AU598456B2 - Scannable beam dual active mirrored laser oscillator system - Google Patents

Description

598456 This "ML'nt~l ccnu~tC 'irmencdonents made vndLr ~cin49 and is correct fl intng o 4 4 4 44 .4 4, 0e1 'sit t 5GO7 3/%3 APPLICANT: LILY HARRIET HUGHES

NUMBER:

FILING DATE: FORM COMMONWEALTH OF AUSTRALIA The Patents Act 1952 COMPLETE SPECIFICATION FOR AN INVENTION ENTITLED: SCANNABLE BEAM DUAL ACTIVE MIRRORED LASER OSCILLATr.i SYSTEM The following statement is a full description of this invention, including the best method of performing it known to me: W. L. THOMSON To: The Commissioner of Patents

ABSTRACT

This invention relates to a single segment scannable beam laser oscillator system which is directly excited through use of its dual active mirrored faces via the narrow band incoherent light generated by a computer switched array of semiconductor light sources.

Although the invention may not attain high operating laser beam flux densities, it nevertheless possesses high power operating capability due to its large a e o* output aperture scalability.

a 10 The invention is capable of producing output laser beams of any cross-sectional shape and intensity distribution and can be scanned via the travelling wave switching of the exciter array or an auxiliary .sweep beam of the same wavelength.

15 The invention has applications in industrial work-stations, medical apparatus, document readers, real time image projection and defence equipment.

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2 This invention relates to a directly excited, scannable beam, dual active mirrored, laser oscillator system. The slab is excited with narrow band light output of incoherent semiconductor light sources, consisting of a single laser oscillator gain medium disc/slab segment, one of the optically polished faces of which is mirrored for 100% reflectivity at the lasing wavelength and minimum reflectivity at the excitation wavelength, the other, output aperture face °being also optically polished and mirrored to allow :ol 0 o 10 the emission of the laser output beam and maximum azt reflection of the excitation light, the emitted laser a 0output beam being normal to said output aperture face or, when said disc/slab medium is swept by an auxiliary laser beam of the same wave-length as the a 15 laser output beam, can be deflected from side to side, ba focussed or diverged, depending on the nature of the a :auxiliary laser beam and the angle at which it enters the disc/slab medium via the optically polished face ca or edge, the said laser output beam then being composed of transmitted portions of the medium sweeping auxiliary beam as it is reflected off the output mirror within said medium. The laser oscillator output beam can also be X-Y scanned via the travelling wave, computer switching of the incoherent semiconductor excitation light emitting array.

~airaPrscr~ 3 The high quality output beam of the invention is scalable to large diameter, high power in various geometrical configurations, particularly beams of circular and elliptical crossksections which can be subsequently amplified in prior art laser amplifier systems when such need arises, without the use of complex lens systems to match said output beams to such prior art systems. In the scanned mode, the o laser oscillator of this invention provides a direct 10 means of focussing, diverging and deflecting its t powerful output beam without the use of complex lens t t systems.

The invention has applications in industrial workstations for cutting, welding, drilling, workA hardening and marking a wide range of materials, has applications in medical systems for surgical and therapeutic treatments, in commercial laser based products for example for scanning documents and reading bar codes, in laser radar systems, target designator systems and as a short range defensive weapon system when scaled to the appropriate levels as described elsewhere.

SUMMARY OF THE PRIOR ART Prior art dual mirrored laser oscillator systems consisted of end pumped laser rods, travelling wave excited slabs and zig-zag slab laser oscillators.

4 w I Ml Dual active mirrored rod laser oscillators provide high quality, low power laser output beams at high efficiency compared with flashtube excited rod laser oscillators of prior art. However prior art laser oscillators are severely limited due to end pumping over a small active mirrored end area and to achieve realistic power outputs had to utilize long o lengths of the rod. Laser oscillators of this type <Oo could not produce a scanned beam output without the 10 use of complex lens systems.

o o o Prior art dual active mirror-,d slab oscillators 0 "0 produced deflected beam outputs resulting from inL direct optical excitation of travelling wave format oo. produced via an opticaal fibre bundle with a spatial 15 gradient, that is, the length of rows of the optical fibre exciters increased systematically to produce a travelling wave excitation beam. However, such a travelling wave exciter bundle could only produce a given deflection angle for the said slab laser oscillator output beam. The zigazag slab oscillator utilized dual active mirrors to improve on prior art zigzag oscillators which depended on Brewster angle internal reflections to propagate the laser oscillator beam within the slab medium. ZigAzag dual mirrored laser oscillators are limited to small diameter laser beams which have to propagate within the gain medium.

In other words, both peak power and continuous wave power of ziguzag dual active mirrored laser oscillator systems are severely limited.

The present invention overcomes the restrictions and problems of prior art dual active mirrored laser oscillator systems by providing large, directly excited, output apertures with short separation QOo between the two large, mirrored faces, operating well ,0 below intensity damage threshold yet providing very o" 9" 10 high power outputs in a high quality laser beam due ao to its large cross* sectioned area. The present o* invention also differs from prior art dual active mirrored laser oscillator systems because it utilises an auxiliary laser beam of identical wavelength to 15 sweep out its volume, a portion of said sweep beam S being transmitted through the output aperture of the invention each time I't is reflected internally by the partially reflecting active mirror deposited on said output aperture of the invention.

By changing the angle at which the sweep beam is reflected off the partially reflective output mirror of the invention, the direction of propagation of the output beam of the invention can be changed giving deflected, focussed or diverging output beams.

The present invention differs from prior art, dual active mirrored laser oscillators due to its I ability to sharply define the cross.sectional shape of its excitation light. For example, the invention can produce a high quality elliptically shaped laser beam output cross4section without the use of lenses. Such an elliptical cross4section output beam can be subsequently amplified in prior art slab amplifier systems without the need for matching lenses.

SGo. It should be noted that the various beam crossA a sections that can be produced by the present invention 10 have controlled intensity distributions. For example, ,o athe elliptical crossNsection output beam can have a o -flat intensity distribution as it is emitted and there is no need to use additional optical elements to .achieve a flat intensity distribution as is the case 15 with the output of prior art laser oscillator pulsed generators used to drive large amplifier systems, for example, the Nova laser system at Livermore, California.

The definition achievable in producing the various beam output crossAsection depends on the collimation of the light output of the semiconductor light source excitation array. With highly collimated, individually switchable exciters in the array the image definition the output of the invention is maximised. The computer switching of the individual elements of the exciter array allows for a ~I r~ 7 real time exciter light image of high definition which is transformed to a high definition laser output beam image in the invention, also in real time.

BACKGROUND OF THE INVENTION During the early 1960's there was considerable activity worldwide aimed at consolidating well estabg lished microwave radar techniques in the optical a 04 region of the electromagnetic spectrum based on the properties of the laser ffirst operated in the United States in 1960.

The inventor joined the team at the Royal Radar Establishment, Malvern, UK in 1963 and was given the task of developing and field testing laser radar systems. To consolidate laser radar technology one '15 required high quality laser beams, high mean powers in both pulsed and continuous wave outputs and a 4$ 4 t capability of precisely scanning powerful laser beams.

$441tAlthough many of the necessary laser techniques for the development of laser radar were identified at that time, they were severely material limited, and, therefore, could not be adequately tested. However, laser rod, disc, slab and optical fibre techniques with a range of excitation schemes were considered and the less esoteric were field tested. on the Lark Hill Proof range in Southern England during the period 1963A21904. it became obvious that the diameter of the 8 of the laser output aperture had to be increased significantly if high power, high quality, scannable laser beams were to be produced. However, since high pulse repetition rates were also of prime importance to the development of laser radar it was also clear that the laser medium had to be segmented. This led to the concept of a scalable exponential amplifier o (Applied Optics, August 1967) developed by the inventor under the auspices of the British/Australian 10 Joint Projects Board at the Weapons Research 69Establishment in South Australia. In the exponential amplifier, the laser medium is segmented whilst at the same time maintaining a constant laser beam intensity through the amplifier during the amplification process. This work led to the development of the multi disc/slab, divergent beam, folded exponential amplifiers which are currently being developed for a wide range of applications.

'414 In addition to the segmentation of the laser gain medium perpendicular to the direction of propagation of the laser beam undergoing amplification, work had also been directed towards the segmentation of the amplifying laser medium parallel to the propagation of the laser beam through the amplifiers. In this approach, the solid glass or crystal laser amplifier media were replaced by 9 sections of optical fibre bundles forming phasedu locked aperture segments. Such phased array laser systems have an inherent capability of acting as laser beam deflectors and complex XkY scanning becomes feasible provided the appropriate time delays can be introduced over such a large number of aperture segments. This invention incorporates many of the o c beneficial aspects of disc, slab and fibre bundle o laser oscillator/amplifier systems at the same time a 10 overcoming many difficult aspects of their scaling so and scanning techniques. Furthermore, the invention *4 utilizes the latest techniques of semiconductor light sources, large crystal segments for high pulse rate operation and a variety of output beam profiles.

A particular problem with large, single segment laser amplifier systems is the parasitic oscillations that are set up perpendicular to the laser beam propagation direction. In the laser oscillator configuration of the present invention, such parasitic oscillations, which diminish the stored energy available for laser beam generation within the gain medium, are minimised by varying the intensity of the semiconductor excitation source with time in such a manner that the excitation above lasing threshold takes place relatively rapidly at the higher excitation efficiency possible in that regime. In w -LIIYI-~.CII~~-C*lfEIY*~ S6 078/86 this way, the onset of parasitics in a direction along the body of the disc or slab laser medium segment is preceeded by the onset of laser action to generate the output beam of the invention. Prior art techniques, such as high absorption edge cladding at the laser wavelength are also utilized. Due to the limited sizes of crystalline segments and the parasitic oscillation in large single segments, the single segment gain medium of the invention can be a 10 composed of many sections, optically polished along I their edges for best fit.

SOBJECTS OF THE INVENTION It is an object of the invention to produce scalable diameter laser beams of high quality from a S 15 single, directly excited laser oscillator segment which has closely spaced dual active mirrors, said laser output beam being pulsed, continuous wave or modulated in synchronism with the excitation light.

Another object of the invention is to utilize secondary laser beams to scan said laser oscillator medium segment so as to produce a scanned output beam.

A further object of the invention is to produce a variable focussed output beam diretly, without the need for external optical components.

Yet a further object of the invention is to I_ o1 56079ft 4 9 6 11 produce output beams of circular and elliptical crossusection, without the use of lenses and associated optical components, which have controllable intensity distributions.

An object of the invention is to provide a simple laser oscillator pumped by semiconductor light sources which can be computer switched both spatially and in a time dependant intensity profile, the properties of the laser output beam in time being related to the S 10 properties of the excitation light above lasing threshold.

i Yet another object of the invention is to excite the single segment dual active mirrored disc or slab laser oscillator so that parasitic oscillators within said laser oscillator are minimised.

An object of the invention is to utilize large, thin segments of doped crystalline, glass or fluid laser media in transparent containers, to produce large diameter laser output beams.

Another object of the invention is to convert a computer generated image on the switched semiconductor excitation light source array and trans# form it onto a single laser beam image in real time.

Yet another aim of the invention is to travelling wave switch the incoherent semiconductor light source array to scan the output beam of the invention in two 12 dimensional format.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the invention may be obtained from the following consideration taken in conjunction with the figures which are not meant to limit the scope of the invention in any way.

Figure 1 shows the invention with a continuous or segmented slab or slab-like container of a laser oscillator medium, excited by a semiconductor light 6 4 a Ssource array, said array being switched wholly, 10 partially or in a time dependent manner by a Scomputer.

S: Figure 2 shows a circular distribution of excitation light from the semiconductor array with a toGaussian intensity distribution.

Figure 3 shows an elliptical distribution of excitation light from the semiconductor array with a flat intensity distribution.

Figure 4 shows the configuration of the invention incorporating auxiliary laser beam deflectors to inject either a line or circular sweep beam into the laser oscillator to both generate and direct the resulting laser output beams.

Figure 5 shows the preferred cov4iSguti~ron .0 tw invention in an industrial work-station.

DETAILED DESCRIPTION OF THE INVENTION In Figure 1, numeral 1 denotes a di L i: i: 13 laser gain medium forming the body of the laser oscillator of the invention. Numeral 2 indicates a dielectric mirror deposited on the optically polished face of said gain medium 1 which reflects the laser oscillator wavelength 100% but allows the passage of the excitation light into said laser medium 1.

Numeral 3 indicates a partially transmitting dielectric mirror deposited on the optically polished output face of said laser oscillator which allows the passage of the laser oscillator output beam indicated by numeral 4. Numeral 5 indicates the narrow optical band excitation light emitted from arrays of semiconductor light sources indicated by numeral 6.

Numeral 7 indicates the electrical leads joining the semiconductor light source array 6 to their power supply indicated by numeral 8 which in turn is switched by the computer system indicated by numeral 9. It should be noted that mirror 3 which is partially transmitting at the lasing wavelength of the laser oscillator is fully reflecting over the ave& length of the semiconductor light source output so that any excitation light not absorbed on the first pass through medium 1 has an additional probability of being absorbed on the second pass, As excitation light 5 enters the laser gain medium 1, laser oscillations are set off between 14 mirrors 2 and 3 within medium 1 generating output beam 4 which may be pulsed, modulated or continuous wave depending on the computer switching of power supply 8 and incoherent diode arrays 6.

In Figure 2, numeral 10 indicates the laser output beam 4 of circular crossAsection and numeral 11 indicates its Gaussian intensity distribution produced A by appropriately varying the intensity distribution p «s of a section of the semiconductor excitation source 6 a 6 OS 4 10 via computer 9.

In Figure 3, numeral 12 indicates the laser S, output beam 4 of elliptical crosssection and numeral 13 indicates its flat intensity distribution produced S• by appropriately setting the intensity distribution of a section of the semiconductor excitation source 6 via computer 9.

In Figure 4, the invention is shown with mirror 2 not extending to the edge of the gain medium 1 to allow a secondary laser beam indicated by numeral 14 and generated in a laser generator indicated by numeral 15 to be deflected by the laser beam deflector indicated by numeraal 16 to be reflected at various arp',es off reflector 3, sequentially through medium 1 where it is reflected back to 3 via reflector 2. Each time beam 14 is reflected off mirror 3, a portion of the laser beam 17 is emitted by the invention.

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However, the remaining portion of beam 14 within gain medium i, which is reflected off 3 via 2 and back to 3, is amplified as it transverses gain medium 1 so as always to maintain its intensity as it encounters reflector 3. By adjusting the thickness of gain medium i, the injection angle of beam 14 and the pulse duration of the sweep beam 14, beam 17 is both generated and directed from within medium i.

o* With beam 14 of line or rectangular j 10 configuration, the output beam 17 is swung from side 44ir to side of output aperture 3. On the other hand, if laser beam 14 is circularly symmetric, then output beam 17 is focussed with a focal length which is proportional to the injection angle of beam 14 onto S 15 mirror 3.

In Figure 5, numeral 18 indicates an industrial workpiece which is being treated with focussed output beam 17 of the invention in the cutting, welding, hole drilling, work hardening or marking modes of operation. To mark an image on workpiece 18, computer 9 must generate the appropriate real time image on semiconductor light array 6 to excite laser medium 1 in such a manner as to transfer the incoherent optical image into a coherent one which can then be focussed onto the surface, or within the body of workpiece 18.

16 In Figure 6, the conically scanned output beam 19 of the invention is used to scan a document indicated by numeral 20. Concentric detectors indicated by numeral 21 placed beneath the document pick up the signals corresponding to the conically scanned laser beam 19 illumination pattern and transL mit them to another unit of the invention at a remote site where they are fed into computer 9 which in turn displays the document in real time as laser image 22.

Although the invention has a configuration which minimises optical, thermal and electrical loading, its overall efficiency at best will be less than so that heat generated will have to be removed either by radiation transfer or fluid cooling prior art techniques. Where the semiconductor exciter is in the form of semiconductor laser arrays, gain medium 1 can become an absorbing portion of the external cavity of individual laser diodes within the exciter array. In this case the 100% reflective coating for the exciter on the output face of the invention becomes the cavity mirror for such external cavity laser diode arrays.

This excitation mode provides the highest possible image definition on the output face of the invention.

If the secondary laser beam is then interchanged for incoherent semiconductor light sources, then the gain medium 1 can be excited to lasing threshold via 17 incoherent light and rapidly excited above lasing threshold by an array of coherent semiconductor diode lasers with external cavities a common part of which is gain medium 1. In this way the output beam of the invention will be the phased locked outputs of the array of external cavity diode pumped sections of the laser medium segment of the invention.

The invention can also be used as a real time, high intensity projector of television and computer g generated images.

10 Modifications may be made to the invention as a Q described herein without departing from the spirit and o scope of the invention. To this extent it is pointed Qar 9 out that the invention is to be given a broad o connotation and is not to be restricted to the 64tc embodiments specifically described.

Claims (7)

1. 2. A system as claimed in claim 1 with an auxiliary laser beam of the same wavelength as the said laser output beam which can be injected into said laser medium segment, via the outer, unmirrored portion of one of the optically polished faces so as to be reflected at various angles internally from the partially reflecting mirror on the said output aperture face, a portion of it being emitted through said output aperture at each reflection, the full sequence of reflections building up a complete output Lv LJ. ULJ ±UI mIaxlmisea. Tne computer switching of the individual elements of the exciter array allows for a 4 19 M laser beam, said auxiliary beam being reamplified between each reflection from the mirrored surfaces.
2. 3. A system as claimed in claim 2 where the auxiliary beam is of rectangular cross-section deflected at various angles into the laser oscillator segment to produce an output beam on either side of the central S"o axis of the said laser gain segment normal to its o o Soutput face. 0 40 9« 04. A system as claimed in claim 2 where the auxiliary 0 0 0 A beam is circularly symmetrical and injected into said laser gain segment via an unmirrored outer portion of o 00 .a.o one of the polished faces at various angles so as to produce a variable focal length output beam, each focal length being related to a particular injection angle of 15 said circularly symmetric auxiliary sweep beam.
3. 5. A system as claimed in claim 1 where the semisconductor light source array is computer switched to provide a circularly cross4sectioned beam of excitation light with a Gaussian intensity distribution which is transferable into a laser output beam of corresponding parameters.
4. 6. A laser oscillator system as claimed in claim 1 where the semiconductor excitation light source array is computer switched over an area of elliptical cross-section to emit said excitation light with a uniform intensity distribution across said elliptical dLLu uile ess esoreric were rie-a tested on the Lark Hill Proof range in Southern England during the period 1963*19 4. It became obvious that the diameter of the 4 20 cross sectional area said excitation light being used to excite the said laser medium to produce a laser output beam from said laser medium with similar characteristics.
5. 7. A laser oscillator system as claimed in claim 1 where the semiconductor light source array exciter is computer switched to emit a long duration pulse which excites said laser medium to its lasing threshold with a much shorter duration pulse superimposed onto it to excite said laser medium well above its lasing threshold thus minimising the effects of any parasitic oscillations within said excited laser medium.
6. 8. A laser oscillator system as claimed in claim 1 where the laser gain medium is a neodymium doped laser o 15 crystal section.
7. 9. A laser oscillator system as claimed in claim 1 where the laser gain medium is neodymium doped glass. I 10. A laser oscillator system substantially as hereinbefore described and illustrated with respect to the drawings. LILY HARRIET HUGHES DATED: 21 SEP 89