Objective Angular-resolved photoemission spectroscopy (ARPES) is a powerful and unique technique used to study the electron structure in condensed matter. It can directly obtain the energy band structure and electron self-energy by momentum resolution. A 1024 nm laser can be used as an ARPES source after multiple frequency doubling, such as the measurement of charge density wave gaps of telluride of rare earth elements. The main emission peak of Yb 3+ ions is near 1064 nm; hence, the emission of the 1024 nm pulse laser is relatively difficult, requiring the fine control of the filter parameters. Thus far, reports on the 1024 nm laser are mainly concentrated on solid-state lasers. A fiber laser has the advantages of compact structure, low cost, and good stability. The low output power of the fiber laser can be made up by the amplifier, which has a good application value.
Methods In this study, pulse evolution in a fiber laser was numerically simulated by the system of the coupled Ginzburg-Landau equation and solved using the split-step Fourier method. The laser passed through each device in turn in the cavity and repeatedly circulated. The pulse formation and evolution were simulated. Experimentally, the seed source was an all-normal dispersion Yb-doped fiber laser mode-locked by nonlinear polarization rotation (NPR). The oscillator consisted of a 976 nm pump, a 980/1030 nm wavelength division multiplexer (WDM), a piece of 54 cm Yb-doped fiber (YDF), an isolator polarization-independent isolator, an output coupler, and an NPR saturable absorber. A master oscillator power amplifier system based on a tunable fiber filter was built to realize the 1024 nm output laser. The seed pulse was filtered by a tunable filter. Subsequently, preamplification and amplification experiments were conducted. The preamplifier contained a pump, a WDM, a piece of YDF, and an isolator, while the amplifier comprised a pump, a WDM, and a piece of YDF. The laser was output by a collimating isolator. The output laser parameters were measured by a 12.5 GHz photoelectric detector, a 6 GHz oscilloscope, a spectrograph (Yokogawa, AQ6370C), an autocorrelator (FR-103XL), and a frequency spectrograph (Agilent, E4447A). The seed laser formation was the NPR mechanism realized by the artificial saturable absorber formed by the wave plate, band pass filter, and polarization beam splitter. The NPR belongs to the fast saturation absorber mode locking mechanism; thus, it was more conducive for the ultrashort pulse generation.
Results and Discussions The dissipative soliton pulses can be generated when the simulated model parameters are appropriate (Fig. 2). The pulses gradually formed with the accumulation of the intracavity energy. Figures 2(b) and (c) depict the evolution of the spectral and pulse shapes at different locations in the cavity. The output pulse of the oscillator was a dissipative soliton with a steep edge spectrum. The established process of the mode-locked pulse was studied herein. The pulse establishment process included the peak power improvement, high peak power, and stable mode-locked stages. The experimental results were consistent with the theoretical simulation results. The seed laser generated by the oscillator can generate a pulse laser with different central wavelengths through the tunable filter (Fig. 6(a)). The tunable range of the central wavelength was 1022--1030 nm. We set the central wavelength of the tunable filter to 1024 nm. The pulse output power was greatly reduced because of the spectral filtering; hence, a preamplifier was added after the tunable filter. To obtain pulses with more concentrated spectral energy and higher peak power, the pump power of the amplifier was set to 200 mW, and the output power of the preamplifier was set to 42.02 mW. Figure 8 shows the output pulse characteristics of the main amplifier. When the pump power of the amplifier was 5 W, the stable output mode-locked laser was obtained with a central wavelength of 1024 nm, an average power of 1.1 W, a pulse energy of 51 nJ, a repetition frequency of 21.5 MHz, and a signal-to-noise ratio of 67.5 dB.
Conclusions This study theoretically simulated the evolution process of the dissipative soliton pulse in the NPR mode-locked Yb-doped fiber laser based on the Ginzburg-Landau equation. We experimentally obtained the dissipative soliton pulses and studied the establishment process of the mode-locked pulses. The results were basically consistent compared with the theoretical simulation results. A series of pulses with 1020--1030 nm central wavelength were obtained by passing the seed laser pulse through a tunable filter. The laser with a 1024 nm central wavelength was selected as the seed laser for amplification. The laser with 8.55 ps pulse width and 42.02 mW average output power was obtained after the preamplifier. After the main amplifier, when the pump power of the amplifier was 5 W, the output average power was 1.1 W, the pulse energy was 51 nJ, and the central wavelength was 1024 nm. A useless wavelength was found in the spectrum, which was mainly caused by the limited filtering effect of the tunable filter. However, compared with the current experimental research reports, the spectral output width of this amplifier is significantly narrowed, and further experiments on spectral compression are under way. The results are helpful in understanding the dynamic characteristics of dissipative soliton mode locking in the NPR mode-locked fiber laser. Furthermore, the pulse laser at this wave band is expected to be applied in ARPES measurement.
