The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power $P_{in}=5P_{cr}$. The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as $0.3\sim 1$ atm) are beneficial to the propagation of the filament and the broadening of the spectra. It is interesting that, for fixing the initial energy $E_{in}=1$ mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure $p=0.3\sim 1$ atm is still longer than that of $p=1\sim 0.3$ atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components.

数值研究了通过在 2 m 长的气室中传播飞秒高斯光束而产生的灯丝的特性，该气室在不同焦距处具有连续变化的压力。模拟结果表明，起始距离和长丝的长度敏感地取决于压力的变化。这些效果随着初始焦距的增加而增强${磷}_{\mathrm{一世}n}=5{磷}_{Cr}$. 主要原因是一些参数（群速度色散、非线性折射率、多光子电离率和中性原子密度）随气压变化。而灯丝的稳定性则是由压力和焦距共同作用决定的。此外，我们讨论了固定压力和连续变化压力之间灯丝特性的差异。通过对时间动力学和超连续谱的分析，我们发现维持一个大压力（1 atm）和改变到一个大压力（如$0.3～1$atm) 有利于灯丝的传播和光谱的展宽。有趣的是，为了固定初始能量${乙}_{\mathrm{一世}n}=1$ mJ，虽然激光脉冲的临界功率在低压下较高，但在连续变化的压力下灯丝长度 $磷=0.3～1$ atm 仍然比那个长 $磷=1～0.3$atm 随着焦距的增加。该研究对大气污染物成分的远程检测具有重要意义。