Effect of native defects and laser-induced defects on multi-shot laser-induced damage in multilayer mirrors

The roles of laser-induced defects and native defects in multilayer mirrors under multi-shot irradiation condition are investigated. The HfO 2 /SiO 2 dielectric mirrors are deposited by electron beam evaporation(EBE) . Laser damage testing is carried out on both the 1-on-1 and S-on-1 regimes using 355-nm pulsed laser at a duration of 8 ns. It is found that the single-shot laser-induced damage threshold(LIDT) is much higher than the multi-shot LIDT. In the multi-shot mode,the main factor influencing LIDT is the accumulation of irreversible laser-induced defects and native defects. The surface morphologies of the samples are observed by optical microscopy. Moreover,the number of laser-induced defects affects the damage probability of the samples. A correlative model based on critical conduction band(CB) electron density(ED) is presented to simulate the multi-shot damage behavior.     

Spatial amplification of a laser wave and the transverse spread of electrons in an undulator with a noncollinear configuration

The spatial amplification of a wave in a magnetostatic undulator with noncollinear electron and laser beams is studied in the framework of the dispersion relation for single-frequency and collective regimes. The dependence of the gain on the electron beam width is estimated with regard to the spatial boundedness of the beams. The laser power threshold at which the selection with respect to the transverse velocity is possible is obtained for a free-electron laser without inversion.     

Continuous wave dye laser pumped by a HeNe laser

Continuous wave operation of a dye laser pumped at 633 nm by a HeNe laser is reported. A jet of a cooled solution of Oxazine 1 in ethanol was used as the laser medium in an asymmetric spherical cavity. With mirrors of high reflectivity a threshold as low as 10 mW could be achieved.     

Optical feedback cavity ring-down technique for accurate measurement of ultra-high reflectivity

A novel cavity ring-down (CRD) technique, based on the optical feedback effect of Fabry–Perot diode lasers, is developed for accurate reflectivity measurement of highly reflective cavity mirrors. The strong optical feedback, including the direct reflection from the front cavity mirror of a linear ring-down cavity, is re-injected into the oscillator cavity of the diode laser, and large resonant peaks are observed in the ring-down cavity output signals. The diode laser is switched off by a threshold circuit when the amplitude of a resonant peak exceeds a pre-defined threshold. Exponentially decayed signals recorded immediately after switching off the laser are used to determine the cavity decay time. The ultra-high reflectivity of cavity mirrors is measured to be 99.99606% with a reproducibility of 0.00003%. Compared with the previous phase-shift CRD technique, the signal-to-noise ratio of the cavity output signals is improved with an enhancement factor of over 100.     

The study of the wake field effects on the self-focusing and the compression of the laser pulse in plasma at the relativistic regime by Lagrangian method

The trial function in the variational-Lagrangian method is a fundamental method to explain the essential features of the laser pulse evolution in the plasma. Self-focusing behavior and compression of the laser pulse in plasma are analyzed, for both low and high intensity regimes. It is shown that the compression threshold depends on both pulse intensity and pulse length. In particular, in both regimes, the compression threshold is directly proportional to the pulse length. This is while this threshold is directly, in the linear regime, and inversely, in the nonlinear regime, proportional to pulse intensity. In the present work, the existence of oscillations is revealed, with a behavior akin to laser pulse width. Finally, the effects of pulse intensity, pulse length, and plasma density on compression are analyzed.     

Albedo and gain threshold of a diffusive Raman random laser

The diffuse reflectance (albedo) and transmittance of a Raman random gain medium are derived from a diffusion equation with power dependent gain. The results show good agreement with the experimental data for barium nitrate powder. Both the Raman albedo A_R and Raman transmittance T_R diverge at a critical gain γ_c, interpreted as the threshold for diffusive Raman laser generation. The parametric dependence of the albedo and threshold gain on the scattering characteristics of the random medium is analysed and the feedback effect of Fresnel reflection at the gain boundaries evaluated. The addition of external mirrors, particularly at the pumped surface, significantly reduces the generation threshold.     

Stimulated brillouin scattering in SF6 with a free-running XeCl laser as pump

A XeCl laser system without dispersive elements is used to investigate near threshold reflectivity and phase-conjugation (PC) fidelity of stimulated Brillouin scattering (SBS) mirrors with SF₆ as the active medium. Using different focal-length lenses to focus the broadband laser radiation into the Brillouin medium, it was found that at threshold the effective interaction length for SBS is equal to the confocal parameter and that the SBS gain is equal to its steady-state value for monochromatic pumping. High PC-fidelity values ranging from 0.8–1.0 were found under most of the experimental conditions investigated.     

Optical characteristics of a turbid medium between two mirrors

Using the one-dimensional Boltzmann equation, we examine the optical scattering properties of a turbid medium that is located between two mirrors with controllable reflectivity. We examine how the mirrors can be used to enhance the total transmission of an intensity modulated laser beam through this system. The analytical results show that, for certain modulation frequencies, the total transmission can be increased if the laser source is placed between the mirrors. This finding could improve diffusive imaging for those highly scattering media that are so thick that the laser light would not penetrate sufficiently deep in the absence of any mirrors.     

Emission Dependence on Length in a 1.48-mum Pumped Erbium-Doped Laser with an All-Fiber Configuration

We report a study on the main features of a 1.48-mum pumped erbium-doped fiber laser, such as threshold pump power, output power, and slope efficiency. In particular we have focused on its dependence on active fiber length. We employed an all-fiber laser configuration, which included mirrors directly coated onto the polished fiber endfaces. Launched pump power and laser output power were introduced and extracted, respectively, from the laser cavity by means of couplers. Our measure ments show the existence of an optimum erbium-doped fiber length, where threshold power reaches minimum and output power maximum.     

Interaction of intense IR laser radiation with a-C:H protective coatings

The results of a determination of the optical breakdown thresholds of air near the surface of copper laser mirrors with a-C:H protective coating by intense pulsed 10.6 μm radiation are analyzed and systematized. It is shown that there is no correlation between the breakdown threshold of a coated mirror, the reflectance of the coating, and the breakdown threshold of the initial copper surface. Experimental dependences of the threshold of optical breakdown of coated mirrors on the a-C:H thickness and deposition rate as well as the storage time of the mirrors are given. Estimates are made of the rise in the surface temperature in the irradiation zone for the case of an ideal adhesive contact and calculations are made of the damage threshold of the coating in the case that the adhesive contact between the mirror and the coating is impaired. The effect of the a-C:H coating properties and the conditions at the polished metal surface-protective coating interface on the optical breakdown threshold is discussed. Zh. Tekh. Fiz. 68, 59–66 (September 1998)     

Direct intensity modulation of a rectangular ring laser with bidirectional lasing characteristics

The direct intensity modulation of a three-guide coupled rectangular ring laser having bidirectional lasing characteristics is reported for the first time. The rectangular laser cavity consists of four low loss total internal reflection mirrors and an output coupler made out of three passive coupled waveguides. The laser is fabricated using a total cavity length of 580 μm with active section lengths of 250 μm. For both the clockwise and counterclockwise circulating directions, a lasing threshold of around 38 mA is obtained at room temperature under continuous wave operation. A maximum 3-dB modulation bandwidth of approximately 3.2 GHz is observed in both circulating directions.     

Laser emission from polished NdP5O14 crystals with directly applied mirrors

We report on crystal processing, laser fabrication, emission spectra and output characteristics of neodymium pentaphosphate lasers. We obtained cw oscillation with an output power of 19 mW from a crystal with dielectric mirrors directly applied on polished (001) planes. We pumped longitudinally with 280 mW of an argon ion laser at 514 nm focussed to a 94-μm diameter. Near threshold our laser operated in the vicinity of 1053 nm; when pumped 1.6 times threshold it operated dominantly in the vicinity of 1063 nm.     

极高反射率氧碘激光腔镜研制

报道了反射率大于９９．９％的氧碘激光腔镜的研制工艺及其反射率测试结果。对该工艺制作的样品进行了激光损伤实验,实验结果表明能够满足目前ＣＯＩＬ激光器的阈值要求。     

极高反射率氧碘激光腔镜研制

报道了反射率大于99.9%的氧碘激光腔镜的研制工艺及其反射率测试结果。对该工艺制作的样品进行了激光损伤实验,实验结果表明能够满足目前COIL激光器的阈值要求。     

From amplified spontaneous emission to laser oscillation: dynamics in a short-cavity polymer laser

We present an experimental study of the input-output characteristics of an ultrashort pumped pi -conjugated polymer in solution. By comparing the results for configurations with zero, one, and two mirrors around the polymer, we show that the physics is driven by amplified spontaneous emission and not by cooperative emission. This finding is substantiated by picosecond-time-scale measurements of the evolution of the emission of the polymer. For the two-mirror configuration a sharply defined threshold for laser oscillation is found; the output of the laser exhibits strong pulsations.     

重复率激光作用下光学薄膜损伤的累积效应

使用脉宽12 ns,频率10 Hz的1064 nm调Q NdYAG激光器,研究了高反射膜在重复率激光作用下的损伤的累积效应.实验发现,高反射膜的损伤阈值随辐照脉冲数增加而降低,表现出明显的累积效应.通过对损伤阈值和损伤概率以及辐照次数的统计性研究,并结合单脉冲辐照的结果,说明了存在于薄膜中微小的缺陷参与了多脉冲激光对薄膜的损伤过程.可用预损伤机制解释实验结果.得到了关于IBS制备的高反射膜的损伤阈值和照射次数的关系式,并用实验结果进行验证,发现具有很好的一致性.实验过程中样品的损伤形貌通过Nomarski偏光显微镜进行了观察,发现是典型的缺陷损伤.     

多脉冲激光作用下光学薄膜损伤的累积效应

研究了高反射膜在多脉冲激光作用下损伤的累积效应.实验中使用1064nm调Q的Nd∶YAG激光器,脉宽是12 ns,频率为10 Hz.实验发现:高反射膜的损伤阈值随辐照脉冲数增加而降低,表现出明显的累积效应.通过对损伤阈值和损伤概率以及辐照次数的统计性研究,并结合单脉冲辐照的结果,说明了存在于薄膜中微小的缺陷参与了多脉冲激光对薄膜的损伤过程,得到了制备IBS高反射膜的损伤阈值和照射次数的关系式,用Nomarski偏光显微镜观察了实验过程中样品的损伤形貌,发现是典型的缺陷损伤.     

极高反射率氧碘激光腔镜研制

 报道了反射率大于99.9%的氧碘激光腔镜的研制工艺及其反射率测试结果。对该工艺制作的样品进行了激光损伤实验,实验结果表明能够满足目前COIL激光器的阈值要求。     

Predicting the performance data for a fiber laser by measuring the emitted fluorescence power of the end-on pumped fiber

A novel method is described that enables the prediction of the main laser parameters (threshold pump power, output power, slope- and extraction efficiency) without having realized the laser itself. The emitted fluorescence power of an end-on pumped fiber is absolutely measured along the fiber. Using specific material parameters of the doped glass and the waveguide attenuation, we calculate the laser properties without taking the resonator losses into account. This approach is extremely useful for fibers with special design parameters. We have used this method to characterize a fiber with a novel design, the M-profile fiber. Combining the results with the measurements on the realized laser, the impact of resonator losses (e.g., tilted fiber endfaces, effects of butt-coupled mirrors) can be inferred and improvements can be undertaken.