Beam diameter thresholds as applying light depolarization for effective submicron and micron root mean square roughness evaluation.

To further study the microscopic mechanism and beam diameter effect during light depolarization (LDP), we developed a compact laser instrument (λ=632.8  nm) with an adjustable beam diameter of ≥18  μm (approximately 28λ). Six nickel plate samples with rms roughness, Rq , of 42 nm to 2.3 μm (i.e., 0.067-3.7λ) fabricated by the fine-honing method are examined. To analyze the beam diameter effect as applying LDP for submicron and micron Rq evaluation, the cross-sectional beam-spot size (BSS) is adjusted from 20 μm to 650 μm during off-specular inspections. The results of BSS ≤40  μm (i.e., 60λ) have a 10-nm-level Rq sensitivity, while those of BSS ≥140  μm (220λ) have about a 100 times weaker sensitivity. It means that BSS of 60λ and 220λ should have instructional significance as applying LDP for precision levels of 10 nm and 1 μm surface roughness analyses, respectively. In addition, since the instrument is simple, portable, stable, and low-cost, it has great potential for both LDP analyses and practical online roughness testing.