6.1         Influence of humidity and temperature on polyimide-coated fiber Bragg gratings

This section is based on the paper presented at the BGPP conference (Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides) organized by the Optical Society of America in Stresa (June 2001). The contributing authors were :

-         Philippe Giaccari, Hans G. Limberger : Institute of Applied Optics, Swiss Federal Institute of Technology

-         Pascal Kronenberg : Institute of Structural Engineering and Mechanics, Swiss Federal Institute of Technology

6.1.1         Abstract

The influence of humidity and temperature on a polyimide coated in-fiber Bragg grating was investigated. The obtained normalized Bragg grating responsivities are 4.36×10^-6 RH%^-1 and 1.06×10^-5 K^-1, respectively.

6.1.2          Introduction

Intra-core fiber Bragg gratings (FBG's) have a huge application potential in telecommunication and sensor networks. The reliability of these devices is crucial for their long-term applications [1]. In addition, a channel spacing of 50 to 100 GHz in dense wavelength division multiplexing systems (DWDM) or precise temperature monitoring requires high wavelength stability over time in standard or harsh environments. Environmental parameters such as temperature and stress are known to alter the filter characteristics of FBG's [2, 3].

Here we report on first results on the sensitivity of polyimide coated FBG's to relative humidity with a cross-sensitivity to temperature. Bare fiber FBG's are insensitive to humidity as bulk glass. However, polyimide polymers are hygroscopic and swell in aqueous media. This coating swelling induces axial and radial strain in the fiber, modifying the Bragg condition of the FBG.