Feedback-based Wavefront Shaping Research Articles

A thorough study on genetic algorithms in feedback-based wavefront shaping

Feedback-based wavefront shaping focuses light through scattering media by employing phase optimization algorithms. Genetic algorithms (GAs), inspired by the process of natural selection, are well suited for phase optimization in wavefront shaping problems. In 2012, Conkey et al. first introduced a GA into feedback-based wavefront shaping to find the optimum phase map. Since then, due to its superior performance in noisy environment, the GA has been widely adopted by lots of implementations. However, there have been limited studies discussing and optimizing the detailed procedures of the GA. To fill this blank, in this study, we performed a thorough study on the performance of the GA for focusing light through scattering media. Using numerical tools, we evaluated certain procedures that can be potentially improved and provided guidance on how to choose certain parameters appropriately. This study is beneficial in improving the performance of wavefront shaping systems with GAs.

Control the normalized polarization ratio of a focal spot in speckle field formed by non-polarization-maintaining multimode fiber

Due to the effects such as mode coupling, bending, mode dispersion, etc, a speckle pattern is generated by passing a coherent laser beam through a multimode fiber. For a non-polarization-maintaining multimode fiber, the randomization also occurs in the polarization distribution. Feedback-based wavefront shaping has been applied in modulating the speckle generated by a scattering medium. Based on the feedback-based wavefront shaping, we propose the combination method to modulate the normalized polarization ratio (NPR) of a focal spot in the speckle field formed by non-polarization-maintaining multimode fiber. The quantitative control of the NPR is achieved by amplitude modulation or phase modulation.

Far-field super-focusing by a feedback-based wavefront shaping method

Abbe diffraction limit has always been an important subject in conventional far-field focusing and imaging systems, where the resolution of an image is usually limited to 0.5λ/NA. Recently, the studies of the optical super-oscillation lens (SOL) enable us to break the limitation in both theory and practice successfully. Here a genetic algorithm was introduced to design the SOL phase more controllably and precisely obtain much better focusing such as the focal spot with 0.105λ/NA (or 79.0% minification) in the simulation and 65.5% minification in the experimental demonstration. This technique is of great significance in advanced optical lithography or biology microscopy, because it promises non-invasive unlabelled imaging from the far field.

Improving Depth Sensitive Fluorescence Spectroscopy With Wavefront Shaping by Spectral and Spatial Filtering

Depth sensitive optical spectroscopy preferentially detects optical spectra from different depths in layered samples, which plays a crucial role in many applications such as the optical diagnosis of epithelial precancer and cancer. In depth sensitive optical measurements, multiple light scattering in tissues significantly degrades the depth sensitivity to a subsurface target layer. To address this issue, feedback based wavefront shaping led by guide stars can be used to refocus light to increase the depth sensitivity to a target layer. However, the lack of intrinsic guide stars in tissues or tissue-like samples often leads to poor enhancement in depth sensitive Raman/fluorescence measurements (~20% in the past literature) from the target layer due to the contribution from the overlaying non-target layer. In this study, we demonstrate that spatial filtering and spectral filtering can significantly improve the performance of depth sensitive fluorescence spectroscopy assisted by feedback based wavefront shaping in tissue-like scattering phantoms. The two filtering techniques work by effectively increasing the relative contribution from the target layer to the feedback signal during wavefront optimization through spatially and spectrally rejecting off-target fluorescence light, which is essentially similar to the role of time or coherence gating. When the filtering techniques are applied, a maximum of three-fold enhancement in fluorescence contribution from the target layer is observed, which is in contrast to nearly no enhancement in case of no filtering. This significant enhancement has not been reported previously for depth sensitive optical spectroscopy in the area of feedback based wavefront shaping. Therefore, our work represents a new advance towards the application of wavefront shaping in depth resolved optical spectroscopy for the characterization of layered structures such as epithelial tissues or drug tablets, in which the creation of an external guide star is challenging or not allowed.

Enhancement of surface plasmon polariton excitation via feedback-based wavefront shaping.

Surface plasmon polariton (SPP) is an electromagnetic excitation with efficient spatial confinement and high local field intensity at a metal/dielectric interface, which has been widely applied in many fields such as nanophotonics, imaging, biosensing, nonlinear optics, and so on. However, the destructive interference, which arises from wavevector mismatching between the spatial components of incident light and SPP, limits the effective excitation of SPP.Here, we experimentally demonstrate the enhancement of SPP excitation via a feedback-based wavefront shaping method in the Kreschmann configuration. After optimizing the phase profile of the incident laser beam, the intensity is enhanced by a factor of 1.58 times even at the resonance angle of the fundamental mode Gaussian beam. Besides, the influences of different conditions for the enhancement of SPP excitation are also analyzed. This work provides a flexible and convenient method to further enhance the SPP excitation, and it may have the application of further enhancement of the interaction between SPP and other physical processes.

Manipulation of the spontaneous parametric down-conversion process in space and frequency domains via wavefront shaping.

The spontaneous parametric down-conversion (SPDC) source of entangled-photon pairs is important for applications in the quantum information process and quantum communication, but suffers from scattering by sample defect and air impurity. Here, we proposed an alternative scheme to manipulate the scattered SPDC process, where only a spatial light modulator was used to control the incident wavefront. The scheme was experimentally tested and also applied on the manipulation of photon pairs through the SPDC process with spectral control. This work proved the feasibility of manipulating nonlinear signals at quantum level with feedback-based wavefront shaping and also indicated applications in long-distance quantum key distribution, quantum communications, and quantum imaging, especially in complex environments.

Recovery of wave-mixing conversion efficiency in weakly scattering nonlinear crystals.

Nonlinear optical wave mixing is a widely used method to produce light with new frequencies that has a significant impact on laser technology and optical imaging. The most important figure of merit in wave-mixing processes, i.e., high conversion efficiency, is always required in laser applications. We demonstrate a method to recover high conversion efficiency of second harmonic generation in a BaMgF4 single crystal with weakly scattering defects via feedback-based wavefront shaping under birefringent phase-matching condition. By optimizing the fundamental wavefront, a typical second harmonic output with an enhancement factor of 1.14 and a corresponding recovery efficiency of 86.3% is displayed. This investigation may modify the wide understanding of scattering in crystals and provide an avenue to recover the nonlinear optical conversion efficiency in crystals with various defects without special fabrications.

Adaptive pumping for spectral control of broadband second-harmonic generation.

Second-harmonic generation (SHG) is always a significant frequency conversion process in nonlinear optics for many great applications but can be limited when broadband spectral laser sources are involved, e.g.,femtosecond pulses. The conversion efficiency can be high, but the spectral control is hard because of the phase-matching (PM) limitation. Recently, a random quasi-phase-matching (QPM) scheme was proposed to make use of highly nonlinear materials that are difficult to be phase matched under traditional configurations. The spectral control is even harder in anisotropic random materials, and the coherence is completely lost. Here, we proposed an approach to solve this problem by coherent light control via feedback-based wavefront shaping. We utilized this method for spectral control of broadband SHG, which can be efficient even in strongly scattering media. Randomly selected wavelengths in the broadband spectra were enhanced with a good selectivity, and the direction was also controlled in a three-dimensional (3D) configuration. This technique paves the way for convenient spatial and spectral control of both linear and nonlinear emissions and a local enhancement of their conversion efficiency, indicating great progress in both random and ultrafast nonlinear optics.

Focusing light through scattering media by transmission matrix inversion

Focusing light through scattering media has broad applications in optical imaging, manipulation and therapy. The contrast of the focus can be quantified by peak-to-background intensity ratio (PBR). Here, we theoretically and numerically show that by using a transmission matrix inversion method to achieve focusing, within a limited field of view and under a low noise condition in transmission matrix measurements, the PBR of the focus can be higher than that achieved by conventional methods such as optical phase conjugation or feedback-based wavefront shaping. Experimentally, using a phase-modulation spatial light modulator, we increase the PBR by 66% over that achieved by conventional methods based on phase conjugation. In addition, we demonstrate that, within a limited field of view and under a low noise condition in transmission matrix measurements, our matrix inversion method enables light focusing to multiple foci with greater fidelity than those of conventional methods.

Second-harmonic focusing by a nonlinear turbid medium via feedback-based wavefront shaping.

Scattering has usually been considered detrimental for optical focusing or imaging. Recently, more and more research has shown that strongly scattering materials can be utilized to focus coherent light by controlling or shaping the incident light. Here, purposeful focusing of second-harmonic waves, which are generated and scattered from nonlinear turbid media via feedback-based wavefront shaping, is presented. This Letter shows a flexible manipulation of both disordered linear and nonlinear scattering signals, indicating more controllable degrees of freedom for the description of turbid media. This technique also provides a possible way to an efficient transmission of nonlinear signal at a desired location in the form of a focal point or other patterns. With the combination of random nonlinear optics and wavefront shaping methods, more interesting applications can be expected in the future, such as nonlinear transmission matrix, multi-frequency imaging, and phase-matching-free nonlinear optics.

Focusing light into desired patterns through turbid media by feedback-based wavefront shaping

We demonstrate that the focusing of light into desired patterns through turbid media can be realized using feedback-based wavefront shaping. Three desired focused patterns—a triangle, a circle, and a rectangle—are used as examples to study this ability. During the process of modulating scattered light, the Pearson’s correlation coefficient is introduced as a feedback signal. It is found that the speckle field formed by the turbid media gradually transforms into the desired pattern through a process of modulation of the input beam wave front. The proposed approach has potential applications in biomedical treatment and laser material processing.

Feedback-based wavefront shaping.

Light scattering was thought to be the fundamental limitation for the depth at which optical imaging methods can retain their resolution and sensitivity. However, it was shown that light can be focused inside even the most strongly scattering objects by spatially shaping the wavefront of the incident light. This review summarizes recently developed feedback-based approaches for focusing light inside and through scattering objects.