WO1990016101A1

WO1990016101A1 - Double slab, diode pumped fibre bundle coupled solid state laser

Info

Publication number: WO1990016101A1
Application number: PCT/AU1990/000259
Authority: WO
Inventors: John Leonard Hughes
Original assignee: Australian Electro Optics Pty Ltd
Priority date: 1989-06-14
Filing date: 1990-06-14
Publication date: 1990-12-27

Abstract

This invention relates to a double slab (1) laser system, fibre bundle (9) coupled to a remotely sited optical power supply (10), said system consisting of two slabs (8) of a given laser medium positioned parallel to each other and separated from each other such that a laser beam (3) propagating between said slabs in a 'zig-zagged' optical path through a plane parallel to both slabs, is reflected through said plane off mirrors (2) on the outside, optically polished faces of each of the said slabs, the inside, anti-reflection coated faces (4) of said slabs allowing for the propagating laser beam (3) to enter and leave each of the said slab laser media so that said laser beam can be amplified as it propagates between said slab pair to emerge from the invention at increased power (7).

Description

- 1 -

Double Slab, Diode Pumped Fibre Bundle Coupled

Solid State Laser

Field of the Invention

This invention relates to a double slab laser system, fibre bundle coupled to a remotely sited optical power supply, said system consisting of two slabs of a given laser medium positioned

5 parallel to each other and separated from each other such that a laser beam propagating between said slabs in a "zig-zagged" optical path through a plane parallel to both slabs, is reflected through said plane off mirrors on the outside, optically polished faces of each of the said slabs, the inside, anti reflection coated faces of ι o said slabs allowing for the propagating laser beam to enter and leave each of the said slab laser media so that said laser beam can be amplified as it propagates between said slab pair to emerge from the invention at increased power.

Both of the said slabs of the invention are optically excited via

15 fibre bundle couplers connected to remotely sited optical power supplies said fibre bundles being positioned relative to said slabs such that the excitation light emitted from the ends of the said fibre bundles enter the slabs via the smaller face pairs so as to confine said excitation- light within said slab via critical angle 0 reflections wherever possible. The slabs are fluid cooled around the open edges and may be Peltier cooled through the larger, mirrored faces.

The invention can be optically switched by switching the pump light and has applications in the industrial, medical and defence 5 fields where either a continuous wave or pulsed laser beam is required.

The mirrors on the outside faces of the slabs of the invention may be positioned away from said faces without affecting the operation of the invention if the optical losses suffered by the passage of the said laser beam through the said slabs are minimal which would be the case, for example, when the said laser beam enters and leaves the slabs at Brewster's angle.

Summary of the Prior Art

Prior art slab laser systems have been of two main types, namely, the "internal cavity" typed pioneered by the General Electric Company of the United States, where the laser beam is amplified along a "zig-zagged" path within the said slab laser medium and the external cavity slab laser pioneered by the inventor, where the laser beam "zig-zagged" partly within the said laser medium and partly outside said medium so as not to limit the size of the laser beam being amplified to the smaller dimensions of the slabs and also to reduce the self-focusing path within said slabs.

Prior art slab laser systems were optically excited via their mirrored surfaces, through their largest optically polished faces. Prior art slab laser systems suffered because the volume of the slab laser medium was limited due to both cooling and self- focusing problems and their active mirror excitation.

This invention overcomes the defects of the prior art slab laser systems in that more laser gain medium is provided without undue limitations on the size of the laser beam undergoing amplification.

This invention also overcomes the problems of active mirror optical excitation of the slab by fibre bundle coupling through the smaller end faces and confining the pump light within the said

5 slabs via critical angle reflections until absorbed within said slabs. A unique advantage of the present invention is the fact that it has a much smaller volume of slab laser medium for a given power output than was the case with prior art systems. In fact, for a given volume of slab laser medium the present invention has up to ι o twice the power output capability of prior art systems, particularly at high peak power output levels.

Background of the Invention

During the early 1960's, it soon became clear that laser rods could not be scaled to higher output power levels independently of 5 the detrimental effects of thermal lensing arising from the fact that the axis of the rod could not be cooled as effectively as its surface, a situation resulting in the output faces of such laser rods having a convex curvature at elevated temperatures, that is, when operated at high continuous wave or high mean power levels. o Firstly, one could slice the rod perpendicular to the direction of propagation of the laser beam through the said rod, resulting in the slab laser development avenue.

Alternatively, we could effectively slice the rod parallel to the direction of propagation of the laser beam through the said rod 5 resulting in the fibre bundle laser development avenue. These two approaches were pioneered by the inventor, initially at the Royal Radar Establishment, Malvern, United Kingdom and later at the Weapons Research Establishment, Salisbury, South Australia. The first publication to emerge on slab lasers by the inventor was in Applied Optics, USA, August 1967 whilst the first on fibre bundle lasers appeared in Applied Optics, USA in July 1979.

The slab laser is far more effective regarding thermal lensing than are rod lasers, whilst the problem is effectively eliminated in fibre lasers due to the very small core diameter of the core of optical fibres. ι o The slab laser offers a different avenue to laser beam generation than that offered by both rod and fibre lasers and the inventor pioneered the avenues which resulted in large, folded path, exponential slab amplifiers, and compact, external cavity slab lasers.

15 This invention provides a double slab external cavity laser oscillator and amplifier system where an effectively cooled double slab laser system can enhance the laser beam generating properties of slab lasers using a compact configuration, without incurring the defects of prior art single slab systems with the same laser

20 medium volume.

The laser beam being amplified by the invention can be amplified further by passage through another configuration of the invention with larger slab dimensions. However, the basic double slab unit remains of the same configuration except that in an

25 oscillator mode, two resonator mirrors are added to define the ends of said resonator. Summary of the Invention

It is an object of the invention to generate a high power, high quality laser beam in* a slab laser system which optimises both the slab and laser beam reflector configuration. Another object of the invention is to provide a slab laser system which is optically excited with excitation light which matches the absorption bands of said slab laser medium and is confined within said slabs via critical angle reflectors until absorbed. It is an object of the invention to optically excite a slab laser system such that the laser beam output profile follows that of the excitation light.

Another object of the invention is to provide fluid cooling of the slab segments via surfaces not utilised for either the optical excitation of the slab or the reflection of the laser beam to be amplified.

It is an object of the invention to avoid critical angle reflections of the laser beam undergoing amplification with said slabs. Another object of. the invention is to minimise optical losses of the laser beam undergoing amplification by directing said laser beam in and out of said slabs at Brewster's angle.

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 schematic layout of the invention with two slabs of laser media positioned parallel to each other, and excited via optical fibre bundles which couple them to remotely sited 5 optical power supplies. The laser beam to be amplified is "zig¬ zagged" between mirrors on the rear faces of both slabs which are fluid cooled along a pair of side faces and Peltier or fluid cooled over the largest pair of faces.

Figure 2 shows a schematic layout of the invention with the ι o mirrors used to "zig-zag" the laser beam path displaced from the rear surfaces of the slabs with the laser beam undergoing amplification entering and leaving the slab at Brewster's angle.

Figure 3 shows a schematic layout of a water cooled slab segment of the invention which is end excited via optical fibre 15 bundles connected to a remotely sited power supply.

Detailed Description of the Invention

In Figure 1 , numeral 1 indicates the laser medium slabs whose optically polished rear surfaces are mirrored as indicated by numeral 2 to fully reflect the laser beam undergoing amplification 20 indicated by numeral 3, out of the said slab 2 via its optically polished and anti-reflection coated inner surface indicated by numeral 4. The angled mirror indicated by numeral 5 directs beam 3 into slab 1 via the inner surface 4 whilst the reflector, indicated by numeral 6 directs the amplified output laser beam indicated by 25 numeral 7 out of the invention.

In Figure 1 , numeral 8 indicates the Peltier cooling stacks used to cool slabs 1 whilst numeral 9 indicates the optical fibre bundles used to convey the excitation light for slab 1 from remotely sited optical power supplies indicated by numeral 10 which generate excitation light which match the absorption bands of slab media 1.

5 In Figure 2, numeral 11 indicates the slab laser media whilst numeral 12 indicates the laser beam reflectors displaced from said slabs 11. Numeral 13 indicates the end section of the fibre bundles used to convey the excitation light from a remotely sited optical power supply (not shown). Numeral 14 indicates the input laser ι o beam which is incident on slab media 11 at Brewster's angle to minimise reflection losses whilst numeral 15 indicates the amplified output laser beam. When used as a laser oscillator, the invention has an optical resonator defined by the 100% reflectin mirror indicated by numeral 16 and a partially transmitting mirror

15 indicated by numeral 17. These mirrors are removed when the invention is used as an amplifier.

In Figure 3, numeral 18 indicates the largest face of the slab laser medium whilst numeral 19 indicates the fibre bundle coupler used to convey the excitation light for slab 18 from a remotely o sited optical power supply generating light which matches the absorption bands of slabs 18 and indicated by numeral 20.

The invention has applications in industrial, medical and defence laser fields where laser beam sources are required which are compact, highly efficient and optically excitable to high power 5 levels for continuous and pulsed output beams.

The invention can be pulsed and optically switched via its excitation light at high repetition rates because the output beam profile in the oscillator configuration follows the profile of the excitation light. When the invention is immersed in a Raman gas, for example methane or deuterium, then the laser output beam can be frequency shifted.

Claims

I claim,

1. A double slab laser amplifier system, which is fibre bundle coupled to a remotely sited optical power supply, said system consisting of two slabs of a given laser medium positioned parallel

5 to each other and separated from each other such that a laser beam propagating between said slabs in a "zig-zagged" optical path through a plane parallel to both of the slabs, is reflected through said plane off mirrors on the outside, optically polished faces of each of the said slabs, the inside, anti-reflection coated faces of ι o said slab allowing for the propagating laser beam to enter and leave each of the said slab laser media so that said laser beam can be amplified as it propagates between said slab pair to emerge from the invention at increased power.

2. A system as claimed in Claim 1 , where the excitation light

15 entering the said slabs from such fibre bundle couplers is confined within said slab via critical angle reflections until absorbed in said slab laser medium.

3. A system as claimed in Claim 1 where the excitation light is pulsed such that the output beam pulsed envelop follows that of the o excitation pulse.

4. A system as claimed in Claim 1 where one of the largest, optically polished faces of the said slabs are mirrored to reflect the laser beam 100%, the other large face anti-reflection coated at the laser beam wavelength, whilst the smallest face pairs are anti- 5 reflection coated at the pump wavelength and accept the output of the fibre bundle couplers whilst the third face pairs are water cooled to extract the heat generated from the non-radiative decays inside the said activated slab laser medium.

5. A system as claimed in Claim 1 where the mirrored faces are fluid cooled.

6. A system as claimed in Claim 1 where the mirrored faces are Peltier cooled.

7. A system as claimed in Claim 1 where no laser beam reflection depends on a critical angle reflection with said slab laser media.

8. A double slab laser amplifier system which is fibre bundle ι o coupled to a remotely sited optical power supply, said system consisting of two slabs of a given laser medium positioned parallel to each other and separated from each other such that a laser beam propagating between said slabs in a "zig zagged" optical path light on plane parallel to both of the slabs, is reflected through said

15 plane off mirrors displaced from and set parallel to the outside, optically polished faces of each of the said slabs, both of the optically polished faces of said slab traversed by the said laser beam being anti-reflection coated to minimise optical losses, so that the said laser beam can be amplified as it propagates along

20 said slab pair to emerge from the invention at increased power.

9. A system as claimed in Claim 8 where the laser beam to be amplified is incident on the said slabs at Brewster's angle to minimise optical reflection losses when the said slab surfaces are not anti-reflection coated.