The Principle And Application Research Of A Sandwich Multimode Fiber Mach-Zehnder Interferometer

Multi-mode fiber optic sensor based on core diameter mismatch has received a lot of attention in the past 20 years because of its unique spectral characteristics,easy production,low cost and stable performance.Singlemode-multimode-singlemode（SMS）is the most basic structure of multimode fiber Mach-Zehnder interferometer.The change of external environmental parameters will affect the optical field distribution of guide modes in multi-mode fiber,and its transmission spectrum will change accordingly.These changes of external environmental parameters can be resolved in real time by tracking wavelength or intensity changes in characteristic peaks.So far,the research on multimode fiber Mach-Zehnder interferometer mainly focuses on stepped index rather than graded index multimode fiber.Due to the special refractive index distribution of GIMMF,there are still many unknown characteristics to be explored in optical fiber sensing.In this paper,we sandwiched the GIMMF between two very short（～1 mm）SIMMFs spliced with input and output single-mode optical fibers（SMF）on the basis of the SMS structure study.A novel sandwich multimode fiber interferometer（SIMMF-GIMMF-SIMMF）was designed and an effective composite interferometer was established in the GIMMF.After the theoretical simulation analysis,the preparation of the sensor and the analysis of temperature,curvature,refractive index and liquid level sensing characteristics were completed through the experiment.The research content of this paper are as follows:（1）The development history of multimode fiber Mach-Zehnder interferometer was summarized in detail,and the advantages and disadvantages of various structures were summarized.On the basis of previous studies,a novel sandwich multi-mode fiber interferometer was designed.The sensing principle and interference spectrum characteristics of the sandwich structure multi-mode fiber interferometer were analyzed in detail.The simulation models of SMS and SIMMF-GIMMF-SIMMF were established in the optical simulation software Rsoft respectively.The optical transmission process in the model and the optical field distribution in the GIMMF were simulated,and the influence of different interference lengths on the transmission spectrum of the sensors were simulated.（2）The sandwich multimode fiber Mach-Zehnder interferometer(L_GIMMF=1 mm)was prepared by using the self-assembled optical fiber precision cutting platform.Based on the theoretical simulation analysis,the response characteristics of the interference（MZI）dips of the fiber core modes and low order cladding modes to curvature and temperature were studied experimentally.The results show that the intensity of interference valleys is highly sensitive to external bending,but almost independent of ambient temperature.In contrast,the sensitivity of the interference dip wavelength to external bending is negligible,but the sensitivity to temperature is high.Therefore,a high sensitivity temperature independent curvature sensor could be realized by tracking the intensity change of interference valley.At the same time,different interference valleys show different curvature sensitivity based on intensity,which provides more options for curvature sensing applications.In the small curvature range of 0-2.36 m^-1,the maximum curvature sensitivity of the sensor can reach up to-78.75 d B/m^-1.（3）A flexible wearable breath sensor was designed by integrating the the sandwich multimode fiber Mach-Zehnder interferometer into the elastic belt and fixed on the human abdomen.The results showed that the designed wearable fiber respiratory sensor could effectively identify different breathing states and breathing rates.（4）The response characteristics of cladding mode interferometer（CMI）to refractive index and liquid level in the sandwich multimode fiber Mach-Zehnder interferometer(L_GIMMF=3,5 and 7 cm)were investigated experimentally.When the length of GIMMF is 3cm,The wavelength of the CMI interference dips is highly sensitive to the external refractive index and liquid level,but almost independent of the surrounding temperature.In the range of 0-30 mm liquid level（for aqueous solution）,the sensor has a maximum level sensitivity of 0.88 nm/mm.The sensor has a maximum refractive index sensitivity of315.21 nm/RIU in the refractive index range of 1.3162～1.3564,higher than previously reported heteronuclear structures,and a temperature sensitivity of only 2.71 pm/℃in the range of 30～45℃.Near 1550 nm,the longer the GIMMF length,the lower the liquid level sensitivity of the structure and the larger the dynamic detection range.It is easy to achieve customized sensing performance through appropriate optical fiber cutting.When the whole GIMMF section was taken as the refractive index sensing region,the refractive index sensitivity of the structure was independent of the GIMMF length.However,refractive index sensitivity was positively correlated with the sensing length of the actual GIMMF when only a partial GIMMF was used.On this basis,a more reliable calculation method for simultaneous demodulation of refractive index and liquid level was proposed by taking advantage of the difference of liquid level and refractive index sensitivity of two CMI characteristic dips,and the experimental verification was carried out.The results show that our measurement results are in good agreement with the actual value.（5）The main frequency component of the experimental spectrum of the sandwich multimode fiber Mach-Zehnder interferometer(L_GIMMF=2 cm)was extracted by band-pass filter and superposed with the reference spectrum obtained by frequency conversion processing of the experimental spectrum by signal processing to form a vernier spectrum.A high sensitivity temperature sensor based on wavelength demodulation was designed by tracing the envelope of the vernier spectrum.In the temperature range of 40℃to 100℃,the temperature sensitivity of the sensor system is 3.884 nm/℃,which is 37.346 times more sensitive than that of a single sandwich multimode fiber interferometer.