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.
We report on the physical observation of mixtures of volume contractional monohydric alcohol solutions interrogated by a planar Bragg grating sensor. Via evanescent field interaction, the sensor detects any changes of the composition of a liquid compound on the sensor surface that are associated with a change of the refractive index. Studying various aqueous solutions of monohydric alcohols, we found a nonlinear relationship of the analytes refractive index on its mixing ratio, indicating the different nature of the mixed phases in the solution and the effect of volume contraction. Analyzing aqueous solutions of methanol, ethanol, and 2-propanol reveals a local maximum of the refractive index for the application of smaller monohydric alcohol molecules, which is displaced in the direction of a higher alcohol content with an increasing number of carbon atoms in the chain of the alcohols. In turn, this sensor can be used for the online monitoring and observation of the water content in aqueous alcohol solutions, including volume contractional solutions.
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.<p> </p>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.
We report on a refractive index sensor based on a planar Bragg grating (PBG) capable to online monitor the water
content in Biodiesel and the amount of ethanol admixture to conventional fuels, respectively. Our results demonstrate the
capability of the sensor to distinguish the transition between about 190 and 500 ppm water in Biodiesel, enabling to
monitor the production process of Biodiesel in the relevant range according to industrial standards. The ethanol content
in petrol has been investigated in the range of 0-100%, covering the entire standardized range of E-5 to E-85 fuel mixing
ratios. These experiments reveal a sensitivity of 112 nm/riu allowing the measurement of the ethanol content with a
resolution of 8.9·10<sup>-6</sup>.
An evanescent field refractive index sensor consisting of a Bragg grating that is written into a silica-on-silicon planar
optical waveguide structure by UV laser radiation is utilized to monitor the composition of liquid binary chemical
systems. We have investigated various selected liquid compounds that are commonly used in the pharmaceutical and
chemical industry, finding sensitivities on the order of 100nm/RIU and minimum detectable index resolution on the
order of 5•10<sup>-6</sup> fulfilling industrial demands on detection limits and partly being superior to other electrical transducer
systems. The planar structure of the sensor chip allows on chip integration of fluidic structures that we have generated by
laser ablation using a pulsed fiber laser, enabling connection to the adjacencies.