Primer
Spectrometer requirements for optical coherence tomography
Line rate, roll-off and sensitivity, the three specifications that determine whether an OCT system produces a usable image.
Optical coherence tomography produces micron-scale cross-sections of a material or a tissue without contact. The spectrometer performs a different task from a conventional spectroscopic measurement: it reads an interference pattern whose fringe spacing encodes depth. Three specifications dominate the design, and wavelength range is not one of them.
Line rate
Every A-scan is one spectrometer readout, and a 2D image is hundreds or thousands of A-scans. Line rate therefore sets the frame rate directly, and frame rate determines whether the system images a living, moving subject or produces a motion-blurred average of it.
This specification makes OCT a CMOS application. A CCD fast enough for clinical OCT would have read noise that makes the sensitivity unusable.
Roll-off
Sensitivity in OCT falls with depth, because deeper structure produces higher-frequency fringes and the finite resolution of the spectrometer cannot resolve them indefinitely. Roll-off, usually quoted in dB per millimeter, describes how quickly the usable image depth is exhausted.
Roll-off is set by the spectral resolution of the spectrometer and by how cleanly the optics image the spectrum onto the array. It is the specification most often omitted from datasheets and the one most worth asking about.
Sensitivity
Quoted in dB, sensitivity is the weakest reflection that can be distinguished from noise. Tissue is a weak reflector, so sensitivity determines whether the image shows structure or speckle.
Trade-offs between the three
All three cannot be maximized at once. Broader bandwidth improves axial resolution but spreads the same light over more pixels, which costs sensitivity. Higher line rate means shorter exposure per A-scan, which also costs sensitivity. Finer spectral resolution improves roll-off but usually costs bandwidth.
The right instrument depends on which of these the application can afford to lose. That is a question about the sample rather than the specification sheet, and it is best settled in a conversation with an applications engineer before a configuration is selected.
