Low-coherence grating characterisation scheme

Volanthen, M., Geiger, H. and Dakin, J.P. (1997) Low-coherence grating characterisation scheme. In Proceedings of IEE Colloquium on Optical Fibre Gratings. IEEE. 6/1-6/6 . (doi:10.1049/ic:19970240).

Abstract

Optical fibre gratings are being produced for an increasing number of applications. With the advances in grating fabrication technology, advances in grating characterisation schemes are required. A few years ago, only the reflectivity, bandwidth and central wavelength of gratings were needed. Today, applications such as dispersion compensation and distributed sensing require the distributed characteristics of a grating, namely the time delay and reflectivity as a function of wavelength. Until recently, distributed grating measurements have been made using a tunable narrowband laser source. The wavelength to be measured was selected by tuning the laser. The reflectivity was obtained from the returning optical power level and the time delay of the chosen wavelength was obtained using phase measurements. The phase measurements were either optically coherent, or optically incoherent (electrically coherent).
These systems only correctly characterise gratings having monotonic wavelength profiles. Their temporal resolution is determined by the local chirp of the grating and the grating wavelength. For dispersion compensation, gratings with a linear, monotonic chirp are required and thus the laser characterisation scheme at first sight appears suitable.
We recently demonstrated a novel grating characterisation system using a broadband source. Low-coherence interferometry selected the time delay to be interrogated and a tunable filter measured the local Bragg wavelength. This system has characterised gratings of arbitrary (including non-monotonic) wavelength profiles.
The latest results of our low-coherence interrogation system are presented together with results from a theoretical analysis of its performance. The ultimate performance characteristics are predicted and examined experimentally.

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