WO1991006139A1

WO1991006139A1 - Diode pumped segmented fibre bundle coupled conical rod laser system

Info

Publication number: WO1991006139A1
Application number: PCT/AU1990/000499
Authority: WO
Inventors: John Leonard Hughes
Original assignee: Australian Electro Optics Pty. Ltd.
Priority date: 1989-10-20
Filing date: 1990-10-19
Publication date: 1991-05-02

Abstract

This invention relates to a conical rod laser medium (1) forming a semi confocal resonator between mirrors (2 and 3) with the sides of said conical rod (1) mirrored (5) to reflect the pump light thus trapping it within said conical rod. Fibre bundles (9) convey the optical outputs of remotely cited laser diodes (10) into the conical rod laser medium (1). The conical rod may be formed by spaced segments (13) which can be side pumped. The invention has applications in any field requiring a compact source of laser light with stable operating parameters and is particularly suitable for generating laser beams very highly absorptive laser media.

Description

Diode Pumped Segmented Fibre Bundle Coupled Conical Rod Laser System

Field of the Invention

This invention relates to a diode pumped, segmented fibre bundle coupled conical rod laser oscillator system consisting of a laser gain medium in the form of a conically shaped rod whose

5 smaller output end face is flat and optically polished whilst its larger end face is curved and optically polished, the side of said conical rod being coated to reflect the excitation wavelength, but being transparent to other wavelengths, said laser system being optically excited via the larger curved end surface with narrow ι o spectral bandwidth optical radiation emitted by an array of optical fibre ends of said segmented optical fibre bundle connected to a remotely sited optical radiation source whose optical radiation output has a narrow spectral bandwidth which matches the optimum absorption bands of the said laser gain medium. The laser

15 resonator of the invention is formed by a 100% reflecting mirror at the laser wavelength being attached to, or positioned near to, the said curved end surface of the laser gain medium and a flat, semi- transparent mirror, attached to or positioned near to the smaller output face of said laser gain medium. The absorption of the 0 excitation light and the cooling of the conical rod laser medium can be enhanced by the segmentation of said medium.

Summary of the Prior Art

Prior art diode pumped, fibre coupled solid state rod lasers did not direct the fibre bundle optical excitation output in such a manner as to confine said optical excitation radiation to the volume of the rod laser gain medium which sustained the lasing action when using a semi-confocal resonator, thus failing to maximise the excitation efficiency of the rod laser medium. Another defect of prior art rod laser systems was the fact that the excitation radiation was partially converted into other radiations within the said rod laser medium which could not selectively escape from within said medium. Another defect of the prior art diode pumped rod laser systems was the fact that their end pumping could not uniformly excite the whole of the ro when the . laser medium was highly absorptive. Also, a severe defect of prior art rod lasers was the fact that it was difficult to uniformly cool them, a defect which led to thermal effects whereby the rod ends curved, causing an increase in laser output beam divergence and a lower beam quality at higher power operating levels.

The present invention overcomes the defects of prior art rod lasers by using a conically shaped rod, which is optically excited via its larger, curved end face using an array of closely coupled optical fibre ends and coating the surface of said conical rod with a reflector which reflects any excitation light, which would otherwise escape from said conical rod, back into said rod, at the same time allowing any other incoherent radiation to escape from said rod. The invention overcomes the defects of prior art diode pumped rod lasers associated with high absorption of the excitation light and thermal lensing effects, by allowing for the segmentation of the said conical rod laser medium into easily coolable sections which can also be end pumped via optical excitation light emitted via fibre end faces of a segmented fibre bundle used to convey said excitation light from a remote site.

Background of the Invention

The tuneable, narrow band emissions of diode lasers offer the

5 best possible means of optically exciting solid state laser media whose absorption bands are of comparable optical bandwidth'. This prospect has been known in the art since the advent of the diode laser during the early 1960's. However, the output power of individual diode lasers is so small that is has taken over 25 years ι o of diode laser developments resulting in laser diode arrays before the output power of these semiconductor devices became adequate for the optical pumping of solid state lasers. Even today, the cost of laser diode pumps for solid state lasers is so high that care is required to optimise the volume of the laser medium being excited

15 with diode laser light and it is also necessary to remove the heat generating diodes away from as far as is possible from the laser medium being excited.

This invention introduces multiple fibre bundle coupling of the laser diode pumps into conical rods instead of cylindrical rods so o as to match the lasing volume and the excitation volume in a manner that also optimises the cooling of said conical rod laser medium at the higher power levels of its operation.

When the laser medium used is highly absorptive of the pump light, that is whenthe thickness of the medium being excited should 5 not exceed a few millimeters, then it is very convenient to segment the conical rod medium of the invention and also end pump each of the resulting segments via fibre bundle couplers between the said segments and the remotely sited laser diode pumps.

It is also convenient to use two types of optical excitation techniques with solid state lasers, namely, one source of

5 excitation light to raise the laser to near lasing threshold and a pulsed source of excitation light to bring it rapidly above its operaing threshold. One way of doing this is to provide several avenues whereby the pump light can enter the said lasing medium. For example, the threshold pumping can be achieved via continuous ι o optical pumping through one end of a rod laser whilst the pulsed pumping can be achieved through the other end.

Such diode pumped solid state lasers exhibit remarkable outpu frequency stability because of the minimal thermally induced distortions that can be achieved in relatively small volumes of the

15 lasing medium.

Summary of the Invention

It is an object of the invention to provide a conically shaped laser medium which is excited by a conical beam of excitation ligh which is coupled from a remotely sited optical power supply by 2o multiple bundles of optical fibres whose output end faces are arranged around the larger, curved end face of said conical rod medium so as to produce a convergent beam of excitation light.

Another object of the invention is to mirror the surface of sai conical rod so as to trap the converging excitation light but allow 25 all other light to escape from said rod. It is an object of the invention to provide threshold excitation using continuous wave excitation from one end of said control rod whilst using pulsed pump light from the other end.

Another object of the invention is to allow the use of highly 5 absorptive lasing media by segmenting said conical rod laser medium and also exciting said anti-reflection coated segments around their rims via excitation light converged from a remotely sited diode laser pump light source via a fibre bundle.

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

Figure 1 shows the layout of the invention with the 100% laser 5 reflecting mirror attached to the curved end face of the conical rod laser medium.

Figure 2 shows a layout of the invention with the 100% laser reflecting mirror separated from the curved end face.

Figure 3 shows the layout of the invention with the conical rod 0 laser medium segmented, each segment being optically polished and anti-reflection coated at the laser wavelength and end pumped via the optical output of a remotely sited, segmented fibre bundle coupled, optical power supply with a narrow spectral bandwidth which matches the absorption bands of 5 said segmented laser medium. Figure 4 shows the layout of the amplifier configuration of the invention with the input laser beam being converged to match the confocal resonator configuration of the invention using a polarizer and quarter wave plate to allow the amplified output beam to be 5 separated from the input beam.

Detailed Description of the Invention

In Figure 1 , numeral 1 indicates the conically shaped laser rod medium. Numeral 2 indicates the curved end surface of 1 which is covered by a 100% reflecting laser mirror with the lowest possible ι o transmission at the optical excitation wavelengths. The flat, partially transmitting output mirror of the invention is indicated by numeral 3 whilst numeral 4 indicates the flat output end of conical rod laser medium 1 which is mirrored 100% at the excitation light wavelength only. Numeral 5 indicates the

15 reflective surface of conical rod laser medium 1 which traps the excitation light within 1 whilst allowing any radiation likely to heat up rod 1 to escape. Numeral 6 indicates the laser beam within the half confocal resonator configuration defined by mirror 2 and output mirror 3. Numeral 7 indicates the diverging laser output

20 beam of the invention.

In Figure 1 , numeral 8 indicates the narrow spectral bandwidth excitation light which is closely coupled to rod 1 via the segmente fibre bundle indicated by numeral 9. Numeral 10 indicates the optical excitation sources with a narrow spectral bandwidth which

25 match the absorption bandwidths of laser medium 1. Numeral 11 indicates the power source for sources 10. ln Figure 2, numeral 12 indicates a 100% reflecting laser mirror which is separated from the curved end 2 of conical rod 1.

In Figure 3, numeral 13 indicates the segments resulting from the segmentation of conical rod 1 whose surfaces are anti-

5 reflection coated as indicated by numeral 14. Numeral 15 indicate the edge pumping of segments 13 with optical radiation conveyed from a remotely sited optical power supply similar to supply 10 (not shown) via a segmented optical fibre bundle.

In Figure 4, numeral 16 indicates the input laser beam for an ι o amplifier configuration of the invention as shown in. Figure 1. Inpu beam 16 is matched to the confocal configuration of the invention using the lens indicated by numeral 17, which transfers said laser beam 16 into the converging beam indicated by numeral 18. Numeral 19 indicates a polarizer/beam splitter which directs the 5 input laser beam to be amplified into conical rod 1 via the quarter wave plate indicated by numeral 20, where it traces out the laser beam indicated by numeral 21. On being reflected by the mirror on surface 2, the amplified laser beam then returns along its incident path, but with its plane of polarization rotated by 90% after 0 passing twice through quarter wave plate 20 to emerge as the diverging, amplified laser beam 22.

The . invention has application in the medical, industrial and defence fields where compact, flexible and powerful sources of laser light are required. The invention can operate in both the 5 continuous wave and pulse modes and can be pulsed by pulsing the optical excitation sources 10 without the need for electro-optics switches. The laser gain medium can take the form of a solid or a solid conductor of a fluid laser medium which can be optically excited.

Claims

claim,

1. A diode pumped, multiple fibre bundle coupled, conical rod laser system consisting of:

(a) A laser medium in the shape of a cone the base of which is

5 spherically curved, optically polished and mirrored to reflect^' the laser wavelength up to 100% and the excitation light wavelength down to 0%, the opposite, smaller end being flat, optical polished and mirrored to partially transmit said laser beam, with the side of said conical rod laser medium being ι o mirrored to reflect the pump light but transmit all other wavelengths, including the laser wavelength spontaneously emitted light.

(b) A multiple fibre bundle to convey the pump light from a remotely sited optical power supply to said conical rod laser 5 medium, the output ends of said multiple fibre bundles being compacted together to form a continuous output face such that the output pump light converges into a conical beam as it enters said conical rod medium through its spherically curved end face. o (c) An array remotely sited laser diodes with each thermally tuned diode emitting its output light into a particular bundle of fibres.

2. A system as claimed in Claim 1 where the smaller, flat output end is anti-reflection coated and positioned near the partially 5 transmitting mirror.

3. A system as claimed in Claim 1 where the conically shaped rod laser medium is segmented and additional fibre bundles direct pump light into said disc segments around the rim of said segments.

4. A system as claimed in Claim 1 where the laser medium is

5 brought up to lasing threshold via continuous wave excitation via the rear, spherically curved base of said core whilst pulsed excitation light is used to pump the said media above threshold via the smaller front face of said conical laser medium.

5. A system as claimed in Claim 1 where the laser medium ι o forming said conical rod is highly absorbing.

6. A system as claimed in Claim 1 whose dimensions and power output are small enough to maintain a single frequency output.