High-order Filters Based On High-Q Microtoroid Cavities

Due to their small mode volume and high optical quality factor,whispering-gallery mode optical microcavities have aroused a wide attention both in basic study and application area.When meeting proper resonance condition,light of certain wavelengths will be coupled into three microcavities,forming the Lorentz type transmission spectrum.The characteristics of optical microcavity can be used to fabricate optical filters.Optical filter is a kind of optical device,which can be used to select or forbid light of certain wavelengths.With the rapid development of micro-and nano-scale fabrication processing,chip-based optical filters,as crucial photonic devices,find many applications in optical communication and optical signal processing networks.A well-performed optical filter should have a box-like channel dropping response with a flat top,high out-of-band signal rejection and sharp rolling off.A common approach is coupling a large number of optical microcavities to realize high order optical filters with sharp rolling off and flat top.Traditional high order filters are mostly integrated structures.The resonant frequencies are difficult to match well due to processing defects.It is difficult to tune the resonant frequencies and the couplings.Meanwhile,an Nth-order filter needs N microcavities,which makes it a big challenge for the integration of the devices.Backscattering-induced splitting is a common phenomenon in high-Q optical microresonators.Due to surface imperfections,impurities or water pollution,single peak modes are easily splitted into double peak modes.Here,we put forward a new kind of scheme to realize the high order filter.By employing optical microcavities with proper backscattering strengths,2Nth-order filters can be realized in an N-cavity system.We experimentally demonstrate sixth-order filters by constructing a triple-cavity photonic molecule with backscattering-induced mode splittings in high-Q silica microtoroid cavities.The realized sixth-order filter has a narrow bandwidth of 149.8 MHz with sharp roll-off slope larger than 80 dB/GHz.Due to high quality factor of the samples,the dropping response is more box-like with flat bandwidth.The rolling off slope is much larger than previous reports.However,the dropping loss is large,up to 11.8 dB,because our experimental principle is based on back-scattering.The microtoroids are placed on nanopositioners independently and both the coupling strengths between the cavities and the resonant frequencies of the individual cavity can be well controlled by tuning the distances between the microtoroids and thermoelectric cooler elements(TEC).The controllable coupled-cavity system serves as a promising platform for integrated tunable high-order filters with ultra-narrow bandwidths.