Optoelectronic techniques for generation and detection of terahertz (THz) signals have been reviewed. The operation principles of THz photomixer sources and THz parametric sources have been studied and recent developments in these areas have been presented. The performances of developed THz optoelectronic sources have been discussed and compared.

Terahertz Photonics: Optoelectronic Techniques for Generation and Detection of Terahertz Waves

This paper reviews recent advances in the development of high-speed 1.55-/spl mu/m traveling-wave p-i-n photodetectors (TWPD) for photonic millimeter-wave and submillimeter-wave local oscillators. We first discuss the basic physics and performances of high-speed 1.55-/spl mu/m TWPD. Next, we present a frequency-domain optical-heterodyne measurement technique for ultra-wide-band characterization of the TWPD and photonic transmitter modules within the frequency range from almost dc up to more than 1 THz. We further demonstrate ultra-wide-band (0.02-0.7 THz) photonic transmitter modules consisting of a high-speed TWPD coupled to a broad-band bow-tie antenna as well as a narrow-band 0.46-THz photonic transmitter module producing output power levels sufficient to operate a superconductor-insulator-superconductor (SIS) astronomical receiver under optimum conditions. Finally, we will report on ultra-wide-band (0.06-1 THz) photonic transmitter modules consisting of high-speed TWPDs coupled to various rectangular metallic waveguides (WR10, WR8, and WR5).

Ultra-wide-band traveling-wave photodetectors for photonic local oscillators

The interaction of laser and photoconductor in an optical heterodyne conversion scheme is studied in detail. A dc biased photoconductor excited by two continuous-wave (CW) laser beams with a difference in their central frequencies falling in the terahertz spectrum is considered as the core element in all photoconductive photomixing structures. For this configuration the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor. It is shown that in a CW terahertz photomixing scheme the resulting photocurrent contains a dc component and a terahertz component. It is also shown that the amplitude and the phase of the terahertz component of the photocurrent are functions of the applied bias, physical parameters of the photoconductor, parameters of the lasers, and photomixer configuration. The dependency of the photocurrent on all of these parameters is explored in detail for a typical photomixer made of low-temperature-grown GaAs photoconductor.

The interaction of laser and photoconductor in a continuous-wave terahertz photomixer

A new terahertz (THz) photoconductive photomixer/antenna device is presented. A continuous-wave THz signal is generated in a dc-biased photoconductive film by employing optical heterodyne scheme, and at the same time, the size of the film on the grounded dielectric substrate is designed to have an efficient broadside radiation. Incorporating the photoconductive film as the photomixing media and the radiation element not only eliminates source to antenna coupling problem but also makes the proposed device attractive for THz array source configurations. Analytical expressions for the photocurrent and the radiation power are derived under high dc bias condition, and the effects of the device configuration on its performance is studied. Two possible photomixer configurations, namely longitudinal and transversal, are introduced and their photomixing efficiencies and radiation power are compared. The typical nW output power is achievable by mW laser pump power for frequencies up to 10 THz.

Analysis and design of a photoconductive integrated photomixer/antenna for terahertz applications

Optical heterodyne millimeter-wave generation using traveling-wave photodetectors (TW-PDs) is examined both experimentally and theoretically. Ultrahigh-frequency InP-based 1.55-/spl mu/m TW-PDs were fabricated and employed in an experimental setup for optical heterodyning. For the first time, optical heterodyne millimeter-wave generation in excess of 160 GHz is experimentally demonstrated in the frequency domain. The maximum electrical power delivered by the TW-PD to a 5052 impedance is -11.5 dBm at 110 GHz with a polarization penalty of only 1.3 dB. Furthermore, a theoretical analysis in frequency domain is presented describing the frequency response of TW-PD including effects of the photogenerated carrier dynamics as well as optical and electrical wave propagation phenomena. A broadband and flat frequency response is found indicating a total rolloff of about 13.1 dB for a frequency span from 25 GHz to 200 GHz. Finally, the detectors responsivity is theoretically investigated to differentiate between the physical phenomena associated with high-frequency limitations.

Optical heterodyne millimeter-wave generation using 1.55-/spl mu/m traveling-wave photodetectors

We have investigated a novel device concept for generating CW millimeter waves with output power in the multiwatt range. The concept involves the utilization of modulated laser radiation to induce in a distributed Schottky-diode structure a traveling-wave current which, in turn, synchronously excites the dominant mode of a waveguide structure to generate millimeter power. The induced traveling-wave current is directly proportional to the laser modulation generated by the interference of two overlapping laser beams of millimeter beat frequency. Detailed analysis indicates that the device has both high-output and frequency-tunable characteristics.

Shi-Kay Yao

Papers

A Laser-Induced Traveling-Wave Device for Generating Millimeter Waves