Conversion of a high-order mode beam into a nearly Gaussian beam by use of a single interferometric element

We present a new, compact, and practical optical mode converter that efficiently transforms a high-order Hermite-Gaussian (HG) laser beam into a nearly Gaussian beam. The mode converter is based on coherently adding different transverse parts of the high-order mode beam by use of a single planar interferometric element. The method, configuration, and experimental results obtained with a pulsed Nd:YAG HG TEM10 laser beam are presented. The results reveal that the efficiency of conversion of a HG beam to a nearly Gaussian beam can be as high as 90%.     

Femtosecond Cr(4+):YAG laser with an L-fold cavity operating at a 1.2-GHz repetition rate

Pulses of 55-fs width and 1.2-GHz repetition rate at 1.52 mum are generated from an L-fold cavity Cr(4+):YAG laser by optimization of the lasing-beam mode inside the gain medium. The pulse width is comparable with that obtained from a standard Z-fold cavity. The pulses are characterized by a second-harmonic-generation frequency-resolved optical gating method. The pulse shape deviates from a sech(2) function because of its broad shoulders. With further miniaturization it should be possible to extend the Cr(4+):YAG mode-locked operation to multigigahertz rates.     

Simple annular-beam generator with a laser-diode-pumped axially off-set power build-up cavity

We have developed a novel, compact-mode selective annular-beam generator that employs a laser-diode (LD)-pumped, passively locked power build-up cavity (PBC). The PBC functions as a flexible and compact light source that is capable of producing TEM_0n high-order Hermite-Gaussian (HG) beams. The generated HG beams can then be converted to LG modes, particularly to annular-beams, using two cylindrical lenses. The system was theoretically studied and a mode-matching condition was derived. The tailoring position of the active semiconductor laser medium relative to the external cavity of the PBC enabled the production of single high-order HG modes. The optimum position was calculated by model calculations of the mode-coupling efficiency. The desired TEM_0n modes of HG beams were generated by translating the axial position of the external cavity of the PBC in increments of ∼100 μm relative to the LD emission surface. High quality TEM₀₁ to TEM₀₁₅ of the HG beams and their conversion to the desired LG modes having powers of the order of 1 mW was successfully demonstrated. This system is suitable for optical tweezers.     

Squeezing by second-harmonic generation in a monolithic resonator

We have measured the intensity fluctuations of the second-harmonic mode generated in a MgO:LiNbO₃ external monolithic cavity pumped by a Nd:YAG laser. The cavity has mirror coatings for both the fundamental and the second-harmonic mode. We scan the cavity using the electro-optic effect of the crystal and observe that the second-harmonic beam of 2 mW exhibits a quantum noise reduction of 40(±5)%. In addition, we report on the active frequency stabilization of the monolithic device used in our squeezing experiments. Several fast tuning parameters such as the electro-optic effect, the photo-elastic effect, and the laser frequency have been investigated. With these tuning parameters the monolithic resonator can be locked on double-resonance at the phase-matching temperature, which is a prerequisit for observing squeezing in a cw-regime.     

Radially polarized laser beam from a Nd:YAG laser cavity with a c-cut YVO4 crystal

We demonstrate the generation of a radially polarized beam by simply inserting an undoped c-cut YVO₄ crystal into a Nd:YAG laser cavity. In a hemispherical cavity, the cylindrically symmetric, positive birefringence of the YVO₄ crystal extends the stability limit of the cavity length for an extraordinary ray (radial polarization) compared to an ordinary one (azimuthal polarization). By adjusting the cavity length, a radially polarized beam with an output power up to 1 W was selectively obtained. In addition, a higher-order transverse mode was also generated by arranging the cavity design. The method demonstrated in this paper can be readily applied to laser systems with an isotropic laser medium. PACS 42.60.Da; 42.25.Lc; 42.25.Ja     

Transverse mode locking of Stokes radiation in diode end-pumped Nd:YVO<Subscript>4</Subscript> laser passively Q-switched by Cr<Superscript>4+</Superscript>:YAG

Transverse mode locking of the Stokes component generated in the mode of stimulated Raman scattering self-conversion by a pulsed diode end-pumped Nd:YVO₄/Cr:YAG laser is implemented for the first time under conditions of frequency degeneracy of cavity modes.     

腔内波片产生的Nd∶YAG激光纵模分裂规律研究

简要介绍了激光纵模分裂的基本原理，研究了腔内含有1个和2个1/4波片时产生Nd∶YAG激光纵模分裂的规律。实验结果表明：在1064nm Nd∶YAG激光的腔内放置一个1/4波片时，每一激光纵模分裂为2个正交的线偏振模，在波片表面垂直于激光光线的条件下，纵模分裂量（即频率差或波长差）恰好等于激光纵模间隔的一半；当在Nd∶YAG激光腔内沿垂直光线方向平行放置2个1/4波片时，也能产生纵模分裂现象，其纵模分裂量取决于2波片快（慢）轴之间的夹角。在0°～ 90°范围内调节角度，可使纵模分裂量在一个激光纵模间隔内线性调谐。实验结果与理论分析相吻合。     

LD端面泵浦Cr~4∶YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr4+∶YAG被动调Q Nd∶YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr4+∶YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr4+∶YAG晶体内光斑半径也随谐振腔长度减小,引起Cr4+∶YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

LD端面泵浦Cr4:YAG被动调Q激光器输出特性研究

对激光二极管端面泵浦Cr⁴⁺:YAG被动调QNd:YAG激光器输出特性进行了实验研究.实验研究发现,激光器输出功率及脉冲重复频率随谐振腔长度增大而增大.为解释这一实验现象,测量了泵浦光斑在激光晶体内尺寸,同时计算了激光晶体及Cr⁴⁺:YAG晶体内的基模激光光斑半径随谐振腔长度变化.分析结果表明:激光晶体内泵浦光斑尺寸远小于激光晶体内基模光斑半径,腔模间交叠效率较低;当腔长增加时,激光晶体内的基模激光光斑减小,腔模间交叠效率增加,从而导致输出功率及脉冲重复频率随腔长增加而增加;另外,Cr⁴⁺:YAG晶体内光斑半径也随谐振腔长度减小,引起Cr⁴⁺:YAG晶体漂白时间缩短,导致脉冲重复频率随腔长增加而增加.     

A thermally-insensitive passively Q-switched Cr:YAG/Nd:YAG laser

A passively Q-switched laser with Cr⁴⁺:YAG/Nd:YAG composite crystal using a corner cube prism cavity has been realized, in which the corner cube prism is the key element as it aids the compensation of the thermal lens effect of laser crystal. Compared with plane–plane mirror cavities under the same conditions, the stability of the laser performance using the corner cube prism cavity was improved remarkably over temperatures ranging from −40 to 65 °C. The decrease of Nd:YAG stimulated emission cross section with temperature was considered to be the main reason for the increase of average output pulse energy under the same cycle for the two different cavities when the ambient temperature changed −40 to 65 °C. The mode properties produced by the prism cavity were analyzed, and the theoretical results were verified by experimental observations.     

Output performance of thermally-insensitive passively Q-switched Nd:YAG laser

We study a Cr⁴⁺:YAG Q-switched Nd:YAG laser with a thermally-insensitive corner-cube-prism cavity where the corner cube prism is the key element. The corner cube prism is insensitive to misalignment in any direction. We demonstrate experimentally that a laser cavity with such a prism provides stable laser performance under violent changes of ambient temperature and the effect of the laser crystal thermal lens. The laser mode properties are analyzed by numerical simulations. We show that the numerically simulated results agree well with the experimental ones.     

Passive mode locking of a Nd：YAG laser with co－doped Nd, Cr：YAG as saturable absorber

We demonstrate the characteristics of relatively low saturation intensity using co-doped Nd, Cr:YAG as saturable absorber for passively mode locking the Nd:YAG laser. The difference of the saturation intensity between Q-switched and mode-locked operation in co-doped Nd, Cr:YAG was only one to two orders of magnitude, while Cr:YAG was generally reported at a difference of five orders of magnitude. More than 80% mode locking modulation depth was achieved at an incident pump power of 4.4W, corresponding to an intracavity intensity of 6×10^4W/cm^2, using a 68cm long plano-concave cavity.     

High-power continuous wave green beam generation by use of simple linear cavity with side-pumped module

High-power continuous wave green radiation has been generated by means of type-II phase-matched frequency doubling in a KTP crystal located in a simple linear cavity incorporating a diode side-pumped Nd:YAG laser module. The cavity was designed to make the fundamental beam radius at the KTP crystal smaller than that at the gain medium, as is required for obtaining large mode volume in Nd:YAG crystal and realizing efficient CW intracavity frequency doubling. Output power of 51.2 W is obtained in the experiment with a diode-to-green optical conversion efficiency of 10.3%. The M²-parameters of the laser are measured at different output powers. For the output power of about 47 W, the power fluctuation is measured less than 1%. The experimental results show that the continuous wave green laser system using this simple linear cavity offers good laser performance and output stability.     

几种新型LD泵浦Nd∶YAG双频激光器

介绍了几种LD泵浦Nd∶YAG双频激光器 ,均应用双折射原理实现。一类是利用自然双折射效应 ,在激光谐振腔内加入自然双折射元件 ;另一类则是对YAG晶片加压力 ,使YAG晶片本身成为应力双折射元件。由于双折射效应使激光在谐振腔内产生偏振方向互相垂直的寻常光 (o光 )和非寻常光 (e光 )两种成分。因为o光和e光在双折射元件中有着不同的折射率 ,因此一个激光谐振腔变成了具有两个物理长度的谐振腔 ,从而产生双频激光。改变自然双折射元件或对YAG晶片施加的压力 ,可调谐频差。实验中获得 10 9Hz量级的大频差 ,合成波长可到几十毫米 ,适用于绝对距离干涉测量     

Single higher-order transverse mode operation of a radially polarized Nd:YAG laser using an annularly reflectivity-modulated photonic crystal coupler

We demonstrate single higher-order transverse mode operation of a radially polarized Nd:YAG laser by using a polarization-selective and reflectivity-modulated output coupler. A narrow annular region with low reflectivity fabricated in a photonic crystal mirror behaves so as to select a higher-order transverse mode of a cylindrically symmetric laser beam. A double-ring-shaped radially polarized TM(02) mode beam is stably generated.     

Spectral and temporal investigations of nonlinearities in a non-polarization preserving single-mode optical fibre

Intense green and red components generated in a non-polarization preserving optical fibre using a Q-switched and mode-locked cw Nd: YAG laser have been examined spectrally and temporally with picosecond resolution. In addition to the red (four-photon interaction) and green (second-harmonic) components, many other frequency mixing processes were detected. From theoretical considerations, the red component (anti-Stokes) which is generated through parametric four photon mixing should be in the same propagation mode as the Stokes component (ir) and this was confirmed experimentally.     

Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm

A cw mid-IR coherent source based on difference-frequency generation is designed and characterized. For mid-IR generation, a periodically poled MgO:LiNbO(3) crystal is placed inside a compact Ti:sapphire laser cavity. This provides high-power pump radiation for the nonlinear process. Optical injection by an external-cavity diode laser ensures single-frequency operation of the Ti:sapphire laser, while signal radiation is provided by a fiber-amplified Nd:YAG laser. Mid-IR radiation can be generated with 3850-4540 nm tuning range, narrow linewidth, Cs-standard traceability, and TEM(00) spatial mode. 30 mW power is obtained at 4510 nm.     

非稳腔主动式直接获取纳秒近似无衍射贝塞尔绿光

通过对基于轴棱锥的贝塞尔谐振腔和贝塞尔-高斯谐振腔的研究,设计了一台腔内倍频Nd:YAG纳秒近似无衍射贝塞尔绿光激光器. 非稳激光器谐振腔由轴棱锥和凸面镜组成. 实验采用单灯脉冲抽运激光增益介质Nd:YAG晶体,倍频晶体选用KTP. 当抽运电压为350 V时,由主动式直接产生纳秒近似零阶贝塞尔绿光,其脉冲宽度为55.1 ns,波长为532 nm,线宽为0.8 nm,近似无衍射零阶贝塞尔绿光的中心光斑直径为192 μm. 利用Fresnel-Kirchhoff 衍射积分和Fox-Li迭代法,通过数值计算得出 关键词： 主动式 纳秒贝塞尔光束 腔内倍频 轴棱锥     

Continuous-wave, 15.2 W diode-end-pumped Nd:YAG laser operating at 946 nm

A high-power continuous-wave (cw) Nd:YAG laser operating at 946 nm by utilizing a quasi-three-level transition is reported. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser and a simple plane-concave cavity. At an incident pump power of 40.2 W, a maximum cw output of 15.2 W at 946 nm is obtained, achieving a slope efficiency of 45%. To the best of our knowledge, this is the highest output at 946 nm ever generated by diode-pumped Nd:YAG lasers. In addition, at an incident pump power of 15.2 W, a 1.25 W blue output at 473 nm is achieved with a simple compact three-element cavity and a type-I lithium triborate (LiB(3)O(5)) crystal as a frequency doubler.     

Passive Q-switching of the diode-pumped Er:YAG laser cavity with the Q-switch based on the Fe<Superscript>2+</Superscript>:ZnSe crystal

Repetitive-pulse generation of the diode-pumped Er:YAG laser (λ = 2.94 μm) in the free mode and in the mode of cavity passive Q-switching was achieved using a Q-switch based on the Fe²⁺:ZnSe crystal. When using pump pulses 3 ms long, the pulse-average output power of the Er:YAG laser in the free generation mode was 0.5W. In the passive Q-switching mode, giant pulses 180 ns long with an energy of 3 μJ were obtained.