Frequency stabilisation 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 spaceborne 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 repetition 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.