Distributed sources on coplanar waveguides: application to photomixers for THz local oscillators

摘要

The lack of compact and frequency-agile local oscillators (LO) constitutes a bottleneck in the performance of submillimeter-wave heterodyne receivers (300 GRz to 3 THz). Traditionally, instruments targeting the lower end of the frequency interval use a chain of frequency multipliers [1, 2]. These multiplier chains perform very well below 1 TRz, but the number of stages required to reach frequencies well into the TRz range renders this solution cumbersome, difficult to design, and eventually inefficient. Since their inception [31, photomixers have been proposed as a competitive alternative to the multiplier chain approach. In these devices, the output power is derived from conductivity modulation in a photoconductive material caused by interference of two impinging lasers having different wavelength. An rf current results due to the application of a dc voltage between electrodes deposited on the surface of the material. Several variations of electrode designs have been proposed and implemented to collect the generated rf current. In some of them the electrodes were localized at the center gaps of dipole antenna elements [31, while other designs make use of distributed coupling to planar transmission lines [4]. In fact, none of these designs has been safisfactory for a variety of reasons. The drawback of gap designs is the concentration of significant optical power on a physically small area. This leads to thermal breakdown of the photomixing materials before any substantial if power could be generated. In the case of the distributed design [41, a lack of electromagnetic modeling tools did not allow the optimization of the various configurations. In particular a coplanar stripline (CPS) design was adopted, which was printed on very thick dielectric substrate. This lead to the generation of a power leaking (radiating) structure and consequently the traveling-wave design suffered large losses along the gain region.