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.
Distributed sensors, based on Brillouin effect in the optical fiber, provide an excellent method for measuring temperature
and strain over long distances. There are two types of such sensors. The first type is based on spontaneous Brillouin
scattering, and is called Brillouin optical time domain reflectometer (BOTDR). It measures the Brillouin frequency shift
or Brillouin power or Brillouin gain bandwidth to get the temperature and strain information. The second type of sensor
is based on stimulated Brillouin amplification. It is called Brillouin optical time domain analyzer(BOTDA). Normally, it
uses one laser at each fiber ends, one as pump and the other as a probe light. The probe light will experience Brillouin
amplification. Through the analysis of Brillouin gain spectrum (BGS), we can get the temperature and strain information.
Both the two types of sensors are attracting attention all over the world, and temperature resolution of less than 1 degree
and strain resolution up to 5 με was reported. The fiber distances of up to 150km was presented while other papers
reported a spatial resolution of the order of 1cm with frequency domain techniques or correlation techniques. We
proposed and analyzed our design, it is an improvement of BOTDA with a single end laser, which make it easy to
implement in field. Through simulations, optimized launch power has been found for a certain design.