Slit Beam Shaping Technique for Femtosecond Laser Inscription of Enhanced Plane-by-Plane FBGs

Abstract

In this work, the development of plane-by-plane (Pl-b-Pl) fiber Bragg gratings (FBGs) by using a single pulse is reported. A slit oriented along the longitudinal axis of the fiber is employed for this. The purpose is to shape the beam so that the focal volume is spatially wider in the transverse direction of the fiber. In this way, it is possible to make 2-D modifications of the refractive index whose width and height depends on the slit width and the pulse energy, respectively. Through an analytical mathematical modelling, the relationship between the diameter of the Gaussian laser beam (Dx) and the slit width (s = Dy) is determined in order to obtain a circular crosssection. In the experimental results carried out, it can be observed that Dx/Dy=3 ratio cause positive and uniform refractive index changes, compared to the negative and inhomogeneous refractive index changes corresponding to point-by-point (PbP) FBGs. Likewise, Pl-b-Pl FBGs, presenting a broader modification of the core cross-section, show significantly lower losses in transmission (0.3 dB), as well as better reflectivity and FWHM. It is observed that these three parameters (losses, reflectivity and FWHM) have a monotonous tendency according to the slit width. Polarizationdependence is also evaluated.

Optical Fiber Sensors by Direct Laser Processing: A Review

“…This improvement can be clearly seen in Figure 4a,c where transmission microscope images of a FBG written without and with slit are respectively depicted. The reported FBGs written with this technique exhibit lower PDL, FWHM, broadband losses, and higher reflectivity than its PbP counterpart at the same effective pulse energy [83].…”

Single Longitudinal Mode Lasers by Using Artificially Controlled Backscattering Erbium Doped Fibers

“…1. In order for each RIC to be induced in the entire core cross-section, the slit beam shaping technique is used [10], with a slit width of 400µm (2.5mm beam diameter).…”

Photonic circuits written by femtosecond laser in glass: improved fabrication and recent progress in photonic devices

“…For this purpose, several beam shaping techniques were introduced, including slit beam shaping, astigmatic beam shaping, simultaneous spatiotemporal focusing (SSTF), and spatial light modulator (SLM) beam shaping. [23][24][25][26][29][30][31][32]…”

“…Although the slit beam shaping technique is simple, it is not efficient, and most of the laser energy is attenuated by the slit. 32,38 To shape the laser beam, astigmatic shaping is also adopted to reshape the beam with an astigmatic cylindrical telescope before focusing, which allows for writing WGs with a circular cross-section and arbitrary size. 29,31,[39][40][41][42][43][44] The basic principle for the astigmatic beam shaping is that the WG size does not depend on the focal size along the sample translation direction in the transverse writing regime.…”