In the 1960s, a waveguide made of quartz was used for the first time to transmit optical signals, which became known as optical fibers. Corning has developed low-loss optical fibers that can transmit optical signals over long distances. This led to a period of rapid development of fiber-optic communication. Subsequently, fiber-optic sensing came into being. Compared with traditional electrical sensors, fiber-optic sensing uses optical signals as the modulation and transmission carrier, which allows it to have many unique advantages ^[1][2][3][4][1,2,3,4], such as strong resistance to electromagnetic interference in the transmission process, thus allowing it to play a very significant role in the power system ^[5][6][5,6], and strong corrosion resistance, which can be measured for highly corrosive analytes [7], as it has a compact structure that can be fabricated according to the needs of the size of very small fiber optic sensors [8]. Simultaneously, several studies have highlighted the advantages of simple fiber optic materials, cost-effectiveness, and broad scale utilization ^[9][10][9,10]. With their own inimitable characteristics, fiber-optic sensors have broad application prospects in a variety of sectors, including environmental monitoring, civil engineering, biomedicine, industrial production, aerospace, energy development, and food safety ^{[11][12][13][14][15][16][17]}[11,12,13,14,15,16,17].

Optical fibers have excellent resistance to interference from the external environment, which enables them to be highly reliable and stable, and to a certain extent expands their application fields. In order to achieve the sensing function and increase the sensitivity, special processing procedures are frequently used to modify the geometry of the fiber in order to disrupt the original total reflection transmission mode. Processing methods include taper pulling, core-offset splicing, laser etching, and side grinding and polishing ^{[8][18][19][20][21][22][23]}[8,18,19,20,21,22,23].

Through advanced optical fiber processing equipment and fabrication techniques, more and more special fiber structures are coming onto the stage of fiber sensing, such as tapered optical fiber (TOF) structure [17], D-shaped structure [24], U-shaped structure [25], S-shaped structure [26], fiber grating structure [27], heterocore structure [28], core-offset structure ^[29][30][29,30], and microsphere structure [31]. These special fiber structures can effectively excite the enhanced evanescent field and expose it to and interact with the surrounding medium, thus promoting the interaction between light and sensing materials and achieving higher sensing sensitivity. Among them, the TOF structure has a smaller radius of the waist taper region, that can generate a larger local electric field in the tapered region, thus generating a higher power evanescent field. The high-power evanescent field has the ability to detect subtle changes on the fiber surface, including biomolecules, temperature, pressure, chemical ions, and gases. This results in a higher level of sensitivity and an improved limit of detection (LOD) for the sensor ^{[32][33][34][35]}[32,33,34,35]. As a result, optical fiber sensors with TOF structures have numerous applications in a variety of domains ^[36][37][36,37].

The TOF sensor also has many other distinguishing features. Firstly, it is compact and can be flexibly installed for measurement and detection in various environments [38]. Secondly, TOF does not require complex processing and expensive materials, and the preparation cost is relatively low [39]. In addition, TOF processing accuracy is high, and the preparation process is relatively easy to control with high reproducibility. At the same time, TOF has a large surface area at the tapered structure, which can better contact the analyte, thus improving the sensing effect [40]. Furthermore, the TOF sensor design is flexible and can realize different functions of the sensor by changing its angular structure, sensing medium, and sensing layer. The optical fiber processed by pulling the taper expands the abrupt field when light is transmitted in the fiber, increases the contact area with the external environment, significantly improves the sensitivity and response speed of the fiber optic sensors, and shortens the size of the fiber-optic sensor ^[41][42][41,42]. The preparation methods of TOF probes include arc discharge technology [43], laser processing technology [44], chemical etching technology [45], etc. TOF structures are commonly found in the following configurations: single tapered structure [46], nano-tapered fiber [47], grating tapered structure [48], multi-tapered cascade structure [49], taper tip [50], and so on. Internal beam splitting and optical path coupling will occur as a result of the change in fiber geometry. Moreover, the evanescent field near the tapered fiber will cause the light to diffuse into the surrounding environment as well as change the beam splitting and coupling ratio of the tapered fiber. The fiber-optic sensor with a tapered structure of can determine the refractive index (RI), curvature, strain, and other physical quantities of the surrounding environment. In conclusion, based on the characteristics and advantages of TOF sensors, it will provide a broader development space and a new platform for optical fiber sensor research.

Tapered optical fiber-based sensors have gained popularity in various fields such as gas sensing, physical sensing, chemical sensing, and biological sensing in recent years. Shaimerdenova et al. [51] reported a shallow tapered reflection-free fiber optic sensor for cancer biomarker detection. The sensor utilizes magnesium oxide nanoparticle-doped optical fibers and is functionalized using a silylation method to detect the breast cancer biomarker CD44 protein. The results show that the proposed sensor prototype is capable of measuring CD44 protein with remarkably low detection limits and high specificity. Similarly, Ayupova et al. [52] proposed a sensor based on a gold-modified shallow-tapered chirped fiber Bragg grating (FBG) to simultaneously monitor RI and temperature.

2. Fabrication Method of Tapered Optical Fiber

Different fabrication methods can be used to manufacture TOF depending on the fabrication requirements of the tapered fiber and the limitations of the equipment. The common fabrication methods include arc discharge technology, laser processing technology, and chemical etching technology. The methods and characteristics of these technologies are summarized in this section.

2.1. Arc Discharge Technology

Arc discharge technology is a processing technique that allows controlled electrodes to periodically heat the fiber layer to a molten state by freely adjusting the discharge power and precisely limiting the discharge time. Compared with the hydroxide flame heating method, the arc discharge technique is able to form a highly uniform high-temperature area around the optical fiber to heat it up, enabling faster softening of the fiber and greatly improving processing efficiency. For instance, the combiner manufacturing system (CMS) machine is used to heat the optical fiber using thermally stabilized plasma technology. The vacuum saturation and the discharge power between the electrodes are adjusted using a program to achieve optimal heating conditions. The important internal components of the CMS mainly composed of electrodes, fiber clamps, camera, and a movable motor platform. The vacuum module uses the gas pressure principle and a peripheral air pressure pump to maintain a constant semi-vacuum around the fiber. The three electrodes discharge synchronously and flexibly to cover uniformly the maximum thermal zone size of the said fiber. When the fiber reaches the molten condition, the platform motor stretches it under the control of a predetermined program, producing a tapered structure. This method can produce high-quality, well-shaped optical fiber structures in a shorter period of time.

2.2. Laser Processing Technology

The laser processing technique is a typical approach for manufacturing TOFs. The fundamental concept is to employ the laser’s high energy and precision to locally erode the fiber surface for constructing a TOF structure. The laser beam is specifically focused on the fiber surface via a lens, causing the material on the fiber surface to melt or peel, resulting in a reduction in fiber diameter and shape modulation. This technology has applications for fabricating TOF of diverse shapes and diameters because of its superior precision, controllability, and reproducibility. 

2.3. Chemical Etching Technology

Chemical etching is a common method for TOF fabrication, which involves etching the fiber surface through a chemical reaction ^[53][88]. The taper angle of the TOF can be controlled by varying the time the fiber is treated in the chemical reagent. The size of the taper angle can affect the efficiency of light transmission in the fiber. The fabrication process has simple equipment and low cost, but it is difficult to control the taper angle and the diameter of TOF precisely. The chemical reaction conditions and operation process need to be strictly controlled to ensure the quality of the fabricated TOF ^[54][89].  Overall, each of the above techniques for fabricating TOF has its own advantages and disadvantages, and the most suitable manufacturing method needs to be selected according to the specific application requirements and experimental conditions.