Extended-range multiwavelength all-fiber interferometer

摘要

This paper presents a design procedure for a fibre interferometer, the optical system and its associated electronic control. Analog and digital circuits were optimized to achieve a low cost compact system. The lock-in amplifier required for phase control was designed using a FPGA. The errors in an interferometric measurement were studied in detail and its results were used to estimate the capabilities of the interferometer. These matched our observed resolution of 40 nm. A stabilization technique of controlling the path length difference between the arms of interferometer nullifies any phase errors. The design and testing of the control circuit are described in detail. In addition, the FPGA was programmed to carry out phase stepping, as this technique is used to calculate the desired phase. The interferometer was used to measure samples with step heights in the hundreds of nanometers, with improvements in accuracy through averaging of data. We verified the successful working of the instrument by measuring a height of 423 nm for a 420 nm structure.