Wave Plate Research Articles

Low-noise 2-GHz figure-9 fiber laser based on passive harmonic mode-locking

We have proposed and demonstrated the generation of a high-repetition-rate ultrashort pulse with long-term stability and low noise based on a harmonic mode-locked (HML) figure-9 fiber laser. Different HML orders from the 2nd to the 13th are generated by adjusting the pump power, net dispersion, and wave plate angles. A 2-GHz HML pulse is obtained with a 155-MHz fundamental repetition rate in a Yb-doped fiber laser, and the corresponding supermode suppression level is as high as 50 dB. The average power of the output pulse is nearly 200 mW, and the compressed pulse duration is 535 fs. To the best of our knowledge, 2 GHz and 196 mW represent, respectively, the highest repetition rate and output power in a figure-9 fiber laser. This work offers a potential pathway to achieve high-repetition-rate ultrashort pulses in the GHz range with high power and low noise levels.

Polarization-insensitive optical coherence tomography using pseudo-depolarized reference light for mitigating birefringence-related image artifacts.

Optical coherence tomography (OCT) images are prone to image artifacts due to the birefringence of the sample or the optical system when a polarized light source is used for imaging. These artifacts can lead to degraded image quality and diagnostic information. We aim to mitigate these birefringence-related artifacts in OCT images by adding a depolarizer module in the reference arm of the interferometer. We investigated different configurations of liquid crystal patterned retarders as pseudo-depolarizers in the reference arm of OCT setups. We identified the most effective depolarization module layout for polarization artifact suppression for a spectral-domain OCT system based on a Michelson and a Mach-Zehnder interferometer. The performance of our approach was demonstrated in an achromatic quarter-wave plate allowing the selection of a variety of sample polarization states. A substantial improvement of the OCT signal magnitude was observed after placing the optimal depolarizer configuration, reducing the cross-polarization artifact from 5.7 to 1.8dB and from 8.0 to 1.0dB below the co-polarized signal for the fiber-based Michelson and Mach-Zehnder setup, respectively. An imaging experiment in the birefringent scleral tissue of a post-mortem alpine marmot eye and a mouse tail specimen further showcased a significant improvement in the detected signal intensity and an enhanced OCT image quality followed by a drastic elimination of the birefringence-related artifacts. Our study presents a simple yet cost-effective technique to mitigate birefringence-related artifacts in OCT imaging. This method can be readily implemented in existing OCT technology and improve the effectiveness of various OCT imaging applications in biomedicine.

Controllable temporal twisting polarization within an ultrafast laser pulse.

We propose a method for controlling the time-varying polarization of optical pulses by introducing a quarter-wave plate into a 4-f pulse shaper and using a spatial light modulator to impose a group delay. This setup enables the polarization state of the incident pulse to vary over time. Specifically, for linearly chirped incident pulses, the polarization ellipse twists uniformly over time, while its ellipticity changes monotonically. As the group delay increases, the pulse intensity gradually splits in the time domain, transitioning from a single pulse with uniformly twisting polarization to two circularly polarized pulses with opposite chirality. We provide a detailed and comprehensive explanation of this modulation process using analytical expressions.

Low-frequency propagating and evanescent waves in strongly inhomogeneous sandwich plates

The paper aims at studying dispersion of elastic waves in a sandwich plate with the parameters, characteristic of aerogel core and hard skin layers, typical for aerospace applications including optimal design of fuselage structural components. The proposed approach relies on multiparametric analysis, taking into account the effect of strong transverse inhomogeneity. It is demonstrated that both an additional low-frequency propagating wave and a slowly decaying evanescent one appear due to a high contrast in geometric and mechanical parameters of the layers. The key findings include the derivation of two-mode asymptotic expansions of the full dispersion relation at the low-frequency limit, as well as elucidation of the non-trivial link between long-wave evanescent and propagating modes. A sophisticated composite nature of the obtained expansions involving various shortened forms is investigated. The range of validity for each of these forms over frequency and wave-number domains is evaluated. Comparison of asymptotic results with the numerical solution of the full dispersion relation is presented.

Modeling and Analysis of Ellipticity Dispersion Characteristics of Lamb Waves in Pre-stressed Plates

Modeling and Analysis of Ellipticity Dispersion Characteristics of Lamb Waves in Pre-stressed Plates

Multichannel vortex beam array based on quarter-wave plate metasurface

Multichannel vortex beam array based on quarter-wave plate metasurface

A high-precision visual detection method for rail profile based on Scheimpflug condition

Rail profile is an important indicator of rail service status, which should be always kept in good condition. How to efficiently collect rail profiles with high precision is a critical issue for their condition-based maintenance. This paper focuses on the method for accurate visual detection of rail full profile (RFP), including optical modeling, visual calibration, and error compensation. The proposed optical unit, featuring double line structured-light sensors, introduces an innovative method under the Scheimpflug condition to avoid the limitation of depth of field in RFP measurement. Subsequently, double visual sensors are calibrated together using the common reference target to extract the internal and external parameters respectively. The dynamic errors resulting from the impact of vehicle vibrations are ultimately investigated to rectify the distorted profiles to the normal. Both laboratory and field tests are performed to verify the effectiveness of the proposed system in terms of precision and efficiency.

Interpretable machine learning study of a collector based on combined twisted-tape and wavy-tape inserts

Nowadays, the efficiency of air collectors for solar thermal applications is still low, and many researchers tend to use machine learning to predict and model the performance of thermal systems, but most of the existing machine learning methods are uninterpretable, which poses a challenge for machine learning applications. In this paper, a new collector insert with enhanced heat transfer in the form of a combination of wave and helical twisted bands is firstly designed for performance test experiments using solar air collectors. Then, based on the test data, three mutually interpretable machine learning methods, PDP, ALE, and SHAP, are explored for predictive studies of collector performance. The results show that the average efficiency of the collector with inserted structure increases by 19.71 %, 12.25 %, and 17.53 % at inlet flow rates of 2.1 m/s, 3.3 m/s, and 4.5 m/s, respectively. The highest collector efficiency was achieved with a wave plate length of 360 mm, a helical twist ratio of 5.14, and an inlet flow rate of 4.5 m/s. Understanding how much the input affects the output through the interpretability of SHAP proves the value of interpretable machine learning, which is useful in guiding the modification of the collector structure.

Error analysis based on a tunable wave plate polarization interferometric imaging spectrometer

Interference imaging spectroscopy combines modern imaging technology with spectral technology, holding significant importance for object imaging and spectral detection. This article introduces the principle of an adjustable wave plate polarization interferometric imaging spectrometer. The example design specifications are set for an observation wavelength range of 450–780 nm and a maximum resolution of 2 nm at 450 nm, with a 0.5 in detector as the base for calculating the specific dimensions of the Soleil–Babinet compensator. An investigation was conducted on the issues of nonuniform sampling, as well as three types of mechanical errors: flatness, wedge angle tolerance, and optical axis orientation accuracy. Emphasis was placed on discussing the impact of these errors on the instrument’s optical path difference and spectral reconstruction accuracy. This research provides theoretical guidance for the design and engineering of this miniaturized imaging spectrometer.

Dielectric metasurface Fresnel zone plates for polarization conversion

Abstract Conventional Fresnel zone plates (FZPs) can only achieve a single focusing function and require the combination with other optical elements to achieve multiple optical functions. This contradicts the development trends for miniaturized, integrated and multifunctional optical devices. However, the emergence of metasurfaces offers new solutions for this problem. In this paper, we design two different types of multifunctional metasurface Fresnel zone plates (MFZPs). One is based on amplitude modulation, and the other is based on phase modulation, both of which can achieve linear polarization conversion and focusing functions. The realization of these functions is based on the ability of silicon diatomic nanopillars to decouple and control the amplitude, phase, and polarization of electromagnetic waves. The designed ultrathin dielectric metasurface effectively combines the functions of conventional half-wave plates and FZPs, thereby reducing the volume of the optical system. The designed MFZPs have enormous potential for application in integrated and compact optical systems.

Polarization self-modulation in a coaxially end-pumped orthogonally polarized laser

A scheme of orthogonally polarized laser (OPL) with polarization self-modulation (PSM) in the coaxial end-pumping configuration is proposed in this work. The gain medium consisted of two Nd:YVO4 crystals with orthogonal optical axes, and a quarter-wave plate served as a polarization-switching device. By rotating the electric vector during each cavity round trip, frequency degeneration and correlated operation of two polarization components were achieved. The polarization eigenstates in two orthogonal directions were analyzed using the Jones matrix and eigenequation. The spectral distribution was also briefly discussed based on thermal effects. The results showed that two eigenmodes located at 45° and 135° to the crystal axis, respectively, with a frequency difference equaled to half the free spectral range of the resonator. In the experiments, it was found for the first time that the modulation of eigenstates can be realized by adjusting pump parameters (including pump power, beam size, focusing position, and pump wavelength) and cavity anisotropy, which enables the manipulation of degree of polarization (DOP) and monitoring the resonator properties. Overall, tuning the laser polarization eigenstate offers a flexible method to switch the operational states of the OPL, including the longitudinal mode frequency difference, DOP, polarization angle, etc. It is believed this scheme provides a valuable reference to realize compact, high-beam-quality, and robust OPLs for applications such as dual-frequency light source development, precision measurement and polarization control.

Meta-optics empowered by all-silicon quarter-wave plates for generating focused vortex beams

The Poynting vector associated with the focused vortex beam (FVB) that carries orbital angular momentum (OAM) is oriented in a twisted manner relative to the principal axis of propagation. This characteristic has significant applications in advanced fields, including high-dimensional information processing, high-resolution imaging, and particle manipulation. Currently, complex optical systems that operate on alignment principles for generating FVBs have been miniaturized by metasurfaces, resulting in the achievement of polarization-dependent vectorized behavior. Nevertheless, generating and manipulating FVBs that carry OAM in the terahertz (THz) range remains a significant challenge. This difficulty arises particularly when utilizing quarter-wave plates (QWPs) that serve both polarization conversion and polarization filtering functions. Here, we experimentally demonstrate a planar all-dielectric array capable of generating FVBs with high power density within a single-handed circularly polarized channel. Engineered QWP meta-atoms are utilized as candidates for the efficient generation of the desired FVB through the application of dynamic phase gradients. A series of samples were fabricated to assess the effectiveness of this design strategy in converting an incident linearly polarized THz beam into arbitrary single-handed circularly polarized FVBs. Leveraging the high degree of freedom inherent in polarization multiplexing coding, the proposed QWP metasurface can generate FVBs exhibiting topological charge evolution along the longitudinal direction. This capability further underscores its robust polarization modulation proficiency. This work presents a generalized framework for the polarization-dependent generation of ultracompact structural optical fields, which may have potential applications in highly integrated THz communication systems.

The Multi-Frame Imaging Detection of Ultrasonic Guided Waves in Welded Structural Plates Based on Arc Sparse Array with Left Rank

The imaging detection of ultrasonic guided waves in plates using arc sparse arrays is highly significant for weld scattering conditions. A novel approach for detecting welded plate structures using the left rank of ultrasonic guided waves in arc sparse arrays was proposed. The relationship between the receiving matrix and the left rank was analyzed, along with the connection between the arc sparse array with the left rank and the receiving aperture. The imaging mechanism of the ultrasonic guided waves in arc sparse arrays with left rank under weld scattering conditions was investigated. The results of imaging experiments demonstrated a downward trend in the gray and background gray of the multi-frame images. As the left rank reaches approximately 64% of the full rank, the slope of the image gray and background gray decreases gradually, leading to the appearance of an inflection point. With an increasing signal-to-noise ratio curve, the imaging improved during the multi-frame imaging process of ultrasonic guided waves for the arc sparse arrays with left rank under weld scattering conditions. This research showed that the multi-frame imaging of ultrasonic guided waves in welded structural plates using arc sparse arrays with left rank effectively characterized scattering information with millimeter-scale wavelength size. The experimental results validated the feasibility of the theoretical analysis. This research provides a crucial foundation for the further exploration and application of the multi-frame imaging detection of ultrasonic guided waves using irregular arrays in welded structural plates.

Herschel-Quincke filters adapted to bending waves in beams and plates

A Herschel-Quincke (HQ) filter is a device for controlling the transmission of guided waves: by adding one or more auxiliary branches to a waveguide, it is possible to construct delay lines, which induce destructive interference effects and consequently zero-transmission phenomena at targeted frequencies. We explore the use of this principle in the context of elastic waves in beams.The dispersive nature of bending waves and their coupling with longitudinal waves introduces complexity into the tuning of the zero transmission frequencies. A model, developed within the framework of Timoshenko's beam theory, provides the scattering matrix of an HQ filter composed of several branches. The model is confirmed by FE and thus demonstrates the interest of the approach for the control of vibrations. The architecture of the HQ device and the modelling approach are extended to a 3 branches configuration. The properties of the filter are analysed, leading to a discussion of the advantages and disadvantages of the HQ strategy.

Topological robust transmission of elastic waves in heterogeneous phononic crystal plates

Topological insulators possessing robust transmissions in the topologically protected edge have attract increasing attention. Conventionally, the topological edge states are highly concentrated at the edge with limited width, putting restrictions on the potential phononic applications. In this work, a broadened topological interface is reported in the phononic crystal plate with Y-shaped perforated holes, by adding a phononic crystal featuring Dirac points between two phononic crystals with different phases. The band structure of topological edge state together with typical displacement field is presented. Using a line source to excite on the left side of the topological interface, the topological transports of elastic waves are visualized within a broadband frequency range. Moreover, based on the broadened interface including multiple cavities at different spatial locations, the robust wave propagation is investigated by displacement fields. These results give a straightforward platform to explore the elastic wave manipulation in heterostructures for versatile advanced phononic devices.

Design of true zero-order half-wave plates using KCsMoP2O9 crystal: A novel molybdophosphate material with extremely small birefringence

The true zero-order half-wave plates (HWPs) with millimeter thicknesses were manufactured using the KCsMoP2O9 (KCMP) crystal with extremely small birefringence (Δn = 0.007 at 1064 nm). The designed KCMP HWPs showed small sensibility (wavelength, temperature, and angle), wide operating waveband (50 nm at >200:1), good thermal stability (>900:1 at 80oC), high extinction ratio (>2500:1), and excellent phase-retardation characteristics. In addition, an optical attenuator assembly of KCMP HWP (A16M0) and polarizers was designed. The transmittance could be adjusted well from 0 to 1 by the designed optical attenuator, comparable to commercial quartz HWP (glued zero-order HWP). All results indicated that the KCMP crystal is a good candidate as a wave plate material with the potential for polarization modulation in optical systems.

Spectral analysis of high-order harmonic generation in carbon plasma using linearly, radially, and azimuthally polarized femtosecond pulses

We demonstrate high-order harmonic generation in carbon plasma using linearly, radially, and azimuthally polarized beams from a femtosecond laser (1030nm, 40 fs, 50kHz). The conditions for carbon plasma formation during laser ablation were optimized by adjusting the heating pulse energy in the cases of the single-color (1030nm) and two-color (1030nm + 515nm) pump schemes to generate harmonics up to the 39th order. The application of radial and azimuthally polarized annular single-color (1030nm) beams at optimal conditions of plasma formation allowed the demonstration of the spectral division and inclination of the vector harmonics. An asymmetric distribution of harmonics was obtained using the vortex beam generated during the propagation of laser radiation through the quarter-wave plate and S-waveplate.

Directional Phase and Polarization Manipulation Using Janus Metasurfaces.

Janus metasurfaces, exemplifying two-faced 2D metamaterials, have shown unprecedented capabilities in asymmetrically manipulating the wavefront of electromagnetic waves in both forward and backward propagating directions, enabling novel applications in asymmetric information processing, security, and signal multiplexing. However, current Janus metasurfaces only allow for directional phase manipulation, hindering their broader application potential. Here, the study proposes a versatile Janus metasurface platform that can directionally control the phase and polarization of terahertz waves by integrating functionalities of half-wave plates, quarter-wave plates, and metallic gratings within a cascaded metasurface structure. As a proof-of-principle, the study experimentally demonstrates Janus metasurfaces capable of independent and simultaneous control over phase and polarization, showcasing propagation direction-encoded focusing and polarization conversion. Moreover, the directionally focused points are utilized with distinct polarization states for advanced applications in direction- and polarization-sensitive detection and imaging. This unique strategy for simultaneous phase and polarization control with direction-dependent versatility opens new avenues for designing ultra-compact devices with significant implications in imaging, encryption, and data storage.

Optimal multi-spectrum polarization detection with full-Stokes in a single channel by using standard retarders

Polarization detection with full-Stokes is one of the important way to obtain spatial distribution and physical information on targets. However, the most full-Stokes multi-spectrum polarization detection system is relatively complex, as the traditional method is generally designed for multi-channel to adapt the different wavelength. In this paper, we propose an optimal polarization detection method to obtain the full-Stokes vector of the incident light in one channel for different wavelengths. This method utilizes two standard retarders for multi-spectrum full polarization modulation, which can obtain accurately the desired retardance to achieve optimal system performance at multi-wavelengths. The numeric analysis validates the feasibility of the dual-retarders detection system based on tunable polarization retardance method. Furthermore, in experiments, just two much more readily available quarter-wave plate (QWP) retarders are utilized, the various special retardance within 0.2° errors and high polarization measurement accuracy below 10-2 level at multi-wavelengths are achieved. This method, with a simple structure, has excellent application prospects in high-precision polarization detection, such as solar magnetic field detection.

Comparative study of analytical models with linear and quadratic pressure drop conditions on a perforated plate in water waves

Proper pressure drop conditions are essential in predicting hydrodynamic performance of perforated structures in the framework of a potential flow theory. In this study, we investigate water waves passing through a perforated plate with various pressure drop models to examine how pressure drop conditions affect the hydrodynamic performance of perforated structures. We derive the analytical solutions of waves across the perforated plate by adopting three commonly pressure drop conditions, including one linear pressure drop condition (LPDC), and two quadratic pressure drop conditions (QPDCs). A series of laboratory experiments are conducted to validate the present theoretical results. We compare the hydrodynamic performance between the experimental data and the theoretical solutions with the three different pressure drop models, typically for wave reflection coefficients, dynamic pressures, and velocity fields measured by the technique of particle image velocimetry in the experiments. It is found that the existing pressure drop models can predict well wave reflection coefficients, dynamic pressure, and velocity profiles at certain conditions. However, the linear wave solutions associated with LPDC or QPDCs generally underpredict the amplitude of particle velocity near the free surface on the perforated plate. This underestimation increases largely as the wave steepness increases. The LPDC performance relies much on the selection of empirical coefficients. For the QPDCs, the analytical models show deviated results from the experimental data for wave reflection coefficients when the wave steepness is relatively low.