2.2        Fiber Bragg grating

2.2.3       FBG fabrication by the phase mask technique

A phase mask is a quartz plate on which a periodic corrugation has been engraved. The period and depth of the mask grating are optimized to maximize the first order of the Bragg reflection for a given light wavelength. The superposition of the ± 1 order generates an interference pattern with a period that is half the mask period. The interference can only occur if the illumination light source exhibits a sufficient coherence length (temporal and spatial). If the fiber is placed in the interference region, a FBG with half the phase mask period can be written. Typical values of 40 % energy in each first order of diffraction are observed. The remaining zero order (less than 1 % to 5 % of the incident beam) reduces the fringe visibility of the interference pattern and induces a constant refractive index change. The realized writing set-up is presented in Fig. 2-4.

Fig. 2-4 FBG writing set-up with the phase mask technique

The maximal photosensitivity depends partly on the laser fluence and for this reason the laser beam height is reduced by a factor three. The beam reducer system is composed of the convex and concave cylindrical lens that can be seen in the side view of Fig. 2-4. The beam is preferably kept parallel to protect the phase mask from beam focusing that could damage the grating, but some experiments have required focalizing the beam to reach the maximal fluence on the fiber.

The illumination source is a pulsed excimer laser operating at 193 nm (ArF) with energy ranging from a few millijoules to 240 mJ per pulse. In addition, another cylinder lens can be introduced in the system to enlarge the beam width from 6mm to 3cm and thus increasing the possible grating length. The laser beam width is limited by a slit limits placed before the phase mask. The phase masks can be changed or removed very easily. A CCD video system is used to align the fiber in front of the phase mask with respect to the laser beam. The setup shows a good mechanical stability since grating erasure was observed only at high total dose. Without the phase mask, the setup allows homogeneous post-exposure of the fiber to increase the mean refractive index and permits hence a wavelength fine tuning.