Terahertz Technologies for Telecommunications
High speed of transmission in fiber optic networks reaching Pbit/s  leads to increased requirements for volume and data transfer rate in wireless access systems. According to Edholm’s law of increased data transfer rates in wireless links , data transfer rates of dozens and hundreds Gbits – not less than 24 Gbit/s will be required for broadcasting UHDTV television data by 2020; and for 100G Ethernet support 100 Gbit/s will be needed [3, 4]. Short-range data exchange technologies  also require increasing data transfer rates between the components of different devices and equipment (Fig.1) .
Use of infrared band for high speed data transfer does not seem possible either: data transfer rate in this band is limited by hundreds of Mbit/s  due to low sensitivity of radiation receivers, diffuse reflection losses, existence of light noise in the environment, as well as radiation power limitation due to the risk of visual impairment . In such a way, there are to possible approaches to achieve data transfer rates of 10–100 Gbit/s in wireless networks; first of them implies increase of communication line spectral efficiency up to dozens of bit/s/Hz; the second approach consists in the increase of frequency band to few dozens of GHz. The last approach seems to be the most feasible and implies using terahertz frequency band (0.1–10 THz), as this very band includes required section of unreserved spectrum.
As THz radiation is significantly lost during dissemination in the atmosphere, as it is shown in the fig. 2, field of THz communication application is mainly limited by local wireless access networks. In case, if it is necessary to transfer THz signal to long distances, THz optical signal modulation technology is used for signals transferred by fiber [14, 16]. Characteristics of absorption and spreading of THz radiation transfer channel indoors are theoretically and experimentally studied, wave diffuse scattering on surface irregularities, multilayer structure return and diffraction are taken into consideration [11, 17].
Approaches to realization
of THz wireless links
THz wireless communication networks are divided into optical, electronic and hybrid networks depending on used components. Nowadays, hybrid (mixed) systems are the most widely used, as at this stage of development of photonics and electronics this very combination of electronic and optical devices allows obtaining records data transfer rates.
Hybrid THz wireless communication link may be realized on the basis of two approaches, depending on the method of THz signal generation. One approach includes THz signal generation by means of photonics technologies, the other one – electronics technologies. It was demonstrated that photonics based approach to THz signal generation was the most feasible regarding used frequency band, possibilities of reconstruction and stability; it may be applied to obtain data transfer rates up to 10 Gbit/s and more due to availability of such telecommunication components as lasers, modulators and photodiodes working at such data transfer rates. Use of fiber optic cables allows transferring high-frequency radio signals at long distances [16, 34]. Besides that, absolute advantage of photonic approach is the fact that fiber optic and wireless communication networks may be smoothly combined regarding data transfer rate and modulation format [15, 19].
This work is devoted to the newest achievements in realizing hybrid and fully optical THz communication wireless systems at carrier frequencies exceeding 100 GHz using photonic technologies for signal generation.
Components and configuration of signal generation schemes
Methods of optical signal generation based on photonic technologies that are the most efficiently used in wireless THz communication systems include generation of THz signals by means of ultrafast pulsed lasers  and heterodyning of optic beats (photomixing) of continuous radiation [3, 19, 21–22, 23]. General scheme of optical signal generation by means of photonic methods is shown in the fig. 3. Optical signal whose intensity is modulated on THz frequencies is at first generated by optical source (with pulsed or continuous radiation), and then it is coded by electro-optical modulator based on Mach-Zehnder interferometer. Finally, optical signal envelope is converted to THz signal by means of photodiode or photoconductor; then it is emitted to open space by antenna. In some case power amplifier and/or frequency multiplier is placed after the photodiode, if these electronic devices are available at required frequencies. Infrared semiconductor lasers are frequently used as optical signal generators, that’s why reliable and feasible telecommunication components – optical fiber, erbium doped fiber amplifiers and semiconductor laser amplifiers – are essential parts of such schemes. Control of converter electrical bias is effectively used for THz signal modulation .
In case of signal generation by means of photonic methods, optoelectronic converter (photodiode or photoconductor) working at wavelengths of 1.3–1.55 µm is the key component additional to optical signal sources; eventually, it determines transmission capacity regarding capacity and power . The most widespread and commercially available are photoconductors based on structures grown at low temperatures – GaAs (LT-GaAs) for 700–900 nm laser wavelengths and LT-InGaAs for 1300–1600 nm wavelengths [23–24]. In practice, photoconductive materials the most often are used as switches surrounded by the structure of metal (for example, golden) antennas, forming photoconductive antennas. During lighting of such structure with femtosecond pulses, photoconductive switch changes its state from isolating to conducting; in the result of such change short current pulse which is the source of THz radiation is formed.
Photoconductors and photoconductive antennas are used with pulsed as well as continuous optical signal sources; photodiodes are more effective in combination with continuous sources from the point of view of output power. Photomixing method is used for generation and registering of continuous THz radiation [3, 19, 21–23]. With such approach, THz radiation generation results from heterodyning and application of two light waves of different frequencies into photoconductive structure. Fig. 4а contains schematic demonstration of such method realization by means of two laser sources with adjustable wavelengths. This generation technique has a number of advantages for use in THz wireless communication system: it provides the largest range of radiation frequency change –from GHz to THz region; besides that, with such approach expensive electronic devices working at frequencies of millimeter and/or THz range are not needed. However, one needs to understand that for efficient photomixing polarization, frequencies and phases of inserted optical beams must be continuous, that’s why it is necessary to add phase synchronization systems to system scheme. In such a way, use of optical mixers for THz generation has a number of advantages – they are cheap, compact, they work at room temperature and allow changing radiation frequency. However, rather narrow bandpass caused by antennas attached to them in order increase radiation efficiency limits application of mixers in wireless communication systems, as for further increase of data transfer rate larger frequency bands are required. The solution may consist in development of mixing antennas with high spectral bandpass and efficiency, as well as significant amplification (for compensation of radiation power losses during signal dissemination with air). For example, in  it was said that antenna based on photonic-crystal structure was developed; such antenna fully covers the range of 275 to 320 GHz. Photomixers with built-in antenna were developed earlier; generation of radiation with changeable frequency at 30–300 GHz range was demonstrated on their basis .
Another heterodyne technique of THz radiation generation is shown in the fig. 4b; such technique allows obtaining signals with low phase noise [27–29]. Key components are: generator of optical frequency comb and optical filter separating two frequencies of optical comb. Optical frequency comb is obtained either by means of laser system with mode synchronization or by means of modulation of continuous laser beam. In such a way, multifrequency optical signals with intervals between them equal to intermodal frequency f0 for lasers with mode synchronization are obtained; at this, all modes are synchronized by phase. Separation and combination of two modes is carried out between modes at the interval of Nf0. Basic frequency f0 for lasers with mode synchronization is within the range of 10–30 GHz; optical modulators and controlling electronic devices are not commercially available for such range, and multiplication index N may reach values exceeding 50. Due to possibility of exact change of f0 by means of synthesized signal generator, output signal frequency may be continuously changed from f0 to Nf0.
As it was mentioned before, optoelectronic converter is the key component of all hybrid systems during generation of THz signal by means of photonics. For high rate data transfer in wireless THz communication systems with carrier frequencies exceeding 100 GHz powerful optoelectronic converters working at high frequencies are needed.
Among photodiodes for 1550 nm range, ultra-rapid travelling wave diodes with separate zones of radiation absorption and separation of photocarriers with extended dynamic range (Uni-Travelling-Carrier or UTC-PD) for analog fiber optic communication lines and their modifications are characterized with the highest output power: power of more than 20mW at the frequency of 100 GHz, over 500 µW – at the frequency of 350 GHz and over 100µW at the frequency range of 350–450 GHz and over 10µW at the frequency of 1 THz was obtained [23–24, 26, 28]. Photodiode is often integrated with flat antenna to increase its efficiency.
For practical use of UTC photodiodes in wireless THz communication systems it is necessary to increase output power at frequencies over 500 GHz more than in sequence. There are three major directions of works on photodiode power: elimination of UTC diode heating; development of efficient connection between photodiode and antenna and creation of photodiode massifs and antennas. Besides that, as power amplifiers operating at these frequency bands are currently commercially unavailable, one of the most effective approach us the use of frequency multiplier. For example, Moeler et al.  obtained output power over 1 MW at the frequency of 625 GHz, having started from 13 GHz, by means of such approach.
For effective connection of photodiodes and antennas THz waveguides are being developed. For example, photodiode modules with hollow rectangular waveguides for W-frequency band (75–110 GHz), F-band (90–140 GHz), D-band (110–170 GHz) and J-band (220–325 GHz) were developed . Works on creation of waveguides for higher frequencies are being carried out. For example, hollow metal-glass waveguide with the lowest attenuation of 0.95 dB/m for the frequency of 2.5 THz was developed .
Basic registration methods
Two basic approaches are used for detection of THz signals – direct detection and heterodyne detection (fig. 5). Schottky diodes (SD) and bolometers are the most frequently used as receiver. Direct detection (fig. 5a) is the most widespread method for measurement of amplitude or power of THz radiation, limit detection frequency of SD may reach 10 THz in case of using GaAs based materials and 1.5 THz in case of using 130 nm silicone CMOS technology. Heterodyne detection based on SD mixer and local oscillator (heterodyne) (fig. 5б) provides higher sensitivity and phase information on THz signal. Heterodyne detection schemes with heterodynes with THz signal generation by photonic methods are shown on fig. 5c and 5d. Advantage of the scheme in the fig. 5в is that THz heterodyne signal may be led by means of optical fiber, and receiver bandpass is increased due to possibilities of more diverse frequency change during signal generation by infrared semiconductor lasers . In combination with photomixers heterodyne scheme (fig. 5г) provides the largest bandpass. Photoconductors, photoconductive antennas and photodiodes are the most frequently used as photomixers.
Demonstration of wireless
THz communication systems
For the first time wireless communication system with carrier frequency over 100 GHz was presented by NNT Company in 2000 . This system presented a wireless line with carrier frequency of 120 GHz with generation and modulation of signals by means of photonics. Obtaining of unprecedented data transfer rate of 10 Gbit/s  led to development of electronic components for wireless communication systems and, consequently, fully electrical wireless CMOS-based system was developed, by means of which real time broadcasting of the Olympic Games in Beijing was carried out in 2008 . After this, development of wireless THz communication systems was carried out quite fast: during next years, the results of experiments on data transfer at carrier frequencies of 75–110 GHz [16, 34, 35], 140 GHz , 200–240 GHz [37–38], 250–400 GHz [7, 29, 39–40,42], 625 GHz [29–30] were published.
A number of scientific groups are engaged in development of wireless communication systems on the basis of photonics at frequency range of 75–110 GHz (W-range). Several such systems were demonstrated: for example, in publication  100 Gbit/s hybrid wireless communication line based on optical heterodyne up-conversion 12.5 Gbit optical 16-CAM signal of basic band with polarization multiplexing at distances up to 120 cm. Further, this team offered data transfer system for W-range based on optical comb generator, by means of which simultaneous THz signal generation with bandwidth of 15 GHz on three channels with orthogonal frequency and special multiplex and data transfer rate of 8.3 Gbit at every channel . Generation of basic frequency band and receiving was carried out according to the method described in publication , and transmission distance reached 2 meters.
Another scientific team presented W-range data transfer system in which transducer and receiver are manufactured on the basis of pulsed radio architecture . This system allowed reaching 10 Gbit/s data transfer rate in air, as well as in fiber.
In publication  wireless data transfer system at carrier frequency of 140 GHz with maximum data transfer rate of 10 Gbit/s at the distance of 1.5 km in non-real time mode was described. Subharmonic mixer and Schottky diode multiplier, H-type band filter, Cassergain antenna and other system components were developed in order to ensure highly efficient transmission and receiving. 16-quadrature amplitude modulation was used for obtaining spectral efficiency of 2.86 bit/s/Hz.
Developments of wireless communication systems for frequency range of 200–240 GHz are actively carried out. In publication  it is said about obtaining transfer rate of 100 Gbit/s with carrier frequency of 235.7 GHz, generated during mixing of two laser subcarriers with mode synchronization by means of UTC photodiode. Different modulation modes are studied; transfer with data transfer rate of 50 Gbit/s was made at quadrature phase modulation; 75 Gbit/s at 8- phase modulation; 50 Gbit/s at quadrature amplitude modulation; 100 Gbit/s at 16-quadrature phase modulation. Transfer distance in all cases was 20 meters. Receiving was carried out by means of electronic single chip converter with frequency lowering based on active millimeter monolithic integrated circuit. The same team of authors further presented data transfer system with the use of 1–3 carriers at frequencies of around 237 GHz for single polarization transfer for the distance of 20 to 40 meters at rates of 75 to 100 Gbit/s . Narrow band signals of THz carrier were generated by photomixing in UTC photodiode of two highly stable laser lines with mod synchronization. Electrical signal at the photodiode» output was emitted by means of cone antenna with lens, receiving module contained monolithic integrated circuits with mixers and amplifiers. It is anticipated that such synergistic use of THz photonics and electronics named teratonics should lead to wireless transfer of several Tbit/s at the distance over 1 km.
300–400 GHz range at this time is the most perspective regarding data transfer rate. In publication  wireless data transfer system working at the rate of 14 Gbit/s at carried frequency of 300 GHz at the distance of 0.5 m is described. THz signal is generated by heterodyning of light from two sources with changeable wavelength, and then modulated with optical intensity modulator based on pulsed code generator; after that optical signal is converted to electrical one by means of UTC photodiode and generated to open space by cone antenna. Schottka diode is used as receiver.
In the article  a team of Japanese scientists informed of accurate transfer at carrier frequency of 300 GHz in real time mode with maximum data transfer rate up to 40 Gbit/s for one channel and up to 48 Gbit/s for channel with polarization multiplexing. It is shown that during increase of capacity of basic frequency of detector scheme it is possible to obtain data transfer rates of 50 Gbit/s and 100 Gbit/s for these channels correspondingly. Also 600 GHz system showing that carrying frequency may be doubled for providing higher data transfer rate was described. Finally, for the use of multilevel modulation schemes for transfer in real time mode at rates over 100 Gbit/s phase stabilized transducer based on optical frequency comb was presented. Operation of such scheme was tested experimentally at carrier frequency of 100 GHz.
In publication  data transfer at carrying frequency of 300 GHz with rate of 12.5 Gbit/s by means of transducer based on photonics and receiver module based on enhanced Schottky diode was demonstrated. It is noted that possibilities of such system are not limited to obtained data transfer rate, further enhancements will allow transferring data at the rate up to 20 and more Gbit/s.
Among the newest developments in this frequency range one may name publication , where authors offered data transfer system for frequencies over 250 GHz with the use of amplitude modulation based on UTC-photodiode equipped with waveguide and Schottky diode with integrated antenna as a receiver. Error-free transmission at the rate of 24 Gbit/s at carrier frequency of 300 GHz at the distance of around 50 cm was demonstrated. Besides that, this team developed receiver on the basis of monolithic integral circuit of microwave range with state-of-the-art InP technology of bipolar transistor at heterojunction. For compactness of the device antenna, radiofrequency amplifier, amplitude detector and amplifier were integrated in one chip. Besides that, for further increase of data transfer rate this team is working on THz transceiver operating at 300 GHZ frequency based on monolithic integral circuit of microwave range with multilevel modulation.
In recent publication  communication line with data transfer rate of 3 Gbit/s based on high level quadrature amplitude modulation (16-CAM) with carrier frequency of 0.34 THz for future local wireless networks is described. In this system heterodyne transceivers and parallel digital data process system is used. By means of two specially developed antennas it was possible to obtain transfer distance of 50 meters. Besides that, prototype of local wireless network at 0.34 THz frequency based on IEEE 802.11 protocol with data transfer rate of 6.536 Mbit/s at the distance of 1.15 m was presented.
Terahertz communications are the most perspective technology for realization of wireless local communication networks taking into consideration growing requirements for data transfer rate. In many countries works on standardization and regulation of terahertz frequency band are carried out ; meanwhile, in Japan, in 2014 frequency range of 120 GHz was already officially assigned for wireless communication line .
The future of terahertz communication systems is without any doubt directly connected with development of photonics. Integration of photonic and electronic devices with the use of modern manufacturing technologies such as silicone photonics for creation of compact and economically efficient THz transceivers is perspective. Recently, development of THz power amplifiers and preliminary amplifiers for transducers and receivers based on silicone technology has started. For example, recently, the first mini-amplifier operating at 1 THz and 1.3 THz frequencies with amplification in 10 and 9 dB correspondingly was developed .
Due to the size of available frequency band in THz range, even on the basis of the simplest modulation methods, data transfer rate of several dozens of Gbit/s was obtained. In the nearest future increase of data transfer rate in wireless networks is connected with the use of complex multilevel modulation systems and different methods of multiplexing. Nowadays, prototypes of wireless terahertz communication systems already exist; record data transfer rate of such systems is 100 Gbit/s.
Further increase of data transfer rate in wireless networks is connected with the use of pulsed THz optics. For example, prototype of installation for data transfer with the use of pulsed THz source with pulse spectrum of 0.1 to 1 THz was developed . Information coding in the spectrum of every wideband THz pulse, as well as pulse sequence was carried out by means of original method described in publication . Besides that, it is possible to increase transmitted pulse spectrum up to 0.1–10 THz which will allow increasing data transfer rate up to 100 Tbit/s in local wireless networks and satellite communication systems.