We present a numerical framework for the simulation of lasers in the time domain. The algorithm is based on the finite-difference time-domain method, which has been extended to include material gain by using auxiliary differential equations for a frequency dependent conductivity. The algorithm is applied to the simulation of micro-disk lasers based on an erbium doped SiO<sub>2</sub> material system in order to obtain a CMOS compatible fabrication process. Lasing behavior and lasing threshold is studied in two and three dimensions for single and multi-disk systems.
We show that by incorporating a filter that removes unwanted stray light, high performance multimode interference couplers with uniformity in splitting ratios of better than 0.03 dB can be designed and fabricated in silicon-on-insulator waveguide technology, allowing the fabrication of very high extinction ratio switches and modulators.
It has recently been shown that a tapered slab glass waveguide (wedge) can be used to make a flat panel display by projecting a video image into the thick edge of the wedge1. In this paper, we present the equations that relate the incident angle to the position where the ray emerges from the wedge. In the analysis, skew rays are also considered. We also present the design and experimental results of an anti-reflection coating that increases the brightness and reduces the blur of the wedge. With the coating, the transmittance of the interface rises from 0 (TIR) to 99% for a 0.35 degree change in incident angle for S-polarised monochromatic light. Without the coating, the transmittance falls below 50%. We also present a chromatic design intended to work with S-polarised rays from an arc lamp.
Compact integrated optical circuits for signal routing and signal processing are currently the subjects of much active research. Multimode interference (MMI) couplers are widely used as splitters and combiners since they possess the desirable attributes of small size, low excess loss, well- defined slitting, dimensional tolerance and ease of fabrication. Recently there has been renewed interest in employing MMI devices within Mach-Zehnder structures to achieve splitting and switching functions. However, the extent of the switching capabilities achieved so far has been quite limited. This paper highlights how the switching capabilities of these Mach-Zehnder switches can be extended and presents design techniques for this type of photonic switch.
We describe a new design of optical fiber surface plasma wave chemical sensor. The sensor consists of a tapered single mode optical fiber with a thin layer of silver evaporated on to the tapered section. The gradually changing diameter of the fiber along the taper results in a distributed coupling between the guided mode of the fiber and the surface plasma wave. As a result, and in contrast to conventional plasma wave sensors, coupling to the surface plasma wave occurs over a broad spectral range, typically several hundred nm. The device shows good sensitivity to changes in the refractive index of the external environment, with refractive index changes of 10<SUP>-4</SUP> being detectable. The device is compact, simple to make, and has applications as a biochemical or immunosensor.
An evanescent wave immunoassay for cholera antitoxin immunoglobulins was performed using a single mode tapered optical fiber loop sensor. The transducer was silanized with 3- glycidoxypropyltrimethoxysilane and chemically modified to link covalently either cholera toxin B subunit or a synthetic peptide derived from it, CTP3. The sensor was exposed to seral fluids, obtained from human volunteers having been exposed to live virulent Vibrio cholerae 01 and shown to produce rice-water stools. Other toxins of interest, such as Clostridium botulinum toxin A, have been tested on similar systems. The bound unlabelled immunoglobulins were then exposed to a mixture of FITC-anti-IgG and TRITC-anti-IgA, without requirement for a separation step. The emanating fluorescent emissions of fluorescein and rhodamine, excited by the input laser light, were coupled back into the guided mode of the tapered fiber, and used to determine the concentrations of the complementary antigens.
A novel single mode tapered optical fiber loop biochemical sensor based on fluorescence spectroscopy has been developed. The fundamental fiber mode propagating through the tapered portion of the waveguide has evanescent fields which penetrate into the aqueous environment where the biochemical recognition event occurs. The model measurands were conjugated with a fluorescent dye, fluorescein isothiocyanate. When excited by the input laser light from the near end of the taper, generated fluorescence is coupled into the guided mode of the fiber and collected at the far end of the taper. Several radioactive and fluorescent quantification methods have been explored to determine the density of available binding sites immobilized on the fiber, and thus the ultimate sensitivity of the device. A generic avidin- biotin system has been tested as a model immunological diagnostic system. The high sensitivity of a single mode tapered loop device combined with a simple immobilization method provides a powerful tool for performing immunoassays.
Adiabatically tapered single-mode fibers provide a novel and effective method of gaining access to the optical field inside an optical fiber. The fabrication of low-loss tapered fibers with waist diameters of less than one micron is described. By surrounding a tapered fiber with laser-dye solution, compact very efficient amplifiers, saturable absorbers, and super- fluorescent sources have been demonstrated.