Dr. Robert E. Setchell Profile

Dr. Robert E. Setchell

at Sandia National Labs

SPIE Involvement:

Author

Publications (10)

Proceedings Article | 13 September 2007 Paper

Challenges in high-intensity laser injection into multiple optical fibers

Robert Setchell, Dante Berry

Proceedings Volume 6662, 666203 (2007) https://doi.org/10.1117/12.736494

KEYWORDS: Fiber lasers, Laser optics, Laser induced damage, Laser damage threshold, Polishing, Diffraction, Optical fibers, Diffraction gratings, Interfaces, Optical design

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Proceedings Article | 30 October 2001 Paper

Evaluation of a diffractive microlens-array beam shaper for use in acceleration of laser-driven flyers

Wayne Trott, Robert Setchell, Jaime Castaneda, Dante Berry

Proceedings Volume 4443, (2001) https://doi.org/10.1117/12.446748

KEYWORDS: Beam shaping, Interfaces, Neodymium glass lasers, Interferometers, Near field optics, Silica, Aluminum, Near field, Optical fabrication, Metals

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Proceedings Article | 25 October 2000 Paper

Laser injection optics for high-intensity transmission in multimode fibers

Robert Setchell

Proceedings Volume 4095, (2000) https://doi.org/10.1117/12.405250

KEYWORDS: Fiber lasers, Laser optics, Optical design, Near field optics, Cameras, Polishing, Diffraction, Integrated optics, Laser induced damage, Laser applications

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Proceedings Article | 7 April 1999 Paper

Effects of accelerated aging on fiber damage thresholds

Robert Setchell

Proceedings Volume 3578, (1999) https://doi.org/10.1117/12.344411

KEYWORDS: Fiber lasers, Laser damage threshold, Silica, Polishing, Cladding, Failure analysis, Interfaces, Laser induced damage, Laser optics, Humidity

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Laser-induced damage mechanisms that can occur during high- intensity fiber transmission have been under study for a number of years. Our particular interest in laser initiation of explosives has led us to examine damage processes associated with the transmission of Q-switched, Nd:YAG pulses at 1.06 micrometers through step-index, multimode, fused silica fiber. Laser breakdown at the fiber entrance face is often the first process to limit fiber transmission, but catastrophic damage can also occur at either fiber end face, within the initial 'entry' segment of the fiber, and at other internal sites along the fiber path. Past studies have examined how these various damage mechanisms depend upon fiber end-face preparation, fiber fixturing and routing, laser characteristics, and laser-to-fiber injection optics. In some applications of interest, however, a fiber transmission system may spend years in storage before it is used. Consequently, an important additional issue for these applications is whether or not there are aging processes that can result in lower damage thresholds over time. Fiber end-face contamination would certainly lower breakdown and damage thresholds at these surfaces, but careful design of hermetic seals in connectors and other end-face fixtures can minimize this possibility. A more subtle possibility would be a process for the slow growth of internal defects that could lead to lower thresholds for internal damage. In the current study, two approaches to stimulating the growth of internal defects were used in an attempt to produce observable changes in internal damage threshold. In the first approach, test fibers were subjected to a very high tensile stress for a time sufficient for some fraction to fail from static fatigue. In the second approach, test fibers were subjected to a combination of high tensile stress and large, cyclic temperature variations. Both of these approaches were rather arbitrary due to the lack of an established growth mechanism for internal defects. Damage characteristics obtained from fibers subjected to each of these aging environments were compared to results from fresh fibers tested under identical conditions. A surprising result was that internal damage was not observed in any of the tested fibers. Only breakdown at the fiber entrance face and catastrophic damage at both end faces were observed. Fiber end faces were not sealed during the accelerated aging environments, and thresholds at these faces were significantly lower in the aged fibers. However, most fibers transmitted relatively high pulse energies before damaging, and a large fraction never damaged before we reached the limits of our test laser. The absence of any observable affect on internal damage thresholds is encouraging, but the current results do not rule out the possibility that some other approach to accelerated aging could reveal a growth mechanism for internal defects.

Proceedings Article | 20 April 1998 Paper

End-face preparation methods for high-intensity fiber applications

Robert Setchell

Proceedings Volume 3244, (1998) https://doi.org/10.1117/12.306994

KEYWORDS: Optical fibers, Surface finishing, Polishing, Carbon monoxide, Gas lasers, Laser damage threshold, Fiber lasers, Laser optics, Laser applications, Explosives

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High laser intensities are being transmitted through optical fibers in a growing number of applications. Our interest in laser initiation of explosives has led us to examine the transmission of Q-switched, Nd:YAG laser pulses through step-index, multimode, fused-silica fibers for a number of years. A common limiting process is a plasma-forming breakdown occurring at the fiber entrance face. The breakdown threshold at this face depends on the surface characteristics that result from the particular method of end-face preparation. In previous studies we examined entrance-face breakdown thresholds for several different mechanical polishing schedules, and also for several schedules of CO₂-laser conditioning following mechanical polishing. In the present study we examined three end-face preparation methods that were base on the recent availability of exceptionally good cleaved surfaces for our fibers of interest. Using test procedures similar to those in past studies, we examined the cleaved fibers directly, fibers with cleaved surfaces that were subsequently flame polished, and fibers with cleaved surfaces that were subsequently conditioned with a CO₂ laser. All of these preparation methods resulted in fibers that showed a broader range of entrance-face breakdown conditions than found in past studies, together with a susceptibility to subsurface exit-face damage. By introducing additional cleaning steps with the cleaved surfaces, we were able to reduce the variability in breakdown thresholds observed after subsequent CO₂- laser conditioning. A consistent location of exit-face damage sites indicates that subsurface fracturing occurs during the cleaving process, and that the subsequent end-face processing steps were not effective in mitigating damage at these sites. Threshold energies for entrance- face breakdown are also affected by the relation between incident laser energy and the resulting peak local fluence at this surface. Laser characteristics and the design of the laser-to- fiber injection optics determine this relation. The present study utilized an improved injection system consisting of a custom diffractive optical element combined with a lens having a very short focal length. This system produced the lowest value for the ratio of peak-to-average fluence at the entrance face that we have observed, and was very successful in inhibiting internal damage mechanisms along the fiber path.

Proceedings Article | 15 May 1997 Paper

Injecting a pulsed YAG laser beam into a fiber

William Sweatt, Robert Setchell, Mial Warren, Michael Farn

Proceedings Volume 3010, (1997) https://doi.org/10.1117/12.274426

KEYWORDS: YAG lasers, Diffractive optical elements, Laser optics, Laser applications, Cladding, Multimode fibers, Fiber lasers, Laser damage threshold, Laser development, Quartz

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Proceedings Article | 13 May 1997 Paper

Optimized fiber delivery system for Q-switched Nd:YAG lasers

Robert Setchell

Proceedings Volume 2966, (1997) https://doi.org/10.1117/12.274264

KEYWORDS: Fiber lasers, Laser optics, Laser damage threshold, Carbon dioxide lasers, Nd:YAG lasers, Polishing, Laser induced damage, Surface finishing, Q switched lasers, Near field

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Interest in the transmission of high intensities through optical fibers is being motivated by an increasing number of applications. Using different laser types and fiber materials, various studies are encountering transmission limitations due to laser-induced damage processes. For a number of years we have been investigating these limiting processes during the transmission of Q-switched, multimode, Nd:YAG laser pulses through step-index, multimode, fused- silica fiber. We have found that fiber transmission is often limited by a plasma-forming breakdown occurring at the fiber entrance face. This breakdown can result in subtle surface modifications that leave the entrance face more resistant to further breakdown or damage events. Catastrophic fiber damage can also occur as a result of a variety of mechanisms, with damage appearing at fiber entrance and exit faces, within the initial 'entry' segment of the fiber path, and at other internal sites due to fiber fixturing and routing effects. System attributes that will affect breakdown and damage thresholds include laser characteristics, the design and alignment of laser-to-fiber injection optics, and fiber end-face preparation. In the present work we have combined insights gained in past studies in order to establish what thresholds can be achieved if all system attributes can be optimized to some degree. Our multimode laser utilized past modifications that produced a relatively smooth, quasi-Gaussian profile. The laser-to-fiber injection system achieved a relatively low value for the ratio of peak-to-average fluences at the fiber entrance face, incorporated a mode scrambler to generate a broad mode power distribution within the initial segment of the fiber path, and had improved fixturing to insure that the fiber axis was collinear with the incident laser beam. Test fibers were from a particular production lot for which initial-strength characteristics were established and a high-stress proof test was performed. Fiber end faces were prepared by a careful mechanical polishing schedule followed by surface conditioning using a CO₂ laser. In combination, these factors resulted in higher thresholds for breakdown and damage than we had achieved previously in studies that utilized a simple lens injection system. Probability distribution functions were fitted to the threshold data, providing a means for estimating the probability for transmission failure at lower laser energies.

Proceedings Article | 14 July 1995 Paper

Reduction in fiber damage thresholds due to static fatigue

Robert Setchell

Proceedings Volume 2428, (1995) https://doi.org/10.1117/12.213734

KEYWORDS: Laser damage threshold, Fiber lasers, Polishing, Silica, Failure analysis, Interfaces, Cladding, Optical fibers, Manufacturing, Fiber characterization

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For a number of years we have been investigating laser-induced damage mechanisms that can occur during the transmission of Q-switched, Nd/YAG laser pulses through fused silica fibers. We have found that fiber end-face characteristics, laser characteristics, and aspects of the laser-to-fiber injection typically determine dominant damage mechanisms. However, an additional damage process has been observed occasionally at internal sites where fibers were experiencing significant local stresses due to fixturing or to bends in the fiber path. A transmission reduction prior to damage was typically not measureable at these sites. Damage would not always occur during initial testing, but sometimes occurred later in time at laser levels that previously had been transmitted without damage. In these cases the time at stress appeared to be more important than the number of transmitted shots prior to damage. A possible relation between internal damage thresholds at stressed sites and the total time under stress is suggested by the fact that silica fibers experience static fatigue processes. These processes involve the slow growth of local defects under tensile stress at rates that depend upon environmental conditions. Defects reaching sufficient size and having appropriate location could be sites for reduced laser-induced damage thresholds. This possibility could have important implications for high-power fiber transmission systems that must satisfy extended lifetime requirements. The needs of the telecommunications industry have motivated extensive studies into initial fiber defect characteristics and their likely growth mechanisms. The present work used the understanding developed in these studies to guide a preliminary experimental investigation into the possibility that static fatigue processes can affect damage thresholds. The experiments used a laser injection and fiber routing configuration that produced significantly elevated fluences within fiber core regions under tensile stress. In one set of experiments, internal damage thresholds were determined in available fiber samples that had been assembled in stress-imposing fixtures for periods up to 24 months. A decline in mean thresholds with time was observed, although measured values showed significant scatter. In order to establish initial strength and fatigue properties for these fibers, a number of additional samples were used to generate time-to-failure data at various stress levels. Based on these results, other fiber samples were subjected to conditions that greatly accelerated fatigue processes. Internal damage thresholds were then measured in these fibers and compared to thresholds measured in fresh fibers. Conclusive comparisons were frustrated by sample-to-sample and lot-to-lot variations in fiber defects.

Proceedings Article | 28 July 1994 Paper

Damage studies in high-power fiber transmission systems

Robert Setchell

Proceedings Volume 2114, (1994) https://doi.org/10.1117/12.180910

KEYWORDS: Polishing, Liquids, Fiber lasers, Surface finishing, Interfaces, Silica, Adhesives, Microscopes, Prototyping, Laser damage threshold

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Proceedings Article | 29 July 1992 Paper

Laser-induced damage studies on step-index multimode fibers

Robert Setchell, Paul Klingsporn

Proceedings Volume 1624, (1992) https://doi.org/10.1117/12.60104

KEYWORDS: Polishing, Laser induced damage, Fiber lasers, Resistance, Step index fibers, Laser damage threshold, Fiber coupled lasers, Multimode fibers, Explosives, Fiber couplers

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A previous investigation of laser-induced damage mechanisms and corresponding thresholds in step-index, multimode fibers was motivated by an interest in optical systems for firing explosives. In the initial study, the output from a compact, multimode Nd/YAG laser was coupled into fiber cores of pure fused silica. End-face polishing steps were varied between successive fiber lots to produce improved finishes, and each fiber was subjected to a sequence of progressively increasing energy densities up to a value of more than 80 J/cm². Essentially all of the tested fibers experienced a 'laser conditioning' process at the front fiber face, in which a visible plasma was generated for one or more laser shots. Rather than produce progressive damage at the front surface, however, this process would eventually cease and leave the surface with improved damage resistance. Once past this conditioning process, the majority of fibers damaged at the rear end face. Other modes of damage were observed either at locations of fixturing stresses or at a location of high static tensile stress resulting from bends introduced to the fiber. Although the previous results were encouraging in terms of achieving useful damage thresholds, a number of areas for further study were indicated. In the present study, a similar experimental procedure was used to address these areas. The relative permanence of front-surface laser conditioning was examined by re-testing fibers that had experienced this process at least a year previously. End-face mechanical polishing was again examined by testing fibers prepared using a refined polishing schedule. Attempts to use a single fixture to hold an entire lot of fibers throughout end-face polishing and damage testing met with mixed results, with fiber positions subjected to fixturing stresses likely sites for initial damage. In an effort to prepare fiber faces with the improved damage resistance observed with front faces following 'laser conditioning,' two schedules for CO₂-laser polishing of end faces were developed and evaluated. Finally, to improve resistance to damage at sites with significant static stresses, fiber samples which passed a much higher tensile proof test during manufacturing were tested. The current experiments were conducted with a new laser having a shorter pulsewidth and a significantly different mode structure. The beam was injected into the fiber using a geometry that had been successful in the previous study in minimizing a damage mechanism which can occur at the core/cladding interface with the first few hundred fiber diameters. However, the different mode structure of the new laser apparently resulted in this mechanism dominating the current results.

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