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16.1 Introduction

An optical fiber sensor is a sensor that uses optical fibers as a sensing element and also, in many cases, as a means of relaying signals from a remote sensor to an electronic module called an interrogator that processes the signals. The fiber sensors have been used because of their small sensing element (sensor head) size and high sensitivity. Moreover, electrical power can be supplied to the remote location of the signal processor and light source, not to the location of sensing head. Another advantage is that many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor head or by measuring the time delay as light transmits along the fiber through each sensing element [1]. In addition to these properties, the fiber sensors have the advantages of light weight, structural versatility, immunity to electromagnetic interference, large bandwidth and environmental ruggedness [2]. To implement much higher sensitivity for some applications, fiber-optic surface plasmon resonance (SPR) sensors with micro- and nano-structures were introduced in 1983 as shown in Fig. 16.1. Since then, surface plasmons, which have strong evanescent field at the interface between metallic substrate and sensing medium, have been employed in a variety of fiber sensors [3–5].

Fig. 16.1
figure 1

The history of fiber-optic and fiber SPR sensors

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In this chapter, we mainly describe the SPR fiber sensor as a sensing element which can measure the refractive index (RI) change of a sample medium. First, the fundamental principle of SPR is briefly explained and then various approaches to improve the sensitivity of conventional SPR fiber sensors are illustrated. For achieving sensitivity improvement in SPR fiber sensors, there are two key agendas—achieving high overlap of plasmonic waves with sample medium (to be measured) and achieving low loss. High overlap and low loss are necessary to attain high sensitivity and enough optical power of guided-light systems [6–8]. In an attempt to increase the overlapping area, the fiber cladding is modified through different approaches such as cladding-off, side-polishing, tapering, and angled fiber tip. Other approaches deal with the plasmonic structures such as metallic gratings, metallic nanoparticles and nanoholes or use gratings made in the core or cladding of optical fiber for efficient coupling of light with plasmonic waves.

Many developments on hybrid fiber SPR sensors can also be found in recent literatures [9–12]. Here we provide an integrated outlook of fiber-optic sensors advanced by micro- or nano-structure optical fibers with regard to physical characteristics, fabrication methodologies and sensing performance.

16.2 Fundamentals of Fiber-Optic Surface Plasmon Resonance Sensors

In this section, we discuss some basic principles of SPR in fiber based SPR sensors. An optical fiber is composed of a core and a cladding which has a slightly lower RI compared with that of the core. Light propagation is confined within the fiber due to total internal reflection (TIR). In typical SPR fiber sensors, the silica cladding is removed to reveal a small portion of the fiber core and a thin metallic layer is coated on it. The metallic layer is enclosed by a sensing layer, i.e., medium to be measured. When a light is launched in a given sensor, the evanescent field from the guided core mode excites surface plasmons at the interface of metal and sensing layers. The coupling efficiency of the core mode to the surface plasmons strongly depends on the wavelength, fiber parameters, sensor structure, and metal layer properties.

16.2.1 Basic Principles of Surface Plasmon Resonance

SPR refers to the excitation of surface plasmon polaritons (SPPs) which are electromagnetic waves coupled with collective electron oscillations and trapped along metal-dielectric interfaces. SPPs are guided along the metal-dielectric interfaces in the same way that light is guided by an optical fiber, with the unique property of subwavelength-scale confinement perpendicular to the interface [13]. Fundamentally, the SPPs are excited by a longitudinal (TM- or p-polarized) wave that has a surface normal electric field component as shown in Fig. 16.2. The magnitude of electric and magnetic fields is at its maximum at the interface between the metal and dielectric layers. The propagation constant of the surface plasmon wave propagating along the metal-dielectric interface is expressed by

$$ k_{sp} (\omega ) = \text{Re} \left( {\frac{\omega }{c}\sqrt {\frac{{\varepsilon_{m} \varepsilon_{s} }}{{\varepsilon_{m} + \varepsilon_{s} }}} } \right), $$
(16.1)

where \( \varepsilon_{m} \) and \( \varepsilon_{s} \) are the permittivity of the metal layer and dielectric (sample) medium, respectively, ω is the frequency of incident light, and c is the speed of light in vacuum.

Fig. 16.2
figure 2

Surface plasmon polaritons propagating along the interface between the dielectric and metal layers. The penetration depths to the dielectric and metal layers are denoted by \( \delta_{d} \) and \( \delta_{m} \), respectively

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16.2.2 Schematic and Sensing Principles of Fiber SPR Sensor

There are a few configurations that can excite surface plasmons. The general approach is the attenuated TIR method based on a prism coupler, called the Kretschmann method, as shown in Fig. 16.3a. Optical SPR fiber sensors are fundamentally analogous to Kretschmann’s prism structure, where the prism is substituted by a fiber core, as represented in Fig. 16.3b. As shown in the schematic of SPR fiber sensor, the cladding has been etched off and the core is symmetrically coated with noble metal film.

Fig. 16.3
figure 3

Schematics of a Kretschmann configuration, b SPR fiber sensor and c sensing principle associated with SPR

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In the configuration of Fig. 16.3a, the component of propagation wave-vector of light along dielectric medium should be equal to the propagation wave-vector of the surface plasmons in order to excite them. A prism with a high dielectric constant is required because the incident light directly from air or sample region cannot excite SPPs at the metal-air or metal-sample interface. When an angle of incident light beam is greater than the critical angle at the prism-air interface, the TIR of light beam can be observed. However, at a resonance angle the incident light excites the SPPs at the boundary of the metal layer and outside sample region. The evanescent tail of plasmonic waves as shown in Fig. 16.3a experiences the RI of the sample region. Hence, the effective propagation constant of the SPPs strongly depends on the RI of the sample region.

The interface-directional propagation constant of the incident wave at the interface is given by

$$ \,k_{i} (\omega )\, = \,\frac{\omega }{c}\sqrt {\varepsilon_{p} } \sin \theta , $$
(16.2)

where \( \varepsilon_{p} \) indicates the dielectric constant of the prism material and θ is the angle of incident beam. The surface plasmon wave can be excited when the  (16.1) and (16.2) are equal:

$$ \text{Re} \left( {\sqrt {\frac{{\varepsilon_{m} \varepsilon_{s} }}{{\varepsilon_{m} + \varepsilon_{s} }}} } \right) = n_{p} \sin \theta , $$
(16.3)

where \( n_{p} \left( { = \sqrt {\varepsilon_{p} } } \right) \) is the RI of the prism in Fig. 16.3a or RI of the fiber core in Fig. 16.3b. Hence, the SPP excitation condition sensitively responds to any change in the boundary condition. For example, the variation of RI in the sample medium, resulting from the biological interactions or reactions, leads to the change in the propagation constant of surface plasmon wave. Another key issue is how far the surface plasmon wave penetrates. As mentioned above, the surface plasmon wave has a maximum field at the interface and decays on both sides of metal and dielectric region. The depth of penetration (defined as the depth for which the field is decreased by exp(−1) when compared with the field at the interface) through a dielectric (sample) medium can be given as

$$ \delta_{d} = \left[ {\text{Re} \left( {j\frac{2\pi }{\lambda }\frac{{\varepsilon_{s} }}{{\sqrt {\varepsilon_{m} + \varepsilon_{s} } }}} \right)} \right]^{ - 1} . $$
(16.4)

If the normalized reflected intensity in Fig. 16.3a is measured as a function of incident angle θ by keeping other parameters and components unchanged, the sharp resonance dip is observed at the resonance angle θ res due to an efficient transfer of energy to surface plasmons. When the wavelength interrogation method is used, the shift of resonance wavelength in response to the variation of RI in sample medium can be observed as in Fig. 16.3c.

To demonstrate the capability of SPR fiber sensor, sensitivity is very important for the evaluation of optical sensors. There are several ways to define the sensitivity. Here follows three examples: The first is to define it as the resonant angle shift with respect to the variation of surrounding RI as follows:

$$ S_{\theta } \, = \,\frac{{\partial \theta_{res} }}{{\partial n_{s} }}. $$
(16.5)

Another definition of sensitivity in the spectral interrogation with wavelength modulation is the ratio of the wavelength shift to the variation of surrounding RI.

$$ S_{\lambda } \, = \,\frac{{\partial \lambda_{res} \,}}{{\partial n_{s} \,}}\,. $$
(16.6)

When using a broadband light source, the figure of merit (FOM) defined as the ratio of (16.6) to the resonance width in the spectrum can be useful:

$$ FOM\, = \,\frac{{S_{\lambda } }}{{\Delta \lambda_{res} \,}}. $$
(16.7)

The third definition of sensitivity is given by

$$ S_{R} \, = \,\frac{\partial R\,}{{\partial n_{s} \,}}, $$
(16.8)

where R is the reflectivity of right at resonance wavelength. The input light wavelength is matched to the SPR wavelength when there is no change in RI of sample. If there is a change in RI of sample, the reflectivity of light is increased because the resonance wavelength is changed. Hence, for an incident light with given wavelength, the reflectivity increases with the change in RI of sample. When this is applied to optical SPR fiber sensor of Fig. 16.3b, the reflectivity becomes transmission coefficient through the optical fiber because the reflected light at the core-metal boundary propagates through the optical fiber. Hence, in that case, the following notation is more appropriate:

$$ S_{T} \, = \,\frac{\partial T\,}{{\partial n_{s} \,}}, $$
(16.9)

where T denotes optical power transmission coefficient through the optical fiber. In some cases the sensitivity is represented as the minimum detectable change in RI. In fact this is the number the user of sensor is most interested.

16.3 Various Micro- and Nano-Structured SPR Fiber Sensors

16.3.1 Micro- and Nano-Structured SPR Fiber Sensors Based on Fiber Shaping

Optical fiber is a useful substitute to waveguide to easily induce the evanescent field and SPR. Compared with the prism SPR sensor structure, shortcomings in fiber SPR sensors including low sensitivity and detection limit are addressed. In addition, it is difficult to control the angle of the incident light, which allows the resonance curve of transmission spectrum broader. Hence, various approaches to overcome the drawbacks of SPR fiber sensors have been reported. There are mainly two categories to distinguish SPR fiber sensors as presented in Fig. 16.4a: One is fiber shaping such as tapering, D-shaping and side-polishing. Another is adding distinct structures featured by fiber gratings, subwavelength metallic gratings or the use of metallic layers in photonic crystal fibers. Reported sensitivity in various types of SPR fiber sensors is presented in Fig. 16.4b, showing the broadband wavelength range from visible to infrared regime. In attempt to resolve low sensitivity issue in conventional fiber sensors, micro- or nano-structured SPR fiber sensors have been continuously exploited.

Fig. 16.4
figure 4

a Various SPR fiber sensors and b reported sensitivity with respect to wavelength and sensing structure (LPG long-period grating, RIU refractive index unit)

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To boost interactions between guided light and sensing materials, the fiber cladding can be taken off by polishing. Substantial overlapping area between sensing material and optical field can be achieved. It is capable of enhancing the sensitivity. In a tapered fiber, the power transmission is reduced in comparison with normal fiber. But, the transmission strongly depends on the outside material, which enables us to use it as a sensor. The small changes in the RI or the thickness of this overlay remarkably influence on the transmission properties in the multimode central region. In the tapered region, a critical factor to be addressed is taper ratio which determines the sensitivity in this sensor system. In Fig. 16.5, we can identify that the sensitivity increases with the taper ratio increments in a given tapered fiber geometry [14].

Fig. 16.5
figure 5

a Tapered fiber and b sensitivity plot according to the taper ratio in SPR fiber sensor. Metal-coated length is a third of whole tapered length in step-index multimode fiber. Given fiber core diameter is 600 µm and gold layer thickness is 50 nm. ©IEEE [14]

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However, the decrease in signal light transmission with the increase of taper ratio would cause deterioration of signal-to-noise ratio in detection. The enlargement of the area of evanescent waves in contact with sensing regions is attributed to tapering fiber. At the metallic layer, incident wave vector and corresponding resonance wavelength shift rely on the taper ratio. As a consequence, sensitivity in the spectral interrogation method directly depends on the taper ratio. The modified fibers also involve side-polished fibers, D-shaped fibers and U-shaped fibers. Figure 16.6 describes the general and advanced configurations of SPR fiber sensors incorporating tapered fibers and the schematic of U-shaped fiber. A typical tapered fiber SPR sensing probe is shown in Fig. 16.6a [15, 16]. Changing the profile of tapered SPR sensing probe can also influence on the sensitivity of the sensor [15]. To improve the sensitivity, the SPR probe of uniform core with metallic coating sandwiched between two unclad tapered fiber regions was reported as shown in Fig. 16.6b. In this unique geometry, all guided rays can propagate up to the output end of the fiber. This is achieved by selecting the minimum value of the radius in the uniform core of the sensing region. Rays in the sensing region propagate close to the critical angle of the region. In Fig. 16.6c, an SPR fiber sensor with uniform metal coated U-shaped probe is adopted. The bending radius in U-shaped probe makes it possible to tune the sensitivity of the probe [17]. Furthermore, one-side metal coated fibers with and without remaining cladding, and structures with modified fiber tips such as flat or angled structures have been proposed [18, 19].

Fig. 16.6
figure 6

SPR fiber sensors with a tapered probe, b uniform sensing probe sandwiched between two tapered fiber regions, and c U-shaped fiber sensing probe

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The side-polished fibers including D-shaped fibers in Fig. 16.7 are usually fabricated by CO2 laser machining or femtosecond laser. Traditionally, the plastic cladding of optical fiber was easily stripped to expose the fiber core. The operating sensing principle in side-polished fibers is based on attenuated TIR via multiple internal reflections along the fiber. The attenuated light intensity linearly responds to the increase of surrounding RI. The loss of light energy caused by the sensing portion of the fiber is detected by a sensor interrogation system [20–24]. The ability of multiple D-shaped zones in multi-mode fiber as a high sensitivity RI sensor was also demonstrated in Figs. 16.7a–b. The sensor resolution can be optimized with respect to the number of D-shaped zones in Fig. 16.7b [22]. When the number of D-shaped zones augments, the sensor resolution improves and then becomes worse after exhibiting the best value at five D-shaped zones in this work. In general, single-mode fiber SPR sensors are more sensitive than multi-mode fibers. Single mode tapered fiber with uniform waist with asymmetric metallic coating in Fig. 16.7c offers advanced immunity to deformation of optical fibers [25]. This desirable performance results from the use of polarization-maintaining optical fiber and thus enables more accurate SPR measurements. Side polished single-mode fiber SPR sensor with a thin metal overlayer in Fig. 16.7d was introduced as well. In this configuration, the main feature is single-polarization single-mode.

Fig. 16.7
figure 7

Shaping fiber geometry with metal layer. a D-shaped fiber SPR sensor and b sensor resolution according to the number of D-shaped zones (Reproduced with permission from sensors; published by MDPI, 2011). c Single-mode tapered fiber with uniform waist made of high birefringence optical fiber. Asymmetric gold layer is deposited on it. d Side-polished single-mode fiber SPR sensors

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Aforementioned, many groups have reported the adjustment of resonance wavelength in the transmission spectrum and the improvement of sensitivity with regard to an overlayer in SPR fiber sensors. The overlayer is basically required to protect the metallic surface against oxidation. But, it needs an improvement to perform the behavior of sensor. For example, bimetallic layers with silver and gold are deposited on the optical fiber [26–28]. A double-clad fiber, characterizing a structure composed of three layers was proposed [29]. It is usually available in the high power fiber laser system with the core doping of rare earth elements and has the jacket made of a polymer which has lower index than the cladding. Thus, it can guide the incident light into the cladding as well as the core. The polymer can act as an outer cladding. These kinds of structures constitute either overlayer or multilayer structures. The overlayer or multilayer make it possible to tune the measurable range in SPR fiber sensor.

16.3.2 Micro- and Nano-Structured Fiber SPR Sensors Based on Gratings

In this section, SPR fiber sensors with various types of gratings are introduced. Gratings can couple light from the core mode into various cladding modes that might induce SPPs on metal-sample interface. In some cases, metallic gratings can compensate the phase mismatch between incident light and SPPs. In Fig. 16.8, fiber optic SPR sensors equipped with various gratings are illustrated. Long-period gratings (LPGs) in Fig. 16.8a typically have grating periods of hundreds of micrometers [30–32]. Although fiber Bragg gratings are widely used for optical sensors detecting strain and temperature, LPGs are more suitable to SPR fiber sensors. The LPG couples light from a guided mode into forward propagating cladding modes. The degree of coupling from the guided mode to cladding modes is wavelength dependent. LPGs have broad resonance spectrum to cladding modes. Meanwhile, other types of gratings advanced from the fundamental gratings have also been studied. Both tilted fiber gratings in Fig. 16.8b and metallic Bragg gratings exhibit multiple dips in the transmission spectrum of SPR fiber sensors [33, 34]. These gratings are capable of producing the sharp resonance dips and a superior value of signal to noise ratio (SNR), though the sensitivity is slightly deteriorated. Titled fiber gratings have a certain tilt angle between grating plane and fiber cross section, giving rise to the occurrence of more complicated mode coupling. It can be modeled as a reflection of core mode to backward propagating cladding modes.

Fig. 16.8
figure 8

Fiber SPR sensors using a long-period grating, b tilted grating, and c subwavelength metallic grating

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In Fig. 16.8c, subwavelength metallic gratings are adopted into the optical fiber to enhance the sensitivity [35]. We numerically investigate the subwavelength metallic gratings in Fig. 16.8c via rigorous coupled wave analysis, to verify the sensing capability. The core is symmetrically enclosed by subwavelength metallic gratings whose dimensions are optimized. The structural parameters of given subwavelength gratings were set to the grating depth of 40 nm, the engraved part of 55 nm, and the period of 239 nm. Incident angle was set to 85° to match the resonance wavelength for the case of multi-mode fiber. Such a large incident angle allows subwavelength metallic gratings to be applied in SPR fiber sensor. TM-polarized plane waves with a fixed incident angle but different wavelengths were incident and reflected at the metallic grating layer. The calculated reflectance depending on the period and incident wavelength is shown in Fig. 16.9a. Strong resonance is observed within the wavelength range between 1,000 and 1,060 nm. As the period length in given subwavelength metallic gratings increases, effective dielectric constant depending on grating variables gets smaller. As a consequence, the resonance wavelength is slightly blue-shifted as the period length increases. For the purpose of examining the sensing capability in a given geometry, the reflectance spectra in response to the variation of surrounding RI from 1.3 to 1.36 are presented in Fig. 16.9b. The resonance wavelength is red-shifted in the near-infrared wavelength range with respect to the increase of corresponding RI. The calculated sensitivity in this subwavelength metallic grating structure shows approximately 3,583 nm/RIU.

Fig. 16.9
figure 9

a Reflectance according to the period of grating and incident wavelength, b reflectance spectra in response to the variation of surrounding refractive index from 1.3 to 1.36

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To identify the electromagnetic field distribution arisen from this given structure, Fig. 16.10 provides the field distributions at the resonance wavelength of 1,000 nm. It implies that the field is coupled in the surface plasmon mode and produces strong evanescent wave in near-field regime. This evanescent field is in charge of the sensitivity in response to surrounding RI change.

Fig. 16.10
figure 10

Electromagnetic field distributions a x-component distribution of electric field, b z-component distribution of electric field, and c y-component distribution of magnetic field in x–z plane of given subwavelength metallic gratings when the incident wavelength is 1,000 nm and surrounding refractive index is 1.33

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In fabrication of metallic nanostructures onto the fiber end shown in Fig. 16.11, a standard optical communication fiber is used for this fiber-optic sensing probe. The diameter of fiber core is 9 μm and the cladding is 125 μm. Typically, gold is chosen as the deposited metal on the fiber end face, because it is sensitive to a change in the RI of the sensing layer and chemically stable. The fabrication procedure is as follows: the optical fiber was stripped of its polymer buffer layer and manually cut with a fiber cleaver for the purpose of realizing a flat and smooth fiber end surface. Next, the fiber end probe was coated with metal by e-beam evaporator. Before depositing metal, a titanium layer of 5 nm was first deposited on the cleaved fiber end. It plays as the adhesion layer between the fiber end face and metal layer. Then, gold is subsequently deposited over the titanium layer, approximately 850 nm thick. In patterning the nano-sized structure, focused ion beam milling is used. By manipulating the shape and size of nanostructures in the metal layer, the location of the resonance wavelength can be adjusted.

Fig. 16.11
figure 11

SEM images of various nanostructures fabricated on an optical fiber tip. a Fiber end tip with a 2D grating structure located in the center of cross section, b one-dimensional slit grating structures on a thick metal layer on a fiber end, c two-dimensional grating structures on the corresponding fiber end, and d split ring structures on the corresponding fiber end. The thickness of metal layer is 850 nm

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Figure 16.11a–d present the SEM images of various periodic nanostructures mounted on the optical fiber tip. Figure 16.11a shows the overall fiber probe end which is patterned in the core region. The fabricated area is 13 μm by 13 μm square, which is enough to cover the fiber core. As mentioned above, these gratings mounted onto the fiber end are capable of enhancing the sensitivity [36]. In addition, the resonance wavelength can be tuned by engineering the dimension of metallic gratings.

16.3.3 Nano-Structured LSPR Fiber Sensors

Not only SPR but also localized SPR (LSPR) has been of interest for chemical and biological sensing applications. LSPR arisen from a metallic nanoparticle or nanostructure refers to collective oscillation of conduction electrons in a metallic nanoparticle or nanostructure at a resonant frequency which is determined by its size, shape, composition, and the RI of surrounding dielectrics. Sharma and Gupta proposed the use of metallic nanoparticle film for the purpose of enhancing the sensitivity in the optical fiber sensor shown in Fig. 16.12a [37]. Metallic nanoparticles are coated on the unclad portion of the fiber as presented in Fig. 16.12. On the other hand, the array of nanoapertures or nanoholes mounted on the tip of gold-coated optical fibers can also be employed as highly sensitive RI sensors as shown in Fig. 16.12b. As the localized RI of the medium in the vicinity of gold film possessing the array of nanoapertures is varied, a shift is detected in the transmission spectrum of metallic nanostructures. This sensor based on the arrays of nanoapertures is beneficial because several extraordinary transmission peaks in the transmission spectrum can be monitored, providing multiple reference points in response to the RI of the medium around the fiber.

Fig. 16.12
figure 12

Nano-structured LSPR fiber sensor a with metallic nanoparticle layer, b metallic nanohole array mounted on a fiber tip, and c hybrid metallo-dielectric nanostructures

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Fundamentally, transmission of light through aperture smaller than the wavelength of incident light was discussed by Bethe [38]. According to him, the transmission coefficient of light at normal incidence to an aperture is proportional to the fourth power of the ratio of aperture radius to wavelength, which means that the transmission of light diminishes rapidly with the decrease of aperture radius. However, when the surface plasmon interaction is taken into account, considerably more light can be transmitted through the array of subwavelength apertures at the wavelength which depends on the periodicity of the array and material parameters. Therefore, the peak transmission and corresponding wavelength are mediated by the surrounding RI and can be made use of as the basis for sensors. Therefore, the binding of organic and biological molecules on the metallic surface could be monitored via the arrays of nanoholes in a gold film [39, 40]. In Fig. 16.12c, the sensing platform based on the integration onto the optical fiber tip of two-dimensional hybrid metallo-dielectric nanostructures supporting LSPR is shown and experimentally demonstrated [41]. This platform provides a sensitivity of ~125 nm/RIU for detecting the changes in bulk RI of different chemicals surrounding the fiber-tip device. For the sake of reliable fabrication of both dielectric and metallic nanostructures directly on the fiber tip, alternative approaches based on direct-write patterning of the fiber tip have been explored.

16.4 Other Structures of SPR Fiber Sensors

Besides the fiber-optic SPR sensors explained above, there are many other proposals and structures for micro- or nano-structured fiber-optic SPR sensors. Here let us take the examples of distinct photonic crystal fiber (PCF)-based or microstructured SPR sensors. PCFs or holey fibers have multiple holes along the axial direction in cladding or core of optical fibers. There have been many researches on the use of holes for gas or liquid sensing because holes filled with gas or liquid affect the properties of guiding optical mode [42, 43]. Metallic layer coated in the holes of PCF or microstructured fiber allows the sensitivity to the RI of surrounding medium to increase due to SPR effect.

Figure 16.13 shows various exemplary structures relevant to PCF-based or micro-structured SPR sensors. The sensor in Fig. 16.13a consists of selectively metal-coated air holes containing analyte channels [44]. It helps to enhance the phase matching between the plasmonic mode and the core-guided more. Desirable sensitivity as high as 5,500 nm/RIU was reported in this sensor. In Fig. 16.13b, two large semicircular metalized channels are integrated into the fiber structure to raise microfluidic flow rate and the efficiency of plasmonic excitation [45]. SPR-based three-hole microstructured optical fiber was proposed and numerically demonstrated with 1 × 10−4 RI resolution for aqueous analytes in Fig. 16.13c [46]. Another PCF based on SPR sensor is found in Fig. 16.13d. PCF with the metal nanometer film coated on large central air core of photonic bandgap and infusion of the glycerin liquid has been proved to be temperature sensor with liquid core [47]. Hence, the performance of PCF-based SPR sensor can take the full advantage of adjusting the thickness of the metal layer, over-layer and hole diameter.

Fig. 16.13
figure 13

Various PCF-based or microstructured SPR sensors: a micro-structured SPR fiber sensor with large metal-coated holes filled with analyte, b micro-structured SPR fiber sensor integrated with large semicircular metalized channes, c SPR-based three-hole micro-structured fiber sensor, and d PCF-SPR temperature sensor with liquid core

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16.5 Conclusion and Prospect

In this chapter, we overviewed micro- and nano-structured fiber optic SPR sensors with regard to the types, characteristics, and geometry. Assorted fiber SPR sensor structures are briefly organized in Table 16.1. The collaboration of SPR technique with optical fiber technologies has brought considerable achievements in sensing applications. Novel technologies will be continuously tried to be combined with conventional fiber SPR sensors for further advancement. Although the fiber SPR sensors have not been competitive to commercial optical sensor yet, many researchers believe that they will be one of the promising sensor technologies, representing the unique properties of real-time and label-free detection.

Table 16.1 Performance of micro- and nano-structured fiber SPR sensors
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