This dissertation describes a novel all-fiber acousto-optic tunable bandpass filter (AOTBF) based on the spatial mode coupling in a few mode fiber. The AOTBF is composed of a few mode fiber, an acoustic transducer, mode strippers and a UV-written long period fiber grating (LPFG) in the middle of acousto-optic interaction region.
In the AOTBF, incident $LP_{01}$ mode is coupled to $LP_{11}$ mode at resonance wavelength by a flexural acoustic wave. At the LPFG, the $LP_{01}$ mode is coupled to $LP_{02}$ mode that is removed at the mode stripper, while $LP_{11}$ mode passes through. The $LP_{11}$ mode is coupled back to the $LP_{01}$ mode in the second part of the interaction region. The resonant peak is tuned by the applied acoustic frequency. In comparison with conventional bandpass filters, the proposed AOTBF has advantages of fast tuning and electrical control.
To get the mode coupling between $LP_{01}$ and $LP_{02}$ modes in the LPFG at wide wavelength range, we used a fiber with the normalized frequency of 4.5 around 1550 nm. The LPFG is made by using an amplitude mask with the period of 138um and an excimer laser. The center wavelength of the laser was 248nm and the output pulse energy was 250mJ/pulse. The LPFG had a resonance peak at 1559nm and 10dB bandwidth of 28nm.
The total insertion loss of the AOTBF made in this experiment was about 2.6 dB. The background loss of the LPFG was 0.8 dB , the splicing loss between a single mode fiber and a few mode fiber was 1 dB. The rest portion of the loss is attributed to imperfect acousto-optic coupling and the attenuation of the $LP_{11}$ mode by the LPFG. The 3 dB bandwidth was about 5.6 nm for acousto-optic interaction length of 25 cm. The tuning range that exhibits the extinction ratio of 10 dB was 28 nm.
This AOTBF shows potential for the application to a WDM channel monitor, an all-fiber optical spectrum analyzer, and a wavelength tunable laser.
