15. Lasers

Question 1

production of laser energy

Answer 1

pumping  spontaneous emission  stimulated emission

Question 2

pumping

Answer 2

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

Question 3

spontaneous emission

Answer 3

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

Question 4

stimulated emission

Answer 4

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

Question 5

set up for lasers

Answer 5

• 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

Question 6

resonance

Answer 6

• 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

Question 7

what does laser stand for

Answer 7

Light Amplification by the Stimulated Emission of Radiation

Question 8

laser light is

Answer 8

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

Question 9

gas active medium

Answer 9

argon, krypton, carbon dioxide

Question 10

liquid active medium

Answer 10

dye

Question 11

solid active medium

Answer 11

Nd:YAG

Question 12

laser modes

Answer 12

transverse, continous wave, q-switching, mode locking

Question 13

cross section of beam

Answer 13

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

Question 14

gaussain curve

Answer 14

o At this point, energy is most concentrated

Question 15

trasnverse laser mode

Answer 15

o Important mode for photo-disruptive lasers

Question 16

continous wave

Answer 16

o Laser energy is produced continuously

Question 17

continous wave

Answer 17

e.g. argon
measured in watts

Question 18

q-switching mode

Answer 18

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

Question 19

pulsed wave

Answer 19

e.g. Nd:YAG
measured in joules

Question 20

q-swtiching mechamism

Answer 20

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

Question 21

mode locking laser mode

Answer 21

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

Question 22

Effect of laser depends on

Answer 22

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

Question 23

o Melanin

Answer 23

RPE and choroid  absorbs most of the visible spectrum

Question 24

o Xanthophyll

Answer 24

macula  strongly absorbs blue

Question 25

* Effects of laser

Answer 25

ionising, thermal, photochemical

Question 26

o Haemoglobin

Answer 26

absorbs blue, green and yellow (not red)

Question 27

Ionisation

Answer 27

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

Question 28

Thermal

Answer 28

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

Question 29

Photo-chemical

Answer 29

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

Question 30

Laser safety

Answer 30

* 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

Question 31

delivery methods for laser

Answer 31

slit lamp, indirect ophthalmoscope, intraocular endolaser probe

Question 32

YAG

Answer 32

yttrium-aluminium-garnet

Question 33

Argon-blue green gas - wavelength

Answer 33

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

Question 34

Argon-blue green gas - characetistics

Answer 34

- 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

Question 35

Argon-blue green gas - uses

Answer 35

Thermal = PRP

Question 36

Helium-neon (He-Ne) - wavelength

Answer 36

632.8nm (visible, red)

Question 37

Helium-neon (He-Ne) - characteristics

Answer 37

- low power gas laser that is visible

Question 38

Helium-neon (He-Ne) - uses

Answer 38

Aiming beam (for diode and YAG)

Question 39

Diode - wavelength

Answer 39

810nm IR in continuous wave mode

Question 40

Diode - characteristics

Answer 40

- only absorbed by melanin - minimal scatter, can also penetrate sclera

Question 41

Diode - uses

Answer 41

PRP Photocycloablation for glaucoma Dacryocystorhintsomy endoscopically

Question 42

Nd:YAG - wavelength

Answer 42

1064nm IR in CW and q-switched

Question 43

Nd:YAG - characteristics

Answer 43

- requires aiming beam as it is invisible - all must be focused on same point

Question 44

Nd:YAG - uses

Answer 44

Ionising = cap, PIs

Question 45

Excimer - wavelength

Answer 45

193nm UV light

Question 46

Excimer - characteristics

Answer 46

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

Question 47

Excimer - uses

Answer 47

Ablation = corneal (PRK, LASIK, PTK)

Question 48

Carbon dioxide - wavelength

Answer 48

10,600nm IR

Question 49

Carbon dioxide - characteristics

Answer 49

- strongly absorbed by water - tissue is coagulated and water is vaporised

Question 50

Carbon dioxide - uses

Answer 50

Thermal No used much in ophthalmology

Question 51

confocal optics

Answer 51

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

Question 52

confocal microscopy

Answer 52

Laser scanning confocal microscopy view depths of cornea

Question 53

Confocal scanning laser ophthalmoscope

Answer 53

Employs confocal optics For optic nerve head and retina

Question 54

Scanning laser polarimetry

Answer 54

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

Question 55

Confocal scanning laser tomography

Answer 55

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

Question 56

Laser interferometry

Answer 56

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

Question 57

Laser microperimetry

Answer 57

Laser beams onto retina Small scotomas

Question 58

Laser doppler flowmetry

Answer 58

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