Chapter 3 - Lasers

Question 1

What is Opthalmology?

Answer 1

Clinical and surgical practices related to the eye

Question 2

What does laser stand for?

Answer 2

Light Amplification by Stimulated Emissions of Radiation

Question 3

What is a mechanical wave and does it involve light?

Answer 3

Mechanical waves involve the periodic motion of particles in a medium. Light is NOT a mechanical wave because it can travel in both mediums and vacuums. Mechanical waves cannot travel in vacuums.

Question 4

What two forces make up light?

Answer 4

Magnetic and Electric forces

Question 5

What type of wave is light?

Answer 5

A transverse wave. (Two perpendicular waves that travel in the same direction)

Question 6

How is the energy of light related to its wavelength

Answer 6

The energy of light is inversely proportional to its wavelength. (The shorter the wavelength, the more energy it is)

Question 7

What is spontaneous emission?

Answer 7

When an electron absorbs energy through random occurrences and transitions back down to its ground state, releasing a photon.

Question 8

What is stimulated emission?

Answer 8

When a photon that has the same energy as the energy difference between ground and excited state passes by an excited state electron and causes it to transition down to its ground state, emitting a photon.

Question 9

What is population inversion?

Answer 9

When there is a higher amount of electrons in their excited state rather than the ground state.

Question 10

What is pumping?

Answer 10

The process of putting electrons in a perpetually excited state

Question 11

What is the active medium (aka lasing medium)?

Answer 11

The medium in which energy is pumped into that puts electrons in their excited state. Is often gas but could be solid or liquid.

Question 12

What is the metastable state?

Answer 12

The state in which electrons stay in their excited energy state.

Question 13

What is the optical resonating cavity?

Answer 13

A cavity consisting of a mirror on either side, one fully reflective, one partially. The lasing medium is placed inside.

Question 14

What are the 5 most important characteristics of laser light?

Answer 14

• Monochromaticity
• Coherence
• Directionality
• Intensity
• Focusability

Question 15

What are the three base components of a laser?

Answer 15

• Energy source (pump)
• Optical resonator (cavity)
• Gain medium

Question 16

What is spatial coherence?

Answer 16

A spatially coherent laser is a laser whose light is emitted in a very well-defined beam

Question 17

What is the power density of a laser (also called intensity)?

Answer 17

The amount of power delivered to a surface per unit area

Question 18

What is fluence?

Answer 18

The total energy emitted by a laser beam, divided by the illuminated area. Best to be thought of as the “dose” of energy per beam.

Question 19

What is coagulation?

Answer 19

The heat-induced disordering of proteins

Question 20

What is photocoagulation?

Answer 20

The process of coagulation (heating tissues to cause disorder) brought on by a laser. i.e., using a laser to coagulate tissue

Question 21

What is a photocoagulation burn?

Answer 21

When tissues surrounding a photocoagulated area die, the dead tissues develop into a burn called a photocoagulation burn

Question 22

What is heat flow (heat transfer)?

Answer 22

The phenomenon in which heat travels from a high-temp area to a low-temp area.

Question 23

What is the Thermal Relaxation Time?

Answer 23

It is the amount of time required for heat to flow into adjacent tissues. So if a laser is on a tissue for less time, the neighboring tissues wont be damaged.

Question 24

What is photovaporization?

Answer 24

Water within tissue is heated to above 100 C and is hit with an extremely high power-density laser that instantly vaporizes the tissue.

Question 25

What is the heat of vaporization?

Answer 25

The energy needed to vaporize different tissues. Different tissues all have varying levels of water content so different energy levels are needed to photovaporize different tissues

Question 26

What is Photobolation?

Answer 26

The use of high-power lasers that use UV light to selectively break chemical bonds in tissues without causing local heating

Question 27

What two modes can lasers operate in?

Answer 27

- CW (Continuous Wave - The laser is constantly providing power) - Pulsed (The laser delivers an intense amount of power for an extremely short duration)

Question 28

What is Q-Switching?

Answer 28

Using a Pockels Cell to build up energy into the laser and releasing it all in an instant to deliver extremely high amounts of power

Question 29

What is the difference between average power and instantaneous power?

Answer 29

Average power is always smaller than instantaneous power.

Question 30

What is photodisruption?

Answer 30

The use of pulsed lasers to create tiny explosions at the base of the laser beam which sends shockwaves that tear apart tissues and breaks up hard deposits.

Question 31

What is laser lithotripsy?

Answer 31

Using lasers to break up gallstones in the urinary tract

Question 32

How does absorption work?

Answer 32

DIfferent things absorb and reflect different colours. The colours that we perceive are those that are reflected off of an object, all other colours are absorbed.

Question 33

How do lasers use absorption?

Answer 33

Since the physical colour of tissue is the colour that's reflected, lasers must be the colour that is absorbed so that the optimal amount of energy can put into the tissue. For example, Oxyhemoglobin appears red so a blue-green laser would be best suited for use for it.

Question 34

How is photocoagulation used to treat diabetic retinopathy?

Answer 34

Photocoagulation covers areas damaged by leaking blood vessels, stopping the bleeding and allowing them to heal

Question 35

What are some procedures done to correct myopia (near-sightedness)? Can they be done to correct hyperopia (far-sightedness)

Answer 35

Photorefractive Keratectomy (PRK) and Laser Assisted In-Situ Keratomileusis (LASIK). In some instances they can be done to correct far-sightedness but the vast majority of procedures correct near-sightedness.

Question 36

What is Photodynamic Therapy? (PDT)

Answer 36

The use of light to target and destroy cells in cancer therapy and other treatments.

Question 37

What are photosensitizers?

Answer 37

Light-activated chemicals that are injected into the bloodstream and accumulate in cells, staying longer in cancer cells. They react strongly to red-light so they are used to locate and analyze the extent of a tumour.

Question 38

What are some advantages of laser surgery?

Answer 38

- Less blood loss - More precise - Targetting specific tissues (wavelength dependant) - Versatility - Faster recovery - Reduced risk of infection - Penetrating without incisions - Cuts to arbitrary depths

Question 39

What are some disadvantages of laser surgery?

Answer 39

- Traditional surgery may heal faster - Accidental ignition of materials - Additional safety training - More expensive - Doctors need additional training - Damage by trial/error - Photovaporization of gases and viruses can release them into air.