JPH0983051A - Circuit for detecting deterioration of laser diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a circuit for detecting deterioration of a laser diode even when the output therefrom is not controlled constantly. SOLUTION: An LD current detection circuit 13 detects the voltage value corresponding to an LD current ILD while a PD current detection circuit 14 detects the voltage value corresponding to a PD current IPD. An increase rate of PD current detection value is initially set to be equal to that of LD current detection value. When a differential amplifier 15 takes a relative difference to the LD current ILD under that state, the PD output has the relative value of threshold current Ith which is invariant regardless of variation in the LD current ILD. When the LD deteriorates with that setting, the threshold current Ith increases to cause increase of output from the differential amplifier 15. A comparator 16 detects an output from the differential amplifier 15 exceeding a reference level and makes a decision that the LD has deteriorated before delivering an alarm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode deterioration detecting circuit for detecting deterioration of a laser diode with time.

[0002]

2. Description of the Related Art As is well known, a laser diode (hereinafter referred to as an LD) is often used as a light source for optical communication, but its lifetime is shorter than that of other parts, so that deterioration with time is detected. It is a common practice to issue an alarm and then replace it before it fails.

An LD is a PD that normally monitors its output.
It is used in the form of a module packaged together with the LD.
As a method of detecting deterioration over time, there has been adopted a deterioration detection method in which a current for driving an LD is detected, and when the current has increased by a certain amount from an initial set value, it is determined to be deterioration.

By the way, recently, an optical amplifier, such as an erbium-doped fiber amplifier (EDFA), which amplifies signal light by exciting with a light source having a wavelength shorter than that of the signal light, has been put into practical use. FIG. 6 shows an example of the optical amplifier.

In FIG. 6, the signal light input to the optical input terminal 21 is amplified by the amplification optical fiber 22 from the LD pumping light source 23 by pumping light having a shorter wavelength than the signal light, and is guided to the optical output terminal 24. The output light of the amplification optical fiber 22 is partly branched by the optical branching coupler 25 and photoelectrically converted by the output light detection PD 26. The output of the PD 26 is supplied to the output detection circuit 27 as an output monitor signal.

The output detection circuit 27 monitors the output level from the monitor level of the output monitor signal from the PD 26,
The LD drive circuit 28 is controlled according to the level fluctuation.
The LD drive circuit 28 increases / decreases the LD output by increasing / decreasing the LD current of the LD excitation light source 23.

L as an excitation light source for such an EDFA
Although the D module is used, the LD driving method in this case is not to make the LD output constant but to control the LD output so that the signal light output of the optical amplifier becomes a predetermined level. .

For example, when the signal light input to the optical amplifier decreases, the pumping LD current is increased to increase the gain of the optical amplifier to keep the signal light output constant. Therefore, the increase of the pump LD current is not necessarily caused by the deterioration of the pump LD, but is also caused by other factors, for example, the decrease of the optical amplifier input and the increase of the optical amplifier loss.
Therefore, even if an increase in LD current is detected, L
It cannot be judged as D deterioration.

[0009]

As described above, in the conventional LD deterioration detection circuit which uses the increase of the LD current to detect the deterioration of the LD, the LD current control does not make the LD output constant, and the excitation LD drive circuit of the optical amplifier is used. In such cases, there is a problem that the deterioration of the LD cannot be determined. An object of the present invention is to solve the above-mentioned problems and to set L even if LD output constant control is not performed.
It is to provide a laser diode deterioration detection circuit capable of detecting D deterioration.

[0010]

An LD deterioration detection circuit according to the present invention which solves the above problems includes an LD current detection circuit for detecting a drive current of a laser diode and a photodiode for monitoring an optical output of the laser diode. A PD current detection circuit for detecting a monitor current by multiplying it by a constant;
A differential amplifier that detects a difference voltage between the output voltage of the current detection circuit and the output voltage of the PD current detection circuit, and an alarm is issued when the difference voltage between the two obtained by this differential amplifier exceeds a reference value. It is configured to include a comparator.

That is, in the LD deterioration detection circuit having this configuration, the current flowing in the LD and the PD output monitoring the LD output in the LD module are detected,
The difference between the voltage corresponding to the LD current and the PD output × constant multiple voltage is obtained, and an alarm is issued when the difference exceeds a certain reference value.

In this case, the current flowing in the LD and the monitor PD output for monitoring the LD output are respectively detected. If the LD current exceeds the threshold value between the LD current and the LD output, the LD The output has a characteristic that it increases almost in proportion to the LD current. Since the monitor PD output is proportional to the LD output, (LD current value) − (monitor PD output) × constant becomes constant even if the LD current changes by appropriately selecting the value of the constant. Here, the LD has a characteristic that when it deteriorates, the threshold current increases and efficiency decreases. Therefore, the monitor PD output for the same LD current becomes smaller as the LD deteriorates, and (the voltage value corresponding to the LD current)-
The value of (monitor PD output voltage) × constant increases. Therefore, the value can be compared with the reference value and an alarm can be generated when the value exceeds the reference value.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the structure of an LD deterioration detection circuit according to the present invention. Reference numeral 11 denotes an LD module in which a laser diode LD and a monitor photodiode PD are packaged. The drive current of the laser diode LD is controlled by the LD drive circuit 12 including the transistor Tr. This drive current is the LD current detection circuit 1
It is converted into a voltage by the resistor R1 of 3 and the operational amplifier OP
1 and the resistors R2 to R5 are averaged by the amplifier circuit and amplified and output.

On the other hand, the light emitted from the LD is converted into a monitor current by the PD. This monitor current is supplied to the PD current detection circuit 14. This PD current detection circuit 14
Is a transimpedance amplifier that converts the current flowing in the monitor PD into a voltage using the operational amplifier OP2.
The amplification factor is determined by the value of the variable resistor VR.

The detection output of the LD current detection circuit 13 and the detection output of the PD current detection circuit 14 are both a differential amplifier 15.
Is supplied to. This differential amplifier 15 is an operational amplifier OP
3 and resistors R6 to R9, and detects and outputs the difference voltage between both inputs. This difference voltage is supplied to the comparator 16. This comparator 16 is composed of an operational amplifier OP4, compares the difference voltage from the differential amplifier 15 with a reference value Vref, and when the difference voltage exceeds the reference value Vref,
The alarm signal ALM is generated. This alarm signal AL
M is output from the output terminal 17.

That is, the LD deterioration detection circuit according to the present embodiment is configured such that the current flowing in the LD and the LD module 11 are
Each of the PD outputs monitoring the LD output is detected, and the difference between the LD current and the PD output multiplied by a constant is output. When the difference exceeds a constant reference value Vref, an alarm signal ALM is issued. It is configured as follows.

In this configuration, the current flowing in the LD and the L
The output current of the monitor PD that monitors the D output is detected. Here, there is a relationship as shown in FIG. 2 between the LD current and the LD output, and when the LD current exceeds the threshold value Ith, the LD output increases substantially in proportion to the LD current.
When the deterioration of the LD deteriorates, the threshold value Ith increases, the proportional constant decreases, and the increase rate of the LD output with respect to the LD current decreases.

On the other hand, since the monitor PD output is proportional to the LD output, (LD current value)-(monitor PD output) × constant can be obtained by appropriately selecting the value of the constant as shown in FIG. Can be kept constant even when changes occur.
When the LD deteriorates, the threshold current Ith increases and the efficiency decreases, so that the monitor PD output for the same LD current becomes small, and the value of (LD current value) − (monitor PD output) × constant increases.

Description will be given along the configuration shown in FIG. First,
In the LD current detection circuit 13, the LD of the LD module 11 is
When a current ILD is applied to the transistor Tr, the current ILD is
Since it flows from the emitter of V.sub.2 to V.sub.EE through the resistor R1 and the voltage drop in the resistor R1 is amplified by the operational amplifier OP1, a voltage value proportional to the LD current ILD can be obtained.

In the PD current detection circuit 14, the current IPD flowing in the monitor PD is converted into a voltage by the operational amplifier OP2. Here, in the initial state, the variable resistor R2
The amplification factor is adjusted by so that the increase rate of the PD current detection value is equal to the LD current detection value.

In this state, the output of the operational amplifier OP2 when the resistance value of the variable resistor VR is changed (PD current detection circuit 1
4), the gradient is large when the resistance value of the variable resistor VR is large, and the gradient is small when the resistance value of the variable resistor VR is small.
Therefore, by adjusting the resistance value of the variable resistor VR, the output of the PD current detection circuit 14 can be a voltage conversion value of (ILD-Ith). Therefore, the differential amplifier 15
Then, if the relative difference with the LD current ILD is taken, the PD output becomes the relative value of the threshold current Ith, and does not change even if the LD current ILD increases or decreases.

Under the above setting, when the LD deteriorates, the threshold current Ith increases, so that the output of the differential amplifier 15 increases. Therefore, the increase in the LD threshold current Ith can be detected from the output of the differential amplifier 15. Therefore, if the comparator 16 detects that the output of the differential amplifier 15 exceeds the reference value, it is determined that the LD is deteriorated,
An alarm can be issued.

When the efficiency is lowered due to the deterioration of LD,
As shown in FIG. 5, since the LD current for producing the same LD output increases, the output of the differential amplifier 15 increases from the initial level Δ1 to the level Δ2. Therefore, also in this case, if the comparator 16 detects that the output of the differential amplifier 15 exceeds the reference value, it can be determined that the LD is deteriorated and an alarm can be issued.

Therefore, in the LD deterioration detection circuit having the above structure, the increase in the LD current for outputting the same output from the LD is monitored, and when the LD current exceeds the reference value, the deterioration alarm is output. If the current-output characteristics of the LD do not change, the output that determines the alarm does not change even if the LD current changes due to other factors. If the LD deteriorates and the current-output characteristics change, the LD at that time changes. The output for judging the alarm increases regardless of the current, and the deterioration can be detected.

[0025]

As described above, according to the present invention, the LD
It is possible to provide a laser diode deterioration detection circuit capable of detecting LD deterioration even when the output is not controlled to be constant.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of an LD deterioration detection circuit according to the present invention.

FIG. 2 is a characteristic diagram showing an LD current-output characteristic of the same embodiment.

FIG. 3 is a characteristic diagram showing monitor PD output characteristics of the LD module of the same embodiment.

FIG. 4 is a characteristic diagram showing output characteristics of the monitor output detection amplifier of the same embodiment.

FIG. 5 is a characteristic diagram showing a change in monitor output when the LD of the embodiment is deteriorated.

FIG. 6 is a circuit diagram showing a configuration of an optical amplifier to which the present invention is applied.

[Explanation of symbols]

 11 ... LD module LD ... Laser diode PD ... Monitor photodiode 12 ... LD drive circuit Tr ... Transistor 13 ... LD current detection circuit OP1 ... Operational amplifier R1-R5 ... Resistor 14 ... PD current detection circuit OP2 ... Operational amplifier VR ... Variable resistance 15 ... Differential amplifier OP3 ... Operational amplifier R6-R9 ... Resistor 16 ... Comparator OP4 ... Operational amplifier 17 ... Alarm signal output terminal ALM ... Alarm signal 21 ... Optical input terminal 22 ... Amplification optical fiber 23 ... LD excitation light source 24 ... Optical output terminal 25 ... Optical branching coupler 26 ... Output light detection PD 27 ... Output detection circuit 28 ... LD drive circuit

Claims (3)

[Claims] 1. An L for detecting a drive current of a laser diode.
A D (laser diode) current detection circuit; a PD (photodiode) current detection circuit that detects a monitor current of a photodiode that monitors the optical output of the laser diode by multiplying it by a constant; and an output voltage of the LD current detection circuit. A differential amplifier for detecting a difference voltage from the output voltage of the PD current detection circuit, and a comparator for issuing an alarm when the difference voltage between the two obtained by the differential amplifier exceeds a reference value. Laser diode deterioration detection circuit. 2. The PD current detection circuit has a magnification of P.
2. The laser diode deterioration detection circuit according to claim 1, wherein the detection efficiency of the D output detection circuit is set to be equal to the detection efficiency of the LD current detection circuit. 3. The laser diode deterioration detection circuit according to claim 2, wherein the LD current detection circuit is a current average value detection.