We report on a new periodically poled lithium niobate grating design with a continuous grating-period change (fan-out). We observed 350 cm(-1) (80 nm at 1.5 mu m) of complete spectral coverage at a constant temperature in a cw optical parametric oscillator. Complete spectral coverage is demonstrated by measurement of an absorption band of CO2. (C) 1998 Optical Society of America. OCIS codes: 140.0140, 300.0300, 300.6360, 300.6340, 190.0190, 190.4970.