The terahertz (THz) frequency band is a fascinating and relatively unexplored part of the larger electromagnetic spectrum (EMS). The terahertz frequency range, lying between electronics and optics (300 to 3000 gigahertz frequencies), can be disruptive force in sectors as diverse as from medical imaging, biological research, pharmaceutical monitoring, manufacturing and quality control, and semiconductor testing to communications, and security and defence. Science and technologies based on terahertz frequency electromagnetic radiation have developed rapidly over the last 30 years.
This radiation does have some uniquely attractive qualities: For example, it can yield extremely high-resolution images and move vast amounts of data quickly. And yet it is nonionizing, meaning its photons are not energetic enough to knock electrons off atoms and molecules in human tissue, which could trigger harmful chemical reactions. Terahertz may revolutionize medical imaging, security screening and manufacturing quality control because of THz’s non-ionizing property and capability to penetrating dielectrics, fabrics and body tissue.
Being non-ionising as well as non-destructive, THz waves can pass through non-conducting materials such as clothes, paper, wood and brick, making them ideal for applications in areas such as cancer diagnosis, detection of chemicals, drugs and explosives, coating analysis and quality control of integrated circuit chips. Current application (as an imaging tool) includes non-destructive testing and quality control (QC) of various materials, including plastics, concrete, and ceramics.
Recently, THz imaging has been used in laboratory tests and security screening to examine the condition of space shuttle components, examine text in books that are too sensitive to examine physically, and to localize illicit drugs based on specific composition. Due to the power constraints, its use has been restricted to an approximate distance of 10 meters or less. THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners.
Terahertz can also provide revolutionary capabilities in defense, including Secure Terahertz communications, Chemical and Biological Agent detection and anti-stealth THz ultra wideband radar. Successful military operations depend upon freedom of action in the warfighting domains of air, space, ground, sea, and cyberspace. Today, effective command and control and situational awareness depend upon radio communications and sensors. Domination of the electromagnetic spectrum (EMS) enables joint force commanders to gain tactical, operational, and strategic advantage over a potential adversary. EMS is broken down into frequency bands defined by certain physical characteristics, which include radio waves, microwaves, millimeter waves, infrared, visible light, ultraviolet radiation, x-rays, and gamma rays.
Terahertz wave has strong penetrability, high safety, good directionality, high bandwidth characteristics, can be applied to national defense, military and other civilian areas, and in the military field. Over the past decade, defense establisments around the world have been assessing the feasibility of sensors, radar, and communications operating in the terahertz (THz) portion of the frequency spectrum. The U.S. Department of Defense’s efforts are particularly focused on technological breakthroughs in the microelectronics that would drive THz emitters. DARPA is utilizing the biologically safe rays for non-invasive diagnosis of a wide range of ailments. The first generation of devices would most likely have the ability to look only a few millimeters beneath the skin — but that’s still an incredible ability.
The PLA has long believed that modern warfare hinges upon “the fifth domain of the EMS space (第五维电磁空间,di wu wei dianci kongjian),” and that THz is “unquestionably” a key technology to dominate the EMS and gain an edge in military competition. (PLA Daily, April 10). EMS domination is seen as the key to “muting the adversary’s communications, blinding its radars, and paralyzing its networks” to win modern wars. (Civilian Staff WeChat, April 11). Military and civilian resources, both in terms of funding and human capital, have been invested in China’s pursuit of THz technologies as early as the 2005 Xiangshan Science Conference (香山科学会议, Xiangshan Kexue Huiyi), although the exact quality of China’s THz research and development (R&D) remains unclear (XSSC, November 21; THz Applications WeChat, January 29). The majority of “outputs” of such R&D programs show promise, albeit with seemingly limited military value. Nevertheless, over the past fifteen years or so, China has created a state-led innovation ecosystem to sustain both basic and applied research of THz.
Terahertz will also key technology in future 6G wireless communications. 6G could also satisfy Military’s growing requirements to gather, analyze, and share information rapidly; to control an increasing number of automated Intelligence, Surveillance, and Reconnaissance (ISR) assets; to command geographically dispersed and mobile forces to gain access into denied areas.
Chinese Terahertz advancements
PLA scholars acknowledge that THz technologies have extensive military applications, and have discussed THz’s possibilities in the following fields: detect and distinguish explosive; assist long-range detection and imaging detection; facilitate battlefield and satellite communications; enhance terminal precision-guided missiles; and facilitate counterterrorism security inspection (PLA Daily, December 22; PLA Daily, May 24). However, THz has been relatively underexplored compared to the rest of the EMS due to technical challenges associated with generating, detecting, and processing signals at these wavelengths.
While acknowledging “gaps” in THz research, Dr. Zeng Yang (曾旸), from the College of Meteorology and Oceanography of the National Defense University of Technology (NUDT/国防科技大学), identified three key military applications: high-speed encrypted communications, high-resolution target detection, and battlefield situational awareness and imaging where THz applications could be useful (PLA Daily, April 10). This echoes an earlier discussion in which PLA analysts explicitly noted that THz technology will play a significant role in “military communications, battlefield reconnaissance, precision guidance, counter-stealth, and electronic countermeasures (ECM)” (PLA Daily, March 31).
Miao Wei (苗圩), former head of the Ministry of Industry and Information Technology (MIIT), said in 2018 that China “has already started looking into 6G development”. Li Shaoqian (李少谦), director of UESTC’s National Key Lab for Communications Counter-Countermeasure Technology (通信抗干扰技术国家级实验室, Tongxin Kang Ganrao Jishu Guojia ji Shiyanshi), has said that “THz communications should be the technology that 6G network is built on,” (Xinhua, March 26). Indeed, Li’s lab was involved in at least one 863 Program project on integrated millimeter wave and THz technology and high-speed baseband signal processing technology research (毫米波和太赫兹总体技术与高速基带信号处理技术研究, Haomibo he Taihezi Zongti Jishu yu Gaosu Jidai Xinhao Chuli Jishu Yanjiu) (UESTC, undated).
Li’s comments echo PLA analysts’ writings about the advantages of THz communications, which include possibilities for high-capacity and highly secure battlefield communications. THz communications can sustain “data transmission that is a hundred times faster than 5G with a latency of microseconds.” (PLA Daily, April 10). Another important consideration is that the decreased angular divergence (that is, the increased directionality) of transmitted signals, owing to the reduced effects of diffraction on THz waves with shorter wavelengths, present a more challenging environment for eavesdroppers compared to the wide-area broadcasts used at lower frequencies.
Li Shaoqian’s lab likely also has been involved in exploring the use of THz technologies in space situational awareness, and inter-satellite communications (Renmin Net, October 10). On November 6, UESTC announced on its official website that a 70kg- satellite that bears the university’s name, “UESTC,” also known as Tianyan-05 (天雁05卫星), was successfully launched from China’s Taiyuan Satellite Launch Center (TSLC) in Kelan, Shanxi, to carry out multiple in-orbit testing including using THz communications equipment developed by UESTC (UESTC website, November 6; Sichuan Daily, November 7).
THz machines have a distinct advantage over X-ray machines or millimeter waves (these tubular devices are often found in airport screening corridors immediately following the metal detectors). The waves also stimulate molecular and electronic motions in many materials—reflecting off some, propagating through others, and being absorbed by the rest. These features have been exploited in laboratory demonstrations to identify explosives, reveal hidden weapons, check for defects in tiles on the space shuttle, and screen for skin cancer and tooth decay.
Detecting reflected THz radiation makes it possible to create spectroscopic information and 3D images with unique spectroscopic signatures – terahertz fingerprints – not found at other wavelengths like optical and infrared. Several THz-wave propagation properties, which microwave and infrared waves lack, make THz signals a good candidate for material and gas sensing. In particular, many biological and chemical materials exhibit unique THz spectral fingerprints. THz signals can penetrate a variety of non-conducting, amorphous, and dielectric materials, such as glass, plastic, and wood.
Moreover, metals strongly reflect THz radiation, which enables detecting weapons. Due to strong molecular coupling with hydrogen-bonded networks, THz signals can be used to observe water dynamics. Similarly, due to unique spectral signatures that arise from transitions between rotational quantum levels in polar molecules, THz signals can be used for gas detection (rotational spectroscopy). They are better suited for detecting explosives and illicit materials and more reliable in identifying almost any chemical composition under the right conditions. Operationalizing this type of screening system in a dense urban environment against enemy state or non-state actors would be especially useful. Much of this has already been demonstrated within controlled environments.
Regarding ground-based THz systems, security corridors or mobile patrols could soon ‘see-through’ structures, clothing, vessels or transport containers, and most other non-liquid material to probe for concealed materials. A future security environment where threats can be detected and countered could thwart criminals and belligerents manufacturing or smuggling explosives. It could also help locate bomb-making materials, weapons/mines, illegal drugs, and rare earth minerals.
In a non-combat environment, it could be used to help detect plastic or minimal metal land mines on current or former battlefields; most anti-personnel mines are a combination of metal and plastic (and manufactured to avoid detection by metal detectors). Current technology for land-mine detection requires analysis of the soil temperature and is measured in three dimensions, then injected into cumbersome software algorithms that make rough estimates with limited confidence. This detection technique uses Field Programmable Gate Array (FPGA) technology. THz spectroscopic imaging is a logical alternative to FPGA, as it can detect almost any material under the right conditions with relatively high confidence.
In June of 2016, China’s People’s Daily Online reported that the State Administration of Science, Technology, and Industry for National Defense completed research and development (R&D) efforts of China’s first solid-state THz imaging system. The Chinese article also proposes applications for urban and anti-terrorist combat, such as scanning behind walls or inside compounds.
Ground-based ISR, radar system integration, airborne and space-based ISR, and precision targeting and communications are capabilities that can offer a massive advantage. As such, THz technology should be considered part of the proverbial arms race that exists for artificial intelligence (AI), quantum computing, and machine learning (ML).
It will enable intelligence analysts with the ability to process satellite images of weapons caches in underground bunkers. Security patrols will have the ability to ‘see-through’ suspected terrorist domiciles or into bomb-making facilities. Unmanned aerial vehicles will be able to positively identify maritime vessels carrying illicit drugs or victims of human trafficking inside shipping containers.
Second, THz radiation could also work in parallel with EO/IR sensors to find and strike targets while moving. EO sensors are the primary cueing mechanism for laser-guided weapons and ideal for destroying high-speed targets. In the EO/IR video image, the pod operator can use a THz overlay to find and target a vehicle known to be transporting weapon-making material. This ability to detect chemical ‘fingerprints’ will help expedite finding targets in dense urban environments when discrimination can be difficult. Hypothetically, it will be possible to outfit an airplane’s SAR and EO/IR pod with a vast catalog of THz filters. Based on mission needs, (multiple) different filter settings will be selectable in any given situation.
Low observable technologies like those found on the F-35 fighter and B-2 bomber will be ineffective, as radar fidelity will outpace low-observable technologies. At the same time, big data updates to the war will be transmitted rapidly to artificial intelligence nodes to produce a common operating picture for military commanders. If correctly developed, terahertz (THz) technology is the key to unlocking this next evolution of warfighting—increasing our ability to find and target adversaries and their capabilities at lightning-fast speed.
Military THz Radar
THz radar can emit tens of thousands of species frequency as well as pico-second and nanosecond pulse at GW level to provide information on the composition of targets and thus target identity, not available in other remote sensing methods. NASA’s Jet Propulsion Laboratory, has built a 675-GHz imaging radar with a peak output power below 1 mW, it can conduct rapid “frisk” or “pat-down” types of searches of persons as far away as 25 m, useful for security screening at airports and other public places.
It will allow intelligence professionals and field operators to quickly discriminate targets based on composition, counter adversary concealment or deception techniques, and identify items of interest based on chemical resonance that is visible only through the use of THz imaging.
New terahertz systems are being planned like DARPA’s Video Synthetic Aperture Radar (ViSAR) that seeks to build a sensor system for aerial platforms that peers through clouds to provide high-resolution, full-motion video for engaging moving ground targets in all weather conditions.
The advent of THz on airborne and space assets could also be a shift in the way the United States conducts ISR, as it can overcome line-of-sight issues and ‘see-through’ structures. Countries that use camouflage, concealment, and deception techniques might no longer be able to depend on current strategies. Specifically, actors that use subterranean structures or bunkers to hide weapon systems will no longer enjoy the benefit of obscurity. In places like Kangwon, located in a remote area of the Democratic People’s Republic of Korea, airbases and surface-to-air weapon systems are commonly protected inside mountains and terrain.
China has claimed to have developed terahertz radar that could unmask stealth fighters like F-35. Earlier a report on the English.cri.cn website claimed that China has completed research and development of a new radar system, which can penetrate walls and provide scanning imagery of objects inside houses. The radiation produced, or ‘T-rays’, can penetrate composite metals to examine underlying aircraft metals. These metals are specific to certain types of aircraft, making the process of finding and positively identifying low observable (LO) aircraft fast and easy.
While communication system designers are preoccupied with readying millimeter wave (mmWave) frequency bands (30 to 300 GHz) to offer multi-gigabit-per-second (Gbps) data rates for 5G mobile devices, the terahertz band (0.3 THz to 30 THz) is the next frontier in wireless communications for its ability to unlock significantly wider segments of unused bandwidth. Thus, wireless Tbit/s communications and the supporting backhaul network infrastructure are expected to become the main technology trend within the next ten years and beyond.
The aim is to enable a network connection in the terahertz frequency range, which is so stable that data can also be transported wirelessly at speeds of up to 400 gigabits per second. As a first intermediate goal the researchers intend to embed terahertz radio solutions in fiber optic networks with high data rates, open up new frequency bands and thus pave the way for a resilient communications infrastructure that will meet the demands of the future.
Li’s comments echo PLA analysts’ writings about the advantages of THz communications, which include possibilities for high-capacity and highly secure battlefield communications. THz communications can sustain “data transmission that is a hundred times faster than 5G with a latency of microseconds.”. Another important consideration is that the decreased angular divergence (that is, the increased directionality) of transmitted signals, owing to the reduced effects of diffraction on THz waves with shorter wavelengths, present a more challenging environment for eavesdroppers compared to the wide-area broadcasts used at lower frequencies.
Like mmWave communications, terahertz bands can be used as mobile backhaul for transferring large bandwidth signals between base stations. Moreover, the increasing number of mobile and fixed users in the private sector as well as in the industry and the service sector will require hundreds of Gbit/s in the communication to or between cell towers (backhaul) or between cell towers and remote radio heads (fronthaul). In such scenarios, Terahertz Wireless Communication can also play an important role.
Another venue for fiber or copper replacement is point-to-point links in rural environments and macro-cell communications. . In the future, the users in rural or remote regions, which are difficult to access (e.g. mountains, islands), should be connected with high data rates up to 10 Gbit/s per user. This is either infeasible or very costly when using solely optical fibre solutions.
High-speed drone-to-drone communications can also be achieved at the THz band. Drones can form flying ad hoc networks (FANETs) to enable broadband communication over a large-scale area. In FANETS, the THz band achieves high capacity at greater flexibility compared to FSO, which has stringent pointing and acquisition requirements. Nevertheless, drones operating at the THz band would require millimeter levels of positioning accuracy, as opposed to less than a centimeter for a drone operating at 30 GHz.
Terahertz wi-fi could in theory support data rates up to 100Gb/s within ranges of about 10m. The Researchers from the Tokyo Institute of Technology, have already demonstrated 3Gb/s transmission at 542GHz. At the heart of the team’s 1mm-square device is what is known as a resonant tunnelling diode, or RTD. Furthermore, THz communications are expected to enable the seamless interconnection between ultra-high-speed wired networks, e.g., fibre optic links, and personal wireless devices, such as laptops and tablet-like devices, achieving full transparency and rate convergence between wireless and wired links
More importantly, terahertz bands can be employed in close-in communications, also known as whisper radio applications. That includes wiring harnesses in circuit boards and vehicles, nanosensors, and wireless personal area networks (PANs). Then, there are applications like high-resolution spectroscopy and imaging and communication studies that require short-range communications in the form of massive bandwidth channels with zero error rate in crucial areas like coding, redundancy, and frequency diversity.
Finally, fully adopting digital networking in industry, commerce and public services, including traffic control and autonomous driving, remote health monitoring services, supply chain, security and safety procedures, large production sites automation and production lines, place stringent requirements for Tbps-class access subject to fast response constraints. These Cyber Physical Systems scenarios, describing the ‘true colours’ of what is commonly known as Tactile Internet, mainly challenge the capability of systems beyond 5G. In all the above scenarios, THz wireless is an attractive complementing technology to the less flexible and more costly optical fibre connections and to the lower data rate wireless technologies (Visible Light Communication, mmWave, WiFi).
The above-mentioned use cases provide a glimpse of how terahertz communications can be a game changer by offering even lower latency than fiber networks. Additionally, terahertz bands can complement mmWave communications in the commercial realization of indoor wireless networks, position localization studies and gigabyte Wi-Fi support of internet of things (IoT) applications.
Though these signals degrade quickly in the atmosphere, there is a significant utility in space where the signals experience almost zero loss. Compared to modern spacecraft S-band, Ka-band, and Ku-band systems that use complex architecture to achieve high data rates, a THz wireless system is much less complex, the components are modular and light weight compared to Ku-band hardware. Satellites or spacecraft using this lightweight payload could be able to communicate easily while on orbit. As the next evolution in competition arises, THz enabled communication would be helpful on Lunar and Martian surfaces due to the lack of atmosphere and moisture.
Terahertz can satisfy the growing exponential demand for wireless bandwidth that is predicted to increase to tens or hundreds of gigahertz. For Military, Gigabit secure battlefield wireless sensor network, which can provide a multi sensor fusion of a wide range of imaging and non imaging sensors, can be developed. It can enable long-range secure Gigabit Aircraft to satellite Communication with very little power, space communications, high speed information processing and computing.
Many challenges need to be addressed prior to the widespread introduction of THz communications. For instance, the THz band’s high propagation losses and power limitations result in very short communication distances, and frequency dependent molecular absorptions result in band-splitting and bandwidth reduction. Signal misalignment and blockage are also more severe at the THz band.
THz signal strength diminishes at an extreme rate. A 1-watt transmission at a frequency of 1 THz diminishes to almost nothing (10-30 percent of original strength) after one kilometer. This degradation can be even more severe if water is present in the atmosphere. Second, the power requirement to overcome this attenuation is not presently realistic outside of a laboratory environment — the immense power requirement is a critical subcategory of common design considerations: Size, Weight, Power and Cooling (SWaP-C).
However, current THz sources and detectors are large, multi-component systems that are heavy and expensive. Such systems are also hard to transport, operate, and maintain. Collectively, SWaP-C presents a barrier known as the ‘terahertz wall.’ Currently, SWaP technology for mobile transport of THz devices does not exist to enable stand-off capabilities. Recently researchers have used Graphene to enabled the development of the first flexible terahertz detector, paving the way for high-powered electronics on flexible materials.
THz-band applications raise many health and privacy concerns. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) considers heating to be the main risk factor of THz radiation. Since THz radiation does not penetrate the body, this risk is confined to the heating of skin tissues.
Soon, technological leaps will enable development of power sources that will prevent THz degradation in the atmosphere and at distance. When this happens, THz will be an exquisite tool to help the United States find and target items of interest. Scientists have successfully developed flexible, high performance and low-power driven terahertz (THz) emitters that could be mass-produced at low cost. This novel invention is a major technological breakthrough and addresses a critical challenge for industrial application of THz technology.
TeraHertz until recently was the last unexploited part of the electro-magnetic spectrum because of the lack of efficient sources, sensors, and detection techniques and strong attenuation of THz radiation through the terrestrial atmosphere. However the TeraHertz “gap” is being rapidly filled up through unprecedented creativity in the development and commercialization of TeraHertz sources, transmission components and detectors. The Market for Terahertz Products is predicted to reach $570 Million by 2021.
Many experiments are underway on terahertz bands for indoor wireless, localization studios, and gigabyte Wi-Fi networks. These fundamental experiments on channel modeling at terahertz bands are mostly focused on propagation measurements, directional path losses, and penetration losses.
Take, for instance, the propagation measurements in the 140 GHz D-band conducted by Aalto University at a shopping mall. The transmitter (TX) and receiver (VX) were placed at 200 m using a channel sounder system and omnidirectional path losses at 140 GHz were compared with the 28 GHz wireless channel. The slope and variations of the path loss data of the two bands were quite similar.
For example, the research team at NYU Wireless has carried out penetration measurements at the 140 GHz band for various building materials. That includes drywall, clear glass, and glass doors. Here, the average penetration loss of clear glass has been 3.2 dB/cm at 28 GHz, while it amounted to 14 dB/cm at 140 GHz, showing how the penetration loss increases with frequencies.
A terahertz communication system processing a multi-GHz channel will demand greater computational capacity in the digital domain where the baseband subsystem encompasses tasks like physical layer (PHY) channel coding. Similarly, the analog domain mandates improvements in analog-to-digital (ADC) and digital-to-analog (DAC) parts to efficiently capture higher frequency signals.
At a time when broadband and high-gain components are not available, a prototype can facilitate the key building blocks of the terahertz communication systems. That includes high-bandwidth baseband processing subsystem, bandwidth-filtered immediate frequency (IF) stage, local oscillator (LO) module, and radio heads to cover multiple frequency spectrums. The 5G dynamo is opening the floodgates of wireless bandwidth, and once mmWave bands move toward wider adoption and proliferation for 5G and other communication use cases, terahertz bands like 140 GHz are inevitably going to be the next in line
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