Optical Sensors of Bulk Refractive Index Using Optical Fiber Resonator Mustafa Eryürek1, Yasin Karadağ2, Moeen Ghafoor1, Nima Bavili1, Kenan Çiçek1 and Alper Kiraz1,3 1. Department of Physics, Koç University, Rumelifeneri Yolu, 34450 Sarıyer İstanbul, Turkey; 2. İstiklal Street Birlik Avenue No:23, Ümraniye, İstanbul, Turkey; 3. Department of Electrical Engineering, Koç University, Rumelifeneri Yolu Sarıyer, 34450 İstanbul, Turkey 1. Motivation 4. Experimental Results of Ethanol / Ethylene Glycol Sensing Experiments Optical detection of changes in bulk liquid refractive index Relatively easy fabrication of the sensor consisting of a tapered fiber and a fiber resonator Due to the compact size and applicability to liquid environment, this system is convenient for many sensing applications 2. Preparation of the Sensor Device: Fiber Tapering and Placement of the Resonator While pulling, the fiber is heated with a hydrogen flame The fiber is pulled with computer controlled translation stages Resonator fiber Tapered fiber After tapering, another fiber is placed on the taper as a the resonator Optical fiber resonator Tapered fiber In (a) and (c), WGM spectra for the beginning of ethanol and EG experiments, respectively In (b) and (d), spectral shifts under different concentrations of ethanol and EG are plotted Dashed lines represent the analytical calculations Experimental results are in a very good agreement with the analytical calculations TM modes show slightly larger spectral shifts than TE modes Slight misalignment of spectral positions of WGMs presented in (a) and (c) is attributed to the deviation of 90° angle between the tapered fiber and the fiber resonator Concentration sensitivity towards EG is higher because of higher refractive index contrast between EG and water Analytically Calculated Values Bulk RI Sensitivity (nm/RIU) Concentration Sensitivity (pm/%) TM TE Around 0% Ethanol 62.9 55.3 25.7 22.6 Around 0% EG 62.6 56.8 50.8 46.1 In the SEM image, the taper region has a width of ~3.2 microns 3. Experimental Setup Experimentally Measured Values Bulk RI Sensitivity (nm/RIU) Concentration Sensitivity (pm/%) TM TE Around 0% Ethanol 52.7 53.0 23.7 23.8 Around 0% EG 69.8 66.8 49.2 47.1 Tunable laser (1500-1620 nm) Detector Sensor chamber Sensor Syringe pump Waste Solution Detector 5. Conclusion Sample chamber Refractive index change of liquid solutions are detected Analytical calculations of WGM shifts are provided [1] The sensor consists of a tapered fiber and a bare fiber [2] Sensing mechanism relies on the spectral shifts of the whispering gallery modes (WGMs) Achieved bulk refractive index sensitivity is as high as ~63 nm / RIU. Limit of detection for bulk refractive index is calculated to be 2.7 x 10-5 RIU. The sensor can be employed for determination of the composition of different fluid mixtures at harsh environments including CO2 / ethanol mixture at high pressures [3] This work will lead to demonstrations of sensitive, compact, easy-to-fabricate optical sensors Syringe pump After the sensor is prepared, it is placed into the sensor chamber Certain concentrations of ethanol or ethylene glycol (EG) in water are pumped into the chamber to set the ambient refractive index to desired values Spectral shifts of optical resonances are monitored for sensing Q-factors in air and in liquid are as much as 63000 and 16000, respectively Decrease in q-factor is attributed to the absorption of water and scattering from particles inside the liquid FSR is measured and calculated to be ~4.16 nm in both air and liquid References: [1] H. C. van de Hulst, Light Scattering by Small Particles, 2nd ed. Dover Publications. Inc, 1981. [2] Aitor Urrutia, Kartheka Bojan, Leonel Marques, Kevin Mullaney, Javier Goicoechea, Stephen James, Matt Clark, Ralph Tatam and Sergiy Korposh, Novel Highly Sensitive Protein Sensors Based on Tapered Optical Fibres Modified with Au-Based Nanocoatings. Journal of Sensors, 2016. 2016: p. 1-11 [3] Mamata Mukhopadhyay and Bhatta Sankara Rao, Modelling of Supercritical Drying of Ethanol-Soaked Silica Aerogels with Carbon Dioxide. J. Chem. Technol. Biotechnol, 2008. 83: p. 1101-1109. Acknowledgements: This work is supported by TÜBİTAK (Grant Numbers. 110T803 and 115F446) Koç University Nano-Optics Research Laboratory, Rumelifeneri Yolu, 34450 Sarıyer, İstanbul, Turkey, Email: meryurek@ku.edu.tr