Research On Fabrication Of Long Period Fiber Grating By Femtosecond Laser And Its Sensing Characteristic

Long Period Fiber Grating (LPFG), whose grating period is approximatelyfrom dozens to hundreds of micrometers, performs the coupling between thecore mode and the forward-propagating cladding modes. It is a kind oftransmission grating. Compared with Fiber Bragg Grating (FBG), the LPFGhas many unique applications that can be used as gain flat in erbium dopedfiber amplifier, band elimination filter, the most important application is thatthe LPFG is very sensitive to the surrounding refractive-index, also can beused as a very ideal refractive index sensor. The fabrication of LPFG bytraditional ultraviolet laser technology is very proficient and also can get thegood spectrum, but the traditional ultraviolet laser induced LPFG also hassome shortcomings, not only need additional method such as hydrogenloading to enhance the photosensitivity, but also the weak temperature stability,the degeneration that induced by UV light was observed even at85℃.Femtosecond laser has ultra-short pulse width and extremely high peakpower (up to1021W/cm2), it has been widely applied in transparent mediummaterial three-dimensional fine processing to fabricate high performanceoptical waveguide、F-P cavity and grating with size of micro-nanometer opticalcommunication and sensing devices. As a new grating carving technology, thetechnology of fabricating LPFG by infrared femtosecond laser attracts moreand more attention in recent years. Compared with the traditional ultravioletlaser LPFG carving technology, the biggest advantage is that the infraredfemtosecond laser can not only write grating in non-sensitized silica fiber (forexample): pure fused silica core optical fiber, photonic crystal fiber andsapphire crystal optical fiber, etc), but also in complex waveguide medium, andthe temperature stable performance is good even at high temperature.Whereas all these mentioned, this paper carried out the fabrication of LPFG byfemtosecond laser、 the mechanism and the sensing application, the maincontents are as follows: First of all, we mainly research the mechanism of interaction between thefemtosecond laser and transparent medium, the phase change mechanismand a brief introduction of the basic principle of the LPFG, these are the basictheory of using800nm infrared femtosecond laser amplifiers to fabricate LPFG.We know two different forms of femtosecond laser interacting with fused silica:nonlinear photoionization and avalanche ionization. Nonlinear photoionizationinclude two different regimes of photoionization: the multiphoton ionization andthe tunnelling ionization. Each kind of ionization has the different characteristic.Through the theoretical model and simple calculation, we know the mainfunction is the multiphoton ionization in our experience using800nm infraredfemtosecond laser to fabricate LPFG. The induced several major types ofdefect and damage are also introduced in the femtosecond laser interactingwith wide band-gap medium material like fused silica, this is the source ofrefractive index modulation. We also make a certain theoretical research onthe theory of the LPFG to master the working principle and design method ofLPFG.Secondly, high performance LPFG was made by using the femtosecondlaser. Two major kinds of method are adopted to fabricate LPFG, one isfocusing the irradiation of infrared femtosecond laser by cylindrical lensthrough amplitude mask, and another method is to fabricate LPFG by focusedirradiation of infrared femtosecond laser through objective. We fabricate460μm-LPFG using amplitude mask technology to measure the relationshipbetween the resonant wavelength shift and axial stress change:D lr es=-0.02446-0.99221e, which means that the stress sensitivity of the LPFGis about0.99nm/N. Using the amplitude mask method to fabricate LPFG israpid, but the amplitude mask is very easy to be broken, and the LPFGtemperature stability is weak. So we select the60×oil immersed objective tofocus the laser to fabricate100μm-LPFG, the stress sensitivity is3.13nm/N,the temperature sensitivity were-15.52pm/℃(20℃~500℃) and-37pm /℃(500℃~700℃), respectively, the refractive index sensitivity can be ashigh as5265nm/RIU (1.433~1.443), can work at high temperature above500℃. For comparison, we fabricate460μm-LPFG, the refractive indexsensitivity is much lower than that of100μm-LPFG, so the100μm-LPFG is agood candidate for RI sensing to reduce the cross sensitivity from temperaturevariation. The calculated transmission spectrum of100μm-LPFG indicate theresonance peak corresponding to the HE1,22&EH1,22, HE1,23&EH1,23,HE1,24&EH1,24and HE1,25&EH1,25at the second order diffraction, thehigher-order cladding mode cause the high refractive index sensitivity.Finally, we presents a new type of LPFG: LPFG based on microchannels(MCLPFG), fabricated by combining with the femtosecond laser direct writingtechnology and4%hydrofluoric acid (HF) solution selective etching method tofabricate periodic microchannels array in ordinary single-mode optical fiber.The fabrication process involves two main steps: firstly, inscription of thedesired straight line shape structure throughout fiber cross section by using afocused femtosecond laser beam through60×oil immersed objective, andthen etching the fiber in4%HF solution for selective removal of thelaser-modified regions. The optical fiber transmission spectrum will graduallychange with HF etching because the etching rate of femtosecond laser-modified regions is much faster than else, so we can control the transmissionspectrum of MCLPFG by control the time of HF etching. The SEM image ofMCLPFG shows that the microchannels throughout the entire optical fibercross section with a width of about5μm. The sensing test of MCLPFG showsthat the transmission spectrum has very good response with the axial tensionand refractive index change, and we can measure the axial tension andsurrounding refractive index by monitoring both the resonance wavelength andthe loss dip.