Mikko Karppinen

Fabrication and characterization of polymer optical waveguides with integrated micromirrors for three-dimensional board-level optical interconnects

This paper describes the fabrication and characterization of optical/electrical printed circuit boards (O/E-PCB) with embedded multimodal step index (MM-SI) waveguides and integrated out-of-plane micromirrors (IMMs) for three-dimensional (3-D) optical interconnects. Optical circuitry is built up on PCBs using UV lithography; 45/spl deg/ input/output (I/O) couplers are fabricated by inclined exposure. Commercial polymers are used as optical core and cladding materials. Critical mirror properties of angle, surface quality, reflectivity, and coupling efficiency are characterized experimentally and theoretically. Optical and scanning electron microscopy, white light interferometry, and fiber scanning method are used in the investigations. Sloping profiles measured as a function of the incident light showed the attainment of mirror angles of /spl alpha/=36/spl deg/-45/spl deg/ with /spl plusmn/2/spl deg/ consistency. Near-field optical imaging with a white light source showed that out-of-plane beam turning was achieved. Topography investigations revealed a rectilinear negative tapering shape regardless of the incoming beam angle or type of substrate. However, higher substrate reflectancy was observed to lower the mirror angle. The average propagation loss measured for 10-cm-long waveguides at /spl lambda/=850 nm by the cut-back method was 0.60 dB/cm; the excess loss calculated for the mirror coupling was 1.8-2.3 dB. The results showed that the IMMs can be incorporated in O/E-PCBs to couple light in and out of planar waveguides. Furthermore, the presented results indicate that optical waveguides with integrated micromirrors for optical 3-D wiring can be produced compatible with volume manufacturing techniques.

Parallel optical interconnect between ceramic BGA packages on FR4 board using embedded waveguides and passive optical alignments

We have studied technologies to design and fabricate high-bit-rate chip-to-chip optical interconnects on printed circuit boards (PCBs) using board-embedded polymer waveguides and surface-mounted component packages or modules. In order to demonstrate the developed technologies, a 4times10 Gb/s optical interconnect was completely integrated on a standard FR4 PCB. The optical link demonstrator consists of 4-channel BGA-mounted transmitter and receiver modules as well as of four parallel multimode optical waveguides fabricated on top of the solder mask of the PCB using lithographic patterning. The transmitters and receivers built on low-temperature co-fired ceramic (LTCC) substrates include 4times10 Gb/s flip-chip mounted VCSEL or photodiode array, wire-bonded driver and receiver ICs, and optical coupling structures. Two microlens arrays and a micro-mirror enable optical coupling between the optoelectronic devices and the waveguide array. The passive optical alignment was based on the marks and structures fabricated both in the LTCC process and in the patterning of the optical layers. The characterized optical alignment tolerances are in the limits of accuracy of the surface-mount technology. The demonstrated technology is suitable for interconnecting CBGA-packaged ICs or multi-chip-modules

Embedded optical interconnect on printed wiring board

Integration of high-speed parallel optical interconnects into printed wiring boards (PWB) is studied. The aim is a hybrid optical-electrical board including both electrical wiring and embedded polymer waveguides. Robust optical coupling between the waveguide and the emitter/detector should be achieved by the use of automated pick-and-place assembly. Different coupling schemes were analyzed by combining non-sequential ray tracing with Monte-Carlo tolerance simulation of misalignments. A modular demonstrator was designed based on three different kind of optical coupling schemes: butt-coupling and couplings based on microlens arrays and on micro ball lenses. The optical front-ends were implemented with PIN and flip-chip-VCSEL arrays as well as 10-Gb/s/channel electronics onto LTCC-based (low-temperature co-fired ceramic) transmitter and receiver modules, which were surface mounted on high-speed PWBs. An electrical simulation model was developed for the design of a VCSEL-based transmitter circuit. Polymer waveguides were fabricated on separate FR-4 boards to allow characterization of alignment tolerances with different waveguides. Optical and adhesion properties of several potential waveguide materials were characterized. The simulations and experiments suggest that, with optimized optomechanical structures and with low loss waveguides, it is possible to achieve acceptable total path loss and yield with the accuracy of automated assembly.

Fiber-Optic Transceiver Module for High-Speed Intrasatellite Networks

High-speed intrasatellite networks are needed to interconnect units such as synthetic aperture radars, high-resolution cameras, and fast image-compression processors that produce data beyond gigabits per second. We have developed a fiber-optic link, named SpaceFibre, which operates up to 3.125 Gb/s and is compatible with the existing SpaceWire network. The link provides symmetrical, bidirectional, full-duplex, and point-to-point communication. It employs 850-nm vertical-cavity surface emitting lasers, radiation-hardened laser-optimized 50/125 mum graded-index fibers, and GaAs p-i-n photo diodes. The transceiver electronics is realized using a multilayer-ceramic-substrate technology that enables the passive alignment of optical fibers to active devices. The SpaceFibre link demonstrator was tested to transfer data at 2.5 Gb/s over 100 m with a bit error rate of less than 1.3middot10-14. Fiber-pigtailed modules were stressed with temperature variations from -40degC to +85degC, vibrations up to 30 g, and mechanical shocks up to 3900 g. The test results of 20 modules show that the SpaceFibre link is a promising candidate for the upcoming high-speed intrasatellite networks

Copper-Core MCPCB With Thermal Vias for High-Power COB LED Modules

To improve thermal performance of high-power chip-on-board multichip LED module, a copper-core metal core printed circuit board (MCPCB) substrate with copper filled microvias is introduced. As a reference, the performance is compared with alumina module with the same layout by means of thermal simulations and measurements. Up to 55% reduction in the thermal resistance from the LED source to the bottom of the substrate is demonstrated. The excellent performance of the Cu MCPCB module is due to copper-filled microvias under the blue LED chips that occupy the majority of the multichip module. The conclusion was verified by measuring increased thermal resistances of red chips without thermal vias on the Cu MCPCB module. However, as the blue LEDs dominate the thermal power of the module, they also dominate the module thermal resistance. The thermal resistance was demonstrated to correspond with the number of vias as lower thermal resistance was measured on modules with larger number of vias. The Cu MCPCB was processed in standard PCB manufacturing and low cost material, FR4, was utilized for the electrical insulation. Thus, the solution is potentially cost-effective despite the higher cost of copper in comparison with aluminum that is the most commonly used MCPCB core material.

Investigation of Environmental Reliability of Optical Polymer Waveguides Embedded on Printed Circuit Boards

In this paper, the effects of environmental stresses on the properties of polymer optical waveguides were studied. Optical multimode waveguides were fabricated on printed circuit boards using commercial polymers. The stability of the optical properties of the guide systems was investigated in damp heat-high humidity, high temperature storage, thermal shock and in environmental flowing multigas tests. Aging at high temperature and temperature cycling reduced the refractive index to largest extent. The optical build-up (o-BU) structure in which the optical layer was fabricated on the board surface was observed to be vulnerable under temperature shock when compared with the structure where the optical layer was laminated inside the FR4/Cu boards. The buffer layer beneath the optical build-up was found to improve the stability of the optical waveguides significantly.

Multi channel in-plane and out-of-plane couplers for optical printed circuit boards and optical backplanes

We present multi channel optical interconnects, based on integrated glass fibers, with passively aligned in-plane and 90° out-of-plane couplers for printed circuit boards. The out-of-plane coupler features micro optics, mechanical alignment structures, and a snap-fit system to assemble the complex coupler out of several simple, self aligning parts. Further we present an optical printed circuit board for on-board transmitter and receiver electronics for four channels using 4×10 Gbit/s transmitter and receiver chips.

Wireless infrared data links: ray-trace simulations of diffuse channels and demonstration of diffractive element for multibeam transmitters

Free-space IR transmission provides high bandwidth and good security with small-sized and low-cost links, for instance, for high- bit-rate wireless LANs. Some robustness against shadowing is achieved using diffuse channels, but disadvantages are high path loss and multipath propagation. We use ray-trace simulation software to analyze IR channels in realistic office rooms. Simulations are performed to test the methods and to specify the bandwidth and power budget requirements of diffuse links. Both diffuse and specular reflections as well as shadowing effects due to furniture are considered. Based on the study, novel Monte Carlo ray-tracing software, such as ASAP (Advanced System Analyses Program), are suitable for analyzing the multipath dispersion and the optical power budget of infrared links in realistic indoor environments. The simulations also reveal the benefits of quasidiffuse link configurations, which are composed of multibeam transmitters with restricted beam divergences as well as of detectors with narrow fields of view. For implementation of multibeam transmitters we design an array- type diffractive element that modifies the beams of a 2-D vertical-cavity surface-emitting laser (VCSEL) array. A single element providing the largest 50-deg illumination angle is fabricated for demonstration. The measured diffraction efficiency is in fair agreement with the calculated one after considering the properties of the real VCSEL beam.

Advances in miniature spectrometer and sensor development

Miniaturization and cost reduction of spectrometer and sensor technologies has great potential to open up new applications areas and business opportunities for analytical technology in hand held, mobile and on-line applications. Advances in microfabrication have resulted in high-performance MEMS and MOEMS devices for spectrometer applications. Many other enabling technologies are useful for miniature analytical solutions, such as silicon photonics, nanoimprint lithography (NIL), system-on-chip, system-on-package techniques for integration of electronics and photonics, 3D printing, powerful embedded computing platforms, networked solutions as well as advances in chemometrics modeling. This paper will summarize recent work on spectrometer and sensor miniaturization at VTT Technical Research Centre of Finland. Fabry-Perot interferometer (FPI) tunable filter technology has been developed in two technical versions: Piezoactuated FPIs have been applied in miniature hyperspectral imaging needs in light weight UAV and nanosatellite applications, chemical imaging as well as medical applications. Microfabricated MOEMS FPIs have been developed as cost-effective sensor platforms for visible, NIR and IR applications. Further examples of sensor miniaturization will be discussed, including system-on-package sensor head for mid-IR gas analyzer, roll-to-roll printed Surface Enhanced Raman Scattering (SERS) technology as well as UV imprinted waveguide sensor for formaldehyde detection.

Investigation of environmental reliability of optical polymer waveguides embedded on printed circuit boards

Purpose – To investigate the influences of environmental stresses on board‐embedded polymeric waveguides.Design/methodology/approach – Optical multimode waveguides were embedded on printed circuit boards using commercial polymers. The optical‐PCBs varying in board structure and in optical build‐up materials were exposed to heat, moisture and ionic‐contaminants in accelerated reliability tests. The influence of stress factors on the structural integrity and functional parameters, namely the refractive index and optical transmissivity, was investigated at the key communication wavelengths.Findings – Isothermal annealing reduced the refractive index to the greatest extent. The optical‐PCB structure with an optical surface build‐up layer was observed to be more vulnerable under temperature shock when compared with the optical‐PCB with optical inner layer. The buffer layer beneath the optical build‐up was found to improve the stability of the optical waveguides significantly. The results indicated of wavelengt...