Fig. 1. Schematic of experimental arrangement. diode: 3-section DBR diode laser, fiber pump: 25W fiber-coupled 976nm diode bar, ISO 1: 60dB optical isolator, ISO 2: 30dB optical isolator, Q: quarter wave plate, H: half wave plates, M₁–M₄: OPO cavity mirrors, PPLN: 40mm PPLN crystal in oven.

Fig. 2. Variation of OPO output wavelengths (lower plot), and corresponding idler output power (upper plot), during pump tuning by seed laser DBR section alone. PPLN grating period was 29.75 µm and temperature was 180.5°C. Calculated tuning data was derived using the Sellmeier equations of Jundt [10].

Fig. 3. Upper trace: Rapid, mode-hop free tuning of the seed laser over 110 GHz observed in the transmission of a Fabry-Perot interferometer with 7.1GHz FSR. Lower trace: corresponding OPO idler tuning observed in the transmission through a 90 cm cell containing 15 mbar CH₄ buffered to 0.52 bar in air (solid line), calculated cell transmission derived from HITRAN data (dashed line). Pump tuning axis was calibrated from the Fabry-Perot transmission and an absolute frequency reference, measured under static conditions, is indicated. A corresponding idler frequency reference is indicated for the measured and calculated plots. The signal frequency remained constant throughout the measurement. The total time scale represents one half of a single period of the 17 Hz tuning function.