Speckle Reduction In Digital Holography Coherence Imaging System

Digital holography is a powerful imaging technique which can simultaneouslyand independently retrieve the amplitude and phase information of the wavetransmitted through the object, which is characterized by its non-invasion,non-contact, high-speed and full wave-field. So DH is widely used in manyapplications such as surface morphology detection. Quantitative phase-contrastimage is of particular interest since the sample detected. Despite its advantages, italso possesses drawbacks similar to those of most coherent methods, such as specklenoise arising from rough surface of the object when its height variations exceed thelight wavelength. The superposition of these scattered waves produces an unevenspeckle pattern in the recording plane, which degrades the image quality andmeasurement accuracy of the reconstructed images. The speckle pattern, as a spatialrandom intensity distribution, is introduced when fairly coherent light either isreflected from a rough surface by random scattering or propagates through a mediumwith random refractive index fluctuation.In this paper we introduced and demonstrated an approach using phase SLM(spatial light modulator) and LED (Light-emitting diode) to reduce speckle noise indigital holography. Many significative researches and results have been proposed asfollows:1. In the paper, we analyzed the SLM device’s working principle and discussedthe speckle reduction method with speckle theory. We analyzed the relationshipbetween the speckle phenomena and the illumination angle. By compared the maxmodulation angle that SLM device can reached with the min angle that the methodrequired, we proved that phase only SLM device can perfect modulate theillumination light and reduced the digital holography’s speckle noise in theory. Anautomatic speckle noise suppression method of digital holography is proposed byangular diversity with phase-only spatial light modulator (SLM). Firstly, the minimalangular change of two illumination beams is analyzed to ensure the irrelevance ofthe speckle patterns. Then, the phase-only SLM is modulated in two-dimensiondirection to accurately produce a large number of tilted illumination beams.117holograms are recorded with different tilted illuminations based on an image planedigital holography system. Finally, the experimental results demonstrate that thesynthesized image significantly reduces the speckle noise in a good agreement with the theoretical value. Comparing with the present methods, the tilted illuminationbeams is controlled without any manual operation, and a large number of hologramscan be recorded. Therefore, the remarkable advantage of the proposed approach isgood effectiveness, repeatability and practicability for the speckle suppression.2. Light-emitting diode (LED) is a kind of short-coherence light source, and itcan generate time gating effect and reduce the speckle noises in digital holography.Meanwhile, it can cut down the cost of the setup since LED is cheaper than laser. Inthis paper, an in-line digital holography setup was designed with LED, and theholograms were reconstructed by four-step phase shifting method. Comparing withthe phase image obtained by the laser-based setup, results show that the phase imageof USAF test target possesses lower speckle noises with the help of LED. Finally, wereconstruct a series of intensity images with different depths of a coin, and theseimages are fused to get a synthesized image with more information.