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Publication Date: 2017-Jan-30
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An optical spectrum analyzer (OSA) is often an indispensable tool for use in a wide variety of applications.  Typically, an OSA includes various bulk optics devices such as a diffraction grating, a beam splitter, a beam combiner, an optical filter, and a polarizer.  However, in some applications, where feasible, bulk optics devices are being replaced by photonics-based integrated circuits that can provide certain advantages related to aspects such as miniaturization, cost manufacturability, and performance.  Several of these photonics-based integrated circuits incorporate high Q resonant structures that can be used for various purposes such as for filtering an optical signal.  Fig. 1 below shows a spectral display corresponding to an output of a three-ring photonic filter fabricated on a photonic integrated circuit.  The three-ring photonic filter can be provided in the form of a tunable Fabry-Perot filter that operates as an optical pre-selector in front of an optical detector.  

Fig. 1 Spectral Display of a three-ring photonic filter fabricated on an integrated circuit

As can be recognized from the stop bands in the spectral display shown in Fig. 1, the three-ring photonic filter provides excellent selectivity.  Unfortunately, the three-ring photonic filter (as well as some other photonic filters with fewer or larger number of rings, or other forms of cavity such as waveguide structures with high-reflectivity end-mirrors) suffers from certain limitations.  One among these limitations is a non-linear response characteristic that is dependent upon the power level of an input optical signal coupled into the photonic filter.  The non-linear response characteristic renders the performance of a high-finesse optical filter unsatisfactory when an input optical signal exceeds a certain power level.  The distortion created as a result of the non-linear response characteristic can be observed in Fig. 2 below where a passband shape factor of the three-ring optical filter, particularly the skirt at the low frequency end, is distorted with respect to increasing power levels.  As is known, the passband shape factor defines how well a photonic filter (located inside an OSA, for example) can pass through different wavelengths without introducing amplitude and/or filter shape distortion.  

The non-linear response characteristic, which can be attributed in part to the nature of the material used for fabricating the integrated circuit (silicon nitride, silicon, indium phosphide etc.), and to the structural design which sets optical mode field intensity as well as absorption losses,  may prove unacceptable in certain applications.  For example, some telecommunication applications require a passband shape factor (or stop band shape factor in the case of a band-stop filter) that is box-like in appearance with a flat top and very steep skirts that provide high out-of-band rejecti...