Abstract
A technique to perform two-wavelengths digital holographic microscopy (DHM) measurements with a single hologram acquisition is presented. The vertical measurement range without phase ambiguity is extended to the micron-range, thanks to the resulting synthetic wavelength defined by the beating of two wavelengths with a separation of about 80nm. Real-time dual-wavelength imaging is made possible by using two reference waves having different wavelengths and propagation directions for the hologram recording. The principle of the method is exposed and experimental results concerning a 1.2mum m high test sample as well as a moving micro-mirror are presented. To the extent of our knowledge, this is the first time that real-time synthetic beat-wavelength digital holography measurements are reported.
Highlights
The technique of digital holography has experienced substantial developments in the past years [1,2,3] as charge coupled device (CCD) and digital image processing technologies progressed
Such a specimen cannot be observed with monochromatic Digital holographic microscopy (DHM) because the phase ambiguity is nearly impossible to solve with the help of unwrapping algorithms, due to the steep transitions compared to the lateral resolution
The two reconstructed quantitative phase-contrast images of the object in Fig. 3(d,e) obtained from the same hologram recording (Fig. 2) illustrate the phase ambiguity for single-wavelength imaging: the smallest step of 375nm is invisible in Fig. 3(e) because it corresponds to an optical path lengths (OPL) of 750nm, which is nearly the value of λ 2 = 760nm
Summary
The technique of digital holography has experienced substantial developments in the past years [1,2,3] as charge coupled device (CCD) and digital image processing technologies progressed. By obtaining the phase or contour map of the sample with this beatwavelength, the phase ambiguity at single wavelength is removed and the vertical measurement range is greatly extended This has been successfully applied in interferometry [7,8,9] and holography [10,11,12] since the early 70’. We show that a dual-wavelength spatial multiplexing in digital holography enables single-acquisition beat-wavelength measurements and provides an ideal solution to compensate for chromatic aberrations. Most importantly, obtaining complex wavefront information for both wavelengths, by recording a single hologram, permits the computation of the phase difference between each wavefront for each acquisition, and provides real time beat-wavelength imaging
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