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OPTICS > Chapter 15: Lasers > Flashcards

Chapter 15: Lasers Flashcards

1
Q

What does LASER stand for?

A

Light Amplificatino by the Stimulated Emission of Radiation

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2
Q

What are 3 key features of all laser light?

A
  • coherent
  • laser beam is collimated i.e. waves of light are parallel
  • monochromatic (of one wavelength)
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3
Q

In one sentence explain how laser light is produced?

A
  • if an atom at a higher energy level is further stimluated by a photon whose wavelength is that which it would naturally emit, resulting emission will be coherent with the stimulating photon, and the atom will drop to a lower energy level.
  • the amount of energy released by stimulated emission can be amplified
  • active laser medium is in a tube with mirrors either end, distance betwen mirrors is a multiple of the wavelengths of light emitted, so resonance (reinforcing itself due to being in phase on subsequent journeys) can occur
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4
Q

What is continuous wave mode?

A

when light is produced continuously from a laser

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5
Q

How does the luminous flux of a laser compare with its brightness?

A

relatively small luminous flux but very bright as flux is not scattered
5 lumens can have intensity of 500 million candela

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6
Q

What are types of lasers named after?

A

their active medium

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7
Q

Why is the active medium important for lasers?

A

contains the atoms or molecules which undergo stimulated emission

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8
Q

What are 3 types of laser active media and examples of each?

A
  1. gas: argon, crypton, carbon dioxide
  2. liquid: dye
  3. solid: neodymium supported by yttrium auminium garnet crystal (Nd:YAG)
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9
Q

What are 3 types of sources of energy that can be pumped into the laser active medium?

A
  1. electrical discharge
  2. second laser
  3. incoherent light
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10
Q

What are transverse electromagnetic modes?

A

points along laser beam path where it is more intense (rather than slightly divergent)

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11
Q

What is the fundamental mode of a laser?

A

the point along its path where it is least divergent and energy is ocused to the smallest spot

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12
Q

At the point of focus of a laser how is energy distributed?

A

most concentrated in centre of beam, diminishes peripherally - described by Gaussian curve

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13
Q

How can non-fundamental modes of lasers be cancelled?

A

by an aperture inside the laser cavity to allow only passage of parallel rays

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14
Q

What is the equation to calculate power from a laser (demonstrating how power can be increased)?

A

Power = energy / unit time
therefore can increase power by delivering same energy over a shorter time

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15
Q

What are 2 methods to deliver a laser in a brief pulse rather than continuous wave to increase its power?

A
  1. mode locking
  2. Q-switching
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16
Q

What is Q-switching?

A
  • shutter is placed in front of one of the 2 mirrors in the laser tube
  • this maximises the energy state of the laser medium by limiting energy loss to spontaneous emission alone
  • opening shutter allows oscillation to occur producing single pulsed surge of stimulated emission (2-3 nanoseconds)
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17
Q

What is mode-locking?

A

refinement of Q switching which synchronises various wavelengths, so they are in phase periodically, and summate as a train of very high pulses

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18
Q

What is free running mode of a laser?

A

when sub wavelengths are not in phase due to wavelength impurities caused by the doppler effect, heat expansion and long tube length

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19
Q

What are 3 reasons causing impurities of laser light wavelength?

A
  1. length of laser tube enormous compared with wavelength of light, so possible for multiples of different wavelengths to fit into its length
  2. heat generated during an operation may cause expansion of the laser crystal, altering mirror distance
  3. Doppler effect - gas molecules have random motion, wavelength depends on whether motion is same as emission
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20
Q

What produces more power, mode-locking or Q-switching?

A

mode-locking (pulse is 1 picosecond vs 2-30 nanoseconds for Q switching)

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20
Q

What produces more power, mode-locking or Q-switching?

A

mode-locking (pulse is 1 picosecond vs 2-30 nanoseconds for Q switching)

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21
Q

What are 3 things that the effects of laser energy on ocular tissues depend upon?

A
  1. Wavelength
  2. Pulse duration
  3. Absorption characteristics of tissue - determined largely by its pigments
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22
Q

What are 3 types of effects of lasers on ocular tissues?

A
  1. ionising
  2. thermal
  3. photochemical
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23
Q

How can lasers produce ionisation of tissue?

A
  • Photon energy delivered in a nanosecond or less may be sufficient to strip electrons from molecules to form a collection of ions and electrons called a plasma.
  • A plasma has a very high temperature and rapidly expands to cause a mechanical shock wave sufficient to displace tissue.
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24
Q

What are 2 types of lasers that can produce ionisation of tissues?

A
  1. Nd-YAG
  2. Argon-fluoride eximer laser
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25
Q

How do lasers produce thermal effects on ocular tissues?

A

light converted into heat energy if wavelength coincides with the absorption spectrum of the tissue pigment on which it falls and if the pulse duratino is between a few microseconds and 10s

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26
Q

What are the 3 important ocular pigments relevant to laser thermal effects?

A
  1. melanin (RPE, choroid)
  2. xanothophyll (macula)
  3. haemoglobin
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27
Q

What wavelengths of light are absorbed by
* melanin
* xanthophyll
* haemoglobin?

A
  • most of visible spectrum
  • blue light
  • blue, green and yellow
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28
Q

What degree of temperature increase do thermal effects of lasers cause in the retina and what does this cause?

A

10-20 degree C rise in tissue temperature
photocoagulation, localised burn

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29
Q

What happens when thermal effects of lasers raise tissue temperature to 100 degrees C?

A

water vaporises and causes tissue disruption

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30
Q

What causes photochemical effects from lasers to tissues?

A

when pulse duration of 10 seonds or more is required to cause damage, mechanism is the formation of free radical ions which are highly reactive and toxic to cells

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31
Q

Which wavelengths of light are more harmful in terms of causing photochemical effects?

A

shorter wavelengths (blue, ultraviolet) - can damage at lower levels of irradiance

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32
Q

What are 3 methods for delivering lasers used in ophthalmology?

A
  1. fibre optic cable to slit lamp
  2. indirect ophthalmoscope
  3. intraocular endolaser probe
33
Q

What are 8 examples of lasers in use in ophthalmology?

A
  1. Argon blue-green gas laser
  2. He-Ne laser
  3. Diode lasers
  4. Nd-YAG laser
  5. Frequency-doubled Nd-YAG laser
  6. Excimer laser
  7. Erbium:YAG laser
  8. Carbon dioxide laser
34
Q

What is the breakdown of blue-green light emitted by the argon blue-green gas laser?

A

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

35
Q

What is the most common use for argon lasers and how does this work?

A

retinal photocoagulation
treats outer retina and spares inner retina to avoid damaging NFL

36
Q

Why is direct use of blue laser light at the macula contraindicated?

A

xanthophyll in the inner layer of the macula absorbs lue light so can cause direct damage to the retina in this region

37
Q

Why do older patients require higher power settings with argon blue-green gas lasers?

A

scatterin of the laser light by the crystalline lens of older individuals affects the focus of the beam

38
Q

What condition can photocoagulation be used to treat with the argon blue-green gas laser?

A

retinopathy of prematurity

39
Q

What oes the He-Ne laser consist of?

A

low power gas laser with visible red 632.8 nm emission

40
Q

What is a function of the He-Ne laser?

A

visible red aiming beam for lasers with invisible output (e.g. Nd-YAG, diode)

41
Q

What do diode lasers consist of?

A

wavelength of 810nm infrared in continuous wave mode, laser energy is generated by a semiconductor diode chip

42
Q

What are 3 benefits of diode lasers?

A
  1. efficient
  2. generate little excess heat
  3. portable
43
Q

What are 5 uses of diode lasers and why?

A
  1. retinal photocoagulation - absorbed only by melanin
  2. penetrates sclera - photocoagulation can be performed by placing probe on surface of eye if retina oscured through pupil
  3. photocycloablation of ciliary body in end-stage glaucoma
  4. create dacryocystorhinostomy (lacrimal sac hole) when used endoscopically
  5. photocoagulation of vascular structures with IV indocyanine green
44
Q

What are 6 possible complications of use of the cyclodiode for ciliary body destruction?

A
  1. Anterior inflammation including hypopyon
  2. Hypotony
  3. Scleral thinning / perforation
  4. Cataract
  5. Lens subluxation
  6. Sympathetic endophthalmitis
45
Q

What type of laser is the Nd:YAG?

A

emits 1064nm infrared radiation; powerful continuous wave laser, usually Q-switched when treating th eye

46
Q

What is required to be used in conjunction with the Nd-YAG laser?

A

He-Ne infrared laser to guide beam (Nd-YAG is invisible)

47
Q

What are 3 uses of the Nd-YAG laser?

A
  1. peripheral iridotomy - narrow angle glaucoma
  2. capsulotomy (disrupt posterior capsule of lens following cataract surgery)
  3. breaking posterior synechiae
48
Q

What wavelengths of radiation are emitted by the Nd-YAG laser vs the frequency-doubled Nd:YAG laser?

A
  • Nd:YAG: 1054nm
  • Frequency doubled: 532nm
49
Q

How is frequency-doubled Nd:YAG achieved?

A

radiation passed from YAG crystal through a potassium titinyl phosphate (KTP) crystal - converting some energy to 532nm radiation

50
Q

What is the function of the frequency-doubled Nd:YAG laser?

A

photocoagulation - effect similar to continuous wave argon green laser (retinal photocoagulation)

51
Q

What is an excimer laser?

A

excited dimer - two atoms form molecule in excited state (dissociate in ground state)
employ argon-fluorine (Ar-F) dimer laser to emit 193nm UV radiation

52
Q

What are the ues of the excimer laser?

A

ablation with precisely determined ablation depth: PRK, LASIK, LASEK, phototherapeutic keratectomy (PTK) (remove abnormal corneal surface tissue)

53
Q

What limits penetration of excimer lasers beyond the cornea?

A

high absorbption of UV y cornea

54
Q

What are the effects of excimer lasers on surrounding tissue during ablation?

A

temperature in a tiny volume of treated tissue becomes very high but the amount of heat produced is very small and there is no significant rise in temperatur eof adjacent tissue

55
Q

What si the Erbium:YAG laser?

A

delivers 2940 nm irared radiaion, absrobed by water and penetrates tissue by <1 micrometre - causes explosive evaporation of tissue, thermal effects limited to surround 5-15 micrometres

56
Q

What is a common use of the erbium:YAG laser?

A

used experimentally to emulsify the lens in cataract surgery

57
Q

What does the carbon dioxide laser consist of?

A

10600 nm mid-infrared wavelength, strongly absored by water and therefore more tissues. only has thermal effects

58
Q

What are carbon dioxide lasers used for?

A

produce nearly bloodless incision in surgery (not in ophthalmology)

59
Q

What happens to surrounding tissue with use of a carbon diozide laser?

A

coaguates adjacent tissues, water vaporisation releases steam

60
Q

What are 10 investigational applications of lasers in ophthalmology?

A
  1. confocal optics
  2. confocal microscopy
  3. confocal scanning laser ophthalmoscope
  4. scanning laser polarimetry
  5. confocal scanning laser tomography
  6. laser interferometry
  7. laser microperimetry
  8. laser doppler flowmetry
  9. holmium laser
  10. Nd: YLF laser
60
Q

What are 10 investigational applications of lasers in ophthalmology?

A
  1. confocal optics
  2. confocal microscopy
  3. confocal scanning laser ophthalmoscope
  4. scanning laser polarimetry
  5. confocal scanning laser tomography
  6. laser interferometry
  7. laser microperimetry
  8. laser doppler flowmetry
  9. holmium laser
  10. Nd: YLF laser
61
Q

What is meant by confocal?

A

when an imaging and illumination system focus on the same small point

62
Q

What is confocal optics?

A

confocal view, contrast and resolution of image increased by minimising scatter (small area of illumination and field of view) by using laser source of illumination and observer view through pinhole/slit

63
Q

What is confocal microscopy?

A

principle of confocal optics used in laser scanning confocal microscopy, a means of looking in microscopic detail at different depth of living cornea

64
Q

What is confocal scanning laser ophthalmoscopy?

A

images optic nerve head and retina
different laser sources used (argon blue, argon green, HeNe red, diode infrared)

65
Q

What is scanner laser polarimetry?

A
  • measures thickness of retinal nerve fire layer (RNFL) by exploiting its birefrigence
  • spot of polarised laser light projected onto retina by CSLO, passes through RNFL to deeper structures and partly reflected back
  • Magnitude of polarisation is called retardation and correlates with thickness of RNFL
66
Q

What can scanning laser polarimetry be used to assess?

A

RNFL damage caused by glaucoma

67
Q

What is confocal scanning laser tomography?

A
  • CSLO diode used to produce a topographic map of the optic nerve head.
  • First image obtained in plane parallel to surface of retina
  • Subsequent images acquired by advancing focal plane posteriorly towards lamina cribrosa
  • Converted into 3D reconstruction
68
Q

What can confocal scanning laser tomography be used for?

A

evaluate disc damage in glaucoma

69
Q

What are laser interferometers?

A
  • project laser light (usually He-Ne) from 2 sources onto the retina; interference occurs where 2 beams meet and causes sine wave grating
  • Reducing separation between the light sources reduces spatial frequency of sine wave grating, allows estimation fo potential visual acuity e.g. in cataract
70
Q

What is laser microperimetry?

A

uses laser beam to determine light sensitivity of very **small areas of retina **and to identify small scotomas

71
Q

What is laser doppler flowmetry?

A

means of measuring retinal capillary blood flow based on doppler pinricple - laser light on moving blood cells reflacted at different frequency. greater shift in freqeuncy indicates greater blood flow velocity

72
Q

What is the Holmium laser used for? 2 things

A
  • used to create sclerostomy to increase aqueous humour outflow in glaucoma treatment
  • thermokeratoplasty to change surface curvature of cornea
73
Q

What is the Nd: YLF laser?

A
  • emits 1053nm with picosecond pulse duration which can be delivered at high repetition rate
  • used experimentally to dissect rather than disrupt tissue
74
Q

What are 3 uses of the Nd: YLF laser?

A

dissects rather than disrupts tissue:
1. create sclerostomies
2. dissects vitreous membranes
3. change refractive errors by intrastromal ablation of cornea

75
Q

Why is the eye at greater risk of damage from lasers than other parts of body?

A

optical propoerties focus beam on the retina, increasing irradiance by as much as 105x

76
Q

Which laser safety categories do all surgical lasers fall into?

A

3b and 4 i.e. risk of significant eye damage/ irreversible damage

77
Q

What are 2 safety measures when using lasers in theatre?

A
  1. filters buit into instrument - absorb laser light but transmit enough to observe effect of laser on target
  2. protective googles should be used if unable to leave laser area
78
Q

What are the 5 clases of laser safety and what does each correspond to?

A

OPTICS (19 decks)

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