GB1572148A - Passive optical range simulator device - Google Patents

Description

(54) PASSIVE OPTICAL RANGE SIMULATOR DEVICE (71) We, WESTINGHOUSE ELECTRIC CORPORATION, of Westinghouse Building, Gateway Center, Pittsburgh, Pennsylvania, United States of America, a company organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an optical range simulator device.

Advanced electro-optical weapons delivery systems require sophisticated test methods to assure measurement integrity.

Techniques are required to confirm accurate ranging of optical radars, proper boresighting of all optical channels, and round trip system performance. A straightforward and convenient means of testing such a system is desirable.

In the past the performance of such systems have been tested primarily by two methods: (1) through the use of a surveyed outdoor optical radar range having targets of known characteristics; and (2) through the use of an active optical transponder directly coupled to the system undergoing test. Method (1) requires considerable terrain area, a fixed plant, and presents weather and safety problems. Method (2) requires a critical optical interface, particularly with respect to boresighting, and it is difficult for the transponder to simulate the signature of returns from real targets. The magnitude of the transponded returns must be known if the receiver sensitivity contribution to overall system performance is to be established.

Such magnitudes are not readily calibrated at the wavelengths of interest, particularly for short pulses and signal levels close to optical receiver thresholds. Timing of the transponded returns is critical to range accuracy, and it is not straightforward to achieve and maintain the required accuracy using inexpensive radiation sources, for example.

It is an object of this invention to provide a novel passive optical range simulator device with a view to overcoming the deficiencies of the prior art.

The invention resides in a passive optical range simulator device comprising an adapter member for mechanical coupling with an optical radar ranger system having a working aperture via which optical radar beams enter and leave in parallel along a path, a focusing lens assembly disposed in said path for transforming said beams between parallel and a focal spot in a common plane, an optical fiber coil having one end for disposition in said common plane in registry with said focal spot and having a reflector at its opposite end for enabling such fiber coil to function as an optical delay medium.

In accordance with the teachings of the present invention, there is no need for a collimator assembly as an optical interface with the ranger system under test. The range simulator device can be attached directly to the ranger system undergoing test via an adapter member. This direct attachment greatly simplifies optical alignment procedure and eliminates the risk of vignetting.

The focusing lens is of wide aperture to focus beams directly on the fiber entrance face.

In accordance with a preferred embodiment of the invention, a tilted-glass-element primary attenuator gives a "free space" characteristic to the beam path by the elimination of first surface reflection of energy back into the system under test, and an oppositely-angled flat glass elements of the fine adjustment attenuator prevents unwanted translation of beam position during transmit therethrough. The optical range simulator has an optical fiber delay line coil within which multiple reflections are induced by reflecting surfaces at each end of such coil.The entrance one of these surfaces is partially reflecting and partially transmitting at the wavelength of interest so that a fraction of the energy stored in the coil may be extracted through this partially reflective surface to furnish the ranger system with a series of "target echoes" equally spaced in range at multiples of the fiber delay line optical length.

This invention will beome readily apparent from the following description of an exemplary embodiment thereof when read in conjunction with the accompanying drawings, in which: Figure 1 is a schematic representation, partly in outline and partly in section, of the passive optical range simulator device of the present invention; and Figure 2 is a partial view of a modification of the Figure 1 embodiment, using a partial mirror at the entrance end of an optical fiber delay line.

Referring to the drawing which shows a preferred embodiment of this invention, the passive optical range simulator device comprises an adapter member 1 that is contoured to mate with a complementary portion of the optical radar ranger system 2 undergoing test. The adapter 1 is provided with openings 3 to accept locating pins 4 projecting from a forward portion of a housing 5 of the device of the present invention.

Construction of the member 1 is such that when in use, a test input aperture for the range simulator device coincides and/or is aligned with the working aperture 7 of the ranger system undergoing test. If the physical characteristics of the tested device become significantly different, the adapter member is changed to suit; so that repeated tests of similar systems may be set up and effectuated expeditiously. From the aperture 7 of the optical radar ranger system undergoing test, pulses of optical laser energy will leave along a boresight axis 8 and will return along a parallel axis 9 or 10 for reentry into the system aperture 7. The other of such parallel axes 9 or 10 may represent a path for input of optical information to a television camera (not shown).Attachment screws 11 affiliated with the adapter member 1 provide for locking the locating pins 4 in the openings 3 when positioned therein.

Other securing means (not shown) may be employed for removably attaching the adapter member 1 to the system 2 undergoing test. It will be appreciated that the locating or alignment pins 4 when in position within the adapter member 1 while mounted on the system to be tested, position the simulator device housing 5 such that the beam system of the system 2 falls completely within the clear aperture, thus avoiding vignetting.

All beams to and from the system 2 undergoing test along the axes 8, 9, and 10 are subjected to primary attenuation by a tilted-glass attenuator 12, inclined at 45 , for example, to deflect any first surface reflec tion downwardly into a radiation absorber consisting of a second parallel tilted glass absorber plate 14 affiliated with optical black surfaces 15 and 16 located to absorb the transmitted and reflected components leaving plate 14. This primary attenuator at the input to the simulator device ensures that such device looks like "free space" to the system 2 undergoing test, thus minimizing "main bang" reflected energy which otherwise tends to overload the return input to such system and possible damage to the laser source herein.

All beams along the axes 8, 9, and 10 pass through a focusing lens 17 which transforms such parallel beams to and from the system 2 into corresponding focused beams entering and leaving one end 18 of a coil 19 of optical fiber 20 having a mirror 21 presented to its opposite end 22. Lens 17 must be of good optical quality so that the focused blur circle at the wavelength of interest is small enough to couple efficiently with the end face 18 of the fiber 20, and it is free of chromatic aberration at the wavelengths of the optical energy in the beams handled by the system undergoing test, such as laser pulse beam for ranging and a visible spectrum beam observed by the television camera. One frequency for the former and a nominal frequency for the latter suffices as a practical matter, in most instances.The focal length of the lens 17 is chosen to provide a good match with the numerical aperture of the optical fiber 20 and a sufficiently small diffraction-limited focused spot size at the fiber end 18 for efficient energy coupling. The choice of focal length is frequently a compromise between these two desiderata. Thus for a given laser transmitter beam divergence, focused spot size may be reduced to increase input coupling efficiency by selecting a short focal length, but in doing so, a greater fraction of the available fiber numerical aperture is used for input coupling and correspondingly less is available for output coupling to the system's receiver input. The compromise focal length for one working embodiment has been chosen as eleven inches.

A fine adjustment attenuator assembly 24 is interposed in the optical path between the focusing lens 17 and the fiber end 18.

The assembly consists of a pair of oppositely tilted glass attenuator plates 25 and 26 through which the focused beams pass enroute to and from fiber end 18. The tilting prevents first surface reflections from reaching the system 2 undergoing test. The directions of tilt are designed to be selfcompensating for zero boresight axis displacement. The adjustment provided by the assembly is obtained by virtue of removability and replacement with similar attenuator assemblies of different absorption values.

The fiber end 18 or entrance face is optically polished and embedded in a terminator which comprises a short section of poly styrene tube 30 partially filled with epoxy adhesive 31 in which the fiber end is centrally located. The terminator is carried in a translation member 32 affiliated with adjusting screws 33, 34, and 35, for example, to provide for fibre end position adjustment in three mutually perpendicular directions X, Y, and Z. Compression springs 36 interposed between the housing 5 and the member 32 assure that such member will follow position adjustment of the screws.

Guide means for member 32 has not been shown. The X-Y axis adjustments allow the fiber end 18 to be positioned for coincidence with the common focal point of all beams along axes 8, 9, and 10 in the final image plane, and the Z axis adjustment permits fine focus.

The coiled length of optical fiber 20 is typically one half to one kilometer in length, wound on a storage spool 40 as a series of fully interleaved single layer winnings using controlled tension to minimize fiber stress and achieve minimum optical loss. The fiber itself can be a low-loss step-index type as furnished by Corning Glass Works, for example, and typically exhibits loss of the order of 2.5db/km at 1064 nanometer wavelength. Core and cladding diameters were 85 and 125 microns, respectively. The numerical aperture fell in the range of 0.14 to 0.18, and pulse dispersion rates of 10 nanoseconds per kilometer was typical. The fiber is mechanically protected throughout its length by an elastic urethane "buffer" coating having a nominal thickness of .005 inch, which results in an overall fiber diameter of .015 inch.

The reflective end of the fiber terminates in a manner substantially identical to that of the end 18 described above, except that the polished fiber end 22 is maintained .001 inch away from the surface of the plane mirror 21 by means of a separator (not shown) and such reflective end need not be adjustable.

The mirror 21 can exhibit a first surface reflectivity of 99.9% at the wavelength of interest coupled with high transmissivity at visible wavelengths. This aspect allows for reflection of the laser transmitter pulses with small loss, while permitting light to be coupled into the fiber from a bulb or source of visible light 41 directly behind the mirror.

It should be noted that attenuators 12, 25, and 26 are selected to be absorptive at the wavelength of interest and relatively transparent at visible light wavelengths. This permits light coupled into the fiber from the source 41 to be viewed on the television monitor of the system undergoing test as such light is emitted from the fiber end face 18. This illuminated fiber end behaves almost as a point source of light, greatly facilitating its positioning on the bore-sight axis 8 and subsequent fine focus adjustment.

The passive optical range simulator as described above uses a fiber optic delay line to furnish a single target echo capable of confirming the range calibration and boresight accuracy of an optical radar ranging system. Range calibration of the simulator may be accomplished by any of the following methods: measurement of fiber delay line length and conversion of this effective optical length using known refractive index of fiber; use of an optical radar ranging system (known to be good) to transfer calibration from surveyed targets on an outdoor range to the simulator being calibrated; or, timing optical pulses propagating on a oneway path through the fiber optical delay line to determine its effective length.

In practice, none of these methods is simple or straightforward, and most simulators have been calibrated by the second method. A target echo from a single known range is sufficient to set or confirm the zero-range adjustment of an optical ranging system under test, provided that the system's range scaling is accurate and behaves linearly. Under these conditions, even though the system has been calibrated only at a single range point, the calibration will be valid at other ranges. In many cases, there is a lack of confidence in system range linearity even though range scale factor may have been set initially by independent electrical measurements.

From the foregoing, it will be apparent that it is desirable to simulate several target echoes separated in range so as to cover the range window of interest A minimum of three such target echoes is required to distinguish between the effects of zero-set and range-scaling adjustments, to to assesss the fundamental ability of a system to time the ranging interval accurately. Thus, zerorange of the system under test may be set or confirmed on any one target echo, rangescaling adjusted (if necessary) to exhibit correct range readout on a second, and proper range readout independently confirmed by means of a third target echo.

There also is a need for multiple target echoes to exercise a ranging system's dynamic response to a multiplicity of targets and change in range of a most distant target.

It is desirable that the multiple target simulator substantially confirm its own calibration in order to avoid the high cost in terms of time and inconvenience associated with the three "independent" calibration techniques identified above. In furnishing the above desired properties, it is important that the ability to confirm boresight accuracy be retained.

In Figure 2, there is shown a modification of the simulator device of Figure 1, which accommodates the foregoing objec tives by the use of a partial mirror at the entrance end 18 of the fiber optic line 20 so that multiple reflections are induced within the fiber optic line by reflecting surfaces at each end of such coil. The partial mirror 37 is formed by an interface surface 37 of an input window 38 mounted on the translation member 32, the interface surface being partially reflective by provision of a dielectric layer thereon. The input window is anti-reflection coated on its exterior surface.

The direct physical contact between the partially reflective surface 37 and the fiber end provides a low loss junction therebetween.

Introduction of laser pulses from the system 2 undergoing test to the fiber end at the partially reflective surface 37 results in transmission of such pulses through the optical fiber 20 including its coil 19 to the total reflector 21 at its terminal end 22 and back to the fiber entrance at the partial mirror 37. Each laser pulse will bounce back and forth between the partial mirror 37 and the total mirror 21 via the coil 19 a number of times. Each time the reflection appears at the partial mirror 37, be it of sufficient strength, it can be observed by the receiver (not shown) within the system 2 along the axis 9 or 10, for example. Preferably, the partial mirror surface 37 has a 63% reflectivity at the laser pulse frequencies of interest.

The entrance one of these surfaces is partially reflecting and partially transmitting at the wavelength of interest so that a fraction of the energy stored in the coil may be extracted through this partially reflective surface to furnish a series of "target echoes" equally spaced in range at multiples of the fiber delay line optical length. The partial transmission property of this surface does not hinder the coupling of optical pulses into the coil from the transmitter of the ranging system under test. In the situations requiring maximum echo amplitudes, a total reflector is necessary at the other end of the delay line length. As in the embodiment of Figure 1, all collimated and parallel beams intercepted by the focusing element find their common focus at a point in a common plane which is adjusted to coincide with the fiber entrance end.The reflecting surface of the partial entrance reflector is also coincident with such plane and in physical contact with the fiber end under controlled pressure as described above in connection with the embodiment of Figure 1. The addition of the partial entrance reflector to achieve multiple reflections does not affect the above-mentioned common focus properties, hence the boresight confirmation feature is preserved.Both reflectors are of dielectric layer construction to acc:mmodate the high energy densities pro kqded by the optical transmitter, and to exhibit the desired properties at the wavelength of interest whilst retaining useful transparency at visible wavelengths to retain the feature of transmitting visible light from an external source though the fiber to illuminate the fiber entrance aperture for ease of positioning in the focal plane.

Change in the number of target echo reflections obtained can be arrived at either by change of optical path attenuation exterior to the reflecting delay line by means of an adjustable attenuator or by controllable separation of the fiber end and the terminal reflector.

Of these methods, the first one is preferred when it is required to change attenuation in discreet calibrated steps, while the second one lends itself to continuous attenuation adjustment. Either technique will produce fewer system-recognizable target echoes with increasing path attenuation. The number of target echoes that can be simulated is a function of the usable dynamic range of the simulator defined as a residue of a total system dynamic range remaining after all sources of less have been subtracted. The usable dynamic range is usually input power limited, because of the need to avoid damage to optical surfaces such as the entrance reflector, and the attenuation introduced into the optical path to limit the focussed power density constitutes the major fraction of the above loss.Thus, a typical system under test may exhibit a dynamic range in excess of 130db of which perhaps 50db is available to the simulator in usable form. A theoretical model has been developed which predicts the usable dynamic range required for visibility of the nth target echo when corrected for the effects of pulse dispersion. For a 0.5 Km delay line (loss 2.5db per Km at wavelength of interest) used with a 63% reflectivity entrance mirror and total terminal mirror, the predictions are as follows: Usable Dynamic Range Requirements Echo Order n db 1 16.5 2 24.2 3 31.1 4 37.7 5 44.0 This tabulation shows that 50db of usable dynamic range should produce five target echoes tapering in ampliture relative to the first echo (n=l) as indicated by the db differences. On this bases, five target echoes can simulate ranges out to approximately 12,000 feet in air using a comparatively short (hence less expensive) fiber optical delay line coil, and confirm range linearity at five equispaced ranges in between.

The equispaced target echoes might also be used to confirm the calibration of both fiber delay line length and system ranging if the system under test embodies an accurate range scale. The accuracy criterion is met, if: a precision time or frequency source furnishes the reference timing intervals; the range interval timing is accomplished in a linear manner; and, the correct range readout scale factor has been pre-set by means of electrical measudements.

Good range-interval-timing-linearity is indicated by quality of target-echo spacing, and this is not influenced by scale factor or zero-set errors, i.e., the "apparent" range of each echo is sufficient to determine equality of spacing. Under these conditions, the range readout corresponding to echo spacing is a measure of the effective length of the fiber optic coil delay line, i.e., the delay line is calibrated to within range scale accuracy.

Small random departures from exact equality of spacing can occur in practice due to corresponding departures from range interval timing linearity. The effect of these errors may be minimized by calculation of average spacing as being equal to the range separation of first and nth target echoes divided by n- 1; the accuracy improving with increasing n. Once the optical fiber coil delay line length has been determined, the accuracy of zero-range setting of the system under test may be established. It is evident that any significant departure from an exact multiple relationship between the first echo range and the ranges of subsequent echoes indicates an error in the 0-range setting.

The magnitude and direction of this error may be determined from the following expression which gives the true range of the nth echo:

where n is an integer > 1 Rnt=true range of nth echo.

R=apparent range of nth echo.

Rl=apparent range of first echo.

The quantity in parentheses furnishes the magnitude and sign of the zero-set error, and this information may be used to correct the zero-range setting of the system under test. When this has been done, the indicated and the calculated (true) ranges will coincide, and both the system under test and the optical fiber coil delay line are calibrated. It is seen that the ability to furnish multiple target echoes is vital to this calibration process even though the basic calibration reference is the precision time or frequency source in the system under test.

Due to the additional distance propagated within the fiber, each target echo exhibits more dispersion (pulse stretching). than its precursor. By selection of a specific target echo, this behaviour may be exploited in order to simulate the degree of pulse stretching exhibited by optical radar echoes when ranging to real targets at oblique angles. In the simulator of Figure 1, the simulation of pulse stretching requires selection of suitable optical fiber properties, hence a change in fiber optic coils. It is evident that target echo selection in the embodiment of Figure 2 is accomplished with relative simplicity. This embodiment is also capable of simulating a wider range of dispersion.

When the simulator of Figure 1 uses ia 0.5 Km optical fiber coil delay line, its single target echo normally arrives at the optical receiver during the latter's recovery from "main bang" desensitization with the result that the receiver's threshold sensitivity is degraded. It follows that such simulator cannot be used to determine round-trip system performance unless the optical fiber coil delay line length is increased to allow full receiver recovery. By contrast, the multiple reflection simulator of Figure 2 using a 0.2 Km optical fiber coil delay line yields several target echoes in the region of full threshold aensitivity.

Round-trip system performance of a system under test (or total system dynamic range) is a direct function of transmitter output power and receiver sensitivity. As for the simulator of Figure 1, the total value of attenuation in the optical path may be used as a measure of this dynamic range.

Thus with a given value of optical attenuation in the path, the visibility of the nth target echo may be used as a system acceptance criterion. If attenuation is introduced in calibrated increments by means of the input attenuator, the dynamic range separation of the target echoes may be determined. This measurement provides a selftest feature in the multiple reflection simulator of Figure 2 because small dynamic range separation of target echoes is indicative of minimal optical loss within the fiber optical coil delay line. Conversely, large intra-coil losses absorb comparable blocks of dynamic range per echo, rapidly consuming the usable dynamic range and yielding a reduced number of target echoes. The number of target echoes visible cannot be used as the sole simulator performance criteria because this is dependent also upon the available total system dynamic range.The dynamic range separation of target echoes is independent of the system dynamic range and is a direct function of fiber optical coil delay line quality; it therefore offers a reliable and readily measured performance criterion and a self test feature. When first commissioned, a multiple reflection simulator may be employed with a ranging system whose total dynamic range is known, omitting any adjustment of fixed attenuation in the optical path to achieve specific echo visibility. Dynamic range separation of the target echoes may then be measured, as previously described, and checked against both theoretical predictions and the number of target echoes expected within the residual (usuable) dynamic range.Good correlation indicates proper simulator functioning, and the measured separations serve as benchmarks to which all future measurements on this simulator may be referenced Special attention to design is required in the multiple reflection simulator if intra-coil losses are to be minimized in order to achieve the maximum number of target echoes within the available usable dynamic range.Thus, a maximum echo yield is obtained when: The partial reflector at the fiber entrance is selected for a transmission/reflection ratio that enhances the higher order echoes at the expense of the lower order ones; The fiber optic waveguide used in the coil exhibits a minimum loss at the lavelength of interest, and is stored in a stress-free manner to preserve such minimum loss: and Fiber entrance/exit, and reflection losses are minimized by careful attenuation to fiber termination and surface preparation coupled with fiber-reflector interface design.

The ability to accommodate high optical power densities is inherent in the design of the present terminator structure, with the result that its incorporation in the simulator permits the use of much higher incident energies than is allowable for the simulator of Figure 1. As was explained earlier, the usable dynamic range is input power limited and, it follows that the ability to accept more energy increases this dynamic range and the associated maximum echo yield. It should be recalled that the increase in echo yield via this mechanism is entirely independent of that achieved by the reduction of intra-coil losses. The success of a practical multiple reflection simulator requires that both improvement mechanisms be exploited in the pursuit of maximum echo yield. This is true because there are limits beyond which echo yield cannot be improved by sheer application of energy.The first barrier is the nonlinear behaviour of the fiber at high energies (stimulated Raman scattering), following by optical damage at critical surfaces.

The simulator of Figure 2 retains all im portent features of the simulator of Figure 1 without compromise, and is an improvement thereof in that it provides the following additional capabilities and features: It is capable of furnishing a plurality of target echoes through a multiple reflection process within the fiber optic coil delay line, this being induced by the addition of a planar partially reflecting surface coincident with the fiber entrance to a coil terminated in a planar reflector The delay associated with each target echo exhibits the stability and precision inherent in the use of a length of optical fiber; Furnishes means to control the number of targe echoes obtained as described hereinbefore, and secures the simulation of change in range of the most distant target;; Provides cost saving by employing a relatively short fiber optic coil delay line to simulate ranges of many times its own length and allow full recovery of optical receiver threshold sensitivity; When operated in conjunction with a ranging system embodying an accurate range scale, is able to calibrate its own fiber optic coil delay line length and subsequently the ranging function of all systems under test.

This technique avoids the need for a separate procedure to calibrate fiber optic coil delay line length, and provides an independent check of system range linearity; Simulates a range of values of real target pulse stretching by a selection of the appropriate target echo; Embodies the self-test feature in the form of a measurement of target echo dynamic range separation. Separation values furnish a performance criterion against which simulator behaviour may be checked during its lifetime; Furnishes a maximum number of target echoes within the available dynamic range through the use of techniques which minimize intra-coil loss; and, Furnishes a maximum number of target echoes through the use of techniques which increase the available dynamic range.

WHAT WE CLAIM IS:- 1. A passive optical range simulator device comprising an adapter member for mechanical coupling with an optical radar ranger system having a working aperture via which optical radar beams enter and leave in parallel along a path, a focusing lens assembly disposed in said path for transforming said beams between parallel and a focal spot in a common plane, an optical fiber coil having one end for disposition in said common plane in registry with said focal spot and having a reflector at its opposite end for enabling such fiber coil to function as an optical delay medium.

2. A passive optical range simulator device according to claim 1, comprising a fineadjustment attenuator disposed in said path

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. delay line quality; it therefore offers a reliable and readily measured performance criterion and a self test feature. When first commissioned, a multiple reflection simulator may be employed with a ranging system whose total dynamic range is known, omitting any adjustment of fixed attenuation in the optical path to achieve specific echo visibility. Dynamic range separation of the target echoes may then be measured, as previously described, and checked against both theoretical predictions and the number of target echoes expected within the residual (usuable) dynamic range.Good correlation indicates proper simulator functioning, and the measured separations serve as benchmarks to which all future measurements on this simulator may be referenced Special attention to design is required in the multiple reflection simulator if intra-coil losses are to be minimized in order to achieve the maximum number of target echoes within the available usable dynamic range.Thus, a maximum echo yield is obtained when: The partial reflector at the fiber entrance is selected for a transmission/reflection ratio that enhances the higher order echoes at the expense of the lower order ones; The fiber optic waveguide used in the coil exhibits a minimum loss at the lavelength of interest, and is stored in a stress-free manner to preserve such minimum loss: and Fiber entrance/exit, and reflection losses are minimized by careful attenuation to fiber termination and surface preparation coupled with fiber-reflector interface design. The ability to accommodate high optical power densities is inherent in the design of the present terminator structure, with the result that its incorporation in the simulator permits the use of much higher incident energies than is allowable for the simulator of Figure 1. As was explained earlier, the usable dynamic range is input power limited and, it follows that the ability to accept more energy increases this dynamic range and the associated maximum echo yield. It should be recalled that the increase in echo yield via this mechanism is entirely independent of that achieved by the reduction of intra-coil losses. The success of a practical multiple reflection simulator requires that both improvement mechanisms be exploited in the pursuit of maximum echo yield. This is true because there are limits beyond which echo yield cannot be improved by sheer application of energy.The first barrier is the nonlinear behaviour of the fiber at high energies (stimulated Raman scattering), following by optical damage at critical surfaces. The simulator of Figure 2 retains all im portent features of the simulator of Figure 1 without compromise, and is an improvement thereof in that it provides the following additional capabilities and features: It is capable of furnishing a plurality of target echoes through a multiple reflection process within the fiber optic coil delay line, this being induced by the addition of a planar partially reflecting surface coincident with the fiber entrance to a coil terminated in a planar reflector The delay associated with each target echo exhibits the stability and precision inherent in the use of a length of optical fiber; Furnishes means to control the number of targe echoes obtained as described hereinbefore, and secures the simulation of change in range of the most distant target;; Provides cost saving by employing a relatively short fiber optic coil delay line to simulate ranges of many times its own length and allow full recovery of optical receiver threshold sensitivity; When operated in conjunction with a ranging system embodying an accurate range scale, is able to calibrate its own fiber optic coil delay line length and subsequently the ranging function of all systems under test. This technique avoids the need for a separate procedure to calibrate fiber optic coil delay line length, and provides an independent check of system range linearity; Simulates a range of values of real target pulse stretching by a selection of the appropriate target echo; Embodies the self-test feature in the form of a measurement of target echo dynamic range separation. Separation values furnish a performance criterion against which simulator behaviour may be checked during its lifetime; Furnishes a maximum number of target echoes within the available dynamic range through the use of techniques which minimize intra-coil loss; and, Furnishes a maximum number of target echoes through the use of techniques which increase the available dynamic range. WHAT WE CLAIM IS:-

1. A passive optical range simulator device comprising an adapter member for mechanical coupling with an optical radar ranger system having a working aperture via which optical radar beams enter and leave in parallel along a path, a focusing lens assembly disposed in said path for transforming said beams between parallel and a focal spot in a common plane, an optical fiber coil having one end for disposition in said common plane in registry with said focal spot and having a reflector at its opposite end for enabling such fiber coil to function as an optical delay medium.

2. A passive optical range simulator device according to claim 1, comprising a fineadjustment attenuator disposed in said path

having a pair of glass plates tilted oppositely to provide first surface reflection deflection and prevent path deflection of beams passing therethrough.

3. A passive optical range simulator device according to claim 1 or 2, comprising a primary attenuator plate disposed in said path at a tilt angle with respect thereto.

4. A passive optical range simulator device according to claim 3, comprising a radiation absorbing means in acceptance of first surface reflection from said primary attenuator plate.

5. A passive optical range simulator device according to claim 4, wherein said radiation absorbing means includes a second glass plate in parallel with said primary attenuator plate and in receipt of first surface reflection therefrom, and optical block surface means in receipt of transmitted and reflected optical energy from said second glass plate.

6. A passive optical range simulator device according to any of the preceding claims, wherein said optical fiber coil is composed of low-loss step-index buffer-coated type optical fiber wound on a storage spool as a series of fully interleaved single layer windings.

7. A passive optical range simulator device according to any of the preceding claims, wherein said reflector is effective at the wavelength of the optical radar beam and transparent to visible light and said device includes a visible light source behind said mirror to light up the aforesaid one end of said optical fiber for alignment with the aforesaid focal spot.

8. A passive optical range simulator device according to any of the preceding claims, comprising a partial optical radar pulse reflector at the aforesaid one end of said optical fiber coil, whereby multiple reflections of an optical radar pulse from the ranger system is generated within said optical fiber coil for observation by such system.

9. A passive optical range simulator device according to claim 8, comprising means for adjusting separation between the aforesaid opposite end of said length of optical fiber and said total optical radar pulse.

10. A passive optical range simulator device substantially as described with reference to, and as shown in, the accompanying drawings.