Field-widened Michelson interferometer system as the spectroscopic filter of high-spectral-resolution lidar

We propose and develop a field-widened Michelson interferometer (FWMI) system to act as a new type of spectroscopic filter in HSRL application. Due to the field widening characteristic, the FWMI can allow relatively large off-axis incident angle, and can be designed to any desirable wavelength. The theoretical foundations of the FWMI are introduced in this paper, and the developed prototype interferometer is described. It consists of a solid arm made of the glass H-ZF52 with the dimension of 37.876 mm, and an air gap with the length of 20.382 mm. These two interference arms are connected to a cube beam splitter to constitute a Michelson interferometer. Due to the matched dimensions and refractive indices of the two arms, the experimental testing results show that the OPD variation of the developed FWMI is about 0.04 lambda and the RMS is less than 0.008 lambda when the incident angle is as much as 1.5 degree (half angle). The cumulative wavefront distortion caused by the FWMI is less than 0.1 lambda PV value and 0.02 lambda RMS value. To lock the filtering frequency of the FWMI to the laser transmitter, a frequency locking system, which is actually an electro-optic feedback loop, is established. The setup and principle of this frequency locking system are also described in detail. Good locking accuracy of the FWMI about 27MHz is demonstrated through the frequency locking technique. All these results validate the feasibility of this developed FWMI system as a spectroscopic filter of an HSRL.