Pentti Karioja

Fabrication of optical waveguides by imprinting: Usage of positive tone resist as a mould for UV-curable polymer

Optical ridge type waveguides based on UV-curable polymer were fabricated by imprinting method. Positive tone resist patterned on a silicon wafer was used as a mould. The characterization of waveguides was carried out by coupling TE-polarized light from a tapered fiber into a waveguide with 30 mm length and mapping the intensity distribution with another tapered fiber at the output facet of a waveguide. Proper single- or multimode operation was observed depending on the waveguide width being either 2 microm or 6 microm. Experimental observations on the mode profiles were also supported by the simulation results. Average power transmissions of 32% at 1530 nm wavelength and 45% at 1310 nm wavelength were characterized. The results suggest that the simple mould fabrication process might be a useful technique for device prototyping and that the performance of replicated waveguides can meet the requirements for certain applications.

Highly sensitive biosensor based on UV-imprinted layered polymeric–inorganic composite waveguides

An evanescent field sensor utilizing layered polymeric-inorganic composite waveguide configuration was developed in this work. The composite waveguide structure consists of a UV-imprint patterned polymer inverted rib waveguide with a Ta2O5 thin film sputter-deposited on top of the low refractive index polymer layers. The results suggest that the polymer based sensor can achieve a detection limit of 3 × 10(-7) RIU for refractive index sensing and corresponding limit of about 100 fg/mm2 for molecular adsorption detection. Besides enhancing the sensitivity significantly, the inorganic coating on the polymer layer was found to block water absorption effectively into the waveguide resulting in a stabilized sensor operation. The ability to use the developed sensor in specific molecular detection was confirmed by investigating antibody - antigen binding reactions. The results of this work demonstrate that high performance sensing capability can be obtained with the developed composite waveguide sensor.

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.

Comparison of distributed fiber optic sensing methods for location and quantity information measurements

A state-of-the-art review of distributed optical fiber sensor technologies has been carried out. The studied methods include polarization-modulation-based, Brillouin-scattering-based, and frequency-modulated continuous-wave (FMCW) methods as well as optical coherence domain reflectometry (OCDR) and interferometric distributed sensing techniques. A classification of the methods with their main properties for measuring quantity information continuously from a long sensor fiber is carried out. The main emphasis is on the comparison of the methods with respect to their spatial resolution, measurement time, and measurement range. The FMCW methods are found to be the most versatile techniques for various applications, due to their short measurement times with measurement ranges extending up to tens of kilometers and location sensing resolution better than 0.1%. The OCDR methods are fast and compete with the FMCW methods in versaility, especially when the measurement range is shorter than 1 to 2 km.

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.

Polymeric slot waveguide at visible wavelength

Polymeric slot waveguide structure, which pushes the mode field toward the surrounding media, was designed and characterized. The slot waveguide was fabricated by using nanoimprint lithography, and the operation of the slot was demonstrated at 633 nm wavelength with an integrated Young interferometer. The experimental result shows that the nanolithography method provides possibilities to fabricate disposable slot waveguide sensors.

Polymeric slot waveguide interferometer for sensor applications

A refractive index sensor based on slot waveguide Young interferometer was developed in this work. The interferometer was fabricated on a polymer platform and operates at a visible wavelength of 633 nm. The phase shift of the interference pattern was measured with various concentrations of glucose-water solutions, utilizing both TE and TM polarization states. The sensor was experimentally observed to detect a refractive index difference of 6.4 × 10(-6) RIU. Furthermore, the slot Young interferometer was found to compensate for temperature variations. The results of this work demonstrate that high performance sensing capability can be obtained with a polymeric slot Young interferometer, which can be fabricated by a simple molding process.

Single-mode tuning of a 1540-nm diode laser using a Fabry-Pe/spl acute/rot interferometer

We realized a wavelength-tunable laser using a 1540-nm Fabry-Pe/spl acute/rot diode laser and a silicon surface micromachined Fabry-Pe/spl acute/rot interferometer device in the short external cavity configuration. This hybrid-integrated system enables the use of standard laser chips and potentially has a low cost. We obtained single-mode tuning of 13 nm, a sidemode suppression ratio of better than 25 dB, and an average single-mode fiber-coupled power of 100 /spl mu/W. The emitter can be employed in optical communication and fiber-optic sensor applications.

Manipulation of optical field distribution in layered composite polymeric-inorganic waveguides

We discuss the manipulation of optical field distribution in a low-refractive index polymeric waveguide by depositing a thin high refractive index Ta2O5 film on top of the waveguide. According to microstructure studies, the sputtered Ta2O5 thin films deposited on the polymer layer were very smooth with root mean square surface roughness value of 0.58 nm, had amorphous phase, and were optimal for integrated optical devices. Both computational and experimental optical studies suggest that the inorganic-polymeric composite waveguide design greatly increases the intensity distribution of the propagating mode at the surface. Consequently, the interaction of the optical field with the ambient surrounding medium is enhanced by a factor of about 1.7 in order of magnitude.

Atmospheric oxidation and carbon contamination of silver and its effect on surface-enhanced Raman spectroscopy (SERS).

Surface-enhanced Raman spectroscopy (SERS) is considered a highly promising technology for different analytical purposes. The applications of SERS are still quite limited due its relatively poor quantitative repeatability and the fact that SERS is very sensitive to oxidation, which is a challenge especially with silver based SERS substrates. Here, the link between these phenomena is investigated by exposing silver SERS substrates to ambient laboratory air. We show that SERS intensity decreases exponentially after the exposure, which consequently leads to an increasing standard deviation (σ) in intensity. Within a five-hour measurement window, the SERS intensity already drops by 60%, while σ triples from 7% to 21%. The SERS results are supplemented by elemental analysis, which shows that oxidation and atmospheric carbon contamination coincide with the rapid SERS intensity decrease. The results emphasize how sensitive SERS is towards atmospheric contamination and how it can also reduce the measurement repeatability – even if the substrates are exposed to air just for a very short period of time.