Abstract

We report on recent investigations regarding ultra-simplified designs for microstructured optical fiber sensors. This minimalist approach relies on the utilization of capillary-like fibers—namely embedded-core fibers, surface-core fibers, and capillary fibers—as platforms for the realization of sensing measurements. In these fibers, guidance of light is accomplished in an embedded or surface germanium-doped core or in the hollow part of capillaries. External stimuli can alter fiber wall thickness and/or induce birefringence variations, allowing, for the embedded-core and capillary fibers, to operate as pressure or temperature sensors. For the surface-core fiber design, the interaction between the guided mode and external medium allows the realization of refractive index sensing either by using fiber Bragg gratings or surface plasmon resonance phenomenon. Also, we report the realization of directional curvature sensing with surface-core fibers making use of the off-center core position. The attained sensitivities are comparable to the ones obtained with much more sophisticated structures. The results demonstrate that these novel geometries enable a new route toward the simplification of optical fiber sensors.

Keywords: fiber optics, fiber optics sensors, fiber Bragg gratings, surface plasmon resonance, microstructured optical fibers

## 1. Introduction

The increasing need for the development of optical sensors motivates intense research in this area. Particularly, great efforts have been observed in the field of optical fiber sensors since they can provide numerous advantages such as high sensitivity and improved resolution. Moreover, fiber optics are immune to electromagnetic interference and suitable to be used in harsh environments.

In this context, microstructured optical fibers have much contributed to the development of sensors due to their huge design freedom. Thus, numerous sensing configurations have been reported in the literature to be able to probe variations of a great diversity of parameters such as temperature, strain, hydrostatic pressure, curvature, and refractive index. Regarding pressure sensors, for example, successful approaches are to use photonic-crystal fibers (PCFs) with triangular-shaped microstructures [1] or side-hole PCFs [2]. In these configurations, the hydrostatic pressure application generates asymmetric stresses distributions within the fiber

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*Applications of Optical Fibers for Sensing*

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s150818666

JLT.0082.921915

structure and changes its birefringence via the photoelastic effect. Additionally, metal-filled side-hole PCFs have been demonstrated to act as high-sensitivity temperature sensors [3, 4]. In this approach, the metal expansion inside the fiber structure induces an asymmetric stress distribution within the same (due to the different thermal expansion coefficients of silica and of the metal). It entails changes in the fiber birefringence which can be optically probed and related to the temperature variation through a suitable calibration.

fiber structures, which generate, by virtue of the photoelastic effect, birefringence

Minimalist Approach for the Design of Microstructured Optical Fiber Sensors

Figure 1d presents a forth structure, which is simply a capillary fiber [12]. Here, we study the guidance of light in the hollow part of the capillary and investigate how the optical response of the fiber is changed when it experiences temperature variations. It is worth saying that, in this investigation, we employed polymethyl methacrylate (PMMA) capillaries. This choice was due to the higher thermal expansion and thermo-optic coefficients of PMMA when compared to

In the next sections, we specifically describe the principle of operation of each configuration. Moreover, we present theoretical and experimental results and com-

The application of pressure to capillary fibers generates displacements on their walls. This, in turn, induces an asymmetric stress distribution within the capillary structure which, due to the stress-optic effect, entails birefringence variations in it. As described in [7], an analytical model can be used to account for the material birefringence variations (ΔBmat) in pressurized capillaries. To do that, we can employ Eq. (1), where C1 and C2 are the elasto-optic coefficients (C1 <sup>=</sup> �0.69 � <sup>10</sup>�<sup>12</sup> and C2 <sup>=</sup> �4.19 � <sup>10</sup>�<sup>12</sup> Pa�<sup>1</sup> for silica), and <sup>σ</sup><sup>x</sup> and <sup>σ</sup><sup>y</sup> are the pressure-induced

ΔBmat ¼ ð Þ C<sup>2</sup> � C<sup>1</sup> σ<sup>x</sup> � σ<sup>y</sup>

The stresses σ<sup>x</sup> and σ<sup>y</sup> can be obtained from Lamé solution inside thick-walled tubes subjected to pressure [15]. The resulting expression for the material birefringence at a position x on the horizontal axis is shown in Eq. (2), where rin and rout are the inner and outer radius of the capillary, and pgauge = pout – pin (pin and pout are the

By observing Eq. (2), we see that, when maintaining rin constant, |ΔBmat| will be greater for higher rin/rout ratios. It means that the analytical model predicts that the change in the birefringence is increased for thin-walled capillaries. Moreover, we see that, for fixed rin and rout values, the change in the birefringence is higher for

rout � �<sup>2</sup> " #�<sup>1</sup>

� � (1)

r2 in

<sup>x</sup><sup>2</sup> (2)

strain levels within the core can be probed by a FBG [9].

pare them with the ones available in the literature.

inner and outer pressure levels) [7].

3. Embedded-core capillary fibers for pressure sensing

stresses on the horizontal and vertical directions, respectively [13, 14].

<sup>Δ</sup>Bmat <sup>¼</sup> <sup>2</sup>ð Þ <sup>C</sup><sup>2</sup> � <sup>C</sup><sup>1</sup> pgauge <sup>1</sup> � rin

positions (x) which are closer to the inner wall of the capillary [7].

Figure 1c shows a diagram of the so-called surface-core fiber [9]. In this structure, the fiber core is placed on the fiber external surface. As here the core directly interfaces the external medium, the evanescent field of the guided optical mode permeates the external environment. Surface-core fibers are, then, a suitable platform for refractive index sensing. A possible approach is to imprint fiber Bragg gratings (FBGs) in the fiber core and measure the sensor spectral response as the external refractive index is altered [9]. A second approach is to perform plasmonic sensing by metal-coating the surface-core fiber with a nanometric metallic layer [10]. Additionally, the off-center position of the fiber core permits the surface-core fibers to act as directional curvature sensors. In this case, the curvature-induced

variations in the fiber core.

DOI: http://dx.doi.org/10.5772/intechopen.81265

silica.

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Microstructured optical fibers can also be used as refractive index sensors. Among those ones, plasmonic sensors acquire great importance due to the high sensitivities they can achieve. In these platforms, selected regions of the fiber structure are coated with a nanometric-thick metallic layer to provide coupling between the optical mode and a plasmonic mode. Possible approaches are to coat the inner holes of microstructured fibers [5] or to open up a channel in the fiber structure to expose the fiber core for metallic nanospheres immobilization [6].

However, the microstructured optical fibers usually employed in the sensing schemes described above are sophisticated, which demand great technical efforts for their fabrication. Here, alternatively, we present sensors which are endowed with ultra-simplified microstructures based on capillary-like fibers (embedded-core fibers [7, 8], surface-core fibers [9, 10], and capillary fibers [11]). As it will be shown in the following, even though these configurations are very simple, the attained sensitivities are high when compared to other fiber sensors based on more complex structures. Thus, we can identify the use of capillary-like fibers as a new avenue for obtaining highly sensitive fiber sensors with simplified fabrication process.
