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FAQ4: Why are many companies that build 50 GHz DWDM systems
abandoning arrayed waveguide gratings (AWG) and fiber Bragg gratings (FBG)
in favor of Thin Film Filters (TFF)?
 Thin
film filter (TFF) technology dominates the 100 GHz and 200 GHz DWDM
markets due to its low cost and high reliability. After a slow start,
it is also proving the most cost-effective and reliable 50-GHz solution.
Indeed, many companies have recently backed away from arrayed waveguide
gratings (AWG) in favor of TFFs that display better insertion loss,
crosstalk and temperature stability. Likewise, fiber Bragg gratings
(FBG) have exhibited performance limitations that cannot be eliminated
without incurring high costs and system complexity. These problems
have only grown more acute in the transition from 100 GHz to 50
GHz systems.
The performance and cost benefits of TFF over AWG and FBG in 50
GHz applications are shown in the table below. The FBG has 100-GHz
filters and a circulator; the AWG is a 50-GHz grating; and the 50
GHz TFF is manufactured by Lightwaves2020.
Table
1. Performance Comparison of TFF, FBG and AWG
Drawbacks
to AWG
- AWGs have
low non-adjacent channel isolation and thus low accumulated isolation.
- AWGs have
poor thermal performance.
- AWGS have
a considerably large PDL compared to other technologies.
- AWGs have
a high cost for add/drop applications.
- Phase errors
arising from manufacturing mistakes, lack of material uniformity,
temperature variation, and stress-related performance degradations
are all commonplace in AWGs.
- AWGs are
cost-prohibitive unless used in large channel count devices.
- In order
to maintain stable overall performance, a TEC is usually embedded
in the AWGs design, which uses considerable power.
Drawbacks to FBG
- Due to its
reflection operating mode, a circulator or coupler is needed to
separate the output from the input signals, which leads to a complex
hybrid structure. The cost of each channel is thus high compared
to other approaches.
- FBG has limited
application due to its large chromatic dispersion, which causes
the broadening of the signal pulse and increases the bit error
rate. Although the dispersion can be compensated for by modifying
the grating structure, this only adds to the design complexity
and cost.
- At 50 GHz,
inaccuracies in the grating cause ripples in the dispersion, which
can degrade system performance.
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