All optical image processing using third harmonic generation
Application: Image Correlation All Optical Image Processing Using Third Harmonic Generation Course Presentation for: Optical Image Processing, under Professor Dorsinville Md. Faisal Halim (Faissal) Department of Electrical Engineering The City College of New York, CUNY Monday, 13 th December, 2010 CCNY
Motive: Computing <ul><li>Extend Moore’s Law (or put it on steroids): </li></ul><ul><ul><li>Optical image processing can increase computing speeds tremendously 1 </li></ul></ul><ul><ul><li>E.g. IBM’s Road Runner Super Computer: 1.7x10 15 FLOPS Occupies: 560 m 2 of real estate. Possible NLO based ultrafast 2D signal processor: 10 16 OPS Occupies: a few square meters for the complete setup. </li></ul></ul><ul><li>Why 2D signal processing (signal has spatial, not just temporal components)? </li></ul><ul><ul><li>Massive parallelism enhances processing speeds: </li></ul></ul><ul><ul><ul><li>E.g., matrix convolutions, multiplications </li></ul></ul></ul>
Background <ul><li>Current Optical Processing is done based on Electro-Optics technology (like LCD) </li></ul><ul><ul><li>Slow limited by electronic switching speeds </li></ul></ul><ul><li>Opto-optic systems are faster. </li></ul><ul><li>THG is inherently ultrafast </li></ul><ul><ul><li>Less efficient than SHG </li></ul></ul><ul><ul><li>Information on amplitude and phase of a pulse preserved </li></ul></ul><ul><ul><ul><li>Regardless of temporal shape of the pulse </li></ul></ul></ul>
What is Image Correlation? <ul><li>Basic Concept: </li></ul><ul><ul><li>Comparison of input image (h) with a previously known object/template (g), in the presence of a reference beam (r) </li></ul></ul><ul><ul><li>Identifies the object, if object in image matches a known pattern. </li></ul></ul>Source: Ref. 1 Source: Ref. 2
Why do we want things Ultrafast? <ul><li>Fast pulses, at high repetition rates (82 MHz+ has been achieved), will enable fast computing (fast frame rates) </li></ul><ul><li>Ultrafast laser pulses avail intensities high enough for nonlinear optical processes, like THG </li></ul><ul><li>THG, owing to very small regions where the intensity, and wave propagation vector, will allow it to happen, will generate very well defined images when autocorrelation happens </li></ul><ul><li>Ultrafast reference source allows picking up “ballistic photons” thus giving clear images even through scattering media, using a gating function. </li></ul>
Third Harmonic Generation Source: Ref 1. Left-hand panels show schematics of: a) the two-beam noncollinear THG; and b) the three-beam noncollinear THG geometries. Right-hand panels show images captured with a Si CCD camera of: a) the four THG beams generated in a two-beam noncollinear THG process labeled with their corresponding wavevectors k ijk (3 ω ) and a diagram of the wavevector combinations of k 1( ω ) and k 2( ω ) that give rise to each beam; and b) the ten THG beams generated through a three-beam noncollinear process labeled with their corresponding wavevectors k ijk (3 ω ).
Characteristics of Fourier Filter Material <ul><li>High value of χ (3) at the excitation wavelength </li></ul><ul><li>Low absorption in the THG wavelength region </li></ul>Source: Ref 1. Spectral dispersion of the complex index of refraction. The shaded areas correspond to the spectral ranges where a pump beam produces a measurable third harmonic (TH). The inset shows the molecular structure of the dipolar chromophore. Pump power dependence of collinear THG. The inset shows the angular dependence of the noncollinear THG power in a two-beam configuration normalized by the power in the collinear beam. The dashed line is a guide to the eye.
Generating the Three Beams Source: Ref. 2 (Color online) Schematics of the joint transform correlator geometry used for the implementation of the noncollinear THG correlator. The inset shows the ten THG beams generated when three noncollinear pulses coincide spatially and temporally in the NLO material along with the fundamental wave vector combinations that give rise to them.
Cleaning up a Pulse That passed a Scattering Medium Source: Ref 1 The upper panel shows an schematics of the time-gated THG imaging configuration. The lower panel shows the images obtained for a Gaussian beam ( h ( r ,t)) propagating in a scattering media (SM) with 14 mfp: A) unfiltered image at the 1550 nm detected with an NIR-sensitive camera in the k h direction; a)–e) Time-gated images captured in the k hrr direction at different delay times, τ , illustrating the time-gated process.
Complete Setup Source: Ref. 1 Diagram of the DOE-based noncollinear THG optical correlator. For clarity, only the THG beam at the k rhg direction is shown. Object and test template masks put here, as ‘shaped’ apertures, into this blocking mask.
Pros of the system <ul><li>Quality of optical image processing obtained is comparable to that obtained by state of the art non-resonant Kerr nonlinearities in other polymers (polyacetylenes) </li></ul><ul><li>System not susceptible to scattering at the fundamental wavelength the signal is filtered spatially and spectrally. </li></ul><ul><li>A modest increase in the THG conversion efficiency could enable operating energies below 1 nJ per pulse (currently 1nJ<energy<1nJ is used) </li></ul><ul><li>System works at eye safe 1550nm regime, with a cheap silicon camera for the 2D detector. </li></ul><ul><li>Phase matching of beams is not required. </li></ul><ul><li>System can be built from processable polymers, using spin coated thin films </li></ul><ul><li>Potential for large spectral and angular bandwidths </li></ul>
Cons of the System <ul><li>When making new, higher χ (3) materials care has to be taken to ensure that the material is highly absorbing at the fundamental wavelength, and transparent at the third-harmonic wavelength </li></ul><ul><li>The kind of material currently being tried, polyacetylines, has its drawbacks along these lines. </li></ul>
Future Work <ul><li>Development of octupolar molecules that will abate some of the disadvantages of higher χ (3) materials 4 </li></ul>
Conclusion <ul><li>All optical image correlation was achieved using a thin film </li></ul><ul><li>Phase matching was not required between the light beams </li></ul><ul><li>Independence from the need for electro-optic SLMs was not yet achieved, however </li></ul><ul><li>A fast system has been created for the detection of previously defined shapes in previously defined orientations. </li></ul>
References <ul><li>"Third-harmonic generation and its applications in optical image processing“ Fuentes-Hernandez, et. al. J. Mater. Chem., 2009, 19, 7394-7401 </li></ul><ul><li>"Ultrafast optical image processing based on third-harmonic generation in organic thin films“ Fuentes-Hernandez et. al. APPLIED PHYSICS LETTERS 91, 131110, 2007 </li></ul><ul><li>"Thick Optical-Quality Films of Substituted Polyacetylenes with Large, Ultrafast Third-Order Nonlinearities and Application to Image Correlation“ Chi, et. al. Adv. Mater. 2008, 20, 3199–3203 </li></ul><ul><li>“ Second-order nonlinear optical properties of octupolar molecules structure–property relationship” Kim, et. al. J. Mater. Chem., 2009, 19, 7402–7409 </li></ul>