Fiber optic Fourier transform spectrometer with a coherent interferogram averaging scheme

A fiber optic Fourier transform spectrometer capable of coherent interferogram averaging is constructed, for the first time to our knowledge. One fiber arm of the interferometer is periodically stretched, and the amplitudes of the fundamental and second harmonics of the modulation frequency in the photocurrent output are simultaneously detected. Successful operation results from the generation of an external clock for sampling the interferogram from the detected outputs varying in phase quadrature. Even when the moving mirror in the interferometer translates nonlinearly and two fiber arms receive random perturbations independently, sampling can be uniformly performed along the interferogram. From the experiments, variation of sampling start position on each interferogram was limited within 100 nm during one hundred repetitive scans. For 100 coherent averages, therefore, the SNR increased by ~10 without deformation.