Double Pass Interferometer Tilted Mirrors Ghost Light
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Solution Overview
Problem
Conventional double pass interferometers suffer from significant interpolation errors due to ghost light interference, which deteriorates the linearity of displacement measurements, especially in applications requiring ultrahigh accuracy, as the ghost light signals superimpose on the primary interference signal, causing errors of several nanometers to tens of nanometers.
Innovation Solution
The interferometer design includes a light splitting device, reference and measurement mirrors tilted to differ from the normal direction, and a reflective device to ensure that reflected lights are irradiated back onto the mirrors at specific angles, resulting in a conjugate relation that minimizes ghost light interference by creating an interference pattern with a controlled pitch, allowing for averaging and reducing interpolation errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional double pass interferometer configuration is used, then displacement measurement capability is achieved, but ghost light interference causes significant interpolation errors deteriorating measurement linearity
Solution Approach 1:
The patent applies asymmetry by tilting the reference mirror and measurement mirror at different angles relative to the optical axis. Specifically, the reference mirror is tilted at a first angle while the measurement mirror is tilted at a second angle, creating an asymmetric optical path configuration. This asymmetric arrangement causes ghost light from different interfaces to diverge at different angles, preventing them from coherently interfering with the primary measurement signal, thereby eliminating the 1/2λ periodic errors while preserving the displacement measurement capability.
2Stability of the object's composition
If multiple reflective surfaces are present in optical path, then complete optical path is formed, but ghost light signals are generated superimposing on primary signal
Solution Approach 1:
The patent introduces an angular separation mechanism as an intermediary effect. By tilting the mirrors at different angles, the system creates angular separation between the primary optical path and ghost light paths. This angular separation acts as a mediator that spatially filters out ghost light signals before they can reach the detector, allowing the complete optical path to function while eliminating harmful ghost light interference.
3Object-affected harmful factors
If ultralow reflective AR coat is used, then ghost light interference intensity is reduced, but measurement error remains significant
Solution Approach 1:
The patent changes the geometric parameters of the optical system by introducing different tilt angles for the reference mirror and measurement mirror. This parameter change transforms the optical path configuration from a symmetric to an asymmetric arrangement, fundamentally altering how ghost light propagates through the system. The angular parameter modification causes ghost light to diverge from the primary signal path, reducing interference intensity to negligible levels and eliminating interpolation errors without relying solely on AR coating reflectivity reduction.
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 the impact of ghost light on the interference signal, improving the linearity of displacement measurements to sub-nanometer accuracy by averaging the interference pattern over a larger light receiving area, thereby minimizing errors to a negligible level even in ultra-high precision applications.
Implementation Method 1
A laser beam 110 having a wavelength of λ (λ=633 nm) emitted from a light source 10 enters a PBS 20 (a polarizing beam splitter) and is split into reference light 120a and measurement light 120b on the PBS surface 20p
Implementation Method 2
The reference light 120a is reflected on the reference mirror 40a and enters the PBS 20 again by passing through the previous optical path
Implementation Method 3
A P wave is converted to an S wave by being transmitted through a 1⁄4λ plate 30a twice
Implementation Method 4
An interference signal having a period of 1⁄4λ in accordance with a displacement of the measurement mirror 40b can be obtained by receiving the multiplexed light 150 by a light receiving device 160
Data Source
AI summary
An interferometer of the present invention includes a PBS2 which splits light into reference light and measurement light, a reference mirror 4a which reflects the reference light entering the reference mirror from a first direction, a measurement mirror 4b which reflects the measurement light entering the measurement mirror from a second direction, a lens system 6 which reflected lights from the reference mirror 4a and the measurement light 4b enter, a reflective device 5 which reflects light from the lens system 6, and a light receiving device 16 which receives multiplexed light, wherein the reference mirror 4a and the measurement mirror 4b are in a conjugate relation with respect to the reflective device 5, and at least one of the reference mirror 4a and the measurement mirror 4b is tilted so that its normal direction differs from the first and the second direction.