Researchers have used software to sharpen a quantum imaging system's depth measurements after its optics blurred the signal. In a laboratory test, the measured response to a reflecting surface narrowed from 5 micrometres to about 2.7 micrometres. This measure, called axial resolution, indicates how well the system can distinguish features lying close together along the direction of the beam. Lower values mean finer depth detail.
Optical coherence tomography, or OCT, finds structures at different depths by comparing light returned from a sample with light travelling along a reference path. Light of different wavelengths travels differently through optical materials, blurring the resulting depth profile. This effect is called chromatic dispersion. Quantum OCT uses correlated photon pairs to cancel some of that distortion automatically, but residual components, called odd-order dispersion, still matter when trying to resolve very fine detail.
The team's Fourier-domain quantum OCT system collects the wavelength information needed to calculate a depth profile in software. It uses paired infrared photons centred at 1,550 nanometres, a wavelength also used in fibre telecommunications. One photon takes the reference path and the other the sample path. Detectors record their paired arrivals after long fibre spools spread different wavelengths into different arrival times.
To remove the remaining distortion, the team first measured a mirror at a known position. It used that calibration to estimate the wavelength-dependent error, then applied the opposite correction before calculating later depth profiles. Across nine mirror positions, corrected responses were sharper, approaching the source's theoretical resolution of 2.7 micrometres. The team also showed clearer boundaries in a glass sample.
Resolution did not remain constant across the whole scan. Beyond roughly 200 micrometres, finite spectral resolution reduced fringe visibility and broadened the peaks. With 5-kilometre fibre spools, the useful imaging depth was about 0.35 millimetres. Replacing them with 10-kilometre spools made the second interface of a 260-micrometre plastic sheet visible and extended the range to about 0.7 millimetres.
The extra depth carried a steep speed cost. Collecting one full spectrum took about 1.5 minutes with the shorter fibre spools and five minutes with the longer ones. The preprint demonstrates the correction on mirrors, glass and plastic, not biological tissue. It improves depth detail under those conditions without resolving the system's slow acquisition.