Side Polished Fiber: A Versatile Platform for Compact Fiber Devices and Sensors

Side polished fiber (SPF) has a controllable average roughness and length of the side-polishing region, which becomes a versatile platform for integrating multiple materials to interact with the evanescent field to fabricate all-fiber devices and sensors. It has been widely used in couplers, filters, polarizers, optical attenuators, photodetectors, modulators, and sensors for temperature, humidity, strain, biological molecules, chemical gas, and vector magnetic monitoring. In this article, an overview of the development history, fabrication techniques, fiber types, transmission characteristics, and varied recent applications of SPFs are reviewed. Firstly, the fabrication techniques of SPFs are reviewed, including the V-groove assisted polishing technique and wheel polishing technique. Then, the different types of SPFs and their characteristics are discussed. Finally, various applications of SPFs are discussed and concluded theoretically and experimentally, including their principles and structures. When designing the device, the residual thickness and polishing lengths of the SPF need to be appropriately selected in order to obtain the best performance. Developing all-fiber devices and sensors is aimed at practical usability under harsh environments and allows to avoid the high coupling loss between optical fibers and on-chip integrated devices.


Introduction
In high-performance integrated circuits, on-chip optical interconnects have the potential for solving the communication bottleneck. At present, on-chip optical interconnects mainly rely on silicon photonics [1,2]. However, the precise coupling between the silicon waveguide and the optical fiber mainly through optical gratings or taper fibers usually requires the long assembly time and cost [3]. Moreover, combining with the few-mode fiber and multi-core fiber is a challenge, and the grating coupling has limited the bandwidth. The development of the optical fiber versatile platform meets the requirements of all-fiber integration and expects to achieve seamless connection between optoelectronic devices and optic fiber networks. The concept of "lab-on-fiber" has attracted a great attention since 2012, which is a new technology that integrates highly functionalized micro/nanostructures and materials on optical fibers [4][5][6]. This technology helps to realize a small and compact optical fiber system and develop the all-in-fiber device. It will allow the "lab" to be combined with transmission mechanisms and integrated into modern optical systems for communication and sensing applications [4]. In addition, the lab-on-fiber has the advantages of resisting electromagnetic interference, small size, geometric versatility, and compatibility with fiber optic systems.
In the past few decades, the emergence of the optical fiber has promoted the development of the communication field, which becomes an important optical waveguide. Therefore, optical fiber-based optoelectronic devices and sensors emerged at the historic moment [7][8][9][10][11]. With the maturity of the microfabrication technology and the emergence of two-dimensional (2D) materials, the foundation has been laid for the fabrication and integration of the nanostructure on optic fibers. Since Bergh et al. [12] polished the first side polished fiber (SPF) in 1980, the research on SPFs has a history of 40 years. By removing part of the fiber cladding, the cross section of the polished area looks like a D-shape and the unpolished fiber section is still cylindrical. The side-polishing region of SPFs provides a flat platform for the integration of the micro-and nanostructures to realize effective light-matter interaction [13]. Hence, SPFs can be a versatile platform for optical fiber compactable devices and sensors.
This paper reviews the fabrication techniques, characteristics, fiber types, and applications of SPFs. The article is organized as follows. Firstly, the fabrication techniques of SPFs are described, including the V-groove assisted polishing technique and wheel polishing technique. Then, different types of SPFs and their characteristics are explained in detail. Finally, various applications of SPFs are introduced in detail, including their principles and structures. The reliable mechanical properties and anti-electromagnetic interference properties make the SPF widely suitable for applications in fiber couplers, filters, polarizers, photodetectors, sensors, etc. The SPF has the advantages of direct compatibility with the theoptical fiber, low insertion loss (0.1 dB), minimized polarization-dependent loss (<0.02 dB), and wide operational bandwidth, which has attracted more interests in the construction of all-fiber devices or functional fiber sensors as a versatile platform.

Fabrication techniques of the side polished fiber
The development of the optical fiber side polishing technology has a history of 40 years. Some reliable polishing techniques of fabricating SPFs without introducing excessive loss and the method of non-destructively estimating the residual thickness have attracted a lot of attention. Generally, the manufacturing process of a standard SPF begins with a rough lapping process to remove the cladding and then a fine polishing process to remove the surface scratches. However, a high-quality SPF with the smooth polished surface usually takes a long side-polishing time. Researchers have continuously tried and improved the fabrication technology of the SPF, mainly including the V-groove assisted polishing technique and wheel polishing technique. Before side-polishing, the complete fiber cladding ensures that the optical wave will be transmitted in the core. When part of the cladding of the fiber is removed, a portion of the optical signal leaks through the fiber core and into the cladding through an evanescent field that decays exponentially with the distance from the polished surface to the core.

V-groove assisted polishing technique
One of the reliable polishing techniques of producing an SPF is the V-groove assisted polishing technique. This technique requires supporting grooves or blocks to support the optical fiber, and the fiber will grind or polish simultaneously with the block substrate, as shown in Fig. 1(a). Tseng et al. [13] first reported a reproducible silicon V-groove assisted polishing technique and estimated the polishing depth by the liquid-drop method. Pei et al. [14] used the arc discharge method to improve the roughness of the polished surface and suppress the large loss caused by microcracks or pits, and they independently developed an equipment to precisely control the polishing depth of the SPFs. Dikovska et al. [15] engraved multiple V-grooves on an oxyfluoride glass, which can achieve simultaneous polishing of multiple optical fibers and greatly improve the efficiency. It usually takes several polishing steps to obtain a standard SPF. Firstly, coarse abrasive powders are used to polish one or more times, and then a few micrometers particles are used to finely polish. The disadvantage of this method is that a groove must be engraved on the substrate first, and each fiber needs a newly engraved groove substrate. In addition, the optical fiber should be fixed with epoxy glue, and after polishing, chemical solvents need to be used to dissolve the epoxy glue. This technique is costly, time-consuming, environmentally unfriendly, and difficult in mass production. What's more, the SPF made by this method is difficult to ensure the consistent polishing depth of the entire polishing region and difficult to make a long side-polishing region [16].

Wheel polishing technique
Another method to produce an SPF without blocks is the wheel polishing technique [17,18], which is suitable for various types of fiber, including the single-mode fiber (SMF-28e), few-mode fiber, multimode fiber, panda fiber, and plastic optical fiber [19][20][21][22]. It has the advantages of simple operation, low cost, and no need to use glue to fix, and the middle of the side-polishing region is completely flat. This technique also allows to polish multiple optical fibers at the same time. The optical fiber with a section of the coating stripped is suspended tightly above the fixed motor-driven wheel by two fiber clampers and then dropped paroline oil onto the abrasive paper as lubrication. By adjusting the size of the polishing wheel to control the length of the polishing region, the polishing depth is estimated by in situ monitoring the optical power online [23,24]. Connecting the light source and optical power meter to both ends of the fiber, by observing the throughput attenuation during the light propagation, can monitor the whole polishing process. The abrasive paper is wrapped around the wheel by the double-sided tape to uniform polishing. The wheel can be driven by a low-cost 6 V direct current (DC) motor, and the stress and the speed of the wheel can be turned [24]. The length of the side-polishing region can be controlled from 2 mm to 60 mm [17]. The manufacture of the SPF starts with the rough lapping process of the coarse abrasive paper, which makes the remaining thickness of the SPF drop rapidly and then starts a fine polishing process to remove scratches on the polishing surface. The roughness of the polishing region is determined by the polishing time and the grit size of the abrasive paper. Zhao et al. [25] demonstrated a fast mechanical wheel lapping technique to fabricate a rough SPF, the surface scratches are discontinuous and malposed as observed by 5 000×SEM, and the schematic diagram of the wheel polishing setup and the surface scratches of the SPF is shown in Fig. 1(b). These scratches will cause a large insertion loss (10 dB ~ 30 dB) by surface scattering. Jinan University has independently developed a fully automated wheel side polishing equipment with a microscopic imaging system [as shown in Fig. 1(c)]. The commercial mass production of the SPF has been achieved. It is equipped with multiple laser sources and power which realiz shape, and and the fib fine polishin paper, the

Variou their char
The SPF lead-out opti flat region, polishing, t optical wave and the eva  [13].
For the wheel polishing technique, the transition region is an arc-shaped area with a gradual cladding thickness whose radius of the curvature depends on the diameter of the wheel. The transitional region provides an efficient way to excite the high-order modes and strong evanescent field of the SPF [28]. There are many methods for measuring the RT, including the use of mechanical comparators, measuring the length of the oval polished section of the fiber [29], the liquid-drop method [29], and in-situ monitoring technique [23]. Among them, the in-situ monitoring technique has the advantage of high efficiency, precision, reproducibility within 1 µm, and no need for refractive index matching fluid [23].

Single-mode fiber and multi-mode fiber
The SMF and multi-mode fiber (MMF) are the most two common optical fiber types in SPFs. Because of their low cost and easy polishing characteristics, they have been widely used in the sensing field and optical fiber communications. By polishing a part of the cladding of the SMF or MMF, the evanescent field can easily leak from the fiber core.
The transmission spectrum of the side-polished SMF is related to the RT, and the transmission spectrum of the original SMF shows nearly none power loss, as shown in Fig. 2(b). As the RT decreases continually, the power loss gradually increases until the RT reaches 71 µm. When polishing close to the fiber core (RT<69 µm), the peaks/dips in the transmitted spectrum can be observed owing to the multimode interference [30], and the extinction ratio of the interference spectrum reaches 5 dB when RT = 68.1 µm [25]. When the RT of the SPF keeps unchanged, increasing the polishing length will increase the loss, as shown in Fig. 2(c) [25]. In addition, there are some studies on the transmitted optical power through the SPF changed with the surrounding refractive index [27,31,32]. The external medium can be an effective mode sink when polished close to the fiber core. As shown in Fig. 2(d), when overlaying the oil with different refractive index (n oil ) values on the polished region, the optical power loss is less than 1 dB under the condition of n oil <1.437 8, the optical power loss swiftly increases from 1 dB to 60 dB (1 310 nm) when n oil increases from 1.437 8 to 1.452 1, and the loss of optical power decreases gradually and levels off at a certain value when n oil > 1.453 2 [27]. The result shows that the loss of optical power at 1 310 nm is larger than that at 1 550 nm.
Figures 2(e) -2(g) show some simulation results, including the theoretical calculation of the optical power loss of the side-polished SMF changed with the refractive index of the overlaying material [32], transmission spectra, and field evolution of the coreless side-polished fiber (CSPF) by the commercial beam propagation method (BPM) module from Rsoft Inc [28]. The simulation results show that some dips/peaks exist in the transmission spectra and the spectra have a red shift when the surrounding refractive index increases from 1.454 to 1.456, as shown in Fig. 2(f). The light enters the SPF in the form of the fundamental mode by the lead-in SMF and is reflected in the polished region to excite the high-order modes. This phenomenon is called the multimode interference (MMI) which will result in interference peaks/dips in transmission spectra due to the constructive/destructive interference. Xiao et al. [33] calculated numerically the effective mode index (EMI) of a graphene-coated SPF modulator. The simulation result shows that the field distribution is related to the residual thickness of the SPF, as shown in Fig. 2(g). With a decrease in the residual thickness, the mode field distribution moves from the fiber core to the cladding which is gradually far from the graphene. This modulator is theoretically predicted to have a 0.007 2 dB/μm modulation depth and 56.2 THz optical modulation bandwidth [3 The side order mode modes cause of speckles been verifie is proportio side-polished Hence, the sensitivity o changes in t

Polarization maintaining fiber
The polarization maintaining fiber (PMF) is popular for its unique characteristics of two orthogonal polarization modes, which can propagate at different phase velocities [20,37]. According to the difference in the shape of the stress zone, the PMF can be divided into the panda fiber, bow-tie fiber, and ellipse fiber. The polishing PMF is more complicated than the polishing SMF, and it needs to measure the polarization stress axis of the PMF first. The side polishing of the PMF is along the slow or fast axis. Only when the polarization can be maintained on the entire fiber system, the polarization maintaining properties of side polished PMFs can be fully exploited. Until now, the azimuthal alignment method for the PMF without the coating includes the polarization observation by the lens-effect-tracing (POL) method [38][39][40], direct monitoring method using the central image [41], five-point eigenvalue method [42], light intensity distributions eigenvalue method with the five-finger profile [43], and ray-tracing method [44]. The principle of the POL method is based on the anisotropy of the lens effect of the PMF. The POL method setup usually consists of the fiber optic lamp, mirror, fiber rotator, objective lens, charge coupled device (CCD) camera, and monitor. A successful side-polished PMF still possesses the birefringence, and the polarization dependent loss (PDL) measured by the liquid-drop method is negligible [45]. Base on the POL method, Wang et al. [39] proposed a new polarization observation by lens-effect-tracing with the middle-line (POLM) method to improve the precision of angle orientation within 1°. Zhen et al. [46] established an on-line setup of the azimuthal position observation by the POL method which realized on-line adjusting axes of the PMF with and without the coating. The side-polished PMFs have been successfully used in asymmetric directional couplers, strain measurement, and optical sensors [47,48]. King et al. [20] used the side-polished panda fiber to fabricate an electric field sensor by the slab-coupled optical sensor technology. Wu et al. [48] first reported Sagnac interferometers for the strain measurement and surface plasmon resonance (SPR) biochemical sensing using two types of side-polished panda fibers. These works indicated that side-polished panda fibers could manufacture promising optical fiber devices for the Internet of things sensing technology.

Micro-structure fibers
The PCF is also called micro-structure fiber, which usually includes the hollow core PCF and solid core PCF, which has endless single mode transmission, high birefringence, and high nonlinearity optical characteristics [49]. In general, the PCF is composed of periodic refractive index distribution in the cladding and/or core. The unique properties make the PCF show better sensing performance than conventional fibers, especially the side-polished PCF has been successfully applied to bio-sensing, refractive index sensing, SPR sensing, and optic coupler [50][51][52][53]. Le et al. [53] designed a symmetrical side-polished PCF structure to solve the problem of temperature cross-sensitivity of the magnetic field sensor by commercial finite element software Comsol Multiphysics. He et al. [54] analyzed optical propagation characteristics of the side-polished PCF with the residual thickness, polished length along the PCF, and azimuth by use of the 3D finite difference beam propagation method. This article shows the optical power transmittance of the side-polished PCF decreases and oscillates as the polishing length increases and residual thickness decreases. When the innermost layer of air holes of the PCF has been side polished, the fundamental mode field is mainly constrained in the central solid area even though the symmetric periodical configuration of the PCF has been destructed. What's more, in the polished area, the originally launched fundamental mode will be coupled to the high order propagation modes and radiation mode [54].

Bragg fibers
Bragg fibers (BFs) are a kind of photonic bandgap fibers proposed in 1978 which has unique dispersion and modal properties [55,56]. BFs can be classified into hollow-core-BFs and solid-core-BFs, which have an alternating cladding of high and low refractive indices. Due to the easy control band gap, BFs can be used as wavelength-dependent distributed filters [56]. Among them, the potential application of hollow-core-BFs is the low loss of CO 2 laser transmission and trace gas sensing in the mid-infrared region [57]. The basic principle of BGs sensing devices is to monitor the wavelength shift of the reflective signal for any changes in the external physical parameters. The side-polished BGs increase the sensitivity of fiber sensors because of a decrease in the distance from the external to the core, and the reflection and transmission spectra can be observed by an OSA. Another optical fiber with periodically changes of core refractive index is the fiber Bragg grating (FBG). By monitor the wavelength shift of the returned Bragg signal to measure the change of the environment parameter, side-polished FBGs have been widely used in hydrogen sensors, magnetic sensors, and refractive index sensors [58][59][60].

Plastic fibers
Plastic fibers (PFs) are also called polymer optical fibers. The first plastic fiber was produced at the end of the 1960s by DuPont, and PFs have the advantages of physical robustness and can operate in the visible light regime [61]. Generally, PFs have a large fiber core with diameters of 0.25 mm − 1 mm which allows to use the lower precision connectors in the optical system to reduce the cost [62]. Some polymers like poly (methyl methacrylate) (PMMA) are the most commonly used to produce PFs, and they have the characteristic of resistance to impacts and vibrations, and large elastic deformation limits (10%) [62,63]. Compared with the silica fiber, the materials of PFs are softer which makes the side-polishing procedure simpler. Although PFs have been successfully applied to curvature and refractive index sensors [22,61], the large attenuation about 125 dB/km (650 nm) has restricted its long-range application.

Multi-core fibers
Multi-core fibers have been considered as a promising way to increase the capacity of communication systems. An effective way to increase the number of cores in a multi-core fiber is by enlarging the cladding diameter and relaxing the requirement for inter-core crosstalk [64]. By the side-polishing approach, we can choose to interact with a core in the multi-core fiber without introducing large additional losses. Side-polished multi-core fibers have become an important platform for the directional coupler [64], phase modulator [65,66], and directional coupler-based relative humidity sensor [67]. Especially, the side-polished twin-core fiber is widely used, and the side-polished core and unpolished core act independently as two arms of the Michelson interferometer, which can realize phase modulation [64,65].

Couplers
Optical fiber couplers are key passive components to realize the light of different channels bonding or splitting in the optical communication system [49]. The coupling ratio, excess loss, and insertion loss are important parameters to characterize fiber optic couplers [68]. The couplers can be made of the SMF, MMF, PMF, and PCF. Among them, the SMF optic coupler is most widely used in optical communications, sensing, and fiber lasers. One of the methods to fabricate fiber couplers is the fused biconical taper method [69], while this method is not suitable for the fabrication of PCF couplers, which will destroy the internal structure of the PCF and induce defects. During the fusing process, the air hole will suffer from collapse and the cladding layer will distort [49]. Another fabrication method is based on the side-polishing technique. Zhang et al. [70] proposed a broadband mode-selective coupler by a side-polished six mode fiber and a tapered side-polished single-mode fiber, and they observed that the coupling ratios were larger than 85%, as shown in Fig. 3(a). Kim et al. [71] fabricated a coupler with a tunable coupling ratio by controlling the parameters of two side-polished PCFs [71]. For PCFs couplers, the side-polishing technique has the following advantages: the coupling efficiency can be easily adjusted by controlling the matching parameters and the pore structure of PCFs will not be deformed. Luo et al. [72] purposed a novel side-adhering technique to fabricate the fiber coupler, which was validated by the fabrication of the side-polished SMF coupler. Some nano silica powders were deposited at the flat polished area, and when encountering oxy-hydrogen flame, the nano silica powders would melt and adhere two side-polished fibers. Zhu et al. [49] fabricated a PCF coupler by the side-adhering technique, and they got the best fabrication parameters (polishing angle = 0˚, residual radius = 5.7 μm) by the theoretical analysis. If the polishing angle and residual radius are not properly selected during the fabrication process, it may result in a significant loss of the side-adhered PCF coupler [73]. This technique orders a direction for further research and development of the side-polished PCF coupler and commercial PCF coupler.

Filters
The optical fiber filter is one of the key components in the wavelength division multiplexing (WDM) optical communication system and sensing system. It is developing towards the tunable wavelength and low insertion loss. The common structural designs of the filters are based on the coupler, fiber grating, fiber interferometers, etc [74][75][76]. Each type of fiber filters has its own filtering and tunable range. In 1994, Archambault et al. [77] used a 2×2 side-polished SMF grating-frustrated coupler to form a low-loss (0.22 dB) channel-dropping filter, which exhibited the 0.7 nm bandwidth and 13 dB isolation, and one of the side-polished SMF cores contained an index grating. Near the Bragg wavelength, the filter would not operate as a conventional coupler, and the grating would frustrate the transfer of the optical power from Fiber 1 to Fiber 2 [77]. Lausten et al. [78] demonstrated an arbitrarily tunable and optically reconfigurable Bragg filter by azopolymer-coated SPF blocks, which could realize the optically write-erase-write cycle. Yu et al. [76] proposed an all-optically reconfigurable and tunable band-rejection filter by overlaying the photosensitive liquid crystals (P-LCs) hybrid film on the SPF, as shown in Fig. 3(b). This filter showed a wider tunable bandwidth of 60 nm, and the extinction ratio was as high as 21.5 dB and 23.4 dB when the P-LCs were reconfigured as short-period and long-period fiber surface gratings, respectively. P-LCs had a large and tunable birefringence characteristic that allowed to construct optically tunable and reconfigurable photonic device by an external field [76]. Yu et al. [79] also reported a wavelength dependent optical fiber add-drop filter based on a micro/nano fiber ring and an SPF, when the micro/nano fiber ring diameter was 580 mm, the add-drop filter reached the maximum extinction ratio of the drop port and the add port were 7.5 dB and 4.8 dB, respectively. The SPF serves as an integrated platform and provides an evanescent field to interact with the guided light mode filed to realize in-line light coupling and can seamlessly connect with a conventional optical fiber system.

Optical resonators
Optical resonators have been explored for many optical applications such as wavelength-selective reflectors and filters, switches, spectrum analyzers, and nonlinear optical devices [80][81][82]. Fabricating resonators on the optical fiber promises to avoid coupling loss, reliability, and cost issues of fiber-to-chip coupling [83]. Compared with the curved plane of an unpolished fiber, the SPF has a flat side-polishing region, making it easier to fabricate micro-structures on this flat region. And the SPF substrate allows the resonators to be easily connected to fiber systems and other fiber resonators. Sherwood et al. [83] used the two-photon polymerization to fabricate polymer ring resonators on an SPF, and the rough surface limited the Q-factor in the range of 300 -400. About ten years later, Shi et al. [81][82][83][84] fabricated whispering gallery mode (WGM) resonators on the SPF by femtosecond laser micromachining. By femtosecond laser ablation, they fabricated the cylindrical resonator cavity in the SPF to achieve a WGM resonator [in Fig. 3(c)] with a Q-factor of 1.44×10 3 at 1 556.52 nm [82]. It is more time-efficiency to machine the micro-structure on the SPF, because the debris accumulated on the SPF is much less than that in the unpolished fiber during the femtosecond laser ablation process. However, restricted by the average surface roughness after femtosecond laser ablation, the maximum Q-factor is limited to 10 3 , which is smaller than those of the tapered fiber coupled WGM resonators.

Polarizers
The fiber-optic polarizer is one of the essential components in both optical communications systems and polarization-dependent sensing systems for selecting the polarization of electromagnetic waves. The asymmetric structure of the SPF produces a large attenuation difference between orthogonal polarizations modes, thus becoming a favorable platform for the preparation of polarizers. Many kinds of overlays, such as birefringent polymer thin films [85], metal films [86], birefringent crystals [87], and optical phase change materials [88], have been used to make in-line SPF polarizers. These overlays have only one guided polarization state, while the other polarization state is not supported and will be absorbed or radiated. With the appearance of 2D materials, some SPF-based graphene polarizers with a high extinction ratio have become promising candidates, because graphene can use the linear dispersion of Dirac electrons to select TE or TM surface plasmon mode to support, as shown in Fig. 3(d) [89][90][91]. Table 1 presents an overview of the SPF polarizers with different overlay materials, including the implementation and performance of each study [85][86][87][88][89]92]. From the comparison result, the SPF polarizers show a low insertion loss, but researchers need to enhance the light-matter interaction to further improve the extinction ratio. Although the extinction ratio of the fiber-optic polarizer is smaller than that of the birefringent crystals polarizer (~10 4 :1) [93], it is simpler, smaller, and with a less loss in connection to optical fiber systems.

Attenuators
Fiber-optic attenuators can regulate the optical power propagated in fiber optical communication and WDM networks. In fiber optical communication networks, the variable optical attenuators (VOAs) are used to dynamically control the optical power from light sources and optimize the optoelectronic responses of high-speed receivers to avoid overload [94][95][96]. With the development of communication networks, the pursuit of the low-cost and high-performance VOA has become the focus of research. All-fiber devices provide an excellent technical solution for this need. The ideal VOA attenuation range is at least 20 dB optical attenuation, 40 nm optical operation bandwidth, and 1 dB wavelength dependent loss [94]. Commercial active VOAs are typically based on severing step index guiding fiber with a thin-film absorption filter on the cutting area [95]. The attenuation can be controlled dynamically by rotating or sliding the filter mechanically, so as to change the optical path length in the absorptive material. Compared with microelectro-mechanical systems (MEMS) optical fiber VOAs, the SPF-based optofluidic VOA used a single mode and continuous optical fiber [ Fig. 3(e)] and achieved a broadband optical attenuation (40 nm bandwidth) with an optical attenuation range up to 26 dB, because it had not any fiber-gaps and fiber lenses between two fibers [94]. The first optically controllable P-LCs SPF attenuator operating at 1.5 μm has been demonstrated [95]. The attenuation was controlled by the photochemical-induced phase change of the P-LCs, which adjusted the evanescent field leaking from the side polishing area. When the environmental temperature was 45 ℃, this attenuator could reach a 15 dB optical attenuation with an optical field of 20 mW [95].

State of polarization controllers
Controlling the state of polarization (SOP) is important to obtain desired performance characteristics, such as polarization conversion and power equalization in sensing and communication systems. So far, the manufacture of the all-fiber SOP controller is mainly based on two types of fibers: one is the PCF and the other is the SPF [97][98][99][100]. Both the PCF and the SPF SOP controllers have the advantages of high extinction ratio and low loss. As mentioned above, P-LCs have been successfully used for creating optically tunable SPF devices. Hsiao et al. [99] reported an optically tunable polarization rotator by covering P-LCs on the SPF as the overlaid birefringent material. Its working principle is to change the SOP of the propagated light and increase the phase shift by changing the power of the light irradiation. Except for the P-LCs, the chalcogenide glass film has also been coated onto the SPF to fabricate an optically switchable, inline, and all-fiber SOP controller [100]. An optically tunable SOP SPF controller based on the amorphous As 2 Se 3 thin films has been demonstrated, as shown in Fig. 3(f). This SOP controller can achieve 70° phase retardation without the optical loss (<0.2 dB) [100].

Optoelectronic devices
In recent years, all-fiber optoelectronic devices have been extensively studied due to the advantages of easy compatibility and high robustness. The SPF provides a flat platform for optoelectronic devices integration, and the excellent optical and electronic properties and flexibility of 2D materials makes "lab-on-fiber" possible. Recently, the 2D materials represented by graphene have been successfully integrated with the SPF to develop photodetectors, electro-optical modulators, optical phase modulators and fiber lasers [10,11,33,[101][102][103]. Among them, the most common applications are SPF-integrated graphene photodetectors and the optical modulators, which have been verified in both theory and experiment [10,11,33]. Zhuo et al. [11] by a s s e m b l i n g a h y b r i d c a r b o n n a n o t u b e s (CNT)/graphene film onto the SPF to achieve an all-fiber integrated photodetector with an ultrahigh responsivity the graphen improved by an exception to terahertz photodetect graphene op as shown in Fig. 3  hybrid film and CSPF-based device can work as both an electro-optical modulator with an amplitude modulation efficiency of 1.13 dB/V at a wavelength of 1 540 nm, and a photodetector with a responsivity of 0.44 A/W in the near-infrared region [10]. SPF-integrated devices provide an in-line photodetection and modulation way to seamlessly connect with optical communication networks and provide a novel solution to break the ultra-weak interaction between light and 2D-materials [105][106][107]. Lee et al. [103] proposed an electric control of the all-fiber graphene device by using ion liquid as an efficient gating medium. By integrating this device into a fiber laser system, it can work as an electrically tunable in-line nonlinear saturable absorber and control the fiber laser operating at different regimes: continuous wave, Q-switching, and passive mode-locking [103]. This all-fiber electrically tunable device opens a hopeful way for actively controlling optoelectronic and nonlinear photonic devices in the SPF platform. These researches show that the SPF can be a reliable platform for realizing online multifunctional optoelectronic devices.

Various types of sensors
Since the 1970s, the development of fiber sensors is revolutionizing our life. Different from the electrochemical sensors, fiber sensors have the advantages of small size, remarkable compatibility, immunity to electromagnetic interference, and chemical inertness. Benefited from the flat polished surface and evanescent window of the SPF, more and more high-sensitive SPF-based sensors have been demonstrated . A lot of studies have been carried out to combine the SPF with different optical materials to seek better sensors. The optical fiber sensors are prepared by coating sensitive materials on the side polished region of the SPF. When the external environment changes, the properties of the material will change, resulting in predictable changes in the optical power transmitted by the optical fiber.

Refractive index sensors
Refractive index (RI) fiber sensors show a great value in biomedical, chemical, and food processing. To date, numerous RI fiber sensors have been reported, such as CSPF-based MMI RI sensors [28,108], D-shaped PCF RI sensors based on the SPR [109][110][111][112], and MMI RI sensors based on the side polished single mode-multimode-single mode fiber (SP-SMSF) [19]. The SPR technique is pivotal to the field of optical sensors, and it is very sensitive to the refractive index of the surrounding environment with the help of the evanescent wave [111]. The SPR occurs at specific wavelengths, and the resonant wavelengths will shift when the environment changes slightly. Table 2 presents an overview of RI sensors, including the structure and performance of each study. From Table 2, the RI fiber sensor based on the SPR technique has the advantage of detecting tiny RI changes because the SPR is very sensitive to the change of the surrounding environment permittivity. The experimental setup of the CSPF-based MMI RI sensor is shown in Fig. 4(a) [28], and the sensitivity of this sensor is enhanced by the efficient excitation of high-order modes and shows ultra-high RI sensitivity. The MMI-based RI fiber sensor is attractive due to the easy fabrication, ultra-high sensitivity, and freedom in tailoring the spectrum.

Humidity sensors
Humidity sensors play an important role in the semiconductor manufacturing process, food storage, structural health monitoring, and meteorology field [113]. The sensitivity and dynamic range are two vital parameters of humidity sensors. Currently, there are multiple sensing techniques to monitor relative humidity (RH) like resistive, capacitive, gravimetric, and optic technologies [114]. Although humidity sensors based on resistive and capacitive techniques have the advantages of low cost, mass producibility, and broad dynamic range, the sensors based on the gravimetric technique have the advantages of low hysteresis and drift [114,115]. However, the optical fiber humidity sensor has the characteristics of the compact size, immunity to electromagnetic interference, and suitability for remote sensing and in line monitoring, and it has an irreplaceable role in hazardous environments. The experimental setup for humidity sensing is described in Fig. 4(b). Researchers have been committed to developing high performance humidity sensors, in which SPF humidity sensors based on 2D materials researched by Jinan University show high sensitivity, broad dynamic range, and fast response [114][115][116][117][118][119]. 2D materials such as transition metal dichalcogenides (TMDs) and graphene oxide (GO), have been applied to the field of sensing because of the high surface to volume ratio and unique optics properties [114,116]. We enumerate the performance of these SPF humidity sensors in Table 2. The MoSe 2 sensor shows a response and recovery time of 1 s and 4 s, which has the capability of monitoring human breath [114].

Temperature sensors
Compared with the traditional electric sensors, fiber temperature sensors have the ability of withstanding harsh environments like high temperature and high pressure and the capability of distributed remote measurement [120]. Temperature sensing is usually based on the wavelength shift of the transmission spectrum or the variation of transmitted power. One efficient way to realize high sensitivity is by combining of a sensitive material with the optical waveguide [30,121,122]. Reduced graphene oxide (rGO), polystyrene microspheres, and polydimethylsiloxane (PDMS) have ultrahigh thermal conductivity and optical absorption ability, which are very sensitive to temperature variation [30,122,123], and the experimental setup of the polystyrene microspheres temperature sensor is shown in Fig. 4(c). The other way to achieve the temperature sensor is by using the Mach-Zehnder interferometer (MZI) structure. We enumerate the performance of the SPF based temperature sensors in Table 2, among them, the MZI-based fiber sensor has ultra-high sensitivity to environmental variations, which is always used for temperature, strain, torsion, and refractive index sensing.

UV/Violet sensors
Ultra-violet (UV)/violet sensors are used to in-line and in-situ monitor the optical power levels of violet light or UV light. P-LCs are sensitive to magnetic, electric, thermal, and optic, especially the UV light [95,96,124,125]. In P-LCs based sensors, the deformation of P-LCs molecules can be transduced and amplified easily [125]. The principle of the UV/violet sensor is that when the light (<450 nm) is incident, the photochemical phase transition of P-LCs generated by the trans-to-cis photoisomerization of azobenzene makes the refractive index of the LCs change. Thanks to the emergence of the SPF, it can be used as a sensing platform to expand the functionalities of P-LCs-based sensing materials. Recently, SPF overlaid hybrid P-LCs (an azobenzene dye, a chiral dopant, and a nematic P-LCs) have been used to UV light sensing which are sensitive to the 380 nm light emitting diode (LED), mercury lamp, and office ceiling lights. The experimental setup is shown in Fig. 4(d) [125]. The sensitivity of this sensor is 0.16 dB/(μW/cm 2 ) with a detection limit of 45 lx when a 380 nm LED light is illuminated. Another fiber optic violet sensor is based on the optical surface grating, which is formed by a P-LC hybrid film covered on the SPF [124]. It shows a loss peak in the transmission spectrum between 1 520 nm and 1 620 nm, and this loss peak shifts linearly toward the shorter wavelength as the 405 nm light power increases from 30 mW/cm 2 to 80 mW/cm 2 with a high sensitivity of 1.154 nm (mW⋅cm −2 ) −1 .

Strain sensors
Strain sensors are widely used in aerospace, civil engineering, and electric power projects. In recent years, wearable and flexible electronic devices have attracted a great attention. However, due to the poor stretchability and fragility of crystalline silicon, the dominance of semiconductor strain sensors in modern strain sensing has been weakened. In order to meet the surge in demand for flexible electronic sensors, some novel strain sensing techniques have been discovered, such as the carbon nanomaterials based films [126], polymer optical fiber [127], fiber loop mirror [128], Raman spectroscopy [129], and FBG [130]. The SPF has been used in strain sensors due to the strong strength, sustainability, and durability. A D-shaped polarization maintaining fiber loop mirror-based sensor has been presented, which can monitor strain and the surrounding temperature simultaneously [128]. This sensor shows high sensitivities of 46 pm/µε and 130 pm/ ℃ for strain and temperature, respectively. The complete experimental scheme for the proposed sensor is shown in Fig. 4(e). The change of the surrounding medium directly affects the propagation constants or the length of the D-shaped polarization maintaining fiber loop mirror, which is reflected in the power loss or the shift of the peak/dip spectral location [128]. Lo et al. [131] proposed an ultrahigh sensitivity (2.19×10 4 deg/ε) polarimetric strain sensor on the D-shaped optical fiber based on the SPR technology. It exhibited significantly higher sensitivity than a non-SPR fiber sensor (5.2×10 2 deg/ε) [131]. Ying et al. [132] further discussed the SPR strain sensing characteristics through numerical simulation. Studies have shown that D-shaped optical fiber sensors have better performance and show a great potential for high-sensitivity stain sensors.

Acoustic sensors
The need of acoustic wave detection and analysis have grown a lot in the past few decades, such as in the applications of navigation (depth sounding/sea-bottom profiling), non-destructive evaluation of structures through detection of emitted ultrasounds (detection and location of cracks and inner stresses), and health sciences (medical imaging and diagnosis) [133]. Whether in the air or water, the acoustic pressure is the most common measurement with amplitudes on the order of mPa (10 −8 bar) [134]. Acoustic methods have the ability to study the structure and properties of materials and their internal physical processes. Fiber-optic acoustic pressure sensors have attracted more interest than common piezoelectric or capacitive sensors due to their advantages of small size, immunity to electromagnetic interference, fast response, and resistance to harsh environments [135]

Biological and chemical sensors
Biological and chemical sensing is closely related to human health and safety. The VOCs gas is a common air pollutant that evaporates easily at room temperature [137,138]. Sensing and monitoring the VOC gas show an important status in human health, industry, and agriculture because of the high flammability and explosion of VOCs. Many sensors have been fabricated to detect VOCs, such as cholesteric P-LCs film sensors [137], rGO sensors [139], metal oxide sensors [140], polymer composite sensors [138,141], and CNTs (carbon nanotubes) sensors [142]. Among them, SPFs provide a platform to integrate these materials for in line monitoring VOCs in real time. Tang et al. [137] designed a wavelength selectively coupling from the SPF to the cholesteric P-LCs film sensor, which was sensitive to tetrahydrofuran, acetone, and methanol gas with the sensitivities of 7.08 nm·L/mmol, 3.46 nm·L/mmol, and 0.52 nm·L/mmol, respectively, as shown in Fig. 4(g). Khan et al. [138] deposited a polymer planar waveguide on the SPF to fabricate a high sensitivity and a wide dynamic range sensor with the response time of 35 s, and it could detect dimethylamine, ethanol, benzene, toluene, and acetic acid with the concentration of 0 ppm -5 ppm. In these wavelength shift VOC sensing systems, the resonance wavelength shifted with the change of the VOC concentration.
SPF-based SPR sensors play a beneficial role in the application of in-situ, real-time, and label-free biosensing applications. Some SPF-based SPR sensors have been realized to detect the bovine serum albumin and phospholipase A 2 (PLA 2 ) in the nM concentration, as shown in Fig. 4(h) [143,144]. A D-shaped PCF biosensor proposed by Wu et al. has shown the sensitivity of 21 700 nm/RIU in the refractive index environment of 1.33 -1.34, and the excellent sensing performance of a D-shaped PCF SPR sensor was experimental demonstrated [50]. Dong et al. [21] demonstrated a side polished few-mode fiber SPR biosensor based on the gold film [ Fig. 4(f)], the sensitivity reached 4 903 nm/RIU, and the figure of merit was 46.1 RIU −1 when the RI ranged from 1.333 to 1.404. These provide a potential way for quantitatively detecting biological molecules in a real-time and online manner.

Vector magnetic field sensors
Magnetic field sensors are important to the areas of industry and military. However, the applications are limited because they can only sense the intensity of the magnetic field and cannot determine the direction [145]. The non-central symmetric structure of the SPF provides a new approach to detect the intensity and the orientation of the magnetic field simultaneously [9,146,147]. Combining SPFs with the SPR technology helps to construct highly-sensitive magnetic-field sensors, and several works have already been published [9,146]. Jiang et al. [9] proposed an SPF-based SPR vector magnetic field sensor, and the sensor showed a high sensitivity of 598.7 pm/Oe to the magnetic field intensity and a sensitivity of -5.63 nm/deg to the orientation of the magnetic field. Chen et al. [146] developed a highly-sensitive vector magnetometer based on the side-polished few-mode-fiber and SPR technology, and it is sensitive to the intensity and orientation of magnetic field (0.692 nm/Oe and -11.917 nm/°, respectively). Chen et al. [147] further proposed a portable vector-magnetometer based on the SPF and smartphone platform, and the sensitivities of this magnetometer to the intensity and orientation of the magnetic field can reach -0.050 dB/Oe and -0.263 dB/°, respectively, as shown in Fig. 4(i). This portable, low-cost, and highly-sensitive vector-magnetometer is suitable for sensing magnetic vector applications that require a portable size and real-time transmission of the measured results.

Conclusions and prospective future
This article reviews and summarizes the fabricating technique, fiber types, characteristics, and applications of SPFs, which have aroused strong interests and exhibited excellent application potentials in optical fiber devices and sensors. The SPF, an excellent versatile platform, has realized the integration with multiple materials such as metals, 2D materials, and P-LCs. The SPF can enhance the light-matter interaction, making the "lab-on-fiber" possible. Now, the fully automatic artificial intelligence fabrication technology and equipment have been successfully developed and commercialized, ensuring the large-scale fabrication and commercial application of the SPF. The applications of the SPF based devices and sensors have been intensively studied in the following areas: optical communication, WDM, health monitoring and wearables, environmental real-time monitoring, and detection of chemical substances and specific biological targets in the food and medical field.