Although, as previously mentioned, the main operation
regime of MWP filters is based on using incoherent sources,
several works [45]–[50] have reported and experimentally
demonstrated that it is possible to overcome the limitations that
source coherence imposes over the operation of MWP filters.
The use of coherent laser sources is interesting as higher input
powers can be injected into the filter thus reducing the RF link
losses. In the context of MWP filtering, a coherent source is that
having a coherence time (i.e., inverse of its linewidth) much
longer that the basic delay between adjacent samples. If such
sources are employed, then optical interference due to mechanical
and environmental fluctuations can severely compromise
the filter performance. An interesting technique is based on
controlling the source coherence by means of the application
of an external phase modulation [47]. Here, a residual phase
modulation applied at the output of the optical source increases
its linewidth thereby reducing its coherence time and, therefore
the effects of interference. Schemes overcoming coherence
limitation have been proposed for single cavity notch type [46],
high-performance amplified bandpass filters [49], and complex
structures based on the cascade of cavities.
Although, as previously mentioned, the main operationregime of MWP filters is based on using incoherent sources,several works [45]–[50] have reported and experimentallydemonstrated that it is possible to overcome the limitations thatsource coherence imposes over the operation of MWP filters.The use of coherent laser sources is interesting as higher inputpowers can be injected into the filter thus reducing the RF linklosses. In the context of MWP filtering, a coherent source is thathaving a coherence time (i.e., inverse of its linewidth) muchlonger that the basic delay between adjacent samples. If suchsources are employed, then optical interference due to mechanicaland environmental fluctuations can severely compromisethe filter performance. An interesting technique is based oncontrolling the source coherence by means of the applicationof an external phase modulation [47]. Here, a residual phasemodulation applied at the output of the optical source increasesits linewidth thereby reducing its coherence time and, thereforethe effects of interference. Schemes overcoming coherencelimitation have been proposed for single cavity notch type [46],high-performance amplified bandpass filters [49], and complexstructures based on the cascade of cavities.
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