Objective Fiber lasers have been widely used in scientific research, industrial processing, biomedical, and other fields because they offer unique characteristics such as good heat dissipation, high light-light conversion efficiency, and excellent beam quality. As a classic fiber laser, the erbium-doped fiber laser with a wavelength of 1.5 μm has been widely used in optical fiber communication, environmental monitoring, medical facility, precision spectroscopy, and other fields. Cascade amplification is a typical technique for achieving high-power pulsed lasers, but group velocity dispersion and the damage threshold of fiber materials attributed to nonlinear effects such as four-wave mixing and self-phase modulation, restrict the high-power ultrashort pulse output. To overcome this limitation, chirped pulse amplification has been proposed. The laser pulse is first stretched temporally, amplified, and compressed again. This method can effectively suppress phenomena such as self-focusing and pulse splitting caused by nonlinear effects and improve the peak power of laser pulse. In this study, a femtosecond laser amplification system based on all-polarization-maintaining erbium-doped fibers using broadband chirped fiber gratings was reported. Using chirped pulse amplification and group velocity dispersion optimization that combines dispersion compensation fibers with the chirped fiber grating pair with different dispersions, a femtosecond laser pulse with pulse width, peak power, and repetition rate of 63 fs, 35.2 kW, and 59.5 MHz, respectively, was experimentally obtained.
Methods The amplification system (Fig.1) proposed herein was divided into two parts, namely, the seed laser and two-stage cascade amplifier. First, an erbium-doped ring fiber laser mode-locked using a graphene saturable absorber was established. Then, the two-stage amplifier was connected to the back of the seed laser and a polarization-maintaining isolator was inserted between the two amplifiers. After preamplification using the first-stage amplifier with a forward pump, the preamplified pulse was stretched using the broadband chirped fiber Bragg grating (CFBG). Next, the group velocity dispersion was compensated for by using the polarization-maintaining dispersion compensation fiber. Subsequently, it was amplified using the main amplifier with a bidirectional pump. The amplified laser pulse was further compressed using another broadband CFBG. Subsequently, we qualitatively analyzed the main influence of the group velocity dispersion difference of the two CFBGs on realizing the high-power ultrashort pulse output.
Results and Discussions For seed laser, the laser was pumped with a 976 nm laser with 140 mW average power. The measured pulse width, spectral bandwidth, and average power of the seed laser output were 480 fs, 6.5 nm, and 4.8 mW, respectively. The time-bandwidth product was calculated to be 0.388, which is close to the Fourier transform limit. Moreover, the fundamental repetition rate with respect to the signal-to-noise ratio of 65 dB was measured to be 59.5 MHz (Fig. 4). For preamplified laser, the pulse width was 500 fs (Fig. 5). In the main amplifier stage, CFBG was used to expand the preamplified output laser pulse and the group velocity dispersion in the system was optimized using the dispersion compensation fiber. Finally, the amplified laser pulse was compressed by splicing the opposite port of another CFBG. The study found that the laser pulse was broadened by splicing Port 1 of CFBG 1, amplified using 2.2 m gain fiber with a total pump power of 900 mW, and compressed by splicing Port 2 of CFBG 2, a laser output with an average power, peak power, and pulse width of 110 mW, 1.6 kW, and 1.107 ps, respectively, were obtained (Fig. 6). Then, using the method in which the amplified lase pulse was broadened using CFBG 1 Port 2, amplified using the gain fiber with the same pump power, and compressed using CFBG 2 Port 1, ultrashort laser pulses with an average power, peak power, and pulse width of 132 mW, 35.2 kW, and 63 fs, respectively, were obtained (Fig. 6). Using analysis, the results of the above two laser pulse outputs were attributed to the difference in the group velocity dispersion parameter between CFBGs 1 and 2.
Conclusions Herein, a stable femtosecond laser amplification system based on all-polarization-maintaining erbium-doped fibers was established. The system comprised a mode-locked fiber laser source and two-stage amplifier. Two CFBGs with different dispersion parameters were used as a pulse stretcher and compressor for chirped pulse amplification. Finally, an ultrashort pulse laser output with an average power, peak power, and pulse width of 132 mW, 35.2 kW, and 63 fs, was realized. The key role of the group velocity dispersion difference between the two CFBGs in the chirped pulse amplification of the ultrashort pulse output was qualitatively analyzed. As a highly stable ultrafast pulse source, the femtosecond fiber laser system with linear polarization characteristics shows potential applications in scientific research, communication engineering, etc.
