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While the thus produced sensors have turned out to suit well for evanescent refractive index sensing in humidity-saturated environments like aqueous solutions, however, the additional deployment as an independent on-chip fluid temperature sensor is widely restricted due to the considerably temperature-dependent moisture uptake and swelling mechanisms of the epoxy materials, which are comparable to the well-known issues in polymethylmethacrylate. These severe mechanisms prevent the physical quantities temperature and relative humidity from being measured independently. Hence, the presented chip-design demands an extra sensor element solely responding to temperature variations.
In this contribution, we therefore propose and evaluate three differing approaches to realize an enhanced optofluidic labon- a-chip-platform with integrated temperature sensor for a corrected RI measurement using multiple Bragg grating sensors. In the first approach, an EpoCore waveguide Bragg grating sensor is symmetrically sandwiched in between two humidity insensitive Topas 6017 sheets by solvent bonding. In the second method, an amount of COC is fully dissolved in cyclohexane solvent and the liquid copolymer is drop coated onto the sensor area in terms of a thin layer aiming for humidity desensitization. The third approach uses the COC substrate directly itself as temperature sensor material. Thereby, besides the epoxy waveguide grating, a separated further waveguide Bragg grating is laser-written into the Topas substrate material being inherently insensitive to humidity influences.
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.
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