Duke Elder: Optics (Alice) > Chapter 15: Lasers > Flashcards

Chapter 15: Lasers Flashcards

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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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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
How do lasers produce thermal effects on ocular tissues?
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
26
What are the 3 important ocular pigments relevant to laser thermal effects?
1. melanin (RPE, choroid) 2. xanothophyll (macula) 3. haemoglobin
27
What wavelengths of light are absorbed by * melanin * xanthophyll * haemoglobin?
* most of visible spectrum * blue light * blue, green and yellow
28
What degree of temperature increase do thermal effects of lasers cause in the retina and what does this cause?
10-20 degree C rise in tissue temperature photocoagulation, localised burn
29
What happens when thermal effects of lasers raise tissue temperature to 100 degrees C?
water vaporises and causes tissue disruption
30
What causes photochemical effects from lasers to tissues?
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
31
Which wavelengths of light are more harmful in terms of causing photochemical effects?
shorter wavelengths (blue, ultraviolet) - can damage at lower levels of irradiance
32
What are 3 methods for delivering lasers used in ophthalmology?
1. fibre optic cable to slit lamp 2. indirect ophthalmoscope 3. intraocular endolaser probe
33
What are 8 examples of lasers in use in ophthalmology?
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
What is the breakdown of blue-green light emitted by the argon blue-green gas laser?
70% blue (488nm) 30% green (514nm)
35
What is the most common use for argon lasers and how does this work?
retinal photocoagulation treats outer retina and spares inner retina to avoid damaging NFL
36
Why is direct use of blue laser light at the macula contraindicated?
xanthophyll in the inner layer of the macula absorbs lue light so can cause direct damage to the retina in this region
37
Why do older patients require higher power settings with argon blue-green gas lasers?
scatterin of the laser light by the crystalline lens of older individuals affects the focus of the beam
38
What condition can photocoagulation be used to treat with the argon blue-green gas laser?
retinopathy of prematurity
39
What oes the He-Ne laser consist of?
low power gas laser with visible red 632.8 nm emission
40
What is a function of the He-Ne laser?
visible red aiming beam for lasers with invisible output (e.g. Nd-YAG, diode)
41
What do diode lasers consist of?
wavelength of 810nm infrared in continuous wave mode, laser energy is generated by a semiconductor diode chip
42
What are 3 benefits of diode lasers?
1. efficient 2. generate little excess heat 3. portable
43
What are 5 uses of diode lasers and why?
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
What are 6 possible complications of use of the cyclodiode for ciliary body destruction?
1. Anterior inflammation including hypopyon 2. Hypotony 3. Scleral thinning / perforation 4. Cataract 5. Lens subluxation 6. Sympathetic endophthalmitis
45
What type of laser is the Nd:YAG?
emits 1064nm **infrared** radiation; powerful continuous wave laser, usually Q-switched when treating th eye
46
What is required to be used in conjunction with the Nd-YAG laser?
He-Ne infrared laser to guide beam (Nd-YAG is invisible)
47
What are 3 uses of the Nd-YAG laser?
1. **peripheral iridotomy** - narrow angle glaucoma 2. **capsulotomy** (disrupt posterior capsule of lens following cataract surgery) 3. **breaking posterior synechiae**
48
What wavelengths of radiation are emitted by the Nd-YAG laser vs the frequency-doubled Nd:YAG laser?
* Nd:YAG: 1054nm * Frequency doubled: 532nm
49
How is frequency-doubled Nd:YAG achieved?
radiation passed from **YAG crystal** through a **potassium titinyl phosphate (KTP) crystal** - converting some energy to 532nm radiation
50
What is the function of the frequency-doubled Nd:YAG laser?
photocoagulation - effect similar to continuous wave argon green laser (retinal photocoagulation)
51
What is an excimer laser?
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
What are the ues of the excimer laser?
**ablation** with precisely determined ablation **depth**: PRK, LASIK, LASEK, phototherapeutic keratectomy (PTK) (remove abnormal corneal surface tissue)
53
What limits penetration of excimer lasers beyond the cornea?
high absorbption of UV y cornea
54
What are the effects of excimer lasers on surrounding tissue during ablation?
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
What si the Erbium:YAG laser?
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
What is a common use of the erbium:YAG laser?
used experimentally to emulsify the lens in cataract surgery
57
What does the carbon dioxide laser consist of?
10600 nm **mid-infrared** wavelength, strongly absored by water and therefore more tissues. only has thermal effects
58
What are carbon dioxide lasers used for?
produce nearly bloodless incision in surgery (not in ophthalmology)
59
What happens to surrounding tissue with use of a carbon diozide laser?
coaguates adjacent tissues, water vaporisation releases steam
60
What are 10 investigational applications of lasers in ophthalmology?
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
What is meant by confocal?
when an imaging and illumination system focus on the same small point
62
What is confocal optics?
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
What is confocal microscopy?
principle of confocal optics used in laser scanning confocal microscopy, a means of looking in microscopic detail at different depth of living cornea
64
What is confocal scanning laser ophthalmoscopy?
images **optic nerve head and retina** different laser sources used (argon blue, argon green, HeNe red, diode infrared)
65
What is scanner laser polarimetry?
* 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
What can scanning laser polarimetry be used to assess?
RNFL damage caused by glaucoma
67
What is confocal scanning laser tomography?
* 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
What can confocal scanning laser tomography be used for?
evaluate disc damage in glaucoma
69
What are laser interferometers?
* 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
What is laser microperimetry?
uses laser beam to determine light sensitivity of very **small areas of retina **and to identify small **scotomas**
71
What is laser doppler flowmetry?
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
What is the Holmium laser used for? 2 things
* used to create **sclerostomy** to increase aqueous humour outflow in glaucoma treatment * **thermokeratoplasty** to change surface curvature of cornea
73
What is the Nd: YLF laser?
* emits 1053nm with picosecond pulse duration which can be delivered at high repetition rate * used experimentally to dissect rather than disrupt tissue
74
What are 3 uses of the Nd: YLF laser?
dissects rather than disrupts tissue: 1. create **sclerostomies** 2. **dissects vitreous membranes** 3. change r**efractive errors** by **intrastromal ablation** of cornea
75
Why is the eye at greater risk of damage from lasers than other parts of body?
optical propoerties focus beam on the retina, increasing irradiance by as much as 105x
76
Which laser safety categories do all surgical lasers fall into?
3b and 4 i.e. risk of significant eye damage/ irreversible damage
77
What are 2 safety measures when using lasers in theatre?
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
What are the 5 clases of laser safety and what does each correspond to?
79
What are 10 investigational applications of lasers in ophthalmology?
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
80
What produces more power, mode-locking or Q-switching?
mode-locking (pulse is 1 picosecond vs 2-30 nanoseconds for Q switching)
Duke Elder: Optics (Alice)
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