R.6.2 Lasery Optoelektronika 2014 Dr hab. inż. B. B. Kosmowski 1

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1. Active material: -Ions in crystals (eg. Fe 2+, Dy 2+, Cr 3+, Ti 3+, Nd 3+, Pr 3+, Dy 3+, Ho 3+, Er 3+, Tm 3+, Yb 3+ ), -Atoms (e.g. Cu, Ne, I), -Ions (e.g. Cd +, Ar + ), -Molecules (e.g. ArF, CO, CO 2 ), -Gases, -Dye molecules in solutions or solids, -Electron-hol pairs in semiconductor material, -Multi-ionized atoms in plasma. 2. Pumping energy source: Pumping requires: - The light from a flash-lamp, - Radiation from another laser, - Electrical discharge, - Chemical reaction, - Electric current to cause electron collisions. 3. Optical (open) resonator – creates optical feedback mechanism essential for formation of laser oscillations inside the resonator: - system of two (or more!) mirrors or other reflecting optical elements (prisms). 9

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17 Laser classification: Active medium lasers can be classified into five groups: solid state, semiconductor, gas, liquid, and plasma lasers, Wavelengths generated by lasers can be divided into: infrared, visible, ultraviolet, and X-ray lasers, Energy levels involved in stimulated emission lasers are distinguished as: electron, ion, and molecular (rotational, vibrational, rotational-vibrational) bands in semiconductor lasers, Pumping methods: the systems are divided into lasers pumped optically, by electric discharge, by electron beam, by expansion of compressed gas, by chemical reaction, by recombination, etc., Time development of the radiation: lasers can be divided into continuous (cw) lasres, pulsed, and quasi-continuous, Regime of operation: lasers can be free-running, Q-switched, and mode- locked, Safety precautions: lasers are divided into four categories.

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19 Properties of laser radiation:  Monochromaticity,M=  /  Directionality - kolimacja, gęstość strumienia  Coherence - droga spójności  Brightness - Luminancja P – moc / strumień, A – powierzchnia,  - kąt bryłowy

20 Laser radiation parametrs

21 Laser radiation parameters -Continuos (cw), -Pulsed, -Free-running, Q-switched, mode-locked.

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24 -free-running regime, -Q-switched regime, -Mode-locked regime -Time characteristic is a significant parameter of the generated output radiation, power of the radiation. The interacting time scale can vary by 15 orders of magnitude. Laser parameters:

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28 Energy and power of Laser radiation Peak power Average power Fluence or energy density Intensity, irradiance, or power density Radiation dose or radiant exposure Radiant intensity

29 Spatial structure and divergence of the laser beam Beam diameter is the width at which the beam intensity has fallen to 1/e 2 (13,5%) of its peak value.

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32 The spot radius at a distance z from the beam waist is: The near field and the far field: The Rayleigh range (legth) is defined as: The beam quality factor or beam propagation factor, M 2, describes how far the real laser beam is from a so-called „perfect Gaussian” one. Non-perfect beam, the value of M 2 > 1

33 Beam parameter product (BPP) (mm x mrad): -The product of beam radius (measured at the beam waist) and the beam divergence half-angle (measured in the far field), -The smallest possibile BPP – Gaussian beam. In this case BPP= / 

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36 Laser beam focusing 2w o ’ 2w o ’ – średnica w ognisku, F – liczba soczewki D – średnica promienia f - ogniskowa soczewki

Wavelength (μm), Wave number k (cm -1 ) k=1 /, Pulse width (duration) , Repetition frequency (repetition rate) f rep (Hz), Pulse energy E(J), Peak power P peak (W) (pulsed laser or quasi-cw laser)- generated energy E(J) per length of the pulse  (s): P peak = E / , Average or mean power P(W) (pulsed laser or quasi-cw laser)- generated single pulse energy E(J) multiplied by repetition frequency f rep (Hz): P ave = E · f rep, 37 Summary:

38 Power (cw laser): P(W) – generated output energy per second, Beam spot size A(cm 2 ), refers to the area of the laser beam on the target (A=  w 2 ), Energy density (fluence or energy dose) F(J/cm 2 ) – the amount of energy E(J) delivered to the treated area A(cm 2 ) F=E/A, Intensity, irradiance, power density I (W/cm 2 ) – power P(W) per irradiated area A(cm 2 ) I = P / A, Laser beam divergence angle  (rad) – is defined by beam quality factor M 2, generated wavelength (cm), and a spot radius at the beam waist w o (cm)  = M 2 / (  w o ).