Transcript Array Waveguide Gratings (AWGs)

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.