Development of a Lamp-Pumped Cr:LiSAF laser operating at 30 Hz

Cr:LiSrAlF6 crystals area a very interesting laser medium due to its spectroscopic characteristics, presenting a broad emission band in the near infrared and being able to be pumped either by flashlamp or by diodes. Up to now, its limitation is mostly due to its poor thermal properties that limit the laser performance either in the repetition rate in pulsed system or output power in CW systems. We have designed and constructed a flashlamp pumped laser using a standard rod pumping cavity that avoided most of the heat generated in the pumping process and allowed the operation in a fairly high repetition rate of 30 Hz with a high average power of 20 W in a conservative operation mode. Introduction Single crystals of Cr:LiSAF (Cr:LiSrAlF6) show very attractive optical spectroscopic properties 1 for a potential laser medium, such as a long lifetime of the upper laser level (~67 μs) at room temperature ,three broad absorption bands and a wide emission band ranging from 650 nm to 1050 nm. Laser action was demonstrated under several pumping schemes 3, 4, , particularly in CW and pulsed regimes. Pulse durations ranging from hundreds of microseconds under free-running pulsed excitation down to nanoseconds in QSwitching and few femtoseconds in Mode-Locking regime were achieved. Flashlamp-pumped Cr:LiSAF tunable lasers 8, 9,10 have been developed reaching pulse energies up to 8.8 J, and flashlamp pumped ultrashort pulse amplifiers 12, 13, 14 reached peak powers up to 8.5 TW. Due to the poor thermal properties of the LiSAF host, the operation repetition rate of these lasers/amplifiers were always confined either to the single pulse regime or up to 12 Hz. The low thermal conductivity leads to crystal cracking due to thermally induced stress, and in the case of a gain medium in the shape of a rod, fracture was observed at 18 Hz. Besides the thermal induced stress that leads to fracture, the lifetime of the Cr:LiSAF laser transition is strongly temperature dependent, dropping from ~67 μs at room temperature to half this value at 69°C, due to thermal quenching. Under flashlamp pumping, the low LiSAF thermal conductivity prevents heat extraction from the laser medium, and if its temperature rises above ~25°C, the nonradiactive decay generates more heat, what in turn increases the nonradiactive decay rate, rapidly increasing the crystal temperature in a catastrophic process that reduces the energy storage capacity of the crystal and can lead to fracture. In order to avoid thermal quenching and crystal fracture due to accumulated heat, flashlamp pumped Cr:LiSAF oscillators have been kept operating at low repetition rates. Shimada et al. reported the highest repetition rate and power on a Cr:LiSAF laser to be 4.5 W at 12 Hz, and Perry et al. reported the highest amplifier repetition rate to be 10 Hz. Alternatively, a slab geometry laser scheme requires small thickness of the gain medium, allowing for better heat extraction and therefore a lower stress in the gain medium and in this case the laser achieved pulse energies as high as 8.8 J, but at 5 Hz repetition rate. Aiming to rise the repetition rate of flashlamp pumped Cr:LiSAF lasers and still keeping its gain and power, we propose a different approach that minimizes the crystal thermal load and temperature gradient by decreasing the heat reaching the gain medium and being generated inside it. This scheme allowed the laser operation at repetition rates as high as 30 Hz, with an average power of 20 W. Experimental Setup We developed a flashlamp pumped pumping cavity, aiming to minimize the rod thermal load and to increase the laser repetition rate. The rod has a 1.5mol% Cr doping, 101.6 mm of length and 6.35 mm of diameter, with Brewster angled faces. The cavity has two 4" arc-length, 7 mm bore, 450 torr Xenon flashlamps, each one independently fed by a power source capable of delivering up to 50 J in ~67 μs pulses. The pulse width was chosen in order to match the laser transition lifetime, consequently decreasing heat generation by pump energy that is lost to spontaneous emission. The cavity is a closed coupled one, with an alumina diffuse reflector, and cooled by deionized water at 11°C in turbulent flow regime. The humidity in the laboratory is kept under 40%, lowering the dew point, avoiding water condensation on the rod surfaces. XXIX ENFMC Annals of Optics 2006 The temperature of the laser medium inside a pumping cavity is determined by how much energy is absorbed by the medium, and the amount of that energy that is not converted into light emission (spontaneous or stimulated), and how this excess energy is extracted. The main heat source for the Cr:LiSAF crystal is the Stokes-shift from the three absorption bands centered at 290 nm, 450 nm and 650 nm to the emission band at 850 nm. For a photon absorbed at the center of the 290 nm band resulting in an emitted photon at 850 nm, about 65% of its energy is converted into heat due to the Stokes-shift. For photons absorbed at the center of the 430 nm and 650 nm bands, these fractions are 50% and 24%, respectively. With the purpose of minimize the heat in the rod, optical filters were inserted into the pumping cavity between the rod and each one of the flashlamps, absorbing all light below 600 nm and above 700 nm. In this way, only the 650 nm band is excited, decreasing the Stokes-shift generated heat to only ~25% of the absorbed power. The pumping cavity was designed in a way that the optical filters divide it in three compartments, isolating the rod from the flashlamps, allowing independent coolant flow around each component. The cooling water flows around the rod, and then refrigerates the flashlamps. Thus, heat transfer from the flashlamps to the rod by the cooling water is avoided. The optical resonator was built using a 1 m curvature radius concave High-Reflector mirror located 21 cm away from one rod end, and a plane output coupler located 12 cm from the other end of the rod, resulting in a stability product g1·g2=0.57 for an empty resonator. Plane output couplers with reflections ranging from 55% to 96% at 850 nm were used to characterize the laser performance Results and Discussions The laser output pulse energy as a function of one of the flashlamps input energy, for eight different output coupler reflectivities (ROC), is shown in figure 1. In Table 1 we present the total (2 flashlamps) pump energy threshold for laser action for each output coupler and the corresponding slope efficiencies. The maximum total measured efficiency is 0.65% for ROC=89.3% and 100 J pumping energy. The laser emission is centered at 851 nm, with a bandwidth of 6.4 nm, and the pulse duration is 65 μs .