Sources of phase noise in homodyne and heterodyne phase modulation devices used for tissue oximetry studies

Abstract

The objective of this study was to characterize sources of phase noise in homodyne and heterodyne phase modulation devices (PMDs) used for tissue oximetry measurements. Each PMD incorporates a laser diode modulated at a radio frequency in the 50–200 MHz range, an optical detector and a homodyne/heterodyne phase sensitive detector. The intensity modulated light which propagates through tissue is attenuated and undergoes a phase shift, which reflects the mean time of flight of the photons through the tissue. The measured amplitude and phase can be used to determine hemoglobin saturation in tissues using equations based on diffusion theory. Four studies were performed to characterize the sources of phase noise. First, the signal to noise ratio was characterized to determine if the PMDs are operating at the shot noise or detector noise limit. Second, the accuracy of the three PMDs for measuring phase shifts in tissue were compared by using them to measure the phase shift as a function of path length change in air, at a constant amplitude, and at signal to noise ratios comparable to that measured from tissue. The third source of noise measured was the phase shift that results from optical attenuation of the signal (phase-amplitude cross talk) at a constant path length, to characterize intensity dependent phase shifts in the PMDs. Finally, the interchannel interference of a dual wavelength PMD which uses radio frequency multiplexing to perform phase measurements at two wavelengths simultaneously was compared to that of a dual wavelength PMD which uses time multiplexing to perform phase measurements at two wavelengths serially to determine the effect of each on phase error.