Abstract
In this work, the behavior of refractive index sensors based on optical micro-ring resonators is studied in detail. Using a result of waveguide perturbation theory in combination with numerical simulations, the optimum design parameters of the system, maximizing the sensitivity of the sensor, are determined. It is found that, when optimally designed, the sensor can detect relative refractive index changes of the order of Δn/n≈3×10−4, assuming that the experimental setup can detect relative wavelength shifts of the order of Δλ/λ≈3×10−5. The behavior of the system as bio-sensor has also been examined. It is found that, when optimally designed, the system can detect refractive index changes of the order of Δn≈10−3 for a layer thickness of t=10 nm, and changes in the layer thickness of the order of λt≈0.24 nm, for a refractive index change of Δn=0.05.
Highlights
Introduction and theoretical analysisOptical ring resonators are interesting optical devices with a plethora of applications especially in optical switching [1,2,3,4], routing [4,5,6,7], and sensing [8,9,10]
An optical ring resonator usually consists of a straight waveguide coupled to a circular one, as shown in figure 1(a), or two straight waveguides coupled through a circular one, as shown in figure 1(b)
A2 r2 2ar cos Tn 1 r2a2 2ar cos where r is the self-coupling coefficient of light between the straight and circular waveguide, and a is a loss parameter related to the power attenuation coefficient through the equation a2 exp L, where L is the length of the circular waveguide
Summary
Optical ring resonators are interesting optical devices with a plethora of applications especially in optical switching [1,2,3,4], routing [4,5,6,7], and sensing [8,9,10]. Following a similar analysis it can be shown that the condition for minimum transmittance through the lower waveguide, and simultaneously maximum transmittance through the upper waveguide, is described again by equation (6) In this case equations (8a), (8b) take the form. As far as the use of micro-ring resonators as refractive index sensors is concerned, the most important result of the above analysis is that, in both configurations, the resonant wavelength is given by equation (6), which practically implies that it is proportional to the effective refractive index of the propagating mode in the circular waveguide. According to the above analysis, the most important parameter regarding the performance of an optical ring resonator when used as a refractive index sensor is the dependence of the effective refractive index of the propagating mode on the refractive index of the environment The parameter S can be numerically calculated quite as described
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