Optical Microfiber Sensors

Abstract

Optical microfiber is a class of specialty fibers, which is featured with wavelength scale diameters. With such small dimensions, the optical microfiber offers large fractions of evanescent fields and high surface field intensities, making it highly sensitive to disturbances in the surrounding medium. Thus, the optical microfiber is an ideal building block for high-performance photonics sensing devices. In this chapter, recent progress in optical microfiber-based sensors is reviewed. It starts with a brief introduction of the fundamental optical properties of optical microfibers and the well-developed fabrication techniques. Then, a brief summarization of the well-established microfiber-based refractive index sensing schemes is given, including working principles and sensing performances. Following this section, the latest progress on new effects and strategies for sensing enhancement is reviewed. In the last, the conclusions and an outlook are presented.

1 Introduction

Optical microfibers have been gaining tremendous attention in a wide range of research fields ever since its first discovery (Tong et al. 2003). With diameters close to the wavelength of light, these tiny optical fibers cannot confine the guided light tightly inside the fiber, like their telecommunication counterparts. Instead, the optical microfibers show a strong evanescent field, which means that a substantial amount of the guiding light can enter and interact with the surrounding medium. Besides, the optical microfibers also offers several unique properties, including small footprint, low optical loss, high nonlinearity, and high mechanical flexibility. All these superior properties make the optical microfiber a promising platform for high efficient light-matter interactions. Applications in fields ranging from optical sensing (Chen et al. 2019) to ultrafast laser (Luo et al. 2015b), nonlinear optics (Vienne et al. 2008), and optomechanics (Zhang et al. 2020) have been explored.

In particular, the small geometry size and the strong evanescent filed make the optical microfibers an ideal platform to perform sensing on the nanoscale (Tong et al. 2004). Any environmental parameter variations, external forces, object movements, as well as the chemical reaction that can alter the optical guiding property of the microfiber, can be easily detected. As such a perspective filed, the optical microfiber sensors have witnessed prosperity during the last decades, along with the improvement of the fabrication techniques. Multifarious novel microfiber sensors with unique advantages of ultra-small footprint, high sensitivity, and fast response have been explored. These sensing devices include microfiber modal interferometers (Yadav et al. 2014), Mach–Zehnder interferometers (MZI) (Hu et al. 2012), microfiber couplers (Tazawa et al. 2007), microfiber resonators (Sumetsky et al. 2006), microfiber gratings (Ran et al. 2011; Wang et al. 2017), and nanomaterial decorated microfibers (Yu et al. 2017; Zhou et al. 2019). These versatile sensors have found applications in temperature sensing, pressure sensing, strain sensing, flow rate sensing, refractive index (RI) sensing, as well as biochemical sensing. In this chapter, the recent progress in microfiber sensors is reviewed, with an emphasis on the basic sensing principles of the well-developed microfiber-based sensing devices and the newly developed strategies for sensing enhancement.

2 Basics of Optical Microfibers

The typical construction of an optical microfiber is depicted in Fig. 1, where the uniform microfiber lies in the middle region connects with two standard optical fibers through two conical tapers. When the standard optical fibers are tapered into microwires with diameters around the operation wavelength, the original core reduces to below 100 nm and becomes negligible. Thus, the whole microfiber works as the core, and the environmental medium (air/liquid) whose RI usually is lower than the fiber works as the new cladding.

Fig. 1

Schematic diagram of the optical microfiber

2.1 Optical Properties

2.1.1 Few-Mode/Single-Mode Operation

As the diameter of the microfiber is on the scale of the wavelength, only a limited number of guided optical modes can be supported. The number of the guided modes are determined by the V number, which is defined as (Tong et al. 2003):

$$V = \frac{\pi D}{\lambda }\sqrt {n_{\text{co}}^{2} - n_{\text{cl}}^{2} }$$

(1)

where D, λ, n_co, and n_cl are the fiber diameter, the wavelength in vacuum, core RI, and cladding RI, respectively. The critical condition for single-mode operation is V < 2.405 when all the high-order modes are cut off (Tong et al. 2003). The propagation constant, effective RI (ERI), and the spatial field distribution of the modes can be obtained by solving the Helmholtz equations (Tong et al. 2003). The modal dispersion curve showing the ERI of the supported modes in optical microfibers in water is depicted in Fig. 2. It is evident that when the diameter of the fiber is smaller than 2 µm, the single-mode operation can be achieved.

Fig. 2

Modal dispersion of an optical microfiber with a water cladding

2.1.2 Strong Evanescent Filed

The unique and attractive optical property of the optical microfiber is the strong evanescent field. It means that a substantial portion of the guided optical power can enter the surrounding medium and propagate along the fiber. The modal field distributions of the HE₁₁ mode for optical microfibers with a diameter of 2 µm and 0.8 µm are shown in Fig. 3. It is evident that the 2-µm-thick microfiber only possesses a small portion of the evanescent field, whereas for the 0.8-µm-thick nanofiber, more than half of the optical power is guided outside the fiber. The evanescent power ratio of the HE₁₁ mode for microfibers with different diameters is displayed in Fig. 4a. With the enhanced evanescent field, the penetration depth also increases significantly. For nanofibers thinner than 0.5 µm, the penetration depth can reach several microns, which is far more significant than the small value of half-wavelength, for the conventional evanescent field in a prism configuration. The strong evanescent field provides a perfect platform for light-matter interactions and further serves as essential building blocks for super sensitive sensing devices.

Fig. 3

Modal field distribution of the fundamental HE₁₁ mode for optical microfibers (in water)

Fig. 4

a Evanescent power ratio of the fundamental HE₁₁ mode. b The penetration depth of the evanescent field of the optical microfiber

2.1.3 Low Optical Loss

The insertion loss is also an essential parameter for the microfiber. A low optical loss facilitates the application of the optical microfiber in the field of lasers, nonlinear optics, and sensing. The optical loss mainly comes from the surface roughness-induced scattering. Utilizing the heating and pulling method, microfibers with surface roughness as low as sub-nanometer can be achieved, and the insertion loss can be as low as 0.06 dB (Hoffman et al. 2014), which can be almost ignored.

2.2 Fabrication Techniques

In order to manufacture optical microfibers with well-defined geometries and optical characteristics in a scalable manner, several methods have been developed. The most common fabrication techniques are the so-called heating and pulling method (Felipe et al. 2012) and the chemical etching method (Li et al. 2018d). As the dimension of these tiny microfibers is generally below 5 µm, less than 4% of the original diameter of the 125-µm-thick standard telecommunication optical fibers, the precise control of the diameter can be challenging. Therefore, in-line monitoring methods have been developed to monitor the diameter or the optical spectra of the fabricated microfibers during the fabrication process. With such thin diameters, the microfibers usually have low mechanical strength and are quite fragile. Thus, proper packaging is essential to keep the microfiber safe and stable.

2.2.1 Heating and Pulling Method

The typical fabrication process of the heating and pulling technique is displayed in Fig. 5a. Firstly, a section of standard optical fiber with the polymer coating removed in the central part is mounted on two translation stages and fixed using fiber holders. Secondly, a heating source is applied to the silica fiber to keep it above the glass transition temperature. Then, the fiber is pulled by the two translation stages, and the dimension of the melting central region reduces gradually as the two translation stages move apart. Figure 5b shows a micrograph of a typical optical microfiber fabricated by this method. The fabrication process usually takes about several minutes. The diameter of the microfiber can be controlled by adjusting the pulling velocity and the pulling duration. The width of the heating source determines the length of the uniform waist region. Various heating elements have been adopted, including oxyhydrogen flame (Felipe et al. 2012), ceramic heater (Ma et al. 2016), and the CO₂ laser beam (Xuan et al. 2009). Typically, the oxyhydrogen flame and the ceramic heater that provide millimeter-long heating regions are favorable for fabricating adiabatic optical microfibers. The CO₂ laser beam provides a heating width of as small as several hundreds of microns and is suitable for producing short and abrupt tapers. So as to obtain optical microfibers with the desired length, a scanning heating source is usually used, which can extend the effective width of the heating source. The most significant merit of the heating and pulling method is the sub-nanometer-scale surface roughness, which renders the microfiber with ultra-low optical loss.

Fig. 5

Adapted with permission Zhang et al. (2018a)

a Schematic diagram of the heating and pulling fabrication method. b A micrograph of an optical microfiber fabricated by the heating and pulling method.

2.2.2 Chemical Etching Method

The chemical etching technique is illustrated in Fig. 6a. Typically, a section of standard single-mode optical fiber with the desired length is stripped off the protective polymer coating and cleaned with acetone. Then, the bare silica fiber is placed inside the fluidic channel of a specially designed silicon chip and fixed by glue. Afterward, the hydrofluoric acid etching solution was added into the fluid cell, and the etching process begins. The diameter of the fabricated microfiber can be tuned by adjusting the etching duration. In order to achieve a proper diameter, a two-step etching strategy is adopted. In the first step, the course etching using high-concentration hydrofluoric acid is applied to the fiber. It takes about 50 min to decrease the 125 µm diameter to about 5 µm. In the second step, low-concentration hydrofluoric acid is added into the fluid cell to fine-etch the 5-µm-thick microfiber to the target dimension (Li et al. 2014). A typical image of a resultant sub-wavelength optical microfiber is depicted in Fig. 6b. The whole manufacture procedure takes about several hours, which is much longer than the heating and pulling method. However, the productivity can be easily improved by a parallel fabrication strategy (Li et al. 2018d). The transmission loss of the fabricated fibers is usually more significant than those fabricated by the heating and pulling method. However, one distinct advantage of the etching method is that the microfluidic channel for the fabrication can readily serve as channels for the infiltration of bio-samples. Thus, a further packaging procedure is eliminated.

Fig. 6

Adapted with permission Li et al. (2018d)

a Schematic diagram of the chemical etching method. b An SEM image of an optical microfiber fabricated using the chemical etching method.

2.2.3 In-Line Monitoring Technologies

The in-line monitoring systems can facilitate the fabrication process to enhance the capability in controlling the dimension and optical properties of the fabricated microfiber in a real-time manner. The widely adopted approach is connecting the two ends of the optical fiber to a light source and a detector, respectively. Thus, the optical loss and the output spectra can be measured in real-time, and the fabrication process can be terminated once the desired parameter is reached. This in-line monitoring system has been applied to both the heating and pulling method and the chemical etching method (Li et al. 2014). In order to control the diameter and the optical properties more precisely, advanced in-line measuring techniques have been developed. The cutoff of a high-order mode occurs at a determined diameter. Thus, by monitoring the cutoff effect of high-order modes and the time interval between two drops, the diameter of the nanofiber can be precisely determined in real-time during the pulling process (deviation <5 nm) (Xu et al. 2017a). An in-line monitoring technique utilizing the harmonic generation was also developed to measure the diameter of the nanofiber during the tapering process. The manufacturing deviation can be lower than 2% (Wiedemann et al. 2010).

2.2.4 Packaging of the Optical Microfibers

The convenient and safe handling of the fabricated microfibers is an essential prerequisite to the practical applications. In order to protect the fiber from mechanical disturbs and airborne dust, different approaches have been developed by groups around the world that are specialized in optical microfibers. For example, the fabricated microfiber can be embedded inside low-refractive-index substrates such as PDMS (Polynkin et al. 2005), Teflon (Xu et al. 2007), and silica aerogels (Xiao et al. 2011) to maintain the low optical loss and enhance the portability and long-term stability. For biosensing and gas sensing applications, the microfiber can be fixed inside a microfluidic channel (Li et al. 2014) or sealed inside silica tubes (Mao et al. 2018) that provide input and output ports to make the sensor accessible to the samples under test.

3 Optical Microfiber-Based Sensing Devices

Owing to the unique advantages of strong evanescent field and mechanically flexibility that are not available in standard telecommunication optical fibers, the optical microfiber sensors have witnessed a flourishing development ever since its invention. Various microfiber-based sensing devices employing diverse mechanisms have been explored, which show considerable promise in fields ranging from fundamental physical parameter measurement to practical biochemical analysis. The number of published research works in this field keeps increasing substantially, and a number of reviews are available. In this section, a brief summarization of the well-established sensing devices with a focus on their schemes and sensing mechanisms is provided to give the readers a general concept of how these sensors work. Then, the latest progress in new effects and strategies for sensing enhancement is reviewed.

3.1 Basic Sensing Devices

The optical interferometric sensors probably account for the largest group of the microfiber-based RI sensors. For a specific guided mode in the microfiber with a determined optical path, a small variation in the RI of the surrounding medium can induce a phase change. Thus, by introducing multi-modes into a single microfiber device or incorporating an additional reference arm, interferometers can be constructed. Based on the structural characteristics, these interferometers can be classified into the following categories: the modal interferometers (Zhang et al. 2018a) and the Sagnac interferometers (Sun et al. 2014). Apart from the interferometers, optical microfiber based resonators and gratings also have been widely explored.

3.1.1 Modal Interferometers

Fig. 7

a Schematic diagram of a biconical tapered microfiber-based modal interferometer. b Schematic diagram of an optical microfiber coupler

The simplest form of the modal interferometer is the biconical tapered microfiber (Fig. 7a). It consists of a uniform waist region that can support multimode propagation and two abrupt tapered regions, which can couple a portion of the fundamental core mode into high-order modes in the microfiber region. The fundamental HE₁₁ mode and the high-order mode possess different propagation constants and ERIs so that a phase difference can be formed between the two modes. The high-order mode can be HE₁₂ mode or other high-order modes. The transmission of a microfiber interferometer can be calculated through

$$I = I_{1} + I_{2} + 2\sqrt {I_{1} I_{2} } \cos \left( {\frac{{2\pi L\Delta n_{\text{eff}} }}{\lambda }} \right)$$

(2)

where I₁ and I₂ represent the optical power of two interference modes. L, λ, and $\Delta n_{\text{eff}}$ denote the lengths of the optical microfiber, the operating wavelength, and the difference between the refractive index of the two modes, respectively. With this compact sensing device, physical or chemical changes that can alter the length or the mode property of the microfiber can be easily detected.

The optical microfiber coupler (OMC) is a traditional fiber-optic device, which has been extensively employed in fiber-optic networks, fiber lasers, and fiber-optic imaging. The tapered fiber coupler also serves as a promising candidate for sensing. The OMC is consists of two input ports, two output ports, two transition regions, and a uniform waist region where two microfibers are placed in parallel, as shown in Fig. 7b. The underlying operating mechanism is the interference between the even supermode and the odd supermode. When light is injected from port 1, the output power at port 3 and 4 can be obtained as (Yang et al. 1998).

$$P_{3} = P_{1} \cos^{2} \left( {\frac{{2\pi L\left( {n_{\text{eff}}^{\text{even}} - n_{\text{eff}}^{\text{odd}} } \right)}}{\lambda }} \right)$$

(3)

$$P_{4} = P_{1} \sin^{2} \left( {\frac{{2\pi L\left( {n_{\text{eff}}^{\text{even}} - n_{\text{eff}}^{\text{odd}} } \right)}}{\lambda }} \right)$$

(4)

where $n_{\text{eff}}^{\text{even}}$ and $n_{\text{eff}}^{\text{odd}}$ are the ERIs of the even supermode and odd supermode, respectively. L and λ denote the coupling length and the working wavelength.

Due to the periodical coupling between the even and odd supermodes, sine wave-like transmission spectra can be obtained from the two output ports. When it is used as a sensor, the variation of the environmental parameter can affect the coupling and eventually can be analyzed through the transmission spectra. The microfiber coupler sensor regularly exhibits equivalent sensing performances to the microfiber interferometric sensors (Tazawa et al. 2007).

3.1.2 Sagnac Interferometers

Fig. 8

a Schematic diagram of a highly birefringent optical microfiber. b Schematic diagram of the Sagnac interferometer. c Cross-sections of typical highly birefringent optical microfibers (rectangular shape, oval shape, H shape, and D shape)

The microfiber-based Sagnac interferometer usually employs the highly birefringent optical microfibers with none circular cross-sections (Fig. 8a). In this sense, the two orthogonally polarization states of the fundamental mode experience different propagation constants and effective RIs. The birefringent optical microfibers are conventionally incorporated into a Sagnac loop to form the Sagnac interference between the two orthogonally polarized optical beams, which are also named the fast beam and the slow beam (Fig. 8b). To date, microfiber Sagnac interference with ellipse cross-sections (Sun et al. 2014), rectangular cross-sections (Li et al. 2011), H-shaped cross-sections (Xuan et al. 2010), and D-shaped cross-sections have been demonstrated (Fig. 8c) (Luo et al. 2016). The transmission of the Sagnac interferometer is obtained as (Li et al. 2011)

$$T = P_{0} \sin^{2} \left( {\frac{\pi BL}{\lambda }} \right)$$

(5)

where $B = n_{\text{eff}}^{x} - n_{\text{eff}}^{y}$ denotes the birefringence of a guided mode, with $n_{\text{eff}}^{x}$ and $n_{\text{eff}}^{y}$ represent the ERIs of the x-polarized mode and y-polarized mode, respectively.

3.1.3 Optical Microfiber Resonators

The optical microfiber resonators can be formed by microfiber loops (Fig. 9a) (Guo and Tong 2008) or microfiber knot resonators (Xiao and Birks 2011), and microfiber coil resonator (Fig. 9b) (Xu et al. 2007) that are wrapped around a rod. Theses resonators can confine the light in a marginal volume and significantly enhance the evanescent field, which can significantly promote the sensing capability. The quality factor of microfiber resonators can reach as high as 10⁶ (Xiao and Birks 2011). Although the sensitivity of these sensors is relatively low compared to microfiber interferometers and microfiber couplers, the detection limit can be several orders of magnitude lower, considering the high-quality factor. Numerous progress has been made both in exploring new resonator schemes and improving the quality factor. Despite the RI sensing performance, these sensors are seldomly employed for biochemical sensing. The relatively low mechanical robustness may limit this. Advanced packaging techniques are expected to promote practice applications.

Fig. 9

Schematic diagram of a optical microfiber ring resonator. b optical microfiber coil resonator

3.1.4 Optical Microfiber Gratings

Fiber-optic gratings are the most classic fiber sensors for strain and temperature measurements, which have found wide applications in the fields of railway monitoring, structure health monitoring, and oil pipeline inspections. Thanks to the pronounced evanescent field, microfiber gratings (Fig. 10) can extend their application to RI sensing and biochemical detection. These gratings are routinely inscribed in the microfiber using the ArF excimer laser (Ran et al. 2011) or the femtosecond laser (Fang et al. 2010). Microstructure-patterned microfiber gratings fabricated by the focused ion beam (FIB) milling (Ding et al. 2011) and the Rayleigh-Plateau instability (Li et al. 2017) have also been reported. Based on the period of the grating, microfiber grating can be divided into two classes: the microfiber Bragg grating (Kou et al. 2012) and the long-period microfiber grating (Li et al. 2017). The merits of the microfiber gratings are the possibility of automatic production with high uniformity. However, the relatively low sensitivity remains a critical problem which needs to be overcome.

Fig. 10

Schematic diagram of the optical microfiber grating

3.2 New Effects and Strategies for Sensing Enhancement

3.2.1 Dispersion Turning Point

Searching for new strategies to enhance the performance of existing sensors is a permanent theme. For microfiber RI sensors, the highest sensitivity is always achieved when the RI of the surrounding medium approaches that of the fiber itself. This can be explained by the fact that the evanescent field becomes pronounced when the external RI comes close to that of the fiber, which can enhance light-matter interaction. However, in practical applications, most of the samples are with low RI. For example, in biosensing, biomolecules are always given in the form of water solutions whose RI is quite close to that of water (around 1.33–1.36). Also, for gas sensing and gas-phase biomarker detections, the working medium usually shows an extremely low RI of ~1. Therefore, it is of great practical importance to develop RI sensors that can achieve high sensitivity in the low RI region.

Fig. 11

Adapted with permission Li et al. (2018c)

a Schematic diagram of an OMC with the interference between the even mode and odd mode. b The microscopic view of an OMC with a waist width of 1.8 µm. c The calculated sensitivity of near the dispersion turning point for OMCs with different waist width. d Positions of the dispersion turning point for OMCs (w = 1.6–2.4 μm) as SRI increases from 1.3329 to 1,4100. e Variation of the transmission spectrum of the OMC along with increasing SRI from 1.3329 to 1.3929. f spectral responses of the OMC in the RI range around 1.3329. g The sensitivities of five dips closest to the dispersion turning point.

By carefully designing the parameters of an OMC, it can work around the modal dispersion turning point, which can significantly improve the sensitivity in a given RI range (Li et al. 2016, 2018b, c). As shown in Fig. 11a, the OMC mainly relies on the interference of the even supermode and the odd supermode. The RI sensitivity can be calculated through (Li et al. 2018c):

$$S_{\text{RI}} = \frac{{\partial \lambda_{N} }}{\partial n} = \frac{{\lambda_{\text{N}} }}{{n_{g}^{\text{even}} - n_{g}^{\text{odd}} }}\frac{{\partial \left( {n_{\text{eff}}^{\text{even}} - n_{\text{eff}}^{\text{odd}} } \right)}}{\partial n}$$

(6)

where $\lambda_{N}$ represents the wavelength of Nth dip on the interference spectrum. $n_{g}^{\text{even}}$ and $n_{g}^{\text{odd}}$ denotes the ERI of the even and odd supermodes, and can be obtained by $n_{g} = n_{\text{eff}} - \lambda_{N} \partial \left( {n_{\text{eff}} } \right)/\partial \lambda_{N}$. $n_{\text{eff}}^{\text{even}}$ and $n_{\text{eff}}^{\text{odd}}$ are the ERIs of the two modes, respectively. It is evident from Eq. (6) that when the group index difference $g = n_{g}^{\text{even}} - n_{g}^{\text{odd}}$ approaches zero, the sensitivity can be significantly enhanced. Numerical simulation results (Fig. 11c) indicate that OMCs with a width ranging from 1.6 to 2.4 µm exhibit dispersion turning points in the wavelength range of 600–1400 nm, along with extremely high sensitivities of above 10,000 nm/RIU. When the waist width of the microfiber coupler is appropriately selected, the dispersion turning points can be readily obtained in a vast range of SRIs, ranging from as low as 1.0 for the gas medium (Li et al. 2018b) to liquid samples as high as 1.44 (Li et al. 2018c). Dual-peaks/dips interference characteristic near the dispersion turning point, which is different from conventional modal interferometers, was discovered and investigated experimentally (Fig. 11f). An exceptional sensitivity as high as 59,624 nm/RIU was achieved (Fig. 11g). In particular, OMCs with sub-micron waist width show clear dispersion turning points in the gaseous environment, and ultra-sensitive gas RI detection has been demonstrated (Li et al. 2018b).

Analogous dispersion turning point and ultra-high sensitivities have also been demonstrated in optical microfiber modal interferometers, which mainly rely on the interference of the HE₁₁ and the HE₁₂ modes (Luo et al. 2015a; Zhang et al. 2018a). By optimizing the waist diameter, dispersion turning points can be achieved in a vast RI range from 1.0 to 1.44. However, one problem with the tapered microfiber modal interferometer is that high-quality interference fringes are challenging to achieve experimentally. This is because the high-order HE₁₂ mode is excited by the LP₀₁ core mode of the standard optical fiber at the down taper. The excitation ratio usually is relatively low, and other high-order modes are also excited simultaneously. This newly discovered sensing effect is not limited to microfiber couplers and biconical tapered optical microfibers. It may be further extended to other microfiber-based sensing devices.

Leveraging on this well demonstrated OMC sensor, ultra-sensitive detection of the cardiac troponin I (cTnI) biomarker has realized (Zhou et al. 2018). The measuring setup is shown in Fig. 12a. The packaging of the OMC is shown in Fig. 12b. The OMC biosensor is fabricated through the heating and pulling method, followed by surface modification with site-specific cTnI antibodies via covalent binding. The results in Fig. 12c, d demonstrate the remarkable performance of this sensor for cTnI biomarker detection. A limit of detection (LOD) as low as 2 fg/mL was achieved, which is much low than previously reported fiber-optic biosensors.

Fig. 12

Adapted with permission Zhou et al. (2018)

a Schematics of the experimental setup for the OMC biosensor. b Scanning Electron Microscopy images of the OMC and a photograph of the sensor chip. c Spectral responses of the OMC (waist width: ~1.0 µm) to cardiac troponin I biomarkers in PBS buffers with concentrations of 2 fg/mL, 4 fg/mL, 6 fg/mL, 8 fg/mL, 10 fg/mL, respectively. d Real-time response of the sensor to PBS, 2 fg/mL, and 4 fg/mL.

3.2.2 Vernier Effect

The Vernier effect is adapted from the Vernier caliper, which is used for length measurement with enhanced resolution. This effect can be applied to fiber-optic sensors with enhanced performance. To realize the Vernier effect, one should obtain and superpose two paths of resonances or interferences with identical but unequal free spectral range (FSR). Such structures could be easily constructed using the optical microfibers. To date, the Vernier effect has been demonstrated within cascaded microfiber ring resonators and the OMCs.

By cascading a θ-shaped microfiber resonator (Fig. 13a) with an additional fiber Fabry–Perot interferometer, the Vernier effect can be generated when the FSR of the resonator and the interference are identical. The experimental measuring setup is shown in Fig. 13b. Benefiting from the Vernier effect, the RI sensitivity of the combined structure is m times higher than the sensitivity of the singular θ-shaped microfiber resonator. Moreover, this sensor is highly tunable, as the researchers have demonstrated in Fig. 13c, d: when the cavity length of the θ-shaped microfiber resonator is adjusted from 9.4 to 8.7 mm, the RI sensitivity can be widely tuned from 311.77 to 2460.07 nm/RIU (Xu et al. 2017b).

Fig. 13

Adapted with permission Xu et al. (2017b)

a Schematic diagram of the θ-shaped ring resonator. b The experimental setup of the θ-shaped microfiber resonator cascaded with the Fabry–Perot interferometer. c Vernier spectra with different periods. d The wavelength shifts of three θ-shaped microfiber resonators to SRI.

The OMC is a naturally birefringent waveguide, which can be utilized to compose the Vernier effect without incorporating an additional structure (Li et al. 2018a). The basic principle is shown in Fig. 14a. Due to the modal birefringence, two paths of orthogonally polarized interferences that vary slightly can be achieved using a single OMC. Figure 14b shows the microscopic images of an OMC with a waist width of 3.2 µm, together with the transmission spectra in both air and water. The pronounced envelopes in the spectra result from the Vernier effect. Benefitting from the Vernier effect, this sensor shows an improvement of one order of magnitude in RI sensing and the highest sensitivity of 35,823.3 nm/RIU (Fig. 14c, d). Biosensing application was also performed for cardiac troponin T (cTnT) biomarker detection, and a detection limit of 1 ng/mL was achieved (Fig. 14e). Compared with other Vernier effect enhanced fiber-optic sensors that utilize cascaded schemes, this sensor is simple and compact in structure and shows high stability. Although the Vernier effect has only been demonstrated in microfiber ring resonators and OMCs, it is applicable to other microfiber sensors to achieve improved performance.

Fig. 14

Adapted with permission Li et al. (2018a)

Vernier effect in OCMs for enhanced sensing. a Basic sensing scheme and mechanism. b An OMC with a waist width of 3.2 µm, and the transmission spectra in both air and water, with the Vernier effect. c Spectral response of the sensor to SRI. d The dependence of wavelength shift on SRI. e Spectral response of the sensor for cardiac troponin T biomarker detection.

3.2.3 Integration with Functional Nanomaterials

The integration of functional nanomaterials with optical microfibers can bring about novel optical properties and provide a versatile platform for various sensing applications. Multifarious nanomaterials with diverse optical, optoelectronic, and photothermal properties can be readily incorporated on the surface of optical microfibers via electrostatic interactions or covalent binding to compose novel sensors (Fig. 15). By decorating silica nanoparticles on the surface of a tapered multimode optical fiber, a Mie scattering enhanced RI sensor can be constructed. The measured sensitivity in terms of transmitted optical intensity raises two orders of magnitude for a 2.8-μm-thick microfiber after modified by silica nanospheres (Liu et al. 2012). Decorating the microfiber surface with plasmonic nanoparticles can make a compact sensing probe with enhanced light-matter interactions (Zhou et al. 2019). The strong evanescent field can exit the localized surface plasmon resonance of the plasmonic nanoparticles and confine the light in narrow spaces of about tens of nanometers over the surface of the nanoparticles. Thus, ultra-high sensitivities can be achieved.

Fig. 15

The basic concept of integration of functional nanomaterials with the optical microfibers

To this end, understanding the optical interaction between the guided light with the nanomaterials is the key to unlocking the potential. Researches have built a model to analyses the strength of the interaction on the single-nanoparticle level (Fig. 16a) by taking the plasmonic gold nanosphere as an example (Li et al. 2018d). The proportional optical power encountered by a single nanoparticle is a valid indicator of the strength of light-nanoparticle interaction. The numerical result in Fig. 16b shows that, when the diameter of the microfiber decreases from 10 μm to sub-micron, the proportional optical power $\eta_{\text{particle}}$ encountered by a 20-nm gold nanosphere dramatically improves and reaches a peak value at d = 0.36 μm. However, when it falls below 0.36 μm, the proportional optical power near fiber surfaces is relatively reduced. Thus, generally, thinner optical microfibers are preferable to achieve better sensing performance as long as d > 0.36 μm. This tendency was proved experimentally using gold nanosphere decorated optical microfibers with different diameters, as shown in Fig. 16c, d.

Fig. 16

Adapted with permission Li et al. (2018d)

a Schematic diagram of the model. b Calculated $\eta_{\text{particle}}$ and ${\text{d}}\eta_{\text{particle}}$ of a 20 nm-sized nanoparticle located on the surface of an optical microfiber as a function of fiber diameter (wavelength: 520 nm). c Absorption spectra of optical microfibers with different diameters as gold nanoparticle binding reaches saturation. d RI sensing performances of optical microfiber-based LSPR sensors with different fiber diameters.

Based on the preliminary analysis, a highly integrated microfiber LSPR sensor decorated with high quality and monodisperse β-cyclodextrin-capped gold nanoparticles (β-CD-capped AuNPs) has been constructed (Fig. 17a, b). The bi-functional gold nanoparticles, which were synthesized in a one-step facile and eco-friendly process, can effectively deliver promising localized surface plasmon resonance properties. Moreover, the β-CD molecule can also work as the receptor for cholesterol, which is of great diagnostic significance for various disorders, including coronary heart disease, atherosclerosis, hypertension, and nephrosis. The proposed biosensor reaches an ultra-low cholesterol LOD of 5 aM (shown in Fig. 17c), which is the most sensitive among the state-of-the-art cholesterol detections (Zhang et al. 2019).

Fig. 17

a Schematic diagram of the integrated microfiber LSPR biosensor capped with β-CD-capped AuNPs. b SEM images showing the profile of the optical microfiber and the nanoparticles on the fiber surface. c Spectral responses of the sensor to cholesterol samples. Adapted with permission Zhang et al. (2019). d Schematic diagram of the microfiber optic biosensor integrated with two-dimensional molybdenum trioxides. e Atomic force microscope (AFM) image of the two-dimensional molybdenum trioxides. f Spectral responses of the sensor to BSA samples. Adapted with permission Zhang et al. (2018b)

Two-dimensional (2D) plasmonic nanomaterials facilitate exceptional light-matter interaction and enable in situ plasmon resonance tunability. By introducing heavily doped MoO3-x nanoflakes that provide strong plasmon resonance located at ~735 nm onto the surface of an optical microfiber, a new sensing platform was constructed (Fig. 17d). The MoO3-x nanoflakes (Fig. 17e) show an excellent affinity to negatively charged biomolecules. A LOD of 1 pg/mL was experimentally demonstrated for the detection of bovine serum albumin (Fig. 17f). It proved the feasibility and prospects of employing 2D plasmonic materials in highly integrated devices compliant with frequently used and a cost-effective optical system (Zhang et al. 2018b).

Aside from working as the sensing element, the nanomaterials can also function as the signal indicator and amplifier in optical microfiber biosensors (Li et al. 2014). The primary sensing scheme of a microfiber biosensor is illustrated in Fig. 18a, in which a sandwiched immunoassay method was adopted. First, a single layer of AFP specific first antibodies was coated on the surface of the optical microfiber. Then, the samples were injected, and the first antibodies captured the biomarkers. Finally, the secondary antibody-coated gold nanoparticles are injected and bind to the surface of the fiber through the antibody-antigen conjugations. The captured gold nanoparticles that laid inside the evanescent field of the microfiber can induce significant loss. The loss is proportional to the quantity of the nanoparticles, and hence the number of the captured biomarkers. This sensing strategy was experimentally verified using a 1-µm-thick optical microfiber and 40-nm gold nanoparticles as the label. The concentration of AFP biomarker was successfully detected with a LOD of 2 ng/mL in bovine serum (Fig. 18b–e). The advantages of this biosensor are simple detection scheme, fast response time, the immunity to other irrelevant proteins in complex samples, which might make this biosensor a promising platform for clinical cancer diagnosis and prognosis.

Fig. 18

Adapted with permission Li et al. (2014)

a Schematic diagram of the microfiber biosensor that utilizes the gold nanoparticles as the signal indicator and amplifier. b Absorption spectra of gold nanoparticle solution and bio-functionalized gold nanoparticle solution. The insets show the absorption spectra of the secondary antibody functionalized gold nanoparticle binding to a biomarker coated microfiber and an SEM image of the fiber decorated with GNPs. c, d Real-time responses of the sensor to AFP biomarkers that are spiked in bovine serum. e Calibration curve of the biosensor as a function of analyte target concentration in bovine serum.

4 Conclusion

This chapter reviewed the fundamental optical properties, fabrications techniques, the well-developed sensing schemes as well as the latest progress on new effects and strategies for enhanced sensing. The optical microfiber, with the unique property of strong evanescent field and compact micron-scale size, can serve as building blocks to construct versatile sensing schemes with unprecedented sensitivities. On the one hand, new sensing schemes with better performances will be continuously developed; on the other hand, the integration of functional nanomaterials with the microfiber has opened new opportunities and will bring prosperity to this field. The optical microfiber sensors are also facing some challenges, which have blocked the path to practical applications. One challenge is the batch production of optical microfibers with high uniformities. The other is effective packaging, which can protect the microfiber-based devices from external continents and provide long-term stability. It is believed that remarkable progress will be made in the near future to address the challenges mentioned above.