Optical Diffraction

Operation Principle of Diffractive Optical Elements

Charged complex formation between the phenylboronic acid functional group and the 1,2- cis -diol glucose resulted in reversible swelling of the inverse opal hydrogel, which was observed through shifts in the optical diffraction wavelength. The hydrogel was sensitive to glucose at physiological concentrations and ionic strength. These resolution limitations are often referred to as the diffraction barrier, which restricts the ability of optical instruments to distinguish between two objects separated by a lateral distance less than approximately half the wavelength of light used to image the specimen. Figure 1 - Resolution Limit Imposed by Wave Nature of Light.

Optical diffraction limit

The different types of DOEs (beam-splitters, pattern generators, kinoforms, beam shapers and gratings) utilize a microstructure surface relief profile for their optical function. Light transmitted by a DOE can be reshaped to almost any desired distribution, just by diffraction and the subsequent propagation. The DOE only encodes the shape of the desired intensity pattern, but maintains other parameters of the incident light source (e.g. beam size, divergence, polarization).

Due to their design flexibility, DOEs can have optical functions that can otherwise not be achieved at all, or only with complicated optical systems. Moreover, compared to refractive optical elements, DOEs are typically much thinner and lighter, making them an attractive replacement in a number of applications.

In most scenarios the incident laser beam is collimated, or focused to a fixed distance behind the DOE. HOLOEYE can offer solutions also for non-collimated or partially collimated light sources. To do so, the DOE microstructure combine pattern generation with focal power. In addition, the focusing properties of HOLOEYE’s DOEs are not limited to fixed working distances. Instead, our pattern generators can provide required focal power to achieve focusing of the laser to strongly inclined target planes. In fact, the best focus location per diffraction direction can be matched to almost arbitrary surfaces.

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Diffractive Beam Splitter

A Diffractive Beam Splitter splits the incident laser beam into a 1-dimensional or 2-dimensional array of beams. Typically diffractive beam splitters are used in combination with a focusing lens. If so, the output beam array becomes an array of focused spots at a certain distance behind the lens.

The arrangement of the spots is not limited to arrays in perpendicular x-y lattices. Also hexagonal or irregular lattices are possible. For more complex arrangement of spots, like for structured light pseudo-random spot patterns, the diffractive beam-splitters can also be referred to as ► Diffractive Pattern generators.

Applications:
  • Multi-channel splitting for 1D or 2D sensors
  • Process parallelization in material processing (laser dicing, laser scribing, …)
  • Multi-Focal Microscopy
  • Coherent beam combination
  • Camera calibration

Diffractive Pattern Generators

With Diffractive Optics complex patterns with a very high depth of field can be created.

The pattern comprises of many spots, which may overlap so that the element could be referred to as a ► Diffractive Diffusor, or still be visibly as individual spots, so that the element could be referred to as a ►Diffractive Beam-splitter.

Due to the high accuracy of the microstructures, the diffraction angles can be extremely precise, in particular when using a frequency stabilized laser source.

Applications:
  • Structured light and pattern projection for 3D sensing applications: pseudo-random spot patterns, fringe patterns, De Brujn patterns
  • Graphics, range and chart projection for alignment and measurements
  • Laser aiming, barcode scanners, POI patterns
Optical Diffraction

Diffractive Diffusers

With Diffractive Diffusers flexible shaping of the emitted angular power distribution of various light sources can be achieved. Diffractive Diffusers can be best used with VCSEL arrays, because they consist of many individual incoherent laser emitters.
As a result, the angular far field diffracted light distribution is much less affected from interference-caused intensity modulations, and more uniform light distributions are obtained.

With tailored diffractive diffusers, HOLOEYE is able to create various light distributions for the application wavelength. By suppressing the zero order diffraction to well below 1% compared to the incident light even for large diffraction angles, the desired profiles can be obtained in very good approximation.

Optical Diffraction Tomography

Applications:
  • Time-of-Flight 3D sensing
  • Laser autofocus
  • 2D sensing with flood illumination
  • Illumination applications
  • LIDAR

Diffractive Beam Shapers

An incident laser beam of ideally Gaussian intensity profile is transformed into a desired intensity profile at the target plane or workpiece. In most cases, the target is a uniform (‘flat-top’) circular or rectangular beam profile. Other shapes and non-uniform profiles can be obtained as well.

For a custom development, precise information about the input beam intensity and phase profile is required. For beams with high beam quality of M²<1.3, the phase profile is sufficiently described by the radius of curvature of its wavefront.

Applications:
  • Laser material processing
  • Lithography
  • Biomedical devices

Key Points

  • Iterative Optical Diffraction Tomography (iODT) approach for optically imaging phase objects with high index contrasts or large optical path differences
  • iODT algorithm provides better accuracy, faster convergence, and better reliability by suppressing artifacts
  • Enables the reconstruction of fiber refractive index profiles accurately and robustly with sub-wavelength resolution
Diffraction

Abstract

Researchers at the University of Central Florida have invented a technique for capturing and reconstructing images of phase objects with greater accuracy and quality than conventional optical diffraction tomography (ODT) inversion methods. Example objects include transparent samples such as biological cells, tissues and optical fibers with high contrast, complicated structure, or sizeable optical path difference (OPD).

With such targets, most non-iterative tomographic reconstruction methods adopt the weakly-scattering assumption, which degrades the imaging quality. In contrast, the UCF Iterative Optical Diffraction Tomography (iODT) method, with its algorithmic process, reconstructs such objects and those with complex permittivity with better accuracy, fast convergence, and sub-wavelength resolution.

Technical Details

The UCF iODT approach improves the reconstruction quality of multiply scattering two-dimensional and three-dimensional phase objects by iteratively reducing the error between the fields diffracted by the reconstructed object and the true fields measured experimentally or obtained through simulations of phantoms for all illumination angles.

In one example application, the first iteration of iODT provides an estimate of the unknown refractive index (RI) profile using the standard linearized ODT inversion algorithm. Subsequent iterations improve the estimate by applying a perturbative correction based on differences between the fields diffracted by the imperfectly reconstructed object and the measured fields diffracted by the true object. The process includes translating this error into an error in the associated complex phase and then computing a correction to the reconstructed object function. The method uses the Rytov approximation in every iteration as it is more applicable to the perturbative function, as opposed to the original function. Since the magnitude (distribution) of the perturbative function becomes smaller (smoother) at higher iterations, the Rytov approximation improves. Further, as expected, the number of phase vortices in the perturbative complex phase is gradually reduced in a self-healing process as the iterations converge. In essence, the embodied iterative algorithm is a nonlinear reconstruction based on perturbative expansion, much like a higher-order Born or Rytov expansion for forward propagation.

Partnering Opportunity

The research team is looking for partners to develop the technology further for commercialization.

Optical Diffraction Microscopy In A Teaching Laboratory

Stage of Development

Prototype available.

Diffraction equation physics

Benefit

  • Heuristic iterative algorithm provides better accuracy and fast convergence, as well as sub-wavelength resolution
  • Can suppress errors resulting from phase-unwrapping failures, which typically occur in field-based inverse problems

Market Application

Diffraction Limited Resolution

  • Quantitative refractive-index imaging of a highly scattering media
  • Index and gain/loss imaging of optical fibers with high-index contrasts or large optical path differences
  • Biological and biomedical imaging