Spectral linewidth and tuning requirements of sources for gas sensing in space-based applications at 935 nm

Abstract

Abstract:The spectral linewidth and tuning requirements of an external cavity laser (ECL) in the Littrow architecture and a distributed feedback (DFB) device for space-based gas sensing were investigated. The wavelengths of both devices were in the 935 nm range and different linewidth measurment techniques were explored for the two sources. The linewidth of the ECL was measured using a short-arm homodyne technique, while the linewidth of the DFB was measured with a heterodyne technique using the ECL as a local oscillator. The spectral linewidth of the ECL was measured to be ,300 kHz with ,5 MHz for the DFB device. The emission wavelength tuning characteristics, including the overall tuning range and the free running stability of the ECL, were also examined. The full frequency drift of the ECL measured over a 36-hour period was found to be approximately 540 MHz, while it showed approximately 0.4 nm continuous mode-hop free wavelength tuning. IntroductionThe objective of this work is the stabilisation of four laser frequencies to four specific predefined wavelengths that are associated with water absorption lines in the 935-940 nm range for space-based gas sensing. A list of these water absorption lines, with their wavelengths and linestrengths, is shown in Table 1. The experimental set-up involves locking one injection seed laser (ISL) to the strongest water line via a water vapour reference gas cell. This laser is then used as a reference to stabilise the other lasers, each to one of the three remaining lines. Locking by means of gas absorption is the preferred technique as the absorption wavelength at a fixed temperature and pressure is constant. The stabilisation technique used for the three remaining lasers employs a Fabry-Perot interferometer to reference from the locked ISL by a fixed detuning. The work performed in this research is a portion of a wider study being carried out by the European Space Agency (ESA). The overall project is termed the WALES [water vapour light detection and ranging (LIDAR) experiment in space] mission and its objective is to provide better insight into water vapour, temperature and aerosol distribution in the upper troposphere and lower atmosphere. Sampling both strong and weak water vapour absorption lines at different wavelengths in the 935 nm region can achieve a complete water vapour absorption profile across an entire altitude range. Further information on the WALES mission can be found in [1]. The required optical characteristics of the ISL specified for this project include, an output power .10 mW, linewidth ,1 MHz, continuous tuning range ,0.5 nm, frequency stability ,50 MHz and free-running frequency drift ,500 MHz. Many devices were initially considered as a possible candidate for the ISL but because of the specified wavelength in the 935 nm region, an external cavity laser (ECL) was chosen as the preferred laser technology for the ISL, as it is capable of meeting all the necessary optical requirements. It has long been recognised that semiconductor lasers in an externa...