GB2198531A

GB2198531A - Optical sensor system

Info

Publication number: GB2198531A
Application number: GB08629820A
Authority: GB
Inventors: Roger Edward Jones
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-12-13
Filing date: 1986-12-13
Publication date: 1988-06-15
Anticipated expiration: 2006-12-13
Also published as: GB8629820D0; GB2198531B

Abstract

An optical sensor, e.g. a pressure or temperature sensor, includes an optically driven vibrating element 13 whose natural resonant frequency is a function of the parameter to be measured. Detection of vibration is effected interferometrically in optical system 11. Feedback to provide drive of the element and the correct frequency and phase is provided in electronic system 12 via a phase locked loop 37 running at double the vibrational frequency. This prevents loss of phase matching. Typically the frequency doubler comprises a precision full wave rectifier 35 (or a signal squarer). Application is to well-logging, chemical processing or nuclear instrumentation. <IMAGE>

Description

OPTICAL SENSOR DEVICE this invention relates to optical sensors, and to signal processing techniques and apparatus for use with optical sensors.

Optical sensors, e.g. for sensing temperature or pressure, have been prepared for use in hazardous or difficuit environments, e.g. in well logging applications.

One for of sensor that has been developed comprises 2 vibrating element whose resonant frequency is a function of the parameter to be measured. The element is excited Into a state of vibration via an optical signal supplied to the element frc- a remote station via an optical fibre link, and is interrogated optical to provide a measure of the vibrational frequency. This gives a value for the perameter that is being measured.A major disadvantage o optical sensors has been the difficulty of transmitting sufficient optical power over long distances, typically several km, to vibrate the sensor at a sufficient amplitude to provide a measurable modulation of the interrogation signal. It 5 necessary to detect this modulation in order to provide feedback to synchronise the optical drive signal with the vibrating element and thus maintain resonance. At present the solution to this problem is to increase the optical drive power to a sufficient level. This is not always practicable as the life of the drive source may be very much reduced. Also the operation cf the resonant device at large amplitude reduces its life.

In an attempt to overcome tis problem, it has been suggested that interferometric detection of vibration may be employed. This is a very sensitive technique which can detect displacement as small as lnm. However, present interferometric techniques do not provide an unambiguous indication of the phase relationship between the vibrating element and the optical drive. If these are not maintained substantially in phase then the element may cease to vibrate. Further, the amplitude of vibration may be unpredictable.

The object of the present invention is to minismise or to overcome this disadvantage.

According to the Invention there is provided an optical sensor system, including a vibratable element whose natural frequency corresponds to a parameter to be measures, optical interferometric means for deriving an oscillatory signal corresponding to vibration of the element, means or multiplying the frequency of said signal an an Integral factor, means for generating further signal cf the same frequency of said multiple frequency signal at a constant phase relationship with said signal, means for drv@ding te frequency of said further signal by the factor, ant optical means for driving said element in synchronisation with said divided frequency signal.

The syster maintains the drive signal in a constant phase relationship with the vibration of the element irrespective of the position of the working point cf the Interferometer. The amplitude of vibration of the element is less than the wavelength of light allowing the use ci a relatively Low drive power. Typically the uibrational amplitude cf the element is from 1 to loud.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: - Fig. 1 is a schematic diagram of the optical sensor system; Fig. 2 shows a phase locked loop circuit used in the sensor system of Fig. 1, and Fig. 3 shows one form of transducer for use with the system of Fig.

Referring to Fig. 1, the sensor system comprises an electronic part 11 ano an optical part 12. The optical part Includes a transducer 13, e.g. a pressure sensor, driven by a laser 1 and coupled thereto through optical fibre links 15 and 16 via a wavelength division multiplexer 17. Typically the laser 14 is a semiconductor laser operating at a wavelength of 850nm. The laser operates in the pulse mode and is synchronised with the natural frequency of the transducer. A further laser 18, operated in the continuous wave (CW) mode, typically at a wavelength of 1300um, is also coupled to the transducer 13 via a beam splitting coupler 19 and the wavelength division multiplexer 17.Interference takes place in the cavity formed bv the end of the fibre and the vibrating element.

The changing intensity corresponding to the vibration modulates tne signal returned via the fibre. The interference condition of the two signals is determined via a photodetector 20 coupled to the beam splitter 19 via an optical filter 21 which rejects light at the wavelength of tee drive laser 14. The beam splitter 19 is coupled to the multiplexer ii, the filter 21 and the laser 18 via optical fire links 22, 23 and 2 respectively.

The electronic part 12 of the system. roludes corcultrp for tracking the frequency of the transducer 13 as this frequency changes in response to changes in the parameter to be measured, and for driving the laser 14 in synchronisation and phase with the transducer Oscillatory signals from the detector 20, corresponding to the periodic variation in the interference condition, are fe vlC a preamplifier 31 to a filter 32 whose pass band corresponds to the vibrational frequency of the transducer.

Typically this frequency is about 100 kHz. The signal from the filter 32 is amplified by amplifier 33 and is then fed via automatic gain control circuit 34 to a frequency doubler. $Typically the frequency doubler $comprises a precision full wave rectifier 35. Alternatively, a signal scuarer Lay be employed. The double frequency signal is fec: via a low pass filter 36 to the input of a phase locked loop 37. The loop maintains a constant phase relationship between the sionais at i Its input and output as these signals track in frequency In response to changes in the transducer frequency.The output of the loop 37 is coupled via a frequency divider circuit 38 to a laser drive circuit 39 whereby the laser is driven at the transducer frequency and at a constant phase relationship with the transducer.

As shown in Fig. 2, the phase locked loop includes an input phase comparator 41, couplec vIa a low pass filter 2 to a voltage controlled oscillator 43 running at the loot frequency. The oscillator output is fec via a unit gatn amplifier 44 to the circuit output. $Feedback from the amplifier output to the phase comparator is provided by a phase shifter circuit 45.The phase comparator determines the phase difference between the input and feedback signalg and, In response to this phase difference, provides a corresponding control voltage from the oscillator 43 such that the oscillator is synchronised in frequency, and not a fixed phase relationship, with the input signal. Thus, following switch-on of the system, the laser drive is maintained in the correct phase relationship with the transducer an all times.

he circuit maintains the tr -r it s state of vinration at its resonant frequency. Any deviation of the drive frequenc@ away from the resonant frequency causes a corresponding variation in phase relationship between the drive signal and the received signal. This causes the phase-locked loop to alter the drive frequency thus maintaining the original phase relationship. The circuit also overcomes the problem of phase reversal when the reference condition drifts through a maximum or minimum.

ThiC condition will of course cause a glitch in the operation ci a conventional system. In the present circuit the frequency doubling ensures that phase reversal has no effect on the signal 'seen' by the phase locked loop and by drive laser, which is thus maintained in synchronism the transducer.

CLAIMS 1. An optical sensor system, including a vibratable element whose natural frequency corresponds to a parameter to be measured, optical interferometric means for deriving an oscillatory signal corresponding to vibration of the element, means for multiplying the frequency of said signal by an integral factor, means for generating a further signal of the same frequency of said multiple frequency signal at a constant phase relationship with said signal, means for dividing the frequency of said further signal by the factor, ano optical means for driving said element in synchron-satior. with said divided frequency signal.

2. An optical sensor system, including a vibratable element whose natural frequency corresponds to a parameter to be measured, a first, pulse mode, optical source for maintaining the element in a state of resonant vibration, optical interferometric means for deriving an osicillatory signal o the same frequency as said resonant frequency, detecter means for converting the oscillatory signal into a corresponding electrical signal, means for doubling the frequency of the electrical signal, a phase-locked loop adapted to oscillate at the doubled frequency and to maintain an output signal in constant phase relationship with the double frequency signal, and means for halving the douhled recuency at the output of the loop t recover a signa oi the same frequency and in constant phase relationship with the vibratable element whereby the pulse mode source is synchronised In frequency and phase with the vibratable element.

3. at optical sensor system as claimed in claim 2, wherein said pulse mode source comprises a solid state laser.

An optical sensor system as claimed in claim 1, 2 or 3, wherein said optical interferometric means includes a timer, continuous mode, laser.

5. An optical sensor system as claimed in any one of claims 1 to 4, wherein said vibratable element comprises the active element of a pressure transducer.

Claims

rig. 3 shows one form of transducer for use wit

the system of Fig. 1. The transducer includes a diaphragn 41 supported on a signal housing 42 to define a cavit 43 therebetween. Typically the cavity 43 is evacuated to provide a constant reference pressure. A flexible resonator element 44 is supported on the diaphragn 41 via mounts 41 such that the element 44 is In a state of tenslon. The resonant frequency of the element 46 is determined by the curvature of the diaphragm 41 which in turn is a function of the pressure differential across the diaphragm. Optical coupling to the element 44 is effected via an optical fibre 46 mounted in the housing 42 and in register with the element.

Typically the diaphragm 41 and element 44 are formed as an integral structure by selectively etching a jpody of single crystal silicon. The element 44 may be coated on one surface with a layer (not snow ) of a material having a relatively large coefficient of thermal expansion. Each laser pulse produces momentary heating of this layer and bence produces a bending force to provide the drive maintaining the element in a state of resonance.

The sensor arrangement described herein is intended for use in well logging applications. It is not however limited to well logging, but may alos be employed e.g. in chemical processing or nuclear instrumentation applications.

6. An optical sensor system substantially as described herein with reference to and as shown in the accompanying drawings.

7. A well iogging apparatus provided with a sensor system as claimed in any one of claims 1 to 6.