Signal optimization in glass-fiber self-mixing intra-arterial laser Doppler velocimetry

Recently a blood velocimeter was developed, based on the principle of self-mixing in a semiconductor laser. This means that the intensity of the light is modulated by feedback from moving scattering particles, which contains the Doppler shift frequency. Upon feedback the characteristics of the laser diode will change. The threshold current will decrease and an instable region may become present just above the new threshold. It turns out that the amplitude of the Doppler signal is related to the difference in intensity between the situations with and without feedback. This amplitude is highest, but also most unstable, just above feedback. The suppression of reflection from the glass fiber facets is of paramount importance. Using an optical stabilization of the feedback, we are able to optimize the performance of the laser-fiber system and the Doppler modulation depth, and to clarify the behavior with a suitable physical model. The velocimeter has been used in vivo with the glass fiber inserted in normal catheters, but in upstream and in downstream situations. For the latter, the fiber facet in the liquid has been provided with a special side-reflecting device.