Generating Frequency-Tunable Continuous-Wave THz Signals

Terahertz signals are being explored for a wide range of applications, including high-resolution imaging, medical science, short-range wireless communications, and homeland security systems. Of course, generating practical THz energy at such high frequencies, typically from 100 GHz to 30 THz, can be challenging. Fortunately, researchers from a number of different facilities in Canada, performing work sponsored by the Natural Science and Engineering Research Council of Canada, have developed an all-fiber approach to THz signal generation using a periodically poled optical fiber. In this approach, a continuous-wave (CW) THz signal is generated at the fiber by beating two optical wavelengths from two laser sources with the wavelength spaced in proportion to the desired frequency of the THz wave.

Terahertz signals have been generated through the use of nonlinear crystal devices, such as those based on gallium arsenide (GaAs) or gallium phosphide (GaP) substrate materials, although such approaches tend to be expensive. As a lower-cost solution, a periodically poled optical fiber was used to generate a tunable terahertz wave by beating two wavelengths with a wavelength spacing corresponding to a THz wave at the periodically poled fiber. The periodically poled fiber is made by a length of twin-hole optical fiber with its fiber core between the two holes. After the twin-hole fiber is drawn, two silver electrodes are inserted into the two holes. The twin-hole fiber is then thermally poled at a high temperature (about +260°C) with a voltage of +3.3 VDC applied to the two silver electrodes to instigate second-order nonlinearities into the nonhomogeneous glass of the fiber. An ultraviolet laser source is then used to periodically erase the thermal poling induced second-order nonlinearity to achieve quasi-phase matching (QPM) to enhance the energy conversion efficiency of the THz source. In this way, a THz CW signal can be produced using a periodically poled optical fiber based on an optical difference-frequency-generation (DFG) approach.

To evaluate this THz signal generation approach, an experiment was conducted in which CW signals at 3.8 THz were produced using incident light waves of 1530.0 and 1560.1 nm. Using a THz detector, signal power of about 0.5 μ W was measured and a conversion efficiency of about  2.9 × 10^-5 was also measured. Although the experimenters admitted that the theoretical signal power levels and conversion efficiency were somewhat higher than the values obtained in the experiment, improved performance could be achieved if the emitted terahertz wave is completely collected and a longer periodically poled fiber is used. The frequency of the generated wave was tunable from 2.2 to 3.8 THz, corresponding to a wavelength in free space from 136.36 to 78.95 μm, in support of a variety of different THz applications. While still an experiment, this use of a periodically poled fiber to generate THz signals shows great promise for future THz applications.

See “Frequency Tunable Continuous THz Wave Generation in a Periodically Poled Fiber,” IEEE Transactions on Terahertz Science and Technology, May 2015, p. 470.