超快光纤激光相干合成技术是突破飞秒激光单根光纤功率提升受限的有效技术手段。基于光纤拉伸器锁相，并结合随机并行梯度下降（SPGD）算法，成功实现了两路超快激光相干偏振合成。该锁相方案不仅避免了采用常规电光相位调制器对脉冲信号造成的光谱调制，而且可有效降低系统的插入损耗，提高相位调制范围、耐受功率以及前级光源系统的紧凑性与鲁棒性。合成的最高功率为10.9 W，最高功率下合成效率为90.1%，闭环状态下锁相残差为λ/31。实验结果表明，采用光纤拉伸器和SPGD算法组成的相位控制系统，在超快激光相干合成领域具有较强的发展潜力。合成的脉冲在最高功率下可压缩至494 fs，压缩效率为73.3%，对应的单脉冲能量为3.99 μJ。

Dual-comb spectroscopy (DCS) has revolutionized numerous spectroscopic applications due to its high spectral resolution and fast measurement speed. Substantial efforts have been made to obtain a coherent dual-comb source at various spectral regions through nonlinear frequency conversion, where the preservation of coherence has become a problem of great importance. In this study, we report the generation of coherent dual-comb sources covering from the ultraviolet to mid-infrared region based on high-order harmonic generation. Driven by high-repetition-rate femtosecond mid-infrared dual-comb pump pulses, up to ninth-order harmonic was generated from the ultraviolet to mid-infrared region using an aperiodically poled lithium niobate waveguide. To investigate the coherence property of the high-order harmonic generation, DCS was performed at every generated spectral region from 450 to 3600 nm. The measured dual-comb spectra with distinctive tooth-resolved structures show the well-preserved coherence without apparent degradation after the cascaded quadratic nonlinear processes. The subsequent methane absorption spectroscopy at multiple spectral regions of different harmonics was carried out to characterize the spectroscopic capability of the system. These results demonstrate the potential of our scheme to generate compact and coherent broadband optical frequency combs for simultaneous multi-target detections.

We report a compact, tunable, self-starting, all-fiber laser-based asynchronous optical sampling (ASOPS) system. Two Er-doped fiber oscillators were used as the pulsed-laser source, whose repetition rate could be set at 100 MHz with a tuning range of 1.25 MHz through a fiber delay line. By employing phase-locked and temperature control loops, the repetition rate offset of the two lasers was stabilized with 7.13 × 10^-11 fractional instability at an average time of 1 s. Its capabilities in the terahertz regime were demonstrated by terahertz time-domain spectroscopy, achieving a spectral bandwidth of 3 THz with a dynamic range of 30 dB. The large range of repetition rate adjustment in our ASOPS system has the potential to be a powerful tool in the terahertz regime.

Gain-parameter-dependent transfer functions and phase-noise performances in a mode-locked Yb-doped fiber laser are measured in this study. It is discovered that the corner frequency in the amplitude and phase domains is determined by the absorption coefficient of the gain fiber, when the total absorption and other cavity parameters are fixed. This shows that an oscillator using gain fiber with higher dopant concentration accumulates more phase noise. Furthermore, we present net cavity dispersion-dependent transfer functions to verify the effect of dispersion management on the frequency response. We derive a guideline for optimizing mode-locked fiber laser design to achieve low phase noise and timing jitter.

Mid-infrared dual-comb spectroscopy is of great interest owing to the strong spectroscopic features of trace gases, biological molecules, and solid matter with higher resolution, accuracy, and acquisition speed. However, the prerequisite of achieving high coherence of optical sources with the use of bulk sophisticated control systems prevents their widespread use in field applications. Here we generate a highly mutually coherent dual mid-infrared comb spectrometer based on the optical–optical modulation of a continuous-wave (CW) interband or quantum cascade laser. Mutual coherence was passively achieved without post-data processes or active carrier envelope phase-locking processes. The center wavelength of the generated mid-infrared frequency combs can be flexibly tuned by adjusting the wavelength of the CW seeds. The parallel detection of multiple molecular species, including C2H2,CH4,H2CO,H2S, COS, and H2O, was achieved. This technique provides a stable and robust dual-comb spectrometer that will find nonlaboratory applications including open-path atmospheric gas sensing, industrial process monitoring, and combustion.

The determination of airflow parameters is essential to the research of critical information on environment monitoring, chemical kinetics, and aerodynamic and propulsion applications. During the past few decades, tunable diode laser absorption spectroscopy has become a common and efficient tool for the flow velocity measurement based on the Doppler shift of the absorption line. Dual-comb absorption spectroscopy (DCAS), as a state-of-the-art Fourier-transform broadband spectroscopic technique, not only can detect multiple trace molecules in parallel but also can extract Doppler shifts to derive the flow velocity through the analysis of dozens of molecular absorption lines simultaneously with high precision. Here, we report a proof-of-principle demonstration of the velocity measurements of acetylene at various flow velocities by means of a high-resolution and broadband DCAS. Mode-resolved Doppler-shifted rotational-vibrational lines in the P branch of acetylene molecules are obtained. A model for multiline Doppler frequency determination is investigated and experimentally verified. The flow velocity measurements with a measuring uncertainty down to the submeter per second over the range from 8.7 m/s to 44.8 m/s at an effective time resolution of 1 s and a measuring uncertainty of 1.97 m/s at 0.1 s are demonstrated. With broadband mid-infrared frequency combs covering atmospheric transmission windows, the open-path measurement for monitoring diffusion of the weak pollutant source would be realized.

We report on the generation of a mid-infrared (mid-IR) frequency comb with a maximum average output power of 250 mW and tunability in the 2.7–4.0 μm region. The approach is based on a single-stage difference frequency generation (DFG) starting from a compact Yb-doped fiber laser system. The repetition rate of the near-infrared (NIR) comb is locked at 75 MHz. The phase noise of the repetition rate in the offset-free mid-IR comb system is measured and analyzed. Except for the intrinsic of NIR comb, environmental noise at low frequency and quantum noise at high frequency from the amplifier chain and nonlinear spectral broadening are the main noise sources of broadening the linewidth of comb teeth, which limits the precision of mid-IR dual-comb spectroscopy.