Recent Optical Solutions With DIFFRACTIVE OPTICAL TECHNOLOGY Tamir Grossinger
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Content
Definition Technology Background Design methods Diffractive Optical Elements Functions Applications
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Definition Diffractive optical element uses a thin micro structure pattern to alter the phase of the light propagated through it. This phase pattern, once properly designed, can manipulate the light to almost any desired intensity profile.
Examples of typical uses of diffractive elements
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Technology Background Invention of holography by D. Gabor followed by Leith and Upatnieks made it possible to perform any arbitrary wavefront transformation.
Concept of digitally generated simulations by A. W. Lohmann in the mid-60’s Advancements in computer capabilities enabling optimizations
Advancements in micro lithography fabrication techniques
Modern flexible diffractive optical elements for various commercial applications including: material processing, medical lasers, 3D imaging, security, etc. 4
Fabrication Techniques Photolithography fabrication of Diffractive Optical element:
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Technology Background Theoretical efficiency for first diffraction order: Blazed
Efficiency (1/)
Continuous profile
100 %
16 levels
98.7 %
8 levels
95 %
Binary - 4 levels
81.1 %
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Design and Simulation Methods Design and Simulation of Diffractive Optical Elements: The design of diffractive optical elements uses many ideas and concepts from conventional optic designs. However a considerable part of it uses computer generated digital simulations and optimization techniques which utilize the up growing power of modern computation.
Design optimization algorithms:
Diffractive simulation techniques:
Iterative Fourier transform algorithm
Fast Fourier transform (FFT). Angular spectrum. Point to point.
(IFTA) Direct search methods. Genetic algorithms. Monte Carlo optimization.
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Project purpose
Input: an array 1024X1024 or 2048X2048. the array will represent the strength of light in each point of the picture (on a screen). Output: an array same size of the input Array. The array will represent the lens plane, each cell will contain a value between 0 to 7 or 15 represents the level we dig in lens.
Fourier Transform
DFT - Discrete Fourier transform
FFT – Fast Fourier Transform
Optimum Algorithms System transform
bidirectional First estimate
Unit cell constraints
MSE
Performance constraints
simulated annealing First estimate
Unit cell constraints
Next estimate
System transform
MSE
Figure of merit
Bidirectional algorithm
local minimum problem
The determinist bidirectional algorithm will gather to a minimum after not more than 7 iteration. But in most of the time it will be a local minimum, it’s depends on our first random estimate.
Can we find the global minimum?
We can use simulated annealing. This algorithm has probability, which decrease every iteration, to reestimate the lens plane. to change things randomly even if it will increase MSE value. It will take us much more iteration and time. Even than we can be sure that we got the global minimum. But in most of the cases local minimum could be good enough.
How will it work?
The program will use condor to run many session of the bidirectional algorithm with, different first estimates. The program will use mpi to split the array to 4 smaller arrays and calculate each one in a different thread. The program will use FFTW to perform FFT efficiently. The result lens and the expected picture of it will be display by VisIt.
Diffractive Optical Elements Functions
Gaussian Beam Shaping (Top Hat): The Top-Hat beam shaper receives as input a gaussian beam. With a specific diffractive profile etched usually on a Plano-convex lens the diffractive element alters the profile of the beam to an uniform top hat like profile. Input beam
Properties of output beam profile:
Uniform Intensity profile Steep transition regions Rectangular or circular shape
Output beam 17
Diffractive Optical Elements Functions
Top Hat beam shaper – optical setup: The Top-hat beam shaper gives the required beam shape at far field. To modify the far field behavior to a certain given distance, the diffractive profile is usually etched on a Plano-convex lens.
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Diffractive Optical Elements Functions
Stable Top-Hat Beam Shaper: To improve the sensitivity to misalignment and the input beam profile a stable TopHat beam shaper was developed. This design starts from the analytical design of the regular Top-Hat and is iteratively optimized for different gaussians and de-centering.
Input beam
Regular Top-Hat
Stable Top-Hat
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Diffractive Optical Elements Functions
Periodic diffractive elements: Each spot is the exact replica of the input beam profile.
Example of 7x7 2D beam splitter
Example of period phase structure
The number and intensity pattern of the spots is determined by the period structure 20
Design properties: These diffractive element are designed as a phase hologram elements. Each portion of the phase projects the entire image. As a result the element is not sensitive to misalignments. However, unlike the multi spots pattern generators these designs are not periodic and therefore there is no spot separation in the projected image.
The design of these diffractive elements is mainly a subject of algorithmic optimization.
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Diffractive Optical Elements Functions
Multi focal Lens: The multi focal lenses exploits the property of a periodic grating to obtain a replica of the image at different orders to give a focused image at various focuses simultaneously. The energy distribution between the focuses and the number of focuses is determined by the design of the profile.
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Diffractive Optical Elements Functions
Application example - Multi focal IOL for ophthalmic surgery: The multi focal IOL lens enables the patient to see both the far field and near field at focus without wearing glasses.
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Diffractive Optical Elements Functions
Dual wavelength beam combiners: The dual wavelength lens uses a diffractive surface etched on a Plano-convex lens to bring the focal points of two wavelength to the same spot.
These type lenses are used in laser surgery were the CO2 laser is used for the treatment of the surface and the HeNe is used as an visible indicator for the surgeon. 26 A
Diffractive Optical Elements Functions
Beam samplers: By diverting a small portion of the beam energy with the exact same profile a the input beam, the beam sampler element enables to inspect the beam energy and intensity profile.
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Diffractive Optical Elements Functions
Anti Reflection diffractive patterns:
By creating a surface relief structure with sub wavelength features, a similar function as anti reflectance coating can be achieved.
This type of anti reflection is highly effective and has high power damage threshold.
