Ultrasensitive Temperature Sensor And Mode Converter Based On A Modal Interferometer In A Two-Mode Fiber

Two-mode fiber is a special few-mode fiber that only supports two transmission modes,LP01 and LP11.The tapered two-mode fiber drawn to the micrometer scale has simple structure,low cost,strong evanescent field and uniform comb interference.Therefore,it has a wide range of applications in the fields of magnetic field sensing,temperature sensing,macromolecular protein detection,refractive index sensing and other fields.Isopropanol is a high-performance thermooptic material with high thermo-optic coefficient,and is widely used in various optical fiber temperature sensors.In this thesis,combining the advantages of tapered two-mode fiber and isopropanol,a temperature sensor with ultra-high sensitivity was fabricated through the interaction of the strong evanescent field of the tapered two-mode fiber and isopropanol,and combined with the vernier effect.A multi-channel mode converter is realized by utilizing the multiple phase matching points of the tapered two-mode fiber.The converter has the advantages of high sensitivity,high mode conversion efficiency,low cost,simple structure and mature fabrication process.Isopropanol-enhanced tapered two-mode fiber device is mainly composed of tapered two-mode fiber and isopropanol encapsulated in the surrounding thermo-optic material.One hand,the tapered two-mode fiber device provides evanescent field and uniform comb interference spectrum,and the strong evanescent field enhances the interaction between transmitted light and Isopropanol.And on the other hand,the uniform mode interference spectrum including only LP01 and LP11 modes overcomes the shortcomings of the traditional interferometric fiber optic sensor that the interference spectrum is not uniform,the sensitivity is low,and cannot be used the vernier effect to amplify the sensitivity.The refractive index of isopropanol with high thermo-optic coefficient changes with temperature,which improves the temperature sensitivity of the device.With the vernier-like effect,the interferometric spectral shift of the temperature sensor is amplified by the vernier spectrum containing a large envelope,achieving a temperature sensitivity far higher than other interferometric fiber optic sensors.At the same time,the multiple phase matching points of the uniform comb spectrum of the tapered two-mode fiber are used to realize the mutual conversion between the LP01 and LP11 modes,and combined with the characteristics of the thermo-optic material isopropanol,the tunable mode conversion is realized.In this thesis,the coupling conditions between the fundamental mode and higher-order modes of the device are firstly analyzed from the optical waveguide theory,and a simulation model based on Rsoft is established to further clarify the evolution of the optical field inside the device,and the properties of device are explored in detail.Next,the temperature sensing properties,mode conversion properties and response time of the isopropanol-enhanced tapered two-mode fiber were fabricated and tested.The relationship between the temperature sensitivity of the device and the wavelength range and free spectral range is emphatically analyzed.The traditional use of vernier effect to achieve sensitivity enhancement requires two interferometers,one of which is used as a reference and the other is a sensor.Since the two interferometers are close to each other,it is difficult to keep one interferometer as a stable reference,and two interferometers are The interferometer will introduce additional insertion loss,so we innovatively propose to amplify the temperature response of the device based on the vernier-like effect of a single device.Using the Vernier-like effect to amplify the temperature sensitivity of the device by a factor of 58.5 without using additional devices,ultrasensitive temperature sensing with a sensitivity of-140.5 nm/℃ is achieved.In the mode conversion test experiment,the mutual conversion between the all-fiber multi-channel LP01 mode and the LP11 mode was realized,and the mode conversion efficiency higher than 99.49%was achieved at the measured conversion wavelength.And measured the mode conversion of the device under the control of external temperature.Finally,the response time of the device was measured to further characterize the device performance.The innovations of this work are as follows:1.The Vernier-like effect based on a single tapered two-mode fiber is proposed for the first time to amplify the temperature response.Compared with other devices using the vernier effect,it does not require an environmentally stable reference device and does not introduce additional insertion loss.Combined with the strong evanescent field of the tapered two-mode fiber,the high thermooptic coefficient of isopropanol,and the Vernier-like effect,it achieves ultra-high Sensitivity temperature sensing.2.Based on the uniform comb spectrum of the LP01 and LP11 mode interference of the device and the characteristics of the thermo-optic material isopropanol,a multi-channel tunable mode converter is realized.