GB2139346A - Optical sensor - Google Patents

Abstract

An optical sensor is provided for sensing variations in a parameter by converting these variations to variations in optical intensity. The sensor is completely passive in that no electrical power is required at the head and can be remotely read over long distances. The sensor comprises light sources (T1 and T2), light detectors (R1 and R2) the sources being arranged to transmit light to the detectors along at least two paths. In one embodiment, a sensor (S) responsive to the said variations in a parameter is arranged to vary the amount of light transmitted through one of the paths but not the other. <IMAGE>

Description

SPECIFICATION Optical sensor This invention relates to an optical sensor for sensing variations in a parameter by converting these variations to variations in optical intensity. The sensor is completely passive in that no electrical power is required at the head and, in a preferred embodiment, it can be remotely read over long distances.

According to the invention there is provided an optical sensor for sensing variations in a parameter by converting these variations to variations in optical intensity, comprising a light source, a light detector, a path comprising at least one optical fibre for directing light from the light source to the light detector, and a sensor responsive to the said variations in a parameter and arranged to vary the amount of light transmitted through the optical fibre path to the detector.

The invention further provides an optical sensor for sensing variations in a parameter by converting these variations to variations in optical intensity, comprising a light source, a light detector, the source being arranged to transmit light to the detector along at least two paths, and a sensor responsive to the said variations in a parameter and arranged to vary the amount of light transmitted through one of the paths but not the other.

Where reference is made herein to "light" it is to be understood as including not only visible but also ultra-violet and infrared radiation.

In the accompanying drawings: Figure 1 shows diagrammatically a first embodiment of the invention; Figure 2 shows diagrammatically a second embodiment in which normalisation is provided; Figure 3 shows a more practical form of the embodiment shown in Fig. 2; Figure 4 shows a third embodiment of the invention; Figures 5a and 5b show two more practical forms of the embodiment of Fig. 3; Figure 6 shows diagrammatically, and on a larger scale, the gratings used in Figs. 3 to 5; and Figure 7 shows how a number of sensors according to the invention may be cascaded together.

The sensor shown diagrammatically in Fig.

1 comprises a light emitting diode (I.e.d.), iaser, or similar optical transmitter T the output of which is guided by an input optical fibre Fj to a sensor head S. The output of the fibre F is collimated by a lens C into a beam which is several mm in diameter, i.e. large compared to the diameter of core of the fibre.

This beam passes through two identical gratings Gm and G,. A second lens C, focuses the beam into a return fibre F, and the intensity is detected by a receiver R in the form of a photodetector.

The gratings are periodic square wave gratings consisting of alternate transparent and opaque lines, the transparent and opaque lines preferably being of equal width to one another. One grating Gf is held stationary with respect to the optical beam and the other is moved relative to the first grating G, by a mechanical displacement to be measured. The intensity of the output beam is thus modified linearly from zero to a maximum, dependent upon the displacement. This variation is periodic and the displacement sensitivity is inversely proportional to the size of the grating lines. There should be several lines across the beam diameter and typically there will be about 10 opaque lines per mm (so in a 5 mm diameter beam there will be 50 opaque lines and 50 transparent lines).

The received intensity produces a current in the receiver R which varies linearly with the mechanical displacement of the grating. By modulating the optical source using either amplitude or frequency modulation (though amplitude modulation is preferred), the current can be measured using conventional lockin amplifier techniques.

The displacement sensor of Fig. 1 requires no electrical power at the sensing element and can be remotely read. The sensor will detect any variable which can be transformed into a displacement, such as temperature, pressure, force, flow, mass and acceleration, or any variable which involves a displacement, such as strain. Also, as mentioned below, the sensor may be of a type in which no displacement is involved and which variations in a parameter cause variations in the optical properties of a sensor by some other means.

The displacement sensor shown in Fig. 1 measures displacement, but is susceptible to drift caused by intensity variations of the source or induced through variations in the fibres. A method of normalising the system is necessary to ensure constant calibration.

One method of recalibrating the system is to drive the sensor through its full dynamic range by moving the gratings relative to each other over a distance of more that one period.

Such a technique in general requires electrical power to scan the gratings, although temperature variations or vibration can be used to produce a scan.

Passive normalisation is therefore preferred, and one way of achieving this is by the use of two additional fibres (Fig. 2). Denoting the power in an output fibre (j) from an input fibre (i) by P, we have: Pl3=LlLAL3Pl (1) P14=L1L4P1 (2) P23 = L2L3P2 (3) P24 = L2L4P2 (4) where: P, and P2 are the optical input powers from transmitters T1 and T2 respectively.

L1, L2, L3, L4 are the losses due to fibres F1, F2, F3, F4 respectively and their associated optical components, and LA is the loss through the gratings and is the quantity to be measured.

Taking the ratios of the output powers gives: P13 LAL3 PA= = (5) P14 L4 P23 L3 PB = =-- (6) P24 L4 and PA -- LA (7) PB By taking the output power ratio it it thus possible to measure the loss through the gratings independent of the loss through the rest of the system, provided the output powers at receivers R and R2 originating from the two inputs can be distinguished, i.e. provided P,4 can be distinguished from P24, and P13 can be distinguished from P23. This is possible, as described below.

It should be mentioned that the above equations only hold good provided the light leaving the ends of the fibres F, and F2 is in each case split equally between the two possible paths. If this is not so the equations must be appropriately modified by incorporating a constant correction factor of the order of unity.

A further refinement to the sensor is to compensate for effects of changes in the grating caused by temperature variations or vibration. This can be accomplished by placing a second pair of gratings, both of which are nominally stationary, in the beam after it leaves fibre F2 and before it enters F3. One practical realisation of this sysem is shown in Fig. 3. The beams from the two input fibres F1 and F2 are collimated by lenses C1 and C2.

From lens Cs half of the beam travels through a movable grating Gm and fixed grating G,, to a lens C3 which launches light into the fibre F3. The other half of the beam is directed into the fibre F4 by total internal reflection in a glass block B1 and a lens C4.

Preferably, to obtain maximum sensitivity, the opaque lines of the grating G,2 are staggered with respect to those of grating Gf3 to just such an extent as to obscure half the width of the transparent lines of grating Gf2.

The output from fibre F2 is split to launch directly into fibre F4 and by total internal reflection launch into fibre F3 after passing through a fixed pair of gratings, G,2 and G,3.

For simplicity, as illustrated in Fig. 6 the gratings G,2 and G,3 are parts of a single fixed grating. In Fig. 6 only two opaque lines are shown (on the grating formed by Gf, and Gf2) and these arts on a greatly enlarged scale. It will be understood that in practice many more lines, smaller in size would be present on all the gratings of Fig. 6. The system of Fig. 3 is a realisation of the system shown in Fig. 2.

The system can also be made using prisms or beamsplitters to provide the beam deflection.

The lenses can conveniently be spherical, graded index or sapphire ball type.

As mentioned above it is necessary to distinguish Pl4 from P24 and P13 from P23. This can be achieved if each transmitter is modulated (preferably amplitude modulated) at a different frequency which can then be detected by filtering the outputs. It should also be mentioned at this point that the mathematical division operation on the power levels, required to compute PA and PB above can be done by using analogue log amplifier techniques or by using A/D conversion and a microprocessor.

The normalised sensor of Fig. 2 is a transmission type and as such requires two optical fibre inputs and two optical fibre outputs. The system is, however, symmetrical and by using a reflection technique only two fibres are required. This form of the sensor is shown diagrammatically in Fig. 4, which is the reflection version of Fig. 2. The return beams can be diverted to the receiver by beam-splitters BS, and BS2 (as shown) or by a fibre splitter. A mirror M is used to reflect the beams. The fibres F1 and F3 of Fig. 2 are replaced by fibre F, 3 and the fibres F2 and F4 by fibre F,,.

In Fig. Sa, which is one practical version of Fig. 4, half od the beam is reflected by total internal reflection in a glass block B2 onto a mirror face M, and then by a second total internal reflection to the output fibre. The other half of the beam is reflected back by mirrors M2 and M3 into the original fibre. The gratings Gm and G" are placed in one of the beams which is reflected directly back to the input. The gratings Gf2 and G,3, which compensate for variations in a single grating, are placed across the beam which suffers total internal reflection.

Figure Sb shows a second practical form of Fig. 4, in which the block B2 of Fig. 5a is replaced by a block B3 in the form of a rightangled prism, and the mirrors M1, M2 and M3 are replaced by a single mirror M.

Throughout the above description of the system illustrated it has been assumed that the conversion of displacement to optical intensity variation has been made by moving two identical periodic gratings with respect to one another. It may in some instances be preferable to use other grating functions to produce the required displacement-intensity conversion. Such functions may be sinusoidal gratings, spatially varying frequency (chirp) gratings or others.

Also, instead of using gratings other forms of intensity modulation can be used, for example using a bire-fringent crystal and polarisers. Providing the parameter to be sensed only alters the intensity in the transmission from output of fibre F1 to the input of fibre F3 in Fig. 2 and in no other beam (or has an equivalent effect in other embodiments) the sensor operates in the same way.

Several of the sensors according to the invention can be cascaded along a length of fibre to produce a passive highway system (Fig. 7). In Fig. 7 four sensors S" S2, S3 and S4 are shown, and the delay time for a signal reaching a receiver R1 or R2 from a transmitter T1 or T2 via a sensor St, S2, S3 or S4 is r1, T2, or or T4 respectively. The signals to the transmitters T, and T2 are each frequency modulated, for example using a sawtooth modulation, by a modulator M, or M2 with the modulations imposed being slightly different from one another. The signal at transmitter T is subjected to an adjustable delay r and compared by a difference circuit D, with a signal received at R1. The signals at T2 and R2 are similarly compared by D2. The delay f can be provided by electrical, acoustical or optical delay lines. In order to isolate the loss produced by a particular sensor the value of 7 is adjusted to be nearly equal to a respective one of the values 71 to 74. The resulting signals are filtered by low pass filters LP1 and LP2. The output of each filter contains two low frequency components, one of which derives from T1 and the other of which derives from T2. These components can then be processed to yield a value for LA (see above in reference to Fig. 2).

As an alternative to what is shown in Fig.

7, the signals from the various sensors may be resolved by intensity modulating the optical source and varying the frequency of this intensity modulation in a predetermined manner.

Claims (18)

1. An optical sensing device for sensing variations in a parameter by converting these variations to variations in optical intensity, comprising a light source,a light detector, a path comprising, at least one optical fibre for directing light from the light source to the light detector, and a sensor responsive to the said variations in a parameter and arranged to vary the amount of light transmitted through the optical fibre path to the detector.

2. An optical sensing device for sensing variations in a parameter by converting these variations to variations in optical intensity, comprising a light source, a light detector, the source being arranged to transmit light to the detector along at least two paths, and a sensor responsive to the said variations in a parameter and arranged to vary the amount of light transmitted through one of the paths but not the other.

3. An optical sensing device for sensing variations in a parameter by converting these variations to variations in optical intensity, comprising first and second light sources, first and second light detectors, a sensor responsive to the said variations in a parameter and arranged to vary the amount of light transmitted therethrough, means defining first, second, third and fourth optical path segments, and means for directing light along the following paths:: (a) from the first light source, along the first path segment, through the sensor, through the third path segment to the second light detector; (b) from the first light source, along the first path segment, bypassing the sensor, through the fourth path segment to the first light detector; (c) from the second light source, along the second path segment, bypassing the sensor, through the third path segment to the second light detector; and (d) from the second light source, along the second path segment, bypassing the sensor, through the fourth path segment to the first light detector, the light received by the detectors being distinguishable as to which originated from the first light source and which originated from the second light source.

4. A sensing device according to claim 3, wherein light which has traversed the first and second path segments is reflected, whereby the third and fourth path segments are identical to the first and second path segments respectively but are traversed by light in the opposite direction.

5. A sensing device according to claim 3 or 4, wherein the distinguishability of light as to source is achieved by modulating the output of the sources differently from one another.

6. A sensing device according to claim 5, wherein the sources are both amplitude modulated but at different frequencies.

7. A sensing device according to any one of claims 1 to 6, wherein the said detector comprises an element which is movable in response to variations in the said parameter.

8. A sensing device according to claim 7, wherein the movable element is a grating arranged to move relative to a fixed grating, both gratings being located so that light passing through the sensor passes through both gratings.

9. A sensing device according to claim 8, wherein at least one of the gratings comprise alternate transparent and opaque lines which extend transversely to the direction of move ment of the movable grating.

10. A sensing device according to claim 9, wherein at least one of the gratings is a square wave grating.

11. A sensing device according to claim 9 or 10, wherein in at least one of the gratings the transparent and opaque lines are of equal width to one another.

1 2. A sensing device according to any one of claims 6 to 9, as dependent on claim 3, 4, 5 or 6, comprising a further pair of gratings located so that light passing along path (c) passes through the further pair of gratings, neither grating of the further pair being movable.

1 3. A sensing device according to claim 12, wherein the fixed grating of the first pair of gratings is integral with one of the gratings of the further pair of gratings.

14. A sensing device according to any preceding claim, comprising a plurality of the said sensors arranged in parallel to one another and connected to a common light source or sources and a common detector or detectors, the sensors being spaced apart from one another so that the time taken for light to reach the detector or detectors is different depending on the sensor via which it has travelled, whereby the variations to which each sensor is responsive can be detected separately from one another.

1 5. A sensing device according to claim 14, wherein light received by the detector or detectors is compared with a signal derived from the light sources but delayed by a predetermined delay time, the predetermined delay time being adjustable to correspond to the time taken for light to reach the detector or detectors via a selected sensor.

1 6. A sensing device according to any preceding claim wherein the parameter to be sensed is temperature, pressure, force, flow, mass, acceleration or strain.

1 7. A sensing device according to any one of claims 2,3,4,5,6,12 and 13, or any one of claims 7,8,9,10,11,14,15 and 16 as dependent thereon, wherein the optical paths are provided by one or more optical fibres.

18. A sensing device substantially as herein described with reference to any one of the embodiments shown in the accompanying drawings.