Abstract

Solid-state lasers emitting in the eye-safe region at around 2 μm are promising candidates for coherent Doppler velocimetry and high-sensitivity Dial measurements of the atmosphere [1]. A frequency stabilized master oscillator is a key component for injection locking of a Q-switched slave oscillator in a coherent laser radar system. In this work we demonstrate the possibility of locking single-frequency tunable Tm-Ho:YAG lasers, by means of the fringe-side-locking technique, to absorption features of different molecules. The lasers are longitudinally pumped by InGaAs diode lasers, and a wavelength tuning from 2085 nm to 2099 nm in single-mode operation was achieved by tilting two intracavity etalons. In particular, we demonstrate frequency locking to the HBr P(12) line, falling at 2097.2 nm, in a particularly suitable transmission window of the atmosphere [2]; to the HBr P(11) line, falling at 2085.7 nm; to the H₂O rotovibrational (0,0,0)-(0,3,0) 2_2,1-2_1,2 line, falling at 2093.9 nm, and to several absorption lines of CO₂, falling in the tunability interval of the Tm-Ho lasers used. We first measured the absorption features by sweeping the operation wavelength applying a ramp voltage to a piezoelectric transducer (PZT) fixed to the output mirror and probing the transmission of multipass cells filled with different gases. We used a differential scheme in order to cancel intensity fluctuations of the laser beam induced by the PZT. Using optical lengths of 7.5 m and 29.5 m and gas pressures of 10-50 torr we found absorption peaks ranging from 15% to 75% with linewidths of a few hundreds of MHz. We measured the frequency fluctuations of the lasers using the edge of such lines as a frequency discriminator, and the spectral density of the frequency noise was determined by Fourier transform of the output voltage signal. A suitably designed electronic filter, consisting in a variable-gain integrator followed by a zero-pole compensating stage, was used to feed back this signal to the PZT and locking the laser to the edge of the molecular feature. The stability of the laser sources was also measured by monitoring the beat note frequency between two stabilized lasers, after frequency to voltage conversion: in Fig. 1, as an example, we show the results obtained in open- and closed-loop operation using the HBr P(12) line at 2097.222 nm. The relative frequency stability in closed-loop operation was maintained within 13 MHz over 60-minutes observation time.

© 2001 EPS