We develop a high-power Er:strengthened-glass laser emitting at the eye-safe 1.535 μm wavelength. To maximize the output parameters of an Er:glass laser, whose power supply must also be used with Nd:YAG, a two-lamp laser head with parallel power forming networks (PFNs) that could be fired with an internal delay is used. The system operates at a slope efficiency of 0.45%, yielding a maximum pulse energy of 330 mJ. Thermal effects are measured, indicating strong thermal lensing, of 16 diopter/kW and mild birefringence induced depolarization of 5% at 200 W electrical power. From this data we determine the radial-tangential average and birefringence elasto-optical coefficients, Cr,ϕ=0.075±0.002 and CB=0.0094, respectively. A TEM00 beam is achieved for a hemispherical resonator configuration. Based on the measurements, a design is presented for improved high-power TEM00 performance using principles based on dynamic compensation of thermal focusing and birefringence correction using a reentrant resonator. © 1998 Society of Photo-Optical Instrumentation Engineers.