Progress in laboratory R & D for fundamental physics space missions - weak light phase-locking, fibre-linked heterodyne interferometry, fibre delay line and picometre real-time motion control

Since 1992, we have engaged in laboratory studies for astrodynamical missions to test relativistic gravity in the solar system. The techniques developed are also relevant to other fundamental space missions such as LISA and the space interferometer for astrometry. Here we report our progress in weak light phase-locking, long fibre-linked heterodyne interferometry, fibre delay line and picometre real-time motion control during the last year. We demonstrated that for two lasers with offset locking up to 2.5 GHz, the heterodyne linewidth after travelling through a 26.27 km fibre-linked interferometer is less than 1 mHz. For weak light phase-locking, we achieved 4.3 nW locking with a 3.4 mW local oscillator. We improved our side-polishing technique to polish more than eight fibres simultaneously and reached a tunable sensitivity as high as 85 - 90 dB in liquid-drop tests. Using these side-polished fibres, we are currently in the process of making tunable directional couplers and fibre delay lines. For laser metrology, we use mid-point cyclic averaging to reduce the nonlinearity error, and use a fitting method to cancel the drift, and have reached 1.5 pm linearity. Currently, with modulation and real-time cyclic averaging, we reach a real-time measuring precision of 560 pm and real-time motion-control precision of 700 pm.