【作者】 王朋朋；

【导师】 常军；

【作者基本信息】 山东大学 ， 光电工程， 2014， 博士

【摘要】 经过几十年的努力和发展,光纤激光器及光纤放大器已经走出实验室,被广泛的应用在了通信、传感、医疗以及工农业等领域。近年来随着掺杂光纤制作技术及光纤光栅刻写技术的发展,在光纤上直接写入光栅而构成线型腔光纤激光器的技术也越来越成熟。而线型腔光纤激光器具有体积小、重量轻、噪声低、线宽窄以及易于大规模组阵复用等优点,而被广泛的应用在光纤激光传感领域。分布反馈光纤激光器(DFB)是在一段掺杂光纤上直接写入一个π相移光栅而构成的线型腔光纤激光器,这类激光器的谐振腔长度都在厘米量级,且输出激光都是窄线宽低噪声的高相干光。DFB光纤激光器由于具有灵敏度高、抗电磁干扰、动态监测范围大、易于大规模组阵复用等优势被广泛应用在光纤传感领域。由于DFB光纤激光器是国内研究起步较晚的一种新型结构激光器,对其特性进行详细的分析和研究,对它的实际应用推广是十分重要的。在本论文中,我们就对DFB光纤激光器的特性进行了详细的理论及实验研究。本文的主要研究成果如下：1.围绕着谐振腔的结构设计阐述了DFB光纤激光器的工作原理,并且介绍了相移位置型非对称DFB光纤激光器以及折射率调制深度型非对称DFB光纤激光器的结构设计,以及可以获得稳定单向激光输出的工作原理。2.介绍了课题组所搭建的基于氩离子紫外激光器和抖动掩模板技术的光纤光栅和光纤激光器的制作系统,该系统能够制作任意复杂的折射率结构,可以精确的控制相移量和相移位置,并定制光栅特性从而结合不同的有源掺杂光纤制作各种光纤激光器。最后还简要介绍了课题组所制作的DFB光纤激光器,以及进行不同封装设计后的DFB光纤激光器。3.利用传输矩阵理论,对DFB光纤激光器的光谱特性,即相移光栅的无源透射谱进行了仿真模拟,从理论上分析了相移光栅的各个参数,包括相移量的大小、相移位置、光纤光栅长度、折射率调制深度等参数对透射谱中间狭缝的半高宽度的影响。实验中提出了新的测试光谱的方法,即DFB-LD电流扫描法,此方法基本实现了连续扫描,具有更高的分辨率和信噪比,能够精确的测试得到相移光栅中间狭缝的半高宽度,分辨率可以达到23.5fm。4.结合理论模拟和具体实验对DFB光纤激光器输出激光的阈值和斜率效率展开了研究。将两种不同类型的DFB光纤激光器,即对称DFB光纤激光器和非对称DFB光纤激光器输出激光的阈值和斜率效率进行了对比。实验得出普通对称DFB光纤激光器两端输出激光功率基本相等,而非对称DFB光纤激光器可以获得稳定的单向输出。经过理论和实验验证得出,这种非对称结构的DFB光纤激光器两端输出激光功率比最大可达到100：1。5.对DFB光纤激光器输出激光的相对强度噪声(RIN)特性进行了理论和实验研究。利用速率方程理论为基础,建立了理论模型,从理论上分析了DFB光纤激光器的结构参数,包括铒离子掺杂浓度,泵浦功率以及耦合系数等参数对RIN特性的影响。在实验中对环境噪声的影响进行了测试,实验发现当引入的外界环境噪声幅度为7dB时,DFB光纤激光器输出激光的噪声水平增大3.6dB／(Hz)1/2。实验中对泵浦功率损耗的影响也进行了测试和分析,并且选取了几个具有不同折射率调制深度的DFB光纤激光器进行了测试研究,分析了耦合系数对RIN特性的影响。6.对DFB光纤激光器的强度响应特性进行了理论和实验研究。以速率方程理论为基础,建立了DFB光纤激光器对外界声压激励强度响应的理论模型,推导得出了外界声压激励扰动的调制函数,并对DFB光纤激光器的强度响应特性进行了仿真和分析。实验中,详细研究了外界自然环境以及泵浦功率对DFB光纤激光器强度响应的影响,并且加入声压信号对DFB光纤激光器输出激光的强度进行调制,在DFB光纤激光器的频率谱中可以明显得出所加入的频率为16.5kHz的声压信号,并将实验结果与理论仿真结果进行了比较,得出结论：DFB光纤激光器具有强度响应特性并且具有应用为调制型传感器的潜力。给出了该类型传感器的声压灵敏度的计算方法,首次通过实验测试和数值计算得出了基于DFB光纤激光器的强度调制型传感器的声压灵敏度,测试频率范围为800Hz到9kHz,声压灵敏度在-180dB re μW/μPa至-165dB re βW/gPa范围内。7.分析了DFB光纤激光器在传感组阵中最常遇到的问题,即阵列中泵浦功率预算,以及阵列中所引入的几种外部光反馈,如熔接点、端面反射和瑞利散射等对阵列输出光特性的影响。首次提出将非对称DFB光纤激光器应用在传感组阵中。实验中选取了四支非对称DFB光纤激光器,并且详细给出了四支DFB光纤激光器输出激光的特性,包括输出功率,相对强度噪声以及驰豫振荡频率对阵列输出激光器的特性进行了测试,实验发现应用非对称DFB光纤激光器组阵可以有效的改善阵列输出激光的平坦性,尤其是阵列中后三元阵元输出激光功率波动在0.5dB以内。8.对四元阵列中的相对强度噪声(RIN)特性进行了测试,分析了各阵元在组阵后,RIN特性受相邻阵元影响后的变化。DFB光纤激光器输出激光的驰豫振荡频率只受泵浦功率影响,而与其他外部因素无关,所以我们利用了这一特点,用驰豫振荡频率对阵列中各阵元的泵浦功率进行校准,由实际的泵浦功率可知DFB的实际RIN,最后将DFB光纤激光器的实际RIN和在阵列中受相邻阵元影响后的RIN进行了对比和分析。实验发现应用非对称DFB光纤激光器进行组阵可以有效的改善DFB光纤激光器后向输出对相邻阵元的影响。更多还原

【Abstract】 After decades of development, fiber lasers and fiber amplifiers have been widely used in many areas, including communications, sensing, medical treatment, and industry and agriculture. In recent years, with the development of doped fiber production and fiber grating fabrication, the technology that fabricating gratings directly in doped fibers to constitute a linear cavity fiber laser has become increasingly more sophisticated. The linear cavity fiber laser is widely used in sensor field due to its small size, light weight, low noise, narrow line-width, and easily multiplexing capability, etc. Distributed feedback fiber laser (DFB) is formed by a π-phase shifted fiber Bragg grating written on a continuous length erbium doped fiber. The kind of laser which cavity length is usually of the order of centimeters is used to provide the coherent light output with narrow line-width and low noise.DFB fiber laser is widely used in the sensor field because of its high sensitivity, anti-electromagnetic interference, dynamic monitoring range, easy multiplexing capability. As the research started later in domestic, the detailed analysis and investigation of its characteristics is very important for its practical application. In this dissertation, through the theoretical and experimental study on the characteristics of DFB fiber lasers, the main research results of this dissertation are carried out as follows:1. The working principle of DFB fiber laser is described around the cavity structural design, and also the design of asymmetrical DFB fiber laser:the phase shift position type and the refractive index modulation depth type. Both the two types can obtain stable single laser output, and its working principle is also introduced.2. The fabrication system based on the UV argon ion laser and dither mask method is introduced. The system is able to produce any structure of the complex refractive index fiber, can accurately control the phase shift size and position, and fabricate multiple fiber lasers based on grating characteristics and different doped fiber. Finally, a brief description of the DFB fiber laser fabricated by our research team is given, as well as different package designs.3. The spectral characteristics of DFB fiber laser is investigated by the transfer matrix theory. The effect of grating parameters, such as the phase shift size, phase shift position, grating length, refractive index modulation depth on the transmission spectrum is analyzed. A novel method based on a distributed feedback laser diode (DFB-LD) continuous wavelength-scanning spectrum for acquiring the precise spectra of phase-shift fiber gratings is presented by experiments. Its main advantage is that it can get nearly continuous transmission spectrum of the phase-shift fiber grating. We experimentally demonstrate measurements of phase-shift fiber grating spectra with a resolution of23.5femtometer. A Signal-to-Noise Ratio (SNR) advantage can also be provided owing to a much higher spectral density of DFB-LD than ASE source.4. The laser threshold and slope efficiency of DFB fiber laser is investigated by theoretical simulation and experimental analysis. The two different types of DFB fiber lasers, symmetric and asymmetric DFB fiber laser thresholds and slope efficiencies are compared. Experimental results show that the output powers at both ends of symmetric DFB fiber laser are substantially equal, and the symmetric DFB fiber laser can obtain a stable one-way output. Through the theoretical and experimental verification, we obtain that the maximum laser output ratio of this asymmetric DFB fiber laser structure can reach to100:1.5. The relative intensity noise (RIN) characteristics of DFB fiber laser is investigated in detail. The theoretical model is deduced using rate equation theory. The contributions of several parameters, such as Er3+doped concentration, pump power and coupling coefficient on the RIN characteristics are analyzed theoretically. In experiments, the influence of external environmental noise is measured; the RIN of DFB fiber laser increases3.6dB/(Hz)1/2when the environmental noise increases7dB. The pump power fluctuation influence is also investigated. Compared to the theoretical analysis, the results obtained from the experiments testify the important role of coupling coefficient in the RIN characteristics of DFB fiber laser.6. The intensity response of DFB fiber laser to external acoustic excitation is investigated theoretically and experimentally. The theoretical model is deduced using rate equation theory. The transfer function for external acoustic excitation modulation has been obtained, and the intensity response characteristics of DFB-FL to external acoustic excitation are described by simulation. In experiments, an intensity modulated sensing system based on DFB fiber laser is constructed. The intensity response property of DFB fiber laser to external acoustic excitation with different pump power has been analyzed. Through experiments we observe a signal of16.5kHz with25dB in the RIN spectrum as the external acoustic pressure is10.25Pa. Both the theoretical and experimental results confirm that DFB fiber laser has potential to be used as intensity-type acoustic sensor. Acoustic pressure sensitivity with an average value between-165dB re μW/μPa and-185dB re μW/μPa at frequencies ranging from800Hz to9kHz of the intensity-type sensor has been obtained by experiments for the first time to the authors’ knowledge.7. Several important technical issues in a sensor array are analyzed, such as the pump absorption and external laser feedback, splice, fiber end reflection and Rayleigh scattering. A very desirable feature of asymmetric DFB fiber laser is undirectionality, and this obvious advantage has important applications to design sensor array. Larger output powers are obtained from shorter ends of all the four asymmetric DFB fiber lasers. The output flatness of the sensor array system presents a good performance with the applications of asymmetric DFB fiber lasers, especially, the back three have mostly the same amplitudes and the power equilibration among them is less than0.5dB.8. The RIN characteristics of the four element sensor array are investigated in detail. Relaxation oscillation frequency plays an important role in the investigation of sensor array. Because the relationship between the relaxation oscillation frequency and the actual absorbed pump power is not affected by external laser injection. By calibrating the relaxation oscillation frequencies, the corresponding pump powers are obtained, and the actural RIN can be obtained. The actual RIN and the measured RIN with different external laser injections of DFB-FL1and DFB-FL4are obtained and compared. The eaperimental results confirm that the RIN induced by the front external laser injections can be effectively reduced by using asymmetric DFB fiber lasers.更多还原