Christof Debaes

The benefits of ultrashort optical pulses in optically interconnected systems

Many properties of an optically interconnected system can be improved through the use of a modelocked laser. The short pulse duration, high peak power, wide spectral bandwidth, and low timing jitter of such a laser lead to these benefits. Timing advantages include simplified synchronization across large chip areas, receiver latency reduction, and data resynchronization. Lower power dissipation may be achieved through improved receiver sensitivity. Additional applications of short optical pulses include time-division multiplexing, single-source wavelength-division multiplexing, and precise time-domain testing of circuits. Several of these concepts were investigated using a high-speed chip-to-chip optical interconnect demonstration link. The link employs a modelocked laser and surface-normal optoelectronic modulators that were flip-chip bonded to silicon CMOS circuits. This paper outlines experiments that were performed on or simulated for the link, and discusses the important benefits of ultrashort optical pulses for optical interconnection.

Receiver-less optical clock injection for clock distribution networks

We present a new technique of injecting clocks optically onto CMOS chips without the use of a receiver amplifier. We discuss the benefits of such a direct approach and present proof-of-principle experiments of the technique. We analytically compare a receiver-less optical clock distribution and an electrical clock distribution in a fan-out-of-four clock tree to evaluate the timing and power benefits of the optical approach for present microprocessors. We also compare receiver-less direct injection of optical clocks to trans-impedance receiver based injection within the same distribution framework.

Plastic microoptical interconnection modules for parallel free-space interand intra-MCM data communication

We design and fabricate a prototype scalable multichannel free-space interconnection module with the potential for Tb/s/spl middot/cm/sup 2/ aggregate bit-rate capacity over inter- and intra-MCM interconnection distances. The component is fabricated in a high quality optical plastic, PMMA, using deep proton lithography, an ion-based rapid prototyping technology. As a feasibility demonstration, data communication is achieved at 622 Mb/s per channel with a bit error rate smaller than 10/sup -13/ for 16 channels with an interchannel crosstalk lower than -22 dB. We perform a sensitivity analysis for misalignments and fabrication errors and study the fabrication issues of these components with injection molding techniques. Finally, we provide evidence that these modules can be mass fabricated with the required precision.

Low-cost microoptical modules for MCM level optical interconnections

A multichannel free-space microoptical module for dense MCM-level optical interconnections has been designed and fabricated. Extensive modeling proves that the module is scalable with a potential for multi-Tb/s/spl middot/cm/sup 2/ aggregate bit rate capacity while alignment and fabrication tolerances are compatible with present-day mass replication techniques. The microoptical module is an assembly of refractive lenslet-arrays and a high-quality microprism. Both components are prototyped using deep lithography with protons and are monolithically integrated using a vacuum casting replication technique. The resulting 16-channel high optical-grade plastic module shows optical transfer efficiencies of 46% and inter-channel cross talks as low as -22 dB, sufficient to establish workable multichannel MCM-level interconnections. This microoptical module was used in a feasibility demonstrator to establish intra-chip optical interconnections on a 0.6 /spl mu/m CMOS optoelectronic field programmable gate array. This optoelectronic chip combines fully functional digital logic, driver and receiver circuitry and flip-chipped VCSEL and detector arrays. With this test-vehicle multichannel on-chip data-communication has been achieved for the first time to our knowledge. The bit rate per channel was limited to 10 Mb/s because of the limited speed of the chip tester.

Elastomeric inverse moulding and vacuum casting process characterization for the fabrication of arrays of concave refractive microlenses

We present a complete and precise quantitative characterization of the different process steps used in an elastomeric inverse moulding and vacuum casting technique. We use the latter replication technique to fabricate concave replicas from an array of convex thermal reflow microlenses. During the inverse elastomeric moulding we obtain a secondary silicone mould of the original silicone mould in which the master component is embedded. Using vacuum casting, we are then able to cast out of the second mould several optical transparent poly-urethane arrays of concave refractive microlenses. We select ten particular representative microlenses on the original, the silicone moulds and replica sample and quantitatively characterize and statistically compare them during the various fabrication steps. For this purpose, we use several state-of-the-art and ultra-precise characterization tools such as a stereo microscope, a stylus surface profilometer, a non-contact optical profilometer, a Mach–Zehnder interferometer, a Twyman–Green interferometer and an atomic force microscope to compare various microlens parameters such as the lens height, the diameter, the paraxial focal length, the radius of curvature, the Strehl ratio, the peak-to-valley and the root-mean-square wave aberrations and the surface roughness. When appropriate, the microlens parameter under test is measured with several different measuring tools to check for consistency in the measurement data. Although none of the lens samples shows diffraction-limited performance, we prove that the obtained replicated arrays of concave microlenses exhibit sufficiently low surface roughness and sufficiently high lens quality for various imaging applications.

Deep proton writing: a rapid prototyping polymer micro-fabrication tool for micro-optical modules

One of the important challenges to deploying the emerging breed of nanotechnology components is interfacing them with the external world, preferably accomplished with low-cost micro-optical devices. In our labs at the Vrije Universiteit Brussel (VUB), we are therefore focusing on the continuous development of a rapid prototyping technology for the fabrication of micro-optical modules. In this technology, which we call deep proton writing (DPW), we bombard polymer samples with swift protons, which will result after chemical processing steps in high quality micro-optical components. The strength of the DPW micro-machining technology is the ability to fabricate monolithic building blocks that include micro-optical and mechanical functionalities which can be precisely integrated into more complex photonic systems. The DPW technology is furthermore compatible with low-cost mass-replication techniques such as micro-injection moulding and hot embossing. In this paper we give an overview of the process steps of the technology and the characteristic qualities we can expect from the components made by DPW. The general overview of the technology is followed by three case studies of different micro-optical components that were fabricated at our labs: (i) two-dimensional fibre connectors, (ii) out-of-plane couplers for optical waveguides embedded in printed circuit boards (PCBs), (iii) intra multi-chip-module (MCM) level optical interconnection via free space optical modules.

Wavelength Conversion Based on Raman- and Non-Resonant Four-Wave Mixing in Silicon Nanowire Rings Without Dispersion Engineering

We propose an efficient wavelength conversion scheme that is based on either Raman-resonant four-wave mixing or non-resonant Kerr-induced four-wave mixing in a silicon nanowire ring, and that does not require dispersion engineering of the nanowire. We rely on the spatial variation of the Raman and Kerr susceptibilities around the ring to quasi-phase match the wavelength conversion processes for TE polarized fields. The flexibility of this quasi-phase-matching scheme can lead to wavelength conversion efficiencies from -26.7 dB to values larger than 0 dB, and in certain circumstances makes it possible to outperform conventional phase-matched conversion in a dispersion-engineered silicon ring by factors exceeding 6 dB.

Discrete Out-of-Plane Coupling Components for Printed Circuit Board-Level Optical Interconnections

We propose discrete out-of-plane coupling components as a versatile alternative to current approaches used to couple light in and out of the propagation plane in waveguide-based printed circuit board (PCB)-level optical interconnections. The out-of-plane couplers feature a 45deg micromirror and are fabricated using deep proton writing as a rapid prototyping technology. Their fabrication is compatible with replication techniques and shows all the potential of low-cost mass fabrication. In a first configuration, we use the component in a fiber-to-fiber coupling scheme. Coupling losses as small as 0.77 dB were achieved. In a second configuration, the out-of-plane coupler is plugged into a laser ablated cavity in optical waveguides integrated on a PCB. Here a total link loss between out-of-plane fiber and in-plane fiber of 3.00 dB was achieved when using it at the transmitter side and 5.69 dB when using it at the receiver side.

High-precision 2-D SM fiber connectors fabricated through deep proton writing

High-precision two-dimensional (2-D) fiber alignment modules would offer great benefits for high-density photonic interconnects at the multichip-module level, where parallel light signals have to be transferred between integrated dense 2-D emitter and detector arrays, or for massive parallel sensing applications. In telecom, the availability of highly accurate low-cost field installable 2-D fiber couplers would boost the further integration of fiber optics in future fiber-to-the-home networks. We present deep proton writing as a prototyping technology for the mastering of small-form-factor 2-D fiber connector components. The alignment components, which we present here, consist of 4 times 8 arrays of circular conically shaped holes for single-mode fibers and feature average insertion losses of 0.062 dB and a maximum loss of 0.15 dB, when used in a fiber butt-coupling configuration

Pump-probe measurements of CMOS detector rise time in the blue

This paper proposes the use of shorter wavelengths and monolithic integration for chip-to-chip and on-chip optical communication. The promise of monolithic detectors for high-speed interconnection is demonstrated through experimental measurements and matching simulations. Responsivities >0.06 A/W and transit-time-limited response can be expected in the blue from planar p-i-n silicon-on-insulator (SOI) detectors.