Microwave opto-electronic device study : traveling wave photodetector and microstrip linear resonator

Abstract

A new type of photodetector, traveling wave photodetector (TWPD), was designed, fabricated, and tested. In a traveling wave photodetector structure the incident light interacts continuously with the optically generated microwave signal while they propagate along their own waveguides. Hence, when the phases of the optical and microwave signals are matched, a strong traveling wave signal can be achieved without losing a high speed and a high quantum efficiency. Three types of TWPD were studied: photoconductive TWPD, p-n TWPD, and guided wave TWPD. The photoconductive TWPD with a coplanar stripline structure was attempted and the p-n TWPD was fabricated by integrating a p-n photodiode in the gap of a coplanar stripline. For an easy phase match and a larger bandwidth-efficiency product, the guided wave TWPD was designed by integrating a p-n photodiode with a GaAs/GaAlAs rib waveguide and employing an asymmetric coplanar stripline structure. For all three devices DC characteristics and RF response were tested. Also the interference effect was proved with multiple input beams on the photoconductive TWPD. A symmetrical microstrip linear resonator integrated with a Schottky photodetector, was investigated for optical excitation and its nonlinear behavior. Harmonic frequencies were generated by optical excitation of the resonator at the first resonance frequency. By mixing a local microwave signal to the optically generated signal in the vicinity of resonance frequencies of the resonator, the sum and the difference frequency of two signals was also generated. Finally a parametric amplification of optical RF signal was characterized with a local microwave pump signal. Both the traveling wave photodetector and the microwave linear resonator demonstrated a potential application on microwave and optical communication areas.