Laser (Light Amplification by Stimulated Emission of Radiation) BY SUMITESH MAJUMDER

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 Mainly used in Single Mode Systems Light Emission range: 5 to 10 degrees Require Higher complex driver circuitry than LEDs Laser action occurs from three main processes: photon absorption, spontaneous emission, and stimulated emission.

Properties of Laser Monochromatic Concentrate in a narrow range of wavelengths (one specific colour). Coherent All the emitted photons bear a constant phase relationship with each other in both time and phase Directional A very tight beam which is very strong and concentrated.

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

Stimulated Emission

Laser Diode Optical Cavity One reflecting mirror is at one end while the other end has a partially reflecting mirror for partial emission Remaining power reflects through cavity for amplification of certain wavelengths, a process known as optical feedback. Construction very similar to the ELEDs.

Mirror Reflections

The operation of the Laser

The operation of the Laser (Pumping the Laser)

The operation of the Laser absorption

The operation of the Laser Spontaneous emission

The operation of the Laser Spontaneous emission Incoherent light Accidental direction

The operation of the Laser

The operation of the Laser Stimulated emission

The operation of the Laser Light: Coherent, polarized The stimulating and emitted photons have the same: frequency phase direction

How a Laser Works

Necessary condition: population inversion Condition for the laser operation 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

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

n1 - the number of electrons of energy E1 Boltzmann’s equation E1 E2 n1 - the number of electrons of energy E1 n2 - the number of electrons of energy E2 example: T=3000 K E2-E1=2.0 eV

E2 E1 Einstein’s relation Probability of stimulated absorption R1-2   Probability of stimulated absorption R1-2 R1-2 = r (n)n1 B1-2 where spectral density is r (n) & Einstein coeff. of absorbtion is B1-2 Probability of stimulated and spontaneous emission : R2-1 = r (n) n2B2-1 + n2A2-1  Einstein coeff. of stimulated and spontaneous emission are B2-1& A2-1 Assumption : For a system in thermal equilibrium, the upword and downword transition rate must be equal : R1-2 = R2-1 n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)

According to Boltzman statistics:   r (n) = =   Planck’s law B1-2/B2-1 = 1

The probability of spontaneous emission A2-1 /the probability of stimulated emission B2-1r(n ):    Visible photons, energy: 1.6eV – 3.1eV. kT at 300K ~ 0.025eV. stimulated emission dominates solely when hn /kT <<1! (for microwaves: hn <0.0015eV)   The frequency of emission acts to the absorption: if hn /kT <<1.     x~ n2/n1

Two-level Laser System Unimaginable as absorption and stimulated processes neutralize one another. The material becomes transparent.

Two level system hn hn =E2-E1 Spontaneous emission E2 E1 absorption Stimulated emission hn E1 E2 hn =E2-E1

Three-level Laser System Initially excited to a short-lived high-energy state . Then quickly decay to the intermediate metastable level. Population inversion is created between lower ground state and a higher-energy metastable state.

Three-level Laser System Nd:YAG laser He-Ne laser

Four-level Laser System Laser transition takes place between the third and second excited states. Rapid depopulation of the lower laser level.

Four-level Laser System Ruby laser

Multimode Laser Output Spectrum (Center Wavelength) Mode Separation g(λ) Longitudinal Modes

Lasing Characteristics Lasing threshold is minimum current that must occur for stimulated emission Any current produced below threshold will result in spontaneous emission only At currents below threshold LDs operate as ELEDs LDs need more current to operate and more current means more complex drive circuitry with higher heat dissipation Laser diodes are much more temperature sensitive than LEDs

Fabry-Perot Laser (resonator) cavity

Modulation of Optical Sources Optical sources can be modulated either directly or externally. Direct modulation is done by modulating the driving current according to the message signal (digital or analog) In external modulation, the laser is emits continuous wave (CW) light and the modulation is done in the fiber

Types of Optical Modulation Direct modulation is done by superimposing the modulating (message) signal on the driving current External modulation, is done after the light is generated; the laser is driven by a dc current and the modulation is done after that separately Both these schemes can be done with either digital or analog modulating signals

Direct Modulation The message signal (ac) is superimposed on the bias current (dc) which modulates the laser Robust and simple, hence widely used Issues: laser resonance frequency, chirp, turn on delay, clipping and laser nonlinearity

Laser Construction A pump source A gain medium or laser medium. Mirrors forming an optical resonator.

Laser Construction

Pump Source Provides energy to the laser system Examples: electrical discharges, flashlamps, arc lamps and chemical reactions. The type of pump source used depends on the gain medium. →A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture. →Excimer lasers use a chemical reaction.

gain medium Major determining factor of the wavelength of operation of the laser. Excited by the pump source to produce a population inversion. Where spontaneous and stimulated emission of photons takes place. Example: solid, liquid, gas and semiconductor.

Optical Resonator Two parallel mirrors placed around the gain medium. Light is reflected by the mirrors back into the medium and is amplified . The design and alignment of the mirrors with respect to the medium is crucial. Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output.

Laser Types According to the active material: solid-state, liquid, gas, excimer or semiconductor lasers. According to the wavelength: infra-red, visible, ultra-violet (UV) or x-ray lasers.

Solid-state Laser Example: Ruby Laser Operation wavelength: 694.3 nm (IR) 3 level system: absorbs green/blue Gain Medium: crystal of aluminum oxide (Al2O3) with small part of atoms of aluminum is replaced with Cr3+ ions. Pump source: flash lamp The ends of ruby rod serve as laser mirrors.

Ruby laser Al2O3 Cr+ Energy rapid decay LASING

How a laser works?

Ruby laser First laser: Ted Maiman Hughes Research Labs 1960

Liquid Laser Example: dye laser Gain medium: complex organic dyes, such as rhodamine 6G, in liquid solution or suspension. Pump source: other lasers or flashlamp. Can be used for a wide range of wavelengths as the tuning range of the laser depends on the exact dye used. Suitable for tunable lasers.

dye laser Schematic diagram of a dye laser A dye laser can be considered to be basically a four-level system. The energy absorbed by the dye creates a population inversion, moving the electrons into an excited state.

Gas Laser Example: Helium-neon laser (He-Ne laser) Operation wavelength: 632.8 nm Pump source: electrical discharge Gain medium : ratio 5:1 mixture of helium and neon gases

He-Ne laser

Semiconductor laser Semiconductor laser is a laser in which semiconductor serves as photon source. Semiconductors (typically direct band-gap semiconductors) can be used as small, highly efficient photon sources.

Applications of laser 1. Scientific a. Spectroscopy b. Lunar laser ranging c. Photochemistry d. Laser cooling e. Nuclear fusion

Applications of laser 2 Military a. Death ray b. Defensive applications c. Strategic defense initiative d. Laser sight e. Illuminator f. Rangefinder g. Target designator

Applications of laser 3. Medical a. eye surgery b. cosmetic surgery

Applications of laser 4. Industry & Commercial a. cutting, welding, marking b. CD player, DVD player c. Laser printers, laser pointers d. Photolithography e. Laser light display