George Stewart

Bio: George Stewart is an academic researcher from University of Strathclyde. The author has contributed to research in topics: Optical fiber & Laser. The author has an hindex of 30, co-authored 132 publications receiving 4590 citations.

Optical waveguide theory

Abstract: In this chapter, after presenting a brief review of the various types of optical waveguides, we outline the key principles and parameters which describe and define the operation of optical waveguides and fibres The ways in which propagation through optical fibres affects the properties of the guided waves are discussed, including dispersion and non linear effects Power transfer between propagating waves is essential to the operation of a number of components and the fundamentals of coupling theory are reviewed In summary, the theory given provides the foundation for understanding the detailed operation of a wide variety of optical components and systems based on optical fibre technology

An investigation of an optical fibre amplifier loop for intra-cavity and ring-down cavity loss measurements

Abstract: We present the design and initial investigation of a fibre optical system which may be used both for intra-cavity and for ring-down measurements of absorption losses. The system consists of a fibre loop containing a length of erbium-doped fibre pumped at 980 nm, with gain adjustment below or above threshold for the two types of operation. The fibre loop is constructed from standard fibre optical components and includes a micro-optical gas cell. The intended application is for measurement of levels of trace gases which possess near-IR absorption lines within the gain bandwidth of the erbium fibre amplifier. We discuss the key issues involved in operation of the system and the level of sensitivity required. Our initial experimental investigations have demonstrated that ring-down times of several microseconds can be obtained, which can be altered through adjustment of the attenuation or gain factor of the loop. Gain control is one of the most important issues and we explain how this may be achieved.

Stable single-mode operation of a narrow-linewidth, linearly polarized, erbium-fiber ring laser using a saturable absorber

Abstract: This paper describes the design and operation of a stable narrow-linewidth linearly polarized fiber ring laser using a polarization-maintaining (PM) erbium-doped fiber as a saturable absorber. The effect of the PM fiber on suppressing mode hopping is experimentally demonstrated and optimum conditions for single-mode operation are identified. Laser output power is /spl sim/ 4.7 mW at 1535 nm for a pump power of 94 mW, the polarization extinction ratio is 24.8 dB, the SNR is larger than 45 dB, the relative intensity noise is below -104 dB/Hz at frequencies above 150 kHz, and the linewidth is less than 1.5 kHz. Potential applications of the fiber laser for interferometric or spectroscopic fiber sensors are briefly discussed.

Design of a portable optical sensor for methane gas detection

Abstract: A detailed investigation has been carried out on the design of a low-cost portable optical sensor for methane detection with a sensitivity of ∼1% of the Lower Explosive Level (LEL) for methane (500 ppm) and able to operate in harsh environments with temperature variation between −20 and 50 °C. The sensor design is based on the use of near-IR LEDs operating around the overtone absorption lines of methane at 1660 nm using a stainless steel tube to direct the light through the gas to the detectors. Various configurations of source/detector layout have been examined to provide appropriate reference and signal paths in order to achieve reliable methane detection at LEL levels in the presence of temperature variation. An optimum design has been identified using two detectors with appropriate optical filtering and with temperature stabilisation of the source and detectors. Based on this design, a prototype instrument has been demonstrated with an ultimate sensitivity of 0.2% LEL methane (100 ppm).

Tunable Diode-Laser Spectroscopy With Wavelength Modulation: A Calibration-Free Approach to the Recovery of Absolute Gas Absorption Line Shapes

Abstract: The principles and implementation of an alternative approach to tunable diode-laser spectroscopy with wavelength modulation are described. This new technique uses the inherent phase shift between diode-laser power modulation and frequency modulation to separate the residual amplitude modulation and the first derivative signals recovered at the fundamental modulation frequency. The technique, through analysis of the residual-amplitude-modulation signal, is absolute, yielding gas-absorption-line-shape functions, concentrations, and pressures without the need for calibration under certain defined operating conditions. It offers the simplicity of signal analysis of direct detection while providing all the advantages of phase-sensitive electronic detection. Measurements of the 1650.96-nm rotation/vibration-absorption-line-shape function for 1% and 10% methane in nitrogen at various pressures are compared to theoretical predictions derived from HITRAN data, and the excellent agreement confirms the validity of the new technique. Further measurements of concentration and pressure confirm the efficacy of the technique for determining concentration in industrial-process environments where the pressure may be unknown and changing. An analysis of signal strength demonstrates that sensitivity comparable to that of conventional approaches is achievable. The new approach is simpler and more robust in coping with unknown pressure variations and drift in instrumentation parameters (such as laser characteristics) than the conventional approach. As such, it is better suited to stand-alone instrumentation for online deployment in industrial processes and is particularly useful in high-temperature applications, where the background infrared is strong.

All-silica single-mode optical fiber with photonic crystal cladding

Abstract: We report the fabrication of a new type of optical waveguide: the photonic crystal fiber. It consists of a pure silica core surrounded by a silica-air photonic crystal material with a hexagonal symmetry. The fiber supports a single robust low-loss guided mode over a very broad spectral range of at least 458-1550 nm. Also see errata - http://eprints.soton.ac.uk/78010/

Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

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A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation

Abstract: The emerging field of nanophotonics1 addresses the critical challenge of manipulating light on scales much smaller than the wavelength. However, very few feasible practical approaches exist at present. Surface plasmon polaritons2,3 are among the most promising candidates for subwavelength optical confinement3,4,5,6,7,8,9,10. However, studies of long-range surface plasmon polaritons have only demonstrated optical confinement comparable to that of conventional dielectric waveguides, because of practical issues including optical losses and stringent fabrication demands3,11,12,13. Here, we propose a new approach that integrates dielectric waveguiding with plasmonics. The hybrid optical waveguide consists of a dielectric nanowire separated from a metal surface by a nanoscale dielectric gap. The coupling between the plasmonic and waveguide modes across the gap enables ‘capacitor-like’ energy storage that allows effective subwavelength transmission in non-metallic regions. In this way, surface plasmon polaritons can travel over large distances (40–150 µm) with strong mode confinement (ranging from λ2/400 to λ2/40). This approach is fully compatible with semiconductor fabrication techniques and could lead to truly nanoscale semiconductor-based plasmonics and photonics. Xiang Zhang and colleagues from the University of California, Berkeley, propose a new approach for confining light on scales much smaller than the wavelength of light. Using hybrid waveguides that incorporate dielectric and plasmonic waveguiding techniques, they are able to confine surface plasmon polaritons very strongly over large distances. The advance could lead to truly nanoscale plasmonics and photonics.

Photonic-Crystal Fibers

Abstract: The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed