We describe a diode-pumped Yb:YAG laser that produces 1080 W of power cw with 27.5% optical optical efficiency and 532 W Q-switched with M(2)=2.2 and 17% optical-optical efficiency. The laser uses two composite Yb:YAG rods separated by a 90 degrees quartz rotator for bifocusing compensation. A microlensed diode array end pumps each rod, using a hollow lens duct for pump delivery. By changing resonator parameters we can adjust the fundamental mode size and the output beam quality. Using a flattened Gaussian intensity profile to calculate the mode-fill efficiency and clipping losses, we compare experimental data with modeled output power versus beam quality.

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A scaleable diode end-pumping technology for high-average-power slab and rod lasers has been under development for the past several years at Lawrence Livermore National Laboratory (LLNL). This technology has particular application to high average power Yb:YAG lasers that utilize a rod configured gain element. Previously, this rod configured approach has achieved average output powers in a single 5 cm long by 2 mm diameter Yb:YAG rod of 430 W cw and 280 W q-switched. High beam quality (M 2 =2.4) q-switched operation has also been demonstrated at over 180 W of average output power 1 . More recently, using a dual rod configuration consisting of two, 5 cm long by 2 mm diameter laser rods with birefringence compensation, we have achieved 1080 W of cw output with an M 2 value of 13.5 at an optical-to-optical conversion efficiency of 27.5%2 . With the same dual rod laser operated in a q-switched mode, we have also demonstrated 532 W of average power with an M 2 <2.5 at 17% optical-to-optical conversion efficiency. These q-switched results were obtained at a 10 kHz repetition rate and resulted in 77 nsec pulse durations. These improved levels of operational performance have been achieved as a result of technology advancements made in several areas that will be covered in this manuscript. These enhancements to our architecture include: (1) Hollow lens ducts that enable the use of advanced cavity architectures permitting birefringence compensation and the ability to run in large aperture-filling near-diffraction-limited modes.(2) Compound laser rods with flanged-nonabsorbing-endcaps fabricated by diffusion bonding. (3) Techniques for suppressing amplified spontaneous emission (ASE) and parasitics in the polished barrel rods.

We describe a diode-pumped Yb:YAG laser producing 1080 W cw with 27.5% optical-optical efficiency and 532 W Q-switched with M2 =2.2 and 17% optical-optical eficiency.The laser uses two composite Yb:YAG rods separated by a 90 degree quartz rotator for bifocusing compensation. A microlensed diode array end-pumps each rod using a hollow lens duct for pump delivery. By changing resonator parameters, we can adjust thefindamental mode size and the output beam quality. Using a flattened gaussian intensity profile to calculate the modefill eficiency and clipping losses, we compare experimental data to modeled output power vs beam quality.High-average-power laser operation with good beam quality has long been an active area of development. Power oscillators offer greater simplicity compared to master-oscillator power-amplifier (MOPA) systems, but the finite TEMoa mode size possible' limits power scaling or results in multimode operation with lower beam quality. Considerable effort has been devoted to studying laser resonators to achieve high power and brightness.2T3A promising method to improve power scaling and reduce thermal effects is to minimize heat generation by the use of Yb3+ doped materials.4'5 lower heating rates compared to Nd3+, Although Yb3+ offers the quasi-three level nature of the laser transition requires careful engineering to achieve high power and efficiency."77%8 One of the challenges of designing a Yb3+ system is delivering pump light of sufficient intensity. In the diode-array end-pumped system described here, pump intensities >50 kW/cm2 enable efficient laser operation far above threshold. Correspondingly, high-average-power operation is achieved with a relatively small cross-section laser rod. This enables the laser rod to be the mode-limiting aperture in order to obtain good beam quality with high efficiency.Recently, very high brightness operation has been demonstrated from Nd:YAG laser oscillators incorporating a symmetric cavity with two identically pumped laser rods separated by a 90 degree quartz rotator for bifocusing compensation.' Here, we describe a similar resonator using two Yb:YAG laser rods. with a hollow lens duct for pump delivery," Diode-array end-pumping is used with the laser beam passing through each hollow lens duct and the two halves of the diode array.The composite laser rods have a 1 x 102' cme3 Yb-doped section, nominally 2 mm in diameter by 50 mm long, and flanged, undoped end caps. For this system, the laser rod diameter tapers along the length for parasitic suppression. l1 The diode arrays are made up of microlensed diode bars 1.5 cm long, mounted on silicon microchannel heatsink packages. l2 Each diode array contains 38 packages, resulting in > 4.56 kW total pump power. The diodes were operated with a cooling water temperature of 30 C to obtain the desired pump wavelength. The cooling water temperature of the laser rods was varied between 0 and 20 C. The resonator is symmetric, with the two laser rods separated by the 90 degree quartz rotator and a negative le...