Polarization and modal filters based on W-fibers Panda for fiber-optic gyroscopes and high-power fiber lasers

Abstract. Polarizing Panda-type W-fibers, practical realizations are presented for fiber-optic gyroscope (FOG) sensing coil (500-m length, 8-μm fundamental mode field diameter) and for polarization/modal filtering at the FOG loop interferometer input (∼1-m length, 10-μm fundamental mode field diameter). Using these fibers, two different mathematical models for light propagation are tested and applied for further dichroism spectral band spreading of polarizing W-fibers with fundamental mode field diameter (MFD) of 6 to 10 μm for FOG coil and at least with MFD=8 to 10 μm for input filtering in loop interferometer. Also, some fibers with large MFD are considered, which may be useful for high-power fiber lasers, requiring the low nonlinear effects. One is a practically realized single-mode 200-m W-fiber Panda with dichroism and MFD=12.4  μm at 1.06 μm, which is bend resistant for 60-mm diameters. For further MFD increase a straight 100-mm length modal and polarization filters are suggested with MFD=30 to 70 μm at 1.06 μm, based on a pure silica core fiber, narrow/shallow fluorine cladding, and stress-applying parts. The latter are used for core refractive index regulation by birefringence (W-profile forming), which is more precise than with core germanium doping.