GB2143634A

GB2143634A - Optical sensors

Info

Publication number: GB2143634A
Application number: GB08318897A
Authority: GB
Inventors: John Stuart Heeks
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-07-13
Filing date: 1983-07-13
Publication date: 1985-02-13
Also published as: GB8318897D0

Abstract

An optical sensor, operating via interferometric techniques, comprises a single mode optical fibre path (12) around which light beams (10a, 10b) are sent in opposite directions before combining and producing interference in the plane of a photodetector (13). A sensing region (14) is disposed asymmetrically in the path (12) and application of a perturbation to be sensed to the sensing region (14) causes corresponding phase modulation of the photodetector output. If tau is the transit time in the path, the phase modulation occurs at perturbing frequencies around 1/(2 tau ). For improved sensitivity (Fig. 5) an offset bias of pi /2 phase between the two light beams is applied, the sensor then being responsive to frequencies around 1/(4 tau ). For fibre path lengths in the range 1 metre to 500 metres, the corresponding sensor response frequencies are 100 kHz to 50 MHz. <IMAGE>

Description

SPECIFICATION Optical sensors This invention relates to optical sensors and in particular to optical sensors operating via interferometric techniques.

Optical sensors operating via interferometric techniques are of interest because of their potential for high levels of performance. The two classic bridge types of interferometers, the Michelson nd the Mach-Zehnder, are illustrated schematically in Figures 1 and 2, respectively of the accompanying drawings. One arm of each interferometer provides a reference, whilst the other arm is arranged to be sensitive to the influence being monitored, for example, a pressure, displacement or magnetic field.

In the Michelson interferometer (Figure 1) a light beam 1 from a source (not shown) is divided into two beams of equal intensity la and ib, the former being directed to a reference arm 2 and the latter being directed to a sensing arm 3, by means of a half-silvered mirror 4. The light beams reflected from the reference arm and the sensing arm recombine at the mirror 4 and fringes are formed which may be observed by an optical detector 5. Figure 2 illustrates a waveguide version of a Mach-Zehnder interferometer. A light beam input at 6 is divided at 7 into two light beam portions which travel along separate paths (arms) until they recombine at 8. In the sensing zone 9 of one arm the light beam portion is affected by the influence to be monitored.In both types of interferometerthe response mechanism is by the phase unbalance of the bridge and leads to a raised cosine of optical intensity against phase deviation. A fundamental problem with such systems arises from the difficulty in keeping the zero balance condition of the bridge under conditions of environmental change. In the case of a displacement sensor, for example, changes in temperature will cause slightly different effects in the reference and sensor arms and thus will change the zero (balance) condition and hence the response of the system.

According to one aspect of the present invention there is provided an optical sensor, operating via interferometric techniques, comprising a single mode optical fibre path, a photodetector, means for splitting an input light beam into two portions and directing each portion along said path in opposite directions prior to recombination and subsequent detection by the photodetector, the optical fibre path including a sensing region asymmetrically disposed along its length, and wherein in use a perturbation to be sensed and applied to the sensing region causes corresponding phase modulation of the photodetector output.

According to another aspect of the present invention there is provided a method of optically sensing a perturbation via interferometric techniques, comprising the steps of splitting an input light beam into two portions, direction each portion in opposite directions along a single mode optical fibre path, the optical fibre path including a region sensitive to the perturbation asymmetrically disposed along its length, recombining the light beam portions after direction along the optical fibre path and detecting the recombined light beam portions by means of a photodetector, the photodetector output being phase modulated in dependence on the perturbation when the latter is applied to the sensing region.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 illustrates schematically the known Michelson interferometer, Figure 2 illustrates schematically the known Mach Zehnder interferometer, Figure 3 illustrates a basic single path interferometer sensor according to the present invention.

Figure 4 illustrates interferometer response, and Figure 5 illustrates schematically a variation on the basic single path interferometer sensor of Figure 3.

The interferometer sensor illustrated in Figure 3 or Figure 5, employs only one optical path, the interference being obtained by employing bidirectional optical signals.

In the interferometer sensor of Figure 3 a light beam 10 from a laser source (not shown) is divided into two beams of equal intensity 10a and lOb by a beam splitter 11, which may be comprised by a half-silvered mirror or a four port 3dB coupler. The light beams 10a and lOb are sent round a single mode optical fibre path 12 in opposite directions before being combined (mutually interfere) in the plane of a photodetector 13, resulting in an interference pattern of concentric interference rings. In a well adjusted optical system only the central fringe is present and this central area is focussed onto the photodetector. If the path 12 is truly single mode this system is balanced for all reciprocal effects and is in fact the configuration used to measure rotation, which reprsents a non-reciprocal influence, by the Sagnac effect.However, if a sensing region or zone 14 is incorporated in an asymmetric position in the loop, then the interferometer sensor will be phase modulated for an input frequency f around f = 1/(2T) where T is the optical transit time in the loop, that is the system is non-reciprocal in the temporal domain.

The sensing zone may comprise a sensor element, for example, an acoustic pick-up, made by winding an end portion of the loop fibre around a PZTformer, or a magnetic field or other type of sensor formed by coating some of the fibre with a corresponding sensitivity material. The phase difference occurring in the two outputs from the fibre give rise to a change of light intensity at the photodetector 13.

A disadvantage with the system in the basic form of Figure 3 is that the phase excursion is about zero phase and hence there is no sensitivity at the fundamental frequency of excitation, though a second harmonic response. This can be overcome by applying an offset bias of Tor/2 phase between the two optical paths so that the sensitivity is maximised. One method of achieving this is described in our co-pending Application No. 8132314 (Serial No.

2108652) (J.S. Heeks 32) and requires the use of an asymmetrically placed phase modulator 15, which may be comprised by an electro-optic device, in the loop (Figure 5) (15' indicates an alternative position for the modulator), the phase modulator 15 is driven with a square wave modulation envelope, obtained from source means 16, at a frequency 11(2r), with synchronous switching of the laser source 17 by means of an optical switch 18. The amplitude of the modulation is set to bias in the tor/2 portion of the interferometer sensor response (Figure 4). The corresponding response of the sensor is at a frequency around 11(4T), a low pass filter at the output of the optical detector photodetector 19 blocking the f = 1I(2T) bias modulation tone.

Taking a convenient range of fibre length from, say 1 metre to 500 metres, with corresponding transit times, the frequency range of the system would span 100 kHz to 50 MHz, which identifies with radio transmission bands. The length of the sensor portion of the delay would represent a fraction of this total.

In its basic form the single optical path interferometer sensor is highly balanced at low frequencues and sensitive to signals occurring at frequencies around l/(2T). For maximum sensitivity however, a "'12 phase bias is effected by phase modulation at l1(2T) (dynamic quadrative bias) and the sensorthen operates at frequencies around l1(4Tl.

Claims

1. An optical sensor, operating via interferometrictechniques, comprising a single mode optical fibre path, a photodetector, means for splitting an input light beam into two portions and directing each portion along said path in opposite directions prior to recombination and subsequent detection by the photodetector, the optical fibre path including a sensing region asymmetrically disposed along its length, and wherein in use a perturbation to be sensed and applied to the sensing region causes corresponding phase modulation of the photodetector output.

2. An optical sensor as claimed in claim 1, wherein transit time in the optical path is T, the sensor being sensitive to perturbing frequencies around lI(2T).

3. An optical sensor as claimed in claim 1, wherein the transit time in the optical path is T and wherein a ITI2 phase bias between the light beam portions is effected by means including a phase modulator asymmetrically disposed along the length of the optical path, wherein the sensor is sensitive to perturbing frequencies around lI(4T).

4. An optical sensor as claimed in claim 3, wherein the phase modulator is driven with a square wave modulation envelope at a frequency of 1I(2T) and a source of the input light beam is synchronously switched, and wherein a low pass filter at the photodetector output blocks the tone corresponding to the l/(2T) bias modulation.

5. An optical sensor as claimed in any one of the preceding claims, wherein the sensing region is in the form of an acoustic pick-up and is comprised by a length of the optical fibre path wound around a former.

6. An optical sensor as claimed in any one of claims 1 to 4,wherein the sensing region iscompris- ed by a length of the optical fibre path provided with a sensitivity material coating.

7. An optical sensor as claimed in claim 6, and comprising a magnetic field sensor.

8. A method of optically sensing a perturbation via interferometric techniques, comprising the steps of splitting an input light beam into two portions, direction each portion in opposite directions along a single mode optical fibre path, the optical fibre path including a region sensitive to the perturbation asymmetrically disposed along its length, recombining the light beam portions after direction along the optical fibre path and detecting the recombined light beam portions by means of a photodetector, the photodetector output being phase modulated in dependence on the perturbation when the latter is applied to the sensing region.

9. A method as claimed in claim 8, wherein a "'/2 phase bias between the light beam portions is applied by means of a phase modulator disposed asymmetrically along the length of the optical path.

10. A method as claimed in claim 9, including the steps of driving the phase modulator with a square wave modulation envelope at a frequency of l/(2T), where T iS the transit time in the optical path, synchronously switching a source for the input light beam, and low pass filtering the photodetector output around a frequency of lI(2T).

11. An optical sensor, operating via interferometrictechniques, substantially as herein described with reference to and as illustrated in Figure 3 or Figure 5 of the accompanying drawings.

12. A method of optically sensing a perturbation via interferometric techniques substantially as herein described with reference to Figure 3 or Figure 5 of the accompanying drawings.