Articles

Effect Of Glycerol On A Diacetone Acrylamide-Based Holographic Photopolymer Material

Dervil Cody et al. — Thu, 24 Jan 2013 05:41:00 PST

The composition of the low-toxicity, environmentally compatible diacetone acrylamide (DA) photopolymer has been modified with the inclusion of different additives. The addition of glycerol to the photopolymer composition is described. Results show that the incorporation of glycerol results in a uniform maximum refractive-index modulation for recording intensities in the range of 1–20 mW/cm2. This may be attributed to glycerol’s nature as a plasticizer, which allows for faster diffusion of an unreacted monomer within the grating during holographic recording. An optimum recording intensity of 0.5 mW/cm2 is observed for exposure energies of 20–60 mW/cm2. The modified photopolymer achieves a refractive-index modulation of 2.2×10−3, with diffraction efficiencies up to 90% in 100 μm layers. Glycerol has also shown to reduce the rate of photobleaching of the DA photopolymer. This is possibly due to more prevalent inhibition effects caused by increased oxygenation of the photopolymer layers. The stability of the photopolymer samples is also improved with the addition of glycerol.

Nanozeolites Doped Photopolymer Layers with Reduced Shrinkage

Mohesh Moothanchery et al. — Tue, 08 May 2012 02:02:47 PDT

An acrylamide based photopolymer doped with pure silica MFI-type zeolite (silicalite-1) nanoparticles has been characterized for holographic recording purposes. The concentrations of the silicalite-1 nanoparticles in the photopolymer layers were 1, 2.5, 5 and 7.5 wt.%. The inclusion of silicalite-1 nanoparticle in the photopolymer has resulted in an increase of the diffraction efficiency by up to 40%, and decrease of the shrinkage from 1.32% to 0.57%. The best results were obtained in layers doped with 5 wt.% silicalite-1 nanoparticles.

New Non-Toxic Holographic Photopolymer Material

Dervil Cody et al. — Wed, 11 Jan 2012 08:49:33 PST

There is an increasing need for environmentally friendly holographic recording materials which can be produced in bulk with little risk to the health of workers in manufacturing. This is why the development of non-toxic photopolymer materials is crucial, and has attracted attention in recent years. Composition and preliminary characterization of a new non-toxic photopolymer material are presented. It operates well at a range of spatial frequencies, and achieves diffraction efficiencies and refractive index modulation comparable to the known acrylamide-based photopolymers.

Studies of Shrinkage as a Result of Holographic Recording in Acrylamide Based Photopolymer Film

Mohesh Moothanchery et al. — Mon, 10 Oct 2011 07:46:30 PDT

We studied the shrinkage in acrylamide-basedphotopolymer by measuring the Bragg detuning of transmission diffraction gratings recorded at different slant angles and at different intensities. Transmission diffraction gratings of spatial frequency 1000 lines/mm were recorded in an acrylamide-based photopolymer film having 60 ± 5 μm thickness. We have obtained the grating thickness and the final slant angles from the Bragg curve and hence calculated the shrinkage caused by holographic recording. The shrinkage of the material was evaluated for three different recording intensities 1, 5 and 10 mW/cm2, while the total exposure energy was kept constant at 80 mJ/cm2. From the experimental results it can be seen that the shrinkage of the material is higher for recording with lower intensities and the corresponding values are 1.9%, 1.3%and 1%.

Study of the Shrinkage Caused by Holographic Grating Formation in Acrylamide Based Photopolymer Film

Mohesh Moothanchery et al. — Tue, 09 Aug 2011 08:41:46 PDT

We study the shrinkage in acrylamide based photopolymer by measuring the Bragg detuning of transmission diffraction gratings recorded at different slant angles and at different intensities for a range of spatial frequencies. Transmission diffraction gratings of spatial frequencies 500, 1000, 1500 and 2000 lines/mm were recorded in an acrylamide based photopolymer film having 60 ± 5 μm thickness. The grating thickness and the final slant angles were obtained from the angular Bragg selectivity curve and hence the shrinkage caused by holographic recording was calculated. The shrinkage of the material was evaluated for three different recording intensities 1, 5 and 10 mW/cm2 over a range of slant angles, while the total exposure energy was kept constant at 80 mJ/cm2. From the experimental results it can be seen that the shrinkage of the material is lower for recording with higher intensities and at lower spatial frequencies.

Electronic Speckle Pattern Interferometer using Holographic Optical Elements for Vibration Measurements

Viswanath Bavigadda et al. — Wed, 26 Jan 2011 13:14:26 PST

A simple and compact electronic speckle pattern interferometry (ESPI) built with holographic optical elements (HOEs) used for the study of out-of-plane vibration is reported. Carefully fabricated reflection and transmission HOEs provide reference and object beams in the interferometer. All the alignment difficulties in conventional ESPI systems are minimized using HOEs. The time average ESPI subtraction method is used to generate correlation fringes. The background speckle noise is removed by introducing a phase shift between sequential images. The amplitude and phase maps are obtained using path difference modulation in an unbalanced ESPI.

Mechancial Characterization of Unplasticised Polyvinylchloride Thick Pipes Using Electronic Speckle Pattern Interferometry

Emilia Mihaylova et al. — Wed, 26 Jan 2011 13:14:22 PST

In this work a number of techniques (electronic speckle pattern interferometry, holographic interferometry, strain gauge and finite element method) are brought to bear in order to establish consistency in the results of strain measurement. This is necessary if optical nondestructive testing methods, such as those used here, are to gain acceptance for routine industrial use. The FE model provides a useful check. Furthermore, ESPI fringe data facilitates the extension of FE models, an approach that is of growing importance in component testing. The use of in-plane and out-of-plane sensitive electronic speckle pattern interferometry (ESPI) for non-destructive material characterization of thick unplasticised polyvinylchloride (uPVC) pipes is presented. A test rig has been designed for stressing pipes by internal pressure. ESPI gives a complete mapping of the displacement field over the area imaged by the video camera. The results for the strain of uPVC obtained from ESPI data and from strain gauges are in good agreement. The value of Young’s modulus has been obtained from the fringe data and compared with results obtained using holographic interferometry and from strain gauge measurements. The FE model also produces fringe data that is consistent with the ESPI results.

Diffraction from Polarization Holographic Gratings with Surface Relief in Side-chain Azobenzene Polyesters

Izabela Naydenova et al. — Wed, 26 Jan 2011 13:14:16 PST

We investigate the polarization properties of holographic gratings in side-chain azobenzene polyesters in which an anisotropic grating that is due to photoinduced linear and circular birefringence is recorded in the volume of the material and a relief grating appears on the surface. A theoretical model is proposed to explain the experimental results, making it possible to understand the influence of the different photoinduced effects. It is shown that at low intensity the polarization properties of the diffraction at these gratings are determined by the interaction of the linear and circular photobirefringences, and at larger intensity the influence of the surface relief dominates the effect of the circular anisotropy. Owing to the high recording efficiency of the polyesters, the ±1-order diffracted waves change the polarization interference pattern during the holographic recording, resulting in the appearance of a surface relief with doubled frequency.

Holographically Recorded Photopolymer Diffractive Optical Element for Holographic and Electronic Speckle Pattern Interferometry

Sridhar Guntaka et al. — Fri, 21 Jan 2011 05:07:13 PST

A diffractive optical element is described that can be used to implement a self-aligning electronic speckle apttern interferometer and holographic interferometer requiring only a laser source and a camera in the optical set-up.

Holographic and Speckle pattern interferometry using a photopolymer recording material

Sridhar Reddy Guntaka et al. — Fri, 21 Jan 2011 04:56:24 PST

White Light Interferometric Surface Profiler

Vincent Toal et al. — Fri, 21 Jan 2011 04:50:15 PST

We describe an optical system for 3-D profilometry based on the white light interferometer. We detail a simple way to construct a profiler that uses two simple algorithms which deal efficiently and quickly with the data. The system has a theoretically unlimited range and can deal with rough and smooth surfaces

Electro-optical Switching of the Holographic Polymer-dispersed Liquid Crystal Diffraction Gratings

Pavani Kotakonda et al. — Thu, 20 Jan 2011 08:07:45 PST

Polymer-dispersed liquid crystal (PDLC) is a material promising for application in optical communications, diffractive optics and optical data storage. Diffraction gratings were optically recorded in a novel PDLC material developed at the Centre for Industrial and Engineering Optics. Details of the fabrication and preliminary results of electro-optical switching of the holographic PDLC (HPDLC) diffraction gratings are presented. The redistribution of LCs was observed by using phase contrast microscopy and confocal Raman spectroscopy.

Application of ESPI-Method for Strain Analysis in Thin Wall Cylinders

David Kennedy et al. — Thu, 20 Jan 2011 08:07:42 PST

The Centre for Industrial and Engineering Optics, DIT, Ireland, have recently developed and applied new optical techniques for the measurement of mechanical strain, one of which was based on electronic speckle pattern interferometry (ESPI). The accuracy of this optical technique when used on flat surfaces is well established. In this research the technique is tested on curved surfaces, and results are compared with the theoretical hoop strain as predicted by the bi-axial strain equation modified for the thin cylinder, and by those obtained by electrical resistance strain gauge (ERSG) undoubtedly the principal method of measuring mechanical strain. For testing procedures, a special unit was designed and produced for holding and loading the thin cylinder. Also the basic equation for hoop strain by ESPI for flat surfaces was modified for applications on curved surfaces. Thin cylinder hoop strain obtained by ESPI and calculated by a modified equation show remarkably good correlation to the predicted theoretical value as well as to the results obtained by ERSG. Since the ESPI theory was originally developed for use on flat surfaces, there are reasonable grounds for further investigating this relationship especially where curved surfaces are involved.

Vibration Phase Measurements using Holographic Optical Elements

Viswanath Bavigadda et al. — Thu, 20 Jan 2011 06:18:58 PST

The application of an out-of-plane sensitive electronic speckle pattern interferometer (ESPI) using holographic optical element (HOE) to vibration amplitude and phase mapping is reported. The novelty of the proposed system is the use of a speckle reference wave stored in a reflection holographic optical element (HOE). The incorporation of a HOE minimizes the alignment difficulties. The HOE based ESPI system is compact containing only a diode laser, HOE and a digital CMOS camera. The measurement technique is a combination of time averaged ESPI and reference beam phase modulation in an unbalanced interferometer. The reference beam phase modulation is implemented by modulating the drive current of the diode laser. The presented HOE based ESPI system is easy to align and compact and thus suitable for industrial non-destructive testing and vibration analysis.

Development of a Panchromatic Acrylamide Based Photopolymer for Multicolour Reflection Holography

Chakrapani Meka et al. — Thu, 20 Jan 2011 06:18:53 PST

The development of a panchromatic acrylamide based photopolymer (ABP) for holographic recording application is presented. The scattering of the recording medium was characterised by measuring the Bidirectional Scattering Distribution Function (BSDF). The dynamic range in reflection mode of recording was evaluated by measuring the diffraction efficiencies of the holographic gratings recorded individually at 633nm, 532nm, and 473nm wavelengths at spatial frequencies of 4200 l/mm, 5000 l/mm and 5700 l/mm respectively. Spectral characterisation of the reflection gratings recorded using a combined single RGB beam was carried out and the reconstructed wavelengths were monitored and compared with the recording wavelengths. The recorded and the reconstructed wavelengths were plotted as points on the CIE chromaticity diagram in order to reveal the shifts due to material shrinkage in the corresponding RGB wavelengths. Finally reflection holograms of an object were successfully recorded at all the three primary wavelengths. The results represent a strong confirmation that this acrylamide based photopolymer can be used as a panchromatic recording material and can be employed in future commercial holographic applications.

Raman Spectroscopy for the Characterization of the Polymerization Rate in an Acrylamide-based Photopolymer

Raghavendra Jallapuram et al. — Thu, 20 Jan 2011 06:18:48 PST

Investigations of polymerization rates in an acrylamide-based photopolymer are presented. The polymerization rate for acrylamide and methylenebisacrylamide was determined by monitoring the changes in the characteristic vibrational peaks at 1284 cm-1 and 1607 cm-1 corresponding to the bending mode of CH bond and CC double bonds of acrylamide and in the characteristic peak at 1629 cm-1 corresponding to carbon-carbon double bond of methylenebisacrylamide using Raman spectroscopy. To study the dependence of the polymerization rate on intensity and to find the dependence parameter, the polymerization rate constant was measured at different intensities. A comparison with a commercially available photopolymer shows that the polymerization rate in this photopolymer is much faster.

Photoinduced Surface Relief Studies in an Acrylamide-based Photopolymer

Pavani Kotakonda et al. — Thu, 20 Jan 2011 06:18:44 PST

Surface relief gratings (SRGs) have been optically recorded in a dry, self-developing acrylamide-based photopolymer. An investigation of the dependence of photoinduced surface relief amplitude and profile on recording intensity, UV post-exposure, thickness of the sample, composition of the photopolymer and temperature at constant spatial frequency was carried out using white light interferometry. Non-sinusoidal surface relief grating profiles which depend on sample thickness were observed at low spatial frequency. The surface relief effect is intended to be applied to the alignment of liquid crystals (LCs) for different applications such as voltage controllable diffraction gratings, lenses, polarizing components and switches.

Investigation of the Light Induced Redistribution of BETA Nanoparticles in an Acrylamide-based Photopolymer

Elsa Leite et al. — Tue, 18 Jan 2011 09:45:42 PST

We report the redistribution of colloidal zeolite Beta nanoparticles during holographic recording in acrylamide-based photopolymers. Using the techniques of confocal Raman spectroscopy and scanning electron microscopy coupled with energy dispersive x-ray analysis (SEM–EDX), we have observed a periodic pattern, whose spacing agrees with the fringe spacing of the recorded holographic diffraction grating. It was estimated that the fraction of nanoparticles redistributed as a result of the holographic recording is 40%. The effect of the nanoparticles on the average refractive index of the nanocomposite layers was studied by UV–visible spectroscopy. It was observed that the addition of zeolite nanoparticles leads to an increase of the photopolymer layer’s thickness, and this is ascribed to the interaction between molecules of monomer and the zeolite nanoparticles, further supported by Raman spectroscopic studies, indicating that these nanoparticles are not an entirely inert additive. The holographic recording properties of the new nanocomposite were characterized and no significant improvement of the net refractive index modulation was observed. This result is explained after taking into account two main factors influencing the final refractive index modulation—the redistribution of the nanoparticles and the change in the monomer’s volume concentration due to change in the thickness of the solid layers. The results presented here contribute to insights about the role of nanoparticles in the mechanism of holographic recording in acrylamide-based photopolymeric systems. The zeolite nanoparticle redistribution achieved in this new nanocomposite could be useful for the fabrication of holographic sensors, as demonstrated by initial studies with toluene.

Electronic Speckle Pattern Shearing Interferometer with a Photopolymer Holographyc Grating

Emilia Mihaylova et al. — Mon, 17 Jan 2011 08:38:28 PST

A photopolymer holographic grating is used to produce the two sheared images in an electronic speckle pattern shearing interferometer. A ground glass screen following the grating serves the purpose of eliminating unwanted diffraction orders and to remove the requirement for the CCD camera to resolve the diffraction grating’s pitch. The sheared images on the ground glass are further imaged onto the CCD camera. The fringe pattern contrast was estimated to be above 90%. A validation of the system was done by comparing the theoretical phase difference distribution with the experimental data from the three point bending test.

Optical and Holographic Characteristics of Photopolymer Layers

Temenujka Yovcheva et al. — Mon, 17 Jan 2011 08:35:41 PST

In the present work the optical and holographic characteristics of acrylamide-based photopolymer layers are studied. For the first time the refractive index change of a liquid acrylamide photopolymer due to exposure at 532 nm is obtained using a critical angle laser micro-refractometer. The 30 mm thick solid photopolymer films are prepared by casting on glass substrates. Bragg holographic gratings with spatial frequencies of 710 mm-1, 1050 mm-1 and 1600 mm-1 are recorded using a diode laser operating at 532 nm wavelength. The diffraction efficiency dependence on the exposure energy is investigated. The obtained results are compared with the Stetson holographic recording method, where two gratings are simultaneously recorded in the same location with spatial frequencies 2020 mm-1 and 3670 mm-1, using a totally reflected reference wave from the air-photopolymer interface. Despite the fact that in the second method the two gratings share the same dynamic range, higher diffraction efficiencies are observed.