All-fiber add-drop filters based on microfiber knot resonators

We demonstrate an all-fiber add-drop filter composed of a microfiber knot (working as a resonator) and a fiber taper (working as a dropping fiber). The dropping taper can be either parallel or perpendicular to the input port of the filter. A quality factor (Q factor) of 13,000 is obtained from a parallel-coupling 308 microm diameter microknot add-drop filter with a free spectral range (FSR) of 1.8 nm. A Q factor of approximately 3300 is obtained from a cross-coupling 65 microm diameter microknot add-drop filter with a FSR of 8.1 nm. This device is particularly easy to fabricate and to connect to fiber systems.     

Waveguide-coupled octagonal microdisk channel add-drop filters

We report planar waveguide-coupled octagonal microdisk channel add-drop filters on silicon nitride-on-silica substrates. Using ray optics, we show that discrete octagonal cavities have wave-front-matched four- and eight-bounce ray orbits. From a planar waveguide-coupled 50-microm-sized octagonal microdisk filter, our experiment demonstrates TM-polarized eight-bounce modes with a coupling efficiency of approximately 56%, Q approximately equal to 1150, and a finesse of approximately 5. Using the two-dimensional finite-difference time-domain method, we obtain qualitative agreement between measurement and simulation of a smaller-sized waveguide-coupled octagonal microdisk filter.     

Ring fiber resonators based on fused-fiber grating add-drop filters:application to resonator coupling

We introduce a fiber ring optical resonator based on adiabatic fused-fiber grating couplers. The coupling of a through fiber to the resonator is controlled by the strength of the fiber Bragg gratings. By using two of these couplers and incorporating erbium-doped (ED) fiber in the ring, we control the internal loss of the ring by pumping the ED fiber. The transmission spectra of the through port and the drop port of a four-port configuration, a ring coupled to two waveguides, are measured. We show that the loss/coupling ratio of the ring-fiber system can be changed and thus that the transmission properties of the fiber can be controlled.     

Novel optical add-drop multiplexer based on dual racetrack resonators

In this paper, we designed and fabricated a four-channel optical add-drop multiplexer (OADM) based on dual racetrack resonators. The size of the fabricated device is only 2400 μm × 500 μm. The fabricated device can effectively and perfectly realize the signals upload and download. The free spectral range (FSR) of OADM is about 15.2 nm. We take the spectral responses near 1555 nm as an example. When the device acts as an optical drop multiplexer, the minimum insertion loss is 4.481 dB and the maximum extinction ratio is 31.931 dB. The maximum adjacent channels crosstalk is -9.845 dB. When the device acts as an optical add multiplexer, the minimum insertion loss is 0.944 dB and all of the extinction ratios are bigger than 25 dB. The maximum crosstalk is -16.531 dB which indicates the crosstalk can be neglected.     

Synthesis of ladder-type filters based on SAW resonators

A synthesis method for ladder-type filters based on SAW resonators is described. SAW devices are designed with modified equations for coupled waves. Nomograms for choosing the geometry of the filters on ST, X-cut quartz and on 36° Y, X-cut LiTaO₃ substrates are presented. Theoretical results are compared with experimental data.     

Metamaterial absorber and extending absorbance bandwidth based on multi-cross resonators

In this paper, we report the design, simulation, and measurements of a broadband metamaterial absorber (MA) based on a periodic array of multi-layer cross-structure resonators. A perfect narrowband MA consists of cross-structure resonator, dielectric substrate, and continuous metal films, and the absorption frequency can be tunable by changing the geometrical parameters based on L-C resonance circuit theory. Furthermore, the absorption band of our design is effectively extended by simply stacking several such structural layers with different geometrical dimensions. Finally, the 4-layer cross-structure MA is only 2 mm, which can achieve a full width at half maximum (FWHM) bandwidth of 2 GHz by numerical simulations, and 90 % bandwidth of 1.9 GHz by experiments.     

Radiation-pressure-driven mechanical oscillations in silica microdisk resonators on chip

We demonstrate radiation-pressure-driven mechanical oscillations from high optical quality factor silica microdisk resonators on chip. Mechanical quality factors of 3520 in air and 12540 in vacuum for the fundamental radial breathing modes are obtained from 73 μm-diameter silica microdisks with mechanical frequencies of ~51 MHz. The measured mechanical oscillation threshold powers for the input light are determined to be 62.5 μW in air and down to 26.6 μW in vacuum.     

Expanding the bandwidth of slow-light photonic devices based on coupled resonators

It is shown theoretically that canceling third-order dispersion can substantially increase the useful bandwidths of linear and nonlinear optical devices based on slow propagation of light. Cancellation on both global and local scales can be achieved by combination of the ring-based coupled resonator lines and all-pass optical filters.     

Optical channel drop filters based on photonic crystal ring resonators

A new optical channel drop filters (CDFs) configuration based on photonic crystals ring resonators (PCRRs) is provided. The transmission characteristics for single-ring and multiple-ring configurations have been investigated by using the two-dimensional (2D) finite-difference time-domain (FDTD) technique in triangular lattice photonic crystal (PC) silicon rods. Both forward and backward dropping were achieved in dual-ring PCRR structures. In this filter, 100% drop efficiency and acceptable quality factor can be obtained at 1550 nm. The present device can be used in the future photonic integrated circuits.     

Mode-coupling analysis of three-dimensional microdisk resonators by the finite-difference time-domain technique

Quality factor enhancement due to mode coupling is observed in a three-dimensional microdisk resonator. The microdisk, which is vertically sandwiched between air and a substrate, with a radius of 1 microm, a thickness of 0.2 microm, and a refractive index of 3.4, is considered in a finite-difference time-domain (FDTD) numerical simulation. The mode quality factor of the fundamental mode HE71 decreases with an increase of the refractive index of the substrate, n(sub), from 2.0 to 3.17. However, the mode quality factor of the first-order mode HE72 reaches a peak value at n(sub) = 2.7 because of the mode coupling between the fundamental and the first-order modes. The variation of mode field distributions due to the mode coupling is also observed. This mechanism may be used to realize high-quality-factor modes in microdisks with high-refractive-index substrates.     

Modeling of racetrack-resonator add-drop filters with arbitrary nonlinear directional couplers

In this Letter we employ the general coupled-mode equations of the nonlinear directional coupler and demonstrate that the switching characteristics of prototypical nonlinear racetrack-resonator structures may differ considerably from those obtained when the standard, generally incorrect, coupled-mode equations are used.     

Microstrip notch filters based on open interconnected split ring resonators (OISRRs)

Two kinds of inorganic/organic hybrid junctions based on ZnO nanorods (NRs), i.e. two-layer planar heterojunction and embedded bulk composite structures, were fabricated on ITO glass substrates. Surface photovoltage (SPV) methods based on a Kelvin probe and a lock-in amplifier were respectively utilized to study the photogenerated charges at the surface and the interface in the ZnO-based hybrid junctions. Results indicate that the lock-in SPV response of the bulk composite structure is much higher than its planar counterpart in terms of intensity and spectral range. Therefore, ZnO NR/PF (poly(9,9-di-n-octylfluorenyl-2,7-diyl)) embedded bulk composite structures are more suitable and preferred for photovoltaic application.     

Simulation of 60-GHz microwave photonic filters based on serially coupled silicon microring resonators

The microwave photonic filters (MPFs) based on serially coupled silicon microring resonators (MRRs) are theoretically analyzed for the application of 60-GHz millimeter wave wireless personal area networks. This is achieved by calculating the improvement of bit error ratio (BER). According to the simulation results, the requirement of signal-to-noise ratio (SNR) of the received data can be reduced by 14 dB for the same BER with and without MPFs. The performance of the MPF with five serially coupled microring structures is better than that of the MPF with a single microring, owing to the improvement of the shape factor.     

Modeling of quasi-grating sidewall corrugation in SOI microring add-drop filters

We build a model to study the sidewall corrugation of fabricated silicon microrings and investigate its impact on the spectral response of the resonator system. From the scanning electron microscope images, the sidewall corrugation can be engineered into certain periodicity and characterized into a group of rectangular gratings, or quasi-gratings, which in turn generate mutual mode coupling between the forward- and backward-propagating modes in the ring and lead to resonance splitting. We find that the reflectivity of the quasi-gratings is proportional to the mutual coupling strength and the resonance splitting only occurs at the resonances where high reflectivity takes place. The model agrees well with the experimental measurements and provides some guideline in applying mutual mode coupling for various functions in the field of optics.     

Azimuthal-order variations of surface-roughness-induced mode splitting and scattering loss in high-Q microdisk resonators

We report an experimental observation of strong variations of quality factor and mode splitting among whispering-gallery modes with the same radial order and different azimuthal orders in a scattering-limited microdisk resonator. A theoretical analysis based on the statistical properties of the surface roughness reveals that mode splittings for different azimuthal orders are uncorrelated, and variations of mode splitting and quality factor among the same radial mode family are possible. Simulation results agree well with the experimental observations.     

Color filters featuring high transmission efficiency and broad bandwidth based on resonant waveguide-metallic grating

A one-dimensional transmission color filter based on a resonant waveguide-metallic subwavelength grating was numerically investigated by employing rigorous coupled-wave analysis (RCWA) and genetic algorithm (GA). The hybrid numerical method is used to determine the optimal parameters (the grating period, filling factor, grating thickness, and waveguide thickness) of two waveguide-grating structures, namely a double-layer resonant waveguide-metallic grating and a triple-layer resonant waveguide-metallic grating. The optical responses of these structures are evaluated and compared in terms of the ideal transmission efficiency aiming at the central wavelengths of 645 nm, 546 nm, and 455 nm of red (R), green (G), and blue (B) lights, respectively, over the visible region (380–780 nm). The results show that the optical performance of the double-layer with silver grating achieves the highest transmission efficiency of 82% (R), 81% (G), and 66% (B); and the largest bandwidth of about 125 nm (R), 118 nm (G), and 85 nm (B). Compared with existing color filters, the proposed device not only obtains a higher transmission and broader bandwidth, but it also suppresses redundant spectral peaks and transmission sidebands.     

Investigation of mode radiation loss for microdisk resonators with pedestals by FDTD technique

<正>Mode radiation loss for microdisk resonators with pedestals is investigated by three-dimensional(3D) finite-difference time-domain(FDTD) technique.For the microdisk with a radius of 1μm,a thickness of 0.2μm,and a refractive index of 3.4,on a pedestal with a refractive index of 3.17,the mode quality(Q) factor of the whispering-gallery mode(WGM) quasi-TE_(7.1) first increases with the increase of the radius of the pedestal,and then quickly decreases as the radius is larger than 0.75μm.The mode radiation loss is mainly the vertical radiation loss induced by the mode coupling between the WGM and vertical radiation mode in the pedestal,instead of the scattering loss around the perimeter of the round pedestal.The WG M can keep the high Q factor when the mode coupling is forbidden.     

Anticrossing of Whispering Gallery Modes in microdisk resonators embedded in an anisotropic environment

We numerically investigate the behavior of Whispering Gallery Modes (WGMs) in circularly shaped resonators like microdisks, with diameters in the range of optical vacuum wavelengths. The microdisk is embedded in an uniaxial anisotropic dielectric environment. By changing the optical anisotropy, one obtains spectral tunability of the optical modes. The degree of tunability strongly depends on the radial (azimuthal) mode order M (N). As the modes approach each other spectrally, anticrossing is observed, leading to a rearrangement of the optical states.     

Characteristics of an add-drop filter composed of a Mach-Zehnder interferometer and double ring resonators

A planar lightwave circuit (PLC) add-drop filter is proposed and analyzed, which consists of a symmetric Mach-Zehnder interferometer (MZI) combined with double microring resonators. A critical coupling condition is derived for a better box-like drop spectrum. Comparisons of its characteristics with other schemes,such as a MZI with a single ring resonator, are presented, and some of the issues about device design and fabrication are also discussed.