# Compact 15 mJ Fiber-Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper
## 3 Experimental Results and Discussion  
### The Spectral Characteristics of Yb-Doped Fiber Master Oscillator  
To match the gain linewidth of the fiber seed source and the Nd:YAG laser amplifier, we use a variety of fiber components to optimize output spectrum, including a fiber Bragg grating (FBG), a bandpass filter (BPF), and a pulse stretcher. The 3 dB bandwidth of FBG at 1064 nm is 0.3 nm. Figure 2a shows the output spectra of the mode-locked fiber laser oscillator and the single-mode fiber preamplifier (preamplifier 1), which were measured using an optical spectrum analyzer (YOKOGAWA-AQ6370D). It illustrates that the spectrum width of fiber laser can be optimized by using the filtering effect of FBG, such that the spectrum after mode-locked oscillator has no significant distortion. Then, the BPF is used to filter the amplified spontaneous emission and residual pump light at 976 nm. After the preamplifier 1, the output central wavelength is 1064.1 nm, and the 3 dB bandwidth is about 0.13 nm. For the Yb-doped all-fiber master oscillator without a pulse stretcher, the output spectrum of preamplifier 2 is shown in Figure 2b. During the power amplification, the power spectrum of preamplifier 1 is gradually broadened. However, the output spectrum of preamplifier 2 under the pump power of 1.5 W is sharply broadened, accompanied by distortion of the spectral profile. The 3 dB spectral bandwidth is broadened from \(\sim\)0.12 nm to \(\sim\)2.43 nm during amplification, and the peak of the spectrum moves to the short wavelength. The main reason of spectrum broadening is due to the self-phase modulation (SPM) effect. SPM is a nonlinear effect produced by picosecond pulsed lasers in the amplification, especially in fiber lasers. The SPM-induced spectrum broadening is proportional to the maximum nonlinear phase shift \(\phi_{max}\), which is given by \(\phi_{max}=\gamma P_{0}L_{eff}\), where \(\gamma\) is the nonlinear coefficient, \(P_{0}\) is the peak power of the incident pulses, and \(L_{eff}\) is the effective length of the fiber [30]. Therefore, the nonlinear phase shift is proportional to the intensity of the incident pulses, which can be significant accumulated in high-peak-power lasers. The broadened spectral bandwidth is mismatched with the gain bandwidth of Nd:YAG, which reduces the amplification efficiency and limits the application of picosecond laser source. To solve the abovementioned problem, we decided to adopt a pulse stretcher to  
Figure 2: Spectrum distribution of (**a**) oscillator and preamplifier 1, (**b**) preamplifier 2 at pump power of 1.5 W without a pulse stretcher, and (**c**) preamplifier 2 at different pump powers with a pulse stretcher. decrease the peak power of the incident pulses, thereby suppressing the degree of spectral broadening. The pulse stretcher is a single-mode passive fiber with a length of 1350 m, which has a group velocity dispersion (GVD) of 20 ps\({}^{2}\)-km\({}^{-1}\) (\(\beta_{2}\)) and a nonlinear coefficient of 0.91 W\({}^{-1}\)-km\({}^{-1}\) (\(\gamma\)) at 1064 nm. Figure 3 shows the measured pulse temporal profiles before and after broadening. The temporal shape before stretching is Gaussian, and the pulse width is approximately 40 ps measured by autocorrelator (Femtochrome, FR-105 XL), as shown in Figure 3a. The hundred-picosecond pulse width of all-fiber master oscillator is monitored by a high-speed photodiode (ALPHALAS, rise time: 35 ps) and a digital oscilloscope (LyCroy, bandwidth: 13 GHz). The pulse width is stretched from 40 ps to \(\sim\)341 ps, which is shown in Figure 3b. The measured pulse shape is characterized by a steep front and a smooth back edge. This is because of the gain saturation effect, whereby the number of inversion particles consumed at the front edge is higher than that at the back edge. Hence, the phenomenon of larger front-edge gain and smaller back-edge gain appears. The corresponding variation in the spectrum distribution of the all-fiber master oscillator with pulse stretcher is shown in Figure 2c.