Terahertz Fiber-Optic Lasers for Detection of Explosives

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

October 1, 2007

Naval Research Laboratory, Washington, DC

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.

A terahertz laser as proposed would include a plastic hollow-core photoniccrystal fiber tailored to support low-loss guiding of both a mid-infrared (e.g., CO2-laser) pump beam and the terahertz beam that one seeks to generate. The voids inside the hollow-core fiber would be filled with a suitable terahertzactive gas. The pump beam would be supplied by a sufficiently powerful gasmatched laser. If, for example, the terahertz- active filling gas were acetylene, a high-power, compact CO2 laser would be used as the pump.

Figure 2. A Proposed Terahertz Laser would include a plastic hollow-core photonic-crystal fiber filled with a terahertz-active gas. The fiber would be part of a terahertz resonator. The terahertz-active gas would be optically pumped by a mid-infrared laser beam.

The gas-filled hollow-core photoniccrystal fiber would be part of a terahertz optical resonator that would also include optical couplers and reflectors next to the input and output ends of the fiber (see Figure 2). An input mid-infrared coupler would enable efficient coupling of the pump beam into the fiber. An input terahertz reflector would be designed to be as highly reflective as possible at the intended terahertz operating frequency. An output terahertz coupler would be designed to optimize laser efficiency. To enhance the overall system efficiency, it may be desirable to design the resonator to provide for additional internal reflection of the pump beam at the input and output ends of the fiber.

Some of the risky and challenging aspects of the proposed development are the following:

It is critical to design hollow-core photonic-crystal fibers to guide both mid-infrared pump and terahertz beams with losses low enough for efficient lasing. While guiding at single wavelengths has been demonstrated, simultaneous guiding at two wavelengths has not yet been realized.
Filling the voids in hollow-core photonic-crystal fibers with suitable terahertz- active gases is a challenging task.
Generation of terahertz beams in gasfilled photonic-crystal fibers has not been demonstrated yet.
Engineering of coupling and reflective structures that provide for efficient and tailorable manipulation of two beams that differ greatly in wavelength is another challenging task.

This work was done by Antonije Radojevic of the C. S. Draper Laboratory, Inc. for the Naval Research Laboratory.