GB2154787A

GB2154787A - Laser stabilisation circuit

Info

Publication number: GB2154787A
Application number: GB08401143A
Authority: GB
Inventors: Roger Edward Jones; Roland Henry Pratt
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1984-01-17
Filing date: 1984-01-17
Publication date: 1985-09-11
Also published as: GB2154787B; GB8401143D0

Abstract

To stabilise the light output from a semiconductor laser (1), that output is split into beams in two optical fibres by a coupler (6). One output beam is the effective output of the laser while the other is a reference output. The reference output is applied via a Fabry Perot fibre optic interferometer (9) to a detector (11) where it is compared with a reference parameter. Any difference results in an error signal which is used to appropriately adjust the laser's current supply to give the light output the required characteristics. As shown the effective output goes to another Fabry Perot optic interferometer (10) used as a sensor. <IMAGE>

Description

SPECIFICATION Laser stabilisation circuit This invention relates to an arrangement for the stabilisation of a semiconductor laser.

For some types of optical fibre sensor, light with a highly stable wavelength is needed.

This is not as readily available from semiconductor diode lasers as it is from gas lasers. In view of the greater convenience of using the relatively small semiconductor lasers as compared with gas lasers this is a disadvantage.

Hence an object of the invention is to produce an arrangement for stabilising a semiconductor laser's wavelength which is adequate for such purposes.

According to the invention, there is provided an arrangement for the stabilisation of a semiconductor laser, in which a proportion of the light output of the laser is applied to a Fabry Perot interferometer the output of which is applied to a light-responsive detector which generates an output appropriate to the characteristics of the light applied to the detector, in which the detector's output is compared with a reference parameter appropriate to what the light output should be, which comparator produces an error signal appropriate to the relation between the laser's light output and what it should be, and in which said error signal is fed back to the laser current supply circuit such as to adjust the laser current in a manner as to cause the laser's output to be altered towards what that output should be.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 represents the optical aspects while Fig. 2 represents the electronic aspects of an arrangement for stabilising a semiconductor laser.

In the arrangement shown in Figs. 1 and 2, the laser 1 is mounted on a support with a temperature sensor 2 and a Peltier cooler, which together co-operate to maintain the temperature of the laser at a suitable level.

This is indicated by the block 4, Fig. 2, which provides the laser temperature control.

The light from the laser 1 passes via launching optics 5 and an optical fibre to a coupler 6, which may be fused or polished, and which splits the light beam into two outputs. Each of these outputs extends via a respective fibre-fibre coupler 7, 8 to a reference Fabry Perot interferometer and a sensing Fabry Perot interferometer 9, 1 0. These interferometers each has its own detector 11, 1 2.

In the detector block, the output of the lightresponsive device, e.g. a photo-diode, is compared with a reference quantity by a comparator. Hence the comparator output is an error signal appropriate to the relation between the light reaching the detector, and what the light should be. This error signal is applied to the laser current supply to influence the latter in the appropriate sense.

As can be seen from Fig. 2, the output of the reference interferometer 9 is fed back to the laser current supply circuit 1 3 to control the current supply for the laser in a sense appropriate to stabilisation of the wavelength of the light. The other Fabry Perot interferometer 10 is the sensing element, and its output is obtained via a discriminator 1 6 and a pulse counter 17, which counts distances between successive peaks in the transmission characteristics of the interferometer 10, which distances vary in accordance with the parameter being sensed.

At this point it is mentioned that each of these interferometers is a length of an optical fibre, such as a single mode fibre with semisilvered optically flat ends, which ends thus act as mirrors for the interferometer. The influence of the parameter being sensed, which can be exercised in various ways, e.g.

by magneto-striction, piezo-electric means, thermal effects, acoustic effects, varies the length of the fibre. Thus detection depends on measuring the changes in the transmitted signals as functions of changes in length due to the parameter being sensed. In one case the sensor interferometer was 40cm long, but the reference interferometer may be shorter.

In the arrangement described herein, a proportion, typically 5% of the laser's total drive current, is controlled by the output of the photo-detector 11 at the remote end of the reference Fabry Perot interferometer. Hence a closed loop is formed and the transmission of the interferometer is retained at a constant value. Isolation of the interferometer from environmental change by suitable protection means that the laser's output wavelength is stabilised. As can be seen, the light output of the laser is also applied to the sensing interferometer, so that the latter thus receives light at a stabilised wavelength.

The light is launched into each of the Fabry Perot interferometers via index-matching material provided by the couplers 7, 8. Each of these couplers may be in the form of a GTE elastomeric splice. Such splices are available from GTE Products Corporation, 2401 Reach Road, Williamsport, PA17701, USA.

In the arrangements described above, the ends of the interferometer are silvered, the reflective coatings can also be applied as multi-layer dielectric coatings.

1. An arrangement for the stabilisation of a semiconductor laser, in which a proportion of the light output of the laser is applied to a Fabry Perot interferometer the output of which is applied to a light-responsive detector which generates an output appropriate to the characteristics of the light applied to the detector, in which the detector's output is compared with a reference parameter appropriate to what the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Laser stabilisation circuit This invention relates to an arrangement for the stabilisation of a semiconductor laser. For some types of optical fibre sensor, light with a highly stable wavelength is needed. This is not as readily available from semiconductor diode lasers as it is from gas lasers. In view of the greater convenience of using the relatively small semiconductor lasers as compared with gas lasers this is a disadvantage. Hence an object of the invention is to produce an arrangement for stabilising a semiconductor laser's wavelength which is adequate for such purposes. According to the invention, there is provided an arrangement for the stabilisation of a semiconductor laser, in which a proportion of the light output of the laser is applied to a Fabry Perot interferometer the output of which is applied to a light-responsive detector which generates an output appropriate to the characteristics of the light applied to the detector, in which the detector's output is compared with a reference parameter appropriate to what the light output should be, which comparator produces an error signal appropriate to the relation between the laser's light output and what it should be, and in which said error signal is fed back to the laser current supply circuit such as to adjust the laser current in a manner as to cause the laser's output to be altered towards what that output should be. An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 represents the optical aspects while Fig. 2 represents the electronic aspects of an arrangement for stabilising a semiconductor laser. In the arrangement shown in Figs. 1 and 2, the laser 1 is mounted on a support with a temperature sensor 2 and a Peltier cooler, which together co-operate to maintain the temperature of the laser at a suitable level. This is indicated by the block 4, Fig. 2, which provides the laser temperature control. The light from the laser 1 passes via launching optics 5 and an optical fibre to a coupler 6, which may be fused or polished, and which splits the light beam into two outputs. Each of these outputs extends via a respective fibre-fibre coupler 7, 8 to a reference Fabry Perot interferometer and a sensing Fabry Perot interferometer 9, 1 0. These interferometers each has its own detector 11, 1 2. In the detector block, the output of the lightresponsive device, e.g. a photo-diode, is compared with a reference quantity by a comparator. Hence the comparator output is an error signal appropriate to the relation between the light reaching the detector, and what the light should be. This error signal is applied to the laser current supply to influence the latter in the appropriate sense. As can be seen from Fig. 2, the output of the reference interferometer 9 is fed back to the laser current supply circuit 1 3 to control the current supply for the laser in a sense appropriate to stabilisation of the wavelength of the light. The other Fabry Perot interferometer 10 is the sensing element, and its output is obtained via a discriminator 1 6 and a pulse counter 17, which counts distances between successive peaks in the transmission characteristics of the interferometer 10, which distances vary in accordance with the parameter being sensed. At this point it is mentioned that each of these interferometers is a length of an optical fibre, such as a single mode fibre with semisilvered optically flat ends, which ends thus act as mirrors for the interferometer. The influence of the parameter being sensed, which can be exercised in various ways, e.g. by magneto-striction, piezo-electric means, thermal effects, acoustic effects, varies the length of the fibre. Thus detection depends on measuring the changes in the transmitted signals as functions of changes in length due to the parameter being sensed. In one case the sensor interferometer was 40cm long, but the reference interferometer may be shorter. In the arrangement described herein, a proportion, typically 5% of the laser's total drive current, is controlled by the output of the photo-detector 11 at the remote end of the reference Fabry Perot interferometer. Hence a closed loop is formed and the transmission of the interferometer is retained at a constant value. Isolation of the interferometer from environmental change by suitable protection means that the laser's output wavelength is stabilised. As can be seen, the light output of the laser is also applied to the sensing interferometer, so that the latter thus receives light at a stabilised wavelength. The light is launched into each of the Fabry Perot interferometers via index-matching material provided by the couplers 7, 8. Each of these couplers may be in the form of a GTE elastomeric splice. Such splices are available from GTE Products Corporation, 2401 Reach Road, Williamsport, PA17701, USA. In the arrangements described above, the ends of the interferometer are silvered, the reflective coatings can also be applied as multi-layer dielectric coatings. CLAIMS

1. An arrangement for the stabilisation of a semiconductor laser, in which a proportion of the light output of the laser is applied to a Fabry Perot interferometer the output of which is applied to a light-responsive detector which generates an output appropriate to the characteristics of the light applied to the detector, in which the detector's output is compared with a reference parameter appropriate to what the light output should be, which comparator produces an error signal appropriate to the relation between the laser's light output and what it should be, and in which said error signal is fed back to the laser current supply circuit such as to adjust the laser current in a manner as to cause the laser's output to be altered towards what that output should be.

2. An arrangement as claimed in claim 1, and in which the Fabry Perot interferometer is a length of a single mode optical fibre with its ends half-silvered to produce two mirrors which are substantially parallel, light entering and leaving the fibre ends via the half-silvering.

3. An arrangement as claimed in claim 1, and in which the Fabry-Perot interferometer is a length of a single-mode optical fibre with its ends provided with multi-layer dielectric coatings.

4. An arrangement as claimed in claim 1, 2 or 3, in which the light from the laser is applied via an optical fibre to an optical fibre coupler where it is split into two beams one of which is applied to said Fabry Perot interferometer whilst the other forms the effective output of the laser.

5. An arrangement as claimed in claim 1, 2, 3 or 4, in which the laser mounting is provided with a temperature sensor and a Peltier cooler, which maintains the temperature of the laser at a desired level.

6. An arrangement as claimed in claim 4 or 5 as appendent to claim 3, in which the laser's effective output is applied to another Fabry Perot interferometer function as a sensor.

7. An arrangement for the stabilisation of a semiconductor laser, substantially as described with reference to the accompanying drawings.

CLAIMS Superseded claims 6 8 7 New or amended claims:- 8 to 1 3

6. An arrangement as claimed in claim 4 or 5 as appendent to claim 3, in which the laser's effective output is applied to another Fabry-Perot interferometer which functions as a sensor.

8. An optical fibre sensing arrangement, which includes a laser, the light output from which is applied to a Fabry-Perot interferometer which is subjected to the parameter to be sensed, so that the parameters of the light negotiating the interferometer are affected by the parameter to be sensed, a light-responsive detector responsive to the light leaving the interferometer and hence to the parameter to be sensed, a further Fabry-Perot interferometer to which a proportion of the light output of the laser is applied, a further light responsive detector responsive to the light leaving the further interferometer, and a source of a reference parameter whose value represents the desired wavelength of the light emitted by the laser, in which the reference parameter is compared with the output of the further detector and hence with the wavelength of the light leaving the further interferometer, in which said comparison produces an error signal appropriate to the difference (if any) between what the laser's light wavelength is and the desired light wavelength, and in which said error signal is fed back to the laser current supply circuit in such a way as to so adjust the laser current as to alter the laser's output wavelength towards the desired value for that wavelength.

9. An arrangement as claimed in claim 8, and in which each said Fabry-Perot interferometer is a length of single-mode optical fibre with its ends half-silvered to produce two mirrors which are substantially parallel to each other, light entering and leaving the fibre ends via the half-silvering.

10. An arrangement as claimed in claim 8, and in which each said Fabry-Perot interferometer is a length of single-mode optical fibre with its ends provided with substantially parallel multi-layer dielectric coatings.

11. An arrangement as claimed in claim 8, 9 or 10, and in which the light from the laser is applied via an optical fibre to an optical fibre coupler where it is split into two beams one of which is applied to the first Fabry-Perot interferometer and the other of which is applied to the further Fabry-Perot interferometer.

1 2. An arrangement as claimed in claim 8, 9, 10 or 11, and in which the laser mounting is provided with a temperature sensor and a Peltier coder which maintains the temperature of the laser at a desired level.

1 3. An optical fibre sensing arrangement substantially as described with reference to the accompanying drawing.