We summarize the results of a European Project entitled WAPITI (Waferbonding and Active Passive Integration Technology and Implementation) dealing with the fabrication and investigation of active/passive vertically coupled ring resonators, wafer bonded on GaAs, and based on full wafer technology. The concept allows for the integration of an active ring laser vertically coupled to a transparent bus waveguide. All necessary layers are grown in a single epitaxial run so that the critical coupling gap can be precisely controlled with the high degree of accuracy of epitaxial growth. One key challenge of the project was to establish a reliable wafer bonding technique using BCB as an intermediate layer. In intensive tests we investigated and quantified the effect of unavoidable shrinkage of the BCB on the overall device performance. Results on cw-operation, low threshold currents of about 8 mA, high side-mode suppression ratios in the range of 40 dB and large signal modulation bandwidths of up to 5 GHz for a radius of 40 μm shows the viability of the integration process.
Microring resonators are excellent candidates for very large scale photonic integration due to their compactness, and fabrication simplicity. Moreover a wide range of all-optical signal processing functions can be realized due to the resonance effect. Possible applications include filtering, add/drop of optical beams and power switching, as well as more complex procedures including multiplexing, wavelength conversion, and logic operations. All-active ring components based in InGaAsP/InP are possible candidates for laser sources, lossless filters, wavelength converters, etc. Our work is based on measurement, characterization and proposal of possible exploitation of such devices in a variety of applications. We investigate the spectral characteristics of multi-quantum well InGaAsP(λ=1.55μm)/InP microring structures of various ring diameters and different configurations including racetracks with one or two bus waveguides and MMI couplers. The latter configuration has recently exhibited the possibility to obtain tunable active filters as well as tunable laser sources based on all-active ring-bus-coupler structures. In the case of tunable lasers single mode operation has been achieved by obtaining sufficiently high side mode suppression ratio. The tuning capability is attributed to a coupled cavities effect, resembling the case of multi-section DBR lasers. However, in contrast to the latter, the fabrication of microring resonators is considered an easier task, due to a single step growth procedure,
although further investigation must be carried out in order to achieve wide range tunability. Detailed mappings of achievable wavelengths are produced for a wide range of injection current values.
Compact integrated microring resonators are promising candidates for various applications in optical signal processing and communication such as optical filters, wavelength (de)multiplexers, single mode sources, dispersion compensators and wavelength converters. Active GaInAsP/InP material is particularly suited for the fabrication of "lossless" filter devices as well as novel laser components with outstanding performance. Design, technology as well as transmission / emission mode characteristics for typical applications are sketched and summarized.
Miniaturization of waveguide circuits is inherently connected with the implementation of highly confined waveguides as one prerequisite for the building of sharp bends and optical microring circuits. Integrated optical ring resonators are promising candidates for compact optical filters and wavelength (de)multiplexers. Their realization in active semiconductor material opens the potential to verify "lossless" filter devices as well as novel laser components with outstanding performance. Since ring resonators do not require facets or gratings for optical feedback they are particularly suited for monolithic integration. Design and technology for device fabrication are summarized. Recent results on transmission and emission mode characteristics of fabricated passive ring resonators as well as gain-included "all-active" resonators in the transmission and emission mode are presented.
Bi-directional, full duplex transceiver InP/GaInAsP-PIC architectures with high crosstalk suppression ratio values for single and dual spectral window operation (1.3 micrometers and 1.5 micrometers ) are reported as well as simulation calculations and preliminary experimental results on its subcomponents.
For the packaging of integrated InP devices we present two approaches, which rely on a passive fiber alignment. In one case the fibers are hold in V-grooves etched directly into the InP, alternatively, V-grooves are etched into Si and the InP chip is assembled by a self- aligning flip chip process.