The Laser

Einsteins realisation?

that from one photon two is gotten so we can amplify light.

L

is distance between mirrors, m is integer.

First step for Laser creation

Absorption: When electrons in the ground state absorb a photon with certain energy then it can move to a higher state, if the energy difference between ground and excited state is same as photons energy.

Spontaneous emission

The electron goes back to the ground state after some time and emits a photon of defined energy.

Population inversion

To get a high intensity light we need to create a lot of numerous excited electrons in a material then invest energy from outside into the material (laser pumping)

Stimulated emission

Einstein that we could induce emission, when a photon with energy that satisfies Bohrs condition for electrons in excited state is emitted by electron in excited state.

LASER

Light amplification by stimulated emission of radiation

Example of three level system

Ruby laser, the medium is chromium doped synthetic sapphire

What do we need for Laser oscillators?

gain medium, intensive electron excitation, positive feedback and optical resonator.

Resonance condition?

2L = m(wavelength)

Explain Monochromatic attribute of Laser

narrow bandwidth

Coherence of Lasers?

create interference even in big path differences.

How is laser Collimated

Has a narrow beam with little divergence

What is Power density for laser lights

has higher power density (E/At) than ordinary light

How is laser polarized?

ultrashort pulses

Pulse length of Q-switched lasers

100 - 200ns

Example of 4 level system

He-Ne laser

Classification of lasers

By gain medium, by operation and by energy

Types of lasers by operation

Pulse lasers and continuous wave

Comparison of pulse lasers and continuous wave

Pulse lasers produce series of pulses at certain width and frequency until stopped

Continuous wave releases constant output over interval of seconds

Medical application of Lasers

Surgical operations, retinal laser surgery, gene surgery

Metastable state

Short lived high energy state that spontaneously drops to a low energy state with long lifespan

How to achieve laser pumping

Thermal excitation, optical excitation and electric discharge