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1

Performance of artificial neural networks in an inverse problem of laser beam diagnostics

Pietrak Karol, Muszyński Radosław, Marek Adam, Łapka Piotr

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2022

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Vol. 70, nr 1

art. no. e140100

EN

The presented results are for the numerical verification of a method devised to identify an unknown spatio-temporal distribution of heat flux that occurs at the surface of a thin aluminum plate, as a result of pulsed laser beam excitation. The presented identification of boundary heat flux function is a part of the newly proposed laser beam profiling method and utilizes artificial neural networks trained on temperature distributions generated with the ANSYS Fluent solver. The paper focuses on the selection of the most effective neural network hyperparameters and compares the results of neural network identification with the Levenberg–Marquardt method used earlier and discussed in previous articles. For the levels of noise measured in physical experiments (0.25–0.5 K), the accuracy of the current parameter estimation method is between 5 and 10%. Design changes that may increase its accuracy are thoroughly discussed.

2

Wigner transform approach to dynamic-variable partially coherent laser beam characterization

Jabczyński J., Gontar P., Gorajek Ł.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2020

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Vol. 68, nr 1

141--146

EN

1) Background: the modeling, characterization, transformation and propagation of high-power CW laser beams in optical (including fiberoptic) trains and in the atmosphere have become hot topics in laser science and engineering in the past few years. Single-mode output is mandatory for high-power CW laser applications in the military field. Moreover, an unstationary, dynamic operation regime is typical. Recognized devices and procedures for laser-beam diagnostics could not be directly applied because of dynamic behavior and untypical non-Gaussian profiles. 2) Methods: the Wigner transform approach was proposed to characterize dynamically variable high-power CW laser beams with significant deterministic aberrations. Wavefront-sensing measurements by means of the Shack-Hartmann method and decomposition into an orthogonal Zernike basis were applied. 3) Results: deterministic aberration as a result of unstationary thermal-optic effects depending on the averaged power of the laser output was found. Beam quality determined via the Wigner approach was changed in the same way as the measurements of the beam diameter in the far field. 4) Conclusions: such an aberration component seems to be the main factor causing degradation in beam quality and in brightness of high-power CW laser beams.

3

Application of Wigner transform for characterization of aberrated laser beams

Jaguś J., Jabczyński J. K., Żendzian W., Kwiatkowki J.

Optica Applicata

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2005

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Vol. 35, nr 1

33-41

EN

The slit scan method was implemented for registration of intensity profiles along the caustics of a laser beam. The inverse Radon transform of spatialy normalized intensity profiles enables direct computation of Wigner transform of real laser beam. The Rayleigh range, divergence angle, beam quality factor, global degree of coherence can be determined in such a simple way. A procedure enabling derivation of the shape of aberrated wavefornt and spherical aberration content was elaborated. This method was applied for investigation of the aberrated laser beams generated by cw and pulsed diode pumped laser.

4

Polarization bistability in a smectic-A liquid crystal.

Pura B., Wierzbicki M., Petykiewicz J., Ratusznik G.J., Wrzesiński Z.

Optica Applicata

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2001

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Vol. 31, nr 3

619-624

EN

We report the measurements the of laser light polarization transmitted through the smectic-A phase of a liquid crystal placed inside the Fabry-Perot optical resonator. All four parameters of polarization were measured simultaneously by an ellipsometer of a special construction. Optical polarization bistability phenomenon was observed, in which polarization parameters of outgoing light depend nonlinearly on the intensity of the input beam. We propose a theoretical explanation of this phenomenon, based on the concept of orientational nonlinearity of liquid crystal's molecules

5

High-performance 980-nm strained-layer InGaAs/GaAs quantum-well lasers

Bugajski M., Regiński K., Mroziewicz B., Kubica J.M., Sajewicz P., Piwoński T., Zbroszczyk M.

Optica Applicata

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2001

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Vol. 31, nr 2

267-271

EN

Reports fabrication of strained-layer InGaAs/GaAs separate-confinement-heterostructure single-quantum-well (SCH SQW) lasers operating in the wavelength range of 980 nm. Design process of the devices involved simulation of their above-threshold operation including all relevant physical phenomena. The lasers were characterized at room temperature in the pulsed operation regime at frequency v=5 kHz and pulse length tau =200 ns. Threshold current densities of the order J/sub th/=280 A/cm/sup 2/ and differential efficiency eta =0.40 W/A were obtained for devices with cavities of 700 mu m in length and broad contacts of 100 mu m in width

6

Focusing large-aperture beams generated by high-peak-power lasers

Jabłoński S., Badziak J.

Optica Applicata

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2001

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Vol. 31, nr 3

625-634

EN

The properties of focusing large-aperture fight beams of parameters typical of the beams produced by picosecond terawatt lasers are analysed numerically with the use of 3-D computer code based on Fresnel-Kirchhoff integral formula. It is shown that, contrary to the case of small-aperture Gaussian-like beams, a peak intensity of light in the focus is a non-monotonic function of f/D with a local maximum at moderate f/D values (5 [left angle bracket]or= f/D [left angle bracket]or= 10), where f is the focal length and D is the aperture of a focusing system. For low f/D values (f/D [left angle bracket] 3) more than 80% of laser energy is scattered after focusing in a low-intensity large-dimension aureole round the central peak of focal intensity distribution. The amount of the scattered energy can be significantly decreased by an increase of f/D, and, as a result, the moderate values of this ratio seem to be optimum for most laser-target experiments. The above features appear for both spherical and aspherical focusing systems.

7

Design considerations for GaAs/(AlGa)As SCH and GRIN-SCH quantum-well laser structures. II. The results

Czyszanowski T., Wasiak M., Nakwaski W.

Optica Applicata

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2001

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Vol. 31, nr 2

325-336

EN

A detailed optical model of complex multi-layered structures of the separate-confinement-heterostructure (SCH) lasers as well as graded-index (GRIN) SCH lasers presented in the first part of the paper is used to discuss some of the possible modifications of their structure to reduce room-temperature thresholds. Recommended design parameters have been found for each structure. Surprisingly, performance of relatively simple SCH lasers is found to be at least comparable with that of much more complex GRIN-SCH lasers.

8

Laser induced diffusion in Ge/Sb-Si films

Szyszko W.

Nukleonika

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1999

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Vol. 44, nr 2

381-392

EN

Melting and solidification of the Ge-Sb multilayer system evaporated onto Si induced by the UV pulsed laser beam are studied together with the diffusion mass transfer. The chemical composition, determined from the numerical simulation of the heat and mass transfer based on Finite Element Method, is compared to the experimental results. The interference effect of the laser beam was also included into the mathematical model of the laser beam interaction with the irradiated target.

PL

W pracy prezentowane są badania procesów zachodzących pod wpływem wiązek laserowych w cienkich, wielowarstwowych strukturach Ge/Sb napylanych na podłoże krzemowe. Redystrybucję oraz końcowy rozkład koncentracji składników tarczy, określany na podstawie symulacji komputerowych rozwiązując numerycznie równanie dyfuzji ciepła i transportu masy w naświetlanych obszarach, porównywano z wynikami doświadczalnymi. W programie komputerowym symulującym zjawiska zachodzące w tarczy pod wpływem zaabsorbowanej energii wiązki laserowej wykorzystano metodę Elementów Skończonych. W modelu matematycznym uwzględniono także zjawisko interferencji światła w cienkich warstwach.