JPH10163555A - Method for controlling light output level and device for controlling light output level - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable an LD to maintain a constant extinction ratio, even when the differentiating efficiency changes due to the deterioration of the LD by monitoring the output difference between two kinds of test signal monitors, having different duty ratios and controlling a pulse modulating current. SOLUTION: A host circuit sets a mode selection circuit 3 to a test mode and outputs two kinds of test signals A and B, having different duty ratios as transmitting data, and the output of a receiving element 13 for monitor is inputted to a storage difference computing circuit 5. Since the test signals A and B have different duty ratios, the light output level of an LD 12 becomes different and the output of a monitor also becomes different. When the difference, computed by means of the circuit 5 becomes smaller than the initial value, the pulse-modulating current is increased, because the differentiating efficiency becomes deteriorated. When the difference becomes zero, the pulse-modulating current is reduced. Therefore, the extinction ratio can be maintained constantly, even when the differentiating efficiency becomes worse by controlling the pulse modulating current.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a semiconductor laser used in an optical transmitter or the like in an optical communication system using an optical fiber as a transmission medium, and more particularly to an optical output compensating circuit for maintaining a constant laser output. About.

[0002]

2. Description of the Related Art Conventionally, as a method of driving a semiconductor laser (hereinafter referred to as LD) module using a semiconductor laser, a DC bias current Ib having a value near a threshold current Ith for starting laser oscillation is applied. In general, a pulse modulation current Ip is superimposed as a signal component. Here, the threshold current Ith changes depending on the ambient temperature. Therefore, when the LD is driven with a constant current, the light output level varies depending on the ambient temperature. Further, the differential efficiency η, which is the efficiency of converting an electric signal to an optical signal, is deteriorated due to the deterioration of the LD, and the light output level and the extinction ratio are deteriorated.

[0003] For this reason, as a method for maintaining a constant optical output, Japanese Patent Application Laid-Open Nos. 7-147446 and 4-82285 (hereinafter referred to as prior arts 1 and 2 respectively) have been proposed.
Etc.) have been proposed.

FIG. 7 is a block diagram showing a configuration of a driving circuit of an optical fiber module according to the prior art 1. As shown in FIG. 7, a light receiving element 52 for detecting the optical output of the LD 51, a pulse current modulation circuit 53 for supplying a pulse current to the LD 51, and a pulse current control circuit 54 for controlling the pulse current value of the pulse current modulation circuit 53 A power monitor circuit 55 for detecting a detection output of the light receiving element; and a bias current control circuit 56 for controlling a bias current Ib for emitting light from the LD 51.

In the LD drive circuit proposed here, in summary, the bias current Ib is changed at two or more points at regular time intervals, and the light emission power of the LD 51 at that time is detected by the light reception power detection means, and the threshold is detected. Value current and calculate the DC bias current I of the LD 51 slightly higher than the threshold current.
b is set in the bias current control circuit 54.

Further, by detecting a change in the threshold current of the LD 51 and the differential efficiency η, the pulse current control circuit 54 sets a pulse current corresponding to the change in the differential efficiency η or the like, thereby setting the light output level. Holds constant.

Next, the prior art 2 will be described. FIG.
FIG. 2 is a diagram showing an optical output compensation circuit proposed in the prior art 2. As shown in FIG. 8, the optical output compensation circuit performs feedback control on the DC component and the signal component (pulse component) of the monitor signal extracted from the output optical signal of the LD, respectively.
Both the average optical power and the extinction ratio of the output optical signal are compensated. Specifically, the monitor signal output from the monitor light receiving element (PD) 61 is
And the peak hold circuit 64. here,
The integration circuit 63 extracts the average value of the output optical signal from the monitor signal. On the other hand, the peak hold circuit is AC-coupled to the monitoring light receiving element 61 and extracts the signal amplitude of the monitoring signal. Therefore, the subtraction circuit 66 and the comparator 6 are added to the drive current of the laser diode drive circuit 65 so that the signal amplitude extracted by the peak hold circuit 64 is constant.
Negative feedback is applied via 7, 68 to stabilize the signal amplitude of the output optical signal.

[0008]

SUMMARY OF THE INVENTION In the prior art,
When the modulation is performed with a constant pulse modulation current Ip, the extinction ratio also deteriorates when the differential efficiency of the LD deteriorates, and the difference between when the "1" level signal is transmitted and when the "0" level signal is transmitted becomes small, so that the temperature fluctuation In the case where the LD is deteriorated due to aging or deterioration over time, the extinction ratio of the LD deteriorates and the reliability of data communication decreases. That is, the error rate increases. Here, the extinction ratio is a ratio of two types of light intensity corresponding to each of the binary signals.

In the prior art 1, in order to avoid the above problem, the bias current Ib is set to 2
It is necessary to control the threshold current and the pulse modulation current Ip by changing the threshold current and the differential efficiency by changing the threshold current and the differential efficiency. This is because, as the degradation modes of the LD, there are slow degradation due to long-time laser oscillation and rapid degradation due to defects in the light emitting element or external factors such as overcurrent and high-temperature energization. If the pulse modulation current Ip is controlled only for a certain period of time (when the power is turned on, at a certain period, when a system error occurs, etc.), the LD
This is because when the deterioration of the chromium rapidly progresses, the extinction ratio cannot be kept constant.

In prior art 2, in order to avoid the above problem, the extinction ratio is stabilized by a negative feedback circuit. However, an amplifier having a wide frequency band and a light-receiving element having good high-speed response are required, and the feedback circuit has a disadvantage that the circuit becomes complicated and the amount of hardware increases.

Therefore, one technical problem of the present invention is to monitor the difference between the monitor outputs of two types of test signals having different duty ratios and control the pulse modulation current Ip, thereby achieving L
An object of the present invention is to provide a light output level control method and a control device capable of maintaining a constant extinction ratio even when the differential efficiency changes due to deterioration of D.

Another technical problem of the present invention is that
An object of the present invention is to provide a light output level control method and a control device capable of coping with rapid LD deterioration by constantly monitoring a change in the bias current Ib.

[0013]

According to the present invention, in a method of driving a laser diode (hereinafter referred to as an LD) in which a pulse modulation current Ip is superimposed on a bias current Ib, duty ratios are different from each other. The optical output of the LD is monitored using the first and second test signals, the average value of the monitor output signals is detected, the average value is stored, and the difference between the average values of the first and second test signals is detected. And the difference between the monitor output average value in the initial state of the LD and the monitor output average value after the oscillation of the LD is taken, and while monitoring the change in the bias current Ib, the pulse modulation current Ip is controlled in accordance with the difference. Thus, a light output level control method is obtained.

Further, according to the present invention, in the optical output level control method, a change in the bias current Ib is monitored with time, and when the change in the bias current Ib progresses in a short time, the LD is controlled. An optical output level control method characterized by outputting a deterioration progress signal is obtained.

According to the present invention, the bias current Ib
In a drive circuit of a laser diode (hereinafter, referred to as an LD) of a type in which a pulse modulation current Ip is superimposed on a pulse, a light output of the LD is monitored using first and second test signals having different duty ratios. Light-receiving element for monitoring, an average value detection circuit for detecting an average value of a monitor output signal of the monitor light-receiving element, and an output of the average value detection circuit, and storing the output of the first and second test signals. A storage difference calculation circuit for calculating a difference between the average value, a difference between the monitor output average value in the initial state of the LD and the monitor output average value after LD oscillation, a bias current monitoring circuit for monitoring a change in the bias current Ib, An optical output level control device comprising a pulse modulation control circuit for controlling a pulse modulation current according to the output of the storage difference calculation circuit is obtained.

Further, according to the present invention, in the optical output level control device, a timer circuit is provided in a bias current monitoring circuit for monitoring a change in the bias current Ib, and the change in the bias current Ib progresses in a short time. In such a case, an optical output level control device characterized by outputting an LD deterioration progress signal is obtained.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration of an optical output level control device according to an embodiment. As shown in FIG. 1, the optical output level control device includes an LD drive circuit 1 and an LD module 2.

The LD drive circuit 1 includes a mode selection circuit 3, an average value detection circuit 4, a storage difference calculation circuit 5, a pulse modulation control circuit 6, a bias control circuit 7, a switching circuit 8, a bias circuit 9, a bias current monitoring circuit 10 , And a pulse modulation circuit 11. The LD module 2 further includes an LD element 12 and a monitoring light receiving element 13 for monitoring the optical output of the LD element 12.

In the LD drive circuit 1, the mode selection circuit 3 is a circuit for setting the LD drive circuit 1 to either a normal operation mode or a test mode for controlling the extinction ratio of the LD. The average value detection circuit 4 detects the average light output of the LD element 12 based on two types of test signals having different duty ratios, that is, the monitor output of the monitoring light receiving element 13 of the first and second test signals. Circuit.

The storage difference calculation circuit 5 is a circuit that stores the output of the average value detection circuit 4 of the first and second test signals having different duty ratios in the initial state and takes the difference. The storage difference calculation circuit 5 also calculates the difference between the average optical output of the test signal in the initial state and the average optical output of the test signal after the deterioration of the LD has progressed.

The pulse modulation control circuit 6 is a circuit for controlling the pulse modulation current Ip when the difference result of the storage difference calculation circuit 5 fluctuates from the initial value.

The bias control circuit 7 includes an average value detection circuit 4
Is a circuit for controlling the value of the bias current Ib so that the average value of the optical output becomes constant based on the output of the bias current Ib.

The switching circuit 8 switches the output of the average value detection circuit 4 to either the bias control circuit 7 or the storage difference calculation circuit 5 according to the setting of the mode selection circuit 3. The bias circuit 9 supplies a bias current I to the LD element 12.
b.

The bias current monitoring circuit 10 is a circuit that monitors the amount of change in the bias current Ib and outputs a deterioration signal to a higher-level circuit when a change occurs.

Further, the pulse modulation circuit 11 is a circuit for pulse-driving the LD element 12 by superimposing and flowing the pulse modulation current Ip on the bias current Ib to the LD element 12.

First, before starting the normal data transfer, that is, in the initial state of the LD, the LD drive circuit 1 is set to the test mode by the mode switching signal from the upper circuit. In the test mode, two types of test signals (first and second test signals) having different duty ratios are input to the LD drive circuit 1 as transmission data from the host circuit. The test signal A is a signal having a duty ratio of 6 to 4, and the test signal B is
This is a signal with a duty ratio of 4 to 6.

The operation of the optical output level control device according to the first embodiment of the present invention will be described.

Before starting the normal operation, the mode selection circuit 3 is set to the test mode by the host circuit. In the test mode, two types of test signals A and B having different duty ratios are output from the upper circuit as transmission data. The output of the monitoring light receiving element 13 at that time is input to the storage difference calculation circuit 5. Since the test signals A and B have different duty ratios, the light output level of the LD 12 is different and the monitor output is also different. In the storage difference calculation circuit 5, the monitor outputs PA and P
Take the difference .DELTA.P = PA-PB of B and store the result. L
D12 deteriorates due to oscillation for a long time, and the gradient (differential efficiency η) of the optical output-bias current characteristic deteriorates. Therefore, the extinction ratio is deteriorated only by controlling the bias current Ib. To prevent this, the test signal is transmitted periodically about once every several months by taking advantage of the property that the LD deterioration gradually progresses, and the difference is stored in the storage difference calculation circuit 5. If the initial value fluctuates, the pulse modulation control circuit 6 controls the pulse modulation current Ip to keep the extinction ratio constant.

That is, when the difference in the storage difference calculation circuit 5 becomes smaller than the initial value, the pulse modulation current Ip is increased because the differential efficiency η has deteriorated. Conversely, when the difference is large, the pulse modulation current Ip is reduced. By controlling the pulse modulation current Ip, the extinction ratio can be kept constant even if the differential efficiency η deteriorates.

The bias current monitoring circuit constantly monitors the change in the bias current Ib. If the bias current Ib increases significantly, it is determined that the deterioration of the LD 12 has rapidly progressed, and the pulse current Ip is similarly controlled. .

Next, an optical output level control device according to a first embodiment of the present invention will be described with reference to FIGS.
This will be specifically described with reference to FIG.

FIG. 2 shows the light output in the initial state of a general LD.
4 is a graph showing current characteristics. As shown in FIG. 2, when the LD is generally pulse-driven, the bias current Ib is set to a value slightly larger than the threshold current Ith, and the pulse modulation current Ip is superimposed thereon.

The test signal input to the LD drive circuit 1 is pulsed by the pulse modulation circuit 12 outputting a pulse modulation current Ip and flowing through the LD element 12 as shown in FIG. At this time, the light output of the LD element 12 is monitored by the monitoring light receiving element 13 and output to the average value detection circuit 4.

The average value detection circuit 4 detects the average value of the light output. The average optical output value (P _A1 ) of the test signal A and the average optical output value (P _B2 ) of the test signal B are output to the storage difference calculation circuit 5, respectively. Test signal A and test signal B are duty
Since the (duty) ratio is different, the light output level is also different. The following equation 1 holds between the outputs P _A1 and P _B1 of the average value detection circuit 4.

[0036]

(Equation 1) In the storage difference calculation circuit 5, the difference between P _A1 and P _A2 (ΔP ₁₁ )
Is calculated and the value is stored. The difference is represented by the following equation (2).

[0037]

(Equation 2) The differential efficiency η, which is the efficiency of converting an electrical signal to an optical signal, is
As shown by the following equation (3), it is represented by the slope of the graph of the light output-current characteristic.

[0038]

(Equation 3) As described above, the test signal A in the initial state of the LD
The average values P _A1 and P _B2 of the optical outputs B and B are detected by the average value detection circuit 4 and the difference ΔP ₁₁ is stored in the storage difference calculation circuit 5, and then the mode selection circuit 3 is set to the normal mode to perform the normal operation. Start.

Here, in a normal operation state, the output of the average value detection circuit 4 is input to the bias control circuit 7 by the switching circuit 8 in the same manner as in the conventional automatic light output control circuit (APC) system using the average light output detection. Is done. The bias control circuit 7 constantly monitors the optical output even during the normal operation, and controls the bias current Ib so that the average value is constant. However,
The pulse modulation current Ip is constant at the initial value.

There are two types of LD degradation modes, slow degradation and rapid degradation. The former slow deterioration is deterioration that progresses due to laser oscillation for a long time in a normal operation state. On the other hand, the latter rapid deterioration is deterioration that progresses rapidly due to external factors such as defects of the light emitting element, overcurrent, high-temperature energization, and the like. As the LD deteriorates, the differential efficiency η changes. This is shown in FIG.

At the same pulse modulation current Ip as the initial state shown in FIG. 2, the differential efficiency η is as shown in the following equations (4) and (5).

[0042]

(Equation 4)

[0043]

(Equation 5) As shown in the above equations (4) and (5), only the conventional control of the bias current Ib deteriorates the extinction ratio and lowers the reliability of data transfer.

In order to prevent this, in the first embodiment of the present invention, the LD drive circuit 1 is set to the test mode periodically (about once every several months), and a test signal is transmitted from the upper circuit. The difference of the average light output at that time is obtained, and when a fluctuation occurs with respect to the initial value stored in the storage difference calculating circuit 5, the pulse modulation control circuit 6 controls the pulse current Ip to change the extinction ratio. To be constant.

As in the initial state, the difference between the output P _A2 of the average value detection circuit 6 when the test signal A is transmitted and the output P _B2 of the average value detection circuit when the test signal B is transmitted is stored in the storage difference calculation circuit 5. Further, a difference from the value ΔP ₁₁ in the initial state is obtained. Since the differential efficiency η is deteriorated, the following equations (6) and (7) are obtained.

[0046]

(Equation 6)

[0047]

(Equation 7) The pulse modulation control circuit 6 increases the pulse modulation current Ip such that ΔP ₁₁ > ΔP ₂₂ , which has fluctuated from the initial value, becomes constant.

After the above operation is completed, the normal operation mode is set. In the normal operation mode, only the control of the bias current Ib by the detection of the optical output average value is performed, and the control of the pulse modulation current Ip is not performed. Thus, the light output level and the extinction ratio can be kept constant. This is shown in FIG.

As shown in FIG. 4, the differential efficiency η is η ₁ > η
_Since it is ₂ , the pulse modulation current Ip is increased to Ip2.

When the deterioration progresses rapidly, the bias current Ib also increases because the threshold current Ith greatly increases. The change in the bias current Ib is monitored by the bias current monitoring circuit 10, and if the bias current Ib increases (for example, 1.3 times) from the initial value, it is determined that the LD has rapidly deteriorated and the upper circuit is determined. An LD deterioration signal is output.
The host circuit receives this signal, sets the LD drive circuit 1 to the test mode, and controls the pulse modulation current Ip to keep the extinction ratio constant in response to the rapid deterioration of the LD, as described above.

Further, the bias current Ib is 2
If it exceeds twice, it is determined that the LD is defective, and a defect signal is output to the host device.

As described above, according to the first embodiment of the present invention, the monitoring of the monitor output of two types of test signals having different duty ratios allows the LD to gradually deteriorate. However, the extinction ratio can be kept constant even when the speed advances rapidly.

(Second Embodiment) Next, a second embodiment of the present invention will be described.
An embodiment will be described.

FIG. 5 is a block diagram showing a configuration of an optical output level control device according to a second embodiment of the present invention. As shown in FIG. 5, in the first embodiment, the test mode and the normal operation mode are switched by the mode selection circuit 3 and the switching circuit 8, but in the second embodiment, the mode selection circuit 3 and the normal operation mode are switched. The second embodiment differs from the first embodiment in that the switching circuit 8 is eliminated.

In the optical output level control device according to the second embodiment, when controlling the extinction ratio in the same manner as in the first embodiment, the mode switching signal and the test signals A and B are transmitted from the host device. Is input to the LD drive circuit 1. The mode switching signal is used as an enable signal for the storage difference calculation circuit 9 and the pulse modulation control circuit 12. When the mode switching signal is enabled {for example, high (HI) level}, the test signals A and B are transmitted, and the storage difference calculation circuit 5 performs two types of test signals as in the case of the first embodiment. The pulse modulation current Ip is controlled by the pulse modulation control circuit 6 when the difference between the initial values is obtained.

In a normal operation state, the mode switching signal is
At the low (LO) level, the storage difference calculation circuit 5 and the pulse modulation control circuit 6 do not operate, but only control the bias current Ib.

In the second embodiment, a signal having a duty ratio of 6: 4 and 4: 6 is used as a test signal. Any number of duty ratios may be used as long as the output average value differs.

(Third Embodiment) Next, a third embodiment of the present invention will be described.
An embodiment will be described. FIG. 6 is a diagram showing a configuration of an optical output level control device according to a third embodiment of the present invention. As shown in FIG. 6, the third embodiment of the present invention has the same configuration as that of the second embodiment except that a timer circuit 14 is built in the bias current monitoring circuit 10. ing. This timer circuit 14 measures the time when the bias current changes.

Here, as the deterioration of the LD progresses,
The threshold current Ith increases, and finally the light output level decreases and becomes unusable. However, by providing the timer circuit 14 in the bias current monitoring circuit 10, when the increase of the bias current Ib is progressing in a short time, the deterioration progress signal is output to the host device, and the deterioration of the LD rapidly progresses. Notification of deterioration, the deterioration can be known in advance, and the countermeasure can be taken at an early stage.

[0060]

As described above, according to the present invention, the difference between the monitor outputs of the first and second test signals having different duty ratios is monitored, and the pulse is controlled so that the extinction ratio becomes constant. In order to control the modulation current Ip, it is possible to provide an optical output level control method and apparatus capable of maintaining the extinction ratio at a constant value even when the characteristics of the LD deteriorate due to temperature fluctuation, aging, and the like.

According to the present invention, the bias current monitoring circuit monitors a change in the bias current Ib and outputs a deterioration signal when the bias current Ib fluctuates from the initial value to control the pulse modulation current Ip. Since the test mode is set to the test mode, even when the LD deteriorates rapidly, it is possible to provide an optical output level control method and apparatus capable of controlling the value of the pulse modulation current Ip and keeping the extinction ratio constant.

Further, according to the present invention, by providing a timer circuit in the bias current monitoring circuit, the time during which the bias current Ib changes is measured. Since the deterioration progress signal is output to the circuit, it is possible to provide an optical output level control method and apparatus capable of grasping in advance that the deterioration of the LD is rapidly progressing.

Further, according to the present invention, since the peak value of the optical output is not detected by the feedback circuit, a high-speed light-receiving element
Since there is no need for an amplifier, the circuit configuration is simpler than that of the prior art (for example, Conventional Technique 2), and an optical output level control method and apparatus that can use general-purpose components can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical output level control device according to a first embodiment of the present invention.

FIG. 2 is a current-light output characteristic diagram of a general LD in an initial state.

FIG. 3 is a current-light output characteristic diagram in an LD degraded state.

FIG. 4 is a diagram for explaining an operation of the optical output level control device according to the first embodiment of the present invention, and shows a change in modulation current.

FIG. 5 is a block diagram showing a configuration of an optical output level control device according to a second embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration of an optical output level control device according to a third embodiment of the present invention.

FIG. 7 is a block diagram showing Conventional Technique 1.

FIG. 8 is a circuit diagram showing a conventional technique 2.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 LD drive circuit 2 LD module 3 mode selection circuit 4 average value detection circuit 5 storage difference calculation circuit 6 pulse modulation control circuit 7 bias control circuit 8 switching circuit 9 bias circuit 10 bias current monitoring circuit 11 pulse modulation circuit 12 LD element 13 light receiving Element 14 Timer circuit

Claims (4)

[Claims] In a method of driving a laser diode (hereinafter, referred to as an LD) of a type in which a pulse modulation current is superimposed on a bias current, the LD is driven by using first and second test signals having different duty ratios. Monitor light output,
The average value of the monitor output signal is detected, the average value is stored, and the difference between the average value of the first and second test signals and L
A difference between a monitor output average value in an initial state of D and a monitor output average value after LD oscillation is obtained, and a pulse modulation current is controlled according to the difference while monitoring a change in the bias current. Light output level control method. 2. The optical output level control method according to claim 1, wherein the change in the bias current is monitored with time, and when the change in the bias current progresses in a short time,
An optical output level control method, comprising: outputting an LD deterioration progress signal. 3. A driving circuit for a laser diode (hereinafter, referred to as an LD) of a system in which a pulse modulation current is superimposed on a bias current, the first and second test signals having different duty ratios are used for the LD. A monitoring light receiving element for monitoring an optical output, an average value detecting circuit for detecting an average value of a monitor output signal of the monitoring light receiving element, and an output of the average value detecting circuit; (2) a storage difference calculation circuit for calculating a difference between the average value of the test signal, a difference between the average value of the monitor output in the initial state of the LD and the average value of the monitor output after the oscillation of the LD, and An optical output level control device comprising: a monitoring circuit; and a pulse modulation control circuit that controls a pulse modulation current according to an output of the storage difference calculation circuit. 4. The optical output level control device according to claim 3, wherein a timer circuit is provided in a bias current monitoring circuit for monitoring a change in the bias current, and the change in the bias current progresses in a short time. An optical output level control device for outputting an LD deterioration progress signal.