Highly-efficient high-power pulsed CO2 laser characterized by transverse-flow laser amplifiers

In this paper, we provide a detailed review of development of a highly-efficient high-power nanosecond pulse CO2 laser using transverse-flow radio-frequency (RF)-exited laser amplifiers, which is for extreme ultraviolet (EUV) light source of a next generation of nano-lithography. High-density excited high-power transverse-flow CO2 lasers were designed and built for the application of laser produced plasma (LPP) EUV source. We carried out an amplification test of the transverse-flow CO2 laser seeded by a nanosecond pulse CO2 laser. A four-amplifier system generated an average output power of 21 kW with an electrical input power of 400 kW for discharges. The electrical-to-optical efficiency was 5.2%. The input pulse laser had an average power of 46 W, the repetition rate was 100 kHz, and the pulse duration was 15 ns. The transverse-flow CO2 laser has strong points in high gain and availability of the multi-fold optical path. A highly-efficient amplification was experimentally proved characterized by transverse-flow CO2 laser amplifiers even with low-power seed. A transverse-flow CO2 laser is a promising candidate for an amplifier in the LPP EUV light source.