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Bandwidth demands are driving operating frequencies constantly higher, with submillimeter-wave and terahertz frequency bands offering tremendous potential for applications in indoor wireless communications, spectroscopy, and imaging systems. Frequencies between 275 and 3000 GHz, which have not yet been officially allocated, offer tremendous growth potential for near-field communications at data rates of 10 Gb/s and higher in the near future.1-4 The challenge facing design engineers is the development of affordable hardware to support applications operating at such high frequencies.

Terahertz imaging and spectroscopy, for example, are among the more interesting applications for medical and industrial applications of terahertz frequencies. Terahertz imaging can be extremely useful for aircraft guiding and landing systems in zero-visibility situations. Millimeter-wave imaging systems can provide two-dimensional images of a landing area.5 Terahertz-based near-field communications can enable transfer of large amounts of data to multiple users within buildings.

During the 2008 Olympic Games, Fuji Television Center used a terahertz system to transmit video signals of a live high-definition (HD) program from a recording studio to the international broadcast center, about 1 km distant. This organization achieved a data rate of 10 Gb/s in recent terahertz transmissions.6 The use of terahertz frequencies has increased in recent years as the output power of transmitters and the sensitivity of receivers at those frequencies has increased.

Terahertz communications devices are based on both photonic and electronic technologies, and different design equations apply to each technology area. Classical equations govern the submillimeter-wave spectrum while quantum equations are generally used for frequencies above the terahertz region. To better understand how they fit for higher-frequency applications, both technologies will be examined and then compared under different circumstances.

Using photonic technology, basic transmit and receive operations are performed in the optical regime with the appropriate devices. The transmitter has the required components for signal generation, modulation, amplification, and optical-to-electrical conversion (Fig. 1). The first step involves optical signal generation and the last step is optical-to-electrical conversion prior to the antenna.

1. This block diagram shows a photonics-based approach for a terahertz transmitter.

Solid-state and vacuum-tube devices have been used for signal generation at lower frequencies within the terahertz region, while beam-wave tubes and optically pumped lasers (OPLs) have been used for higher-frequency terahertz applications. Beam-wave tubes include gyrotrons and free-electron lasers (FELs). These sources provide high output-power levels and are often used for plasma heating, high-power radar systems, and remote-sensing systems. Vacuum electronic devices—such as FELs, electron cyclotron lasers, and Cherenkov wave devices—produce output signals to 1 THz, with cyclotron tubes capable of terahertz signals at power levels to 1 kW. In FELs, the electron beams oscillate at high speeds to release photons via a strong magnetic field. These photons are then directed, by means of a reflector, through the electron beam for added gain. Solid-state lasers provide sources of high-frequency energy,8 as do OPLs.7

Optical modulators include devices that work by modulating signals provided by optical sources, such as absorptive and refractive modulators. In an absorptive modulator, modulation is based on the variation of the absorption coefficient due to the Fermi level or free-carrier concentration changes. In a refractive modulator, operation is based on refractive index variations.9 Decreasing the frequency of an optical signal following generation from an optical source and prior to modulation is another method for producing terahertz and millimeter-wave modulation4; this approach can be implemented in a number of different ways.10-12 In one simple approach, millimeter-wave and terahertz signals are generated by means of a subharmonic mode-locked laser diode and an arrayed waveguide grating (AWG) filter (see Fig. 2).10 AWGs are commonly used as optical demultipliexers in wavelength-division-multiplex (WDM) systems.

2. This block diagram shows an electronics-based approach for a terahertz transmitter.

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