GB2117896A

GB2117896A - Optical microswitch system

Info

Publication number: GB2117896A
Application number: GB08209715A
Authority: GB
Inventors: David George Dalgoutte
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-04-01
Filing date: 1982-04-01
Publication date: 1983-10-19
Also published as: GB2117896B

Abstract

An optical microswitch system has a source 1 and photodetectors 6, 7 linked with a shutter 5 by means of a single optical fibre 3. The photodetectors detect light of different wavelengths, and the switch shutter 5 obstructs the reflection of one of these, the other being either always reflected or never reflected. The two photodetector outputs are compared in a differential amplifier 12 which operates to cancel out the effect of spurious reflections which are otherwise liable to mask the reflection signal provided by the microswitch. Variable gain amplifier 11 is used to zero the differential amplifier when the first one of the wavelengths is not reflected, and the output of the differential amplifier is divided by the signal for the unaffected wavelength at 13 to compensate for drift of the source 1. <IMAGE>

Description

SPECIFICATION Optical microswitch system This invention relates to an optical microswitch system with remote read-out.

For certain applications such as general process control in hazardous areas it is convenient to monitor the position of some integer by means of a microswitch that operates optically rather than electrically. This may be for instance because the microswitch has to operate in an environment where arcing electrical contacts could cause an explosion. The particular type of optical microswitch system with which the present invention is concerned is one in which light from an optical source is directed to the microswitch along an optical fibre to be selectively reflected back into the fibre by the microswitch for return to a detector located at the same end of the fibe as the source.

In a simple system of this type, if the fibre length is such that its attenuation is significant, the strength at the detector of the return signal from the microswitch may be comparable with a spurious signal generated by unwanted and insufficiently suppressed reflections at the fibre ends or at any connection or splice between the ends. The present invention is concerned with an optical microswitch system employing light of two different wavelengths in such a way as to resolve this type of ambiguity.

According to the present invention there is provided an optical microswitch system with remote read-out which system includes; a transmitter adapted to launch light of first and second wavelengths into one end of an optical fibre; a shutter assembly at the opposite end of the fibre having a construction such that in one shutter position light of the first wavelength but substantially none of the other is reflected back into the fibre, and, in another position, either substantially no light of either wavelength is reflected back into the fibre or light of both wavelengths is reflected back into the fibre in substantially equal quantitities; first and second optical detectors optically coupled with said one end of the fibre and adapted to provide output signal strengths at said first and second wavelengths, at least one of which detectors incorporates a variable gain amplifier or other means for varying its sensitivity; and a differential amplifier connected to the outputs of the detectors.

There follows a description of an optical microswitch system embodying the invention in a preferred form. The description refers to the accompanying drawing which shows a schematic of the system.

The output of a light source 1, which may be a composite source rather than a single emitter, is directed through a semi transparent beam splitter or 3 dB optical fibre beam splitting coupler 2 into one end of an optical fibre 3. This light source emits at least at two wavelengths A, and 2, and typically may be provided by two light emissive diodes of different band-gap material, such as a Galas diode operating at 0.85 microns and an InGaAsP diode operated at 1.3 microns, whose outputs are combined typically with a dichroic reflector or by means of a 3 dB optical fibre beam splitter (not shown). The optical fibre extends to a remote location and terminates in a microswitch housing (not shown) of a reflex-type optical microswitch.This microswitch contains a mirror 4 for reflecting light emerging from the fibre and back into the fibre, and the shutter element 5 of the microswitch is located between the mirror and the end of the fibre. In one position of the shutter substantially no light of either wavelength is reflected back into the fibre, while in the other position light of wavelength A, is reflected back into the fibre, but substantially none of wavelength A2. This can be achieved by use of a dichroic mirror for mirror 4 which does not reflect at the wavelength i2, or by making the shutter 5 in two parts, one of which is opaque to light of both wavelengths, and the other of which is opaque to light of wavelength i2, but substantially transparent to light of wavelength A,.

Alternatively the separate mirror can be dispensed with in favour of the use of a dichroic reflecting shutter. In one position of this reflecting shutter light of wavelength A, only is reflected, while in the other position light of neither wavelength is reflected.

At the first end of the optical fibre the output of the third part of the 3 dB optical fibre splitter 2 is optically coupled with two photodetectors 6, 7 by way of dichroic beam splitter 8 so that photodetector detects light of wavelength Aw, and photodetector detects light of wavelength i2.

If there were no spurious reflections then photodetector 7 should receive no signal since in neither position of the shutter 5 should light of wavelength 2 be reflected. In practice however, light of both wavelengths is liable to be reflected by imperfections in the system. Reflection is particularly likely to occur at the terminations represented at 9 and 10 at both ends of the fibre 3. The spurious reflection at termination 9 is particularly liable to be significant in comparison with the signal reflected at the microswitch if the length of the fibre 3 is such that its transmission loss is significant.

The output of photodetector 6 is fed via a variable gain amplifier 11 to the non-inverting input of a differential amplifier 1 2 while the output of photodetector 7 is fed direct to its inverting input. The gain of amplifier 11 is set to provide a balance providing zero output from the differential amplifier when the shutter 5 is in the position in which the microswitch is intended not to reflect light of wavelength A, back into the fibre. It will be apparent that the variable gain amplifier can be dispensed with if alternative means are used to adjust the relative sensitivity of the two photodetectors.

Optionally in order to take into account of possible changes in total power of emission from the source 1, the output of the differential amplifier may be fed to a divide circuit 13 where this output is divided by the output of the photodetector 7.

The output of the divider, or, if the divider is not being used, the output of the differential amplifier, may optionally be fed to a comparator, Schmitt trigger or similar threshold sensing device (not shown).

The above described system provides a way of cancelling the effects of spurious reflections at the wavelength A, which might otherwise mask the microswitch signal reflection by generating a compensating output derived from spurious reflections at the wavelength 2. This compensating signal is used to balance the spurious reflections signal at the wavelength 2.

Alternatively the system can be modified so that the microswitch always reflects at wavelength 2 while reflection at wavelength A, is dependent upon shutter position. The operation is now in essence the inverse of that previously described insofar as the system is now adjusted to balance the differential amplifier inputs when the microswitch is reflecting at both wavelengths in substantially equal quantities, and the unbalanced output signal is provided when wavelength A, is not reflected by the microswitch.

Claims

1. An optical microswitch system with remote read-out which system includes; a transmitter adapted to launch light of first and second wavelengths into one end of an optical fibre; a shutter assembly at the opposite end of the fibre having a construction such that in one shutter position light of the first wavelength but substantially none of the other is reflected back into the fibre, and, in another position, either substantially no light of either wavelength is reflected back into the fibre or light of both wavelengths is reflected back into the fibre in substantially equal quantities; first and second optical detectors optically coupled with said one end of the fibre and adapted to provide output signal strengths at said first and second wavelengths, at least one of which detectors incorporates a variable gain amplifier or other means for varying its sensitivity; and a differential amplifier connected to the outputs of the detectors.

2. An optical microswitch system as claimed in claim 1, wherein the output of the differential amplifier is connected to the input of a divide circuit adapted to divide the differential amplifier output by the output of the second photodetector.

3. An optical microswitch system substantially as hereinbefore described with reference to the accompanying drawing.