Interferometer Ghost Beam Mitigation via Segmented DPBS

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Conventional interferometers suffer from ghost beams that degrade the signal-to-noise ratio of displacement measurements, as tilting surfaces to mitigate these beams also reduces the amplitude of the desired interference signal.

Innovation Solution

The interferometer design includes a double polarizing beam splitter (DPBS) with tilted beam-splitting surfaces and reflective surfaces, ensuring ghost beams are angularly separated from desired beams without degrading the signal-to-noise ratio by maintaining collinearity of the reference and measurement beam axes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If surfaces are tilted to mitigate ghost beams, then ghost beam interference is reduced, but the amplitude of the desired interference signal is reduced

Engineering Contradiction:

Improveghost beam interferenceVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Object-affected harmful factorsVSMeasurement precision

Solution Approach 1:

The single polarizing beam splitter is divided into two separate beam splitters with different orientations. The first beam splitter is tilted to deflect ghost beams away from the measurement path, while the second beam splitter is oriented to maintain proper alignment of the desired measurement and reference beams. This segmentation allows each beam splitter to perform its specific function without compromising the other, thereby mitigating ghost beam interference while preserving signal amplitude.

Inventive Principle:

Principle #1Segmentation

2Device complexity

If a single polarizing beam splitter is used, then the interferometer structure is simple, but ghost beams are generated that degrade measurement quality

Engineering Contradiction:

Improveinterferometer structureVSAvoidghost beams

Core Design Contradiction:

Device complexityVSObject-affected harmful factors

Solution Approach 1:

The single polarizing beam splitter is segmented into two separate beam splitters with different orientations. The first beam splitter is tilted to deflect ghost beams away from the measurement path, while the second beam splitter is oriented to maintain proper alignment of the desired measurement and reference beams. This segmentation allows each beam splitter to perform its specific function without compromising the other, thereby mitigating ghost beam interference while preserving signal amplitude.

Inventive Principle:

Principle #1Segmentation

3Object-affected harmful factors

If beam-splitting surfaces are tilted, then ghost beams are angularly separated, but the collinearity of reference and measurement beam axes is compromised

Engineering Contradiction:

Improveghost beam separationVSAvoidbeam axis collinearity

Core Design Contradiction:

Object-affected harmful factorsVSStability of the object's composition

Solution Approach 1:

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the optical path are treated with different beam splitter orientations. The first beam splitter is tilted in the region where ghost beam separation is needed, while the second beam splitter is oriented to maintain collinearity where desired beam alignment is critical. This local differentiation allows the system to address ghost beams without compromising beam axis collinearity.

Inventive Principle:

Principle #3Local quality

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 design effectively mitigates ghost beams, maintaining the signal-to-noise ratio and maximizing the amplitude of the desired interference component, thus enhancing the accuracy of displacement measurements.

Implementation Method 1

Beam-splitting surface 22 reflects s-polarized light and transmits p-polarized light

Methodology Applied

Scientific EffectPolarisation: Polarisation

Implementation Method 2

Quarter-wave plate 44 is interposed between PBS 20 and movable mirror 40

Methodology Applied

Scientific EffectPhase retardation:

Implementation Method 3

Retroreflector 30 is located on the other side of PBS 20 from fixed mirror 40 and faces the fixed mirror

Methodology Applied

Scientific EffectRetroreflection: Retroreflector

Implementation Method 4

A measurement beam 72 and a reference beam 74 derived by interferometer 10 from incident light beam 62 emerge from PBS 20 and illuminate a light sensor 70

Methodology Applied

Scientific EffectInterference: Interference

Data Source

PatentUS7652771B2Interferometer with Double Polarizing Beam Splitter

Publication Date: 2010.01.26 KEYSIGHT TECHNOLOGIES INC

AI summary

An interferometer has a first reflective surface having a nominal orientation; a second reflective surface having a nominal orientation orthogonal to the nominal orientation of the first reflective surface; a retroreflector facing the first reflective surface; a double polarizing beam splitter (DPBS) between the first reflective surface and the retroreflector; and a respective quarter-wave plate between the DPBS and each of the reflective surfaces. The DPBS has first and second beam-splitting surfaces each having a nominal orientation with respect to the first reflective surface. At least part of at least one of the first reflective surface, the second reflective surface and the beam-splitting surfaces is effectively tilted relative to the respective nominal orientation of such surface, and constitutes a respective tilted surface.