At present, there are some problems in the digital scanning imaging methods of multimode optical fiber, such as poor quality of focused spot, large amount of calculation and long time in the forming process of focused spot. In order to solve the above problems, a parallel phase compensation method based on liquid crystal spatial light modulator (LCSLM) is proposed to achieve fast point-focused scanning of multimode fiber (MMF). The parallel algorithm, which includes the two processes of collecting online specklegrams and calculating offline phase masks, calculates the compensation phase masks so that the phase masks can be loaded on SLM in turn to generate a series of focused spots at different predefined positions, which greatly saves time. Experiments demonstrate that this method doesn’t need many iterations, and the phase compensation mask used for focusing spot at a predefined position on the MMF output facet can be obtained by using a two-step phase-shifting technique, and a series of phase compensation masks can be quickly obtained by using the parallel method, so as to form focused spots at different predefined positions. Experimentally, we obtained 100 focused spots, the average focused efficiency was 30.12%, the average focused diameter was 2.3382μμm, and the quality of the focused spot was improved compared with previous reports.
The image distortions caused by the inherent mode dispersion and coupling of the multimode fiber (MMF) lead its output light field to be scattered and prevent it from applicating in endoscopy. Although various wavefront shaping methods have been proposed to overcome these image distortions and form the focused spots through the MMF, they a re usually time-consuming due to the multiple iterations and tedious calculation. In this paper, we present a binary amplitude-only modulation parallel coordinate algorithm for focusing and scanning light through a multimode fiber (MMF) based on the digital micro-mirror device (DMD) in a reference-free multimode fiber imaging system. In principle, our algorithm is capable of efficiently calculating the masks to be added to DMD for yielding a series of tightly focused spots; and for the same number of modulation sub-regions, our method is more than M (the number of focused spots) times faster than the amplitude iterative optimization algorithm. In the experiment, efficient light focusing and scanning at the distal end of the MMF without the iteration process are demonstrated. Furthermore, we demonstrate that the proposed method can also be extended to focus and scan light at multiple planes along the axial direction by just modifying the input wavefront accordingly. We predict the high-speed focusing method through the MMF might have the potential application for fast spot-scanning imaging.
The image distortions caused by the inherent mode dispersion and coupling of the multimode fiber (MMF) lead its output light field to be scattered and prevent it from applicating in endoscopy. Although various wavefront shaping methods have been proposed to overcome these image distortions and form the focused spots through the MMF, they are usually time-consuming due to the multiple iterations and tedious calculation. In this paper, based on the phase-only modulation of the spatial light modulator (SLM) and the parallel algorithm, we experimentally demonstrate the highspeed focusing through the MMF without the iteration process. Our parallel algorithm, including the online speckle recording process and the offline compensated phase calculation process, allows it possible to calculate the compensated phase masks for generating several focused spots at different predefined positions at one time, which is quite timesaving. Experiment shows that a series of focused spots at arbitrary target positions at the MMF output facet are efficiently generated by just one-loop accessing the SLM and the speckle capturing-device and one-time phase retrieval. Besides, focusing at different depths along the output light axis of the MMF is demonstrated by modifying the compensated phase masks. We predict the high-speed focusing method through the MMF might have the potential application for fast spotscanning imaging.
Imaging by a multimode optical fiber (MMF) has attracted much attention because single MMF could be used as a thinner endoscope probe than traditional ones. One essential feature of the MMF endoscope probe is its endoscopic imaging mode. It is well known that clearly imaging through an MMF is somewhat difficult, this is due to the fact that light waves transmitting in an MMF will be modulated randomly because of the mode dispersion in it. What’s worse, for endoscopic imaging case, such a random modulation effect will be suffered twice: on the way in for illuminating and on the way out for imaging. In this paper, by measuring the monochromatic transmission matrix (TM) of the MMF in the case of working under the endoscopic imaging mode, we realize an MMF-based round-trip imaging through phaseshifting interferometry measurements by use of a spatial light modulator. Experimental results indicate that the images of objects can be effectively recovered using the presented approach. It is found that this approach has several advantages. Firstly, it avoids scanning objects, thus leading to quickly imaging; Secondly, it can also recover 3D information of the objects from seriously distorted optical fields emerging from the MMF. We predict that this approach might be applied for single-fiber rapid 3D endoscopic imaging.
KEYWORDS: Scattering, Transmittance, Scattering media, Image segmentation, Digital micromirror devices, Light scattering, Real time imaging, Modulation, Laser scattering, Binary data
The inhomogeneity of scattering media will distort the propagation of the waves, which is detrimental to the performance of optical imaging. Although various approaches have been proposed recently to overcome this problem, they are not suitable to image the rapid-movement objects as a long sequence of measurement steps are required. In this paper, based on the optical transmission matrix of the scattering medium, we show that the information of the object can be recovered directly from the distorted output optical field. Especially, our method is effective to the thick scattering medium. We predict it might have the potential application for real-time imaging.
Light wave becomes extremely distorted when it passes through a turbid medium. Indeed, the inhomogeneity of scattering medium and the mode dispersion of multimode optical fiber (MMF) always distort the propagation of light waves since they divert the propagation direction and disorder the spatial relationship of rays from the object. This becomes a big challenge for the applications of biological tissues endoscopic imaging. To overcome this problem, many methods based on computational optical imaging schemes have been reported and such a research has become a hot topic in recent years. These methods include the computational ghost imaging, the digital phase conjugation, the speckle correlation, the wavefront shaping, and the optical transmission matrix, etc. In this paper, we report our recent works on computational optical imaging based on digital wavefront modulation, which might be useful for the applications of endoscopy. On one hand, we propose a round-trip imaging method based on the optical transmission matrix of scattering medium, where the light wave is distorted twice. The object is recovered directly from the distorted output wave, while no scanning is required during the imaging process; one the other hand, by modulating the amplitude instead of the phase of the incident light wavefront, we propose a high-speed binary amplitude-only modulation method to focus and scan light through an MMF based on the digital micro-mirror device (DMD). This method can also be extended to focus and scan light at multiple planes along the axial direction by just modifying the input wavefront accordingly.
The inhomogeneity of scattering medium distorts the propagation of the waves, which has been detrimental to the performance of optical imaging. The operating time of the traditional solutions will be very long as the scanning is necessary during the imaging. A recovery solution based on spatial optical transmission matrix has been proposed. With the acquiring of the spatial optical transmission matrix, the incident object wave will be recovered directly from the distorted transmitted wave, in this way, only a single shot is needed during the imaging. The effectiveness of this method has been proved by the simulation and experiment, the principle is simpler and the algorithm is more efficient, which are beneficial to the imaging through the scattering medium.
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