耗散孤子和束缚态脉冲可切换的超连续谱产生

当超短脉冲进入高非线性光纤时,在色散和非线性效应的共同作用下,脉冲频谱中会产生一些新的频率分量,使得输出频谱比输入频谱宽得多。这种光谱被称为超连续谱。超连续谱光源具有光谱范围宽、方向性好、亮度高、空间相干性好等优点。在锁模激光器中,传统孤子、耗散孤子和类噪声脉冲可以作为种子源产生超连续谱。文中,笔者建立了一个NPR被动锁模光纤激光器来产生脉冲激光。然后,添加一段DCF以补偿腔中的色散,从而产生耗散孤子。同时,通过调节腔内PC,可以实现束缚态和耗散孤子的状态切换。输出脉冲经10 m单模光纤压缩后注入部分拉锥后的高非线性光纤以产生超连续谱。实验中,我们得到了脉宽为5.6 ps、重复频率为32 MHz、信噪比为52 dB的耗散孤子锁模脉冲,压缩后的脉冲宽度为868 fs,用作超连续谱产生。超连续谱的覆盖范围约为1200~2200 nm,其20 dB谱宽为357 nm。通过调节偏振控制器,实现耗散孤子脉冲与束缚态脉冲之间的切换,束缚态脉冲持续时间为1.4 ps,脉冲间隔为14 ps,信噪比为51 dB,产生1600~1870 nm的超连续光谱,20 dB的光谱宽度为135 nm。     

单脉冲锁模光纤激光器输出特性的数值研究

为了研究不同腔内净色散下单脉冲被动锁模光纤激光器的输出特性,采用以非线性薛定谔方程为数学模型和分步傅里叶的方法,对激光脉冲在腔内的演化进行了理论分析。获得了在保持单脉冲稳定输出时激光器一些参量与腔内净色散的变化关系,并针对净色散为正的情况,对输出脉冲进行了腔外解啁啾压缩,压缩比达到10倍以上,分析了压缩所需的负色散值以及压缩后脉宽的情况。结果表明,小信号增益系数最大值与净腔色散大体上成正比关系,且当小信号增益系数达到最大值时,输出脉冲的脉宽以及相应的时间带宽积呈现逐渐增加的趋势,3dB带宽则呈现出先增加后减少的趋势。该研究结果为优化被动锁模光纤激光器提供了参考。     

Analysis of Gain Competition Suppression in Multiwavelength Actively Mode-Locked Erbium-Doped Fiber Lasers Incorporating a Highly Nonlinear Fiber

A numerical model for multiwavelength actively mode-locked erbium-doped fiber lasers (AML-EDFL) is presented. Using the model, the influences of four-wave mixing (FWM) and phase modulation effects (i.e., self-phase modulation (SPM) and cross-phase modulation (XPM)) can be studied, respectively. We use it to investigate AML-EDFLs incorporating a highly nonlinear fiber (HNLF) and explore the mechanisms for gain competition suppression (GCS). The net dispersion of the cavity is found critical for GCS. In the small dispersion regime, FWM dominates GCS, but SPM and XPM deteriorate the output pulses. In the large anomalous dispersion regime, SPM and XPM, however, enhance GCS and produce high-quality pulses. Degree of GCS is also quantitatively evaluated. The simulation results are at last confirmed by experiments.     

偏振控制锁模光谱边带偏移量的计算及实验验证

通过对光纤中传输的孤子波演化及其色散波相互作用的动力学方程分析,详细推导了环形腔非线性偏振旋转(NPR)锁模光纤激光器中色散波与孤子波相互干涉而产生的脉冲光谱边带偏移量的理论计算公式.并将各级边带波长与脉冲中心波长的偏移量理论值与相应的实验测量值进行了对比,理论值与实验值符合得非常好,其最大误差在4.1%以下.从而解释了在偏振控制被动锁模条件下,环形腔光纤激光器光谱边带产生的原因.     

呼吸脉冲锁模的光子晶体光纤飞秒激光器

报道了一种掺Yb偏振型大模场面积光子晶体光纤(LMA-PCF)飞秒激光器。作为增益介质的光子晶体光纤的单模场面积比传统光纤高一个数量级,有效地降低了非线性系数,使激光器获得高能量输出。激光器基于线形腔结构,利用半导体可饱和吸收镜实现自启动锁模。光纤激光器利用光栅对进行腔内色散补偿,使其运转在呼吸脉冲锁模状态,即在谐振腔的零色散点附近实现锁模。当腔内净色散呈反常色散时,激光器获得了平均功率为400mW,重复频率为47MHz(对应于8.5nJ的单脉冲能量),脉冲宽度为500fs的稳定的锁模脉冲输出,经腔外色散补偿,脉冲压缩至98fs。当腔内净色散呈正常色散时,激光器输出的单脉冲能量为10.6nJ,脉冲宽度为1.76ps,经腔外色散补偿,脉冲压缩至160fs。     

Multistable self-trapping of light and multistable soliton pulse propagation

It is demonstrated that a nonlinear Schrödinger equation with certain nonlinearities allows for an existence of multistate single solitons (i.e., single solitons with the same carried power but different propagation parameters). In nonlinear optics, these solitons may exist either in the form of short bistable pulses, or bistable self-trapping (both two- and three-dimensional).     

Improved Optical Pulse Characterization Based on Feedback-Controlled Hilbert Transformation Temporal Interferometry

We propose an accurate ultrashort optical pulse reconstruction method based on Hilbert transform temporal interferometry with a feedback control technique. The use of this feedback control allows us to minimize the phase measurement errors associated with instabilities in the fiber-based interferometry system, e.g., caused by environmental fluctuations. The method is experimentally demonstrated by precisely characterizing picosecond pulses after linear dispersion through short sections (30 ~ 700 m) of conventional single-mode fiber. A three-fold improvement in the phase measurement accuracy has been achieved using the feedback control technique.     

超低重复频率全光纤被动锁模掺铒光纤激光器

报道了一种新颖的全光纤超长环形腔掺铒光纤激光器。该激光器采用非线性偏振旋转(NPR)技术实现自启动锁模,除了NPR元件外,其余全部由单模光纤(SMF)元件组成。通过加入4km的SMF构成长达4.046km的环形腔,产生了重复频率为50.92kHz的稳定锁模脉冲,最大平均输出功率为2.73mW,对应的单脉冲能量为53.61nJ。     

长腔被动锁模掺镱光纤激光器的方波脉冲产生

报道了一种基于非线性光纤环形镜(NOLM)、工作在耗散孤子共振(DSR)区的长腔被动锁模掺镱光纤激光器,该激光器谐振腔的总长度约为1502 m,可以输出重复频率为133.18 kHz的高能量方波脉冲,且输出脉冲的宽度和单脉冲能量均随泵浦功率的增大而呈线性增大。当泵浦功率增大到414.47 mW时,输出的方波脉冲具有最大宽度(761.6 ns),同时单脉冲能量达到了最大值(60.2 nJ)。通过改变NOLM中单模光纤的长度,进一步研究了谐振腔长度对输出方波脉冲特性的影响,结果表明:谐振腔越长,所得DSR方波脉冲越宽,脉冲峰值功率越低。     

输出大啁啾脉冲的展宽脉冲锁模光纤激光器

报道了一种大啁啾脉冲输出的全光纤展宽脉冲锁模激光器,以非线性偏振旋转(NPR)实现自启动锁模。激光器其余部分为全单模光纤(SMF)结构,提供很大的正色散,光栅对提供色散补偿,输出展宽脉冲。实验中得到了重复频率36.96MHz,单脉冲能量1.81nJ的稳定锁模脉冲序列,使用频谱分析仪观测得到脉冲序列一次谐波信噪比(SNR)达到80dB。直接输出脉冲有很大的正啁啾,脉宽为2.17ps,经过腔外压缩可获得70fs的脉冲。这种能压缩到百飞秒量级的大啁啾脉冲非常适用于光纤啁啾脉冲放大(CPA)系统。     

基于光谱控制与色散优化的飞秒啁啾脉冲放大系统

飞秒激光在工业加工、精密测量、军事国防、科学研究等领域具有广阔的应用前景。报道了基于光谱控制与色散优化的高功率、高脉冲质量飞秒啁啾脉冲放大系统。利用与压缩器色散量相匹配的色散可调啁啾布拉格光纤光栅(CFBG)作为展宽器,通过微调CFBG色散量补偿系统的残余色散使整个系统的净色散趋于零;同时引入光谱滤波等手段,保证入射到主放大器之前的脉冲光谱形状不发生畸变,避免了放大过程中脉冲质量的劣化。最终获得了重复频率为50 MHz、平均功率为24 W、脉冲宽度为198 fs的高脉冲质量飞秒激光输出。     

Stable multiwavelength dispersion-tuned actively mode-locked erbium-doped fiber ring laser using nonlinear polarization rotation

We demonstrate a stable multiwavelength actively mode-locked erbium-doped fiber laser at 10 GHz based on a dispersion cavity and nonlinear polarization rotation (NPR) effect. The dispersion cavity allows simultaneous multiple wavelengths to satisfy the round-trip condition and smooth wavelength tuning, while the NPR ensures the stable operation and supermode noise suppression. A simple model is developed to explore the optimal setting of the cavity. Simultaneous dual- and five-wavelength operation is experimentally investigated.     

入射光强对多个亮孤子相干相互作用的影响

为了研究入射光强对多个亮屏蔽孤子之间相干相互作用的影响,采用数值模拟的方法求解孤波演化方程。在特定的相互作用距离和相对相位条件下,改变入射孤子的光强,分别对2个孤子和3个孤子的相干相互作用做了分析。结果表明,在同相位条件下,2个孤子的相互作用表现出融合-分开-融合的周期性;对3个孤子,这种周期性消失;同时表现出能量的定向转移特性。改变入射光强的大小,能够控制能量转移的方向。在相反相位的情况下,2个孤子和3个孤子类似,都表现出相互排斥的相互作用,入射光强的大小会影响相互作用的程度。当孤子之间相位差为/2时,2个孤子和3个孤子都表现出能量的定向转移,能量转移方向由相对相位差的符号决定;对3个孤子,随着入射光强的增加,伴随着能量的转移,还会表现出一定的排斥作用。此研究结果说明入射光强对孤子的相互作用具有一定的调节作用。     

1 015~1 046 nm可调谐飞秒掺镱光纤激光器

近年来,超快掺镱锁模光纤激光器由于其转换效率高、操作方便、免维护、尺寸紧凑等优点,被广泛应用于工业加工、医疗外科、多光子成像等领域。在激光器中补偿群速度色散是获得皮秒甚至飞秒脉冲的有效方法。通过利用光栅对与光谱滤波器,实现了对激光波长、腔内色散和光谱宽度的灵活调节。该激光器能输出稳定的锁模脉冲,对应的基本重复频率为19.41 MHz。在+ 0.0127 ps2色散时中心波长1015~1037 nm可调;在+0.007 ps2色散时中心波长1015~1045 nm可调以及在?0.0127 ps2色散时中心波长1020~1046 nm可调。同时,当净腔色散从反常色散到近零色散变化时,光谱带宽可从1.40 nm调到19.38 nm,对应的压缩后脉冲宽度可从1.03 ps调至175.9 fs。该方案具备连续调整激光器状态的能力,有望用于高功率大能量飞秒激光前端,可以满足对激光器有多种谱宽及波长的应用需求。     

Characterization and optimization of optical pulse differentiation using spectral interferometry

A simple fiber-based spectral interferometry setup is implemented for characterizing and monitoring the amplitude and phase of ultrafast temporal waveforms generated by optical differentiation with a long-period fiber grating (LPFG). In particular, the system is applied to characterize subpicosecond odd-symmetry Hermite-Gaussian (HG) pulses, consisting of two /spl pi/ phase-shifted temporal lobes, obtained by temporal differentiation of Gaussian-like pulses. This technique is ideally suited for optimizing the experiment conditions (e.g., wavelength shifting between the input pulse and LPFG transmission characteristic) so as to achieve a nearly ideal odd-symmetry HG temporal waveform (with a sharp discrete /spl pi/ phase shift at its center), of potential interest as a higher order soliton in dispersion-managed optical communication systems.     

Soliton stability in optical fibers with polarization-modedispersion

This letter presents a detailed investigation of how solitons broaden in the presence of polarization-mode dispersion. For a given amount of polarization-mode dispersion and pulsewidth there exists an optimal choice of group-velocity dispersion, for which solitons are most resistant to the effect of polarization-mode dispersion. We also optimize the soliton power enhancement factor and find that solitons can propagate roughly twice the distance compared to linear pulses in the absence of group-velocity dispersion     

Interactions between dispersion managed solitons inoptical-time-division-multiplexed system

Nonlinear interactions between neighboring pulses in optical-time-division-multiplexed (OTDM) systems may cause a timing jitter of the signal pulse, which results in a limitation of the transmission distance and bit-rate. We show that alternating the phases or amplitudes of pulses in the time sequence is effective for reducing the interactions between dispersion managed solitons as in the case of ideal solitons, but only for a relatively weak dispersion management     

Tunable picosecond frequency-shifted feedback fiber laser at 1550 nm

A novel frequency-shifted feedback (FSF) fiber laser cavity is proposed and experimentally verified. The laser generates stable mode-locked pulses of <2 ps, mainly limited due to chirp induced by the frequency-shifting element (i.e., the acoustooptic modulator) on the pulses. Fourier transform-limited subpicosecond pulses were obtained by compensating this excess chirp using a piece of single-mode fiber SMF-28 outside the laser cavity. The unique cavity design also allow us to tune the pulse wavelength throughout the Er-gain band and the ability of the laser to self-start gives us the opportunity to characterize transient behavior of mode-locked pulses in an FSF fiber laser cavity.     

高阶色散和非线性效应下三阶孤子对传输的研究

高阶色散及高阶非线性效应是制约光孤子稳定传输的重要因素。基于光孤子传输的非线性薛定谔方程,综合考虑三阶色散和五阶非线性的影响,采用分步傅里叶算法,数值研究了三阶孤子对间的相互作用。结果表明：三阶色散导致三阶孤子对无规则地裂变,裂变后的光脉冲形状发生畸变,脉冲中心位置出现偏移;考虑负五阶非线性作用后,在一定程度上抑制了孤子裂变,但是光脉冲在传播过程中伴有能量转移。而正五阶非线性作用使三阶孤子对传输图形进一步恶化。适当地选取负五阶非线性参数值,可以完全消除三阶孤子对的裂变和相互作用,改善了脉冲中心位置的线性偏移。     

Dispersion-managed solitons for 160-Gb/s data transmission

The use of dispersion-managed (DM) solitons for 160-Gb/s data transmission in fiber lines where the period of dispersion management is much shorter than the amplifier spacing is investigated through numerical simulations. It is shown that DM solitons outperform both standard solitons and “quasi-linear” return-to-zero (RZ) pulses. The dispersion tolerance is limited by pulse-to-pulse interactions and Gordon-Haus jitter and decreases with increasing transmission distance