Mach - Zehnder Interferometer Based On Dual - Core Fiber And Its Application In Sensing

Twin core fiber(TCF) is a novel special optical fiber and because of the special refractive index(RI) distribution of TCF, a lot of optical passive devices based on TCF have been widely used in optical fiber communication and sensing fields. Nowadays, many fabricating method of Mach-Zehnder interferometer sensors based on TCF have been reported. The common fabricating method is fusion splicing by fusion splicer, but it is difficult to achieve a high accuracy and precise sensing location in sensing applications. We proposed the use of femtosecond laser micromachining technology to fabricate TCF-based Mach-Zehnder interferometer. The technology uses the characteristics of high peak power of laser pulse, short pulse and micron-scale structures in the media surface or interior. Because femtosecond laser is nonlinear interaction with matter, the heat affected zone is small. Compared with traditional processing, the quality of femtosecond laser micromachining is much higher. Therefore, the study of the use of femtosecond laser micromachining technology to fabricate TCF-based Mach- Zehnder interferometer is a new and significative subject.In this paper, the theory and experiment of TCF-based Mach-Zehnder interferometer have been done to study the sensing characteristics. The main work and innovative achievements are as follows:1) The research of twin core fiber, optical fiber Mach-Zehnder interferometer, femtosecond laser micromachining have been introduced.2) We design and build a femtosecond laser micromachining system, which includes a femtosecond laser, beam splitter, half-wave plate, Glan prisms, reflectors, microscope systems and high-precision three-dimensional electric-shift platforms. We have successfully fabricated some types of interferometers by using this system in ordinary single-mode fiber(SMF), TCF, photonic crystal fiber(PCF).3) We have experimentally studied the sensing characters of RI and temperature of TCF-based Mach-Zehnder interferometer. The experiments illustrated an ultra-high RI sensitivity of-10981 nm / RIU and ultra-low temperature cross-sensitivity of 3.96×10-6 RIU/°C. So the influence of temperature cross-talk can be ignored. Theoretical analysis of the sensor can obtain the RI sensitivity of-10690 nm / RIU. It can be seen, the experiment agreed with the theory.4) We have experimentally studied the sensing characters of gas pressure of TCF-based Mach-Zehnder interferometer. A good linear response with an ultra-high pressure sensitivity of-9.6 nm/MPa was obtained. Compared with other sensors, the sensitivity of this gas pressure sensor has nearly been enhanced by one order of magnitude. There are two factors to affect the gas response of the gas pressure sensor which are the changes of the microchannel length and the air RI into the microchannel. In order to further study the factors, we use the soft of ANSYS to simulate the change of microchannel length in gas pressure environment. When the gas pressure was 1 MPa, the length of the microchannel increased by 0.449 nm which resulted in the spectrum shifting by 14 pm, but the minimum resolution of OSA was 20 pm. So the change of microchannel length can be ignored. Changes in the RI of air caused by changes in air pressure has become a major factor in the pressure sensitive. the pressure sensitivity of the TCF-based MZI can be calculated to be ~9.52 nm/MPa which is very close to the experimental result.5) We have experimentally studied the sensing characters of magnetic field of TCF-based Mach-Zehnder interferometer. The TCF with one micro-channel was immerged into a water-based Fe3O4 magnetic fluid that was sealed in a capillary tube, thus achieving a magnetic sensing element. When the magnetic field changes, the RI of magnetic fluid would vary immediately which leads to the change of phase of light. The sensor senses the changes and transmits to the OSA which reflects the shifting of spectrum. Such a Mach-Zehnder-based magnetic field sensor exhibits a linear magnetic response with a measurement range from 0 to 8 m T and an ultrahigh sensitivity of-37.1 nm/m T, to our best knowledge, which is two orders of magnitude higher than other magnetic sensors reported previously. Then we studied the temperature response of magnetic fluid-filled TCF-based Mach-Zehnder interferometer. A high temperature sensitivity of 1.36nm/°C is obtained.6) We have experimentally studied the sensing characters of RI and temperature of in-fiber Michelson interferometer(MI) created by splicing a section of thin core fiber between two standard SMFs. The experiential data present that the intensity and wavelength of reflection spectrum have a variation. Such an in-fiber MI could be used to develop a promising intensity-modulated refractive index sensor that exhibits an ultrahigh sensitivity of-208.24 and 125.44 d B/RIU at the RI of 1.440 and 1.500, respectively. Moreover, the MI-based refractive index sensor solves the cross-sensitivity problem between temperature and surrounding refractive index and could find potential applications in the fields of chemical and biomedical sensing, and environmental monitoring.