Sai Tak Chu

Microring resonator arrays for VLSI photonics

The analytic theory governing the complete scattering response of two-dimensional (2-D) microring resonator arrays is developed, The method is applicable to arbitrary interconnections of general four-port, single polarization nodes. An 8/spl times/8 cross-grid array of vertically coupled glass microring resonators is fabricated for test purposes.

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

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.

Second-order filter response from parallel coupled glass microring resonators

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.

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

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.

Precise control of wavelength channel spacing of microring resonator add-drop filter array

The vertically coupled microring resonator filter is an attractive add-drop wavelength filter due to its functionality and compactness and the possibility of dense integration resulting from the cross-grid configuration. However, in the concatenated 1 /spl times/ 8 filter array in which the ring radii are changed with the increments of 50 nm, the wavelength channel spacing was limited to 5.7 nm due to the resolution limit of photomask (50 nm). Therefore, we developed a new technique to control precisely the channel spacing by an ultraviolet (UV)-trimming technique using polysilane as the over-cladding layer. As a result, we successfully decreased the channel spacing to 0.5 nm, and also controlled the channel spacing precisely to 1.0 nm using the same trimming technique.

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

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.

Ultrashort optical pulse transmission characteristics of vertically coupled microring resonator add/drop filter

The ultrashort optical pulse transmission characteristics of a vertically coupled microring resonator (VCMRR) filter were measured at 1.55 /spl mu/m wavelength, and the transmission capacity per channel was evaluated. The pulsewidth at the output was measured to be 3.4 ps, when 1.2 ps input pulse with a sech pulse shape from a fiber laser was incident on the microring resonator filter with a fullwidth at half-maximum (FWHM) of 0.75 nm (94 GHz). From this result, the maximum transmission capacity of the filter was estimated to be approximately 50 Gb/s. The pulse broadening through the filter was theoretically analyzed, and it was confirmed that the pulse broadening evaluated from the bandwidth of the filter spectrum response were in good agreement with the measured results.

High-coupling efficiency vertical ARROW coupler with large tolerance and short coupling length for three-dimensional optical interconnects

A compact ARROW-type vertical coupler for three-dimensional optical interconnects was demonstrated. The refractive index and thickness of intermediate cladding were optimally designed to achieve large fabrication tolerance and short coupling length with high coupling efficiency. Very short coupling length of 800 /spl mu/m and high coupling efficiency of 96% were experimentally demonstrated.

ARROW-type vertical coupler filter: design and fabrication

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.

Coupling-loss reduction of a vertically coupled microring resonator filter by spot-size-matched busline waveguides

To improve the input-output coupling loss of a vertically coupled microring resonator filter, we fabricated microring resonators on an antiresonant reflecting optical waveguide (ARROW) with a large spot size and on the rectangular busline waveguide with a spot-size transformer. The spot size and the tapered structure were optimally designed from the viewpoint of spot-size matching to single-mode fibers and the reduction of radiation loss. Clear dropping responses were demonstrated for the ARROW-based microring resonator filters, and the coupling loss was successfully reduced by 22 dB.