15 & 16 Flashcards
Laser operation is a 3-state system.
What are these states?
Excited state
Ground state
Metastable state
What is metastable state?
Excited state with long lifetime
What is population inversion?
A larger population of excited is in the excited state than ground state
=> attenuation coefficient < 0
=> more light comes out of the material than go in.
(N1 < N2
=> Occupancy levels of state E1 is smaller than that of E2)
What is optical pumping?
energy input from an external source (electrical, optical, chemical energy) with appropriate frequency
(a process in which light is used to raise (or “pump”) electrons from a lower energy level in an atom or molecule to a higher one.)
Describe induced emission
A coherent process that induced radiation is emitted in the same direction with identical phase, polarization, frequency to inducing radiation.
Relaxation of electrons from metastable state
Describe spontaneous emission
The incoherent process that the probability of spontaneous light emission from excited state and metastable state is low
3 things that a laser oscillator require
(1) Pumping
(2) Positive feedback
(3) Resonator
How does an optical resonator work?
(1) Inversion is achieved by pumping
(2) Light waves from spontaneous emission pass through laser material
=> these waves are amplified by stimulated emission
(3) Waves which are parallel with optical axis with mirror (optical resonator) get back to laser material after reflection with mirror
=> they are then amplified
(Intensity of light grows until saturation value)
4 properties of laser light
(1) Monochromatic
(2) Coherent
(3) Polarized
(4) Parallel beam (small divergence)
Why is laser radiation monochromatic (single-colored)?
Narrow spectral width
Why is laser radiation coherent? (light generated by stimulated emission)
It is phase identity with 2 types (able to observe interference pattern)
- temporal coherence - phase identity of photons emitted at different times
- spatial coherence (phase identity across beam diameter)
Why is laser radiation polarized? (light generated by stimulated emission)
Electron field vector oscillates in single plan
Why does laser radiation have high power? (light generated by stimulated emission)
It has large spatial power density
What are 4 types of lasers
(1) Solid state lasers
(2) Gas lasers
(3) Dye lasers
(4) Semiconductor (diode) lasers
Describe solid state lasers
It contains metal dopoing crystals (Ruby,…)