Time-division multiplexing in a multi-wavelength photon density wave system

A patient monitor comprising:

• one or more light sources configured to emit a plurality of wavelengths of light;
• a driving circuit configured to modulate the one or more light sources at one or more modulation frequencies that exceed the product of a mean absorption coefficient of a pulsatile tissue of the patient multiplied by the speed of light and are sufficient to produce resolvable photon density waves in the pulsatile tissue of the patient to produce a plurality of single-wavelength photon density wave input signals at non-overlapping intervals;
• a fiber coupler configured to combine the plurality of single-wavelength photon density wave input signals into a multi-wavelength photon density wave input signal in which each of the single-wavelength photon density wave signals is alternatingly the sole wavelength active in the multi-wavelength photon density wave input signal for periods of time brief enough to enable each of the single-wavelength photon density wave signals to pass through pulsatile tissue of the patient such that there is no perceptible change in the pulsatile tissue of the patient between the start of each of the multi-wavelength photon density wave input signals and the start of each subsequent multi-wavelength photon density wave input signal;
• a sensor cable connector configured to provide the multi-wavelength photon density wave input signal to a first optical cable of a medical sensor configured to be attached to the patient and to receive a multi-wavelength photon density wave output signal from a second optical cable of the medical sensor;
• and
• data processing circuitry configured to time-division demultiplex the multi-wavelength photon density wave output signal into a plurality of single-wavelength photon density wave output signals and to determine a physiological parameter of the patient based at least in part on a comparison of one of the plurality of single-wavelength photon density wave output signals to a corresponding one of the plurality of single-wavelength photon density wave input signals, wherein the data processing circuitry is further configured to determine an absorption property of the patient based at least in part on a change in amplitude between the one of the plurality of single-wavelength photon density wave output signals and the corresponding one of the plurality of single-wavelength photon density wave input signals, and to determine the mean absorption coefficient based on the determined absorption property.

A method comprising:

• generating a first photon density wave signal by modulating a light source of a first wavelength using a driving circuit at a frequency that exceeds the product of a mean absorption coefficient of a pulsatile tissue of the patient multiplied by the speed of light;
• generating a second photon density wave signal by modulating a light source of a second wavelength at the modulation frequency that exceeds the product of the mean absorption coefficient of the pulsatile tissue of the patient multiplied by the speed of light by using the driving circuit;
• combining, using a fiber coupler, the first photon density wave signal and the second photon density wave signal into a multi-wavelength photon density wave input signal in which each of the first and second photon density wave signals is alternatingly the sole wavelength active in the multi-wavelength photon density wave input signal for periods of time brief enough to enable each of the first and second photon density wave signals to pass through pulsatile tissue of a patient such that there is no perceptible change in the pulsatile tissue of the patient between the start of each of the multi-wavelength photon density wave input signals and the start of each subsequent multi-wavelength photon density wave input signal;
• emitting the multi-wavelength photon density wave input signal through the pulsatile tissue of the patient for a period of time using a sensor coupled to an input optical cable and attached to the patient;
• receiving a multi-wavelength photon density wave output signal that results when the multi-wavelength photon density wave input signal is emitted through the pulsatile tissue of the patient into an output optical cable using the sensor attached to the patient;
• determining an absorption property of the pulsatile tissue of the patient based at least in part on a comparison between the multi-wavelength photon density wave output signal and the multi-wavelength photon density wave input signal;
• and
• determining the mean absorption coefficient of the pulsatile tissue of the patient based on the determined absorption property.