Optical fiber sensors which have many merits such as compactness, fast response, and strong anti-electromagnetic interference are used in monitoring of electrical properties [1], [2], refractive index [3], [4], temperature [5], [6], and strain [7], [8] in many fields including health monitoring and biomedical engineering. There are two kinds of strain sensors. The first type is based on fiber gratings such as fiber Bragg fratings (FBGs) [9], [10] and long period gratings (LPGs) [11] and the second kind is based on interference sensors such as Mach–Zehnder interferometers (MZIs) [12], [13] and Fabry–Pérot​ interferometers (FPIs) [14], [15], [16], [17], [18], [19], [20], [21]. Compared to other sensors, the air cavities in the optical fibers of FPIs [22] have smaller thermal expansion coefficients rendering them relatively insensitive to the environmental temperature leading to lower temperature-strain cross-sensitivity and more reliable strain measurement.

So far, various methods have been proposed to fabricate FPIs with high strain sensitivity by means of electrode discharge fusion splicing technique. For example, Li et al. have prepared a circular air cavity by fusing the single-mode fiber (SMF) and hollow-core photonic crystal fiber (HC-PCF) [14]. The strain sensitivity of the sensor is 3.36 pm/μ ɛ and the interference contrast is 4 dB. Duan et al. have preprocessed optical fiber tips to obtain two SMFs with cleaved flat tips and fusion induced hemispherical tips and then fused them to form an air cavity [15]. The strain sensitivity of the sensor increases to 4 pm/μ ɛ and the interference contrast is 7 dB. Cai et al. have proposed an asymmetrical air-cavity Fabry–Pérot​ interferometer (UAFPI) [21]. The air cavity is formed by welding and chemical etching of the erbium-doped fiber (EDF) and discharging multiple times on one side to form the taper. Compared to the conventional air-cavity Fabry–Pérot interferometer (AFPI), this sensor exhibits a high sensitivity of 10.15 pm/μ ɛ and interference contrast of 5 dB. However, these sensors require expensive PCFs and special optical fibers and the fabrication involves complex pretreatment and dangerous chemicals. For electrode discharge fusion splicing technique, manufacturing process parameters such as discharge time, discharge power and applied tensile stress have great influences on the cavity structures of the sensors, thus leading to different sensing performances. For example, the discharge time determines the radial expansion of air cavity and the flatness of the reflection surface on the air cavity wall. Therefore, further investigations are needed to improve strain sensitivity and interference contrast.

In this work, a simple and economical fabrication method is designed to prepare the Fabry–Pérot​ interferometer (RRAFPI) with a rounded rectangular air cavity. Two SMFs are fused to form the tiny air cavity and then different stresses are applied and discharged to produce the RRAFPI. The sensor has not only the unique characteristics of AFPI, but also a good reflection surface on the air cavity which improves the interference contrast. The selected parameters in this work can make the reflection surface of the cavity wall flatter and smoother, and bring the angle of incident light closer to 90°, which in turn allows the sensor to have an interference contrast of up to 25 dB. The sensor has a strain sensitivity of 8 pm/μ ɛ at 1550 nm. In the temperature range of 20–80 °C, the temperature sensitivity of the sensor is 4.79 pm/°C and the strain-temperature cross-sensitivity [23], [24], [25] is only 0.59 μ ɛ/°C. The interference contrast of the sensor is 25 dB that is almost four times that of AFPI.