Single-crystal (SC) fibers have the potential of delivering extremely high laser energies. Sapphire fibers have been
the most commonly studied SC fiber and the losses for sapphire fibers have been as low as 0.4 dB/m for a 300-
micron core-only fiber at 3 microns. In this study we report on the growth of SC yttrium aluminum garnet, Y3Al5O12(YAG) fibers from undoped SC source rods using the Laser Heated Pedestal Growth (LHPG) technique. The
advantage of YAG over sapphire is the slight improvement in IR transmission of YAG. The IR transmission of bulk
YAG has been shown to extend to 5 μm where the absorption coefficient is 0.6 cm-1. The garnet family of crystals
is one of the most commonly used oxide crystal hosts for lasing ions in high power solid-state lasers, with the most
commercially common laser host being YAG. Thus, it is reasonable to assume that YAG fibers will have high laser
damage thresholds. The optical losses for 400-μm diameter YAG fibers have been measured to be about 3 dB/m at
2.94 μm. The longest length of YAG fiber grown has been about 60 cm.
Bidirectional (mutual) injection locking was demonstrated with solid-state lasers, producing significant improvements over traditional single-direction injection locking. Each laser element shares part of its output with other elements in bidirectional locking, distinct from single-direction (traditional) injection locking where one master laser provides the locking signal for a number of slaves. In a phase-locked array, the individual laser outputs add coherently, and the brightness of the entire array scales with the square of the number of elements, as if the active material diameter were increasing. Benefits of bidirectional locking, when compared to traditional injection locking, include reduced laser threshold, better output beam quality, and improved scaling capability. Experiments using two Nd:YVO4 lasers confirmed that mutual injection locking reduced lasing threshold by a factor of at least two and increased the output beam quality significantly. The injection locking effects began with 0.03% coupling between lasers and full-phase locking for coupling exceeding 0.5%. The 0.5% requirement for full phase-locking limits traditional injection-locked arrays to fewer than 100 elements, while mutually injection-locked arrays have no such limit. Mutual injection locking of an array of lasers can lead to a new architecture for high-power laser systems.
Pure yttrium vanadate crystals (YVO4) have optical properties ideal for polarizers in the mid-IR spectrum. YVO4 is highly transmissive from 400 to 3800 nm and is optically useful to 5 micrometers . YVO4 is a positive uniaxial crystal with a birefringence larger than the negative uniaxial crystal calcite. The positive birefringence of YVO4 gives ordinary polarization to the beam transmitted by the polarizer, producing a desirable symmetric field-of- view for all standard designs. Glan, Wollaston, and Rochon polarizers were fabricated from YVO4 and tested. The preferred Glan design for YVO4 is Glan-Taylor as for calcite. YVO4 has advantages in large aperture size, excellent optical quality, high laser-damage threshold, and excellent optical working ability with only a slight tendency to cleave. New monolithic crystal polarizer designs will be described. It is also believed that YVO4 would be an excellent synthetic substitute for calcite in the visible spectrum.
Surface damage thresholds of polished calcite crystals were measured with a pulsed ruby laser at the wavelength of 694.3 nm. Three grades of calcite used for laser polarizers were tested: Grade A, Schlieren-Free, and Scatter-Free, as characterized by the manufacturer, Karl Lambrecht Corporation of Chicago. The test samples were cut and polished with the surfaces parallel to the optic axis. Sample dimensions were 1 cm X 1 cm and 2 to 3 mm thick. The ruby laser oscillator was passively Q-switched to produce 12-ns pulses in a single-transverse and longitudinal mode. Samples were irradiated at near-normal incidence to the 1-cm2 surfaces with a laser 1/e2 spot diameter of approximately 0.12 mm. Tests were conducted with a single shot per site, and photoelectric detection of a laser- induced surface spark was used as a sensitive method for detecting the onset of damage. Damage threshold of 30, 60, and 50 mJ/cm2, were determined for the three grades, respectively. No significant threshold differences were observed between test with the laser polarization parallel and perpendicular to the optic axis. Laser-induced damage, visible by scattering of a collinear He-Ne laser beam, was most frequently observed at the rear surfaces, although it sometimes occurred at the front surface alone or in the interior. On the basis of standing wave electric-field analysis, the predicted energy density at the rear surface was more than twice that at the front surface, and laser damage at the rear surface was expected to occur at a correspondingly lower fluence. Early damage at randomly distributed surface imperfections appeared to preclude such a consistent correlation.
A pulsed erbium garnet laser (30 Hz at 2.78 micrometers ) was used to characterize cutting on various soft tissues. Bovine heart, porcine liver and chicken breast were cut, and the resultant craters cut were microscopically examined. The variation of crater depth with average power settings and number of pulses was studied in detail. The second part of the study was to evaluate the coagulation effect of the erbium laser on surgical cuts induced into mucosal soft tissues. Conclusions of this study will be provided from qualitative analysis of the histopathological data.
Thermal and stress lensing effects have been measured in a Cr,Er:YSGG rod by observing a transmitted 1064 nm Nd:YAG beam diverging from an operating Cr,Er:YSGG laser. The results compare favorably with theory and estimated thermal-optic properties of YSGG, which is intermediate between YAG and GSGG. Numerical simulations of the laser agree substantially with the threshold and power observed and show a heat generation rate which is consistent with our observations of lensing. Thermal fracture of the rod has been observed on several occasions, leading to an estimate of the thermal fracture figure of merit which is also intermediate between YAG and GSGG. Back focal distances of less than 20 cm occur in the vicinity of half the thermal rupture limit.
Human enamel and dentin were cut using a 2.8 micrometers erbium laser beam in conjunction with water spray. Ablation rates of the order of 0.14 mm3/s were achieved for both tissues at 4 W average power. An ablation threshold of 0.28 W was determined for dentin when using the water spray. When above threshold, the volume of tissue ablated per joule was discovered to be nearly constant, not related to the average laser power. Comparative microscopy between tissue irradiated with and without water spray demonstrated a lack of thermal effects when the water spray was utilized.
A pulsed erbium glass laser operated at 1533 nm using transverse pumping by four 12-bar laser diode subarrays at 932 nm. The laser ran in free-running and Q-switched modes, using both electro-optical and mechanical switches. With a LiNbO3 Pockels cell Q-switch, the erbium laser achieved 57 mJ energy per pulse, 26 ns pulsewidth and 3 Hz repetition rate.
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