Using a simplified fabrication process, we present the experimental verification of the performance of a 3-D photonic crystal optical transmission filter. Inherent to this unique fabrication approach to the realization of narrow line width, highly efficient optical transmission filters, is the ability to spatially vary the transmission characteristics across the filter aperture. This differentiates this type of filter from conventional dielectric based space variant optical transmission filters which require additional processing at intermediate steps within the dielectric film deposition process. The multilayer stack consists of alternating high and low refractive index dielectric material grown on either side of a high index dielectric spacer layer, which produces a narrow transmission notch in the center of a large stop band. The nano-structuring of a square lattice array of holes and subsequent etching of the pattern through a dielectric stack provides the ability to spatially vary the location of the narrow transmission peak within the wide stop band based off of variation of the hole diameter or lattice constant of the array.
We present a novel mode selective coupling technique for step index fiber. This technique utilizes phase matching for excitation of higher-order modes while suppressing the fundamental mode. Using this technique, a phase element is fabricated and tested to demonstrate the high coupling efficiency to the LP11 mode. In addition, we derive an analytical expression of the coupling efficiency of the LP11 using a single phase element.
Bragg gratings have been used relatively extensively in recent years due to their highly dispersive and wavelength selective nature. Typically used as a reflective structure, the gratings reflect specific wavelengths at specific locations along the structure based on the grating periodicity to spatially shape an incident pulse of light according to its spectral components. Usually the purpose is to either compress or stretch the pulse.
Unfortunately, fabrication tolerances severely limit the amount of chirp per unit of waveguide length that can be placed on a Bragg grating. For some applications, a few nanometers of chirp over a meter or more of waveguide would be ideal, yet placement accuracy of individual features is usually far less than is needed for such a task. We propose an alternative fabrication method which would provide a long grating with substantially increased placement accuracy. Instead of fashioning the grating in the typical linear manner, a waveguide is fabricated in a spiral shape. This has been done for delay lines and amplifier structures in the past. However, we propose to incorporate a radial grating underneath it. This provides us an additional degree of freedom, since the period of the grating changes very linearly with its radius, and a waveguide can be accurately positioned on top of it so as to gradually spiral inwards (or outwards) and change radius (and, hence, grating period) very slowly along its length. We present fabrication results, optical comparisons between similar linear and spiral structures, and preliminary theoretical modeling of the structures.
An integrated fiber-optic displacement sensor based on multimode interference is characterized through analysis of experimental performance in comparison to the expected behavior that is theoretically predicted. Multimode interference and re-imaging theory applied to the specific fiber properties and geometry of the device can be used to design and predict the performance of the device. Essentially, the sensor consists of a multimode fiber of a specific length fusion spliced to a single mode fiber used in conjuction with an 80/20 splitter, source, and detector that are used to inject and detect reflected signals from targets. The sensor is fabricated into a robust, compact, and single arm device capable of operating as a calibrated displacement sensor over a large displacement range. This is achieved through analysis of power reflected off of a surface and back through the device over a finite wavelength range.