Chapter 4 : FBG characterization by optical low coherence reflectometry

Precise characterization of the grating parameters is essential and the knowledge of the local distributions (Dn_ac, Dn_dc and L) is often desired during the writing process or in distributed sensing. It has been shown that optical low coherence reflectometry (OLCR) is a powerful method to characterize the position, length, and index modulation of homogeneous FBGs [4-1].

Measurements using interferometers can be affected by small perturbations, like temperature variations or vibrations, in both interferometer arms that modify the optical path length difference (OPLD). Corresponding drifts of the interference signal phase have to be compensated or the measurement has to be performed in a few milliseconds for assuring high accuracy. A high speed OLCR set-up has been developed to retrieve the complex spectral properties of chirped gratings [4-2]. The drawback is a limited S/N (shot noise) of typically -80 dB [4-3]. In this work, we built a new OLCR set-up, where the S/N is only limited by the Rayleigh scattering in the fiber and the phase drifts are compensated. The OLCR phase information of a FBG is precisely measured by comparing it with the phase of a tunable laser at the same wavelength.

In this chapter, we will show that the OLCR response is mainly the impulse response of the measured FBG. Moreover, the interference of partially coherent light is presented and its application to the OLCR case is developed. Two interfereometers have been conceived and realized, where the reference laser was either wavelength multiplexed or time multiplexed. The reconstruction process of the complex coupling coefficient from the measured OLCR response is exposed. Several FBGs with different properties were measured and their local properties reconstructed.