Peter Van Daele

Direct writing of microlenses in polycarbonate with excimer laser ablation.

A method for fabricating microlenses in polycarbonate material is reported. Using a direct-write technique based on scanning excimer laser ablation with a circular beam, we can etch an arbitrary shape in the polymer material. The beam is obtained by imaging a circular aperture onto the polymer surface, and scanning is realized by the translation stage carrying the sample, which makes successive contours with well-chosen diameters and scan velocities. Afterward, to smooth the ablated surface and release it from debris, a large beam aperture covering the full lens area is used to ablate the lens deeper into the substrate. The fabrication process and the characterization method are described, including calculation of the contour set for a desired lens shape. The optical performance is evaluated by Mach-Zehnder interferometry, showing that aberrations below lambda/10 are possible for slow lenses.

Optical fiber sensors embedded in flexible polymer foils

In traditional electrical sensing applications, multiplexing and interconnecting the different sensing elements is a major challenge. Recently, many optical alternatives have been investigated including optical fiber sensors of which the sensing elements consist of fiber Bragg gratings. Different sensing points can be integrated in one optical fiber solving the interconnection problem and avoiding any electromagnetical interference (EMI). Many new sensing applications also require flexible or stretchable sensing foils which can be attached to or wrapped around irregularly shaped objects such as robot fingers and car bumpers or which can even be applied in biomedical applications where a sensor is fixed on a human body. The use of these optical sensors however always implies the use of a light-source, detectors and electronic circuitry to be coupled and integrated with these sensors. The coupling of these fibers with these light sources and detectors is a critical packaging problem and as it is well-known the costs for packaging, especially with optoelectronic components and fiber alignment issues are huge. The end goal of this embedded sensor is to create a flexible optical sensor integrated with (opto)electronic modules and control circuitry. To obtain this flexibility, one can embed the optical sensors and the driving optoelectronics in a stretchable polymer host material. In this article different embedding techniques for optical fiber sensors are described and characterized. Initial tests based on standard manufacturing processes such as molding and laser structuring are reported as well as a more advanced embedding technique based on soft lithography processing.

Integration of multimode waveguides and micromirror couplers in printed circuit boards using laser ablation

Integration of optical interconnections on a Printed Circuit Board (PCB) is very challenging, as it should remain compatible with existing PCB manufacturing technology based on laminated FR4-substrates and making use of solder-reflow and well-known placement and assembly techniques. In this paper we will describe different technologies being used for integration of such optical interconnections in PCBs. As we will demonstrate, the use of laser ablation, already used in PCB manufacturing for microvias, is a suitable technique for the fabrication of multimode waveguides and micromirrors to provide optical coupling. Laser ablation is a very flexible technology that is particularly well suited for structuring of polymers because of their excellent UV-absorption properties and highly non-thermal ablation behavior. One of the most critical problems on the integration of optical interconnections in PCBs is coupling the light in and out of the optical plane. Because in our set-up the excimer laser beam can be tilted, the 45 degrees micromirrors can be easily fabricated using laser ablation. The focus is on ablation of waveguides using a frequency tripled Nd-YAG laser and on ablation of 45 degrees facets using a KrF excimer laser. It is shown that these structures can be defined in one single processing step, resulting in a very accurate alignment.

Fabrication of microgrooves with excimer laser ablation techniques for plastic optical fiber array alignment purposes

Laser ablation is extremely well suited for rapid prototyping and proves to be a versatile technique delivering high accuracy dimensioning and repeatability of features in a wide diversity of materials. In this paper, we present laser ablation as a fabrication method for micro machining in of arrays consisting of precisely dimensioned U-grooves in dedicated polycarbonate and polymethylmetacrylate plates. The dependency of the performance on various parameters is discussed. The fabricated plates are used to hold optical fibers by means of a UV-curable adhesive. Stacking and gluing of the plates allows the assembly of a 2D connector of plastic optical fibers for short distance optical interconnects.

4-mW microcavity LED at 650 nm on germanium substrates

We report on the realization of red micro cavity LEDs on germanium substrates, offering a significant cost advantage compared to GaAs wafers. The MCLED structure, grown by LP- MOVPE, consists of 3 GaInP quantum wells within a (detuned) 1- (lambda) AlGaInP cavity, enclosed by Al95GaAs/Al55GaAs DBRs, with a current spreading layer on top. MCLEDs with a 200 micrometer aperture, exhibit a quantum efficiency up to 4.35% (at 10 mA) and an optical power higher than 4 mW (at 80 mA), without any packaging. The optical spectrum was centered at 650 nm, with a FWHM of plus or minus 13 nm. Because of the detuning the opening angle of these structures was as much as 120 degrees. Rudimentary packaging resulted in a luminous intensity of 2.5 cd at 30 mA, with an opening angle of plus or minus 13 degrees. Initially the electrical performance was not optimal, but additional tests and a new processing have indicated that forward biases as low as 2.0 V (at 20 mA) can be obtained for LEDs on Ge-substrates. The new processing further resulted in an improved optical output with 5 mW at 80 mA. We feel there is room for further improvement, but already we have demonstrated the feasibility of germanium substrates for commercial red (to orange/yellow) LED applications.

Optical interconnections on PCBs: a killer application for VCSELs

As a result of the constant improvement of performances and reliability of VCSEL-fabrication, parallel short distance optical interconnections are becoming more and more important. Integrating optical interconnections on a board level, covering distances from a few centimeters to a few meters, is however very challenging as the optical interconnection and mounting technology has to be integrated in existing printed circuit board manufacturing technology. Fiber based interconnections, using technologies as Fiber-In-Board and Fiber-based optical backpanels are already available, but new solutions for integrating a guided-wave based optical interconnection layer in a standard FR4-based electrical printed circuit board are emerging. These technologies are based on either organic materials or glass sheets integrated in the FR4-stack. Examples of both technologies will be presented and optical interconnections showing the feasibility of both technologies will be described. The interconnections will be realized using VCSEL-arrays and photodetector arrays which are flip-chip, mounted on the printed circuit boards. The coupling of light in and out of the optical layer in the FR4-stack is done using deflecting micro-optics realized in the optical layer, e.g. using laser-ablation.

Substrate-removed 850-nm RCLEDs and small core (63/125 um) plastic optical fibers for optical data communication

Current developments in computer technology give rise to increasing data communication over relatively short distances at backplane- and inter MCM interconnect level. It is foreseen that electrical interconnect will not be able to accommodate the necessary data traffic in advanced data processing systems in the future and hence a bottleneck will be created. A potential remedy for this interconnect problem is the use of parallel optical datalinks. In this paper we propose small diameter step index plastic optical fiber ribbons in combination with high efficient resonant cavity LEDs as a cheap and feasible option for these optical links. A design for such an optical link is presented with special attention for the optical pathway. Experimental results on the optical properties of the used POF are shown. We describe the development of RCLEDs at 850 nm specially designed for efficient coupling into POF. We measured a RCLED to POF coupling efficiency up to 40%. Additionally we report on the technologies used for the fabrication and assembly of the optical pathways and finally some experiments were carried out on the first realized assemblies.

Simple-to-fabricate and highly efficient spot-size converters using antiresonant reflecting optical waveguides

We report on a new concept for InGaAsP-InP 1.55 μm lasers with integrated spot-size converters based on antiresonant reflecting optical waveguides (ARROW). The mode expanders consist of a laterally tapered active region on top of a fiber-matched passive slab waveguide. The large slab mode is laterally confined by an antiresonant configuration of a couple of lateral waveguides defined in the same fabrication process as the active ridge. This feature makes the presented spot-size transformer as simple to fabricate as a standard waveguide, only requiring a planar growth step and a single conventional etch process. The fabricated tapers exhibit a low transformation loss and reduce the coupling loss to standard single-mode fibers from 8 to 4 dB. We also analyze by simulation two variants of the concept proposed in this work, including a taper structure for a buried waveguide, which are expected to show better performance. Simulation results show fiber-coupling efficiencies as low as 2.4 and 1.1 dB for both variants.

OPTOELECTRONIC AND PHOTONIC INTEGRATED CIRCUITS: Modelling and Technology

Publisher Summary This chapter discusses the modeling and technology of optoelectronic and photonic integrated circuits. Optoelectronic devices are fairly complex structures, in which electromagnetic waves interact with materials, often in a nonlinear way. When a number of such devices—laser diodes, passive waveguide structures, modulators, or detectors—are combined on one chip, the complexity obviously increases. Contrary to the situation of electronic integrated circuits (IC), where there are only few different elements on one chip, the variety of components on optoelectronic integrated circuits or photonic integrated circuits is obviously large. Therefore, although the number of devices in todays optoelectronic ICs is still low, the overall complexity is nevertheless high. The design and modeling of optoelectronic devices and circuits requires a range of modeling tools, some of which require specialist knowledge on waveguide theory. The validity range of some of these models is not always clearly established. Efforts are afoot to increase the user-friendliness of these modeling tools.