Speckle noise reduction in coherent spatial frequency domain imaging

This work applies spatial frequency domain imaging (SFDI) to extract optical absorption and reduced scattering values (μa and μs’) of tissue-like samples, with a coherent light source. Here, we evaluate SFDI illuminated by a multi-wavelength vertical-cavity-surface-emitting-laser (VCSEL) array, with selectable wavelengths of 680nm, 795nm and 850nm. The speckle noise is mitigated with a rapid current sweep and with the overlapping of up to three diode lasers per wavelength. The current sweep spans over the single- and multi- transverse mode regimes, to achieve a shorter coherence length and therefore reduce speckle-related noise. Results show a more accurate measurement for the reduced scattering than the absorption coefficient (average error of 1.8% and 11.5% respectively, with 850nm diodes) on tissue phantoms, with the current sweep applied, which is comparable to an incoherent LED source (4.4% and 13.6%). Further reduction in speckle-related noise is observed by illuminating with multiple diodes of the same wavelength. At 850nm, illumination with three diodes causes a noise of 11.5±3.3% in absorption measurements, which is lower than the noise when using a single diode (14.9±4.3%).