Ultrashort-pulse fiber ring lasers

Received: 18 February 1997/Revised version: 22 April 1997     

Ultrashort-pulse child-langmuir law in the quantum and relativistic regimes

This Letter presents a consistent quantum and relativistic model of short-pulse Child-Langmuir (CL) law, of which the pulse length tau is less than the electron transit time in a gap of spacing D and voltage V. The classical value of the short-pulse CL law is enhanced by a large factor due to quantum effects when the pulse length and the size of the beam are, respectively, in femtosecond duration and nanometer scale. At high voltage larger than the electron rest mass, relativistic effects will suppress the enhancement of short-pulse CL law, which is confirmed by particle-in-cell simulation. When the pulse length is much shorter than the gap transit time, the current density is proportional to V, and to the inverse power of D and tau.     

Hartmann wave-front scanner

A wave-front sensor is described that uses a programmable moving aperture to scan an incoming wave front. The position of the diffraction spot is recorded behind an objective lens with a two-dimensional sensor and gives an estimate of the local slope at the aperture position. Then the wave front is reconstructed by processing of the slope data. The device is basically a programmable Hartmann wave-front sensor. Compared with a microlens Shack-Hartmann wave-front sensor, its much longer focal length provides higher resolution, although real-time operation is lost. A practical implementation of the new scanner with a liquid-crystal television as the programmable aperture is presented.     

Ultrashort-pulse sources based on single-mode rare-earth-doped fibers

An overview of ultrashort-pulse sources based on single-mode rare-earth-doped fibers is given. A wide range of pulse-generation schemes comprising mode-locked fiber lasers, parametric pulse sources and hybrid diode-fiber amplifier sources are discussed. Both actively and passively mode-locked fiber lasers are described and their specific merits and operation regimes are elucidated. Techniques for improving the spectral quality and the output powers of diode-based systems based on amplification in rare-earth-doped fibers are also reviewed. Finally, applications are discussed and directions for future research are indicated.     

Ultrashort-pulse generation by lasers with saturable absorbers

A theoretical model for the ultrashort-pulse (USP) generation by lasers with saturable absorbers is presented. The gain medium is assumed to be a two-level system whereas the absorber is described by a four-level system which is characterized by a two-stage absorption process and the pertinent relaxation times. Laser dynamical equations are developed in the rate-equation approximation and boundary conditions appropriate for an unidirectional ring cavity are established. Evolution of USP is investigated for different combinations of parameter values appropriate for solid-state laser systems by computer simulation, employing a finite difference approximation for the dynamical equations. It is shown that USP output is attainable even if the cross-section of the excited-state absorption isgreater than that of the ground-state absorption and the laser is operating just above threshold. In fact, it is found that through the participation of a strong excited-state absorption the discrimination against satellite pulses is enhanced so that single-pulse output is more achievable. Furthermore, it is proposed that single picosecond pulses may be obtainable from relatively broad initial peaks by utilizing the high pulse-selection and pulse-shortening efficiency of the absorber due to the contribution of the excited-state absorption. The applicability of the present model to singlet-triplet crossing and photoisomer formation is also discussed. Work supported in part by the National Research Council of Canada under grant No. A6005. A preliminary version of this paper was presented at the Joint Congress of CAP-APS-SMF, Université Laval, Québec, Canada, 14–17 June 1976. This author is now with Welwyn Canada Limited, London, Ontario, Canada.     

Ultrashort-pulse modulation in adiabatically prepared Raman media

We demonstrate the efficient modulation of an approximately 100-femtosecond pulse in a Raman medium coherently prepared by nanosecond pulses. Raman sidebands of the ultrashort pulse spanning 360 THz are generated with an efficiency of >5%. We show that the mechanism permitting the sidebands to be generated is the preparation of a significant vibrational coherence in the medium that is robust to disturbance by an intense short pulse. If the observed sidebands were phase compensated, they would form a short train of approximately ten 3-fs pulses. Focusing would permit the realization of a peak intensity of >10(13) W cm(-2).     

高分辨力哈特曼传感器的快速波面重构和拼接

建立哈特曼传感器的模型,证明在高空间分辨力下,可以采用Hudgin模型进行波面重构,避免了采用Fried模型带来的复杂性。对哈特曼子孔径缺失破坏连续性的问题进行了分析,介绍了相应的边缘处理算法。完成了基于离散傅里叶变换的波面重构算法数值模拟,实现了波面的无损重构。针对实际应用中输入波面在被遮挡处不连续的问题,提出了基于最小二乘解的拼接方案,实现了非连续波面拼接。分析了影响波面重构速度的主要因素,提出了提高波面重构性能的方法。     

Ultrashort-pulse laser ablation of indium phosphide in air

Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φ_th(1)=0.16 J/cm². The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 23 August 2000     

Ultrashort-pulse dual-wavelength source for digital holographic two-wavelength contouring

Digital holographic shape measurements using femtosecond laser pulses are reported. For contouring of very fast moving objects, the simultaneous generation of at least two spectrally separated ultrashort pulses is required. To deliver this particular spectral signature at high pulse energies, a chirped-pulse Ti:sapphire laser amplifier was modified to emit two spectrally separated pulses with energies above 1 μJ each. The wavelength separation of these pulses was adjustable within the 50 nm gain bandwidth, cutting out two distinct wavelength peaks by a variable double-slit assembly in a prism sequence. A Michelson-type interferometer was employed to perform the two-wavelength contouring. The phases of the holograms and the phase differences are calculated numerically, which allow us to deduce the contour lines of the topology of the object. The suitability of the light source for digital holography is demonstrated with contouring of stationary objects and the potential for high-speed applications is indicated. PACS 42.40.-i; 42.60.By     

Frequency-resolving spatiotemporal wave-front sensor

We report on a high-resolution wave-front sensor that measures the complete spatial profile of any frequency component of a laser field containing multiple frequencies. This probe technique was developed to address the necessity of measuring the spatial overlap of the carrier field with each sideband component of the field exiting the output port of a gravitational-wave interferometer. We present the results of an experimental test of the probe, where we were able to construct the spatial profile of a single radio-frequency sideband at the level of -50 dBc.     

Ultrashort-pulse laser ellipsometric pump-probe experiments on gold targets

Ultrashort-pulse laser pump-probe ellipsometry has been performed on gold targets at intensities 2 x 10(12)-5 x 10(13) W/cm(2). We measured time-resolved p- and s-polarized reflectivity (r(p) and r(s)) and the s-p phase difference (delta). When plotted as Y=[2|r(s)||r(p)|sin((delta)]/(|r(s)|(2)+|r(p)|(2)) versus X=|r(p)|(2)/|r(s)|(2), the experimental data follow approximately the same curve in X-Y space, even for different pump intensities. Although the input energy density is about 40 eV/atom and the plasma expansion is rapid (up to 10 km/sec), our data are consistent with a partially transparent blowoff having an atomic polarizability (-1.75+0.2i)x 10(-24) cm(3). We attribute this behavior to recombination in the expanding plasma. The Saha equation predicts recombination and formation of negative ions and a low density of free electrons in the low-density vapor, and solutions of the Maxwell equations approximately reproduce the data.     

Generalized method for wave-front analysis

We suggest and demonstrate a new method for wave-front analysis based on common-path phase-shift interferometry. We introduce a formalism and an iterative mathematical algorithm in which the wave front is transformed, modified, and inversely transformed. The resulting intensity data are sufficient to reconstruct the entire wave front. In a more restricted case, in which the wave-front modifications are arbitrarily applied over arbitrary spatial regions of the wave front, the wave front is reconstructed semianalytically by use of a model that allows a local solution, followed by an iterative algorithm. Measurement results indicating that the suggested approach has an improved measurement accuracy with respect to existing quantitative phase measurement methods are presented.     

Harnessing caustics for wave-front sensing

Scintillation in measured wave fronts adds spurious dislocations and deformations to their reconstruction. The source of the problem is caustics formed by aberrations in intermediate planes. I propose to use intentional caustics to measure wave fronts under severe conditions such as low light level, fast scale variations, large aberrations, and discontinuities in the wave front. A simple realization is based on the Hartmann-Shack sensor, which samples the wave front with a lenslet array. Movement of the lenslets' foci is linear with slope changes. Here the lenslets are effectively formed in an acousto-optic device: Two standing waves are launched perpendicularly to the light beam and to each other. At some distance down the beam, each wave creates a comb of caustics, and the two orthogonal combs add up to an array of caustic spots. The spatial frequency of the array is linear with the temporal frequency of the standing sound waves. A simple Fourier demodulation scheme supplies the two wave-front gradients.     

Spectral anomalies at wave-front dislocations

In a recent Letter, Gbur, Visser, and Wolf [Phys. Rev. Lett. 88, 013901 (2002)] predict that remarkable spectral changes can take place in the neighborhood of phase singularities of a diffracted focused wave. We report here the experimental observation of this anomalous spectral behavior and show that this is a general characteristic of optical vortices. Using a high-resolution interferometric technique, we are able to measure directly both the spectral intensity distribution and the spectral phase at the singular points of optical waves.     

畸变波前相位校正效果分析

以随机相位屏构造波前畸变相位,运用高通滤波的方法模拟变形镜对光束波前畸变相位的校正作用,模拟分析了畸变波前的相位校正效果,定量分析了校正效果与低频相位畸变和高频相位畸变之间的关系,并进一步讨论了变形镜的校正位置对校正效果的影响。研究结果表明:对于给定单元尺寸的变形镜,随着畸变波前中高频相位畸变所占比例的增大,经校正后的远场光束质量明显降低,校正效果也越来越差。此外,光学变形镜所在位置对校正效果存在明显影响,校正效果随校正位置的变化呈现出起伏变化。     

畸变波前相位校正效果分析

 以随机相位屏构造波前畸变相位,运用高通滤波的方法模拟变形镜对光束波前畸变相位的校正作用,模拟分析了畸变波前的相位校正效果,定量分析了校正效果与低频相位畸变和高频相位畸变之间的关系,并进一步讨论了变形镜的校正位置对校正效果的影响。研究结果表明:对于给定单元尺寸的变形镜,随着畸变波前中高频相位畸变所占比例的增大,经校正后的远场光束质量明显降低,校正效果也越来越差。此外,光学变形镜所在位置对校正效果存在明显影响,校正效果随校正位置的变化呈现出起伏变化。     

Generalized phase diversity for wave-front sensing

Phase diversity is a phase-retrieval algorithm that uses a pair of intensity images taken symmetrically about the wave front to be determined. If these images are taken about the system input pupil this is equivalent to a curvature-sensing algorithm. Traditionally a defocus aberration kernel is used to produce the phase-diverse data. We present a generalization of this method to allow the use of other functions as the diversity kernel. We discuss the necessary and sufficient conditions that such a function must satisfy for use in a null wave-front sensor. Computer simulations were used to validate these results.     

Femtosecond Kerr-lens autocorrelation

An autocorrelation measurement of femtosecond laser pulse duration using the Kerr-lens mechanism is demonstrated. This technique can also be used as a sensitive and absolutely calibratable method for measuring ultrafast optical nonlinearities. A method that uses an electronic spectral-filtering scheme is proposed for determining the frequency chirp of pulses by interferometric autocorrelation.     

Linear phase retrieval for wave-front sensing

Phase retrieval from one or more intensity measurements is a potentially powerful and appealing technique for real-time adaptive-optics wave-front sensors. Under the assumption of small wave-front phase excursions, one is able to derive an exact solution to the inverse problem given three or more intensity measurements with known phase offsets. Applications include a high-order wave-front sensor to correct for residual aberrations in an adaptive-optics system in tandem with a low-resolution Hartmann-Shack wave-front sensor. The formula can also furnish mathematical insights into the full nonlinear phase-retrieval task.     

Hybrid curvature and gradient wave-front sensor

A new wave-front sensor for adaptive optics that gives signals for the curvature and the two gradient components of the wave front simultaneously is proposed. The sensor uses quad cells placed at the foci of an array of astigmatic lenslets, giving three intensity-normalized differential signals. The parameters of a single subdetector are optimized to maximize the curvature signal. The performance of such a sensor designed for a membrane curvature mirror is analyzed. The good match between sensor and mirror is exhibited by an almost diagonal response matrix between the mirror actuator controls and the curvature signals.