All-fiber molecular frequency reference at 2 μm based on a versatile laser modulation sideband locking and a hollow-core fiber gas cell

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.