Inscription of Long Chirped Fiber Bragg Gratings in a Nonuniform Fiber for Wide-Range Tunable Delay Line

A wide-range tunable delayline with long delay time is achieved based on a 50 cm long chirped fiber Bragg grating (FBG) in a nonuniform hybrid fiber. The chirped fiber Bragg grating is inscribed by a standing wave formed by two counter-propagating continuous-wave lights, which induces a periodical refractive index change due to high photosensitivity of As2Se3 core and creates a standard FBG along the whole nonuniform taper section. A 50 cm long chirped FBG is obtained from the inscribed standard FBG by streching the nonuniform As2Se3-PMMA tapered fiber. The delay time is larger than that using traditional chirped FBGs due to the long inscribed section and the high refractive index of the fiber core and the large tunable range is attributed to the the low stiffness of the micron diameter As2Se3 core and PMMA cladding. The maximum delaytime is ~10 ns and the maximum tunable delay time is 0.417 ns at the wavelength of 1553.5 nm, which is, to the best of our knowledge, the largest tunable delay time ever reported in a chirped FBG. The delayed performance can be tailored by designing the nonuniform profile of the inscribed tapered fiber.