GB2045516A

GB2045516A - Limiter circuit for a semiconductor laser

Info

Publication number: GB2045516A
Application number: GB8009442A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-03-26
Filing date: 1980-03-20
Publication date: 1980-10-29
Also published as: FR2452807A1; GB2045516B; FR2452807B3; DE2911858A1; JPS55130190A; DE2911858C2

Abstract

The specification concerns a limiter circuit for a semiconductor laser 2 whose drive current is regulated by a control amplifier 13 according to a comparison between a reference value and an actual value representing the power developed by the laser. The limiter circuit comprises an actual value current measuring circuit 6 and a threshold value circuit 8 arranged between the input of the control amplifier and the reference value source 7. <IMAGE>

Description

SPECIFICATION Limiter circuit for a semiconductor laser The invention relates to a limiter circuit for a semiconductor laser.

Glass fibre optical systems using semiconductor lasers as a light source require a control circuit whereby ageing effects and temperature fluctuations of the laser can be compensated so that the output power of the laser is maintained substantially constant. Normally for this purpose a portion of the laser light is branched off to a photodiode, and the photocurrent of the latter is compared with a predetermined reference value. The control circuit then removes any errors between the actual value of the photocurrent and the reference value.

In such a case it may happen than the operating point of the laser becomes incorrectly adjusted, for example because of a spurious reference value or because of maladjustment or short circuit of the reference value source. As a result of this the very sensitive and comparatively expensive laser may become damaged or even destroyed.

The invention therefore takes as its basic purpose the provision of a limiter circuit for a semiconductor laser, which limits the light output of the laser and thereby protects the laser from damage.

Accordingly, the present invention provides a limiter circuit for a semiconductor laser whose drive current is regulated by a control amplifier in dependence upon a comparison between an adjustable reference value and an actual value representing the power developed by the laser, the limiter circuit comprising an actual value current measuring circuit and a threshold value circuit arranged between the input of the control amplifier and the adjustable reference value source.

The invention has the advantage that without requiring substantially increased capital expenditure on circuit components it ensures reliable overload protection for the expensive laser.

An embodiment of the invention will now be described with reference to the accompanying drawings, wherein; Figure 1 is a block schematic diagram of a semiconductor laser circuit including a limiter circuit according to the embodiment of the invention, Figure 2 is a more detailed circuit diagram of the semiconductor laser circuit of Fig. 1.

In both figures equivalent components are indicated by the same reference characters.

In Fig. 1 a light guide 1 is optically coupled to a semiconductor laser 2, which is supplied with current by a driving stage 3. The driving current is controlled by a control circuit 4.

Furthermore, the laser 2 is connected to a current measuring circuit 6 which delivers to the control circuit 4 a quantity (actual value) representative of the instantaneous power developed by the laser. An adjustable reference value source 7 is also connected to the control circuit 4, and a comparison of the reference and actual values is performed in the control circuit 4, whereby the current of the driving stage 3 is controlled.

For protecting the laser 2, a threshold value circuit 8 is connected to the current measuring circuit 6 and to the control circuit 4. If the actual value delivered to the control circuit 4 increases above a certain threshold, then the threshold value circuit 8 responds and influences the control circuit 4 in such a manner that the control circuit correspondingly reduces the driving current, whereby the laser is protected from damage.

The functioning of the threshold value circuit 8 will now be more particularly described with reference to the detail circuit diagram of Fig. 2.

Under normal operation conditions the laser 2 delivers a portion of its light output to a photodiode 11 of the current measuring circuit 6. The current delivered by the photodiode 11 will vary in like sense with changes in the intensity of the light received by the photodiode 11, so that with increasing light output power of the laser the photodiode current also increases and brings about a greater voltage drop across a resistance 1 2.

This latter, representing the instantaneous photodiode current and thus the instantaneous power of the laser, in combination with the current delivered by the reference value source 7, is effective at the input of a control amplifier 1 3 in such a manner that at the output of the control amplifier 1 3 a control current is available which regulates a transistor 1 4 in the driving stage 3 in the sense of maintaining substantially constant the light output delivered by the laser 2.

If the laser light output becomes excessive, and accordingly if the photodiode current and thereby the voltage drop across the resistance 1 2 exceeds a predetermined threshold value, then a Zener diode 1 6 connected at the junction point 1 4 of the photodiode 11 and the resistance 1 2 becomes conducting. This in turn causes a normally blocked transistor 1 7 to become conducting, so that current additionally flows from the reference value source 7 through a resistance 1 8 and the transistor 1 7. This action inhibits an increase in the laser light output, in that the current delivered to the input of the control amplifier 1 3 is correspondingly reduced.

The result is that the control current at the output of the amplifier 1 3 influences the driving current in such a manner as to cause a limitation in the laser light output, thereby avoiding damage to the laser 2.

It is to be understood that although the foregoing has used a Zener diode as the threshold value switch controlling the transistor 17, other arrangements are possible. In particular it is envisaged that a comparator could be used which compares the voltage at 14 with a reference threshold voltage.

Claims

1. A limiter circuit for a semiconductor laser whose drive current is regulated by a control amplifier in dependence upon a comparison between an adjustable reference value and an actual value representing the power deveioped by the laser, the limiter circuit comprising an actual value current measuring circuit and a threshold value circuit arranged between the input of the control amplifier and the adjustable reference value source.

2. A limiter circuit according to claim 1, wherein the actual value current measuring circuit comprises a photodiode which is irradiated by the semiconductor laser and is connected in series with a resistance, wherein the junction between the photodiode and the resistance is connected to a current amplifier through a threshold value switch, and wherein the current delivered to the current amplifier is arranged to influence the comparison between the reference value and the actual value in such a manner that the control amplifier regulates the drive current so that the light power delivered by the laser does not exceed a predetermined level.

3. A limiter circuit according to claim 2, wherein the threshold value switch comprises a Zener diode.

4. A limiter switch according to claim 2, wherein the threshold value switch comprises a comparator.

5. A limiter circuit for a semiconductor laser substantially as described with reference to Fig. 2 of the accompanying drawings.