This PDF file contains the editorial “Could Photonics Kill Optics?” for OE Vol. 39 Issue 03

This PDF file contains the editorial “Guest Editorial: Special Section on Information Optics Using Polymeric Materials” for OE Vol. 39 Issue 03

Translation of the large molecular hyperpolarizability of chro- mophores into large macroscopic electro-optic activity by electric field poling of chromophore-containing polymers is opposed by molecular- shape-dependent intermolecular electrostatic interactions. Modification of chromophore structure (shape) to minimize the deleterious effect of such interactions leads to significant improvement in electro-optic activity. Drive voltage requirements for polymeric modulator devices are reduced to values ranging from 0.7 to 5 V. Optical loss of electro-optic polymer materials, at the communication wavelengths of 1.3 and 1.55 ?m, is defined by C-H, O-H, and N-H vibrational absorptions and by scattering from index of refraction gradients in the material. The former are significantly reduced by partial deuteration and halogenation to values slightly less than 1 dB/cm. A major source of optical loss encountered in the utilization of polymeric electro-optic modulator devices is coupling losses associated with the mode mismatch between silica fiber waveguides and polymeric modulator waveguides. Recently, alternative coupling strategies based on utilization of tapered transitions and vertical transitions have dramatically reduced coupling losses. Total insertion losses comparable to those for lithium niobate devices are realized. Finally, a phototrimming technique is developed that enables fine-tuning of the performance of circuit elements such as power splitters.

We review a polymeric waveguide technology developed to produce affordable high-performance optical components that address the needs of both the telecom and the datacom industries. We engineer advanced organic polymers that can be readily made into planar singlemode, multimode, and micro-optical waveguide structures of controlled numerical apertures and geometries. These materials are formed from highly crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, optical loss, and environmental stability. These monomers are intermiscible, providing for precise continuous adjustment of the refractive index over a wide range. In polymer form, they exhibit state-of-the-art loss values, suppressed polarization effects, and exceptional stability, enabling their use in a variety of demanding applications. Waveguides are formed photolithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser direct writing. A wide range of rigid and flexible substrates can be used. The devices we describe include a variety of passive and thermo-optically active elements that achieve a variety of coupling, routing, and filtering functionalities. These devices can be either individually pigtailed and packaged components or they can be part of a massively parallel photonic integrated circuit on the multichip module (MCM), board, or backplane level.

The photochemical behavior of dichromated gelatin films is investigated on irradiation at 365 nm and 491.6 nm. Fourier transform infrared (FTIR) spectroscopy confirms the two-step reduction of chromi- um(VI) into chromium(III). Prior to the irradiation, the molar extinction coefficient ? of chromium(VI) in the film is measured as a function of the pH of the starting solution with the result that e appears to be pH independent in the range 2 to 10 contrary to what is observed in solution. Chromium(V) resulting from a charge transfer between the polymeric matrix and chromium(Vi) species in the excited state appears to be surprisingly stable in the gelatin matrix. Accordingly, the subsequent slow reduction of chromium(V) into the final chromium(III) does not interfer in a significant way in chromium(VI) reduction. As a result, it is possible to determine the UV-visible spectrum of chromium(V) and to quantify its formation. So far no quantitative results were reported in the literature concerning the chromium(V) spectrum and its formation in a film of dichromated gelatin. FTIR spectroscopy confirms the two-step reduction of chromium(VI) into chromium(III).

We demonstrate theoretically and experimentally that an initially Gaussian red light beam sent through a ?step phase mask and launched into a thin film of polymer poly(methyl methacrylate) doped with laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)4H- pyran known as DCM evolves into a spatial structure similar to a dark spatial soliton. This takes place due to the Kerr-type time-delayed nonlinearity associated with the mechanism of up-converted photobleaching of the dye-doped polymer. The proposed theoretical model is nonlocal in time and is based on the Shrodinger-type nonlinear equation for the amplitude of the propagating beam complemented by the rate equation for the light-induced decrease of the refractive index. The result of the structuring of the beam is the formation of a permanent pattern of the refractive index of the film that acts as a channel waveguide, trapping a weak Gaussian probe beam close to it. The probe beam can propagate along or against the soliton. We also demonstrate the tolerance of trapping to a possible shift or tilt of the probe beam with respect to the soliton. This makes the proposed approach potentially useful for interconnections between individual fibers, fiber ribbons, bundles, and multicore fibers as well as between fibers and planar integrated optical devices.

A twin-core polymer optical fiber doped with a conjugated biochemical ,?-carotene is fabricated for all-optical fiber switching. Nonlinear coupling and optical switching are observed between the two cores of the fiber. Experimental results are analyzed based on the coupled-mode theory and the nonlinear effects of this fiber were examined.

In the past, four-wave mixing for real-time holography storage based on ground-state absorption at the resonant wavelength has been extensively studied in azo-dye-doped polymer films. We report theoretical and experimental studies on the four-wave mixing based on the excited-state absorption at a nonresonant wavelength using ethyl- orange-doped polyvinyl alcohol films. In our experiments, we use two laser beams: a 632.8-nm He-Ne laser as a nonresonant light source for excited-state four-wave mixing and a 488-nm Ar+ laser as a preexcited light source to enhance the excited-state absorption. To explain the experimental results, we propose a four-level model for the azo-molecule system. The steady state and dynamic behavior of phase conjugation signal in four-wave mixing are simulated by solving rate equations. The theoretical simulations, which are in accord with the experimental results, show that using the preexcited method, four-wave mixing holography storage can be realized by using a weak laser beam with a nonresonant wavelength based on the excited-state process.

The provision of a range of wavelength division multiplexing (WDM) network component functionalities by ferroelectric liquid crystal (FLC) spatial light modulators (SLMs) has already been demonstrated in both transmissive and reflective arrangements. The FLC-SLM reconfiguration rate is limited to ~100 kHz by the liquid crystal response time. x(2) nonlinear optical polymers (NLOPs) might form the material basis of a future generation of faster and less costly SLMs for WDM applications, with response time limited only by electronic RC product. We present a theoretical analysis and the experimental characterization of single-pixel SLMs based on an ~2-?m film of commercially available x(2) NLOP in an asymmetric Fabry-Perot (AFP) cavity. Rigorous modeling enables optimization of the generic pixel design and specific pixel parameters, with the design guidelines and constraints dictated by the context of wideband WDM operation ~1.55 ?m. Polymer films are poled at a low field of ~35 V/?m and a 10-V modulating signal is applied. A modulation depth of 1.4% is provided by an AFP device of optimal pixel structure. Further characterization reveals a 3-dB width of transmitted intensity modulation ~10.8 nm near 1.55 ?m. The modeled behavior is extrapolated to yield two significant performance indicators. One indicator is that the holographic diffraction efficiency n of a full-scale pixelated AFP NLOP-SLM will be principally phase-modulation-dependent if the polymer Pockels coefficient r13 is of the order of 5 pm/V. For the preferred reflective geometry, the diffraction efficiency has a maximum value of ~13.5% cf. ~40% for a binary phase FLC-SLM. The second indicator is that for experimental devices, the 3-dB bandwidth of n~1.55 ?m will be only ~5 nm, but a~190 nm-thin cavity-optimized structure will have y3dB of ~30 nm.

The optical true-time-delay line is a key building block for modern broadband phased-array antennas, which have become one of the most critical technologies for both military and civilian wireless communications. We present our research results in developing an optical polymer-based waveguide true-time-delay module for multilink phased- array antennas by incorporating wavelength-division multiplexing (WDM) technology. The demonstrated optical polymeric waveguide circuits can provide a large number of optical true-time delays with a dynamic range of 50 ns and a time resolution of 0.1 ps. Various fabrication techniques are investigated for producing ultralong low-loss (0.02 dB/cm) polymeric channel waveguides with tilted waveguide grating output couplers. Fast photodiode arrays are fabricated and rf signals with frequencies of 10 to 50 GHz are generated through the optical heterodyne technique. A detailed study of waveguide amplification to achieve loss-less polymeric waveguide is conducted. The optical amplification of 3.8 dB/cm is achieved at a wavelength of 1064 nm in a Nd3+-doped polymeric waveguide. WDM techniques are also employed for potential multilink applications. The presented methodologies enable hybrid integration with a reduced cost in optoelectronic packaging and an increased reliability and decreased payload for the next generation of phased-array antennas.

Thin films of poly(2-decyloxy-1,4-phenylene) (DO-PPP) are studied, especially in terms of their structure and properties. DO-PPP has thermal stability up to 384°C. The indium-tin-oxide (ITO)/DO-PPP/Al device emitted 425-nm blue light. The optimum thickness for the device is about 100 nm. For the ITO/DO-PPP (100-nm)/Al device, the EL quantum efficiency was about 0.01%. For the ITO/DO-PPP (100-nm)/Al device with a heat-treated polymer layer, the electroluminescence (EL) intensity clearly decreases with an increase of crystallinity. This result may be due to low carrier mobility in the crystalline region.

A prototype projection display consisting of optically addressed polymer-dispersed liquid crystal (PDLC) light valves and a Schlieren optics is designed, built, and evaluated. This projection display, which controls light by a scattering effect in the PDLC film, can be configured without requiring a polarizer and can create a brighter high- definition picture display with which high optical efficiency is possible. The relationship between the contrast ratio and the optical efficiency of a Schlieren projection system is discussed. To design a projection display with both high brightness and a high contrast ratio, the scattering characteristics of a reflection-mode PDLC cell is evaluated and the optical efficiency of the total projection system is studied using a simple model with perfect diffusion of a light source. Using these results, we design and prototype PDLC light valves for a full-color display and construct a projection display. We succeed in displaying pictures with a contrast ratio of 80:1 and a brightness greater than 1800 ANSI lumens.

The electro-optic property of electron-irradiated poly- (vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) copolymer is presented. It is found that this special treated polymer not only has a giant electrostriction and relaxor ferroelectric behavior but also has a relatively good electro-optic effect. Thus, this copolymer may be a good candidate for making fast speed electro-optic modulators and switches, which are critical components for information optics.

An optical backplane for high bit rate data transmission between processor boards is presented. The backplane consists of low- loss polymer multimode waveguides, integrated micromirrors and lenses for focusing/collimation. Free space transmission is used for the coupling between processor boards and the backplane. Due to an expanded beam, high alignment tolerances between the boards and the backplane are obtained. Backplanes are realized for a 55 cm (21 in.) interconnection length, for a ring network and for a 4 x4 star network. For each backplane, data transmission with a bit rate > 1 Gbit/s is demonstrated and first environmental tests are successfully passed.

A Y-branch waveguide directional coupler modulator based on domain-inverted electro-optic polymers is demonstrated for highly efficient analog modulation. A detailed theoretical analysis is given to show its working principles based on coupled-mode theory. Domain-inverted electro-optical polymers are obtained by using a high-voltage pulsed poling technique. The demonstrated domain-inverted polymers represent a promising technology for creating a new class of electro-optic modulators with superior performance over conventional counterparts. By using domain inversion in a /-branch waveguide directional coupler, the polymeric electro-optic waveguide modulator provides a highly linear modulation transfer curve that shows a ~100% modulation efficiency with an intermodulation distortion less than -30 dB. The demonstrated modulator also features a switching voltage of 10 V and an extinction ratio of -28 dB at operation wavelength of 1.32 ?m. A linear modulation transfer curve and a 3-dB operating point are obtained without a bias voltage or an asymmetric device structure through a single uniform electrode.

It is necessary to improve optical and mechanical properties in optical disk substrates as information storage devices with high storage density using short-wavelength lasers are being developed. Injection compression molding is regarded as the most suitable process to manufacture optical disk substrates with high dimensional accuracy, low residual stress, and superb optical properties. Polycarbonate magnetooptical disk substrates are fabricated by injection compression molding, and the birefringence, regarded as one of the most important optical properties for optical disk, is measured. The effects of various processing conditions on the development of birefringence distribution are examined experimentally. It is found that the values of the birefringence distribution are very sensitive to the mold wall temperature history. By controlling the compression process, the overall birefringence value can be reduced.

In medical applications, the detection and outlining of boundaries of organs and tumors in computed tomography (CT) and magnetic resonance imaging (MRI) images are prerequisite. A two-layer Hopfield neural network called the competitive Hopfield edge-finding neural network (CHEFNN) is presented for finding the edges of CT and MRI images. Different from conventional 2-D Hopfield neural networks, the CHEFNN extends the one-layer 2-D Hopfield network at the original image plane a two-layer 3-D Hopfield network with edge detection to be implemented on its third dimension. With the extended 3-D architecture, the network is capable of incorporating a pixel's contextual information into a pixel-labeling procedure. As a result, the effect of tiny details or noises will be effectively removed by the CHEFNN and the drawback of disconnected fractions can be overcome. Furthermore, by making use of the competitive learning rule to update the neuron states, the problem of satisfying strong constraints can be alleviated and results in a fast convergence. Our experimental results show that the CHEFNN can obtain more appropriate, more continued edge points than the Laplacian- based, Marr-Hildreth, Canny, and wavelet-based methods.

The area of automatic target classification has been a difficult problem for many years. Many approaches involve extracting information from the imagery through a variety of statistical filtering and sampling techniques, resulting in a reduced dimension feature vector that is the input for a learning algorithm. We introduce the support vector machine (SVM) algorithm, which is a wide margin classifier that can provide reasonable results for sparse data sets and whose training speed can be nearly independent of feature vector size. Therefore, we can avoid the feature extraction step and process the images directly. The SVM algorithm has the additional features that there are few parameters to adjust and the solutions are unique for a given training set. We apply SVM to a vehicle classification problem and compare the results to standard neural network approaches. We find that the SVM algorithm gives equivalent or higher correct classification results compared to neural networks.

The focus of this paper is recognizing articulated vehicles and actual vehicle configuration variants in real synthetic aperture radar (SAR) images. Using SAR scattering-center locations and magnitudes as features, the invariance of these features is shown with articulation (e.g., rotation of a tank turret), with configuration variants, and with a small change in depression angle. This scatterer-location and magnitude quasiinvariance is used as a basis for development of a SAR recognition system that successfully identifies real articulated and nonstandard- configuration vehicles based on nonarticulated, standard recognition models. identification performance results are presented as vote-space scatterplots and receiver operating characteristic curves for configuration variants, for articulated objects, and for a small change in depression angle with the MSTAR public data.

This paper introduces an algorithm designed to segment black-and-white documents for the purpose of compression. A single document is segmented into two documents suitable for pattern-based and run-length-based compression. With some modification the same algorithm may also be used for optical character recognition. The segmentation is performed in two main steps: pattern extraction and classification. Patterns are extracted using a fast scan method that does not need to scan every pixel, and classification uses pattern characteristics such as spread and pattern context to segment the patterns. Documents may be segmented with an accuracy of at least 98%, depending on the content. Furthermore, text of any size and orientation may be successfully classified without the need for skew estimation or correction. This paper presents the segmentation algorithm and discusses the complete compression system.

A method of calculating the gain and the output signal based on the beam propagation method and rate equations is presented. It employs the complex atomic susceptibility in the paraxial wave equation to calculate light propagation in integrated optics devices. As an example, an erbium-doped waveguide planar splitter is analyzed.

A compact self-aligned noncontact range finder is described; it uses the self-mixing effect inside a three-electrode distributed Bragg reflector laser diode. The obtained resolution of the device is 0.5 mm at a distance of 55 cm, instead of 5 mm with a classical Fabry-Perot laser diode in the same operating conditions.

A new interferometric setup for the determination of the concentration distribution in a diffusion experiment is proposed. The method is applied to the determination of the diffusion coefficient D in a binary saline solution. A thin laser beam scans the diffusion cell along the concentration gradient, which is assumed to be one-dimensional. This beam interferes with the reference beam in a Mach-Zehnder interferometer, and the variation of intensity is recorded by means of photomultiplier tube, digitized, and stored in a computer for further analysis. The phase of the intensity function, modulo 2?, is determined by applying the Fourier-transform method. Assuming linear dependence of the refraction index on the concentration, the unwrapped phase gives a curve similar to the theoretical concentration distribution function. The value of the diffusion coefficient D is obtained by means of an iterative process, comparing experimental and theoretical curves until the best fit is achieved between them, by adjusting the parameter D.

A technique for increasing the displacement measurement range in electronic speckle-shearing pattern interferometry is proposed. The technique is based on the introduction of carrier fringes and the use of a temporal-phase-unwrapping method to obtain larger measuring ranges of out-of-plane displacements of an object without sign ambiguities. A larger measuring range is obtained by dividing the desired maximum-phase range into several subranges where the spatial synchronous detection method can be successfully applied. These phase subranges correspond to small out-of-plane displacements of the object, and a speckle pattern is recorded for each displacement. The carrier fringes are obtained by the subtraction correlation of two consecutive speckle fields with a translation of the illuminating beam between recordings. The method allows the measurement of object displacements of about 216 ?m.

Second-harmonic generation (SHG) of CO2 lasers has been investigated since the early 1970s, resulting in the development of a number of efficient nonlinear crystals. Among the most suitable candidates for laser systems one finds are AgGaSe2, ZnGeP2, CdGeAs2, and AgGaS2. For these crystals measured conversion efficiencies of frequency doubling have reached levels of 35%, while basic principles and damage threshold of materials pose an upper limit well above that, at 50%. In the quest for optimal conversion we studied the SHG of a transferred-electron amplifier CO2 laser in an AgGaSe2 crystal, measuring a value of 20%, which is further scalable to 50%. In this paper we compare the various studies and layout the design parameters for an optimal second-harmonic generator for the 4.5- to 5.5-?m range.

A comparison between two models of light scattering on an infinite, nonabsorbing cylinder modeling a fiber in a composite material is presented. In the first model the incident light is modeled by a monochromatic plane wave, in the second by a monochromatic wave with a Gaussian distribution of intensity over the cross section. The results of numerical simulations for the two cases are presented.

By applying energy conservation to the effect of a laser beam on a resist, a modified proximity function is proposed. The measured data on energy absorption in a photoresist are fitted well with the modified function. By using the new model, an effective precompensation method can be used to correct proximity effects in laser direct writing. Experimental results have been obtained on a laser direct writing machine with line width of 0.6 ?m.

In microlithography, the necessity of using bottom antireflective coatings (BARCs) to reduce linewidth variations resulting from reflective notching and thin film interference effects has been largely demonstrated. We report a new methodology, based on a 2-D swing curve concept, to predict the lithographic performance of resist/BARC bilayers in a topographic situation to optimize the critical dimension (CD) range. Due to inherent planarization effects when coating on topography, both the resist and organic BARC thicknesses vary, as do the final reflectivity and CDs. As a consequence, it is not easy to determine the optimal BARC thicknesses and to predict the lithographic performance, taking into account topography effects over the whole chip. Usually, lithographic performance (CD swing curves) is measured or calculated using modeling over plane wafers. Over the topography of a real chip, however, the resist thickness can cover up to four periods of the swing curve, which means that lithographic performance over a real chip cannot be predicted using swing curves calculated or measured on plane wafers. We therefore propose a new method of representing the reflectivity and CD swing curves in two dimensions to evaluate the lithographic performance of the BARC/resist bilayer. A simulation algorithm is performed that enables the determination of both the optimal BARC thicknesses and the lithographic performance window over the whole chip. Practical examples are given demonstrating the role of such a simulation.

Power transfer is analyzed in an optical recording application. We compare using only a collimating lens to deliver light from a laser diode to the write/read objective versus using a lens and a singlemode fiber that is butt-coupled to the laser diode. The system that employs an intermediate singlemode fiber is shown to deliver more power in an optical configuration that exhibits a large object plane field of view (collector numerical aperture ?0.11). Furthermore, the fiber/lens system delivers a circular, anastigmatic beam to the write/read objective. Several potential advantages of fiber/lens systems are outlined.

This paper presents an approach to designing all-reflective, projection cameras for EUV lithography. We make a comparison of fourmirror cameras with numerical apertures ranging from 0.1 to 0.2 and ring-field widths from 1.0 to 3.6 mm. Additionally, we present two theoretical models that allow for the phase change introduced by multilayer coatings

A lens axicon (a converging lens with spherical aberration) illuminated by a Gaussian beam is studied. It is demonstrated that, when a backward-type lens axicon is illuminated with an appropriate beam radius of the incident Gaussian beam, one can achieve a focal segment with uniform axial intensity, which is similar to that generated by the logarithmic axicon. It is shown that this approach is very simple and efficient. A forward-type lens axicon illuminated by a Gaussian beam is also investigated.

A lens axicon (a converging lens with spherical aberration) illuminated by a Gaussian beam is studied. It is demonstrated that, when a backward-type lens axicon is illuminated with an appropriate beam radius of the incident Gaussian beam, one can achieve a focal segment with uniform axial intensity, which is similar to that generated by the logarithmic axicon. It is shown that this approach is very simple and efficient. A forward-type lens axicon illuminated by a Gaussian beam is also investigated.

An optical encoder based on the Lau effect is presented. It is shown that this system has several advantages over standard optical encoders based on the moire effect, such as smaller sensitivity to variations in the gap between gratings and higher design simplicity. Problems due to the quality of the collimated beam, which are present in moire- effect-based encoders, are also avoided. Two different optimized designs for different resolutions are presented. Experimental measurements, in good agreement with the model developed by us, are presented.

Plasma-enhanced chemical vapor deposition is very attractive for the fabrication of optical filters on plastic substrates. We found, using ex situ and in situ spectroellipsometric analysis, that plasma deposition of silicon nitride as a high-refractive-index material, on bare and plasma-pretreated polycarbonate (PC) substrates, gives rise to the formation of a 50- to 100-nm-thick interfacial region (or interphase). We show that plasma pretreatment can reduce the thickness of this interphase by 50% from that obtained with deposition on untreated PC, possibly due to interface stabilization. An optical model based on real-time in situ measurements is proposed to describe the interphase refractive index profile. This model is fully supported by complementary crosssectional transmission-electron-microscopy observations. Important consequences for the mechanical performance (adhesion and scratch resistance) and optical design of optical coatings on plastics are discussed.

The use of optical fiber sensors to study the drag force in molding flow is described. Research has proved that this novel method can be used to study the drag force of the molding flow in IC packaging over a perfectly elastic circular cylinder. The result of this study is found to be useful for characterizing the behavior of the wire sweep in encapsulation of semiconductor chips. An inline fiber etalon (ILFE) as a sensor is designed, constructed, and implemented in the middle of the optical fiber for accurate strain measurement. This is done by first laying an optical fiber in the midplane of a simple rectangular mold cavity, and then measuring the strain at the midspan of the fiber when it is subjected to the flow of a homogeneous fluid. For a given flow field, several dragforce models have been tried to calculate the drag force on the optical fiber, and the resulting strain of the optical fiber has been calculated by finite-element modeling and compared with the experimental results. From the comparison, the Takaisi model to calculate the drag force exerted on the optical fiber by the flow of homogeneous fluid is modified according to the experimental data. This modified model can be applied to study the wire sweep in the encapsulation of semiconductor chips. It is concluded that this measurement system can be used to verify the dragforce model for a molding flow on a perfectly elastic circular cylinder. Even under the more complex conditions in actual molding flow, the proposed sensing system can be used to measure the axial strain accurately. In addition, the transient deformation of the optical fiber characterized by the ILFE sensor provides important information for developing a drag-force model that can be used in IC packaging.

An existing high-speed optical tomography system built at the Air Force Research Laboratory uses Hartmann wavefront sensors to obtain the optical path projections necessary for two-dimensional tomographic reconstruction of the temperature (or equivalently the index of refraction) in a heated jet of air. Conventional optical tomography algorithms require optical-path-length projection data obtained at a number of angles through the flow. However, Hartmann sensors detect the first derivative of the path length. Conventional reconstruction algorithms thus require integration of the measured gradients. New iterative algorithms can reconstruct directly from the measured gradients. Using computer simulation, we compare the accuracy of reconstructions made directly from optical-path-length gradients with the accuracy of reconstructions based on the optical path length obtained by integrating gradient measurements for several iterative tomographic algorithms employing local and global basis functions. For a data-limited tomography system, our studies have shown that accuracy of reconstructions depends on both the choice of algorithm and the structure of the flow under observation.