Laser Wavefront Research Articles

Photonic streaking of attosecond pulse trains

High harmonic radiation, produced when intense laser pulses interact with matter, is composed of a train of attosecond pulses. Individual pulses in this train carry information on ultrafast dynamics that vary from one half-optical-cycle to the next. Here, we demonstrate an all-optical photonic streaking measurement that provides direct experimental access to each attosecond pulse by mapping emission time onto propagation angle. This is achieved by inducing an ultrafast rotation of the instantaneous laser wavefront at the focus. We thus time-resolve attosecond pulse train generation, and hence the dynamics in the nonlinear medium itself. We apply photonic streaking to harmonic generation in gases and directly observe, for the first time, the influence of non-adiabatic electron dynamics and plasma formation on the generated attosecond pulse train. These experimental and numerical results also provide the first evidence of the generation of attosecond lighthouses in gases, which constitute ideal sources for attosecond pump–probe spectroscopy. An all-optical photonic streaking measurement is demonstrated that provides direct experimental access to individual attosecond pulses. The effects of non-adiabatic electron dynamics and plasma formation on the generated attosecond pulse train are directly observed when the pulse train is applied to harmonic generation in gases.

Generation of Laser-Driven Higher Harmonics from Grating Targets

The first experimental evidence of the higher-order harmonic radiation generated by periodically modulated targets (gratings) irradiated by relativistic, ultrashort (<30 fs), high intensity [Iλ(2)=10(20) (W/cm(2)) μm(2)] laser pulse is presented. The interference effects on the grating surface lead to the emission of high harmonics up to 45th order along the target surface when the laser beam is focused onto a grating target close to normal incidence (5°). By means of numerical simulations we demonstrate the possibility of controlling the composition of the higher harmonic spectrum and we prove the influence of the laser pulse parameters in the interaction area (laser focusing and wavefront curvature) on the emission angle of a certain high harmonic order.

Weighted Fried reconstructor and spatial-frequency response optimization of Shack–Hartmann wavefront sensing

Two modified Fried wavefront reconstructors are proposed, both based on an enhanced geometry that combines and balances those of Fried and Hudgin with an additional weight. The optimal weights for both of them are derived with the analytical frequency response functions, which can provide near-unity spatial frequency response over broad bandwidth to the Shack-Hartmann wavefront sensing system. Comparisons between the proposed reconstructors and the classical ones are presented in the frequency domain, and simulations have further confirmed the frequency characteristic and the reconstruction performance of the new reconstructors. It is expected that the optimized reconstructors can help improve the performance of adaptive optics systems for high-power laser wavefront control and other relevant optical systems that require high-accuracy wavefront sensing.

Shack–Hartmann type laser wavefront sensor for measuring two-dimensional electron density distribution over extinguishing arc discharge

Shack–Hartmann type laser wavefront sensors were developed for measuring a two-dimensional electron density distribution over an arc channel in an extinguishing phase by only a single measurement with high temporal and spatial resolutions. The sensors were applied to pulsed arcs in a 3 mm air gap between rod-to-rod tungsten electrodes with a diameter of 1mm. Electron densities in the arc discharges with currents of several tens of amperes were 2 × 1023 m−3 around the gap centre, which were more than twice lower than those near the anode and cathode.

Manufacturing and Characterization of a Deformable LTCC Membrane with Integrated Temperature Sensors, Strain Gauges and Heating Structures

The wavefront of high energy lasers can be corrected by using a deformable mirror. This mirror is a complex multimaterial device based on a LTCC membrane (DP 951) with a galvanically deposited and polished copper surface. In the membrane temperature sensors, strain gauges and heaters are integrated to measure deformation and temperature and to influence the latter. The laser wavefront is corrected by deforming the membrane with screen printed, segmented PZT thick film actuators. In this paper the challenging manufacturing process of the mirror is described. During processing the LTCC membrane with integrated sensors must endure the postfired sintering of PZT at temperatures of up to 900°C without any degradation of the sensors' characteristics. In order to optimize the sensors, various combinations of resistor and conductor pastes and different geometries are characterized. The usability and the performance of the sensor elements after temperature treatment are investigated by measuring the resistance and its resistance temperature characteristic. For the detection of the membrane deformation, an array of miniaturized strain gauges is used. To achieve a controlled temperature distribution across the membrane different buried heater concepts are manufactured and characterized. In order to improve the mechanical stability of the membrane, a rim of fired LTCC (width: 6 mm, diameter: 52 mm, thickness: 600 μm) is bonded to the multilayer membrane using a sealing glass.

A Shack–Hartmann laser wavefront sensor for measuring electron density in low‐current arc

AbstractTo determine electron densities in low current arcs, a Shack–Hartmann laser wavefront sensor was developed, improved in terms of measurement accuracy, and applied to atmospheric arcs with a peak current of 0.8 kA. The electron density in the pre‐extinguishing phase was 5 × 1023 m−3 at the gap center when the instantaneous current decreased to 0.1 kA. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 181(4): 1–8, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22319

Wavefront pre-compensation for thermal deformations in a high power inner optical system

The phase aberration caused by thermal deformations in a high power inner optical system is simulated and the wavefront pre-compensation for correcting the aberration is studied experimentally. Choosing Strehl ratio (SR) and the root-mean-square (RMS) of distorted phase as the characteristic parameters, the beam quality of outgoing laser is calculated when the wavefront of incident laser is compensated. Based on the computed results, the laboratory experiment is designed to model the thermal deformations aberration and the correction process by using two Liquid Crystal on Silicon Spatial Light Modulators (LCoS-SLM). The negative influence of astigmatism and defocusing on laser beam quality in far field is obvious and the astigmatism induces the non-axisymmetrical divergence of intensity distribution. After wavefront pre-compensation the power in the bucket (PIB) value of outgoing laser beam is close to the flat wavefront laser source. Both simulated and experimental results indicate that the method of wavefront pre-compensation can greatly correct the aberrations caused from thermal deformation, and the laboratory experiment is a feasible method to modeling the inner optical system with phase aberration.

Multiple regression analysis in myopic wavefront laser in situ keratomileusis nomogram development

To develop a systematic method for quantifying pretreatment adjustments to the treatment sphere in patients having myopic wavefront laser in situ keratomileusis (LASIK). Moorfields Eye Hospital, London, United Kingdom. Comparative case series. Preoperative and 3-month postoperative data from consecutive cases of myopic wavefront LASIK (<10.0 diopters [D] sphere; <4.5 D cylinder) treated in 2008-2009 were tabulated. Multiple regression modeling was used to derive a weighted relationship between the achieved manifest refraction spherical equivalent (MRSE) and several possible influences. A new nomogram was derived from the resultant regression equation in which the percentage treatment boost was based on the partial correlation coefficient for the 4.0 mm pupil wavefront refraction spherical equivalent (SE), and pretreatment adjustment to the treatment sphere was based on the sum of the other weighted independent variables. The MRSE results in eyes treated using the new nomogram in 2009-2010 were compared with those from the original data set. The variance in postoperative MRSE error was significantly reduced using the new nomogram (0.10 D in 2009-2010 versus 0.16 D in 2008-2009; P<.0001, Bartlett test). The R(2) measure of linear fit increased from 0.967 in 2008-2009 to 0.983 in 2009-2010. Multiple regression modeling can be used to take account of several possible influences in enhancing the accuracy of wavefront LASIK. Nomograms developed through multiple regression modeling can be used to derive eye-specific pretreatment adjustments to the treatment sphere. Neither author has a financial or proprietary interest in any material or method mentioned.

Limitations of atom interferometry for gravitational wave observations in space

One of the atom interferometer gravitational wave missions proposed by Dimopoulos et al.1 in 2008 was called AGIS-Sat. 2. It had a suggested gravitational wave sensitivity set by the atom state detection shot noise level that started at 1 mHz, was comparable to LISA sensitivity from 1 to about 20 mHz, and had better sensitivity from 20 to 500 mHz. The separation between the spacecraft was 1,000 km, with atom interferometers 200 m long and shades from sunlight used at each end. A careful analysis of many error sources was included, but requirements on the time-stability of both the laser wavefront aberrations and the atom temperatures in the atom clouds were not investigated. After including these considerations, the laser wavefront aberration stability requirement to meet the quoted sensitivity level is about 1\times10-8 wavelengths, and is far tighter than for LISA. Also, the temperature fluctuations between atom clouds have to be less than 1 pK. An alternate atom interferometer GW mission in Earth orbit called AGIS-LEO with 30 km satellite separation has been suggested recently. The reduction of wavefront aberration noise by sending the laser beam through a high-finesse mode-scrubbing optical cavity is discussed briefly, but the requirements on such a cavity are not given. Unfortunately, such an Earth-orbiting mission seems to be considerably more difficult to design than a non-geocentric mission and does not appear to have comparably attractive scientific goals.

Loss of spatial coherence and limiting of focal plane intensity by small-scale laser-beam filamentation

We describe a nonlinear optical mechanism that leads to a decrease of the degree of (transverse) spatial coherence of a laser beam as a function of propagation distance. This prediction is in direct contrast with those of the van Cittert-Zernike theorem, which applies to propagation through a linear, homogeneous material. The mechanism by which coherence is lost is the growth of small phase irregularities initially present on the laser wave front. We develop a detailed theoretical model of this effect and present experimental results that validate this model. The practical importance of this result is that by being able to controllably decrease the spatial coherence of a laser beam, one can limit the maximum intensity that is produced in its focal region. By limiting the intensity, one can prevent laser damage to bulk optical components or to sensitive photodetectors. This mechanism thus provides an alternative to current approaches of sensor protection based on optical power limiting.

Beam engineering of quantum cascade lasers

AbstractThis paper reviews beam engineering of mid‐infrared and terahertz quantum cascade lasers (QCLs), based on two approaches: designer plasmonic structures and deformed microcavities. The plasmonic structures couple laser emission into surface waves and control the laser wavefront in the near‐field, thereby greatly increasing beam collimation or introducing new functionalities to QCLs. The plasmonic designs overall preserve laser performance in terms of operating temperature and power output. The deformed microcavity QCLs operate primarily on whispering‐gallery modes, which have much higher quality factors than other modes, leading to lower threshold current densities. Cavity deformations are carefully controlled to greatly enhance directionality and output power.

Three-dimensional in vivo scanning microscopy with inertia-free focus control

The acquisition of high-resolution images in three dimensions is of utmost importance for the morphological and functional investigation of biological tissues. Here, we present a laser scanning two-photon microscope with remote and motionless control of the focus position. The movement of the excitation spot along the propagation direction is achieved by shaping the laser wavefront with a spatial light modulator. Depending on the optical properties of the objective in use, this approach allows z movements in a range of tens to hundreds of micrometers with small changes of the point spread function. We applied this technique for the three-dimensional (3D) imaging of fluorescent cells in the mouse neocortex in vivo. The presented system bypasses the limitations of microscopes based on moving objectives, enabling high-resolution inertia-free 3D imaging.

Comment on “Atomic gravitational wave interferometric sensor”

The use of laser interferometers for detecting and studying gravitational-wave signals from many types of astronomical sources is being pursued actively by a number of groups in different countries. However, it has been suggested recently that cooled atom clouds in atom interferometers could be used to replace the test masses in space-based gravitational-wave detectors and the end mirrors in ground-based detectors [S. Dimopoulos, P. W. Graham, J. M. Hogan, M. A. Kasevich, and S. Rajendran, Phys. Rev. D 78, 122002 (2008).]. Some new error sources that apparently have not been included in proposals of atom interferometer gravitational-wave detectors will be discussed in this comment. They are based on additional effects of aberrations in the laser wave fronts that interact with the atom clouds.

Compensation for time fluctuations of phase modulation in a liquid-crystal-on-silicon display by process synchronization in laser materials processing

We demonstrate the adverse influence of temporal fluctuations of the phase modulation of a spatial light modulator (SLM) display device on nanosecond laser micromachining. We show that active cooling of the display reduces the amplitude of these fluctuations, and we demonstrate a process synchronization technique developed to compensate for these fluctuations when applying the SLM to laser materials processing. For alternative SLM devices developed specifically for laser wavefront control (which do not exhibit such flickering problems), we show that our process synchronization approach is also beneficial to avoid machining glitches when switching quickly between different phase profiles (and hence beam patterns).

Analysis of a digital phase retrieval method for wave-front reconstruction

A phase retrieval algorithm which only needs to measure the intensity distribution at two positions to be effective is used to reconstruct the laser wave-front. Results are obtained from the phase retrieval algorithm in the visible band and the effects of the measurement error on the phase retrieval process are simulated. The algorithm is not sensitive to absolute amplitude measurement error, but is sensitive to the relative distribution of light intensity.

Shack-Hartmann Type Laser Wavefront Sensor for Measuring Electron Density in Low Current Arc

To know electron densities in low current arcs, Shack-Hartmann type laser wavefront sensor was developed, improved in terms of measurement accuracy, and applied to atmospheric arcs with a peak current of 0.8kA. An electron density in pre-extinguishing phase was 5×1023m-3 at gap centre when an instantaneous current decreased to 0.1kA.

Laser beam shape and curvature radius of the laser wavefront passing through ZnO nanostructures

In the present work, we focus on synthesis and optical properties of ZnO nanostructures prepared by the chemical vapour deposition method. The nonlinear optical properties of ZnO nanostructures in ethanol solution were studied by the Z-scan technique using CW He–Ne laser at 632.8nm. The results show a negative nonlinearity and a large nonlinear refractive index. The magnitude and the sign of nonlinear refractive index, n2, were measured using the single beam Z-scan technique. The effect of morphology on nonlinear optical properties of ZnO nanowires is investigated. The laser beam shape and variation of the curvature radius of the wavefront have been simulated when the Gaussian laser beam passes through ZnO nanowires in ethanol. The effect of concentration is investigated in the nonlinearity. We have reported the curvature radius of the wavefront to the nonlinearity for ZnO nanowires as nonlinear sample. Also we found good agreement between the results of theoretical laser beam shape passing through ZnO nanostructure and the experimental data.

Visual acuity loss with healthy ageing: can it be reversed by wavefront laser?

Abstract Purpose To consider what levels of “super acuity” might be achieved by the correction of ocular aberrations in older patients by wavefront corrected ophthalmic surgery. Note that when comparing visual acuity (VA) of older patients with VA in the young, the average optimal monocular visual acuity of a young subject is about 6/4 (decimal VA 1.50) and not the often quoted ‘normal’ figure of 20/20 (6/6 or 1.0 decimal). Methods Studies that attempted to isolate the cause(s) of deterioration in visual function with age in normal, healthy eyes were reviewed. Results The majority of studies suggest that the deterioration in visual function with age is primarily due to changes within the neural system rather than optical factors. In addition, several studies have shown increases in ocular aberrations with age, but this is only found when comparisons are made across age groups with fixed pupil sizes. When natural pupil sizes are considered, there is no change in aberrations with age because of age‐related pupillary miosis Conclusion There appears to be little scope for ocular aberration correction to be used to counteract the loss of vision with age. Reduced vision in patients with cataract is primarily due to increased forward light scatter, and aberrations play a minor role in reducing vision. Intra‐ocular lenses (IOLs) should be designed to keep ocular aberrations at a minimum after cataract surgery, but given that vision loss with age appears to be primarily due to neural changes, there seems little scope for IOLs to improve on the vision of phakic subjects under natural pupil conditions.

Magnetic Field Diffusion and Enhanced Resistivity in 12-cm-Diameter 200-ns 3.5-MA &lt;formula formulatype="inline"&gt; &lt;tex Notation="TeX"&gt;$Z$&lt;/tex&gt;&lt;/formula&gt;-Pinch Implosions

Investigations of magnetic field diffusion and plasma resistivity in 12-cm-diameter triple-gas-puff Ar <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinch implosions were carried out by using planar laser-induced fluorescence (PLIF), a laser shearing interferometer (LSI), and a laser wavefront analyzer (LWA) on a 3.5-MA 200-ns generator. The PLIF measurements gave the initial Ar gas distributions. The implosion velocity and electron density profiles were measured from LWA and/or LSI. From these, the implosion plasma sheath thickness, ion density, mean ion charge states, temperatures, and implosion velocity are obtained, which allows us to calculate the classical plasma resistivity. A 1-D analytic magnetic field diffusion model is constructed and used to predict the imploding plasma sheath thickness and its resistivity. Based on comparisons of the experimental measurements and the diffusion model prediction, we found out that plasma resistivity is enhanced by the cross-field diffusion above the classical value, as high as 60 times the Spitzer's value. Details are given in this paper.

CCD非线性效应对剪切干涉法波前检测的影响

When radial shear interferometer is used in high-power laser wavefront detection, due to the influence of the nonlinearity of CCD, there exist not only the first order frequency with wave-front information but also the second and the third order frequency. They may lead to overlapping spectrum, which makes it difficult to extract useful information and reduces the detection accuracy of wavefront. A theoretical analysis on mechanisms of high-order spectral component and spectrum aliasing caused by the nonlinearity of CCD is established, and the condition to avoid spectrum aliasing is pointed out. Finally, a method by increasing the spatial carrier frequency to reduce or eliminate the spectrum aliasing is proposed. Computer simulation and experimental results have verified the feasibility of