Modeling of iterative time-reversed ultrasonically encoded optical focusing in a reflection mode.

Time-reversed ultrasonically-encoded (TRUE) optical focusing is a promising technique to realize deep-tissue optical focusing by employing ultrasonic guide stars. However, the sizes of the ultrasound-induced optical focus are determined by the wavelengths of the ultrasound, which are typically tens of microns. To satisfy the need for high-resolution imaging and manipulation, iterative TRUE (iTRUE) was proposed to break this limit by triggering repeated interactions between light and ultrasound and compressing the optical focus. However, even for the best result reported to date, the resolutions along the ultrasound axial and lateral direction were merely improved by only 2-fold to 3-fold. This observation leads to doubt whether iTRUE can be effective in reducing the size of the optical focus. In this work, we address this issue by developing a physical model to investigate iTRUE in a reflection mode numerically. Our numerical results show that, under the influence of shot noises, iTRUE can reduce the optical focus to a single speckle within a finite number of iterations. This model also allows numerical investigations of iTRUE in detail. Quantitatively, based on the parameters set, we show that the optical focus can be reduced to a size of 1.6 µm and a peak-to-background ratio over 104 can be realized. It is also shown that iTRUE cannot significantly advance the focusing depth. We anticipate that this work can serve as useful guidance for optimizing iTRUE system for future biomedical applications, including deep-tissue optical imaging, laser surgery, and optogenetics.

Characterization of the spectral memory effect of scattering media.

The optical memory effect is an interesting phenomenon exploited for deep-tissue optical imaging. Besides the widely studied memory effects in the spatial domain to accelerate point scanning speed, the spectral memory effect is also important in multispectral wavefront shaping. Although being theoretically analyzed for decades, quantitative studies of spectral memory effect on a variety of scattering media including biological tissue were rarely reported. In practice, quantifying the range of the spectral memory effect is essential in efficiently shaping broadband light, as it determines the optimum spectral resolution in realizing spatiotemporal focus through scattering media. In this work, we analyze the spectral memory effect based on a diffusion model. An explicit analytical expression involves the illumination wavelength, the diffusion constant, and the sample thickness is derived, which is consistent with the one in the literature. We experimentally quantified the range of spectral correlation for two types of biological tissue, tissue-mimicking phantoms with different concentrations, and diffusers. Specifically, for tissue-mimicking phantoms with calibrated scattering parameters, we show that a correction factor of more than 20 should be inserted, indicating that the range of spectral correlation is much larger than one would expect. This finding is particularly beneficial to multispectral wavefront shaping, as stringent requirements on the spectral resolution could be alleviated by at least one order of magnitude.

Autosetting soliton pulsation in a fiber laser by an improved depth-first search algorithm.

Soliton pulsation is one of the most fascinating phenomena in ultrafast fiber lasers, owing to its rich nonlinear dynamics and potential generation of high peak power pulse. However, it is still a challenge to efficiently search for pulsating soliton in fiber lasers because it requires a fine setting of laser cavity parameters. Here, we report the autosetting soliton pulsation in a passively mode-locked fiber laser. The parameters of electronic polarization controller are intelligently adjusted to search for pulsating soliton state by the improved depth-first search algorithm. Moreover, the intensity modulation depth of pulsating soliton could be flexibly controlled. These findings indicate that the intelligent control of a fiber laser is an effective way to explore on-demand soliton dynamics and is also beneficial to the optimization of ultrafast laser performance.

Feedback-assisted transmission matrix measurement of a multimode fiber in a referenceless system.

Recent development in wavefront shaping shows the promise to employ multimode fibers (MMFs) to deliver images in endoscopy. In these applications, retrieving the transmission matrix (TM) of the MMF is especially important. Among existing non-holographic approaches, feedback-based wavefront shaping requires a large number of measurements, while directly measuring the TM can be easily trapped into local optimums if the constraints are insufficient. To reduce the required number of measurements, we combine the concepts of these two approaches and develop a scheme termed feedback-assisted TM measurements. We show that under such a hybrid scheme, less than 3N intensity measurements are sufficient to accurately retrieve one row of the TM that contains N unknown complex elements. As a proof of concept, we experimentally demonstrated retrieving multiple rows of the TM of an MMF using the proposed scheme with high fidelity. In particular, a single focus and dual foci through the MMF with enhancements larger than 75% of the theoretical values were reported.

Adaptive intensity transformation-based phase retrieval with high accuracy and fast convergence.

Phase-retrieval (PR) receivers can reconstruct complex-valued signals from two de-correlated intensity measurements, without the assistance of any optical carriers. However, both the calculation complexity with hundreds of iterations and the limited PR accuracy prevent it from being applied to high-speed photonic interconnections. Here we propose and demonstrate a PR receiver based on adaptive intensity transformation, with the capability of both fast convergence and high accuracy. Then we numerically reconstruct 56 GBaud QPSK signals after the 80 km standard single-mode fiber transmission by using our proposed PR receiver with only 50 iterations. In comparison with the recently reported PR receiver with the phase reset, our proposed PR receiver can reduce the required optical signal-to-noise ratio by 1.95 and 1.89 dB, in terms of 20% soft-decision and 7% hard-decision forward-error correction, respectively.

Experimental demonstration of a broadband optoelectronic chaos system based on highly nonlinear configuration of IQ modulator.

We experimentally investigated a novel broadband optoelectronic chaos generation scheme. The proposed system is achieved by adopting the highly nonlinear operation of an electro-optical exclusive-NOR (XNOR) logic gate and two delayed feedback loops that refer to the Boolean chaos model. The XNOR gate is established by a commercial use inphase and quadrature-phase (IQ) modulator that works at a specific bias point. The resulting power spectrum of the broadband chaos signal extends from DC to 29.1 GHz (10 dB bandwidth), and the probability density distribution is Gaussian distribution like. Owing to the strong nonlinearity of XNOR operation, the conditions to enter the chaos region are more relaxed compared to traditional optoelectronic oscillator (OEO) chaos systems, and the time delay signature (TDS) of the feedback loop is also suppressed. Moreover, to further enhance the performance of the generated chaos signal, we injected the optoelectronic chaotic signal into a semiconductor laser. Experimental results indicate that after the cascade optical injection, the bandwidth of the output chaos signal is extended to 38.4 GHz and the TDS is completely concealed; meanwhile, a perfect Gaussian distribution can be obtained.

[Posterolateral minimal incision, poking reduction, and simple internal fixation in treatment of collapsed fractures of posterolateral tibial plateau].

Objective: To explore the effectiveness of limited incision, poking reduction, and simple internal fixation in the treatment of collapsed fractures of the posterolateral tibial plateau. Methods: Between October 2010 and January 2016, 16 patients with collapsed fractures of the posterolateral tibial plateau underwent posterolateral incision, poking reduction, and simple internal fixation. There were 10 males and 6 females with the age of 22-63 years (mean, 43.5 years). The injury was caused by falling in 5 cases, traffic accident in 7 cases, and falling from height in 4 cases. All cases had closed fractures. The left knee was involved in 9 cases and the right knee in 7 cases. The injury-to-admission time was 2 hours to 3 days (mean, 10 hours). X-ray films showed that the articular surface collapsing was more than 2 mm. According to Schatzker criteria, 6 cases were rated as type II and 10 cases as type III. Twelve cases had fracture of fibular head. The incision length, operation time, intraoperative blood loss, and incision healing were recorded; fracture healing was observed, and tibial plateau angle and posterior slope angle were measured on X-ray films; loss of articular surface reduction was observed by CT scan; and American Hospital for Special Surgery (HSS) score was used to evaluate the knee joint function. Results: The incision length was 7-10 cm (mean, 8.6 cm); operation time was 35-55 minutes (mean, 46 minutes); intraoperative blood loss was 10-35 mL (mean, 28 mL). Primary healing of incision was obtained. Skin pain occurred in 1 case at 2 months because Kirschner wire retracted. Fifteen cases were followed up 8-21 months (mean, 13.5 months). The fracture healing time was from 3 to 6 months (mean, 4.8 months). There was no significant difference in tibial plateau angle and posterior slope angle between at immediate after operation and at last follow-up ( t=-1.500, P=0.156; t=-1.781, P=0.097). The anatomic reduction rate of articular surface was 93.8% (15/16) at immediate after operation. At last follow-up, the recollapse height of articular surface was 0.1-1.2 mm (mean, 0.36 mm). According to the HSS score system, the results were excellent in 12 cases, good in 2 cases, and fair in 1 case, and the excellent and good rate was 93.3%. Conclusion: The limited incision by posterolateral approach, poking reduction, and simple internal fixation have the advantages of small injury, full exposure, and easy operation in the treatment of simple posterolateral tibial plateau fractures; bone graft support and simple internal fixation can prevent recollapse of the articular surface and achieve satisfactory knee function.