Dual wavelength digital holography for improving the measurement accuracy

Abstract

In dual wavelength digital holography, a synthetic wavelength is obtained by using two lasers with different wavelengths to expand the measurement range of samples' step heights from nanometers to micrometers. However, its measurement accuracy reduces along with the expansion of measuring range and significant noise is introduced at the same time. For cases where the sample's height is smaller than the wavelength of illumination light, the measurement accuracy is very important. In this paper, a new approach of dual wavelength digital holography is presented. The synthetic wavelength is smaller than the wavelength of the two different lasers. Higher measurement accuracy can thus be achieved. The analysis and experimental results show the validity of this method.

Multi-wavelength phase-shifting interferometry based on a two-step phase-shifting phase retrieval algorithm with a color CMOS

“…The relationship between them is expressed as Λ ij_jk ⩾ Λ ij_sub > λ i > Λ ij_add . In accordance with both the traditional method [15][16][17] and the shorter synthetic wavelength method [18][19][20], the phase noise of the three-synthetic wavelength proposed in this paper decreases successively to the level of a two-synthetic long wavelength, a single wavelength, and a two-synthetic short wavelength.…”

“…Taking three-wavelength interferometry as an example, the most commonly used noise reduction method is to decrease the phase noise in the synthetic wavelength phase map to a synthetic two-wavelength level, and then to a single-wavelength level [15][16][17]. A new wavelength synthesis method has recently been introduced, which further reduces phase noise from the single-wavelength level to a short-synthetic wavelength level, which is shorter than either of the other wavelengths [18][19][20].…”