Nature gives photonic crystal fibres a boost

12 Dec 2008

optics.org speaks to the researchers who believe that patterns found in nature could inspire a better class of photonic crystal fibre.

By mimicking a simple spiral design found in nature, UK researchers have designed a 1.55 micron photonic crystal fibre (PCF) that exhibits high birefringence and tunable dispersion. The fibre is said to be cheap to make and could be used to generate supercontinua, or broadband, large bandwidth signals for use in medical sciences, metrology and telecommunications (Optics Letters 33 2716).

"To our knowledge no-one else has directly applied designs from nature to create a PCF," B M Aziz Rahman, a professor at City University London, told optics.org. "This research opens up another design approach that leads to significant improvements in PCF properties and could open up an entirely new generation of dielectric optical devices that are non-reciprocal."

Rahman and colleagues modelled their PCF on the arrangement of seeds found in a sunflower. Their PCF contains 43 identical air holes arranged in a golden spiral pattern at nearly equal distances apart. Each 0.36 µm diameter hole is rotated by approximately 1.618 (the golden ratio) revolutions within a 5-6 microns diameter fibre core.

"The spiral pattern is the most efficient use of space and allows a large air-filling fraction and better light confinement," explained Rahman. "In the design, the arrangement of air holes is asymmetric, which results in no degeneracy of modes."

PCFs with a birefringence of the order of 10_-2 have previously been constructed, however, they require a complicated arrangement of air holes of different sizes and shapes. In comparison, the PCF developed by Rahman and colleagues has a far simpler design, a birefringence of 0.016 and should be cheaper to fabricate.

"We gain high birefringence from the structural asymmetry rather than through using an intricate pattern of air holes," explained Rahman. "In addition, the spiral PCF has polarization-dependent dispersion, which means it could be used for efficient supercontinuum generation instead of having to employ bulky optical sources such as Ti:Sapphire lasers."

Efforts are now underway to fabricate the golden spiral PCF and the team also hopes to explore other spiral patterns that exist in nature. "We are investigating equiangular spiral designs for application in the optical and terahertz regime," concluded Rahman. "We are also looking into fabricating these designs in materials such as compound glasses."