JPS58107692A - Protecting method for semiconductor laser - Google Patents

Abstract

PURPOSE:To prevent the breakdown of a laser chip due to an excessive driving current, by changing a reference thershold level for cutting a driving voltage or the driving current. CONSTITUTION:A variable voltage source 1 supplies a constant voltage VS, whose voltage is controlled by an APC method to the laser chip 2. A comparator 5 compares the constant voltage VS and an output VR of a threshold level generator 6, and outputs an H level when the constant voltage VS is smaller than VR. An AND circuit 7 imparts a modulating signal S to a transistor 4 when the output of the comparator 5 is the H level, and controls the laser 2. It is designed that the voltage VR of the threshold level generator 6 is varied with the temperature of the laser chip. When the constant voltage VS exceeds the reference voltage VR of the level generator VS and the excessive current flows in the laser 2 owing to a certain reason, the output of the cmparator 5 becomes L, the output of the AND gate 7 becomes L, and the transistor 4 is turned OFF. Therefore the driving current to the laser 2 is instantaneously cut off.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a semiconductor laser that reliably prevents damage to the laser chip due to excessive drive current in a semiconductor laser that has been compensated for by an automatic optical power control (APC) method. How to protect lasers: 11ch ・ Conventional fIII lasers are known, but semiconductor lasers have advantages such as being able to continuously change the oscillation light output depending on the drive current value, and having a high conversion efficiency of input to output. Recently, it has become widely used due to its irritating properties.

However, the optical output of a semiconductor laser changes for the same driving current when the chip mf changes, that is, the optical output tends to decrease as the temperature rises, as shown in Figure 1. Therefore, when using a semiconductor laser, Just as its light output remains constant regardless of temperature and other factors, (
(1) A method is used in which the drive current is adjusted in order to keep the chip temperature constant and (2) to keep the optical output constant. As the method (1), for example, a method is used in which a semiconductor laser chip tm is mounted on a Peltier element and the temperature of the chip is always kept constant by controlling the current flowing through the Peltier element, and as the method (2), is an automatic light output control method, or APC (Aulomalic Power
Control method.

A method is used in which the optical output of a semiconductor laser is detected by a photodiode or the like, and the driving current is controlled based on changes in the detected value of the optical output to keep the optical output constant.

By the way, in semiconductor lasers, excessive drive current flows due to external noise and fluctuations in power supply voltage, and the end face of the chip may deteriorate due to the light generated by itself, which may lead to 1llllk, so please take precautions before such a situation occurs. Therefore, the above method (1) must be used for protection by cutting off the drive current.
), since the drive current is normally constant, the drive current or voltage tube is monitored, and when the current value or voltage exceeds a certain threshold, the drive current is cut off. This makes it easy to protect semiconductor lasers. However, in the case of the above method (2), since the drive current changes with temperature change, the above method (2)
It cannot be protected in the same way as in case 1). In other words, when the set value of the drive current is determined as Ic as shown in Fig. 2, the optical output characteristic at fM # Tm K is IIK
However, under the optical output characteristics at temperature [T, ((T)k), the output of the semiconductor laser has already broken down with the same drive current IC. The area PK is included, and conversely the temperature T3 (>TI
), the driving W flow is cut off in a region where the rated output cannot be obtained, which is a meaningless result, and complete protection cannot be achieved.

On the other hand, in the APC method described above, since the output voltage of the photodiode that detects the optical output is weak, it is necessary to use a broadband amplifier with a large gain and a frequency response of several gigahertz in order to amplify it economically. Generally, when the response of an amplifier is improved, a phenomenon known as ringing, such as six-waveform "overshoot" or "swelling" occurs, which causes the problem of erroneously cutting off the drive current. On the other hand, if the response of this amplifier is poor, there is a risk that the cut-off of the driving l11il will be delayed and the chip will be destroyed.

The present invention has been made in view of the above points.
In order to completely prevent damage to the laser chip due to excessive drive current in a semiconductor laser temperature-compensated by the method, the drive voltage or drive current [threshold value that serves as a reference for cutting off] is changed depending on the temperature. .

The present invention will be explained below with reference to the drawings. Figure 3 shows an embodiment of the semiconductor protection method according to the present invention. In Figure 6o, 1 is an APC.
2 is a semiconductor laser, 3 is a current limiting resistor, 4 is a variable M
6 transistors control the drive current of the semiconductor laser l by the signal S, 5 is a predetermined variable reference voltage, that is, the threshold value v
Compare B with constant voltage ■S supplied by variable constant voltage source l, and if Vm < Vm, a, 1 level, vS≧V
If l, 1L# level signal [output comparator,
6 is a threshold generator that determines the threshold value in advance by taking into account the wrinkle characteristics of the semiconductor laser, and 7 is the output of the comparator 5 and the modulation signal S.
This is an AND gate that takes the argument 1illIi.

The 4th vlJ is an embodiment of the 3rd v4 threshold generator,
What is shown here is an example in which a semiconductor laser having the overflow IF characteristic shown in the fifth WK is used.

(2) In the middle, 8 is a temperature/voltage converter that converts the semiconductor laser chip temperature into voltage, and 9 is a polygonal line approximation circuit.
Increase #M Tuna 9 Hatake and Resistance 'R1eR Snow IR.

and a diode D, and the temperature and voltage a! are connected to the inverting input terminal of the operational amplifier 90. I! 01G is a variable off-off circuit that variably adjusts the threshold level, and 11 is the output of the polygonal line approximation circuit 9 and the output of the variable offset circuit (10). It is an addition path for adding e, and its output terminal XK generates a threshold voltage vB. Before explaining the operation of the third bet, the operation of the threshold generator shown in FIG. 4 will be explained.

Semiconductor laser chip I! When the temperature is below the predetermined temperature fTM, the diode D is not conducting, so the output of the temperature/voltage converter 8 is transmitted only through the resistor R and the operational amplifier 9aK.
is input, and the amplification factor is -i R. However, when the chip temperature exceeds 1M, the diode D becomes conductive, so the output of the temperature-to-pressure converter-8 is input to the operational amplifier 9aK through the parallel connection circuit of resistors R1 and R1, and the amplification degree becomes 7.
It increases when it is less than M. In this way, the output of the polygonal line approximation oil passage 9 is i! It becomes a polygonal line characteristic whose slope increases with fTM as the boundary (FIG. 5). This resistance R1e FLm a
By passing R1 excessively, it is possible to match the slope of the polygonal line characteristic and the il f TM whose slope changes with the temperature-current characteristic of the semiconductor laser. For example, the set level is added in the addition circuit 11, and the characteristics at each temperature T+ w T Hirz Ts K shown in FIG.
It is possible to generate a comparison voltage corresponding to the driving current ID before entering the breakdown region P of the semiconductor laser at . If you combine a diode and a resistor that approximates a broken line in parallel, a more ** open tf- can be generated, so
Any 1! Of course, it is possible to deal with semiconductor lasers having longitudinal characteristics.

Now, returning to Fig. 3, we will explain the method for preserving rice cakes according to the present invention.
! v# to do.

It is now assumed that the semiconductor laser 2 is supplied with a constant voltage vs controlled by the APC method from the variable voltage source 1.

This constant voltage Vg is generated by a threshold generator 6 in a comparator 5.
It is compared with the threshold voltage vR outputted from. When the chip concentration of the semiconductor laser 2 is below the predetermined temperature fTM, the threshold value changes with a slightly gentle slope as shown in the first sWJ, but when the chip temperature is above the predetermined temperature 14 f:7M, the threshold value increases at a rate of becomes somewhat steep. When the constant voltage ■s is smaller than the threshold voltage Vl, a signal of V level is output from the comparator 5, so the 1-AND condition of the AND gate 7 is satisfied, and the modulation signal is is sent directly to the transistor 4.
given on the basis of. As a result, the drive current flowing through the semiconductor laser 2 is modulated by the modulation signal S. However,
If for some reason the constant voltage V exceeds the threshold voltage V and an overcurrent flows into the semiconductor relay f2, the output of the humpator 5 becomes 1L level, so the AND condition of the AND gate 7 does not hold, and therefore the transistor 4 It becomes non-conductive regardless of the change of heart @8K, and the driving current! D is instantly cut off. In this way, the semiconductor laser 2 may be destroyed by excessive drive current! be protected. Also, Chip I! Even if the temperature changes, the Vl
changes, so no inconvenience as shown in FIG. 2 occurs.

Sugar 6v! J is another embodiment of the semiconductor laser protection method according to the present invention, which is a method using current control. In the figure, 12 is a variable constant current source that supplies a constant current Is that is current-controlled by the APC method, 13 is a detection resistor that detects the drive current ID flowing to the semiconductor laser 2, and 14 is the semiconductor laser 2
A transistor that turns on/off the driving W current flowing to VT
h is a reference voltage that determines the switching threshold of the transistor 14. The other components indicated by reference numerals 2 to 6 are the same as the elements in the embodiment shown in FIG. 3, and τ is an OR gate. If the semiconductor laser used has a linear temperature characteristic, a simple WIL line approximation (b) as shown in FIG. 7 may also be used.

The dark value generator shown in Fig. 7 is obtained by omitting the polygonal line approximation excitation path from the clear value generator shown in Fig. 4. 1-degree voltage converter 8, variable offset circuit 10, addition path 11 and K
The circuit operation tube of the actual 1 hour example shown in FIG. 16 will be explained below.

Similarly to the embodiment shown in Figure 3, the semiconductor laser 2
The drive IE flow ID is detected as a 6 voltage drop by the detection resistor 13, and the potential V at point E and the threshold voltage vR generated from the threshold generator 6 are compared by the comparator 5, ■! If ≧vB, a signal of 1L level is output from the humparator 5, so only the modulation signal S is applied to the transistor 4 via the OR gate γ. As a result, the drive current In of the semiconductor laser 2 is controlled by the modulation signal B.

However, if the current to the semiconductor laser increases for some reason and Vt < Vi, the output of comparator 5 becomes 1H.
Since the output of the OR gate τ becomes 1H level, the transistor 4 becomes conductive and current flows through the circuit on the left (resistor 3 and transistor 4), so that no drive current flows to the semiconductor laser 2. This is from K.
The semiconductor laser 2 is protected at 6° or more.Although two implementations have been described, in each example, the drive current of the semiconductor laser is detected and the M value to be compared with it is changed depending on the temperature. This makes it possible to reliably detect when the drive current should be cut off even if the temperature changes, making it possible to cut off the drive current before the semiconductor laser is destroyed.

According to the present invention, in a semiconductor laser temperature-compensated by the APC method, the drive voltage or drive current of the semiconductor laser is detected, and the drive voltage or drive current is predetermined to change with the chip mix of the semiconductor laser. When the drive voltage or drive current exceeds the threshold value, the drive voltage or drive DC is cut off, so that drive #J1! Even if the voltage or drive current changes with temperature, the drive voltage or drive current can always be compared with the threshold value, and the drive voltage or drive current can be shut off, making it possible to ensure reliable maintenance of the semiconductor laser. I can do it. According to the present invention, the drive voltage and the
To cut off the current, we do not use a photodiode to obtain the optical output of the semiconductor laser, but instead directly detect the drive voltage or drive current. Furthermore, there is no need to use an expensive wideband amplifier, and therefore malfunctions due to ringing can be avoided and the semiconductor laser can be reliably protected.

[Brief explanation of the drawing]

Figure 1 is a diagram of the optical output characteristics of a semiconductor laser with respect to mt changes of the laser chip, and Figure 2 is a diagram of the optical output characteristics of a semiconductor laser with respect to temperature changes similar to Figure 1I1.
Figure 3 is an example of a protection circuit implementing the semiconductor laser protection method according to the present invention, and Figure 4 is the protection circuit shown in Figure 3. Figure 5 shows the output characteristics of the threshold generator, and Figure 6 shows other embodiments of the protection circuit that implements the haze protection method for semiconductor lasers by Akira Motomoto. Figure 7 shows another example of the orchid medulla growth benefit used in the method of the present invention. l...Variable constant current source, 2...Semiconductor laser, 3...
・Electric a koji resistance, 4.14...transistor, 5...
・Comparator, 6...Threshold generator, 7...AND gate, 7'...OR gate, 8...m interval/voltage conversion, 9...broken line approximation circuit, 9m...operation Amplifier, 10... Variable offset circuit, 11... Adder circuit, 12... Variable constant current source, 13... Detection resistor. Patent applicant: Konishiroku Photo Industry Co., Ltd. Agent: Patent attorney: Hiroshi Suzuki Fig. 2c γ Seven Electric I. Figure 3 Figure 4 Figure 5 Figure 3, □ Figure 7

Claims (2)

[Claims] (1) Detect the drive voltage or drive current of the semiconductor laser, compare the drive voltage or drive current with a predetermined threshold value that changes with the laser chip temperature based on the temperature characteristics of the semiconductor laser, and Alternatively, a method for protecting a semiconductor laser, characterized in that the drive voltage or drive current is cut off when the drive current exceeds the threshold value. (2) Claims characterized in that the threshold value changes in a tapered manner as the semiconductor laser chip is fabricated.
How to protect semiconductor lasers in Article 1 IIK.