AU598457B2 - Structured scannable beam very high power laser system - Google Patents

Description

598457 5607)/36o 9$ C C* C Thbis djocument cont1ain h atmendments madeude L'.ctiori 49 and is correct foi r inting. APPLICANT: LILY HARRIET HUGHES NUMB3ER: FILING DATE: 9 C FORM COMMONWEALTH OF AUSTRALIA The Patents Act 1952 COMPLETE SPECIFICATION FOR AN INVENTION ENTITLED:

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STRUCTURED SCANNABLE BEAM VERY HIGH POWER LASER SYSTEM The following statement is a full description of this inven-tion, including the best method of performing it known to me: -1- K

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ABSTRACT

This invention relates to a repetitively switched, very high power, structured, scannable beam laser oscillator/regenerative amplifier system, directly excited via one coated, optically polished face via the patterned excitation light from a structured array of semiconductor light sources which are computer switched. The invention has application in advance particle physics research and as a laser radar weapon system.

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CrC I; I 02 FIELD OF THE INVENTION This invention relates to a repetitively switched high power regenerative laser amplifier consisting of one passive and one active confocal laser beam reflector between which is placed, near the common focal region, a flat laser beam reflector, with a I central hole, positioned at 45 degrees to the central s e axis defining said confocal mirror cavity and t1** ,c surrounding their common focus region. The active mirror allows the laser to be repetitively switched by directing a portion of said laser beam undergoing amplification to be reflected out of said laser S* oscillator, via the said flat mirror. The beam switching is achieved by rapidly modifying the 15 structure of the Fresnel Zone structure of the amplified laser pulse during a period shorter than the "round trip time of the cavity.

The invent i on has the capability of generating Svery high laser pulse powers and has wide applications 20 in laser radar weapons systems as well as in powerful laser based industrial work-stations. The invention Shas particular application in generating an intense laser photon centre-of-momentum distribution in which advance particle-photon and photon-photon interaction studies can be undertaken.

i' 03 SUMMARY OF THE PRIOR ART Prior art confocal cavity lasers generate laser beams with smooth intensity profiles and have passive laser mirrors forming the resonant optical cavity and cannot be single pulse repetitively switched at high laser peak powers due to electro-optical switch damage.

ne Prior art confocal resonator lasers were not scalable to high laser pulse powers because of the difficulties of injecting and extracting laser pulses into and out of said lasers respectively. The switchlng problem Swith prior art confocal cavity laser systems arises because one of their mirrors has to be reconfigured in a period shorter than the light transit time of the *said laser optical cavity if successful in and out 15 switching is to be achieved. However, the peak power I 44 capabilities of confocal cavity lasers demand large optical apertures for the electro-optic switches and the larger the aperture, the slower becomes the switching speeds. Unfortunately, no electro-optic switch exists with optical aperture diameter more than To deform one of the mirrors mechanically would So need a prohibitively long optical cavity to ensure that the round trip time exceeded the mirror deformation time.

The present invention overcomes the defects of I -i i prior art confocal cavity laser systems by using an active, switchable mirror as one end of the optical cavity. Not only does the said active mirror trap the initial laser pulse to activate the confocal cavity laser but allows for the rapid conversion of a tocussed laser beam into a divergent laser beam so that the laser energy confined within the confocal cavity laser can be extracted.

:BACKGROUND OF THE INVENTION re A need exists in particle physics to study the E; c- 10 interaction of a very large number of photons within the smallest possible volume. In this manner any collective behaviour of photons can be assessed experimentally. Since any such collective photon-particle and photon-photon interactions are going to be minute the apparatus required for such experimental studies will inevitably be complex. It is r well known that energetic photons can be created either by the annihilation of particle-antiparticle pairs or by the interaction of energised electrons and protons with matter. However, both the total number and the number density of such generated photons are relatively small because the higher the photon energy, the harder they are to produce and the more difficult they become to handle, that is, direct into a small interaction volume.

With the advent of the laser in 1960, the experimental situation changed dramatically. Laser photons, despite being of relatively low energy individually, can be produced in very large numbers and can be directed into a relatively small interaction volume, with precision, to form an optical centre-of--momentum region. The potential of focussing intense laser beams in physics was pointed out by the inventor in Nature, May 1963.

However, the lasers available at that time were over ten billion times too weak to make any search for photon collective effects worthwhile. It so happened that the military requirement was for similar lasers to those required for photon interaction physics and were Sct,' along apparently two different routes as viewed by the 15 military authorities of that time, namely, laser weapons and laser radar. Of the two, the laser radar approach was the most sophisticated simply because the effort was based on the microwave experience whereas there was no experience whatever in laser weaponary where brute force techniques were the order of the day.

However, one of the reasons for developing laser radar was its increaAed precision and imaging capabilities over microwave radar;?,>i. It was clear that S. 25 such a radar would be required to pin-point any laser 06 weapon system onto target so it appeared logical to develop laser radar rather than laser weapons because it would automatically provide for all the basic techniques of laser weapons anyway.

The two fundamental goals of laser radar are high mean power and a beam scanning capability. It so happened that these are also the fundamental oo requirements of the laser apparatus for experiments in 0 ooa 0 particle physics. Several technological approaches to i 10 developing the correct approach to high mean laser oa powers and beam scanning were developed from 1963 onwards by the inventor first at the Royal Radar Establishment, Malvern UK, then at the Weapons Research o' 1 Establishment, South Australia under the auspices of S00 .so 15 the British/Australian Joint Projects Board and subsequently from 1970 at The Australian National University in Canberra where the Oliphant homopolar generator was assessed as a source of one second excitation at one gigajoule per second for a repetitively pulsed lased. (Inall and Hughes, Nature 1968.) It should be noted that the low voltage, high current characteristic of the Oliphant homopolar generator matches the same operating characteristics of semiconductor light sources. Key developments in the S1 25 above areas were reported by Hughes et al in Applied 07 Optics, August 1967, Journal of Scientific Instruments E, November 1968, Nature 1968 and Applied Optics 1978.

The technologies have also been described in a series of issued and pending patent applications from 1973.

The difficulty that limits the application of prior art confocal cavity laser systems to high power c rr applications such as particle physics, laser radars and laser weapon systems is the rapid and repetitive injection and extraction of laser beam energies from N 10 such systems at high power levels. On the other hand, the unique property of confocal cavity lasers in these fields is the fact that they have no material media at ti the highest intensity regions, for example, the ,t electron-photon energy transfer in free electron lasers 15 occuring in a vacuum which cannot be affected until the t photon density exceeds 10 watts cm wherein the oscillating electrons start to produce :electron-position pairs and have to be replaced with ion beams. The fundamentally new aspect of the present invention is a structured active mirror and a switch capable of being activated in a period of nanoseconds and capable of withstanding very high peak laser powers. Prior art free electron, or any other type of prior art regenerative amplifier laser system, have completely failed in this respect, namely, they cannot 08 be repetitively switched at high peak output powers.

Both lenses and Fresnel zone plates were considered as laser beam focussing elements by the inventor at the Royal Radar Establishment. To achieve the unique switching process of this invention, the Fresnel zone pattern of exciting the active mirror of the invention is changed during the transit period of the confocal cavity, regenerative laser amplifier in such a manner that the reflected beam convergence is S, changed into a divergent mode allowing the laser pulse energy to be reflected out of the otherwise closed confocal mirrored, regenerative laser optical cavity.

rt During this process, the active mirror segment adds the required aamount of laser beam energy to the cavity pulse pattern, on its last multipass traverse of the laser gain medium being much less than the energy of the amplified pulse, said excitation of laser gain medium on said last pass being confined to those portions required to modify the Fresnel zone pattern.

OBJECTS OF THE INVENTION It is an object of this invention to produce a high power scannable beam, concentrically structured active mirror laser oscillator system. It is also an object of this invention to repetitively inject powerful starting pulses into a confocal cavity, regenerative laser amplifier systenm one or both of whose end mirrors is active and concentrically structured.

Another object of the invention is to repetitively switch out a large amount of laser pulse energy from a confocal cavity regenerative laser amplifier system by adding a relatively small amount of laser energy to o«o selected portions of the Fresnel zone pattern of such a oa g laser pulse via a structured active mirror which is 0nso directly excited via a computer switchable array of S 10 incoherent light emitters. An object of the invention Sis to provide a variable beamwidth laser radar/weapon system.

Yet another object of the invention is to al'ow Q o the repetitive extraction of laser pulses from a Soo 15 confocal mirror laser amplifier cavity after it has been amplified to high power levels in both divergent and convergent beams.

g Another object of the invention is to provide an o optical centre of momentum region where photon-photon and photon-particle interactions can be studied.

BRIEF DESCRIPTION OF THE INVENTION I A better understanding of the invention may be obtained from the following considerations taken in j conjunction with the figures which are not meant to S 25 limit the scope of the invention in any way.

l: 1 Figure i, shows a schematic of the directly excited, structured, dual active mirrored laser oscillator/amplifier, scanned beam end mirror of the confocal cavity of the invention which can also act as a scannable, high power laser beam generator in its own right.

Figure 2 shows the layout of the invention with one, scalable structured, active mirror and a scalable C passable concave mirror forming a confocal optical cavity with a scalable 45 degree flat beam reflector with a central aperture positioned over the common focus region of the two mirrors.

S" DETAILED DESCRIPTION OF THE INVENTION t In Figure i, numeral 1 indicates the central 15 portion of the solid laser medium, or slab like container of a fluid laser medium, structured in the form of a Fresnel zone plate but of dimensions sufficient to hold a range of different Fresnel zone patterns corresponding to convergent, parallel and divergent laser beams. It should be noted that the structured components 1 can be reversed in the spaces between them forming Fresnel zone plate structures with a hollow central portion. Numeral 2 indicates a dielectric mirror with 100% reflectivity to the lasing 25 wavelength but minimum reflectivity at the excitation 11 wavelength band. Numeral 3 indicates a partially reflecting mirror at the lasing wavelength and a fully reflecting mirror over the excitation light band.

Numeral 4 indicates a converging beam output corresponding to its particular Fresnel lasing pattern whilst numeral 5 indicates a diverging beam output tt corresponding to its particular Fresnel lasing Spattern.

Numeral 6 indicates the incoherent light output of a structured array of semiconductor equivalent light sources indicated by numeral 7. Numeral 8 indicates the connecting leads between 7 and its power supplies S* indicated by numeral 9. Numeral 10 indicates the i t4 computer required to switch 9 to 7 to produce the excitation light pattern 2 to generate the required range of Fresnel lasing patterns in medium 1 which allows for the conical scanning of the high power laser beam outputs 4 and In Figure 2 the invention configuration of Figure 1 is positioned as the scalable active end mirror of a confocal optical cavity the other end of which is formed by the scalable passive concave mirror indicated by numeral 11. Numeral 12 indicates a flat dielectri laser beam reflector with a cettral hole indicated b numeral 13 allowing said flat, scalable reflector be positioned around the common focus regipn of te 1 l 12 confocal optical cavity indicated by numeral 14.

To start the lasing operation in the invention, computer 10 switches power supply 9 and semiconductor diode array 7 to give a lasing Fresnel pattern in 1 corresponding to convergent laser beam 4, with the reflectance of mirror 3 at the lasing wavelength being very low. The active end mirror output laser pulse in beam 4 is then trapped in the confocal cavity and passes to and fro gaining energy as it double passes the active mirror laser amplifier medium 1. The structured laser beam builds up to very high power levels via multiple passages through 1 and computer S* ensures, via optical detector monitors (not shown) that I beam structure 4 is maintained via adjustments to 15 excitation light pattern 6. This laser pulse amplification process can be duplicated so as to amplify two equal, but oppositely directed pulses within the invention which form an increasingly intense Sotiral centre of momentum region 14 as they overlap.

20 Under these operating conditions, region 14 can be used for the study of photon-photon and photon-particle collective effects, said selected particles (and antiparticles) being injected into 14 at the o appropriate times. In this mode of operation, the laser pulses within the invention would be amplified in 13 excess of 10 15 watts peak power at some 1,000 to 100,000 times the peak output power capability for single pulse amplification of prior art laser amplifier systems. The laser photon energy density in 14 would then exceed 102 ergs cm3 and can approach 103 ergs cm- 3 It should be noted that the invention can also operate as a free electron laser whose output is switchable.

00 Numeral 15 indicates the output of the invention D0 0 10 in the laser radar/beam weapon mode. During a single round trip time of a laser pulse in the invention, computer 10 adjusts the excitation light 6 emitted by semiconductor emitter array 7 such that the Fresnel pattern lasing medium 1 corresponds to beam 5 which can be conically scanned as output beam To seek a long range target such as an intercontinental. nuclear missile, located roughly by microwave radar or satellite surveillance, the laser output 15 is diverged to operate in the laser radar mode. This divergence can either be achieved via the change of format from beam 4 to beam 5 or by more sharply focussing beam 4 so that it diverges onto mirror 14" and is emitted as. it divergent beam 15. As the target is detected via the optical echo of beam by optical detection system (not shown) computer It 1' t:

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C I SCt progressively converges beam 15 resulting in a higher level echo and greater positioning accuracy. In this manner the target can be progressively located with increasing accuracy until the output 15 is converged into the weapons mode so that the whole pulse output energy can be pin-pointed onto the target destroying it in the process. Being a conically scanned laser radar/weapon system, the invention can track several targets within its diverged beamwidth simultaneously and can destroy them in a rapid sequence. It should also be noted that the laser oscillator configuration of Figure 1 can also be used as a laser radar/weapon system in its own right but of lower output capacity.

The invention has particular advantage over prior art laser radar/weapons systems when operating within the earth's atmosphere because computer 10 can easily and very rapidly compensate the output beam intensity distribution to match the distortion effects of the atmosphere. Prior art laser radar systems required complex mechanical movements to accomplish this feat, inevitably slowing the activity of such laser radar/weapon deployment. In the laser radar/weapon mode of operation the invention is capable of delivering between 10 and 10 joules of laser beam energy onto target in pulse durations from 100 x 10 seconds (one A m__7 I V i 'U 1 4r** 4 44 *g 4 44 tC 4 4 44 4 4-44 4 hundred picoseconds) to a hundred nanoseconds seconds).

Numeral 16 indicates the excitation of the structured laser medium 1 internal to the body of the structured medium. Cooling facilities can also be installed within these openings in structured medium 1 allowing the invention to operate at high pulse repetition rates up to 1,000 HZ, or one thousand times per second.

10 In Figure 3, numeral 16 indicates a more tightly focussed beam than beam 15 which diverges as the beam indicated by numeral 17 into the concave annular reflector indicated by numeral 18 which couples laser pulse out of the cavity as the focussed beam indicated 15 by numeral 19.

Modifications may be made to the invention as described herein without departing from the spirit and scope of the invention. To this extent it is pointed out that the invention is to be given a broad connotation and is not to be restricted to the embodiments specifically described.

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Claims (9)

1. A very high power, conically scannable output beam, concentrically structured active mirror laser oscillator system whose gain medium is in the form of concentric solid rings surrounding a solid central portion or enclosed within containers whose walls take the form of solid, concentric rings, surrounding a hollow container which acts as a central portion, the diameter of said solid rings being much greater than their width, the faces of said solid rings perpendicular to their central axis being optically polished, with one of said faces being coated to reflect the laser output wavelength 100% but having minimum reflectivity at the excitation light wavelength, said other, opposite face being coated so as to reflect 100% of the excitation light but having minimum reflectivity at the laser is output wavelength consistent with the generation of the output laser beam of a given power level, said concentrically segmented laser oscillator medium being excited by a concentric r *r distribution of ,ght rings emitted from a concentrically structured semiconductor light source array which has a jse* S 20 computer controlled power supply to switch said semiconductor excitation light array into particular Fresnei excitation light patterns, and, by changing said excitation light patterns to cause the conical scanning of the laser oscillator output beam.

2. A system as claimed in claim 1 where the gaps between the concentrically ringed structure of said laser oscillator gain medium are utilized for both the excitation and cooling of said laser oscillator gain medium. 16 I r; f

3. A system as claimed in claim 1 where the gaps in the concentrically ringed structure of the semiconductor light source are used for cooling and cabling of said excitation source.

4. A confocal, scaleable end mirrored, regenerative amplifier system one of whose end mirrors is an active mirror as claimed in claim 1 the other end mirror being active or passive, with an annular plane mirror positioned at 45 degrees to the axis of the confocal optical cavity formed by said end mirrors, directly over the confocal region, the pulse to be amplified being generated within the structured gain medium of the active mirror in a converging Fresnel pattern which matches the configuration of said confocal optical cavity so that the said laser pulse is trapped between said mirrors and is amplified each time it traverses the said active end mirror whereby when the required is amplification has been achieved the computer switched excitation array is switched to generate a less converging Fresnel zone pattern so that the said laser pulse can be switched S40:4. out of the said optical cavity via the said annular plane mirror positioned at

5. A confocal, generative amplifier system as claimed in claim 4 where energy is added to the trapped laser pulse via electron ,a beams converting said regenerative amplifier into a switchable, high power solid state free electron laser hybrid system.

6. A confocal system as claimed in claim 4 where the output laser beam is conically scanned at very high power levels in the manner of a high precision laser radar system by varying the 17 focal length of the output beam to produce a range of conically diverging laser beams in the far-field such that when a target has been precisely located via optical echoes, the system is converted into its weapon mode of operation by precisely focussing its output beam onto said target after its coordinates have been determined by the radar mode, in this way the invention can both track and destroy a target over very long distances in excess of 10,000 kilometers.

7. A confocal system as claimed in claim 4 where one or both 1o end mirrors are active emitting one or more laser pulses in such a time sequence that they overlap within the common focus region to form an optical centre-of-momentum region where advance photon-photon and photon-particle/antiparticle interactions can be studied.

8. A system as claimed in claim 1 where the laser medium is of fluid form and is flowed through optically polished containers ove arranged in concentric ring sections. Ut S 5 9. A system as claimed in any one of claims 1 to 8 substantially as hereinbefore described and illustrated with reference to the S baccompanying drawings. ,LILY HARRIET HUGhES DATED THIS SIXTH DAY OF APRIL 1990 JLH/cg

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