GB2218588A

GB2218588A - Optical range simulator devices

Info

Publication number: GB2218588A
Application number: GB8811344A
Authority: GB
Inventors: Michael Benjamin Darlow
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1988-05-13
Filing date: 1988-05-13
Publication date: 1989-11-15
Anticipated expiration: 2008-05-13
Also published as: GB2218588B; GB8811344D0

Abstract

An optical range simulator 20 for use with a rangefinder includes first and second light transmitting ports connected to the respective first ports of first and second 4-port couplers 34, 39. Two fibre optic delay devices formed by two loops 35, 36 of differing loop length have their input ends connected to the second and third ports of coupler 34 and their output ends connected to the second and third ports of coupler 39. The fourth ports of coupler 34, 39 are interconnected. This arrangement produces a succession of return pulses to the rangefinder for each output pulse, each return pulse representing a different range. <IMAGE>

Description

OPTICAL RANGE SIMULATOR DEVICES This invention relates to optical range simulator devices.

In order to test the ranging function of a laser rangefinder it is necessary to fire the transmitter portion of the rangefinder at distant targets of known range to obtain a reflection therefrom which is collected by the receiver portion of the rangefinder. The distant target may be located on a real range but this requires uninhibited use of large tracts of land which is expensive and inconvenient.

Alternatively the distant target may be provided by a range simulator device which is compact and takes the form of an attachment to the rangefinder. Various forms of simulator devices are known and are discribed, for example, in US Patent Specifications 4068952, 4167328 and 4189233 and in UK Patent Specification 2141891.

It is an object of the present invention to provide an optical range simulator device which is particularly adapted for testing the ability of a laser rangefinder to discriminate between two or more adjacent targets which are distantly located.

According to the present invention there is provided an optical range simulator device comprising optical means for receiving ouput pulses from the rangefinder to be tested and for delivering return pulses to the rangefinder, said optical means including first and second light transmitting ports which are connected to the respective first ports of first and second 4-port couplers, and fibre optic delay means including first and second fibre optic loops of differing loop length the input ends of which are respectively connected to the second and third ports of the first coupler and the output ends of which are respectively connected to the second and third ports of the second coupler, the fourth ports of said couplers being interconnected.

By virtue of the present invention each rangefinder output pulse received by said optical means gives rise to a plurality of delayed return pulses including at least two return pulses which are separated by a range difference represented by the differing loop length of said delay means. Accordingly the discrimination function of the rangefinder can be tested.

Preferably the first and second fibre optic loops each have a loop length which is a multiple of the range discrimination accuracy of the rangefinder under test, whereby said differing loop length in also a multiple of said range discrimination accuracy.

The optical means may define a single optical axis for transmission of pulses to and from the rangefinder in which case the first and second light transmitting ports are connected to the single optical axis by a 3-port coupler and pulses are bi-directionally transmitted around said fibre optic loops. Alternatively the optical means may define a first optical axis for receiving pulses from the rangefinder and a second optical axis for transmitting pulses to the rangefinder in which case the first and second light transmitting ports are respectively connected to the first and second optical axes and pulses are uni-directionally transmitted around said fibre optic loops.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which Fig 1 schematically illustrates a first form of optical range simulator device according to the present invention; and Fig 2 schematically illustrates a second form of optical range simulator device according to the present invention.

The device 20 which is shown in Fig 1 is adapted for use with a laser rangefinder 50 having separate transmit and receive channels. Accordingly device 20 is provided with optical means 21 defining a first optical axis 22 and a second optical axis 23 arranged so that pulses 51 emitted by rangefinder 50 are delivered along axis 22 and return pulses 45 are emitted along axis 23. Axis 22 is defined by a lens 32 and the end 33A of an optical fibre link 33 the other end of which is connected to the first port of a 4-port coupler 34. Axis 23 is defined by a lens 44 and the end 43A of a fibre optic link 43 the other end of which is connected to the first port of a 4-port coupler 39.

Couplers 34 and 39 have their respective fourth ports interconnected by a fibre link 42 and the second and third ports of coupler 34 are connected to the input ends of a pair of fibre optic delay loops 35, 36 (wound on spools 37,38) the output ends of which are connected to the second and third ports of coupler 39.

Loop 35 wound on spool 37 has a different length from loop 36 wound on spool 38 and this difference in preferably equal to the nominal range discrimination accuracy of the range finder 50.

In operation, the rangefinder 50 emits an output pulse 51 which is focussed by lens 32 onto the end 33A of fibre link 33. Since end 33A is relatively easily damaged by high power pulses an attenuator 31 is preferably mounted in advance of lens 32. The received pulse is delivered by link 33 to coupler 34 which splits the pulse, preferably equally, to provide half intensity pulses to the two loops 35,36. At the output of loops 35,36 the emergent pulses which are time separated due to the differing loop lengths are delivered to the coupler 39 which splits each emergent pulse so that a minor portion is delivered along link 43, collimated by lens 44 and output as return pulse 45 to the rangefinder 50 whilst the major portion of each emergent pulse is recirculated to the input of the loops 35,36 via link 42 and coupler 34.

Accordingly a single output pulse 51 delivered by the rangefinder 50 gives rise to at least two return pulses, namely the first emergent pulses from loops 35,36, which are separated by a range difference represented by the differing loop length. Additionally of course further return pulses to the rangefinder 50 will be produced by the successive recirculating major portions of each emergent pulse and these further return pulses will contain at least two successive pulses separated by the differing loop length.

By way of example if loop 35 has a length representing a range of 190m and loop 36 has a length representing a range of 220m then the succession of return pulses will represent targets at ranges (in meters) of 190, 220, 380, 410, 440, 570, 600, 630, 660, 760, 790, 820, 850, 880 etc.

In practise the length of optical fibres on spools 37, 38 would be chosen to be multiples of the ranging accuracy of the rangefinder 50, typically 5m or 10m and with each loop length a multiple of the differing loop length which in turn is a multiple of the range discrimination accuracy of the rangefinder 50, typically 30m,so as to avoid return pulses too close together to be separately identified (or discriminated) by the rangefinder 50. Such a nondiscrimination situation arises for example after 7 circuits around the spool 37 and 6 circuits around spool 38 (ignoring the numerous intermediate return pulses) when return pulses at 1330 and 1320m occur for the 190m and 220m loop lengths previously discussed.

Fig. 2 illustrates a second form of optical range simulator device 30 which is the same as device 20 of Fig. 1 except that links 33, 43 are connected to a 3-port coupler 15 the other port of which is connected by a fibre optic link 14 with a fibre end 14A to a single optical axis 9 defined by lens 13 (and end 14A) for adapting the device 30 to a rangefinder 40 having a single transmit/ receive channel 11 along which both ouput pulses 51 and return pulses 45 traverse. In this case when an output pulse 51 is received by the device 30 coupler 15 splits the pulse essentially equally so that half intensity pulses are synchronously delivered to each end of each fibre optic loop 35,36 resulting in synchronous half intensity emergent pulses at each end of the loops 35,36 which are recombined by coupler 15 for delivery through collimating lens 13 to form the (or each) return pulse 45. It will of course be appreciated that as previously the emergent pulses from loop 36 are time-spaced from the emergent pulses from loop 35.

Spools 37,38 although shown separately in the interests of clarity may be in the form of a single bobbin on which both loops 35,36 are wound. Loops 35,36 provide emergent pulses which are non-polarised.

In the devices 20, 30, the splitting ratio of couplers 34, 39, affects the number of different detectable return pulses since there is an attenuation loss on each circuit made by the pulses traversing the loops 35, 36. Typically the loss at each coupler 15, 34, 39, is of the order of 20%.

Preferentially coupling into one of the two loops 35, 36, will filter out higher order returns of the less preferentially coupled route from the pulse train, thus reducing the multiplicity of returns generated by the two fibre optic spools.

Claims

1. An optical range simulator device comprising optical means for receiving output pulses from the rangefinder to be tested and for delivering return pulses to the rangefinder, said optical means including first and second light transmitting ports which are connected to the respective first ports of first and second 4-port couplers, and fibre optic delay means including first and second fibre optic loops of differing loop length the input ends of which are respectively connected to the second and third ports of the first coupler and the output ends of which are respectively connected to the second and third ports of the second coupler, the fourth ports of said couplers being interconnected.

2. An optical range simulator device as claimed in claim 1, wherein the first and second fibre optic loops each have a loop length which is a multiple of the range discrimination accuracy of the rangefinder under test, whereby said differing loop length is also a multiple of said range discrimination accuracy.

3. An optical range simulator device as claimed in either preceding claim, wherein the optical means defines a single optical axis for transmission of pulses to and from the rangefinder and the first and second light transmitting ports are connected to the single optical axis by a 3-port coupler and pulses are bi-directionally transmitted around said fibre optic loops.

4. An optical range simulator device as claimed in claim 1 or 2, wherein the optical means defines a first optical axis for receiving pulses from the rangefinder and a second optical axis for transmitting pulses to the rangefinder and the first and second light transmitting ports are respectively connected to the first and second optical axes and pulses are uni-directionally transmitted around said fibre optic loops.

5. An optical range simulator device as claimed in claim 1, and substantially as hereinbefore described with reference to either of the embodiments illustrated in the accompanying drawing.