Abstract

DECIGO (DECI-hertz Interferometer Gravitational wave Observatory) is a space gravitational wave antenna which consists of three 1000-km Fabry-Perot interferometers to detect space distortion with the strain sensitivity of l/l<10^-23 around 0.1 Hz[1]. Before planned launch of DECIGO in 2027, DECIGO pathfinder (DPF) is also planned to launch in 2015 for establishing technical feasibility[2]. In order to obtain such high strain sensitivity, the high frequency stability of f<1Hz/√Hz, and the intensity stability of I/I<10^-8/√Hz at 1Hz, and high power are required for the light source of DECIGO/DPF, whose frequency level is 10 times better than that of LISA project. We have developed a space-borne frequency-stabilized laser around 1m for DPF. In stead of using an optical cavity, the molecule absorption is chosen as a optical frequency reference because it is less sensitive to external perturbations. In 1m band, the iodine (I₂)-stabilized Nd:YAG lasers with the wavelength of 1064 nm have been studied in many groups as the optical frequency standards. However, the frequency stability of the I₂-stabilized Nd:YAG laser was limited at 20Hz/√Hz at 1 Hz due to the signal to noise ratio of the saturated absorption signal[3], whose level is worse than the required level of DECIGO/DPF. Since the absorption linewidth of I₂ at 515nm is much narrower than that at 532nm[4], the frequency of Yb:YAG laser at 1030nm was stabilized to the saturated absorption of I₂ for the light source of DPF. The I₂-stabilized Yb:YAG laser for short-term frequency stability has hardly been tried before. Fig.1 shows the schematic diagram of the I₂-stabilized Yb:YAG laser. The light source is an LD-pumped monolithic Yb:YAG laser (Yb:NPRO) with the output power of 100 mW. The output of the Yb:NPRO was frequency-doubled by a wave-guide PPLN crystal, and the saturated absorption signal was obtained by using modulation transfer technique. The counter-propagating signal and pump beams were transmitted 4-fold through a 400-mm iodine cell, whose total interaction length was 1.6m. A bread-board model (BBM) for DPF was assembled on a 400mm by 400mm aluminum plate. The shot-noise-limited signal was obtained by balanced detection technique, and the frequency noise of the Yb:NPRO was suppressed down to 1 Hz/√Hz at 1Hz which is evaluated from error signal of the servo circuit and SNR of the saturated absorption signal(Fig.2). The frequency stability of our I₂-stabilized Yb:YAG laser is now evaluated more precisely by using a Cryogenic Sapphire Oscillator (CSO)-stabilized femtosecond mode-lock optical frequency comb. The intensity noise of the laser was suppressed down to 1x10^-8 /√Hz at 1Hz by controling the injection current of the pumpLD.

© 2011 Optical Society of America