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Recent Optical Solutions
With DIFFRACTIVE OPTICAL TECHNOLOGY
Tamir Grossinger
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Definition
Technology Background
Design methods
Diffractive Optical Elements Functions
Applications
Content
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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
Advancements in computer capabilities enabling optimizations
Modern flexible diffractive optical elements for various commercial applications including: material processing, medical lasers, 3D imaging, security, etc.
Advancements in micro lithography fabrication techniques
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
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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.
Fast Fourier transform (FFT). Angular spectrum. Point to point.
Diffractive simulation techniques:
Iterative Fourier transform algorithm (IFTA) Direct search methods. Genetic algorithms. Monte Carlo optimization.
Design optimization algorithms:
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
bidirectional
simulated annealing
System
transform
Performance
constraints
Unit cell
constraints First
estimate
MSE
First
estimate
Unit cell
constraints System
transform MSE
Figure of
merit
Next
estimate
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 re-
estimate 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.
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Diffractive Optical Elements Functions
Uniform Intensity profile Steep transition regions Rectangular or circular shape
Gaussian Beam Shaping (Top Hat):
Input beam
Output beam
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.
Properties of output beam profile:
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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 Top-Hat 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:
The number and intensity pattern of the spots is determined by the
period structure
Example of period
phase structure
Each spot is the exact replica of the input beam profile.
Example of 7x7 2D
beam splitter
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Diffractive Optical Elements Functions
1D Beam Splitter 2D Beam Splitter
Examples of multi spot patterns:
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Diffractive Optical Elements Functions
Beam splitters applications:
Skin resurfacing, tattoo removal, hair removal.
Parallel processing.
Hole drilling.
3D camera.
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Diffractive Optical Elements Functions
Design properties:
The design of these diffractive elements is mainly a
subject of algorithmic optimization.
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.
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Diffractive Optical Elements Functions
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.
Multi focal Lens:
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Diffractive Optical Elements Functions
The multi focal IOL lens enables the patient to see both the far field and near field at focus without wearing glasses.
Application example - Multi focal IOL for ophthalmic surgery:
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Diffractive Optical Elements Functions
A
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.
Dual wavelength beam combiners:
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.
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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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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.
Diffractive Optical Elements Functions
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Encryption in free space laser communication:
Diffractive Optical Elements Functions
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The End
Question ?
