The research carried out in the Microwave Photonics Research Laboratory covers the following topics:1. Photonic generation of microwave, mm-wave and THz signals
2. All-optical microwave mixing and filtering
3. True time-delay beamforming for phased array antennas
4. UWB and Radio over fiber
5. Fiber lasers and amplifiers
6. Fiber optic sensors
1. Photonic generation of microwave signals
RF signals are conventionally generated using electronics by multiplying a low frequency to a high frequency with several stages of multipliers and amplifiers. Consequently, the system is bulky, complicated, inefficient, high phase noise and costly. In this research, we investigate different microwave-photonic achitectures to generate RF signal, in which two phase-correlated wavelengths are generated with a wavelength difference that falls within the frequency range of microwave, millimeter-wave or THz, depending on the applications. The RF signal is thus obtained by beating the two waves at a photodetector. The key problem to be solved in this research is to generate two wavelengths that are phase-correlated and stable with large tunability.Microwave generation using a single-longitudinal-mode dual-wavelength fiber ring laser Movie
2. All-optical microwave filtering
Photonics offers unique capabilities for performing advanced signal processing functions such as convolution, correlation and microwave frequency filtering. In addition to the small size, low-weight, fiber-optic microwave filters can process wideband signals directly in the optical domain, by eliminating the need for intermediate electro-optic conversions. The speed and bandwidth capabilities of these photonic microwave filters extend well beyond those realizable with state-of-the-art digital electronics. In this research, the emphasis will be on the study of microwave filters to achieve different functions, such as low-pass, and band-pass filtering, and code identification. In addition, we will also investigate the algorithm for photonic microwave filter synthesis. The basic modules of such filtering achitectures include optical amplifiers, optical couplers or circulators, and fiber Bragg gratings. The synthesis of fiber Bragg grating based all-optical microwave filters using genetic algorithm has been developed recently in the Lab.
3. True time-delay beamforming
High-performance radars and wireless communication systems require the use of phased-array antenna (PAA) composed of elemental monolithic T/R modules. PAAs offer many advantages, including beam steering without mechanical movement, very accurate beam pointing, increased scan flexibility in two dimensions, and reduced power consumption and weight. Many defense applications require PAAs to cover a large portion of the electromagnetic spectrum. The steering of output beam of a PAA can be achieved by using either phase shifters or time delay units. For wide band applications, PAAs using phase shifters encounter the squint problem where different beams are formed for different frequency components. This squint phenomenon effectively broadens the antenna beam. To overcome the squint problem, photonic true time-delay (TTD) units may be used for wideband applications. For photonic TTD units, the delay times for all the frequency components are the same, resulting in the same steering direction for all the frequency components. In the proposed research, we will investigate different techniques, including fiber Bragg gratings 1, 2, 3, AWG arrays and MEMS switches, to achieve accurate and controllable time delays.
4.UWB and Radio over fiber
The concept of fiber radio has been investigated for numerous applications such as antenna remoting and intelligent transport system. In a broadband wireless network, this concept is very appealing since high frequency signals (up to 60 GHz) are required to obtain the high data rates required by modern multi-media applications. The cost of this millimeter-wave equipment in the base stations raises the expense of the system to a prohibitive amount. Fiber radio allows a unique millimeter-wave generation technique, located in a central office, to produce and optically distribute a signal to the proper base stations. The same distribution network is also used on the return path, forming a bi-directional wireless communications system. The general goal of this research is to develop optical transceivers for a low-cost, hybrid photonics-wireless base station for broadband access. The transceiver will be the interface between the terrestrial fiber-optics network, which will carry microwave signals between a central office and a multiplicity of base stations, and the wireless link connecting the base station to the end users.
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