Diode laser frequency stabilisation for water-vapour differential absorption sensing

Abstract

We describe a low-power continuous-wave laser system for water-vapour sensing applications in the 935-nm region. The system is based on extended-cavity diode lasers and distributed-feedback lasers and delivers four single-mode frequency-stabilised optical signals. Three lasers are locked to three water-vapour absorption lines of different strengths, whereas the fourth lies outside any absorption line. On-line stabilisation is performed by wavelength-modulation spectroscopy using compact water-vapour reference cells. An offset-locking technique implemented around an electrical filter is applied for the stabilisation of the off-line slave laser to an on-line master laser at a frequency detuning of 18.8 GHz. Stabilities in the order of 15 MHz over one day were observed for the strongest lines, at the detection limit of the measurement instrumentation. The developed techniques and schemes can be applied to other wavelength ranges and molecular species. Differential absorption lidar instrumentation can in particular benefit from such a system when the stabilised lasers serve as injection seeders to pulsed power oscillators.PACS 42.62.Fi; 42.68.Wt IntroductionWater vapour is a key component in the atmosphere. As part of the global water cycle it strongly influences the surface/atmosphere interaction processes. Its phase transitions determine the transport and redistribution of energy around the globe, affecting the weather. It plays a part in many atmospheric chemistry processes and, as a major greenhouse gas, plays a central role in the climate. Improved water-vapour measurements are needed in numerous disciplines and scientific issues [1]. For meteorology, weather forecasting, climatology as well as atmospheric process studies, it is important to accurately monitor both the temporally and spatially variable water-vapour concentration, not only through point or integrated column measurements, but also with a high spatial and temporal resolution.The water-vapour differential absorption lidar (DIAL) technique provides an independent and direct manner to retrieve the atmospheric humidity profile in a continuous way, u Fax: +41-32-722-04-20, E-mail: renaud.matthey@ne.ch with a high temporal and spatial resolution [2][3][4]. It is based on the comparison of the lidar signals at two wavelengths, one coinciding with a water-vapour absorption line of adequate strength and the second sufficiently far away from any line not to undergo substantial absorption. The quality of the data obtained from ground-based [5,6] and airborne [7][8][9][10] instruments in terms of vertical resolution, precision and accuracy indicates the feasibility of a space-borne instrument [11], from which major advances in humidity profiling with global coverage are expected [12]. While ground-based water-vapour DIALs operate at two wavelengths only (one on-line and the other off-line), space-borne instruments may operate at a set of different on-line wavelengths corresponding to absorption lines of different strengths, thus providing different p...

Optical frequency locked loop for long-term stabilization of broad-line DFB laser frequency difference

“…Applications range from quantum optics, cold atomic physics and off-resonant light-atom interfaces [1][2][3][4][5], through frequency comb stabilization [6][7][8][9] to precision spectroscopy and sensing [10,11].…”

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In multiple applications, phase coherence of the two laser fields locked at a frequency offset is not required [2][3][4][5][6][7][8][9][10][11] and a mere frequency lock is a sufficient solution. Nevertheless, one of the most commonly used solutions is the optical phase locked loop (OPLL) [12][13][14][15].

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Laser offset-frequency locking up to 20 GHz using a low-frequency electrical filter technique

“…However, some applications require the stabilization of a laser source at a frequency located away from any absorption line, which cannot be accomplished using the above-mentioned techniques. This is, for example, the case in the differential absorption lidar (DIAL) technique for which one or several on-line wavelengths as well as one off-line wavelength must be precisely controlled and stabilized [3,9].…”

“…Besides its simplicity, the main advantage of this method is to essentially use lowfrequency electrical components (apart from a fast photodetector and a frequency downconversion stage) to achieve offset frequencies up to several tens of gigahertz (20 GHz experimentally demonstrated). This offset-locking scheme has been developed for the realization of a multiple-frequency reference unit to be used for future injection seeding in a fourwavelength spaceborne water vapor DIAL system operating in the 935 nm range [9]. In the following, we present a detailed description of the offset-locking principle, including both simulations and experimental demonstrations.…”

Seed laser frequency stabilization for Doppler wind lidar

“…Based on the above technologies, the frequency of the seed laser source is highly stable and tunable in a wide range. This scheme can also be extended to the area of differential absorption lidar (DIAL) for water-vapor [11] , CO 2 concentration [12] sensing, and so on. Our absolute frequency locking setup for the ML is shown in Fig.…”