Output Polarization State Research Articles

Experimental demonstration of the time-dependent source side-channel and countermeasures in polarization-based BB84 QKD systems

High-speed is one development tendency of the practical and commercial quantum key distribution (QKD) systems, and the gain-switched semiconductor laser (GSSL) and Sagnac interferometer have been popular components for high-speed commercial polarization-based BB84 QKD systems thanks to their stability, compactness and low cost. However, due to the finite extinction ratio (ER) of the GSSL, the time-dependent source side-channel, which is theoretically presented in [Phys. Rev. A 106(6): 062618 (2022)10.1103/PhysRevA.106.062618], also exists in these QKD systems. In this article, we experimentally investigate this side-channel in the polarization-based BB84 QKD system. The results show an obvious correlation in the output polarization states between the weak leakage light and its adjacent pulses, which will cause information to leak. More importantly, we have proposed several countermeasures, retested the side channel, and developed a mathematical framework to quantitatively assess the impact of the time-dependent side channel and these countermeasures on the secure key rate. Finally, we offer a trade-off analysis that considers defense effectiveness, pulse impact, and cost for various countermeasures in QKD. This provides a practical recommendation for improving the security of QKD systems.

Spatiotemporally modulated full-polarized light emission for multiplexed optical encryption

Spatiotemporal control of full freedoms of polarized light emission is crucial in multiplexed optical computing, encryption and communication. Although recent advancements have been made in active emission or passive conversion of polarized light through solution-processed nanomaterials or metasurfaces, these design paths usually encounter limitations, such as small polarization degrees, low light utilization efficiency, limited polarization states, and lack of spatiotemporal control. Here, we addressed these challenges by integrating the spatiotemporal modulation of the LED device, the precise control and efficient polarization emission through nanomaterial assembly, and the programmable patterning/positioning using 3D printing. We achieved an extremely high degree of polarization for both linearly and circularly polarized emission from ultrathin inorganic nanowires and quantum nanorods thanks to the shear-force-induced alignment effect during the protruding of printing filaments. Real-time polarization modulation covering the entire Poincaré sphere can be conveniently obtained through the programming of the on-off state of each LED pixel. Further, the output polarization states can be encoded by an ordered chiral plasmonic film. Our device provides an excellent platform for multiplexing spatiotemporal polarization information, enabling visible light communication with an exceptionally elevated level of physical layer security and multifunctional encrypted displays that can encode both 2D and 3D information.

Passively Broadband Tunable Dual Circular Dichroism via Bound States in the Continuum in Topological Chiral Metasurface.

Dynamic control for a strong circular dichroism (CD) response is essential in engineering applications such as polarization manipulation, sensing, and imaging. Here, we propose and experimentally demonstrate a broadband tunable dual CD response via bound states in the continuum (BICs) in two-dimensional topologically protected metasurfaces composed of all-dielectric Si chiral grating structures that generate a pair of mixed and degenerated BIC mode and circular dichroic mode (CDM) as an additional degree of freedom in CD manipulation. It is found that a singular CD peak of nearly 100% at 1.6 μm can be achieved by CDM when BIC is hidden under normal incidence, while the CD peak can be split into two in which peak wavelengths can be precisely and linearly tuned over a bandwidth of 180 nm by the incident angle when the BIC mode is excited under oblique incidence. Additionally, dynamic modulation of output polarization states from linear to circular can be arbitrarily achieved at the split CD peaks by controlling the incident angle when asymmetry perturbations on chiral gratings are introduced due to the decoupling of various polarization states at Γ point by BIC to different positions in K space. The proposed chiral grating metasurface exhibits unique angle-sensitive tunable CD spectral characteristics, making it ideal for hyperspectral and spin-selective wavefront shaping, and holds significant promise in various applications such as optical security, angle sensors, chiral lasers, nonlinear filters, and other active chiral optical devices.

Pseudo coherent-perfect-absorption approach toward perfect polarization conversion

Polarization is one of the essential properties of light. Thereby, its manipulation is important for numerous applications. When employing a resonance in a mirror-symmetry system to manipulate polarization, non-zero residual light in the excited polarization channel leads to the shrink in the scope of the polarization manipulation, and a perfect polarization conversion cannot occur. In this work we show that the concept of coherent perfect absorption can be applied to perfect polarization conversion for circular polarization states. We find that the only requirement to achieve a perfect polarization conversion is that the working frequency is the resonant one. More importantly, the range of the output polarization states can be efficiently enlarged, and can span the entire Poincare sphere by combining the momentum dependent radiative coupling rate driven by the bound states in the continuum (BIC) and the phase delay. When applied to realistic design, we adopt a guided mode resonance driven from the symmetry protected BICs in a dielectric photonic crystal slab. Numerical results are in good agreements with our theoretical predictions. We believe this work can deliver important benefits for a variety of applications based on the efficiently light polarization control and management.

Wavelength-tunability of arbitrary transmittance functions in first-order fiber comb filters based on heterogeneous waveplate combination

We investigated the wavelength tunability of arbitrarily chosen transmittance functions (ACTFs) in first-order fiber comb filters using heterogeneous waveplate combinations: specifically, a combination of dual quarter-wave plates (QWPs) and a set of a half-wave plate and a QWP in a polarization-diversified fiber loop (PDFL). PDFL comb filters studied here create polarization interference using two polarization-maintaining fiber (PMF) segments of equal length and a polarization beam splitter. We considered two comb filters with different waveplate combinations to scrutinize the effect of waveplate combination order. By analyzing the effect of the waveplates and PMF on the output polarization state (OPS) of the filter, we identified the orientation angles of the waveplates that resulted in a redshift of the ACTF by one free spectral range (λS) by making all polarization states on the OPS trace of the second PMF move in the direction of wavelength decrease. For each filter, we theoretically derived 360 sets of waveplate angles that enabled the tuning of its ACTF by λS. Among these sets, we picked eight sets inducing a wavelength shift of λS/8 for each increase in set order and displayed wavelength-shifted comb spectra at these eight sets, which corroborate the wavelength tunability of the ACTFs of the filters.

Optical coherent combining of high-power optical amplifiers for free-space optical communications.

Highly efficient coherent beam combining (CBC) of two very-high-power optical amplifiers (VHPOA) with applications to long-range FSO communications such as ground-to-space feeder links is presented. The CBC setup is designed to minimize the telecom signal degradation, with a polarization beam splitter used to minimize the power fluctuations and to control the output polarization state of the beam. The system delivers 80 W output power and is proven to be compatible with 25 Gb/s telecom signals with a less than 1 dB power penalty.

Sequential unsharp measurement of photon polarization

We propose a general experimental scheme based on binary trees of partially polarizing beam splitters (PPBSs) for realizing sequential unsharp measurements of photon polarization. The sharpnesses and the bases of the particular photon polarization measurements can be chosen arbitrarily by using corresponding PPBSs and phase plates in the setup. In the limit of low sharpnesses the scheme can realize sequential weak measurements, too. We develop a general formalism for describing sequential unsharp measurements of photon polarization in which the particular unsharp measurements are characterized by appropriate measurement operators. We show that a straightforward experimental realization of this model is the proposed scheme. In this formalism the output polarization states after the sequential measurement and any correlation functions characterizing the measurement results can be easily calculated. Our model can be used for analyzing the consequences of applying postselection and reselection in the measurement. We derive the anomalous mean value for an unsharp polarization measurement with postselection and the anomalous second-order correlation function for the sequential unsharp measurement of photon polarization with reselection. We show that these anomalies can be easily measured using the proposed scheme.

Arbitrary Achromatic Polarization Control with Reconfigurable Metasurface Systems

AbstractDynamic control over the polarization state of light is foundational for many scientific and technological applications, yet it remains a challenge to dynamically tailor responses with arbitrary polarization bases over a broad bandwidth. Broadband metasurface systems that utilize microscale displacements between two metasurfaces to enable reconfigurable polarization responses within a predefined polarization basis are reported. The metasurface pairs form an interferometer, and the lateral displacements produce detour phase shifts within the interferometer beam paths that mediate polarization state tuning. It is shown how the metasurface systems can be designed using freeform topology optimization to enable tailorable elliptical birefringence responses over a large bandwidth and how cascaded metasurface systems can enable the mapping of input and output polarization states between any two points on the Poincare sphere. It is anticipated that these concepts will have utility in imaging, display, communications, and metrology applications in classical and quantum optical domains.

Polarization variable terahertz metasurface along the propagation path

Conventional metasurfaces for terahertz polarization are limited to performing lateral (in the x-y plane) polarization control of the output wave. In such cases, the polarization state remains unchanged in each output plane along the propagation path. Herein, we propose a terahertz polarization metasurface that operates longitudinally (i.e., along the z-axis direction of propagation), which modifies the polarization state of each output plane throughout the propagation path. Our designed metasurface can control the phase delays of the left-handed and right-handed circularly polarized (LCP and RCP) components of the incident terahertz wave. This enables the LCP beam and RCP beam to converge to the z-axis through distinct paths, creating a Bessel beam. The proposed design achieved a linearly polarized terahertz wave including both LCP and RCP components with a precise phase difference Δφ at each point within a certain range along the z-axis. The Δφ varies as the propagation distance, resulting in a rotated linearly polarized output wave along the propagation path, while the rotation angle ranges from 0 to π. Based on the variable property of longitudinal polarization, we propose an application concept of dielectric refractive index sensing, in which an additional medium is placed in the terahertz propagation path and the unknown refractive index is determined by detecting the rotation angle of the output polarization state. Theoretically, the device might find potential applications in variable media excitation, terahertz communication, and terahertz radar ranging.

Paper-folding-based terahertz anti-resonant cavity.

Recently, the concept of core-anti-resonant reflection (CARR) has been proposed, greatly expanding the options of cladding materials and morphologies for Fabry-Perot-type (F-P) cavities. For instance, a single-layer tube made of A4 paper can be a precision resonator in the terahertz (THz) band, which seemed counterintuitive before. More importantly, thanks to the involvement of paper-like materials as the cavity plates, it is possible to equip the CARR cavity with the currently popular origami functionality. Following this clue, in this work we combined a simple octagonal paper tube with different origami patterns and realized the programmable adjustment for the distance between two parallel surfaces of the tubular cavity. Accordingly, the combination of the CARR cavity and the origami property offers a new degree of freedom and flexibility to vary the cavity distance, tune the resonant frequency, and explore related applications. For applied examples, we carried out pressure sensing with this foldable structure and achieved a high sensitivity (S = 57.9 kPa-1). Meanwhile, the origami cavity could also act as a THz polarization converter, and the output polarization state of the cavity mode was easily modulated from the original linear to circular polarizations with different chiralities. In future works, besides the pressure-driven method used here, heat and magnetism, etc., can further be employed to tune the CARR cavity, benefiting from four dimensional (4D) or soft-magneto origami materials as the cavity wall.

Cylindrical vector beam generation from a passively mode-locked Raman fiber laser

We experimentally investigate cylindrical vector beam (CVB) generation from a passively mode-locked Raman fiber laser. The mode-locked Raman fiber lasers based on nonlinear polarization rotation (NPR), utilize the Raman gain resulted from the stimulated Raman scattering (SRS) effect in passive fibers, instead of stimulated emission amplification of active fibers in conventional mode-locked fiber lasers. With a mode-selective coupler (MSC) incorporated into the mode-locked Raman fiber laser, the generation of CVBs has been realized. In experiment, using a Raman pump source of 1454 nm, mode-locked CVB output with central wavelength of 1562 nm was obtained. The output polarization state can be switched between the radial and azimuthal polarization states, and the beam purity is above 90%. Due to the broadband Raman gain in optical fiber, the CVB output with the desired wavelength is available in principle by selecting a suitable pump source. This kind of flexible wavelength CVB Raman fiber laser may have potential applications in optical communications and fiber optics.

Thermal-stress-induced birefringence management of complex laser systems by means of polarimetry

The novel method of the thermally-induced polarization changes driven power losses (TIPCL) analysis in the complex laser systems has been developed. The measurement has been tested on the amplifier chain of the 100 J / 10 Hz laser system ‘Bivoj’ operated at HiLASE Centre. By the usage of the measured non-uniform Mueller matrix of the amplifier chain, the optimization of the ideal input and output polarization state has been calculated numerically. The results of the optimization have been applied to the laser system, thus reducing the TIPCL from originally observed more than 33% to 7.9% for CW beam and to 9% for pulsed laser beam, respectively. To the best of our knowledge, this result represents the most efficient TIPCL suppression method for complex laser systems so far. The method also allows the definition of the ideal fully polarized non-uniform pre-compensation of input beam consequently suffering from zero TIPCL.

Full Poincaré Mueller Polarimetry Using a CCD Camera

A new method is proposed to perform Mueller matrix polarimetry using a Full Poincaré beam (i.e., a non-uniformly polarized beam presenting all polarization states across its section) as a parallel polarization state generator and a charge-coupled device (CCD) camera as a detector of the polarization state analyzer. In this way, the polarization change is measured for all possible input states simultaneously. To obtain the Mueller matrix of the sample, the overdetermined system of equations that relates the input and output states of polarization is solved by means of the Moore–Penrose pseudo-inverse. Preliminary numerical simulations are performed to identify and exhaustively analyze the main sources of error. In order to test the method, experimental measurements are presented for several known samples, showing an excellent agreement between the experimentally obtained Mueller matrices and the theoretically expected ones.

Tunable Full‐Color Vectorial Meta‐Holography

AbstractVectorial metasurface holograms ar realized through segmenting or interleaving nanostructures to modulate spatial polarization of the reconstructed holography on the imaging plane, but it is still a challenge to achieve full‐color vectorial meta‐holograms with subwavelength meta‐molecules and independent spatial polarization control of reconstructed images. Here, this problem is solved by multiplexing the hologram phase for one circularly polarized light and the conjugated hologram phase for another orthogonal circularly polarized light using only a single nanostructure and then interleaving three kinds of metasurfaces for red, green, and blue light in subwavelength meta‐molecules. By independent control of the phase difference of two overlapped circularly polarized holographic images for each color, the vectorial field on the imaging plane can be freely manipulated at the pixel scale and dynamically changed with the incident polarization. Therefore, the local color and even contrast of the reconstructed full‐color image can be tuned by selecting the incident and output polarization states, demonstrating great potentials in secure optical encryption and dynamic color display.

All-optical input-agnostic polarization transformer via experimental Kraus-map control

The polarization of light is utilized in many technologies throughout science and engineering. The ability to transform one state of polarization to another is a key enabling technology. Common polarization transformers are simple polarizers and polarization rotators. Simple polarizers change the intensity depending on the input state and can only output a fixed polarized state, while polarization rotators rotates the input Stokes vector in the 3D Stokes space. We experimentally demonstrate an all-optical input-agnostic polarization transformer (AI-APT), which transforms all input states of polarization to a particular state that can be polarized or partially polarized. The output state of polarization and intensity depends solely on setup parameters, and not on the input state, thereby the AI-APT functions differently from simple polarizers and polarization rotators. The AI-APT is completely passive, and thus can be used as a polarization controller or stabilizer for single photons and ultrafast pulses. To achieve this, we, for the first time, experimentally realize complete kinematic state controllability of an open single-qubit by Kraus maps put forth in Wu et al. (J Phys A 40:5681, 2007). The AI-APT may open a new frontier of partially polarized ultrafast optics.

Versatile Integrated Polarizers Based on Geometric Metasurfaces.

We propose versatile integrated polarizers based on geometric metasurfaces. Metasurface polarizer consists of an L-shaped hole array etched on a silver film, and it can simultaneously generate several polarization states, including linear polarization, circular polarization, elliptical polarization, or even hybrid polarization. Meanwhile, the combination of output polarization states changes with the illumination polarization type. The theoretical analysis provides a detailed explanation for the generation of the integrated polarization states. The well-designed metasurface polarizers may generate more complex polarization modes, including vector beams and vector vortex beams. The theoretical and simulated results confirm the polarization performance of the proposed integrated metasurface polarizers. The compact design of metasurface polarizers and the controllable generation of versatile polarization combinations are a benefit to the applications of polarization light in optical imaging, biomedical sensing, and material processing.

Active terahertz liquid crystal device with carbon nanotube film as both alignment layer and transparent electrodes

Liquid crystal (LC) materials are a good candidate for active phase and polarization devices. However, they also encounter significant challenges in the lack of transparent electrodes and the difficulty of LC alignment due to the thick LC cell for sub-mm wavelength in the terahertz (THz) regime. Here, we presented a strategy to overcome these difficulties, that is, using a super-aligned carbon nanotube (CNT) film with dual functions for both transparent electrodes and the aligning layer for THz LC cell. The LC molecules are uniformly aligned along the orientation direction of the CNT film because of its surface anchoring effect. The experiment results show that the active THz polarization conversions are controlled by the bias voltage tuning from 0 to 30 V between the upper and lower CNT films, and the output polarization state is converted between linear polarization and circular polarization. This work may pave a simple and flexible path toward the development of various active THz liquid crystal devices for spatial light modulation, dynamic imaging, and wavefront control.

A Filterless Photonic Approach for DFS and AOA Measurement Using a Push-Pull DPol-MZM

We report a novel simple photonic approach for measuring the Doppler frequency shift (DFS) and the angle of arrival (AOA) of a microwave signal in a radar detection system. The echo signals combined with a reference signal are applied to a dual-polarization Mach-Zehnder modulator (DPol-MZM), and a low-frequency electrical signal is generated after beating between the two signals in a low-speed photodetector. The DFS without direction ambiguity can be calculated by the value of the low-frequency signal. A phase difference is introduced between two echo signals which are sent to two sub-MZMs of the DPol-MZM respectively. The AOA is determined by the peak power of the down-convert signals, and a simple calibration is used to remove the amplitude of the echo signals and the reference signal by switching the output polarization states of the system. Compared with previously reported works, the proposed method carries out a comprehensive and multifunction measurement system based on a filterless and simple structure. Experimental results demonstrate the DFS errors within ± 0.5 Hz for a 15 GHz echo signal with a frequency offset of ± 100 kHz, and AOA measurement errors are within ± 1° for a range of 16° – 82°

EDF laser temperature sensor using double-pass cascaded-chirped long-period fiber grating in sigma-cavity configuration

We propose a novel Erbium-doped fiber laser sensor in a sigma-cavity configuration (EDFσL) for temperature sensing. In this configuration, the sigma-branch of the EDFσL, or the double-pass cascaded-chirped long-period fiber grating (C-CLPG), consists of an optical circulator, a C-CLPG, and a Faraday rotator mirror (FRM), and works as a reflective sensor device as well as a wavelength selection element with a comb-like spectrum, namely a channeled spectrum. The double-pass C-CLPG has the advantage of not only a flexible sensor arrangement, especially for remote sensing, but also the compensation effect for the polarization fluctuations in the sigma-branch caused by the measurand. In the experiment, it is shown that the FRM reflection scheme can stabilize the output polarization state of the double-pass C-CLPG during the temperature change around the C-CLPG, and it is found that the EDFσL enables a more stable and precise measurement than the previous simple EDF ring laser using the C-CLPG. Furthermore, it is also demonstrated that the oscillation is stabilized by the polarization compensation effect of the double-pass C-CLPG even if the polarization state is intentionally changed by applying the mechanical deformation near the C-CLPG.

Electrically Driven Tunable Broadband Polarization States via Active Metasurfaces Based on Joule‐Heat‐Induced Phase Transition of Vanadium Dioxide

AbstractBroadband tuning of polarization states is pivotal yet challenging in modern photonics technologies, especially for miniaturized or integrated systems. Metasurfaces potentially provide an effective approach to resolve this challenge. However, once a metadevice is fabricated, its functionalities are determined, and it is hard to actively tune the polarization states. Here, the electrically tunable broadband polarization states by combining phase‐change material (vanadium dioxide) and dispersion‐free metasurface are demonstrated for the first time. The polarization states are modulated through the electrically driven, Joule‐heat‐induced phase transition of vanadium dioxide, where the output polarization state can be continuously tuned from horizontal one to vertical one, or from circular polarization to linear polarization. With accurate on‐chip control of the phase transition, continuous and reversible modulation of polarization is verified in a scanning display. Moreover, a proof‐of‐concept demonstration for dynamically independent control of multiple polarization display is carried out. Different images are produced by applying electrical currents in N separate channels to generate a dynamic multiplexing polarization display with 2N encoding states. Such an active metasurface can be readily integrated with electronics and has potential applications in display, encryption, camouflage, and information processing.