In this work, brief theoretical modeling, analysis, and novel numerical verification of a photorefractive polymer based four wave mixing (FWM) setup for defect detection has been developed. The numerical simulation helps to validate our earlier experimental results to perform defect detection in periodic amplitude and phase objects using FWM. Specifically, we develop the theory behind the detection of isolated defects, and random defects in amplitude, and phase periodic patterns. In accordance with the developed theory, the results show that this technique successfully detects the slightest defects through band-pass intensity filtering and requires minimal additional post image processing contrast enhancement. This optical defect detection technique can be applied to the detection of production line defects, e.g., scratch enhancement, defect cluster enhancement, and periodic pattern dislocation enhancement. This technique is very useful in quality control systems, production line defect inspection, and computer vision.
The photorefractive joint transform correlator (JTC) combines two features. The first is embedded semi-adaptive optimality which weighs the correlation against clutter and noise in the input and the second is the intrinsic dynamic range compression nonlinearity which improves several metrics simultaneously without metric tradeoff. The performance of this two-beam coupling joint transform correlator scheme is evaluated against several other well-known correlation filters that have been developed during the last three decades. The result shows that the two-beam coupling joint transform scheme is a very robust correlator with respect to standard evaluation metrics for different sets of data.
In prior work, we demonstrate optical correlation via dynamic range compression in two-beam coupling using
thin-film organic materials. In this paper, we continue the effort; characterize the performance of this correlator
for variety of input. We successfully demonstrated correlation results almost free of cross- correlation and noise
for extremely complicated noisy image were the signal image consist of several targets and reference image
superposed of many templates.
In prior work, we exploited the nonlinearity inherent in four-wave mixing in organic photorefractive materials for
adaptive filtering. In this paper, we extend our work further and demonstrate new applications which involve:
dislocation, scratches and defect enhancement. With the availability of the organic photorefractive materials with
large space-bandwidth product, it should open the possibility of using the adaptive filtering techniques in quality
control systems.
Photorefractive composites derived from conducting polymers offer the advantage of dynamically recording holograms
without the need for processing of any kind. Thus, they are the material of choice for many cutting edge applications,
such as updatable three-dimensional (3D) displays and 3D telepresence. Using photorefractive polymers, 3D images or
holograms can be seen with the unassisted eye and are very similar to how humans see the actual environment
surrounding them. Absence of a large-area and dynamically updatable holographic recording medium has prevented
realization of the concept. The development of a novel nonlinear optical chromophore doped photoconductive polymer
composite as the recording medium for a refreshable holographic display is discussed. Further improvements in the
polymer composites could bring applications in telemedicine, advertising, updatable 3D maps and entertainment.
In this paper, we demonstrate optical correlation via dynamic range compression in two-beam coupling using thin-film
organic materials. In contrast to the first demonstration, in which it was not possible to demonstrate correlation with
complicated input, here we demonstrate correlation with extremely challenging cases involving finger prints, images in
clutter, and SAR images. Our correlation results outperform many correlation results, including ones based on optimal
filters.
Imaging in atmospheric turbulence and target recognition in cluttered environments have been research topics for many
years. Currently, there are some well-established techniques for image restoration and recognition; however, if the
atmospheric turbulence becomes a severe scattering medium and the surrounding environment is very cluttered, most
conventional methods, such as inverse filtering and Wiener filtering, will be inadequate for correcting and recognizing
the captured images. In this paper, we experimentally demonstrate nonlinear dynamic range compression techniques for
image restoration and correlation via two-beam coupling and four wave mixing in organic photorefractive films.
In this paper, we exploit the nonlinearity inherent in four-wave mixing in organic photorefractive materials and demonstrate edge enhancement, contrast conversion, and defect enhancement in a periodic structure. With the availability of these materials, which have large space-bandwidth products, edge enhancement, contrast conversion and defect enhancement are possible. Some simulation results also are provided.
In this paper, we exploit the nonlinearity inherent in four-wave mixing in organic photorefractive materials and
demonstrate edge enhancement, contrast conversion, and defect enhancement in a periodic structure. With the
availability of these materials, which have large space-bandwidth products, edge enhancement, contrast conversion and
defect enhancement are possible.
The electron transporting molecule tris(8-hydroxyquinoline) aluminum (Alq3) was introduced into a photorefractive
composite in a low density to study the effects of electron traps on the performance. Compared to a control sample, Alq3
samples exhibited higher dielectric strength, over-modulation at reduced voltage, and increased writing speed. Transient
measurements indicated grating revelation via decay of a competing grating. The dynamics are consistent with a bipolar
charge transport model. Overall, Alq3 improves the sensitivity, trapping, and breakdown voltage without significant
losses in absorption or phase stability.
The very first demonstration of our refreshable holographic display based on photorefractive polymer was published in
Nature early 20081. Based on the unique properties of a new organic photorefractive material and the holographic
stereography technique, this display addressed a gap between large static holograms printed in permanent media
(photopolymers) and small real time holographic systems like the MIT holovideo. Applications range from medical
imaging to refreshable maps and advertisement. Here we are presenting several technical solutions for improving the
performance parameters of the initial display from an optical point of view. Full color holograms can be generated
thanks to angular multiplexing, the recording time can be reduced from minutes to seconds with a pulsed laser, and full
parallax hologram can be recorded in a reasonable time thanks to parallel writing. We also discuss the future of such a
display and the possibility of video rate.
Ultrasound-modulated optical tomography uses a well focused ultrasound beam to modulate diffuse light inside soft
biological tissues. This modality combines the advantages of ultrasound resolution with optical contrast. However,
because of the low ultrasound modulation efficiency, the large background of un-modulated photons gives a low
signal-to-noise ratio. Here we report a technique for detection of ultrasound-modulated light using a phase conjugated signal
generated by four-wave mixing in a photorefractive polymer. The experimental results demonstrate the potential of this
method to detect ultrasound-modulated optical signals in a highly scattering media with an excellent signal-to-noise
ratio.
We report the photorefractive properties of tetraphenyldiaminobiphenyl (TPD) based polymer composites
that have been developed for single pulse laser operation at 532 nm. With an optimized composite, we
demonstrate more than 50% diffraction efficiency using 4 mJ/cm2 single shot writing and 633 nm
continuous wave (cw) beam reading. The present devices showed a 300 μs fast response time. This
reveals the potential for these polymer devices in applications which require fast writing and erasure. Since
the writing pulse-width is in nanosecond time scale, the recording is totally insensitive to vibrations. These
devices can also be used as a stepping stone to realize all-color holography since they are sensitive to both
green (532nm) and red (633nm) wavelengths. The holograms can be written with either of these two
wavelengths and can be read by the same wavelength or the other wavelength with high diffraction
efficiency. This demonstrates that these devices have the advantage of performing two-color holography, a
step closer to a dynamic full-color holographic recording medium.
We propose and demonstrate a novel technique for efficient local fixing of photorefractive polymer hologram using a
laser beam. In the new technique, a CO2 laser beam is used to heat the sample and a local hologram can be fixed easily.
By using glass and sapphire with particular thickness as the substrates for the photorefractive device, the hologram can
be fixed efficiently and at much faster speed. The fixation efficiency can be greater than 80% and the hologram can be
fixed in a few seconds. This technique is critical for dynamic holographic 3D display and holographic data storage.
We propose an optically controllable device Photorefractive Connection Module (PRCM) for free-space optical interconnection between boards. The optical signal composed of a large number of spatially divided channels passes through the photorefractive material which is core of PRCM. In the photorefractive material, diffraction gratings are induced only where the control beam and the pump beam are illuminated. The signal beams in desired spatial channels can be diffracted by setting an appropriate pattern of the control beam and extracted by separating from the control beam with a beam splitter. In this study, we use organic photorefractive polymer PATPD as the photorefractive material. Diffraction efficiency of PATPD is comparable with thick inorganic crystals although the thickness of PATPD is less than 1/10 of these crystals. Thin gratings have a large advantage for PRCM because the thickness of the gratings causes large crosstalk between spatial channels. In addition, the constraint of phase matching is reduced because the phase mismatch is proportional to the thickness of the material. The decrease of the phase mismatch provides large allowance for misalignment of incident angle of beams; therefore it becomes easy to adjust incident angles. We reveal the relation of diffraction efficiency to angular difference of PATPD by analysis and experiment. Diffraction efficiency decreases by half at the difference of 0.5o, which is about ten times larger than that of thick crystals. We demonstrate that it is possible to extract the signal beams in desired channels and reconfigure the extraction pattern according to optical control by using PATPD.
We have demonstrated an optical novelty filter based on the two-beam coupling effect in photorefractive polymers. The photorefractive polymer composition was optimized for response time and two-beam coupling gain by changing the ionization potential and polarizability of various components. In this study, a photorefractive polymer composition was simultaneously optimized for response time and gain, and employed as a key element in a two-beam coupling novelty filter with a high contrast ratio and a limiting frequency of 14Hz, considerably higher than any previously reported in a two-beam coupling photorefractive novelty filter.
We report on the photorefractive properties of two polymer composites that utilize a new bis-triarylamine side-chain polymer matrix. Correctly locating the frontier orbitals of the new transport manifold with respect to the HOMO levels of chromophores, allows stable continuous operation over exposure levels of more that 4 kJ/cm2 when samples are electrically biased at 57 V/μm. This operational stability is combined with video-rate compatible grating build-up times and a dynamic range that allows index modulations of 3 x 10-3 and gain coefficients on the order of 100 cm-1 at moderate fields. The thermal stability of one of the composites reported is excellent, showing no signs of phase separation even after one week at 60°C. A comparison with the stability of composites where the new matrix was replaced by PVK is also presented.
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