Frequency stabilization of Q-switched Nd:YAG oscillators for airborne and spaceborne lidar systems

Abstract

Lidar Systems for the measurement of three-dimensional wind or cloud and aerosol formations in the earth atmosphere require highly stable pulsed single frequency laser systems with a narrow line width. The lasers for ESAs ADM-Aeolus and EarthCARE missions require frequency stabilities of 4 and 10 MHz rms at a wavelength of 355 nm and a line width below 50 MHz at 30 ns pulse duration[1]. Transferred to the fundamental wavelength of the laser systems the stability requirement is 1.3 and 3.3 MHz, respectively. In comparison to ground based lidar systems the vibrational load on the laser system is much higher in airborne and spacebome systems, especially at high frequencies of some hundred Hertz or even some kHz. Suitable frequency stabilisation methods have therefore to be able to suppress these vibrations sufficiently. The often used Pulse-Build-up method is not suitable, due to its very limited capability to suppress vibration frequencies of the order of the pulse repeti tion frequency. In this study the performance of three frequency stabilisation methods in principle capable to meet the requirements, the cavity dither method, the modified Pound-Drever-Hall method and a modified Ramp-Fire method - named Ramp-Delay-Fire - is theoretically and experimentally investigated and compared. The investigation is performed on highly efficient, passively cooled, diode end-pumped q-switched Nd:YAG oscillators, which are breadboard versions of the A2D (ADM-Aeolus) and possible ATLAS (EarthCARE) oscillators. They deliver diffraction limited output pulses with up to 12 mJ pulse energy at a pulse duration of 30 ns and 100 Hz pulse repetition rate

Airborne wind lidar observations over the North Atlantic in 2016 for the pre-launch validation of the satellite mission Aeolus

“…In addition to the strict requirements in terms of frequency stability, a further challenge is imposed by the necessity to trigger the receiver electronics about 60 µs before the laser pulse emission with an error of less than 100 ns. Therefore, a dedicated active frequency stabilization technique was developed which is based on the ramp-delay-fire method (Nicklaus et al, 2007). Fast detection of the master oscillator cavity resonances with the seed laser frequency enabled effective compensation of higher-frequency vibrations, while providing a sufficiently early trigger for the detector electronics with a timing stability of around 80 ns .…”

Doppler Rayleigh/Mie/Raman lidar for wind and temperature measurements in the middle atmosphere up to 80 km

“…Due to remaining imperfections in the seeding process (thermal perturbations etc.) the wavelength of the outgoing laser pulse can show an offset to the seed laser (Nicklaus et al, 2007). Therefore we measure the emitted wavelength during the atmospheric measurements with an I 2 pulse spectrometer .…”

Airborne wind lidar observations over the North Atlantic in 2016 for the pre-launch validation of the satellite mission Aeolus

“…In addition to the strict requirements in terms of frequency stability, a further challenge is imposed by the necessity to trigger the receiver electronics about 60 μs before the laser pulse emission with an error of less than 100 ns. Therefore, a dedicated active frequency stabilization technique was developed which is based on the Ramp-Delay-Fire method (Nicklaus et al, 2007). Fast detection of the master oscillator cavity resonances with 5 the seed laser frequency enabled effective compensation of higher-frequency vibrations, while providing a sufficiently early trigger for the detector electronics with a timing stability of around 80 ns (Lemmerz et al, 2017).…”