A parametric study of negative feedback Nd:YAG laser

The output pulse parameters of a mode-locked Nd:YAG laser with a passive negative feedback element were studied experimentally. The pulse evolution within the train was experimentally recorded using a modified second harmonic generation autocorrelator. By comparing the laser operation with and without an acousto-optic modulator, we found that with the later there is a significant increase in the mode locking probability and the pulse acquires a temporally Gaussian symmetric pulse shape. Further with the active modulator there is a relaxation in alignment tolerances and increase in the range of permissible dye concentrations for stable mode locking. It was also observed that the pulse width of the negative feedback laser depends on the saturation intensity of the mode locking dye and reduces for a dye with higher saturation intensity. The pulse width was also found to reduce linearly as the initial transmission of the dye is reduced.     

The SNR improvement for quartz-enhanced photoacoustic spectroscopy using wavelength calibration and fiber reflector

Wavelength calibration technique combined with a fiber reflector was used to improve the signal to noise ratio (SNR) of quartz-enhanced photoacoustic spectroscopy (QEPAS). A distributed feedback laser diode (DFB-LD), driven by sawtooth wave and high frequency sinusoidal wave, was used to excite the second harmonic signal of a quartz tuning fork (QTF) through laser-gas molecular interaction. Two collimators conducted the laser alignment through the spacing gap of QTF forks. Central wavelength of the DFB-LD was locked to the target gas absorption center by identifying the second harmonic signal maximum and applying calibration feedback on the driving current. The gas absorption center calibration and gas concentration measurements are conducted at a specific interval. The SNR of the photoacoustic signal was further acoustically enhanced by using a pair of on-beam acoustic resonators through increasing the photo-acoustic conversion efficient, and optically enhanced by using a fiber reflector to improve the laser power for photoacoustic signal excitation. The experimental results show that the SNR in wavelength calibration mode is 15 times higher than the conventional wavelength scanning mode and QEPAS signal with fiber reflector is 1.37 times stronger compared with that without a fiber reflector.     

Optimal control of nonadiabatic alignment of rotationally cold N2 molecules with the feedback of degree of alignment

Nonadiabatic alignment of rotationally cold N2 molecules is optimally controlled by shaping femtosecond pump pulses with the feedback of degree of alignment evaluated by an ion imaging technique. The alignment is optimized by doubly peaked pulses with approximately equal intensities. A doubly peaked pulse with an appropriate interval can be regarded as a single pulse with a center trough based on the considerations from both time and frequency domains, suggesting that the effective duration of a doubly peaked pulse rather than its structure is crucial to nonadiabatic molecular alignment.     

Linewidth narrowing caused by optical feedback in a multi-mode vertical-cavity surface-emitting laser

We have studied experimentally spectral characteristics of a multi-mode vertical-cavity surface-emitting laser that is subject to optical feedback. Appropriate alignment of the feedback mirror can suppress higher-order modes and significantly decrease the spectral linewidth of the laser.     

一种质子交换铌酸锂波导与保偏尾纤对轴方法

提出了一种用宽谱光源及偏光干涉效应为质子交换铌酸锂波导与其保偏尾纤的对轴方法.将宽谱光源与待对轴的保偏光纤相熔接,当宽谱光通过保偏光纤和质子交换铌酸锂波导后,由于偏光干涉效应,相干函数上新增不同的次峰.利用各次峰的位置和幅值信息,可解算出质子交换铌酸锂波导与其保偏尾纤的对轴角度,将解算出的角度作为反馈控制信号即可实现在线对轴.     

Automatic alignment of a high-power CO2 laser resonator

An on-line, electronic feedback approach for automatic alignment of a laser resonator is described. A measure of the resonator alignment error is derived by partitioning a 1% sample of the laser output into quadrants, and using standard electronic circuits to compare the average power present in each quadrant. A motorized X — Y alignment system is then used to implement the alignment changes required to maintain optimum output quality from the laser. The system is shown capable of stabilizing the output power in each quadrant of the beam to within about 2% of the optimum level.     

Optimal alignment sensing of a readout mode cleaner cavity

Critically coupled resonant optical cavities are often used as mode cleaners in optical systems to improve the signal-to-noise ratio (SNR) of a signal that is encoded as an amplitude modulation of a laser beam. Achieving the best SNR requires maintaining the alignment of the mode cleaner relative to the laser beam on which the signal is encoded. An automatic alignment system that is primarily sensitive to the carrier field component of the beam will not, in general, provide optimal SNR. We present an approach that modifies traditional dither alignment sensing by applying a large amplitude modulation on the signal field, thereby producing error signals that are sensitive to the signal sideband field alignment. When used in conjunction with alignment actuators, this approach can improve the detected SNR; we demonstrate a factor of 3 improvement in the SNR of a kilometer-scale detector of the Laser Interferometer Gravitational-Wave Observatory. This approach can be generalized to other types of alignment sensors.     

基于EKF的水下LD通信精对准控制算法

高速激光通信中接收机与光斑中心很难处于精对准状态,导致水下光通信链路难以稳定建立.首先采用蒙特卡洛仿真统计法分析激光光子在海水中传输的接收光强分布规律,再通过实验对接收端的光斑图像进行采样分析,利用曲线拟合得到接收器位置与接收光强的关系.仿真与实验结果表明:光束经过25 m的水下传输,接收光强分布仍近似为高斯分布.采用非线性估计算法(扩展卡尔曼滤波)与基本状态控制反馈理论,根据接收光强度估计接收器当前位置与最大光强处的距离,通过反馈算法实现接收端与光斑中心的主动跟踪对准.算法仿真结果显示,接收端对准误差在2 mm以下,稳定后接收效率超过98%.     

Laser beam alignment by fast feedback control of both linear and angular drifts

A new way to enhance the directional stability of laser beams with alternating intensity by fast feedback control of both linear and angular drifts has been proposed for alignment of laser beams at higher accuracy. Both linear and angular drifts of laser beams, processed through light intensity modulation and primary alignment using single-mode optical fiber (SMOF), are separated using light path arrangement and detected using phase-lock technique, and are controlled using fast feedback control mechanisms according to their detected magnitudes, so that both linear and angular drifts are suppressed to enhance the directional stability of the emitting laser beams. Theoretical analyses and preliminary experimental results indicate that the approach proposed can be used to achieve an alignment accuracy of more than 10⁻⁸ rad.     

用于波导阵列——光纤阵列自动对接的多目标演化算法的研究

开发设计了一种新的列阵自动对接方法,该方法将多目标演化法导入光纤—光波导列阵—光纤列阵的自动对接,并行操作次数较常规遗传法大幅减少.数值仿真表明,对于模场非对称因子为0.4%的单模波导列阵与光纤列阵的双芯对接,能实现0.04 dB的平均端面耦合损耗.用于1×8波导分支耦合器与通道间距误差在0.35 μm以内的光纤列阵对接,自动耦合仿真达到了小于0.1 dB的平均端面耦合损耗,最大值与最小值的差小于0.06 dB.     

Strong optical feedback in birefringent dual frequency laser

Strong optical feedback in a birefringent dual frequency He--Ne laser with a high reflectivity feedback mirror has been investigated for the first time. The output characteristics of two orthogonally polarized modes are demonstrated in two different optical feedback cases: one is for both modes being fed back and the other is for only one of the modes being fed back. Strong mode competition can be observed between the two modes with strong optical feedback. And when one mode's intensity is near its maximum, the other mode is nearly extinguished. When both modes are fed back into the laser cavity, the mode competition is stronger than when only one mode is fed back. The difference in initial intensity between the two orthogonally polarized modes plays an important role in the mode competition, which has been experimentally and theoretically demonstrated.     

基于组合模理论设计的激光二极管阵列外腔

针对激光二极管阵列提出了"组合模"概念,计算了组合模的远场分布.每个组合模在远场的空间分布呈双瓣结构.基于激光二极管阵列组合模的远场分布特征,设计了离轴外腔反馈的激光二极管阵列.运用此装置所获得激光二极管阵列的远场分布有了明显变化.与自由运转的激光二极管阵列相比,离轴外腔反馈的激光二极管阵列远场宽度减少了4.3倍.在抽运电流为16 A时,测得输出激光的功率1.82 W,这相当于相同电流下自由运转激光器输出功率的79%.组合模理论不仅可以用来指导设计一维离轴外腔反馈激光二极管阵列,而且也可以用于设计二维离轴外腔反馈激光二极管阵列.二维离轴外腔反馈激光二极管阵列可以产生高功率,高光束质量输出的激光.     

Experimental characterization of DFB and FP chaotic lasers with strong incoherent optical feedback

We have experimentally investigated chaotic power oscillations in single-longitudinal mode DFB and multi-longitudinal mode FP lasers as a function of incoherent optical feedback strengths of up to 42%. We have demonstrated the existence of chaos in the output oscillations of both laser types using classical experimental tools such as RF spectrum, standard deviation, and maximum Lyapunov exponent, which all increase with increasing of feedback strength for both in single-longitudinal mode DFB lasers and multi-longitudinal mode FP lasers. It is also shown that power switching among longitudinal modes of multimode FP semiconductor laser is a considerable portion of the chaotic power oscillations for both strong and weak incoherent optical feedback.     

光电负反馈下单向耦合注入垂直腔表面发射激光器的混沌同步特性研究

基于自旋反转模型（SFM）,对光电负反馈下单向耦合注入垂直腔表面发射激光器（VCSEL）的混沌同步特性进行了数值仿真和理论分析.研究结果表明：随着反馈强度的增加,激光器输出功率呈现两个不同的混沌区域.在单一偏振模区域,能实现很好的同步性,但接近两模共存时,同步质量急剧下降;在两模共存区域,偏振模的连续性差,同步质量不理想.两模共存时的同步质量比单一偏振模差的这一特性,与已报道的基于光反馈的情景相反.产生这一特殊现象的原因是光电负反馈诱发了偏振模竞争效应.此外,还发现内部参数的变化对两个混沌区域的选取有较大 关键词： 线性偏振模 光电负反馈 垂直腔表面发射激光器（VCSEL） 混沌同步     

Pulse shaping control of alignment dynamics in N2

Control on the alignment transients of impulsively aligned ensembles of N₂ molecules has been demonstrated by the use of laser pulses shaped by a spatial light modulator. An alignment experiment has been inserted in the feedback loop of an evolutionary algorithm that found optimum pulse shapes for a set of criteria. Optimum pulse shapes for the maximization of total alignment and for the control of certain aspects of the revival structures are given. The physical mechanisms responsible for the control are analysed with the help of single‐parameter control schemes and numerical simulations, which allowed us to explore the low‐temperature region. This approach sheds light on the role played by different control mechanisms for the alignment dynamics of a molecular ensemble. Copyright © 2006 John Wiley & Sons, Ltd.     

Collective alignment of polar filaments by molecular motors

We study the alignment of polar biofilaments, such as microtubules and actin, subject to the action of multiple molecular motors attached simultaneously to more than one filament. Focusing on a paradigm model of only two filaments interacting with multiple motors, we were able to investigate in detail the alignment dynamics. While almost no alignment occurs in the case of a single motor, the filaments become rapidly aligned due to the collective action of the motors. Our analysis shows that the alignment time is governed by the number of bound motors and the magnitude of the motors’ stepping fluctuations. We predict that the time scale of alignment is in the order of seconds, much faster than that reported for passive crosslink-induced bundling. In vitro experiments on the alignment of microtubules by multiple-motor covered beads are in qualitative agreement. We also discuss another mode of fast alignment of filaments, namely the cooperation between motors and passive crosslinks.     

Longitudinal mode competition in semiconductor lasers under optical feedback: Regime of short-external cavity

This paper introduces a theoretical study of longitudinal mode competition in semiconductor lasers subject to optical feedback. The study is based on a model of time-delay multimode rate equations taking into account both symmetric and asymmetric suppressions of modal gain. The model is numerically solved and applied to the case of a short-external cavity. Mode competition is characterized along the feedback-induced period-doubling route to chaos as well as under chaotic dynamics. Contributions of symmetric and asymmetric gain suppressions to both mode dynamics and modal operation under OFB are clarified. The results show that under chaotic dynamics, mode competition induces multimode hopping giving rise to asymmetric multimode output spectra. In regimes of continuous-wave operation, mode competition supports single-mode oscillation, and the side-mode suppression ratio improves with the increase of feedback. In the regime of strong feedback, the lasing mode moves to either long- or short-wavelength side in a seemingly random fashion, which is strongly related to asymmetric gain suppression.     

A two-dimensional self-aligning system driven by shape memory alloy actuators

The two-dimensional automatic alignment system with an optical aperture is developed. The optical aperture with four integrated photodiode cells for feedback control signal is fabricated by MEMS techniques. The translation stage of the system is driven by four shape-memory-alloy positioning actuators to realize two-dimensional (XY-plane) movement.     

Self-mixing interference effects in orthogonally polarized dual-frequency Nd:YAG lasers at different feedback levels

The self-mixing interference effects are investigated in birefringence dual-frequency Nd:YAG lasers at different optical feedback levels. For weak feedback level, both of the intensities of the two modes are sinusoidally modulated by external cavity length with a feedback period of half laser wavelength. While these two modes can only coexist in a part of feedback period at moderate feedback level, i.e., one mode reaches its maximum intensity, the other one extinguishes. At high feedback level, complete polarization switching (one mode oscillating, the other one extinguishing) between two polarized modes is observed, and every switching corresponds to quarter-wavelength variation of external cavity length. Furthermore, two feedback schemes, i.e., isotropic optical feedback (IOF) and polarized optical feedback (POF) are taken into account. The theoretical analysis based on a model of two-mode-coupled laser is presented, which is in good agreement with experimental results. Our results can advance the research of self-mixing interferometer of orthogonally polarized dual-frequency lasers.     

Fano resonance and spectral compression in a ring resonator drop filter with feedback

We propose a ring resonator drop filter with feedback loop which can form the other ring cavity to produce an asymmetric Fano-resonance line shape and mode compression. By properly adjusting the feedback loop, the asymmetric line shapes of the transmission spectra can be controlled and selectively mode compression of the circuiting intensity in the feedback loop is achieved. Such characteristics are useful for applications in ring resonator-based photonic devices such as all-optical switches, sensors and intracavity frequency selection single-mode laser.