Surface Cleanliness Maintenance with Laminar Flow Based on the Characteristics of Laser-Induced Sputtering Particles in High-Power Laser Systems

Abstract

In high-power laser systems, the primary cause of contamination of optical components and degradation of spatial cleanliness is laser-induced sputtering of particles. To mitigate this problem, laminar flow is frequently utilized to control the direction and transport of these particles. This study characterizes the properties of laser-induced sputtering particles, including their flying trend, diameter range, and velocity distribution at varying time intervals. A time-resolved imaging method was employed to damage the rear surface of fused silica using a 355 nm Nd: YAG pump laser. The efficacy of laminar flow in controlling these particles was then assessed, with a particular focus on the influence of laminar flow direction, laminar flow velocity, particle flight height, and particle diameter. Our results indicate that the optimal laminar flow velocity for preventing particle invasion is highly dependent on the maximum particle attenuation distance (or safety distance), which can vary by up to two orders of magnitude. Furthermore, a laminar flow velocity of 0.5 m/s can effectively prevent particle sedimentation. Future research will aim to optimize laminar flow systems based on these findings to achieve high surface cleanliness in high-power laser systems with minimal energy consumption.

Keywords: cleanliness; fused silica; high-power laser systems; laminar flow; laser-induced damage; motion behavior; particle.

Figure 1

Schematic of time-resolved imaging for capturing transient particles after a laser-induced breakdown event. PD: photon detector, PL: polarized lens, SP: sampler, HWP: half-wave plate, BS: beam splitter, FL: focus lens, NBF: narrowband filter, MS: microscope, HR: high reflector, EM: energy meter, FOV: field of view.

Figure 2

Two methods preventing particulate invasion by laminar flow, (a) avoid direct impact for particles with a longitudinal velocity of V₀; (b) sedimentation control after V₀ decreases to 0.

Figure 3

Transient characteristics of laser-induced particles acquired by comparing two images at different time delays at the laser fluence of 79 J/cm². (a,b), (i,j), and (k,l) at 3982 ns to 5504 ns, (c,d), and (e,f) at 3985 ns to 5997 ns, (g,h), and (m,n) at 5987 ns to 7495 ns.

Figure 4

Longitudinal velocity V₀ of particles with different diameters vs. probe time delays ranging from 500 to 12,000 ns, where (a) represents particles with diameters <10 μm, (b) 10–20 μm, (c) 20–30 μm, (d) 30–40 μm, (e) 40–50 μm, and (f) >50 μm, red curves represent fit decay functions.

Figure 5

(a) Particle’s maximum attenuation distance vs. time calculated by Equation (5) using velocities fitted at 13,000 ns; (b,c) are plots of particle velocity and distance vs. time governed by Allen and Stokes regime, respectively, where particle diameter range >50 μm.

Figure 6

Protection efficiency of laminar flow (η) vs. particle height ratio (L/W) in preventing particle invasion during free-sinking, where particle diameter ranges from <10 μm to >50 μm.