Yb-doped fibers are widely used in applications requiring high average output powers and high power pulse
amplification. Photodarkening is one limiting factor in these fibers. In this paper, characterization of photodarkening in
large-mode-area (LMA) fibers is presented building upon our previous work, which indicated that meaningful
comparison of photodarkening properties from different fibers can be made as long as care is taken to equalize the
excited state Yb concentration between samples. We have developed a methodology that allows rapid and reproducible
photodarkening measurements to be performed and that enables quantitative comparison of the photodarkening
propensity among fibers with different compositions and under different operating conditions. We have shown that this
measurement technique can be used effectively for LMA fibers by employing cladding pumping rather than the more
standard core pumping. Finally, we observe a seventh-order dependence of the initial photodarkening rate on the excited-state
Yb population for two different Yb-doped fibers; this result implies that photodarkening of a Yb-doped fiber source
fabricated using a particular fiber will be strongly dependent on the device configuration.
A robust, real-time, dynamic eye tracker has been integrated with the short pulse mid-infrared laser scanning delivery system previously described. This system employs a Q- switched Nd:YAG laser pumped optical parametric oscillator operating at 2.94 micrometers. Previous ablation studies on human cadaver eyes and in-vivo cat eyes demonstrated very smooth ablations with extremely low damage levels similar to results with an excimer. A 4-month healing study with cats indicated no adverse healing effects. In order to treat human eyes, the tracker is required because the eyes move during the procedure due to both voluntary and involuntary motions such as breathing, heartbeat, drift, loss of fixation, saccades and microsaccades. Eye tracking techniques from the literature were compared. A limbus tracking system was best for this application. Temporal and spectral filtering techniques were implemented to reduce tracking errors, reject stray light, and increase signal to noise ratio. The expanded-capability system (IRVision AccuScan 2000 Laser System) has been tested in the lab on simulated eye targets, glass eyes, cadaver eyes, and live human subjects. Circular targets ranging from 10-mm to 14-mm diameter were successfully tracked. The tracker performed beyond expectations while the system performed myopic photorefractive keratectomy procedures on several legally blind human subjects.
Photospallation is proposed as the mechanism behind recent animal studies involving corneal ablation by nanosecond-pulse mid-IR laser beams. Following a brief summary of earlier work directed to refractive procedures in the mid-IR, a preliminary analysis is performed based on simple 1D models of thermoelastic expansion developed previously. The results of the analysis indicate that front surface spallation is consistent with the striking tissue ablation characteristics observed in the most recent work with short pulse mid-IR radiation, including very small ablation rates and submicron thermal damage zones. This is because spallation is a mechanical -- rather than a thermal -- process, allowing tissue to be removed in thin layers at fluences far lower than those used in the earlier corneal studies with mid-IR beams, resulting in minimal heating. We conclude that the existing theoretical basis supports the use of nanosecond pulses as an effective approach to achieving controlled ablation in the presence of very high absorption. We further suggest that such domain of operation may in fact be preferred over shorter pulses, both from a practical standpoint and to mitigate against potential damage from shock waves. Additional validation of the precise nature of corneal ablation with mid- IR nanosecond pulses was obtained from recent ablation rate experiments conducted in gel models, which resulted in submicron ablation rates of magnitudes very similar to those achieved with excimer. A brief summary of these preliminary results is given.
Purpose: This study first evaluated the corneal ablation characteristics of (1) an Nd:YAG pumped OPO (Optical Parametric Oscillator) at 2.94 microns and (2) a short pulse Er:YAG laser. Secondly, it compared the histopathology and surface quality of these ablations with (3) a 193 nm excimer laser. Finally, the healing characteristics over 4 months of cat eyes treated with the OPO were evaluated. Methods: Custom designed Nd:YAG/OPO and Er:YAG lasers were integrated with a new scanning delivery system to perform PRK myopic correction procedures. After initial ablation studies to determine ablation thresholds and rates, human cadaver eyes and in-vivo cat eyes were treated with (1) a 6.0 mm Dia, 30 micron deep PTK ablation and (2) a 6.0 mm Dia, -5.0 Diopter PRK ablation. Cadaver eyes were also treated with a 5.0 mm Dia, -5.0 Diopter LASIK ablation. Finally, cats were treated with the OPO in a 4 month healing study. Results: Ablation thresholds below 100 mJ/cm2 and ablation rates comparable to the excimer were demonstrated for both infrared systems. Light Microscopy (LM) showed no thermal damage for low fluence treatments, but noticeable thermal damage at higher fluences. SEM and TEM revealed morphologically similar surfaces for low fluence OPO and excimer samples with a smooth base and no evidence of collagen shrinkage. The Er:YAG and higher fluence OPO treated samples revealed more damage along with visible collagen coagulation and shrinkage in some cases. Healing was remarkably unremarkable. All eyes had a mild healing response with no stromal haze and showed topographic flattening. LM demonstrated nothing except a moderate increase in keratocyte activity in the upper third of the stroma. TEM confirmed this along with irregular basement membranes. Conclusions: A non- thermal ablation process called photospallation is demonstrated for the first time using short pulse infrared lasers yielding damage zones comparable to the excimer and healing which is also comparable to the excimer. Such Infrared sources are, therefore, potentially attractive competitors to the excimer to perform PRK and LASIK.
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