This PDF file contains the editorial “Electronic Publishing” for OE Vol. 43 Issue 09

The single-mode condition for silicon rib waveguides with large cross sections is obtained using a numerical method based on the finite-difference beam propagation method (FD-BPM). An ultrawide computation window is used to contain the wide mode profile near cutoff. Comparison with previous results shows that the formula predicted by the mode-matching technique is in a better agreement with our results.

A novel wideband polarization-insensitive semiconductor optical amplifier (SOA) gate containing compressively strained InGaAs quantum wells and tensile-strained InGaAs quasi-bulk layers is developed. The fabricated SOA gates have a wide 3-dB optical bandwidth of 102 nm, less than 0.8-dB polarization sensitivity, more than 50-dB extinction ratio, and less than 75-mA fiber-to-fiber lossless operating current.

This PDF file contains the editorial “Volume Holographic Optical Elements” for OE Vol. 43 Issue 09

We present an overview of imaging systems that incorporate a volume hologram as one of the optical field processing elements in the system. We refer to these systems as volume holographic imaging (VHI) systems. The volume hologram is recorded just once, and the recording parameters depend on the functional requirements of the imaging system. The recording step offers great flexibility in designing application-specific imaging systems. We discuss how a VHI system can be configured for diverse imaging applications ranging from surface profilometry to real-time hyperspectral microscopy, and summarize recent developments in this field.

Following the realization of photonic crystals in the late 1980s and fueled by the vision of photonic "highways" and all-optical switching, the field has grown into one of the most exciting and fertile areas of cross-disciplinary research spanning the scientific and engineering communities. We focus on the state of the art in holographic fabrication of photonic crystals by providing a unified framework of design principles for the creation of two- and three-dimensional optical lattices, with an emphasis on how all five two-dimensional and all fourteen three-dimensional Bravais lattices may be formed with three and four coherent laser beams, respectively. Additionally, we discuss quasiperiodic crystal interference profiles and aspects of the relevant holographic materials. After firmly establishing the general relationship between the holographic parameters and the resulting optical interference profiles, we offer a comprehensive literature review and set forth the most pressing research issues in holographic photonic crystals.

We focus on a photorefractive response under the influence of temperature in conjunction with reduction. A series of reduced BaTiO₃:Rh crystals is made. Their optical absorptions, dark decays, and light-induced decays are investigated. The temperature-dependent dark decay and light-induced decay are performed by an argon ion laser operated at 514.5 nm. The experimental results show that the photorefractive response can be improved by reduction in cooperation with the elevation of temperature. For different types of Rh-doped barium titanate crystals, the elevation of temperature has different influences in their photorefractive responses. The sample reduced at 10^–14 atm has the best response of 15 ms at 100°C.

The photoproducts in phenanthrenequinone (PQ)-dissolved methyl methacrylate (MMA) liquid samples and PQ-doped poly(methyl methacrylate) (PQ/PMMA) solid photopolymer samples have been analyzed by various chemical measurements. A mechanism for holographic recording in our PQ/PMMA photopolymer is proposed. By UV-VIS transmission and photoluminescence spectral measurements, we find that under light exposure the molecular structure of PQ is transformed to be less conjugated. The measured results of mass spectra, Fourier transform infrared spectra, NMR spectra, and gel permeation chromatograph analyses provide some evidence for recognizing the molecular structure of the photoproducts in our PQ/PMMA photopolymers. The results show that under light exposure the PQ and MMA form new molecules, mainly an adduct of one PQ molecule with one MMA molecule. In addition, PQ also reacts as a photoinitiator to form PMMA oligomers in our samples. The structure change of the PQ molecule induces a strong change of the refractive index in the material. It provides a mechanism to record a phase hologram in our PQ/PMMA photopolymer. Holographic recordings in the samples are demonstrated, and the dynamic range of the sample is investigated.

An analysis of volume holographic long period gratings in photonic nanostructured fibers and waveguides is presented. It is found that the unique photonic nanostructure could substantially enhance the tuning capability of the spectral response of the grating, which could result in many practical applications in optical communications and sensors, such as wide tuning range wavelength filters and ultrahigh sensitivity optical sensors.

A spectral device based on multiple holographic gratings is proposed. Several volume holograms are superimposed into a common volume of photorefractive crystal Fe:LiNbO₃ using the wavelength and angular multiplexing method. This device is implemented by only one single wavelength laser using a two-lambda method. The imaging function of the device is realized with convergent recording beams. It can reduce the spectral system because the focusing lens is removed. Because the diffraction efficiency and the spectral resolution of the superimposed volume holographic gratings could be very high, the spectral device could achieve attracting characteristics of the diffraction efficiency and spectral resolution. Theoretical analysis and model design are presented and a discrete spectral device working at four wavelengths is demonstrated. The device has a uniform diffraction efficiency of 10% and the resolution of 1 to 2 nm. In addition, it has the advantages of wide spectral coverage, simple structure, and flexible design.

We propose and demonstrate a widely tunable optical filter, realized by angle tuning a volume holographic grating. The volume holographic grating selectively drops a narrow portion of the signal bandwidth into a fiber while passing through the rest of the signals. The demonstrated 1510- to 1590-nm tuning range covers the entire erbium-doped fiber amplifier (EDFA) C band, with small bandwidth variation and low insertion loss (<1 dB). Group delay, polarization-dependent loss, and polarization mode dispersion are measured and investigated for optimizing the filter characteristics.

A Gaussian apodization technique applied to a transmission volume hologram for a holographic demultiplexer is proposed. A Gaussian apodized grating 15×11 mm in size, 38 μm thick, and with a 3.2-mm horizontal standard deviation of the modulating index profile is fabricated. A 22-channel demultiplexer based on that grating has been optically demonstrated. Channel spacing, interchannel crosstalk level, and channel uniformity of 0.8 nm, –30 dB, and 1.5 dB, respectively, are obtained. In addition, an insertion loss of 13 dB, which is caused by the primary limitation of this technique, is observed.

The design of a 2048-channel dense-wavelength division multiplexer (DWDM) with channel spacing of 0.05 nm (a 3-dB bandwidth) using reflection holograms in LiNbO₃ is reported. We use an innovative approach to combine angular multiplexing with wavelength multiplexing to keep the bandwidth constant within the S, C, and L band of the fiber transmission window. The key to the design is to calculate the half width at first zero (HWFZ) for the Bragg selectivity of reflection holograms derived from the VOHIL model. Thus, one can enable the channel spacing for each center wavelength of the diffraction peak to be constant and maximize the number of channels in DWDM accordingly.

We present a setup that transfers 2-D images between two ports. By using a photorefractive LiNbO₃ crystal and a BaTiO₃ crystal, we are able to transfer images either way without making any change in the experimental layout. The resolutions of the device are 10.1 and 6.4 lp/mm for the two transferring directions, respectively.

We propose and characterize the remote multiplexing of holograms with random pattern references from a multimode fiber bundle. The random pattern reference is modeled by using the superposition and concatenation of propagation modes of multimode fibers and free space. We compare two laser-coupling methods to the fiber bundle, i.e., direct coupling and lens coupling, for angle, shift, and wavelength remote multiplexings. The optimum laser-coupling method for each multiplexing is analyzed based on the theoretical model and experimental results. Retrieved hologram images show that 1-D intensity variation of the image might occur for the 90-deg geometry holograms in relation to the random pattern properties. Environmental variation effects such as vibration and temperature variation are also discussed and examined. These remote multiplexing methods can be applied to general multimode waveguide arrays to construct compact and integrated optical systems, where multiplexing can be performed at remote places.

We propose a novel method for optical security by using a randomly selected binary phase code. The phase code is recorded with holographic volume gratings, and the recording material can be regarded as an optical lock, while the original phase code acts as the key. When an arbitrary phase code is used for decryption, the decryption probability is less than 10^–10 when the phase code contains only five rows. The decryption probability decreases dramatically as the row number of the code or the threshold value increases. Furthermore, the security system performs good tolerance for data loss of the key. Both the theory and experiment are demonstrated.

When a prerecorded cross-beam hologram is reconstructed (so-called edge-lit readout) with a uniform plane wave and a point source, the resulting exact solutions reveal Bessel-function-type diffracted beam profiles, which are fundamentally modified under weak propagational diffraction. The case of a profiled beam readout with propagational diffraction may be analyzed using a transfer function approach based on 2-D Laplace transforms. In a second series of investigations, dynamic readout from a cross-beam volume hologram recorded with two orthogonal uniform plane waves is considered for various dependences of the refractive index modulation with intensity. Typically, refractive index profiles that are proportional to the intensity (as in the case of Kerr-type media or photorefractives with predominantly photovoltaic effect) and to the derivative of the intensity (as in diffusion-dominated photorefractives) are considered. Two-dimensional nonlinear coupled equations are developed for the two (Bragg) orders for both cases. Closed form solutions are obtained for the first case, indicating only nonlinearly induced self and cross-phase coupling. A simple experiment involving simultaneous recording and readout using photorefractive lithium niobate crystal indicates beam profile distortion, which may be expected in such 90-deg geometries.

To reduce computation time, a new approach for designing computer-generated holograms is proposed. In this new method, we utilize artificial neural networks to initiate the genetic algorithm, so that the high computation cost of genetic algorithms for synthesizing holograms is significantly reduced, while the high diffraction efficiency and uniformity are still ensured. Designed binary phase holograms have a diffraction efficiency of 75.8% and uniformity of 5.8%, which are proven in computer simulation and experimentally demonstrated.

A laser beam automatic alignment system is applied in a multipass amplifier of the SG-III prototype laser. Considering the requirements of the SG-III prototype facility, by combining the general techniques of the laser beam automatic alignment system, according to the image relayed of the pinholes in the spatial filter, and utilizing the optical position and the spatial distribution of the four pinholes of the main spatial filter in the multipass amplifier of the SG-III prototype, a reasonable and optimized scheme for automatic aligning multipass beam paths is presented. It is demonstrated on the multipass amplifier experimental system.

Hyperspectral sounder data is used for retrieval of atmospheric temperature, moisture and trace gas profiles, surface temperature and emissivity, and cloud and aerosol optical properties. This large volume of data is 3-D in nature with many scan lines containing cross-track footprints, each with thousands of IR channels. Unlike hyperspectral imager data compression, hyperspectral sounder data compression is desired to be lossless or near-lossless to avoid substantial degradation of the geophysical retrieval. For this new class of data for compression studies, a lossless compression algorithm combining the context-based adaptive lossless image codec (CALIC) and a novel bias-adjusted reordering (BAR) scheme is presented. The 3-D data are arranged into two dimensions with the original 2-D spatial domain converted into one dimension using a continuous scan order. In the BAR scheme, the data are reordered such that the bias-adjusted distance between any two neighboring vectors is minimized. The result is then encoded using the CALIC algorithm with significant compression gains over using the CALIC algorithm alone.

Novel demultiplexer configurations based on concatenated Mach-Zehnders and a ring resonator are presented. These new devices exhibit high channel isolation, flexibility, no loss, and megahertz-adjustable full width at half maximum (FWHM) suitable for narrow channel spacing. General design equations are described. Examples of a 1×4-2.5-GHz demultiplexer and a 2.5-GHz/5-GHz interleaver are reported. These designs have channel isolation >40 dB, 128 MHz FWHM, 10 dB gain. These devices can be conveniently implemented using the silicon or InP integrated optic technologies. A fabrication tolerance analysis is carried out. The worst figures for the previous designs are 220 MHz FWHM, 23-dB channel isolation, and 5.7-dB gain and can be readjusted with a gain control.

High coupling efficiency and large misalignment tolerance are two important optical requirements of packaging a laser diode [or vertical-cavity surface-emission laser (VCSEL)] for various applications in optical fiber communication. An integrated lens at the end of a single-mode fiber can increase the coupling efficiency between fibers and VCSELs. We present a theoretical analysis of a coupling scheme, which involves a combination of both a convex lens and a hemispherical lensed fiber configuration. The theoretical analysis is conducted to include the influence of lens aberration. The results show that lens aberration reduce coupling efficiency from 40 to 34% for a VCSEL operating at a 850-nm wavelength under the TO52H package. On the other hand, the lateral tolerance of the two lens coupling scheme improved compared to the TO can-butt coupling fiber scheme.

We propose a scale-based image enhancement method using a modified anisotropic diffusion filter that employs sensor noise estimation and scale space methods based on a minimum reliable scale. Based on the relationship between the local gradient and the critical value function, an image is classified into various scales representing the region's complexity. The degree of enhancement is then adjusted according to each region's complexity. When compared to the existing linear unsharp masking, adaptive unsharp masking, and warping-based methods, the proposed algorithm produces negligible noise amplification in homogeneous regions and superior edge enhancement.

We propose a new image-downsampling method for interlaced-to-progressive transcoding. The proposed image-downsampling method is performed in the discrete cosine transform (DCT) domain. We develop a downsampling filter matrix that transcodes the interlaced picture into the progressive picture. The method shows a good peak signal-to-noise ratio (PSNR) compared with spatial-domain downsampling.

In rational progressive transmission, the spatial orientation trees define the relationship on the pyramid that results from the wavelet transformation. Each tree is associated with one subregion of the original image. The trees may by grouped together into a reduced number of quantizers that convey structural information about the picture to the transmission system. We propose a quantizer formation that will give neither tree a cause for "reasonable regret" in rational progressive transmission. In this sense, we call it a "just" quantizer formation. We introduce the basic concept of "reasonable regret" for justice and give conditions for a just quantizer formation. We show that the definition of a priori importance of a spatial orientation tree is pivotal in the development of a theory for just quantizer formation. The estimation of the a priori importance of a tree must be solved using a large number of insignificant wavelet coefficients—as measured by their magnitudes—that are still effective via coalitions for the reconstruction of the tree. This problem falls into the realm of cooperative game theory with one exogenously given communication graph that results from the wavelet transformation. We provide a cooperative game in this particular communication situation, named after the "SOT-Restricted Game" that naturally induces a game-theoretic solution to the estimation problem: the "zerotree" solution of an SOT-restricted game. Experimental results illustrate the comparative performance of the rational progressive transmission with a just quantizer formation against the state of the art in progressive transmission.

We present a novel optical current sensor architecture based on the Faraday effect. In the proposed sensor, the light source is an incandescent lamp driven by the electric current induced in a small solenoid placed near the conductor carrying the current to be measured. After passing through the sensor head, the light travels in the free space, reaching a photodetector placed some distance away from the conductor carrying the current. The proposed sensor head with its self-induced light source can be constructed as a compact unit. It works autonomously without requiring a precise alignment with additional optics (e.g., polarization-maintaining fibers to carry light to or from the magneto-optical material).

We describe a diffraction-grating interferometer that enables us to obtain a high contrast ratio of 120 between the test wave and the reference wave by optimizing the groove shape of a blazed-type diffraction grating. The high contrast provides good visibility for the resulting interference fringes when testing aspheric mirrors using computer-generated holograms (CGHs). Generally CGHs perform wavefront nulling, but greatly attenuate the intensity of the test wave. We present experimental results for an aspheric concave mirror of 600 mm diameter obtained using a blazed diffraction grating with a 1/1200 mm groove spacing and a 17-deg blazed angle. The binary-type CGH used for the experiments provided low transmittance efficiency but was cost effective.

Negative photoresist films (HNR-80 and HNR-120) are fabricated and characterized for waveguiding purpose. Various waveguiding parameters, namely, refractive index, thickness, propagation losses, and birefringence of the photoresist films are evaluated. Propagation losses are optimized to the value of 0.29 dB/cm for both materials. These photoresist materials show excellent transmission with almost no absorption in the wavelength range of 400 to 900 nm. The waveguides are polarization insensitive for various temperatures. Considering these properties, these materials offer a good option to use as waveguiding materials in integrated optics.

Polyvinyl alcohol (PVA)-based thin film optical waveguides are fabricated and characterized. Various waveguide parameters, namely, refractive index, propagation losses and depth, birefringence, and polarization conversion properties of dip-coated thin film optical waveguides are evaluated. The effective refractive index of PVA film is found to be between 1.506 and 1.531 for various concentrations. The propagation losses are 2.35 to 0.30 dB/cm for various thicknesses. The propagation loss reduces when the thickness of the film is increased. The low propagation losses, birefringence insensitivity, high transmission in the aged and annealed waveguides, control over film refractive index, the good adhesiveness of the film and good compatibility with substrate show that PVA-based waveguides are promising candidates for the optoelectronic field.

We previously proposed the hybrid integration of photonic crystals (PhCs) and conventional index-guided waveguides (CWGs) as a potentially attractive method of realizing compact waveguide elements for large-scale planar lightwave circuits (PLCs). We now examine 90-deg bends and beamsplitters in PhC/CWG structures in which the waveguide core has a high refractive index (3.25) and yet a low refractive index contrast (1.54%) with the clad material. A PhC structure composed of a triangular or square array of air holes is placed at the intersection of input and output waveguides to obtain high efficiency 90-deg bends. We find that diffraction from the boundary of the PhC region with CWG limits the optical efficiency of the bend. To overcome this we use a rigorous design tool based on a microgenetic algorithm (µGA) and a finite difference time domain (FDTD) method to optimize the boundary layer to suppress the unwanted diffraction. We find that this approach yields improvements in the bend efficiency at a wavelength of 1.55 µm from 56.2 to 92.5% (for a triangular PhC structure, TE polarization) and from 72.0 to 97.4% (square PhC structure, TM polarization).

Photonic technology is gradually replacing traditional electronic functionality, particularly in all-optical communications network. Optical filtering, switching, and amplification and optical memory are already proven functions and more optical functionality is emerging. Optical pattern detection is a key function for optical signal processing and it is applicable to optical packet identification, error control, wavelength assignment, and routing. All optical pattern detection eliminates optical-electrical-optical (OEO) conversions that add significantly to complexity and cost of system and network. We describe a direct optical pattern detector that in real time recognizes patterns in optical binary streams of synchronous frames or asynchronous packets. We demonstrate the detection of start of frame of an optical signal, and we describe the applicability of the method to all-optical digital-to-analog conversion. Simulation results support the detector optical circuitry.

Future spaceborne interferometric arrays must meet stringent optical performance and tolerance requirements while exhibiting modularity and acceptable manufacture and integration cost levels. The Massachusetts Institute of Technology (MIT) Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) is a wide-angle Fizeau interferometer spacecraft testbed designed to address these research challenges. Designing a space-based stellar interferometer, which requires tight tolerances on pointing and alignment for its apertures, presents unique multidisciplinary challenges in the areas of structural dynamics, controls, and multiaperture phasing active optics. In meeting these challenges, emphasis is placed on modularity in spacecraft subsystems and optics as a means of enabling expandability and upgradeability. A rigorous theory of beam-combining errors for sparse optical arrays is derived and flown down to the design of various subsystems. A detailed elaboration on the optics system and control system is presented based on the performance requirements and beam-combining error tolerances. The space environment is simulated by floating ARGOS on a frictionless air-bearing that enables it to track both fast and slow moving targets.

Speckle interferometry is used to measure the deformation of an object with a rough surface. In particular, the precise measurement can be easily performed with electronic-speckle-pattern interferometry (ESPI) using fringe scanning technology. Then, the measurement accuracy is influenced by the ratio between the speckle size and the pixel size of a CCD. Sometimes, this causes a problem concerning optical dislocations. An in-plane displacement is measured by the optical measurement arrangement using the two collimated beams. The measurement is performed by ESPI technology with the spatial fringe analysis method under an optimal measurement condition. Then, the optimum condition is discussed as the measurement parameters concerning the speckle's size and the passband of the bandpass filter. In the experimental measurement of in-plane displacement, it is shown that the optimal measurement condition can evade the problem of optical dislocations. At the same time, it is confirmed that a precise measurement can be performed.

A novel real-time-invariant target tracking algorithm is proposed for forward-looking IR (FLIR) image sequences based on a modified synthetic discriminant function (SDF)-based fringe-adjusted joint transform correlator (JTC) algorithm. The proposed approach utilizes fringe-adjusted JTC-based tracking, which efficiently compensates camera motion and accommodates 3-D distortions. To further increase the tracking accuracy, the target model is continuously updated using a modified version of the synthetic discriminant function (SDF) algorithm. The proposed technique can track small objects comprising only a few pixels and is capable of compensating the high egomotion of the sensor. We demonstrate the robustness of the proposed technique by computer simulation performed on sequences of real-life FLIR imagery taken from an airborne moving platform.

The effects of gamma radiation on vertical-cavity surface-emitting laser (VCSEL) characteristics are investigated experimentally. Three groups (batches) of oxide-confined VCSELs are investigated. It is found that radiation can change (increase or decrease) the quantum efficiency of VCSELs considerably, depending on their production batch. Small changes in the threshold current are observed for some devices. Shifts to a longer wavelength in the peak-emission profile are noticed after irradiation, which is found to be reversible in some VCSELs by simply increasing the radiation dose. It is also discovered that gamma radiation affects the total number of competing transverse modes depending on the radiation dose. Self-pulsation characteristics in some devices are not affected by the radiation exposure. Knowledge of the behavior of VCSELs under gamma exposure would be useful for their applications in hostile environment.