Compact tunable filter based on microfiber Bragg grating

Tunable filters for wavelength-division multiplexing transmission system are essential. Fiber Bragg grating (FBG) is one of the best candidates for this application. The wavelength shift of FBGs can be tuned by applying different temperature, hydrostatically pressure and strains (in traction or/and compression). A comprehensive study of using these three parameters as being tunable filters was reported [1]. Either traction [2] or compression [1, 3] or hybrid [4] could practically achieve long range tuning. The tensile breaking strength of FBGs is about 700~1000-MPa [1, 5]. This corresponding brittle failure gives a strain of ~1% in tension, i.e. ~12-nm wavelength shift. For conventional optical fiber with diameter of 125-μm, the force required to reach the tensile brittle failure is about 8.6-N. This force requirement can only be achieved by using high voltage driven piezoelectric transducer, actuator or step motor. One simple way to minimize the applied force is to use microfibers, which has small diameters and ease of handling. However, it lead to another problem that was the optical guiding become multi-modes [6, 7]. The optical guiding was no longer between the core and the cladding, but instead, between the silica and air. Having the single reflective mode characteristics, with designated taper, a microfiber Bragg grating (MFBG) is a potential alternation as a tunable filter with a key feature of small force driven, as a result, leading to a low voltage driven and compact filter design feasibility. In this paper, we present the result of a tunable filter based on an effectively single mode MFBG, followed by the discussion of the study of the MFBG as a tunable filter.