强飞秒激光脉冲的水下传输特性及调控

doi:10.19596/j.cnki.1001-246x.8829

摘要/Abstract

摘要：

通过数值模拟研究强飞秒激光在水下传输的特性, 利用输入脉冲能量、透镜焦距及束腰宽度对其进行调控。结果表明: 当系统参数选取合适时, 可以通过透镜焦距有效地调控光丝在水下1 m到10 m范围内形成, 且传输长度达到米级。随着透镜聚焦能力变弱(如f=10 m), 产生的等离子体丝会发生剧烈的振荡, 不利于波谱的水下探测, 此时通过增加光束的束腰宽度, 实现光丝在水下较远目标位置较稳定地传输。通过增加输入功率来平衡海水中杂质对激光能量的衰减效应, 实现光丝在水下远距离的传输。

关键词: 非线性光学, 强飞秒激光, 等离子体, 光丝, 衰减系数

Abstract:

The propagation characteristics of intense femtosecond laser pulses underwater are numerically investigated and modulated by the input energy, lens focal length and beam waist width.The results indicate that, when the system parameters are appropriately selected, the generation of filament can be effectively controlled by the focal length of lens in range of 1 meter to 10 meters underwater, and the filament length reaches the meter scale. With increase of the focal distance (such as f=10 m), the generated plasma filament will oscillate strongly, which is disadvantage to the underwater detection of spectrum. At this time, by increasing the waist width of the beam, the filament can be transmitted more stably at a distant target position underwater. The attenuation effect of the impurities in seawater on pulse energy can be balanced by increasing input power, so as to realize the long-distance transmission of filaments.

Key words: nonlinear optics, intense femtosecond laser, plasma, filament, attenuation coefficient

冯志芳, 刘丽娜, 刘勋, 李维, 于承新, 叶地发. 强飞秒激光脉冲的水下传输特性及调控[J]. 计算物理, 2025, 42(1): 38-46.

Zhifang FENG, Lina LIU, Xun LIU, Wei LI, Chengxin YU, Difa YE. Underwater Transmission Characteristics and Regulation of Intense Femtosecond Laser Pulses[J]. Chinese Journal of Computational Physics, 2025, 42(1): 38-46.

图/表 5

表1 数值模拟中所用物理参数值[31]

Table 1 Physical parameters in numerical simulation[31]

Magnitude	Symbol/units	Value
Vacuum wavelength	λ₀/nm	800
Linear refractive index	n₀	1.334
Nonlinear index	n₂/(cm²·W^-1)	1.9×10^-16
MPA(Keldysh)	β^(K)/(cm^2K-3·W^1-K)	3.6×10^-50
Group veolcity dispersion	β₂/(fs²·cm^-1)	248
MPA order	$ K=\left\langle\frac{U_{\mathrm{i}}}{\hbar \omega_0}+1\right\rangle$	5
Critical electron density	n_e/cm^-3	1.7×10²¹
Plasma absorption	σ/cm²	6.3×10^-18
Neutral atom density	n_at/cm^-3	6.7×10²²
Ionization potential	U_i/eV	6.5
E-ion collision time	τ_k/fs	3
Speed of light in vacuum	c/(m·s^-1)	3
Vacuum permittivity	ε/(F·m^-1)	8.85×10^-12
Attenuation coefficient	C/m^－1	2.42

图1 激光输入能量和焦距对光丝的调制效应(a)等离子体峰值密度(黑线为左坐标)和峰值强度(红线为右坐标)随传播距离z的变化；(b)与图(a)对应的不同输入能量和焦距的峰值等离子体密度和峰值强度的最大值；(c)归一化能量随传播距离z的演化(黑色、蓝色、红色、紫色、绿色实线分别代表Ein=1, 3, 6, 9, 12 μJ的输入脉冲能量。)

Fig.1 Modulation effects of laser input energy and focal length on the filament (a) evolution of peak plasma density (black line represents left coordinate) and peak intensity (red line represents right coordinate) with propagation distance z; (b)maximum value of the peak plasma density and the peak intensity with different input energy and focal length corresponding to (a); (c) evolution of normalized energy with propagation distance z (The black, blue, red, purple, green solid lines represent the input pulse energy of Ein=1, 3, 6, 9, 12 μJ, respectively.)

图2 轴上光强(W ·cm-2)的时间分布随传输距离z的演化

Fig.2 Temporal distribution of on-axis intensity (W ·cm-2) with propagation distance z

图3 束腰宽度和焦距对光丝特性和超连续波谱的调控(a)~(f)不同束腰宽度和焦距下等离子体密度和峰值强度随传播距离z的变化，(g)在固定光束腰宽和(h)固定焦距下，光丝初始距离处的光谱强度

Fig.3 Modulation of optical filament properties and supercontinuum spectrum by waist width and focal length (a)~(f) evolution of peak plasma density and peak intensity with propagation distance z under different beam waist width and focal length, the intensity of the spectra at the initial distance of filament under (g)fixed beam waist width and (h)fixed focal length

图4 衰减系数对光丝特性和能量演化的影响(a)~(j)不同衰减系数下，峰值等离子体密度和峰值强度随传播距离z的变化；(k)不同焦距和(l)不同输入能量下的归一化能量随传播距离z的变化

Fig.4 Effect of attenuation coefficient on optical filament properties and energy evolution (a)~(j) evolution of peak plasma density and peak intensity with propagation distance z for different attenuation coefficients; evolution of normalized energy with propagation distance z for (k) different focal lengths and (l) different input energies

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