Laser diode drive scheme for noise reduction in photoplethysmographic measurements
First Claim
1. A photoplethysmographic measurement apparatus for determining a blood analyte level in a tissue under test, said apparatus comprising:
- a plurality of laser diodes operable to transmit a corresponding plurality of light signals centered at different predetermined wavelengths through the tissue under test in response to a corresponding plurality of drive signals;
a detector positionable to detect at least a portion of said light signals after transmission through the tissue under test and operable to output a multiplexed signal indicative of an intensity of said detected portion of said light signals;
a drive signal generator operable to supply said drive signals to said laser diodes, wherein each said drive signal includes a modulation frequency and a modulation depth, wherein said modulation frequency and modulation depth of each said drive signal are set to achieve operation of its corresponding laser diode at a desired laser intensity noise level; and
a demodulator operable to demodulate said multiplexed signal to obtain signal portions corresponding with each of said light signals, wherein said signal portions are employable to determine a blood analyte level in the tissue under test.
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Abstract
The present invention discloses a photoplethysmographic measurement apparatus and related method for determining a blood analyte level in a tissue under test employing an inventive laser diode drive scheme to achieve noise reduction. Noise reduction is achieved by driving a plurality of laser diodes with modulated drive signals to cause emission of light signals from the laser diodes that are directed through the tissue under test and from which various blood analyte levels are determinable based upon the intensities of the transmitted light signals. Each drive signal is modulated at an appropriate modulation frequency that causes its corresponding laser diode to operate in a low noise regime wherein laser intensity noise is reduced, and the modulation depth of each drive signal is set to broaden the line width of the laser diode and thereby reduce the potential for optical feedback noise. In this regard, the modulation frequency and depth of each drive signal may be set to achieve operation of its corresponding laser diode at a desired laser intensity noise level. The desired laser intensity noise level may be near that (e.g., within the same order of magnitude) of the independent laser RIN level.
119 Citations
32 Claims
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1. A photoplethysmographic measurement apparatus for determining a blood analyte level in a tissue under test, said apparatus comprising:
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a plurality of laser diodes operable to transmit a corresponding plurality of light signals centered at different predetermined wavelengths through the tissue under test in response to a corresponding plurality of drive signals;
a detector positionable to detect at least a portion of said light signals after transmission through the tissue under test and operable to output a multiplexed signal indicative of an intensity of said detected portion of said light signals;
a drive signal generator operable to supply said drive signals to said laser diodes, wherein each said drive signal includes a modulation frequency and a modulation depth, wherein said modulation frequency and modulation depth of each said drive signal are set to achieve operation of its corresponding laser diode at a desired laser intensity noise level; and
a demodulator operable to demodulate said multiplexed signal to obtain signal portions corresponding with each of said light signals, wherein said signal portions are employable to determine a blood analyte level in the tissue under test. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, 31, 32)
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12. A photoplethysmographic measurement apparatus for determining a blood analyte level in a tissue under test, said apparatus comprising:
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a plurality of laser diodes for transmitting a corresponding plurality of light signals centered at different predetermined wavelengths through the tissue under test, wherein each said laser diode is modulated by a corresponding drive signal having a modulation frequency between a lower frequency limit corresponding to a −
3 db point on a 1/f noise versus frequency curve of said photoplethysmographic measurement apparatus and an upper frequency limit corresponding to a relaxation oscillation frequency of its corresponding laser diode;
a detector for detecting at least a portion of said light signals after transmission through the tissue under test and outputting a multiplexed signal indicative of an intensity of said detected portion of said light signals; and
a demodulator for demodulating said multiplexed signal to output signal portions corresponding with each of said light signals, wherein said signal portions are employable to determine a blood analyte level in the tissue under test.
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17. A method for use in photoplethysmographic measurement of a blood analyte level in a tissue under test, said method comprising:
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transmitting a plurality of light signals at different predetermined center wavelengths through the tissue under test by driving a corresponding plurality of laser diodes with a corresponding plurality of drive signals, wherein each drive signal has a modulation frequency and a modulation depth, and wherein the modulation frequency and modulation depth of each drive signal are set to achieve operation of its corresponding laser diode at a desired laser intensity noise level;
detecting at least a portion of the light signals;
outputting a multiplexed signal indicative of an intensity of the detected portion of the light signals; and
demodulating the multiplexed signal to output signal portions corresponding with each of the light signals, wherein the signal portions are employable to determine a blood analyte level in the tissue under test.
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28. An apparatus for driving a plurality of laser diodes in a photoplethysmographic probe, said apparatus comprising:
a drive signal generator operable to supply each of the laser diodes with a corresponding drive signal, wherein each said drive signal has a modulation frequency and a modulation depth, wherein said modulation frequency and modulation depth of each said drive signal are set to achieve operation of its corresponding laser diode at a desired laser intensity noise level.
Specification