Laser sensor module for particle detection with offset beam

A method of measuring a particle density of particles with a size of less than 20 μm, the method comprising:

• emitting a laser beam to a mirror rotatable about a rotation axis, wherein the laser beam is focused to a focus region,
• dynamically redirecting the laser beam by a movement of the mirror, wherein a direction of the redirected laser beam defines an optical axis moving in accordance with the movement of the mirror, wherein the laser beam is displaced with respect to the rotation axis of the mirror such that the focus region of the laser beam is moving with a velocity comprising components normal and parallel to the optical axis of the redirected laser beam such that an angle α between the parallel velocity component with the normal velocity component is at least a threshold angle of 2°,
• determining a self mixing interference signal of an optical wave within a laser cavity of the laser, the self mixing interference signal being generated by laser light of the laser beam reflected by at least one of the particles,
• analyzing the self mixing interference signal based on the shift of the self mixing interference signal to higher frequencies.

A computer readable medium comprising a computer program product comprising instructions, which when executed by a computer processor cause the processor to perform the following operations:

• emit a laser beam to a mirror rotatable about a rotation axis, wherein the laser beam is focused to a focus region,
• dynamically redirect the laser beam by a movement of the mirror, wherein a direction of the redirected laser beam defines an optical axis moving in accordance with the movement of the mirror, wherein the laser beam is displaced with respect to the rotation axis of the mirror such that the focus region of the laser beam is moving with a velocity comprising components normal and parallel to the optical axis of the redirected laser beam such that an angle α between the parallel velocity component with the normal velocity component is at least a threshold angle of 2°,
• determine a self mixing interference signal of an optical wave within a laser cavity of the laser, the self mixing interference signal being generated by laser light of the laser beam reflected by at least one of the particles,
• analyze the self mixing interference signal based on the shift of the self mixing interference signal to higher frequencies.