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In this paper, we measure and analyze the pump power and wavelength dependent gain of an optically pumped infrared 1050-nm vertical external-cavity surface-emitting laser vertical external-cavity surface-emitting laser (VECSEL) developed for an application as a frequency doubled green laser in mobile projectors. Increasing the reflectivity of the chip coating from 3% to 20% for 1050-nm is found to result in a maximal gain of 7.9%, more than twice as high as that of the 3% coating, and in a spectral narrowing of the wavelength-dependent gain as a consequence of the enhanced coupling between the optical field and the quantum well gain medium. From our gain measurements, the wavelength dependent laser threshold can directly be obtained and compared to the VECSEL device characteristics. A very good agreement between the data extracted from the gain measurements and the measured laser threshold in VECSELs with different output mirrors is found.