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.
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.<sup>1,2</sup> 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 sensor's signal is linear to temperature changes and shows a sensitivity of -306 nm/K, allowing high resolution temperature measurements.
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 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.
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.
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.
The successful fabrication of several freeform optical elements by ultraprecision micromilling is presented in this article.
We discuss in detail the generation of the tool paths using different variations of a computer-aided manufacturing
(CAM) process. Following a classical CAM approach, a reflective beam shaper was fabricated. The approach is based
on a solid model calculated by optical design software. As no analytical description of the surface is needed, this
procedure is the most general solution for the programming of the tool paths. A second approach is based on the same
design data. But instead of a solid model, a higher order polynomial was fitted to the data using computational methods.
Taking advantage of the direct programming capabilities of state-of-the-art computerized numerical control units, the
mathematics to calculate the polynomial based tool paths on-the-fly during the machining process are implemented in a
highly flexible CNC code. As another example for this programming method, the fabrication of a biconic lens from a
closed analytical description directly derived from the optical design is shown. We provide details about the different
programming methods and the fabrication processes as well as the results of characterizations concerning surface quality
and shape accuracy of the freeform optical elements.
We present our research on integrated optical Talbot interferometers for particle mass concentration measurements. For
optimum integration of highly sensitive optical measurement systems we apply a planar emitter-receiver-unit with a
vertical cavity surface emitting laser (VCSEL) at 850 nm as light source. The optical system is integrated into a planar
transparent PMMA (polymethylmethacrylate) substrate. We suggest a planar integrated free-space optical system for
monitoring the particle mass concentration of polydisperse suspensions. Thorough simulations of the optical sensor show
that for the required regime of particle concentration and particle size distribution (PSD) turbidity measurements where
the attenuation of a light beam is evaluated for example at different wavelengths do not provide the required
measurement precision. We therefore propose a system where the probe beam even though sent through the system is
blocked before reaching the detector by an interferometric setup. The stray light originating from the particles is
exploited for the measurement. For our application we focus on particles with sizes in the range 1 - 120 μm and particle
mass concentrations in the range of 1-10 mg/L. In this case significant strength of the scattering signal only appears in
small angles relative to the incident probe beam (forward scattering). The probe beam and the stray light thus overlap to
a large extent.
Our sensor concept is based on a monolithically integrated Talbot interferometer. Two properly aligned diffraction
gratings are used to remove the primary beam. We use a stripe detector as second grating. The stray light causes
perturbations within the formation of the self image of the grating. These perturbations are visualized as speckles on a
detector and exploited for particle concentration measurements. The potential of the sensor concept is presented on the
example of a modular Talbot interferometer using a HeNe laser at 633 nm to measure particle mass concentrations
between 1 mg/L and 250 mg/L of Arizona test dust. We present the results of our investigations concerning the
generation of Talbot self images in the planar configuration using a diverging multimode VCSEL light source.
Furthermore we discuss the design and demonstrate the fabrication of a planar optical test system containing the
integrated passive optical elements necessary for forming an integrated Talbot interferometer. Light source and sensor
are positioned on a separate chip.