GB2323680A - Focussing laser beams - Google Patents

Abstract

A laser transmitting and/or receiving system includes an astronomical telescope with an element 12 having at least two constituent lenses 20 and 21, together serving partially to define an hermetically sealed unit in which a laser beam 15 to traverse the system 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, for example, air at a pressure of less than 15 newtons per square metre, or nitrogen, possibly at a pressure less than atmospheric.

Description

Laser Transmitting and/or Receiving Systems This invention relates to laser transmitting and/or receiving systems, and in particular to such systems each employing a telescope.

In some such systems, for example, surveillance aids, it is desirable for the telescopes to have as wide an angle of view as conveniently may be provided.

However, if a high power laser beam is brought to a focus within a system, for example, in an astronomical telescope, the intense electric field so produced would cause breakdown of the air dielectric, were the focus to occur in air, with consequent attenuation of the radiation.

It is an object of the present invention to provide a laser transmitting and/or receiving system employing a telescope and in which system the laser beam is brought to a focus, but the possible adverse effects of so bringing the laser beam to a focus are obviated.

According to the present invention a laser transmitting and/or receiving system at least includes an astronomical telescope having a field element comprising 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 telescope, in which unit the laser beam to traverse the system 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.

An astronomical telescope employing a field element is advantageous in that it is compact. The laser beam traversing such a telescope, in either direction, inherently is required to be brought to a focus at the field element 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.

The present invention will noer be described by way of example with reference to the accompanying drawing, which is a section of an astronomical telescope comprising part of a laser transmitting and/or receiving system, and comprising one embodiment according to the present invention.

The illustrated astronomical telescope, for a laser transmitting and/or receiving system, has an objective 10, a field element, indicated at 12, and an piece element indicated at 13.

The axis of the telescope is indicated at 14, A transmitted laser beam, to traverse the astronomical telescope, in either direction, is indicated at 15.

In some laser systems including a telescope, it is required that the telescope has as wide an angle of view, a, as conveniently may be provided, for example, if the telescope is included in a surveillance aid. The illustrated astronomical telescope with the field element 12, is advantageous in this respect. The transmitted laser beam is shown, in dotted line form, at 16, at both extremes of the angle of view. For simplicity, only approximate paths for constituent rays of the laser beam traversing the astronomical telescope are illustrated.

Irrespective of the angle of incidence of the transmitted laser beam at the objective 10, the laser beam is brought to a focus in a plane 17 at the field element 12.

The source of the laser beam, and the means to detect the laser beam, if provided within the system, are not shown.

The illustrated astronomical telescope, with the field element 12, is advantageous in that it is a compact arrangement with a wide angle of a view, a enabling the sizes of the refracting elements following the telescope to be less than otherwise would be the case. However, the laser beam is to be brought to a focus at the field element 12. In focussing a 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 these effects producing severe attenuation of the laser beam radiation. Hence, if a high power laser beam is to be brought to a focus within an astronomical telescope, with a field element, according to the present invention, the field element 12 comprises two spaced constituent lenses 20 and 21, with the plane 17 at which the laser beam is to be focussed being intermediate between the lenses 20 and 21. The lenses 20 and 21 are mounted in a tube 23, and the tube 23 and the lenses together define an hermetically sealed unit. The interior of the unit is of an atmosphere which does not cause attenuation of the incident radiation when subjected to a focussed laser beam therein, for example, comprising either ai-r at a pressure of less than 15 newtons per square metre or nitrogen, possibly at a pressure less than atmospheric. The lenses 20 and 21 are spaced apart by an amount sufficient to ensure that the lens material, or materials, can withstand the maximum anticipated power density at their positions in the laser beam, under normally encountered operating conditions for the astronomical telescope.

The lenses 20 and 21 may be of any convenient material, and may be of different materials and of specific shapes to set, or to correct, pupil aberration.

It is required that each constituent refracting element, 10, 12 and 13 of the astronomical telescope is of a material, or materials, capable of transmitting all the radiation of the different wavelengths with which the telescope is intended to be employed. For example, for radiation of wavelengths in the range 2 to 23 micrometres, germanium may be used for any constituent lens of the telescope.

When the radiation with which the astronomical telescope is intended to be used has wavelengths within the range-0.5 to 14 micrometres, the refracting elements each may at least include one constituent lens of zinc selenide, and/or one constituent lens of zinc sulphide. Lenses of zinc sulphide and zinc selenide are described and claimed in our co-pending British patent application number (Edinburgh Case 481.) An astronomical telescope according to the present invention may be included in the common optical system of apparatus comprising the combination of a laser transmitting and/or receiving system such as a surveillance aid, and an infra-red imaging system, as described and claimed in our co-pending British patent application Number (Edinburgh case 483). In any such arrangement, it is usually not important for any refracting element of the astronomical telescope to be substantially corrected for chromatic aberration for a laser beam to traverse the telescope, as the laser radiation is monochromatic. Infra red radiation of wavelengths in the range 7.5 to 14 micrometres is emitted by objects in a terrain, and this radiation can be employed to cause a display of the objects. hen such a display is to be provided by an infra-red imaging system, in combination with a laser transmitting and/or receiving system, and with the illustrated astronomical telescope with a field element, it is required that the eye-piece element and the objective element are substantially free from chromatic aberration for the radiation to cause the display, in order to form an acceptable image, and which radiation is to be transmitted by the astronomical telescope, but it may not be required that the field element be so constructed. For example as described and claimed in our co-pending patent application No, (Edinburgh Case 481), referred to above, and as illustrated in present application, a refracting element, for example, comprising the illustrated eye-piece element 13, has two constituent lenses 24 and 25, axially spaced apart by a predetermined distance, in any convenient way, the arrangement being such that the eye-piece element 13 is substantially free from chromatic aberration for infra-red radiation of wavelengths in the range 7.5 te 14 micrometres. One constituent lens, 24, is of zinc selenide and the other lens, 25, is of zinc sulphide, these two materials having different dispersions, and the appropriate combination of lenses is obtained by a suitable choice for the powers of the constituent lenses. Both these materials are highly dispersive. Similarly, the arrangement may be such that the illustrated objective 10 of the telescope has two constiUJent lenses 26 and 27, such that the objective 10 also is substantially free from chromatic aberration for infra-red radiation wavelengths in the range 7,5 to 14 micrometres, by a suitable choice for the powers of the constituent lenses.

The construction of the objective 10 differs from that of the eye-piece element 13, in that the two constituent lenses 26 and 27 are formed from a block provided by depositing either zinc sulphide or zinc selenide, on one refracting surface, of one constituent lens 26 or 27, and of the other of these two materials, the common refracting surface having already been ground and polished.

The two lenses 26 and 27 are then completed by grinding and polishing the outer surface of the deposited material, and possibly also the material of the lens on which the other material is deposited.

The eye piece element may have a similar construction to the illustrated objective, or the objective element may have a similar construction to the illustrated eye piece element.

More than two constituent lenses may he provided in the objective, and/or the field element, and/or the eye-piece element.

It is not essential that the objective and/or the eye-piece element of the astronomical telescope each have at least two constituent lenses, and only one lens may be provided if it is not required that these refracting elements are to be substantially free from chromatic aberration.

Claims (4)

What we claim is:

1. A laser transmitting and/or receiving system at least including an astronomical telescope having a field element comprising 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 telescope, in which unit the laser beam to traverse the system 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.

2. A system as claimed in claim 1 in which the atmosphere within the hermetically sealed unit is air at a pressure of less than 15 newtons per square metre.

3. ~ A system as claimed in claim 1 in which the atmosphere within the hermetically sealed unit is nitrogen, possibly at a pressure less than atmospheric.

4. A laser transmitting and/or receiving system, at least including an astronomical telescope, substantially as described herein with reference to the accompanying drawing.

4. A laser transmitting and/or receiving system, at least including an astronomical telescope, substantially as described herein with reference to the accompanying drawing.

Amendments to the claims have been filed as follows What we claim is: 1, A laser transmitting and/or receiving system at least including an astronomical telescope having a field element comprising 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 telescope, in which unit the laser beam to traverse the system 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.

2. A system as claimed in claim 1 in which the atmosphere within the hermetically sealed unit is air at a pressure of less than 15 newtons per square metre.

3. A system as claimed in claim 1 in which the atmosphere within the hermetically sealed unit is nitrogen, possibly at a pressure less than atmospheric.