Solid-State Laser Wavelength Scanning for Secondary Hologram Control
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Solution Overview
Problem
Solid-state lasers used in holographic optical elements lack a method to control and reduce their effective coherence length, leading to the formation of spurious secondary holograms and artifacts in holographic displays.
Innovation Solution
The coherence length of a single-frequency, solid-state laser is controlled by varying its emission wavelength over a small range, using a feedback loop to maintain the phase of the holographic interference pattern, thereby reducing secondary hologram formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the coherence length of the laser is increased to improve hologram recording efficiency, then the primary hologram quality improves, but secondary holograms and artifacts are generated
Solution Approach 1:
The patent applies parameter changes by dynamically varying the laser wavelength during exposure. The wavelength is modulated according to a predetermined pattern that changes over time, which effectively reduces the coherence length integration over the exposure period. This allows the system to achieve high primary hologram efficiency while suppressing secondary hologram formation, as the varying wavelength prevents coherent buildup of secondary interference patterns.
2Productivity
If a single-frequency laser is used to achieve long coherence length, then hologram exposure efficiency improves, but control over coherence length is lost
Solution Approach 1:
The patent implements dynamics by transitioning from a static single-frequency laser to a dynamically varying wavelength laser. The wavelength modulation is controlled by a feedback system that adjusts the laser frequency in real-time during exposure. This dynamic approach provides adaptability and control over the effective coherence length while maintaining high exposure efficiency, as the system can optimize the wavelength variation pattern for different hologram recording conditions.
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 method effectively reduces the coherence length of the laser to minimize secondary holograms, enhancing the clarity and reducing artifacts in holographic recordings, particularly in large-aperture applications like head-up displays.
Implementation Method 1
The coherence length of a laser is a measure the propagation length of the laser's beam over which coherence degrades significantly.
Implementation Method 2
a laser beam is split along two paths and recombined to form an interference pattern in a holographic film
Implementation Method 3
the latter being enabled by an internal Fabry-Pérot etalon that selects a single frequency from among the many otherwise available within the gain curve of the laser tube
Data Source
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AI summary
The effective coherence length of a single-frequency, solid-state laser is limited to reduce spurious, secondary holograms in conjunction with a holographic recording. The wavelength of the laser is varied or 'scanned' with high precision over a very small wavelength range. In an embodiment, the temperature of the laser's resonant cavity optical bench is altered, causing the dimension of the cavity to change and the emission wavelength to move in a controlled manner. The changing wavelength is monitored at high resolution, and a feedback control loop updates the temperature set-point to keep the monitored laser wavelength moving at a desired rate of change through a desired range. As the wavelength of the laser is scanned, the phase of the holographic interference pattern is locked at a position of maximum coherence/contrast within the holographic film aperture.