Gap FBG and its application in tunable narrow linewidth fibre laser

●The gap FBG has the characteristics of both a phase shifted FBG and a Fizeau interferometer.●The phase wavelength of the gap FBG can be tuned by adjusting the air-gap longitudinally or transversely.●A tunable fibre ring laser with a narrow line-width was attained using the gap FBG.

A gap fibre Bragg grating (g-FBG) is fabricated by cutting a uniform FBG in the middle to introduce a small air gap between the two sections. Numerical and experimental investigations show that the g-FBG has the characteristics of both a phase shifted FBG and a Fizeau interferometer. The peak transmission wavelength of the g-FBG can be finely tuned by adjusting the air-gap longitudinally or transversely with respect to the fibre central axis. With a g-FBG as tunable intra-cavity filter, a tunable fibre ring laser with a narrow line-width is demonstrated. The wavelength tuning range of the laser is 0.12 nm with an output power flatness better than 0.9 dB. The 3 dB spectral line-width is less than 12 pm and output power is 3.68 dBm.