High-Power RF Uni-Traveling-Carrier Photodiodes (UTC-PDs) and Their Applications

Research on exploring millimeter waves and/or terahertz (THz) waves, which cover the frequency range from 30 GHz to 10 THz, has lately increased since the nature of these electromagnetic waves is suited to spectroscopic sensing as well as to ultra-broadband wireless communications. One of the obstacles to developing applications of millimeter waves and terahertz waves is a lack of solid-state signal sources. For the generation of millimeter waves and terahertz waves, photonic techniques are considered to be superior to conventional techniques based on electronic devices with respect to wide frequency bandwidth, tunability, and stability. Moreover, the use of optical fiber cables enables us to distribute high-frequency RF signals over long distances. In this scheme, optical-to-electrical (O-E) converters, or “photodiodes”, which operate at long optical wavelengths (1.3-1.55 μm), play a key role, and high output current is required in addition to large bandwidth for practical applications. Among various types of long-wavelength photodiode technologies, a uni-traveling-carrier photodiode (UTC-PD) and its derivatives have exhibited the highest output powers at frequencies from 100 GHz to 1 THz, with improvement in layer and device structures since its debut in 1997 (Ishibashi et al., 1997). This paper describes recent progress in the high-power RF UTC-PDs, which operate at millimeter-wave and terahertz-wave frequencies. In Section 2, we discuss how its operation mode differs from that of the conventional photodiode. Next, some typical characteristics of the UTC-PD, such as output RF powers and their frequency characteristics, as well as some techniques for enhancing the output powers from the UTC-PD, such as resonant design, antenna integration, and packaging, are presented. In Section 3 we show recent analog applications of the UTC-PD such as wireless communications, spectroscopy, and imaging, which have not been realized with conventional electronics or photodiode technologies.