such as opto-electrical negative feedback. Along with this, the light from the light receiving element -
Distortion during electrical conversion has become a problem. This is a major problem particularly in baseband transmission of wideband signals such as video signals or transmission of frequency multiplexed signals. However, a clear solution to this problem has not yet been found.

[Purpose of the invention]

An object of the present invention is to improve the above-mentioned drawbacks of conventional optical receiving circuits and to provide a low distortion optical receiving circuit suitable for signal transmission.

[Summary of the invention]

FIG. 2 shows the relationship between the second harmonic distortion suppression ratio of the APD and the average received light power using the multiplication factor M as a parameter. As shown in the figure, the distortion generated by APD is closely related to the multiplication factor. In the figure, the difference between the case of M=10 and the case of M=50 is small because the distortion of the light emitting element is dominant. When the strain of the light emitting element is small, the strain characteristics have a dependence on the multiplication factor that is somewhat different from that shown in FIG. However, if the multiplication factor is increased unnecessarily, the distortion characteristics will deteriorate and the influence of shot noise of the APD will increase, which is not preferable. Considering the above points, it can be said that there is a multiplication factor that minimizes the amount of distortion.

As is clear from this, if the bias of the APD is controlled so that the multiplication factor always minimizes distortion, it is possible to construct an optical receiving circuit with much lower distortion than in the past. Further, even when the received light power changes, more stable low-distortion reception is possible by controlling the APD bias so as to minimize the amount of distortion.

In the present invention, the distortion level of the transmission signal is constantly monitored,
By controlling the bias of the APD so as to minimize this, a low distortion optical receiving circuit is constructed.

In particular, as a method for monitoring the amount of distortion, if the distortion component of the pilot signal conventionally used for pilot AGC is to be monitored, the optical receiver can be This is advantageous because low-distortion transmission can be achieved with only improvements.

An example is shown in FIG. (a) is a case of a baseband wideband signal, and (b) is a case of a frequency multiplexed signal. fP
is the pilot signal, and 2fP is the second harmonic distortion of fp. By monitoring this 2fP signal level and controlling the APD bias level to minimize it, a low distortion receiving circuit as described above can be constructed.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in more detail based on examples.

The transmission signal is a video signal as shown in FIG. 3(a). However, it goes without saying that other signals are also possible.

FIG. 4 shows a circuit block configuration of an embodiment of the present invention.

The optical signal from the optical fiber F is converted into an electrical signal by APDI. After this electrical signal is amplified by an amplifier 2, a low-pass filter 5.passes the video signal up to fv. It is input to the fp bandpass fortalo and the 2fp bandpass filter 7. This allows the video signal,
Separate popular beliefs into the pilot signal and the second harmonic distortion component of the pilot signal. The video signal is input to variable gain amplifier 8. The pilot signal that has passed through the fP bandpass filter 6 is converted by the detection smoothing section 9 into a DC signal corresponding to the pilot signal level. This DC signal controls the variable gain amplifier 8 and applies AGC.

On the other hand, the second harmonic distortion component is converted into a DC signal according to the distortion level by the detection smoothing section 10, and the APD bias voltage is controlled by the bias control section 3' so as to minimize this. Fluctuations in signal output due to changes in APD bias are compensated for by the AGC. As described above, according to this embodiment, it is possible to provide a low distortion receiving circuit by simply adding a few partial circuits to the conventional pilot AGC type optical receiving circuit.

Another embodiment according to the invention is shown in FIG.

The incident optical signal from the optical fiber F is converted into an electrical signal by the APDl, and is amplified by the preamplifier 2 to a level required for subsequent signal processing. The amplified electrical signal is separated into signal components by a low-pass filter 5 for video signals, a band-pass filter 6 for pilot signals, and a band-pass filter for second-order harmonic distortion components of pilot signals. The pilot signal is converted into a DC signal according to the signal level by the detection smoothing section 9, and becomes a control signal for the variable gain amplifier 8.

AGC is applied by controlling the variable gain amplifier 8 to keep this DC signal constant.

A portion of the pilot signal is passed through a phase comparator 11° and a low pass filter 12 . DC amplifier 13. voltage controlled oscillator 14,
A phase-locked loop (hereinafter referred to as PL) consisting of a 1/2 frequency divider 15
(abbreviated as L). With this PLL part,
The output of the voltage controlled oscillator 14 is phase-synchronized with the double wave of the pilot signal. This signal is also phase synchronized with the second harmonic distortion component of the pilot signal.

On the other hand, the distortion component separated by the 2fP bandpass filter 7 is amplified by the amplifier 16, and then the voltage controlled oscillator 14
The phase is adjusted by the phase shifter 17 so that it is in the same phase as the output of. The output is input to the phase discrimination detector 18,
Only signal components in phase with the output of the voltage controlled oscillator 14 are extracted. The extracted distortion component is converted into a DC signal corresponding to the distortion level by the detection smoothing section 10, and the APD bias is controlled by the bias control section 3' so as to minimize this.

According to the method of this embodiment, even if the distortion component level is at the same level as noise, the distortion component can be extracted with a good S/N ratio by phase detection, so it is possible to create a low distortion receiving circuit with high noise resistance. It is.

In the above embodiment, the phase shifter 17 is placed on the distortion signal side. Even if this is inserted into the input of the voltage controlled oscillator side of the phase discrimination detector 18, exactly the same effect can be obtained.

Furthermore, the APD multiplication factor that provides the optimum distortion characteristic is significantly different from the optimum multiplication factor for the noise characteristic, and there may be cases in which noise increases extremely when trying to minimize the distortion component. In such cases, if the APD bias variation range is determined within the allowable noise range and the APD bias is controlled to minimize distortion within this variation range, low distortion and low noise optical reception can be achieved. It can be a circuit.

Furthermore, the center frequency of the bandpass filter 7 is changed from 2fP to 3fp, and the 1/2 frequency divider 15 of the PLL section is changed to a 1/3 frequency divider, thereby suppressing third harmonic distortion. It is also possible to do so. Furthermore, an integrating circuit may be included in the PLL in order to improve the S/N ratio of the PLL.

The present invention can be applied to both analog and digital transmission.

〔Effect of the invention〕

As described above, according to the present invention, a low-distortion optical receiving circuit can be constructed by detecting the distortion component generated by the APD and controlling the APD bias so as to minimize it. This is based on the fact that the distortion characteristics of the APD depend on the multiplication factor of the APD, and that there is a multiplication factor that minimizes the distortion.
The PD multiplication factor is changed.

According to this method, it is possible to control the distortion generated by the APD, and at the same time, it is also possible to suppress the distortion of the preamplifier that occurs due to the APD output being too large.

In addition, with this method, distortion characteristics can be improved by simply adding a few parts to the optical receiving circuit, and other optical transmitting parts can be the same as before, making it easy to improve performance. It has the advantage of being able to measure

[Brief explanation of drawings]

Figures 1 (a) and (b) are conventional optical receiver circuits, Figure 2 is a graph showing the dependence of received signal distortion characteristics on APD multiplication factor, and Figure 3 is a graph showing the dependence of received signal distortion characteristics on APD multiplication factor.
The figure is an explanatory diagram showing the principle of the invention, and FIGS. 4 and 5 are circuit block configuration diagrams showing embodiments of the invention. 1...APD, 2...Preamplifier, 3,3'...
APD bias control section, 4...detection smoothing section, 5...
Low-pass filter, 6, 7...Band-pass filter, 8
...Variable gain amplifier, 9, 10...Detection smoothing section, 1
1... Phase comparator, 12... Low pass filter, 1
3... DC amplifier, 14... Voltage controlled oscillator, 15
...Thorax divider, 16...-amplifier, 17... Phase shifter,
18...Phase discrimination detection'% 3 Figure (L) (b)

Claims (1)

[Scope of Claims] 1. In an optical receiving circuit that uses an avalanche photodiode to perform optical-to-electrical conversion of an optically modulated signal and outputs an electrical signal, means for detecting a distortion component in the electrical signal, and the detecting means and means for controlling a multiplication factor of the avalanche photodiode using an output signal from the avalanche photodiode. 2. In the optical receiving circuit according to claim 1,
An optical receiving circuit characterized in that a harmonic component of an AGC pilot signal is detected as a detection signal. 3. In the optical receiving circuit according to claim 1,
An optical receiving circuit characterized in that a signal component phase-synchronized with a harmonic of a transmission signal is detected as a detection signal.