Current status of commercialized one-box type femtosecond lasers for material processing is summarized, showing a wide variety of available pulse energy and repetition rate. Depending on the irradiated fluence, the femtosecond laser pulses give us distinctive phenomena such as ablation, LIPSS (Laser-Induced Periodic Surface Structure), phase transition of the semiconductor. In a viewpoint of industrial applications, lower manufacturing cost is generally expected for lower process fluence. Higher manufacturing speed is expected for higher average power with high repetition rate, about 100μJ pulse energy, and multiple beam output.

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All solid-state high power visible and ultraviolet laser oscillators are expected to use in various industrial applications such as fine material processing, annealing process of amorphous-silicon, and so on. We have developed two types of visible and UV laser oscillators with wave length conversion technique. One is high power type (Nd: YAG) and the other is high repetition rate type (Nd: YVO₄). Maximum THG output power of high power type has reached 125W with 6.4% pumping efficiency at 10kHz, that is the highest power output using the intra cavity THG technique currently known in the world. Maximum single mode THG output power of the high repetition rate type has reached 6W at 50k Hz. The life time of THG crystal is expected over 400 hours at one position of THG crystal and total life time of that is expected over one year.

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Substantial progress has been made overseas in high power laser diode arrays. Recent progress in high power laser diodes and two-dimensional high power laser diode arrays is reviewed. The technical development of high power, high efficient, one-and two-dimensional diode arrays has enabled the development of several kilowatt class laser diode systems for direct materials processing. In the near future, a centimeter long diode bar array of l k W power, with 80% efficiency, and a lifetime in excess of 40,000 hours will be realized.

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Recent progress of CO₂ laser sheet-metal cutting systems is described. The CO₂ laser is still the front-runner for industrial laser market, however more than two decades have passed since the CO₂ laser cutting system entered the market. The power scaling of laser oscillator has enhanced the productivity of systems, and the advance of the processing technology has drastically improved the cutting quality. We believe that the CO₂ laser cutting systems will be more practical industrial tools by continuous developments for wide range of applications.

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Although lasers are now widely used in many application fields, such as communications, data storage, materials processing, biology, medicine, basic sciences, etc., owing to their excellent features such as spatial and temporal highly coherent behaviors and outstanding beam controllability, there are still a lot of R&D activities on new laser technologies in the world due to the fact that progress in laser technologies is very rapid and new laser technologies are key to the success for the valuable innovations in many fields. This article briefly introduces some trends in the national or multi-national major R&D projects on high power lasers in Europe and USA, and emphasizes the importance of strengthening such a nation-wide R&D activities on high power lasers and their applications also in Japan.

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With the invention of large-mode-area fiber and cladding pump technique, output power of rare-earth doped fiber lasers were significantly enhanced in recent years. Fiber-based systems have many attractive features such as constant beam quality independent of laser power, high optical efficiency, and easy heat removal. However, “bulk solid-state-laser” is still a choice of geometry in high-pulse-energy regime. Scaling of pulse energy with a fiber-based system is limited mainly due to damage threshold, nonlinear effect, and energy dissipation induced by parasitic lasing. To overcome these issues, we selected a combination of a Nd: YAG laser oscillator and a large-core fiber amplifier. The system producing 24m J pulse energy with repetition rate of 2k Hz was demonstrated. The pulse energy is, to our knowledge, the highest number obtained at more than 1k Hz range.

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We presented high quality, high power pico-second output from a diode-side-pumped Nd: YVO4 laser system with a photorefractive phase conjugate mirror. Peak output powers in the range of 2.8-6.8MW were achieved at a pulse repetition frequency range of 0.33-1.0MHz. The corresponding average powers of 18-26W were obtained.

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This paper reports the results of high-order harmonic generation (HHG) observed by changing the angleαbetween polarization directions in the pump and probe experiment using high-intensity femtosecond laser pulses. Theα-dependent HHG from an ensemble of aligned O₂ molecules was observed to be very similar to that from N₂, being contrary to what was expected from the higher occupied molecular orbital of a single O₂ molecule. It is shown that the characteristic angle dependence for O₂ is due to low degree of alignment of molecules that can be achieved in the pump-probe experiment. The experimental results for N₂ and O₂ are in good agreement with those calculated using the recently developed theory. This demonstrates the validity of the theory in describing the characteristic properties of HHG from aligned N₂ and O₂.

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