Elimination of amplitude-phase crosstalk in frequency domain near-infraredspectroscopy

Changes in phase that occur with changes in amplitude impose severe limitat ions on the accuracy of frequency domain near-infrared spectrometers. Phase is related to the photon pathlength in tissue and phase errors introduced by the instrument can be interpreted as changes in tissue oxygenation. The instrument described in this article employs a reference radio frequency mo dulated laser diode to eliminate the effects of amplitude-phase crosstalk a nd requires no feedback. Light from the reference laser diode does not pass through the medium under investigation but passes directly onto the detect or. The reference and medium signals follow a common path through the detec tor and so the same phase error is imposed on both. Summing the reference a nd medium phase eliminates the crosstalk and enables the resultant to be at tributed only to the photon pathlength within the medium. It is also demons trated that elimination of amplitude-phase crosstalk is a natural consequen ce of a phased array configuration. (C) 2001 American Institute of Physics.