Showing papers in "Advances in Optics and Photonics in 2010"

Stimulated Brillouin scattering in optical fibers

Abstract: We present a detailed overview of stimulated Brillouin scattering (SBS) in single-mode optical fibers. The review is divided into two parts. In the first part, we discuss the fundamentals of SBS. A particular emphasis is given to analytical calculation of the backreflected power and SBS threshold (SBST) in optical fibers with various index profiles. For this, we consider acousto-optic interaction in the guiding geometry and derive the modal overlap integral, which describes the dependence of the Brillouin gain on the refractive index profile of the optical fiber. We analyze Stokes backreflected power initiated by thermal phonons, compare values of the SBST calculated from different approximations, and discuss the SBST dependence on the fiber length. We also review an analytical approach to calculate the gain of Brillouin fiber amplifiers (BFAs) in the regime of pump depletion. In the high-gain regime, fiber loss is a nonnegligible effect and needs to be accounted for along with the pump depletion. We provide an accurate analytic expression for the BFA gain and show results of experimental validation. Finally, we review methods to suppress SBS including index-controlled acoustic guiding or segmented fiber links. The second part of the review deals with recent advances in fiber-optic applications where SBS is a relevant effect. In particular, we discuss the impact of SBS on the radio-over-fiber technology, enhancement of the SBS efficiency in Raman-pumped fibers, slow light due to SBS and SBS-based optical delay lines, Brillouin fiber-optic sensors, and SBS mitigation in high-power fiber lasers, as well as SBS in multimode and microstructured fibers. A detailed derivation of evolutional equations in the guided wave geometry as well as key physical relations are given in appendices.

Ghost imaging: from quantum to classical to computational

Abstract: Ghost-imaging experiments correlate the outputs from two photodetectors: a high-spatial-resolution (scanning pinhole or CCD array) detector that measures a field that has not interacted with the object to be imaged, and a bucket (single-pixel) detector that collects a field that has interacted with the object. We give a comprehensive review of ghost imaging—within a unified Gaussian-state framework—presenting detailed analyses of its resolution, field of view, image contrast, and signal-to-noise ratio behavior. We consider three classes of illumination: thermal-state (classical), biphoton-state (quantum), and classical-state phase-sensitive light. The first two have been employed in a variety of ghost-imaging demonstrations. The third is the classical Gaussian state that produces ghost images that most closely mimic those obtained from biphoton illumination. The insights we develop lead naturally to a new, single-beam approach to ghost imaging, called computational ghost imaging, in which only the bucket detector is required. We provide quantitative results while simultaneously emphasizing the underlying physics of ghost imaging. The key to developing the latter understanding lies in the coherence behavior of a pair of Gaussian-state light beams with either phase-insensitive or phase-sensitive cross correlation.

Two-photon absorption: an overview of measurements and principles

Abstract: The range of organic compounds whose degenerate two-photon absorption (2PA) spectrum has been reported has increased rapidly in recent years, in parallel with the growing interest in applications based on the 2PA process. The comparison of results from different techniques is not always straightforward, and experimental conditions employed may vary significantly. We overview the concepts underlying 2PA measurements and the common assumptions and approximations used in the data analysis for various techniques. The importance of selecting appropriate excitation regimes under which measurements should be performed and of avoiding contributions from absorption mechanisms in addition to 2PA will be emphasized.

Nonlinear refraction and absorption: mechanisms and magnitudes

Abstract: We provide an in-depth treatment of the various mechanisms by which an incident light beam can produce an intensity- or flux-dependent change in the refractive index and absorption coefficient of different materials. Whenever possible, the mechanisms are initially traced to single-atom and -molecule effects in order to provide physical understanding. Representative values are given for the various mechanisms. Nine different mechanisms are discussed, starting with the Kerr effect due to atoms and/or molecules with discrete states, including organic materials such as molecules and conjugated polymers. Simplified two and/or three-level models provide useful information, and these are summarized. The nonlinear optics of semiconductors is reviewed for both bulk and quantum-confined semiconductors, focusing on the most common types II–VI and III–V. Also discussed in some detail are the different nonlinear mechanisms that occur in liquid crystals and photorefractive media. Additional nonlinear material systems and mechanisms such as glasses, molecular reorientation of single molecules, the electrostrictive effect, the nuclear effect (vibrational contributions), cascading, and the ever-present thermal effects are quantified, and representative tables of values are given.

Photonic technologies for angular velocity sensing

Abstract: Photonics for angular rate sensing is a well-established research field having very important industrial applications, especially in the field of strapdown inertial navigation. Recent advances in this research field are reviewed. Results obtained in the past years in the development of the ring laser gyroscope and the fiber optic gyroscope are presented. The role of integrated optics and photonic integrated circuit technology in the enhancement of gyroscope performance and compactness is broadly discussed. Architectures of new slow-light integrated angular rate sensors are described. Finally, photonic gyroscopes are compared with other solid-state gyros, showing their strengths and weaknesses.

Momentum of Light in a Dielectric Medium

Abstract: We review different expressions that have been proposed for the stress tensor and for the linear momentum of light in dielectric media, focusing on the Abraham and Minkowski forms. Analyses of simple models and consideration of available experimental results support the interpretation of the Abraham momentum as the kinetic momentum of the field, while the Minkowski momentum is the recoil momentum of absorbing or emitting guest atoms in a host dielectric. Momentum conservation requires consideration not only of the momentum of the field and of recoiling guest atoms, but also of the momentum the field imparts to the medium. Different model assumptions with respect to electrostriction and the dipole force lead to different expressions for this momentum. We summarize recent work on the definition of the canonical momentum for the field in a dielectric medium.

Slow light in various media: a tutorial

Abstract: I consider the physical basics of slow light propagation in atomic media, photonic structures, and optical fibers. I show similarities and differences between all of the above media and develop set of criteria that are then used to compare different media. Special attention is given to dispersion of group velocity and loss, which are shown to limit the bandwidth and delay capacity of all the slow light schemes.

Self-assembled quantum-dot superluminescent light-emitting diodes

Abstract: The development of low-cost, compact, high-power and broadband superluminescent light-emitting diodes is an important research subject for a wide range of applications. We describe how self-assembled quantum-dot structures can provide an efficient means of realizing such devices utilizing a number of their unique physical properties. Such quantum dot superluminescent diodes are leading to a revolution in the development of broadband emitters for widespread medical, biological and telecommunications applications.

Angular dispersion: an enabling tool in nonlinear and quantum optics

Abstract: The dispersive properties of materials, i.e., their frequency-dependent response to the interaction with light, in most situations determines whether an optical process can be observed. Although one can always search for a specific material with the sought-after properties, this material might be far from optimum or might not even exist. Therefore, it is of great interest to develop methods that could tune the dispersive properties of a medium independently of the working frequency band. Pulses with angular dispersion, or pulse-front tilt, precisely allow us to achieve this goal. In this tutorial, we show the basics of how angular dispersion can manage to tune the dispersion parameters that characterize the propagation of light in a medium, thus permitting the observation and application of various optical processes in nonlinear and quantum optics that could not be realized otherwise. To keep the focus on first principles, the list of topics addressed is not exhaustive. More specifically, we consider the role of angular dispersion for pulse stretching and compression, broadband second-harmonic generation, the generation of temporal solitons in nonlinear χ(2) media, the tunable generation of terahertz waves by means of optical rectification of femtosecond pulses, and the tuning of the frequency correlations and of the bandwidth of entangled paired photons.

Coherence and entanglement in a two-qubit system

Abstract: Entanglement is a fundamental concept in quantum mechanics. In this review, we study various aspects of coherence and entanglement, illustrated by several examples. We relate the concepts of loss of coherence and disentanglement, via a model of two two-level atoms in different types of reservoir, including cases of both independent and common baths. Finally, we relate decoherence and disentanglement, by focusing on the sudden death of the entanglement and the dependence of the death time with the distance of our initial condition from the decoherence-free subspace. In particular, we study the sudden death of the entanglement in a two-atom system with a common reservoir.