Abstract
Two slightly shifted wavelength Bragg gratings are inscribed one over the other in a single mode fiber, by moving the phase-mask (PM) between two positions. The inscription setup includes a NIR femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. A first fiber Bragg grating (FBG) is inscribed, while the second overlapping FBG is inscribed only after a slight movement of the PM, enabling a slight wavelength shift. The transmission spectrum of the complex structure is like that of a phase-shifted fiber Bragg grating (PSFBG), while the inscription process is very fast and simple compared to other standard methods. A high-quality PSFBG with two −22 dB transmission dips, and a central 16 dB transmission peak with a 30 pm transmission bandwidth at 3 dB is achieved. It is also observed that this PSFBG structure is birefringent. Finally, the temperature and strain sensitivities of the PSFBG are experimentally measured and are 10.8 pm/°C and 0.8 pm/µɛ, respectively.
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