Optical particle sensor module

1. A laser sensor module for detecting a particle density of small particles with a particle size between 0.05 μm and 10 μm, the laser sensor module comprising:

a first laser configured to emit a first measurement beam;

a second laser configured to emit a second measurement beam;

an optical arrangement configured to focus the first measurement beam to a first measurement volume and to focus the second measurement beam to a second measurement volume, the optical arrangement including a first numerical aperture corresponding to the first measurement beam and a second numerical aperture corresponding to the second measurement beam, wherein the first numerical aperture and the second numerical aperture are arranged to detect a predetermined minimum particle size at a reference velocity, wherein the reference velocity is chosen within a predetermined velocity range between 0.01 m/s and 7 m/s comprising the reference velocity, and wherein the first measurement beam and the second measurement beam mutually enclose an angle ϕ between 10 0 and 160°;

a first detector configured to determine a first self-mixing interference signal of a first optical wave within a first laser cavity of the first laser;

a second detector configured to determine a second self-mixing interference signal of a second optical wave within a second laser cavity of the second laser; and

an evaluator configured to:

receive detection signals generated by the first detector and the second detector in reaction to the determined self-mixing interference signals,

determine a first average velocity of particles detected by the first detector and a second average velocity of particles detected by the second detector by using the detection signals received in a predetermined time period,

determine a first number of particles based on the detection signals provided by the first detector in the predetermined time period and a second number of particles based on the detected signals provided by the second detector in the predetermined time period, and

determine a particle density based on an average particle velocity determined by the first average velocity and the second average velocity, the first number of particles and the second number of particles, and

correct the determined particle density by a factor comprising a cube root of the ratio between the reference velocity and the determined average particle velocity.

2. The laser sensor module according to claim 1 , wherein the first measurement beam encloses a first angle β 1 with a reference surface, wherein the second measurement beam encloses a second angle β 2 with the reference surface, wherein a projection of the first measurement beam on the reference surface and a projection of the second measurement beam on the reference surface enclose an angle γ between 20° and 160°.

3. The laser sensor module according to claim 1 , wherein the first and the second numerical aperture are between 0.01 and 0.06 with respect to the measurement beams, wherein the reference velocity of the detected particles is less than 1 m/s.

4. The laser sensor module according to claim 1 , wherein the reference velocity is chosen such that error minimization within the predetermined velocity range comprising the reference velocity is symmetric with respect to the reference velocity.

5. The laser sensor module according to claim 1 , wherein the first measurement volume is linearly extended in a direction of the first measurement beam,

wherein the second measurement volume is linearly extended in a direction of the second measurement beam,

wherein the evaluator is further configured to determine a first relative likelihood for detection of particles in the first measurement volume and to determine a second relative likelihood for detection of particles in the second measurement volume, and

wherein the evaluator is further configured to correct the determined particle density by using the first relative likelihood and the second relative likelihood.

6. The laser sensor module according to claim 1 , wherein the evaluator is further configured to determine a first particle count rate at a first signal to noise ratio threshold level and a second particle count rate at a second signal to noise ratio threshold level different than the first signal to noise ratio threshold level, and wherein the evaluator is further configured to correct the determined particle density by using the first particle count rate and the second particle count rate.

7. The laser sensor module according to claim 1 , further comprising:

a third laser configured to emit a third measurement beam, wherein the optical arrangement is further configured to focus the third measurement beam to a third measurement volume, wherein the first measurement beam, the second measurement beam, and the third measurement beam mutually enclose an angle between 10° and 110°; and

a third detector configured to determine a third self-mixing interference signal of a third optical wave within a third laser cavity of the third laser,

wherein the evaluator is further configured to:

receive detection signals generated by the third detector,

determine at a third average velocity of particles detected by the third detector within the predetermined time period,

determine a third number of particles by using the detection signals generated by the third detector in the predetermined time period, and

determine a particle density based on an average particle velocity determined by using the first average velocity, the second average velocity and the third average velocity and the first number of particles, the second number of particles, and the third number of particles.

8. The laser sensor module according to claim 1 , wherein the laser sensor module is further configured to detect the particle density in a first mode and to detect a proximity of an object with a size of at least 1 mm in a second mode.

9. The laser sensor module according to claim 1 , wherein the first measurement beam is emitted from a first transmissive area of a reference surface,

wherein the second measurement beam is emitted from a second transmissive area of the reference surface, and

wherein the laser sensor module is configured to detect the particle density of the small particles in a particle flow parallel to the reference surface.

10. The laser sensor module according to claim 9 , wherein the laser sensor module is configured to detect the particle density of the small particles in a particle flow parallel to the reference surface and at a range of detection distances from the reference surface, and

wherein the first numerical aperture and the second numerical aperture are selected to provide detection distances between 3 and 10 mm.

11. The laser sensor module according to claim 9 , wherein the first measurement beam encloses a first angle β 1 with the reference surface,

wherein the second measurement beam encloses a second angle β 2 with the reference surface, and

wherein a projection of the first measurement beam on the reference surface and a projection of the second measurement beam on the reference surface enclose an angle γ between 20° and 160°.

12. The laser sensor module according to claim 11 , wherein the laser sensor module is configured to detect the particle density of the small particles in a particle flow parallel to the reference surface and at a range of detection distances from the reference surface, and

wherein the first numerical aperture and the second numerical aperture are selected to provide detection distances between 3 and 10 mm.

13. The laser sensor module according to claim 9 , wherein the first laser and the second laser comprise semiconductor layers provided on a single semiconductor chip.

14. A laser sensor module for detecting a particle density of small particles with a particle size between 0.05 μm and 10 μm, the laser sensor module comprising:

a first laser configured to emit a first measurement beam;

a second laser configured to emit a second measurement beam;

an optical arrangement configured to focus the first measurement beam to a first measurement volume and to focus the second measurement beam to a second measurement volume, wherein the optical arrangement includes a first numerical aperture corresponding to the first measurement beam and a second numerical aperture corresponding to the second measurement beam, wherein the first numerical aperture and the second numerical aperture are arranged with respect to the measurement beams to detect a predetermined minimum particle size at a reference velocity, wherein the reference velocity is chosen within a predetermined velocity range between 0.01 m/s and 7 m/s comprising the reference velocity, and wherein the first measurement beam and the second measurement beam mutually enclose an angle ϕ between 10° and 160°;

a first detector configured to determine a first interference signal,

a second detector configured to determine a second interference signal,

an evaluator configured to:

receive detection signals generated by the first detector and the second detector in reaction to the determined interference signals,

determine a first average velocity of particles detected by the first detector and a second average velocity of particles detected by the second detector from the detection signals received in a predetermined time period,

determine a first number of particles based on the detection signals provided by the first detector in the predetermined time period and a second number of particles based on the detection signals provided by the second detector in the predetermined time period,

determine a particle density based on an average particle velocity determined by the first average velocity and the second average velocity, at least the first number of particles and at least the second number of particles, and

correct the determined particle density by a factor comprising a cube root of the ratio between the reference velocity and the determined average particle velocity.

15. The laser sensor module according to claim 14 , wherein the laser sensor module is configured to provide a first reference beam based on the first measurement beam by partial reflection of the first measurement beam and a second reference beam based on the second measurement beam by partial reflection of the second measurement beam, wherein the first detector is configured to determine the first interference signal based on interference of reflected light of the first measurement beam and the first reference beam, and wherein the second detector is configured to determine the second interference signal based on interference of reflected light of the second measurement beam and the second reference beam.

16. The laser sensor module according to claim 14 , wherein the first detector is separated from the first laser, wherein the second detector is separated from the second laser, wherein the laser sensor module further comprises a first beam splitter configured to provide the first reference beam, and wherein the laser sensor module further comprises a second beam splitter configured to provide the second reference beam.

17. A mobile communication device comprising the laser sensor module according to claim 1 , wherein the mobile communication device is arranged to present measurement results provided by the laser sensor module.

18. A method for detecting particles with a particle size between 0.05 μm and 10 μm, the method comprising:

emitting a first measurement beam by a first laser,

emitting a second measurement beam by a second laser,

focusing the first measurement beam with a first numerical aperture, wherein the first numerical aperture is arranged to detect a predetermined minimum particle size at a reference velocity, wherein the reference velocity is within a predetermined velocity range between 0.01 m/s and 7 m/s,

focusing the second measurement beam with a second numerical aperture, wherein the second numerical aperture is arranged to detect a predetermined minimum particle size at a reference velocity, wherein the reference velocity is within a predetermined velocity range,

determining a first interference signal or first self-mixing interference signal of a first optical wave within a first laser cavity of the first laser,

determining a second interference signal or second self-mixing interference signal of a second optical wave within a second laser cavity of the second laser,

determining a first average velocity based on the first interference signals or the first self-mixing interference signals determined in a predefined time period,

determining a second average velocity based on the second interference signals or the second self-mixing interference signals determined in a predefined time period,

determining a first number of particles by the first interference signals or the first self-mixing interference signals determined in the predefined time period,

determining a second number of particles by the second interference signals or the second self-mixing interference signals determined in the predefined time period,

determining an average velocity based at least in part on the first average velocity and the second average velocity,

determining a particle density based at least in part on the determined average velocity, the first number of particles and the second number of particles, and

correct the determined particle density by a factor comprising a cube root of the ratio between the reference velocity and the determined average particle velocity.

19. A computer program product comprising computer executable instructions stored on at least one memory device, wherein the computer executable instructions include instructions for carrying out the method according to claim 18 .