Optical particle sensor
The invention describes a laser sensor module. The laser sensor module comprises at least a first laser ( 111 ) being adapted to emit a first measurement beam ( 111 ′) and at least a second laser ( 112 ) being adapted to emit a second measurement beam ( 112 ′). The laser sensor module further comprises an optical device ( 150 ) being arranged to redirect the first measurement beam ( 111 ′) and the second measurement beam ( 112 ′) such that the first measurement beam ( 111 ′) and the second measurement beam enclose an angle between 45° and 135°. The laser sensor module comprises one detector ( 120 ) being adapted to determine at least a first self-mixing interference signal of a first optical wave within a first laser cavity of the first laser ( 111 ) and at least a second self-mixing interference signal of a second optical wave within a second laser cavity of the second laser ( 112 ). This configuration enables determination of an average velocity of the particles despite of the fact that it is not possible to determine the components of the velocity vector. The introduced error by means of statistical variations is acceptable because the number of detected particles scales with the cubic root of the particle velocity. The invention further describes a particle sensor ( 100 ) comprising such a laser sensor module, a corresponding method and computer program product. The invention enables a simple and low-cost particle sensor ( 100 ) for detecting small particles based on laser self-mixing interference.
1. A particle sensor comprising: a laser sensor, the laser sensor comprising:
at least a first laser, wherein the first laser is arranged to emit a first measurement beam;
at least a second laser, wherein the second laser is arranged to emit a second measurement beam;
an optical device, wherein the optical device is arranged to redirect at least the first measurement beam such that the first measurement beam and the second measurement beam enclose an angle between 45° and 135°,
a detector arranged being adapted to determine at least a first self-mixing interference signal of a first optical wave within a first laser cavity of the first laser and at least a second self-mixing interference signal of a second optical wave within a second laser cavity of the second laser; and
an evaluator circuit,
wherein the evaluator is arranged to receive detection signals generated by the detector in reaction to the determined self-mixing interference signals,
wherein the evaluator circuit is arranged to determine at least one average velocity of particles using the detection signals received in a predetermined time period,
wherein the evaluator circuit is arranged to determine a particle density based on a number of self-mixing interference signals determined in the predetermined time period and the at least one average velocity.
2. The particle sensor according to claim 1 ,
wherein the laser sensor module comprises at least a third laser,
wherein the third laser is arranged to emit a third measurement beam,
wherein the optical device is arranged to redirect at least two of the three measurement beams such that the redirected measurement beams mutually enclose the same angle,
wherein the detector is arranged to determine at least a third self-mixing interference signal of a third optical wave within a third laser cavity of the third laser.
3. The particle sensor according to claim 2 , wherein the three measurement beams mutually enclose an angle of 90°.
4. The particle sensor according to claim 1 , wherein the at least first laser and the at least second laser comprise semiconductor layers provided on a common semiconductor chip.
5. The particle sensor according to claim 4 , wherein the detector is integrated in the semiconductor layers.
6. The particle sensor according to claim 1 , wherein the optical device comprises a grating.
7. The particle sensor according to claim 4 , wherein the optical device comprises a surface grating integrated in the semiconductor layers.
8. The particle sensor according to claim 2 ,
wherein the optical device comprises micro-optical components,
wherein the micro-optical components redirect the three measurement beams emitted by the three lasers, and
wherein each micro-optical component is attached to one of the three lasers.
9. The particle sensor according to claim 8 ,
wherein the optical device further comprises at least one focusing element associated with each of the three measurement beams,
wherein the at least one focusing element is arranged to focus the respective measurement beam on a focus region.
10. The particle sensor according to claim 2 further comprising an electrical driver, wherein the electrical driver is arranged to electrically drive the three lasers such that the each of the three lasers emit the three measurement beams.
11. An air purifier, a sensor box or a wearable device comprising the particle sensor according to claim 1 .
12. A method of particle detection, the method comprising the steps of:
emitting at least a first measurement beam using a first laser,
emitting at least a second measurement beam using a second laser,
redirecting at least the first measurement beam such that the first measurement beam and the second measurement beam enclose an angle between 45° and 135°,
determining at least one self-mixing interference signals of a first optical wave within a first laser cavity of the first laser and at least a second optical wave within a second laser cavity of the second laser using a detector within a predetermined time period,
determining at least one average velocity based on the determined self-mixing interference signals,
determining a particle density based on the determined average velocity and a number of self-mixing interference signals determined within the predetermined time period.
13. The method according to claim 12 :
determining at least a first average velocity based on self-mixing interference signals determined in a first time period,
determining at least a second average velocity based on self-mixing interference signals determined in a second time period,
determining the number of determined self-mixing interference signals in the predetermined time period comprising the first and the second time period,
determining the particle density based on the at least first average velocity, the at least second average velocity and the corresponding number of determined self-mixing interference signals.
14. A computer program product comprising computer code wherein the computer code is arranged to perform the method according claim 12 .
15. The method according to claim 12 , emitting at least a third measurement beam using a third laser,
wherein an optical device is arranged to redirect at least two of the three measurement beams such that the redirected measurement beams mutually enclose the same angle; and determining at least at least a third self-mixing interference signal of a third optical wave within a third laser cavity of the third laser.
16. The method according to claim 15 , wherein the three measurement beams mutually enclose an angle of 90°.
17. The method according to claim 12 , wherein the at least first laser and the at least second laser comprise semiconductor layers provided on a common semiconductor chip.
18. The method according to claim 12 , wherein the detector is integrated in the semiconductor layers.
19. The method according to claim 15 , further comprising redirecting the three measurement beams emitted by the three lasers.
20. The method according to claim 15 , further comprising focusing each measurement beam on a focus region.