Visualisation of human subcutaneous blood vessels by increasing coherence probing depth

An improved Fourier-domain rapid-scanning optical delay in the reference arm of a Michelson interferometer is described which allows the use of a low-power superluminescent diode (down to 0.2 mW) and of a few wavelengths simultaneously. The method of raster scanning and averaging in the sample arm of the interferometer within a pixel provides an additional increase in the signal-to-noise ratio by 4–10 dB and, in combination with optical clearing, an increase in the coherence probing depth for the human skin in vivo up to 1.5–1.6 mm. As a result, subcutaneous blood vessels are visualised for the first time and a signal from the bloodstream is detected in the transitional regime between reflected and diffusively scattered photons. The images of subcutaneous blood vessels of the human palm and finger in vivo are presented. The possibility of construction of a low-coherence Doppler spectrograph with a variable coherence length is discussed.