Position Detection Device Stray Light Suppression
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing optical displacement measurement devices using grating interferometers face issues with stray light intrusion due to higher order diffracted light reflections on the protective layer, which degrades detection accuracy and S/N ratio.
Innovation Solution
By setting the angle of incidence of the coherent light beam on the diffraction grating to satisfy specific relationships, such as d < 2λ0/n and |sin θ0| < (2λ0/dn) − 1, the generation of ±2nd order or higher order diffracted light is suppressed, using only the 1st order diffracted light for position detection and preventing stray light intrusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a transparent protective layer is formed on the surface of the diffraction grating to protect it from contamination or grazing, then the protection of the diffraction grating surface is improved, but higher order diffracted light may be reflected by the protective layer to generate stray light that intrudes into the optical path and deteriorates detection accuracy
Solution Approach 1:
The patent changes the physical parameters of the protective layer by controlling its thickness to be between 1/10λ to 1/2λ (where λ is the wavelength of light), and by selecting materials with specific refractive indices. This parameter optimization allows the protective layer to minimize reflection of higher order diffracted light while maintaining its protective function, thereby resolving the contradiction between protection and measurement precision.
2Measurement precision
If the angle of incidence is set to prevent 0th order light from intruding into the optical path, then the intrusion of 0th order light is prevented, but higher order diffracted light may still be generated and reflected as stray light
Solution Approach 1:
The patent optimizes the angle of incidence parameter to satisfy specific mathematical relationships that simultaneously prevent both 0th order light and higher order diffracted light from entering the optical path. By carefully selecting and adjusting this angular parameter, the system eliminates multiple sources of stray light while maintaining effective position detection.
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 S/N ratio and position detection accuracy by eliminating unnecessary higher order diffracted light and stray light, improving the overall performance of the optical displacement measurement device.
Implementation Method 1
a coherent light beam forms an optical image on a grating surface of a diffraction grating... A first diffracted light beam is collimated by a second image-forming means and illuminated on a reflector
Implementation Method 2
Higher order diffracted light may turn out to be stray light due to reflection on, for example, a boundary surface of the protective layer of the diffraction grating
Implementation Method 3
This invention relates to a position detection device in which a position of an object is detected by changes in the phase of light caused by movement of a diffraction grating... detects the displacement of a diffraction grating position, as recorded on a movable scale, by taking advantage of light interference
Data Source
AI summary
A position detection device in which 2nd-order or higher order diffracted light as well as stray light is suppressed from being generated to improve the S/N ratio of a position detection signal as well as to improve detection accuracy. With a pitch d of a diffraction grating 11, whose grating surface 11a is covered with a protective layer 12 of a refractive index n, and with a wavelength λ0 in vacuum of coherent light illuminated, d<2λ0/n is set. An angle of incidence θ0 of the coherent light on the diffraction grating 11 is set to satisfy the following relationship: |sin θ0|<(2λ0/dn)−1. The 1st order diffracted light is used for position detection.