Microwave signals distributed over optical fibre are of great interest for many applications. There are many
advantages of all-optical microwave filters for the direct processing of microwave signals in the optical domain, such as,
large time-bandwidth products, insensitivity to electromagnetic interference, low loss, and lightweight. A number of low
pass photonic microwave filters have been reported, where it is required to achieve optically incoherent summing of two
light beams. To overcome the optical coherence problem, either a laser array is used, or the coherence length of the light
source is kept smaller than the minimum delay time of the filter. Incoherent summing in bandpass filter has also been
achieved; however, they require very long length of Hi-Bi fibre.
We propose here all-optical low pass and bandpass microwave photonic filters configurations, together with their
application in a 20 km radio-over-fibre (RoF) link. The key problem when using a narrow linewidth source is the
coherent operation because of the narrow laser source. High differential group delay (DGD) will be induced by Hi-Bi
linearly chirped fibre Bragg grating (LCFBG), the optical interference is avoided because the two orthogonal state of
polarizations (SOPs). Meanwhile, the positive or negative chromatic dispersion (CD) will also be provided by the
chirped Hi-Bi LCFBG. The bandpass resonance is eliminated by the use of phase modulation. The CD value also can be
compensated or increased by the chirped LCFBG in the RoF link for both low pass and bandpass filters. Measured
results agree well with the theoretical results.
A full-vectorial analysis of photonic crystal fibers based on a compact two-dimensional finite-difference
time-domain method (C2D-FDTD) is presented. The model with material dispersion incorporation is
formulated and validated. The Sellmeier equation is implicitly included into the model to account for
the material dispersion of silica. In this paper we use a formulation of Maxwell's curl equations by
electric flux density and magnetic field intensity, with auxiliary differential equations; and we
demonstrate the flexibility and robustness of this approach in treating general material in PCF. We
have good agreement with multipole method.
The fabrication of a tunable all-solid photonic bandgap fiber coupler based on side-polishing technique is reported.
The all-solid photonic bandgap fiber is set into a silica block and then polished to access the evanescent field. The
photonic bandgap fiber coupler was assembled by mating two identical half-blocks with each other. By
longitudinally adjusting the relative position between the mated pair, the tunable coupling ratio as much as 92.5% at
1550 nm is achieved. The investigation of the spectrum properties shows that the coupler has excellent tunability
properties, for which the coupling ratio can be smoothly and continuously tuned.
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