## 2 Experimental Setup  
As depicted in Figure 1a, the femtosecond laser system at 515 nm consisted of two parts: a home-made 20-MHz Yb-doped master-oscillator-power-amplifier (MOPA) laser system and the SHG module. The central wavelength of the MOPA laser system was 1030 nm, and the maximum output power can boost up to 2 W with pulse duration of 280 fs (nearly Fourier transform limited). A 5 mol% MgO:PPLN (CTL Photonics Corp. Ltd., Fujian, China) with 6.3 \(\upmu\)m poling period, 3 mm length, and 2 \(\times\) 0.5 mm\({}^{2}\) aperture area was adopted in the experiment. Both surfaces of MgO:PPLN crystal were anti-reflection coated at 515 nm and 1030 nm. LEN1 and LEN2 were both 50 mm lenses, but with B-coating (650-1050 nm) and A-coating (400-700 nm), respectively. Two plane mirrors (M1 and M2) with high reflection coating (1020-1120 nm) were used to adjust the position of the focused beam. A polarizing beam splitter (PBS) and a half-wave plate (HWP1) were composed to control the incident fundamental average power. Another half-wave plate (HWP2) was used to adjust the polarization state of incident beam before MgO:PPLN crystal. The MgO:PPLN was placed in an oven for controlling the temperature. Following LEN2, two dichroic mirrors (DM1 and DM2) were employed to separate the 515 nm light and the residual 1030 nm fundamental light. The whole laser system was integrated into a portable home-made aluminum alloy case, to avoid any potential environment disturbance for a long-term power monitoring. Figure 1b shows the configuration of the fiber laser. The all-PM NALM oscillator was operated in all normal dispersion regime [27], and a 1030 nm fiber Bragg grating with 0.8 nm bandwidth (Raysung Photonics Inc., Shaanxi, China) was employed as a spectral filter. As the pump power increased to \(\sim\)300 mW, the mode-locked pulses were stably generated with an average power of \(\sim\)5 mW. The optical spectrum of the oscillator was recorded by an optical spectrum analyzer (Yokogawa, AQ6370C, Shanghai, China) with a resolution of 0.02 nm, shown as the pink curve in Figure 2a. Its central wavelength was 1029.9 nm, and the 3-dB spectral bandwidth was 0.18 nm, corresponding to the transform-limited pulse duration of \(\sim\)8.8 ps for the Gaussian shape pulse. Then, the average power of the laser was first amplified to \(\sim\)160 mW by a YDFA. By using a segment of 15-m-length PM single-mode  
Figure 1: (**a**) Experimental configuration of the SHG module. (**b**) Schematic diagram of the MOPA laser system. Oscillator: all-PM fiber oscillator; YDFA: Yb-doped fiber amplifier; ISO1 and ISO2: optical isolators; LD: laser diode; Combiner: pump-signal combiner; PCF: Yb-doped photonic crystal fiber; Compressor: pulse compressor. fiber (PM980-XP, Nufern) for spectral broadening, the 3-dB spectral bandwidth extended to 10.3 nm at central wavelength of 1029.9 nm attributed to the self-phase modulation (SPM) effect, shown as the blue curve in Figure 2a. The main amplifier was composed of a segment of 1.5 m long PCF (DC-135/14-PM-Yb, NKT Photonics, Shenzhen, China), a combiner (AFR Inc., Guangdong, China), and a 9 W diode laser (K976 AB2 RN-9.00W, BWT Inc., Beijing, China). Figure 2b shows the slope efficiency of the main amplifier (red dot line). With 9 W pump power, the main amplifier produced up to 3.36 W average power, corresponding to a slope efficiency of \(\sim\)39%. The violet curve in Figure 2c shows the amplified optical spectrum with 3.36-W output power, and no obviously spectral broadening is observed comparing to the blue curve in Figure 2a. The Fourier transform limited duration of that was about 266 fs. A 1030 nm free-space optical isolator (ISO2, AFR Inc.) with 85% transmission efficiency was employed to block the backward light. A pair of transmission gratings of 1250 lines/mm (PCG-1250-10 XX-986, Ibsen Photonics, Farum, Denmark) with totally 70% efficiency was used to compress the pulse from 12.5 ps to 280 fs with a 2 W average power for the compressed pulse (also named as incident fundamental laser).