The Next Generation Heterodyne Laser Interferometer in Joule Balance

In the joule balance, it is important to measure the relative displacement between the exciting magnet and the suspended coil. So, we have designed and applied a compact dual-axis heterodyne interferometer, which is vacuum compatible and has been carefully aligned for dual parallel measurement arms. However, with the wear of the linear translation stage and the coupling of three-axis relative displacement locking, the periodic nonlinearity error may drift during the displacement measurement of joule balance, and so the performance of the existing laser interferometer system is not so stable. In order to solve this problem, in this article, we have designed and employed a new heterodyne interferometer based on spatially separated beams (SPBs) to restrain the periodic nonlinearity error caused by the double-frequency mixing. As to this SPB heterodyne interferometer, the input and output (I/O) beams can share the same optical space, which will contribute a lot to a compact structure in the joule balance. Meanwhile, the spatial attitude of the interferometer is precisely controlled to restrain the error caused by ghost reflections. The experimental result shows the validation of this SPB heterodyne interferometer for the displacement measurement in the joule balance.