B.SC.II PAPER-B (OPTICS and LASERS) Submitted by Dr. Sarvpreet Kaur Assistant Professor PGGCG-11, Chandigarh Unit-IV Lasers and Fiber optics LASERS History of the LASER Invented in 1958 by Charles Townes (Nobel prize

in Physics 1964) and Arthur Schawlow of Bell Laboratories Was based on Einsteins idea of the particlewave duality of light, more than 30 years earlier Originally called MASER (m = microwave) Laser: everywhere in your life Laser printer Laser pointer What is Laser? Light Amplification by Stimulated

Emission of Radiation A device produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher energy levels When they return to lower energy levels by stimulated emission, they emit energy. Properties of Laser The light emitted from a laser is monochromatic, that is, it is of one color/ wavelength. In contrast, ordinary white light is a combination of many colors (or wavelengths) of light.

Lasers emit light that is highly directional, that is, laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as from a light bulb, is emitted in many directions away from the source. The light from a laser is said to be coherent, which means that the wavelengths of the laser light are in phase in space and time. Ordinary light can be a mixture of many wavelengths. These three properties of laser light are what can make it more hazardous than ordinary light. Laser light can deposit a lot of energy within a small area.

6 Monochromacity Nearly monochromatic light Example: He-Ne Laser 0 = 632.5 nm = 0.2 nm Diode Laser 0 = 900 nm = 10 nm Comparison of the wavelengths of red and blue light

Directionality Conventional light source Divergence angle (d) Beam divergence: d= /D ~ 1 = f(type of light amplitude distribution, definition of beam diameter) = wavelength D = beam diameter Coherence Incoherent light waves Coherent light waves

Incandescent vs. Laser Light 1. Many wavelengths 1. Monochromatic 2. Multidirectional

2. Directional 3. Incoherent 3. Coherent 10 Basic concepts for a laser Absorption

Spontaneous Emission Stimulated Emission Population inversion Absorption Energy is absorbed by an atom, the electrons are excited into vacant energy shells. Spontaneous Emission The atom decays from level 2 to level 1 through the emission of a photon with the energy hv. It is a completely random process.

Stimulated Emission atoms in an upper energy level can be triggered or stimulated in phase by an incoming photon of a specific energy. Stimulated Emission The stimulated photons have unique properties: In phase with the incident photon Same wavelength as the incident photon Travel in same direction as incident photon Population Inversion A state in which a substance has been energized, or excited to specific energy levels.

More atoms or molecules are in a higher excited state. The process of producing a population inversion is called pumping. Examples: by lamps of appropriate intensity by electrical discharge Pumping Optical: flashlamps and high-energy light sources Electrical: application of a potential difference across the laser medium Semiconductor: movement of electrons in junctions, between holes

Two level system E2 h h=E2-E1 E2 h h E1

absorption E1 Spontaneous emission Stimulated emission Boltzmanns equation E2 n2 ( E2 E1 )

exp n1 kT n1 - the number of electrons of energy E1 n2 - the number of electrons of energy E2 Population inversion- n2>>n1 E1 example: T=3000 K eV

E2-E1=2.0 n2 4 4.4 10 n1 Resonance Cavities and Longitudinal Modes Since the wavelengths involved with lasers and masers spread over small ranges, and are also absolutely small, most cavities will achieve lengthwise resonance

L = n Plane c f parallel resonator Concentric resonator Confocal resonator c: center of curvature, f: focal point f

Hemifocal resonator c Hemispheric al resonator Unstable resonator Transverse Modes Due to boundary conditions and quantum mechanical wave

equations TEM00: I(r) = (2P/dd2)*exp(-2r2/d2) (d is spot size measured to the 1/e2 points) Einsteins coefficients E2 Probability of stimulated absorption R1-2 R1-2 = () B1-2 E1

Probability of stimulated and spontaneous emission : R2-1 = () B2-1 + A2-1 assumption: n1 atoms of energy 1 and n2 atoms of energy 2 are in thermal equilibrium at temperature T with the radiation of spectral density (): n1 R1-2 = n2 R2-1 n1 () B1-2 = n2 ( () B2-1 + A2-1) A2 1 / B2 1 = n1 B1 2

1 n2 B2 1 According to Boltzman statistics: n1 exp( E2 E1 ) / kT exp(h / kT ) n2 A2 1 / B2 1 () = B1 2 h

exp( ) 1 B2 1 kT 8h 3 / c 3 = exp(h / kT ) 1 Plancks law B1-2/B2-1 = 1 A2 1 8h 3 3 B2 1

c The probability of spontaneous emission A2-1 stimulated emission B2-1(: /the probability of A2 1 exp(h / kT ) 1 B2 1 ( ) 1. Visible photons, energy: 1.6eV 3.1eV. 2.

kT at 300K ~ 0.025eV. 3. stimulated emission dominates solely when h/kT <<1! (for microwaves: h <0.0015eV) The frequency of emission acts to the absorption: x if h/kT <<1. n2 A2 1 n2 B2 1 ( ) A2 1 n2 n2 [1 ]

n1B1 2 ( ) B2 1 ( ) n1 n1 x~ n2/n1 Condition for the laser operation E2 E1 If n1 > n2 radiation is mostly absorbed absorbowane spontaneous radiation dominates. if n2 >> n1 - population inversion

most atoms occupy level E2, weak absorption stimulated emission prevails light is amplified Necessary condition: population inversion How to realize the population inversion? Thermal excitation: E2 n2 E

exp n1 kT impossible. The system has to be pumped Optically, electrically. E1