The use of hollow-core fused silica fibres has assisted the development of lasers in the mid-infrared spectral range.
The mid-infrared part of the electromagnetic spectrum ranges between wavelengths of between 2 and 5 microns (micrometres). This is the wavelength range of radiation emitted by heart-seeking missiles, active volcanoes, fires, explosions and planets in outer space when viewed from Earth. Lasers that emit pulses or a continuous beam in the mid-infrared range are used in medicine as scalpels and diagnostic instruments, in space communications and in plastics manufacture, among a growing list of applications.
However, there has always been a challenge in producing these lasers, because in the conventional laser design, the beam’s power falls off dramatically once its wavelength exceeds about 2.8 microns. Researchers at Bath University, England, have come up with a solution by using fused silica hollow-core fibres to guide the beam.
Conventional lasers use solid optical glass fibres to confine the beam. The fibre effectively acts as a waveguide. Fused silica is easily drawn into fibres and has a very high mechanical strength as well as high transparency. But once electromagnetic wavelengths exceed about 2.8 microns, the silica begins to absorb the radiation. This has been a big drawback in laser technology, as the silica material is relatively inexpensive to manufacture.
A result of the Bath University research is the design of a new fibre laser. Acetylene gas was used as the source of the mid infrared radiation and pumped into hollow-core fibres that trapped the gas and light. These interact within the confines of the hollow core and can remain in place for wavelengths up to 10 or 11 microns. In this way, the researchers managed to overcome silica’s tendency of absorbing light of 2.8 micron wavelength and higher.
The researchers moved on to another innovation by producing a feedback fibre. This takes some of the light produced by the acetylene gas and uses it to seed another light cycle. As a result, the laser itself required a lower pumping pressure, making it more economical to use. The feedback can also be employed to increase the laser’s power.
The researchers believe that this is just the start of mid infrared laser development. They expect to use a variety of other gases in addition to acetylene. By producing a series of interacting laser beams, they hope to produce lasers emitting up to wavelengths of 5 microns or even more.
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