All-fiber tunable filters using flexural and torsional acoustic wave

Abstract

This dissertation describes two types of all-fiber tunable filters based on flexural and torsional acoustic waves. The principles of operation and the performance of the each device are discussed in detail. First, an all-fiber acousto-optic tunable bandpass filter (AOTBF) exhibiting narrow optical bandwidth and negligible polarization dependence is demonstrated by employing a novel ultraviolet (UV)-induced core-mode blocker written in a high numerical aperture (NA) fiber. The device exhibits the measured 3-dB optical bandwidth of 0.65 nm, the polarization dependent center-wavelength splitting of 0.05 nm, and the extinction ratio of -22dB at the wavelength around 1550 nm. The details of the transmission characteristics of the filter and the loss mechanism of the core-mode blocking element inscribed in the high NA fiber are discussed. Second, a first practical all-fiber tunable polarization filter with high coupling efficiency based on acousto-optic (AO) coupling between two optical polarization modes of the $LP_{01}$ mode propagating in a highly birefringent (HB) single mode optical fiber is demonstrated. An over-coupling between the two polarization modes is realized over the wavelength range from 1530 nm to 1610 nm using traveling torsional acoustic wave. The measured 3-dB optical bandwidth of the filter was 4.8 nm at the wavelength around 1550 nm. The details of the filter transmission and the coupling characteristics are discussed. Finally, the first demonstration and analysis of the AO coupling between two optical polarization modes of the LP01 mode propagating in a HB photonic crystal fiber (PCF) are reported. The coupling is realized in an all-fiber tunable polarization filter based on wavelength selective AO coupling by traveling torsional acoustic wave. The dispersion properties of the torsional acoustic wave in the PCF and the influence of axial non-uniformity in the modal birefringence on the filter transmission are discussed in detail.