Off-axis interferometer with grating for low-noise holography
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Solution Overview
Problem
Existing off-axis interferometers require highly coherent light sources, which lead to coherent noise and high costs, and are limited by the coherence length, making it difficult to record phase information with partially coherent light sources, especially for fast and dynamic applications.
Innovation Solution
An off-axis interferometer design that uses a grating to split incident light into diffracted and non-diffracted beams, allowing interference fringes to be recorded independently of position, even with partially coherent light sources, by maintaining temporal coherence and using a 4f optical system to ensure parallel diffracted beams reach the recording plane simultaneously.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If highly coherent light sources (lasers) are used in off-axis interferometers, then interference fringes can be observed with path length differences, but coherent noise (speckle field) appears and costs increase
Solution Approach 1:
The patent changes the temporal coherence parameter of the light source from high (laser) to partial coherence (LED with narrow bandwidth filter), thereby eliminating speckle noise while maintaining sufficient coherence for off-axis interferometry with controlled path length differences
2Reliability
If highly coherent light sources are used, then interference can be observed over larger path length differences, but the cost increases
Solution Approach 1:
The patent replaces expensive laser sources with inexpensive LED sources combined with narrow bandwidth optical filters, achieving a low-cost solution that provides sufficient temporal coherence for the application while eliminating the high costs associated with laser systems
3Object-generated harmful factors
If partially coherent light sources are used, then coherent noise is reduced and costs decrease, but the coherence length is insufficient to maintain interference over the recording plane
Solution Approach 1:
The patent optimizes the bandwidth parameter of the LED source (using narrow bandwidth filters) to extend the temporal coherence length sufficiently across the recording plane, and controls the off-axis angle to keep path length differences within the coherence length, thereby maintaining interference with partially coherent light
Solution Approach 2:
The patent uses partial coherence (rather than full coherence) which is sufficient for the application, achieving the desired interference pattern without the excess coherence that would generate speckle noise, representing an optimal partial action approach
4Measurement precision
If on-line configuration with phase stepping is used, then complex amplitude can be obtained, but acquisition speed is limited by camera frame rate
Solution Approach 1:
The patent introduces a preliminary off-axis angle between object and reference beams before recording, which pre-separates the spectral components in the Fourier domain, enabling direct complex amplitude extraction from a single snapshot without requiring subsequent phase stepping operations
Solution Approach 2:
The patent replaces the mechanical phase-stepping system (requiring multiple sequential measurements) with an optical off-axis configuration that encodes phase information spatially, allowing parallel acquisition of complex amplitude data in a single camera exposure
5Productivity
If off-axis configuration is used, then acquisition speed increases with single-shot recording, but highly coherent light is required which causes speckle noise
Solution Approach 1:
The patent changes the coherence parameter of the light source from high (laser) to partial coherence (filtered LED), maintaining sufficient temporal coherence for off-axis interferometry while eliminating speckle noise, thereby achieving fast single-shot acquisition without coherent artifacts
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
Enables fast and low-noise digital holographic recording with partially coherent sources, such as LEDs, allowing for full-color holography without coherent noise and reducing costs, while maintaining interference fringe quality across the recording plane.
Implementation Method 1
a grating for splitting an incident light beam into a plurality of diffracted light beams
Implementation Method 2
a first lens for focusing the diffracted light beams to infinity, a second lens for focusing the parallel diffracted light beams on a recording plane
Implementation Method 3
the object beam and the reference beam interfere, producing interference fringes
Data Source
AI summary
An Interferometer for off-axis digital holographic microscopy includes a recording plane, and a grating located in a plane optically conjugated with the recording plane. The grating defining a first and a second optical path, the optical path corresponding to different diffraction order.