Next generation optical fibres

OPTICAL FIBRES have been produced from two materials with different refractive indices, the higher index material forming the solid core and the lower index material (called cladding) surrounding it.

The former carries the light, while the latter confines the light to the core by total internal reflection.

The rate of innovation in fibre design slowed down in the early 1980s as performance approached the fundamental limitations of technology.

In 1991, a new idea was proposed: `could light be trapped inside a hollow fibre core by creating a periodic lattice of microscopic holes in the cladding glass, which is known as `photonic crystal'?

Light cannot normally be guided down an air hole.

For such guiding requires cladding material with a lower refractive index than the core; none lower than that of air exists.

However, surrounding the air hole with a photonic crystal, frequency ranges corresponding to the band gap of the crystal would be trapped within and guided along the core.

This is the seed of the innovation.

Devising a fabrication technique was a challenge as the precedant known was the glass nanocrystals (Science Vol 258, 1992) but these were only a few hundreds of micrometers thick.

After several false starts, knight et al. University of Bath U.K. hit upon the stack-and-draw procedure by which silica capillaries could be stacked, fused together and drawn successfully to photonic crystal structure (Opt. Letters Vol 21, 1996)

Russell mentions that his team had chanced upon a technology first used by the Egyptians to make mosaic glasses (third-to-first century BC).

The diversity of new features beyond that offered by conventional optical fibre find applications in the following fields of science and technology:

— Gas-based nonlinear optics

— Atom and particle guidance

— Supercontinium (namely, millions of individual frequencies) generation measurements that used to take hours and involve counting individual photons can now be made in a fraction of a second.

Photonic crystal fibres (PCF) represent a next generation, radically improved version of a well-established and highly successful technology.

So many new applications and developments have emerged from the PCF concept that there is now a need to rewrite the text books on fibre optics (Philip Russell, Science 17 January 2003).