GB2246258A - Optical sensor - Google Patents

Abstract

Calibration information describing the functional dependence of the optical response of an optical sensor (10) upon its measurand is stored in optically coded form as the spectral characteristic of a filter such as a diffraction grating (19) formed by refractive index variations or by microbending in the optical fibre pigtail (18) of the sensor (10). An interrogating beam is sent along an optical fibre 11 to the sensor and, after being modulated by the filter 19, returns along the fibre 11 and is split at 15 to two detectors. One detector 17 is a spectrometer 17 which detects gaps in the spectrum created by the filter 19. The sensor may be a resonator, the frequency of which varies with applied pressure or temperature, and the light may be sent and returned along different optical fibres. <IMAGE>

Description

OPTICAL SENSORS ! This invention relates to optical sensors.

Utilisation of optical techniques and advanced materials gives optical sensors the potential for improved accuracy and performance-in comparison with their electrical counterparts. To fully realise this potential, calibration information that is specific to each sensor is required which will substantially characterise the functional dependence of the sensor output upon the measurand to which the sensor is responsive. Such calibration information may be stored electronically, but if the full hostile environment capability of an optical sensor is to be exploited, this storage should not be at the sensor, but at a remote processing unit in order to obviate the need for electronic components at the sensor and more particularly the need for an electrical feed from the processing unit to such components at the sensor.

Nevertheless, interchangeability of optical sensors, which may be required in the event of sensor failure, would be considerably enhanced if the calibration information could be optically stored at the sensor head, as in these circumstances access to the processing unit would not be necessary, and there is a much reduced risk of a sensor being used with calibration information pertaining to another sensor. The present invention is particularly concerned with such optical storage of calibration information.

The optical storage facility provided by the invention can however be used for other purposes, either additonal to the storage of calibration information, or in place of it.

According to the present invention there is provided an optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent upon the magnitude of a measurand to which the sensor is responsive, which sensor includes a spectrally selective filter through which the interrogation signal or the output signal, or both signals are transmitted, the spectral characteristics of which filter provides information that characterises the sensor.

The invention further provides an optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent upon the magnitude of a measurand to which the sensor is responsive, which sensor includes a spectrally selective filter through which the interrogation signal or the output signal, or both signals are transmitted, the spectral characteristics of which filter substantially characterise sad functional dependence.

In instances where either or both signals are coupled with a sensor by way of an optical fibre pigtail, a preferred form of spectrally selective filter is provided by a grating formed in the optical fibre pigtail of that sensor.

There follows a description of a telemetry system including at least one sensor embodying the invention in a preferred form optically coupled with a sensor information processing unit at a location remote from that sensor. The description refers to the accompanying drawing which is a schematic diagram of the system.

The telemetry system of the drawing comprises an optical sensor head indicated generally at 10 connected by way of an optical fibre 11 to a sensor information processing unit indicated generally at 12.

Although not shown in this drawing, a single processing unit may be operated with more than one sensor either on a time division or wavelength division multiplexing basis.

In the processing unit 12 light from an optical source 13 is directed by way of a first beam splitter 14 into the optical fibre 11. The return signal from the sensor is directed by way of that beam splitter 14 to a second beam splitter 15 which shares the return signal between a detector 16 and a spectrometer 17.

The optical sensor 10 is basically a conventional optical sensor, for instance one in which the measurand such as pressure or temperature affects the frequency of oscillation of a resonator. Examples of such sensors are for instance described in United Kingdom Patent Application Specifications Nos.

GB 2 215 840A and 2 215 055A. The special feature of this sensor 10 is its fibre pigtail 18 by means of which the sensor is optically coupled with the fibre 11. This fibre pigtail 18 incorporates at least one optical grating 19 having a periodicity designed to absorb or scatter specific wavelengths. Such a grating can be formed by modifying the optical fibre itself, for instance by suitable doping to change its refractive index or pressing it against a mechanical grating to induce microbending-induced mode-coupling between the fundamental mode-and one or more other modes that are significantly attennuated. Alternatively the grating can be provided by inserting a micro-optic or integrated optic component into the fibre.

The source 13 is broad band, and so the return signal from the detector, which has made a double pass through the grating 19, will have gaps in its spectrum corresponding to the wavelengths that have been selectively scattered or absorbed by the grating. These are analysed by the spectrometer 17. The grating 19 is designed so that the gaps in the spectrum provide a coded representation that substantially characterises the functional relationship between the sensor measurand, typically pressure or temperature, and the optical output of the sensor, typically modulation frequency. In order to achieve greater precision with this characterisation it may be more convenient to use an arrangement of two or more optical gratings optically in series, each having a relatively simple periodic structure, rather than a single grating with a more complex periodic structure.

The need to use a spectrometer for analysing the wavelengths selectively removed from the return signal by the grating 19 can be avoided by using instead of the broad-band source 13 a narrow-band source whose emission frequency is swept through the spectral range of interest. Another possible modification of the system is to employ separate fibres for the interrogation signal and the return signal. Under these circumstances the beam splitter 14 is dispensed with, and source 13 is connected with the sensor via one fibre to provide the interrogation signal, while the return signal is fed directly to beam splitter 15. The grating 19, or gratings, may be on the interrogation side of the sensor 10 or on its return side, or may be shared between the two sides.

Claims (2)

CLAIMS.

1. An optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent upon the magnitude of a measurand to which the sensor is responsive, which sensor includes a spectrally selective filter through which the interrogation signal or the output signal, or both signals are transmitted, the spectral characteristics of which filter provides information that characterises the sensor.

2. An optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent upon the magnitude of a measurand to which the sensor is responsive, which sensor includes a spectrally selective filter through which the interrogation signal or the output signal, or both signals are transmitted, the spectral characteristics of which filter substantially characterise said functional dependence.