JPS6328519B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical output stabilizing circuit when a semiconductor laser is used as a light source in an optical communication system.

Semiconductor lasers are used as light sources in optical communication systems due to their small size, high efficiency, high output, and high-speed response, but their threshold current is greatly affected by temperature, making their optical output unstable. For this reason, conventional optical output stabilization circuits detect a part of the optical output of a semiconductor laser (hereinafter referred to as monitor light) using a photoelectric conversion element such as a photodiode, and feed back changes in this detected output negatively to a bias current. It has been known.

This circuit is shown in FIG. 1, and its operation will be briefly described. An electrical signal inputted from a terminal 7 is converted into a voltage-current by a pulse drive circuit 1, and is superimposed with a bias current in a current superimposition circuit 2. This current drives the semiconductor laser 3 to obtain optical output at the terminal 8. On the other hand, the monitor light output from the terminal 3-1 is converted into a current by the photoelectric conversion element 4. After the signal component is detected by the signal component detection circuit 5 (or after the average value is detected by the average value detection circuit), this current is compared with a reference voltage and amplified, and the bias current is controlled to produce an optical signal. Keep output constant.

In this circuit, the signal pulse current is constant and the bias current is controlled to stabilize the optical output, so when the temperature rises or the semiconductor laser deteriorates, the threshold current of the semiconductor laser increases. However, the optical output when the signal pulse is "0" increases. This is equivalent to an increase in backlight for the optical receiver, and deteriorates the light receiving characteristics of the optical receiver, so that extra optical input power of the optical receiver is required to obtain the same characteristics. This extra required optical power (hereinafter referred to as power penalty) is determined by the ratio of the optical output at "1" and the optical output at "0" (hereinafter referred to as extinction ratio), and as the extinction ratio decreases, , the power penalty increases.

FIG. 2 shows an example (calculated value) of the power penalty with respect to the extinction ratio when the optical output pulse occupancy is 50% and an avalanche photodiode with an excess noise figure of 0.3 is used as the light receiving element. The horizontal axis is the extinction ratio, and the extinction ratio γ is expressed by the following equation.

γ=10logP ₁ /P ₀ (dB) Here, P ₁ is the optical output when it is “1”, and P ₀
is the optical output when it is "0". The vertical axis shows the power penalty of the optical receiver when the extinction ratio is ∞, and in this example, when the extinction ratio decreases from 20 dB to 13 dB, the power penalty increases by about 1 dB.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical output stabilizing circuit that can improve the conventional drawbacks as described above and prevent deterioration of the light receiving characteristics of an optical receiver due to a reduction in the extinction ratio of the optical output of a semiconductor laser.

The optical output stabilizing circuit of the present invention detects a signal component of monitor light from the semiconductor laser in an optical output stabilizing circuit that drives a semiconductor laser with a superimposed current of a signal pulse current and a bias current, and outputs a detected signal component. The bias current is controlled to keep constant
The present invention is characterized in that the average value of the monitor light is detected, and the signal pulse current is controlled so as to keep the difference between the detected output and the average value of the electric signal pulse constant.

Examples will be described in detail below.

FIG. 3 is a block diagram of an embodiment of the present invention;
1 is a pulse drive circuit, 2 is a current superimposition circuit, 3 is a semiconductor laser, 4 is a photoelectric conversion element, 5 is a signal component detection circuit, 6 is a bias current control circuit, 7 is an electric signal input terminal, 8 is an optical signal output terminal , 9 is an average value detection circuit, 10 is an average value detection circuit, 11 is a signal pulse current control circuit, and 12 is a DC bias voltage.

The electrical signal input to the terminal 7 is subjected to voltage-to-current conversion in the pulse drive circuit 1, and is superimposed on the bias current of the semiconductor laser 3 in the current superimposition circuit 2. This current drives the semiconductor laser 3 to obtain optical output at the terminal 8. On the other hand, the monitor light from the semiconductor laser outputted from the terminal 3-1 is converted into a current by the photoelectric conversion element 4. This current is the signal component detection circuit 5
After the signal component is detected by the bias current control circuit 6, it is compared with a reference voltage and amplified, thereby controlling the bias current. If the monitor light output to the terminal 3-1 is large, the bias current is controlled to decrease, and if it is small, the bias current is controlled to increase, thereby stabilizing the optical output of the semiconductor laser. On the other hand, the average value of the current electrically converted by the photoelectric conversion element 4 is detected by the average value detection circuit 9 and is applied to the signal pulse current control circuit 11.
The signal pulse current control circuit 11 compares the average value detection voltage with the signal voltage passed through the average value detection circuit 10 and to which the DC bias voltage 12 is added, and then amplifies it. If the detection output decreases, the signal pulse current is controlled to increase, and if the detection output decreases, the signal pulse current is decreased to keep the extinction ratio constant. In other words, the detection output of the average value detection output circuit 9 is the average value of the optical output when the optical output is "1" and the optical output when the optical output is "0", and the detection output of the average value detection circuit 10 is "1". The average value of the electric signal at the time of , and the DC bias voltage 12 correspond to the average value of the electric signal at the time of "0", respectively, so that the optical output at the time of "0" is kept at a constant amount corresponding to the DC bias voltage 12. Controls signal pulse current.

As explained above, since the present invention maintains the extinction ratio constant while keeping the optical output of the semiconductor laser constant, the receiving characteristics of the optical receiver can be maintained even when the temperature rises or the semiconductor laser deteriorates. It is possible to transmit signals without deteriorating them.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an example of a conventional optical output stabilization circuit, Figure 2 is a characteristic diagram showing an example (calculated value) of optical receiver characteristic deterioration (power penalty) with respect to extinction ratio, and Figure 3 is 1 is a block diagram showing an embodiment of a light output stabilizing circuit according to the present invention. FIG. 1...Pulse drive circuit, 2...Current superimposition circuit,
3... Semiconductor laser, 4... Photoelectric conversion element, 5...
... Signal component detection circuit, 6 ... Bias current control circuit, 7 ... Electric signal input terminal, 8 ... Optical signal output terminal, 9 ... Average value detection circuit, 10 ... Average value detection circuit, 11 ... Signal Pulse current control circuit, 12
...DC bias voltage.

Claims (1)

[Claims] 1. In an optical output stabilization circuit that drives a semiconductor laser with a superimposed current of a signal pulse current and a bias current, the signal component of the monitor light from the semiconductor laser is detected, and the bias current is adjusted to keep the detected output constant. 1. A light output stabilizing circuit comprising: controlling the monitor light, detecting an average value of the monitor light, and controlling a signal pulse current so as to maintain a constant difference between the detected output and the average value of the electric signal pulse.