JPH0337871B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical output stabilizing device for stabilizing the optical output of a semiconductor laser.

[Technical background of the invention and its problems]

The oscillation threshold and differential quantization efficiency of semiconductor lasers vary greatly depending on temperature. Therefore, in order to obtain a constant optical output pulse from the semiconductor laser, it is necessary to optimally control the bias current Ib and the amplitude Ip of the drive pulse depending on the temperature of the semiconductor laser.

In order to cope with this, the optical output from the semiconductor laser (LD) is received by a monitoring photodiode (PD), and the bias current Ib of the LD or the amplitude value Ip of the driving pulse is controlled according to the amount of received light. Various output stabilizing devices have been developed.

As a light output stabilizing device that does not require a high-speed monitoring PD, one that controls the bias current Ib based on the average value of optical pulses received by the PD, for example, is known. As shown in FIG. 5, this device, for example, changes the bias current to Ib1ave at temperature T1.
The bias current is controlled to Ib2ave at temperature T2.
It is intended to be controlled.

However, this device is limited by the amplitude value of the driving pulse.
Since Ip is not controlled according to temperature, if the pulse amplitude Ip is selected to be optimal at temperature T2 (Fig. 5 I), the "0" level of the drive pulse will exceed the oscillation threshold Ith1 at temperature T1. Significantly lower, 5th
As shown in the figure, the light emission delay becomes large. As a result, the optical pulse becomes narrower and the reliability of pulse transmission decreases. Since the amount of light emission delay is determined by the relationship between the pulse current value at the "0" level and the threshold value, the above problem becomes more serious as the pulse transmission speed increases, and becomes a major problem in high-speed pulse transmission.

Conversely, if the pulse amplitude value is determined at temperature T1 (Fig. 5), at temperature T2, as shown in Fig. 5, the optical output does not become zero even if the drive pulse is at the "0" level, and the extinction ratio deteriorates. occurs, and the reception S/N
There was a problem that it caused deterioration.

Therefore, a device has been proposed that controls the amplitude value Ip of the drive pulse as well as the bias current Ib.

The system shown in FIG. 6 is a system in which the peak value of the optical output is distributed to the bias current Ib and the pulse amplitude Ip at a constant ratio. That is, the optical output from the LD11 is received by the monitor PD12, and the preamplifier 1
3 and clamped by the clamp circuit 14,
The peak value is detected by the peak detection circuit 15.
The distribution circuit 16 then multiplies the peak value by a predetermined coefficient to obtain Ib and Ip, and distributes Ib to the variable bias current source 17 and Ip to the pulse current amplitude adjuster 18, respectively.

This device can optimally control Ip and Ib as long as there is no deterioration in the characteristics of the semiconductor laser, but it has the drawback that if the ratio of Ip and Ib changes due to element deterioration, it becomes no longer controllable. .

Moreover, what is shown in FIG. 7 is an upper peak detector 21 that detects the peak value when the optical output is at the "1" level, and a lower peak detector 21 that detects the peak value when the optical output is at the "0" level. A detector 22 is provided, and the respective peak values are compared by comparators 23 and 24, and the upper peak value controls the pulse amplitude Ip, and the lower peak value controls the bias current Ib. be.

However, with this device, it is difficult to detect errors when the optical output is "0", and the extinction ratio cannot be increased.

In addition, in the field of optical communications, transmission rates are several hundred MHz.
As mentioned above, the speed is becoming extremely high, and in order to make the above two devices compatible with this, two points are required: the monitoring PD must be able to operate at high speed, and the peak value can be detected at high speed.

In order to operate a monitoring PD at high speed, it is necessary to reduce the PD's light receiving diameter and capacitance. However, if we reduce PD,
The optical output from the LD cannot be detected sufficiently,
Not only does this result in a decrease in the received light level, but also distortion in the pulse waveform occurs when only a portion of the light emitted from the LD is received. For this reason, there was a limit to increasing the speed of PD.

Furthermore, since the technology for detecting a specific peak point at high speed and with high precision is sophisticated, there are currently problems in terms of circuit scale and cost.

As described above, the conventional optical output stabilizing device has a problem in that it is extremely difficult to stabilize the optical output during high-speed transmission.

[Purpose of the invention]

The present invention was made based on these problems, and it is possible to control the optical output of a semiconductor laser with high precision in response to temperature fluctuations without requiring high-speed PD or peak hold technology, resulting in an extremely stable system. An object of the present invention is to provide a light output stabilizing device that can obtain light output.

[Summary of the invention]

The present invention combines the output of a monitoring photodetector that receives optical output from a semiconductor laser and a signal obtained by band-limiting the input pulse with a low-pass filter having approximately the same transfer characteristics as the monitoring photodetector. the bias current of the semiconductor laser is controlled by the integral value of this error signal, and the pulse amplitude of the driving pulse is controlled by the integral value of the product of the error signal and the weight at which the input pulse is present. It is characterized by the fact that it is made to do so.

〔Effect of the invention〕

According to the present invention, an error signal is obtained by taking the difference between the output of a monitoring photodetector and an input pulse passed through a low-pass filter having approximately the same transfer characteristic as that of the monitoring photodetector. , based on this error signal
Since Ib and Ip are controlled, even if the bandwidth of the monitoring photodetector is somewhat narrow, the error signal will not be affected by the bandwidth limitation. Therefore, according to the present invention, even when a low-speed operating photodetector is used, no deterioration in control performance is caused.

In addition, in the present invention, the bias current Ib and the pulse amplitude Ip are controlled by the integral value of the error signal, and there is no need to sample a specific point as in the past, thereby avoiding complication of the circuit. Furthermore, since Ib and Ip are detected individually by the first and second integrators, highly accurate optical output control is possible.

[Embodiments of the invention]

Hereinafter, details of the present invention will be explained with reference to one embodiment.

FIG. 1 is a diagram showing the configuration of a light output stabilizing device according to this embodiment.

The pulse current amplitude control regulator 31 input pulse Ip
A driving pulse is generated by controlling the amplitude Ip of . Further, variable bias current source 32 controls bias current Ib. The LD 33 is supplied with these bias currents and drive pulses and outputs light. The light output is
The light is received by the monitor PD 34. PD34 for monitor
The output is amplified to a predetermined level by a preamplifier 35 and then applied to one input of a subtracter 36. On the other hand, the input pulse IP is amplified to a predetermined level by an amplifier 37, band-limited by a low pass filter (LPF) 38, and given to the other input of the subtracter 36 as a comparison signal.

The LPF 38 is set to have approximately the same transfer characteristics as the monitor PD 34 and the preamplifier 35.
This is to eliminate the influence of errors caused by differences between the two signal waveforms to be compared. In addition, the amplifiers 35 and 37 are connected so that the phases of the two signals are exactly matched.
At least one of them has been adjusted. Subtractor 36
The error signal from is integrated in the first integrator 39 and detected here as the average value of the error signal. This integrated value is then compared with the reference voltage Ref1 in the comparator 43, and is fed back as a control input to the variable bias current source 32 only when it exceeds the reference voltage Ref1. On the other hand, the error signal from the subtracter 36 is also given to one input of the multiplier 40. The other input of this multiplier 40 has the input pulse IP
A signal delayed by a delay circuit 41 is provided. The delay circuit 41 takes into account that the error signal from the subtracter 36 is slightly delayed from the input pulse IP, and delays the phase of the input pulse IP by that amount. The output of the multiplier 40 is equivalent to multiplying only the portion of the error signal where the pulse exists by a weighted function, and is
It is possible to obtain a signal in which the error signal at the level is emphasized. The output of this multiplier 40 is integrated by a second integrator 42. This integrated value is compared with the reference voltage Ref2 in the comparator 44, and is fed back as a control signal to the pulse current amplitude regulator 31 only when it exceeds the reference voltage Ref2. Note that in this example, the time constant of the integrator 39 is set smaller than the time constant of the integrator 42.

As shown in FIG. 2, the optical output stabilizing device configured in this way is stabilized at a bias current of Ith1 and a pulse amplitude of Ip1 when the temperature is T1.
When the temperature is T2, the bias current is stabilized at Ith2 and the pulse amplitude is stabilized at Ip2, and as a result, the optical output can be stabilized so that both the pulse width and pulse amplitude are constant as shown in the figure.

Figure 3 shows the temperature T1 of the IL characteristic shown in Figure 2.
Figure 4 shows the process of stabilization immediately after the state changes from T2 to T2.
Immediately after changing from the state to T1, the process toward stabilization is shown.

In the case of Figure 3, both Ib and Ip are initially low, and LD33
The light output of will be smaller. As the light output decreases,
Since the amount of light received by the monitor PD 34 also decreases, the output of the preamplifier 35 also decreases as shown.
Note that since the monitor PD 34 operates at a low speed, the output of the preamplifier 35 is also slow. Furthermore, the output of the LPF 38 is also band-limited and becomes dull. Initially, the difference between these two signals, ie the output of the subtracter 36, is large. Therefore, the output of the first integrator 39 also rises relatively rapidly at the beginning. The variable bias current source 32 receives this integrated value and gradually increases the bias current Ib as shown by the dotted line in the figure. This increase in bias current Ib continues until it reaches the oscillation threshold Ith2 at temperature T2.

On the other hand, since the output of the delay circuit 41 is in phase with the output of the subtracter 36, the output of the multiplier 40 has a waveform that emphasizes the portion of the output of the subtracter 36 where the pulse is exactly present. This waveform is initially a pulse waveform with a large pulse amplitude, and the integrator 42
The output also increases relatively rapidly at the beginning, and the degree of increase gradually becomes smaller. Note that the second integrator 4
Since the time constant of 2 is larger than the time constant of the first integrator 39, the bias current Ib reaches the stable point first, and the shortfall is compensated for by the pulse amplitude Ip. In this way the light output is stabilized.

In the case of FIG. 4, as the temperature decreases, the light emission level increases and the pulse amplitude also increases, so the optical output is initially large both in amplitude value and zero level. Therefore, in this case, since the output of the subtracter 35 becomes negative, the output of the integrator 39 gradually decreases, reducing the bias current Ib. Also, integrator 4
Since the output of No. 2 also gradually decreases, the pulse amplitude Ip also decreases. In this way the light output is stabilized.

In this way, according to this embodiment, high-speed monitoring
without using PD and peak hold circuit.
Optical output can be stabilized. Note that the present invention is not limited to the embodiments described above.

For example, the time constant of the first integrator may be made larger than the time constant of the second integrator, and after the pulse amplitude Ip is controlled first, the bias current Ib may be controlled. By varying the time constants in this way, it is possible to quickly converge to a stable point.

Furthermore, the same operation can be obtained by replacing the first and second integrators with low-pass filters whose cut-off frequency is lower than that of the input pulse. Even when the two integrators are replaced with low-pass filters, the above-mentioned effect can be obtained by making their cutting frequencies different.

As described above, the present invention can be implemented with various modifications without departing from the gist thereof.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an optical output stabilizing device according to an embodiment of the present invention, FIG. 2 is a diagram showing the relationship between the drive pulse waveform of the device, bias current, and optical output, and FIG. 3 4 is a waveform diagram of each part to explain the operation of the device, FIG. 5 is a diagram to explain the drawbacks of the conventional device, and FIGS.
The figures are block diagrams showing conventional optical output stabilizing devices. 11, 33... Semiconductor laser, 12, 34...
Monitoring photodiode, 13, 35... Preamplifier, 14... Clamp circuit, 15... Peak detection circuit, 16... Distribution circuit, 17, 32... Variable bias current source, 18, 31... Pulse current Amplitude adjuster, 21... Upper peak detection circuit, 22...
...lower peak detection circuit, 24, 25, 43, 44
... Comparator, 36 ... Subtractor, 37 ... Amplifier,
38... Low pass filter, 39... First integrator, 40... Multiplier, 41... Delay circuit, 42...
...Second integrator.

Claims (1)

[Claims] 1. In an optical output stabilizing device that stabilizes optical output by controlling the amplitude of a driving pulse for a semiconductor laser generated from an input pulse having a constant amplitude value and the bias current of the semiconductor laser. , a monitoring photodetector that receives the optical output from the semiconductor laser;
A low-pass filter that has approximately the same transfer characteristics as the circuit for obtaining a monitor signal using this monitoring photodetector and limits the band of the input pulse, and an output of this low-pass filter and an output of the photodetector. a first integrator that integrates the output of the subtracter; and means for adjusting the input pulse to the output timing of the subtracter;
a multiplier for multiplying the input pulse obtained by this means by the output of the subtracter; a second integrator for integrating the output of the multiplier; An optical output stabilizing device comprising: means for controlling a bias current of a semiconductor laser; and means for controlling an amplitude of the drive pulse based on the output of the second integrator. 2. The integration time constant of the first integrator is
2. The optical output stabilizing device according to claim 1, characterized in that the integration time constant of the integrator is longer than the integration time constant of the integrator. 3. The integration time constant of the first integrator is
2. The optical output stabilizing device according to claim 1, wherein the integration time constant of the integrator is shorter than the integration time constant of the integrator. 4. At least one of the first and second integrators is comprised of a low-pass filter whose cutoff frequency is set sufficiently lower than the input pulse frequency. The optical output stabilizing device according to item 1. 5 The first and second integrators each include a low-pass filter, and the cutoff frequency of the low-pass filter constituting the first integrator is set to a low pass filter constituting the second integrator. 5. The optical output stabilizing device according to claim 4, wherein the optical output stabilizing device is set to a lower frequency than the cutoff frequency of the pass filter. 6. The first and second integrators each include a low-pass filter, and the cutoff frequency of the low-pass filter constituting the first integrator is set to a low pass filter constituting the second integrator. 5. The optical output stabilizing device according to claim 4, wherein the optical output stabilizing device is set at a frequency higher than the cutoff frequency of the pass filter.