Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

We describe the effect of temperature variations on supercontinuum (SC) generation in optical fibers using a continuous-wave (CW) Raman fiber laser as a pump. We achieve supercontinuum generation by pumping only ∼2 W of power into a 7 km-long nonzero dispersion-shifted fiber (NZDSF) in the region of small anomalous dispersion. In these conditions, the supercontinuum builds up basically on modulational instability and Raman. At room temperature, the supercontinuum covers effectively the S, C and L transmission bands defined by the International Telecommunication Union (ITU). Temperature tuning of the fiber environment provides a means of tuning the fiber dispersion, and thus a means of changing the width and shape of the supercontinuum spectrum. We demonstrate a 27% increase in the 10-dB SC width. We believe that the application of this new tuning mechanism to other experimental configurations using pulsed sources might be used to produce extremely broad supercontinuums.     

1040nm泵浦的光子晶体光纤超连续光谱产生性能退化研究

光子晶体光纤已经被广泛应用于由飞秒脉冲激光源产生超连续光谱。当激光源的重复频率较低时,由光子晶体光纤产生的超连续光谱随时间的变化过程较为缓慢,通常不被注意到。而在天文光谱仪定标等应用中,需要使用GHz至几十GHz量级的高重复频率激光源。此时,可观察到光子晶体光纤的超连续光谱产生性能在有限时间内产生显著的退化。在1 040 nm飞秒激光泵浦条件下,通过测试三种不同气孔占空比的光子晶体光纤的超连续光谱产生性能演化,发现超连续光谱的退化进程随光纤气孔占空比的增大而加速。观察发生光谱退化后的光子晶体光纤样品,发现在光纤上超连续光被产生的区段出现多个不同颜色的亮点,呈现有方向性的光泄露现象。针对光泄露现象,通过测量光纤的吸收光谱线,证实了实验中超连续光谱退化的主因并非是光纤熔融石英材料中大量非桥氧色心产生。针对光泄露具有方向性这一特征,提出了经由多光子吸收作用在光纤纤芯中形成长周期光栅的理论。为探究影响光子晶体光纤超连续光谱产生性能的退化的因素,以达到光谱退化抑制的目的,首先通过改变了光纤的拉锥参数,期望增强光纤熔融石英材料的光子耐受性。实验结果证实了该方法的有效性较为有限。其次,从保持激光源的平均功率,降低激光脉冲的峰值功率和保持激光脉冲的峰值功率,降低激光源平均功率两个方面入手,对激光源进行调制。实验结果证明,光纤单位时间内接受的高峰值功率脉冲总量是影响其超连续光谱产生性能的最重要因素。在天文光谱仪定标的应用中,对超连续光谱光功率的需求并不高,使用斩波器降低光子晶体光纤入射光的平均功率是减缓超连续光谱产生性能退化过程的有效且简单可行的方法。     

基于光子晶体光纤和拉锥式单模光纤的超连续光谱产生的实验研究

本文使用重复频率为250 MHz、脉冲宽度为135 fs、最大功率为2.2 W的锁模掺镱光纤激光作为种子源,利用光子晶体光纤和自制的拉锥式单模光纤两种高非线性光纤研究了超连续光谱的产生特性,通过对比两种光纤的结构、色散等特性,分析了拉曼孤子、色散波及其他非线性效应对产生的超连续谱形状的影响,并均得到了大于一个倍频程的超连续光谱,特别是拉锥式单模光纤产生的超连续光谱,耦合效率达到60%,这为众多研究领域,尤其是光学频率梳的建立提供了实用的超连续光源.     

石英光子晶体光纤中高功率中红外超连续谱的产生

非石英光纤在产生大功率超连续谱方面存在难以克服的局限性.本文首次报道了采用石英光纤产生大功率中红外超连续谱.精心设计光纤结构使色散有利于超连续谱向中红外波段展宽,同时保证相对较大的芯径以承受较高的泵浦功率.合理选择光纤长度,在保证光谱展宽到3.4 μm的情况下使光纤损耗的影响降低到最小限度.研究表明,在1.95 μm皮秒脉冲泵浦下,采用色散适宜的石英光子晶体光纤可以产生20 dB带宽覆盖1 550~3 420 nm的超连续谱.超连续谱的平均功率可达56.6 W.     

高阶模抑制型MILO慢波结构高频特性分析北大核心CSCD

理论分析了高阶模抑制型磁绝缘线振荡器(MILO)慢波结构,推导了其各分区电磁场分布.通过对场分布的进一步分析可以发现,角向开槽使得原始MILO结构中的两个简并的HEM11模式去简并,分裂成为极化方向与开槽方向垂直或平行的两个模式;通过改变慢波结构叶片间开槽角向位置相对关系,可以破坏高阶模式之间π模谐振关系,从而抑制高阶模式的起振,使器件稳定工作在基模.     

Supercontinuum generation in single crystal sapphire fibers

In this paper, we report supercontinuum generation by launching ultra-short femtosecond laser pulses into single crystal sapphire fibers. The major advantages of using sapphire fiber for supercontinuum generation are: (1) high transparency up to 5 μm, (2) low material dispersion in the 0.8-5 μm spectral range, and (3) a higher laser damage threshold (500 times higher than that of silica). Thus, a very high power, super broadband [from visible to middle IR (up to 5 μm)], supercontinuum source can be realized by employing sapphire fiber for supercontinuum generation. Our experimental results also confirm that sapphire fiber can offer a broader supercontinuum spectrum than that of bulk sapphire counterpart under the same exciting conditions. This work opens the door to new opportunities in generating high power supercontinuum radiation (in particular, at the middle-IR regime), and will have a great impact on many applications, including sensing and broadband multi-spectrum free space communications.     

Different supercontinuum generation processes in photonic crystal fibers pumped with a 1064-nm picosecond pulse

Picosecond pulse pumped supercontinuum generation in photonic crystal fiber is investigated by performing a series of comparative experiments. The main purpose is to investigate the supercontinuum generation processes excited by a given pump source through the experimental study of some specific fibers. A 20-W all-fiber picosecond master oscillator-power amplifier （MOPA） laser is used to pump three different kinds of photonic crystal fibers for supercontinuum generation. Three diverse supercontinuum formation processes are observed to correspond to photonie crystal fibers with distinct dis- persion properties. The experimental results are consistent with the relevant theoretical results. Based on the above analyses, a watt-level broadband white light supercontinuum source spanning from 500 nm to beyond 1700 nm is demonstrated by using a picosecond fiber laser in combination with the matched photonic crystal fiber. The limitation of the group velocity matching curve of the photonic crystal fiber is also discussed in the paper.     

高激光损伤阈值Ge-As-S硫系玻璃光纤及中红外超连续谱产生

测量了Ge-As-S系列硫系玻璃在中红外波段的飞秒激光损伤阈值,研究了它与玻璃化学组成的关系.基于优化的玻璃组成,采用棒管法制备了芯径为15μm的阶跃折射率非线性光纤.采用飞秒脉冲抽运光纤,研究了光纤中超连续谱(supercontinuum,SC)的产生特性.在研究的Ge-As-S硫系玻璃中,具有化学计量配比的Ge0.25As0.1S0.65玻璃显示出最高的激光损伤阈值.以该玻璃作为纤芯材料、以与其相匹配的Ge0.26As0.08S0.66玻璃作为包层材料制备的光纤的数值孔径约为0.24,背景损耗<2 dB/m.采用4.8μm的飞秒激光抽运长度为10 cm的光纤,获得了覆盖2.5-7.5μm的SC.这些结果表明,Ge-As-S硫系玻璃光纤是一种有潜力的中红外高亮度宽带SC产生的非线性介质.     

Wideband supercontinuum generation in tapered tellurite microstructured fibers

Enhanced soliton trapping of dispersive waves in a tapered tellurite microstructured fiber pumped by a 1556 nm femtosecond fiber laser is demonstrated. The short wavelength edge of supercontinuum light is extended from 960 to 600 nm after tapering the tellurite microstructured fiber, which is caused by the enhanced soliton trapping of dispersive waves owing to the changing group velocities in tapered fibers. Wide-band supercontinuum light source spanning from 600 to >2400 nm is generated in tapered tellurite micro-structured fibers. Our experimental and simulated results show that short length (several centimeters) zero-dispersion-wave length decreasing highly nonlinear fiber has a potential for generating wideband supercontinuum light source expanding from visible to mid-infrared region.     

Numerical analysis of the As2Se3 chalcogenide glass multi-ring photonic crystal fiber

A novel kind of As₂Se₃ chalcogenide glass multi-ring photonic crystal fiber is proposed. The characteristics of multi-ring photonic crystal fiber have been studied by the finite element method. Compared with another two kinds of single-core photonic crystal fibers, the multi-ring PCF can make the perfect combination of the effective mode area, the nonlinearity coefficient and multi-zero dispersion. It can achieve high power supercontinuum generation in middle infrared band; It also can be used in high-power pulsed laser fiber amplifier and high-power ultra-short pulse laser transmission in far infrared band.     

非均匀微结构光纤中双折射现象的研究

报道了利用飞秒脉冲激光与非均匀微结构光纤相互作用中产生超连续光谱后在非均匀微结构光纤传输中双折射拍频现象的研究.利用35?fs的飞秒激光脉冲在高双折射微结构光纤中的传输过程中直接观察到了拍频现象.并利用有限元方法对该光纤进行了模拟计算分析，计算得出在600?nm处拍频长度为毫米量级.所得结果与实验一致. 关键词： 双折射效应 微结构光纤 超连续光谱 有限元法     

Characterization of a supercontinuum generated from a photonic crystal fiber and its application to coherent Raman spectroscopy

The temporal and spectral profiles of supercontinuum radiation generated from a photonic crystal fiber are evaluated with a polarization-gate frequency-resolved optical gating technique. The supercontinuum is then applied to coherent inverse Raman spectroscopy. A stimulated Raman signal of cyclohexane is observed as an induced absorption signal with an instantaneous response. The Raman signal has a peak at a slight negative delay time, which is explained by perturbed Raman-induced coherence.     

不同零色散点光子晶体光纤的超连续谱产生

本文实验研究了飞秒脉冲在不同零色散点光子晶体光纤中传输时产生超连续谱的现象。首先,我们通过非线性薛定谔方程理论计算了激光脉冲分别在正、负色散光子晶体光纤中传输时产生的超连续谱;计算结果表明在正色散光子晶体光纤产生的超连续谱远远大于在负色散中产生的超连续谱。其次,在实验上采用零色散点分别为800 nm、1 060 nm和2 000 nm的光子晶体光纤,将脉宽为130 fs,中心波长800 nm,脉冲重复频率为80 MHz的脉冲输入这些光纤中产生超连续谱并研究其特性,实验结果表明光子晶体光纤的零色散点越小,在其中产生的超连续谱越宽越平坦。同时产生的超连续谱也与激光脉冲的能量和中心波长相关。     

Supercontinuum Generation with High Birefringence SF6 Soft Glass Photonic Crystal Fibers

A kind of high birefringence SF6 soft glass photonic crystal fiber （HBSF6-PCF） is proposed. The properties of birefringence, dispersion, nonlinear coefficient and the transmission characteristics are studied by the multipole method and the adaptive split-step Fourier method. The numerical results show that the birefringence and the nonlinear coefficient reach the orders of 10^-2 and 10^-1, respectively. In addition, the HBSF6-PCFs can generate very smooth supercontinuum spectra when illuminated with femtosecond pulsed light of 1064 nm. It is found that up to 800nm spectral width （evaluated at -5dB from the peak） is achieved. Therefore, the advantage of the HBSF6-PCFs is such that a high birefringence, a high nonlinearity and a smooth supercontinuum are perfectly combined in them.     

基于低非线性系数光子晶体光纤的全光纤高效超连续谱光源

 报道了一种基于低非线性系数光子晶体光纤的全光纤高效率超连续谱产生系统。将光纤锁模激光器输出的脉宽5 ps、重复频率20 MHz、平均功率50 mW的脉冲,输入到15 μm的大模场光纤中进行放大,通过与两级芯径较小的短光纤模场匹配缩小输出的模场直径后,输入到20 m低非线性系数的光子晶体光纤,获得的超连续谱波长覆盖范围宽于650～1 700 nm。输入光子晶体光纤的泵浦光功率为740 mW,输出超连续光功率为670 mW,转换效率大于90%。实验研究了超连续光谱展宽的过程,从理论上进行了分析解释。     

The mode structure and spectral properties of supercontinuum emission from microstructure fibers

The mode structure and spectral properties of supercontinuum emission generated by femtosecond pulses of Ti: sapphire laser radiation in microstructure fibers are studied. The long-wavelength (720–900 nm) and visible (400–600 nm) parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber modes, which can be isolated with an appropriate spectral filtering. The spatial modes thus isolated in spectrally sliced supercontinuum emission possess a spatial quality sufficient for further efficient frequency conversion. The possibility of achieving a high spectral quality of supercontinuum emission is also demonstrated. We explore the ways to control the spectrum of supercontinuum emission by matching parameters of the pump pulse with the parameters of a microstructure fiber and by tuning the initial chirp of the pump pulse. The results of our studies show that supercontinua produced in microstructure fibers offer new approaches to designing a new generation of optical parametric amplifiers and broadband radiation sources for spectroscopic, metrological, and biomedical applications.     

Design of solid core photonic bandgap fibers for visible supercontinuum generation

We present a numerical study of two dimensional solid core photonic bandgap fiber design criteria for their particular application to blue/visible supercontinuum generation. By exploiting their strong frequency-dependent dispersion when compared to index guiding micro-structured fibers, we highlight the design of solid core photonic bandgap fibers to fulfill group index matching conditions between the first ejected optical soliton and the trapped dispersive wave generated in the visible wavelength range. We study how these matching conditions depend on the opto-geometrical parameters of the micro-structured cladding, and we use frequency-domain numerical simulations to determine the expected supercontinuum spectral characteristics for selected cases. We investigate design criteria to generate short wavelengths by pumping in such photonic bandgap fibers in different pulse duration regimes and we identify a novel class of short wavelength (blue/visible) supercontinuum generation in the 3rd bandgap of a typical structure by pumping into the 2nd bandgap through a high attenuation spectral region.     

Random-hole optical fiber evanescent-wave gas sensing

Research on development of optical gas sensors based on evanescent-wave absorption in random-hole optical fibers is described. A process to produce random-hole optical fibers was recently developed that uses a novel in situ bubble formation technique. Gas molecules that exhibit characteristic vibrational absorption lines in the near-IR region that correspond to the transmission window for silica optical fiber have been detected through the evanescent field of the guided mode in the pore region. The presence of the gas molecules in the holes of the fiber appears as a loss at wavelengths that are characteristic of the particular gas species present in the holes. An experimental setup was constructed with these holey fibers for detection of acetylene gas. The results clearly demonstrate the characteristic absorptions in the optical spectra that correspond to the narrow-line absorptions of the acetylene gas, and this represents what is to our knowledge the first report of random-hole fiber gas sensing in the literature.     

Supercontinuum generation in ultraviolet-irradiated fibers

We demonstrate that UV exposure of highly nonlinear, germanosilicate fibers causes a strong change in their chromatic dispersion and can significantly alter the infrared supercontinuum generation in these fibers. By varying the level of UV exposure to the fiber, we show that the dispersion zero and the short-wavelength edge of the supercontinuum can be changed by more than 100 nm. A nonlinear Schr?dinger equation model of the continuum generation in the nonlinear fiber shows that the short-wavelength behavior of the continuum is primarily controlled by changes in the fiber dispersion caused by the UV-induced change in the refractive index of the fiber core.     

光纤色散对超连续谱产生的影响

对1550nm波长附近具有不同色散特性的光纤产生超连续谱进行了详细的计算和分析。结果表明，在反常色散区和零散区，由于内脉冲拉曼散射效应和三阶色散效应的影响，不能产生平坦、宽带的超连续谱。而在正常色散区，可以产生平坦光滑的超连续谱。进一步研究表明，具有较小正常色散的色散平坦光纤对于产生平坦、宽带的超连续谱极为有效。通过增强脉冲抽运功率，可以得到谱强起伏小于10dB、带宽达300nm以上的平坦超宽超连续谱。