Particulate matter sensors based on split beam self-mixing interferometry sensors

1. A particulate matter sensor, comprising:

a self-mixing interferometry sensor configured to generate a self-mixing interference signal;

a set of one or more optical elements positioned to receive an optical emission of the self-mixing interferometry sensor, split the optical emission into multiple beams, direct each beam of the multiple beams in a different direction towards a respective measurement region, and redirect received reflections or backscatters of the multiple beams from the respective measurement regions back into the self-mixing interferometry sensor to generate the self-mixing interference signal, wherein each of the multiple beams is separated from other beams of the multiple beams by 120 degrees in a plane perpendicular to an axis of the optical emission and each of the multiple beams has a secondary axis that diverges from the axis of the optical emission by a same angle θ; and

circuitry configured to:

extract, from the self-mixing interference signal, particle speed information for particles passing through any of the multiple beams;

estimate a particle speed using the angle θ and using the particle speed information for particles passing through any of the multiple beams interchangeably;

estimate, using the particle speed, an air flow volume through respective measurement regions of the multiple beams;

count a number of time-domain disturbances in the self-mixing interference signal over a period of time, the number of time-domain disturbances corresponding to the number of particles passing through the respective measurement regions over the period of time; and

estimate a particulate matter concentration using the number of particles, and the air flow volume.

2. The particulate matter sensor of claim 1 , wherein the particle speed information comprises Doppler frequency shifts.

3. The particulate matter sensor of claim 2 , wherein the circuitry is further configured to:

perform a frequency domain analysis to extract the Doppler frequency shifts from the self-mixing interference signal.

4. The particulate matter sensor of claim 2 , wherein the circuitry is further configured to:

perform a time-frequency domain analysis to extract the Doppler frequency shifts from the self-mixing interference signal.

5. The particulate matter sensor of claim 2 , wherein the Doppler frequency shifts comprise unsigned Doppler frequency shifts.

6. The particulate matter sensor of claim 1 , wherein the angle θ is selected such that cos 2 (θ)=⅓.

7. The particulate matter sensor of claim 1 , wherein the circuitry is further configured to detect an existence of particulate matter using the particle speed.

8. The particulate matter sensor of claim 1 , wherein the multiple beams consist of three beams.

9. The particulate matter sensor of claim 1 , wherein the self-mixing interferometry sensor comprises an electromagnetic radiation source integrated with a photodetector.

10. The particulate matter sensor of claim 1 , wherein:

the self-mixing interferometry sensor comprises an electromagnetic radiation source and a photodetector;

the electromagnetic radiation source has a resonant optical cavity bounded by first and second mirrors, with each of the first mirror and the second mirror being at least partially transmissive to a wavelength of electromagnetic radiation; and

the electromagnetic radiation source is stacked on the photodetector.

11. The particulate matter sensor of claim 1 , wherein:

the set of one or more optical elements is a first set of one or more optical elements; and

the particulate matter sensor further comprises a second set of one or more optical elements configured to receive the set of multiple beams and redirect the set of multiple beams toward a set of overlapping or consonant measurement regions.

12. A method of sensing particulate matter, comprising:

splitting an optical emission received from a self-mixing interferometry sensor into multiple beams;

directing each beam of the multiple beams in a different direction towards a respective measurement region, wherein each of the multiple beams is separated from other beams of the multiple beams by 120 degrees in a plane perpendicular to an axis of the optical emission and each of the multiple beams has a secondary axis that diverges from the axis of the optical emission by a same angle θ;

redirecting received reflections or backscatters of the multiple beams from the respective measurement regions back into the self-mixing interferometry sensor to generate the self-mixing interferometry signal;

extracting, from the self-mixing interference signal, particle speed information for particles passing through any of the multiple beams;

estimating a particle speed using the angle θ, and the particle speed information for particles passing through any of the multiple beams interchangeably;

estimating, using the particle speed, an air flow volume through respective measurement regions of the multiple beams;

counting a number of time-domain disturbances in the self-mixing interference signal over a period of time, the number of time-domain disturbances corresponding to the number of particles passing through the respective measurement regions over a period of time; and

estimating a particulate matter concentration using the number of particles, and the air flow volume.

13. The method of claim 12 , wherein the particle speed information comprises Doppler frequency shifts, and the method further comprises:

performing a frequency domain analysis to extract the Doppler frequency shifts from the self-mixing interference signal.

14. The method of claim 12 , wherein: the angle θ is selected such that cos 2 (θ)=⅓.

15. The method of claim 12 , wherein the particle speed information comprises Doppler frequency shifts, and the method further comprises:

performing a time-frequency domain analysis to extract the Doppler frequency shifts from the self-mixing interference signal.

16. The method of claim 15 , wherein the Doppler frequency shifts comprise unsigned Doppler frequency shifts.

17. The method of claim 12 , further comprising:

receive the set of multiple beams and redirect the set of multiple beams toward a set of overlapping or consonant measurement regions.