Coherence and OCT Flashcards

1
Q

What is OCT

A

a 3d imaging technique with high spatial resolution and large penetration depth even in highly scattering media

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2
Q

What principles is OCT based on

A
  • 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
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3
Q

What are some of the uses for OCT

A
  • functional imaging
  • guided surgery
  • embryology, endoscopy, dermatology, opthalmology
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4
Q

What does interferometry measure in OCT

A

small time delays of scattered photons

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5
Q

Describe the set up for OCT

A
  • 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
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6
Q

What are the characteristics of coherent sources

A
  • monochromatic
  • definite and constant phase relation
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7
Q

What are two methods for obtaining two coherent sources

A
  • wavefront splitting
  • amplitude splitting
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8
Q

What can be said about photon sources of light

A
  • radiate wavetrains of finite length
  • more than one wavelength (spectral bandwidth)
  • fixed phase relation only within individual wavetrain
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9
Q

Define coherence

A

correlation of light wave at two points in space-time : πšͺ (π’“πŸ, π’•πŸ; π’“πŸ, π’•πŸ) =< 𝑬 (π’“πŸ, π’•πŸ) 𝑬 (π’“πŸ, π’•πŸ) >

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10
Q

Define temporal coherence

A

correlation of light wave along the light propagation direction : πšͺ =< 𝑬𝒂 (𝒕) 𝑬𝒃
βˆ— (𝒕) >=< 𝑬 (𝒕 + 𝒕𝒃𝒂) π‘¬βˆ— (𝒕) >

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11
Q

Define coherence length

A

the length of the wavetrain where there is definite phase relation

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12
Q

Define coherence time

A

the time for the elementary wavetrain to pass a single point

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13
Q

what is the relationship between the coherence length and time

A

𝐿𝑐 = 𝑐 πœπ‘

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14
Q

What is temporal coherence a measure of

A

spectral bandwidth

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15
Q

How does temporal coherence relate to the spectral bandwidth

A
  • high temporal coherence gives a narrow spectral bandwidth
  • they are a fourier transform pair
  • 1/delta nu = πœπ‘
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16
Q

what is the effect if the path length difference is much less than the coherence length

A

the same wavetrain overlap and an interference fringe is observable

17
Q

what happens if the path length difference is much greater than the coherence length

A

different wavetrain overlap and no interference fringe is observable

18
Q

describe a partially coherent source

A
  • a broad spectrum band (like incoherent sources)
  • definite phase relation within coherence length
19
Q

What is the phase change for interference with partially coherent light sources

A

2πœ‹Ξ”πΏπœˆ

20
Q

What is the interference for light with continuous spectra

A

Ξ“ (Δ𝐿) β‰ˆ 2𝐼0 integral from 0 to ∞ of 𝑆(𝜈) cos (2πœ‹Ξ”πΏπœˆπ‘–)
where S(v) is the spectral density

21
Q

How are the source spectrum and coherence function related for interference with a partially coherent light source

A

they are linked by the fourier transform

22
Q

What is the intensity formula for interference of partially coherent sources

A

𝐼 = 𝐼1 + 𝐼2 + 2 sqrt(𝐼1𝐼2) exp[βˆ’4 ln 2 (Δ𝐿/𝐿𝑐)^2]cos(2πœ‹Ξ”πΏπœˆ)

23
Q

What are the conditions for seeing interference fringes with partially coherent sources

A

optical path length are matched within the coherence length of the source
broader bandwidth = shorter Lc and higher resolution

24
Q

What are the common light sources for OCT

A
  • pulsed lasers
  • scanning sources
25
What are the advantages of scanning sources for OCT
the reference arm is not scanned and fast scanning is feasible
26
How is an OCT image constructed
scanning - contrast comes from refractive index variations
27
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
28
What is swept source OCT
- no scanning of reference arm but wavelength of source changes