Kin Seng Chiang

Construction of refractive-index profiles of planar dielectric waveguides from the distribution of effective indexes

From a set of effective indexes, a continuous effective-index function is defined, which is then used to construct a refractive-index profile by numerically solving a WKB equation. The method applies to synthesis of profiles from prescribed propagation constants. Profiles for spatial image transmission are presented as an example. The method can also be applied to recovery of Smooth profiles from measured effective indexes. Effects on the recovery of profiles due to errors in measured effective indexes are demonstrated and procedures are given to alleviate those effects.

Laser-micromachined Fabry-Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index

We propose and demonstrate a Fabry-Perot (F-P) optical fiber tip sensor for high-resolution refractive-index measurement fabricated by using 157-nm laser micromachining, for the first time to our knowledge. The sensor head consists of a short air F-P cavity near the tip of a single-mode fiber and the fiber tip. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. Such a sensor can provide temperature-independent measurement of practically any refractive index larger than that of air and offers a refractive-index resolution of ~4 x 10(-5) in its linear operating range. The experimental data agree well with the theoretical results.

Review of numerical and approximate methods for the modal analysis of general optical dielectric waveguides

AbstactAbstactThis paper reviews numerical and approximate methods for the modal analysis of general optical dielectric waveguides with emphasis on recent developments. Six groups of methods are reviewed, covering (1) the finite-element method, (2) the finite-difference method, (3) the integral-equation method, (4) methods based on series expansion, (5) approximate methods based on separation of variables, and (6) methods that do not fit into the above groups, ranging from the specialized ones for analysing restricted classes of waveguides to the most general ones for analysing inhomogeneous, arbitrarily-shaped, anisotropic waveguides. Some suggestions with regard to the selection of methods for particular applications are given.

High-sensitivity pressure sensor using a shielded polymer-coated fiber Bragg grating

We demonstrate a novel high-sensitivity pressure sensor, which is based on the use of a fiber Bragg grating (FBG) embedded in a polymer-filled metal cylinder with an opening on one side to enhance the pressure sensitivity. The measured pressure sensitivity of the fractional change in the Bragg wavelength of our experimental sensor is -3.41/spl times/10/sup -3/ MPa/sup -1/ which is approximately 1720 times higher than that can be achieved with a bare FBG. The linearity of our sensor is also good. This sensor should find applications in the area of low-pressure measurement.

Graphene-coated microfiber Bragg grating for high-sensitivity gas sensing

A graphene coated microfiber Bragg grating (GMFBG) for gas sensing is reported in this Letter. Taking advantage of the surface field enhancement and gas absorption of a GMFBG, we demonstrate an ultrasensitive approach to detect the concentration of chemical gas. The obtained sensitivities are 0.2 and 0.5 ppm for NH3 and xylene gas, respectively, which are tens of times higher than that of a GMFBG without graphene for tiny gas concentration change detection. Experimental results indicate that the GMFBG-based NH3 gas sensor has fast response due to its highly compact structure. Such a miniature fiber-optic element may find applications in high sensitivity gas sensing and trace analysis.

Intermodal dispersion in two-core optical fibers.

The switching characteristics of a two-core optical fiber can be described by the beating of the even and odd modes of the composite two-core structure. It is shown that the group-delay difference between these two modes can be as large as 10 ps/m. This intermodal dispersion sets a limit on the shortest duration of the optical pulse that can be switched effectively in devices that use long lengths of two-core fibers, such as wavelength-division multiplexers, polarization splitters, and nonlinear directional couplers.

Long-period gratings in planar optical waveguides

We present a theoretical analysis of light propagation in a four-layer planar waveguide that consists of a long-period grating (LPG) having a period of the order of 100 microm. By means of the coupled-mode theory, we show that such a structure is capable of coupling light from the fundamental guided mode to the cladding modes at specific wavelengths (resonance wavelengths) and thus results in sharp rejection bands in the transmission spectrum of the waveguide. Our numerical results show that the resonance wavelengths as well as the transmission spectrum can be significantly changed with the waveguide and grating parameters. A waveguide-based LPG should provide a useful approach to the design of a wide range of integrated-optic devices, including wavelength-tunable filters, switches, and environmental sensors.

Analysis of optical fibers by the effective-index method

The effective-index method for determining waveguide dispersion is derived from the scalar wave equation and applied to optical fibers of arbitrary cross-sectional shapes. In the simplest use of the method, the optical fiber is replaced by an equivalent slab waveguide with an index profile derived from the geometrical shape of the fiber. Results from analyzing circular, elliptical, and cusp-shaped fibers are used to illustrate the general features of the method. A procedure is also given for improving the accuracy of the method applied to a class of single-mode fiber.

Dual effective-index method for the analysis of rectangular dielectric waveguides

Two different approximations for the propagation constant of a guided mode in a rectangular dielectric waveguide can be obtained by applying the conventional effective-index method to the waveguide in two different ways. By combining these two approximations with the other two approximations for a complementary mode in a complementary waveguide so that the errors in the approximations can be largely canceled, a more accurate approximation can be derived. Numerical results from analyzing several basic rectangular structures reveal that in most cases the new technique is considerably more accurate than the conventional effective-index method and does not generate ambiguous results as the conventional effective-index method does in the analysis of square dielectric rods.

Widely tunable long-period gratings fabricated in polymer-clad ion-exchanged glass waveguides

Long-period-grating filters were fabricated in polymer-clad ion-exchanged BK7 glass waveguides. The transmission spectra of the filters exhibited strong polarization dependence. A contrast as high as 25 dB at the resonance wavelength was obtained. The temperature sensitivity of the filters was measured to be /spl sim/9.0 nm//spl deg/C, which allows potential wavelength tuning over the entire S+C+L band of /spl sim/180 nm with a temperature control over a range of /spl sim/20/spl deg/C.