Focusing Polarization Diversity Grating Couplers in Silicon-on-Insulator

We report on compact focusing polarization diversity grating couplers in silicon-on-insulator, which can be used to overcome the polarization dependence of nanophotonic integrated circuits. The minimum fiber-to-fiber polarization dependent loss is 0.4 dB and the focusing grating couplers are as performant as standard 2-D-grating couplers without focusing. In addition, the focusing property of the gratings results in an 8-fold length reduction of the coupling structure as compared to standard nonfocusing 2-D-grating versions.     

Compact spot-size converters with fiber-matched antiresonant reflecting optical waveguides

We propose a concept for InGaAsP-InP 1.55-microm lasers integrated with spot-size converters based on modal interference between the modes of the structure formed by an active waveguide and an underlying fiber-matched antiresonant reflecting optical waveguide. Simulation results show that the spot-size converters exhibit low transformation loss, and narrowed far-field emission patterns (10 degrees x 20 degrees) and reduce the coupling loss to standard single-mode fibers from 8 to 2.6 dB over lengths approximately 200 microm shorter than the adiabatic concept. A tolerant design to fabrication variations is also proposed, which could be realized by standard processing techniques.     

Photonic-crystal surface-emitting laser near 1.55 /spl mu/m on gold-coated silicon wafer

An InP/InGaAs-based photonic band-edge laser bonded on silicon operating near 1.55 mum is presented. A gold reflector positioned below the slab containing the active layer reduces the optical losses of the Bloch-mode resonator. As a result, a quality factor exceeding 8000 is obtained at transparency leading to a laser threshold as low as 3.4 muJ/cm²     

All-Optical Flip-Flop Based on an SOA/DFB-Laser Diode Optical Feedback Scheme

We report on the dynamic all-optical flip-flop (AOFF) operation of an optical feedback scheme consisting of a semiconductor optical amplifier (SOA) and a distributed feedback laser diode (DFB-LD), bidirectionally coupled to each other. The operation of the AOFF relies on the interplay between the optical powers in both the DFB-LD and the SOA. Switching times as low as 150ps for switch pulse energies of around 6 pJ and a repetition rate of 500MHz have been measured. The contrast ratio was measured to be above 12 dB     

Control of widely tunable SSG-DBR lasers for dense wavelengthdivision multiplexing

This paper presents a general discussion on the control of widely tunable super structure grating distributed Bragg reflector (SSG-DBR) lasers. A feedback control scheme is presented that ensures frequency stability and accuracy (better than ±0.5 GHz), as well as high side mode suppression ratio (>35 dB). The active section voltage is monitored to maintain mode stability and a highly stable Fabry-Perot etalon is used as a reference to lock the laser frequency to a specific ITU channel. It is shown that stability can even be maintained when directly modulating the laser at 1.244 Gb/s. Furthermore, a characterization scheme is demonstrated that uses the voltage monitoring to generate a look-up table of operation points very efficiently and accurately. For all operation points, the frequency accuracy is better than ±0.5 GHz and the side mode suppression ratio is above 35 dB     

Experimental demonstration of all-optical regeneration using anMMI-SOA

A novel all-optical 2R regenerator based on a multimode interference coupler (MMI) semiconductor optical amplifier is presented. Static measurements of the transfer function reveal a digital transfer characteristic and a high increase in extinction ratio. The experiments are in good agreement with simulations, which have been done using a modified beam propagation method program. The device has a high tolerance to the MMI length. It has been fabricated in an all-active layout, avoiding the need for active/passive integration and is very compact     

Modelling the electro-osmotically enhanced pressure dewatering of activated sludge

The mechanical dewatering of biological materials, such as activated sludge, is troublesome due to their high compressibility. The dewatering can be improved drastically by the use of electro-osmosis, a technique in which a direct current electric field is applied to the filter cake, inducing an electro-kinetic displacement of the liquid phase. In this article, a model is presented which describes this process for a one-dimensional, one-sided filter press. The model starts from two existing models, each describing a part of the process [Iwata, M., Igami, H., Murase, T., 1991. Analysis of electroosmotic dewatering. Journal of Chemical Engineering of Japan 24(1), 45-50; Sørensen, P.B., Moldrup, P., Hansem, J.A., 1996. Filtration and expression of compressible cakes. Chemical Engineering Science 51(6), 967-979]. A robust solution scheme for the basic flux and continuity equations is used to describe filtration and expression. Regarding the electro-osmosis, an adapted form of the Darcy equation, expressing electro-osmotic induced flow as well as pressure induced flow, is integrated into the former solution scheme. This permits the simulation of the overall process using a single model. Validation of the model revealed that the experimental piston height during dewatering can be described accurately provided that temperature effects are taken into account.     

Properties of poly(butylene terephthatlate) polymerized from cyclic oligomers and its composites

The high viscosity of thermoplastic matrices hampers fiber impregnation. This problem can be overcome by using low viscous polymeric precursors such as cyclic butylene terephthalate (CBT^® resins), which polymerize to form a thermoplastic matrix. This allows thermoset production techniques, like resin transfer molding (RTM), to be used for the production of textile reinforced thermoplastics. Due to the processing route and more specifically the time-temperature profile, inherent to the RTM process, the crystallites of the matrix consist out of well-defined, thick and well-oriented crystal lamellae. Together with a high overall degree of crystallinity and a low density of tie molecules, these large and perfect crystals cause polymer brittleness. Matrix brittleness lowers the transverse strength of unidirectional composites to below the matrix strength, but leaves the mechanical properties in the fiber direction unaffected. Although not a valid option for the RTM production route, crystallization from a truly random melt and at a sufficiently high cooling rate would substantially improve the ductility.     

Design and Optimization of Electrically Injected InP-Based Microdisk Lasers Integrated on and Coupled to a SOI Waveguide Circuit

We have performed a numerical study involving the design and optimization of InP-based microdisk lasers integrated on and coupled to a nanophotonic silicon-on-insulator (SOI) waveguide circuit, fabricated through bonding technology. The theoretical model was tested by fitting it to the lasing characteristics obtained for fabricated devices, which we presented previously. A good fit was obtained using parameter values that are consistent with numerical simulation. To obtain optimized laser performance, the composition of the InP-based epitaxial layer structure was optimized to minimize internal optical loss for a structure compatible with efficient current injection. Specific attention was paid to a tunnel-junction based approach. Bending loss was quantified to estimate the minimum microdisk diameter. The coupling between the InP microdisk and Si waveguide was calculated as function of the bonding layer thickness, waveguide offset and waveguide width. To study the lateral injection efficiency, an equivalent electrical network was solved and the voltage-current characteristic was calculated. Based on these results, the dominant device parameters were identified, including microdisk thickness and radius, coupling loss and tunnel-junction p-type doping. These parameters were optimized to obtain maximum wall-plug efficiency, for output powers in the range 1-100 W. The results of this optimization illustrate the potential for substantial improvement in laser performance.