We present a continuous, narrow-linewidth, tunable laser system that outputs up to 14.0 W at 770 nm. The light is generated by frequency doubling 18.8 W of light from a 1540 nm fiber amplifier that is seeded by a single mode diode laser achieving >74% conversion efficiency. We utilize a Lithium Triborate Crystal in an enhancement ring cavity. The low intensity noise and narrow linewidth of the 770 nm output are suitable for cold atom experiments.Near-infrared(NIR) lasers around 760 -780 nm have many applications including laser cooling and trapping of atoms, atomic state manipulation, and spectroscopy of Oxygen(760.8, 763.8 nm)[1], Potassium(766.7, 770.1 nm) [2] and Rubidium(780.2 nm) [3]. Iodine I 2 rovibrational lines[4] at 770.7 nm provide excellent frequency references. In the context of quantum computing, magic wavelengths for optical trapping of cold alkali atoms [5, 6] near this wavelength exist, and the light can be used to rapidly drive Raman transitions between Potassium and Rubidium hyperfine ground states.Common diode lasers at these wavelengths are typically power limited up to ∼ 100 mW. Semiconductor optical amplifiers can boost the power to ∼ 3 W, although typically with a poor spatial profile that reduces the usable power when diffraction-limited performance is required. Furthermore, amplified spontaneous emission generates incoherent frequency noise extending over tens of nm, which leads to significant reduction of atomic coherence when the light interacts with atoms. For atom trapping applications at IR wavelengths, fiber lasers are typically used for their high optical power and low frequency noise [7].An alternative route to high power tunable sources for atomic spectroscopy is based on second harmonic generation (SHG) or sum frequency generation(SFG)[8] of a high power fiber laser. This approach benefits from the availability of tunable, single mode laser diode sources, and high power fiber amplifiers that utilize mature industrial technology in telecom wavelength bands. A variety of nonlinear crystals have been explored for this application. Early efforts utilized the high nonlinearity of periodically-poled Lithium Niobate(PPLN) to achieve high power output[9-15] without introducing an enhancement cav-ity. Examples include 11 W at 780 nm for Rb cooling[9], 313 nm for Be ion cooling and quantum gates[10], and 319.8 nm for metastable Helium trapping[11]. Other efforts have utilized Magnesium Oxide doped PPLN (PPMgO:LN) for Rb/Cs cooling[16] and K cooling[17]. Periodically-poled Potassium Titanyl Phosphate(PPKTP) in a Fabry-Perot enhancement cavity was used to achieve 95% conversion efficiency[18] and an output power of 1.05 W at 775 nm.Periodically-poled crystals are engineered to exhibit high optical non-linearity utilizing quasi-phase matching. Typically a single or double pass interaction is sufficient to reach high conversion efficiency with high power sources. Despite their high optical nonlinearity, these crystals are prone to thermal effects [19][20][21][22] leading to photorefracti...
