Near-diffraction-limited 561 nm self-frequency-doubled laser based on a millimeter-scale Nd:GdCOB crystal

We report, to the best of our knowledge, the first continuous-wave 561 nm self-frequency-doubled (SFD) laser based on a millimeter-scale Nd:GdCOB crystal under diode pumping. To realize this compact configuration, the 1.5 (length) × 1.0 × 0.7 mm³ Nd:GdCOB crystal was directly coated on both end faces to form a monolithic resonator, combining laser gain and nonlinear frequency conversion within a single 1.05 mm³ crystal. With this integrated configuration, a maximum output power of 10.23 mW at 560.9 nm was obtained with the crystal temperature set to 25℃. Despite the millimeter-scale crystal size, the laser maintained near-diffraction-limited beam quality at the maximum-output-power operating point, with Mx² = 1.20 and My² = 1.16. The 561 nm output power remained relatively high over a broad range of crystal temperatures near the optimum, while still exhibiting a dependence on crystal temperature. Long-term measurements further showed that the beam remained stable after thermal equilibration and subsequently reached a new stable state following a change in crystal temperature. Polarization measurements further indicated a substantial linearly polarized component in the 561 nm output. Together, these results demonstrate that the millimeter-scale Nd:GdCOB crystal enables compact 561 nm SFD operation while maintaining near-diffraction-limited beam quality and stable laser performance, providing a promising route toward miniaturized visible laser sources for compact photonic systems.