Optical imaging through scattering media such as ground glass, fog, biological tissues, etc. has always been a widely used and challenging task in the optical field. Compared with traditional imaging methods such as transmission matrix and optical phase conjugation, deep learning has shown great potential in this field because of its simple device and fast reconstruction speed. In this article, we developed an algorithm based on convolutional neural network to realize imaging through scattering media and applied this algorithm to recover complex images. The speckle images of the original images are obtained through a speckle generation program, and then the speckle images and the original images are input into the neural network in pairs for training. Finally, the reconstructed speckle images can be obtained by using the trained neural network. In the numerical simulation, we proposed two indicators, peak signal-to-noise ratio (PSNR) and structural similarity (SSIM), to evaluate the quality of reconstructed images. The results show that our method can restore highfidelity images. This new image reconstruction method provides new ideas for research in the fields of astronomy and biomedicine.
The research of light focusing and imaging through scattering media is currently a popular topic. Many focusing technologies have been developed, such as transmission matrix method, phase conjugation method, iterative optimization method, etc. However, these methods have some limitations. At present, deep learning methods have been widely used in the field of image restoration, and have strong inverse restoration capabilities. Yet, the application of this method in the field of optical focusing is still relatively limited, and the performance is not ideal. In this letter, a method based on complex neural network is proposed, and the multi-point focusing of light passing through the scattering medium is numerically simulated. Since the complex information of the network is not reduced, compared with the real number neural network method, this method more accurately simulates the recovery process of light scattering, and can obtain multiple light focal points with high enhancement at the same time.
When light passes through the scattering medium, disordered speckles are formed. The wavefront shaping technology can adjust the amplitude or phase of the incident light to achieve focusing and imaging. In recent years, the research on single-point focusing of wavefront shaping has been relatively thorough, while multi-point focusing has been few researched. In order to achieve controllable multi-point light focusing, we combined the phase superposition method with the Hadamard encoding algorithm to develop a new feedback optimization algorithm. Using the constructed transmission matrix model, we made numerical simulations of the algorithm, and finally successfully achieved multipoint uniform focusing and customized proportional focusing, and proved that the algorithm has strong anti-noise and light control capabilities in multi-point light focusing. Our research will have broad application prospects in the fields of optical manipulation and optogenetics in the future.
Ultrashort pulses have been found to have important applications in many fields, such as ultrafast diagnosis, biomedical engineering, and optical imaging. Passively mode-locked fiber lasers have become a tool for generating picosecond and femtosecond pulses. In this paper, the evolution of a picosecond laser pulse in different stable passively mode-locked fiber laser is analyzed using nonlinear Schrödinger equation. Firstly, different mode-locked regimes are calculated with different net cavity dispersion (from ~-0.3 ps2 to ~+0.3 ps2 ). Then we calculate the maximum small-signal gain on the different net cavity dispersion conditions, and estimate the pulse width, 3 dB bandwidth and time bandwidth product (TBP) when the small-signal gain coefficient is selected as the maximum value. The results show that the small signal gain coefficient is approximately proportional to the net cavity. Moreover, when the small signal gain coefficient reaches the maximum value, the pulse width of the output pulse and their corresponding TBP show a trend of increase gradually, and 3dB bandwidth shows a trend of increase firstly and then decrease. In addition, in the case that the net dispersion is positive, because of the pulse with quite large frequency chirp, the revolution to dechirp the pulse is researched and the output of the pulse is compressed and its compression ratio reached more than 10 times. The results provide a reference for the optimization of passively mode-locked fiber lasers.
Focusing light through strongly scattering media plays an important role in biomedical imaging and therapy. Here, we experimentally demonstrate light focusing through ZnO sample by controlling binary amplitude optimization using genetic algorithm. In the experiment, we use a Micro Electro-Mechanical System (MEMS)-based digital micromirror device (DMD) which is in amplitude-only modulation mode. The DMD consists of 1920×1080 square mirrors that can be independently controlled to reflect light to a desired position. We control only 160 thousand mirrors which are divided into 400 segments to modulate light focusing through the scattering media using advanced genetic algorithm. Light intensity at the target position is enhanced up to 50±5 times the average speckle intensity. The diameters of focusing spot can be changed ranging from 7 μm to 70 μm at arbitrary positions and multiple foci are obtained simultaneously. The spatial arrangement of multiple foci can be flexibly controlled. The advantage of DMDs lies in their switching speed up to 30 kHz, which has the potential to generate a focus in an ultra-short period of time. Our work provides a reference for the study of high speed wavefront shaping that is required in vivo tissues imaging.
Kilowatt Ytterbium-doped fiber laser is found widespread application in medical technology, industry and military areas. At present, most of the multi-kilowatt single-mode fiber lasers are achieved by tandem-pumped master oscillator power-amplifier (MOPA) system. When the laser output power reaches kilowatt, the output will be strongly affected by nonlinear effects in the amplifier. The Stimulated Raman Scattering effects is known as the major restrictions to the increase of output signal power. Up to now, Raman effects in conventional diode-pumped amplifier have been well studied while the Raman effects in tandem-pumped has not yet been thoroughly analyzed. In this paper, a theoretical analysis of Raman effects using numerical solution of steady-state rate equations in kilowatts tandem-pumped ytterbium-doped fiber amplifiers is presented. The numerical simulation describing output power characteristics and laser distribution along the fiber is carried under the co-directional end-pumping. Furthermore, an optimization of Raman effects is discussed, which provides a solid foundation for achieving a higher fiber laser output.
The noncooperative and high sensitivity optical displacement measurement technology is very relevant to the study and the determination of high-precision thermal expansion coefficients (TECs) of materials. This paper describes a measurement technology based on Nd:YVO4 laser feedback systems, which can realize fully non-
contact measurement of many kinds of materials with surface reflectivity greater than 10-5. A muffle furnace is
designed with two coaxial holes opened on the opposite furnace walls. This length determination technique is
based on the frequency-shifted optical feedback effects and the heterodyne phase measurement technique. For
validation, the samples are determined in the temperature range 298 to 748K, confirming high sensitivity non-
contact measurement of the materials and demonstrating TEC-measurement capabilities with uncertainties in the range of 10-7 or less.
A method is presented in this paper for resolution calibration of the laser feedback displacement sensor based on Fabry-Perot (F-P) high-order feedback cavity using a conventional feedback system to measure the same displacement with it simultaneously. By calibrating the ratio of the intensity modulated curves gotten by this two different systems, the accurate optical resolution of the integrated system can be obtained. Without using any other subdivision method, the optical resolution can be about 11nm, which is traceable to light wavelength. By adding 20 times electric subdivision, the final resolution of this system is about 0.55nm. The integrated system can fit the requirement of the industrial application, and can also be used for nanometrology.
When light is stored in an optical fiber via stimulated Brillouin scattering (SBS) process, data-pulse
from the storage to the restoration through two processes relates to acoustic wave, so the effects of an
acoustic wave diffraction have to be considered. We numerically solve SBS coupled wave equations
containing acoustic diffraction and study the effects of acoustic wave diffraction on stored light when
the radius of fiber as storage medium is different. The results of light storage are obtained in the two
situations presence and absence of acoustic wave diffraction when the control-pulse and the data-pulse
have different temporal distribution profile. The results show that the acoustic diffraction influence on
the light storage is more and more small with the fiber radius increasing, and when the fiber radius is 6
micron, the effects is disappear. The results also show that acoustic diffraction effects are minimum
when data-pulse and control-pulse are rectangular and Gaussian distribution profile respectively.
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