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New fibre solves Mid-IR issues, see it on show at Photonex London.

The mid-IR (2-5µm) has historically been a difficult region for optically transparent glasses. The few glasses available are often extremely fragile, difficult or impossible to apply anti-reflective coatings to, very sensitive to exposure to moisture and sometimes suffer from poor transparency and low optical damage thresholds at key wavelengths.

The Mid-IR part of the spectrum is important in the latest environmental sensing, defence and medical applications. The issue of obtaining practical optical fibre for these wavelengths is a significant problem.

A new mid-IR transport fibre based on Tellurite oxide glass has recently been launched. This new fibre is broadly transparent from the visible through to ~4.5µm. The visible transparency is helpful to applications where both visible and mid-IR light is required, for example, where a visible aiming beam is needed.

Although Tellurite glass has been in existence for years, it was not useful due to the large OH (water) content in the glass, which causes large losses in the mid-IR. Without the careful removal of OH, the transmission loss shown in figure 1 would increase by ten times between 2 µm and 4 µm. The new fibre has a minimum loss between 3 and 4 microns.

Many types of Mid-IR glass, such as fluoride and chalcogenide, are very fragile. Tellurite, however, has a much higher tensile strength and a low sensitivity to moisture. Fluoride fibres are particularly sensitive to water exposure, requiring extreme care to be taken to isolate the material from the environment.

The new Tellurite glass structure is optimised so that it can be easily handled. A statistical analysis (Weibull plot) of thirty bare, uncoated fibres stretched until broken showed that the mechanical strength is greater than 60 kpsi. Testing the fibre with average power ~10W at ~2 µm and peak power >2kW over extended periods did not have any effect. The damage threshold is expected to be around 1-5GWcm-1.

See Laser 2000 at Photonex London on stand 4.


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