Optical particle sensor module

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.

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.