An all-fibre-optic Fourier transform spectrometer

A novel fibre-optic Fourier transform spectrometer was constructed. The optical retardation between the arms of a Mach-Zehnder-type interferometer made of single-mode optical fibre is modulated by variation of the fibre temperature. Spectra of a multimode laser diode and a broad-band light source were recorded. Frequency shifts within the observed spectra can be explained with the frequency dispersion of the coefficient of the dependence of the refractive index on temperature, dn/dT. An oversampling scheme utilizing a phase-locked loop circuit was successfully tested. This allows for the acquisition of spectra with shorter wavelength components than the wavelength of the reference laser if a fibre with sufficiently low cut-off wavelength is employed. Thus, the accessible wavelength region may be extended to the near UV using a conventional silica fibre. Coherent averaging of multiple interferograms was successfully applied to enhance the signal-to-noise ratio of interferograms.