M. Rosenberger

Planar Bragg grating in bulk Polymethylmethacrylate

We report on a one-step writing process of a planar waveguide including a Bragg grating structure in bulk Polymethylmethacrylate (PMMA). A KrF excimer laser and a phase mask covered by an amplitude mask were used to locally increase the refractive index in PMMA and thereby generate simultaneously the planar waveguide and the Bragg grating. Our results show a reflected wavelength of the Bragg grating of about 1558.5 nm in accordance to the phase mask period. The reflectivity of the grating is about 80%. Initial characteristics of the Bragg grating structure towards humidity are investigated.

Compressive and tensile strain sensing using a polymer planar Bragg grating.

A polymer planar Bragg grating sensor is used for measuring both mechanical compressive and tensile strain. The planar waveguide with integrated Bragg grating is fabricated in bulk Polymethylmethacrylate in a single writing step using combined amplitude and phase mask technique. After butt coupling of a single-mode optical fiber the planar structure can be applied for measuring both mechanical tensile and compressive strain alongside the integrated waveguide without the need of further modifications. In this respect, we particularly report for the first time compressive strain measurements using a polymer Bragg grating. Furthermore, the sensitivity of the sensor against tensile and compressive strain, its reproducibility and hysteresis are investigated and discussed.

Highly Sensitive Detection of Naphthalene in Solvent Vapor Using a Functionalized PBG Refractive Index Sensor

We report an optical refractive index sensor system based on a planar Bragg grating which is functionalized by substituted γ-cyclodextrin to determine low concentrations of naphthalene in solvent vapor. The sensor system exhibits a quasi-instantaneous shift of the Bragg wavelength and is therefore capable for online detection. The overall shift of the Bragg wavelength reveals a linear relationship to the analyte concentration with a gradient of 12.5 ± 1.5 pm/ppm. Due to the spectral resolution and repeatability of the interrogation system, this corresponds to acquisition steps of 80 ppb. Taking into account the experimentally detected signal noise a minimum detection limit of 0.48 ± 0.05 ppm is deduced.

Simultaneous 2D strain sensing using polymer planar Bragg gratings.

We demonstrate the application of polymer planar Bragg gratings for multi-axial strain sensing and particularly highlight simultaneous 2D strain measurement. A polymer planar Bragg grating (PPBG) fabricated with a single writing step in bulk polymethylmethacrylate is used for measuring both tensile and compressive strain at various angles. It is shown that the sensitivity of the PPBG strongly depends on the angle between the optical waveguide into which the grating is inscribed and the direction along which the mechanical load is applied. Additionally, a 2D PPBG fabricated by writing two Bragg gratings angularly displaced from each other into a single polymer platelet is bonded to a stainless steel plate. The two reflected wavelengths exhibit different sensitivities while tested toward tensile and compressive strain. These characteristics make 2D PPBG suitable for measuring multi-axial tensile and compressive strain.

Polymer planar Bragg grating sensor for static strain measurements.

We report on a new optical strain sensor based on a polymer planar Bragg grating (PPBG). The sensor consists of commercially available bulk Polymethlymethacrylate with a UV-inscribed optical waveguide as well as a Bragg grating, both of which are fabricated simultaneously in a single writing step. Upon axial strain, the Bragg wavelength reveals a quasi-instantaneous spectral red shift that depends linearly on the mechanical load with a sensitivity of 2.95 pm/με. The relative reflected intensity of the PPBG remains constant in the investigated load region.

Allylated cyclodextrins as effective affinity materials in chemical sensing of volatile aromatic hydrocarbons using an optical planar Bragg grating sensor

We report on the application of perallyl-substituted α-, β- and γ-cyclodextrins to an optical planar Bragg grating refractive index sensor for the effective sensitization of the sensor for airborne volatile aromatic hydrocarbons. Thereby, the emphasis of this work lies on the comparison of the different cyclodextrin types regarding their suitability as affinity material assessed by the sensors sensitivity and response behavior. The opto-chemical sensor device showed an immediate and quick response to the application of the investigated analytes benzene, toluene and m-xylene as well as a linear dependence on the concentration of those analytes. Studies on the sensors sensitivity depending on the applied cyclodextrin types revealed a generally higher sensitivity for the sensor sensitized with perallyl-substituted β-cyclodextrins. Here, the sensor systems detection limit was found to 60±4 ppm for benzene, 18±3 ppm for toluene and 3.8±0.5 ppm for m-xylene. The response time and recovery time were found to approximately 30s and 40s, respectively, depending on the applied cyclodextrin and the chosen analyte.

Polymer planar Bragg grating for sensing applications

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.

Detection of biochemical reaction and DNA hybridization using a planar Bragg grating sensor

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.

Influence of the UV dosage on planar Bragg gratings in cyclo-olefin copolymer substrates

We report on the influence of the applied UV dosage on the fabrication of planar Bragg gratings in cyclo-olefin copolymers. Integrated waveguides with inscribed gratings are fabricated simultaneously using a single writing step technique and characterized by measuring the reflected and transmitted spectra. In addition, the effect of length and width of both waveguide and Bragg grating on the reflectivity and attenuation are studied using the cut-back method. The results show that in this material class, integrated waveguides with an attenuation as low as 1.2 dB/cm and Bragg gratings with a reflectivity of up to 99% can be realized. Most strikingly, we demonstrate that Bragg gratings with a length of only 2.1 mm still exhibit a distinct Bragg reflection facilitating miniaturized Bragg grating applications. Moreover, we demonstrate that the refractive index sensitivity of the planar Bragg gratings strongly depends on the UV dosage, in turn facilitating to tailor grating properties according device requirements in sensing applications.

Flexible Polymer Shape Sensor Based on Planar Waveguide Bragg Gratings

A newly developed concept for a miniaturized optical shape sensor is demonstrated. The sensor consists of a flexible polymer bending beam with two integrated optical waveguide Bragg gratings spatially staggered to each other. We report on the basic layout and the fabrication process and characterize the fundamental sensing response by an elaborate testing scheme. The polymer shape sensor reveals a pronounced spectral shift of the Bragg wavelength of several nanometers upon deformation with a distinct directionality. In conjunction with an undiscernible hysteresis, these results highlight the general applicability of the polymer Bragg grating-based bending beam as a shape sensor.