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

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 LP 01 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 LP 11 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 LP 01 has almost no polarization dependence. Moreover, we report that the reflected mode patterns of the cross-coupling of the LP 01 and LP 11 are linked to the direction of linear polarization, through which one can selectively excite an arbitrarily oriented LP 11 by merely altering the polarization.