Microwave generation based on fiber Bragg grating.
Date of Issue2011
School of Electrical and Electronic Engineering
Photonics Research Centre
Microwave generation in an optical way is of great interests since it has unique advantages over the traditional electrical methods. Using electric circuitry with several stages of frequency doubling to generate a microwave signal requires complicated and costly system setup. Moreover, the transmission of the electric signals also poses a problem due to large losses in the coaxial cables during data delivery. Optical fiber, because of its well-known advantages such as low transmission loss, light weight, immunity to electromagnetic interference, and low cost, has become an ideal medium to transfer data. The main objective of this thesis is the optical generation of microwave signals, by heterodyning the output from a dual-wavelength laser at a photodetector. The dual-wavelength laser was realized by incorporating a fiber Bragg grating based dual-passband filter within a fiber laser cavity. Fiber Bragg grating (FBG), a significant optical component for such microwave generation, has been widely utilized in fiber optics communication and fiber optics sensors since its first demonstration in 1978. Six different types of FBG structures, namely, uniform, apodized, chirped, phase-shifted, sampled, and tilted gratings have been well studied in the past three decades. In this thesis, a novel FBG structure which we name as inverse-Gaussian apodized FBG (IGAFBG) was designed and used as a dual-passband filter in a fiber laser. The principle, property, advantages, and application of the IGAFBG are explained and explored in this report. Microwave at the designed frequency was realized using this IGAFBG filter. In addition, frequency tunable microwave signals with a range of 20.16-24.196 GHz were also generated using IGAFBG by bonding the IGAFBG onto a cantilever and applying a displacement at the end of the cantilever, which forms a linear chirp to change the wavelength spacing of the two passbands in the filter. To obtain a higher microwave frequency tuning range of 8.835-24.36 GHz, phase-shifted FBG bonded onto a similar cantilever was used. To further extend the generated microwave frequency range beyond what the phase-shifted FBG mounted on the cantilever can achieve, a phase-shifted chirped FBG comprising a permanent π phase shift and a temporary π phase shift was used. The temporary π phase shift was induced by a platinum thin film heater. A tuning range from 6.88 to 36.64 GHz for the generated microwave signals was demonstrated. This range is limited only by the bandwidth of the stopband of the phase-shifted chirped FBG.
DRNTU::Engineering::Electrical and electronic engineering::Antennas, wave guides, microwaves, radar, radio