Some Studies Of Devices For Optical Communications And Sensing Based On Fiber Gratings

Information technology changes our life. As a main carrier of information transmission medium optical fiber plays an important role in the process of information. Various types of optical fiber-based photonics devices make up a modern information highway. Fiber gratings, including fiber Bragg gratings (FBGs) and long-period gratings (LPGs), are critical components due to their spectral selectivity. Furthermore, fiber-based sensors are also widely used in the information collection. Fiber grating-based sensors have turned into one of the most important class of optical fiber sensor due to their inherent self-referencing and multiplexing capability.In this thesis, several fiber grating-based photonics devices for optical communications and sensing applications have been investigated theoretically and experimentally. The main contents are listed as follows:Firstly, the spectra of various fiber gratings, including uniform FBG, apodised FBG, chirped FBG and LPG, were theoretically calculated with the coupled mode theory and transfer matrix method by C++ programming. The spectral properties of such gratings were also analyzed. During the project we have established a fiber grating fabrication system. Furthermore, a program was developed to automate the grating fabrication system and data acquiring. We have also designed and developed a high-pressure hydrogen-loading system, which made the grating fabrication on various types of fiber possible. The grating fabrication system and fiber hydrogen-loading system provide a good experimental platform for the followed research.LPGs are essentially band-rejection filters and the transmission spectra can be tailored according to the grating parameters. We proposed a novel structured LPG as gain-flattening devices of erbium-doped fiber amplifer (EDFA) based on the spectral tailoring capability of LPG. The structured LPG is composed of three sections, each section of which has different grating parameters. In order to obtain a gain-flattening spectrum for an EDFA with a gain bandwidth of over 80 nm, we have optimized the grating parameters with a self-developed genetic algorithm. We also have fabricated the proposed structured LPG with the grating parameters. By concatenating the LPG and EDFA a gain-flattening spectrum with flatness within ±0.69 dB is achieved.Two LPGs separated with an interval in a same fiber compose an in-fiber Mach-Zehnder interferometer (MZI), which can be used as multiwavelength filter. The issue of how to enlarge the effective bandwidth of such LPG-based MZI is to enlarge the bandwidth ofcomposed LPGs. We proposed a novel step-changed LPG, in which the multi-path interference is suppressed but the spectral tailoring capability has been improved. With genetic algorithm we optimized the grating parameters of such step-changed LPG to obtain an LPG with a wide flat-bottom transmission spectrum. We have fabricated the wideband multiwavelength filter based on the step-changed LPG and demonstrated its spectra properties.A multiwavelength erbium doped fiber (EDF) laser using multimode fiber Bragg gratings (MFBGs) was proposed. The amplification principle of EDF was introduced with the two-level model. Due to its excellent amplification EDF has also been used for fiber laser. However, EDF is essentially a homogeneous broadening medium at room temperature, which leads to mode competition in the multiwavelength fiber laser when using EDF as gain medium. Several methods to suppress the mode competition in the EDF laser, including the polarization hole-burning (PHB), were introduced. The properties of MFBG were also analyzed and MFBG is a polarization-dependent loss element, which provides PHB. By combining PHB and overlapped gain cavities we realize a four-wavelength fiber laser, and the wavelengths can be switched by controlling the polarization controllers. The grating fabrication and how the fiber laser works were detailed introduced.The spectra of LPGs are sensitive to the surrounding refractive-index (RI), which is determined by the mode-coupling mechanism of LPGs. Compared with a single LPG, LPG-based MZI processes sharper resonance peaks, which help to improve wavelength measurement. The theoretical analysis shows that the sensitivity to RI of LPG-based MZI can be improved by changing the interval section between two LPGs of MZI. In the thesis we proposed two schemes to improve the RI sensitivity: the etched interval and the tapered interval schemes. The experimental results show that the RI sensitivity can be effectively improved with such schemes.Discrimination between RI and temperature is a critical issue in RI sensing. We proposed a sensor based on a dual-LPG structure for simultaneous measurement of RI and temperature. The two LPGs possessed similar temperature sensitivity but different RI sensitivity. If the wavelength shifts of two LPGs were obtained, using sensitivity-matrix the changes of RI and temperature can be calculated, respectively. Such scheme for simultaneous measurement of RI and temperature is relative simple and easy-fabricated.