Temperature insensitive refractometer using TE and TM modes in open top ridge waveguides

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In our previous work, a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed (1). We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index (2). In this work, a technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide design is presented. By using the TE mode resonance as a temperature reference, the relative shift of the TM mode can be monitored in order to measure the refractive index of liquids under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 1×10 -4 of measured index change, with a temperature sensitivity For temperature insensitive sensors based on fiber Bragg gratings, several techniques have been proposed to discriminate between Bragg resonance spectral shifts associated with refractive index measurements and those induced by fluctuations in temperature. These techniques are implemented by using: a second Bragg grating in a side-polished fiber Bragg grating refractometer (3-4), higher order modes in an etched-core of a fiber Bragg grating sensor (5-6), and higher order modes in a tilted fiber Bragg grating sensor (7-12). We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index. The relative shift of the core mode resonance to cladding mode resonance is used to measure the refractive index of substances under test. The device fabricated produced a relative resonance shift of 1 pm for every 5×10 -4 of measured index change, with a temperature sensitivity ~ 0.5 pm/°C (2). Taking a similar approach, here, we reported another technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide architecture in order to discriminate between changes in temperature and refractive index. In our previous work (1), a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed. The structure of the open-top ridge waveguide is as shown in Fig.1. The center ridge waveguide with the Bragg grating is tested as a refractometer by coupling the light source into the end of the waveguide. The function of the two adjacent waveguides is to act as a barrier and to partially prevent the liquid from flowing away from the waveguide containing the grating. The guided light of the center waveguide couples evanescently into the surrounding liquid through the top and sides of the waveguide. When a Bragg grating is induced in the core of an open-top ridge waveguide with a larger birefringence, TE and TM resonances are observed when the light guided by the core is phase matched by the grating structure. Both TE and TM resonances are sensitive to the liquid refractive index on the top layer of the open-top ridge waveguide. The TE and TM sensitivities to temperature fluctuations however, are more closely matched. These characteristics can be used to decouple fluctuations of the Bragg resonance of the core mode due to temperature from those changes that are due to variation in the refractive index of the analyte liquid. In the experiments presented here, the variation of TE and TM resonances are investigated as a function of temperature and the external refractive index nt. A theoretical model is developed to investigate the performance of some potential waveguide structures. Relationships between the waveguide core size, refractive index distribution, as