WO1990015462A1

WO1990015462A1 - Diode pumped, fibre bundle coupled laser oscillator, phase-locked laser amplifier system

Info

Publication number: WO1990015462A1
Application number: PCT/AU1990/000239
Authority: WO
Inventors: John Leonard Hughes
Original assignee: Australian Electro Optics Pty Ltd
Priority date: 1989-06-01
Filing date: 1990-06-01
Publication date: 1990-12-13

Abstract

A system for generating a powerful, scaleable phase-locked, and steerable laser beam (9), by paralleling together rod laser amplifier chains (6) driven from a common rod or slab laser oscillator (1). The oscillator (1) and the parallel array of rod amplifiers (6) are coupled via bundles of single mode optical fibres (4) and excited with the optical output of arrays of semiconductor light sources (3). These are coupled to said rod oscillator and paralleled rod amplifiers via bundles of optical fibres (2, 8).

Description

DIODE PUMPED, FIBRE BUNDLE COUPLED LASER OSCILLATOR, PHA

LOCKED LASER AMPLIFIER SYSTEM

Field of the Invention

This invention relates to a system for generating a powerful, scaleable, phase-locked and steerable laser beam, by paralleling together rod laser amplifiers driven from a low power, diffraction limited output beam, common oscillator, said oscillator and parallel array of rod amplifiers being laser beam coupled via single mode optical fibres and excited with the optical output of arrays of semiconductor light sources, themselves coupled to both of the said rod oscillator and paralleled rod amplifiers via bundles of optical o fibres.

The output faces of said laser amplifier rods forming the output aperture of the said phase-locked array of paralleled rod amplifiers have a diameter which is comparable to their separation from each other. s Said paralleled rod faces may be packed into a hexagonal pattern uniformly across the output aperture of the invention with the centres of said rods being equidistant, or into groups such that the diameter of each group of said rod amplifier ends are comparable to their separation. ⁰ The invention can be scaled to high power output levels simply b adding more rod laser amplifier modules to said parallel array of sai rods.

The rods used in the invention can be replaced with slabs of las media to produce a paralleled array of slab faces as the phase-locked ⁵ output aperture, the single, phase-locked output beam being

SUBSTITUTE SHEET • 2 -

equivalent in power to the rod format of the invention for a given area of the output aperture.

The invention has applications in the industrial, medical and defence fields where scaleable laser beam powers are required in high quality laser beams.

Summary of the Prior Art

Prior art rod and slab laser oscillator-amplifier systems have in general being of the seriesed format where the laser beam was generated in an oscillator and used directly, or amplified in a series of amplifier rods or slabs so that the said laser beam was amplified in stages, passing from one amplifier stage to the other in the process.

Although large laser systems used in laser fusion studies did involve parallel amplifier chains, their outputs were independent of each other and were not phase-locked into a single beam. Also, these prior art systems suffered from being very large, limited to a few shots per hour at best, and were extremely sensitive to vibrations due to their size and distribution. Such prior art systems were limited to pulsed operation and did not possess a beam steering capability. This invention overcomes the defects of prior art rod and slab oscillator-amplifier systems by providing a compact, scaleable, phase-locked single beam generator. Each of the individual amplifier modules of the invention is compact and can thus be fully optimised relatively simply in terms of its excitation, cooling, rigidity, and laser beam path alignment.

The present invention is also superior to prior art systems in that it can be scaled as a continuous beam oscillator system. The output beam of the present invention can be steered by adjusting the phases of the individual rod and slab transmitters.

Background of the Invention

There are many applications of lasers which demand very high peak powers and high continuous wave powers. Experience has shown that it soon becomes impractical to simply add amplifiers of increasing diameters in long, expensive chains as has been done, for example, in large laser fusion and defence related laboratories. In particular, industrial applications demand powerful, but compact laser beam generators.

As early as 1963, whilst working on Laser Radar at the Royal Radar Establishment, Malvem, UK, I began to pioneer the use of phase- locked arrays of laser amplifiers to produce scaleable and steerable laser beam outputs. This invention aims to use a fully optimised rod or slab laser amplifier module, in as compact a form as possible, and use them in phase-locked arrays to scale laser beam output powers under both continuous wave and pulsed output operating conditions. By generating the initial laser beam in a single master oscillator and feeding portions of it along single mode optical fibres to a scaleable array of diode pumped rod or slab laser amplifiers, a single, scaleabl and beam steerable output laser beam can be produced from a very compact system which is more characteristic of a shoe box than a vast laser fusion laboratory. From the commercial viewpoint, my invention can be produced from only five key components, namely, laser crystals, laser diode pump arrays, optical fibres, precision optical components and compact power supplies. In this way, powerful, scaleable. and beam steerable laser beam generators can be produced from a narrow manufacturing base of only five key components yet the resulting product can address large segments of the world market for high power lasers and laser systems, in particular the industrial and defence markets where compact, portable and scaleable laser beam generators with long operating lifetimes and minimal servicing requirements are in increasing demand.

The invention also offers an attractive choice to researchers requiring an affordable but scaleable source of high quality laser beams. In the past, anyone who needed access to a powerful laser beam was forced to secure very expensive operating time on large, laser fusion type facilities whose output beams were invariably far from optimum as far as the requirement of a particular researcher was concerned. This invention, offers the lone researcher access to very high quality laser beams which can be frequency tuned across the electromagnetic spectrum at a price they can afford. If the power output is not quite high enough in a given year, a few more modules can be purchased the following year, so the output beam can be effectively scaled without upsetting national budgets as has been the past experience with national laser facilities requiring approval for hundreds of millions of dollars and also requiring large numbers of technicians to maintain them. This invention puts powerful, scaleable laser beams into small budget laboratories where they can be used most effectively in as wide a range of experiments as possible. Summary of the Invention _*

It is an object of the invention to provide a scaleable power laser beam output using a phase-locked array of compact rod laser amplifiers whose end faces form the single output aperture. Another object of the invention is to provide means of adjusting the relative phases of the scaleable output beam so that the said output beam can be steered without the use of any moveable optical components.

It is an object of the present invention to maximise the output power of rod laser amplifiers consistent with the highest quality output laser beam from said rod amplifiers and to combine the outputs of an array of said amplifiers into a single laser beam. Another object of the invention is to optimise the output characteristics of single slab laser amplifier and then to combine the output beams of a phase-locked array of said slab amplifiers so as to produce a single output laser beam which can be steered via the appropriate changes in the phase of the laser beams emitted by the individual slab lasers forming said array.

It is an object of the invention to space the individual rod or slab amplifier modules of the scaleable array of said modules so as to maximise the space between said modules consistent with the achievement of a coherently phase-locked output beam across the whole of the said array aperture, thus facilitating both the excitatio and cooling of said amplifier modules. Another object of the invention is to provide a mix of rod and slab laser modules in a phase-locked array of said modules.

It is also an object of the invention to provide the laser output beams of individual laser amplifier segments of the array of said amplifiers with an intensity profile consistant with the intensity profile characterising the phase-locked output beam emitted by the array as a whole.

Brief Description of the Drawings A better understanding of the invention may be obtained from the following considerations taken in conjunction with the drawings which are not meant to limit the scope of the invention in any way.

Figure 1 shows a cross-sectional layout of the invention with the laser oscillator fibre coupled from a remotely sited semiconductor light source supply which also provides fibre coupled excitation energy to the array of amplifiers the end surfaces of each individual amplifier forming part of the array of said end surfaces which in turn form the phase-locked output aperture of the invention. The oscillator and amplifier sections of said invention are coupled via a bundle of single mode optical fibres, the ends of selected fibres being coupled to a selected amplifier input face of the optically polished end surfaces of the rods or slab segments. It should be noted that the ends of the fibres connecting the excitation diode arrays to the amplifier array need not be terminated along the circumference of the array but could penetrate the said array of amplifiers and be used to side excited individual rods or slabs.

Figure 2 shows the output array formed from the end faces of laser amplifier rodS_'.of circular cross-section.

Figure 3 shows the output array formed from end faces of laser amplifier slabs of elliptical cross-section.

Figure 4 shows a schematic layout of a segment of the invention in a beam steering mode with the phases of the laser beams traversing different rod amplifier segments being adjusted so as to steer the output beam in two mutually perpendicular directions.

Detailed Description of the Drawings

In Figure 1 , numeral 1 indicates a fibre coupled, diode pumped rod or slab laser oscillator with the coupling fibre bundle indicated by numeral 2 whilst numeral 3 indicates a semiconductor diode array optical excitation source. The output beam of laser oscillator 1 is coupled to the amplifier array via a bundle of single mode optical fibres indicated by numeral 4. Numeral 5 indicates the optical lens system used to match the output from the end of the individual fibres of fibre bundle 4 into the laser amplifier chains indicated by numeral 6. Numeral 7 indicates the hollow tubes used to connect the ends of laser amplifier sections in amplifier chain 6 and prevent the cooling fluid from interfering with the propagation of the laser beam through the said amplifier chains 6. Numeral 8 indicates optical fibres used to transport excitation energy from source 3 to laser amplifier chains 6. Numeral 9 indicates the phase-locked output beam of the invention, a single beam formed by phase-locking together all of the oirtputs of the amplifier chains of said array of said chains. The invention is enclosed in the casing indicated by numeral 10. Numeral 11 indicates the cooling fluid which circulates through said array of amplifier chains 6.

Numeral 12 indicates the frame into which the segments of said amplifier chains 6 are sealed and interconnected via tubes 7. Numera 13 indicates the seals used to seal the ends of said laser amplifier segment end into frame 12, said output end being indicated by numeral 14. In Figure 2, numeral 15 indicates the end face 14 of circular cross-section.

In Figure 3, numeral 16 indicates the end face 14 of elliptical cross-section. In Figure 4, numeral 17 indicates the single mode optical fibres connected to phase shifters indicated by numeral 18. Numeral 19 indicates the phase-locked outputs with a tilted wavefront resulting in steered output beam indicated by numeral 20.

The invention has application in material processing and laser radar systems. The oscillator and amplifier gain media may be of any material that can be excited with the optical output of semiconductor light sources.

Claims

- S -I claim,

1. A semiconductor optical diode pumped, rod laser oscillator, phase-locked rod amplifier array, scaleable and beam steerable laser system consisting of: (a) A diode pumped, fibre bundle coupled rod laser oscillator whose output face is optically matched into the input face of a bundle of single mode optical fibres, said rod laser oscillator operating at low power, and emitting a diffraction limited output beam and is fluid cooled, with said fibre bundle conveying optical energy which matches the absorption bands of said laser rod, said optical energy being generated in a remotedly sited power supply, (b) A phase-locked bundle of single mode optical fibres whose input end is compacted into an optically polished face and positioned in front of the output face of said rod laser oscillator such that the diffraction limited output beam of said rod oscillator is matched into the said aperture face of the said optical fibre bundle, the opposite ends of the said fibres being optically connected to its respective rod laser amplifier module forming ' part of a phase-locked array of such modules. o (c) A phase-locked, paralleled array of rod laser amplifiers which are optically excited via bundles of fibres connecting a remotely sited optical power supply which generates optical energy in a bandwith which matches the absorption bands of the said laser rods in said array leading to its laser beam amplification 5 properties.

2. A system as claimed in Claim 1 where the laser rod is operted a a single frequency.

3. A system as claimed in Claim 1 where the said input face of said single mode fibre bundle coupler consists of a phase-locked array of single mode optical fibres whose core diameters is comparable to the thickness of their cladding this providing the largest possible core area to said incident diffraction limited laser output of the rod laser oscillator.

4. A system as claimed in Claim 1 where the diameter of the laser amplifier rods forming said array of said rods are comparable to their separation. ι o

5. A system as described in Claim 1 where the laser amplifier rods forming said phase-locked rod amplifier array are grouped together such that the separation of the amplifier rods forming the said group are comparable to their diameter whilst the separation of the said groups from each other are comparable to their diameter.

15 6. A system as described in Claim 1 where the ends of the optical fibre bundle couplers connecting the amplifier rods to said remotely sited optical power supply are used for the side excitation of said rod amplifiers forming said rod amplifier array.

7. A system as claimed in Claim 1 where laser beam phase shifters 20 insterted into the path of each laser beam undergoing amplification such that the phase of the individual laser beam can be adjusted so as to steer the phase-locked output beam of the invention in a desired direction.

8. A system as described in Claim 1 where some of the rod

25 amplifier modules are replaced with slab laser amplifier modules.

9. A system as claimed in Claim 1 where all of the rod amplification modules are replaced with slab amplifier modules.

10. A system as claimed in Claim 1 where the phase-locked output beam can be increased in power by adding more amplifier modules to the phase-locked rod amplifier array.