Chapter 15: Lasers Flashcards
What does LASER stand for?
Light Amplificatino by the Stimulated Emission of Radiation
What are 3 key features of all laser light?
- coherent
- laser beam is collimated i.e. waves of light are parallel
- monochromatic (of one wavelength)
In one sentence explain how laser light is produced?
- 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
What is continuous wave mode?
when light is produced continuously from a laser
How does the luminous flux of a laser compare with its brightness?
relatively small luminous flux but very bright as flux is not scattered
5 lumens can have intensity of 500 million candela
What are types of lasers named after?
their active medium
Why is the active medium important for lasers?
contains the atoms or molecules which undergo stimulated emission
What are 3 types of laser active media and examples of each?
- gas: argon, crypton, carbon dioxide
- liquid: dye
- solid: neodymium supported by yttrium auminium garnet crystal (Nd:YAG)
What are 3 types of sources of energy that can be pumped into the laser active medium?
- electrical discharge
- second laser
- incoherent light
What are transverse electromagnetic modes?
points along laser beam path where it is more intense (rather than slightly divergent)
What is the fundamental mode of a laser?
the point along its path where it is least divergent and energy is ocused to the smallest spot
At the point of focus of a laser how is energy distributed?
most concentrated in centre of beam, diminishes peripherally - described by Gaussian curve
How can non-fundamental modes of lasers be cancelled?
by an aperture inside the laser cavity to allow only passage of parallel rays
What is the equation to calculate power from a laser (demonstrating how power can be increased)?
Power = energy / unit time
therefore can increase power by delivering same energy over a shorter time
What are 2 methods to deliver a laser in a brief pulse rather than continuous wave to increase its power?
- mode locking
- Q-switching
What is Q-switching?
- 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)
What is mode-locking?
refinement of Q switching which synchronises various wavelengths, so they are in phase periodically, and summate as a train of very high pulses
What is free running mode of a laser?
when sub wavelengths are not in phase due to wavelength impurities caused by the doppler effect, heat expansion and long tube length
What are 3 reasons causing impurities of laser light wavelength?
- length of laser tube enormous compared with wavelength of light, so possible for multiples of different wavelengths to fit into its length
- heat generated during an operation may cause expansion of the laser crystal, altering mirror distance
- Doppler effect - gas molecules have random motion, wavelength depends on whether motion is same as emission
What produces more power, mode-locking or Q-switching?
mode-locking (pulse is 1 picosecond vs 2-30 nanoseconds for Q switching)
What are 3 things that the effects of laser energy on ocular tissues depend upon?
- Wavelength
- Pulse duration
- Absorption characteristics of tissue - determined largely by its pigments
What are 3 types of effects of lasers on ocular tissues?
- ionising
- thermal
- photochemical
How can lasers produce ionisation of tissue?
- 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.
What are 2 types of lasers that can produce ionisation of tissues?
- Nd-YAG
- Argon-fluoride eximer laser
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
What are the 3 important ocular pigments relevant to laser thermal effects?
- melanin (RPE, choroid)
- xanothophyll (macula)
- haemoglobin
What wavelengths of light are absorbed by
* melanin
* xanthophyll
* haemoglobin?
- most of visible spectrum
- blue light
- blue, green and yellow
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
What happens when thermal effects of lasers raise tissue temperature to 100 degrees C?
water vaporises and causes tissue disruption
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
Which wavelengths of light are more harmful in terms of causing photochemical effects?
shorter wavelengths (blue, ultraviolet) - can damage at lower levels of irradiance
What are 3 methods for delivering lasers used in ophthalmology?
- fibre optic cable to slit lamp
- indirect ophthalmoscope
- intraocular endolaser probe
What are the 5 clases of laser safety and what does each correspond to?
