Uses of Terahertz Measurement in Broadband Connections

Terahertz measurement is widely used to generate terahertz broadband pulses and terahertz narrowband continuous-wave (cw) signals. Since their demonstration as practical THz sources and detectors, THz-PCAs have been the subject of a vast number of scientific and industrial reports investigating their application as terahertz wave transmitters and receivers.

In continuous-wave mode, two cw laser beams, with their frequency difference in the THz range, combined either inside an optical fibre or properly overlapped in space, are mixed in a photo-absorbing medium (photomixer) and generate a beat frequency signal. Terahertz signals with the frequency linewidth as low as a few kHz can be generated by photomixers. The frequency of the terahertz signal can be tuned by tuning the wavelengths of the lasers. The output power in conventional photomixers falls from 2 μW at 1 THz to below 0.1 μW at 3 THz.

The generation of pulses

Broadband terahertz measurement pulses can be generated by exciting THz-PCAs with a femto-second short pulse laser. Using a femto-second laser with ~ 100 fs optical pulse duration, terahertz pulses with their frequency content extended up to around 5 THz and an average power of a few μW can be achieved. Broadband terahertz pulses can also be generated in electrooptic crystals excited by femto-second short pulse lasers.

Terahertz signals can be generated via parametric interaction of near-infrared photons and optical vibration modes inside an optical crystal. Using this technique, generation of a quasi-cw terahertz signal with pulse duration of 3.6 ns, an average power 9 nW and a frequency tuning range of 0.7 to 2.4 THz has been reported.

Terahertz measurement technologies utilize electromagnetic radiation in the frequency range between 300 GHz and 10 THz. Potential applications for terahertz technology in biology, chemistry, medicine, astronomy and security are wide ranging. THz wavelengths have several properties that could promote their use as sensing and imaging tool. There is no ionization hazard for biological tissue and Rayleigh scattering of electromagnetic radiation is many orders of magnitude less for THz wavelengths than for the neighboring infrared and optical regions of the spectrum.

THz radiation in nonmetallic materials

THz radiation can also penetrate nonmetallic materials such as fabric, leather, plastic which makes it useful in security screening for concealed weapons. The THz frequencies correspond to energy levels of molecular rotations and vibrations of DNA and proteins , as well as explosives , and these may provide characteristic fingerprints to differentiate biological tissues in a region of the spectrum not previously explored for medical use or detect and identify trace amount of explosives.

THz wavelengths are particularly sensitive to water and exhibit absorption peaks which makes the technique very sensitive to hydration state and can indicate tissue condition. THz radiation has also been used in the characterization of semiconductor materials, and in testing and failure analysis of VLSI circuits. In an unconventional context, THz techniques allowed art historians to see murals hidden beneath coats of plaster or paint in centuries-old building, without harming the artwork