Fibre interferometer for multi-wavelength interferometry with a femtosecond laser

In this paper we propose a novel fibre interferometer for the simultaneous measurement of optical phase at multiple wavelengths generated within the spectral envelope of a femtosecond laser. The use of narrow bandwidth fibre Bragg gratings allows particular wavelengths to be selected with sufficiently narrow bandwidth to give the necessary coherence length for interferometric phase measurements over practical distances exceeding 30 mm. Multi-wavelength interferometry provides a means of making absolute measurements by extending the unambiguous measurement range beyond the wavelength of light. Preliminary experimental results are presented for a two-wavelength sensor where an absolute range of 0.13 mm with 3 nm resolution has been achieved producing a dynamic range of 43000:1. This approach has the potential to be applied to high speed transient events.