15. Lasers Flashcards
production of laser energy
pumping spontaneous emission stimulated emission
pumping
ground state, energy delivered via pumping atoms absorb and go to a higher energy level
spontaneous emission
excited electrons accumulate in the upper laser energy level, inversion of population (excited electrons > unexcited electrons), these are unstable and emit incoherent light spontaneously
stimulated emission
an excited electron stimulated by a further photo (wavelength equals the wavelength that would be emitted in spontaneous emission) coherent emission
set up for lasers
- Set up for lasers: active laser medium in a tube with mirror at each end – one mirror is paritially transparent to allow some light to leave
- Distance between the two mirrors is equal to multiple wavelengths of the light
resonance
- Most are still doing spontaneous emission, can use resonance to amplify
- Light is reflected and re-reflected
- So that light gets stronger and stronger while remaining in the same phase
what does laser stand for
Light Amplification by the Stimulated Emission of Radiation
laser light is
coherent (wavefronts in same phase), monochromatic (of one wavelength) and collimated (all rays are paralell)
gas active medium
argon, krypton, carbon dioxide
liquid active medium
dye
solid active medium
Nd:YAG
laser modes
transverse, continous wave, q-switching, mode locking
cross section of beam
o Cross section of laser beam = very slightly divergent, in that it is more intense in certain points
o Where it is least divergent, this point is where energy can be focussed on the smallest spot fundamental mode
gaussain curve
o At this point, energy is most concentrated
trasnverse laser mode
o Important mode for photo-disruptive lasers
continous wave
o Laser energy is produced continuously
continous wave
e.g. argon
measured in watts
q-switching mode
o Same amount of energy over shorter time increased power
o Mode locking / Q-switching = brief pulse rather than continuous wave
o Maximases erngey by limiting energy loss to spontaneous emission alone
pulsed wave
e.g. Nd:YAG
measured in joules
q-swtiching mechamism
shutter is place in front of one of the mirros between the oscilaiton of the beam
Various shutters = rotating mirros, dyes, electro-optic switching
Opening the shutter also oscillation to occur and produce a single pulse surger of stimulated emission
Duration = 2-30 nanoseconds
mode locking laser mode
o Refinement of Q-swtiching which syncohroizes the various wavelengths so that they are released in phase and summated very high energy pulses
o Lasts about 30 picoseconds but much higher energy
Effect of laser depends on
wavelength, duration of exposure, absorption characteristics of the tissue (largely dependant on pigment)
o Melanin
RPE and choroid absorbs most of the visible spectrum
o Xanthophyll
macula strongly absorbs blue
- Effects of laser
ionising, thermal, photochemical
o Haemoglobin
absorbs blue, green and yellow (not red)
Ionisation
Duration: < nanosecond
Effect: Strip electrons from molecule to form plasma (ions + electrons), rapidly expands to cause mechanical shock
Uses: Nd-YAG and excimer
Thermal
Duration: A few micro-seconds 10s
Effect:
-Occurs if wavelength coincide with absorption spectrum of tissue and pulse duration correct
- 10-20 rise = photocoagulation and burns
- 100 rise = tissue disruption (water vaporisation)
Uses: Argon and carbon dioxide laser
Photo-chemical
Duration: >10 seconds
Effect:
- Formation of free radicals highly reactive and toxic
- Shorter wavelengths (blue, UV) cause more damage
Uses: Undesirable, most lasers have a blue filter to correct for this
Laser safety
- Eye focuses the beam on the retina, increasing the irrance by as much as 105
- All ophthalmic lasers = 3b (significant damage) or 4 (irreversible damage)
- Safety = shutters, filters and protective googles
delivery methods for laser
slit lamp, indirect ophthalmoscope, intraocular endolaser probe
YAG
yttrium-aluminium-garnet
Argon-blue green gas - wavelength
Mixture:
70% 488nm (blue)
30% 514nm (green)
Argon-blue green gas - characetistics
- treats outer retina and spares inner retina (does not damage NFL)
- argon green = absorbed by melanin and haemoglobin
- xanthophyll absorbs blue light therefore use of blue on macular is contra-indication
Argon-blue green gas - uses
Thermal = PRP
Helium-neon (He-Ne) - wavelength
632.8nm (visible, red)
Helium-neon (He-Ne) - characteristics
- low power gas laser that is visible
Helium-neon (He-Ne) - uses
Aiming beam (for diode and YAG)
Diode - wavelength
810nm IR in continuous wave mode
Diode - characteristics
- only absorbed by melanin
- minimal scatter, can also penetrate sclera
Diode - uses
PRP
Photocycloablation for glaucoma
Dacryocystorhintsomy endoscopically
Nd:YAG - wavelength
1064nm IR in CW and q-switched
Nd:YAG - characteristics
- requires aiming beam as it is invisible
- all must be focused on same point
Nd:YAG - uses
Ionising = cap, PIs
Excimer - wavelength
193nm UV light
Excimer - characteristics
- two atoms (Ar-F) forming a molecule in excited state
- high absorption of UV by the cornea prevents significant penetration
Excimer - uses
Ablation = corneal (PRK, LASIK, PTK)
Carbon dioxide - wavelength
10,600nm IR
Carbon dioxide - characteristics
- strongly absorbed by water
- tissue is coagulated and water is vaporised
Carbon dioxide - uses
Thermal
No used much in ophthalmology
confocal optics
when an imaging and illumination system focuses on the same small point = confocal
confocal microscopy
Laser scanning confocal microscopy
view depths of cornea
Confocal scanning laser ophthalmoscope
Employs confocal optics For optic nerve head and retina
Scanning laser polarimetry
Exploits RNFL birefringence – axons are arranged parrealel
Polaisred light (780nm) onto retina – partly reflected back, magnitude of polarisation – retardation and correlates to RNFL thickness
Measure RNFL thickness
Confocal scanning laser tomography
CSLO uses diode (670)
Produces 3D image of the optic nerve head
Laser interferometry
Uses He-Ne laser from two sources – interference occurs where they meet producing a sine wave grating
Spatial frequency of the grating estimates visual potential even in the presence of cataract
Laser microperimetry
Laser beams onto retina
Small scotomas
Laser doppler flowmetry
Doppler principle – laser is incident on moving RBCs greater shift = greater blood flow
Measure retinal capillary blood flow
