9.4 Digital holography and computer-generated holograms
3 min read•Last Updated on July 22, 2024
Digital holography revolutionizes traditional techniques by using digital sensors and algorithms to record and reconstruct holograms. This enables flexible post-processing, quantitative phase imaging, and precise measurements, opening up new possibilities in microscopy and metrology.
Computer-generated holograms take it a step further, using numerical simulations to design custom wavefronts. These can be displayed on spatial light modulators, creating dynamic holograms for applications like 3D displays and augmented reality.
Digital Holography and Computer-Generated Holograms
Principles of digital holography
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Digital holography records and reconstructs holograms using digital sensors and numerical reconstruction algorithms
Replaces traditional photographic film with digital cameras or sensors for recording holograms
Employs numerical reconstruction algorithms to simulate the optical reconstruction process digitally
Basic setup for digital holography includes a coherent light source (laser), beam splitter to divide light into object and reference beams, digital sensor to record interference pattern, and computer to store and process recorded data
Recorded digital hologram contains both amplitude and phase information of object wave
Enables numerical focusing and reconstruction of object at different depths by manipulating recorded data
Enables quantitative phase imaging by capturing phase information
Measures optical path length differences and surface topography using phase data (interferometry)
Computer-generated hologram techniques
Computer-generated holograms (CGHs) are created using numerical algorithms and simulations to design arbitrary wavefronts and light distributions
Fourier transform serves as fundamental tool in generating CGHs
Complex amplitude of desired wavefront is Fourier transformed to obtain hologram pattern
Inverse Fourier transform of hologram reconstructs desired wavefront
Iterative algorithms like Gerchberg-Saxton algorithm optimize CGHs by iteratively refining hologram to minimize difference between desired and reconstructed wavefronts
CGHs can be displayed on spatial light modulators (SLMs) to physically reconstruct designed wavefront
SLMs modulate phase or amplitude of incident light based on CGH pattern (liquid crystal displays, digital micromirror devices)
Digital vs traditional holography
Advantages of digital holography:
Eliminates need for photographic film processing by recording and reconstructing holograms digitally
Enables flexible post-processing and analysis through numerical focusing and reconstruction
Allows precise measurements of surface topography and optical path length differences via quantitative phase imaging
Facilitates easier storage, transmission, and manipulation of digital holograms compared to physical holograms
Limitations of digital holography:
Suffers from limited resolution due to pixel size and number of digital sensor (megapixels)
Demands high computational power for numerical reconstruction and processing (graphics processing units)
Susceptible to noise and artifacts introduced by digital sensor and numerical algorithms (quantization noise, speckle noise)
Requires specialized and expensive equipment such as high-resolution cameras and powerful computers
Applications in microscopy and metrology
Digital holographic microscopy (DHM) enables quantitative phase imaging of biological samples and microstructures
Provides label-free, non-invasive, and real-time imaging of living cells and tissues (cell culture monitoring)
Allows numerical refocusing and reconstruction of 3D object volumes (tomography)
Metrology and surface characterization applications measure surface topography and deformations with nanometer-scale precision using digital holography
Useful for quality control, material characterization, and non-destructive testing (semiconductor wafer inspection, MEMS characterization)
Virtual and augmented reality applications use CGHs to create realistic 3D displays and holograms
Generates complex, dynamic, and interactive 3D scenes for gaming, education, training, and visualization (holographic displays, head-up displays)