Strain sensors can convert mechanical deformation into light/electrical signals and play an important role in fields such as robotics, human-computer interaction, electronic skin, human health, aircraft structures, and ground deformation monitoring. Fiber optic sensors offer many outstanding advantages, such as electromagnetic interference resistance, corrosion resistance, miniaturization, fast response speed, remote operation, and real-time monitoring. These sensors are a unique sensing method in specific situations, such as electrical hazards and explosive environments. Based on the above advantages, optical fiber sensors have great potential and application value in the field of strain detection. Photonic crystal fibers (PCF) have characteristics that traditional fibers do not possess. First, while traditional fibers are typically made of solid material, PCFs contain many pores, which makes it easier to deform when subjected to mechanical forces. Therefore, strain sensors based on PCF are more sensitive. Second, the background material of the PCF consists of pure silica and air holes, which results in very low thermal dependence. This helps to avoid crosstalk between strain and temperature, as the thermal sensitivity coefficient of the core in traditional optical fibers is much higher than that of pure silica due to the doping of other materials. Numerous reports on strain sensors based on PCFs exist, and their sensing types mainly include Fabry?Perot interferometers, Mach?Zehnder interferometers, Sagnac interferometers, long-period gratings, mode coupling, and others. These sensors are typically used as single-point sensors. By connecting multiple sensors in series to form a sensor array, a single interrogation unit can measure multiple sensing points simultaneously. This approach can significantly reduce the cost and complexity of sensors and enable the creation of sensor networks.

The PCF used in this experiment consists of six layers of air holes arranged in a hexagonal pattern. Compared with ordinary single-mode PCF, it has more air holes. Although it is also a refractive index-guided PCF, the more complex cladding structure can excite higher-order modes, which enables the interferometer to have a richer output spectrum. The welding model number is FITEL s179c. Since there is no welding procedure set for this type of PCF in the welding machine, the welding of the single-mode fiber and PCF is completed manually. The preparation of the interferometer involves only simple cutting and welding, which can be done using cutting knives and welding machines. The main preparation steps are as follows: before welding, remove the coating layer of the fiber and wipe it with alcohol; place the PCF and single-mode fiber at both ends of the welding machine, align them manually, and select the appropriate discharge intensity for welding. The porous structure of the PCF collapses under the stimulation of a high-intensity current to form a solid silicide. Light is introduced from the left end of the single-mode fiber, then into the solid silicide. The light is diffracted and broadened in the silicide, then enters the core and cladding of the PCF, meeting at the collapse region at the other end of the PCF, where it is coupled to form Mach?Zender interference. The position and depth of the valley are closely related to the length of the collapse area. A deeper valley improves the contrast and is more conducive to the detection of the sensor. The optical fiber sensor with different valley positions is obtained by tuning the length of the collapse area, and then the sensor is cascaded to realize the optical fiber sensor array.

The total output spectrum of the three interferometers after cascading is a simple superposition of the output spectrum of each individual interferometer (Fig. 6), and the position of the resonance wavelength remains basically unchanged. As shown in this figure, the valleys of the three PCF interferometers are effectively separated, and the spacing between them is relatively large, which means that a sufficient range of spectrum movement is available. This provides the basis for the sensor to measure multiple sets of parameters or multiple points simultaneously. We perform strain experiments on the cascaded interferometers. First, the total strain applied to the interferometer with a PCF length of 10.5 mm is 1800 με, with a strain interval of 300 με. The corresponding valley of the 10.5 mm interferometer lies within the range of 1570?1600 nm, and the interference valley shifts to shorter wavelengths as the strain increases. Meanwhile, the valleys of the other two PCFIs remain almost unchanged, which indicates that the spectral movement caused by the detection of the interferometer does not affect the detection of the other two interferometers [Fig. 7(a)]. The relationship between the valley of the three interferometers and the strain is shown [Fig. 7(b)], and the valley corresponding to the 10.5 mm interferometer is fitted linearly. The linear fitting coefficient reaches 0.99905, the fitting equation is y=-0.00143x+1585.11, and the sensitivity is -1.43 pm/με. The other two valleys shift by up to 0.042 nm. From the results of the measurements and linear fitting, the cascaded use of the interferometer does not affect its own performance or measurement accuracy. We conduct a similar experiment on another interferometer and obtain similar results, where the wavelength corresponding to the valley of the interferometer shifts towards shorter wavelengths as the strain increases, and the sensitivity reaches -1.55 pm/με. The difference in interferometer length leads to differences in sensitivity, while the valley positions of the other two interferometers, which are not subjected to strain, change very little.

We present an interferometric fiber sensor formed by fusing a PCF into two single-mode fibers. Strain is measured by observing the movement of the spectrum. By optimizing the collapse region length of the PCF, the appropriate cladding and core modes are excited. This coupling results in a narrow and deep valley appearing across a wide wavelength range, which greatly reduces the difficulty of sensor cascading and demodulation. An interesting phenomenon is observed during the experiment: when the input and output terminals are connected in a positive-negative configuration, the spectrum waveform remains almost unchanged. We first perform strain experiments on a single sensor, which shows a sensitivity of -1.36 pm/με and a linearity of 0.98920. Then, we conduct strain experiments on the cascaded PCF interferometer. There is almost no crosstalk between the sensors, which indicates that simultaneous measurement of different parameters or multi-point measurement of the same parameter could be realized. With further study of optical fiber mode excitation, we anticipate that the sensor’s loss will be reduced, and the number of sensors in the array will increase, thereby expanding the application potential of optical fiber sensors.