Abstract
A digital hologram-optimizing method was proposed to improve the imaging quality of dual-wavelength digital holographic microscopy (DDHM) by reducing the phase noise level. In our previous work, phase noise reduction was achieved by dual-wavelength digital image-plane holographic microscopy (DDIPHM). In the optimization method in this paper, the phase noise was further reduced by enhancing the real-image term and suppressing effects of the zero-order term in the frequency spectrum of a digital hologram. Practically, the carrier frequency of the real-image term has the correspondence with interference fringes in the hologram. Mathematically, the first order intrinsic mode function (IMF1) in bidimensional empirical mode decomposition (BEMD) has similar characteristics to the grayscale values of ideal interference fringes. Therefore, with the combination of DDIPHM and BEMD, by utilizing the characteristics of IMF1, the digital hologram was optimized with purified interference fringes, enhancing the real-image term simultaneously. Finally, the validity of the proposed method was verified by experimental results on a microstep.
Highlights
With various advantages such as the real-time performance, noninvasive property, and easy processing by mathematical computing, digital holographic microscopy (DHM) has experienced substantial development in surface profile measurement of microstructures [1,2,3]
Dual-wavelength digital holographic microscopy (DDHM) extends the measurement range of single-wavelength digital holographic microscopy when DHM is applied to measure high aspect-ratio structures, especially the step structures with the micron step height [5, 6]
The experimental results of dual-wavelength digital image-plane holographic microscopy (DDIPHM), DDIPHM with bidimensional empirical mode decomposition (BEMD), and DDHM are compared to demonstrate that BEMD can achieve a lower phase noise level
Summary
With various advantages such as the real-time performance, noninvasive property, and easy processing by mathematical computing, digital holographic microscopy (DHM) has experienced substantial development in surface profile measurement of microstructures [1,2,3]. The phase noise, especially in the recording process, is amplified when the measurement height range is amplified simultaneously, resulting in a loss of axial resolution in the measurement [7, 8]. Except for image processing methods [9,10,11], several noise-reducing approaches aimed at DDHM have been proposed in the last decades, such as the mathematic methods, the dual-wavelength unwrapping algorithms [7, 8]. Error points occur when the dual-wavelength unwrapping algorithms are applied. We analyzed the reasons for occurrence of error points and proposed a much safer method, namely, dual-wavelength digital image-plane holographic microscopy (DDIPHM) [12, 13] to suppress the phase noise in DDHM. An optimization method based on combination of bidimensional empirical mode decomposition (BEMD) and DDIPHM was put forward to improve the imaging quality of DDHM
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