14 Nov 2006
Computer modelling reveals that a simple photonic crystal structure could be an effective way of coupling light into optical fibers, ridge waveguides and lenses.
A cascaded photonic crystal could be just the answer for coupling light into optical fibers, ridge waveguides and lenses, according to finite-difference time-domain (FDTD) analysis performed in China. The team from Harbin Institute of Technology (HIT) claims that its simple, self-collimating design transmits around 80% of the energy entering the system into a directional beam. (Appl. Phys. Lett. 89 131120)
"The photonic crystal structure creates self-collimating beams that interfere when emitted into free space," Weiqiang Ding of HIT's physics department told optics.org. "It is the interference of these beams that results in the highly directional emission."
Ding and his co-workers accept that they are not the first to group to come up with the concept, but believe that their design could be the easiest to make.
Firstly, a line of cylinders is removed from a square lattice to create a photonic crystal waveguide (see image). To complete the design, the exit surface is then covered with an enlarged version the initial lattice.
As Ding explained, the success of the structure lies in its ability to excite multiple side lobe self-collimating light beams that can interfere and cause directional emission. According to the team, the output is comparable to the directional emission from a photonic crystal structure with periodic corrugations on its transmission surface - a more complicated design in terms of fabrication.
The researchers used a FDTD algorithm to determine the field pattern and transmission coefficients of their design. "Computational time is our biggest challenge," revealed Ding. "Presently, we use a two-dimensional model to speed up the process."
Determined to make the modelling as realistic as possible, the team plans to switch to a more practical three-dimensional version with a finer mesh grid following an upgrade in computer hardware.
The work forms part of a larger project entitled "Exploring the periodic world of photonics" and complements research on surface modes of periodic structures and properties such as negative refraction.