【作者】 张玲；

【导师】 隋青美；

【作者基本信息】 山东大学 ， 检测技术与自动化装置， 2012， 博士

【摘要】 长周期光纤光栅(LPFG)作为一种重要的新型光纤光栅滤波器件,凭借其独特优势,如插入损耗小、阻带宽、后向反射低、传感灵敏度高,抗腐蚀、体积小等,在光纤通讯、光纤传感等领域得到了广泛应用和发展。不同的光栅制作方法影响着光栅的性能,从而制约着其应用。如传统紫外曝光法利用光纤的光敏性诱导折变,刻写的LPFG温度稳定性低；CO2激光脉冲诱导光纤折变的机理主要是热效应引起的残余应力释放,刻写的LPFG具有较大的插入损耗,在刻写超长周期光纤光栅时容易导致光纤变形。近年来,飞秒激光技术成为光纤光栅制作方法上的重大突破。飞秒激光诱导折变为多光子吸收过程,不但对刻写光纤的光敏性没有要求,而且刻写的光栅具有高的温度稳定性、窄的线宽和大的折射率调制等特性,在高温传感和光纤激光器等方面具有潜在的应用价值。与传统紫外曝光法等相比,飞秒激光技术在制备光纤光栅等光纤传感器方面具有突出的技术优势和应用价值。其中,飞秒激光逐点法因其简单灵活性已成为制备LPFG的主要研究方法。本文在深入研究飞秒激光脉冲制备新型光纤传感器技术的基础上,对飞秒激光脉冲逐点法写制LPFG技术、及用该方法写制的LPFG的光谱特征和传感特性进行了研究,具体工作内容包括以下几个方面：1.介绍了本课题的研究背景和意义；介绍了飞秒激光微加工技术；阐述了飞秒激光制备光纤光栅的方法；总结了飞秒激光逐点法刻写长周期光纤光栅的国内外研究现状和水平；总结了利用飞秒激光直写技术制备新型光纤干涉传感器的研究现状；归纳了飞秒激光制作的光纤传感器的传感特性。2.讨论了飞秒激光与透明固体电介质相互作用的机理,包括光致电离、雪崩电离、等离子体形成和自聚焦效应。总结了解释飞秒激光诱导介质折射率发生改变的现有机理模型,包括热模型、色心模型、结构改变模型和微爆模型。3.建立了光纤光栅的非均匀折射率调制和温度分布模型,理论仿真了不同曝光时间下的光谱变化,并实验研究了低脉冲能量下飞秒激光对光纤光栅的作用,与理论分析吻合良好。实验采用聚焦的红外飞秒激光对传统紫外曝光法刻写的Type Ⅰ型光纤布拉格光栅(FBG)分别进行了单点和扫描式曝光,研究发现,激光单点照射栅区任意位置时,照射过程中的光谱有较大红移,且光谱结构不再是单透射峰而是不规则的多透射峰；然而照射结束后的布拉格波长蓝移且光栅透射率增加,随着曝光时间的增加该变化逐渐趋于饱和。虽然激光脉冲能量低,但是聚焦光斑的高斯能量分布导致被照射光栅的曝光区处在一个非均匀温度场中。把聚焦的激光脉冲看作一系列热源,分析了热源扩散过程对光栅折射率的影响,给出了激光脉冲单点照射光栅过程中的非均匀温度分布模型及照射结束后折射率的调制轮廓模型,理论仿真了不同曝光时间后和曝光过程中的光谱变化,与实验结果非常吻合。研究结果表明在低能量情况下,焦点的热场是引起光栅折射率改变的主要因素。4.设计了基于水平、垂直双CCD对准的飞秒激光长周期光纤光栅逐点写制系统,提高了激光与光纤对准的准确度和效率,在未作增敏处理的单模光纤内刻写了后向反射损耗低、强度大、线宽窄且光谱平滑的长周期光纤光栅。详细说明了该刻写系统的设计原理和光刻过程。介绍了刻写系统搭建和元器件选择上的一些要点和难点问题。实验研究了光栅刻写过程中的光栅周期、光栅长度、激光脉冲能量等刻写参数对光栅形成和光谱的影响。5.在对飞秒激光诱导光纤折变轮廓综合分析的基础上,理论研究了长周期光纤光栅光谱中的低幅值谐振峰现象,并给出了飞秒激光写制LPFG的光谱分析方法。首先从三层光纤模型出发,以耦合模方程为基础,对余弦折射率调制LPFG的光谱特性进行了理论分析,数值仿真分析了光栅长度(即光栅周期数)、折射率调制深度对光栅光谱的影响。其次针对飞秒激光诱导折射率改变的横截面轮廓和轴向矩形分布,分横向折变在纤芯均匀、横向折变非均匀、横向折变轮廓中心偏离光纤中心三种情况,对飞秒激光脉冲逐点法刻写的LPFG的光谱特征及分析方法,进行了总结分析。6.对飞秒写制LPFG的温度和折射率传感特性及应用进行了实验研究。在对LPFG的波长和损耗峰幅度的温度灵敏度进行理论分析之后,对飞秒激光写制LPFG的温度特性进行了实验研究。分别对未发生过耦合光栅和过耦合光栅的光谱随温度的变化进行了分析；与高频CO2激光脉冲和传统紫外曝光法制作的LPFG的温度特性进行了对比；并对飞秒写制LPFG的高温传感特性进行了实验研究。理论分析了LPFG的折射率灵敏度影响因素,搭建实验装置,对飞秒激光刻写的LPFG进行了氯化钠和蔗糖浓度测量,实验结果表明飞秒写制LPFG用于溶液浓度的传感测量是可行的,且响应快。最后给出了提高其折射率灵敏度的三种方法。最后对全文进行了总结,并对下一步的研究工作进行了展望。更多还原

【Abstract】 The long-period fiber grating (LPFG), as one of the important new fiber grating filter, has been widely used in the fiber sensing and communication aspects for its unique characteristics, such as low insertion loss, wide stopband, low backward reflection, high sensitivity, resistant to corrosion and small size. However, the application of LPFGs is restricted by their performances which are mainly related to the fabrication methods. The LPFG fabricated by traditional ultraviolet(UV) exposure method, which utilizes the photosensitivity to induce refractive-index change, has a low temperature stability. The gratings fabricated by CO2laser pulses,which based on releasing of residual stress caused by thermal effect, usually have bigger insertion loss and it is easy to cause fiber deformation when fabricating ultra-long period gratings.The refractive-index change induced by femtosecond (fs) laser is based on multi-photon absorption process. Compared with the traditional fabrication methods, fs laser irradiation can remove the need for fiber photosensitivity at the wavelength of the writing laser, and gratings inscribed by fs laser pulses have superior temperature performances, narrow bandwidth and large refractive index modulation, so they are potentially useful for high temperature sensor. And the point-by-point (PbP) irradiation of an optical fiber by a focused fs laser with suitable intensity is the most popular one used for recording a long-period fiber grating (LPFG).Based on thoroughly studying the fabrication technology of new fiber optic sensors using fs laser pulses, the PbP writing LPFG technology, the spectral features and sensing properties of LPFG fabricated by fs laser were mainly studied in this dissertation. The detailed research contents are as following:1. The research background and significance of this subject were introduced. The fs laser micromachining technology was also introduced. The grating fabrication methods using fs laser pulses were elucidated. The research developments in fabrication of LPFG with PbP method and fabrication of miniature fiber-optic interferometer sensors by fs laser were reviewed and discussed. The sensing characteristics of fiber sensors fabricated by fs laser were summarized at last.2. The interaction mechanism of fs laser and transparent bulk dielectrics were discussed, including the photoionization, avalanche ionization, plasma formation and self-focusing effect. The explain model of refractive index change induced by fs laser pulses were also summarized, including thermal model, color center model, structural change model and microexplosion model.3. The fiber grating non-uniform refractive index modulation and temperature distribution model were established. The spectral changes under different exposure time were simulated, and the effects of fs laser with low pulse energy on fiber Bragg grating were experimentally studied. The experimental results fit well with the theoretically simulated ones.The single point and scan exposure experiments on ultraviolet laser inscribed type Ⅰ fiber Bragg grating by focused infrared fs laser is performed. Experimental results show that the grating spectrum during irradiation will move to the long wavelength direction, and the spectral structure is no longer a single transmission peak but more irregular transmission peaks. However after stopping exposure, the resonant wavelength will show blue shift phenomemon and the transmissivity will increase, which tend to be saturated with the time increasing. Although the laser pulse energy is low, the Gaussian energy distribution of the focused spot lead to the grating exposure region in a non-uniform temperature field. Let the focused laser pulse as a series of heat source, analyzed the effects of the heat source diffusion process on the grating refractive index, given the non-uniform temperature distribution model and the modulation profile model of the refractive index after irradiation in the single-point laser pulse irradiation raster process, theoretically simulated the spectral changes in different exposure time and exposure process, it is in good agreement with experimental results. The results show that in the case of low pulse energy, the grating refractive index change is mainly caused by the focus thermal field.4. One LPFG fabrication system with fs laser pulses was designed, which is based on vertical and horizontal dual-CCD alignments. The system improved the accuracy and efficiency of the laser and fiber alignment. LPFGs were successfully inscribed in single-mode fiber with the absence any sensitizing treatment. These LPFGs have high spectra quality such as low backward reflection loss, strong resonance strength, narrow line width and smooth spectra. The system design principles and inscription process were explained in detail. The effect of grating period, grating length and laser pulse energy on LPFG transmission spectra studied.5. Based on the comprehensive analysis of the refractive-index change profile induced by fs laser, the low amplitude resonant peak phenomenon of grating spectrum was theoretically studied, and the spectral analysis method of LPFG inscribed by fs laser were given. First, according to the three layers fiber structure model, and on the basis of the coupled mode equations, the spectral characteristics of LPFG with cosine refractive index modulation were numerically analyzed. Second, according to the cross-sectional profile and the axial rectangular distribution of refractive-index change induced by fs laser, the spectral characteristics and analysis methods of the LPFG were analyzed for three cases:with uniform refractive index changes in the core, with non-uniform refractive index changes in the transverse, and with mean inscription position deviated from the fiber center.6. The temperature and refractive-index (RI) sensing characteristics of LPFGs fabricated by fs laser are researched. First, the temperature sensitivities of loss-peak wavelength and amplitude with over-coupled and under-coupled gratings were analyzed and measured, including the high temperature properties. The temperature sensitivitis of LPFGs fabricated by fs laser, CO2laser pulses and traditional UV-light exposure were measured. Second, the influencing factors of LPFG RI sensitivity were theoretically discussed. Further solution concentration sensing experiments of sodium chloride and sucrose were performed, The results show that it is feasible to measure the solution concentration with fast response using LPFGs fabricated by fs laser. Finally, three methods to improve refractive sensitivity are proposed.Finally, the main contents and conclusions in this thesis were summarized and the contents for next study were pointed out.更多还原