Investigation And Demonstration Of The Mach-Zehnder Interferometer Based Optical Fiber Filter And Its Sensing Characteristics

Modal interference based all-fiber MZI sensors have been reported lately due to their simple structure. SMS fiber structure and MSM fiber structure is two representative modal interference based fiber devices. In this paper, we focus on axial strain, temperature, refractive index sensing characteristics of the SMS and MSM fiber structure. On basis of that, we proposed a fiber ring cavity laser sensing system based on modal interference effect. Also, two-path Mach-Zehnder interferometer based distributed fiber vibrate sensing system is theoretically and experimentally demonstrated. The main achievements of this thesis are listed as follows:1. The dependency of output SMF core and cladding modes excitation coefficients to the MMF length of the SMS fiber structure is firstly established when the input field is injected from a section of SMF. The mathematical expression of the center SMF cladding modes coupling coefficients is derived. On basis of this, the MMF length dependency of the MSM transmission spectrum is investigated and demonstrated. We only calculated the transmission spectrum of MSM fiber structure with AFS50/125MMF for its smaller core diamete. The results shows that the power will center in two interfaced cladding modes and the core mode in the center SMF with MMF length of1.4mm, which possibly bring the MSM fiber structure a better transmission spectrum with short length of SMF.2. A fiber ring laser sensing system based on modal interference effect occurs in multimode fiber is proposed. An axial strain and a refractive index laser sensor are established, respectively. Compared to conventional SMS based sensing system, the proposal have higher output peak intensity, higher optical signal to noise ratio (more than40dB), and narrower3dB bandwidth (about0.05nm), while not complicated the conventional sensing system. The proposed sensing system have not improved the reponse sensitivity.3. A polarization maintaining fiber (PMF)-MMF-PMF (PMP) fiber structure is firstly proposed as a fiber twist sensor. The operation principle can be explained by the different behavior of the optical field vector in the PMF and circular-symmetric fiber, when the fibers are exposed to twist around their longitudinal axes. Simutaneously, the MMF in the PMP fiber structure performs band-pass filter, and thus the transmission spectrum can be changed when there is a twist around the MMF longitudinal axes. We mapped the twist angle to an interference peak intensity. 4. A MSPSM fiber structure based fiber twist sensor is firstly proposed and experimentally demonstrated. The proposed sensor is constructed by spliced a short section of PMF in the MSM fiber structure. Compared to the PMP based twist sensor, the transmission spectrum of this structure have higher extinction ratio.5. A method to accurate measured the phase modulation coefficient of a PZT based optical phase modulator and a phase generate carrier demodulation method based distributed fiber vibration sensor is proposed. The function relationship between the driving signal amplitude and the phase modulation coefficient at the resonant frequency is built. Utilized the measurements, a PGC demodulation method based vibration sensor constructed by a Sagnac combined with a MZI is proposed. And a new algorithm is applied to located the vibration position in the sensing system.