G.L. Bona

Apodized surface-corrugated gratings with varying duty cycles

We present the first realization of a Bragg grating apodization based on a concatenation of different duty cycles. The gratings were fabricated in SiON planar waveguides. The reflected signal exhibits a sidelobe suppression better than -20 dB outside a bandwidth of 1.36 nm. At the same time the reflection is stronger than 99.9%, i.e., the resonant light is suppressed better than -30 dB in transmission over 0.22 nm. The gratings are perfectly suited to serve as the wavelength-selective element of a Mach-Zehnder add/drop filter.

Development of a low-cost low-loss polymer waveguide technology for parallel optical interconnect applications

We report on the material evaluation, design, fabrication, and characterization of low-loss multimode polymer waveguides that are compatible with standard PCB manufacturing processes for use in large-area high-density high speed optical backplane interconnects.

Wavelength tunable optical add-after-drop filter with flat passband for WDM networks

A tunable optical add-after-drop filter is presented with add and drop functions optimized individually. The add function exhibits a conventional passband, whereas the drop passband is flattened (less than 0.1 dB ripple over more than 0.5 nm). The design is based on the resonant coupler principle. The free spectral range is 6.4 nm and the channel spacing 1.6 nm. The device is realized in a newly developed, high-refractive-index-contrast SiON technology that permits small bending radii of 1.5 mm. Thermo-optic heaters are used to tailor and tune the filter characteristic. The on-chip losses are about 3.5 dB, the wavelength channel isolation is better than 27 dB, and the TE-TM polarization shift is smaller than 0.1 nm.

Characterization of parallel optical-interconnect waveguides integrated on a printed circuit board

The development of optical interconnects in printed circuit boards (PCBs) is driven by the increasing bandwidth requirements in servers, supercomputers and switch routers. At higher data rates, electrical connections exhibit an increase in crosstalk and attenuation; which limits channel density and leads to high power dissipation. Optical interconnects may overcome these drawbacks, although open questions still need to be resolved. We have realized multimode acrylate-polymer-based waveguides on PCBs that have propagation losses below 0.04 dB/cm at a wavelength of 850 nm and 0.12 dB/cm at 980 nm. Transmission measurements at a data rate of 12.5 Gb/s over a 1-m-long waveguide show good eye openings, independent of the incoupling conditions. In the interconnect system, the transmitter and receiver arrays are flip-chip-positioned on the top of the board with turning mirrors to redirect the light. The coupling concept is based on the collimated-beam approach with microlenses in front of the waveguides and the optoelectronic components. As we aim for large two-dimensional waveguide arrays, optical crosstalk is an important parameter to be understood. Accordingly, we have measured optical crosstalk for a linear array of 12 optical channels at a pitch of 250 um. The influence of misalignment at the transmitter and the receiver side on optical crosstalk will be presented as a function of the distance between waveguide and transmitter/receiver.

Integrated all-optical switch in a cross-waveguide geometry

An all-optical switch for future computer optical interconnect systems based on an optical cavity with a high quality factor and a nonlinear material is computionally investigated in two dimensional with a finite-difference time domain method. The signal and control bus are perpendicular to each other and can couple into a high-Q cavity consisting of a nonlinear material. It is designed in such a way that the control bus switches the signal bus on and off. Owing to the nonlinearity in the cavity, the resonance is shifted in frequency when increasing the power in the control bus so that the signal can pass through the resonator. The high Q of the cavity maximizes the interaction with the nonlinear material, and the symmetry of the cavity mode is designed in such a way that the cross talk between the signal bus and the control bus is minimized.

Compact tunable FIR dispersion compensator in SiON technology

We present a tunable dispersion compensator, based on a sixth-order finite impulse response lattice filter. The filter has a free spectral range of 100 GHz and can be tuned for linear group delay slopes between -100 and 100 ps/nm with less than 1-ps ripple over a usable bandwidth of more than 60 GHz. Within this usable bandwidth, the average polarization-mode dispersion is low, reaching 2.4 ps only for extreme group delay slopes. The filter can also generate higher order group delay curves, for example for dispersion slope compensation.

Tunable optical add/drop components in silicon-oxynitride waveguide structures

Ultra-compact tunable add/drop components, based on the resonant coupler concept, are realized in high index-contrast silicon-oxynitride waveguides. Thermo-optic heaters are used to tailor and tune the filter characteristic.

Wavelength tunable 1-from-16 and flat passband 1-from-8 add-drop filters

Tunable add-drop filters for wavelength-division multiplexing networks are presented. The compact devices are realized in high-index-contrast silicon-oxynitride planar waveguide technology and are based on the resonant coupler principle. The first device selects one wavelength channel from a comb of up to eight wavelengths and exhibits a flat passband. Scalability of the filter concept to a larger number of wavelength channels is illustrated with a 1-from-16 add-drop filter. Tunability is obtained using the thermo-optic effect and chromium heater stripes. The devices show negligibly small polarization sensitivity. The isolation of the transit channels to the drop port is between 15 and 20 db and the rejection of the drop channel ranges from 18 to 23 dB. The design and scalability trade-offs are discussed.

Polarization-independent thermooptic phase shifters in silicon-oxynitride waveguides

The effective refractive index of dielectric waveguides can be tuned using the thermooptic effect. In general, the tuning efficiency is polarization-dependent owing to temperature-induced stress in the layers, which causes polarization-dependent loss in optical devices. These stress issues are analyzed and tested for a high-index-contrast waveguide structure based on a silicon-oxynitride core. Experimental results are in agreement with simulations. The relative difference in tuning efficiency for transverse electric and transverse magnetic polarized light can be tuned from -3% to +3% by appropriate waveguide technology control. The optimized thermooptic phase shifters show tuning efficiency differences below 0.25%, which are reproducible from wafer to wafer.

Versatile silicon-oxynitride planar lightwave circuits for interconnect applications

Low-cost silica (SiO/sub 2/) on silicon integrated planar lightwave components for applications in interconnect and optical networks have been developed. Using silicon-oxynitride (SiON) as the optically guiding core layer, a great flexibility in the design of planar optical waveguide structures is obtained as the refractive index of SiON can be varied by changing its nitrogen content. This enables the fabrication of weakly guiding waveguides with an optical mode matched to standard single mode fibers as well as more strongly guiding waveguides for compact integrated lightwave circuits. This technology can also be used as a silicon motherboard for advanced hybrid packages for optical interconnects. As an example, with a refractive index of the SiON core layer of 1.50, waveguide structures with bending radii of 1.5 mm can be achieved. The strength of this planar waveguide technology is illustrated by the example of a thermo-optically tunable space switch that can be used either as a single 1-to-8 switch or multiple parallel 1-to-4, 1-to-3 or 1-to-2 switches.