Broadband Tunable All-Fiber Polarization Interference Filter Based on 45$^{\circ}$ Tilted Fiber Gratings

We have theoretically and experimentally designed and demonstrated an all-fiber polarization interference filter (AFPIF), which is formed by a polarization-maintaining (PM) fiber cavity structure utilizing two 45
$^{\circ}$
tilted fiber gratings (45
$^{\circ}$
-TFGs) inscribed by UV laser on the PM fiber. Such a filter could generate modulated transmission of linear polarization status. It has been revealed that the modulation depth of the transmission depends on the coupling angle between the 45
$^{\circ}$
-TFGs and the PM fiber cavity. When the two 45
$^{\circ}$
-TFGs in PM fiber are oriented at 45
$^{\circ}$
to the principal axis of the PM fiber cavity, the maximum modulation depth is achievable. Due to the thermal effect on birefringence of the PM fiber, the AFPIF can be tuned over a broad wavelength range just by simple thermal tuning of the cavity. The experiment results show that the temperature tuning sensitivity is proportional to the length ratio of the PM fiber cavity under heating. For 18 and 40 cm long cavities with 6 cm part under heating, the thermal tuning sensitivities are 0.616 and 0.31 nm/
$^{\circ}$
C, respectively, which are almost two orders of magnitude higher than normal fiber Bragg gratings.