All‐fiber versatile laser frequency reference at 2 μm for CO2 space‐borne lidar applications

Sensing atmospheric gas traces is crucial for climate monitoring and to predict the importance of global climate changes. Among the various atmospheric constituents, carbon dioxide (CO2), methane (CH4) and water vapor (H2O-v), the most prominent greenhouse gasses, have a major impact on climate. Advanced monitoring techniques are necessary to measure these gas species on a global scale all around the Earth. In the case of CO2, improved accuracy and precision would enable determining more correctly its source and sink locations, its amount and variability, to better understand its fluxes and exchanges between the atmosphere, the lands and the oceans, and, hence, its global cycle. The global coverage and spatial resolution that are generally required for such a monitoring have driven the studies of space-borne active remote sensing Light Detection and Ranging (lidar) instruments [1]. A lidar relies on a time-resolved analysis using short laser light pulses scattered back to the instrument by atmospheric constituents or by a solid surface (e.g., the ground surface for airborne lidars) to measure the density profile of the studied species or its integrated column density [2]. The Differential Absorption Lidar (DIAL) approach offers the advantage of selective detection of the target species by sensing the difference in light absorption at two close wavelengths, one of which is chosen to coincide with an absorption line of the species under study, whereas the second one is chosen sufficiently far off the line to avoid substantial absorption [3]. Being the most important anthropogenic greenhouse gas and the prominent contributor to the total anthropogenic change in the Earth radiation budget, CO2 is of particular interest in the context of global climate warming. This is why space agencies have been studying future CO2 spaceborne integrated-path differential absorption (IPDA) lidar Abstract We present a frequency stabilized laser at 2051 nm based on a versatile all-fibered stabilization setup. A modulation sideband locking technique is implemented to lock the laser at a controlled frequency detuning from the center of the CO2 R(30) transition envisaged for spaceborne differential absorption lidar (DIAL) applications. This method relies on the use of a compact all-fibered gas reference cell that makes the setup robust and immune to mechanically induced optical misalignments. The gas cell is fabricated using a hollow-core photonic crystal fiber filled with pure CO2 at a low pressure of ~20 mbar and hermetically sealed at both ends by splices to silica fibers. Different configurations of this fibered cell have been developed and are presented. With this technique, frequency stabilities below 40 kHz at 1-s integration time and <100 kHz up to 1000-s averaging time were achieved for a laser detuning by around 1 GHz from the center of the CO2 transition. These stabilities are compliant with typical requirements for the reference seed source for a space CO2 DIAL.