GB2328575A

GB2328575A - Range detection

Info

Publication number: GB2328575A
Application number: GB8723037A
Authority: GB
Inventors: Herbert Alan French; Philip Sutton
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1986-10-01
Filing date: 1987-10-01
Publication date: 1999-02-24
Anticipated expiration: 2007-10-01
Also published as: GB2328575B; GB8723037D0; GB8623612D0

Abstract

Radiation from a self-illuminating or an illuminated target 51 is used to determine the target range by using an interferometer such as Mach Zehnder to split the radiation into two such that in an image plane light is received emanating from two apparent sources, the separation of which is determined by the target range. Radiation is split by a beam splitter 53 to follow two substantially identical paths 54,55 via mirrors 56,57 to a combining beam splitter 57. Converging lenses 58 and 59 of differing power are placed in the respective light paths and these result in the superimposition of two wavefronts of differing curvature in the plane of the camera 511. Fringes in the resulting interference pattern are measured by means of the camera 511 and the target range is calculated from these measurements.

Description

IMPROVEMENTs IN OR RELATING TO RANGE DETECTION The invention relates to detectors for the measurement of distance and in particular to passive range measurement of objects without requiring cooperative transmission of em radiation by the detector system.

A passive non-invasive ranging technique is desirable particularly in the military environment for covert observation since any form of transmission associated with an active range asuring system, eg radar or laser, can be detected and appropriate counter-measures taken.

Passive ranging conventionally involves binocular techniques.

Slectromagnetic radiation emitted or reflected from an object is viewed from two positions with a suitable baseline separation between them. Then by a method of triangulation the position or range of the object can be determined. For distant objects therefore it is necessary to use a long baseline.

The object of the present invention is to provide a passive ranging device operating on a monocular principle, thereby obviating the need for separate views of the object.

The present invention provides in one form a method of passive determination of the range of an object which is self-radiating or reflecting radiation from a remote source comprising the steps of dividing the radiation received from the object into two components, recombining the components with a path difference therebetween so as to form a pattern of interference fringes at an interference plane, and seauring the fringes to determine the range of the object.

In one form the fringe number and size or width of the fringe may be measured to determine the range. The invention arises from the realisation that by determining the wavefront structure of the radiation from an object the range can be determined0 Providing there is a sufficient degree of temporal and spatial coherence in the radiation from the object then the dimensions of the resulting fringe pattern depend in an analytically solvable manner upon the wavelength of the radiation and the range of the object. Thus if the wavelength is known, the range can be determined. Preferably the method includes the step of passing the radiation through a narrow band-pass filter prior to dividing the radiation. This serves to define the wavelength parameter required to derive the range.

Advantageously the band-pass filter is selected from a plurality of filters so as to maximise the resolution of the interference fringes.

In order to ensure sufficient temporal coherence in the radiation received the spectral bandwidth of the filters is made sufficiently small such that the coherence length of the radiation is greater than the path difference between the two interfering components of the radiation. Spatial coherence of the radiation from the object can be achieved by using a sufficiently narrow field of view.

In another form of the method the path length of one interference component of the divided radiation is cyclically varied and the range may be determined by measuring the phase difference in the radiation at two spatially separated positions in the interference plane.

In an alternative form the invention provides a passive rangefinder for determining the range of an object comprising means for dividing radiation from the object with a sufficient degree of temporal and spatial coherence into two components, means for combining the components with a path difference therebetween less than the coherence length of the radiation so as to form a pattern of interference fringes in an interference plane, radiation detecting means responsive to tho radiation in the interference pattern, means to determine the wavelength of the radiation and mPans connected to tho wavnlength dPtPrmiining means and the radiation detecting means for determining the range of the object.

The radiation detecting means may comprise an array of radiation detectors placed in the interference plane and arranged so as to count the number of a fringe and the size or thickness of the fringe.

Conveniently there are included a number of selectable band-pass filters, a filter being selected to maaimise the signal to noise ratio measured by the radiation detecting means. Advantageously the field of view is small such that there is a sufficient degree of spatial coherence in the radiation received from the source.

In an alternative arrangement a phase modulator is included in the path of one of the two interfering radiation components and there is means to measure the phase difference between two spatially separated positions in the interference plane. Advantageously the phase modulator is an electrically driven Pockels cell. The phase difference may be measured by providing an opaque screen placed in the interference plane, the screen having two apertures and a radiation detector positioned behind each aperture and responsive to the radiation transmitted therethrough and a circuit connected to the outputs from the two detectors for measuring phase difference.

Advantageously a beam splitter may be used to divide the radiation and subsequently recombine the separate components.

The two components after division may be reflected back to the beam splitter by respective planar mirrors. As in the Michelson interferometer the resulting interference pattern consists of circular fringes.

Advantageously the rangefinder may be provided as a narrow fielo-of-view sensor within a broader fielc-of-view device such as in binoculars or a periscope.

In a particularly advantageous arrangement a Mach Zehnder interferometer arrangement is used.

The invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 shows a Michelson interferometer arrangement in schematic form; Figure 2 is an illustration of the interference pattern produced by the Figure 1 arrangement; Figure 3 is a schematic drawing of a modification of the Figure 1 arrangement; Figure 4 is a schematic block diagram of apparatus for fringe pattern analysis; Figure 5 is an alternative optical arrangement to that shown in Figure 3; and Figure 6 is a graph showing a comparison of experimental and theoretical results.

Figure 1 shows a Michelson interferometer arrangement which may be used, according to the invention, to determine the range of a source of light. Light from a point source 1 is separated by a beam splitter 2 into two components 3,4 which are respectively reflected from planar mirrors 5,6 such that they are recombined by the beam splitter 2 to form a beam 7 incident on a detector array b. For convenience the light source is assumed to be a monochromatic coherent source such as a laser.

The range R is given by the expression: R = Z + 2 11 + S (1) where Z is the source to beam splitter distance, 11 is the beam splitter to mirror 5 distance, and S is the beam splitter to screen distance, and the path difference r between the two component beams 3, 4 on recombination is given by: r = 2 (12 - 11) (2) where 12 is the beam splitter to mirror 6 distance. The mirrors 5, 6 are placed such that a path difference r occurs, small compared with the coherence length of the light from the source 1, such that the range R geometry of the wavefront of the light from the source 1, is revealed in the interference fringe structure formed on the detector array 8 by the recombined beam 7.

For on-axis operation the interterence pattern produced by a distant point source appears as shown in Figure 2. The diameter d or the n th circular fringe is given by dn = L8 Rn A (1 + Wr) ] (3) where R, r are given by expressions (1), (2) respectively and X is the wavelength of the illumination.

For R r dn = R [8n #/r] (4) and thus there is a linear relationship between fringe diameter and range.

When R becomes large, d also becomes large and then it might be more n conttniont to obtain the range R by measuring the width of the fringes.

The width of the n th fringe when n is large is given by: W = Wa [ 2 # /nr] (5) n Rere W is also linearly related to the range R but the proportional n constant includes n ≈making W < d and thus easier to quantify n n than dn.

Thus if the number of the fringe (n), the path difference (r) between the two interfering beams, ar.d the wavelength (X) of the point source are all known, R can be calculated by measuring dn or Wn. By connecting the outputs 9 from the detector array to a signal n processor 10 the range R can be calculated and provided as an automatic read-out for display or further processing. A second output from the signal processor may be provided to give the range accuracy.

Figure 3 shows an alternative arrangement for measuring the fringe structure. An electronically controlled Pockels cell 11, or other phase modulator is placed in one of the beams 4 so as to superimpose on the fringe structure formed at the image plane 12 a variation dependent on the phase structure of the wave front from the point source. A screen 13 having two small apertures 14, 15 is placed in the image plane 12 and two detectors 16, 17 are positioned behind the respective apertures 14, 15. The phase modulator 11 is driven by a suitably alternating current signal and the phase difference between the outputs from the two detectors 16, 17 is measured. This phase difference can be shown to be dependent on the wave front structure and hence the range R of the point source.

Although the invention has been described in relation to the detection of a self-emitting laser source, it may also be applied to any source which is self-emitting or reflecting radiation. A spectral filter unit 18 may be placed in the path of the illumination from the source. This includes selectable band-pass filters, for example red, green and blue. In use the rangefinder is directed at an object and a filter is selected giving the maximum signal to noise ratio.

Thus the wavelength is defined by the centre-band wavelength of the filter selected. In addition the spectral width of the filter determines thF lower limit of the coherence length of the illumination and this coherence length is arranged to be greater than the path diffPr -nCP r between the two interfering beams 3, 4. Thus if an object were remitting only reflected msn-light, narrow-band filters would be needed to obtain an "apparent source" of suitable long coherence length for the rangefinder to work.

Where the object is not a true point source the detailed geometry of the wave front at the rangefinder is given by the convolution of the point target wave fronts (the impulse response) and the target geometry.

This can be simplified by introducing a field stop (not shown) into the rangefinder system such that only a small part of the object is considered. By this means the "spatial coherence" of the object is kept high. This also serves to eliminate ambiguities created by having light enter the system from a number of different sources, all at different ranges. The rangefinder thus adapted as a narrow field of view (row) sensor can be bore-sighted with a wider FOV device, Sor example binoculars. Narrow spectral band and narrow FOV requirements reduce the available illumination and consequently high detection sensitivity and a good technique for counting and measuring the interference fringes must be available.

Figure 4 illustrates the equipment which can be used to record and analyse the fringe pattern produced by the interferometer systems and thus calculate the curvature of the wavefront and hence the range of a source. The fringe pattern 40 is viewed by a camera 41 and the picture stored in a video recorder 42. A video line generator 43 is connected to the video recorder such that a selected line profile can be provided across the fringe pattern. This line signal is then connected to a transient recorder 44 for analysis of a selected section of the fringe pattern. An output of the transient recorder is connected to an XY plotter 45 and to an oscilloscope 46. The complete fringe pattern is analysed by connecting the output from the video line generator 43 to a monitor 47 and a rastor hard copier 48. A polaroid camera 49 is provided to view the oscilloscope 46 or the monitor 47.

An alternative Mach Zehnder interferometer arrangement is shown in Figure 5. A remote source 51 is scanned by a steering mirror 52. Light from the beam splitter 53 in to first and second beams 54 and 55. After reflection from angled mirrors 56 and 57 the first and second beams are recombined by a second beam splitter 57. Converging lenses 58 and 59 are placed in the respective paths of light beams 54 and 55 and a diverging lens 510 is placed in the path of the recombined beams in front of a zoom lens camera 511. The lenses image the source in each arm of the interferometer close to the interferogram plane. There is lettle optical path difference between the two arms of the interferometer along their respective optic axes so the demand on the degree of coherence of the source for a fringe pattern to be produced is small. Most of the incident light is used in the formation of the fringe pattern. Where the optical path difference between the two arms of an interferometer is large it may be necessary to provide a small enough input stop and/or a spectral filter to produce the required degree of coherence, thus cutting down the available light and decreasing the sensitivity of the detector.

The Macb Zehnder arrangement thus described places less stringent limitations on the source spectral profile than the Michelson interferometer arrangements described. In addition for a given aperture of the instrument the fringe pattern is compacted and the range capability is enhanced. The focal lengths of the two lenses 58 and 59 are different and it is this difference which allows the interference pattern to be formed within the instruments output field-of-view. A neutral density filter 512 is provided in front of the fringe counting camera 511 to provide protection for the camera photocathode. A second neutral density filter 513 is provided in one arm of the interferometer to improve the contrast of the fringes by balancing the energy density of the light received at the camera from the two arms. Alternatively the energy balance in the two beams can be obtained by tailoring the reflectance/transmission of the beam splitter 53. An optical flat of suitable thickness can be introduced into the other arm of the interferometer to balance the effect of the neutral density filter 513 on the optical path length.

Figure 6 shows a comparison with predicted valves of first fringe experimental measurements against range. Thus there has been found good agreement with theory.

In military use the rangefinder may be used to detect lasers, vehicle lights or sun glints. In addition the device may use reflected light from flares or search lights remotely placed from the rangefinder so as not to give away the rangefinder position. Furthermore the rangefinder may be used to range on objects scattering solar illumination. The range information acquired passively can be used in weapon fire controllers so that weapons can be launched without the need to first switch on detectable radars or laser rangefinders etc.

The rangefinder also has potential use in military aircraft and ships. In the latter case passive ranging via the periscope optics of submarines would be particularly beneficial.

It will be apparent to those skilled in the art that the arrangements described are only exemplary of the invention and that other variations are possible. The Michelson interferometer is only one convenient way of dividing the emitted illumination and then coherently recombining the divided illumination so as to be able to investigate the wavefront of the illumination. Other interferometers include Smarty, Billets and Jamin. Each interferometer splits an incoming beam of radiation to produce separate wavefronts emanating from two apparent sources inside the interferometer.

The separation of the two apparent sources is a funtion of the range of the source emitting the incoming beam of radiation.

The technique used for extraction of the range information will depend upon the choice of interferometer arrangement.

Furthermore the technique could be applied to any part of the electromagnetic spectrum where appropriate components are available, for example infra-red, microwave or ultra-violet.

In addition to conventional relatively long range rangefinder applications the technique could be adapted to give range, and therefore positional information about radiation sources in laboratory experience, for example the positions of scintillations produced by atomic collisions in nuclear physics or cosmic ray experiments. For optical applications a light collecting telescope may be used with the range-finder.

Claims

1. A method of passive determination of the range of an object which is self-radiating or reflecting radiation from a remote source comprising the steps of dividing the radiation received from the object into two components, recombining the components with a path difference therebetween so as to form a pattern of interference fringes at an interference plane, and measuring the fringes to determine the range of the object.

2. A method as claimed in claim 1 wherein the fringe number and width of the fringe are measured to determine the range.

3. A method as claimed in claim 1 wherein the fringe number and size of the fringe are measured to determine the range.

4. A method as claimed in claim 3 wherein there is included the step of passing the radiation through a narrow band-pass filter prior to dividing the radiation.

5. A method as claimed in claim 4 wherein the band-pass filter is selected from a plurality of filters so as to maximise the resolution of the interference fringes.

6. A method as claimed in claim 1 wherein the path length of one interference component of the divided radiation is cyclically varied and the range is determined by measuring the phase difference in the radiation at two spatially separated positions in the interference plane.

7. A passive rangefinder for determining the range of an object comprising means for dividing radiation from the object with a sufficient degree of temporal and spatial coherence into two components, means for combining the components with a path difference therebetween less than the coherence length of the component radiation in the instrument so as to form a pattern of interference fringes in an interference plane, radiation detecting means responsive to the radiation in the interference pattern, means to determine the wavelength of the radiation and means connected to the wavelength determining means and the radiation detecting means for determining the range of the object.

8. A passive rangefinder as claimed in claim 7 wherein the radiation detecting means comprises an array of radiation detectors placed in the interference plane and arranged so as to count the number of a fringe and measure the thickness of the fringe.

9. A passive rangefinder as claimed in claim 7 wherein the radiation detecting means comprises an array of radiation detectors placed in the interference plane and arranged so as to count the number of a fringe and measure the size of the fringe.

10. A passive rangefinder as claimed in claim 9 wherein a band-pass filter is provided to filter the radiation received from the object.

11. A passive rangefinder as claimed in claim 10 wherein there is included a plurality of selectable band-pass filters of different centre frequencies, a filter being selectable to maximise the signal to noise ratio measured by the radiation detecting means.

12. A passive rangefinder as claimed in claim 11 wherein a phase modulator is included in the path of one of the two interfering radiation components and there is included means to measure the phase difference between two spatially separated positions in the interference plane.

13. Apassive rangefinder as claimed in claim 12 wherein the phase difference is measured by providing an opaque screen placed in the interference plane, the screen having two apertures and a radiation detector positioned behind each aperture and responsive to the radiation transmitted therethrough and a circuit connected to the outputs from the two detectors for measuring phase difference.

14. A passive rangefinder as claimed in claim 13 wherein a Mach Zehnder interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

15. A passive rangefinder as claimed in claim 14 wherein converging lenses of differing focal length are placed in the respective paths of the radiation components.

16. A passive rangefinder as claimed in claim 9 wherein a Michelson interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

17. A passive rangefinder as claimed in claim 16 wherein a Mach Zehnder interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

18. A passive rangefinder as claimed in claim 17 wherein converging lenses of differing focal length are placed in the respective paths of the radiation components.

19. A passive rangefinder as claimed in claim 10 wherein a Michelson interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

20. A passive rangefinder as claimed in claim 19 wherein a Mach Zehnder interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

21. A passive rangefinder as claimed in claim 20 wherein converging lenses of differing focal length are placed in the respective paths of the radiation components.

22. A passive rangefinder as claimed in claim 11 wherein a Micbelson interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

23. A passive rangefinder as claimed in claim 22 wherein a Mach Zehnder interferometer arrangement is used to divide the radiation into separate components and subsequently recombine the components.

24. A passive rangefinder as claimed in claim 23 wherein converging lenses of differing focal length are placed in the respective paths of the radiation components.