*2.2. Testing the Waveguide Sensor*

The testing of a PW sensor was performed by varying the RI of the liquid medium, i.e., by injecting into the cell initially filled with water aqueous solutions of NaCl of different concentrations having different RIs and recording the corresponding multi-periodic output waveforms [15,16]. The number of periods of signal oscillation were roughly estimated from these waveforms and presented in Table 1 along with corresponding changes in the RI, which allows estimating the refractive index sensitivity (RIS) of the PW sensors as:

$$RIS = \frac{2 \times \pi \times N}{\Delta n}$$

where *N* is the number of periods of oscillation, and ∆n is the difference between the RI of aqueous sodium chloride (NaCl) and water i.e., ∆*n* = *nNaCl* − *nwater*, where *nwater* = 1.332. The obtained RIS values are also shown in Table 1.

**Table 1.** Evaluation of the refractive index sensitivity (RIS) of the planar waveguide sensor as a function of the number of periods of oscillation (N) and differences in the refractive index (4n) in aqueous sodium chloride (NaCl) solutions at different concentrations (NaCl%).


An average RIS around 5100 radians per RI unit was calculated and is quite remarkable, since it is much higher than in traditional optical methods, e.g., surface plasmon resonance or total internal reflection ellipsometry. Relatively large standard deviation values are due to a rough estimation of a number of periods of oscillations in the output waveforms.
