GB2343509A

GB2343509A - Laser detectors

Info

Publication number: GB2343509A
Application number: GB8612678A
Authority: GB
Inventors: Martin Lawrence
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1986-05-23
Filing date: 1986-05-23
Publication date: 2000-05-10
Anticipated expiration: 2006-05-23
Also published as: GB2343509B; GB8612678D0

Abstract

A laser radiation detector includes a branched light guide (3) to divide incoming radiation into two light paths (4) and optically active (5) means in each light path adapted to rotate the plane of polarisation of a monochromatic, plane polarised laser beam through a different amount and comparison means (7,9 and 8,10) for differentiating the polarisation characteristics of the radiation emerging from the two light paths.

Description

LASER DETECTORS This invention relates to laser detectors which use optical* !Il' active or birefringent materials to distinguish incident laser light from background light.

According to one aspect of this invention there is provided a laser radiation detector including means for splitting input radiation into at least two component beams, light guide means adapted to pass each of said component beams and comparison means for comparing the polarisation characteristics of each of said component beams after leaving said light guide means.

According to another aspect of this invention there is provided a laser radiation detector comprising branched light guide means having an input for radiation and spaced outputs to define first and second light paths each adapted to rotate the plane of polarisation of a monochromatic, plane-polarised incoming beam through a different amont, and comparison means. for determining the polarisation characteristics of the radiation emitted from each of said outputs.

According to yet another aspect of this invention there is provided a laser radiation detector comprising beam splitting means for splitting an input beam into first and second component beams, first and second light guide means each adapted to rotate the plane of polarisation of a monochromatic plane-polarised incoming beam through a different amount, and comparison means for determining the polarisation characteristics of the radiation emitted from each of said light guide means.

According to a further aspect there is provided a laser radiation detector comprising a light guide element of birefringent material for receiving an input beam of radiation, beam splitting means for splitting radiation output by said light guide element into two component beams, and means for comparing the polarisation characteristics of each of said beams.

According to yet another aspect there is provided a laser radiation detector comprising first and second light guide means each adapted to rotate the plane of polarisation of a monochromatic plane-polarised incoming beam through substantially the same amont and comparison means for determining the polarisation characteristics of the radiation emitted from each of said light guide means.

Four specific embodiments of the invention will now be described by way of example : Figure 1 is a schematic diagram of a first embodiment of laser detector using a bifurcated optical fibre and sections of optically active material ; Figures 2a and 2b are schematic diagrams of the planes of polarisation of laser light and background light as they pass through the apparatus shown in Figure 1; Figure 3 is a schematic diagram of a second embodiment of laser detector using a twin core optically active fibre; Figure 4 is a schematic diagram of a third embodiment of laser detector using a material of high birefrinaence ; Figures 5a and 5b are schematic drawings representing the planes of polarisation of laser and background radiations, respectively, as they pass through the apparatus in Figure 4; Figure 6 is a schematic diagram of a fourth embodiment of laser detector which uses optically active or birefringent materials to rotate incident light through the same angle; Figures 6a and 6b represent an enlarged and transverse section view respectively of the fibre in the detector of Figure 6; Figure 7a and 7b are schematic diagrams of background and laser radiation respectively passing through the apparatus of Fig. 6 in which optically active light guides are used.

Referring to Figure 1, the first embodiment of laser detector comprises a front polarising filter 2, and a fibre optic light guide 3 which bifurcates at 4 into two limbs 5 and 6. Each of the limbs 5 and 6 has a section of optically active material at its end. Two second polarising filters 7 and 8 are placed between the optically active sections 5 and 6 and photo detectors 9 and 10 and are aligned with their polarisation planes in the same disposition with respect to the limbs 5 and 6. A differential amplifier 11 detects the signals from the two photo detectors 9 and 10. The two pieces of optically active material in limbs 5 and 6 are chosen to rotate light through different relative angles and this can be achieved either by incorporating different lengths of material of the same optical activity into limbs 5 and 6 or by making each of the limbs from material or materials of different optical activity. The operation of the apparatus is now described by referring to Figure 1 and the corresponding Figures 2a and 2b.

Figure 1 shows background radiation 1 incident on front polarising filter 2. The radiation is polarised into the same plane as filter 2 and passes through fibre optic light guide 3 to be split into two identical beams by bifurcation 4.

The two beams pass through optically active sections 5 and 6.

It is a property of optically active materials that they rotate the plane of polarisation of light by an angle which is dependent upon the wavelength of light, the optical activity of the material and the length of the optically active material.

Because background light contains components of all visible wavelengths, each of the wavelengths present rotates through a different angle and the result is the light beam becomes randomly polarised as shown in figure 2b. Both beams will contain a plane of polarisation which can pass through the second polarising filters 7 and 8 to illuminate photo detectors 9 and 10. Both photo detectors are evenly illuminated and the comparator does not detect a difference signal.

Figure 2a represents the case when laser radiation is incident on the apparatus. The laser radiation 1 passes through the front polarising filter 2 and travels along the fibre optic light guide 3 until it is split into two identical beams at bifurcation 4. The two beams pass through optically active sections 5 and 6 respectively and their planes of polarisation rotate through different angles. Chlike background radiations the laser radiation is strongly monochromatic and does not spread into random polarisations. The two beams maintain distinct planes of polarisation when they impinge on the second polarising filters 7 and 8. In the embodiment shown the planes of polarisation of both beams are rotated through different angles.

One of the beams is attenuated when it passes through filter 7 to illuminate photo detector 9. The other beam has its plane of polarisation oriented so it is able to pass unattenuated through filter 8 to illuminate photo detector 10. The comparator 11 thus detects a difference signal which indicates the presence of laser radiation. Figures 2a and 2b show the two polarising filters arranged with their polarisation axes parallel to each other; though other arrangements could be devised in which the two filters are oriented at different angles with respect to each other.

Specifically, an arrangement could be constructed where a specific angle between the polarisers would enable optically active rods of the same length to show differential transmission.

Figure 3 shows a twin core arrangement in which the incoming beam illuminates two fibre optic light guides and is split into two beams as a matter of course. The arrangement comprises a front polarizer 12 and twin core elements 18 and 19 of the same length but of materials of different optical activities. The core elements are supported by cladding 13 and a second polarising filter 14 is secured to the end of the cores and cladding remte from the polarising filter 12. A pair of detector elements 15 and 16 abut the second filter 14 to receive radiation passed by the core elements 18 and 19. The outputs from the detection elements are, as before, supplied, to a differential amplifier 17 which emits a signal when the two inputs are not the same. As before two beams illuminate the photo detectors.

Laser radiations illuminate the photodetectors with different intensities, whereas background radiations illuminate the photo detectors with an equal intensity and produce a difference signal between the two photo detectors to be detected.

Referring to the arrangement of Figures 4,5a and 5b there will now be described a third embodiment of laser detector which employs a single light guide having high bifringence.

Referring to Figure 4, the birefringent laser detector consists of: a front polarising filter 21, a high birefringence fibre 22, a beam splitter 23, two polarising filters 26 and 27 and two photo detectors 28 and 29. A differential amplifier 30 compares the signals from the two photo detectors 28 and 29.

The operation of the apparatus can be described with reference to Figures 5a and 5b; for clarity these two diagrams have been labelled with numbers which correspond to those in figure 4.

Figure 5a illustrates the case when linearly polarised laser light is incident on the apparatus. The laser light 20 is incident on the front polarising filter 21 and passes into the birefringent material 22. The beam splits into two orthogonally polarised cocponents A and B. One of the components B is brighter than the other A. The intensity of the two components is represented by the size of the arrows, i. e. the larger the arrow the more intense the component of light The plane of polarisation of the light is indicated by the direction in which the arrow is pointing. The phenomenon of bi-refringence occurs when light passing through a material splits into two orthogonally polarised components. The beam is split by a beam splitter 23 into two separate beams 24 and 25 and each of the beams contains the same orthogonally polarised components A and B. The two beams beams and 25 are incident on two secondary polarising filters 26 and 27. The two polarising filters are oriented at different angles with respect to each other. The angle in the diagram is shown as 45 but any angle except 90 or 180 can be used. One of the filters 27 has its plane of polarisation oriented so that both components A and B are attenuated to about half of their original intensities. Attenuated components A and B pass through the filter 27 and impinge on photo-detector 29. The plane of polarisation of filter 26 is oriented at 45 to filter 27 so it coincides with the plane of polarisation of the brighter of the two components B of beam 24 while blocking out component A. All of component B of beam 24 passes through filter 26 and illuminates photo-detector 28. A comparator 30 detects the difference signal between the two photo-detectors 28 and 29 and indicates the presence of a laser.

When background light 20 is incident on the apparatus shown in Figure 4, it undergoes the transition shown in Figure 5b. The background light 20 passes through polarising filter 21 and emerges with a plane of polarisation which corresponds to the plane of the filter. As the light passes into the birefringent material its plane of polarisation experiences a slight rotation which, although not as pronounced as optically active materials, is sufficient to cause background radiation to spread into a series of randomly oriented planes. The angle through which the plane of polarisation of radiation rotates, in the material, is frequency dependent. Because background radiation is of a relatively broadband nature, the vertically polarised beam at filter 21, consists of more than one wavelength. Each wavelength rotates through a different angle depending upon its frequency and the overall result is that background radiation becomes randomly polarised again, despite having passed through polarising filter 21. Ibis is represented in Figure 5b by the arrows pointing in all directions.

Because the rod 22 is made of birefringent material the incident laser beam is split into two orthogonally polarised components A and B. The relative intensities are represented by the length of the arrows. The beam passes through beam splitter 23 and divides into two identical beams 24 and 25. Each of these beams contains the same components A and B. Beams 24 and 25 impinge on polarising filters 26 and 27. Because the components A and B are randomly polarised they both contain planes of polarised radiation which coincide with the planes of polarisation of filters 26 and 27. Both components A and B will pass through polarising filters 24 and 25 irrespective of how the filters are orientated. Figure 5b shows how both components A and B of beam 24 pass through filter 26 to illuminate photo detector 28 and how both components pass through filter 27 to illuminate photo detector 29. The two photo detectors 28 and 29 are equally illuminated and comparator 30 does not detect a difference signal.

In the case where laser radiation is incident on the apparatus it will of course also experience slight rotation as it passes through the birefringent material but because it is strongly monochromatic the plane of polarisation will not spread out to the same extent as background radiation.

Referring to Figures 6 to 7b, the laser detector comprises two lengths of optically active or bi-refringent material 32 and 33 each adapted to impart the same degree of rotation to radiation passing therethrough. The arrangement comprises four polarising filters 35, 36,37,38 and all four polarising filters are arranged at different angles with respect to each other in such a way that they produce a difference signal when laser light is incident on them and no difference signal when background light is incident on the apparatus.

For example in the arrangement illustrated the outlet polarising filters 37,38 are each shown as being at 45 degrees with respect to the associated inlet filter 36,35 respectively; though the filters can be arranged at any number of different angles to produce similar results.

Upon passing through the two front polarising filters each of the beams is polarised into a plane which corresponds with the front polarising filters. The two polarised beams 39 and 40 are rotated by the optically active properties of the light guides to become beams of different orientation at 41 and 42 respectively. The two resultant beams 41 and 42 impinge on polarising filters 37 and 38 and energe with different intensities in the case of laser radiation and with equal intensities in the case of background radiation.

The difference signal is detected by two photodetectors 43 and 44 and amplifie by a comparator circuit 45 in the same way as described before. It will be appreciated that a birefringent material can be used in place of an optically active material to also produce a difference signal, in the presence of laser light.

Although four specific embodiments are shown in the diagrams other arrangements would be possible without departing from the scope of the invention. For example, the incoming light beam might be split into more than two beams before being fed to a comparator.

Claims

1. A laser radiation detector including means for splitting input radiation into at least two component beams, light guide means adapted to pass each of said component beams and comparison means for comparing the polarisation characteristics of each said component beams after leaving said light guide means.

2. A laser radiation detector as claimed in claim 1 in which said laser detector comprises a branched light guide having an input for radiation and spaced outputs to define first and second light paths each adapted to rotate the plane of polarisation of a monochromatic, plane-polarised incoming beam through a different amount, and comparison means for determining the polarisation characteristics of the radiation emitted from each of said outputs.

3. A laser radiation detector as claimed in claim 1 comprising beam splitting means for splitting an input beam into first and second component beams, first and second light guide means each adapted to rotate the plane of polarisation of a monochromatic plane polarised incoming beam through a different amount, and comparison means for determining the polarisation characteristics of the radiation emitted from each of said light guide means.

4. A laser radiation detector as claimed in claim 1 comprising a light guide element of birefringement material for receiving an input beam of radiation, beam splitting means for splitting radiation output by said light guide element into two component beams, and means for comparing the polarisation characteristics of each of said beams.

5. A laser radiation detector conprising first and second light guide means each adapted to rotate the plane of polarisation of a monochromatic plane polarised incoming beam through substantially the same amount and comparison means for determining the polarisation characteristics of the radiation emitted frcm each of said light guide means.

6. A laser radiation detector substantially as hereinbefore described with reference to and as illustrated in any of the accompanying drawings.

6. A laser radiation detector substantially as bereinbefore described with reference to and as illustrated in any of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A laser radiation detector including means for splitting input radiation into at least two component beams, light guide means operable to pass each of said component beams, which light guide means includes at least one length of optically active fibre or rod or of high birefringence material rod, and comparison means for comparing the polarisation characteristics of each said component beams after leaving said light guide means.

2. A laser radiation detector as claimed in claim 1, in which said laser detector comprises a branched light guide having an input for radiation and spaced outputs to define first and second light paths each adapted to rotate the plane of polarisation characteristics of the radiation emitted from each of said outputs.

3. A laser radiation detector as claimed in claim 1, comprising beam splitting means for splitting an input beam into first and second component beams, first and second light guide means each adapted to rotate the plane of polarisation of a monochromatic plane polarised incoming beam through a different amount, and comparison means for determining the polarisation characteristics of the radiation emitted from each of said light guide means.

4. A laser radiation detector as claimed in claim 1, comprising a light guide element of birefringent material for receiving an input beam of radiation, beam splitting means for splitting radiation output by said light guide element into two component beams, and means for comparing the polarisation characteristics of each of said beams.

5. A laser radiation detector comprising first and second light guide means each adapted to rotate the plane of polarisation of a monochromatic plane polarised incoming beam through substantially the same amount and comparison means for determining the polarisation characteristics of the radiation emitted from each of said light guide means.