JP2000031914A - Optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical receiver with an excellent frequency characteristic and less fluctuation in group delay. SOLUTION: This receiver is provided with an avalanche photodiode (APD) light receiving element 2 that converts an optical signal into an electric signal, a preamplifier element 3 that amplifies the electric signal outputted from the APD light receiving element 2, a low pass filter 4 that eliminates an undesired high frequency component included in an output of the preamplifier element 3, a main amplifier element 5 that amplifies a signal passing through the low pass filter 4, a level detection circuit 6 that detects an output level of the main amplifier element 5, a voltage setting circuit 7 that sets a voltage applied to the APD light receiving element 2 based on the result of the level detection circuit 6, and a voltage generating circuit 8 that applies the voltage set by the voltage setting circuit 7 to the APD light receiving element 2. The low pass filter 4 has a Bessel characteristic. The voltage setting circuit 7 adjusts the voltage applied to the APD light receiving element 2 so that an amplification factor by the APD light receiving element 2 is constant when the light receiving level of the APD light receiving element 2 is within a prescribed range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal device for converting a received optical signal into an electric signal, and more particularly, to a technique for automatically adjusting a voltage applied to an avalanche photodiode.

[0002]

2. Description of the Related Art In an optical transmission system for transmitting an optical signal through an optical fiber or the like, a light receiving level input to an optical receiver fluctuates due to a difference in an output level of an optical transmitter, a transmission distance, or a change with time. I do. Since a circuit for processing a digital signal is generally connected to the output side of the optical receiver, it is desirable that the optical receiver outputs an electric signal of a constant level regardless of the light receiving level.

For this reason, in order to widen the dynamic range of the optical receiver with respect to the light receiving level, the conventional optical receiver has a main part having an automatic gain control function capable of adjusting the gain according to the light receiving level. It had an amplifying element.

This type of main amplifying element increases the gain when the light receiving level is low, and decreases the gain when the light receiving level is high, thereby deteriorating the S / N ratio of a signal in the main amplifying element. Saturation is prevented and good transmission characteristics are obtained for a wide range of light receiving levels.

FIG. 5 is a block diagram of a conventional optical receiver incorporating such a main amplifying element. The optical receiver in FIG. 5 includes an avalanche photodiode (hereinafter, APD light receiving element) for converting an optical signal into an electric signal, a preamplifier 3 for amplifying the electric signal output from the APD light receiving element 2, and a preamplifier 3. The low-pass filter 4 removes unnecessary high-frequency components included in the output of the preamplifier 3 and a variable gain main amplifying element 5 that adjusts the gain of a signal passed through the low-pass filter 4.

The main amplifying element 5 shown in FIG. 5 controls the output amplitude of the main amplifying element 5 to be substantially constant irrespective of the light receiving level by reducing the gain as the light receiving level increases. That is, the optical receiver of FIG. 5 maintains stable transmission characteristics with respect to fluctuations in the light receiving level due to the gain characteristics of the main amplification element 5.

However, since the dynamic range of the received light level extends over 25 dB or more, it is difficult to keep the output amplitude constant at all times with only the main amplifying element 5, and a sufficient gain is obtained depending on the received light level. However, there is a possibility that the S / N ratio of the signal cannot be sufficiently ensured, or that the output signal is saturated and the output waveform is distorted.

In order to avoid such a problem, an AP
The voltage applied to the D light receiving element 2 is changed to adjust the amplification factor of the main amplifying element 5 so as to minimize the fluctuation of the signal level in the optical receiver, thereby deteriorating the signal S / N ratio and distortion due to saturation. There has been proposed an optical receiver that prevents the above.

[0009]

FIG. 1 is a diagram showing a conventional example of an optical receiver incorporating a circuit for adjusting the voltage applied to the APD light receiving element 2. As shown in FIG. The optical receiver shown in FIG. 1 has, in addition to the configuration shown in FIG. 5, a level detection circuit 6 for detecting the output level of the main amplifying element 5, and a voltage applied to the APD light receiving element 2 based on the detection result by the level detection circuit 6. And a voltage generating circuit 8 for applying the voltage set by the voltage setting circuit 7 to the APD light receiving element 2.

In the optical receiver shown in FIG. 1, a signal input to the APD light receiving element 2 is converted into an electric signal, amplified, and further amplified by a preamplifier 3. Removed. The main amplifying element 5
The gain of the signal passing through the low-pass filter 4 is variably controlled in accordance with the light receiving level, and the output amplitude is adjusted to be as constant as possible.

If the signal level cannot be adjusted by the main amplifying element 5, the output signal level of the main amplifying element 5 is detected by a level detection circuit 6 for detecting the output signal level of the main amplifying element 5, and the detection result is obtained. Based on this, the voltage applied to the APD light receiving element 2 is set by the voltage setting circuit 7.

The light receiving level of the APD light receiving element 2 and the multiplication factor M
Is as shown by the dashed line in FIG. That is, in the conventional optical receiver shown in FIG. 1, as the light receiving level of the APD light receiving element 2 increases, the multiplication factor M is reduced to suppress the fluctuation of the signal level input to the preamplifier 3. With such control, the output of the optical receiver is always controlled to be constant with respect to the light receiving level. Further, the conventional optical receiver maintains a stable transmission characteristic with respect to a large light receiving level by changing the gain between the main amplifying element 5 and the APD light receiving element 2.

On the other hand, the dashed-dotted waveform in FIG. 3 shows the relationship between the multiplication factor M of the APD light-receiving element 2 and the frequency characteristic (cut-off frequency). Is shown. As shown in these figures,
When the multiplication factor M of the APD light receiving element 2 changes, the frequency characteristic changes, and accordingly, the group delay characteristic also changes. For this reason, the signal waveform is deteriorated and the signal waveform is easily affected by high frequency noise.

In order to suppress the change in the cutoff frequency of the APD light receiving element 2, the cutoff characteristic may be sharpened by using a low-pass filter 4 having Butterworth characteristics or Chebyshev characteristics, but such a low-pass filter 4 is used. Then, ripples occur in the group delay in the pass band of the signal, and as a result, the signal waveform deteriorates.

SUMMARY OF THE INVENTION The present invention has been made in view of such a point, and an object of the present invention is to provide an optical receiver having a flat frequency characteristic over a wide band and a small fluctuation of a group delay.

[0016]

In order to solve the above-mentioned problems, the present invention is directed to an avalanche photodiode for outputting an electric signal corresponding to a received optical signal;
An amplifier that amplifies the electric signal, a level detector that detects an output level of the amplifier, and a voltage adjustment circuit that adjusts a voltage applied to the avalanche photodiode based on a detection result of the level detector. With
The voltage adjusting circuit is configured to control a voltage applied to the avalanche photodiode so that a multiplication factor of the avalanche photodiode becomes substantially constant when a light receiving level of the optical signal in the avalanche photodiode is within a predetermined range. To adjust.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical receiver according to the present invention will be specifically described with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the optical receiver according to the present invention.
The optical receiver shown in FIG. 1 has the same block configuration as the conventional optical receiver shown in FIG. 5, and has an APD light receiving element 2 for receiving an optical signal passing through an optical fiber 1 and converting it into an electric signal;
A preamplifier 3 for amplifying an electric signal output from the D light receiving element 2, a low-pass filter 4 for removing unnecessary high-frequency components included in the output of the preamplifier 3, and a signal passed through the low-pass filter 4 A main amplifying element 5 for amplifying, and a level detecting circuit 6 for detecting an output level of the main amplifying element 5
And APD based on the detection result by the level detection circuit 6.
Voltage setting circuit 7 for setting the voltage applied to light receiving element 2
And the voltage set by the voltage setting circuit 7
And a voltage generation circuit 8 for applying the voltage to

The low-pass filter 4 in FIG. 1 has a gradual characteristic close to the Bessel characteristic so that ripple (variation) does not occur in the group delay in the pass band of the signal. Voltage setting circuit 7
Sets the voltage applied to the APD light receiving element 2 such that the multiplication factor M of the APD light receiving element 2 becomes substantially constant when the frequency of the optical signal is within a predetermined range, as described in detail later.

However, if a low-pass filter 4 having a characteristic close to the Bessel characteristic is used, the frequency characteristic of the APD light receiving element 2 appears as the frequency characteristic of the optical receiver. Therefore, with the conventional configuration, as shown by the one-dot chain line in FIG. 3, as the light reception level increases, the multiplication factor M decreases and the cutoff frequency increases. When the light reception level exceeds a certain value, the multiplication factor M increases. At around 10, the cutoff frequency begins to decrease. When the cutoff frequency increases and the peaking increases, the group delay characteristic changes greatly, causing waveform deterioration.

Therefore, in the present embodiment, the voltage applied to the APD light receiving element 2 is adjusted by the voltage setting circuit 7 so that the cutoff frequency becomes flat and the fluctuation of the group delay is reduced.

2 shows the relationship between the light receiving level of the APD light receiving element 2 of FIG. 1 and the multiplication factor M. The solid line waveform of FIG. 3 shows the light receiving level and the cutoff frequency of the APD light receiving element 2 of FIG. fc is shown.

As shown by the solid line waveform in FIG. 2, in this embodiment, when the light receiving level of the APD light receiving element 2 gradually increases from La which is close to the lowest level, the multiplication factor M is gradually reduced accordingly. Then, when the light receiving level reaches Lb, the multiplication factor M is sharply reduced, and the light receiving level becomes Lb to Lc.
During the period, the multiplication factor M is kept almost constant. When the light reception level exceeds Lc, the multiplication factor M is sharply reduced again, and thereafter, the multiplication factor M is gradually reduced in accordance with the decrease in the light reception level.

The range in which the multiplication factor M is desired to be kept constant (the light receiving levels Lb to Lc in FIG. 2) differs depending on the requirements of the system. For example, the range can be set and changed by the voltage setting circuit 7 in a programmable manner. Is also good.

On the other hand, the cutoff frequency of the APD light receiving element 2 is
As shown by the solid waveform in FIG. 3, the multiplication factor M is changed from Ma to Mbc.
Gradually increases while the multiplication factor M decreases from Mbc to Md. In FIG. 3, the multiplication factor M is the cutoff frequency of Ma, Mbc, and Md, respectively, fc
(A), fc (BC) and fc (D). Where fc (A), f
c (D) <fc (BC).

FIG. 4 is a diagram showing the frequency characteristics and the group delay characteristics of the optical receiver of the present embodiment. The solid line waveform shows the characteristics of the present embodiment, and the dotted line shows the conventional characteristics. As is clear from FIG. 4, the optical receiver of the present embodiment can suppress a change in cutoff frequency and a change in group delay as compared with the related art.

As described above, in the present embodiment, the low-pass filter 4 having the Bessel characteristic is used and the AP
APD when the light receiving level of D light receiving element 2 is within a predetermined range
By adjusting the applied voltage of the APD light receiving element 2 so that the multiplication factor M of the light receiving element 2 becomes substantially constant, the fluctuation of the cutoff frequency and the group delay over a wide dynamic range is reduced. As a result, deterioration of the output waveform of the optical receiver is reduced, and good transmission characteristics with a wide dynamic range are obtained.

The low-pass filter 4 may have a characteristic close to the Bessel characteristic in addition to the one having the Bessel characteristic itself.

As described above, the APD light receiving element 2
Is preferably set in accordance with the configuration of the optical transmission system and the like.

[0029]

As described above in detail, according to the present invention, when the light receiving level of the optical signal in the avalanche photodiode is within a predetermined range, the avalanche photodiode is used.
Since the multiplication factor of the photodiode is made substantially constant, it is possible to suppress the fluctuation of the cutoff frequency of the optical receiver, flatten the frequency characteristics over a wide band, and also suppress the fluctuation of the group delay.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of an optical receiver according to the present invention.

FIG. 2 is a diagram showing a relationship between a light receiving level of an APD light receiving element and a multiplication factor in the embodiment.

FIG. 3 is a diagram showing a relationship between a multiplication factor of an APD light receiving element and a cutoff frequency.

FIG. 4 is a diagram showing frequency characteristics and group delay characteristics of the optical receiver according to the embodiment.

FIG. 5 is a block diagram of a conventional optical receiver including a main amplifying element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber 2 APD light receiving element 3 Preamplifier 4 Low-pass filter 5 Main amplifier 6 Level detection circuit 7 Voltage setting circuit 8 Voltage generation circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/28 10/26

Claims (3)

[Claims] An avalanche photodiode for outputting an electric signal corresponding to a received optical signal; an amplifier for amplifying the electric signal; a level detector for detecting an output level of the amplifier; A voltage adjustment circuit that adjusts a voltage applied to the avalanche photodiode based on the detection result, wherein the voltage adjustment circuit is provided when the light reception level of the optical signal in the avalanche photodiode is within a predetermined range. Wherein the voltage applied to the avalanche photodiode is adjusted so that the multiplication factor of the avalanche photodiode becomes substantially constant. 2. The voltage adjustment circuit according to claim 1, wherein
The voltage applied to the avalanche photodiode is adjusted such that, when the light receiving level of the optical signal in the photodiode is out of the predetermined range, the gain increases as the light receiving level increases. An optical receiver as described. 3. An amplifier, comprising: a preamplifier for amplifying the electric signal; a low-pass filter for removing unnecessary high-frequency components included in an output of the pre-amplifier; and an amplifier for amplifying a signal passed through the low-pass filter. A low-pass filter having Bessel characteristics or a characteristic close thereto, and the level detector detects an output level of the main amplification device. An optical receiver according to claim 1.