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Optics > 15. Lasers > Flashcards

15. Lasers Flashcards

1
Q

production of laser energy

A

pumping  spontaneous emission  stimulated emission

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2
Q

pumping

A

ground state, energy delivered via pumping  atoms absorb and go to a higher energy level

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3
Q

spontaneous emission

A

excited electrons accumulate in the upper laser energy level, inversion of population (excited electrons > unexcited electrons), these are unstable and emit incoherent light spontaneously

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4
Q

stimulated emission

A

an excited electron stimulated by a further photo (wavelength equals the wavelength that would be emitted in spontaneous emission)  coherent emission

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5
Q

set up for lasers

A
  • 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
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6
Q

resonance

A
  • 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
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7
Q

what does laser stand for

A

Light Amplification by the Stimulated Emission of Radiation

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8
Q

laser light is

A

coherent (wavefronts in same phase), monochromatic (of one wavelength) and collimated (all rays are paralell)

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9
Q

gas active medium

A

argon, krypton, carbon dioxide

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10
Q

liquid active medium

A

dye

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11
Q

solid active medium

A

Nd:YAG

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12
Q

laser modes

A

transverse, continous wave, q-switching, mode locking

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13
Q

cross section of beam

A

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

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14
Q

gaussain curve

A

o At this point, energy is most concentrated

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15
Q

trasnverse laser mode

A

o Important mode for photo-disruptive lasers

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16
Q

continous wave

A

o Laser energy is produced continuously

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17
Q

continous wave

A

e.g. argon
measured in watts

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18
Q

q-switching mode

A

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

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19
Q

pulsed wave

A

e.g. Nd:YAG
measured in joules

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20
Q

q-swtiching mechamism

A

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

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21
Q

mode locking laser mode

A

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

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22
Q

Effect of laser depends on

A

wavelength, duration of exposure, absorption characteristics of the tissue (largely dependant on pigment)

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23
Q

o Melanin

A

RPE and choroid  absorbs most of the visible spectrum

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24
Q

o Xanthophyll

A

macula  strongly absorbs blue

25
Q
  • Effects of laser
A

ionising, thermal, photochemical

26
Q

o Haemoglobin

A

absorbs blue, green and yellow (not red)

27
Q

Ionisation

A

Duration: < nanosecond
Effect: Strip electrons from molecule to form plasma (ions + electrons), rapidly expands to cause mechanical shock
Uses: Nd-YAG and excimer

28
Q

Thermal

A

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

29
Q

Photo-chemical

A

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

30
Q

Laser safety

A
  • 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
31
Q

delivery methods for laser

A

slit lamp, indirect ophthalmoscope, intraocular endolaser probe

32
Q

YAG

A

yttrium-aluminium-garnet

33
Q

Argon-blue green gas - wavelength

A

Mixture:
70% 488nm (blue)
30% 514nm (green)

34
Q

Argon-blue green gas - characetistics

A
  • 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
35
Q

Argon-blue green gas - uses

A

Thermal = PRP

36
Q

Helium-neon (He-Ne) - wavelength

A

632.8nm (visible, red)

37
Q

Helium-neon (He-Ne) - characteristics

A
  • low power gas laser that is visible
38
Q

Helium-neon (He-Ne) - uses

A

Aiming beam (for diode and YAG)

39
Q

Diode - wavelength

A

810nm IR in continuous wave mode

40
Q

Diode - characteristics

A
  • only absorbed by melanin
  • minimal scatter, can also penetrate sclera
41
Q

Diode - uses

A

PRP
Photocycloablation for glaucoma
Dacryocystorhintsomy endoscopically

42
Q

Nd:YAG - wavelength

A

1064nm IR in CW and q-switched

43
Q

Nd:YAG - characteristics

A
  • requires aiming beam as it is invisible
  • all must be focused on same point
44
Q

Nd:YAG - uses

A

Ionising = cap, PIs

45
Q

Excimer - wavelength

A

193nm UV light

46
Q

Excimer - characteristics

A
  • two atoms (Ar-F) forming a molecule in excited state
  • high absorption of UV by the cornea prevents significant penetration
47
Q

Excimer - uses

A

Ablation = corneal (PRK, LASIK, PTK)

48
Q

Carbon dioxide - wavelength

A

10,600nm IR

49
Q

Carbon dioxide - characteristics

A
  • strongly absorbed by water
  • tissue is coagulated and water is vaporised
50
Q

Carbon dioxide - uses

A

Thermal
No used much in ophthalmology

51
Q

confocal optics

A

when an imaging and illumination system focuses on the same small point = confocal

52
Q

confocal microscopy

A

Laser scanning confocal microscopy
view depths of cornea

53
Q

Confocal scanning laser ophthalmoscope

A

Employs confocal optics For optic nerve head and retina

54
Q

Scanning laser polarimetry

A

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

55
Q

Confocal scanning laser tomography

A

CSLO uses diode (670)
Produces 3D image of the optic nerve head

56
Q

Laser interferometry

A

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

57
Q

Laser microperimetry

A

Laser beams onto retina
Small scotomas

58
Q

Laser doppler flowmetry

A

Doppler principle – laser is incident on moving RBCs greater shift = greater blood flow
Measure retinal capillary blood flow

Optics (18 decks)

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