We provide the first experimental observation of structure tuning of the electromagnetically induced transparencylike spectrum in integrated on-chip optical resonator systems. The system consists of coupled silicon ring resonators with $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ diameter on silicon, where the coherent interference between the two coupled resonators is tuned. We measured a transparency-resonance mode with a quality factor of 11 800.

Experimental realization of an on-chip all-optical analogue to electromagnetically induced transparency

We review photonic applications of dielectric whispering-gallery mode (WGM) resonators-tracing the growth of the technology from experiments with levitating droplets of aerosols to ultrahigh-Q solid state crystalline and integrated on-chip microresonators.

Optical resonators with whispering-gallery modes-part II: applications

Storing light on-chip, which requires that the speed of light be significantly slowed down, is crucial for enabling photonic circuits on-chip. Ultraslow propagation1,2,3 and even stopping4,5 of light have been demonstrated using the electromagnetically induced transparency effect in atomic systems1,3,4,5 and the coherent population oscillation effect in solid-state systems2. The wavelengths and bandwidths of light in such devices are tightly constrained by the property of the material absorption lines, which limits their application in information technologies. Various slow-light devices based on photonic structures have also been demonstrated6,7,8,9,10; however, these devices suffer a fundamental trade-off between the transmission bandwidth and the optical delay. It has been shown theoretically11,12,13 that stopping light on-chip and thereby breaking the fundamental link between the delay and the bandwidth can be achieved by ultrafast tuning of photonic structures. Using this mechanism, here we report the first demonstration of storing light using photonic structures on-chip, with storage times longer than the bandwidth-determined photon lifetime of the static device. The release time of the pulse is externally controlled.

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Breaking the delay-bandwidth limit in a photonic structure

An eight-channel add-drop cross-grid vertically coupled microring resonator (VCMRR) filter is proposed and demonstrated. The cross grid comprises a grid-like array of buried channel waveguides which perpendicularly cross through each other, VCMRRs at each of the cross-grid nodes serve as the wavelength selective add-drop filters. Measured crosstalk levels at the crossings are typically less than -30 dB. Rings with a nominal radius of 10 /spl mu/m are used to achieve a free-spectral range of 20 nm and optical bandwidths of 1 nm, while changes of the radii in increments of 50 nm lead to a nominal channel spacing of 5.7 nm.

An eight-channel add-drop filter using vertically coupled microring resonators over a cross grid

We address the trade-offs among delay, loss, and bandwidth in the design of coupled-resonator optical waveguide (CROW) delay lines. We begin by showing the convergence of the transfer matrix, tight-binding, and time domain formalisms in the theoretical analysis of CROWs. From the analytical formalisms we obtain simple, analytical expressions for the achievable delay, loss, bandwidth, and a figure of merit to be used to compare delay line performance. We compare CROW delay lines composed of ring resonators, toroid resonators, Fabry-Perot resonators, and photonic crystal defect cavities based on recent experimental results reported in the literature.

/pdf/designing-coupled-resonator-optical-waveguide-delay-lines-2i3br72bmz.pdf

Designing coupled-resonator optical waveguide delay lines

Ring resonators are coupled in parallel in order to obtain a second order type of wavelength response. The phase relationship between the rings determines the details of the spectral response. The rings used in the experiment have radii of 25 /spl mu/m and are fabricated from compound glass having an index of 1.539. Several resonances are observed to have double peaked box-like response.

Second-order filter response from parallel coupled glass microring resonators

Trimming of the resonant wavelength of a vertically coupled glass microring resonator channel dropping filter with a photo-induced refractive index change in a dip coated polymer overlay is reported. In this letter, the glass microring resonator has a radius of 19 /spl mu/m, a free-spectral range of 10 nm, and a Q value of 800. The maximum wavelength shift observed is 9 nm, which is yielded through an absolute bulk index change in the polymer of 0.044. Trimming is a continuous function of exposure time and dose.

Wavelength trimming of a microring resonator filter by means of a UV sensitive polymer overlay

The role of cascaded filters for spectrum cleanup and crosstalk reduction in add-drop filters is addressed experimentally for microring resonator cross-grid technology. The fabricated devices consist of glass microring resonators 10 /spl mu/m in radius vertically integrated above a cross-grid waveguide array. Sharper linewidth rolloff and flattened passband responses are observed. The ratio of 15 to 3 dB bandwidth of a cascaded double filter is reduced by a factor of 3 compared with the single-filter case. Because of the very small dimensions of the resonators, the use of multiple filters to improve the spectral response consumes negligible chip space.

Cascaded microring resonators for crosstalk reduction and spectrum cleanup in add-drop filters

The temperature dependence of vertically coupled glass microring resonator add/drop filters is investigated. Measurements show that the passband of the air-clad microring resonator filter increases with temperature at a rate of 0.0137 mm//spl deg/C. Using a PMMA-TFMA polymer that has an opposite refractive index temperature gradient than glass as the overlay of the microring resonator, the temperature dependence of the filter is compensated to -0.0025 nm//spl deg/C in the operation range of 25/spl deg/C to 55/spl deg/C.

Temperature insensitive vertically coupled microring resonator add/drop filters by means of a polymer overlay

Vertical coupler filters (VCF) exhibiting narrow bandwidth and low sidelobe levels have been designed and demonstrated. Narrow bandwidth filter response is achieved due to the strong asymmetry between the waveguides of the filter and the nondispersive characteristics of the anitresonant reflecting optical waveguide (ARROW) structure. An ARROW-type VCF with a conventional parallel coupled directional coupler configuration with a full width at half-maximum (FWHM) of 1.36 nm and a maximum sidelobe level of -8.5 dB was fabricated using a compound glass consisting of SiO/sub 2/ and SiO/sub 2/-Ta/sub 2/O/sub 5/. The filter sidelobe levels were then further suppressed by using an X-crossing arrangement to provide coupling strength apodization along the device. The sidelobe levels of this modified X-crossing filter were suppressed to below -23 dB and the measured FWHM was 3.9 nm. The central wavelength of the reported filters are in the 1.55 /spl mu/m region. The measured results are in good agreement with theoretical results from an analysis procedure that combines the coupled mode theory with the finite difference complex mode solver.

/pdf/arrow-type-vertical-coupler-filter-design-and-fabrication-f0cfb07hh1.pdf

B.E. Little

Papers

Second-order filter response from parallel coupled glass microring resonators

Wavelength trimming of a microring resonator filter by means of a UV sensitive polymer overlay

Cascaded microring resonators for crosstalk reduction and spectrum cleanup in add-drop filters

Temperature insensitive vertically coupled microring resonator add/drop filters by means of a polymer overlay

ARROW-type vertical coupler filter: design and fabrication