Principles of Lasers. Cheng Wang. Phone: Office: SEM 318

Transcription

1 Principles of Lasers Cheng Wang Phone: Office: SEM 318

2 The course 2 4 credits, 64 credit hours, 16 weeks, 32 lectures 70% exame, 30% project including lab Reference: O. Svelto, Principles of Lasers, Springer, 2010 (main) B. Saleh and M. Teich, Fundamentals of Photonics, Wiley, 2007 William T. Silfvast, Laser Fundamentals, Cambridge, 2004

3 Electromagnetic spectrum 3

4 Ref: Wikipedia Electromagnetic spectrum 4

5 Photons 5 A photon is an elementary particle, the quantum of all forms of electromagnetic radiation including light. 1. Photons exhibit wave-particle duality, both waves and particles 2. Photon has zero rest mass 3. Photon energy E hv hc / p k k 2 / 4. Photon momentum wave number 5. Photon has 2 possible polarization states 6. Photons obey the Bose-Einstein statistics, rather than the Fermi Dirac statistics (electrons, neutron, proton>> Pauli exclusion principle: two identical fermions cannot occupy the same quantum sate simultaneously)

6 6 LASER: Light Amplification of Stimulated Emission of Radiation

7 Lasers in our life 7

8 Approaches 8 Approach Classical Appr. thoery Semi-classical Appr. theory Quantum Quantum Appr. theory (Quantum Electrodynamics) Matter Light Classical, Newtonian mechanics Classical, Maxwell s equations Quantized, Quantum mechanics Classical, Maxwell s equations Complexity Simple middle complex Quantized, Quantum mechanics Quantized, Quantum field thoery

9 Chapter 1 Introduction_L1 9 Laser history Laser concept Laser properties

10 Discovery of stimulated emission in Ref: S. Domsch, Basics of Laser Physics, at Univeritaetsmedizin Mannheim

11 Maser in 1950s 11 Charles Hard Townes Jim Gordon

12 First Laser in 1960 (Ruby) nm

13 Nobel prize in physics in

14 Light intensity Lasers and LEDs on p n n junctions January 1962: observations of superlumenscences in GaAs p-n junctions (Ioffe Institute, USSR). 14 Sept.-Dec. 1962: laser action in GaAs and GaAsP p-n junctions (General Electric, IBM (USA); Lebedev Institute (USSR). Wavelength Cleaved mirror p n + GaAs E n F Eg L p D L n D h E p F Condition of optical gain: Ref: Z. Alferov, Semiconductor Revolution in the 20th Century, St Petersburg Academic University E n F Ep F > E g 1

15 The Nobel Prize in Physics 2000 "for basic work on information and communication technology" 15 for developing semiconductor heterostructures used in high-speed- and opto-electronics for his part in the invention of the integrated circuit Zhores I. Alferov b Herbert Kroemer b Jack S. Kilby

16 Laser-related Nobel prizes in Physics 16

17 Laser-related Nobel prizes in Physics 17

18 Laser-related Nobel prizes in Physics 18 Charles H. Townes, How the Laser Happened, Oxford, 1999

19 International Year of Light 19 In proclaiming an International Year focusing on the topic of light science and its applications, the UN has recognized the importance of raising global awareness about how light-based technologies promote sustainable development and provide solutions to global challenges in energy, education, agriculture and health. Light plays a vital role in our daily lives and is an imperative cross-cutting discipline of science in the 21st century. It has revolutionized medicine, opened up international communication via the Internet, and continues to be central to linking cultural, economic and political aspects of the global society.

20 Chapter 1 Introduction_L1 20 Laser history Laser concept Laser properties

21 Absorption of light 21 When light passes through materials it is usually absorbed. In certain circumstances light may be amplified. This was called gain (negative absorption) It is the basis of laser action

22 Interaction of light and an atom 22 Absorption Spontaneous emission E2 E2 hv hv hv=e2-e1 E1 hv=e2-e1 E1 Stimulated emission hv hv=e2-e1 E2 E1 Stimulated emission produces photons in the same phase and direction, different to spontaneous emission Non-radiative decay

23 Probability of the processes 23 Spontaneous emission dn dt 2 2 sp N sp Non-radiative decay (no photon) dn dt 2 2 nr N Stimulated emission dn dt 2 st Absorption dn dt 1 a nr F N 21 s 2 F N 12 s 1 1 A sp W W s F F s Rates relation Cross section relation N g X g : Carrier population (number) F : Photon flux (number) g W g s sp nr X g W : Spontaneous emission lifetime : Nonradiative decay lifetime : Cross section of stimulated emission : Cross section of absorption : Degeneracy of the energy level

24 t/ N2 t N sp 2 e ( ) (0) sp Population vs. time 24 N(0) Population N 2 N(0)e -1 sp Time t Carrier decay due to spontaneous emission

25 Boltzman distribution 25 Thermal equilibrium: A system is said to be in thermal equilibrium if the temperature within the system is spatially and temporally uniform (constant), where the motion of atoms reach a steady state, and the atom fluctuations are, on average, invariant to time. Nm P( Em) exp( Em / kt ) N N2 g2 E2 E1 exp N g kt 1 1 Under thermal equilibrium g2 N2 N1 g 1 Population inversion (non-equilibrium) g2 N2 N1 g 1 kt = 25.7 K Ref: B. Saleh and M. Teich, Fundamentals of Photonics, Wiley, 2007

26 Photon Fs Amplification or absorption of light 26 Photon generation dfs 21FS N2 12FS N1dz g F N N dz 2 21 S 2 1 g1 F S dz F S +df S Gain coefficient: the material capability of amplifing light 1 dfs g2 g 21 N2 N 1 Fs dz g1 F ( z) F (0) e gz S S g 0 Amplifier g 0 Absorber Length z

27 The way to laser 27 Population inversion Active material, gain medium Amplifier Oscillator Laser (Maser) Loss coefficient i 1 dfs F s dz 2( gi ) L FS(2 L) FS(0) e R1R 2 Threshold condition F (2 L) F (0) S e 2( g ) L i S g ln( R R ) / 2L th i R R 1 2 Length L Laser components: Pump Active medium Resonant Cavity (mirrors) Mirror R1 Active medium Mirror R2 Pump source

28 Pump 28 Pump is the process to lift atoms from a low state to a high state. It can be realized by intense light source or electrical source. Two-level system is impossible to lase. (Best is N 2 =N 1 ) Two-level system Three-level system can lase due to the long lifetime of level 2, but still needs strong pump. (Threshold N 2 =N 1, Ruby laser, pulsed) Four-level system can lase easily, due to the quasi-empty level 2. Three-level laser system Four-level laser system

29 Chapter 1 Introduction_L1 29 Laser history Laser concept Laser properties

30 Laser fundamental properties 30 Monochromaticity Coherence (phase correlation) Directionality Brightness 1. Monochromaticity--- optical linewidth vol 2. Temporal coherence --- coherence length L c L c c / v c c ol 3. Spatial coherence --- coherence area D c 4. Directionality--- beam divergence D c Laser Difraction limit d D ~ 10 rad (0.57 ~ ) D is the mirror diameter 1.22 for plane wave

31 Note: Wave optics 31 In the framework of wave optics, Wavefront is the collection of points characterized by propagation of position of the same phase: a propagation of a line in 1d, a curve in 2d or a surface for a wave in 3d. The wavefronts of a plane wave are planes. A lens can be used to change the shape of wavefronts. Here, plane wavefronts become spherical after going through the lens. The wavefronts of a spherical wave are planes.

32 Note: Huygens principle 32 Huygens principle: Each point at the wavefront becomes a source for the secondary spherical wave. At any subsequent time, the wavefront can be determined by the sum of these secondary waves.

33 Brightness of laser 33 Brightness: The light power per unit projected surface area of the light source per unit solid angle. B ds dp cos d unit: W/ 2 cm sr Normal to the emission surface B P S 2 2 R sin P P D /2 2 Laser D Difraction limit Bd 2 2 P

34 Ref: A. V. Arecchin, T. Messadi; R. J. Koshe, Field Guide to Illuminatin, SPIE, 2007 Note: Solid angle 34

35 Types of lasers 35 Physical state: solid state, liquid, and gas lasers. (free electron lasers) Wavelength: infrared, visible, UV, and X-ray lasers. (1 mm---1 nm) Power: CW laser from nw to a few MW; Pulsed laser peak power up to PW (10 15 W) Pulse duration: from ms down to fs (10-15 s) Cavity length: from nm up to km

36 Homework 36 Page 15: