J. Van Campenhout

Electrically pumped InP-based microdisk lasers integrated with a nanophotonic silicon-on-insulator waveguide circuit

A compact, electrically driven light source integrated on silicon is a key component for large-scale integration of electronic and photonic integrated circuits. Here we demonstrate electrically injected continuous-wave lasing in InP-based microdisk lasers coupled to a sub-micron silicon wire waveguide, fabricated through heterogeneous integration of InP on silicon-on-insulator (SOI). The InP-based microdisk has a diameter of 7.5 mum and a thickness of 1 mum. A tunnel junction was incorporated to efficiently contact the p-side of the pn-junction. The laser emits at 1.6 mum, with a threshold current as low as 0.5 mA under continuous-wave operation at room temperature, and a threshold voltage of 1.65 V. The SOI-coupled laser slope efficiency was estimated to be 30 muW/mA, with a maximum unidirectional output power of 10 muW.

A high-resolution sensor based on tri-aural perception

The authors present a high-resolution sensor composed of three ultrasonic sensors, one transmitter/receiver and two extra receivers, which allows a significant improvement in the information-extraction process. With this sensor the position (distance and bearing) of all isolated objects in an approximately 25 degrees field of view can be determined using information contained in one single snapshot of a moderately complex scene. Within limits, the sensor system can also discriminate between different types of reflectors, in particular, walls and edges, based on their radius of curvature. These results are all based on the determination of the arrival times of the echoes present at the three receivers. A noise model that accounts for the measured variations of the arrival times is used to derive limits on the resolution of the results provided by the sensor. Based on this model it is shown that, to a large extent, the sensor results are impervious to measurement variations common to all three receivers. Results obtained in a realistic environment are compared with those obtained from a conventional time-of-flight sensor. >

Basic structures for photonic integrated circuits in Silicon-on-insulator

For the compact integration of photonic circuits, wavelength-scale structures with a high index contrast are a key requirement. We developed a fabrication process for these nanophotonic structures in Silicon-on-insulator using CMOS processing techniques based on deep UV lithography. We have fabricated both photonic wires and photonic crystal waveguides and show that, with the same fabrication technique, photonic wires have much less propagation loss than photonic crystal waveguides. Measurements show losses of 0.24dB/mm for photonic wires, and 7.5dB/mm for photonic crystal waveguides. To tackle the coupling to fiber, we studied and fabricated vertical fiber couplers with coupling efficiencies of over 21%. In addition, we demonstrate integrated compact spot-size converters with a mode-to-mode coupling efficiency of over 70%.

Isolated word recognition with the liquid state machine : a case study

The Liquid State Machine (LSM) is a recently developed computational model with interesting properties. It can be used for pattern classification, function approximation and other complex tasks. Contrary to most common computational models, the LSM does not require information to be stored in some stable state of the system: the inherent dynamics of the system are used by a memoryless readout function to compute the output. In this paper we present a case study of the performance of the Liquid State Machine based on a recurrent spiking neural network by applying it to a well known and well studied problem: speech recognition of isolated digits. We evaluate different ways of coding the speech into spike trains. In its optimal configuration, the performance of the LSM approximates that of a state-of-the-art recognition system. Another interesting conclusion is the fact that the biologically most realistic encoding performs far better than more conventional methods.

III-V/Si photonics by die-to-wafer bonding

Photonic integrated circuits offer the potential of realizing low-cost, compact optical functions. Silicon-on-insulator (SOI) is a promising material platform for this photonic integration, as one can rely on the massive electronics processing infrastructure to process the optical components. However, the integration of a Si laser is hampered by its indirect bandgap. Here, we present the integration of a direct bandgap III-V epitaxial layer on top of the SOI waveguide layer by means of a die-to-wafer bonding process in order to realize near-infrared laser emission on and coupled to SOI.

Accurate ranging of multiple objects using ultrasonic sensors

The authors propose a measurement setup consisting of a number of ultrasonic sensors used in parallel to perform triangulation measurements. The sensor system is based on two ideas. The first idea was to use signal processing techniques borrowed from existing radar and sonar systems. This allows the accurate determination of the position of multiple objects. Processing data in real time demands a fairly powerful processing system. The second idea was to assign a microprocessor to each transducer. To support the use of multiple sensors in the final measurement setup, transputers were used as processing elements as they offer easy scalability because of their serial links. This sensor measured the distance to multiple objects very accurately and with a resolution of 2 cm. It is shown that these techniques could be implemented in a cost-effective manner.<<ETX>>

Heterogeneous integration of electrically driven microdisk based laser sources for optical interconnects and photonic ICs

A new approach for an electrically driven microlaser based on a microdisk transferred onto Silicon is proposed. The structure is based on a quaternary InGaAsP p-i-n junction including three InAsP quantum wells, on a thin membrane transferred onto silicon by molecular bonding. A p++/n++ tunnel junction is used as the p-type contact. The technological procedure is described and first experimental results show a laser emission in pulsed regime at room temperature, with a threshold current near 1.5 mA.

BSA, a fast and accurate spike train encoding scheme

In this paper we introduce a new algorithm for encoding analog information into spike trains, given that the reconstruction will take place using a FIR filter. An older technique called HSA is reviewed and an optimal threshold value is found. A new technique called BSA is introduced. These methods are then compared experimentally.

Pruning and regularization in reservoir computing

Reservoir computing is a new paradigm for using recurrent neural network with a much simpler training method. The key idea is to use a large but fixed recurrent part as a reservoir of dynamic features and to train only the output layer to extract the desired information. We propose to study how pruning some connections from the reservoir to the output layer can help on the one hand to increase the generalization ability, in much the same way as regularization techniques do, and on the other hand to improve the implementability of reservoirs in hardware.

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.