Michael Vervaeke

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.

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.

Plastic Microlens Arrays by Deep Lithography with Protons: Fabrication and Characterization

In this paper, we present the state-of-the-art of deep lithography with protons (DLP), a technology that we have adopted and optimized to rapidly prototype three-dimensional micro optical components and high-aspect-ratio micro mechanical structures in poly(methyl methacrylate). In particular, we focus on the fabrication of individual plastic refractive microlenses featuring a wide range of numerical apertures, diameters and pitches and on their 2-dimensional arrays. We provide a detailed description of the microlens fabrication technique and its calibration procedure. We highlight the quantitative geometrical and optical characteristics of these DLP microlenses and we demonstrate the reproducibility of their fabrication process. We also illustrate the prototyping flexibility of DLP by fabricating arrays featuring microlenses with different sags, pitches and diameters. As a conclusion, we analyze and discuss the strengths and weaknesses of this technology.

Demonstration of a multichannel, multiresolution imaging system

In conventional multichannel imaging systems, all channels have similar imaging properties [field-of-view (FOV) and angular resolution]. In our approach, channels are designed to have different imaging properties which add multiresolution capability to the system. We have experimentally demonstrated, for the first time to our knowledge, a three-channel imaging system which simultaneously captures multiple images having different magnifications and FOVs on an image sensor. Each channel consists of four aspherical lens surfaces fabricated from four PMMA plates by ultraprecision diamond tooling and of a baffle made from a titanium (Ti) and aluminum (Al) based metal alloy. The integrated imaging system is able to record a FOV of 7.6° with the first channel and 73° with the third channel while having a magnification ratio of about 6 between them. The experimental and simulation results, specifically the FOV and magnification ratios, are comparable, and this paves a way for low-cost, compact imaging systems which can embed smart imaging functionalities.

SPAD arrays and micro-optics: towards a real single photon spectrometer

This study aims at proving that single photon sensing can be made accessible in the form of cheap off-the-shelf micro-devices with micro-optical/micro-mechanical coupling systems. In order to achieve this challenging goal, use is made of different micro-technologies, not yet fully established but promising and innovative in and of themselves. It is planned to combine them into a more challenging micro-technology capable of making single photon handling off-the-shelf. Moreover, the technology to be implemented should make it possible to provide photonic sensors in a ready-to-go fashion.

Optomechanical Monte Carlo Tolerancing Study of a Packaged Free-Space Intra-MCM Optical Interconnect System

We report on the performance of an intra-multichip-module free-space optical interconnect that integrates microlenses and a deflection prism above a dense optoelectronic chip, under various fabrication and assembly errors. This paper describes the results of a combination of mechanical Monte Carlo analysis and optical simulations. Both the technological requirements to ensure a high process yield, and the specifications of the technology we use at our laboratories to fabricate the microoptical and micromechanical components, deep lithography with protons (DLP), are discussed. Therefore, we first conduct a sensitivity analysis that is subsequently used to set the variances of the random perturbations of the Monte Carlo simulation. By scaling these variances, we are able to investigate the effect of a technology accuracy enhancement on the fabrication and assembly yield. We estimate that 40% of the systems fabricated with DLP will show an optical transmission efficiency above -4.32 dB, which is -3.02 dB below the theoretical obtainable value. In this paper, we also discuss our efforts to implement an optomechanical Monte Carlo simulator. It allows us to deal with specific issues not directly related with the microoptical or DLP components, such as the influence of gluing layers and structures that allow for self-alignment, by combining mechanical tolerancing algorithms with optical simulation software. In particular, we determine that DLP provides ample accuracy to meet the requirements of a high manufacturing yield (around 91% meet an optical transmission that is -0.75 dB below the theoretical maximum). The adhesive bonding of optoelectronic devices in their package, however, is subject to further improvement to enhance the tilt accuracy of the devices with respect to the optical interconnect modules

Increased lumens per étendue by combining pulsed LED's

Led based projectors have numerous advantages compared to traditional projectors, such as: compact, larger color gamut, longer lifetime, lower supply voltage, etc. As LEDs can switch rapidly, there is the possibility to pulse. However, there is also an important disadvantage. The optical power per unit of etendue of a LED is significantly lower than e.g. an UHP-lamp (approximately 50 times). This problem can be remedied partly by pulsing of the LED’s. If one drives a LED with a pulsed current source, the peak luminance can be higher, albeit that the average luminance will not increase. By pulsing X LEDs alternately, their increased flux can be added up in time and will generate a higher average flux within the same etendue. This can be carried out in a number of different configurations. The first configuration uses moving components where a number of LEDs (e.g. 8) are mounted on a carrousel and consecutively the pulsed LED is brought in the light path of the projector to fill up the time with its peak flux. An alternative without moving components can be reached with 2 LEDs which are combined with a PBS. By alternately pulsing the LEDs with 50% duty cycle and changing the polarisation of one LED with a switchable retarder, one can combine the flux of both LEDs in the same etendue. Because of its fast switching time ferro-electric retarders are used here. This can be extended further to 4,8,16... LEDs, at the price of a larger and more complicated optical architecture.

Ion micro-beam diagnostics with photodetectors

We have developed two techniques for microscopic ion beam imaging and profiling, both based on scintillators, particularly suitable for applications in deep lithography with protons (DLP) or with heavier ions. The first one employs a scintillating fiberoptic plate and a CCD camera with suitable lenses, the second makes use of a small scintillator optically coupled to a compact photomultiplier. We have proved the possibility of spanning from single beam particles counting up to several nA currents. Both the devices are successfully exploited for on-line control of proton beams, down to a beam size of less than 50 μm, in the framework of DLP application.

Free-Form Optics Enhanced Confocal Raman Spectroscopy for Optofluidic Lab-on-Chips

We present an optofluidic lab-on-chip for confocal Raman spectroscopy, which can be used for the analysis of substances. The device strongly suppresses unwanted background signals because it enables confocal detection of Raman scattering thanks to a free-form reflector embedded in the optofluidic chip. We design the system using non-sequential ray-tracing combined with a mathematical code to simulate the Raman scattering behavior of the substance under test. We prototype the device in polymethyl methacrylateby means of ultraprecision diamond tooling. In a proof-of-concept demonstration, we first show the confocal behavior of our Raman lab-on-chip system by measuring the Raman spectrum of ethanol. In a next step, we compare Raman spectra measured in our lab-on-chip with spectra measured with a commercial Raman spectrometer. Finally, to calibrate the system we perform Raman measurements on urea solutions with different concentrations with our proposed experimental proof-of-concept setup. We achieved a detection limit that corresponds to the noise equivalent concentration of 20 mM.

A single photon spectrometer for biomedical applications

SINPHOS is a monolithic micro-device, able to measure simultaneously time distribution and spectrum of photons coming from a weak source like Delayed Luminescence of biological systems. In order to achieve this challenging goal, we use: Deep Lithography with Ions (DLI) and microelectronic technologies for the fabrication of dedicated passive micro-optical elements and for the realization of Single Photon Avalanche Diode (SPAD) detectors, respectively