Changes in phase that occurs with changes in amplitude impose severe limitations 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 employs a reference RF modulated laser diode to eliminate the effects of amplitude- phase crosstalk and requires no feedback. Light from the reference and medium channels follow a common path through the detector and so the same phase error is imposed on both. Summing the reference and medium phase eliminates the crosstalk and enables the resultant to be attributed only to photon pathlength within the medium. It is also demonstrated that elimination of amplitude-phase crosstalk is a natural consequence of a phased array configuration.

Date Published: 29 June 2001
PDF: 7 pages
Proc. SPIE 4250, Optical Tomography and Spectroscopy of Tissue IV, (29 June 2001); doi: 10.1117/12.434536

Published in SPIE Proceedings Vol. 4250:
Optical Tomography and Spectroscopy of Tissue IV
Britton Chance; Robert R. Alfano; Bruce J. Tromberg; Mamoru Tamura; Eva Marie Sevick-Muraca, Editor(s)