JPH08204268A - Semiconductor laser driving circuit - Google Patents

Abstract

PURPOSE: To always monitor the deterioration of a semiconductor laser by making a bias current follow the change of the threshold of the laser even in such a state that the light output of the laser is forcibly interrupted or no signal is outputted from the laser. CONSTITUTION: Even when a laser diode 1 receives a light output forcedly interrupting state (stand-by state) command, a bias current control section 51 makes a bias current IB follow the change of the threshold of the diode 1 by increasing the bias current IB by supplying the control signal corresponding to the amplitude a decrement in a low-frequency modulated component into a bias current source 21. Since the current IB can be made to follow the threshold change of the diode 1 by using the method which does not rely on the pulse current IP corresponding to a data signal in such a way, the rise of the threshold which is the deterioration indicator of the diode 1 can be monitored always by watching the current IB even in such a state that the light output of the diode 1 is forcedly interrupted (stand-by state) or the diode 1 does not output any signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit for driving a semiconductor laser for transmitting an optical signal in accordance with a data signal to emit light, and more particularly to a technology for enhancing a function of monitoring a deterioration state of the semiconductor laser.

[0002]

2. Description of the Related Art A conventional semiconductor laser drive circuit is basically constructed as shown in FIG. In FIG. 3, 1
Is a laser diode represented by a semiconductor laser. The laser diode 1 is driven by the drive current ID from the drive current generator 2.

Here, the laser diode 1 is shown in FIG.
It has a drive current vs. light intensity characteristic as shown in (a), and a linear characteristic cannot be obtained unless the drive current is equal to or more than the threshold value Ith. Therefore, as shown in FIG. 4B, the drive current generator 2 generates a bias current IB having a threshold value Ith and a pulse current IP corresponding to the input data signal Din, and combines both currents IB and IP. The drive current ID of the laser diode 1 is generated. Thereby, the output light intensity of the laser diode 1 is, as shown in FIG.
The amplitude changes with a predetermined level or higher in accordance with the data signal.

Further, the threshold value Ith of the laser diode 1 fluctuates according to changes in ambient temperature and the degree of deterioration. Therefore, if the bias current IB is constant, the output light intensity of the laser diode 1 may not have an amplitude of a predetermined level or more, or the waveform may deteriorate.

Therefore, in the drive circuit shown in FIG. 3, a photodiode 3 is arranged in the vicinity of the laser diode 1, and a part of the output light of the laser diode 1 is received by the photodiode 3 to obtain the laser output light intensity. The monitor current IM corresponding to the change is taken out. The light output level detector 4 smoothes the monitor current IM to detect the average level of the light output, and the drive controller 5 biases the bias current IB of the drive current generator 2 so that the average level becomes constant. I am trying to increase or decrease.

For example, if the threshold value Ith of the laser diode 1 rises due to an increase in ambient temperature, secular change, etc., the amplitude level of the output light intensity of the laser diode 1 drops. At this time, since the amplitude level of the monitor current IB of the photodiode 3 also decreases, the average level of the optical output is detected from this monitor current, and the bias current IB is increased by an amount corresponding to the decrease in the detection level, whereby the bias current IB is increased. Is the threshold Ith
To keep the amplitude level of the output light intensity constant.

Further, since the deterioration of the laser diode 1 appears as an increase in the threshold value Ith, conventionally, the deterioration is detected by monitoring the bias current IB. In this case, in order to distinguish from the rise of the threshold value Ith due to the ambient temperature, the bias current monitor unit performs temperature compensation to prevent erroneous detection of deterioration.

By the way, when constructing an optical transmission system using an optical transmitter having a semiconductor laser drive circuit as described above, generally, as shown in FIG. (Or redundant system) A and redundant system (or normal system) B optical transmitters 11A and 11B are prepared, and optical switch 12 is used to provide optical transmitters 11A and 11B.
The optical output of either one of B is selectively derived.

However, in the currently used optical switches, the isolation between A and B is poor (about 20 dB), and for example, even if the A system is selected, the output light of the B system leaks and the transmitted optical signal is transmitted. Have an adverse effect. Therefore, the optical output of either one of the optical transmitters 11A and 11B, which is a redundant system, must be forcibly turned off.

As the method, in the example of FIG. 3, the pulse current IP of the drive current generator 2 is passed through the drive controller 5.
Various methods have been considered, such as applying squelch to, or applying squelch to both bias current IB and pulse current IP.

However, whichever method is adopted,
In the conventional semiconductor laser drive circuit, when it is specified to forcibly set the light output to the OFF state, the change in the amplitude of the light output is completely eliminated. It does not change. Therefore, in the conventional method in which the bias current IB is controlled so that the average level of the optical output becomes constant based on the monitor output, the bias current I
B cannot follow the change in the threshold Ith due to deterioration.

From the above, in the conventional semiconductor laser drive circuit, deterioration of the semiconductor laser could not be detected even if the bias current IB was monitored in the standby state (light output forced disconnection setting state). For the same reason, even in the normal system, the deterioration of the semiconductor laser could not be detected in the no signal output state.

[0013]

As described above, the conventional semiconductor laser drive circuit cannot detect the deterioration of the semiconductor laser in the light output forced cutoff state or the no-signal output state. The present invention has been made to solve the above-mentioned problems, and it is possible to make the bias current follow the threshold change of the semiconductor laser even in the optical output forced disconnection state or the no-signal output state. An object of the present invention is to provide a semiconductor laser drive circuit capable of constantly monitoring deterioration.

[0014]

In order to achieve the above-mentioned object, the present invention generates a pulse current corresponding to a bias current and a data signal of a threshold value of a semiconductor laser, and synthesizes both currents to generate a pulse current of the semiconductor laser. In a semiconductor laser drive circuit including a drive current generation unit that generates a drive current, and a light receiving element that receives a part of the output light of the semiconductor laser and converts the change in light intensity into a change in current to obtain a monitor current, At a frequency sufficiently lower than the frequency band of the data signal,
And a low-frequency signal source that generates a low-frequency signal whose amplitude is sufficiently lower than the amplitude of the pulse current, and a bias current that is generated by the drive current generator according to the low-frequency signal output from this low-frequency signal source. Bias current modulation means for amplitude modulation, low frequency modulation component extraction means for extracting a low frequency modulation component from the monitor current obtained by the light receiving element, and detecting the amplitude level of the low frequency modulation component extracted by this means, A drive control unit that increases or decreases the bias current generated by the drive current generation unit so that the amplitude level becomes constant, and has a function of monitoring the deterioration state of the semiconductor laser by detecting the bias current. I had it.

In particular, the drive control unit has a function of squelching the pulse current of the drive current generation unit when the output light of the semiconductor laser is designated to be forcibly turned off. .

Further, the bias current modulating means sets the modulation degree to such an extent that it does not affect the optical output intensity amplitude of the data signal, and the bias current is always a low frequency signal regardless of the presence or absence of the data signal input and the mark ratio. It is characterized by modulating.

[0017]

In the semiconductor laser drive circuit having the above structure, the bias current is amplitude-modulated by the low-frequency signal whose frequency is sufficiently lower than the frequency band of the data signal and whose amplitude is sufficiently lower than the amplitude of the pulse current. The semiconductor laser is constantly driven, the light output of the semiconductor laser is monitored by the light receiving element, and the low frequency modulation component is extracted from the monitor current obtained by the light receiving element.

When the threshold of the semiconductor laser increases, the amplitude level of this low frequency modulation component decreases. Therefore,
By increasing or decreasing the bias current so that the amplitude level of this low-frequency modulation component becomes constant, the bias current is made to follow the threshold change of the semiconductor laser, and by checking the bias current, the threshold that is a barometer of semiconductor laser deterioration. I am able to monitor the increase.

In this case, when the drive control unit specifies that the output light of the semiconductor laser is forcibly turned off, the pulse current of the drive current generation unit is squelched, thereby making the optical switch. Therefore, it does not affect the optical signal of the common system.

Further, by setting the modulation degree of the bias current to an extent that does not affect the optical output intensity amplitude by the data signal,
The bias current can always be modulated with a low-frequency signal regardless of the presence or absence of the data signal input, so that the bias current can follow the threshold change of the semiconductor laser even in the optical output state of the data signal, and the increase of the threshold can be monitored. It is possible.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. In FIG. 1, the same parts as those in FIG. 3 are designated by the same reference numerals, and different parts will be mainly described here.

FIG. 1 shows the configuration of a laser diode drive circuit according to the present invention, which is the drive signal generation section 2 described above.
Is a bias current source 21 that generates a bias current IB.
And a pulse current source 22 for generating a pulse current IP corresponding to "1" and "0" of the input data signal Din, and both currents I
A current adder 23 for combining B and IP to generate a drive current ID is provided, but here, a mixer 24 is further provided in the bias current path. A low frequency signal from the low frequency signal source 6 is supplied to the mixer 24.

This low frequency signal source 6 generates a low frequency signal IL having a frequency sufficiently lower than the frequency band of the data signal Din and sufficiently lower than the amplitude of the pulse current IP. By supplying this low-frequency signal to the mixer 24 of the drive signal generator 2, the bias current IB is amplitude-modulated by the low-frequency signal IL and is added and synthesized with the pulse current IP.

On the other hand, the monitor current IM obtained by the photodiode 3 is input to the low frequency extraction unit 7. The low frequency extraction unit 7 extracts the low frequency modulation component from the monitor current IM by passing the monitor current IM through a low pass filter, and the low frequency modulation component obtained here is sent to the drive control unit 5. To be

The drive controller 5 is a bias current controller 5
1 and a pulse current controller 52. The bias current control unit 51 detects the amplitude level from the low frequency modulation component and increases or decreases the generated current with respect to the bias current source 21 of the drive current generation unit 2 so that the amplitude level becomes constant. Specifically, the extracted low-frequency modulation component is integrated and smoothed, a variation is obtained by comparing with the reference voltage, and a control signal corresponding to this variation is sent to the bias current source 21.
The generated current is increased or decreased.

On the other hand, the pulse current controller 52 cuts off the current output of the pulse current source 22 of the drive current generator 2 when the optical output forced disconnection state is instructed, thereby squelching the pulse signal IP.

The operation of the above arrangement will be described below with reference to FIG. Note that, in FIG. 2, the cycle of the low-frequency signal is shortened to simplify the drawing. First, the laser diode 1 has the drive current vs. light intensity characteristic shown in FIG. 2A, and in the bias current source 21 of the drive current generation unit 2, the bias current IB becomes substantially equal to the threshold Ith of the laser diode 1. Is generated. Now, the bias current IB is changed by the low frequency signal IL as shown in FIG.
If modulation is performed as shown in (c), the pulse current IP corresponding to the data signal Din is added to the bias current IB '.
Is added, a drive current ID as shown in FIG. 2 (b) is obtained.

When the laser diode 1 is driven by this drive current ID, an optical output signal as shown in FIG. 2D is obtained. A low-frequency signal is superposed on this optical output signal, but its frequency and amplitude are sufficiently lower than the optical intensity amplitude, so that it does not affect its transmission.

In addition, the pulse current I
When P is not added, or when there is no signal, the low frequency modulated bias current IB 'is supplied as it is to the laser diode 1 as the drive current ID, so that its optical output signal is as shown in FIG. 2 (e). Changes to. However,
Since this optical output signal has an extremely long cycle and low-level amplitude, the redundant system output shown in FIG. 5 does not affect the regular system optical signal by the optical switch.

When the laser diode 1 outputs an optical signal as shown in FIG. 2 (d), the low frequency extraction component 7 extracts the low frequency modulation component from the monitor current IM obtained by the photodiode 3. And a low frequency component modulating the bias current IB is obtained. Here, if the threshold value Ith of the laser diode 1 increases, the amplitude level of the low frequency modulation component decreases.

Therefore, the bias current control unit 51 detects a decrease in the amplitude level from the low frequency modulation component obtained by the low frequency extraction unit 7, and outputs a control signal corresponding to the decrease in the drive current generation unit. 2 to the bias current source 21 to increase the bias current IB. As a result, the bias current IB follows the change in the threshold Ith of the laser diode 1.

Further, when the command for the forcible light output state (standby state) is received, the pulse current control unit 5 of the drive control unit 5
2 outputs a squelch control signal, and the drive current generator 2
The current output of the pulse current source 22 is squelched. At this time, the optical output signal shown in FIG. 2E is obtained from the laser diode 1. When this optical signal is monitored by the photodiode 3 and the low frequency modulation component is extracted by the low frequency extraction unit 7, the same low frequency modulation component as in the above case can be obtained.

Therefore, in this case as well, the bias current controller 5
At 1, the control signal corresponding to the decrease in the amplitude level of the low frequency modulation component is sent to the bias current source 21, and the bias current I
By increasing B, the bias current IB can follow the change in the threshold Ith of the laser diode 1.

As described above, the semiconductor laser drive circuit having the above-described configuration can make the bias current IB follow the change in the threshold Ith of the laser diode 1 by a control method that does not depend on the pulse current IP corresponding to the data signal. Therefore, even if the light output is forcibly cut off (standby state) or no signal is output, the threshold value Ith, which is a barometer of deterioration of the laser diode 1, can be observed by checking the bias current IB.
The rise of can be monitored at all times.

Furthermore, since the bias current IB follows the threshold value Ith of the laser diode 1 very well, the amplitude of the optical output signal can always be optimized, and the efficiency is excellent. The present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

[0036]

As described above, according to the present invention, the bias current can be made to follow the threshold change of the semiconductor laser even in the state where the optical output is forcibly cut off or the signal is not output, whereby the deterioration of the semiconductor laser is caused. It is possible to provide a semiconductor laser drive circuit capable of constantly monitoring the temperature.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a configuration of an embodiment of a semiconductor laser drive circuit according to the present invention.

FIG. 2 is a diagram showing characteristics and input / output waveforms of a laser diode for explaining the operation of the above embodiment.

FIG. 3 is a block circuit diagram showing a configuration of a conventional semiconductor laser drive circuit.

4 is a diagram showing characteristics and input / output waveforms of a laser diode in the circuit configuration of FIG.

FIG. 5 is a block circuit configuration diagram showing an example of use of the semiconductor laser drive circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Laser diode, 2 ... Drive current generation part, 21 ... Bias current source, 22 ... Pulse current source, 23 ... Current adder, 24 ... Mixer, 3 ... Photodiode, 4 ... Optical output level detection part, 5 ... Drive control unit, 51 ... Bias current control unit, 52 ... Pulse current control unit, 6 ... Low frequency signal source, 1
1A, 11B ... Optical transmitter, 12 ... Optical switch, IB,
IB '... bias current, IP ... pulse current, ID ... drive current, IM ... monitor current, IL ... low frequency signal, Din ... input data signal, Ith ... laser diode threshold.

Claims (3)

[Claims] 1. A drive current generator for generating a bias current corresponding to a threshold value of a semiconductor laser and a pulse current corresponding to a data signal, and combining the two currents to generate a drive current for the semiconductor laser; In a semiconductor laser drive circuit including a light-receiving element that receives a part of output light and converts a change in light intensity into a change in current to obtain a monitor current, wherein the frequency is sufficiently lower than the frequency band of the data signal and the pulse A low-frequency signal source that generates a low-frequency signal whose amplitude is sufficiently lower than the amplitude of the current, and an amplitude modulation of the bias current generated by the drive current generator according to the low-frequency signal output from this low-frequency signal source. Bias current modulation means, low frequency modulation component extraction means for extracting low frequency modulation components from the monitor current obtained by the light receiving element, and low frequency modulation components extracted by this means. And a drive control unit that detects the amplitude level of the modulation component and increases or decreases the bias current generated by the drive current generation unit so that the amplitude level becomes constant. The semiconductor is obtained by detecting the bias current. A semiconductor laser drive circuit having a function of monitoring a deterioration state of a laser. 2. The drive controller has a function of squelching the pulse current of the drive current generator when it is designated to forcibly turn off the output light of the semiconductor laser. The semiconductor laser drive circuit according to claim 1. 3. The bias current modulating means sets the modulation degree to such an extent that does not affect the optical output intensity amplitude of the data signal, and the bias current is always a low frequency signal regardless of the presence or absence of the data signal input and the mark ratio. The semiconductor laser drive circuit according to claim 1, wherein the semiconductor laser drive circuit modulates.