Limitations to beam quality of mid-infrared angled-gratingdistributed-feedback lasers

The far-field characteristics of mid-infrared angled-grating distributed-feedback (α-DFB) lasers with W active regions are studied as a function of etch depth, stripe width, and optical pumping intensity. Whereas near-diffraction-limited output is obtained for 50 μm stripes at ten times threshold, the beam quality degrades rapidly when either the stripe width or the pump intensity is increased. A key finding is that most of the degradation may be attributed to the onset of Fabry-Perot-like lasing modes that propagate along the direct path normal to the facets. We further show that these parasitic modes may be effectively eliminated by using ion-bombardment to create angled virtual mesas surrounded by loss regions. The bombarded structures show substantial improvement of the beam quality for wide pump stripes and high pump intensities, although in this first demonstration the efficiency decrease was greater than expected     

Novel Multiwavelength L-Band Brillouin–Erbium Fiber Laser Utilizing Double-Pass Brillouin Pump Preamplified Technique

We experimentally demonstrate successful operation of an enhanced multiwavelength L-band Brillouin-erbium fiber laser and analyze its performance under various operating conditions. This scheme utilizes double-pass amplification technique to preamplify the Brillouin pump (BP) power within the laser cavity before entering the single-mode fiber. Owing to this double-pass pre-amplification within the erbium gain medium, the proposed laser structure is able to operate at low pumping power and exhibits a low-threshold power of 15.9 mW. Moreover, the double-pass pre-amplification of BP is able to shift the unstable operation of the laser to a higher pump power, enabling us to generate high-power laser signals. We experimentally show that the proposed novel setup can produce up to 30 channels at 40 and 0.035 mW of 1480 nm pump and BP powers, respectively. An obvious suppressant for unstable self-lasing cavity modes because of the effect of homogenous saturation of the erbium-doped fiber gain due to the high intensity of BP is observed in the laser cavity configuration.     

抽水蓄能机组泵工况分阶段启动及经济运行

在仙游抽蓄电站机组调相转抽水的过程中,由于导叶开启过快,使得电机功率改变量较大,导致电网频率波动较大。对此,本文提出了在泵工况采用导叶分阶段线性启动的解决方法。基于高扬程水泵水轮机模型转轮特性数据,研究了不同导叶开度时机组运行的稳定性;分析了水泵流量、轴功率、效率与扬程和导叶开度的关系,提出了确定泵工况高效经济运行最大导叶开度的方法。通过对水泵和水轮机工况区压力脉动的比较分析,得出水轮机工况的压力脉动比水泵工况相同位置的大得多,所以水泵工况也可以采用导叶开度分阶段开启方式的重要结论。对水泵零流量工况的分析表明,当水泵启动时,采用缓慢开启导叶的方式有利于避免发生较大的零流量压力脉动现象。同时,选择较小的导叶开启目标开度有利于减小零流量压力脉动强度。最后,水力过渡过程计算示例表明,在水泵启动过程每个阶段导叶保持不变的过程中,水压和轴功率波动幅度较小且很快衰减,机组能够稳定运行,发生超过水轮机工况压力脉动的风险微小。由于高水头或高扬程水泵水轮机转轮单位流量和力矩特性及压力脉动具有类似特点,上述结论也可供其它抽蓄电站参考。     

Optical parametric amplification of femtosecond ultraviolet laser pulses

Broad-band amplification of femtosecond laser pulses using the scheme of noncollinear optical chirped pulse parametric amplification is modeled. The effect of two-photon absorption at the pump wavelength was also taken into account. The signal pulses range from 220 to 410 nm with pump pulses at 267, 248, and 213 nm. The best four crystals chosen among 12 possible ones are BBO, KDP, CLBO, and LB4. In an experiment, 30-fs laser pulses at 400 nm were amplified in a BBO crystal pumped by 267 nm pulses, exhibiting a single pass gain of 3550. The gain was found spectrally flat within the available 17-nm bandwidth of the signal pulse.     

Generation of mid-infrared pulses suitable for two-colorIR-spectroscopy with 300-fs time resolution, temporal analysis viatwo-photon absorption in germanium and first application

The generation of two independently tunable light pulses in the mid-infrared spectral region is reported here. Synchronously pumping two optical parametric oscillators in parallel with a flashlamp-pumped, pulsed Nd:YLF laser delivers tunable radiation in the range 1.2-1.7 μm with duration of 300 fs. Subsequent parametric amplification in AgGaS₂ crystals yields intensive light pulses in the wavelength range of 2.7-7 μm with duration of approximately 1 ps and energies up to 10 μJ. Pulse analysis determining also the zero delay time and the temporal resolution of the experimental system is performed via two-photon absorption in germanium, utilizing the direct band gap. The temporal resolution of the two-color IR-system is determined to be approximately 300 fs and first application to time-resolved spectroscopy in the condensed phase is demonstrated     

A 380-mW 7-MHz cerium LiLuF laser pumped by the frequency doubledyellow output of a copper-vapor-laser

We report a high average power Ce:LiLuF laser pumped by the second harmonic of the yellow output from a copper vapor laser. This cerium laser yielded up to 380 mW at a pulse repetition frequency of 7 kHz at the peak lasing wavelength of 309.5 nm, with a slope efficiency of 50%. In addition, single prism tunability was obtained from 305.5 to 316 nm and from 322 to 331.8 nm, Preliminary investigation into color center behavior has been performed involving crystal cooling, He-Ne pump probe experiments and antisolarant pumping     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

Slow light produced by far-off-resonance Raman scattering

The authors survey the theoretical and experimental aspects of generation of slow light in a far-off-resonance Raman medium driven by a strong coupling field. When material dispersion is negligible, the propagation of two coupled sidebands can be described in terms of two normal modes that propagate independently at different group velocities, one at the vacuum speed of light and one at a reduced velocity. They use solid hydrogen as a Raman medium to demonstrate the generation of slow light. The numerical calculations and experimental observations show that, due to high density, narrow Raman width, and small two-photon detuning, far-off-resonance Raman scattering in solid hydrogen can slow down the pulse-peak velocity of the Stokes and anti-Stokes fields to the order of c/10000. This velocity reduction affects the amplitudes of the Stokes and anti-Stokes fields via the beating between the normal modes. The double-peak structure observed in the intensity temporal profiles of the sideband fields is a signature of the splitting of the copropagating normal modes.     

Lasing mechanism of InGaN-GaN-AlGaN MQW laser diode grown on SiC bylow-pressure metal-organic vapor phase epitaxy

We studied the lasing mechanism of an InGaN-GaN-AlGaN multiquantum-well (MQW) laser diode by making various optical characterizations on the diode. Excitation power dependence of photoluminescence (PL) intensity was obtained to investigate the carrier recombination process of the laser. Surface emission and edge emission were compared by optical pumping to clarify where the lasing lines were located in relation to the absorption continuum. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers. PL mapping was also taken on the same laser chip to examine the in-cavity bandgap inhomogeneity. We found a very large bandgap scattering of 100 meV. We also found that the wavelength distribution has a periodic modulation. We clarified that the various stimulated emission lines observed in our lasers are caused by the in-cavity spatial bandgap inhomogeneity of the InGaN MQW     

Ultrafast all-optical switching in multiple-quantum-well Y-junctionwaveguides at the band gap resonance

An integrated Y-junction switch is used to demonstrate all-optical switching due to two mechanisms in GaAs-AlGaAs multiple quantum well structures at room temperature. Carrier-induced nonlinearities are compared to nonlinearities due to the optical Stark effect for ultrafast operation. In the first case, device geometry is exploited by a two-pulse switching process, whereby one control pulse turns the device on and a second control pulse turns the device off. Time gating of signal pulses within an 8 ps window was realized in our experiments. In the second case, the nonlinearity is ultrafast, and hence switching and recovery take place during the control pulse evolution. Consecutive switching events spaced 1.7 ps apart have been achieved. In our measurements, two-photon absorption played a significant role in the switching characteristics of the device. In order to access the carrier-induced nonlinearity, the pulse energy that is needed for switching (9 pJ in this case) results in a very high peak intensity that leads to two-photon absorption. This is observed as a strongly induced absorption exactly at the precise zero time delay between the control and signal pulses. In the second case, two-photon absorption competes directly with the optical Stark effect since both are instantaneous intensity dependent effects. Furthermore, the optical Stark effect appears to saturate at a low intensity level (0.9 pJ or 200 MW/cm² ); consequently only incomplete switching with a 2:1 contrast ratio was observed, whereas a 9:1 switching contrast was achieved with the carrier-induced nonlinearity     

Optical phenotyping of human mitochondria in a biocavity laser

We report a new nanolaser technique for measuring characteristics of human mitochondria. Because mitochondria are so small, it has been difficult to study large populations using standard light microscope or flow cytometry techniques. We recently discovered a nanooptical transduction method for high-speed analysis of submicrometer organelles that is well suited to mitochondrial studies. This ultrasensitive detection technique uses nanosqueezing of light into photon modes imposed by the ultrasmall organelle dimensions in a semiconductor biocavity laser. In this paper, we use the method to study the lasing spectra of normal and diseased mitochondria. We find that the diseased mitochondria exhibit larger physical diameter and standard deviation. These morphological differences are also revealed in the lasing spectra. The diseased specimens have a larger spectral linewidth than normal, and have more variability in their statistical distributions.     

竖井位置对双向贯流泵装置水力特性的影响

双向竖井贯流泵作为一种可实现双向抽引的低扬程泵装置形式,在平原城市地区应用广泛,而竖井布置位置一直是工程实际中需要考虑的问题之一。本文通过三维造型软件建立模型,并使用全结构化网格划分方法划分竖井流道、直管流道、叶轮段及导叶段网格,数值计算控制方程为连续性方程、k-ε湍流模型方程和雷诺时均方程并采用稳态计算对双向竖井贯流泵装置进行模拟。计算分别模拟了竖井贯流泵的竖井前置、后置两种情况下泵装置叶轮正向反向运行共计四种运行情况的水力性能特性。结果表明,设计工况下无论竖井后置或是前置,泵装置正向运行效率皆高于反向运行。无论泵装置正反向运行,竖井流道和直管式流道作为进水流道时流态皆较为平顺,水力性能差异不大;泵装置正向运行时竖井流道和直管式流道作为出水流道时尽管因造型不同内部流态有所差异但总体水力损失较小。反向运行时出水流道环量不能被导叶回收,产生能量损失较正向运行大,而此时竖井流道作为出水流道水力性能要劣于直管式出水流道。     

Compensation of Laser Thermal Depolarization Using Free Space

Thermally induced birefringence in a single-transverse-mode laser for moderate pumping is well compensated when the rod is double-passed after reflection from a mirror one Rayleigh range away from the rod. The depolarized light is in a spatial (1, 1) mode and the relative Gouy phase shift between the fundamental and depolarized modes in propagating a Rayleigh range leads to good compensation.     

Miniature erbium:ytterbium fiber Fabry-Perot multiwavelength lasers

We demonstrate stable simultaneous lasing of up to 29 wavelengths in miniature 1- and 2-mm-long Er³⁺:Yb³⁺ fiber Fabry-Perot lasers. The wavelengths are separated by 0.8 (100 GHz) and 0.4 nm (50 GHz), respectively, corresponding to the free spectral range of the laser cavity. The number of lasing wavelengths and the power stability of the individual modes are greatly enhanced by cooling the laser in liquid nitrogen (77 K). The polarization modes and linewidth of each wavelength are measured with high resolution by heterodyning with a local oscillator. The homogeneous linewidth of the Er³⁺:Yb ³⁺ fiber at 77 K is determined to be ~0.5 nm, from spectral-hole-burning measurements, which accounts for the generation of a stable multiwavelength lasing comb with wavelength separations of 0.4 nm     

Electrohydrodynamic induction pumping of a stratified liquid/vapormedium in the presence of volumetric and interface electric charges

Electrohydrodynamic (EHD) pumping of two-phase media has potential applications related to both pure pumping as well as heat transfer enhancement. Utilization as pumping mechanism is attractive for terrestrial and outer space systems. Augmentation of mass flow rate in two-phase heat transfer applications, such as boiling and condensation, leads to enhanced heat transfer coefficients, thus making EHD pumping an attractive candidate for these situations. A theoretical model is presented for EHD induction pumping of a stratified liquid/vapor medium accounting for electric shear stresses due to interfacial and bulk charges. Bulk charges are a result of temperature gradient in the liquid phase of the fluid. Conditions are established under which the bulk effect plays a significant role in the pumping process     

High-power continuous-wave intracavity frequency-doubled Nd:GdVO/sub 4/-LBO laser under diode pumping into the emitting level

The continuous-wave high power laser emission of Nd:GdVO/sub 4/ at the fundamental wavelength of 1.06 /spl mu/m and its 531-nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 808 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 5/2/ transition) and 879 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 3/2/ transition). It is shown that, in spite of a lower absorption at 879 nm, the infrared emission is comparable under these two wavelengths of pump. The green emission performances were, however, improved by the 879 nm pump: 5.1 W at 531 nm with M/sup 2/=1.46 and 0.31 overall optical-to-optical efficiency was obtained from a 3-mm-thick 1-at.% Nd:GdVO/sub 4/ laser medium and a 10-mm-long LBO nonlinear crystal in a Z-type cavity for 16.5 W pump power. In similar conditions, the maximum green power for the 808 nm pump was 4.4 W, with 0.26 overall optical-to-optical efficiency and M/sup 2/=3.40 beam quality; at this pump wavelength the green emission shows evident saturation for pump power in excess of 9.9 W. This behavior is connected with the enhanced heat generation under 809-nm pumping, as evidenced by the increased thermal lensing of the fundamental emission. A careful alignment of the laser enables emission almost free of chaotic intensity fluctuations.     

A very convenient setup to generate intense VUV coherent light at125 nm with use of nonlinear effects in mercury vapor at roomtemperature

Using only one dye laser, efficient generation of VUV radiation (10¹³ photons/pulse) is demonstrated at 125.140 nn and 125.053 nm by four-wave sum-frequency mixing in a room-temperature mercury vapor. The emission at 125.053 mn, which is out of two-photon resonance, but near three-photon resonance, has been observed for the first time and is carefully analyzed. In particular, numerical calculations have been carried out for Gaussian pump beams, taking into account absorption of VUV photons and optical Kerr effect. The results of the calculations reproduce the VUV emission lineshape and power saturation effects measured in the experiment     

Power Scaling on Efficient Generation of Ultrafast Terahertz Pulses

We have investigated power scaling for the efficient generation of the broadband terahertz (THz) pulses. These THz short pulses are converted from ultrafast laser pulses propagating in a class of semiconductor electrooptic materials. By measuring the dependence of the THz output on the pump beam in terms of incident angle, polarization, azimuthal angle, and pump intensity, we have precisely determined the contributions made by optical rectification, drift of carriers under a surface or external field, and photo-Dember effect. When a second-order nonlinear material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, the three mechanisms mentioned earlier often compete with one another, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients can be resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also analyzed how the THz generation is affected by some competing processes such as two-photon absorption.     

Production Operation of Submersible Pumps with Closed-Loop Adjustable Speed Control

Submersible pump performance in a given installation is determined by the intersections of the pumping load line with the pump capability curves. With fixed pump speed, the operating point on the head-capacity curve moves as the well productivity changes, and operation may be then outside the desired range. Efficiency and reliability are thereby reduced. With adjustable speed pumping, the operating point may be held within the desired range at high efficiency, while production is changed to match the well capability. Cycling or throttling may be avoided. Production is optimized if the well fluid level is held as low as possible, while avoiding pump-off, cavitation or gas-lock conditions. This may be achieved by using closed-loop control of pump speed with the feedback of either downhole pressure or motor current data or both. Practical experience has demonstrated the reliability of these techniques and the resulting ability to confidently operate with a lower fluid level, which is maintained constant through changing productivity.     

Nd-YAG and Yb-YAG rotary disk lasers

The unconventional rotary disk laser holds promise in the development of new laser systems that have low gain and that are currently below threshold due to thermal effects at the high pump intensity required for lasing. It can be made to operate in almost any solid-state laser gain medium and can significantly increase the single-mode output power in CW or pulsed operation over the current state of the art. This article reports on the efficient single-mode operation of a Nd-YAG rotary disk laser and presents the first experimental data on its beam quality, pointing error, and power fluctuation. It also presents preliminary results from a single-mode Yb-YAG rotary disk laser.