Asynchronous equal optical path difference sampling method for MEMS-based Fourier transform spectrometers

In a MEMS micromirror-based Fourier transform (FT) spectrometer (FTS) working in the near-infrared (NIR) range, a laser with a NIR wavelength is usually used to accurately detect the position of the MEMS mirror plate in real-time. The interference signal generated by the NIR laser for position measurement is acquired synchronously with the broadband interference signal from the sample under test, causing strong coupling of the laser signal into the sample signal. In this work, an asynchronous equal optical path difference (AEOPD) sampling method is proposed to avoid the optical coupling due to the synchronous signal acquisition. In this method, the sample interference signal is acquired when the laser is turned off, i.e., the laser is turned on periodically. A MEMS FTS system based on an electrothermal MEMS micromirror has been built to verify the feasibility of this method. Experimental results show that the period of the laser pulses can be as long as 12.5 minutes when maintaining the MEMS FTS with its spectral repeatability within 0.0005. Experiments also demonstrate that the laser coupling is eliminated and the signal-to-noise ratio (SNR) is increased. Thus the method proposed in this work can be applied in MEMS-based FTS spectrometers in the future.