WO1991003710A1

WO1991003710A1 - Method and sensor for optical measurement of displacement

Info

Publication number: WO1991003710A1
Application number: PCT/US1990/005059
Authority: WO
Inventors: Peter L. Fuhr; William B. Spillman, Jr.
Original assignee: The University Of Vermont And State Agricultural College
Priority date: 1989-09-07
Filing date: 1990-09-07
Publication date: 1991-03-21

Abstract

An optical displacement sensor is provided in which a pair of matched optical bandpass filters are used to encode displacement measurements in a broadband optical signal. The filters move with an object under measurement, and as the filters move the input optical signal illuminates varying amounts of each filter. Determination of the relative intensities at each filter pair's pass band is based on measurements acquired with matching filters and photodetectors. Because the displacement measurements are based on relative intensity measurements, the system is intensity independent.

Description

Method and sensor for optical measurement of displacement

Background of the Invention

Fiber optic sensors offer a number of potential advantages over conventional types of sensors, such as increased sensitivity, geometric versatility in that sensors can be configured into nearly any shape, and immunity from the effects of electromagnetic interference (EMI). Recent developments in fiber optic technology have generated a considerable amount of interest in exploiting these advantages, and over the past few years, more than 6.0 different types of sensors have been

developed. Fiber optic sensors have been developed to measure such parameters as temperature, pressure,

magnetic fields, radiation, and rotation. Another measuring parameter for which fiber sensors have been developed is linear displacement. Many techniques have been examined in such sensor designs, including

heterodyne systems with frequency modulated light

sources, zero - crossing counting electronics, FMCW optical ranging, polarization modulation with polarization maintaining fiber, multi-bit grey code optical encoders, and many electrical signal processing techniques applied to intensity transducers. While such systems have been shown to yield good displacement information, the

coherence and system requirements necessary for

heterodyne signal recovery are at times very burdensome.

Summary of the Invention

In accordance with the present invention, a

displacement sensor measures the displacement of an object from a fixed reference frame. A light source generates light which is directed toward first and second optical bandpass filters. The filters are

adjacent one another and move with the object. The first input filter has a known frequency response R1, and the second filter has a known frequency response R2, and R1 and R2 are distinctly different from one another. The filter pair is positioned such that light from the light source, at a predetermined position, passes through a dividing line separating the input filters.

An optical coupler receives light passing through the input filters and divides the received light into a first and a second output beam of equal intensity. The first beam is directed toward a first output filter which has a frequency response identical to R1. Similarly, the second output beam is directed toward a second output filter which has a frequency response identical to R2.

A first photodetector aligned with the first output filter is illuminated by the light passing through the first output filter, and generates an electrical signal indicative of the received light intensity. A second photodetector aligned with the second output filter is illuminated by light passing through the second output filter, and generates and electrical signal indicative of the intensity of the light with which it is illuminated. A signal processor then receives the electrical signals from the photodetectors and generates a displacement output signal. The displacement output signal is formed by taking the ratio of the difference of the

photodetector intensities, to the sum of the

photodetector intensities. This relative intensity measurement makes the signal processor output signal intensity independent. One variation of the present invention is a

displacement sensor using a first and a second light source. The light sources operate in alternation with each other, and each directs light toward a pair of optical bandpass input filters which is moveable with the object. Each filter pair has one filter with a known frequency response R1 and one filter with a known

frequency response R2. The first pair of filters are aligned adjacent each other in a first direction, and are positioned such that light from the first light source at a predetermined position passes through a dividing line separating the filters. The second pair of filters are aligned adjacent one another in a direction perpendicular to the adjacency direction of the first filter pair.

Similar to the first filter pair, the second filter pair is positioned such that light from the second light source at a predetermined position passes through a dividing line separating the two filters.

After filtering, an optical coupler receives light from both the first filter pair and the second filter pair, and divides the light it receives into a first and a second output beam of equal intensity. The first output beam is directed to a first output filter which has a frequency response identical to R1. The second output beam is directed to a second output filter which has a frequency response identical to that of R2.

As with the previous configuration, the outputs of the two filters are each detected by a photodetector, and the photodetectors generate electrical signals indicative of received light intensity. A signal processor receives the output signals from the photodetectors and generates a displacement output signal from the photodetector output signals. In addition, the signal processor also controls the alternate powering of the light sources.

Brief Description of the Drawings

Figure 1 shows a linear displacement sensor in accordance with the present invention.

Figure 2 is a two-dimensional version of the

displacement sensor of Figure !.

Figure 3 is a two-dimensional displacement sensor using two input filter pairs with synchronized light sources.

Figure 4 shows the displacement sensor of Figure 1 having an additional light source.

Figure 5 is a linear displacement sensor similar to Figure 1 which uses a dichroic beam splitter in place of an optical coupler.

Figure 6 shows the displacement sensor of Figure 1 configured with a portion of the sensor components housed inside a casing.

Figure 7 is an alternative embodiment of the present invention which measures rotational displacement.

Detailed Description of the Preferred Embodiments

Referring to Figure 1, an optical displacement sensor system 10 is shown comprising in general a

broadband light source 12, filters 18, 20 and

photodetectors 38,42. In this embodiment, the light source 12 is a light - emitting diode (LED), but other light sources may be used as well. The LED is coupled into a high bandwidth multimode optical fiber 14 which contains and transmits the light from LED 12 to

collimating lens 16. The lens is shown in Figure 1 as being separate and enlarged relative to the fiber. However, it is preferred that the lens 16 is actually formed at the end of the fiber 14 through appropriate grinding and polishing of the fiber. Forming the lens 16 at the end of the fiber 14 removes the necessity of coupling the light from the fiber 14 into the lens 16. Nonetheless, adequate optical coupling can be provided using a separate standard lens.

The lens 16 focuses the light from fiber 14 toward an adjacent pair of optical bandpass input filters 18, 20. The filters 18,20 are rigidly connected to a

displacement mount 22 which is free to move linearly in a direction normal to the direction of incident light, as indicated by the two directional arrows A and B shown in Figure 1. The linear movement of mount 22 is the linear displacement which is measured by the system. The mount 22 is therefore rigidly connected to any device or object for which linear displacement is to be measured.

Filters 18,20 are optical bandpass filters each of which have a distinctly different frequency response. The frequency response of each filter is such that it will not pass any of those frequencies passed by the other filter. At an initial displacement position of the mount 22, the light from fiber 14 is focused to a beam by lens 16, and directed toward the dividing boundary 24 between the filters 18,20. The focused spot of the light beam overlaps the boundary 24, and the light is thereby divided between the two filters 18,20 in equal amounts.

The light reaching the filters 18,20 passes through the filters 18,20, but is selectively filtered by the filter through which it passes. That portion of the light passing through filter 18 has all frequencies removed but those allowed by the filter 18 pass band. Similarly, the portion of the light passing through the filter 20 has all frequencies removed but those allowed by the filter 20 pass band. The output light beam leaving the filter pair 18,20 therefore consists of light in two distinct frequency bands. The relative Intensity of the light in each band is dependent on how much of the light incident on the filter pair 18,20 passes through each filter. Since at the initial position of the mount 22, equal amounts of light pass through each filter, the relative light intensity is also equal in each of the frequency bands of the output light 18,20.

After being filtered by the filter pair 18,20, the output light beam is coupled by converging lens 26 into optical fiber 28. Optical fiber 28 is a high bandwidth multimode fiber similar to optical fiber 14. The output light is transported to a 1:2 fiber optic coupler 30, which serves as a beam splitter by dividing the input light into two optical paths, i.e., the two optical fibers 32,34. The coupler 30 is a 50/50 intensity coupler such that 50% of the light is transported along fiber 32 and 50% is transported along fiber 34. The coupler divides by intensity only, and no separation by frequency is performed.

The light transported by fiber 32 is focused by lens 33 toward bandpass filter 36 which has a frequency response identical to the response of filter 18. Lens 33 is also preferably formed at the end of the transporting fiber 32. The light focused by lens 33 is filtered by filter 36 to pass those frequencies within the filter 36 pass band. Since the frequency response of filter 36 is identical to that of filter 18 and excludes those

frequencies passed by filter 20, the only light passing through the filter 36 is that which previously passed through filter 18. The light passing through the filter 36 then illuminates photodetector 38 which generates an output signal indicative of the intensity of the

illuminating light from filter 36. As apparent from the configuration of the system, the intensity of the light reaching photodetector 38 is one half the intensity of the light passed by filter 18.

The light coupled into fiber 34 is transported to lens 35 and focused on bandpass filter 40. Filter 40 has a frequency response identical to the frequency response of filter 20. Filter 40 therefore only passes those frequencies which are passed by filter 20, and excludes those passed by filter 18. The light passing through filter 40 illuminates photodetector 42 which generates an output signal indicative of the intensity of the

illuminating light from filter 40. The intensity of the light reaching photodetector 42 is one half the intensity of the light passed by filter 20.

The outputs of both photodetector 38 and

photodetector 42 are input to a signal processor such as computer 44. The signal processor compares the

electrical output signals of both photodetectors to determine the relative quantity of light reaching the photodetectors. At the initial zero position, both fiber 32 and fiber 34 carry an equal amount of light intensity, and the light in each frequency band is distributed equally between both fiber 32 and fiber 34. The ratio of the output light intensity illuminating photodetector 38, to the output light intensity illuminating photodetector 42 is a unitless quantity which is intensity independent. This ratio is equal to the ratio of the light intensity passed by filter 18 to the light intensity passed by filter 20. It can be seen that the division into equal intensity signals by the coupler 30 is necessary to preserve the intensity

independence of the system.

Linear motion of the mount 22 in the directions indicated by the arrow of Figure 1 increases the relative quantity of light incident on one of the filters 18', 20, while decreasing the relative quantity of light incident on the other filter. The light output of the filter pair 18,20 therefore contains corresponding relative

intensities of light in the frequency bands passed by respective filters 18,20. For example, as the mount 22 moves in the direction of arrow A, the light intensity passed by filter 18 increases, and the light intensity passed by filter 20 correspondingly decreases. Since there is an increase in the ratio of light filtered by filter 18 to that filtered by filter 20, the output of the filter pair 18,20 contains a correspondingly higher ratio of light in the pass band of filter 18 to that in the pass band of filter 20. These relative intensities are detected by the photodetectors 38,42 and computed by computer 44. Since it is the movement of mount 22 which causes the relative changes in the photodetector outputs, these output fluctuations are used to determine the position change of the mount, and hence the corresponding displacement of any object to which the mount is fixed.

The present embodiment is configured to sense linear position as shown in Figure 1. It is possible to develop a pair of equations describing the system throughput intensity, or photodetected signal, for each optical path through the displacement sensor. The system throughput equation for the output of photodetector 38, PD is:

PD₃₈ = I_o(ω)G(ω,x)½G(ω-ω₁)η (1) Similarly from photodetector 42, PD ₄₂ , the output is

PD₄₂ = I_o ( ω ) G ( ω , x ) ½ G ( ω - ω ₂ ) η (2)

Where I_o(ω) is the light source's spectral di stribution, and G(ω,x) is the filter pair's transfer function.

G(ω,x) is a function of both the filters' bandpass characteristics and the linear one-dimensional

displacement position of the filter holder (x):

G(ω,x) = h(x-x')G(ω-ω₁) + (1-h(x-x'))G(ω-ω₂) (3)

G(ω-ω₁) and G ( ω - ω ₂) are the filter functions for each interference filter. Each filter has been designed to have a Lorentzian filter distribution. Substitution of Equation (3) into (1) and (2) yields the overall transfer equation for each photodetector

and O

h(x-x') is the system transfer equation's positional dependence and affects the detected signal intensity. h(x-x') takes on values between 0 and 1 in proportion to the amount of the light field that is illuminating each filter. In the apparatus described here, h(x-x')=1 when all of the light is illuminating filter 18. In reference to the system diagram depicted in Figure 1, as the stacked filter holder is moved in the direction of arrow A, the fiber's collimated output will illuminate more of interference filter 18, resulting in a larger PD₃₈ signal with an associated decrease in PD₄₂. A figure of merit, the detected signal's Visibility, V, is thus position dependent and is used to compute the filter mount's actual position. The Visibility is defined in the classical sense (13) as

(5)

Substitution of Equations (4. a) and (4.b) into (5) yields )

The input light field's spectral distribution function, I_o(ω), is not explicitly included in the Visibility function. However, the light field must have spectral emission within the interference filters' passband to have any photo-detectable signal. It is assumed in

Equation (6) that photodetectors 38 and 42 have the same quantum efficiency, η, in converting light to current. It is also assumed that the collimated illuminating beam has a uniform intensity distribution.

The filters' passbands are narrow and do not overlap. Therefore a modified delta function sifting property is used to operate on Equation (6), since light passing through both filter 18 and filter 40 or both filter 20 and filter 36 will be suitably extinguished. This point is mathematically expressed as

G(o>-ω₁) G(ω-ω₂)≅0 (7)

Equation (6) may then be reduced to

The position dependent term, h(x-x'), may then be

determined from Equation (8) and is found to be

It is therefore apparent from Equation (9) that for various values of Visibility, V, and fixed interference filter characteristics, G(ω-ω₁) and G(ω-ω₂), the filter position may be determined.

Shown in Figure 2 is an embodiment of the

displacement sensor which measures displacement in two dimensions. The input portion of the system, LED 12, fiber 14 and lens 16, is the same as in Figure 1.

However, four adjacent optical bandpass filters

50,52,54,56 are now connected to mount 22. Each of the filters 50,52,54,56 has a different pass band which excludes those frequencies passed by the other filters. The light beam from lens 16 is focused in the center of the four filters such that the light passes through all four of the filters when the mount is in an initial displacement position.

The light passing through the filters 50,52,54,56 is collimated by lens 26 into optical fiber 28. This light is divided by 1:2 coupler 30 into fibers 32 and 34, as in the embodiment of Figure 1. However, the light on each of the fibers 32,34 is then divided again by 1:2 couplers 58 and 60, respectively. The four equal portions of light are transported by four different fibers to four different lenses. Each lens focuses one light portion toward one of four output bandpass filters 62,64,66,68. The frequency response of filters 62,64,66,68 are exclusive of one another, each having a frequency

response identical to the frequency response of one of the filters 50,52,54,56. Thus, each output filter

62,64,66,68 passes only that frequency band of light which was passed by the filter 50,52,54,56 having the same frequency response.

The outputs of filters 62,64,66,68 illuminate phodetectors 70,72,74,76, respectively. Each

photodetector correspondingly generates an output signal indicative of the intensity of the light with which it is illuminated. The output signals from the photodetectors 70,72,74,76 are input to computer 44 which correlates them and makes a determination of positional displacement of the mount 22. The four relative intensity

measurements correspond to the relative amount of light passing through each filter 50,52,54,56, and thereby contain the information necessary to determine absolute position of the mount 22 in the filter plane.

In Figure 3 is shown an alternative embodiment of a two-dimensional displacement sensor. The embodiment of Figure 3 is similar to that of Figure 1, but a second pair of bandpass filters 72,74 is connected to the mount 22 in addition to the filter pair 18,20. The frequency response of filter 72 is identical to that of filter 18, and the response of filter 74 is identical to that of filter 20. However, the filters 72,74 lie adjacent one another in a direction perpendicular to the adjacency direction of filters 18,20. In other words, if filter 18 was said to be "above" filter 20, then the filters 72,74 could be considered "side by side".

In addition to LED light source 12, which is coupled to input optical fiber 14, a second wideband LED 76 Is included with the embodiment of Figure 3. LED 76 couples its light into input optical fiber 78, which transports the light to lens 80. Lens 80 then focuses the light onto a center region of filter pair 72,74. The light exiting filter pair 72,74 is focused by converging lens 82 into optical fiber 84. Fiber 84 transports the light to a 2:1 fiber optic coupler 86, which combines the optical fibers 28 and 84 such that light on either fiber 28,84 is transported to 1:2 coupler 30. The optical system after coupler 30 is identical to that of Figure 1.

The system of Figure 3 functions by alternately powering LED 12 and LED 76. A two position switch 88 is controlled by computer 44, which synchronizes switching of the LEDs 12,76 with the correlation of the output signals from the photodetectors 38,42. When switch 88 is positioned as shown in Figure 3, LED 12 is activated and light passes through filter pair 18,20. This light is then transported to the output stage of the system to be divided, filtered, detected, and correlated in the same manner as that of the Figure 1 embodiment. Computer 44, being in control of switch 88, computes the relative displacement and stores it as being with reference to the linear dimension indicated by arrows A and B.

Once the first linear measurement is complete, the switch 88 is then moved to its other position to activate LED 76 and deactivate LED 12. The light from LED 76 passes through filter pair 72,74 and is coupled into the output stage of the system through 2:1 coupler 86. The correlation measurement is repeated, but since the filters 72,74 are aligned "side by side", the results correlated by computer 44 are stored as being with reference to the linear displacement directions indicated by arrows C and D. Once computer 44 has obtained a displacement measurement in each of the two linear dimensions, the absolute position of the mount In the filter plane is determined.

Figure 4 shows a variation of the embodiment of Figure 1 in which a backup LED 89 is added in the case of failure of the LED 12. LED 89 is coupled into the input stage of the optical system through 2:1 fiber optic coupler 90. In the event that LED 12 burns out or otherwise fails, LED 89 is activated to keep the system operational. Alternatively, both LEDs may be operated simultaneously, such that if one fails the other

continues to operate. This prevents the need to detect the failure of the first LED in order to activate the backup LED. The system is intensity independent as long as at least one of the LEDs is operational. Therefore, if the input intensity is halved by the failure of one of the LEDs, there will be no corresponding loss of system resolution.

The embodiment of Figure 5 is another variation on the linear displacement sensor of the Figure 1

embodiment. In Figure 5, the 1:2 coupler 30 is replaced with dichroic beam splitter 92. The refractive qualities of dichroic beam splitter 92 are chosen with regard to the pass bands of filters 18,20. The color separation of the dichroic beam splitter 92 is selected such that it transmits light in the frequency range of the filter 18 pass band and refracts light in the frequency range of the filter 20 pass band. Lenses 93,94,95 are provided to couple light between the dichroic beam splitter 92 and optical fibers 28,32,34. The color separation of the beam splitter 92 provides the necessary output filtering for separating the filter 18 pass band frequencies from the filter 20 pass band frequencies. Thus, the output filters 36,40 of the Figure 1 embodiment are not

necessary.

Due to system housing problems such as harsh

environments, it is often desireable to keep electronic components away from the actual location of the

positional displacement. Figure 6 shows the displacement sensor of Figure 1 configured to allow a large portion of the system to be kept separate from the displacement environment. The LED 12 and the output portion of the system, including the coupler 30, filters 36,40, and photodetectors 38,42 are housed in casing 96. Modular inputs 97 provide optical coupling to fibers 14 and 28 which lead to the actual displacement region. Fibers 14 and 28 may be made as long as necessary to isolate the contents of casing 96 from the displacement environment. Data processor 44 is shown as being acessed through electrical inputs 98, but may be alternatively housed in the casing 96. Battery 99 provides on board power to the LED 12. The battery may also provide power to the signal processor 44.

Figure 7 shows an embodiment of the present

invention which directs itself toward the measurement of rotational displacement. The system components are identical to those of Figure 1, except that the optical bandpass filters 18,20 are wrapped around a cylindrical mount 100. The cylindrical mount 100 responds to

rotation about its axis, which causes the amount of light passing through filters 18 and 20 to increase and

decrease in relative amounts. The measurement procedure is identical to that of the Figure 1 embodiment, and the signal processor 44 is programmed to output a

displacement value in rotational terms. The embodiment of Figure 7 may be alternately configured to determine rotation about other angular directions by changing the orientation of the cylindrical mount 100.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. In particular, the disclosed measurement techniques may be used for making displacement measurements other than those specificly demonstrated in the embodiments shown. The particular number of bandpass filters may be varied in each of the one-dimensional, the two-dimensional, and the rotational embodiments. Light sources other than broadband sources may be used, and in particular light sources with frequency outputs coordinated with the frequency response of the bandpass filters may be used.

A displacement sensor may be conditioned such that intensity independence is not necessary. If the input signal intensity is measured and used in calculating an output displacement measurement, the output power of the system may be normalized. This technique is analogous to the electronics concept of automatic gain control. Such a technique could effectivey eliminate the need for the optical coupler and one of the output filters. A single output filter/photodetector combination could be directly coupled to the output of the input filters, the

photodetector output being normalized to the input power.

An additional modified embodiment of the invention involves the use of mechanical linkage to modify the total range of displacement of an system under test

Such a linkage could be attached to the mount 22 of the displacement sensor to customize the sensor to the particular range of displacement that is to be measured

Claims

CLAIMS 1. A displacement sensor for measuring the displacement of an object from a fixed reference frame,

comprising:

a light source for generating light; first and second optical bandpass input filters adjacent one another and moveable with said object, the first input filter having a first known

frequency response R1, and the second input filter having a second known frequency response R2

distinctly different from R1, the positioning of the input filters being such that light from the light source at a predetermined position passes through a dividing line separating the input filters;

an optical coupler receiving light passing through the input filters and dividing the received light into a first and a second output beam of equal intensity;

a first output filter having a frequency response identical to R1 for filtering said first output beam;

a second output filter having a frequency response identical to R2 for filtering said second output beam;

a first photodetector positioned such that it is illuminated by the light passing through the first output filter, the first photodetector

generating a first electrical output signal

indicative of the intensity of the light by which it is illuminated;

a second photodetector positioned such that it is illuminated by the light passing through the second output filter, the second photodetector generating a second electrical output signal

indicative of the intensity of the light by which it is illuminated; and

a signal processor for generating a

displacement output signal from said first and second electrical output signals.

2. The displacement sensor of Claim 1 wherein the light source is an LED.

3. The displacement sensor of Claim 1 wherein the first known frequency response R1 and the second known frequency response R2 are substantially adjacent in frequency.

4. The displacement sensor of Claim 1 wherein the input filters are positioned such that at an initial displacement of said object, an equal amount of light from the light source is incident on each of the input filters.

5. The displacement sensor of Claim 1 wherein the size and shape of the input filters are gauged to allow detection across the entire range of possible displacements of the objects.

6. The displacement sensor of Claim 1 wherein the

optical coupler is a beam splitter.

7. The displacement sensor of Claim 1 wherein the input filters are rectangular in shape.

8. The displacement sensor of Claim 7 wherein the input filters share a common boundary.

9. The displacement sensor of Claim 1 wherein the

optical coupler is a dichroic beam splitter.

10. The displacement sensor of Claim 1 further

comprising an additional light source generating light.

11. The displacement sensor of Claim 1 further

comprising a battery for powering the light source.

12. The displacement sensor of Claim 11 wherein the

battery also powers the signal processor.

13. The displacement sensor of Claim 1 further

comprising a casing in which the light source, the output filters and the photodetectors are housed.

14. The displacement sensor of Claim 1 further

comprising an optical fiber transporting light from light source to the input filters.

15. The displacement sensor of Claim 14 further

comprising optical fibers transporting light from the optical coupler to the output filters.

16. The displacement sensor of Claim 15 further

comprising an optical fiber transporting light from the input filters to the optical coupler.

17. The displacement sensor of Claim 16 further

comprising optical lenses coupling light into and out of the optical fibers.

18. The displacement sensor of Claim 17 further

comprising a casing in which the light source, the output filters, and the photodetectors are housed.

19. The displacement sensor of Claim 18 further

comprising optical fiber connectors on the casing for making optical connection with system components housed in the casing.

20. A linear displacement sensor for measuring the

displacement of an object from a fixed reference frame, comprising:

a broadband light source generating light in the fixed reference frame;

first and a second optical bandpass input filters adjacent one another and fixed in position relative to said object, the first input filter having a first known frequency response, and the second input filter having a second known frequency response distinctly different from the first known frequency response, the positioning of the filter pair being such that light from the light source passes through at least one of the input filters at any linear displacement of said object;

an optical coupler receiving light passing through the input filters and dividing the received light into a first and a second output beam of equal intensity; a first output filter having said first known frequency response, the first output filter

filtering said first output beam;

a second output filter having said second known frequency response, the second output filter

filtering said second output beam;

generating a first electrical output signal

indicative of the intensity of the light by which it is illuminated;

indicative of the intensity of the light by which it is illuminated; and

a signal processor for generating a

corresponding displacement output signal from said first and second electrical output signals.

21. A displacement sensor for measuring the displacement of an object from a fixed reference frame,

comprising:

a light source generating light;

a plurality of adjacent optical bandpass input filters each having a distinctly different frequency response, the input filters being moveable with said object and being aligned such that light from the light source passes through at least one of the input filters at any displacement position of the object; a plurality of optical couplers aligned to divide the light passing through the input filters into a plurality of separate light beams of equal intensity, the total number of separate beams being at least equal to the total number of input filters;

A plurality of optical bandpass output filters, the total number of which at least equals the number of input filters, each output filter having a distinctly different frequency response which is identical to the frequency response of one of said input filters, and each output filter being

positioned to filter one of said separate beams;

a plurality of photodetectors, each of which is illuminated by one of the filtered separate beams, each photodetector generating an electrical output signal indicative of the intensity of the light of the separate beam with which it is illuminated; and a signal processor for generating a

displacement output signal from the electrical output signals of the photodetectors.

22. The displacement sensor of Claim 21 wherein the

light source is a broadband light source.

23. The displacement sensor of Claim 22 wherein the

light source is an LED.

24. The displacement sensor of Claim 21 wherein the

input filters are positioned such that at an initial displacement of said object, an equal amount of light from the light source is incident on each of the input filters.

25. The displacement sensor of Claim 21 wherein the input filters are rectangular in shape.

26. The displacement sensor of Claim 21 further

27. The displacement sensor of Claim 21 further

comprising an optical fiber transporting light from light source to the input filters, from the input filters to the optical coupler, and from the optical coupler to the output filters.

28. The displacement sensor of Claim 27 further

comprising optical lenses coupling light into and out of the optical fibers.

29. The displacement sensor of Claim 28 further

30. A displacement sensor for measuring the displacement of an object from a fixed reference frame,

comprising:

a first light source periodically generating light;

a second light source generating light in alternation with the light generated by the first light source;

a first pair of optical bandpass input filters moveable with said object and having a first known frequency response, R1, and a second known frequency response, R2, respectively, R1 and R2 being distinctly different from one another, the first input filter pair being aligned adjacent one another in a first direction and being positioned such that light from the first light source at a predetermined position passes through a dividing line separating the filters of the first filter pair;

a second pair of optical bandpass input filters moveable with said object, the filters of the second filter pair having a frequency response Identical to R1 and a frequency response identical to R2, respectively, the second input filter pair being aligned adjacent one another in a direction

perpendicular to the adjacency direction of the first input filter pair and being positioned such that light from the second light source at a

predetermined position passes through a dividing line separating the second filter pair;

an optical coupler receiving light passing through the first input filter pair and the second input filter pair and dividing the received light into a first and a second output beam of equal intensity;

a first photodetector positioned such that it is illuminated by the light passing through the first output filter, the first photodetector generating a first electrical output signal indicative of the intensity of the light by which it is illuminated;

indicative of the intensity of the light by which it is illuminated; and

a signal processor for alternately powering the first light source and the second light source, and for computing and generating a displacement output signal from said first and second electrical output signals.

31. The displacement sensor of Claim 30 wherein the

light sources are broadband sources.

32. The displacement sensor of Claim 30 wherein the

filters of the first and second input filter pairs are rectangular in shape.

33. The displacement sensor of Claim 30 wherein the

optical coupler is a dichroic beam splitter.

34. The displacement sensor of Claim 30 further

comprising optical fibers for transporting light between optical components of the system.

35. The displacement sensor of Claim 34 further

comprising optical lenses coupling light into and out of said optical fibers.

36. The displacement sensor of Claim 35 further

37. A method of measuring the displacement of an object from a fixed reference frame, the method comprising: providing a light source which generates light; filtering said light with first and second optical bandpass input filters adjacent one another and moveable with said object, the first input filter having a first known frequency response R1, and the second input filter having a second known frequency response R2 distinctly different from R1, the positioning of the filter pair being such that light from the light source at a predetermined position passes through a dividing line separating the input filters;

dividing light passing through the input filters into a first and a second output beam of equal intensity with an optical coupler;

filtering said first output beam with a first output filter having a frequency response identical to R1;

filtering said second output beam with a second output filter having a frequency response identical to R2;

detecting the intensity of light passing through the first output filter with a first photodetector which generates a first electrical output signal indicative of the intensity of the light by which it is illuminated;

detecting the intensity of light passing through the first output filter with a second photodetector which generates a second electrical output signal indicative of the intensity of the light by which it is illuminated; and

generating a displacement output signal from said first and second electrical output signals with a signal processor.