Graded-index time-lens implementation

The propagation of a laser pulse in a graded-index (GRIN) space lens and in an electro-optic crystal with an appropriate refractive index modulation is studied. It is shown that the crystal functions as a GRIN time lens if the propagation's second-order dispersion is included and if the phase velocity of the modulating wave equals the group velocity of the laser pulse.     

热效应对克尔自锁模激光腔的影响

讨论了克尔透镜锁模激光腔中热效应对锁模的影响，用数值解法给出了存在热效应时腔中光斑半径随归一化功率的变化曲线。同时给出了存在热效应时光斑半径对归一化功率的相对敏感度参数。     

一种新型GRIN透镜光纤加速度计的研究

研究了一种新颖的光纤加速度计,阐述了三光纤GRIN透镜加速度敏感元件的工作原理及其结构设计.该传感器包括三根光纤、一个GRIN透镜和一个质量块.根据光功率耦合原理,对透镜倾斜时的误差进行了定量分析.该加速度计具有结构新颖、体积小和灵敏度高等优点,是一种具有多用途的加速度传感器.     

Focusing effect of a graded index photonic crystal lens

We present a new graded-index photonic crystal (GRIN PhC) structure which can be made by laser interference lithography (LIL). It can be applied as miniaturized focusing lens. Numerical simulations are carried out on the structures with different widths, lengths, and modulations. In the qualitative analysis of the focal length, focusing of a GRIN PhC was proved to originate from both the graded sizes of the rods and multimode interference. Multimode interference dominates the focusing effect when a GRIN PhC has a small length or a small difference in the rod sizes. Intensity distributions in different cases were given to support the conclusion.     

Anamorphic gradient index (GRIN) lens for beam shaping

In this short communication we are reporting a new kind of rod lens with toric power with moderately large power difference. These rods can be directly used in coupling power from a semiconductor laser to optical fiber or in free space communication to convert the beam shape. This rod may directly be butt-jointed to the laser, which may attract many application scientists. Moreover, anamorphic power in a GRIN lens can be generated by proper selection of geometry of the substrate for ion exchange. This may lead to a new kind of optical system that needs further exploration.     

Model of pulsed Nd:GSAG laser at 942?nm considering rate equations with cavity structure

The laser resonator structure and beam diameter change with the pump power due to the thermal lens effect. This influences the output power. With increasing pump power, the output power sharply decreases when the cavity becomes unstable. In order to describe this situation, rate equations considering the cavity structure and the thermal lens effect are presented; numerical methods are used to solve them. This modeling can be used in repetitively pulsed laser systems when the thermal lens depends on the average pump power, while the peak value of the output power and pump power are considered by the rate equations. Experiments are performed with 50 Hz LD diode pumped Nd:GSAG laser crystal operating at the quasi-three-level laser transition with 942 nm wavelength. Fitting the calculated results to experimental data allows to determine the cavity loss. PACS  42.55.Xi; 42.60.Da     

Zonal model of nonstationary self-focusing of femtosecond laser radiation in air: effective beam characteristics evolution

The generic scenario of intense femtosecond laser pulse propagation in the air from the viewpoint of evolution of its integral effective parameters (energy transfer coefficient, effective radius, effective duration, limiting angular divergence) is considered. The analysis of variation of the effective parameters along the propagation path in the single and multiply filamentation scenarios based on numerical calculations is presented. It is shown that the process of self-action of the ultrashort radiation is characterized by the formation in a medium of the nonlinearity layer, after which optical pulse propagates quasi-linearly with the limiting angular divergence that depends mainly on initial pulse power. The effective pulse temporal duration and the effective beam radius increase after the passage through the nonlinearity layer, and their values are mostly determined by the initial beam power also. The coefficient of energy transmission of femtosecond laser radiation is lower than in the linear medium and has a tendency to decrease with the increase of the pulse power.     

Propagation of Gaussian beams family through a Kerr-type left-handed metamaterial

In this paper the propagation of elegant Hermite-cosh-Gaussian, elegant Laguerre Gaussian, and Bessel Gaussian beams through a Kerr left-handed metamaterial(LHM) slab have been studied. A split-step Fourier method is used to investigate the propagation of laser beams through this media. Numerical simulation shows that Gaussian beams have different focusing behaviors in a Kerr LHM slab with positive or negative nonlinearity. Beam focusing happens in slabs with positive nonlinearity and not in slabs with negative nonlinearity; however, negative nonlinearity is required for a Kerr LHM slab to act like a lens. Additionally, the focusing properties of beams can be controlled by controlling the thickness of the slab or the input power of the incident beam. A multilayer structure is also proposed to have beam focusing by thinner slabs and passing longer distances.     

Efficient diode-pumped passively Q-switched laser with Nd:YAG/Cr:YAG composite crystal

Diffusion bonded Nd:YAG/Cr:YAG composite laser crystal has been employed to perform a compact diode-pumped passively Q-switched laser. At the incident pump power of 3.3 W and pulse repetition rate of 16.3 kHz, the maximum average output of 592 mW has been obtained, corresponding to an optical-to-optical conversion efficiency of 18%. Stable passively Q-switched operation with peak power of 6.5 kW and pulse duration of 6 ns was also achieved. It has been experimentally revealed that Nd:host/Cr:YAG composite structure is a promising material for compact cost-effective Q-switched laser sources with high-peak power and short pulse duration. In addition, thermal lens effect (TLE) in the active medium and its impact on the Q-switched laser performance have been analyzed.     

梯度折射率透镜在口腔内窥镜中的设计与应用

作为口腔内窥镜的摄像镜头，利用梯度折射率透镜质量轻、体积小、结构简单、分辨率高、成像景深大的优点，研制出一套适合医患交流及进行口腔检查与治疗的医疗装置。采用修正折射率剖面的梯度折射率透镜, 降低系统渐晕，减小畸变,获取口腔内部图像，经CCD光电转换装置转换成电信号，再将电信号经过数字信号处理以后，将该数字信息存储于计算机内部或在显示器上显示出来。口腔内窥镜是集梯度折射率透镜、光学原理、光学材料与加工、微电子、数字成像技术等为一体的新型医疗设备。特别是采用经修正的梯度折射率透镜制造的口腔内窥镜成像清晰, 在图像畸变、景深等方面优于现有产品，而且便于对患者的各种信息进行管理、查询、输出等。     

Laser shaping of a relativistic intense, short Gaussian pulse by a plasma lens

By 3D particle-in-cell simulation and analysis, we propose a plasma lens to make high intensity, high contrast laser pulses with a steep front. When an intense, short Gaussian laser pulse of circular polarization propagates in near-critical plasma, it drives strong currents of relativistic electrons which magnetize the plasma. Three pulse shaping effects are synchronously observed when the laser passes through the plasma lens. The laser intensity is increased by more than 1 order of magnitude while the initial Gaussian profile undergoes self-modulation longitudinally and develops a steep front. Meanwhile, a nonrelativistic prepulse can be absorbed by the overcritical plasma lens, which can improve the laser contrast without affecting laser shaping of the main pulse. If the plasma skin length is properly chosen and kept fixed, the plasma lens can be used for varied laser intensity above 10(19) W/cm(2).     

Designing a coupler for the intersatellite optical communication system

Coupling a single-mode laser diode with 200 mW to a single-mode fiber (SMF) through an orthonormal aspherical cylindrical lens and a GRIN lens for the intersatellite optical communication system is proposed and demonstrated. We experimentally studied how the coupling efficiency changes with the SMF's position displacement and axial angle variation, and obtained 80 mW output power at the end of the SMF, which shows that the coupling units have satisfied the designed request.     

自聚焦透镜的三维相干成像特性分析

运用傅里叶光学方法，分析并推导了自聚焦透镜的三维相干成像公式。研究表明，虽然自聚焦透镜中介质折射率是径向渐变折射率分布，不同于薄透镜中的均匀分布，但是由于它的三维相干成像性质却类似于薄透镜，从而使自聚焦透镜可以在成像和光学信息处理系统中有效地代替薄透镜，实现光学系统的微型化。     

Coupling coefficients of gratings with graded refractive index waveguides

In this paper, the effective coupling coefficient κeff and the self-coupling coefficient ζ1 are introduced to describe the characteristic of gratings in a resonant situation when the effects of radiation and other partial waves coupling are considered. The dependence of these two coupling coefficients on grating tooth shapes and depths and the dimensions of graded refractive index (GRIN) waveguides is numerically analysed. The results show that the gratings with linear GRIN waveguides have the largest |κeff|. The possibility of realizing a complex-coupled DFB laser, even a pure gain or loss coupled DFB laser, employing only a real refractive index coupled grating is also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

采用GRIN透镜的微胶囊内窥成像技术

研究了采用新型的GRIN透镜的微胶囊内窥镜成像的工作原理.根据近轴子午光线方程,用矩阵分析法分析GRIN透镜成像原理,并提出自动调焦、CCD光电转换、图像传送和无线传送电能等的实现方法.为研发微型生物医学仪器提供了必要的理论基础和重要的技术方法.     

GRIN透镜干涉仪光学系统模型和分析

较完整地提出GRIN透镜干涉仪的数学模型,分析了光学系统处于六种不同模态下的特点,讨论了有关结构因素,特别是GRIN透镜长度对干涉仪工作性能的影响,有关分析和相关结论可作为GRIN透镜干涉仪系统设计和测量计算的依据。     

Electrostatic forward-viewing scanning probe for Doppler optical coherence tomography using a dissipative polymer catheter

A novel flexible scanning optical probe is constructed with a finely etched optical fiber strung through a platinum coil in the lumen of a dissipative polymer. The packaged probe is 2.2 mm in diameter with a rigid length of 6mm when using a ball lens or 12 mm when scanning the fiber proximal to a gradient-index (GRIN) lens. Driven by constant high voltage (1-3 kV) at low current (< 5 microA), the probe oscillates to provide wide forward-viewing angle (13 degrees and 33 degrees with ball and GRIN lens designs, respectively) and high-frame-rate (10-140 fps) operation. Motion of the probe tip is observed with a high-speed camera and compared with theory. Optical coherence tomography (OCT) imaging with the probe is demonstrated with a wavelength-swept source laser. Images of an IR card as well as in vivo Doppler OCT images of a tadpole heart are presented. This optomechanical design offers a simple, inexpensive method to obtain a high-frame-rate forward-viewing scanning probe.     

A stable 10 kHz–1.3 MHz high repetition rate continuously controllable AO Q-switched Nd:YVO4 laser

A stable AO Q-switched Nd:YVO₄ laser using diode-end-pumped structure is presented. By optimizing laser crystal, pumping laser and controlling electric parameters to eliminate thermal lens difference, this AO Q-switched laser can reach 1.3 MHz repetition rate and achieve stable pulse in large range from 10 kHz continuously to 1.3 MHz. The important relationship between repetition rate and electric parameters, including pumping current, duty cycle of electric pulse, are also presented. Output average power and pulse width at 1.3 MHz are 4 W and 35 ns, respectively.     

多脉冲激光对胶合透镜热破坏效应研究

具有一定强度的激光辐照胶合透镜时可造成透镜的破坏.建立了多脉冲激光与材料相互作用的一维非稳态温度场模型，计算了瞬态温度场分布，并对胶合透镜前表面的熔融和胶合材料的软化进行了数值模拟研究.利用频率为10Hz，脉冲宽度为200ns，峰值功率为20MW的CO₂激光对胶合透镜进行辐照实验研究.实验表明，当激光辐照时间为12s时，胶合透镜前表面发生熔融破坏；当照射时间为30s时，胶合材料发生软化并出现彩色斑纹，透镜完全破坏，理论分析与实验结果相符. 关键词： 胶合透镜 激光辐照 激光破坏