Dual-wavelength squeezing interferometry with a novel double phase shift strategy: synthetic-wavelength interferometric fringe extraction by the conjugate complex function coupling

In simultaneous dual-wavelength interferometry (SDWI), we present a dual-wavelength squeezing interferometry for the extended measuring range. Combining with single-wavelength interferometric conjugate complex functions coupling and double phase shift strategy, the proposed method can retrieve the longer synthetic-wavelength phase with the lower spatial carrier. Based on the novel double phase shift strategy, several groups of phase-shift dual-wavelength interferogram are acquired with every four frames in each group. And the phase-shift step is designed as π/2 at one single wavelength in each group, while π/2 at synthetic wavelength between the adjacent groups. For the dual-wavelength squeezing interferometry, the temporal phase shift among each group is converted into spatial carrier in the generated single spatialtemporal fringe (STF). Therefore, it does not need extra spatial carrier in the initial dual-wavelength interferogram for the spectral separation. The single-wavelength interferometric complex functions could be easily obtained by the Fourier transform and appropriate spectral filter performed on the dual-wavelength STF. The obtained single-wavelength interferometric conjugate complex functions are coupled by multiplication to extract the needed synthetic-wavelength interferogram from every group. And then the extracted synthetic-wavelength interferogram from each group with π/2 phase shift are demodulated by conventional phase-shift algorithm directly. Finally, the method is tested in experiment data for recovering phase from dual-wavelength fringe patterns with closed fringes.