Chua 4 Flashcards
How is laser energy produced
Pumping - atoms absorb energy to elevate electrons from ground state to higher energy level
Spontaneous emission
Excited electrons accumulate at the upper laser energy level
Excited electrons spontaneously return to ground state by emitting light energy - incoherrent
Stimulated emission
If excited electron is stimulated further by photon whose wavelength is that which the excited electron would naturlaly emit, resulting emission is coherent
Resonance
Active laser medium in tube with mirror at each end
One mirror is paritally transparent
Distance between mirrors equals multiple of wavelength of light emitted
What are properties of laser light
Coherent (wavefronts in phase)
Monochormatic ( mostly of one wavelength)
Collimated (parallel)
What does 5 lumen light from laser have intensity?
500 million candela - as light is in fine parallel beam (not scattered)
What is laser transverse mode?
Comparison of cross section of laser beam at different points
Point along laser beamwhere light is least divergent - energy focused to smallest spot - fundamental mode
What is Q switching?
Delivering energy over a shorter time means increased power
Brief pulse of laser light
Pulsed laser such as NdYAG has peak of power
Shutter is placed infront of one of the mirrors in laser tube. This limits energy loss.
Opening shutter allows oscillation to occur and produce single pulsed surge of stimulated emission
What is mode locking
Refinement of Q switching which synchronises various wavelengths so that periodically they are in phase and summate
Pulse lasts 30 picoseconds and produces up to 100 times as much pwoer for the same energy compared with Q swiching
duration of pulse shorter than Q switch
What is ionising laser? Example
Pulse duration < nanosecond
Strip electrons to form plasma
High temp - suffiecient to displace tissue
NdYAG and Excimer lasers
What are thermal lasers, example
Light energy converted into heat energy if wavelegnth conincides with absoption spectrum of tissue pigment and microseconds pulsee
MElanin - RPE and choroid - absorbs visible spectrum
Xanthophyll - macula - absorbs blue light
REsults in photocoagulation and thermal burns
CO2 and Argon laser
What are classes of laser safety?
1 Blue/green/red <0.024mV safe
2 Visible <1mV safe
3a 1-5mV Safet
3b 5-500 damage
4 >500 irreversible damage
What lasers are deterimental to human eye?
> 5mV
Class 3b or above
What does Argon laser emit
Blue 488nm
Green 514nm
What laser is used for photocoagulation? What do xanthophyl at macula absorb?
Argon green laser
Xanthophyll absorb blue light - contraindicated
What is diode laser used for?
Infrared continuous wave laser
Photocoagulation
Photocycloablation - transparency of sclera allows ablation of ciliary body
Dacrocystorhinostomy
What does ndYAG laser emit? What mode is used to treat the eye?
neodymium, yttrium, aluminium, gaarnet
1064nm infrared
Q switched to trat eye
Requires He-Ne laser red aiming beam
What is frequency doubled YAG used for?
532 nm
Photocoagulation
