# Fabry-Perot Interferometer Based on Suspended Core Fiber for Detection of Gaseous Ethanol
## 2 Materials and Methods  
The suspended core fiber (SCF) used in this work, whose cross-section is shown in Figure 1, was fabricated at the Leibniz Institute of Photonic Technology. It consists of a pure silica core with a diameter of \(\sim\)2 \(\upmu\)m, surrounded by three air holes with a dimension of \(\sim\)26 \(\upmu\)m. Between the holes, there is a bridge with a thickness of \(\sim\)360 nm, which acts as a supporting structure for the suspended core. The silica cladding of this optical fiber has a thickness of \(\sim\)36.4 \(\upmu\)m and an outer diameter of \(\sim\)125 \(\upmu\)m. The optical fiber was fabricated via stack-and-draw technique, and coated with a standard acrylate.  
To fabricate the sensing head, the SCF was firstly stripped and spliced to a section of a commercial single mode fiber (SMF28, from Corning, New York, USA) using the manual mode of the fusion splicer. The splicing power and time were set to 10 arb. units and 500 ms, respectively. For comparison purposes, the splicing parameters in the automatic mode are 20 arb. units and 2000 ms. The splicing was done with a lateral offset to minimize potential damage produced by the arc discharge to the suspended core. Therefore, the arc discharge was mainly applied in the SMF, as evidenced in Figure 2a. After the splicing procedure, the SCF was cleaved under a magnifying lens, assuring a balanced compromise between the sensor compactness and spectral readability (Figure 2b). Several sensing heads were produced and characterized to achieve the optimal parameters (both splicing parameters and cavity length). Figure 2c presents the micrograph of the sensing head, which has a length (\(L_{\text{FP}}\)) of 365 \(\upmu\)m.  
Figure 1: Microscope photograph of the SCF cross-section.  
Figure 2: Schematic representation of the sensor fabrication process: (**a**) SMF and SCF samples spliced by electric arc discharges; (**b**) Cleaving of the SCF; (**c**) microscope photograph of the final sensor, with a SCF length of 365 \(\upmu\)m.  
The sensor was then connected in a typical reflection scheme, has shown in Figure 3. The broadband optical source (Amonics LS--CL-17-B-FA, Hong Kong), centered at 1570 nm, with a bandwidth of 80 nm, was connected to the first port of an optical circulator. The sensor was connected to the second port, whereas the optical spectrum analyzer (OSA, Anritsu MS9740A, Kanagawa, Japan), operating with a resolution of 0.02 nm, was connected to the third port.  
The sensor reflection spectrum, shown in Figure 4, shows an interferometric pattern typical of a Fabry-Perot interferometer. For simplicity purposes, the two-wave interferometer was considered, which allows us to consider the wavelengths of two adjacent maxima, and the expression:  
\[n_{\text{eff}}=\lambda^{2}/(2L_{\text{FP}}\Delta\lambda), \tag{1}\]  
to calculate the effective refractive index of light propagating in the suspended core, obtaining a value of 1.433, was used.  
To attain atmospheres with different concentrations of ethanol molecules, a set of 11 ethanol-water mixtures was done, starting with pure ethanol (99.9% purity from Sigma-Aldrich, St. Louis, MO, USA) up to 100% deionized water, in steps of 10 wt.% of ethanol. The mixtures were prepared with a magnetic stirrer and let to stabilize during 24 h at room temperature (\(\sim\)19 \({}^{\circ}\)C). The experiments were carried out at constant room temperature (\(\sim\)19 \({}^{\circ}\)C) by placing the sensor head in the gas atmosphere of the closed beakers, each one containing different ethanol concentrations. For each ethanol mixture, there was a waiting time of 90 min to achieve stabilization, recording several spectra during this time. Between  
Figure 3: Schematic representation of the sensor characterization to different concentrations of gaseous ethanol. The insets depict the sensor when in the presence of VOCs molecules.  
measurements, the sensor head was left in the ambient atmosphere for 10 min to eliminate cumulative fluctuations between solutions, mainly due to possible condensation effects.