Tunable electro-optic devices for fiber optical RF signal processing

In order to provide the flexible coherent or incoherent RF signal processing in the optical domain, the tunable optical RF filters are very important elements to investigate. For coherent optical RF signal processing, optical resonators are extensively used as filters or modulators. In the case of multiple resonators working together, accurate phase or frequency tunability is very essential to compensate the fabrication error, to align the carrier frequency to the passbands, and to fine-tune the RF response function shape. In this dissertation, we have proposed and demonstrated a loop resonator based on Titanium diffused lithium niobate waveguide which can be accurately tuned over wide frequency range with fast tuning speed. A special waveguide structure called Y- junction reflector has been adopted to change the shape of the loop from a full circle to two slightly curved lines, so that the bending loss of the weakly-guided waveguide is avoided and the device is compact in size. The measurement has demonstrated a frequency tunability of 33.3 MHz/V for the testing device which has a 1.9 mm long phase tuning electrode. It is equivalent to 175 MHz/(V cm) by normalizing the tunability to the electrode length. With reasonable assumptions, we predicted that the device can have a frequency tunability of 260 MHz/(V cm) with an improved overlap between the optical and electrical fields. The design issues of the optical structure and electrode have been theoretically and experimentally investigated. The device also has a unique advantage of high speed tunability. It has been evaluated as a modulator. As a modulator a 3 dB link gain bandwidth of 5 GHz has been measured and a -42 dB normalized link gain is estimated. For incoherent optical RF signal processing, we have introduced and demonstrated a novel notch filter based on a tunable differential group delay (DGD) module with 6-bit resolution. The notch filter can provide 60 dB rejection level and the notch frequency is digitally tuned according to the DGD delay time setting. The bandpass filter based on two DGD modules also has been investigated experimentall