Two-wavelength digital holography using frequency-modulated continuous-wave technique for multiplexing in the time–frequency domain

In this study, we have proposed a digital holographic technique in which the frequency-modulated continuous-wave technique is introduced as a novel implementation of wavelength multiplexing in the time–frequency domain. In the proposed technique, the holograms are recorded with two wavelengths and the information of each hologram can be separated in the time–frequency domain by modulating the frequencies of two laser diodes at different modulation widths. Therefore, a temporal Fourier analysis is performed on each pixel of the time-series holograms whose intensity is modulated with two beat frequencies. And then, the holograms corresponding to the two wavelengths are extracted independently. Initially, a holographic system with two close wavelengths of 782.43 nm and 782.50 nm was designed to measure the surface profile of metallic gauge blocks with a known step-height of 1.16 mm in both experimental and numerical calculations. In addition, the measurement accuracy of the proposed system was investigated using both the experimental and the numerical results. Furthermore, the numerical calculation was conducted to investigate the origin of the periodic noise superimposed on the experimental results. Finally, the reduction method of the periodic noise was proposed, and the effect of the method was demonstrated using numerical calculations.