Meike Hofmann

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.

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.

Technology for polymer-based integrated optical interferometric sensors fabricated by hot-embossing and printing

Integrated optical Mach-Zehnder interferometers (MZI) may be used as high-sensitivity sensors by taking advantage of the interaction of the waveguide evanescent field with liquids or gases surrounding the sensor. We present here the design and simulation of polymer-based MZIs fabricated using printing technologies. Based on simulations of an integrated MZI system with regard to variations of waveguide cross-section and refractive indices of core and cladding to optimize high sensitivity to external refractive index changes of analytes, waveguides with single mode behavior are fabricated by hot-embossing and ink-jet or flexographic printing technologies. Finally, a hybrid combination of a laser diode with the printed MZI will be shown.

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.

Design and simulation of integrated optical interferometers fabricated in polymer foils

Integrated optical Mach-Zehnder interferometers (MZI) can be used as high sensitivity sensors through the interaction of the evanescent field of the waveguide with liquids or gases surrounding the sensor. We present here the design of polymer-based MZIs fabricated by hot-embossing and printing technologies. Simulations of an integrated MZI system with regard to variations of the waveguide cross-section and the refractive indices of the core layer are carried out to guarantee single mode behavior and optimize high sensitivity to external refractive index changes of analytes. The simulation of propagation losses induced by the Y-coupleres is also presented. Furthermore, transmission as a function of different interaction window lengths are also simulated on the entire MZI structure using a mixture of water and ethanol as an analyte on the sensing arm. Finally, we calculate the coupling efficiency of a laser diode into a tapered waveguide and estimate that a value of 30% is possible.

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.

Integrated all-polymer Mach-Zehnder interferometers without interaction window in asymmetric configuration

Integrated Mach-Zehnder interferometers (MZI) based on semiconductors or glasses have been widely used as evanescent field sensors for the monitoring of liquid or gas concentrations. In these systems the upper cladding of the sensing arm is removed partially to form an interaction window by means of subtractive fabrication techniques like etching. The use of polymer materials implicates new options and challenges. Polymers are tunable in terms of refractive index and viscosity offering a great flexibility in design and fabrication in a certain range. They enable a cost-efficient and large-scale roll-to-roll manufacturing of integrated optics on flexible foils as substrate material. The foils can be pre-patterned for example by hot-embossing. Additive steps such as printing a pattern or dispensing a homogeneous layer of liquid monomer material followed by a UV induced polymerization can be used to define the optical structure. However, when a large scale fabrication is required, the reliable production of small lateral structures and thin layers is challenging. Thus the fabrication according to the classical MZI design including an interaction window is difficult so that new design approaches are required. We present here the design and systematic evaluation of MZI sensors without interaction window based on polymer materials. The phase shift at the recombining Y-splitter of the MZI upon a refractive index change of an analyte, which serves as upper cladding of the entire system, is generated by a geometrical asymmetricity of the MZI. The waveguides in the sensing and the reference arm have different width leading to different effective refractive indices and sensitivities. We consider theoretically the expected interference signal and show results from numerical simulations of the whole system using commercial software. The simulations include the material as well as propagation losses and give an overall optimal system length.

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.