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。     

Multiwavelength picosecond optical pulse generation using anactively mode-locked multichannel grating cavity laser

A multichannel grating cavity (MGC) InGaAsP-InP ridge diode laser is found to be suitable for multiwavelength ultrashort optical pulse generation using active mode-locking techniques. By using different configurations of the MGC laser, dual wavelength picosecond pulses are successfully produced with good spectral quality, either simultaneously or with a programmable relative delay between the channels. For a channel separation of 2.2 nm, pulses with a duration of 60 ps and a spectral width of 11.6 GHz have been obtained. The experimental results are compared with those from a theoretical analysis of the multiwavelength mode-locking process using a set of modified coupled-cavity rate equations. The minimum achievable pulsewidths generated by the actively mode-locked MGC laser are shown to be limited by the resolution bandwidth of grating used. The dependence of pulsewidth on RF drive frequency detuning is examined and it is found that the ultrashort pulses can be obtained over a wide range of frequency detuning. The interchannel cross-talk originating from gain saturation and carrier depletion is also discussed     

Self-phase modulation and spectral broadening of optical pulses insemiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems     

Spectral broadening of pulsating AlxGa1-xAs double heterostructure lasers

The longitudinal mode spectrum of stripe geometry double heterostructure (DH) lasers exhibits a spectral broadening of ∼ 0.8 Å when the laser emits pulsations. The broadening results from a chirp to longer wavelengths which occurs during the ∼ 0.4 ns long pulse. The chirp is due to an increase of the refractive index resulting from the reduction in the carrier concentration during the pulse. Both plasma and band to band interactions affect the index change with the latter mechanism giving the dominant contribution. A comparison of the wavelength shift obtained during the pulse with the current dependent shift of the longitudinal modes at currents well below threshold confirms that the change in carrier concentration is responsible for the chirp.     

双曲正割脉冲在光纤中传输的非线性效应

高斯型脉冲在光纤中的传输已有解析解,而对于一些锁模激光器发射的双曲正割脉冲在光纤中的传输特性尚未有解析解。为了研究高功率双曲正割脉冲在单模光纤中的传输特性,文中借助于分步傅里叶方法分析了峰值功率5 000W的双曲正割脉冲在初始啁啾为1.7 rad/s,色散长度0.64 m,非线性长度1.53 m 时1 m 长的光纤中的脉冲演化过程。结果表明,其色散感应脉冲展宽的定性特征与高斯脉冲近似一致,但色散感应的频率啁啾沿脉冲不再是纯线性变化,若输入脉冲峰值功率过大,会引起频谱振荡。     

Generation of orthogonal UWB shaping pulses based on compressed chirp signal

This study investigates a novel method to numerically generate orthogonal ultrawide band (UWB) shaping pulses based on compressed chirp signal. First, a pulse template with less than 1 ns duration time, which is used to construct a Hermitian matrix, is produced with a compressed chirp pulse. Sub-nanosecond orthogonal pulses are then generated for UWB by using the Hermitian matrix eigenvectors. The simulation results show that the power spectral density distribution of the UWB shaping pulses met the constraint of Federal communications commissions (FCC) spectral mask. The shaping pulses not only have higher spectrum utilization ratio and very short time duration but also have excellent autocorrelation and cross-correlation properties, which is an advantage to reduce the interference between multiusers. Especially, a method to produce sub-nanosecond orthogonal UWB shaping pulses by using a relatively longer duration chirp signal is presented.     

利用非线性偏振旋转锁模技术产生0.7nJ,1.5ps光脉冲

为了研究锁模光纤激光器以增益平坦型掺铒光纤放大器作为增益介质对输出特性的影响,采用增益平坦型掺铒光纤放大器结合光纤偏振控制器、偏振相关光隔离器组成锁模光纤激光器,基于非线性偏振旋转锁模技术,实现稳定、自起振锁模运转,得到了中心波长1560nm、重复频率6.495MHz、单脉冲能量0.7nJ、脉宽1.5ps的超短光脉冲。同时实验观察到峰值波长为1557nm和1570nm的双峰值波长锁模脉冲的产生。结果表明,采用增益平坦型掺铒光纤放大器替代普通掺铒光纤组成锁模光纤激光器,可获得较高单脉冲能量的超短光脉冲,锁模脉冲的输出光谱可能出现双峰结构,从而可为超短脉冲光纤激光器设计及实用化提供参考。     

短脉冲在色散平坦光纤中传输前后波形、相位和啁啾测量的实验研究

二次谐波频率分辨光学门(SHG-FROG)是能够准确测量短脉冲多项特性参量的新技术。利用二次谐波频率分辨光学门脉冲分析仪对在色散平坦光纤中传输前后的短脉冲进行了测量,得到了待测光脉冲的频率分辨光学门(FROG)图、自相关曲线、自相关频谱曲线、波形和相位曲线以及脉宽、谱宽、啁啾等反映短脉冲特性的信息,对实验结果进行了分析,并与高斯脉冲在单模光纤中的线性传输理论进行了比较。结果表明,激光器输出的短脉冲是具有负线性啁啾的近变换极限高斯脉冲,经过12.7 km色散平坦光纤传输后仍然为具有负线性啁啾的高斯脉冲,其谱宽在传输过程中基本保持不变,脉宽展宽了3.1倍,啁啾增大了4倍。实验测量结果和理论预期一致。     

Calculation of the colliding pulse mode locking in CW dye ring lasers

A calculation of the influence of a transient absorber grating on the pulse parameters of a dye laser in the "colliding pulse mode-locking" (CPM) regime is given. It is shown that, compared to other passive methods, considerably shorter pulses with a more favorable stability region can be obtained. Within the stability range of single pulse operation the shortest pulses in the CPM are obtained by operating far above threshold where the saturation energy of the amplifier is much greater than that of the absorber. Deviation from an amplifier position that is symmetric with respect to the amplification of the counter-running pulses leads to different energies of the pulses and to a strong decrease of the stability range. Taking the dispersive properties of absorber and amplifier into consideration, equations for the time dependent phase of the laser pulse are established and solved by an analytical ansatz. The resulting pulse chirp is shown to be mainly dependent on the detuning of the pulse frequency from the center frequency of the saturable absorber. The estimated values for the pulse duration and the down-chirp for typical experimental conditions of a rhodamine 6G-DODCI CPM laser agree well with the measured ones.     

光子晶体光纤中频率啁啾对超连续谱的影响

基于光子晶体光纤(PCF)中脉冲演变遵循的非线性演化方程,用数值方法研究了反常色散情形下PCF中脉冲初始啁啾对超连续谱产生的影响,探讨了利用脉冲啁啾控制超连续谱产生的方法。结果表明：正、负啁啾均使谱展宽的速率变小,尤其是负啁啾情形,展宽速率更小,说明啁啾对谱展宽是不利的;在一定的初始条件下,光谱展宽存在一个最佳光纤长度,在这个长度处,光谱展宽最大,进一步增加光纤长度,谱宽基本保持不变;相对于变换极限脉冲来说,初始正、负啁啾脉冲对应的最佳光纤长度分别缩短和延长。     

Novel Analytic Theory for Laser Mode-Locking

The time-domain ABCD matrix formalism is based on the propagation of a Gaussian pulse which is characterized by the pulsewidth and chirp. In this paper, we will perfect this method and derive time-domain ABCD matrices for more optical devices. Especially, we extend this perfected theory to analyze the principle of passive mode-locking by reasonable approximation though, as we know, the output pulse generated from passive mode-locking fiber laser is of sech-shape. It is attractive because it can present a fairly apparent view to understand the mechanism of fiber lasers as well as analytic results to understand the pulse characteristics.     

用干涉自相关包络宽度测量超短激光脉冲啁啾

为了测量超短激光脉冲啁啾值,提出了一种用2次干涉自相关包络宽度测量啁啾值的简单方法。利用2次干涉自相关包络宽度对啁啾有很高的灵敏度、含有不同啁啾量的超短激光脉冲有不同的包络宽度的特性,通过对高斯型强度分布的线性、平方及立方啁啾的干涉自相关包络函数进行了理论分析,得到包络宽度与啁啾量值之间的对应关系。采用干涉自相关2次谐波检测系统对加浓染料激光器输出的含有啁啾的脉冲进行测量,其干涉自相关包络宽度为1.15,被测超短激光脉冲啁啾为1.0。结果表明,根据含有啁啾的干涉自相关曲线两翼的特征,可判断啁啾阶数,再根据包络宽度与啁啾值的对应关系,可估定啁啾量值;用干涉自相关包络宽度能容易地测量超短激光脉冲的啁啾量值。     

Algorithm for complete and rapid retrieval of ultrashort pulseamplitude and phase from a sonogram

We describe an algorithm based on principal-component generalized projections that is capable of unambiguous retrieval of the amplitude and phase of an ultrashort pulse from a measurement of its sonogram. Running on a personal computer, the algorithm can complete more than 50 iterations per second which enables real-time display of an ultrashort pulse intensity and phase profile. Retrieval is demonstrated for a range of pulses with commonly encountered phase errors including linear chirp, self-phase modulation, and cubic spectral phase, and experimental results illustrate good agreement between the retrieved pulse characteristics and independent spectral measurements     

10GHz ps光孤子在G652光纤中的绝热传输实验

利用二次谐波频率分辨光学门(FROG)脉冲分析仪,实验研究了一阶啁啾ps光孤子在普通单模光纤(SMF)中长距离绝热传输时脉冲波形、脉宽和啁啾的变化规律。实验结果表明,在色散较大的标准SMF中,一阶啁啾孤子的稳定传输长度超过1000个色散长度,即很好地保持双曲正割波形不变(啁啾不影响孤子的波形),但啁啾孤子的脉宽展宽速度高于非啁啾孤子绝热传输的微扰结果;采取预加重措施,可以减小孤子脉宽的增大。因此,对于占空比较小的ps孤子脉冲,可以不必采用复杂的消啁啾和色散管理措施,就可以利用绝热方式进行稳定传输。     

10 GHz超低抖动光短脉冲源

报道了一种新型的重复频率为10 GHz,脉宽为5.3 ps,抖动为184 fs的高稳定光短脉冲源.将大信号调制半导体激光器产生的10 GHz光脉冲,先送入LiNbO3电光相位调制器增强负啁啾,并使光谱进一步展宽,再通过色散补偿光纤(DCF)压缩脉冲啁啾,可得到光短脉冲.由于大信号调制激光器输出的光脉冲本身具有负啁啾,而通过相位调制器的光脉冲在不同的时间间隔内既有正啁啾也有负啁啾,通过适当调整进入相位调制器的光脉冲时延,使其通过相位调制器后累加产生更大的负啁啾,再利用正色散光纤压缩啁啾,从而得到低抖动且无基座的光短脉冲.     

Compressible frequency chirping due to slow and fast self-phasemodulation of ultrashort pulses and high-gain dye laser amplifiers

A theoretical investigation was conducted of the amplitude and phase changes in high-gain ultrashort pulse dye laser amplifiers for cases when the beam area is rather small and the pulse spectral center is not located at the amplifier gain peak. Due to high energy densities as well as high intensities, both slow (saturation-induced) and fast (Kerr-type) self-phase modulation can take place. Under certain conditions, these effects can together produce a considerable amount of mostly linear carrier chirp, resulting in compressible bandwidth enhancement. Compression of the amplified pulses is also calculated using quadratic and quadratic-plus-cubic phase compensation     

啁啾指数脉冲的非线性传输研究

数值研究了初始线性啁啾指数光脉冲在单模光纤反常色散区的非线性传输特性,并将这种特性与双曲正割脉冲的相关特性作了比较.脉冲振幅A=1时,负啁啾对脉冲时域宽度展宽的影响比正啁啾大;指数脉冲受啁啾影响比双曲正割脉冲更敏感.A＞1.2时,脉宽在演化过程中呈周期性衰减振荡,振荡周期和振幅随啁啾|C|(|C|≤2)的增大而增大,最后能演化成光孤子.脉冲演化成光孤子所需的最小振幅A比啁啾双曲正割脉冲的大.     

Chirp compensation in mode-locked DFB laser diodes with extendedcavity

The spectro-temporal behavior of actively mode-locked fiber-external-cavity DFB lasers at 1.55 μm is analyzed in detail. Experiments are performed with multiquantum-well InGaAsP lasers of different lengths and different phase-amplitude coupling factors. Transform-limited picosecond pulses are generated with pulsewidth adjustable over half a decade by changing the RF conditions and the device length. As in the case of conventional laser diodes with external grating, the smallest time-bandwidth products are obtained for modulation frequencies to the high-frequency side of the mode-locking band. This result is presently described in terms of chirp compensation by the DFB cavity dispersion. An analytical expression of the pulse compressibility is established from a steady-state mode-locking equation including refractive index variations and cavity dispersion effects. Gain parameters entering the expression are separately evaluated from standard rate equations. A good agreement is found between experimental and theoretical results     

Synchronized transform-limited operation of 10-GHz colliding pulse mode-locked laser

We report a 10-GHz colliding pulse mode-locked laser fabricated with integrated active-passive waveguides. The laser fabrication adopted a deep reactive ion etching and single-step metal-organic vapor phase epitaxy regrowth process for forming the buried heterostructure waveguide. Clean output pulses resulted from laterally tilting the active-passive interface and effectively suppressing residual back-reflections at the interface. Hybrid mode-locking resulted in a synchronized transform-limited sech/sup 2/optical waveform. Pulsewidth, chirp, timing jitter, and frequency-locking range were investigated through systematic device biasing condition optimization.     

Photonic Generation of Chirped Microwave Pulses Using Superimposed Chirped Fiber Bragg Gratings

A novel approach to generating linearly chirped microwave pulses in the optical domain based on spectral shaping and linear frequency-to-time mapping is proposed and experimentally demonstrated. In the proposed system, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is spectrum-shaped by an optical filter that consists of two superimposed chirped fiber Bragg gratings (SI-CFBGs) with different chirp rates. The SI-CFBGs form a Fabry-Perot cavity with a cavity length linearly dependent on the resonance wavelength, thus a spectral response with an increased or decreased free spectral range is generated. A chirped microwave pulse with the pulse shape identical to the shaped spectrum is obtained at the output of a high-speed photodetector thanks to the frequency-to-time mapping in a dispersive device. The proposed technique is experimentally demonstrated, a linearly chirped microwave pulse with a central frequency of 15 GHz and a chirp rate of 0.0217 GHz/ps is experimentally generated.