Introduction to Lasers 자연과학부 나종훈

목 차 LASER 의 시초 Atomic Structure Transitions between Laser states Population Inversion Pulsed Operation Power and Energy The elements for Lasers Transverse & Longitudinal Modes

The Beginning of LASER Microwave Amplification by the Stimulated Emission of Radiation -1954, 24-GHz ammonia device Light Amplification by Stimulated Emission of Radiation -1957, First idea of LASER(using Fabary Perot cavity as part of LASER)

ATOMIC STRUCTURE

Spontaneous · Stimulated Emission E=hν=E 2 -E 1 h: Planck’s constant ν: frequency of the laser light Ideal laser light are at exactly the same frequency (wavelength) & all the photons are in phase.

Population Inversion N 2 /N 1 =Exp-(E 2 -E 1 )k B T Population ratio between two states N2: population in the upper state N1: population in the lower state kB: Boltzman’s constant T: temperature (K) Gain 달성 조건 : Population in the upper laser state is greater than the population in the lower state. A pulse of light, an electrical spark or a chemical reaction can all be used to populate the upper laser state. → pumping

Pulsed Operation Q-swicthing To concentrate the laser energy into the pulse Mode locking To shorten the width of the pulse in time Effective for reducing the laser’s heating (cf: semiconductor diode lasers)

Power and Energy Average Power : The power of a continuous wave laser P avg =E pulse *R reprate Peak power : E pulse /t pulse Average energy density : The energy(J) per unit area Average power density : The energy power per unit area Peak power density : The peak power per unit area

Monochromaticity & Coherency Monochromaticity -A group of photons at exactly one frequency Coherency - A group of photons with the same relative phase

The elements for Lasers Gain Material solid(Nd:YAG,Ruby,GGG,GSGG,alexandrite,emerald,Cr:Sapphi re,Ti:Sapphire,AlGaAs) Liquid(dye,chelate) Gas(Krypton,argon,nitrogen,helium-neon,CO2,KrF,XeCl) Plasma(X-ray,free-electron) Pumping source To provide the energy to set up the energy states so that stimulated emission can occur optically to be pumped using other lasers(most solid-state lasers) electrically to be pumped using a pn-junction(semiconductor diodes) Resonant cavity physically shorten the laser tailor the profile of the electromagnetic mode

Transverse & Longitudinal Modes TEM(transverse electromagnetic mode) The frequencies of Longitudinal electromagnetic modes of the cavity(v=p*c 0 /2nL) A laser can only lase at those wavelengths(longitudinal mode) for which an integral multiple of wavelengths fit into the cavity.