Multimode Laser Beam Research Articles

BER evaluations for multimode beams in underwater turbulence

In underwater optical communication links, bit error rate (BER) is an important performance criterion. For this purpose, the effects of oceanic turbulence on multimode laser beam incidences are studied and compared in terms of average BER (<BER>), which is related to the scintillation index. Based on the log-normal distribution, <BER> is analysed for underwater turbulence parameters, including the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, the parameter that determines the relative strength of temperature and salinity in driving index fluctuations, the Kolmogorov microscale length and other link parameters such as link length, wavelength and laser source size. It is shown that use of multimode improves the system performance of optical wireless communication systems operating in an underwater medium. For all the investigated multimode beams, decreasing link length, source size, the relative strength of temperature and salinity in driving the index fluctuations, the rate of dissipation of the mean squared temperature and Kolmogorov microscale length improve the <BER>. Moreover, lower <BER> values are obtained for the increasing wavelength of operation and the rate of dissipation of the turbulent kinetic energy in underwater turbulence.

Numerical simulation of sensorless adaptive phase correction of regular, vertical, and incoherent multimode laser beams

Numerical simulation of sensorless adaptive phase correction of regular, vertical, and incoherent multimode laser beams

Multimode Laser Beam Scintillations in Non-Kolmogorov Turbulence

Employing the Rytov solution, the scintillation index at the origin of the receiver plane is evaluated in non-Kolmogorov weak atmospheric turbulence when multimode laser incidence is used. The solution presented can be used when the multimode is composed of even modes. The novelty of this work lies in the theoretical combination of multimode laser beam excitation and non-Kolmogorov turbulence in the scintillation evaluations, which is not known both theoretically and experimentally. The study involves mathematical rigor but no experimental results. Being valid for any power law exponent of the non-Kolmogorov turbulence, the scintillations of the multimode beams are found to be smaller than the scintillation index of a single Gaussian beam. For the multimode laser beam excitation, the scintillation index is smaller at smaller power law exponent values. If the multimode content is formed by beams with larger mode numbers, the scintillations decrease for any non-Kolmogorov realization. When large sized beams are used in the multimode, the scintillations increase as compared to small sized content, and the scintillations become almost the same as the Gaussian beam scintillations. Comparing the multimode structures that have the same number of beams, the ones with higher order modes yield smaller scintillations, and for such multimode structures, very similar scintillation index behaviour versus the power law exponent can be obtained by varying the amplitudes of the modes composing the multimode. The main contribution of this paper is the formulation and evaluation of the scintillation noise in order to understand whether the use of multimode laser excitation will improve the link performance of optical wireless communication systems operating in a non-Kolmogorov atmosphere.

Intensity fluctuations of multimode laser beams in underwater medium.

In an oceanic optical communications link, the received intensity fluctuations, quantified by the scintillation index, are formulated and evaluated when a multimode laser is used. The variations of the scintillation index versus the oceanic turbulence parameters are examined for different multimode laser structures. Oceanic turbulence parameters used are the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, the Kolmogorov inner scale, and the parameter w that defines the ratio of temperature to salinity contributions to the refractive index spectrum. The results in this paper can be used to improve performance in the design of oceanic optical communications links.

Continuous Wave Diode Laser Surface Texturing of Austenitic and Pearlitic Steels

Microstructuring of steel resulting in directional solidification and texturing, previously observed in various metallic materials during pulsed laser processing, melt-spinning, high-gradient liquid metal melting, zone melting etc., is reported for the first time in continuous wave diode laser processing of steels. Influence of laser interaction time on surface morphology/topology of austenitic manganese and pearlitic steels is investigated utilizing a wide rectangular multi-mode diode laser beam. X-ray diffraction analysis of the laser treated austenitic steel surface showed strong texturing influence, with preferred crystallographic orientation of γ-Fe crystals in the (200) plane, which increased with interaction time. In case of pearlitic steel, no such texturing influence could be observed. The free surface topologies were also observed to be different in each case, with well-aligned domes of γ-Fe observed in laser treated austenitic steel as compared to randomly oriented fine domes of metal oxides in pearlitic one. In situ surface temperature measurement during laser irradiation indicated higher temperature on pearlitic steel than in austenitic manganese steel owing to its lower effective thermal conductivity associated with higher oxide film formation.

Laser beam homogenizing system design for photoluminescence.

Based on waveguide coupling technology, we set a kaleidoscope homogenizer to get an intensity redistributed uniform multimode laser beam and then shape the uniform beam into a large area square beam with an imaging system. The simulation results of overall light intensity uniformity are both greater than 89% under the irradiation areas of 3.5 cm × 3 cm and 10 cm × 10 cm. In the practical application, it is also greater than 85%. After that, we apply this technology to the photoluminescence imaging detection of semiconductor devices, proving that this technology can be widely used in a variety of research requiring large areas of uniform laser illuminating spots.

Reflective terahertz imaging with the TEM01 mode laser beam

Reflective terahertz imaging with a first-order Hermite-Gaussian laser beam was experimentally investigated. High spatial resolution targets prepared by direct laser microprocessing were used to evaluate the performance. The reflection imaging system at 2.524 THz frequency demonstrated up to diffraction limited resolution using the single focusing mirror with the numerical aperture not smaller than 0.6. The TEM(01) mode laser beam was also applied for practical samples such as silicon solar cell terahertz (THz) imaging. It is shown that usage of appropriate optics enables us to obtain high-quality THz images with the multimode laser beam.

Generating function approach for creation of coherent multimode beams by diffractive optics

Tailored compositions of transverse modes provided by mathematical generating functions are exploited for the synthesis of multimode laser beams in free space. We show that analytical equations which are available for the generating functions provide physical insight into modal phase and power balance in multimode coherent light beams. Multimode coherent beams were created by methods of diffractive optics implementing the generating functions. Experimental and computer simulated results demonstrate a good match.

Multimode laser beam analyzer instrument using electrically programmable optics

Presented is a novel design of a multimode laser beam analyzer using a digital micromirror device (DMD) and an electronically controlled variable focus lens (ECVFL) that serve as the digital and analog agile optics, respectively. The proposed analyzer is a broadband laser characterization instrument that uses the agile optics to smartly direct light to the required point photodetectors to enable beam measurements of minimum beam waist size, minimum waist location, divergence, and the beam propagation parameter M(2). Experimental results successfully demonstrate these measurements for a 500 mW multimode test laser beam with a wavelength of 532 nm. The minimum beam waist, divergence, and M(2) experimental results for the test laser are found to be 257.61 μm, 2.103 mrad, 1.600 and 326.67 μm, 2.682 mrad, 2.587 for the vertical and horizontal directions, respectively. These measurements are compared to a traditional scan method and the results of the beam waist are found to be within error tolerance of the demonstrated instrument.

Aberrational synthesis of optical systems intended for the conversion of laser beams

This paper presents a technique of aberrational synthesis of laser optical systems with small wave aberrations for maintaining similarity of the fields when multimode laser beams are being transformed. The minimization of the wave aberrations of laser optical systems is based on the detected connection of its design parameters with the expansion coefficients of the wave aberration. Examples are given of the automatic synthesis of laser optical systems to illustrate the effectiveness and high response rate of the proposed algorithm.

πShaper – Refractive Beam Shaping Optics for Advanced Laser Technologies

Laser beam shaping brings to various industrial and scientific laser techniques effects that improve their performance comparing to what can be achieved with using Gaussian or Gaussian-like laser beams: more stability, less tough positioning tolerances make the technologies easier to use, higher efficiency of using of costly laser energy, etc. The task of such a transformation is solved by series of refractive beam shaping optics of field mapping type. This solution is important in irradiating the cathode of Free Electron Lasers, confocal microscopy, biomedical fluorescence techniques, many industrial technologies like welding, cladding, hardening, various laser techniques in photovoltaics, homogenizing of pump radiation by building powerful femtosecond lasers, etc. The refractive beam shapers can be used with TEM00 and multimode laser beams, achromatic design provides the same conditions of beam shaping for several lasers of a certain spectrum range simultaneously, low inherent losses allow to use them with powerful laser sources, particular models can be implemented as Galilean Telescope without internal focusing or as Collimators. This paper will describe the principles of operation, design features of the achromatic refractive beam shapers; there will be presented examples of beam intensity transformation and effects on material processing achieved in several industrial applications.

Influence of the statistical and spatio-temporal features of laser pulse on its depolarization in a Kerr medium

Due to nonlinear rotation of polarization, the stochastic intensity and the spatio-temporal inhomogeneity of a multimode laser beam results to its depolarization when propagating in a nonlinear media. The ensemble and spatio-temporal averaging of Stokes parameters was performed analytically for an isotropic media and numerically for an anisotropic one. The experimentally measurable Stokes parameters show monotonic or oscillating decreasing dependence on the propagation distance; the characteristic depolarization length is evaluated. With respect to depolarization the anisotropic media is shown to behave like the isotropic one if the radiation intensity is sufficiently high. The measured value of depolarization agrees with the presented calculations.

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).

On formation of a partially coherent beam in a stable-resonator laser

A new method involving the expansion of the field coherence function in partially coherent modes — the eigensolutions of the problem for the second-order coherence function in a stable resonator — is proposed for the theoretical description of the process of multimode laser beam formation. The method for solving the problem for arbitrary partially coherent modes is formulated and the expressions for these modes are derived in the general form. The characteristics of the fundamental partially coherent mode, which coincides with the coherence function of a Gaussian partially coherent beam, are analysed in detail. The partially coherent modes are shown to possess two spatial scales — the effective radius and the coherence radius, which makes them a convenient tool for solving the problem of generation of a partially coherent beam. It is found that the unambiguous relation between the characteristics of partially coherent modes and the stable-resonator parameters is achieved by involving into consideration not only the process of the beam formation by the resonator mirrors but also the process of interaction of radiation with the active laser medium.

Description, calculation, and analysis of the distortions of multimode laser beams

This paper discusses questions of the description of the distortion of multimode laser beams and introduces laser-radiation-distortion functions. An expression is presented for calculating the amplitude-phase field distribution at the output of an actual laser optical system, and an analytic expression is obtained for its approximate calculation and analysis. This expression is useful at the initial stages of synthesizing laser optical systems with small distortions.

Laser beam profile influence on LIBS analytical capabilities: single vs. multimode beam

Single vs. multimode laser beams have been compared for laser ablation on steel samples. Laser plasma properties and analytical capabilities (precision, limit of detection) were used as key parameters for comparison. Peak fluence at focal spot has been observed to be higher for Gaussian beam despite ∼14-fold lower pulse energy. A comparison of Gaussian and multimode beams with equal energy was carried out in order to estimate influence of beam profile only. Single mode lasing (Gaussian beam) results in better reproducibility of analytical signals compared to multimode lasing while laser energy reproducibility was the same for both cases. Precision improvements were attributed to more stable laser ablation due to better reproducibility of beam profile fluence at laser spot. Plasma temperature and electron density were higher for Gaussian laser beams. Calibration curves were obtained for four elements under study (Cr, Mn, Si, Cu). Two sampling (drilling and scanning procedures) and two optical detection schemes (side-view and optical fiber) were used to compare Gaussian and multimode beam profile influence on analytical capabilities of LIBS. We have found that multimode beam sampling was strongly influenced by surface effects (impurities, defects etc.). For all sampling and detection schemes, better precision was obtained if Gaussian beam was used for sampling. In case of single-spot sampling better limits of detection were achieved for multimode beam. If laser sources have the same wavelength and equal energy then quality of laser beam becomes a crucial parameter which determined laser ablation and analytical capabilities of LIBS.

Gerchberg—Saxton algorithm: experimental realisation and modification for the problem of formation of multimode laser beams

An original method is proposed to calculate the formation of specified far-field intensity distributions by a bimorph mirror in the case of initial transverse-multimode beams. The method is based on the Gerchberg—Saxton algorithm with replacement of the phase function in the plane of the control element by a function that takes into account both the intensity and phase distributions of each mode. The numerical results on the formation of a beam with the third-order super-Gaussian intensity distribution from beams composed of two or four lowest transverse modes are discussed. The experimental results on using the conventional Gerchberg—Saxton algorithm to form a desired intensity distribution from single-mode laser beams using a liquid-crystal modulator are presented.

Passive Laser Beam Combining With Intracavity Interferometric Combiners

In this paper, we present an approach for passive laser beam combining using plane-parallel intracavity interferometric combiners. With this approach, efficient coherent combining of Gaussian laser beams, transverse single high-order-mode laser beams, and even transverse multimode laser beams is possible. We experimentally obtained a combining efficiency of about 90% when coherently combining 16 solid-state laser channels, as well as when coherently combining four fiber laser channels. We also present an arrangement for simultaneous coherent combining and spectral combining, which could possibly overcome current upscaling limits. We present the basic coherent combining approach, review our past and recent investigations and results with both solid-state and fiber laser configurations, and discuss the possible upscaling and future prospects of this approach.

MAPK-activated Protein Kinase-2 (MK2)-mediated Formation and Phosphorylation-regulated Dissociation of the Signal Complex Consisting of p38, MK2, Akt, and Hsp27

The p38 MAPK and heat shock protein 27 (hsp27) form a signaling complex with serine/threonine kinase Akt and MAPK-activated protein kinase-2 (MK2), which plays an important role in controlling stress-induced apoptosis and reorganizing actin cytoskeleton. However, regulation of the complex is poorly understood. In this study, the interaction between p38 and hsp27 was visualized in single living L929 cells using fluorescence resonance energy transfer technology, while their association with Akt was examined by immunoprecipitation analysis. Under normal growth conditions, p38 kinase constitutively interacts with hsp27. When cells were exposed to H(2)O(2) or stimulated by arachidonic acid, this interaction was disrupted. However, inhibition of the activation of p38 and Akt by selective inhibitors or overexpression of the kinase-dead mutant of p38 diminished such effects. Furthermore, mutation of phosphorylation sites of hsp27 renders the interaction resistant to H(2)O(2) and arachidonic acid. It was interesting to find that the interaction disappeared in the cells from MK2-knock-out mice or the cells treated with lemptomycin B that blocks export of MK2 from nucleus to cytosol. However, MK2 is not required for the association of hsp27 with Akt. This study suggests that MK2 mediates the incorporation of p38 into the pre-existing complex of hsp27 with Akt. Phosphorylation of hsp27 finally breaks the signaling complex.

Coherence effects in surface roughness induced by vacuum ultraviolet F_2 laser ablation

We analyze statistical fluctuations in the pulse-to-pulse spatial fluence distribution of a highly multimode F2 laser beam and the influence that coherence effects have on the attainable smoothness of an ablated surface. The magnitude of fluctuations is predicted to be a few percent, with associated roughness increasing as m1/2, where m is the number of ablation pulses. By use of a white-light optical interferometer, measurements have been made of roughness induced on N-BK7 glass surfaces ablated with a 157-nm laser, and reasonable agreement with predictions based on this roughening mechanism has been found.