Showing papers in "Advances in Optics and Photonics in 2014"

Space-division multiplexing: the next frontier in optical communication

Abstract: Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Advances in optical security systems

Abstract: Information security with optical means, such as double random phase encoding, has been investigated by various researchers. It has been demonstrated that optical technology possesses several unique characteristics for securing information compared with its electronic counterpart, such as many degrees of freedom. In this paper, we present a review of optical technologies for information security. Optical security systems are reviewed, and theoretical principles and implementation examples are presented to illustrate each optical security system. In addition, advantages and potential weaknesses of each optical security system are analyzed and discussed. It is expected that this review not only will provide a clear picture about current developments in optical security systems but also may shed some light on future developments.

Diffraction phase microscopy: principles and applications in materials and life sciences

Abstract: The main obstacle in retrieving quantitative phase with high sensitivity is posed by the phase noise due to mechanical vibrations and air fluctuations that typically affect any interferometric system. In this paper, we review diffraction phase microscopy (DPM), which is a common-path quantitative phase imaging (QPI) method that significantly alleviates the noise problem. DPM utilizes a compact Mach–Zehnder interferometer to combine several attributes of current QPI methods. This compact configuration inherently cancels out most mechanisms responsible for noise and is single-shot, meaning that the acquisition speed is limited only by the speed of the camera employed. This technique is also nondestructive and does not require staining or coating of the specimen. This unique collection of features enables the DPM system to accurately monitor the dynamics of various nanoscale phenomena in a wide variety of environments. The DPM system can operate in both transmission and reflection modes in order to accommodate both transparent and opaque samples, respectively. Thus, current applications of DPM include measuring the dynamics of biological samples, semiconductor wet etching and photochemical etching processes, surface wetting and evaporation of water droplets, self-assembly of nanotubes, expansion and deformation of materials, and semiconductor wafer defect detection. Finally, DPM with white light averages out much of the speckle background and also offers potential for spectroscopic measurements.

Ultrafast laser direct writing and nanostructuring in transparent materials

Abstract: An overview of recent achievements in the field of femtosecond laser writing in transparent materials is presented. Thanks to the unique properties of light–matter interaction on the ultrashort time scale, this direct writing technique has led to observation of unique phenomena, including nonreciprocal writing and anisotropic photosensitivity, in transparent materials and allowed engineering of novel photonic devices.

The circular Bragg phenomenon

Abstract: Exhibited by structurally chiral materials—such as Reusch piles, cholesteric liquid crystals (CLCs), and chiral sculptured thin films (STFs)—due to their helical nonhomogeneity along a fixed axis, the circular Bragg phenomenon is the almost total reflection of the incident light of the co-handed circular-polarization state but very little reflection of the incident light of the cross-handed circular-polarization state. Manifesting itself in spectral regimes that depend on the angle of incidence, the structural period, and the relative permittivity dyadic, the phenomenon amounts to the formation of a light pipe that bleeds energy backward under appropriate conditions. Mild dissipation and dispersion do not significantly affect the circular Bragg regime. Every structurally chiral material of sufficient thickness is essentially a circular-polarization-sensitive band-rejection filter. Cascades of these materials with or without structural defects can be used to satisfy complex filtering requirements, such as multiband, narrowband, and ultra-narrowband filtering. A shift in the circular Bragg regime due to infiltration of a chiral STF by a fluid enables optical sensing. Sources of circularly polarized light can be fabricated by embedding emission sources in CLCs and chiral STFs.

Optics of the eye and its impact in vision: a tutorial

Abstract: The human eye is a relatively simple optical instrument. This limits the quality of the retinal image affecting vision. However, the neural circuitry seems to be exquisitely designed to match the otherwise limited optical capabilities, providing an exceptional quality of vision. In this tutorial article, the main characteristics of the eye’s geometry and optics will first be reviewed. Then, their impact on vision under a variety of normal conditions will be discussed. The information gathered here should serve both those readers interested in basic vision and physiological optics and those more interested in related applications. © 2014 Optical Society of America

Real photonic waveguides: guiding light through imperfections

Abstract: Real photonic waveguides are affected by structural imperfections due to fabrication tolerances that cause scattering phenomena when the light propagates through. These effects result in extrinsic propagation losses associated with the excitation of radiation and backscattering modes. In this work, we present a comprehensive review on the extrinsic loss mechanisms occurring in optical waveguides, identifying the main origins of scattering loss and pointing out the relationships between the loss and the geometrical and physical parameters of the waveguides. Theoretical models and experimental results, supported by statistical analysis, are presented for two widespread classes of waveguides: waveguides based on total internal reflection (TIR) affected by surface roughness, and disordered photonic crystal slab waveguides (PhCWs). In both structures extrinsic losses are strongly related to the waveguide group index, but the mode shape and its interaction with waveguide imperfections must also be considered to accurately model the scattering loss process. It is shown that as long as the group index of PhCWs is relatively low (ng<30), many analogies exist in the radiation and backscattering loss mechanisms with TIR waveguides; conversely, in the high ng regime, multiple scattering and localization effects arise in PhCWs that dramatically modify the waveguide behavior. The presented results enable the development of reliable circuit models of photonic waveguides, which can be used for a realistic performance evaluation of optical circuits.

Subwavelength semiconductor lasers for dense chip-scale integration

Abstract: Metal-clad subwavelength lasers have recently become excellent candidates for light sources in densely packed chip-scale photonic circuits. In this review, we summarize recent research efforts in the theory, design, fabrication, and characterization of such lasers. We detail advancements of both the metallo-dielectric and the coaxial type lasers: for the metallo-dielectric type, we discuss operation with both optical pumping and electrical pumping. For the coaxial type, we discuss operation with all spontaneous emission coupled into the lasing mode, as well as the smallest metal-clad lasers to date operating at room temperature. A formal treatment of the Purcell effect, the modification of the spontaneous emission rate by a subwavelength cavity, is then presented to assist in better understanding the quantum effects in these nanoscale semiconductor lasers. This formalism is developed for the transparent medium condition, using the emitter-field-reservoir model in the quantum theory of damping. We show its utility through the analysis and design of subwavelength lasers. Finally, we discuss future research directions toward high-efficiency nanolasers and potential applications, such as creating planar arrays of uncoupled lasers with emitter densities near the resolution limit.

Vector Fourier optics of anisotropic materials

Abstract: The Fourier optics technique is founded on the transfer function derived from the scalar wave equation and thus has traditionally been applied to monochromatic scalar fields propagating paraxially in isotropic lossless materials. We review scalar and vector diffraction theory to show that the scalar Fourier optics technique can be seamlessly extended to the case of time-dependent nonparaxial and evanescent vector fields propagating in anisotropic and/or absorbing materials. The missing piece is shown to be the Fourier-domain vector boundary conditions that have recently been derived from the real-domain vector Rayleigh–Sommerfeld diffraction integral. These boundary conditions, complemented by the anisotropic transfer functions provided by the roots of the Booker quartic, combine the rigor of Green’s function integrals with the intuitive simplicity and general applicability of Fourier optics. We illustrate the power of this technique via analytically simple solutions to traditionally complex problems such as Fresnel transmission between arbitrary media, uniaxial conoscopy, and biaxial conical refraction. The accuracy of vector near-field diffraction is validated via comparison with the finite-difference time-domain method.