Evaluation of Uni-Traveling Carrier Photodiode Performance at Low Temperatures and Applications to Superconducting Electronics

Abstract

High-speed photodiodes are useful devices for optical-telecommunication systems and scientific applications. A uni-traveling carrier photodiode (UTC-PD), has extremely wide band performance of over 300 GHz and used for many high-frequency or high-speed applications. Signal transmission using optical fibers, which has several advantages such as its wide band transmission and low transmission loss, is an indispensable technology that forms the foundation of the Internet. Optical fibers also exhibit low thermal conductance and are capable of electrical isolation. These features are useful for interfacing between low-temperature and room-temperature electronics. Superconducting devices and circuits are attractive for high-speed, low-power, and quantum mechanical operations. However, such devices and circuits have to be cooled below the critical temperatures of superconducting materials, Tc. For high-temperature superconducting materials such as YBCO, the operating temperature is around that of liquid nitrogen, 77 K, and for lowtemperature metal-based superconducting materials, such as Nb and NbN, the operating temperature is around that of liquid helium, 4 K. Input/output links are one of the bottle necks preventing practical application of superconducting devices and circuits. In particular, devices and circuits using low-temperature superconductors exhibit serious problems because the high-frequency electrical I/O cables consume a large amount of cooling power. However, cooling power, especially at around 4 K and below, is quite small, typically less than 1 W, though the input AC power is as large as several KW. The amount of AC input power can be reduced by reducing the cooling power. Our goal is to use a compact cryocooler. Such a cryocooler has limited cooling capability; however, it is enough for most applications of superconducting devices due to their low power requirements. Optical I/O has potential to overcome the problem by using optical fibers and photo devices such as photodiodes. A UTC-PD seems to be the most attractive device because of its high-speed performance and is required to operate at low temperatures for application in superconducting systems. In this chapter, we describe UTC-PD performance at low temperatures and its applications in superconducting systems.