The coupling characteristic of long-period fiber grating (LPFG) is the power exchange between the fundamental modes transmitted in the core and the cladding modes transmitted through the cladding. Currently, LPFG is widely used in communication and sensing fields due to the advantages of high sensitivity, strong anti-interference ability, and high integration [1], [2], [3], [4], [5], [6], [7]. It can measure temperature, RI curvature, strain, etc. However, LPFG requires fixed equipment to be prepared, and the production costs are relatively high [8], [9]. The SNS structure based on multi-mode interference has the advantages of simple fabrication and high sensitivity. It has gradually become a substitute for LPFG in the field of sensing. To further improve the sensor’s sensitivity, the researchers have proposed a variety of sensors. In 2013, a compact temperature sensor used an MMF tip for temperature sensing with an average sensitivity of 0.0114 nm/°C [10]. In 2017, a sensor formed with the periodic arrangement of MMF and SMF was proposed for curvature and temperature measurement with a temperature sensitivity of −0.015 nm/∘C [11]. In the same year, the team proposed to replace MMF with NCF to form quasi LPFG with a sensitivity of −0.00643 nm/∘C [12]. In 2019, a miniature SMS-LPG bending sensor with high sensitivity created through periodically splicing of SMF and MMF was proposed. Compared with the traditional LPFG bending sensor scheme, the MMF in this sensor can not only reduce the sensor size, but also significantly improve bending response. In addition, the experiment results show that the temperature response of the sensor is insensitive, which helps obtain an accurate curvature value [13]. In the same year, Rao et al. proposed an RI modulated single mode-multimode-single mode (RMSMS) fiber sensor with a temperature-insensitive refractometer structure. The results show the maximum RI sensitivity of the fabricated sample is as high as 206.96 nm/RIU [14]. At present, most of the LPFG production processes require a high-frequency CO2 laser to periodically scan the fiber or a femtosecond laser to engrave the fiber [15]. However, both engraving methods will cause the micro-bending phenomenon of the grating. The expensive apparatus limits femtosecond laser to engrave the grating with asymmetric mode distribution.

Considering sensor miniaturization while ensuring sufficient RI modulation to form a clear resonance peak, a sensor called TNT fiber structure is proposed in this paper. This sensor mainly comprises no core fiber (NCF) and thin core fiber (TCF). TCF has a small fiber core diameter, and optical power in the optical fiber when propagating is constrained to a smaller range, so it is more sensitive to the change of external environmental parameters. NCF not only has a low thermal coefficient, but also efficiently avoids temperature cross-sensitivity [16]. The proposed sensor can exhibit a temperature sensitivity of approximately −0.066 nm/°C in the experiment. The maximum RI sensitivity can reach 880 nm/RIU. Due to high sensitivity, low cost, and simple preparation, the TNT fiber structure has potential application value in wide fields.