Publication
Sensing of atmospheric trace gases is crucial for climate monitoring and to predict global climate changes. The required global coverage and spatial resolution have driven the studies of space-borne differential absorption lidar (DIAL) instruments to remotely monitor atmospheric gases from a satellite to ground. The performance of such instruments is notably determined by the frequency stability and accuracy of a low-power continuous-wave laser that seeds the pulsed laser transmitter. For a CO2 DIAL, this reference laser needs to be stabilized with an adjustable frequency-detuning from the center of the probed molecular transition and the 2.05-μm spectral range is of high interest from a spectroscopic point-of-view [1].We have developed an all-fiber modulation sideband locking set-up enabling a laser to be locked at a controlled frequency detuning from the center of the CO2 R(30) transition at 2050.97 nm, selected for DIAL applications. The offset frequency can be directly tuned over a span ranging from some hundred MHz up to at least 3 GHz, which is the typical requirement for a space-borne CO2 DIAL. The method is depicted in Fig. 1a. It consists of a distributed feedback (DFB) laser, followed by an intensity electro-optic modulator (EOM) driven by a radio-frequency signal at fEOM provided by an amplified voltage-controlled oscillator (VCO). The EOM generates a pair of sidebands shifted by ±fEOM that are coupled into a reference gas cell. The sidebands are dithered by modulating the VCO at a frequency fm 40 kHz to implement wavelength modulation spectroscopy (WMS). An error signal is produced by demodulating the reference cell transmission signal to servo-lock one of the sidebands at the center of the transition. As a result, the unmodulated laser carrier is detuned from the transition linecenter by the frequency offset fEOM, which can be easily varied, thus making the system versatile.
Olga Fink, Gaëtan Michel Frusque