We propose a spatially chirped quasi-phase-matching (QPM) scheme that enables ultrabroadband second-harmonic-generation (SHG) by using a fan-out QPM grating to frequency-convert a spatially chirped fundamental wave. A “zero-dispersion” 4f system maps the spectral contents of ultrabroadband fundamental onto different spatial coordinates in the Fourier plane, where the fundamental is quasi-monochromatic locally in picosecond duration, fundamentally canceling high-order phase mismatch. A fan-out QPM grating characterized by a linear variation of the poling period along the transverse direction exactly supports the QPM of the spatially chirped beam. We theoretically demonstrate the SHG of an 810-nm, 12.1-fs pulse into a 405-nm, 10.2-fs pulse with a conversion efficiency of 77%.

论证了单晶体光参量放大（OPA）过程在特定边界条件下满足频域宇称-时间（PT）反对称性。归一化的数值求解结果显示，OPA系统PT对称阈值点附近呈现增益跃变性质。对于存在位相失配的OPA，通过实时调控泵浦光强，即可控制系统PT对称性，论文基于相位失配OPA中可超快调控PT对称性的特性构建了超快光开关，一方面光开关与周期性幅度调制的泵浦光联合使用，可直接将连续激光转换为超短脉冲序列输出；另一方面，构建的光开关也可用于脉冲激光再压缩，有望用于中红外波等长波段超短种子源。论文提出的基于超快光开关直接产生超短脉冲序列的方案，由于不需要光学谐振腔，易于实现大于10 GHz的超高重复频率。

提出并研究了一种宽带、大量程、方向可控的超短脉冲群速度调控方法。利用级联光参量放大，将泵浦光时域的线性强度调制转移至啁啾信号光频域的线性位相调制，压缩后信号光脉冲将获得延时或提前。通过调控泵浦时域强度调制的斜率大小和符号，可对信号光群速度调控的量程和方向进行灵活操控。级联光参量放大过程不需要满足位相匹配，响应带宽很大，原理上支持周期量级脉冲的群速度调控。分别研究了锯齿型和高斯型脉冲泵浦的级联光参量放大过程，演示验证了该方法的调控效果。该方法可在常规非线性晶体中实施，具备应用价值。

提出了一种可同时实现高效率和大带宽的频率上转换技术，可将高能钕玻璃基频激光（波长1 054 nm）高效率地转换为紫蓝光波段的宽带2.5倍频激光（波长421.5 nm）.将窄带钕玻璃的倍频激光与中心波长为2 100 nm的宽带中红外激光以能量比4∶1进行非线性和频，并通过非共线构型实现位相匹配和群速度匹配，实验产生了能量为12.4 J、带宽为10.4 nm（相对带宽大于2%）的宽带2.5倍频激光.实验结果能够支撑紫蓝光波段百太瓦强激光的产生，为未来开发短波长的超短超强激光提供借鉴.

文中首次提出并验证了基于腔内色散管理实现飞秒光参量振荡器（OPO）光谱净化和稳定性提升的方法。对于高功率飞秒OPO，输出脉冲通常具有随时间无序变化的宽带不规则光谱，输出功率波动较大。利用铌酸锂（LiNbO₃）晶体在腔内引入额外的负色散，通过泵浦脉冲的时间滤波效应实现了干净平滑的窄光谱近转换极限的飞秒脉冲输出，光谱稳定性和功率稳定性得到了极大改善。该方法是一种实现飞秒OPO光谱净化和稳定性提升的灵活简便的方法，对于发展高功率的超短脉冲OPO具有重要的应用价值。

We demonstrated a femtosecond mode-locked Er:ZrF₄-BaF₂-LaF₃-AlF₃-NaF (Er:ZBLAN) fiber laser at 2.8 μm based on the nonlinear polarization rotation technique. The laser generated an average output power of 317 mW with a repetition rate of 107 MHz and pulse duration as short as 131 fs. To the best of our knowledge, this is the shortest pulse generated directly from a mid-infrared mode-locked Er:ZBLAN fiber laser to date. Numerical simulation and experimental results confirm that reducing the gain fiber length is an effective way to shorten the mode-locked pulse duration in the Er:ZBLAN fiber laser. The work takes an important step towards sub-100-fs mid-infrared pulse generation from mode-locked Er:ZBLAN fiber lasers.

Separating lights into different paths according to the polarization states while keeping their respective path’s polarizations with high purification is keen for polarization multiplex in optical communications. Metallic nanowire gratings with multi-slits in a period are proposed to achieve polarized beam splitters (PBSs) in reflection and diffraction. The setting of multi-slits largely reduces the reflection of photons with a transverse magnetific field via the plasmonic waveguiding effect, which leads to highly polarized output lights with extinction ratio larger than 20 dB in each channel. The proposed reflection/diffraction PBSs enrich the approaches to control the polarization states with the advantages of wide incident angles and flexible beam splitting angles.

Quasi-parametric chirped-pulse amplification (QPCPA) can improve the signal amplification efficiency and stability by inhibiting the back-conversion, in which the idler absorption plays a critical role. This Letter theoretically studies the impacts of idler absorption on the QPCPA performance in both the small-signal and saturation regimes. We demonstrate that there exists an optimal idler absorption that enables the achievement of maximum pump depletion within a minimum crystal length. To overcome the reduction in small-signal gain induced by idler absorption, the configuration of gradient idler absorption is proposed and demonstrated as a superior alternative to constant idler absorption. The results provide guidelines to the design of state-of-the-art QPCPA lasers.

High-power ultrafast fiber lasers operating at the 2 μm wavelength are extremely desirable for material processing, laser surgery, and nonlinear optics. Here we fabricated large-core (LC) double-cladding Tm-doped silica fiber via the sol-gel method. The sol-gel-fabricated Tm-doped silica (SGTS) fiber had a large core diameter of 30 μm with a high refractive index homogeneity (Δn=2×10 4). With the newly developed LC SGTS fiber as the gain fiber, high-power mode-locking was realized. By using a semiconductor saturable absorber mirror (SESAM) as a mode locker, the LC SGTS fiber oscillator generated mode-locked pulses with an average output power as high as 1.0 W and a pulse duration of 23.9 ps at the wavelength of 1955.0 nm. Our research results show that the self-developed LC Tm-doped silica fiber via the sol-gel method is a promising gain fiber for generating high-power ultrafast lasers in the 2 μm spectral region.

Back conversion is an intrinsic phenomenon in nonlinear frequency down-conversion processes. However, the physical reason for its occurrence is not well understood. Here, we theoretically reveal that back conversion is the result of a π-phase jump associated with the depletion of one interacting wave. By suppressing the idler phase jump through a deliberate crystal absorption, the back conversion can be inhabited, thus enhancing the conversion efficiency from the pump to the signal. The results presented in this Letter will further the understanding of nonlinear parametric processes and pave the way toward the design of highly efficient down-conversion systems.