Off-axis holography using thick Bragg gratings for vibration immunity
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Solution Overview
Problem
Off-axis digital holography microscopy devices are sensitive to external disturbances such as ambient air fluctuations and vibrations, affecting measurement accuracy due to the sensitivity of the reference beam.
Innovation Solution
The use of one or more thick Bragg gratings to form the reference wave and signal wave, which interfere directly at the output of the gratings, reducing the device's susceptibility to external disturbances and enhancing robustness and compactness by providing a low-pass spatial filtering of the deflected wave.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Mach-Zehnder interferometer with separate reference and object arms is used, then off-axis digital holography can be implemented, but the device becomes sensitive to external disturbances such as ambient air fluctuations and vibrations
Solution Approach 1:
The patent merges the reference wave and signal wave paths by having both waves pass through the same objective lens and interfere in the same detection plane. This common-path configuration eliminates the sensitivity to external disturbances that plagues separate-path interferometers, as both waves experience identical environmental conditions. The harmful effect of external disturbances is neutralized by making the optical paths common rather than separate.
2Device complexity
If beam splitters and mirrors are used to form reference and signal waves, then interferometry can be implemented, but the device complexity increases and compactness is reduced
Solution Approach 1:
The patent extracts the beam splitter and mirror components from the optical path by using a single objective lens to generate both the reference and signal waves directly. The reference wave is obtained by blocking part of the illumination beam with an aperture, eliminating the need for complex beam splitting and recombining optics. This reduction in components directly addresses the contradiction by simplifying the device while maintaining measurement stability.
3Length of stationary object
If the distance between the object and detection device is reduced for compactness, then the device becomes more compact, but the angular selectivity and imaging quality may be affected
Solution Approach 1:
The patent changes the angular parameters of the interfering waves by controlling the aperture position and size, which adjusts the angle between the reference and signal waves. This parameter adjustment allows the system to maintain high angular selectivity and imaging quality even at reduced distances between the object and detection device. The aperture acts as a spatial filter that preserves the necessary angular separation for high-quality holographic imaging in a compact configuration.
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 arrangement improves the robustness and compactness of the device, allowing for high-quality imaging with reduced distance between the object and detection device, and enhances the angular selectivity of the deflected reference wave, leading to improved measurement accuracy and sensitivity.
Implementation Method 1
a thick Bragg grating intended to receive an illumination wave coming from an object, and to form a reference wave and a signal wave
Implementation Method 2
the reference wave and the signal wave interfere with each other in a detection plane of a two-dimensional detector
Data Source
Figure 1A~1B
Figure 2
Figure 3A~3C
AI summary
According to one feature, the present description relates to a device for optical imaging (20) of an object (OBJ) by means of off-axis holography comprising a light source (21) suitable for transmitting an illumination wave (EI) of the object, in transmission or in reflection, and an assembly formed of one or a plurality of volume Bragg gratings (22) configured to receive a wave (EO) from the object thus illuminated and to deflect a first component (ER) of the wave from the object, called the reference wave, and to allow to pass without deflection a second component (ES) of the wave from the object, called the signal wave, such that the deflected reference wave has predetermined deflection angles relative to the undeflected signal wave defined in two perpendicular planes. The imaging device according to the first feature further comprises a two-dimensional detection device (23) for acquiring an interferogram resulting from the interference between said deflected reference wave and said signal wave and a calculation unit for determining, from said interferogram, an amplitude and phase distribution of the signal wave in the object plane (hologram).