Research On Fabrication Of Fiber Bragg Grating By Femtosecond Laser And Its Application

Modern optical fiber gratings are not only applied in various filters of optical communication network, but also possess potential application prospects and business values in high power lasers and high temperature optical fiber sensors. But optical fiber gratings fabricated through traditional ultra-violet laser technology are hard to satisfy these needs due to inherent deficiency. The induced refractive (RI) index is highly dependent on UV photosensitivity of optical fibers and easily erased in high temperature condition. This stimulates researchers to improve existed fabrication methods and introduce new ones.Femtosecond laser system has been widely used in micro-nanofabrication for transparent dielectric materials. Its ultra-short pulse width and ultra-high peak power are able to induce bigger RI change in these dielectric materials in order to realize fabrication of micro-optical structures and photonic devices. This provides brand new prospects and research basis for fabrication fields of optical fiber gratings. Fabricating fiber Bragg gratings (FBGs) using femtosecond laser is as simple as writing gratings through ultra-violet laser and even have some advantages over ultra-violet laser technology. It can write Bragg gratings in non-sensitized silica fiber. The induced RI modulation and temperature stability is higher. Besides, it only needs a few seconds of exposure time to get high quality grating structures if laser power has reached a certain value. The mechanism of this technology is thought to be direct band gap transition stimulated by multiphoton absorption. The induced RI variation is dependent on incident laser intensity. Above the ionization threshold intensity I th, multiphoton ionization (MPI) can induce localized melting and material compaction causing an permanent RI change which can sustain even at the glass transition temperature Tg of the material. This kind of FBGs is called Type II-IR FBGs. Below I th, multiphoton absorption results in higher nonlinear defect formation. This kind of FBGs which has low temperature stability is called Type I-IR FBGs.Based on the theory above, the research on fabrication of FBGs using infrared femtosecond laser combined with phase template method and application of it is conducted. This paper firstly expounds in detail the analysis method of FBGs'spectral characteristics including coupled-mode theory of waveguide in perturbation and Rouard's method and then simulates the influence inflicted on spectral characteristics by grating length and coupling coefficient's product ? L through coupled-mode theory. For IR FBGs fabricated by infrared laser, the RI modulation ?n mod is non-sinusoidal, which makes Rouard's method appropriate for simulating this kind of spectral response characteristics.Then I analyzed the diffracted-order walk-off effect after femtosecond laser transmitting through phase template. The obvious advantage of this effect is that pure two-beam interference pattern can be made even without zero-order suppression. Moreover, thorough utilizing tight focusing cylindrical lens and selecting bigger distance between template and optical fibers, avoiding impairment of templates to extend the lifetime and forming high power density in fiber core can be achieved. Based on this, Type I-IR and Type II-IR FBGs are fabricated in SMF-28 optical fibers using infrared femtosecond laser with wavelength of 800nm and pulse width of 120fs. The laser repetition of 4 ~1000Hz and pulse energy of 0.400 ~1.000m J /pulse are adopted. The reflectivity of the two gratings fabricated are 91.15% and 99.85%. The RI modulations are estimated to be 2.14 ? 10?4 and 4.50 ? 10?4. In the research of Isochronal annealing, the former is erased below 1000℃while the latter can be stable until 1000℃. And the reflectivity which is the RI modulation ?n mod slightly increases with temperature. It might be because the light intensity corresponding to the groove region in the interference field is lower than the impairment threshold. As a result, the RI variation with lower stability was formed. This variation will be erased when temperature gets higher, which leads to slight increase of ?n mod. In addition, the research on axial stress and temperature sensing characteristics of Type II-IR FBGs shows that stress sensitivity is approximately 1.474 nm /N and the temperature sensitivity is approximately 14.01 pm /℃. The relation of resonant wavelength and the two is linear.At last, a thermal effect method utilizing Type II-IR FBGs temperature sensitivity characteristics to detect interaction region of femtosecond laser and optical fibers was put forward. The theoretical basis is that the energy accumulation in the interaction region can form equivalent temperature. Because thermal-optical effect and thermal expansion effect can inflict influences on optical fiber's property, if Bragg grating structure is written into laser affected optical fiber region, based on temperature sensitivity of FBGs ? ??? B ? ?B ? ? ?? T, the resonant wavelength shift ?? B brought by effective temperature can be detected and the effective temperature can be further gained. This thermal effect is determined by femtosecond laser repetition and pulse energy. Finally, through experimental study, the effective temperature cure of interaction region of femtosecond laser and optical fibers can be given. This method is hoped to provide a new prospect for research on thermal effects in femtosecond laser"cold"fabrication process.