Optical-fiber finger photo-plethysmograph using digital techniques

This paper presents digital techniques to design an optical-fiber photo-plethysmograph. This noninvasive method is based on the measurement of the intensity of an infrared-light beam, which penetrates the index finger. Sinusoidal modulation of the light-beam by driving an infrared LED is proposed. The method of direct digital synthesis is used to produce a very stable sinusoidal waveform. Optical fibers are used to transfer the light to and from the sampling point. Synchronous amplitude demodulation is used here instead of using analog low-pass filters, so the information concerning the amplitude is almost noiseless. Off-line frequency analysis of the peripheral volume pulse-wave signal is performed. The power content in the harmonics of this pulse wave may be a useful measure of aging and vascular disease.