An investigation into femtosecond pulse formation in acontinuously-pumped passively-mode-locked CPM ring dye laser

A detailed experimental investigation of the formation of ultrashort pulses in a CW-pumped passively mode-locked CPM dye laser is described. It is found that it takes ~10⁴ round trips for the circulating energy in the cavity to reach the level at which the passive mode-locking process is initiated, and a subsequent ~10³ round trips for the ultrashort pulse evolution to reach a steady state. Sequences of autocorrelation profiles and spectra are presented showing the dynamic pulse evolution, and the influence of intracavity group velocity dispersion on pulse formation is examined     

圆对称高斯光束在吸收克尔介质中的稳态自聚焦行为

由光束在吸收克尔介质中传输的非线性近轴波方程 ,推导出具有二次相位分布的圆对称高斯光束在线性吸收介质中的耦合传输方程以及光束束腰的演化方程。数值计算的方法证实了光束束腰变化的几种可能 ,分析了稳态自聚焦的阈值特性。在给定参数的情况下 ,得到了稳态自聚焦的阈值入射光强和阈值聚焦长度     

环激光中Langmuir流效应的理论分析

气体增益介质的流动是激光陀螺中的一项主要误差源.本文定量地讨论了在增益介质中激活原子和非激活原子的流动所带来的反向行波频差漂移.给出了在三阶理论下的Langmuir流的理论公式及计算值.本文重点在于用统计的方法处理了高斯光束通过增益场和流速场时的频差漂移.并给出各种情况下的计算公式.     

势函数分析高斯光束在克尔介质中的传输

从标准的非线性薛定谔方程出发,导出了光束宽度参量所满足的方程,并用物理上较为直观简洁的势函数分析方法研究了一维高斯光束在克尔介质中的传输行为,得到了光束可能的3种传输行为,较为成功地说明了数值计算的结果所预测的基本特征。     

On the saturation effects and start jump of Gaussian modes in oscillators

The present paper deals with the interaction of a Gaussian mode with a homogeneous and an inhomogeneous laser transition. The interaction of the beam with the medium in a laser is treated by taking into account the spatial distribution of both radiation and gain. The intensity characteristics are very different from those obtained for a one-dimensional interaction of a plane wave with a saturating medium. In the presence of a small-signal gain profile the threshold condition requires much higher inversion densities along the optic axis than by ignoring this profile. For gas lasers, for instance, having a small-signal gain profile that is approximately described by a zero-order Bessel function, the threshold inversion density can be about 50 percent higher. For high-power systems the saturation of the medium by the Gaussian intensity distribution results in a considerable amount of radial radiation transport. For homogeneous transitions this amount is about equal to the stimulated emission and for inhomogeneous transitions it is about half of it, independent of the beamwidth. Further, it is found that if one slowly passes the threshold condition for laser action, the intensity jumps from zero to a certain value and vice versa. This effect has also been verified experimentally.     

基于半导体光放大器的可调谐环形腔激光器的激光建立过程

基于半导体光放大器(SOA)的环形腔激光器可实现高达兆赫兹的高速调谐,在光纤通信和光纤传感领域有广泛应用。采用稳态模型和分段算法研究SOA自发辐射特性,并基于此分别讨论了使用高斯型滤波器和法布里-珀罗(F-P)滤波器的两种可调谐环形腔激光器,研究了激光建立过程的特性,包括输出光谱和输出光功率随绕行圈数的变化,讨论了波长调谐的速度。结果表明,F-P滤波器的环形腔可以更快建立激光振荡,便于实际高速调谐应用。     

Effects of hard apertures on mode properties of resonators with gaussian reflectivity mirrors

The effects of hard apertures on the energy distribution, the far-field beam quality, the mode volume, and the discrimination against high-order modes in Cassegrain resonators with Gaussian reflectivity mirrors have been investigated both theoretically and experimentally. It has been shown that, in the far field, the fraction of energy in the secondary lobes remains small as long as the ratio of the design beam waist to the radius of the gain mediumw_{b}/ais <0.40. Forw_{b}/a > 0.7, deterioration of the beam quality and diffraction losses counterbalanced a more efficient filling of the gain medium to limit the resonator efficiency. At low magnifications, resonators with Gaussian mirrors out perform resonators with Standard mirrors.     

Analysis of passive additive-pulse mode locking with eigenmodetheory

The equation of motion for the coupled cavities of additive-pulse mode locking is solved by the eigenvalue method. The pulse evolution and self-starting condition for passive operation is analyzed. It is shown that the laser configuration is equivalent to an intracavity interferometer, and a phase modulation across the pulse is the essential element for mode locking. With the method developed, transient pulse evolution of an initial seed pulse can be calculated and thereby optimized. It is found that the pulse shortening rate is linear with respect to the number of round trips. The duration for the initial pulse to shorten to a steady state is proportional to its initial width and inversely proportional to the nonlinear coefficient     

Gaussian beam optical systems with high gain or high loss media

Coherent electromagnetic wave amplifiers with non-negligible gain per wavelength are included in the Gaussian beam matrix formalism and a procedure is developed for propagating Gaussian beams in optical systems that may include unsaturated amplifiers and similar absorbers. Standard formulas for beam spotsize and radius of curvature in a uniform medium are generalized in a new way to include gain or loss. An asymmetric focal shift and a potentially infinite spotsize are predicted. These dramatic effects are interpreted physically     

双镜环行行波激光器

提出了一种全新的固态双镜立体环行行波激光器.此类激光器的谐振腔仅由两面反射镜组成,其中一面反射镜直接镀制在激光介质上,激光束在腔内沿立体环形光路行进,形成闭合谐振回路.利用Nd:YAG作为工作介质和双凹稳定谐振腔型,实现了用激光二极管(LD)抽运的多种双镜环行行波激光的稳定运转.实验结果表明此类激光器具有结构简单、调整容易、稳定性好的优点,并可望通过多点纵向抽运实现热效应低的高效率、高光束质量、大功率全固态激光器,单向输出的基频光和/或倍频光的单频激光器,以及高重复频率、短腔长的锁模激光器.     

高脉冲稳定性的100kHz皮秒再生放大器

报道了一种具有高脉冲稳定性的100kHz皮秒脉冲再生放大装置。该放大装置采用激光二极管(LD)端面抽运的Nd:YVO4晶体作为增益介质,RTP晶体作为电光晶体。再生腔的腔型为对称W型,总长1.8m。分析了皮秒脉冲在再生放大腔中往返次数和再生腔损耗对放大脉冲倍周期分叉现象以及稳定放大时输出功率的影响。抽运功率为30W时,通过选取最优的往返次数获得了功率为5.3W的高脉冲稳定性的再生输出,脉冲稳定性均方根(RMS)值小于2%。放大后皮秒脉冲脉宽为13.78ps,脉冲峰值功率3.84MW,再生腔输出的光束质量因子M2≤1.5。     

All-fibre, diode-pumped recirculating-ring delay line

A Nd³⁺-doped optical fibre is used as an amplifier in a 35 m fibre recirculating delay line to overcome the round-trip losses experienced by injected pulses. Dichroic fused tapered couplers are used to couple light from a semiconductor source into the ring, to pump the amplifier. Injected pulses have been maintained for more than 300 round trips     

内腔连续波光学参量振荡器:高斯光束理论

建立了描述内腔连续波单共振光学参量振荡器(ICCWSRO)的功率特性的高斯光束理论。     

对卡塞格林式多程放大器中环形光束传输的数值模拟

利用光线追迹法以及分步傅里叶-贝塞耳变换法,考虑了光阑对光束的拦截和通光作用以及激光放大过程中的增益饱和效应,对环形腔调Q钕玻璃激光系统振荡器输出的光束经卡塞格林式放大器多程放大后的光场演化过程进行了数值模拟,并进行了实验验证。分析得出现有系统中输入输出镜的阶跃反射率分布是导致输出环形光束质量不好的主要原因,并重点对放大器的腔镜部分进行了优化设计,提出了反超高斯反射率分布的输入镜与超高斯反射率分布的输出镜相组合的腔镜对结构。数值模拟结果表明,该腔镜对的使用可大大改善输出环形光束的质量。     

Mode structure of a tapered-wiggler free-electron laser stable oscillator

The transverse mode structure of a tapered-wiggler free-electron laser (FEL) oscillator operating at full saturated intensity is analyzed numerically. The unique features of the FEL geometry, and particularly the refractive property of thee-beam, which acts like a series of small focusing lenses, tend to produce a steady-state mode structure different from the TEM00mode of the cavity. The higher order mode content is especially evident in confocal cavities, since higher order modes constructively interfere on different round trips. Constructive addition of higher-order modes is also expected in practical FEL's operating at substantial power levels using near-concentric cavities. The presence of higher order modes has unusual effects on the intracavity intensity distribution of a confocal cavity, including different spot sizes on the front and rear mirrors and different transverse structure on forward and reverse passes. Nevertheless, the output beam quality is nearly diffraction limited for the low gain, low Fresnel number cases studied. Noticeable higher order mode content is also expected for near-concentric cavities.     

PAC vs. MAC for Decentralized Detection Using Noncoherent Modulation

In decentralized detection, local sensor observations have to be communicated to a fusion center through the wireless medium, inherently a multiple-access channel (MAC). As communication is bandwidth- and energy-constrained, it has been suggested to use the properties of the MAC to combine the sensor observations directly on the channel. Although this leads to an array-processing gain if the sensors' transmissions combine coherently on the channel, it has been shown that this is not the case when they combine noncoherently. We review known results for the coherent case and then analyze the noncoherent case based on a simple on/off scheme combined with optimal sensor “censoring.” Since the optimal forwarding function is not available, we also bound the performance using an equivalent communication problem and a centralized estimator to verify trends. We find that for noncoherent modulation, there is no processing gain using the MAC for decentralized detection, but compared to parallel-access channels (PACs) the MAC avoids the noncoherent combining loss. Still the performance of the MAC approach is only of diversity one, as the output of the MAC is approximately a zero-mean complex Gaussian random variable for a large number of sensor. The MAC performance can be increased by using multiple independent channels, each used as a MAC by all sensors, which we term diversity-MAC. This approach always outperforms the PAC scheme on Rayleigh fading channels, where the output is exactly Gaussian, but has inferior performance across random phase channels when few sensors are used, as the PAC does not create “artificial” fading.      

Thermal Microwave Emission from a Three-Layer Random Medium with Three-Dimensional Variations

The radiative transfer theory has been used to study the thermal microwave emission from a three-layer random medium. The random medium is characterized by correlation functions that are Gaussian in the horizontal direction and exponential in the vertical direction. Gaussian quadrature methods are used to solve for the brightness temperatures. The spectral and angular dependence of the brightness temperature are illustrated numerically. In particular we compare the case of a two-layer random medium whose brightness temperature decreases with frequency with the case of a three-layer random medium whose brightness temperature increases with frequency.     

译码转发中继系统基于校验分裂的IRA码设计

在译码转发中继系统中,非规则重复累积(IRA)码的设计本质是设计码率兼容IRA码.由于通过校验分裂得到的码字的不同部分经历不同的信噪比,因此给出了基于校验分裂的修正的高斯近似的密度进化算法来分析IRA码的门限并且用差分进化算法对其优化.实验仿真证明与传统的密度进化算法得到的IRA码相比,当误帧率是10~(-3)时,优化得到的码字大约有0.5dB的增益.     

高阶调制系统下LDPC码优化设计

为了提高编码调制系统的整体编码增益,提出一种高阶调制系统下LDPC码的度分布优化方法.根据高阶调制符号中不同比特的误比特特性,将调制符号所经历信道建模为一组对称二进制输入加性高斯信道.在此基础上,推导了高阶调制系统下LDPC码高斯近似密度进化分析方法,并得到译码收敛条件.结合度分布约束关系及译码收敛条件,提出高阶调制系统下LDPC码的度分布优化问题及差分进化实现方法.仿真结果表明,设计的LDPC码在高阶调制系统中的渐进性能和误码性能优于基于比特优化映射的编码调制方案.