The objectives of reactive chemical and nonreactive thermal processing with laser radiation are outlined giving indication that processing with laser radiation is governed by a hierarchy of time constants originating from photon-matter interaction, phase transition dynamics, laser source excitation fluctuations, and optical feedback in combination with the influence of beam delivery systems, processing/shielding gas flow configurations, robotics, production lines and environment. The minimization of losses by heat flow, reflection and transmission and the stringent need for quality assurance require as first approach the control of processing, which is mainly due to the capability of laser radiation source. The current status of laser radiation sources is reported giving information on the state of the art of processing with laser radiation in combination with subsequent demonstration of future trends and developments with respect to radiation sources, beam delivery, beam shaping, materials, processing and quality assurance.

We extracted 386 W average power from a moving slab glass laser with a pumped area of 300 cm2. The total efficiency was 3.1 %. The beam was linearly polarized and had constant divergencies of 3 mrad in the small dimension (6 mm) and 13 mrad in the large dimension (20 mm), independent of the average input power. The repetition rate was 10 pps. A new method to measure the thermal load of the laser glass is presented. We found that 6.6 % of the electrical input power is dissipated as heat in the glass slab, which is of the same magnitude as the storage efficiency.

The performance characteristics of a Nd:YAG-Laser cooled by mechanical contact with two transparent high heat conduction solid-state sheets are presented. The size of the slab-crystal was 0.6 cm (height h) x 1.5 cm (width w) x 10 cm (length 1). 300 W average laser power in multimode were generated in free running repetitively pulsed (30 pps) operation; the slope efficiency was 3.3%. Some particular properties applying to this laser for material processing will be mentioned.

Several lasers are, or have been, developed at the HOYA Laser Laboratory: Conventional YAG-rod lasers, glass fiber bundle lasers and moving glass slab lasers. Slab lasers are considered the engineering answer to the demand of higher average power. We obtained 386 W with a moving glass slab laser. Parts of the program are also erbium doped YAG and glass lasers. We developed a stable and reliable 10-W output 3-μm Er:YAG laser. All models have been developed with a specific application in mind,

Glass compositions containing rare earth elements such as neodymium offer many of the properties desired of the active materials in high average power laser systems. The most popular laser glasses for this application are neodymium doped phosphate compositions which have high cross sections for stimulated emission, long fluorescence lifetimes, and moderate non-linear refractive index values, while still being available in large sizes with high optical quality and with large neodymium concentrations. The predominant disadvantages of glass, in comparison to crystalline materials, are efficiency and thermo-mechanical limitations. We discuss advancements which address both of these problems as well as which offer improvements in other properties relevant to glass utilization in high average power laser systems.

The active material is one of the crucial components of high average power solid state lasers. The maximum average power of present-day commercial solid state lasers is in the order of 1 kW. Several kW are expected to be available in the near future. In order to achieve this design objective, one has to understand which parameters of the active materials are responsible for the limitations of the maximum average power. As long as the lasers are pumped by flashlamps in a broad band, the most important limiting property is the maximum acceptable thermal stress due to temperature differences in the active solid material. Crystal-line active materials have an advantage as compared to laser glasses because of their larger coefficients of thermal conductivity and of their larger surface strength. On the other hand, active laser glasses can be produced in larger volumes and with better homogeneity than crystals. In addition, the surface strength of laser glasses can be increased by ion excange. In the paper, several possibilities especially with laser glasses will be discussed how the thermal load of the active material can be reduced in order to overcome the present limitations and to increase the maximum average power of solid state lasers.

The lasing properties of a number of Nd:Cr;GGG laser crystals with different Cr-concentrations were investigated under pulsed flashlamp excitation. For an optimum average concentration of 2.4*10 20 Nd-at/cm3 and 3.7*10 19 Cr-at/cm3 an increase in slope efficiency by a factor of 2.5 was observed, compared to a standard Nd:YAG laser rod. A 79 mm x 4 mm diameter rod was operated quasi-continuously with pump powers up to 2 kW, delivering an average output power of 50 W. From the onset of resonator instability a thermal lensing power of the GGG rod of 2 diopters/kW was deduced.

Free running laser efficiency measurements were performed with Cr:Nd:GGG and Nd:YAG laser crystals. The round trip loss factor has been determined using the Findley Clay method. The slope and intrinsic lasing efficiencies of Cr:Nd:GGG were a factor of 1.6 higher than for Wd:YAG. Absorption and photoluminescence excitation spectroscopy data reveal that the Cr 3+ sensitization is highly efficient.

Properties of alexandrite, a newly availabe tunable solid-state laser, will be briefly reviewed. These properties provide for its ready employment as high energy and high repetition rate laser hosts. Specific systems developed which exploit these characteristics and their performances are discussed. Potentials of scaling to even higher energy and repetition rates will be addressed.

We report on the new 1.06 micron laser material Cr3+,Nd3+:Y3(ScxAl2-x)Al3012(Cr,Nd:YSAG). To find a congruent melting composition for this mixed garnet, boules were grown with Sc concentrations between x=0.9 and x=1.6. With rods prepared from these Cr-sensitized Nd-crystals flashlamp pumped laser experiments have been performed. Until now slope efficiencies up to 6.4% are obtained at a repetition rate of 10 Hz.

In this paper 2 Am laser action of YAG:Cr,Tm,Ho under flashlamp excitation is demonstrated and analyzed. Room temperture laser performance of the Ho3+ 5I7 -> 5I8 transition is made possible this material by an efficient Cr-Tm-Ho pump mechanism. The excellent overlap of the Cr3+ absorption spectrum in YAG with the flashlamp emission spectrum and the efficient downconversion process yield slope efficiencies as high as 4.3 %. In this context concentration limitations of the optically active ions are discussed. The increase of the laser threshold energy with increasing rod temperature is mainly due to thermal occupation of the Stark-levels. A decrease of output energy with increasing repetition rate is due to the same effect.

High power solid-state lasers are still being pumped mainly by gas discharge lamps filled with krypton, xenon or some mixture of gases. The radiation of these lamps is trans-ported to the laser rod by closed coupling or by the reflecting properties of an elliptical reflector in the laser cavity.

Extraction efficiency, beam quality, misalignment sensitivity and thermal lensing of stable and unstable resonators were investigated, both experimentally and theoretically. The theoretical description was based on a model combining diffraction theory with the amplification of the electric field inside the active medium. Saturation effects on mode vo-lume and losses are discussed. Experiments were performed with a pulsed Nd:YAG system. A detailed comparison of the two kind of resonators, indicates several advantages of the unstable one, such as low misalignment sensitivity and high far-field brightness. Furthermore, several methods of improving the beam quality of unstable resonators were investigated.

Optical beams of fundamental mode shape were obtained from a pulsed Nd:YAG laser using an output mirror with a parabolic reflectivity profile in an unstable Cassegrain resonator. When the laser is operated in the free-running regime, the near field is smooth and the far field has no significant secondary lobe. In the passive mode-locking regime, the beam quality is maintained; however, the beam waist is reduced compared to the free-running regime. This observation is attributed to the ability of a nonlinear element such as a saturable absorber to reshape a laser beam. Emission of pulse trains with a 96% reproducibility has been obtained with such a resonator.

A method of designing unstable telescopic resonators is considered in which the thermolens of laser rod is taken into account. Examples of two tested resonators with good perfermence are given.

As an attempt to improve the efficiency of a solid state laser cavity, a non-elliptical cavity is proposed. Efficiency was calculated by the ray trace method and the cavity was simulated using a novel approach with splines. Computation shows that substantial gain in efficiency can be achieved for a close coupled configuration.

In this paper we present the results of a ray tracing analysis of some involute-type reflector profiles, using a Montecarlo simulation. The simulation show that also this profile gives a non uniform slab illumination if a finite reflector surface reflectivity is considered. The same happens if the emission from the generating circle of the involute is not Lambertian, even if the reflector still has a 100% transfer efficiency. These results are compared with measurements of reflectors built for a zig-zag Nd:Glass slab laser.

A physical nature of smoothing of the thermooptical inhomogeneities (STOI effect) in the active media of solid-state lasers, while increasing absorption of the optical pumping, lies in the suppression of heat-conducting gradients by inhomogeneous pumping energy release. This leads to reduction of thermooptical stresses, provides inhomogeneity for refractive index of the active medium, and decreases laser radiation divergence, thus, making it possible to enhance the average power of generation /1-3/. This effect is more distinct if the optical excitation conditions of the active medium correspond to quasi-grey excita-tion and absorption spectra (optical density should be significantly more than unit), i.e. in the optically dense active media /4/. Later we shall regard the main occurence of the STOI effect in the solid-state lasers with optically dense active media, and present the results of experiments in chromium and neodymium co-doped rare-earth scandium garnets (RESG). In the pulsed solid-state lasers, these media (say, yttrium-scandium-gallium garnet with chromium and neodymium - YSGG:Cr3+, Nd3+ ) agree well with optically dense active medium, especially at high chromium concentration. Moreover, in these lasers the optical density can be varied widely due to a variation of chromium ion concentration, not changing the physics of the active medium from the point of view of spectrally-luminescent characteristics /5/.

This is a report on the characteristics of the large aperture Nd:YAG disk laser with dimensions of 6x40x80 mm. The product of small signal gain coefficient and the long axis the disk without clading absorption glasses was limited to less than 1.36 because of the severe parasitic oscillations. The Nd:YAG crystal boule grown by the temperature gradient technique (TGT) has 75-80 mm in diam. and 133 mm long. The Nd doped concentration can be up to 2.3 Wt%. Now the clading absorption coating is considered to be used in the disk laser system and further results will be reported.

We determined the excitation and extraction efficiencies of rod and slab laser systems as a function of the small signal gain and the resonator losses. Taking into account the beam overlap and beam enlargement in slabs, it turns out that higher extraction efficiencies can be achieved in slab systems compared to equivalent rod systems. This could be proven by experimental investigations made at a comparable Nd:YAG slab and a Nd:YAG rod system. Provided that the pumping arrangement of slab systems is optimized similar to rod systems, higher output powers and overall efficiencies should be possible in slab systems.

This paper presents data about the output characteristics of kilowatt Nd:YAG laser systems, developed by NEC. Since a big part of the publication regarding YAG lasers deal with SLAB lasers, it seems difficult to explain why results about laser systems using YAG rods, are still interesting. However, the answer is simple: one, laser systems with YAG rods and with more than one kilowatt output power are available today commercially. The second point is, that industry needs high powered systems for material processing. It cannot wait any longer. The technique is ripe, and industry needs more practical experience with high power YAG laser radiation, because results have proven that the key to material processing in the future lies with YAG lasers. Industry must awaken to this fact. Even if rod lasers she higher beam divergency than SLAB lasers, the technology provides a lot of other practical benefits, especially when fiber cables are used for resolving beam handling problems. There are sufficient applications in material processing, fully satisfied by rod lasers: cutting of sheet metal and 3 dimensional parts up to several mm thick, welding and surface hardening.

As part of the Eureka Eurolaser definition phase programme Lumonics Ltd have undertaken an experimental work programme with supra-kilowatt lasers. The work, which started early in 1987 has been directed at identifying the practical advantages that might result from the realisation of high power solid-state lasers and which would justify their commercial development. The specific tasks within the Lumonics programme have included, (i) establishing a processing data base with available CO2 and YAG laser sources to provide 'benchmarks' against which to assess improvements, (ii) building experimental pulsed YAG lasers of average power to greater than 2 kW for advanced processing trials, (iii) investigating the feasibility of developing fibre optic beam delivery systems compatible with supra-kilowatt average power lasers, and (iv) determining the practical power limits to solid-state laser technology, eg component life, efficiency, etc. The paper will review the progress of these four key areas within the Lumonics programme. It will cover the processing results obtained with experimental YAG lasers of up to 2.3 kW average power and 35 kW peak power in a beam of approx 170 mm-mrad diameter-divergence product. In particular, the performance in deep penetration welding of stainless steel and mild (low carbon) steel will be reported and will be compared with that obtained with other power sources, eg CW CO2 and YAG beams, e-beams, etc. The conclusions which can be drawn from the work programme will be summarised.

In this paper, the deeply thermo-stable telescopic resonators whose g* parameters satisfy the condition g1*g2*≈1/2 are theoretically investigated in detail. A formula of the: telescope defocusing which can compensate the thermal lensing of laser rod is obtained, using an equivalent thick lens and transfer matrix method. It has been shown that under appropriate assumptions the simplified approximate expressions of the telescope defocusing derived by Hanna and Z. Lu can be obtained simply from our formulae. The computer simulations and numerical calculations have been carried out to confirm our theoretical results and have given a series of curves, which is useful for the telescope-resonator design.

A zig-zag optical path slab geometry Nd: glass laser cooled through flowing air-water is developed by us. Theoretical studies on temperature distribution of slab and rod configurations in the unsteady state clarify the advantages of the slab geometry laser. The slab design and processing are also reported. In our experiments main laser output characteristics, e. g. laser efficiency, polarization, far-field divergence angle as well as resonator misalignment are investigated. The slab phosphate glass laser in combination with a crossed Porro-prism resonator demonstrates a good laser performance.

High power green laser has a lot of application. We have got 34.2-watt average power of laser at 532nm from a intracavity frequency doubled YAG laser. KTP crystal was used as the frequency doubler. The advantage of Quasi-CW pumping for frequency doubling was taken, the output power of harmonic wave with Quasi-CW pumping is higher than CW pumping. The influence of the polarization property of YAG laser on frequency doubling efficiency was theoretically analysed, and the optimum orientational angle for frequency doubling crystal was found inaddition to the traditional phase matching condition. Some experimental work, the parameters and properties of the laser was reported.

Spectroscopic and lamp pumped laser action characteristics comparative study of emerald and alexandrite grown by flux and Czochralsci methods respectively were carried out. In spite of considerably higher (approximatly 6 times) losses level emerald has better energy characteristics due to its higher laser transition cross section. The perspectives of the emerald as an active medium for tunable solid state lasers are discussed.

Since the invention of the slab-geometry solid-state laser in 1972, 1, this geometry has been proven to be more suited for high average power operation.2,3 The zigzag configuration eliminates the thermal induced beam distortions and increases the average power capability. The moving-slab laser was first proposed by Kane and Byer for the low thermal conductivity Nd:glasses. By moving a piece of glass medium, it is possible to spread the thermal load over a large area while maintaining high gain in the small volume pumped by the lamps. A hollow cylinder of Nd:glass rotating around its axis, which can realize uniform rotating simply, overcomes the disadvantage of the non-uniform motion of moving-slab at the turning points. In this paper, we give theoretical analysis of thermal effects in rotating hollow cylinder lasers and some useful performance scaling relations for guiding our initial design.