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,…)
4 types of non-radiative processes (absorption of light)
(1) Photochemical reactions
(2) Atomization
(3) Ionization
(4) Thermal effects
Describe gas lasers
It contains laser materials such as gases (He-Ne, CO2<…)
Describe DYE lasers
Containing dilute solution of organic dyes (rhodamine, coumarin)
Describe semiconductor lasers
Combination of p- and n-type dopes semiconductors
3 subtypes of thermal effects
(!) Coagulation
(2) Vaporization
(3) Carbonization
What is coagulation effect?
Triggers bleeding to reduce bleeding and form scar
What is photo ablation? which type of radiative process it is?
The process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam.
At high laser flux, the material is typically converted to a plasma. (pulsed UV laser)
Photochemical reactions
What is photo disruption? which type of radiative process it is?
The molecules of tissue are ionized by laser beam, which then causes acoustic shock wave. The tissue ruptures as a result of the vapor bubble.
PHOTODISRUPTION
Application of coagulation (thermal effect)
Retina treatment by form scars that inhibit further retinal detachment
What is atomization in laser therapy?
The application of lasers to refractive surgery of eye
The role of Er:YAG lasers
Induction of shock waves (cause small circular hole) to remove formation of bone tissue in excess on bone surface
Dental application of dentistry.
Soft laser therapy
(1) Biostimulation (thermal effect)
(2) Low power
(3) Faster wound healing
(4) Antimocrobial effect
(5) Bone restoring, implantology
Dental application of dentistry.
ErYAG - caries removal
Absorption in water and hydroxyapatite
- > Vaporization and mechanical shockwave
- > caries removal
Dental application of dentistry.
YAG - oral surgery
Frenectomy
Gingivectomy
Dental application of dentistry.
Argon laser
Teeth whitening
- a rubber dam is put over your tooth to protect gums
Describe penetration of light into skin
(1) Light intensity is attenuated due to absorption, reflection and refraction
(2) Penetration depth depends on wavelength
What is photodynamic therapy?
a photochemotherapeutic procedure aimed at the treatment of tumors
Describe photodynamic therapy
- Administration of photosensitizing agent (porphyrins or 5-aminolaevulinic acid)
- The photosensitiser concentrated in cancer tissue
- . Activation of photosensitiser with light
- The excited photosensitizer produces free radicals
and reactive oxygen species (ROS) tissue reaction neighbouring healthy tissues remain intact
