Uwe Gleissner

Asymmetric Mach-Zehnder interferometers without an interaction window in polymer foils for refractive index sensing.

We report on the fabrication and characterization of integrated Mach-Zehnder interferometers in polymer foil without an interaction window. The interferometers are based on inverted rib waveguides, which allow single mode behavior even for waveguide widths larger than a few micrometers. The phase change between the two interferometer arms upon a refractive index change of the analyte that serves as the upper cladding is generated by the asymmetricity of the two interferometer arms. A difference of the waveguide width in the straight part of the interferometer leads to different effective refractive indices and thus to a change in the interference signal. We show in small scale the process chain, which is compatible with a cost-effective roll-to-roll fabrication process. For a proof of principle we apply deionized water and a glucose solution as analytes to the sensor foils and detect the transmitted intensity as a measure of the induced phase change. A detection limit of 3·10⁻³ refractive index units is reached for homogeneous sensing at a total system length of 9.3 mm and a total waveguide core thickness of 3 μm.

Cladded self-written multimode step-index waveguides using a one-polymer approach.

Low-loss optical-coupling structures are highly relevant for applications in fields as diverse as information and communication technologies, integrated circuits, or flexible and highly-functional polymer sensor networks. For this suitable and reliable production methods are crucial. Self-written waveguides are an interesting solution. In this work, we present a simple and efficient one-polymer approach for self-written optical connections between light-guiding structures such as single-mode and multi-mode optical fibers or waveguides that relies on self focusing of the light inside a photopolymerizing mixture. The optical connections are produced in a two-step process by writing into monomer resin using cw laser light in the blue wavelength range and subsequent UV curing. Since only one photopolymerizing resin is required, we reduced the fabrication complexity compared to previous approaches to obtain a waveguide embedded in a rigid cladding material. We discuss the production method, the results obtained as function of relevant process parameters such as writing speed or curing time, and evaluate optical properties and coupling efficiencies.

Ink-jet printed fluorescent materials as light sources for planar optical waveguides on polymer foils

Abstract. Polymer-based optical sensor networks on foils (planar optronic systems) are a promising research field, but it can be challenging to supply them with light. We present a solvent-free, ink-jet printable material system with optically active substances to create planar light sources for these networks. The ink is based on a UV-curable monomer, the fluorescent agents are Eu(DBM)3Phen or 9,10-diphenylantracene, which fluoresce at 612 or 430 nm, respectively. We demonstrate the application as light source by printing a small area of fluorescent material on an optical waveguide fabricated by flexographic printing on PMMA foil, resulting in a simple polymer-optical device fabricated entirely by additive deposition techniques. When excited by a 405-nm laser of 10 mW, the emitted light couples into the waveguide and appears at the end of the waveguide. In comparison to conventional light sources, the intensity is weak but could be detected with a photodiode power sensor. In return, the concept has the advantage of being completely independent of any electrical elements or external cable connections.

Inkjet printed single-mode waveguides on hot-embossed foils

We report on the fabrication of all-polymer inverted rib waveguides by hot-embossing and inkjet printing. Inkjet printing as an additive fabrication technique is well suited for a fast, selective and automated patterning of large areas. In general, the lines that can be printed with polymer inks can serve as waveguides themselves but the dimensions are too big to form single-mode waveguides. To overcome this limitation we apply hot-embossed grooves as assist structures to ensure the lateral confinement of the guided wave. We show the waveguide design, spin-coated single-mode waveguides as an intermediate result and finally inkjet printed all-polymer waveguides and their optical performance.

Automated Misalignment Compensating Interconnects Based on Self-Written Waveguides

Optical interconnects are the key components for integrated optics to link photonic integrated circuits or to connect external elements such as light sources and detectors. However, misalignment of the optical elements contained and its compensation is a remaining challenge for integrated optical devices. We present a novel method to establish rigid interconnects based on a 2-wavelength self-written waveguide process which automatically compensates for misalignment. We exemplarily demonstrate the capability of our process by writing interconnects between two multimode fibers as well as hot-embossed integrated polymer waveguides and a bare laser diode chip. The coupling efficiency of the interconnects obtained is analyzed with respect to misalignment. We found that coupling losses are as low as 1.3dB if a lateral misalignment lies within a 10μm interval, which is achieved by commercially available pick-and-place machines. Our approach is easily combined with high-throughput techniques such as hot embossing and enables low-cost production of interconnects even for mass fabrication in future applications.

Optical temperature sensing on flexible polymer foils

In contrast to established semiconductor waveguide-based or glass fiber-based integrated optical sensors, polymerbased optical systems offer tunable material properties, such as refractive index or viscosity, and thus provide additional degrees of freedom for sensor design and fabrication. Of particular interest in sensing applications are fully-integrated optical waveguide-based temperature sensors. These typically rely on Bragg gratings which induce a periodic refractive index variation in the waveguide so that a resonant wavelength of the structure is reflected. 1,2 With broad-band excitation, a dip in the spectral output of the waveguide is thus generated at a precisely-defined wavelength. This resonant wavelength depends on the refractive index of the waveguide and the grating period, yet both of these quantities are temperature dependent by means of the thermo-optic effect (change in refractive index with temperature) and thermal expansion (change of the grating period with temperature). We show the design and fabrication of polymer waveguide-integrated temperature sensors based on Bragggratings, fabricated by replication technology on flexible PMMA foil substrates. The 175 μm thick foil serves as lower cladding for a polymeric waveguide fabricated from a custom-made UV-crosslinkable co-monomer composition. The fabrication of the grating structure includes a second replication step into a separate PMMA-foil. The dimensions of the Bragg-gratings are determined by simulations to set the bias point into the near infrared wavelength range, which allows Si-based detectors to be used. We present design considerations and performance data for the developed structures. The resulting sensors signal is linear to temperature changes and shows a sensitivity of -306 nm/K, allowing high resolution temperature measurements.

Development and characterization of high refractive index and high scattering acrylate polymer layers

The aim is to develop a polymer layer which has the ability to diffuse light homogeneously and exhibit a high refractive index. The mixtures are containing an acrylate casting resin, benzylmethacrylate, phenanthrene and other additives. Phenanthrene is employed to increase the refractive index. The mixtures are first rheologically characterized and then polymerized with heat and UV radiation. For the refractive index measurements the polymerized samples require a planar surface without air bubbles. To produce flat samples a special construction consisting of a glass plate, a teflon sheet, a silicone ring (PDMS mold), another teflon sheet and another glass plate is developed. Glue clamps are used to fix this construction together. Selected samples have a refractive index of 1.585 at 20°C at a wavelength of 589nm. A master mixture with a high refractive index is taken for further experiments. Nano scaled titanium dioxide is added and dispersed into the master mixture and then spin coated on a glass substrate. These layers are optically characterized. The specular transmission and the overall transmission are measured to investigate the degree of scattering, which is defined as the haze. Most of the presented layers express the expected haze of over 50%.

Ink-jet printing of host-guest systems based on acrylates with fluorescent dopants

We present two ink-jet printable, optically active material systems that point towards flexible foil-based optical systems independent of any electrical elements or physical connections. The materials are based on a UV-curable monomer doped with europium and diphenylantracene, resulting in red (610 nm) and blue (430 nm) fluorescence excited by UV light. Additionally, ethylene glycol dimethacrylate (EGDMA) is used to tune the viscosity to 10 mPas via a print-head temperature of 50 °C, as required by the ink-jet print-head. When illuminated with 1.5 W/cm 2 , the measured intensity of the europium is in the range of 1 mW/cm 2 . By printing these materials on PMMA foil, we can create fluorescent tracks with a feature size well below 100 μm that could serve as light sources within a planar optronic system.

Temperature characterization of integrated optical all-polymer Mach-Zehnder interferometers

Two new design concepts for all-polymer-based integrated optical Mach-Zehnder interferometers in foil as chemical or bio-chemical sensors are presented. Fabricated with hot-embossing and printing techniques, these all polymer optical components are designed for low-cost fabrication and yield highly sensitive response to external refractive index changes. Compared to traditional semiconductor based systems, these polymer sensors do not need the interaction window and do not require a cleanroom for fabrication. The optical response of the asymmetric interferometers to temperature variations is determined theoretically and compared for two designs. Using the designed asymmetric interferometer, a chemical micro-fluidic test system with temperature controller experimentally demonstrates the sensors’ temperature characteristics.

Multimode interference structures as sensing elements integrated into Mach-Zehnder interferometers in polymer foils

Integrated Mach-Zehnder interferometers (MZIs) based on flexible polymer materials have been demonstrated as evanescent field sensors for the detection of refractive indices and molecule concentrations. The used application of a measurement window in classical MZIs is difficult in a roll-to-roll fabrication process. We have previously demonstrated foil-based asymmetric MZIs with different widths in sensing and reference arm which do not need a measurement window. Here we present the use of a multimode interference structure (MMI) inserted into the sensing arm of the interferometer to increase the sensitivity. We consider the expected interference signal from numerical simulations and optimize the system in terms of sensitivity, dimensions and absorption losses. The fabricated MMI-MZI foils are tested experimentally to demonstrate the function of the MMI-MZI system by applying water/glucose solutions with different refractive indices.