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

2. 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:

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

4. 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)

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

6. 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.

7. Experimental setup

8. 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

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

10. 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.

11. 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.

12. Formation scheme for the cascaded two LPFGs

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

14. 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.