High power Nd:YAG laser rods may easily exceed their fracture strength as a result of thermally induced tensile stress. Strengthening of such rods is achievable by wet chemical etching in concentrated orthophosphoric acid, or in a mixture of phosphoric and sulfuric acids, at elevated temperatures. In the present study, the etching rates of Nd:YAG in both etchants were determined as a function of temperature. The calibration curves thus obtained enabled the controlled removal of the outer "damaged" layer of lasing elements. Four-point flexture strength measurements were performed on deep etched and on non-etched YAG slabs. The measurements showed a 3.5 fold increase in the strength of the etched slabs. Weibull analysis treatment was done on the basis of our fracture data.
Diffusion-bonded surfaces inside a cavity usually have little effect on a laser beam when oriented perpendicular to the beam direction. They may have a severe effect on lasing efficiency and mode structure when oriented parallel to the beam. Characterization of the interface between diffusion bonded elements and the understanding of its effect on lasing is, Therefore, important. Isolating the effect of the bonding interface from effects such as an index difference between the bulks, we used bonded BK7 slabs as the bonded element. We measured the reflections arising from the bonding interface, and then put the slabs as a passive element inside a hemispherical resonator. It was found that even for similar materials, with no refraction index difference, significant reflections occur at the bonding interface for oblique angles of incidence. When put intracavity parallel to beam axis the bonded slabs caused a significant power loss and forced higher order lasing modes. The power loss depended on both, the transmission through the interface and the size of the interface cross section relative to beam diameter.
Composite laser devices of passively Q-switched Nd:YAG were prepared by optical contacting of a Ca,Cr:YAG crystal onto a Nd:YAG wafer, followed by thermal treatments at elevated temperatures. Thermal treatments of the composite devices under reducing and/or oxidizing environments enabled to control the Cr4+ ion concentration, which affects its absorption saturation behavior. Optical absorption saturation measurements on partialy reduced Ca,Cr:YAG crystal revealed that residual absorption of the saturable absorber at 1064 nm results from excited-state absorption. Laser damage measurements at the gain/absorber interface of the monolithic device showed that the damage threshold at the gain/absorber interface of the composite device is higher than at the entrance interface. The high optical quality device thus obtained was end-pumped by a fiber-optic-coupled diode laser, and exhibited a high repetition-rate oscillations of short pulses, with fair beam quality.
ZnGeP2 crystallizes congruently through an (beta) > (alpha phase transition. The high temperature (beta) -phase has a diamond-like cubic structure, with Zn and Ge ions randomly distributed on the metal sublattice. The (alpha) - ZnGeP2 crystal has an ordered tetragonal structure, in which the Zn and Ge ions occupy different crystallographic sites. In the present study, good quality ZnGeP2 crystals have been grown by means of the Horizontal Gradient Freezing technique. A small amount of coherently oriented domains of the cubic (beta) -ZnGeP2 structure was identified, however, within the tetragonal ZnGeP2 crystals, indicating an incomplete transition. The (beta) - ZnGeP2 phase was partially retained by rapid quenching from the melt.
Solar pumped lasers are candidates for wireless power transmission in space, free space optical communication and photochemistry. The present paper describes the progress on a joint project to develop solar pumped lasers, focusing on the temporal and spectral shaping of the laser beam, which comply with those applications. The program emphasizes the design, fabrication and testing of laser rods, passive and active Q-switches and non-linear crystals. This work is carried on in four projects, the first involves a study of possible applications of such a laser, which will determine the required solar pumped laser specifications. In the second project, the solar tower facilities at Weizmann Institute are used for studying the performance of the above mentioned laser components. The third project includes the construction of an innovative 7.5 kW solar concentrator at TEMED Industrial Park facility, to be used for the further study of laser components. The fourth project is dedicated to the development of a master oscillator-power amplifier chain, in which a diode-pumped laser serves as the initial oscillator, while the amplifier is solar pumped.
Q-switched, solar-pumped, high power Nd:YAG lasers are attractive for a variety of applications requiring high instantaneous peak power density. The Q-switching can be obtained by an acousto-optic, electro-optic or passive device. Passive Q-switching seems an excellent choice for space as well as for other applications since it neither requires an external driver nor an electrical power supply. In recent years Cr+4:YAG single crystals were extensively used as passive Q-switches for flashlamp-pumped high power Nd:YAG lasers, demonstrating their superior thermal superior thermal characteristics and durability. In this work we report the first operation of passive Q- switched, solar-pumped, high power Nd:YAG lasers. The concentrated solar energy for he optical pumping of the laser was obtained by a 3-stage combination of imaging and non-imaging optics. It included: i) Weizmann Institute solar tower heliostats, ii) 3D compound parabolic concentrator, and iii) 2D compound parabolic concentrator in which the laser rod was placed. 72 mm long laser rods with either 3 mm or 4 mm diameter were used. The passive Q-switch was made from a Cr$=+4):YAG single crystal having a low- intensity transmission of 72 percent at 1.06 (mu) . Its rear surface was coated by a high reflectivity coating, serving as the rear mirror of the cavity. Output coupling mirrors with various reflectivities were used. The passive Q-switch demonstrated excellent durability and reliability during all the experiments. Repetition rates of 6-39 kHz were measured, showing higher repetition rates at higher laser power levels. The pulses demonstrated shorter full width at half maximum (FWHM) time for higher laser power elves, and the FWHM time range was 190-310 nsec. The maximal measured average power was 14 W. Thermal lensing was measured as a function of the absorbed solar power in the laser rod. It is estimated that laser peak power densities of approximately 100 kW/cm2 were achieved in the experiments. It is believed that near-future modifications may improve this value appreciably.
Intracavity second harmonic generation of a passively Q- switched cw Nd:YAG laser is studied. The saturable absorber used was a Cr4+:YAG crystal, and the second harmonic generator was a flux grown KTP crystal. The results were compared to a case of acousto-optic Q-switched case. To allow for a unidirectional second harmonic output beam, a folded laser cavity was constructed. However, due to a partially transmitting mirror in the cavity, two output counter propagating second harmonic beams, were generated. Unexpectedly, 180 degrees out of phase periodic power variations of the two output beams were observed, as a function of the distance from the nonlinear crystal to the mirror.
A comparative research into the use of Cr4+-doped YAG, YSGG, and LuAG (Lu3Al5O12) as saturable absorbers for the 1.06 micrometer emission of Nd:YAG laser has been conducted. Q-switched pulses with temporal duration ranging from 52 to 23 nsec (FWHM) were observed for the different garnets under investigation. Cross-sections for ground state and excited state absorption were evaluated by fitting our saturation data to Frantz-Nodvik equation. A brief discussion on the use of Cr4+ as a tunable mid-IR laser also is presented.
The success in lasing of Cr4+:forsterite in the near IR had motivated a search for new crystalline hosts for Cr4+. Silicates and garnets containing distorted tetrahedral crystalline sites are suitable for accommodating tetravalent chromium ions. In the present paper we compare the structural and spectral characteristics of Cr4+ doped YAG, Mg2SiO4, Ca2Ga2SiO7, Ca2Al2SiO7 and Y2SiO3 crystals, as well as their growth conditions. The applications of these materials as tunable solid state lasers and passive Q-switches for pulsed Nd lasers are discussed.
The Czochralski crystal growth of pure Cr, Er and Ho doped as well as Cr,Er and Cr,Ho codoped yttrium silicate Y2SiO5 (YSO) is reported. The growth conditions for producing the inclusion free single crystals of high optical quality are discussed. Spectroscopic properties of the singly doped and codoped material at room temperature are compared. Energy transfer processes from Cr4+ ions to Ho3+ and Er3+ ions in YSO host have been demonstrated for the first time.
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