Browsing by Author "Lally, Evan M."
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- Fourier Transform Interferometry for 3D Mapping of Rough and Discontinuous SurfacesLally, Evan M. (Virginia Tech, 2010-04-28)Of the wide variety of existing optical techniques for non-contact 3D surface mapping, Fourier Transform Interferometry (FTI) is the method that most elegantly combines simplicity with high speed and high resolution. FTI generates continuous-phase surface maps from a projected optical interference pattern, which is generated with a simple double-pinhole source and collected in a single snapshot using conventional digital camera technology. For enhanced stability and reduced system size, the fringe source can be made from a fiber optic coupler. Unfortunately, many applications require mapping of surfaces that contain challenging features not ideally suited for reconstruction using FTI. Rough and discontinuous surfaces, commonly seen in applications requiring imaging of rock particles, present a unique set of obstacles that cannot be overcome using existing FTI techniques. This work is based on an original analysis of the limitations of FTI and the means in which errors are generated by the particular features encountered in the aggregate mapping application. Several innovative solutions have been developed to enable the use of FTI on rough and discontinuous surfaces. Through filter optimization and development of a novel phase unwrapping and referencing technique, the Method of Multiple References (MoMR), this work has enabled surface error correction and simultaneous imaging of multiple particles using FTI. A complete aggregate profilometry system has been constructed, including a MoMR-FTI software package and graphical user interface, to implement these concepts. The system achieves better than 22µm z-axis resolution, and comprehensive testing has proven it capable to handle a wide variety of particle surfaces. A range of additional features have been developed, such as error correction, particle boundary mapping, and automatic data quality windowing, to enhance the usefulness of the system in its intended application. Because of its high accuracy, high speed and ability to map varied particles, the developed system is ideally suited for large-scale aggregate characterization in highway research laboratories. Additionally, the techniques developed in this work are potentially useful in a large number of applications in which surface roughness or discontinuities pose a challenge.
- Method Of Multiple References For 3D Imaging with Fourier Transform InterferometryLally, Evan M.; Gong, Jianmin; Wang, Anbo (Optical Society of America, 2010-03-01)This letter presents an improved phase referencing technique, called Method of Multiple References, for optical profilometry. Based on a lookup table, the method eliminates several major drawbacks of single-reference Fourier Transform Interferometry by enabling surface error correction for steep slopes and step discontinuities, and by allowing mapping of multiple discrete objects using a single image set. The algorithm is tested using a fiber optic coupler-based FTI system and shown to have RMS surface error less than 0.03mm. (C) 2010 Optical Society of America
- A Narrow-Linewidth Laser at 1550 nm Using the Pound-Drever-Hall Stabilization TechniqueLally, Evan M. (Virginia Tech, 2006-08-11)Linewidth is a measure of the frequency stability of any kind of oscillator, and it is a defining characteristic of coherent lasers. Narrow linewidth laser technology, particularly in the field of fiber-based infrared lasers, has progressed to the point where highly stable sources are commercially available with linewidths on the order of 1-100 kHz. In order to achieve a higher level of stability, the laser must be augmented by an external frequency stabilization system. This paper presents the design and operation of a frequency locking system for infrared fiber lasers. Using the Pound-Drever-Hall technique, the system significantly reduces the linewidth of an input laser with an un-stabilized linewidth of 2 kHz. It uses a high-finesse Fabry-Perot cavity, which is mechanically and thermally isolated, as a frequency reference to measure the time-varying frequency of the input laser. An electronic feedback loop works to correct the frequency error and maintain constant optical power. Testing has proven the Pound-Drever-Hall system to be highly stable and capable of operating continuously for several seconds at a time.
- ZrO2 Thin-Film-Based Sapphire Fiber Temperature SensorWang, Jiajun; Lally, Evan M.; Wang, Xiaoping; Gong, Jianmin; Pickrell, Gary R.; Wang, Anbo (Optical Society of America, 2012-10-01)A submicrometer-thick zirconium dioxide film was deposited on the tip of a polished C-plane sapphire fiber to fabricate a temperature sensor that can work to an extended temperature range. Zirconium dioxide was selected as the thin film material to fabricate the temperature sensor because it has relatively close thermal expansion to that of sapphire, but more importantly it does not react appreciably with sapphire up to 1800 degrees C. In order to study the properties of the deposited thin film, ZrO2 was also deposited on C-plane sapphire substrates and characterized by x-ray diffraction for phase analysis as well as by atomic force microscopy for analysis of surface morphology. Using low-coherence optical interferometry, the fabricated thin-film-based sapphire fiber sensor was tested in the lab up to 1200 C and calibrated from 200 degrees to 1000 degrees C. The temperature resolution is determined to be 5.8 degrees C when using an Ocean Optics USB4000 spectrometer to detect the reflection spectra from the ZrO2 thin-film temperature sensor. (C) 2012 Optical Society of America OCIS codes: 060.2370, 120.6780, 310.1620.