Laser Self-Mixing Device Diffractive Element Speckle Suppression
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Solution Overview
Problem
Laser self-mixing devices face challenges in accurately measuring displacement-related parameters when dealing with randomly reflecting surfaces or surfaces that cause strong speckle patterns, leading to low or vanishing signal levels and reduced measurement accuracy.
Innovation Solution
Incorporating a diffractive element with a periodic structure in the optical path to redirect a portion of the laser beam back into the laser cavity, which eliminates or suppresses speckle-related phenomena and enhances measurement accuracy by providing a consistent and strong self-mixing signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a laser self-mixing device is used to gauge the velocity or distance of an object having a randomly reflecting surface, then the measurement can be performed without additional optical filters or complex devices, but the reflected signal strongly depends on the spatially variable reflectivity causing very low reflected signal levels and low measurement accuracy
Solution Approach 1:
A diffuse reflecting surface is introduced as an intermediary between the laser and the target object. This intermediary surface receives the laser beam and redirects it toward the target, ensuring that even objects with randomly reflecting surfaces return sufficient light to the laser cavity for accurate measurement, thereby resolving the contradiction between device simplicity and measurement accuracy.
2Device complexity
If a laser self-mixing device measures displacement-related parameters of a surface causing strong speckle patterns, then the measurement can be performed without complex interferometers, but the speckle pattern gives rise to strong random intensity variations modulating the self-mixing signal amplitude, frequency and phase
Solution Approach 1:
A diffuse reflecting surface acts as a mediator that scrambles the coherent laser light before it reaches the target surface. This scattering effect eliminates the formation of speckle patterns by ensuring that the light reflected from the target returns to the laser cavity with randomized phase relationships, thereby stabilizing the self-mixing signal while maintaining device simplicity.
Solution Approach 2:
The introduction of a diffuse reflecting surface changes the optical parameters of the light path, specifically transforming the coherent laser beam into scattered light with randomized directions and phases. This parameter change in the light propagation characteristics eliminates speckle-related signal modulations while preserving the core self-mixing measurement capability.
3Ease of operation
If the specular beam is not reflected back into the laser cavity, then the measurement geometry can be simplified, but the reflected signal levels become very low or even vanishing on absorptive or black surfaces
Solution Approach 1:
A diffuse reflecting surface is positioned between the laser and the target to act as a mediator that redirects light toward the target and ensures return paths to the laser cavity. This intermediary enables measurement geometries where the specular beam does not need to return directly, while simultaneously ensuring sufficient signal levels by scattering light in multiple directions including back toward the laser.
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
The solution enables high-accuracy measurement of displacement, velocity, and rotation by stabilizing the laser intensity and reducing speckle effects, allowing for precise detection of displacement-related parameters without the need for complex devices like interferometers.
Implementation Method 1
a diffractive element arranged in the optical path of the laser, which redirects a portion of the laser light along the optical path back into the laser cavity. The diffractive element has a periodic structure, which diffracts the incident laser light into partial beams
Implementation Method 2
Laser self-mixing occurs if an external reflecting surface is arranged within the optical path of a laser so that an external cavity is obtained. Tuning of the external cavity results in a readjustment of the laser equilibrium conditions and thus to detectable changes in the laser output power
Implementation Method 3
a laser self-mixing device is known from U.S. Pat. No. 6,707,027 B2. This device makes use of the Doppler phase shift occurring if the reflecting surface has a component of movement along the optical path
Data Source
AI summary
A laser self-mixing measuring device is provided, comprising a laser with a laser cavity and a surface arranged along the optical path of the laser beam which redirects incident laser light back into the laser cavity. The surface comprises a periodic structure which diffracts the laser light into partial beams.