Optical Coherence Tomography: Coherence-Gated Depth Imaging

Source & Sample

Center wavelength λ₀ 850 nm

Source bandwidth Δλ 50 nm

Lateral scan speed 15 col/s

Tissue sample

Live A-scan (current column)

Readouts

Axial res. Δz (free space)

— µm

Axial res. Δz (in tissue)

— µm

B-scan progress

How it works

OCT is low-coherence Michelson interferometry. A broadband source (center wavelength λ₀, bandwidth Δλ) is split between a reference mirror and the tissue sample; light only interferes when the two arms' optical path lengths match to within the source's coherence length — this is coherence gating, and it is what lets the device pick out one depth at a time without any moving parts inside the tissue.

Coherence length (axial resolution):
  Δz = (2·ln2 / π) · λ₀² / Δλ

Coherence-gated echo from an interface at optical
path Δz_i, detected at scan depth z:
  A(z) = Σᵢ √Rᵢ · exp(−4·ln2·((z − Δz_i)/Δz)²)

Fresnel reflectivity at a refractive-index step:
  R = ((n₁ − n₂) / (n₁ + n₂))²

λ₀ / Δλ — a wider source bandwidth shortens the coherence length, sharpening axial resolution but also lowering signal per gate; this is the fundamental OCT resolution–sensitivity trade-off.
Scan speed — the probe sweeps laterally column by column; each column is a full axial (A-)scan, and the stack of A-scans is the B-scan cross-section shown on the right.
Tissue sample — each preset sets the refractive indices and layer thicknesses; every visible boundary in the B-scan is a genuine Fresnel reflection computed from the index mismatch, with speckle noise typical of real OCT.
Real-world relevance: this exact mechanism resolves retinal layers to ~5 µm in ophthalmology, and intravascular OCT catheters use it to image coronary plaque and stent apposition in cardiology.

Scanning beam (A-scan) Live B-scan image

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