Abstract
Speckle noise in digital holography has a great influence on image quality, and one of the reasons for the formation of speckle noise is that the degree of coherence for the reference light and the object light is too high and unnecessary noise is introduced to the recorded hologram. In this paper, the spatial depolarization and the temporal domain depolarization are analyzed to reduce the speckle noise. The spatial depolarization utilizes the depolarization performance of the depolarizer. By adding a quartz depolarizer to the object light, the linearly polarized light in the object light becomes randomly polarized light, thereby reducing the degree of coherence with the reference light, realizing the suppression of speckle noise, and by utilizing the characteristics of pupil parameters towards the depolarization degree, it is to realize the control for the degree of coherence. The temporal domain depolarization uses two quarter-wave plates and adjustable liquid crystal to realize the depolarization superposition. By controlling the phase delay of the liquid crystal phase variable retarder, the multi-depolarization accumulation in a short time is realized, and the experimental results show that this method preserves the detail portion of the image while better removing speckle noise. Finally, the Non-Local Means filtering algorithm is used to make the reconstructed image smoother. The method proposed in this paper has great applicable value for improving the image quality of digital holography system.
Highlights
Digital holography has been widely used in various fields
When passing through the 1/4 wave plate again, the component propagation speed on the fast axis is faster, and the components in the two directions are superimposed to form non-linearly polarized light after the depolarization.The signal generator is used to frequency sweep the liquid crystal, in order to continuously change the phase delay amount generated by the liquid crystal, so that the linear polarization achieves different depolarization effects in the temporal domain
EXPERIMENT AND RESULTS The off-axis optical path of the experimental light path is shown in Fig. 6(a) and 5(b), where in Laser is a laser light source with a wavelength of 532 nm; PBS is a polarizing beam splitter, which acts as a beam deflection; M1, M2 are plane reflectors; BS is a beam splitter; ND is an optical rotating gradient sheet for adjusting the intensity of reference light; BE1, BE2 are beam expansion collimation modules, the objective lens size is 10×, the aperture size is 20um; and the DQ used for spatial depolarization is a quartz depolarizer
Summary
Digital holography has been widely used in various fields. It contains a large amount of information and has a large practical value, but it has shortcomings. It is proposed to effectively suppress such coherent noise by using optical scanning multi-view method combined with 3D. The laser illumination system of multimode fiber bundle can be used to reduce speckle noise [21].the correlation length of the incident laser beam increases by a factor equal to the magnification of the optical system Another method is to use a piezoelectric transducer with multimode fiber [22], the dynamically modulated fiber structure produces a time average smoothing effect on the output field distribution in a short time to reduce speckle. Where COR∗ contains the wavefront information of the object, but the holographically recorded light source has a high coherence, which makes the light wave have different scattering effects on different objects when it is reflected on the surface of the object. Where I0 is the ideal object light intensity and In is the speckle intensity
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