JPH0440668B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention can deal with variations in the drive current of a laser diode (LD), and the LD at that temperature.
The present invention relates to a laser diode drive current monitoring circuit that accurately issues an alarm for an increase in drive current and allows easy setting of a reference voltage for alarm in response to temperature changes.

FIG. 1 shows the temperature characteristics of the driving current I of the laser diode. In Fig. 1, P is the optical output, I is the drive current, P ₀ is the reference optical output, T ₁ , T ₂ ,...T _o (T ₁
<T ₂ <...<T _o ) indicates the temperature, and I ₁ , I ₂ ,...I _o indicate the drive current required to obtain the reference optical output P ₀ at each temperature.

As shown in FIG. 1, the drive current for obtaining the reference optical output P ₀ varies from I ₁ to I _o depending on the temperature, and this value also varies considerably depending on the laser diode (LD).

Figure 2 shows a basic system diagram of a conventional LD drive current monitoring circuit. In Figure 2, LD is a laser diode, R is a resistor, LDDRV is a laser diode (LD) drive circuit, PD is a photodiode,
APC is automatic light output control circuit, CURDET is current detection circuit, TH is thermistor, COMP is comparison circuit,
ALMOUT is the alarm output.

Generally, the laser diode LD is connected to the LD drive circuit.
The optical output of the laser diode LD is monitored by the photodiode PD, and the automatic optical output control circuit APC controls the output of the LD drive circuit LDDRV.
The LD drive current is controlled so that the optical output of the laser diode LD becomes a constant specified output.

A conventional monitoring circuit compares the output v _a of the current detection circuit CURDET with a reference voltage V _r created by a temperature sensor composed of a thermistor TH, etc., which replaces temperature changes in the laser diode LD with a resistance value.
Compare with COMP and output of current detection circuit CURDET
Alarm output when v _a exceeds reference voltage V _r
Get ALMOUT. i.e. laser diode
When the optical output becomes weak due to deterioration of the LD, the LD drive current increases in order to obtain the specified output. When the current exceeds the allowable range of the LD drive current to obtain the reference optical output P ₀ corresponding to the operating temperature, an alarm output ALMOUT is issued.

In this case, the laser diode LD drive current changes depending on the temperature as shown in Figure 1, and there are also variations depending on the element, so in order to compensate for these, a circuit that creates a reference voltage such as a thermistor is used. It is necessary to make it adjustable and match it to each LD element. In addition, unless the temperature fluctuation characteristics of the laser diode LD correspond to those of the thermistor TH within the operating temperature range, it will not be possible to generate an accurate alarm in response to an increase in the LD drive current, and the laser diode LD element will not be compatible. There were problems in selecting a thermistor with appropriate temperature characteristics, and at what temperature the alarm generation point could be set to cover the entire temperature range, and the drawback was that adjustment was difficult.

The aim of the invention is to overcome the above-mentioned drawbacks.

The configuration of the present invention includes means for monitoring the drive current of the laser diode and using it as one input of the comparison circuit;
a storage circuit that stores in advance the temperature characteristics of the drive current of the laser diode and outputs drive current information corresponding to the temperature sensor input; and a comparison circuit that creates a predetermined alarm reference voltage from the drive current information. A laser diode drive current monitoring circuit is characterized in that it has a coefficient determination circuit which takes the other input of the laser diode drive current monitoring circuit. Hereinafter, embodiments will be described in detail with reference to the drawings.

FIG. 3 is a system diagram of an embodiment of the present invention,
THSEN is a temperature sensor and uses a thermistor. A/D is an analog-to-digital conversion circuit,
MEM is a memory circuit and uses ROM. COEF
is a coefficient setting circuit composed of, for example, a potentiometer. D/A is a digital-to-analog conversion circuit, and other symbols are the same as those in FIG.

LD drive current is usually controlled by automatic light output control circuit
APC causes a value to flow that keeps the optical output from the laser diode LD constant. Further, as can be understood from FIG. 1, the LD drive current value I for obtaining a constant optical output P ₀ varies depending on the temperature T.

First, connect the circuit shown in Figure 4 to the temperature sensor.
The temperature of the laser diode LD is detected by THSEN, and the LD drive current value at each temperature is stored in advance in the memory circuit MEM. In Figure 3, the temperature during operation is detected by the temperature sensor THSEN, and the analog/digital conversion circuit A/
It is input to the memory circuit MEM via D. memory circuit
The MEM reads out the reference LD drive current value corresponding to the temperature addressed by the input from the temperature sensor, and converts the digital-to-analog conversion circuit D/
Pass through A and input to the coefficient setting circuit COEF. The coefficient setting circuit COEF is the reference LD drive current at each temperature (previously written in the memory circuit MEM).
The output of this coefficient setting circuit COEF is input to the comparator circuit COMP as the alarm reference voltage _Vr . On the other hand, the actual LD of the laser diode LD is controlled by the current detection resistor R.
The drive current is detected, and the voltage V _a that can be applied to the comparator circuit COMP at a voltage proportional to the LD drive current is detected by the current detection circuit CURDET.
This is one of the inputs. Comparison circuit COMP compares both voltages and generates an alarm when V _a exceeds V _r . In other words, as the temperature of the laser diode changes, the automatic optical output control circuit APC works to increase the LD drive current that obtains the reference optical output P ₀ as shown in the temperature characteristic curve in Figure 1, so the current detection circuit CURDET When the output v _a becomes larger than the voltage corresponding to the standard LD drive current, and when V _a becomes larger than the alarm reference voltage V _r that takes into account the allowable range, the deterioration of the laser diode is notified by an alarm output. It is.

As described above, according to the present invention, the reference voltage of the comparator is automatically set in response to temperature changes during operation, and a correct alarm generation point can be maintained for each temperature. Further, since the coefficient setting can be easily set using a variable resistor or a potentiometer, it is possible to deal with variations in laser diodes and to perform accurate alarm settings. As described above, since the present invention has a function of automatically adjusting the reference voltage V _r of the comparator in response to temperature fluctuations, it can be used in the optical transmission section of an optical transmission device.

[Brief explanation of drawings]

Fig. 1 is a characteristic diagram of temperature change in laser diode drive current versus light emitting output, Fig. 2 is a conventional laser diode drive current monitoring circuit, Fig. 3 is a system diagram of an embodiment of the present invention, and Fig. 4 is FIG. 3 is a diagram showing writing to a memory circuit. LD...Laser diode, R...Current detection resistor, PD...Photodiode, APC...Automatic output control circuit, THSEN...Temperature sensor, A/D...
...analog-digital conversion circuit, MEM...memory circuit, D/A...digital-analog conversion circuit,
COEF...Coefficient setting circuit, CURDET...Current detection circuit, COMP...Comparison circuit, ALMOUT...
Alarm output.

Claims (1)

[Claims] 1. Means for monitoring the drive current of a laser diode and using it as one input of a comparison circuit, and a drive that stores in advance the temperature characteristics of the drive current of the laser diode and corresponds to the temperature sensor input. A laser diode drive current monitoring circuit comprising: a storage circuit that outputs current information; and a coefficient determination circuit that creates a predetermined alarm reference voltage based on the drive current information and inputs the same to the other input of the comparison circuit. . 2. A laser diode drive current monitoring circuit according to claim 1, which uses a ROM as a memory circuit. 3. The laser diode drive current monitoring circuit according to claim 1, which uses a potentiometer as the coefficient determination circuit. 4. A laser diode drive current monitoring circuit according to claim 1, which uses a thermistor as a temperature sensor.