Plastic Makes Low-Loss Terahertz Fiber

Kevin Robinson

Low-loss optical fiber transparent in the terahertz region has moved a step closer to reality, thanks to researchers at the Center for Terahertz Photonics at Pohang University of Science and Technology in South Korea. The group fabricated a plastic optical fiber that performs well in that spectral region. The fiber uses high-density polyethylene (HDPE), a common plastic used for soda bottles and other consumer products.

To create terahertz fiber, researchers arrange tubes of high-density polyethylene (HDPE) around a single solid HDPE filament.

The terahertz region, which corresponds to the spectral range from 30 µm to 3 mm, is becoming extremely important to research and applications in biomedicine, sensors and communications. Despite this, the technology remains relatively underdeveloped, largely because of a lack of efficient compact sources as well as waveguides and cavities.

Silica fiber is an obvious waveguide candidate, but it suffers from two problems. The first is that silica has high absorption in the terahertz region. Second, to transmit terahertz pulses, silica fibers must be large and mechanically inflexible. According to Haewook Han of the university's department of electrical and computer engineering, most terahertz components in use rely on free-space propagation. HDPE, however, is relatively transparent at these frequencies, and when fabricated as a photonic crystal structure, it makes a functional waveguide.

Depending on its size and periodicity, the matrix of a photonic crystal creates a bandgap at certain frequencies. By introducing a defect into the crystal, photons can be guided through the matrix without loss.

Such fibers can incorporate either a high-index defect, such as a solid HDPE filament, or a low-index defect, such as air. The high-index core confines terahertz radiation using total internal reflection and can transmit broadband signals; the low-index core guides photons through the photonic bandgap effect and enables what Han described as a "virtually lossless terahertz fiber."

Modeling and testing

In its recent work, the research team arranged 500-µm HDPE tubes in a triangular structure around a solid high-index HDPE filament and fused it in a conventional furnace. Testing revealed that the arrangement compared well with calculated performance.

Han said that plans will include further work on air and vacuum-guided terahertz fibers. In addition, the researchers plan to develop testing regimens and modeling tools to continue improving the fiber. In the long run, their work could prove extremely useful in biomedical sensing and imaging applications.