Bridging the Terahertz Gap: Photonics-Assisted Free-Space Communications From the Submillimeter-Wave to the Mid-Infrared

Abstract

Since about one and half centuries ago, at the dawn of modern communications, the radio and the optics have been two separate electromagnetic spectrum regions to carry data. Differentiated by their generation/detection methods and propagation properties, the two paths have evolved almost independently until today. The optical technologies dominate the long-distance and high-speed terrestrial wireline communications through fiber-optic telecom systems, whereas the radio technologies have mainly dominated the short-to medium-range wireless scenarios. Now, these two separate counterparts are both facing a sign of saturation in their respective roadmap horizons, particularly in the segment of free-space communications. The optical technologies are extending into the mid-wave and longwave infrared (MWIR and LWIR) regimes to achieve better propagation performance through the dynamic atmospheric channels. Radio technologies strive for higher frequencies like the millimeter-wave (MMW) and sub-terahertz (sub-THz) to gain broader bandwidth. The boundary between the two is becoming blurred and intercrossed. During the past few years, we witnessed technological breakthroughs in free-space transmission supporting very high data rates, many achieved with the assistance of photonics. This paper focuses on such photonics-assisted freespace communication technologies in both the lower and upper sides of the THz gap and provides a detailed review of recent research and development activities on some of the key enabling technologies. Our recent experimental demonstrations of highspeed free-space transmissions in both frequency regions are also presented as examples to show the system requirements for device characteristics and digital signal processing (DSP) performance.

Wireless communications sensing and security above 100 GHz

“…Of course, there are valid reasons for concern; the challenges discussed in this article are indeed formidable. Nevertheless, we feel that the research results of the last few years have established that THz technologies are a promising foundation for future needs in wireless networks, which seem likely to exploit these frequencies for at least some of their key functions 86 . While many open questions remain, there is at this point a clear and compelling motivation to pursue the goal of THz wireless.…”

High capacity terahertz communication systems based on multiple orbital-angular-momentum beams

“…Recently, THz wireless communications with the carrier frequency of 0.1-1 THz have attracted increasing attention and have been demonstrated using Gaussian beams [17,20,24,25,29,39]. Challenges of implementing OAMbased THz communications to further increase data capacity and spectral efficiency includes efficient generation/detection of high-data-rate THz beams and THz OAM beams.…”

“…Challenges of implementing OAMbased THz communications to further increase data capacity and spectral efficiency includes efficient generation/detection of high-data-rate THz beams and THz OAM beams. Recently, photonic-assisted THz signal generation and electrical THz signal detection approaches have been utilized for the demonstration of OAM-based MDM links [20,24,25,39].…”

“…In terms of other frequency regimes, an area of increased recent interest is the free-space communications in the terahertz (THz) frequency regimes [17][18][19][20][21][22][23][24][25][26][27][28]. This is partially motivated by the following issues: (a) compared to millimeter waves, THz can potentially provide a more usable spectrum for communications and has lower beam divergence for longer distance transmission [27] and (b) compared to optical waves, THz generally has a lower interaction with matter and is less affected by deleterious link conditions (e.g.…”

Broadband Terahertz Near-Field Excitation and Detection of Silicon Photonic Crystal Modes

“…As a result, much effort has been directed toward the development of increasingly efficient, compact, and reliable terahertz interconnect solutions. 10,11 In the context of free-space communication, terahertz radiation uniquely provides a bridge between optics and electronics, 12 sharing the advantages of both IR waves (high directivity, large bandwidth) and microwaves (reduced diffraction). However, the strong dependence on atmospheric conditions, 13 the large absorption of water and vapor, 14 signal interference, 15 the loss of the long-range free space path, 16 and the need for precise alignment between the source and receiver 17 limit the practicality of THz wireless links.…”