Two-octave spectral broadening of subnanojoule Cr:forsterite femtosecond laser pulses in tapered fibers

Spectral broadening of femtosecond Cr:forsterite laser pulses is enhanced due to the use of tapered fibers. Supercontinuum generation with unamplified subnanojoule femtosecond Cr:forsterite laser pulses is observed for the first time. With 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation coupled into a tapered fiber having a taper waist diameter of about 2 μm and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber in the regime of anomalous group-velocity dispersion. This result opens the way for the creation of compact femtosecond Cr:forsterite laser plus tapered fiber systems for optical metrology and biomedical applications. Received: 23 October 2001 / Accepted: 16 January 2002 / Published online: 14 March 2002     

飞秒光脉冲在光子晶体光纤不同色散区产生超连续谱的数值分析

采用预测校正分步傅里叶方法数值模拟了飞秒光脉冲在光子晶体光纤中的非线性传输和超连续谱产生,分析了初始光脉冲的中心波长、峰值功率和光纤长度在光子晶体光纤正、反常色散区对超连续谱形状和带宽的影响。结果表明,当初始光脉冲的中心波长在光纤的反常色散区时,产生的超连续谱宽要远宽于正常色散区,但是光谱的平坦性较差;当光脉冲中心波长在靠近零色散波长的反常色散区且其他脉冲参数不变的情况下,存在一个产生宽且平坦的超连续谱的最佳峰值功率和最佳光纤长度。对于超连续谱系统的优化设计与实际应用具有参考意义。     

局部高斯变迹布拉格光纤光栅中的调制不稳定性

利用激光脉冲在光纤光栅中传播时所遵守的相干耦合非线性薛定谔方程,研究了激光脉冲在局部高斯变迹布拉格光纤光栅中传输时,在反常色散区和正常色散区所产生的调制不稳定性。结果表明在反常色散区和正常色散区都能产生调制不稳定性;在反常色散区,当输入功率达到一定数值、传输距离一定时,当f=-1禁带之上能带底时,调制不稳定性增益的强度最强、宽度最窄;当远离能带底时强度减弱、宽度变宽;在正常色散区,在产生调制不稳定性功率区域,调制不稳定性存在并从给定值一直持续到无穷;并且,在反常色散区和在正常色散区,增益谱都受到局部高斯变迹函数的制约。     

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。     

Tailoring the dispersion properties of photonic crystal fibers

Photonic crystal fibers (PCFs) have had a substantial impact on nonlinear fiber optics and shortpulsed fiber laser systems due to their novel dispersion properties. The large normal or anomalous waveguide dispersion available in such fibers opens up a number of new opportunities not accessible with standard fiber technology. In this contribution, the fundamentals of PCF dispersion are briefly reviewed along with earlier results. In addition, some of our recent work on dispersion tailoring to facilitate nonlinear processes, and dispersion control in lasers will be presented.     

Multiwavelength actively mode-locked fiber ring laser with a dispersion compensated cavity

A multiwavelength actively mode-locked fiber ring laser that utilises a single semiconductor optical amplifier as both a gain and mode-locking element is presented. It is verified experimentally that the number of mode-locked wavelengths may be maximised by compensating for the anomalous intra-cavity dispersion. This is achieved through the inclusion of an appropriate length of normal dispersion fiber in the laser cavity. Using this technique, the number of wavelengths simultaneously mode-locked at 10 GHz was increased from 2 to 5.     

10 GHz增益开关激光器输出脉冲的三级压缩实验研究

通过理论分析,数值模拟和实验,研究了10GHz增益开关半导体激光器输出脉冲的几种压缩技术,提出了一种新型的三级压缩技术,包括1）正色散光纤补偿;2）正色散光纤加负色散光纤的脉冲压缩;3）梳状色散光纤压缩,实验上得到脉冲宽度为2.8ps的压缩脉冲,总的压缩比为9。此压缩技术对实验80Gb/s光时分复用（OTDM）系统具有重要意义。     

Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers

We report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to ?1.6 ps² is experimentally studied. Strongly positively chirped pulses from 4.3?ps (0.01?ps²) to 39?ps (2.5?ps²) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8?ps (?0.07 ps²) to 3.9?ps (?1.6 ps²). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped-pulse regimes are discussed.     

反常色散锥形微结构光纤中高效率脉冲压缩研究

设计了一种沿光纤轴向线性变化的锥形微结构光纤,该光纤在1.55 μm波长处具有反常色散.利用自适应分步傅里叶方法求解非线性薛定谔方程,对中心波长为1.55 μm、初始脉宽为1 ps的脉冲在锥形微结构光纤中的传输进行了模拟,利用1 m长的锥形微结构光纤实现了光脉冲的高效压缩,获得了压缩因子为56.9、品质因子为27的脉冲压缩效果.从脉冲在光纤中的演化发现,在反常色散区基于非线性渐增、色散渐减的锥形微结构光纤有利于实现脉冲的高效率压缩.与普通非线性光子晶体光纤相比,锥形微结构光纤粗端具有较大的有效模式面积,这     

Generation of a compact high-power high-efficiency normal-dispersion pumping supercontinuum in silica photonic crystal fiber pumped with a 1064-nm picosecond pulse

Broadband normal dispersion pumping supercontinuum （SC） generation in silica photonic crystal fiber （PCF） is investigated in this paper. A 1064-nm picosecond fiber laser is used to pump silica PCF for the SC generation. The length of PCF is optimized for the most efficient stimulated Raman scattering process in the picosecond pump pulse region. The first stimulated Raman Stokes peak is located in the anomalous dispersion regime of the PCF and near the zero dispersion wavelength; thus the SC generation process can benefit from both a normal dispersion pumping scheme and an anomalous dispersion pumping scheme. The 51.7-W SC spanning from about 700 nm to beyond 1700 nm is generated with an all-fiber configuration, and the pump-to-SC conversion efficiency is up to 90%. In order to avoid the output fiber end face damage and increase the stability of the system, an improved output solution for the high power SC is proposed in our experiment. This high-efficiency near-infrared SC source is very suitable for applications in which average output power and spectral power density are firstly desirable.     

高斯变迹布拉格光纤光栅中的调制不稳定性

利用激光脉冲在光纤光栅中传播时所遵守的相干耦合非线性薛定谔方程,研究了激光脉冲在高斯变迹布拉格光纤光栅中传输时，在反常色散区和正常色散区所产生的调制不稳定性.结果表明在反常色散区和正常色散区都能产生调制不稳定性；在反常色散区，当输入功率达到一定数值时，产生明显的有规律的增益谱；在正常色散区，在产生调制不稳定性功率区域，调制不稳定性存在并从给定值一直持续到无穷；并且，在反常色散区和在正常色散区，增益谱都受到高斯变迹函数的制约. 关键词： 高斯变迹 布拉格光栅 调制不稳定性 增益     

Sub-100 kHz repetition rate erbium-doped fiber laser in anomalous dispersion regime

We propose and demonstrate a simple but efficient way to generate low repetition rate pulses in an erbium-doped fiber ring laser by using self-pulsing technique. About 2.25 km long single mode fiber is used to elongate in the cavity, providing large anomalous cavity dispersion in the wavelength region around 1550 nm. Self pulses at a repetition rate of 85.3 kHz with single pulse energy as high as 110 nJ are efficiently obtained on the nanosecond time scale. The experimental results demonstrate that the self-pulse fiber laser operating in the anomalous dispersion regime can also realize high-energy pulses at a very low pulse repetition rate.     

超短光脉冲在双折射光子晶体光纤中产生超连续谱的偏振特性数值分析

采用矢量耦合非线性薛定谔方程描述了超短光脉冲在双折射光子晶体光纤中的传输过程,并利用分步傅里叶方法求解该方程。数值模拟了中心波长为1550nm的超短光脉冲在不同色散参量的双折射光子晶体光纤中超连续谱的产生及其偏振特性。分析了光纤在不同色散区时,高阶色散和非线性效应对超连续谱及其偏振态的影响。结果表明,当超短光脉冲波长位于近光纤零色散点的反常色散区时,比其在光纤正常色散区和远离光纤零色散点的反常色散区更容易产生宽且平坦的超连续谱,所得到的光谱显示出了复杂的偏振态特性。     

A Raman laser intensity sensor induced by the cooperative Rayleigh scattering in a ring configuration

In this work, a laser sensor is described that uses the multipath interference produced inside a ring cavity to measure the power loss induced by a moving taper intensity sensor. The laser is created due to the virtual distributed mirror formed by the Rayleigh scattering produced in a dispersion compensating fiber when pumped by a Raman laser. Two laser peaks were formed, one of them is obtained by the Raman gain (1555 nm) inside the ring and the second is created by the combination of the Raman gain and the Rayleigh scattering (1565 nm). A taper sensor is used as displacement sensor and when the losses is applied in the taper the second laser peak is reduced and the first peak is maintained constant and can be used as reference level.     

Impact of spectral filtering on a weak breathing laser based on AS-Yb-PBGF with large net normal dispersion cavity

We have investigated numerically the effects of the spectral filtering on the temporal and spectral properties of the pulse with large net normal cavity dispersion in a weak breathing passively mode-locked ring laser based on all-solid Yb-doped photonic bandgap fiber (AS-Yb-PBGF), which gives the laser gain and anomalous dispersion simultaneously. The specific range of spectral filter bandwidth is indispensable for obtaining high-peak-power pulse in a given highly chirped pulse laser system under a fixed pump level. The shortening of the pulse durations and the shaving of the sharp peaks at the steep edges of the spectrum by the spectral filter are demonstrated. Moreover, the advantage of the weak breathing fiber laser by introducing the AS-Yb-PBGF for scaling up the peak power and shortening the pulse width is investigated, and the effect of third-order dispersion induced by AS-Yb-PBGF on the pulse shape is also discussed.     

Modulation instability at propagation of narrowband 100-ns pulses in optical fibers of various types

The instability of the propagation of narrowband 100-ns laser pulses in the transparency window of a single-mode fiber in the presence of continuous broadband noise is demonstrated. The modulation instability developed due to the nonlinearity and anomalous dispersion leads to a rapid decay of the narrowband component after passing through the fiber. The modulation-instability threshold is shown to be sensitive to the noise level in a spectral interval of about 100 GHz near a central wavelength of about 1.55 μm and is inversely proportional to the fiber length (varied from 1.6 to 21.0 km in the experiments). The effect is completely eliminated owing to the application of a dispersion-shifted fiber with normal dispersion.     

Tunable Supercontinuum Generated in a Yb~(3+)-Doped Microstructure Fiber Pumped by Ti:Sapphire Femtosecond Laser

We experimentally demonstrate that a tunable supercontinuum(SC) can be generated in a Yb3+-doped microstructure fiber by the concept of wavelength conversion with a Ti:sapphire femtosecond(fs) laser as the pump.Experimental results show that an emission light around 1040 nm in an anomalous dispersion region is first generated and amplified by fs pulses in the normal dispersion region. Then, SC spectra from 1100 to 1380 nm and 630 to 840 nm can be achieved by combined effects of higher-order soliton fission and Raman soliton self-frequency shift in the anomalous dispersion region and self-phase modulation, dispersive wave, and four-wave mixing in the normal dispersion region. It is also demonstrated that the 20 nm change of pump results in a 280 nm broadband shift of soliton and the further red-shift of soliton is limited by OH-absorption at 1380 nm.     

非线性高双折射阶跃光纤中单一频率传输区域调制不稳定性

利用光脉冲在高双折射非线性光纤中传播时所遵守的相干耦合非线性薛定谔方程,研究了单一频率的线偏振光,当偏振方向沿两个双折射轴的分量强度相等时,其在阶跃式光纤中反常色散区和正常色散区所产生的调制不稳定性。结果表明,调制不稳定性这一过程不仅产生在反常色散区,而且产生在正常色散区,对于阶跃式光纤,矢量调制不稳定性还随级数不同发生变化。并且,反常色散区产生的增益谱不同于正常色散区,较之多一个增益谱区域。     

Long-cavity passively mode-locked fiber ring laser with high-energy rectangular-shape pulses in anomalous dispersion regime

We report on a long-cavity passively mode-locked fiber laser in the anomalous dispersion regime. The nonlinear polarization rotation technique is employed to achieve mode locking. The output pulse from the fiber laser has a rectangular shape and a corresponding gaussian-shape spectral profile. Stable mode-locked pulses at a repetition rate of 278 kHz with single pulse energy as high as 715 nJ are obtained under equal bidirectional pumping power of 500 mW in cavity. The experimental results demonstrate that the passively mode-locked fiber laser operating in the anomalous regime can also realize a high-energy pulse, which is different from the conventional low-energy soliton pulse.     

具有高阶色散项的交叉相位调制不稳定性分析

以包含了三、四阶色散项的耦合非线性薛定谔方程为基础,重点研究了三、四阶色散对交叉相位调制不稳定性的影响。结果表明：三阶色散对调制不稳定性不起作用;当满足一定条件时,由于四阶色散的影响,不仅在光纤的反常色散区,而且在光纤的正常色散区,交叉相位所致的调制不稳定性均发生在两个频谱区。当光纤各参量及两束入射光波功率一定时,光纤反常色散区第一区域的增益谱要比正常色散区第一区域的增益谱宽;同时,反常色散区第二区域的增益谱比正常色散区第二区域的增益谱更靠近零点;进一步对比反、正常色散区的这两个频谱区,发现两个频谱区的范围有确定的对应关系。