探测距离是红外系统应用的重要评价指标，随着制冷型红外探测器的发展，红外系统自身热辐射已成为探测距离提升的重要限制性因素，冷光学设计是抑制自身热辐射的必然选择，因此对冷光学制冷温度指标进行评估和优化成为红外系统设计分析的新问题。文中从红外系统自身热辐射和经典探测距离理论出发，推导了包含系统噪声项的红外系统探测距离计算公式，提出了分布式探测距离的分析方法。以透射式光学系统为例，进行了影响因素灵敏度分析。通过对探测器焦平面进行分区域数据处理，得到了对应探测距离的主要影响表面。在此基础上，分析了在对主要影响表面进行低温处理前后（293.15 K制冷到173.15 K）探测距离的变化。结果表明，探测距离最大提升量达到43.32%，提升效果显著。该方法可为红外系统冷光学设计和评估提供参考。

We report here an ultra-broadband linearly polarized (LP) LP₀₁ LP₁₁ mode converter operating at 1 μm based on a long period fiber grating (LPFG) fabricated in a conventional two-mode fiber (TMF) by a line-focused CO₂ laser. The measured 3 dB bandwidth is about 240 nm, which is the broadest bandwidth for such fiber mode converters. The maximum conversion efficiency between the LP₀₁ and LP₁₁ modes is >99% over the range of 1000 nm to 1085 nm, almost covering the whole emission band of Yb³⁺, which is useful for further power scaling of high-power fiber lasers operating at the 1 μm band.

The average power of diode-pumped fiber lasers has been developed deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting the further power scaling. Here, we have demonstrated the mitigation of SRS in kilowatt-level diode-pumped fiber amplifiers using a chirped and tilted fiber Bragg grating (CTFBG) for the first time. The CTFBG is designed and inscribed in large-mode-area (LMA) fibers, matching with the operating wavelength of the fiber amplifier. With the CTFBG inserted between the seed laser and the amplifier stage, an SRS suppression ratio of ${\sim}10~\text{dB}$ is achieved in spectrum at the maximum output laser power of 2.35 kW, and there is no reduction in laser slope efficiency and degradation in beam quality. This work proves the feasibility and practicability of CTFBGs for SRS suppression in high-power fiber lasers, which is very useful for the further power scaling.

The average power of fiber lasers has been scaled deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting further power scaling. Here, we have demonstrated for the first time, to the best of our knowledge, the suppression of SRS in a half 10 kW tandem pumping fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). With specially self-designed and manufactured CTFBGs inserted between the seed laser and the amplifier stage, a maximum SRS suppression ratio of >15 dB in spectrum is observed with no reduction in laser efficiency. With one CTFBG, the effective output power is improved to 3.9 kW with a beam quality M2 factor of ～1.7 from <3.5 kW with an M2 factor of >2; with two CTFBGs, the effective laser power reaches 4.2 kW with an increasing ratio of 20% and an M2 factor of ～1.8, and further power improvement is limited by the power and performance of the 1018 nm pump sources. This work provides an effective SRS suppression method for high-power all-fiber lasers, which is useful for further power scaling of these systems.