Terahertz-active gases inside hollow-core optical fibers would be optically pumped.

Fiber-optic lasers of a proposed type would serve as sources of coherent radiation at frequencies between 0.5 and 4.0 THz and output power levels ≥100 mW. In the original application envisioned in the proposal, these lasers would be parts of compact, man-portable instruments for detection of explosives.

The development of the proposed terahertz fiber-optic lasers would take advantage of, and extend, prior developments in the specialties of (1) generation of terahertz radiation by optical pumping of gases and (2) hollow-core photonic-crystal fibers.

• 

  Figure 1. These Scanning Electron Micrographs show cross sections of four hollow-core plastic photonic-crystal fibers produced by the Optical Fiber Technology Center in Sydney, Australia.
  Optical pumping of gases has been used for several decades as a means of generating terahertz radiation. Efficient tabletop sources have been demonstrated and employed in many important applications. In a typical example of a source of this type, a midinfrared beam from a CO2 laser excites methanol molecules to a higher vibrational energy level, and terahertz photons are emitted in a radiative decay from the higher level to a lower level. At a pump power level of the order of 10 W, a source of this type is capable of producing tens of milliwatts of coherent terahertz optical power.
• Hollow-core photonic-crystal fibers have been fabricated from diverse materials, including plastics (see Figure 1). Hollow-core photonic-crystal fibers can be tailored to act as waveguides in selected frequency ranges.