Cavity-enhanced frequency doubling for preparing the 455.6  nm single-frequency blue laser: LBO versus PPKTP

The 455.6 nm single-frequency blue laser corresponds to the cesium $6{{\rm S}_{1/2}}-7{{\rm P}_{3/2}}$ transition line, which is crucial for applications such as high-resolution cesium spectroscopy, cesium ladder-type Rydberg excitation systems (455.6 nm + 1060 nm), and cesium laser cooling and trapping. In this study, a continuous-wave Ti:sapphire laser (911.2 nm) was used as the fundamental source, and a bow-tie-type four-mirror ring frequency-doubling cavity combined with Hänsch-Couillaud (HC) locking technology was constructed to stabilize the doubling cavity. We systematically investigated the core parameters (output power, beam quality, and power stability) of 455.6 nm single-frequency blue lasers generated via cavity-enhanced resonant frequency doubling using PPKTP and LBO crystals. The nonlinear coefficient (9.4 pm/V) and nonlinear conversion coefficient (1.03%/W) of Type-0 PPKTP are significantly higher than the nonlinear coefficient (0.84 pm/V) and nonlinear conversion coefficient ($8.4 \times {10^{- 5}}\% /{\rm W}$) of the LBO crystal. Under a mode-matched fundamental power of 318.3 mW, Type-0 PPKTP crystals achieve 130.1 mW of the blue laser output with a frequency-doubling efficiency as high as 40.8%, even when the input transmittance is below the impedance-matching condition. While PPKTP delivers excellent beam quality, its long-term stability is compromised by thermal effects. In contrast, LBO yields the 51.2 mW blue laser output with a 15.5% frequency-doubling efficiency under a 329.5 mW mode-matched fundamental power. Although LBO shows lower efficiency at low powers and requires prism-based beam shaping, it offers superior long-term stability.