Laser Sensor Module With Offset Beam For Particle Detection
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Solution Overview
Problem
Existing laser sensor modules face challenges in accurately detecting particle density of small particles (less than 20 µm) due to 1/f noise and difficulty in detecting particles at rest, especially in low airflow conditions, which reduces detection accuracy and sensitivity.
Innovation Solution
A laser sensor module with a dynamically redirecting mirror that shifts the self-mixing interference signal to higher frequencies by adjusting the geometric relation between the laser and mirror, ensuring an angle of at least 2° between particle trajectories and the redirected laser beam's velocity vector, thereby increasing detection volume and count rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the laser beam is directed orthogonally through the focus, then the detection setup is simple, but particles at rest with 90° phase shift cannot be detected and 1/f noise dominates
Solution Approach 1:
The patent introduces an asymmetric angular offset α between the laser beam trajectory and the normal to the optical axis. This asymmetric configuration ensures that particle trajectories are not orthogonal to the laser beam, preventing the 90° phase shift condition that makes particles undetectable. The asymmetric setup shifts the self-mixing interference signal to higher frequencies, avoiding 1/f noise dominance while maintaining detection capability for all particles including those at rest.
2Productivity
If the detection volume is increased to improve particle count rate, then more particles are detected, but the system complexity and noise interference increase
Solution Approach 1:
The patent employs a dynamic mirror that can change the direction of the laser beam in real-time. This dynamic adjustment allows the system to sweep through a larger detection volume without requiring a physically larger or more complex static detection apparatus. The dynamic angular adjustment of the laser beam enables increased particle count rate by accessing more detection volume while maintaining relatively simple system configuration.
3Device complexity
If the self-mixing interference signal is kept at DC frequency, then the measurement system is simple, but 1/f noise reduces detection accuracy
Solution Approach 1:
The patent introduces a mechanical vibration equivalent by dynamically adjusting the angle of the laser beam through mirror movement. This dynamic angular adjustment modulates the self-mixing interference signal, shifting it from DC frequency to higher frequencies. The frequency shift eliminates 1/f noise dominance while keeping the signal processing relatively simple, as the frequency modulation is achieved through mechanical mirror adjustment rather than complex electronic signal processing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the detection accuracy and sensitivity of particle density, particularly for particles smaller than 20 µm, by shifting self-mixing interference signals to higher frequencies, reducing 1/f noise and improving the count rate of detected particles, even in low airflow conditions.
Implementation Method 1
the detector is arranged to 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
Implementation Method 2
The mirror is arranged to dynamically redirect the laser beam. A direction of the redirected laser beam defines an optical axis.
Data Source
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AI summary
The invention describes a laser sensor module (100) for detecting a particle density of particles (10) with a size of less than 20 μm, preferably less than 10 μιη in a fluid, wherein the laser sensor module (100) comprises a laser (111), a detector (121) and a mirror (160), wherein the laser (111) is arranged to emit a laser beam (112) to the mirror (160), wherein the mirror (160) is arranged to dynamically redirect the laser beam (112), wherein a direction of the redirected laser beam (112) defines an optical axis (51), wherein the detector (121) is arranged to determine a self mixing interference signal of an optical wave within a laser cavity of the laser (111), the self mixing interference signal being generated by laser light of the laser beam (112) reflected by at least one of the particles (10), wherein a geometric relation between the laser (111) and the mirror (160) is arranged such that the self mixing interference signal is shifted to higher frequencies, wherein the laser sensor module (100) is arranged such that an angle a between trajectories of particles (10) at rest with respect to a velocity vector (55) of the redirected laser beam (112) normal to the optical axis (51) is at least a threshold angle of 2°. The invention further relates to a particle detector (200) comprising such a laser sensor module (100) and a mobile communication device (190) comprising such a laser sensor module (100) or particle detector (200). The invention further relates to a method of measuring a particle density of small particles. The invention finally relates to a corresponding computer program product.