Abstract

Wavelength selection and wavelength tuning in the bands of XeCl (308 nm), KrF (248 nm), ArF (193 ran| were investigated with the aim of achieving low amplified spontaneous emission, i.e., high-locking efficiency, and of extending the tuning range into the wings as far as possible. Wavelength selection and tuning were performed by a grating, the beam being expanded by three prisms. An etalon could be added to increase spectral brightness. The tuned radiation was injected into a regenerative amplifier which was designed as an unstable resonator of the Cassegrain type. Proper time delay between oscillator and amplifier had to be fulfilled. Figure 1 shows the spectrum of the free-running ArF laser (bottom) and in comparison narrowbanded emission over the tuning range when the grating is tilted. Typically the bandwidth is 0.3 cm^{- 1}. Note that the tuning range is considerably broader than the spectral emission of the free-running laser. This indicates that a sufficient number of round trips occurs in the oscillator so that the spectral narrowing element is an active part of the resonator, resulting in a rather high-locking efficiency even in the spectral wings (Fig. 2). Tuning to the band maxima yields a locking efficiency of more than 98 %. Pulse energy (grating tuned to band maxima) is 100 mj (ArF), 160 mj (KrF), 120 mj (XeCl). Inserting an etalon the bandwidth could further be reduced to <0.05 cm^-1 (KrF). (12 mln) TUR3 Fig. 1. Spectral tuning of an ArF laser. The structure in the spectrum of the free-running laser is due to 0₂ absorption (Schumannn-Runge bands). TUR3 Fig. 2. Locking efficiency of the ArF laser spectrum; the dashed line connects the data points of Fig. 1.

© 1986 Optical Society of America