Performance of a 80 W copper vapor laser with “alignment free” unstable CAT-EYE resonator and other configurations using intra-cavity apertures

Performance of a kinetically enhanced copper vapor laser (KE-CVL) with various stable/unstable “alignment free” CAT-EYE resonator configurations are presented here in this paper. The laser used in the experiment was a 45 mm bore (∼2 l discharge volume) kinetically enhanced copper vapor laser developed in our laboratory and capable of generating maximum power of ∼80 W (at ∼9.8 kHz). The efficiency of the laser was ∼1.4% and beam divergence of ∼3.5 mrad in a plane-plane standard multimode cavity. For the first time performance of unstable CAT-EYE resonator is demonstrated with a CVL/KE-CVL. On using unstable CAT-EYE resonator the divergence of the laser beam reduced to ∼0.22 mrad (∼20-fold reduction as compared to conventional plane-plane cavity), ∼40 W output power and with excellent misalignment tolerance. The laser output power was found to be within ∼5% drift/decline with misalignment angle of about 4 mrad between the mirrors. This is a significant improvement in comparison to standard conventional unstable resonator (M ∼ 50) CVL where ∼0.5 mrad divergence is achieved with power drift/decline of about 45% at ∼4 mrad misalignment angle.Off-axis unstable CAT-EYE unstable resonator was also demonstrated for the first time with further reduction in beam divergence to ∼0.13 mrad and with output power of ∼28 W. The misalignment tolerance was found to be highest in case of off-axis unstable CAT-EYE resonator with decline/drift in laser power of only ∼10% for misalignment angle as high as ∼8 mrad. Performance with intra-cavity apertures in plane-plane type CAT-EYE resonator for transverse mode control is also presented for the first time in CVLs. It is observed that the laser beam divergence reduces significantly to 1.25 mrad (a factor of 2) on using an aperture of ∼3.5 mm at the CAT-EYE reflector as compared to its normal (R = F = d) configuration without aperture. In case of stable CAT-EYE resonator the average beam divergence reduces from 8 mrad to 4 mrad (factor of 2) on using intra-cavity aperture of about 3 mm. It was also observed that high misalignment tolerance was retained on using intra-cavity apertures in almost all the CAT-EYE resonators. Use of intra-cavity mesh was also demonstrated for the first time with stable CAT-EYE resonator for improving the beam focus-ability. Average beam divergence was reduced by a factor of 2.5 (from 8 mrad to 3 mrad) on using intra-cavity mesh. These new configurations in CAT-EYE resonators in KE-CVLs are found to be effective in improving and controlling the laser beam divergence significantly with additional characteristic of high misalignment tolerance.     

100 W Kinetically enhanced copper vapor laser at 10 kHz repetition-rate, high (∼1.5%) efficiency with low (∼1.6 kW/l) specific input power and performance of new resonator configurations

It is established here that kinetically enhanced copper vapor lasers (KE-CVLs) based on large-bore discharge tubes can provide high (>1.4%) efficiency at ∼9-10 kHz rep-rate with very low (<2 kW/l) specific input power. Comparative performance of various large-bore kinetically enhanced copper vapor lasers in the range 45-70 mm is presented for most suitable discharge tube. Maximum output power of ∼100 W was achieved with efficiency of about 1.55% at 10 kHz rep-rate from the 60 mm bore KE-CVL. The pulse to pulse efficiency of the KE-CVL was ∼2%, tube efficiency ∼2.1%, and laser pulse energy was about ∼10 mJ. These results show significant advancement in the laser system as an elemental high temperature CVL due to volumetric scaling and KE-effects combined with very low specific input power of ∼1.65 kW/l as compared to 8-9 kW/l normally required in other kinetically enhanced copper vapor lasers to generate ∼100 W output power. Performance of the KE-CVL with 3 new cavity configurations namely, (1) CAT-EYE resonator (2) hybrid resonator, and (3) modified diffraction coupled resonator with dot mirrors are presented for the first here. CAT-EYE resonator was demonstrated to achieve high misalignment tolerance without significant loss of power. A typical drift in power of ∼5% was observed with misalignment responsible for 40% decline in power in case of standard plane-plane cavity. Effect of resonator misalignment on amplifier output drift was also investigated using CAT-EYE resonator in oscillator-amplifier configuration. In case of using stable-unstable hybrid resonator, high magnification of M ∼ 1500 was realized resulting in extremely low divergence (∼0.08 mrad) beam with modest (∼20%) loss in average power. In case of modified DCR cavity, record power of about 48 W was achieved with beam divergence of about 0.1 mrad on using intra-cavity 2 × 2 array of 4 dot mirrors.     

An experimental study and theoretical analysis of the mode characteristics of a narrow line-width,pulsed dye laser oscillator at different pump powers

This paper presents an experimental study and theoretical analysis on the effect of pump power on the divergence, pointing, line-width and wavelength stability of a narrow line-width, high repetition rate dye laser oscillator. The dye laser oscillator, based on the hybrid multiple-prism grazing-incidence grating cavity, was pumped by copper vapor laser (CVL). The dye laser mode characteristics were studied at CVL average pump powers of 2, 4 and 8 W. The single pulse dye laser divergence/pointing stability was studied by far-field intensity distributions. The line-width and wavelength stability were studied using a high resolution wave-meter. The experimental results were theoretically analyzed in terms of dye laser mode structure drift induced by beam pointing instability, beam divergence and thermally induced optical path length fluctuation. A comprehensive theoretical treatment on pointing stability of dye laser beam and its influence in the dye laser wavelength stability is presented. The theoretical and experimental trends were found to be in good agreement.     

The effect of varying the hydraulic parameters of a flow cell in a dye laser pumped by a copper vapor laser

A simple method for measuring hydrodynamic parameters that influence the copper vapor laser (CVL) pumped dye laser stability is reported. A specially designed converging diverging curved flow duct is employed. The flow gap at the laser pump region is varied from 0.4 to 0.6 mm and the Reynolds number is varied from 1000 to 20,000. As a result, the channel aspect ratio and effective curvature ratio are varied from 41.6 to 62.5, and 69 to 102, respectively, at the CVL pump region. The variation in the intensity of a He-Ne laser beam transmitted through the water flowing in the test dye cell at different flow rates correlated with the numerically estimated hydrodynamic parameters. The impacts of these hydrodynamic parameters on a 5.5 kHz CVL pumped dye laser output are discussed.     

Laser-generated surface acoustic waves in a ring-shaped waveguide resonator

Surface acoustic wave (SAW) waveguide resonator is formed by a ring-shaped strip of copper 10 μm wide and ∼130 μm in diameter embedded into a 0.8 μm thick layer of silica on a silicon wafer. SAWs are excited at one side of the copper ring by a short laser pulse focused into a spatially periodic pattern and detected via diffraction of the probe laser beam overlapped with the excitation spot. SAW wavepackets with central frequency 460 MHz travel around the ring and are detected each time they make a full circle and pass trough the probe spot. Potential applications of ring resonators for SAWs are discussed.     

Analysis on the laser beam pointing instability induced fringe shift and contrast dilution from different interferometers used for writing fiber Bragg grating

This paper presents an analysis on the effect of laser beam pointing instability on the fringe shift and hence the contrast dilution of superposed fringes from two beam interferometers. The interferometers analyzed are those commonly used in writing fiber Bragg grating (FBG) such as phase mask, bi-prism and phase mask based Talbot/Holographic type. The beam pointing instability is incorporated as slight change in the angle between two interfering beams. The relative immunity of different interferometers to laser beam pointing is discussed vs location of FBG writing plane from the beam splitter. The effect of the beam pointing was minimum in proximity FBG writing by phase mask. The effect, in terms of fringes contrast dilution, was worst in case of large length interferometers e.g. phase mask based-Talbot interferometer. For intermediate length prism interferometers, the effect was moderate. For a given length interferometer, the fringe shift was directly proportional to beam pointing angle and inversely proportional to fringe separation. Theoretical analysis is verified experimentally by studying the fringe instability of interference pattern formed by a bi-prism of angle 2^o with the copper vapour laser (CVL, λ = 510 nm) beams of different beam pointing instabilities.     

36.5 W high beam quality 532 nm green laser based on dual-rod AO Q-switched resonator

A high power, high beam quality, compact green laser based on dual-rod AO Q-switched resonator was designed, fabricated and tested. The laser provided a maximum 532 nm average power of 36.5 W at a repetition rate of 65 kHz with the beam quality factor of M² = 1.55 and the optical frequency conversion efficiency from NIR to green laser was as high as 51%. The pulse repetition rate was tunable from 50 kHz to 200 kHz and the overall dimension of the laser was within 500 × 300 × 150 mm³.     

注入控制铜蒸气激光方向性时间过程测量

实验研究了注入控制对铜蒸气激光非稳腔输出光束质量的影响，从输出光发散角时间分辨过程的测量结果说明注入加速被注入腔内激光发散角的减小过程。由于被注入腔内放大自发辐射的存在，为得到较好的注入控制效果，注入光的脉宽和进入被注入腔的延时必须恰当。     

Nonlinear frequency conversion of copper vapor laser radiation using convergent and parallel laser beams

Results of experiments are presented on the comparison of the efficiency of radiation generation at the sum frequency (λ=0.271 μm) of copper vapor laser (CVL) in a nonlinear DKDP crystal using either convergent or parallel laser beam. An intensity of UV radiation of 0.34 W and a conversion efficiency of 6% obtained under conditions of strong focusing occur to be substantially lower as compared to those obtained using a parallel laser beam of the same power (0.73 W and 12%, respectively). Experimental data on the structure of a CVL beam generated with an unstable resonator are presented, and physical reasons for the limitation of the efficiency of nonlinear frequency conversion are analyzed.     

Study of gain dynamics and temporal jitter effects in copper vapor laser oscillator-amplifier chain

Effect of temporal jitter on the performance of copper vapor laser (CVL) master oscillator power amplifier (MOPA) chain using Monte-Carlo techniques was studied. The MOPA chain comprised an oscillator and five amplification stages. Amplification of CVL signal in each amplifier stage was modeled using rate equation based optical model. The effect of temporal jitter on the pulse energy stability in CVL MOPA chain was estimated for various jitter values. Our studies show that the pulse energy spread and the pulse energy instability increases with increase in jitter which in turn depends on the gain pulse width and the jitter considered. Pulse to pulse gain variation in CVL MOPA chain was studied along with temporal jitter. The acceptable tolerance limit on temporal jitter is ±4 ns for a pulse energy stability of 5% of CVL MOPA chain.     

Generalised ABCD matrix treatment for laser resonators and beam propagation

A generalised ABCD matrix treatment for laser resonators and beam propagation is developed for computer programming. In this treatment, imaginary parts are introduced into the matrixes for all optical elements, and the beam quality factor M² and the index of the medium are also taken into account. When the imaginary parts of the complex matrix are zero and the beam quality factor M² and medium index are unity, the stated method is transformed back into the more classic ABCD matrix format in which the fundamental-mode Gaussian beam transmits through real elements in the vacuum. Based on this method, laser resonator software is realised by the VB programme language. The software can be used to analyse and design stable/unstable standing/travelling cavities, phase-conjugate cavities and beam transformations.     

Ultra high Q crystalline microcavities

We report on the fabrication technique of ultra high Q optical crystalline whispering gallery mode microresonators and discuss their properties. The technique is suitable for the majority of available optical crystals and for production of resonators with small size. To validate the method, we made CaF₂ resonators with Q factors exceeding 4 × 10⁸ and a diameter smaller than 100 μm. A single mode resonator has also been fabricated. Possible utilization of these new resonators in quantum optics is discussed.     

环状输出UR90束旋转非稳腔的实验研究

 提出一种适合于高功率超音速横流氧碘化学激光器的环状输出的UR90束转非稳腔,采用FFT 方法模拟了具有非均匀分布的小信号增益时最低阶模的光场强度分布。利用大口径铜蒸汽激光器, 通过实验研究了在放大倍率为1.21和1.13的情况下,环状输出UR90腔正向模与反向模的基本特性, 得到较好的实验结果。     

Measurement of spatial coherence of a copper vapour laser beam using a reversal shear interferometer

We measured the spatial coherernce of a copper vapour laser (CVL) beam with an unstable resonator by the reversal shear interferometer. By this method, we can evaluate the spatial coherence function from a single-shot measurement. The spatial coherence width was 5 mm when an unstable resonator with a magnification factor of 60 was used. Moreover we verified the result by the theoretical calculation on the basis of the passive resonator model.     

Fast wavelength-switching of a diode-pumped solid-state disk laser

The functionality of two experimental setups for fast wavelength-switching of an Yb:YAG disk laser is tested and characterized. The first setup consists of two resonators sharing one disk. Both resonators are alternately opened by a chopper disk. Each resonator is tuned independently by a set of a two-stage Lyot filter and an etalon. Up to 1500 Hz the setup can switch between two freely selectable wavelengths (linewidth (fwhm) ≪1 pm) within the complete Yb:YAG tuning range (from 1020 nm to 1055 nm) emitting a maximum average power of 105 mW. The power of both resonators differs by a factor of 0.7. Their pointing stability amounts to 20 μrad. The second setup consists of a single tunable resonator (two-stage Lyot filter, etalon) additionally equipped with an electro-optical device (Faraday rotator or Pockels cell) providing a high pointing stability of better than 1 μrad. The wavelength switches occur stepwise and are predetermined by the etalon (about 90 pm or multiples thereof). The Faraday rotator suffers from its high thermal load and operates for some few 10 s only at 0.5 Hz. The resonator with the Pockels cell provides long term stability at 1000 Hz, switching steps up to 1.1 nm and a power ratio of 0.83 for the selected wavelengths.     

大功率TEA CO2激光器非稳腔的设计与实验

相比于传统稳腔高功率TEA CO2激光器,如何在不降低功率的基础上,改善其光束质量一直研究者关心的问题。本文在原有稳腔激光器的基础上,设计了三组望远镜虚共焦腔镜,给出了完整的设计过程并进行了理论分析。同时对不同组的非稳腔激光器进行了对比实验研究,结果表明非稳腔能够在保证高单脉冲能量的基础上极大地改善激光远场发散角,其中最佳腔镜组合能够达到单脉冲能量13.7J,其发散角为1mrad（稳腔为2.4mrad）,激光脉宽50ns（稳腔为98ns）。这就为此种类型激光器应用于激光制造领域奠定了基础。     

均匀化位相的常规非稳腔的分析与实验

对常规非稳腔的输出镜面进行改造,在原来不透光的中央区域引入增透膜,使之能部分透过,使输出耦合能的比例得到提高;同时加敷适当的位相膜,使中央区与环带区波面位相拉平些.着重报导了均匀化相位常规非稳腔的系列实验和分析,从而使常规非稳腔的环带输出引起的中央不透明区与外环带输出波面的位相突变得到改善,克服了远场能量分布出现较严重离散的问题,使远场中央亮斑能量更集中.     

激光谐振腔自再现模的波型特征

本文用光传输矩阵理论给出分析激光光腔模式波型特性统一处理方法.导出激光谐振腔自再现模的波型函数.讨论了在各类激光谐振腔中激光束的波型特征.     

强激光腔镜温升对腔几何参数和远场光强的影响

 利用交替方向隐式有限差分法分析了高能激光器虚共焦非稳腔反射镜的温度场及热变形的数值计算方法。以单晶硅为例,计算了反射镜由于吸收激光能量而形成的温度场分布以及由此引起的热变形对谐振腔几何结构参数的影响,并在此基础上数值模拟了热畸变情况下正支虚共焦非稳腔的远场光强分布。计算结果表明:在激光束辐照的开始阶段,温升场主要集中在激光束辐照的中心区域内,在整个镜面上远未拉平,由此引起的厚度方向温升分布也是如此,很不均匀。     

1 W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz

A novel unstable external cavity for a broad area laser diode is presented. The cavity is based on a V-shaped setup that improves the slow axis beam quality by coupling the internal modes of a gain guided laser diode. The novelty here is the compact unstable resonator design without lenses in direction of the slow axis. For frequency stabilisation and to narrow the line width of the laser diode emission a diffraction grating in a Littrow configuration is used. With this setup up to 1 W of near diffraction limited light with a beam quality of M² ? 1.3 and a line width of 1.7 MHz could be achieved. The external cavity laser was tunable over a range of 35 nm (FWHM) around the center wavelength of 976 nm.