Long-distance laser positioning system by using acousto-optic deflector

Long-range three-dimensional laser positioning system is the essential technique for precision formation flying in space, especially for 3 or more satellites large scale formation flying mission. We propose a laser positioning system based on an acousto-optic deflector (AOD) for Japanese space gravitational wave detector DECIGO which can measure two-dimensional angle and distance between satellites. DECIGO is an equilateral triangle-shape Fabry-Perot laser interferometer with the arm length of 1000 km whose arm-length difference between satellites should be less than 0.1% [1] . Since it plans to orbit at high altitude where global navigation satellite system cannot be used, the long-range positioning system is indispensable for initial alignment and keeping it formation. Such angle measurement systems have never reported before. In our system, a laser beam is diffracted by AOD1 in the local site whose diffraction angle is scanned by sweeping its driving frequency, which is called signal beam. The local beam is also propagated with collinearly overlapping with signal beam whose frequency is shifted by an acousto-optic modulator AOM1 in the local arm1( Fig. 1 ). The remote site detects swept diffraction beam, and knows its position (angle) from the frequency shift of the detected diffraction beam as the beat note between the signal and local bean. In our preliminary test in 10.3-m optical path, the angle precision and resolution of 0.0037 mrad and 0.017 mrad are obtained which would be improved in the long-distance optical path under the vacuum condition ( Fig. 2 ). Two-dimensional scan of the beam is performed by adding another AOD (AOD2) located just after ADO1 whose crystal axis is set in orthogonal to that of AOM1, and each AODs are driven by independent frequency, f m and f n . In order to detect 2-D information at the remote site, another AOM (AOM2) is also set at the local arm2 which is driven at f n , and both 0 th and 1 st diffraction beam are selected in double-pass configuration so that the two frequency components are carried in local beam. The remote site detects two frequency beats, f m and f n from one detected diffraction beam from the local satellite to know two-dimensional angle information.