GB2323681A

GB2323681A - Combined laser system and infra-red imaging system

Info

Publication number: GB2323681A
Application number: GB7846039A
Authority: GB
Inventors: Evan Stuart Cameron; Richard Lawrence Fisher
Original assignee: GEC Ferranti Defence Systems Ltd; Ferranti PLC; GEC Marconi Avionics Holdings Ltd
Current assignee: Ferranti International PLC; Leonardo UK Ltd
Priority date: 1978-11-24
Filing date: 1978-11-24
Publication date: 1998-09-30
Anticipated expiration: 1998-11-24
Also published as: SE7909707L; AU697324B1; GB7846039D0; FR2929718A1; FR2930999A1; DE2948831C1; GB2323681B

Abstract

A laser transmitting and/or receiving system, for example, a surveillance aid employing a laser beam 40 of radiation of a wavelength within the range 0.5 to 7.5 micrometres, and an infra-red imaging system employing radiation of wavelengths within the range 7.5 to 14 micrometres, have a common optical system comprising a telescope having an objective and an eyepiece 10 and 11, substantially free from aberration for the infra-red radiation causing the display.

Description

Combined Laser Transmitting and/or Receiving System and Infra-red Imaging System.

This invention relates to apparatus each comprising a combination of a laser transmitting and/or receiving system, for example, a surveillance aid, and an infra-red imaging system.

It is an object of the present invention to provide in a convenient manner apparatus comprising a combination of a laser transmitting and/or receiving system, for example, a laser surveillance aid, and an infra-red imaging system.

According to the present invention, the apparatus comprises a combination of a laser transmitting and/or receiving system, in operation, requiring a laser beam of radiation of a wavelength within the range 0.5 to 14 micrometres to traverse the laser transmitting and/or receiving system in either direction, and an infra-red imaging system arranged to employ radiation of wavelengths within the range 7.5 to 14 micrometres, from objects required to be displayed, there is provided in the apparatus a common optical system including a telescope, through which telescope both the laser beam, and the radiation from objects to cause the display, are to traverse, the telescope being arranged to be substantially free from chromatic aberration for the radiation of wavelengths within the range 7.5 to 14 micrometres.

Usually the laser beam is of radiation of a wavelength less than 7.5 micrometres, but since the laser beam is monochromatic it is usually not important for any refracting element of the telescope to be substantially corrected for chromatic aberration for the laser beam.

Thus, there is provided the combination of a laser transmitting and/or receiving system, and an infra-red imaging system, having a common optical system. It is not apparent that a common optical system conveniently can be provided for such different systems, and which optical system can operate satisfactorily with the different radiations employed in both the constituent laser transmitting and/or receiving system , and the infra-red imaging system of such apparatus.

In order to be substantially corrected for chromatic aberration at least the objective and the eyepiece refracting elements of the telescope each comprise at least two constituent lenses.

There may be parts of the common optical system, other than the telescope, through which both the laser beam and the radiation from the objects required to be displayed, are to traverse, and any refracting element included in these parts of the common optical system is required to be able to transmit the laser beam, and to be substantially free from chromatic aberration for the radiation of wavelengths within the range 7.5 to 14 micrometres, and also to be transmitted thereby.

If the laser beam is of radiation of a wavelength in the range 2 to 14 micrometres, germanium may be used for any constituent lens of the telescope.

If the laser beam is of radiation of a wavelength in the range 0.5 to 14 micrometres, the refracting elements of the telescope each may at least include one constituent lens of zinc selenide, and/or one constituent lens of zinc sulphide.

Lenses of zinc selenide and zinc sulphide are described and claimediri our co-pending British patent application number (Edinburgh Case 481).

There may be parts of the common optical system, other than the telescope, through which only either the laser beam, or the radiation from the objects required to be displayed, are to traverse, and any refracting element included in these parts of the common optical system are required to be capable of transmitting radiation of wavelengths to traverse these parts. Any such refracting element is required to be able to transmit the radiation from the objects required to be displayed, also is required to be substantially free from chromatic aberration for the radiation of wavelengths within the range 7.5 to 14 micrometres. Any such refracting element required to be able te transmit the laser beam may comprise only one constituent lens.

Both zinc sulphide and zinc selenide are highly dispersive, but when combined in, for example, a doublet, such that a positive lens is constructed from one of these materials, and a negative lens of the other material, the dispersive powers of each such constituent lens are of different senses. Thus, by a suitable choice of powers for each such constituent lens, a refracting element substantially free from chromatic aberration for infra-red radiation of wavelengths within the range 7.5 to 14 micrometres, also as described and claimed in our co-pending British patent application number (Edinburgh case 481), and required,at least, for the eyepiece and the objective refracting elements of the telescope of the common optical system, comprises an appropriate combination of such two lenses. Such two constituent lenses of such a refracting element may be held spaced apart by a predetermined axial distance, or the two constituent lenses of such a refracting element may be formed from a composite block of the deposition of one of the materials zinc sulphide and zinc selenide, on one refracting surface of a constituent lens of the other such material.

Zit may be desirable for the telescope to have as wide an angle of view as conveniently may be provided, for example, if the laser transmitting and/ or receiving system comprises a surveillance aid.

A desirable, compact, form for such a telescope comprises an astronomical telescope having a field element. The astronomical telescope so formed is advantageous in that it provides a wide angle of view, whilst enabling the sizes of the refracting elements following the telescope to be smaller than otherwise would be the case. However, it is required that the radiation traversing the telescope is to be brought to a focus at the field element. If a high power laser beam is brought to a focus within the common optical system, for example, in an astronomical telescope, the intense electric field so produced would severely damage any glass, or other solid material, at the focus, or would cause breakdown of the air dielectric were the focus to occur in air, both of these effects producing severe attenuation of the laser beam radiation. However, the field element of the astonomical telescope may comprise at least two constituent lenses, said at least two constituent lenses being spaced apart along their common axis, and together serve partially to define an hermetically sealed unit provided within the astronomical telescope, in which unit the laser beam to traverse the system is to be brought to a focus5 the interior of the unit comprising an atmosphere which does not cause attenuation of the incident radiation when subjected to a focussed laser beam therein, as described and claimed in our co-pending British patent application number (Edinburgh case 482).

The constituent lenses of the field element may be of the same material, as it is not required that the field element should be substantially free from chromatic aberration for radiation from objects to be displayed.

The atmosphere within the hermetically sealed unit may comprise air at a pressure of less than 15 newtons per square metre, or nitrogen, possibly at a pressure less than atmospheric.

Usually a received laser beam does not have sufficiently high power for the focussing of the received laser beam to cause damage of any glass, or other solid material, at the focus, or to cause breakdown of the air dielectric were the focus to occur in air.

Even if a received laser beam is to traverse the telescope, a laser source may be included in the apparatus, the laser source to provide a high power laser beam to traverse the telescope.

When a laser beam to traverse the common optical system is a received laser beam, and because the radiation from the objects to be displayed is to traverse the telescope in the same direction as the received laser beam, beam splitting means , to separate the received laser beam from the radiation from the objects to be displayed, is provided within the common optical system, following the telescope.

The present invention will now be described by way of example, with reference to the accompanying drawing, which is a section of a common optical system of apparatus comprising nne embodiment according to the present invention, the apparatus comprising a combination of a laser transmitting and/or receiving system, and an infra-red imaging system.

The illustrated optical system includes an astronomical telescope with an objective 10, an eyepiece lens 11, and a field lens 12. The axis of the telescope is indicated at 14, and mirrors 16 and 17 are provided between the objective 10 and the field element 12 to fold the optic axis 14 of the telescope.

The optical system is required to be associated with a laser transmitting and/or receiving system, and the refracting elements of the optical system are required to transmit radiation of a wavelength in the range 0.5 to 14 micrometres, and usually less than 7.5 micrometres, and comprising a laser beam.

Further, because the optical system is required to be associated with an infra-red imaging system, the refracting elements of the optical system are required to transmit infra-red radiation of wavelengths within the range 7.5 to 14 micrometresS comprising radiation emitted by objects in a terrain, which are required to be displayed0 Thus, the refracting elements each may comprise at least one constituent lens of zinc sulphide, or of zinc selenide, as described and claimed in our co-pending British patent application number (Edinburgh Case 481). Both these materials transmit radiation of wavelengths in the range 0.5 to 14 micrometres.

Infra-red radiation emitted from objects within the angle of view of the optical system is received by the objective 10, and traverses the telescope to an infra-red imager of a conventional construction, and indicated generally at 20. The infra-red radiation to cause the display is indicated generally at 21. For simplicity, only approximate paths for constituent rays of such radiation traversing the astronomical telescope are illustrated, these rays being the paraxial rays forming the edge of the axial beam. Because such infra-red radiation is to cause a display of the objects from which the radiation is emitted, the telescope must be substantially corrected for chromatic aberration for radiation of wavelengths in the range 7.5 to 14 micrometres. In particular, the objective 10 and the eyepiece 11 must both be substantially corrected for chromatic aberration, and must therefore comprise an appropriate combination of at least two constituent lenses. In the illustrated arrangement, one constituent lens, of each refracting element 10 and 11, is of zinc sulphide, and another constituent lens is of zinc selenide. Such refracting elements are also described and claimed in our co-pending British patent application number (Edinburgh Case 481), and the refracting elements each comprise an appropriate combination of lenses obtained by a suitable choice for the powers of the lenses. Hence, in the illustrated astronomical telescope, the objective 10 has two constituent lenses, 30 and 31, formed from a block provided by depositing either zinc sulphide, or zinc selenide, on one refracting surface, of one constituent lens 30 or 31 and of the other of these two materials. The two lenses 30 and 31 are then completed by grinding and polishing the outer surfaces of the deposited material, and possibly also the material of the lens on which the other material is deposited. The illustrated lens 30 is of zinc sulphide, and the lens 31 is of zinc selenide. The eyepiece 11 has two constituent lenses, 36 and 37, axially spaced apart by a predetermined distance, in any convenient way. The lens 36 is of zinc sulphide, and the lens 37 is of zinc selenide.

If is not required that the field element 12 is substantially corrected for chromatic aberration, because the radiation traversing the telescope is inherently brought to a focus at the field element, and only pupil aberration results.

A high power laser source (not shown) provides a required laser beam, indicated only partially at 40, to impinge on a mirror 41, and to enter the astronomical telescope eyepiece 11 via a refrecting surface 42. The laser beam is transmitted from the apparatus via the objective 10, and a reflected part of the emitted laser beam is received back by the objective, for convenience, this received laser beam also being considered to be a laser beam traversing the common optical system. Both the laser beam emitted by the source, and the reflected laser beam, traverse the astronomical telescope along the same path, but in different directions. A detector 50 for the reflected laser beam is also provided in the apparatus, the detector being arranged to receive that part of the reflected beam transmitted around the edge of the mirror 41, and focussed onto the detector 50 by a lens 51. The lens 51 is of a material such that it transmits the laser radiation, and does not transmit infra-red radiation to cause the required display. Such material may be of a suitable glass composition. That part of the reflected beam that is not allowed to pass around the mirror 41 is reflected along a common transmi/receive path, by the mirror 41, to a laser receiver (not shown) and is detected by a detector. It is not required that the telescope is substantially free f rc chromatic aberration for radiation comprising the laser beam, as the laser beam is monochromatic.

The laser transmitting and/or receiving system comprises both a range finder and a tracking system, which tracking system has a detector to provide directional information and may have a radiation sensitive surface divided into quadrants. The illustrated detector 50 may comprise the detector of the tracking system, and range information is gathered from the unillustrated detector.

Because the laser receiving and/or transmitting system of the illustrated apparatus includes a tracking system, this system, together with the infra-red imaging system, requires the apparatus to have as wide an angle of view as conveniently may be provided. Consequently, an astronomical telescope with a field element, as has been described, is advantageously compact. Such a telescope is advantageous in that it provides a wide angle of view, whilst enabling the sizes of the refracting elements following the telescope to be smaller than otherwise would be the case.

Because a high power laser beam is to traverse the astronomical telescope with a field element 12, inherently the laser beam is brought to a focus at the field element of the telescope. However, in focussing the high power laser beam, the intense electric field so produced would severely damage any glass, or other solid material, at the focus, or would cause breakdown of the air dielectric were the focus to occur in air, both of these effects producing severe attenuatIon of the laser beam radiation. Thus, the illustrated field element 12 comprises two constituent lenses 60 and 61 spaced apart by a predeter mined distance along their common axis, this distance being such that the lens material can withstand the power density at their positions in the laser beam.

Together the lenses 60 and 61 serve partially to define an hermetically sealed unit, as described and claimed in our co-pending British patent application number (Edinburgh Case 482). The laser beam is brought to a focus at a plane 62 within the unit, The lenses 60 and 61 are mounted in the appropriate manner, in a tube 63, which tube 63 serves to complete the definition of the hermetically sealed unit. The interior of the unit comprises an atmosphere which does not cause attenuation of the incident radiation when subjected to a focussed laser beam therein, for exampleS the atmosphere being of air at a pressure of less than 15 newtons per square metre, or nitrogen, possibly at a pressure less than atmospheric0 Because it is not required that the field element 12 is chromatically corrected, both lenses 60 and 61 may be of the same material, i.e.

being of either zinc selenide or of zinc sulphide.

When, as illustrated, both a received laser beam and the radiation to cause the display are to traverse the telescope in the same direction, a director for the received laser beam being included in the system, it is required that beam splitting means, to separate the received laser beam from the radiation to cause the display, is provided between the telescope, and both the detectors and the infrared imager. The illustrated beam splitting means is required to be a reflector for the laser beam, and to transmit infra-red radiation to cause the display.

Alternatively, the beam splitting means is required to be a reflector for the infra-red radiation to cause the display, and to transmit the laser beam.

The illustrated beam splitting means is a plate 70 of zinc selenide, or of zinc sulphide, or of germanium, and having its reflecting surface t2 coated with a material suitable for reflecting the desired wavelength.

The objective 10 and the mirror 16 may be mounted for rotation about two axes, one axis 80 being shown, and comprising the part of the optic axis 14 normal between the mirrors 16 and 17. The other axis is/to the plane of the paper. Rotation of the objective and mirror 16 about these two axes will allow for sightline stabilisation and steering,and such movement of the objective must be arranged to allow for the double angle effect of reflection.

Various other modifications of the apparatus described above are possible.

If the angle of view of the apparatus is not required to be as large as conveniently may be provided, a Galilean telescope may be employed. Thus it is not required to provide an hermetically sealed unit within the telescope, as the laser beam is not required to be brought to a focus.

If a high power laser beam is not to traverse the telescope, for example, if the apparatus is only to receive a laser beam, and radiation emitted by objects required to be displayed, and a laser beam is not to be emitted by the apparatus, it is not necessary to provide an hermetically sealed unit within the telescope, and the field element, if included, may comprise only a single lens.

If a received laser beam is not to traverse the telescope in the same direction as the radiation to cause a display, the beam splitting means may not be required within the apparatus.

If the laser beam is radiation of a wavelength in the range 2 to 14 micrometres a germanium lens may be used in any of the refracting elements of the common optical system.

The eye element may have a similar construction to the illustrated objective, or the objective may have a similar construction to the illustrated eyepiece element The construction of the illustrated objective is optically inferior to the construction of the illustrated eye element, but has the advantage of being more compact and robust.

More than two lenses may be provided in the objective, and/or the field element, and/or the eye element, all constituent lenses being of material transparent at the required wavelengths.

The constituent elements of the common optical system may be at least substantially free from chromatic aberration for radiation of wavelengths within the range 7.5 to 14 micrometres, and be capable of transmitting radiation of wavelengths within the range 0.5 to 14 micrometres, by comprising any suitable materials, other than zinc sulphide, and/or zinc selenide, and/or germanium.

The common optical system may have other unillustrated parts, possibly including refracting elements.

It is required that each refracting element to transmit the infra-red radiation from objects to be displayed is substantially free from chromatic aberration for radiation of wavelengths within the range 7.5 to 14 micrometres. However, it is not required that any refracting element to transmit only the laser beam is to be so corrected, and each such refracting element may comprise only a single lens, as the lens 51. Any such part of the common optical system is required to be able to transmit only the radiation incident thereon In the illustrated arrangement only the telescope is to transmit both infra-red radiation from the objects required to be displayed, and the laser beam, but there may be other illustrated parts of the common optical system required to transmit both such forms of radiation.

Any refracting element after the telescope, for example, the lens 51, may be arranged to correct for different focussing effects.

The mirrors 16 and 17 of the illustrated optical system may be omitted.

The roles of the two detectors of the laser receiving and/or transmitting system may be reversed, or only one detector may be included in the system, the sole detector being arranged to provide both the required range and directional information.

The laser receiving and/or transmitting system may comprise only either a range finder, or a tracking system.

A television camera, operating in the visible region of the range of va > e-'engths o. -D < 'iation trans- mitted by the common optical system, also may be included in the apparatus, suitable beam splitters being provided at convenient places within the common optical system.

Claims

What we claim is:

1. Apparatus comprising a combination of a laser transmitting and/or receiving system, in operation, requiring a laser beam of radiation of a wavelength within the range 0.5 to 14 micrometres to traverse the laser transmitting and/or receiving system in either direction, and an infra-red imaging system arranged to employ radiation of wavelengths within the range 7.5 to 14 micrometres,from objects required to be displayed, and the apparatus having a common optical system including a telescope, through which telescope both the laser beam, and the radiation from objects to cause the display, are to traverse, the telescope being arranged to be substantially free from chromatic aberration for the radiation of wavelengths within the range 7.5 to 14 micrometres.

2. Apparatus as claimed in claim 1 in which at least the eyepiece or the objective refracting element of the telescope comprises a combination of at least one lens of zinc selenide, and another lens of zinc sulphide, held spaced apart by a predetermined axial distance.

3. Apparatus as claimed in claim 1 or claim 2 in which at least the objective or the eyepėce refracting element of the telescope at least includes a combination of two constituent lenses formed froze composite block of the deposition of one of the materials zinc selenide and zinc sulphide, on one refracting surface of a constituent lens of the other such material

4. Apparatus as claimed in any one of the preceding claims in which the telescope of the con uson optical system comprises an as'ronom-cal telescope having a field eleven.

5. Apparatus as claimed in claim 4 in which the field element comprises at least two constituent lenses, said at least two constituent lenses being spaced apart along their common axis, and together serve partially to define an hermetically sealed unit provided within the astronomical telescope, in which unit the laser beam to traverse the telescope is to be brought to a focus, the interior of the unit comprising an atmosphere which does not cause attenuation of the incident radiation when subjected to a focussed laser beam therein.

6. Apparatus as claimed in claim 5 in which said at least two constituent lenses of the field element are of the same material.

7. Apparatus as claimed in claim 5 or claim 6 in which the atmosphere within the hermetically sealed unit is air at a pressure of less than 15 newtons per square metre.

8. Apparatus as claimed in claim 5 or claim 6 in which the atmosphere within the hermetically sealed unit is nitrogen, possibly at a pressure less than atmospheric.

9. Apparatus as claimed in any one of the preceding claims in which, when a laser beam to traverse the common optical system is a received laser beam, and because the radiation from the objects to be displayed is to traverse the telescope in the same direction as the received laser beam, beam splitting means, to separate the received laser from the radiation from the objects to be displayed, is provided within the common optical system, following the telescope.

10. Apparatus comprising a combination of a laser transmitting and/or receiving system, and an infra-red image converter, substantially as described herein with reference to the aco:panying drawing.

Amendments to the claims have been fizzed as follows What we claim is: 1. Apparatus comprising a combination of a laser cransmittlng and/or receiving system, in operation, requiring a laser beam of radiation of a wavelength within the range 0.5 to 14 micrometres to traverse the laser transmitting and/or receiving system in either direction, and an infra-red imaging system arranged to employ radiation of wavelength within the range 7.5 to 14 ffllcrometres, from objects required to be displayed, and the apparatus having a common optical system including a telescope, through which telescope both the laser beam, and the radiation from objects to cause the display, are to traverse, the telescope being arranged to be substantially free from chromatic aberration for the radiation of wavelength within the range 7.5 to 14 micrometres.

3. Apparatus as claimed in claim 1 or claim 2 in which at least the objective or the eyepiece refracting element of the telescope at least includes a combination of two constituent lenses formed from a composite block of the deposition of one of the materials zinc selenide and zinc sulphide, on one refracting surface of a constiteunt lens of other such material.

4. Apparatus as claimed in any one of the preceding claims in which the laser beam to traverse the common optical system is to be transmitted from the apparatus.

5. Apparatus as claimed in claim 4 in which the laser beam is to traverse the common optical system in both directions, comprising both a laser beam to be transmitted from the apparatus, and a laser beam to be received bv the apparatus.

6. Apparatus as claimed in claim 4 or claim 5 in which the telescope of the common optical system comprises an astonomical telescope having a field element.

Apparatus as claimed in claim 6 in which the field element comprises at least two constituent lenses, said at least two constituent lenses being spaced apart along their common axis, and together serve partially to define an hermetically sealed unit provided within the astonomical telescope, in which unit the laser beam to traverse the telescope is to be brought to a focus, the interior of the unit comprising an atmosphere which does not cause attenuation of the incident radiation when subjected to a focussed laser beam therein.

8. Apparatus as claimed in claim 7 in which said at least two constituent lenses of the field element are of the same material.

9. Apparatus as claimed in claim 7 or claim 8 in which the atmosphere within the hermetically sealed unit is air at a pressure of less than 15 newtons per square metre.

10. Apparatus as claimed in claim 7 or claim 8 in which the atmosphere within the hermetically sealed unit is nitrogen, possibly at a pressure less than atmospheric.

11. Apparatus as claimed in any one of the preceding claims in which the laser beam to traverse the common optical system has a wavelength in the range 0.5 to 7.5 micrometres.

12. Apparatus as claimed in claim 11 in which, when a laser beam to traverse the common optical system is a received laser beam, and because the radiation from the objects to be displayed is to traverse the telescope in the same direction as the received laser beam, beam splitting means, to separate the received laser beam from the radiation from the objects to be displayed, is provided within the common optical system, following the telescope.

13. Apparatus as claimed in any one of the preceding claims in which at least the objective of the telescope of the common optical system is mounted for rotation.

14. Apparatus comprising a combination of a laser transmitting and/or receiving system, and an infra-red image converter, substantially as described herein with reference to the accompanying drawing.