Off-axis Digital Holography Object Wave Reconstruction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Off-axis digital holography faces challenges in accurately extracting the object wave due to overlapping artefacts caused by the DC-term in the spatial frequency domain, limiting the quality of amplitude and phase images, especially when the object wave intensity is not constant.
Innovation Solution
The method involves acquiring two digital holograms using reference waves with distinct spatial modulation directions, allowing for spatial separation and replacement of overlapping regions in the spatial frequency domain, enabling effective filtering and extraction of the object wave without manual intervention or additional hardware.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single reference beam is used in off-axis digital holography, then the setup is simple and real-time measurement is enabled, but the DC-term overlaps with the object wave in the spatial frequency domain causing reconstruction artefacts
Solution Approach 1:
The patent divides the spatial frequency domain into multiple quadrants and uses two different reference beams with distinct spatial modulations to record two holograms. This segmentation allows the DC-term and object wave to be separated in different quadrants of the spatial frequency spectrum, enabling artefact-free reconstruction by combining information from both holograms.
Solution Approach 2:
The patent introduces an additional dimension by using two reference beams with different spatial modulations (different tilt angles or directions). This creates two separate holographic recordings that, when processed in the spatial frequency domain, provide complementary information that resolves the overlap problem without requiring complex temporal or spatial filtering.
2Measurement precision
If spatial filtering is applied to separate the object wave from the DC-term, then reconstruction quality improves, but available spatial frequency pixels are limited reducing measurement accuracy
Solution Approach 1:
The patent merges information from two different holograms recorded with distinct spatial modulations. By combining the spectral information from both holograms in the spatial frequency domain, the method recovers the complete object wave spectrum that would otherwise be lost due to DC-term overlap in a single hologram, thereby improving measurement precision without additional hardware.
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 significantly reduces artefacts and improves the quality of amplitude and phase images by replacing overlapping regions with non-overlapping areas, resulting in clearer and more accurate 3D surface height maps.
Implementation Method 1
the measurement beam and the first reference beam together generate an interference pattern on the sensor
Implementation Method 2
subjecting the digital hologram to a Fourier transform in the spatial frequency domain to obtain a two dimensional spectrum
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
The invention provides a method and an apparatus for preparing a digital hologram representing an object (9), comprising generating a measurement beam (11) and two reference beams (10,12), irradiating the object (9) by the measurement beam and guiding the reflected measurement beam to an optical sensor (7), guiding the reference beams to mirror means (5) extending under a slanting angle and guiding the reflected beams to the sensor (7) to make the beams generate an interference pattern on the sensor, providing a digital signal representing the pattern on the sensor, processing the signal to obtain a digital hologram, subjecting the digital hologram to a Fourier transform in the spatial frequency domain to obtain a spectrum comprising a DC-term, two image terms (R1*O,R2*O), and two conjugate image terms (R1O*,R2O*), filtering the spectrum to obtain a term representing the object and replacing a section of an image term overlapped by the DC-term by the corresponding section of the other image term.