Simultaneous frequency stabilization of four laser diodes in the 935-nm water vapour spectrum for space application

Water vapour is a major atmospheric gas in terms of impact on climate and weather. Since its highly variable distribution in time and space is not covered satisfactorily with current instruments, improved water vapour measuring systems with global coverage are strongly demanded by the user community. For this reason, the European Space Agency (ESA) has been planning a satellite-borne H2O differential-absorption lidar (DIAL) instrument. The DIAL technique offers high vertical resolution and selective gas detection by sensing the difference in light absorption at two close on-line and off-line wavelengths. In the frame of an ESA-ESTEC supported study, we are developing a frequency detection unit (FDU) delivering four stable cw optical signals. They will be optically injected into the pulsed highpower oscillator of the transmitter laser, in order to set its spectral properties. This FDU is made of four injection seed lasers (ISL) precisely stabilized on four different wavelengths in the 935.4-nm range. Two different types of ISLs are used in the set-up: ECDL and DFB lasers. Three lasers are directly locked on three H2O absorption lines of different strength, whereas the fourth laser wavelength lies outside of any absorption line (off-line wavelength). This off-line stabilization is achieved by an offsetlocking technique. The FDU thus combines various stabilization schemes in order to grant an ISL stability better than 60 MHz over the 2-year expected lifetime of the system.