Coupled Solid State Laser

Field of the Invention

This invention relates to an optically switched, fluid cooled, fibre coupled, diode pumped solid state laser system which can be frequency stabilised, consisting of three sections, two of which are connected together for safety reasons whilst the third can be detached from the other two, said system having the first section in the form of a portable, fluid cooled head containing an optically switched laser rod, which is permanently connected to the second section which is in the form of a flexible umbilical cord containing 0 an optical fibre bundle which conveys the excitation light for said laser rod, cooling fluid tubes to cool said laser rod and, when frequency stabilised operation is required, a single mode optical fibre to convey the stabilised frequency from the remotely sited oscillator to the said rod laser amplifier head. The umbilical cord 5 connects said head section to a remotely sited service module which contains the diode arrays for generating the excitation light that is conveyed to excite the laser rod via the fibre bundle, the cooling fluid unit, the switchable power supply to drive the diode arrays and the stabilised oscillator when the system is operated in 0 the frequency stabilised mode.

The invention has applications in the industrial, medical and defence fields, in the industrial field the applications of the invention include laser marking and laser scribing. In the medical field the applications include nerve fusion, laser surgery and laser therapies. In the defence fields the applications include laser

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range finding, target designation and communications both on land and underwater.

Summary of the Prior Art

End pumped, fibre bundle coupled rod lasers have been pioneered by the inventor (see for example, Hughes, US Patent No. 4,757,515, issued July 1988).

The switching of this type of prior art laser has been accomplished using miniature electro-optic switches placed within the optical head. Such electro-optic switches have limited pulse repetition rates which are usually well below 1 ,000 Hz and require electrical leads connected to the heads from a remotely sited electrical power supply.

Prior art frequency stabilised solid state lasers were of the oscillator configuration and were of limited power output capability.

Prior art end pumped, fibre bundle coupled lasers did not allow for the profiled excitation of the rod laser medium.

The present invention overcomes the defects of prior art, diode pumped, fibre coupled rod lasers in that the switching of the output beam is achieved optically without the need for the insertion of electro-optic switches in the laser head section. The head of the present invention is fluid cooled via a closed looped cooling circuit passing through the said head and connected to a remotely sited cooling unit via the umbilical cord. The rod of the present invention is excited in any desired excitation profile within the capability of the light outputs of the diode arrays within the remotely sited

optical power supply. For example, if one requires a ^* Gaussian ^v intensity distribution of the output laser beam then the rod has to be excited with an excitation light beam of Gaussian distribution emitted by the fibre bundle coupler said excitation light propagating along the rod before being absorbed by the laser ions ¹ the required Gaussian profile. On the other hand, if we require a "flat topped" intensity distribution of the output beam, then a "flat topped" intensity distribution of the light output of the fibre* bundle coupler is required and produced via the appropriate adjustment of the intensity of the light output of the diode pump arrays in the remotely sited optical power supply.

The present invention also differs from the prior art in that the laser rod in the said head is also side excited, which is particularly effective when operating in the stabilised frequency mode or at high power levels where the rod has to be ^* strongly excited well above the limits available through end pumping.

Summary of the Invention

It is an object of the invention to provide a compact, minimally heated laser rod head which allows for both the end and sictø - excitation of laser rods to produce high quality output beams, the

•f minimal heating of said head being achieved by coupling the heat generating optical diode arrays from a remote site to said head via a low loss optical fibre bundle. ^** * .-

Another object of the invention is to provide an intensity distribution of the excitation light emitted by the fibre bundle couplers which generates a distribution of excited laser ions

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within said laser rod which is comparable to the intensity distribution of the emitted laser beam.

It is an object of the present invention to amplify low power, frequency stabilised laser beams generated at a remote site and conveyed to the rod amplifier head via a single mode optical fibre. Another object of the invention is to firmly couple the rod laser head and the optical fibre bundle coupler but allow for the decoupling of the said fibre bundle coupler from the remotely sited optical power supply and cooling unit. In this way there is minimal danger of the emission of laser excitation light from the fibre bundle coupler.

It is an object of the invention to optically switch the output of the laser rod so that the output beam is modulated at the same rate and form as the excitation light into the rod which in turn is modulated at the same rate and with the same emission envelope as that of the diodes forming the diode array within the remotely sited optical power supply. The shorter the excitation light pulse, the shorter the rod laser output pulse.

Another object of the invention is to produce output pulses at high repetition rates exceeding 1000 Hz by switching the laser diode arrays in the remotely sited optical power supplies at these high rates.

It is an object of the invention to produce diffraction limited laser output beams in excess of 1 watt continuous power. Another object of the invention is to produce diffraction limited output beams of stabilised frequencies.

Brief Description of the Drawings

A better understanding of the invention will be gained from the following description taken in conjunction with the accompanying drawings. It is emphasized that the ensuing teachings are exemplary and not limitative of the scope and the applicability of the invention.

In the drawings:

Figure 1 is a schematic layout of the invention with its three main components, namely, a portable laser beam generating head fibre coupled to a remotely sited optically power supply/fluid cooling module. It should be noted that the umbilical connecting tube is detachable from said remotely sited optical power supply/fluid cooling module, said module containing the switchable, arrays of laser diodes whose optical outputs match the optical absorption bands of the laser rod and which can be selectively switched either simultaneously or sequentially to bring said laser beam generating medium above its lasing threshold with a series of increasingly shorter duration excitation pulses, the shortest duration pulse bringing the said laser medium rapidly above its lasing threshold resulting in a short duration laser beam output pulse.

Figure 2 is a schematic layout of the optically switched laser beam generating head of the invention and the interconnecting umbilical connector which is detachable from the remotely sited optical power supply/fluid cooling module (not shown).

Figure 3 is a schematic layout of the laser beam generating head used for high power laser beam generation via both end and

side excitation of the laser medium which may be of any solid format including a rod and slab format as known in the art.

Figure 4 shows a schematic layout of the cross-section of a side pumped laser rod of the invention where the annular 45° reflector surrounding the rod directs the fibre bundle delivered pump light into said rod.

Figure 5 shows a schematic layout of the cross-section of a side pumped laser rod of the invention where a 180° degree annular reflection reflects and cylindrical lenses axially focus the output of the fibre bundle couplers into the said rod such that said excitation light which is not absorbed is reflected back into said rod off a reflector positioned around half of the circumference of said rod.

Figure 6 is a schematic layout of the stabilised frequency configuration of the invention showing its three sections, namely, the fluid cooled amplifier head, detachable umbilical cord and remotely sited service module containing the frequency stabilised oscillator.

Figure 7 is a schematic layout of the amplifier head showing the fibre coupling to both the remotely sited, stabilised laser beam generator and the diode pump array (not shown).

Figure 8 is a schematic layout of the double-pass amplifier configuration of the said head of the invention.

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Detailed Description of Drawings

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In Figure 1 , numeral 1 indicates the outer, tubular thermally and electrically insulating casing which, in a preferred configuration, consists of two identical halves sealed* together, 5 whilst numeral 2 indicates the umbilical cord which connects head

1 to a remotely sited optical power supply/cooling unit indicated by numeral 3. Numeral 4 indicates a solid state laser medium which can take the form of a rod or slab as known-^n the art.

^*** *_ Numeral 5 indicates the thinly clad optical fibre bundle used to

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10 transport the excitation radiation for medium 4 frcrra the remdfely sited optical power module 3 to head 1 the ratio of fibre cladding ^* cross-sectional area to fibre core area at the input end into , ^* medium 4 being as small as possible to ensure" maximum transfer efficiency of the excitation light from fibre bundle 5 into rod 4, in

15 a beam which is as parallel as possible to the axis of medium 4 along which the laser beam indicated by numeral 6 is propagating. It should also be noted that the excitation light is trapped in

*_* medium 4 by critical angle reflection and reflections off the appropriately coated end face of said medium.

20 Numeral 7 indicates the electrical power supply for the laser diode array indicated by numeral 8 which generates the opticaf excitation energy which matches the absorption bands of laser medium 4. Numeral 9 indicates the ends of the optical fibres forming bundle 5 which are optically matched to the emission

25 apertures of diode array 8 so as to ensure the best possible optical transfer efficiency from said diode aperture into said fibre cores.

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Numeral 10 indicates the fluid recirculating and cooling unit with numeral 11 indicating the outflowing cooling fluid to laser head 1 via umbilical cord 2 and numeral 12 indicates the return path from head 1 to unit 10 for said heated fluid. In any long length of umbilical cord 2 it is important to thermally insulate channels 11 and 12 from each other to prevent their temperature equalization.

In Figure 2, numeral 13 indicates the detachable joint which allows umbilical cord 2 to be separated from module 3. Numeral 14 indicates the inner tube casing of head 1 of the invention which houses laser medium 4 and the end of both fibre bundle 5 and outlet 11. Numeral 15 indicates the "O-rings" used to locate inner tube 14 within head 1 and to seal the decoupling joint 13, whilst numeral 16 indicates channels in the body of 1 which allows the cooling fluid for medium 4 within inner case 14 to be recirculated back to unit 10 via umbilical 2. Numeral 17 indicates a 100% laser beam reflecting mirror on the optically polished end of rod 4 whilst numeral 18 indicates a partially transmitting laser beam reflecting mirror through which laser beam 6 is emitted without infringing onto the output tip of 1 indicated by numeral 19. Numeral 20 indicates a groove for locating detachable optical components such as lenses and frequency converters onto body 1. Numeral 21 indicates a recess in the near portion of outer casing 1 which allows an anchoring piece to be located around umbilical cord 2 to prevent it moving relative to head 1 to which it is permanently attached.

In Figure 3, numeral 23 indicates an inner optical fibre bundle which conveys light from unit 3 for the side excitation of medium 4. Numeral 24 indicates a similar optical fibre bundle containing the same number of fibres but is of larger diameter than 23. Both fibre bundles 23 and 24 are connected to the annular optical couplers indicated by numeral 25 which direct the outputs of 23 and 24 into laser medium 4 to uniformly excite said laser medium. By combining fibre bundle 5 and bundles 23 to 24 for the optical excitation of medium 4 it can be achieved at high power levels, levels which imply that a greater amount of heat will be generated in medium 4 and will have to be removed via fluid flow 11 which is returned into unit 3 via flow 12 between the inner cylinder 14 (not shown) and outer casing 1. In addition to providing more excitation power for medium 4, fibre bundles 23 to 24 via optical couplers 25 allow the laser medium to be sequantially excited via a seris of diode light pulses from array 8 to bring said medium above lasing threshold, said sequence of excitation pulses being of increasingly shorter duration. For example, the first excitation pulse into media

4 could be of one hundred microseconds (10-6 seconds) duration whilst the last excitation pulse, within the first pulse duration, being of only a few nanosecond (10-9 seconds) duration. In this way, laser media 4 and hence the invention itself can be optically switched at very high repetition rates corresponding to the rapid switching periods of diode array 8. If diode array 8 is rapidly pulsed in a single pulse, then the output beam 6 from laser media 4 will be of the same format. If diode array 8 is operated quasi- continuously then output beam 6 will be emitted in a quasi-

continuous mode. The invention can, therefore, emit a pulsed output laser beam 6 without the use of complex and pulse rate limited electro-optic switches of prior art systems. Furthermore, such pulsed output laser beams can be emitted from the invention at high mean power levels.

In Figure 4, numeral 4 indicates the laser rod cross-section whilst numeral 25 indicates the 45° annular reflector used to direct the output of the fibre bundles used to convey the excitation light from a remotely sited optical power supply into said laser rod.

In Figure 5, numeral 26 indicates cylindrical lenses positioned parallel to the axis of the laser rod so as to focus the output of the fibre bundles conveying the excitation light from the remotely sited optical power supply to said laser rod, said focussed light, not absorbed being reflected off the half circular reflector indicated by numeral 27 back into said rod so that it can be further absorbed.

In Figure 6, numeral 28 indicates a stabilised frequency laser oscillator which emits a beam which matches the amplification characteristics of rod 4 of the invention. Numeral 29 indicates a single, single mode optical fibre which conveys the output of oscillator 28 to amplifier rod 4 into output end indicated by numeral 30 being matched into rod 4 via the lens indicated by numeral 31. The laser beam is then amplified via a single lens rod 4 to produce stabilised output beam 6.

In Figure 7, numeral 32 indicates a phase-locked fibre bundle which conveys the laser beams to be amplified in the invention

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from a remotely sited laser oscillator and whose output laser beam indicated by numeral 33 is matched to the laseτ amplifier rod via the telescope indicated by numeral 34. ^{' *}

In Figure 8, numeral 35 indicates a beam reflector which turns ' the input laser beam 33 into the polarizer indicated by numeral 36 then through the quarter wave plate indicated by numeral 7 into rόύ

4 where it undergoes a double pass amplification to emerge from

^* the said head as output laser beam 6.

The invention has application in the industrial, medical and s defence fields where a powerful, switched, portable, high quality, laser beam generating head is required. The head 1 of the in-vintfbn can be mounted onto robotic arms and is deal for use in the scribing of semiconductor chips. Any type of solid state las§r medium 4 can be used in the invention for which matching pump diode arrays exist. _* .