Fiber lasers have attracted substantial research interest due to their high stability, excellent beam quality and system compactness. Furthermore, lasers generating high-energy ultrafast pulses and operating at the 1 550 nm region are widely developed due to the low optical attenuation at the first communication window and more cost-effective than other laser sources in a variety of applications such as ultrafast spectroscopy, precision material processing and terahertz-wave generation. To achieve high-energy pulses, an Erbium-doped fiber amplifier was employed to amplify seed pulses. However, pulses will accumulate large nonlinear effects such as Self-Phase Modulation (SPM) and Stimulated Raman Scattering (SRS) during direct amplification, thus degrading the pulse quality. One common solution is to widen the pulse width by introducing a chirp before amplification. The peak power intensity is significantly attenuated, avoiding excessive nonlinearity. The amplified pulse is then de-chirped by a compressor. This method is called Chirped Pulse Amplification (CPA). Several high-power CPA systems operating at 1.56 mu m have been demonstrated in recent years. However, all of these sources produced a pulse with spectral width between 5 nm and 15 nm. Broadband fiber laser plays an important role in optical frequency combs, optical coherent tomography, optical coherence radar and fiber optical sensing systems. There is a lack of high-energy devices capable of generating pulses with spectral width above 30 nm. Several approaches have been utilized to generate broadband pulses. A noise-like mode-locked fiber laser was demonstrated based on the precise adjustment of intracavity dispersion. However, this laser regime was seldom applied in ultrashort pulses due to its incompressibility. A Mamyshev oscillator is able to generate broadband pulses as shorter than 100 fs at the expense of complicated intracavity structure and accurate pulse evolution. The extra-cavity generation method relies on Highly Nonlinear Fibers (HNLFs), such as photonic crystal fibers, whose complexity of design is increased by demanding careful selection of parameters for the seed pulse. In addition, the nonlinear effect induced by SPM generates a nonlinear chirp on both sides of pulses which degrades the beam quality in CPA systems. Note that self-similar pulses are nonlinear optical structures whose amplitudes and widths could be altered by dispersion, nonlinearity, gain and other system parameters, while maintaining the overall shapes. Since the self-similar pulse has a strict linear frequency chirp induced by the balance between SPM and normal group velocity dispersion in the erbium-doped fiber, it could be effectively compressed by grating pairs to obtain a high-power ultrashort pulse. Therefore, the combination of self-similar amplification and CPA is a promising solution to generating broadband watt-level pulse. High-energy ultrafast pulses based on parabolic evolution in ytterbium-doped lasers have been reported. Nevertheless, the Erbium-Doped Fiber Amplifier (EDFA) based on self-similar amplification operates at an anomalous dispersion region, which is less applicable to generating pulses with the average power above watt-level high-energy pulses comparing to Ytterbium-Doped Fiber Amplifier (YDFA). At the same time, high-energy CPA systems operating at 1 550 nm significantly lag behind Yb-doped lasers due to high quantum defect, thermal effects and nonlinearity. At present, there is no report on a broadband high-energy CPA system based on parabolic evolution operating at 1 550 nm. Here, we demonstrated an all-fiber Er-doped chirped-pulse amplification laser, which generates Watt-level broadband pulse with the application of self-similar amplification. Numerical simulations of the model laser were performed by following the propagation of the pulses and considering every action of cavity components on the pulses. We use the results of one round-trip circulation as the input of the next round of calculation until the optical field becomes self-consistent. For this context, pulse propagation equation is given by the nonlinear Schrodinger equation. The parameters of each element of the laser are optimized according to theoretical simulations. In our experiment, the seed source is a dispersion-managed passively mode-locked fiber laser with a Gaussian-spectral profile, which evolves into a parabolic shape after self-similar amplification, achieving a broadband pulse bandwidth with the full-width at a half-maximum of 44.8 nm under 400 mW pump power. The spectral width and energy of the pulse increase rapidly during amplification. The pulses are stretched in Dispersion Compensating Fiber (DCF) to reduce peak power, avoiding excessive nonlinearity. Then a Double-Clad Er/Yb co-Doped Fiber (DC-EYDF) is used as the main amplifier. The spectral width of the pulse is narrowed down to 30 nm with the effect of gain filtering during amplification. The pulse is amplified to 1.3 W with the pump power of 9 W. The amplifier delivers 32 nJ pulses at a repetition rate of 40.1 MHz, which can be compressed down to 587 fs through a pair of transmission gratings. We believe that the narrower pulses could be achieved by switching to fiber Bragg gratings to adjust the dispersion between the stretchers and compressors precisely. The robust, broadband, and watt-level 1 550 nm fiber laser source can be used for nonlinear frequency conversion, solar cell micromachining and ophthalmology due to its compact size.
