Vapor Sensing with Polymer Coated Straight Optical Fiber Microtapers Based on Index Sensitive Interference Spectroscopy of Surface Stress Birefringence.

Alexandra Blank,Gabriel Guendelman,Yoav Linzon

doi:10.3390/s20092675

Abstract

Optical microfiber tapers provide an advantageous platform for sensing in aqueous and gas environments. We study experimentally the photonic transmission in optical fiber tapers coated with polymethyl methacrylate (PMMA), a polymeric material widely used in optical applications. We demonstrate a durable and simple humidity sensing approach incorporating tapered microfibers attached to silicon (Si) substrate coated with active polymer layer. A model is described for the load stress effect on the birefringence giving rise to interferences in the transmission spectra, strongly dependent on the coating layer thickness, and disappearing following its slow uniform removal. The sensing approach is based on characterization of the interference patterns observed in the transmission spectra of the taper in the NIR range. The device demonstrated persistent detection capability in humid environment and a linear response followed by saturation to calibration analytes. In each analyte of interest, we define principal components and observe unique calibration plot regimes in the principal component space, demonstrating vapor sensing using polymer coated microtapers.

Highlights

Sensing using optical methods has been an active field of research in recent years [1,2,3], due to the inherently superior capabilities offered by complex photonic structures [4,5]
We propose and demonstrate that in straight rather than folded microtapers, uniformly coated with active polymer layer, the swelling of the top layer exposed to the ambient vapor as a function of humidity in the chamber gives rise to interference patterns attributed to surface stress birefringence effect, with significant phase shifts linear with relative humidity (RH) below saturation, and parameters unique to different volatiles used
We have monitored the changes in the transmission spectrum of the device in terms of its baseline and interference patterns that have become smaller in dynamical range after coating thinning

Summary

Introduction

Sensing using optical methods has been an active field of research in recent years [1,2,3], due to the inherently superior capabilities offered by complex photonic structures [4,5]. We propose and demonstrate that in straight rather than folded microtapers, uniformly coated with active polymer layer, the swelling of the top layer exposed to the ambient vapor as a function of humidity in the chamber gives rise to interference patterns attributed to surface stress birefringence effect, with significant phase shifts linear with RH below saturation, and parameters unique to different volatiles used. Using the parameters: taper waist diameter 7 μm; taper waist length L = 7.62 cm; coating of PMMA A4 of mass density 1.18 (g cm−3 ); stress-optic coefficient C = 3.184 × 10−12 ; maximum polymer film thickness t = 450 nm, we obtain for stress-induced birefringence the value of ∆n = 2.3 × 10−6 (Figure 1d). In coated MKRs or MLRs, we estimate that a combination of the spectral effects (both resonances and stress-induced interferences) will simultaneously be observed

Device and Experiment Details

Results and Discussion

Conclusions