Induced Bragg Gratings in Optical Fibers and Waveguides Using an Ultrafast Infrared Laser and a Phase Mask

Abstract

Since its development in 2003, the technique of Bragg grating inscription in optical fibers and waveguides with ultrafast infrared radiation and a phase mask has proven to be as simple as the standard UV-laser grating writing techniques but far more versatile. The ultrafast IR laser-based process allows for the creation of grating structures in glassy and crystalline materials that are not typically UV photosensitive. In this article, we will review the studies that have been performed at the Communications Research Centre Canada on the grating formation processes as well as applications of the ultrafast laser technique to fabricate gratings in various optical fibers and waveguides.

Femtosecond pulse written fiber gratings: a new avenue to integrated fiber technology

“…In such a setup, the length of the FBG is limited by the beam diameter, the diffraction angle of the phase mask and the distance of the fiber from the phase mask. The first inscription of femtosecond FBG by side illumination was achieved in this way [44] and it is now a common approach [44,46,52,[70][71][72][73][74]. It turns out that this approach actually benefits from the short coherence length of the femtosecond laser because of the order-walk-off effect firstly observed and numerically calculated by Smelser et al [75,76].…”

“…Defocusing effects due to the curvature of the fiber occur that further elongate the focus [46] as well as filamentation [128]. In experiments, low-NA setups yielded elongated structures ranging from 10 [62,99] to 50 micron [46,59,70] in length. This allows to inscribe FBGs with very large cross sections that cover almost half of the fiber [77].…”

“…In contrast, direct ultrafast laser inscription relies on nonlinear absorption and also works in nonphotosensitive glasses and crystals [31,32]. Thus, grating inscription in almost any optical fiber is possible [70]: FBGs could be realized in heavily doped phosphate glass fibers [97] and in highly nonlinear bismuth oxide fibers [98] as well as ZBLAN fibers [99]. FBGs can be inscribed in active fibers as well, which are heavily doped with erbium, ytterbium or thulium [51,56,57,97,[100][101][102][103][104][105][106][107][108].…”

Low loss Type II regenerative Bragg gratings made with ultrafast radiation

“…By optimizing exposure times so that type II index change begins to dominate over type I, high reflectivity thermally stable gratings with low scattering loss can be fabricated after thermal treatment. Such gratings are very useful for distributed temperature sensing in harsh environments [11,12].…”

Radiation tolerant fiber Bragg gratings for high temperature monitoring at MGy dose levels

“…The ultrafast laser was focused on the optical fiber core with the help of a cylindrical lens having a focal length of 10 mm. A scan of the beam along the whole fiber core was performed at a frequency of 25 mHz to homogenize the induced refractive index by the technique reported by Mihailov et al [7]. The laser power was adjusted between 400 and 600 mW as a function of the desired grating strength.…”