Two-wavelength Digital Holography Research Articles

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

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

Two-wavelength digital holography through fog

Interferometric detection enables the acquisition of the amplitude and phase of the optical field. By making use of the synthetic wavelength as a computational construct arising from digital processing of two off-axis digital holograms, it is possible to identify the shape of an object obscured by fog and further increase the imaging range due to the increased sensitivity in coherent detection. Experiments have been conducted inside a 27 m long fog tube filled with ultrasonically generated fog. We show the improved capabilities of synthetic phase imaging through fog and compare this technique with conventional active laser illumination imaging.

Single-shot two-wavelength slightly-off-axis flipping digital holography using polarization-splitting modulation

Two-wavelength digital holography (TWDH) can extend the unambiguous range of phase measurements. However, state-of-the-art TWDH is achieved either by using the on-axis or off-axis layout to create interference or by using a Bayer mosaic-filtered color CCD camera to record the hologram. This leads to a tradeoff among the acquisition rate, detector bandwidth, and measurement depth. To solve this issue, inspired by the slightly off-axis single-wavelength digital holography, a single-shot slightly off-axis TWDH is proposed by combining flipping and polarization-splitting modulation. The interferences are yielded by flipping the relative positions of the sample and reference beams of both wavelengths. Two holograms at each wavelength can be simultaneously recorded by a monochromatic CCD camera with the help of polarization-splitting modulation. After subtraction between the two holograms, a correlative filtering division algorithm is used to perform the phase reconstruction. Experimental validation was conducted by measuring step targets and alcohol evaporation.

Color-image reconstruction for two-wavelength digital holography using a generalized phase-shifting approach.

We propose a color-image reconstruction method for two-wavelength digital holography using generalized phase-shifting digital holography (GPSDH). In this method, color interference fringes are captured by a digital camera with a Bayer array color filter, and phase shifting is simultaneously performed for all wavelengths. Color interference fringes are separated into three monochromatic interference fringes using a color-separation method that suppresses the color-filter crosstalk. The object wave is extracted from each monochromatic interference fringe using GPSDH, which prevents problems due to the phase shift wavelength dependence. Image reconstruction is performed using a shifted Fresnel transform-based method, in which the color reconstructed image is obtained by directly superposing the reconstructed images for all wavelengths. We verify the proposed method through optical experiments with a two-wavelength digital holography system. The results show that the dual-color image can be successfully reconstructed without chromatic aberration.

Multiwavelength off-axis digital holography with an angle of more than 40 degrees and no beam combiner to generate interference light.

We propose single-shot multiwavelength digital holography with an extremely large incident angle and show the digital recording of multiple object waves at multiple wavelengths with an angle of more than 40 degrees and no beam combiner to generate interference light. Both the avoidance of the crosstalk between the object waves at different wavelengths and the space-bandwidth extension are simultaneously achieved with a single-shot exposure of a monochromatic image sensor and a reference beam even when the wavelength difference between the object waves is small. An extremely large angle can be set by utilizing the signal theory. An angle of up to 40.6 degrees was introduced, interference fringes with an 818 nm period at the wavelength of 532 nm were generated, and an image sensor recorded a two-wavelength-multiplexed hologram. Resolution improvement was experimentally demonstrated using two-wavelength digital holography with the wavelengths of 640 and 532 nm.