Theory And Sensing Technology Investigation Of Long-period Fiber Bragg Grating

With the optical fiber and photon device fabrication technology improving continually, fiber grating has become one of the most representative and promising fiber-optic passive components. And it has been widely used in optical communication, optical sensing and other fields. The long-period fiber grating (LPFG) as a new kind of fiber grating has been studied by many researchers all around the world recently, and has gotten great progresses in a few years. It attracts home and abroad scholars for its advantages, such as low insert-loss, broad-band, low back-reflection, sensitive to the changes of environments, low-cost and easy to be fabricated. It has shown a very broad application prospects within recent development. Futhermore, new structure, new features, and multi-function photonic device design have become an inevitable trend as the requirements of the fiber Bragg grating functional standard improves. The fiber Bragg grating (FBG) and LPFG, which are fabricated in polarization fiber, gain important applications depending on there unique spectral characteristics.LPFG is mainly used as EDFA gain equalizer, ASE noise filter, optical fiber coupler, beam filter, and WDM channel isolator in fiber communication. In the sensing field, LPFG has more advantages than FBG. The phase matching conditions are met by the gundermental core mode and cladding modes in the same direction due to its large period. So its resonance wavelength and peak strength are all very sensitive to external environmental changes. And LPFG has higher sensitivity than FBG to temperature, strain, bend, twist, lateral load, concentration and refractive index.In this paper, based on the early research of FBG and other photon devices, the coupler-mode theory and fabricated methods of LPFG are investigated in-depth. Furthermore, its temperature and stress characteristics are experimentally studied, and its application in vibration detection, lateral load testing, and sensitivity tunable system as well as in tunable laser manufacture are designed and implemented. Additionally, physical properties and sensing technique applications of PM-FBG and PM-LPFG are also discussed.The main contents of this article include: 1. According to three-layer cylindrical fiber model, the fiber mode distribution is analyzed. And the coupling of fiber core-mode and low first-order cladding mode is also further studied. Then coupling constants are used to describe the strength of mode coupling. Meanwhile, long period fiber grating characteristics analysis and numerical simulation provide a theoretical basis for LPFG fabricated and sensing application development.2. A variety of LPFG fabricated methods are described, including UV-fabrication method, high-frequency CO2 laser-fabrication method, corrosion-groove method, arc discharge method, ion beam implantation method, mechanical-induced micro-bend method and et al. UV-fabrication method, CO2 laser-writing method and mechanical-induced micro-bend method are adopted in this paper. The relation of fabricated period, grating length, refractive index modulation depth and resonant wavelength, resonant peak, bandwidth is researched. And the temperature and stress characteristics are experimentally studied.3. A tunable L-band ring Er(3+)-doped fiber laser (EDFL) is designed through inserting a mechanically induced long-period fiber grating (MLPFG) into Fiber ring cavity. The gratings period is altered by adjusting the angle between the periodic pressure plate and fiber. Above operation will directly influence the highest gain point of the fiber ring cavity, and make the output laser tunable. Optical fiber laser wavelength-tunable range 42nm (1562.465nm-1604.280nm) is achieved. Output laser's 3dB line-width is less than 0.04nm and the 20dB line-width is less than 0.08nm. While the side mode suppression ratio (SMSR) is more than 45dB. Prolonged observation, laser power stability is better than 0.3dBm.Experimental results show that the tunable EDFL has the advantages of wide bandwidth, narrow line-width and stable performance.4. A novel FBG vibration sensor demodulation system is designed through demodulation methods comparison. From the validity of calibration simplicity, reusability, and cost considerations, a special long period fiber grating is chosen as demodulation filter. Based on theoretical analysis of the cantilever beam vibration sensing, the relationship between beam length, width, thickness and the sensor resonant frequency is discussed. And then high-frequency and low-frequency FBG vibration sensor is designed respectively. Not only high-frequency but also low-frequency vibration signals achive exact detection without any distortion, combind with photoelectric conversion, ampliation, and filter circuit design. Furthermore, the sensor's frequency, amplitude and impulse response performance are texperimented. The low-frequency vibration sensor is effective in range of 20-200Hz. The results are compared with vibration sensors based on other technologies. And high-frequency vibration sensor is effective in range of 1～5 kHz. The frequency detection error is less than 0.5%.5. A novel transverse load sensing system based on MLPFG is designed based on the research of transverse load characteristic of LPFG fabricated by CO2 laser-irradiation method and UV-exposure method. Mechanical-line processing technology is used to make period stainless steel pressure grooved plate. The relationship between lateral load and LPFG resonant peak is measured. Finally, the high-precision LPFG lateral load demodulation system is realized in virtue of special designed FBG The practicality of detection system is further enhanced. Experiments show that within the range of 0-60N, the lateral pressure and the MLPFG transmission peak strength have a good linear relationship. The linearity is up to 0.9950, and the sensitivity is about 0.35dB/N. Maintaining the pressure of 45 N for 20 hours, we found the fluctuation of the MLPFG resonant peak is less than 0.2dB. A special chosen FBG with center wavelength 1542.890nm is adoted to realize system demodulation. And the demodulation sensitivity 0.12μw/N is achieved.6. A demodulation sensitivity tunable system is designed depending on the easy tenability of MLPFG resonant slope. A special designed FBG which is fabricated by UV laser technology is used to make a detection sensor. The system is achieved through tuning MLPFG resonant side-band (FBG center wavelength is in the range of MLPFG resonant slope). Sensitivity tunable experiments are carried out when the pressure applied to the LPFG from 20N to 60N, and the axial strain imposed on the FBG sensor from 0μεto 3000μεwith the step interval 200με. The experiment results show that, FBG strain sensing system detection sensitivity of optical power changes from 0.802 nW/μεto 1.204nW/με.7. A linear cavity switchable dual-wavelength erbium-doped fiber laser is designed using a polarization maintaining fiber grating and a polarization controller. Through adding polarizing beam splitter into the formal system, single-polarization laser is derived, corresponding to the PM-FBG fast axis and slow axis, respectively. In addition, PM-LPFG is studied theoretically and fabricated adopting mechanical-induced method. Through temperature characteristic analysis, its application in transverse load and temperature simultaneous measurement, as well as tunable polarization is discussed.