INDEX overlayer effects

Rees and co-workers23 first studied the effect of the deposition of thin HRI overlays onto LPGs by analyzing the wavelength shift of the attenuation bands. The Langmuir-Blodgett deposition technique was exploited to deposit an incrementally thicker overlay (up to few hundreds of nanometers) of an organic material whose refractive index was 1.58. 

In this chapter, we will numerically analyze the effects of the HRI overlay on the cladding modes distribution with particular attention to its influence on the device sensitivity to the SRI changes and to the overlay refractive index changes. The structure to which we refer is depicted in Fig. 3.2. 

In this section, we will analyze the effects of a nano-sized HRI overlay on the distribution of the cladding modes (transversal fields, effective indices, coupling coefficients) to changes of its thickness and refractive index and to changes of the SRI. 

Fig. 3.5 Effective refractive index of the LP02 LP08 cladding modes vs. the SRI in HRI coated fiber with (a) 150 nm (b) 200 nm (c) 250 nm, and (d) 300 nm thin overlays...

The mode transition is shown in Fig. 3.5b-d for different overlay thicknesses, namely 200,250, and 300 nm. As observed in the figures, the transition region moves to lower SRIs as the overlay thickness increases, approaching the ambient index of 1.33 where bare LPGs demonstrate a significantly lower sensitivity. As a secondary effect, the SRI sensitivity becomes smaller as the overlay thickness increases. However, even if the absolute sensitivity decreases, the relative sensitivity in comparison to the bare device still increases as will be shown experimentally. 

The coupling coefficients vs. the SRI are reported in Fig. 3.6 b-d for overlay thicknesses of 200, 250, 300 nm, respectively, always with the same coating index of 1.578. From these figures, it can be clearly inferred that increasing the overlay thickness the coupling coefficients curves shift toward lower SRIs as it happens for the effective refractive index curves. 

Fig. 3.8 Effective refractive index sensitivity to overlay RI changes vs. overlay thickness for different cladding modes...

The presence of the HRI coating induces an increase in the effective refractive index of the cladding modes and thus a decrease in the resonance wavelengths. In addition, the spatial shift of the cladding mode field profile toward the HRI overlay promotes a decrease of the overlap integral with the core mode and so of the transmission loss peak. The subsequent evaporation of the solvent molecules from the nano-cavities results in a reduction of its refractive index. This explains the partial... 

When chloroform was added, the equilibrium response of the sensor progressively decreased. This is probably related to the combination of the nonlinear behavior of the effective refractive index of the coupled cladding mode on the overlay refractive index with the nonlinear relationship between adsorbed mass of... 

Cusano et al. [11, 12] studied theoretically and experimentally the responses and sensitivities of overlayed LPGs for optochemical sensing applications. The numerical results showed that the HRl coating leads to an increase in the effective refractive index of the cladding modes and in turn, a decrease in the resonance wavelengths and in the peak loss which are mode dependent as illustrated in the Fig. 12 and Fig. 13. 

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