Phase shifted and cascaded long-period fiber grating written by using focused high-repetition-rate CO2 laser pulses

Mode-couplings in a LPG Long period fiber grating (LPG) Mode-couplings in a LPG Phase matching condition: Core Cladding core-mode Forward cladding modes Resonant wavelength:

Spectral response of the LPG Transmission (dB) Wavelength (nm) 1600 1400 1800 LP11 LP13 LP15 LP17 LP19

Applications of the LPG Various kinds of filters used in fiber optic communications: (Band-pass filter, band-rejection filter, gain flatten filter….) All-fiber-type environmental parameters sensor All-fiber-type biochemical sensor All-optical signal processing devices (Optical switching, optical differentiator and optical pulse shaper)

Fabrication techniques for the LPG Ultraviolet (UV) exposure The electric-arc discharge The mechanical pressure The focused CO2 laser radiation

Purpose of this study A simple scheme to write the LPFGs is proposed, which is based on the directly writing technique realized by using a high-repetition-rate CO2 laser pulses. Two particular gratings: The phase shifted LPFG The cascaded LPFGs where both the complex phase-mask and the time-tedious post-processing procedures can be avoided.

Experimental setup

Principle scheme for the formation of a LPFG Temporal profile of the laser output at a site just after the shutter The ideal profile of the resulted index-change of LPFG along the fiber

Principle scheme for the formation of a phase-shifted LPFG Index-change profile with a phase-shift inserted and (b) without phase-shift inserted.

Theoretical Results Theoretical results for the transmission spectra of LPG with and without a phase-shift. (a) Without the phase shift inserted, (b) with the phase-shift of p, (c) with the phase-shift of 0.75p, and (d) with the phase-shift of 0.5p.

Experimental results Experimental results for the measured transmission spectrum of LPG with and without phase-shift. (a) Without the phase shift inserted, (b) with the phase-shift of p, (c) with the phase-shift of 0.75p, and (d) with the phase-shift of 0.5p.

Formation scheme for the cascaded two LPFGs

Principle analysis for the cascaded LPGs Grating pitch Λ M-Z Interferometer Incident d Core mode Cladding mode T (dB) Wavelength (nm)

Experimental results Experimental results for the measured transmission spectra the cascaded LPGs with a separation of (a) 0 cm, (b) 10.6 cm, (c) 13.0 cm, and (d) 37.0 cm.