Experimental investigation of point-by-point off-axis Bragg gratings inscribed by a femtosecond laser in few-mode fibers

Abstract

Off-axis Bragg gratings with varied horizontal and vertical distances off the center in a step-index two-mode fiber were fabricated by 800 nm infrared-femtosecond laser pulses through a point-by-point technique. In this article, we experimentally investigate these gratings via measuring the transmitted power and the reflected intensity profiles under different input polarization, with multiple characteristics reported for the first time to the best of our knowledge. To highlight, we find that the birefringence induced to the LP01 reaches its maximum magnitude at an intermediate offset, followed by the fast and slow axes switching at a further slightly increased offset. We also show that the peak reflectivity of the LP11 exhibits strong polarization dependence, with the much stronger peak reflectivity constantly corresponding to the polarization perpendicular to the damage-point-to-center line, whereas the peak reflectivity of the LP01 has almost no polarization dependence. Moreover, we report that the reflected mode patterns of the cross-coupling of the LP01 and LP11 are linked to the direction of linear polarization, through which one can selectively excite an arbitrarily oriented LP11 by merely altering the polarization.

Non-destructive technique for characterization fiber Bragg gratings via diffraction profile

“…It is attributable to the refractive index modification (RIM) induced under each grating point. This RIM is a consequence of aberrations introduced by the focusing system of the femtosecond laser during the FBG fabrication process 35,24,36 . The vertical displacement of the FBG brings the RIM closer to the central region of the fiber core.…”

“…The study discusses the influence of the grating microstructure on the observed diffraction patterns. Later, researchers analyzed side diffraction patterns experimentally [21][22][23][24] and theoretically using Volume Current Method [25][26][27][28] , Coupled Mode Analysis, 29 Free Space Model, 30,31 and Antenna Theory 31 .…”

Controllable Light Scattering on Fiber Bragg Gratings in Multimode Fibers: Tailoring Angular Emission for Advanced Fiber-Based Light Sources

“…32 In all reported cases, the maxima in ϕ formed perpendicular to the direction of the writing laser that imprinted the structure in the fiber. It was attributed to an irregular refractive index modification of the fiber core cross section; 19,31,33 Qui et al 34 even used this effect to find out the transverse orientation of the grating.…”