Array Waveguide Gratings (AWGs)

Optical fiber is a popular carrier of long distance communications due to its potential speed, flexibility and reliability. Attenuation and dispersion problems in fiber, which limit the practical speed and distance of communications, were partially resolved. However, the dispersion qualities of an optical fiber still force a compromise between transmission distance and bandwidth, making it necessary to refresh high-speed signals at intervals using optoelectronic repeaters. A more elegant solution is found using AWGs, which effectively increases the useable bandwidth in a system without electronic repeaters

AWG Principle

Coupling from the slab waveguide to the waveguide array is the most significant source of loss in an AWG, because of the mismatch between the field distributions of the slab waveguide and the arrayed waveguides. The light couples from the input combiner into the array of waveguides that start along an arc centered at the input waveguide-combiner junction, with a radius equal to the focal length of the combiner. This ensures that the light injected from the central input waveguide arrives at the beginning of each of the arrayed waveguides with the same phase.

AWG Related Problems Among various targets of current AWG research, the following topics appear particularly relevant: miniaturization, increasing channel numbers, decreasing channel spacing, reduction of insertion loss, crosstalk and chromatic dispersion, flattening and widening the passband, elimination of polarization and temperature sensitivity, and improving spectral tuning capabilities including advanced add–drop, cross-connect, and filtering functions, and integration with photodetectors, lasers, modulators, variable optical attenuators, optical amplifiers, switches, and other photonic elements. One of the difficulties in large-scale AWGs is crosstalk deterioration caused by phase errors arising from variations in the arrayed waveguide width, thickness, material composition, and stress. Because the influence of such errors increases with the size of the waveguide array, the effect can be severe for densely spaced AWGs.

The crosstalk can be reduced by adjusting the phase delays in the individual arrayed waveguides, but this is rather tedious task. The phase can be measured for each waveguide of the array by low coherence interferometry. The phase correction can be achieved by uv-induced refractive index changes in the glass. All the waveguides are exposed at the same time by using a metal mask with different opening lengths over each waveguide that are proportional to the phase errors to be compensated. Effective index birefringence in the arrayed waveguides produces a polarization-dependent shift of in the demultiplexer central wavelength. The AWG central wavelength changes with temperature. Active temperature stabilization by a heater or Peltier cooler is often used, but it requires continuous power consumption of several watts and temperature control electronics. This can be avoided with an athermal design, with substantially reduced temperature sensitivity.

A typical AWG has a symmetric intensity distribution across the waveguide array, and as such its chromatic dispersion D is negligible. However in a practical AWG this symmetry is disturbed by phase and amplitude errors that are randomly distributed in the arrayed waveguides. This increases chromatic dispersion. Because the errors increase with decreasing channel separation, the chromatic dispersion increases similarly. Recently, AWG devices made of polymeric materials have been gaining a great deal of attention because of their excellent particular features such as easier optical integrating, lower transmission loss and easier control of the refractive index, compared to other AWG devices. Many search groups have focused on the development of polymeric AWG multiplexers, and have fabricated such optical devices using polymeric materials.