World-Record Deep-Ultraviolet to Mid-Infrared Supercontinuum Light Generated in Photonic Crystal Fibre

17.02.2015, 14:19

Bright light sources delivering continuous bands of wavelength from the ultraviolet (UV) to the infrared (IR) are of great importance in advanced optical microscopy, spectroscopy and many other fields. A proven approach to generating broad-band visible to near-IR light is microstructured "photonic crystal" fibre (PCF) pumped by ultrashort IR pulses. PCF is formed from a single strand of glass with a periodic array of microscopic hollow channels running along its entire length (see picture).

By varying the design of this structure, remarkable control over the light-glass interaction is possible, in particular the wavelength dependence of the velocity of light - the chromatic dispersion. This makes it possible to convert pulses of narrowband IR light into a so-called "supercontinuum" - a broadband pulse of bright white light. Up to now, most supercontinuum-generating PCFs have been made from fused silica glass, offering smooth and flat spectral power densities from the near-UV (wavelengths > 380 nm) to the mid-IR (wavelengths < 2500 nm). Outside this spectral range, however, silica glass suffers from strong material absorption and for wavelengths shorter than 380 nm it is susceptible to "solarisation" - UV-related optical damage that causes the glass to darken, rendering it unusable.

Supercontinuum generation in silica-based PCF is therefore only possible between these limits, as can be seen in the specification sheets of commercially available sources. Very recently [Nature Photonics 9, 133-139, 2015], scientists at the Max-Planck Institute for the Science of Light (MPL) in Erlangen, Germany, reported the world's first PCF made from fluoride-based glass, known by its acronym "ZBLAN". ZBLAN glass offers an impressively broad window of transparency, extending from the deep-UV to the mid-IR (wavelengths from 200 nm to 7800 nm). It consists of a mixture of five fluoride salts (ZrF4, BaF2, LaF3, AlF3 and NaF) and is much more difficult to work with than silica, because its viscosity changes dramatically with temperature. This makes it essential to control the furnace temperature to within a few degrees C (compared to a range of ~300°C for silica glass) for successful PCF drawing. This was perceived to be so difficult by many fibre experts, that it was commonly considered to be impossible to draw ZBLAN PCF with features small enough for supercontinuum generation. Despite this, MPL scientists have been able to draw high quality ZBLAN PCF and by coupling IR laser pulses at a wavelength of 1 µm into a very small core (diameter ~1 µm) have generated bright supercontinuum light from the deep-UV (200 nm) to the mid-IR (2500 nm). In contrast to silica-based PCF, the fibre shows no sign of UV-related degradation over many hours of operation. Prior to this breakthrough, the stable generation of broad-band deep-UV light in the wavelength range 200 nm to 380 nm was deemed impossible.

The Max Planck scientists therefore believe that this unique new light source is set to have a major impact in many areas of science and technology, and may even quite quickly emerge in commercial products.

 

Contact: Dr. Xin Jiang

Group: Russell Division

Reference:
X. Jiang, N. Y. Joly, M. A. Finger, F. Babic, G. K. L. Wong, J. C. Travers and P. St. J. Russell, "Deep-Ultraviolet to mid-infrared supercontinuum generated in solid-core ZBLAN photonic crystal fibre," Nature Photonics 9, 133-139 (2015).
doi:10.1038/nphoton.2014.320