This dissertation describes a novel all-fiber acousto-optic tunable bandpass filter(AOTBF) based on the spatial mode coupling in an optical fiber. When compared to former bandpass filters based on Fabry-Perot etalon or fiber Bragg grating, this AOTBF has advantages of broad operating bandwidth and fast tuning by means of rf signal.
The AOTBF has an acoustic transducer composed of PZT and a horn, and a core mode blocker in the middle of acousto-optic interaction region. A fundamental core mode at resonance wavelength is coupled to a cladding mode by a flexural acoustic wave generated by the acoustic transducer. The cladding mode passes through the core mode blocker with small transmission loss and is coupled back to the core mode. On the other hand, the uncoupled core mode at the wavelengths where the phase-matching condition is not satisfied is effectively removed by the core mode blocker. The core mode blocker, which selectively attenuates the core mode except cladding modes, is fabricated by fusion splicing two fibers whose end facets were etched with HF solution to construct an air bubble within the core of the fiber.
When a conventional single mode fiber is used, the insertion loss and the extinction ratio of the AOTBF are about 1 dB and 12.5 dB respectively at the wavelength range of over 60 nm. The 3 dB bandwidth is about 3 nm for an interaction length of 20 cm. The insertion loss mainly comes from the cladding mode loss caused by the deformation of the fiber at the splicing point. The extinction ratio has a periodicity in the spectrum domain, which can be attributed to the interference coming from the air bubble of the core mode blocker. The on-off transition time is about 83 μs for an 18 cm-long AOTBF, which corresponds to the traveling time of the acoustic wave in the fiber. The return loss is about 47 dB.
This AOTBF will be useful for the WDM channel monitoring, all-fiber optical spectrum analyzer, and a wavelength tunable fiber laser