Yingchun Ding

Focusing light through strongly scattering media by controlling binary amplitude optimization using genetic algorithm

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.

Superpixel-Based Complex Field Modulation Using a Digital Micromirror Device for Focusing Light through Scattering Media

We present a digital micromirror device (DMD) based superpixel method for focusing light through scattering media by modulating the complex field of incident light. Firstly, we numerically and experimentally investigate focusing light through a scattering sample using the superpixel methods with different target complex fields. Then, single-point and multiple-point focusing experiments are performed using this superpixel-based complex modulation method. In our experiment, up to 71.5% relative enhancement is realized. The use of the DMDbased superpixel method for the control of the complex field of incident light opens an avenue to improve the enhancement of focusing light through scattering media.

Numerical study on the maximum small-signal gain coefficient in passively mode-locked fiber lasers

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.

Excitation of random intense single-cycle light-pulse chains in optical fiber

Excitation of intense periodic single-cycle light pulses in a stochastic background arising from continuous wave stimulated Brillouin scattering (SBS) in a long optical fiber with weak optical feedback is found experimentally and modeled theoretically. Such intense light-pulse chains occur randomly and the optical feedback is a requirement for their excitation. The probability of these forms, among the large number of experimental output signals with identifiable waveforms, appearing is only about 3%, with the remainder exhibiting regular SBS characteristics. It is also found that pulses with low period numbers appear more frequently and the probability distribution for their occurrence in terms of the pulse power is roughly L-shaped, like that for rogue waves. The results from a three-wave-coupling model for SBS including feedback phase control agree well qualitatively with the observed phenomena.