摘要
超连续谱在光学相干层析成像、未来高速波分复用通信系统、光谱学、光度量学等方面的广泛应用,使其成为近年来人们的研究热点。以前由于很难找到高非线性的介质,只能用飞秒激光脉冲作为泵浦源才能观察到明显的超连续谱现象。随着光子晶体光纤的出世,由于其高的非线性系数和可控的零色散波长等优点,使得皮秒和纳秒脉冲甚至连续光源作为泵浦光也能让初始光谱得到很大的展宽。由于实验室已经对基于飞秒脉冲和连续光源泵浦的超连续谱做了大量的研究,本文主要对基于皮秒和纳秒脉冲泵浦的超连续谱进行了实验研究并对实验结果做出了分析,为超连续谱产品化做了铺垫。主要内容概括如下:
1、介绍了超连续谱的发展和应用以及产生机理。
2、对光子晶体光纤的概念、导光原理、分类、特性、计算方法和应用前景进行了综述。
3、采用皮秒脉冲泵浦光子晶体光纤获得了最大输出功率1.7 W、谱宽1700 nm(500-2200 nm)的超连续谱,其中5 dB谱宽超过1000 nm,分析了光纤长度、零色散波长和色散特性等光纤参数对超连续谱的影响,实验结果表明要获得高效率的超连续谱必须选择最佳长度的光纤;光纤的零色散波长也要与脉冲的中心波长相匹配,脉冲中心波长处在光纤正常色散区且离光纤的零色散波长很远的时候,受激拉曼散射起着主要的作用,脉冲中心波长处在光纤反常色散区的时候调制不稳定现象导致孤子的产生,光谱展宽到正常色散区的时候引起四波混频等效应,脉冲内拉曼散射导致孤子自频移;光纤的色散特性能影响光谱的宽度和平坦度。
4、采用纳秒脉冲泵浦光子晶体光纤来分析重复频率和脉冲宽度等脉冲参数对超连续谱的影响。重复频率变低的时候对应的峰值功率变高了,能提高非线性效应。脉冲宽度的大小与光纤的长度要匹配,脉宽窄的时候很短的光纤就使光谱展宽,而脉宽很宽的时候则需要相当长的光纤来增强非线性效应。
Supercontinuum has been the hot topic of research owe to its broad application in optical coherence tomography, future high-speed dense wavelength-division-multiplexing optical networks, spectroscopy, optical metrology and so on. In the past, obvious supercontinuum was obtained by femtosecond pulses because it was difficult to find a media with high nonlinear coefficient. With the emergence of photonic crystal fiber, which has high nonlinear coefficient and controllable zero dispersion wavelength, supercontinuum can be generated using the picosecond and nanosecond pulses even continuous lasers. Since a mountain of work about femtosecond supercontinuum have done in the laboratory, we mainly focus on the research of picosecond and nanosecond supercontina. Main works in this thesis are summarized as follows:
1. The development, application and theory of supercontinuum are introduced.
2. The concept, basic principle of light-transfering, classification, kinds of characteristic, analytical methods and application prospect of photonic crystal fiber were summarized.
3. In the condition of picosecond pulses, a supercontinuum source with output power up to 1.7 W and spectral width of 1700 nm(500-2200 nm) was obtained, the 5 dB spectral width is about 1100 nm(1100-2200 nm). The influence of parameters of the photonic crystal fiber such as fiber length, zero dispersion wavelength and dispersion characteristics to supercontinuum was analyzed. High efficiency supercontinuum generation needs suitable fiber length. The center wavelength of pump pulse must be matched with the zero dispersion wavelength of fiber. Suitable dispersion characteristic can also affect the spectral width and flatness of supercontinuum.
4. In the condition of nanosecond pulses, the influence of repetition rate and pulse width of pump pluse to supercontinuum was analyzed. The peak power of pulse is increasing with the decreasing of repetition frequency. When the duration of pulse is very short, we can obtain obvious spectrum broadened only using a photonic crystal fiber with the length of a few centimeters. In contrast, we need a very long photonic crystal fiber to generate supercontinuum when the pulse width is broad.
引文
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