Objective Optical coherence tomography (OCT) is a non-invasive optical imaging technology. In recent years, it has become a research hotspot of biological imaging and has been widely used in the field of medical diagnosis. As an imaging technology based on the principle of low coherent light interference, OCT is susceptible to speckle noise. Speckles will destroy details of OCT images and reduce image quality, which imposes significant limitations on the clinical application potential of OCT. Superposition is a common method to reduce additive white noise. However, speckle noise belongs to the multiplicative noise. In order to suppress speckles more effectively, it is necessary to use speckle decorrelation technology to reduce the speckle correlation between images for superimposition, which is called the decorrelation superimposition method. Decorrelation superimposition can improve the speckle signal- to-noise ratios of superimposed images. Up to now, researchers have proposed a variety of speckle decorrelation technologies, but they face the following limitations: the number of available decorrelation images is limited; the overall system is bulky, complex, and expensive; the transmitted light power is greatly lost, and illumination variability during two-dimensional scanning is introduced. In this study, a speckle decorrelation OCT system using a pure random phase plate (PSD-OCT) is reported. The system uses a tailored pure random phase plate (PRPP) to achieve speckle decorrelation, which can avoid the loss of light power and the introduction of illumination variability, and has a simple and low- cost structure. PSD-OCT helps obtain low-noise imaging results. Methods The PSD-OCT system is built based on swept- source OCT. The system uses a PRPP to modulate the wavefront phase of the sample light, and changes the gray value characteristics of speckles when images for superimposition are collected to realize speckle decorrelation, so as to provide low correlation images for decorrelation superposition method and reduce OCT speckle noise. PRPP is a specially designed binary diffractive optical element used to modulate the wavefront phase of OCT, with periodic random phase distribution of 0-2p in the radial direction. The PRPP is placed on the focal plane between the scanning lens and the subsequent lens of a sample arm. Due to the conjugate relationship, the object plane of the imaging sample and the modulation plane of the PRPP are object-image conjugate planes. When the sample is continuously collected at the same imaging position, PRPP moves on the plane perpendicular to the optical axis to change the wavefront phase distribution of the illumination and scattered light of the sample arm, and then realizes the random phase modulation of the object plane through the conjugate relationship. The phase shift of PRPP with time changes the OCT speckle pattern, which makes each image have different speckle patterns, realizes decorrelation superimposition, and reduces speckle noise. Results and Discussions The simulation results of the three-layer scatterer (Fig. 1) show that when the image is decorrelated and then superimposed, the speckle phenomenon of superimposed images is well suppressed, and the tomographic boundary between layers is clearer. In addition, the speckle signal- to-noise ratio is increased by 1.5 times. The result demonstrates that decorrelation superimposition can reduce noise. It can be seen from the imaging results of the scatterer model that PRPP reduces the correlation coefficient between multiple images used for superimposition from 0.93 to 0.49 ( Fig.5), which is close to the ideal result of the simulation (0.42). Such results demonstrate that the PSD- OCT system has a nearly ideal speckle decorrelation effect. After speckle decorrelation, the signal-to- noise ratios of superimposed images are significantly improved, and speckle noise is smoothed. By comparing the imaging results of human nails and fingertip skin of traditional OCT and PSD- OCT ( Figs. 7 and 8), it can be seen that compared with traditional OCT, the granular speckle noise in the superimposed images obtained by PSD-OCT is suppressed, and tomographic structure features between tissues are clearer. To sum up, our experimental results show that PSD- OCT can reduce speckle correlation, improve the speckle signal- to-noise ratio, and observe finer and clearer biological structures. Conclusions In this study, a speckle decorrelation OCT technology using PRPP as a phase modulator is proposed, and a low speckle OCT imaging in vivo is successfully realized by using the decorrelation superposition method. PRPP modulates the wavefront phase distribution of the sample light by generating a time- varying phase shift, and it has excellent speckle decorrelation ability, which makes superposition effectively reduce the impact of speckles on imaging and thus enhances the visual visibility of OCT images. The low signal loss rate introduced by PRPP can avoid the image contrast reduction and illumination variability caused by additional optical devices during operation. Our study shows that compared with traditional OCT, PSD- OCT has achieved a remarkable speckle suppression effect with a simple and compact structure. This system can reveal details of samples originally covered and damaged by speckles, and can more clearly show the fine structure and chromatographic characteristics of biological tissues. PSD-OCT has a wide application prospect in the biomedical imaging field, which will enable doctors to diagnose related diseases more accurately and reduce the difficulty of algorithms including OCT image enhancement, contour extraction, and so on.
