Beam Shaping in High-Power Laser Systems with Using Refractive Beam Shapers

Transcription

1 - 1 - Beam Shaping in High-Power Laser Systems with Using Refractive Beam Shapers Alexander Laskin, Vadim Laskin AdlOptica GmbH, Rudower Chaussee 29, Berlin, Germany ABSTRACT Beam Shaping of the spatial (transverse) profile of laser beams is highly desirable by building optical systems of highpower lasers as well in various applications with these lasers. Pumping of the crystals of Ti:Sapphire lasers by the laser radiation with uniform (flattop) intensity profile improves performance of these ultrashort pulse high-power lasers in terms of achievable efficiency, peak-power and stability, output beam profile. Specifications of the solid-state lasers built according to MOPA configuration can be also improved when radiation of the master oscillator is homogenized and then is amplified by the power amplifier. Features of building these high power lasers require that a beam shaping solution should be capable to work with single mode and multimode beams, provide flattop and super-gauss intensity distributions, the consistency and divergence of a beam after the intensity re-distribution should be conserved and low absorption provided. These specific conditions are perfectly fulfilled by the refractive field mapping beam shapers due to their unique features: almost lossless intensity profile transformation, low output divergence, high transmittance and flatness of output beam profile, extended depth of field, adaptability to real intensity profiles of TEM00 and multimode laser sources. Combining of the refractive field mapping beam shapers with other optical components, like beamexpanders, relay imaging lenses, anamorphic optics makes it possible to generate the laser spots of necessary shape, size and intensity distribution. There are plenty of applications of high-power lasers where beam shaping bring benefits: irradiating photocathode of Free Electron Lasers (FEL), material ablation, micromachining, annealing in display making techniques, cladding, heat treating and others. This paper will describe some design basics of refractive beam shapers of the field mapping type, with emphasis on the features important for building and applications of high-power laser sources. There will be presented results of applying the refractive beam shapers in real installations. Keywords: beam shaping, flattop, tophat, supergauss, Ti:Sapphire, laser pumping, MOPA, ultrashort, high power laser, homogenizing. 1. INTRODUCTION Shaping the spatial (transverse) profile of laser beams is often considered as a way of improving optical design of high-power lasers as well in various applications with these lasers. There exist several approaches of beam shaping of laser radiation that are widely used in scientific and industrial applications to transform the intensity distribution of a beam from a typical Gaussian profile to flattop, super-gauss or other profiles. Modern techniques, like refractive field mapping beam shapers Shaper, allow to provide not only a simple homogenizing of a beam in a certain field but create various intensity distributions with using the same beam shaper. This flexibility in generation of various profiles can serve as a very fruitful mean of improving the design of various solid state lasers, for example, optimizing conditions of pumping the ultrashort pulsed lasers or amplification in MOPA laser designs, reducing thermal load of a crystal and avoiding thermal lensing, etc. Beam shaping is also important in plenty of laser applications where uniform or other intensity distributions are highly desirable. This paper will describe some design basics of Shaper - refractive beam shapers of field mapping type, with emphasis on the features important for building and applications of high-power laser sources. There will be presented results of applying the refractive beam shapers in real research installations. 2. THEORETICAL CONSIDERATIONS By building the optical systems of high power lasers there are several optical tasks that can efficiently solved with using the refractive beam shapers transforming the intensity profile of a laser beam. One of these tasks is pumping of crystals of solid-state lasers, for example Ti:Sapphire crystal with multimode radiation of second harmonic of Nd:YAG. A common problem this case is in high peak intensity in the centre of that pumping beam that leads to thermal lensing and danger of damage of the Ti:Sapphire crystal. As result there are limitations of quality and achievable output laser power of ultrashort pulse laser, instability of laser operation. Transforming of that pumping beam to a

2 - 2 - beam of uniform of intensity or even providing a concave (or inverse Gauss ) intensity distribution helps to overcome those problems and provide more efficient optical systems of solid-state lasers. Similar positive effect of beam shaping is important in high power lasers built according to MOPA configuration. With flattop or super-gauss intensity profile of a beam after seed laser it is possible to achieve higher amplification of the radiation in amplifier due to more efficient use of energy from pump source, reduce the influence of thermo lensing. As result the higher levels of output power as well as more reliable laser operation can be achieved. The beam shaping becomes a mandatory technique when the output beam of the high power laser has to have a flattop profile. Transformation of Gaussian or close to Gaussian intensity profile to flattop one is also important in plenty of applications of high-power lasers. For example, irradiating photocathode of Free Electron Lasers (FEL) with a laser beam of flattop or another intensity profile makes it possible to bring special features of electron beam. Other applications examples are laser material ablation, laser-induced incandescence, annealing in display making techniques and others. To make the above mentioned effects realizable the beam shaping optics to be applied in optical systems of high power lasers has to meet some requirements: - conserving the structure and low divergence of a laser beam, - realizing beam shaping in general sense, i.e. not only beam homogenizing but also providing other profiles, like super-gauss, inverse Gauss, - it is highly desirable that the variety of these profiles can be realized with the same beam shaper, - capability to work wit TEM 00 and multimode laser sources, - adaptability to conditions of particular laser optical system, - high transmission, - high resistance to powerful laser radiation, etc. As an efficient solution to these tasks it is suggested to apply the refractive refractive field mapping beam shapers Shaper capable to provide various intensity distributions, Fig. 1. The design principles of refractive beam shapers of the field mapping type are well-known and described in the literature 1,2,3,4. These beam shapers have two optical components and transform the laser beam profile in a controlled manner, by accurate inducing of wave aberration by the first component and further its compensation by the second one, Fig.1, top. The resulting collimated output beam has a uniform intensity and flat wave front. It also has low divergence - comparable to that of the input beam. In other words, the field mapping beam shapers, like Shaper, transform the beam profile without deterioration of the beam consistency and without increasing its divergence. For the purpose of further considerations let us summarize main optical features of Shaper systems being used in this work: - refractive optical systems transforming Gaussian, or close to Gaussian intensity distribution of source laser beam to a flattop (or top-hat, or uniform) one; - transformation is realized through the phase profile manipulation in a controlled manner - accurate inducing by the first component of spherical aberrations to achieve the energy re-distribution and further compensation of the aberration by the second optical component; - the output beam is free of aberration, the phase profile is maintained flat and low beam divergence is provided; - TEM 00 or multimode beams applied; - collimated output beam, - the resulting beam profile is kept stable over large distance; - achromatic optical design, hence the beam shaping effect is provided for a certain spectral range simultaneously; - Galilean design, no internal focusing. Example of beam shaping for Nd:YAG laser with using Shaper is presented in Fig.2. Figure 1 Refractive field mapping beam shaper Shaper The experimental data of measured profiles show that the Shaper not only converts the intensity profile but improves also the spot shape one can see the slightly distorted input beam is transformed to a flattop output beam with regular round spot.

3 - 3 - Figure 2 Beam shaping with Shaper: Left Input TEM 00 beam, Right - after the Shaper (Courtesy of InnoLas Laser GmbH) One more important feature of the refractive field mapping beam shapers is that their operational principle presumes the input beam has a certain size (usually defined as diameter at 1/e 2 intensity level) and a certain intensity profile (Gaussian or similar profiles with peak intensity in the centre). If an input beam size deviates from the pre-determined one the resulting profile varies as well. In other words, a variation of input beam size corresponds to variation of output beam intensity distribution, and the size of the output beam stays almost invariable. For example, when a Shaper is intended to convert the Gaussian beam to the flattop one and the input beam is essentially smaller, say 2-3 times less than a specified value, the beam shaper operates as an ordinary beam-expander and the resulting profile stays almost the same like at the entrance i.e. Gaussian. This effect is discussed thoroughly in paper 4 and is demonstrated in Fig. 3 where results of theoretical calculations as well as measured in real experiments beam profiles are shown. The data relate to the Shaper 6_6 which design presumes that a perfect Gaussian beam with 1/e 2 diameter 6 mm to be converted to a beam with uniform intensity (flattop) with FWHM diameter 6.2 mm. When the input beam has a proper size, Fig.3 a), the resulting beam profile is flattop, Fig.3 b). But change of input beam size results in changing of the output beam profile: increasing of diameter leads to downing of intensity in the beam centre, Fig.3 c), sometimes this distribution is called inverse Gauss ; beams size reduction allows to get a convex profile that approximately can be described by super Gauss functions, Fig.3 d). (a) (b) (c) (d) Figure 3 Experimental and theoretical intensity profiles: a) TEM 00 Input beam, D in = 6 mm (1/e 2 ), b) Flattop output profile when by D in = 6 mm (1/e 2 ), c) Concave output profile ( Inverse Gauss ), D in = 6,5 mm (1/e 2 ) d) Convex output profile ( supergauss ), D in = 5,5 mm (1/e 2 ) Courtesy of IPG Photonics) Evidently, a simple variation of laser input beam size allows to generate various profiles and this can be done on with the same beam shaper. To vary the beam diameter the ordinary beam expanders, can be applied. With using a zoom beamexpander one can steady vary the resulting beam profile and provide the intensity distributions being optimum for particular optical systems of high power lasers.

4 EXAMPLES OF BEAM SHAPING One of important applications of the refractive beam shapers is homogenizing multimode radiation from 2nd Harmonic Nd:YAG laser when pumping Ti:Sapphire crystals to generate powerful ultrashort pulses. The well known problem of developing powerful femtosecond lasers is high central peak of intensity distribution of a 532 nm multimode pumping laser that leads to destroying of central part of a Ti:Sapphire crystal, this limits, practically, a maximum power level. Evidently, downing of intensity in the center of the multimode pumping beam would allow to overcome this obstacle in reaching higher power of a femtosecond laser. Just this task can be successfully solved by a field mapping beam shaper; result of realisation of this approach by one of users is illustrated in Fig. 4. a) Multimode original beam, = 532 nm b) Beam profile after Shaper 12_12_532 Fig. 4 Comparison of beam profiles of pumping laser (Courtesy of LOTIS) Intensity distribution of the original multimode beam is quite far from the Gaussian function, it is rather flattop but with a pronounced peak in centre of the beam, just this intensity peak is a source of such a problem like damaging of Ti:Sapphire crystal. Applying of the beam shaper effects downing of intensity in central part of the beam and, hence, eliminating of the central peak and providing more smooth beam profile being optimized for pumping of Ti:Sapphire crystal. As result the output laser power as well as its stability were seriously enhanced. As an example of applying the refractive beam shapers in MOPA lasers one can consider the advanced laser system LIFE Lifetest Facility in Lawrence Livermore National Laboratory used for testing the damage threshold of optical components of NIF. To provide correct measurements of damage threshold it was necessary to realize several laser beams with various wavelengths, pulse energies and durations but with just flattop intensity profiles. A complex laser optical system was applied where the radiation emitted by the oscillator was transformed with using the Shaper to a beam of uniform intensity distribution and was then amplified with using high power amplifiers to provide necessary testing modes of the optical components Another application example relates to the task of irradiating the photocathode of a Free Electron Laser (FEL) by short-pulse laser radiation with a flattop beam profile. Providing uniform intensity distribution on the photocathode improves essentially the performance of entire FEL. The refractive beam shapers were used for these purposes in several scientific installations based on femtosecond IR and UV lasers, measured beam profiles caught during experiments at one of these installations are presented in Fig. 5. a) Original beam, = 262 nm b) Beam profile after Shaper 12_12_266 Fig. 5 Comparison of beam profiles, Courtesy of Elettra Sincrotrone The measurement results show how effective can be a refractive beam shaper when operating with beams which intensity profiles are far from the perfect Gaussian distribution. The resulting profile of output collimated beam was then relay imaged, with certain magnification, onto a photocathode, finally the task of creating a round flattop spot was successfully solved.

5 CONCLUSION Applying of beam shaping optics in optical systems of high power lasers makes it possible to improve their performance due to reduction of thermal load of crystal and less influence of thermal lensing, more efficient pumping, and enhanced amplification in MOPA lasers. The specific requirements of optical designs of MOPA and ultrashort pulse lasers, like low divergence, conservation of beam consistency, variable beam profiles, operation with TEM00 and multimode beams, can be successfully met by refractive field mapping beam shapers. Being originally designed to transform Gaussian beams to beams of uniform intensity these beam shapers show high level of flexibility in generation of various intensity profiles like inverse Gauss, super Gauss, and this variety of transformations can be realized with using the same field mapping beam shaper. 5. REFERENCES [1] Dickey, F. M., Holswade, S. C., [Laser Beam Shaping: Theory and Techniques], Marcel Dekker, New York, (2000). [2] Hoffnagle, J. A., Jefferson, C. M., Design and performance of a refractive optical system that converts a Gaussian to a flattop beam, Appl. Opt., vol. 39, (2000). [3] Laskin, A., Laskin, V., Variable beam shaping with using the same field mapping refractive beam shaper Proc. SPIE 8236, Paper 82360D (2012). [4] Laskin, A., Laskin, V. Applying of refractive beam shapers of circular symmetry to generate non-circular shapes of homogenized laser beams Proc. SPIE 7913, Paper (2011). 6. ACKNOWLEDGEMENTS The authors grateful to users of pshaper in IPG Photonics, InnoLas Laser GmbH, LOTIS, Elettra Sincrotrone and Lawrence Livermore National Laboratory for their active and patient work with optics discussed in this paper and kind permission to publish some results achieved during their experiments.