Coherence and OCT Flashcards
What is OCT
a 3d imaging technique with high spatial resolution and large penetration depth even in highly scattering media
What principles is OCT based on
- on measurements of the reflected light from tissue discontinuities
- on the interference between the reflected light and the reference beam, using it as a coherence gate to isolate light from a specific depth
What are some of the uses for OCT
- functional imaging
- guided surgery
- embryology, endoscopy, dermatology, opthalmology
What does interferometry measure in OCT
small time delays of scattered photons
Describe the set up for OCT
- low coherence light source travels through beam expander then beam splitter
- some beams are reflected onto the sample, others are reflected off the moveable reference mirror
- the beams recombine and the signals are processed
What are the characteristics of coherent sources
- monochromatic
- definite and constant phase relation
What are two methods for obtaining two coherent sources
- wavefront splitting
- amplitude splitting
What can be said about photon sources of light
- radiate wavetrains of finite length
- more than one wavelength (spectral bandwidth)
- fixed phase relation only within individual wavetrain
Define coherence
correlation of light wave at two points in space-time : πͺ (ππ, ππ; ππ, ππ) =< π¬ (ππ, ππ) π¬ (ππ, ππ) >
Define temporal coherence
correlation of light wave along the light propagation direction : πͺ =< π¬π (π) π¬π
β (π) >=< π¬ (π + πππ) π¬β (π) >
Define coherence length
the length of the wavetrain where there is definite phase relation
Define coherence time
the time for the elementary wavetrain to pass a single point
what is the relationship between the coherence length and time
πΏπ = π ππ
What is temporal coherence a measure of
spectral bandwidth
How does temporal coherence relate to the spectral bandwidth
- high temporal coherence gives a narrow spectral bandwidth
- they are a fourier transform pair
- 1/delta nu = ππ
what is the effect if the path length difference is much less than the coherence length
the same wavetrain overlap and an interference fringe is observable
what happens if the path length difference is much greater than the coherence length
different wavetrain overlap and no interference fringe is observable
describe a partially coherent source
- a broad spectrum band (like incoherent sources)
- definite phase relation within coherence length
What is the phase change for interference with partially coherent light sources
2πΞπΏπ
What is the interference for light with continuous spectra
Ξ (ΞπΏ) β 2πΌ0 integral from 0 to β of π(π) cos (2πΞπΏππ)
where S(v) is the spectral density
How are the source spectrum and coherence function related for interference with a partially coherent light source
they are linked by the fourier transform
What is the intensity formula for interference of partially coherent sources
πΌ = πΌ1 + πΌ2 + 2 sqrt(πΌ1πΌ2) exp[β4 ln 2 (ΞπΏ/πΏπ)^2]cos(2πΞπΏπ)
What are the conditions for seeing interference fringes with partially coherent sources
optical path length are matched within the coherence length of the source
broader bandwidth = shorter Lc and higher resolution
What are the common light sources for OCT
- pulsed lasers
- scanning sources
What are the advantages of scanning sources for OCT
the reference arm is not scanned and fast scanning is feasible
How is an OCT image constructed
scanning - contrast comes from refractive index variations
What is spectral domain OCT
- no scanning
- depth intensity profile calculated from spectra due to fourier transform relationship between the autocorrelation and the spectral power density
What is swept source OCT
- no scanning of reference arm but wavelength of source changes
