GB2237872A

GB2237872A - Temperature sensor

Info

Publication number: GB2237872A
Application number: GB8925222A
Authority: GB
Inventors: James Mark Naden; Julie Ann Clements
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-11-08
Filing date: 1989-11-08
Publication date: 1991-05-15
Anticipated expiration: 2009-11-08
Also published as: GB8925222D0; CA2022697A1; GB2237872B

Abstract

A temperature sensor comprises a Fabry-Perot cavity formed between the end of the single mode fibre 14 and the surface of a silicon reflector 15 is formed as one sidewall of a channel 17 in a block of silicon 18. The other sidewall 19 of the channel 17 is of lesser height from the base of the channel and terminates a portion 20 of the block which provides an anchorage for the end of the fibre 14. Wavelength modulated light is propagated down the fibre and changes of temperature alter the optical path length of the cavity. The light reflected in the fibre will form interference fringes and the relative magnitudes of the harmonic components in the reflected light are detected and used to determine the temperature changes. The sensor may be formed as a bathythermograph. <IMAGE>

Description

Temperature Sensor This invention relates to a fibre optic temperature sensor device and an arrangement for the utilisation of such a device.

Temperature sensors find application in many fields. One application for a temperature sensor is in a bathythermograph, an instrument having an expendable sensor used to obtain a temperature profile of the sea.

Currently expendable sensors used in bathythermography consists of a thermistor located in a probe which is fired from a ship. An electrical connection is maintained between data processing equipment on board the ship and the probe through a wire which is unreeled from the probe as it descends through the water.

The probes are designed to give a predictable rate of descent. As the water temperature changes the resistance of the thermistor varies. This data is conveyed along the wire to the data processing system, where it is converted into measurements of absolute temperature. Data is recorded and displayed in real time as the probe falls. When the probe reaches its rated maximum depth (a function of ship speed and quantity of wire) the wire breaks and the profile is completed.

According to the invention there is provided a fibre optic temperature sensor arrangement including a length of optical fibre and means for forming a Fabry-Perot cavity between one end of the fibre and a reflecting surface positioned from the end of the fibre, means for illuminating the cavity with wavelength modulated light and means for monitoring the relative magnitudes of the harmonic components in the reflected optical signal whereby information relating to the optical path length is obtained.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of an optical bathythermograph system, Fig. 2 illustrates a fibre optic temperature sensor device, Figs. 3 and 4 illustrate graphical plots of optical intensity versus time recorded at a photodetector resulting from interference fringes created at a fibre optic temperature sensor.

The fibre optical bathythermograph arrangement shown in Fig. 1 consists of a 1.3um semiconductor laser 10, a photodetector 11, a 3dB coupler 12 and signal processing electronics (not shown) all housed on ship and which are connected to a temperature sensor 13 via a single mode fibre 14.

The temperature sensor, Fig. 2, consists of a Fabry-Perot cavity formed between the end of the single mode fibre 14 and the surface of a silicon reflector 15. The silicon reflector is formed as one sidewall of a channel 17 in a block of silicon 18. The other sidewall 19 of the channel 17 is of lesser height from the base of the channel and terminates a portion 20 of the block which provides an anchorage for the end of the fibre 14.

The gap of the Fabry-Perot cavity formed across the channel 17 is of the order of 100um. The end of the fibre 14 is precisely aligned with the silicon reflector surface 15 by placing the fibre in a groove 21 in the silicon. The groove is formed by anisotropic etching of the silicon portion 20.

In operation the wavelength L of the laser 10 is sinusoidally modulated at a frequency f such that L=L + psin(2ft).

where L is the mean wavelength, p is the wavelength 0 modulation index and t is time.

Consider continuous wave light of wavelength L incident on the silicon surface 15 from the fibre end.

A proportion of the light reflected from the silicon will re-enter the fibre and interfere with the light reflected from the fibre end. For a balanced interferometer, in which the two reflections (from the silicon and from the fibre end) are equal, the total reflected intensity I received at the photodetector is given by: I α Io {1+cos(4#x/[Lo+sspsin(2#ft)])} ...(1) where I is the total light intensity propagating 0 towards the fibre end and x is the optical path length between the silicon surface and the fibre end. The interference fringes received at the photodetector 11 (given by Equation 1) are plotted in Figure 3. The quadrature points, which are the points of maximum sensitivity for the interferometer (where dI/dt is a maximum), are also shown.

At the start of temperature measurements the mean wavelength L0 of the laser is tuned; so that the wavelength of the laser is equal to L at the 0 quadrature point of the interference fringes. This ensures that the interference fringes exhibit half-wave symmetry which can only be achieved with odd harmonics.

Hence the interference fringes cannot contain any even (2f) harmonic components; but odd (3f) harmonic components are present and their magnitude is proportional to the wavelength modulation index B.

When the ambient temperature of the Fabry-Perot cavity changes the optical path length x between the silicon surface and the fibre end alters such that: x = Ll+a(T-T0)]nx0 ... (2) where a represents the linear expansivity of silicon, T the ambient temperature, To the initial temperature, n the refractive index of the Fabry-Perot cavity and xO the initial difference between the silicon surface and fibre end.

The equation for the interference fringes received at the photodetector when a change of temperature occurs is therefore obtained by combining equations (1) and (2): I Cc 10l+cos[4if(l+a(T-T0 ))nx0/(L0+Bsin(2 fit)) 1 The change in optical path length with temperature alters the intensity pattern of the interference fringes such that they no longer exhibit half-wave symmetry (see Figure 4). This introduces a 2f component into the fringes where the magnitude of the 2f is proportional to the change in temperature.

The interference fringes received at the photodetector 11 are converted into an electrical signal which then passes into a signal processing circuit (not shown) which extracts the 2f and 3f harmonic components, and then determines the ratio of the components. Note the absolute magnitude of the 2f and 3f components is subject to intensity fluctuations occurring within the system, however the ratio of the components is not subject to intensity fluctuations. It should also be noted that the 3f harmonic is monitored rather than the f harmonic, because the f harmonic is subject to intensity variations (reflections) which do not affect the 2f and 3f signals.

It can be seen that by monitoring the ratio of the 2f and 3f components the ambient temperature of the Fabry-Perot cavity can be determined.

Claims

1. A fibre optic temperature sensor arrangement including a length of optical fibre and means for forming a Fabry-Perot cavity between one end of the fibre and a reflecting surface positioned from the end of the fibre, means for illuminating the cavity with wavelength modulated light and means for monitoring the relative magnitudes of the harmonic components in the reflected optical signal whereby information relating to the optical path length is obtained.

2. A sensor arrangement according to claim 1 wherein the cavity forming means comprises a body having a channel, one side wall of the channel providing the reflecting surface and the other side wall of the channel terminating a portion of the body to which the fibre end is attached.

3. A sensor arrangement according to claim 2 wherein the body is silicon.

4. A sensor arrangement according to claim 3 wherein the portion of the body to which the fibre is attached is formed with a groove normal to the reflecting surface, the fibre end being laid in the groove and secured therein.

5. A fibre optic temperature sensor arrangement substantially as described with reference to Fig. 2 of the accompanying drawings.

6. A bathythermograph arrangement including a sensor arrangement according to any one of claims 1 to 5.

7. A bathythermograph arrangement including a semiconductor laser and a photodetector coupled via a length of optical fibre to a fibre optic temperature sensor having a Fabry-Perot cavity, means for periodically modulating the laser wavelength and m-eans for detecting changes in optical intensity at the photodetector of light reflected from the sensor.