Terahertz Wave Applications with Photonic Technologies # Ho-Jin Song 1 , Katsuhiro Ajito 1 , Tadao Nagatsuma 2 , and Naoya Kukutsu 1 1 Microsystem Integration Labs., NTT Corporation 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan E-mail: song.hojin@lab.ntt.co.jp 2 Graduate School of Engineering Science, Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Abstract We present a potential of photonic technologies originally developed for fiber-optic communications for use in contemporary terahertz-wave applications and demonstrate several examples, including a high- precision time-continuous terahertz-wave signal and terahertz-wave band noise generators, imaging with CW and noise signals, and wireless communications. Keywords : Terahertz-wave, Terahertz imaging, Terahertz coimmunications, Uni-travelling photodiode Terahertz (THz) waves, which lie in the frequency range of 0.1 ~ 10 THz, have long been investigated in a few limited fields, such as astronomy, because of a lack of devices for their generation and detection. Several technical breakthroughs made over the last couple of decades now allow us to radiate and detect terahertz waves more easily, which has triggered the search for new uses of THz waves in many fields, such as bio-science, security, and information and communications technology [1]. However, today’s THz technologies still rely on complex and bulky equipment, which is not suitable for practical use, especially in outdoor sensing and wireless communications applications. For compact and reliable THz-wave applications, we have been investigating the possibility of using photonic technologies, which were originally developed for fiber-optic communications and are therefore inexpensive, compact, and reliable. In this report, we present our recent progress in THz-wave applications using photonic technologies, spectroscopy and imaging systems with the CW and noise signals, noise characterization of electronic devices, and wireless communications. In conventional fiber-optic communications systems, photodiodes (PDs) are used to recover data carried on optical signal. However, because their electrical outputs are, in principle, determined by the autocorrelation function of the input optical field, they can also be used to generate arbitrary radio frequency signals. Figure 1 shows several examples of photonic generation of electrical signals. As shown in Fig. 1(a), when two monochromatic light waves at optical frequencies of ν 1 and ν 2 are input to a PD, electrical currents are induced at DC and radio frequency of |ν 1 − ν 2 |. Therefore, the frequency, phase, and intensity of the output electrical signal can be tuned simply by tuning those of the input optical signals. As can be seen in Figs. 1(b) and (c), a band-limited arbitrary signal can also be generated. Because photonic technologies inherently exhibit very low loss and wide bandwidths, there is no additional loss for tuning optical signals even if one generates THz-waves. Unfortunately, because conventional p-i-n PDs do not provide enough conversion efficiency at THz-wave frequencies, the photonic generation of THz-waves illustrated in Fig. 1 has not been used in practical applications. In 1997, a new sort of PD, called the uni-traveling-carrier photodiode (UTC-PD), was invented by NTT. The UTC-PD shows superior performance in high frequency applications because of its unique operation principle: the photoresponse of the UTC-PD is dominated by the electron transport in the whole structure, resulting in much shorter transit time and faster photoresponse [2]. In addition to the fast response,