GB2039381A - Improvements in laser beam deflection systems - Google Patents

Abstract

A system for the controlled, rapid deflection of a high power laser beam, or a sequence of such beams, utilises a coherent bundle of single mode optical fibres packed together in such a manner that the ends of the said fibres, through which laser beams emerge, form an array of optical transmitters across the output face of said optical fibre bundle. By choosing fibres of different lengths, or otherwise, the phase of light emerging from the output fibre ends is varied progressively to vary the angle of the output beam. A scanning beam may be produced e.g. for radar.

Description

SPECIFICATION Improvements in laser beam deflection systems This invention relates to a system for the controlled, rapid deflection of a high power laser beam, or a sequence of such beams, using a coherent bundle of single mode optical fibres packed together in such a manner that the ends of the said fibres, through which said laser beams emerge, form a phased array of optical transmitters across the output face of said optical fiber bundle. Such phased arrays of electromagnetic wave transmitters are well known in the microwave region of the electro-magnetic spectrum where the wavelength of the radiation is about one centimetre, or about ten thousand times larger than laser wavelengths.

Prior art laser beam scanners can be divided into two classes, namely, those with no moving parts, such as acousto and electro-optic scanners, and those utilizing the movement of mirrors and/or refractive optical components, such as prisms and lenses. All these prior art systems suffer because they can only handle low power laser beams of relatively small diameter. Prior art laser beam steering systems developed for mass storage system studies have incorporated electro-optic phased array techniques restriced to low power laser beams.

The advent of the laser (ligh amplification by the stimulated emission of radiation) in 1960 heralded a new era in the development of radar because for the first time it became possible to transmit through the atmosphere pulses of light of nanoseconds (10-9 seconds) duration, within a very narrow beam width of less than 10-3 radians from modest transmitter apertures of less than 10 cm diameter. During the period 1963-1964, John Leonard Hughes, a joint inventor, was in charge of one of the first systematic field trails of a laser-radar system ever undertaken, details of which were published in a Royal Radar Establishments (Malvern, U.K.) Technical memorandum July, 1966.In addition to confirming the then disputed radar equation, these early field trials of laser-radar confirmed the need for techniques to scan the narrow width transmitted laser beams to ease the problems of target acquisition. However, bearing in mind that even those early laser-radar systems operated on peak powers well in excess of 10 megawatts, the problems associated with scanning such powerful beams utilizing optical components of centimetres dimensions were formidable, and well beyond the capabilities of state of the art beam scanning systems.

During the period 1977-1979 the experiences of the joint inventors in the laser-radar and optical fibre fields were combined resulting in new approach to high power laser beam scanning which led to the present invention.

It is an object of the present invention to achieve a rapid, fully controllable deflection of a high power laser beam.

Another object of the invention is to scan a laser beam in a given pattern in a reproducable manner.

A further aim of the invention is to avoid laser beam induced damage to optical components.

A still further object is to dispense with all moving parts.

Another object of the invention is to provide means of deflecting laser beams of identical or different wavelengths either simultaneously or sequentially.

Another object of the invention is to provide means for scanning a laser beam at a high pulse transmission rate.

Yet another object of the invention is to combine an output consisting of a large number of laser beams into one output beam.

The invention provides a system for deflecting a laser beam over angles in excess of 100, whose peak power can range from less than one milliwatt (10-3 watts), to more than 1015 watts and whose diameter can be less than 1 mm or greater than five metres with a response time which depends on the fastest rate at which one or more laser beam generators can be activated, either together, or in a sequence, to provide the input for the invention.

The invention comprises a coherently packed bundle of single-mode optical fibres, one end of which, the output beam end, is packed tightly together and the surface optically polished utilizing techniques known in the art. The other end of the said optical fibre bundle is loosely packed with each fibre, or small groups of fibres, being couples to a laser beam generator, for example a semiconductor laser, via a coupling device well known in the art of laser-based optical communications using optical fibres.

To achieve a phased array of optical transmitters, the fibre ends forming the polished face of the optical fibre bundles must be located within a distance of about 100 microns (1 0-2 cm) to each other and the laser light must emerge sequentially from row to row of the fibres across the whole polished face of the optical fibre bundle hence the need for coherently packed bundle. For example, if our invention has one hundred rows of fibres, each row consisting of one hundred individual fibres, then the laser light has to emerge simultaneously from the hundred fibres in the first row, followed by the emission from the hundred fibres forming the second row and soon until all the rows are emitting after completion of the said emission sequence.This leads to phased array operation of the optical fibre transmitters which leads to the output beam being propagated at an angle to the optically polished face of the fibre bundle which depends on the sequence that light emerged from the rows of fibres forming said face.

The sequential row by row emission of light along the optically polished output end face of the invention can be achieved in either of two ways. Firstly, the laser beam generators can be fired so as to emit laser pulses with the required delay sequence from row to row. Secondly the laser imput can be in the form of the output from a single laser beam generator, or an array of such generators fired simultaneously with the lengths of rows of fibres differing from each other by a given amount, all the fibres in a particular row being of equal lengths.

For example, if we consider a square array of single-mode optical fibres, thenwith the fibres in a given row being of equal length the fibres in consecutive rows differing by an amount L with the fibres in the first row exceeding those in the second row by a length Land those in turn exceed those in row three by a length Land so on. If a laser beam is passed through such fibre bundle, the radiation emerging from the output end along consecutive rows differs by a constant phase of 2 z 'AlL where X is the laser wavelength.Thus using standard diffraction theory as discussed by Ajoy Ku mar Ghatak, (a joint inventor), "Optics" Tata McGraw Hill, New Delhi, 1977, the angle 0 at which the radiation will propagate relative to the end face of the said bundle will be given by sinO = LA where A is the spacing between the rows of fibres composing the end face of the fibre hundle. Therefore, for a given bundle of optical fibres packed in this manner the emerging laser beam can be directed to propagate at a particular angle. In order to scan the laser beam using this form of the invention it is necessary to combine several bundles into a single bundle so that different groups of fibres can have different values of L or A. This can be achieved because of the small size of individual fibres.Taking individual fibres of 10-3 cm diameter, about 100 such fibres can be packed into a bundle whose crossectional area is about 10-2 x 102 cm or 10-4 sq cm. Such a composite, coherent, single mode optical fibre bundle could deflect a sequence of 100 laser pulses along 100 different angles realtive to the end surface of the said fibre bundle. In this configuration, this invention may have 100 different beam settings or a hundred spot scan.

In the first approach referred to above, all the fibres may be of the same length, but each row of said fibres would be fed with laser pulses having delays in time corresponding to the requirement of a particular row. Thus if fibres in row 1 receive pulses at time t = 0, fibres in rows 2,3 etc would receive pulses at time t = ;, 2 ;, etc. the duration of the pulses being greaterthan. On the basis that the pulses have definite phase relationships with respect to each other, the phase difference between consecutive rows is 2n c;; + (where c is the velocity of light) and the angle at which the radiation prop agates is given by sin 0 = 0- A better understanding of the invention will be gained from the following description taken in conjunction with the accompanying drawings. It is emphasised that ensuing techniques are exemplary and not limitative of the scope and applicability of the invention.

In the drawings: Figure 1 is a schematic layout of a preferred arrangement of the optical fibre ends forming the closed - packed, optically polished endface of the optical fibre bundle of the invention.

Figure 2 is a schematic layout of a preferred arrangement of the optical fibre ends forming the closedpacked, optical polished end-face of the optical fibre bundles of the invention when the said fibres are grouped together into 5 x 5 arrays.

Figure 3 is a schematic layout showing the optical fibre bundle of the present invention composed of rows of fibres, the fibres in each row being the same length but differing in length from the fibres in the other rows, the input pulse being generated in a single source.

Figure 4 is a schematic layout showing laser pulses from an array of sources being injected into rows of different fibres, the firing sequence of the said laser sources being such that the output beam can be deflected on either side of the central axis.

Figure 5 is a schematic layout showing a preferred configuration of the invention with fibres all of identical length which can be fired in sequences that result in the deflection of the output laser beam on either side of the central axis.

Figure 6 shows another configuration of the invention with all fibres being of the same length in a particular group, all the groups being compacted together so as to achieve different beam settings via sequential firing of the laser sources.

Figure 7 shows yet another configuration of the invention where precise time delays between a sequence of pulses can be achieved using a single laser source.

Figure 8 shows the use of the invention in a LIDAR (light detection ranging) system for monitoring the flight path of an aircraft landing onto a runway.

Figure 9 shows the use of the invention in a information retrieval system incorporating a static video record.

Now having particular regard to the numerals on the drawings, 1 indicates the output ends of the single mode optical fibres compacted into a coherent arrangement, utilizing techniques known in the art, to form the optically polished output face of the invention indicated by numeral 2. Similarly, numeral 3 indicates a grouping of the said fibres with one fibre from each of the said groups being connected to a particular laser pulse generator. Laser pulse generators are indicated by numeral 4. Numeral 5 indicates an optical fibre bundle where the lengths of the fibre rows differ progressively from top to bottom of the bundle. Numeral 6 indicates the deflected laser beam whilst numeral 7 indicates the central, undeflected beam axis. Numeral 8 indicates the power supply for the laser beam generators 4' whilst numberal 9 indicates the timing system for firing said power supplies. Numeral 10 indicates an optical fibre bundle where all the fibres are of the same length and each row of fibres is connected to its own laser pulse generator 4'. Numeral 11 indicates the computer control for the timing system indicated by numeral 9.

Numeral 12 indicates an optical fibre bundle which is composed of a large number of bundles all of the optical fibres being of the same length and connected to their own laser beam generators 4'.

Numeral 6' indicates the deflected beam position when the array of laser beam generator 4' are fired in the reverse sequence to that which deflects beam 6.

Numeral 13 indicates a beam splitter array for generating a sequence of identical laser pulses shifted in time by a very precise amount using only one laser beam generator 4. (Numeral 14 indicates coupling devices to couple beam splitter outputs into optical fibre rows). Numeral 15 indicates the invention as a LIDAR transmitter located near an airport runway indicated by numeral 16 tracing the scan pattern indicated by numeral 17 so asto monitor the approach of an aircraft indicated by numeral 18 along the curved path indicated by numeral 19 with detector system 20.

Numeral 21 indicates the computer control for the invention indicated by numeral 15 when used for information retreived from a static video record 22 placed in front of the array of optical detectors 23 which feeds the video display 24 with the information retrieved via scan pattern indicated by numeral 17.

It is well known in the art that optical fibres can now be manufactured with negligible transmission losses over distance of several metres which is the case with the present Invention where the length of fibre bundles 5,10 and 12 range from about 10 cm to about 10 metres. Such short lengths are also beneficial from the viewpoint of non-linear optical effects at high radiation loadings.

As shown in Figure 1, the single-mode, optical fibres 1 are arranged in rows, each row being separated by a distance A, with the fibres in each row, being of 10-20 microns (10-3 cm to 2 x 10-3 cm) in diameter and being packed to touch each other.

Ideally, the row separation should equal the fibre diameter, the separation of the said rows being measured between the central axis of the fibres. The resulting fibre bundles 5, 10 and 12 are assembled utilizing techniques well known in the art, particularly those developed in conjunction with the application of such optical fibre bundles in the medical and optical communications fields. Such bundles of optical fibres are referred to as coherent bundles because an image can be transmitted through the bundle. The compacted end 2 of the bundle of optical fibres 1 is optically polished to an accuracy of about 10-5 cm across the face when using a laser wavelength of one micron (10-4 cm).

Figure 2 is a schematic layout of groups of single mode optical fibres arranged over an area of about 10-4 cm2 and packed together to form the coherent bundle face 2. The purpose of sub-bundles 3 within the main bundle is to allow more than one deflected beam setting. For example, one optical fibre 1 from each of the sub-bundles 3 making up face 2 is brought together at their other end so that a laser pulse injected into the said end of the sub-bundle will emerge via end 2 and be deflected at a particular angle 0 corresponding to the A and L values of that particular sub-bundle. Therefore with twenty-five optical fibres for sub-bundle 3 there would be twenty-five beam setting in the deflection pattern achieved with this particular version of the invention detailed below.

Turning now to Figure 3 the coherent single-mode bundle 5 is composed of rows of optical fibres 1 which are of equal lengths within a given row but differ in length by an amount Lfrom one row to another. If the separation between rows is A then the angle 0 through which beam 6 is deflected from the central axis 7 after emerging from end face 2 is given by Sin e = LIA. Laser pulse generator 4 injects a pulse into optical fibre bundle 5 via face 2'. Since the shortest optical path is along the bottom of bundle 5, the pulse emerges first of all from the bottom of end face 2. Since the optical path along top of bundle 5 is the longest, the pulse emerges last of all from the top of end face 2.In this manner, each of the optical fibre ends facing face 2 behave as a phased locked optical transmitter and the whole of end fact 2 behaves as a phased array of optical transmitters leading to the output laser beam behaving as a single beam 6 which is deflected via an angle 0 relative to the central axis 7. To realise a range of 0 values, either the bundle 5-has to be composed of sub-bundles 3 or a number of bundles 5 have to be used together. In this way a scanning pattern composed of more than 100 x 100 spots can be achieved.

Figure 4 is a schematic layout of optical fibre bundle 5 connected to an array of laser beam generators 4'.

Each of the laser beam generators in the array 4' is coupled into one or more rows of optical fibres 1 in bundle 5. The power supply 8 which drives the laser beam generates in array 4' is controlled via the timing electronics 9. A signal from module 9, discharges power supply 8 to simultaneously activate the laser beam generator array 4'. The varying row lengths in bundle 5 provided the time delays required to ensure that the fibre ends 1, forming end-face 2 are a phased array of optical transmitters leading to the deflected beam 6. By incorporating sub-bundles 2 in bundles 5 many beam settings can be achieved.

Figure 5 is a schematic layout of a configuration of the invention with all the fibres in the bundle 10 being of equal length. Each row of optical fibres in bundle 10 are connected to their own laser pulse generator 4', power supply 8 and timing electronics 9, within the arrays 4', 8' and 9'. Overall control of the operation of this configuration of the invention is via computer module 11. By firing the laser beam generators in array 4' from bottom to top, beam deflection 6 will result; by firing the array 4' from top to bottom, beam deflection 6' will result; by adjusting the delay between the pulses in a given firing sequence any beam setting between that of 6 and 6' can be achieved.

Figure 6 is a schematic layout of the configuration of the invention described in Figure 5 with the optical fibre bundle 10 replaced by a bundle containing sub-bundles 3 and designated 12.

Figure 7 is a schematic layout of the invention utilizing fibre bundle 10 with beam splitter array 13 providing accurately spaced, pulses of equal amplitude from a single laser pulse generator 4, powered by a single power supply 8 and trigger module 9. The pulse sequence generated by beam splitter array 13 is coupled its bundle 10 via optical coupling devices 14, which are well known in the art and used extensively in the optics communications field.

Figure 8 is a schematic layout showing the invention incorporated as a LIDAR transmitter 15 with deflected beams 6, 6' and 6" defining the scanning pattern 17 which allows detector module 20 to determine the flight path parameters 19 of aircraft 18 as it lands on runway 16. It is well known in the art that a unique advantage of LIDAR (Laser-Radar) over prior art radars such as micro-wave radars, as its characteristic narrow beam width, which avoids background "noise " which arises with wide beams width radars. However, prior art LIDAR systems have been restricted to a single beam stems because no suitable beam scanning system was available. With single beam systems, it is an extremely complex process to maintain the approach path of an aircraft, particularly if other aircraft are in the vicinity because the aircraft fills the laser beam.However, by using the present invention, scanning pattern 17 allows LIDAR to sample a much larger area whilst at the same time maintaining the unique advantages of narrow beam width tracking.

Figure 9 shows a schematic layout of the present invention 15, controlled by electronics module 21, being used via scanning pattern 17, to retrieve information from a static video record 22 via detector array 23 and display unit 24. Prior art techniques consisting of FOURIER transform optics can be inserted between module 15 and detector 23 to increase the information storage density of the system.

The electronics control module 21, programms the invention 15 to scan the video record 22 in a predetermined pattern which was used to record the information on video record 22 initially.

The invention has a wide range of applications in LIDAR and information storageiretrieval systems including optical computers. Medical aplications include the scanning of internal organs which can be illuminated with prior art optical fibre bundles, but viewed with only relatively low resolution. Other applications are in commerce, e.g. banking, printing and super-markets.

Although the above description has concentrated on examples of one dimensional scanning, it is clear to those skilled in the art that the techniques can be easily extended to two dimensional scanning.

Finally, to recapitulate the present invention in a preferred form provides a laser beam deflection system with no moving parts consisting of a bundle of single mode optical fibres of the said fibres being of equal length and coherently packed into uniformly separated rows, one end of said bundle being compressed, adhered together (in any manner known per se) and optically polished with the other end of said bundle being loosely bound with each row of said optical fibres being connected to its respective laser beam generator, said generator being powered by its respective power supply and triggered by its respective electronic control module, said generator, power supply and control modules being part of a large array of such modules, said array being fired in particular sequences via a master computer module such that the laser beam emerging from said optically polished face of said bundle is scanned in a reproducible pattern with respect to the central axis of said optically end face of said bundle.

Modifications may be made within the purview of the above described subject matter without departing from the spirit and scope of the invention.

Claims (9)

1. A laser beam deflection system with no moving parts consisting of a bundle of single mode optical fibres, coherently packed into uniformly separated rows of said fibres, the fibres in each row being of equal length but differing in length from one row to another by equal amounts progressively through the said fibre bundle one end of said bundle being compressed, adhered and optically polished with the other end of said bundle being loosely bound with each of said optical fibres being coupled to a laser beam generator so as to accept a portion of the output beam of said generator and transport said beam to the optically polished, end face of said bundle where each optical fibre end behaves as an optical transmitter, the output of which is phase locked with the output of the other optical fibre ends across the said end face in such a manner that the laser beam emerging from said end face propagates at an angle to it.

2. A laser beam deflection system as claimed in claim 1 where the optical fibres forming said optically polished end face are packed in groups, with one fibre from each group being part of one optical fibre bundle in an array of similar bundles with a commonly bound, optically polished end face, the loosely bound ends of each of the said bundles being connected to their respective laser beam generators such that when said laser beam generators are activated in a predetermined sequence, the laser beams emerging from the said common end face of said bundles are deflected at different angles relative to said end face, the said angle being a function of the parameters of the respective bundles.

3. A laser beam deflection system as claimed in claim 1 where the laser beam energy input into the optical fibre bundle is generated in a single laser beam generator.

4. A laser beam deflection system as claimed in claim 1 where the laser beam energy input into the optical fibre bundle is generated in an array of laser beam generators.

5. A laser beam deflection system with no moving parts consisting of a bundle of single mode optical fibres of the said fibres being of equal length and coherently packed into uniformly separated rows, one end of said bundle being compressed, adhered together and optically polished with the other end of said bundle being loosely bound with each row of said optical fibres being connected to its respective laser beam generator, said generator being powered by its respective power supply and triggered by its respective electronic controle module, said generator, power supply and control modules being part of a large array of such modules, said array being fired in particular sequences via a master computer module such that the laser beam emerging from said optically polished face of said bundle is scanned in a reproducible pattern with respect to the central axis of said optically end face of said bundle.

6. A laser beam deflection system as claimed in claim 5 wherein the laser beam generators are in the form of semiconductor lasers.

7. A laser beam deflection system as claimed in claim 5 where the lengths of the said optical fibres range from 10 cms to 10 meters.

8. A laser beam deflection system as claimed in claim 5 where the diameter of the optically polished end face ranges from 1 mm to 5m.

9. A laser beam deflection system substantially as described with reference to, and as illustrated in, any one or more of the Figures of the accompanying drawings.