多芯光纤可饱和吸收体的数值研究

发布时间：2018-03-24 14:01

本文选题：超短脉冲　切入点：锁模激光器　出处：《北京交通大学》2017年硕士论文

【摘要】：超短激光脉冲由于具有功率高、脉冲窄、光谱宽等一系列的优点,在高容量通信,微观探测,受控热核聚变,生物医学等领域有重要的应用价值。当前,被动锁模光纤激光器被认为是产生超短脉冲最好的光源。可饱和吸收体是被动锁模激光器实现锁模的关键器件,其作用是实现对光脉冲的可饱和吸收。常见的锁模器件包括:半导体可饱和吸收镜、碳纳米管、石墨烯、克尔透镜以及非线性偏转锁模。本文主要工作是对一种新的可饱和吸收体—多芯光纤可饱和吸收体的研究,分别对纤芯数、耦合系数、非线性系数进行讨论,研究这些因素对于可饱和吸收特性的影响。具体内容为:首先,分别对双芯、三芯、五芯和七芯光纤进行仿真,通过计算光纤耦合系数和非线性系数,然后数值求解非线性耦合模方程组后发现,对于不同的光纤结构,多芯光纤总是表现为对低功率密度光脉冲吸收和高功率密度光脉冲透过的饱和吸收特性,且不同纤芯数对于这种特性的影响相差不大。因此,在后面的讨论中,均是采用的双芯光纤结构。其次,光纤结构和入射波长均会影响其耦合系数,从而影响到可饱和吸收特性。研究发现,双芯光纤作为可饱和吸收体所需的饱和功率随耦合系数的减小而降低。此外,当选取合适的长度时双芯光纤有较好的滤波特性。对于文中所选双芯光纤结构,当光纤长度为81倍半拍长度时,在波长范围1530nm～1570nm可以实现带通滤波特性。通过向光纤纤芯中注入高非线性液体能够有效的提高光纤的非线性。研究表明,光纤非线性的提高能有效地降低其作为可饱和吸收体的饱和功率。针对双芯光纤研究了由于芯径和折射率差导致的不对称性对可饱和吸收特性的影响,结果表明不对称性不仅会降低耦合效率,而且增大了饱和功率。最后,将所研究的双芯光纤可饱和吸收体置于全正常色散的超短脉冲锁模光纤激光器中进行数值仿真,通过对激光器输出脉冲在时域和频域的分析,验证了这类双芯光纤可饱和吸收体可以被用于锁模光纤激光器中。
[Abstract]:The ultrashort laser pulse has a series of advantages such as high power, narrow pulse and broad spectrum, so it has important application value in the fields of high capacity communication, micro detection, controlled thermonuclear fusion, biomedicine and so on. Passive mode-locked fiber laser is considered to be the best light source to produce ultrashort pulse, and saturable absorber is the key device to realize mode-locking in passive mode-locked laser. Common mode-locked devices include semiconductor saturable absorbers, carbon nanotubes, graphene, Kerr lens and nonlinear deflection mode-locking. The main work of this paper is to study a new saturable absorber, multi-core fiber saturable absorber, and discuss the core number, coupling coefficient and nonlinear coefficient, respectively. The effects of these factors on the saturable absorption characteristics are studied. Firstly, the fiber coupling coefficient and nonlinear coefficient are calculated by simulating the fiber with two cores, three cores, five cores and seven cores, respectively. After solving the nonlinear coupled mode equations numerically, it is found that for different fiber structures, the multi-core fiber always exhibits the characteristics of saturation absorption of low power density optical pulse and high power density optical pulse transmission. And the influence of different core number on this characteristic is not different. Therefore, in the later discussion, the two-core fiber structure is adopted. Secondly, the coupling coefficient is affected by the fiber structure and the incident wavelength. It is found that the saturation power required by the dual-core fiber as a saturable absorber decreases with the decrease of the coupling coefficient. The dual-core fiber has better filtering characteristics when the appropriate length is chosen. For the two-core fiber structure, when the length of the fiber is 81 times the length of half-beat, In the wavelength range, 1530nm~1570nm can realize the band-pass filtering characteristics. The nonlinear properties of the fiber can be effectively improved by injecting high nonlinear liquid into the fiber core. The enhancement of fiber nonlinearity can effectively reduce the saturation power of the fiber as a saturable absorber. The effect of asymmetry caused by the difference of core diameter and refractive index on the saturable absorption characteristics of dual-core fiber is studied. The results show that asymmetry not only reduces the coupling efficiency, but also increases the saturation power. Finally, the saturable absorber of the dual-core fiber is numerically simulated in a fully normal dispersion ultrashort pulse mode-locked fiber laser. By analyzing the laser output pulse in time and frequency domain, it is proved that this kind of saturable absorber can be used in mode-locked fiber laser.
【学位授予单位】：北京交通大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TN248;TN253

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