1. 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, Sarıyer İstanbul, Turkey;
2. İstiklal Street Birlik Avenue No:23, Ümraniye, İstanbul, Turkey;
3. Department of Electrical Engineering, Koç University, Rumelifeneri Yolu Sarıyer, 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
( 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 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, : 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, : p
Acknowledgements:
This work is supported by TÜBİTAK (Grant Numbers. 110T803 and 115F446)
Koç University Nano-Optics Research Laboratory, Rumelifeneri Yolu, Sarıyer, İstanbul, Turkey,