a line narrowed laser having a grating based line narrowing module and etalon - based line narrowing output coupler and a technique for tuning the laser . the etalon based output coupler is adjusted to preferentially reflect light at or near the spectral maximum produced by the grating . substantial improvement in line narrowing results from the amplification in the laser resonance chamber of light at or near the grating spectral maximum which is preferentially reflected by the etalon based output coupler . a special tuning technique takes advantage of the fact that a grating based line narrowing module naturally produces a laser beam having spacial variation in wavelength along a beam cross sectional direction . by comparing profiles of the beam into and out of the etalon in the output coupler , the grating and / or the etalon can be adjusted to achieve desired spectral performance .