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
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.
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:
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= /
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 ).