Method and apparatus for converting a modulated optical signal to an electrical signal with correction for modulation jitter

#This invention relates to applications of photodetectors and particularly to accurate conversion of detected optical information to electrical signals.

#Modulated light sources are used in a variety of applications involving the detection of light reflected from or transmitted through an object, to determine certain properties or characteristics of the object. By way of example, a modulated near infrared light source may be used in connection with the determination of concentrations of various substances, or analytes, in the bloodstream of animals.

#A common method of providing a modulated optical signal is to provide a continuous optical source and periodically interrupting the light from the continuous optical source with a chopper. The chopper may be, for example, a rotating segmented disk driven by an electric motor.

#FIG. 1 is a graph showing the ratio of signal out to light in against frequency for a typical lead-sulfide photodetector.

#FIG. 2 is a schematic illustration of a system according to the invention.

#FIG. 3 is a circuit diagram of an exemplary pre-emphasis circuit in a system according to the invention.

#Referring now to FIG. 1, there is shown the typical response of a lead-sulfide photodetector, as a ratio of electrical signal out to light signal received, as a function of the signal frequency in Hertz. Both axes are shown plotted on a base 10 logarithmic scale. As noted above, lead-sulfide photodetectors are the preferred detectors for converting near infrared optical signals to electrical signals.

#Referring now to FIG. 2, there is shown, schematically, the method and apparatus for irradiating an object, receiving optical radiation emitted from the object, converting the emitted radiation to electrical signals, and compensating the output of the electrical signals. Continuous light source 10 may be an infrared radiation source. In the application in which the body of a mammal is irradiated with near infrared radiation to determine the concentration of analytes in the blood, the infrared radiation source is preferably a tungsten filament bulb which is maintained thermally isolated, with a constant current provided through the filament, and a infrared filter, to minimize the transmission of radiation outside the desired wavelength range. The infrared radiation source may, for example, provide radiation in wave lengths between about 1,000 nm and about 2,500 nm.

#Light from continuous light source 10 is periodically interrupted by chopper 20. Chopper 20, may be, for example, a conventional rotating segmented disk. A simple electrical motor is typically provided to cause chopper 20 to rotate. As a result, the radiation emitted by continuous light source 10 is converted to a modulated near infrared signal. As will be readily appreciated, the use of a modulated signal permits separation of the signal from noise and background radiation. The modulated signal is then transmitted to object 30. In a typical application of the technique, the signal is transmitted by one or more optical fibers. As noted above, the object may be the skin of a mammal, although other applications of the method of the invention are possible.