Optical Displacement Sensor Using Talbot Self-Imaging to Reduce Light Loss
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Solution Overview
Problem
Optical displacement sensors using diffraction gratings suffer from significant light loss due to diffraction, which impairs their performance in terms of sensitivity and signal-to-noise ratio.
Innovation Solution
The optical path length between the diffraction grating and reflective surface is configured to satisfy the relationship L = Tz*n^2 within 20% of Tz^2, where Tz is the Talbot length, ensuring that the diffracted light patterns coincide with the grating position, thereby reducing light loss and enhancing sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a diffraction grating is used to separate light into different diffraction orders, then the signal corresponding to separation between the reflective surface and diffraction grating can be produced, but significant light is lost due to diffraction in directions not collected by photo detectors
Solution Approach 1:
The patent applies preliminary action by pre-configuring the optical path length L to satisfy the specific relationship L = Tz*n^2 (where Tz is the Talbot length) before the measurement process begins. This pre-configuration ensures that diffracted light patterns coincide with the grating position, so that when measurement occurs, maximum light is already positioned to be collected by photo detectors, eliminating the need for real-time adjustment and ensuring optimal light collection from the outset
Solution Approach 2:
The patent applies parameter changes by establishing a specific quantitative relationship between the optical path length L and the Talbot length Tz (L = Tz*n^2). This parameter configuration transforms the optical system to produce self-images of the grating at specific intervals, ensuring that diffracted light is directed toward photo detectors. By changing and optimizing this critical parameter, the system achieves both high measurement precision and reduced light loss
2Reliability
If the optical path length is increased to improve signal-to-noise ratio, then sensitivity is enhanced, but light loss due to multiple reflections and absorption in the air gap increases
Solution Approach 1:
The patent converts the potentially harmful effect of multiple reflections and absorption in the air gap into a beneficial outcome. By configuring the optical path length to satisfy L = Tz*n^2, the system causes diffracted light to form self-images that coincide with the grating position, ensuring that even light undergoing multiple reflections and traversing the air gap is redirected back toward photo detectors. This transforms light that would normally be lost into useful signal light, improving signal-to-noise ratio without the usual penalty of increased light loss
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 configuration significantly reduces light loss and enhances the sensitivity of the optical displacement sensor by ensuring more light is coupled towards the photo detectors, improving the sensor's performance.
Implementation Method 1
A second portion is transmitted through the grating, which diffracts the radiation
Implementation Method 2
The radiation passes through the grating and the two portions of light interfere to create an interference pattern
Implementation Method 3
a collimating optical arrangement arranged to at least partially collimate the light between the light source and the diffraction grating(s)
Data Source
AI summary
An optical displacement sensor comprises a reflective surface and one or more diffraction gratings which, together with the reflective surface, each define a respective interferometric arrangement. The reflective surface is moveable relative to the diffraction grating(s) or vice versa. Light from a light source propagates via the interferometric arrangement(s) to produce an interference pattern at a respective set of photo detectors. Each interference pattern depends on the separation between the reflective surface and the respective grating. A collimating optical arrangement at least partially collimates the light between the light source and the diffraction grating(s). For the or each interferometric arrangement, when the reflective surface or the diffraction grating is in a zero-displacement position, the optical path length L of the light propagating between the diffraction grating and the reflective surface satisfies the relationship:L=Tzn2,to within 20% ofTz2, where n is an integer; where Tz is the Talbot length, defined by:Tz=λ1-1-λ2p2,where λ is the wavelength of the light, and where p is the grating period of the respective diffraction grating.