Chapter 15: Lasers Flashcards
What does LASER stand for
Light amplification by the stimulated emission of radiation
What are the basic properties of the waves of light from a laser?
They are coherent (all waves are in phase)
They are collimated, all waves are parallel
Monochromatic due to being the same wavelenght
Using a laser, describe energy pumping
Energy is delivered to an active laser medium, exciting its electrons to a higher energy state. Atoms in higher energy states are unstable when they drop to a lower state or ground state, they release incoherent light in all directions.
However, if an excited atom is excited by a photon of the same wavelength of that which is released when the atom goes from a high to low energy state, coherent light is released (coherent to the stimulating photon)
Regarding lasers, what is resonance?
The laser medium is housed in a box with 2 mirrors on either side. The distance between the mirrors is a multiple of the wavelength of light emitted. This ensures light is always coherent. Therefore, light can:
1) undergo contstructive interference
2) stimulate excited atoms to release coherent light of the same wavelength as the incident light, thus amplifying the process.
When using a laser, what is continuous wave (WC) mode and how is it produced.
The tube that houses the laser medium has 1 semi transparent mirror. Some light is reflected and contributes to resonance, but some passes through, allowing coherent, collimated and monochromatic light to go through.
In a laser, what is used to pump energy?
- Electricity
- Second laser
- Incoherent light
What are transverse electromagnetic modes
In reality, lasers aren’t perfectly collimated and are in fact divergent. The point at which the laser is most intense is known as the transverse electromagnetic mode (TEM).
They are particularly important for photo distribution
In a laser, what is the fundamental mode?
The point as which the laser beam is least divergent and produces the most focused beam. Laser is less concentrated in the peripheries and follows Gaussian distribution.
What are the units of power?
Energy per unit time (joules/ second) =r watts
In lasers, what is q switching/ mode locking
A shutter is placed infront of 1 of the 2 mirrors around the laser material. This limits energy loss by spontaneous emission. Opening the shutter allows oscillation between the 2 mirrors allowing a single pulsed laser surge for 2-30 nanoseconds.
In reality, why are lasers from a single unit not always the same wavelength?
How do we get around this?
1) the tube is long, and multiple wavelengths can fit inside.
2) in solid state media, heat can cause changes in crystal size, thus changing distance between mirrors
3) in gas media, Doppler effect changes wavelength of light depending on the direction the gas molecule is travelling in relative to the incident light
Via mode locking, summarise various wavelengths so that they are in phase.
What are the 3 effects lasers have on tissue
- Ionisation
- Thermal
- Photochemical
Which lasers work by ionisation
Nd yag
Argon fluoride excimer laser
Which laser works via thermal effects?
Carbon dioxide laser. Causes vaporisation for photocoagulation
What are the photochemical effects of lasers?
With prolonged exposure (>10s). Free radicals can be produced which are toxic to cells.
Argon lasers have a filter to protect the operator from such effects
What are the uses of a diode laser?
Has a wavelength of 810nm which is able to go through the sclera to be absorbed by the melanin in the ٍٍRPE.
Can be placed on sclera to ablate ciliary body in end stage glaucoma.
Can also be used endoscopically for dacrocystorhinostomy
What is scanning laser polarimetry?
A confocal scanning laser ophthalmoscope (CSLO) projects polarised light of 780nm onto the retina. Using the principles of bifringence, we estimate the thickness of the RNFL based on the change in polarisation of light passing through it (called retardation)
Remember: bifrigence = when light is split into 2 beam, each polarised perpendicular to each other
