上海科技大学知识管理系统

Objective: Mid-infrared (mid-IR) fiber lasers have important applications in gas spectroscopy, material processing, laser surgery, and other fields, since several molecular vibrational absorption bands reside in the mid-IR "fingerprint" region, boosting the strength of light-matter interaction by more than one order of magnitude. However, it is challenging to develop high-performance mid-IR ultrafast sources, because both electrical and optical components at mid-IR wavelengths are far from maturity. In recent years, both continuous-wave (CW) and pulsed lasers in the mid-IR region have been successfully demonstrated, based on several laser techniques, which provide useful means of generating high-power pulses. In the field of mid-IR fiber laser systems, for example, one of the most widely-used gain materials is the ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fiber, which shows several advantages, such as long-term stability, and high-power capability. With different doping rare-earth ions, stable lasing, ranging from 2 μm to 4 μm, has been reported in this type of gain fiber in room temperature. ZBLAN fibers doped with Er3+ ions are the most widely-used, with two radiation wavelengths (2.8 μm and 3.5 μm): they can be efficiently pumped with well-developed 980 nm high-power laser diodes, with a maximum power output of ~40 W under CW operation. However, due to the limit of fiber nonlinearity, the highest output power for ultrafast mid-IR laser is at a Watt level, thus restricting practical applications of such laser systems. In this paper, we design a high-power ultrafast fiber laser system at 2.8 μm with a multi-stage scheme, in order to boost the laser output power by an order of magnitude comparing to the best results ever-reported. Our design is based on rigorous numerical simulations, solving the non-linear Schrodinger equation (NLSE) of the pulse spectra-temporal evolution under different amplification conditions. Results demonstrated here can provide references for designing and constructing high-power ultrafast mid-IR fiber laser systems. Methods: In the present research, we apply the seed oscillator, a dispersion-managed mode-locked fiber laser. Using split-step Fourier method, we are able to apply NLSE to simulate pulse propagation, under different nonlinear and dispersive sections. To obtain enough gain and high-power outputs over 10 W, we designed the laser system as composed by three stages: seed oscillator, pre-amplifier, and main amplifier. After the pre-amplifier stage, we obtained high-quality laser pulse with smooth spectral and temporal profiles, by gradually changing the dispersion values in both the seed oscillator and the pre-amplifier. In the main amplifier, we introduced chirped-pulse amplification (CPA) method to mitigate non-linear pulse distortions, which mainly originate from interplays between strong nonlinearity, gain effects, and dispersion. We analyzed the laser output performances of the compressor and the stretcher at different settings in the CPA set-up. By choosing two different groups of stretcher settings, and comparing the resulting laser performances, including output power, pulse width, spectral width, and pulse shape, we were able to optimize the design of the CPA system. Results and Discussions: With the help of the systematic simulation and optimization, we obtained output pulses from the seed oscillator with ~10 nJ pulse energies, and several-ps pulse durations (Figs. 2 and 3). These output pulses were amplified in the pre-amplifier stage by an order of magnitude, with tens of nJ pulse energies and several-pJ level pulse durations (Fig. 4). In order to analyze the performance of the third stage of the main amplifier, we firstly compared two different amplifier designs: one is the conventional CPA scheme, in which the chirped pulses output of the gain fiber were launched into a compressor; the dispersion value of the stretcher was carefully selected to match the gain fiber length, so as to avoid the use of pulse compressor after the gain fiber. In the second scheme, strong pulse distortions due to excessive nonlinearity in the gain fiber were observed when the amplified pulse energy reached ~250 nJ, leading most pulse energies to be located within pulses' pedestals (Fig. 5). In contrast, by using the conventional CPA scheme, high-quality amplified pulses with both higher pulse energies and peak powers can be obtained at the output port of the main amplifier stage. In our studies, different normal dispersion values of the stretcher in the main amplifier stage, ranging from 0.85 ps2 to 1.40 ps2, were used to optimize the design of this stage. To evaluate the results, we examined the pulse duration, bandwidth, peak power, and temporal shape of the output pulses from the main amplifier at different settings. When keeping the output power of the main amplifier, we obtained anomalous dispersion values provided by the compressor under different normal dispersion values when the energy located in the pedestal was the smallest. Meanwhile, other parameters, such as the pulse bandwidth, and output power, have been recorded. We found that, under the same average power, the quality of the output pulse increases as the normal dispersion value of the stretcher increases (Fig. 6). Finally, two normal dispersion values (0.85 ps2 and 1.25 ps2) have been chosen to investigate the spectral distortion, which is directly impacted by the counterbalance, between self-phase modulation and gain-narrowing effect. Although the pulse duration under 0.85 ps2 is shorter, the proportion of central pulse is smaller, and the shape of pulse spectrum appears worse than those under 1.25 ps2 (Fig. 7): this is mainly caused by the combined effect of nonlinearity and gain-narrowing effect in the gain fiber. When the pulse has smaller bandwidth, the pulse peak power is stronger when propagating in the gain fiber and the Kerr effect would also be stronger, while the gain-narrowing effect prevents the broadening of the pulse. Due to these reasons, both temporal and spectral shapes of the amplified pulse were highly distorted by the excessive nonlinearity in the gain fiber. Conclusions: In summary, we have demonstrated the design of a three-stage ultrafast mid-IR fiber laser system, which can directly deliver 10 W level average power, hundreds of nJ pulse energy, and hundreds of fs pulse duration. We believe that the design and analysis demonstrated here can provide useful information to the construction and optimization of real high-power, ultrafast, mid-IR fiber laser systems. © 2022, Chinese Lasers Press. All right reserved.