Abstract
In this work, the development of plane-by-plane (Plb-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 (D ) and the slit width (s = D,) is determined in order to obtain a circular cross-section. In the experimental results carried out, it can be observed that D /D, = 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. Polarization-dependence is also evaluated.
Highlights
F IBER Bragg gratings (FBGs) written with femtosecond lasers exhibit a great interest given the increased temperature resistance and a better flexibility of wavelength and composition, thanks to their characteristic high peak power densities that makes possible to achieve nonlinear ionization at arbitrary locations [1, 2]
Any refractive index change must precisely coincide with the longitudinal axis of the optical fiber, requiring precise alignment
This can be explained through the Coupled Mode Theory (CMT) and x the transversal coupling constant between two modes [9]: κab = 2 0∆nm(x, y)n(x, y)ETa (x, y)ETb ∗(x, y) · dxdy. (1)
Summary
F IBER Bragg gratings (FBGs) written with femtosecond (fs) lasers exhibit a great interest given the increased temperature resistance and a better flexibility of wavelength and composition, thanks to their characteristic high peak power densities that makes possible to achieve nonlinear ionization at arbitrary locations [1, 2]. It usually exhibits strong broadband losses [7, 8], lower grating strength and higher order modes coupling To limit the latter, any refractive index change must precisely coincide with the longitudinal axis of the optical fiber, requiring precise alignment. Instead of a single inscription, several lines are inscribed perpendicular to the longitudinal axis This method is significantly slower than PbP, but covers more cross-section of the core, reducing the amount of cladding modes coupled. The Pl-b-Pl method involves the inscription of a plane that produces a quasi-homogeneous 2D refractive index change in the core (or cladding) cross-section This method involves flexible control of the width (direction of the Y axis in Fig. 1) and height (Z axis) of the planes, since the thickness of the plane (X axis) is defined by the laser wavelength and the numerical aperture of the objective lens. The transmission and reflection spectra are presented, as well as the polarization dependent loss, in order to study the improvements achieved with this new method
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
