Yehoshua Shimony

Solar-pumped Er,Tm,Ho:YAG laser.

Direct solar illumination was used to pump a 5-mm-diameter 62-mm exposed-length rod of Er,Tm,Ho:YAG to achieve a quasi-cw lasing of the Ho ion at an average power of 12 W. The solar radiation was chopped at a 20% duty cycle to avoid overloading of the cooling system. The peak power output was more than 65 W during the choppers open times. The slope efficiency is 3.8%, and the threshold input energy is approximately 100 W. The laser was operated for long times (up to hours) while maintaining its performance. This is, to our knowledge, the first directly solar-pumped laser operating at 77 K.

Passive Q-switching in Nd:YAG/Cr4+:YAG monolithic microchip laser

Abstract Composite laser devices of passively Q-switched Nd:YAG were prepared by optical contacting between Nd:YAG and Cr,Ca:YAG crystal wafers followed by prolonged heating at elevated temperatures. Heating of the composite devices under reducing and/or oxidizing environments allowed to control the Cr4+ ion concentration in the Cr,Ca:YAG, thus affecting its absorption saturation behavior. Optical absorption saturation measurements on partially reduced Cr,Ca:YAG crystal were performed. Residual absorption of the saturable absorber at 1064 run results from the Cr4+ ion excited-state absorption. Laser damage threshold at the gain/absorber interface of the composite device, 14.7 J/cm2, is higher than at the entrance face. The device thus obtained was end-pumped by a fiber-optic-coupled diode laser, and exhibited short (∼5 ns), high repetition-rate pulsing.

Some optical properties of Cr4+-doped crystals

We summarize briefly some of our past studies, and report of preliminary recent new results concerning the optical properties of Cr 4+ -doped crystals that are important for utilization as passive Q-switching devices in Nd:YAG laser systems. The host crystals involved are YAG, YSGG, GGG, LuAG and forsterite. Excited-state lifetimes of 4.0, 1.0 and 1.7 μs were measured by the Cr 4+ fluorescence decay (1.3-1.7 μm) following pulsed excitation at 1064 nm in YAG, YSGG and GGG, respectively. The ground- and excited-state absorption (ESA) cross-sections at λ = 1064 nm were estimated from transmission saturation measurements. For [Cr 4+ ,Mg 2+ ]:YAG the respective results were σ gs = (3.25 ± 0.15) × 10 -18 cm 2 and σ es = (6.25 ± 0.5) × 10 -19 cm 2 . In the orthorhombic forsterite, the cross-sections were polarization dependent. We got σ gs < (3.3 ± 1) × 10 -19 cm 2 for E∥a∥, σ gs = (23 ± 2) × 10 -19 cm 2 and σ es = (9.0 ± 0.7) × 10 -19 cm 2 for E∥b, and σ gs = (16 ± 1) × 10 -19 cm 2 and σ es = (5.7 ± 0.4) × 10 -19 cm 2 for E∥c. Polarised ESA spectra were measured between 680 and 960 nm using the pulsed pump/probe technique. At 750 nm we got σ gs = (110 ± 10) × 10 - 19 cm 2 and σ es = (25 ± 3) × 10 -19 cm 2 for E∥b. Passive Q-switching performance of a flashlamp-pumped Nd:YAG laser using an intracavity [Cr 4+ ,Ca 2+ ]:GGG sample is also demonstrated.

Cr4+ doped garnets: novel laser materials and non-linear saturable absorbers

Abstract The use of the spectral properties of Cr 4+ -doped garnets as passive Q-switches for 1.06 μm laser for both pulsed and CW operating modes is addressed. A comparative research into the use of Cr 4+ -doped YAG, YSGG, and LuAG (Lu 3 Al 5 O 12 ) as saturable absorbers for the 1.06 μm emission of Nd:YAG laser has been conducted. Q-switched pulses with temporal duration ranging from 52 to 23 ns (FWHM) were observed for the different garnets under investigation. Cross-sections for ground state and excited state absorptions were evaluated by fitting our saturation data to Frantz-Nodvik equation. A complete analysis of the saturation transmission curves will be presented. The laser performance of Cr 4+ : YAG and Cr 4+ :LuAG (Lu 3 Al 5 O 12 ) lasing in the mid-IR (1.35–1.55 μm) pumped by the 1064 nm of Nd:YAG will also be reported.

Excited-state absorption at 1.57 μm in U2+:CaF2 and Co2+:ZnSe saturable absorbers

Abstract Appreciable excited-state absorption (ESA) in U2+:CaF2 and Co2+:ZnSe saturable absorbers was measured at λ=1.573 μ m by optical transmission versus light fluence curves of 30–40 ns long pulses. The ground- and excited-state absorption cross-sections obtained were (9.15±0.3)×10−20 and (3.6±0.2)×10 −20 cm 2 , respectively, for U2+:CaF2, and (57±4)×10−20 and (12.5±1)×10 −20 cm 2 for Co2+:ZnSe. Thus, ESA is not negligible in U2+:CaF2 and Co2+:ZnSe, as previously estimated.

Cr4+ doped garnets: their properties as non-linear absorbers

Abstract A comparative research into the use of Cr 4+ -doped YAG, YSGG and LuAG (Lu 3 Al 5 O 12 ) as saturable absorbers for the 1.06 μm emission of Nd:YAG laser has been conducted. Q-switched pulses with temporal duration ranging from 23 to 52 ns (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.

Visible solar-pumped lasers

Abstract We report on two visible solar pumped lasers; a red alexandrite laser with a potential tunability range between 700 and 818 nm, and a frequency doubled Nd:YAG laser operating at 532 nm. Highly concentrated solar energy was obtained by a 3-stage concentrator. An output power of up to 12 W was obtained from the alexandrite laser. The Q-switched Nd:YAG laser employed a type II KTP crystal for intracavity frequency doubling. Q-switching was obtained either passively or actively. In the passive Q-switching version, a water cooled Cr+4:YAG crystal was introduced into the laser cavity acting as both saturable absorber and laser rear mirror, and enforcing high repetition rate operation between 10 and 50 kHz. The average output power at 532 nm was 4.1 W. With an active acousto-optic modulator introduced between the laser rod and the high reflection rear mirror, the average output power at 532 nm and 7.3 kHz repetition rate was 8.7 W.

Passive Q switching of a solar-pumped Nd:YAG laser

Passive Q switching is a preferable choice for switching the Q factor of a solar-pumped laser because it requires neither a driver nor an electrical power supply. The superior thermal characteristics and durability of Cr(4+):YAG single crystals as passive Q switches for lamp and diode-pumped high-power lasers has been demonstrated. Here we report on an average power of 37 W and a switching efficiency of 80% obtained by use of a solar-pumped Nd:YAG laser Q switched by a Cr(4+):YAG saturable absorber. Concentration of the pumping solar energy on the laser crystal was obtained with a three-stage concentrator, composed of 12 heliostats, a three-dimensional compound parabolic concentrator (CPC) and a two-dimensional CPC. The water-cooled passive Q switch also served as the laser rear mirror. Repetition rates of as much as 50 kHz, at pulse durations between 190 and 310 ns (FWHM) were achieved. From the experimental results, the saturated single-pass power absorption of the Cr(4+):YAG device was estimated as 3 ? 1%.

Depolarization induced by pump edge effects in high average power laser rods

Non-radially symmetric residual birefringence in laser rods due to pump edge effects is analyzed both theoretically and experimentally. For cubic crystals such as yttrium aluminum garnet (YAG), this depolarization has a unique sixfold symmetry that is closely connected to the crystals cubic symmetry. While this depolarization is small compared to that observed with linear or circular polarizations, it is measurable and important when using radial or azimuthal polarizations in rods generating heat powers in excess of 70 W/cm. A simple criterion was defined in order to help estimate the amount of depolarization in a high-power laser rod system.

Solar pumped solid state laser program

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.