Chin-Lin Chen

Side-polished fibers

We report a reproducible technique of making side-polished fibers by embedding fibers in silicon V grooves and by polishing them mechanically. Details of V grooves and polishing techniques are described. The attenuation characteristics of polished fibers were measured by a liquid-drop method; the results are in excellent agreement with existing theoretical predictions. To facilitate comparisons, we cast expressions for the attenuation constant in terms of three generalized parameters: the V and b parameters for the fiber and a new generalized parameter V(ex) for the external medium. By using these generalized parameters, we can study the effects of the external medium on the attenuation constant of side-polished fibers in great detail, including in particular the region where the attenuation changes precipitously.

Optical directional couplers with variable spacing.

Optical directional couplers with variable separation are introduced and analyzed. Closed form solutions to the coupled-mode equations with variable coupling coefficient are presented. A procedure for the design of the best geometry of a variety of applications utilizing variable spacing couplers is also presented.

Diffused optical waveguides with exponential profile: effects of metal-clad and dielectric overlay.

Diffusion techniques have provided an alternative to thin film technology for the fabrication of LiNbO(3) and LiTaO(3) light guiding layers. The effects of metal and dielectric film overlays on the propagation of TE and TM modes in diffused LiNbO(3) optical waveguides with expontential profile are investigated. Also examined are strip loaded metal-clad optical waveguides providing 2-D confinement to the light beam.

Fiber polarimetric stress sensors

Simple polarimetric force sensors based on high birefringence fibers are investigated theoretically and experimentally. The sensor consists of a linearly polarized light source, a birefringent fiber, an analyzer or polarizing prism and two photo detectors. At the input, one of the polarization modes is excited. At the output, power in the orthogonal polarization mode is detected. In the theoretical investigation, the sensor is modeled as a concatenation of three birefringent fiber sections with the short middle section as the stress sensing element. The high birefringent fibers are treated as anisotropic media and the effects of stress on the fiber birefringence and the rotation of birefringence axes are studied. In particular, the presence of protective jackets is taken into account in the theoretical consideration. The sensor response is expressed as a function of the applied force, the interaction length, and the direction of force relative to the birefringence axes of the fiber. Experimental results based on high birefringent fibers agree very well with the theoretical predicted behavior.

Optical fiber Fabry-Perot sensors

A new optical fiber Fabry-Perot (F.P.) temperature sensor is proposed and demonstrated. In particular, it can discern the temperature rise from the temperature drop. With this sensor, the temperature change and the direction of temperature change can be determined as a function of time. The basic concept has been verified experimentally. The results of fiber F.P. sensor measurement compare well with an independent thermocouple measurement. To provide further insight, details of the experimentally observed interference fringes have also been compared to a computer simulation. Possible real-time implementations with highspeed electronics are suggested.

Optical-fiber strain sensors with asymmetric etched structures

Optical-fiber strain gauges with asymmetric etched structures have been analyzed, fabricated, and tested. These sensors are very sensitive with a gauge factor as high as 170 and a flat frequency response to at least 2.7 kHz. The gauge factor depends on the asymmetry of the etched structures and the number of etched sections. To understand the physical principles involved, researchers have used structural analysis programs based on a finite-element method to analyze fibers with asymmetric etched structures under tensile stress. The results show that lateral bends are induced on the etched fibers when they are stretched axially. To relate the lateral bending to the optical attenuation, we have also employed a ray-tracing technique to investigate the dependence of the attenuation on the structural deformation. Based on the structural analysis and the ray-tracing study parameters affecting the sensitivity have been studied. These results agree with the results of experimental investigations.

An etched two-mode fiber modal coupling element

When a fiber with topological structures built directly on the cladding is compressed longitudinally, lateral bends are induced. The lateral bends lead to mode coupling. We have built several etched two-mode fiber modal coupling elements to take advantage of this effect of bending. The resulting modal coupling elements are compact. More importantly, the percentage of power converting from one mode to another is variable by varying the axial compression. In this paper, we present a theoretical model describing the mode conversion in the etched fiber elements. Also discussed are the fabrication and testing of the modal coupling elements and the experimental confirmation of the theoretical model.

Wavelength dispersion of optical waveguides

Coefficients that characterize the contribution to the total waveguide dispersion from guide geometry and from material dispersion are introduced. These are cast in terms of the normalized parameters of normalized frequency, asymmetry measure, and effective guide index. This allows plotting of universal curves for the dispersion coefficients for step thin film and exponentially graded slab waveguides that are applicable to all such structures.

Nonreciprocal TM-mode thin-film phase shifters

TM modes guided by thin-film waveguides with one or more magnetooptic and anisotropic layers (i.e., substrate, film, and cover layers) have been studied with emphasis on its nonreciprocal characteristics. Our analysis shows that the nonreciprocity of TM modes is the result of nonreciprocal Goos-Haenchen phase shifts at the cover–film and/or film–substrate interfaces. Contributions to the nonreciprocity defined as (N+ − N−) from various regions are identified. Here N± are the effective indices of refraction of two counterpropagating modes. An accurate algebraic approximation for (N+ − N−) has been obtained to provide physical insight into the problem and facilitate numerical calculations. Guidelines for designing nonreciprocal TM mode phase shifters are also listed.

Excitation of higher order modes in optical fibers with parabolic index profile

A large number of modes can be supported by multimode fibers. There are applications where higher order modes are preferred. Microbend intensity sensors are good examples. The sensitivity of these sensors is greatly increased if higher order modes are excited. In this work, a simple method to excite higher order modes preferentially is suggested. It consists of thin-film gratings deposited directly onto the fiber end. By controlling the film thickness or transparency of the grating structure, a desired transmission coefficient T(r,Phi) is synthesized. The desired mode can be excited preferentially by incident Gaussian beams without the aid of additional optical components. Binary intensity and binary phase gratings have been studied. Numerical investigation reveals that the phase gratings are more effective for the preferential excitation of higher order modes than the intensity gratings. In fact, by using binary phase gratings and in optimal excitation conditions as much as 81.1, 76.9, 74.6, 73.3, and 72.3% of the power in the incoming, linearly polarized, fundamental Gaussian beam can be converted to LP(02), LP(03), LP(04), LP(05), and LP(06) modes, respectively, excluding Fresnel loss.