Active wavefront shaping for controlling and improving multimode fi ber sensor

Wavefront shaping(WFS)techniques have been used as a powerful tool to control light propagation in complex media,including multimode fibers.In this paper,we propose a new application of WFS for multimode fber-based sensors.The use of a single multimode fiber alone,without any special fabrication,as a sensor based on the light intensity variations is not an easy task.The twist effect on multimode fiber is used as an example herein.Experimental results show that light intensity through the multimode fiber shows no direct relationship with the twist angle,but the correlation coefficient(CC)of speckle patterns does.Moreover,if WFS is applied to transform the spatially seemingly random light pattern at the exit of the multimode fiber into an optical focus.The focal pattern correlation and intensity both can serve to gauge the twist angle,with doubled measurement range and allowance of using a fast point detector to provide the feedback.With further development,WFS may find potentials to facilitate the development of multimode fber-based sensors in a variety of scenarios.

A comparative study of optimization algorithms for wavefront shaping

By manipulating the phase map of a wavefront of light using a spatial light modulator,the scattered light can be sharply focused on a specific target.Several iterative optimization algo-rithrns for obtaining the optimumn phase map have been explored.However,there has not been a comparative study on the performance of these algorithms.In this paper,six optimization algorithms for wavefront shaping inchuding continuous sequential,partitioning algorithm,transmission matrix estimation method,particle swarm optimization,genetic algorithm(GA),and simulated annealing(SA)are discussed and compared based on their efficiency when introduced with various measurement noise levels.

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 wavelfront 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 siuperior 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.

Three-dimensional spatial multi-point uniform light focusing through scattering media based on feedback wavefront shaping

We use feedback wavefront shaping technology to realize the multi-point uniform light focusing in three-dimensional(3D) space through scattering media only by loading the optimal mask once.General 3D spatial focusing needs to load the optimal mask multiple times to realize the spatial movement of the focal point and the uniformity of multi-point focusing cannot be guaranteed.First,we investigate the effects of speckle axial correlation and different axial distances on 3D spatial multi-point uniform focusing and propose possible solutions.Then we use our developed non-dominated sorting genetic algorithm suitable for 3D spatial focusing(S-NSGA) to verify the experiment of multi-point focusing in 3D space.This research is expected to have potential applications in the fields of optical manipulation and optogenetics.

Overcoming the penetration depth limit in optical microscopy:Adaptive optics and wavefront shaping

Despite the unique advantages of optical microscopy for molecular specific high resolution imaging of living structure in both space and time,curent applications are mostly limited to research settings.This is due to the aberrations and multiple scattering that is induced by the inhomogeneous refractive boundaries that are inherent to biological systems.However,recent developments in adaptive optics and wavefront shaping have shown that high resolution optical imaging is not fundamentally limited only to the observation of single cells,but can be significantly enhanced to realize deep tissue imaging.To provide insight into how these two closely related fields can expand the limits of bio imaging,we review the recent progresses in their performance and applicable range of studies as well as potential future research directions to push the limits of deep tissuse imnaging.

Lens-free wavefront shaping method for a diffuse non-lineof-sight link in visible light communication

In this Letter,we propose and experimentally demonstrate a lens-free wavefront shaping method that utilizes synchronized signal block beam alignment and a genetic algorithm(SSBGA)for a diffuse non-line-of-sight(NLOS)visible light communication(VLC)system.The proposed method effectively controls the position and mobility of visible light beams by partitioning spatial light modulator pixels and manipulating beams to converge at distinct spatial positions,thereby enhancing wavefront shaping efficiency,which achieves a significant 23.9 dB optical power enhancement at+2 mm offset,surpassing the lens-based continuous sequence(CS)scheme by 21.7 dB.At+40°angle,the improvement reaches up to 11.8 dB and 16.8 dB compared to the results with and without lens-based CS,respectively.A maximum rate of 5.16 Gbps is successfully achieved using bit-power loading discrete multi-tone(DMT)modulation and the proposed SSBGA in an NLOS VLC system,which outperforms the lens-based CS by 1.07 Gbps and obtains a power saving of 55.6%during the transmission at4 Gbps.To the best of our knowledge,this is the first time that high-speed communication has been realized in an NLOS VLC system without a lens.

Wavefront shaping of infrared light through a subwavelength hole

Light passing through a subwavelength hole in an opaque plate is a fundamental concern in both optical science and applications.Using both simulations and experiments,we show that,when a subwavelength hole in a silver thin film is surrounded by well-designed patterns of grooves,the wavefront of the infrared light through it can be shaped into a preset complicated pattern such as a Latin letter‘L’or‘O’at a given position instead of being diffracted in all directions.The design is created via the surface-wave-holography method,which allows direct determination of the surface plasmonic structure for a given wavefront-engineering functionality without the need to solve complex inverse problems.The results will deepen current understanding of this enduring issue and will find applications in many fields such as wave manipulation and sensing.

Reconstitution of optical orbital angular momentum through strongly scattering media via feedback-based wavefront shaping method

Orbital angular momentum(OAM)is a fundamental physical characteristic to describe laser fields with a spiral phase structure.Vortex beams carrying OAMs have attracted more and more attention in recent years.However,the wavefront of OAM light would be destroyed when it passes through scattering media.Here,based on the feedback-based wavefront shaping method,we reconstitute OAM wavefronts behind strongly scattering media.The intensity of light with desired OAM states is enhanced to 150 times.This study provides a method to manipulate OAMs of scattered light and is of great significance for OAM optical communication and imaging to overcome complex environment interference.

An ultrafast reconfigurable nanophotonic switch using wavefront shaping of light in a nonlinear nanomaterial

We demonstrate a new concept for reconfigurable nanophotonic devices exploiting ultrafast nonlinear control of shaped wavefronts in a multimode nanomaterial consisting of semiconductor nanowires.Femtosecond pulsed laser excitation of the nanowire mat is shown to provide an efficient nonlinear mechanism to control both destructive and constructive interference in a shaped wavefront.Modulations of up to 63%are induced by optical pumping,due to a combination of multimode dephasing and induced transient absorption.We show that part of the nonlinear phase dynamics can be inverted to provide a dynamical revival of the wavefront into an optimized spot with up to 18%increase of the peak to background ratio caused by pulsed laser excitation.The concepts of multimode nonlinear switching demonstrated here are generally extendable to other photonic and plasmonic systems and enable new avenues for ultrafast and reconfigurable nanophotonic devices.

Shaping the Wavefront of Incident Light with a Strong Robustness Particle Swarm Optimization Algorithm

We demonstrate a modified particle swarm optimization(PSO) algorithm to effectively shape the incident light with strong robustness and short optimization time. The performance of the modified PSO algorithm and genetic algorithm(GA) is numerically simulated. Then, using a high speed digital micromirror device, we carry out light focusing experiments with the modified PSO algorithm and GA. The experimental results show that the modified PSO algorithm has greater robustness and faster convergence speed than GA. This modified PSO algorithm has great application prospects in optical focusing and imaging inside in vivo biological tissue, which possesses a complicated background.

Optical scanning endoscope via a single multimode optical fiber

Optical endoscopy has become an essential diagnostic and therapeutic approach in modern biomedicine for directly observing organs and tissues deep inside the human body,enabling non-invasive,rapid diagnosis and treatment.Optical fiber endoscopy is highly competitive among various endoscopic imaging techniques due to its high flexibility,compact structure,excellent resolution,and resistance to electromagnetic interference.Over the past decade,endoscopes based on a single multimode optical fiber(MMF)have attracted widespread research interest due to their potential to significantly reduce the footprint of optical fiber endoscopes and enhance imaging capabilities.In comparison with other imaging principles of MMF endoscopes,the scanning imaging method based on the wavefront shaping technique is highly developed and provides benefits including excellent imaging contrast,broad applicability to complex imaging scenarios,and good compatibility with various well-established scanning imaging modalities.In this review,various technical routes to achieve light focusing through MMF and procedures to conduct the scanning imaging of MMF endoscopes are introduced.The advancements in imaging performance enhancements,integrations of various imaging modalities with MMF scanning endoscopes,and applications are summarized.Challenges specific to this endoscopic imaging technology are analyzed,and potential remedies and avenues for future developments are discussed.

Artificial intelligence assisted light control and computational imaging through scattering media

Coherent optical control within or through scattering media via wavefront shaping has seen broad applications since its invention around 2007.Wavefront shaping is aimed at overcoming the strong scattering,featured by random interference,namely speckle patterns.This randomness occurs due to the refractive index inhomogeneity in complex media like biological tissue or the modal dispersion in multimode fiber,yet this randomness is actually deterministic and potentially can be time reversal or precompensated.Various wavefront shaping approaches,such as optical phase conjugation,iterative optimization,and transmission matrix measurement,have been developed to generate tight and intense optical delivery or high-resolution image of an optical object behind or within a scattering medium.The performance of these modula-tions,however,is far from satisfaction.Most recently,artifcial intelligence has brought new inspirations to this field,providing exciting hopes to tackle the challenges by mapping the input and output optical patterns and building a neuron network that inherently links them.In this paper,we survey the developments to date on this topic and briefly discuss our views on how to harness machine learning(deep learning in particular)for further advancements in the field.

Photoinjector beam quality improvementby shaping the wavefront of a drive laser with oblique incidence

To increase the quantum efficiency （QE） of a copper photocathode and reduce the thermal emit- tance of an electron beam, a drive laser with oblique incidence was adopted in a BNL type photocathode rf gun. The disadvantageous effects on the beam quality caused by oblique incidence were analyzed qualitatively. A simple way to solve the problems through wavefront shaping was introduced and the beam quality was improved.

Beam shaping in the high-energy kW-class laser system Bivoj at the HiLASE facility

A fully automatic fail-safe beam shaping system based on a liquid crystal on a silicon spatial light modulator has been implemented in the high-energy kilowatt-average-power nanosecond laser system Bivoj.The shaping system corrects for gain nonuniformity and wavefront aberrations of the front-end of the system.The beam intensity profile and the wavefront at the output of the front-end were successfully improved by shaping.The beam homogeneity defined by the beam quality parameters was improved two to three times.The root-mean-square value of the wavefront was improved more than 10 times.Consequently,the shaped beam from the second preamplifier led to improvement of the beam profile at the output of the first main cryo-amplifier.The shaping system is also capable of creating nonordinary beam shapes,imprinting cross-references into the beam,or masking certain parts of the beam.

Programming nonlinear propagation for efficient optical learning machines

The ever-increasing demand for training and inferring with larger machine-learning models requires more efficient hardware solutions due to limitations such as power dissipation and scalability.Optics is a promising contender for providing lower power computation,since light propagation through a nonabsorbing medium is a lossless operation.However,to carry out useful and efficient computations with light,generating and controlling nonlinearity optically is a necessity that is still elusive.Multimode fibers(MMFs)have been shown that they can provide nonlinear effects with microwatts of average power while maintaining parallelism and low loss.We propose an optical neural network architecture that performs nonlinear optical computation by controlling the propagation of ultrashort pulses in MMF by wavefront shaping.With a surrogate model,optimal sets of parameters are found to program this optical computer for different tasks with minimal utilization of an electronic computer.We show a remarkable decrease of 97%in the number of model parameters,which leads to an overall 99%digital operation reduction compared to an equivalently performing digital neural network.We further demonstrate that a fully optical implementation can also be performed with competitive accuracies.

Second-harmonic generation in periodic fork-shaped χ^((2)) gratings at oblique incidence

Three-dimensional(3D) nonlinear photonic crystals have received intensive interest as an ideal platform to study nonlinear wave interactions and explore their applications. Periodic fork-shaped gratings are extremely important in this context because they are capable of generating second-harmonic vortex beams from a fundamental Gaussian wave, which has versatile applications in optical trapping and materials engineering. However, previous studies mainly focused on the normal incidence of the fundamental Gaussian beam, resulting in symmetric emissions of the second-harmonic vortices. Here we present an experimental study on second-harmonic vortex generation in periodic fork-shaped gratings at oblique incidence, in comparison with the case of normal incidence. More quasi-phase-matching resonant wavelengths have been observed at oblique incidence, and the second-harmonic emissions become asymmetric against the incident beam.These results agree well with theoretic explanations. The oblique incidence of the fundamental wave is also used for the generation of second-harmonic Bessel beams with uniform azimuthal intensity distributions. Our study is important for a deeper understanding of nonlinear interactions in a 3D periodic medium. It also paves the way toward achieving highquality structured beams at new frequencies, which is important for manipulation of the orbital angular momentum of light.

Nonlinear harmonic wave manipulation in nonlinear scattering medium via scattering-matrix method

Scattering of waves, e.g., light, due to medium inhomogeneity is ubiquitous in physics and isconsidered detrimental for many applications. Wavefront shaping technology is a powerful tool to defeatscattering and focus light through inhomogeneous media, which is vital for optical imaging, communication,therapy, etc. Wavefront shaping based on the scattering matrix (SM) is extremely useful in handling dynamicprocesses in the linear regime. However, the implementation of such a method for controlling light in nonlinearmedia is still a challenge and has been unexplored until now. We report a method to determine the SM ofnonlinear scattering media with second-order nonlinearity. We experimentally demonstrate its feasibility inwavefront control and realize focusing of nonlinear signals through strongly scattering quadratic media.Moreover, we show that statistical properties of this SM still follow the random matrix theory. The scattering-matrix approach of nonlinear scattering medium opens a path toward nonlinear signal recovery, nonlinearimaging, microscopic object tracking, and complex environment quantum information processing.

Ultra-thin, planar, Babinet-inverted plasmonic metalenses

We experimentally demonstrate the focusing of visible light with ultra-thin,planar metasurfaces made of concentrically perforated,30-nm-thick gold films.The perforated nano-voids—Babinet-inverted(complementary)nano-antennas—create discrete phase shifts and form a desired wavefront of cross-polarized,scattered light.The signal-to-noise ratio in our complementary nano-antenna design is at least one order of magnitude higher than in previous metallic nano-antenna designs.We first study our proof-of-concept‘metalens’with extremely strong focusing ability:focusing at a distance of only 2.5 mm is achieved experimentally with a 4-mm-diameter lens for light at a wavelength of 676 nm.We then extend our work with one of these‘metalenses’and achieve a wavelength-controllable focal length.Optical characterization of the lens confirms that switching the incident wavelength from 676 to 476 nm changes the focal length from 7 to 10 mm,which opens up new opportunities for tuning and spatially separating light at different wavelengths within small,micrometer-scale areas.All the proposed designs can be embedded on-chip or at the end of an optical fiber.The designs also all work for two orthogonal,linear polarizations of incident light.

Generation of Lommel beams through highly scattering media

Lommel beams have been potential candidates for optical communication and optical manipulation,due to their adjustable symmetry of transverse intensity distribution and continuously variable orbital angular momentum.However,the wavefront of the Lommel beam is scrambled when it transmits through highly scattering media.Here,we explore the construction of Lommel beams through highly scattering media with a transmission matrix-based point spread function engineering method.Experimentally,various Lommel beams with different parameters were generated through a ZnO scattering layer by use of a digital micromirror device.The construction of Lommel beams under high scattering is expected to benefit the optical applications behind highly scattering media.