Lasers

light amplification by stimulated emission of radiation

what does LASER stand for?

narrow beam of collimated, monochromatic, coherent & sometimes polarized radiation

describe the light typically produced by lasers?

1. no energy introduced, atoms in ground state

2. energy introduced, atoms transition to excited state

3. spontaneous emissions start

4. photons capture extra energy in electrons: simulated emission

5. opposing parallel mirrors repeatedly reflect photons traveling along the length of the tube, amplifying the laser beam through repeated stimulated emissions

6. laser beam passes through mirror & leaves laser chamber

what are the principles of laser?

1.5um

because of high absorption by the cornea, the ArF Excimer laser only penetrates ______ into the eye

1. power

2. energy

3. flux density

what 3 things define laser strength?

power

Watts or mW; used for continuous output lasers

energy

Joules; used for short duration pulsed lasers

flux density

Joules/cm² or watts/cm²; critical for determining impact on tissue

1

grade/class of laser: lowest power & are eye-safe irrespective of exposure duration

2

grade/class of laser: low risk & will not damage eye if normal aversion responses are used, which should limit ocular exposure to 0.25sec

3

grade/class of laser: can cause damage even if aversion responses are employed, but will not burn skin

4

grade/class of laser: highest power output & are hazardous to eye & skin

continuous wave laser

must continuously generate excited atoms

pulsed lasers

often turned on & off by the auxiliary light source use to excite the atoms

Q switched lasers

turned on & off very rapidly using a fast shutter or optical elements in the optical path

mode locked lasers

use interference w/in the lasing cavity to generate laser pulses of pico or femto sec durations

1. photo-ionization

2. photo-chemical

3. photo-thermal

what are the 3 general effects of lasers on tissue?

photo-ionization

high energy creates plasma (ex: lightning)

photo-chemical

breaks individual chemical bonds in material absorbing the energy

photo-thermal

absorption generates heat; damage caused by elevated temperature

ionizing

what effect does a YAG laser use?

1. iridotomy

2. posterior capsulotomy

what are 2 common uses of YAG?

photochemical

what effects does Excimer lasers have?

Excimer lasers

photons have sufficient energy to break individual chemical bonds & produce very clean/precise photoablative effects on the cornea

spontaneous emission

results from release of the excess energy from an atom upon returning to the much more stable ground state; omni-directional & has the same frequency as that of the incident light

stimulated emission

results in release of a photon that is identical in radiation frequency, phase, & direction of travel w/ the incident photon

1. reflection

2. absorption

3. scattering

4. transmission

what are the 4 things light can do when it arrives at any tissue?

absorbed

photons must be _____ by tissue to transfer energy

0

short wavelength has ____ transmission through the cornea

IR

____ laser has some absorption in the cornea but also passes partially through & allows us to treat the capsule or the retina

corneal photoablation

what is the ArgonFluoride Excimer laser used for?

ionization for photodisruption

what is the YAG laser used for?

retinal photocoagulation

what is the Argon & Krypton lasers used for?

ArF Excimer laser

• only penetrates 1.5um into cornea

• all of its energy is absorbed at the corneal surface

• energy level breaks molecular bonds & tissue is vaporized

1. broad beam w/ iris diaphragm

2. flying spot lasers

3. variable flying spot lasers

what are the types of delivery systems with the Excimer laser?

Excimer laser flying spot technology

• laser spots are moved over the cornea to create ablation profile

• consecutive spots not aimed adjacent

• minimizes thermal shock

• allows plume to dissipate before returning to location

• rate of pulse = ~12ns & 5-10/sec

• collateral damage: 0.25um

Excimer laser custom treatment

• wavefront guided technology

• utilizes map of optical system

• assumes eyes are not regular

• attempt lower & higher order aberration correction

• attempts to keep a prolate corneal shape