We report on the fabrication of an optical, highly-flexible thin film bending sensor which is based on diffused channel waveguide Bragg gratings inscribed into sheets of OrmoStamp hybrid polymers. The inorganic-organic Ormocer thin films are prepared by non-structured UV-enhanced imprint lithography which allows the fabrication of sheet-like slab substrates with a desired thickness. By this approach, 120 μm thin and highly-flexible plane-parallel substrates are achieved. For the inscription of the diffused channel waveguide Bragg gratings, a fast and efficient single writing step concept is applied, which allows the simultaneous inscription of both waveguide and Bragg grating in only a few seconds. The accordingly fabricated waveguide Bragg gratings feature a defined Bragg reflection peak that lies within the telecom wavelength range and is well-suited for sensing applications that require a reliable detection and tracking of the reflected Bragg wavelength. The applicability of the thus achieved devices as highly-flexible thin film bending sensors is investigated by means of deflection measurements. Here, we found a quasi-instantaneous and highly-reproducible response of the diffused channel waveguide Bragg gratings reflected Bragg wavelength to even small deflections which features a linear dependency of 6.05 × 10-4 nm/μm on the sensors displacement.
In this contribution, we give a comprehensive overview of the fabrication, characterization, and application of integrated planar waveguide Bragg gratings (PPBGs) in cyclo-olefin copolymers (COC). Starting with the measurement of the refractive index depth profile of integrated UV-written structures in COC by phase shifting Mach-Zehnder- Interferometry, we analyze the light propagation using numerical simulations. Furthermore, we show the rapid fabrication of humidity insensitive polymer waveguide Bragg gratings in cyclo-olefin copolymers and discuss the influence of the UV-dosage onto the spectral characteristics and the transmission behavior of the waveguide. Based on these measurements we exemplify that our Bragg gratings exhibit a reflectivity of over 99 % and are highly suitable for sensing applications. With regard to a negligible affinity to absorb water and in conjunction with high temperature stability these polymer devices are ideal for mechanical deformation sensing. Since planar structures are not limited to tensile but can also be applied for measuring compressive strain, we manufacture different functional devices and corroborate their applicability as optical sensors. Exemplarily, we highlight a temperature referenced PPBG sensor written into a femtosecond-laser cut tensile test geometry for tensile and compressive strain sensing. Furthermore, a flexible polymer planar shape sensor is presented.
We demonstrate the application of humidity insensitive polymer planar Bragg gratings written into planar TOPAS substrates for measuring temperature as well as multi-axial tensile and compressive strain. The polymer planar Bragg gratings are realized by a rapid fabrication process using a KrF excimer laser which illuminates a stacked mask configuration consisting of an amplitude mask and a phase mask with a grating period of 1036.79 nm. We determine the UV-induced refractive index modification of the integrated waveguide by phase shifting Mach Zehnder interferometry and investigate its light guiding properties. The integrated polymer planar Bragg grating (PPBG) reflects a wavelength of 1576.5 nm with a reflectivity of about 93% and shows almost no sensitivity against relative humidity. Therefore, the TOPAS-PPBG is well-suited for measuring temperature as well as tensile and compressive strain. Temperature measurements are feasible up to 116°C. An observed temperature hysteresis disappears after three consecutive temperature cycles, after which the TOPAS-PPBG is characterized by a sensitivity of -6.5 pm/°C. Furthermore, tensile and compressive strain is applied at different angles showing an angle-dependent sensitivity of the PPBG making it a suitable candidate for multi-axial strain sensing.
Bragg gratings have become indispensable as optical sensing elements and are already used for a variety of technical applications. Mainly silica fiber Bragg gratings (FBGs) have been extensively studied over the last decades and are nowadays commercially available. Bragg grating sensors consisting of other materials like polymers, however, have only recently come into the focus of fundamental and applied research. Polymers exhibit significantly different properties advantageous for many sensing applications and therefore provide a good alternative to silica based devices. In addition, polymer materials are inexpensive, simple to handle as well as available in various forms like liquid resists or bulk material. Accordingly, polymer integrated optics attract increasing interest and can serve as a substitute for optical fibers.
We report on the fabrication of a planar Bragg grating sensor in bulk Polymethylmethacrylate (PMMA). The sensor consists of an optical waveguide and a Bragg grating, both written simultaneously into a PMMA chip by a single writing step, for which a phase mask covered by an amplitude mask is placed on top of the PMMA and exposed to the UV radiation of a KrF excimer laser. Depending on the phase mask period, different Bragg gratings reflecting in the telecommunication wavelength range are fabricated and characterized. Reflection and transmission measurements show a narrow reflection band and a high reflectivity of the polymer planar Bragg grating (PPBG). After connecting to a single mode fiber, the portable PPBG based sensor was evaluated for different measurands like humidity and strain. The sensor performance was compared to already existing sensing systems. Due to the obtained results as well as the rapid and cheap fabrication of the sensor chip, the PPBG qualifies for a low cost sensing element.
We report on an optical planar Bragg grating evanescent wave refractive index sensor functionalized by a simple
method against aromatic hydrocarbons such as benzene, toluene and xylene (BTX) in solvent vapor. To
functionalize the sensor against BTX, substituted cyclodextrins are applied to the sensor surface using dip
coating. Cyclodextrins have a hydrophobic cavity, which favors the accommodation of an organic molecule of
appropriate dimensions leading to a non-covalent inclusion complex. The temporal sensor response reveals a
multi-exponential rise towards an equilibrium state, whose level is found to be linearly related to the exposed
analyte concentration. Taking into account the spectral resolution of the interrogation system we find a minimum
concentration threshold of 200 ppm for benzene, 70 ppm for toluene and 20 ppm for m-xylene, respectively.
We report on the application of a silica on silicon based planar Bragg grating (PBG) evanescent field sensor as a
refractive index biosensor. Our results demonstrate that typical biochemical reactions such as the binding between Biotin
(vitamin H) and Streptavidin can be traced in real time on the sensor surface. For the detection of Streptavidin, Biotin
was attached to the silanized surface of the planar Bragg grating sensor followed by the immobilization of Streptavidin
with a concentration of 7.5nM, 15nM and 30nM, respectively. Real time monitoring capability is highlighted by
interrupting the biochemical reaction by applying PBS solution and restarting the reaction several times showing a quasi
instantaneous spectral response of the PBG sensor. In addition, applying the same bio-functionalized sensor we have
investigated the detection of DNA hybridization. For this purpose, biotinylated single stranded DNA was linked to the
sensor surface via Streptavidin. Using this functionalized PBG sensor surface, the DNA hybridization of unlabeled
complementary single stranded DNA with a concentration of 5μM can be observed.
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