Lossless Polarization Research Articles

Metasurface-enabled non-orthogonal four-output polarization splitter for non-redundant full-Stokes imaging

Imaging polarimetry plays an essential role in various fields since it imparts rich information that cannot be obtained through mere intensity and spectral measurements. To retrieve full Stokes parameters, at least four sensor pixels are required, each of which projects incident light to a different polarization state in the Stokes space. Conventional full-Stokes division-of-focal-plane (DoFP) cameras realize this function by integrating angled polarizers and retarders on top of image sensors. Due to the inevitable absorption at the polarizers, however, the maximum efficiency of these schemes is limited to 50% in theory. Instead of polarizers, three sets of lossless polarization beam splitters can be used to achieve higher-efficiency polarimetry, however, at the cost of reduced spatial resolution due to the need for six redundant sensor pixels. In this paper, we reveal that low-loss four-output polarization splitting (without filtering) is possible using a single-layer dielectric metasurface. Although these four states are not orthogonal to each other, our metasurface enables simultaneous sorting and focusing onto four sensor pixels with an efficiency exceeding 50%. The designed metasurface composed of silicon nanoposts is fabricated to experimentally demonstrate complete retrieval of full Stokes parameters at a near-infrared wavelength range from 1500 to 1600 nm with −0.67-dB (85.8%) transmission and −2.28-dB (59.2%) overall efficiency. Finally, multi-pixel polarimetry is demonstrated using a 3×4 superpixel array.

General Lossless Polarization and Phase Transformation Using Bilayer Metasurfaces

AbstractMetasurface optical elements enable wavefront control and polarization manipulation with subwavelength resolution. Metasurfaces made of linearly birefringent meta‐atoms such as rectangular nanoposts are commonly used to control phase and polarization, but a single layer of these meta‐atoms cannot implement transformations with elliptically polarized or circularly polarized eigenstates (i.e., chiral transformations). Here, it is shown that two cascaded metasurface layers comprising linearly birefringent meta‐atoms provide sufficiently many degrees of freedom to implement arbitrary unitary phase and polarization transformations. A systematic design method for such metastructures is described and used to design a bifocal metalens that cannot be implemented by a single layer of birefringent meta‐atoms. The presented design and implementation techniques introduce a systematic approach for realizing the most general form of lossless polarization and phase transformations using metasurfaces with high efficiency.

Laser cooling of rubidium atoms in a 2D optical lattice

Lossless polarization gradient cooling of atoms in a far-detuned 2D optical lattice is demonstrated. Temperatures down to K and phase space densities as high as 1 / 1000 are achieved in a total duty cycle of . It is shown that utilizing the vector component of the optical lattice allows lower temperatures to be achieved when compared with pure scalar lattices.

Coherent active polarization control without loss

We propose a lossless active polarization control mechanism utilizing an anisotropic dielectric medium with two coherent inputs. Using scattering matrix analysis, we derive analytically the required optical properties of the anisotropic medium that can behave as a switchable polarizing beam splitter. We also show that such a designed anisotropic medium can produce linearly polarized light at any azimuthal direction through coherent control of two inputs with a specific polarization state. Furthermore, we present a straightforward design-on-demand procedure of a subwavelength-thick metastructure that can possess the desired optical anisotropy at a flexible working wavelength. Our lossless coherent polarization control technique may lead to fast, broadband and integrated polarization control elements for applications in imaging, spectroscopy, and telecommunication.

Dynamic lossless polarization gate using a coherently prepared atomic medium.

We propose a dynamic lossless all-optical polarization gate using coherently prepared atomic media. We show that the loss/gain of two circularly polarized components of a linearly polarized probe field can be simultaneously eliminated by locking the power of the pump field and the external magnetic field intensity simultaneously. Using the polarization selective Kerr phase shift method, we can write π/2 (-π/2) phase shift to the right (left) circularly polarized component of the linearly polarized probe field with a choice of "magic" wavelength for the probe field. Consequently, the linear polarization state for the probe field acquires a 90° rotation at the exit of the medium. The scheme proposed in this Letter is helpful for applications in optical and quantum information processing and computation.

Dynamics of lossless polarization attraction

We study a nonlinear lossless polarizer (NLP), a fiber-based device able to control the polarization of an optical signal while preserving its energy. The NLP exploits the lossless polarization attraction (LPA) generated by the Kerr interactions between the signal and a fully polarized continuous wave (CW) pump. By employing a copropagating pump, we show that the effectiveness of LPA depends on the joint action of the Kerr nonlinearity and the mutual delay between signal and pump. We find the optimal pump wavelength placement and demonstrate that true LPA occurs only within a limited range of delay values. Thus, we explain why the copropagating NLP is more flexible and power efficient compared with the traditional counterpropagating NLP.

Polarization conversion from a thin cavity array in the microwave regime.

Linearly polarized microwave radiation is shown to have its plane of polarization converted to the orthogonal state upon reflection from an ultrathin (λ/25) cavity array. The structure benefits from an uncomplicated design consisting of a metallic grating closely separated from a ground plane by a dielectric spacer. A single set of periodically spaced slits (monograting) exhibits polarization conversion when the normally incident electric field is aligned at 45° to the slits. Two orthogonal sets of slits (bigrating) allows this narrow-band effect to be broadened when the two orthogonal resonances are separated in frequency. We optimise the design and experimentally demonstrate near loss-less polarization conversion (95% of the incident intensity) across a 3.1 GHz frequency band. Finally, we study the dependence of the structure's performance on incident angle and slit width.

Lossless polarization attraction of telecom signals: application to all-optical OSNR enhancement

We propose an all-optical fiber-based device that is able to accomplish polarization control and OSNR enhancement of an amplitude modulated optical signal at the same time. The proposed device is made of a nonlinear lossless polarizer (NLP), which performs polarization control, followed by an ideal polarizing filter, which removes the orthogonally polarized half of additive noise. The task of the NLP is to impose signal polarization aligned with the transparent eigenstate of the polarizing filter. In order to effectively control the polarization of an amplitude modulated signal in the presence of additive noise, we show how one of the two different NLP configurations (with counter- or co-propagating pump laser) is needed, as a function of the signal polarization coherence time. We demonstrate that an OSNR gain close to 3 dB can be achieved by using the proper NLP configuration. Finally, we show how the achievable OSNR gain can be estimated theoretically.

Performance characterization and guidelines for the design of a counter-propagating nonlinear lossless polarizer

We characterize the performance of a nonlinear lossless polarizer, an all-optical fiber-based device that allows for the control of the state of polarization of an optical signal. The device relies on the lossless polarization attraction generated by the nonlinear interaction between the controlled signal and a controlling pump. Choosing a counter-propagating pump, we quantify its performance by introducing the degree of attraction (DOA), which highlights the trade-off between the average attraction of the signal polarization and the unavoidable degradation of its degree of polarization (DOP). We investigate, by numerical simulations, the dependence of the DOA on the injected power and on the fiber length, thus providing the design guidelines to reach the desired performance. We find that an effective attraction can occur even for strongly unbalanced signal and pump power levels, and that fibers longer than a few kilometers yield only a marginal improvement of the DOA.

Lossless polarization attraction of copropagating beams in telecom fibers

We study the performance of a nonlinear lossless polarizer (NLP), the device that transforms an input arbitrary state of polarization (SOP) of a signal beam into one and the same SOP toward the output and, unlike conventional passive polarizers, performs this transformation without polarization-dependent losses. The operation principle of this device is based on the nonlinear rotation of the SOP of the strong signal beam under the interaction with a copropagating strong pump beam in a Kerr medium, which in our case is a telecom fiber. We quantify the performance of this NLP by introducing the notion of instantaneous degree of polarization, which is a natural extension of the conventional notion of the degree of polarization appropriate for CW beams to the case of pulses whose SOP is not constant across the pulse. We pay particular attention to the regime when signal and pump beams experience a walk-off in the dispersive medium. In particular, we demonstrate that a signal pulse experiences much stronger repolarization when the walk-off effect is present as compared with the case of no walk-off. We also study the degradation of the efficiency of the NLP in the presence of polarization mode dispersion.

Lossless polarization attraction simulation with a novel and simple counterpropagation algorithm for optical signals

We introduce a simple and fast iterative algorithm, named SCAOS (simple counterpropagation algorithm for optical signals), for simulating the counterpropagation of optical signals within a nonlinear fiber. Being based on the split-step Fourier method, the algorithm is easily implementable in many traditional optical simulators. Applications of the SCAOS algorithm to the vectorial nonlinear counterpropagation of a polarized pump and a probe signal demonstrate the phenomenon of lossless polarization attraction. The evolution of the signal polarization along the fiber, obtained by simulation, reveals that polarization attraction always entails a certain amount of degradation of the signal’s degree of polarization. Two different setups are studied, involving different types of fibers with Kerr nonlinearity, and highlighting the dependence of the attraction phenomenon, as well as of its effectiveness, on the fiber type.

Theory of lossless polarization attraction in telecommunication fibers: erratum

An erroneous procedure of averaging the components of the Stokes vector of a polarization scrambled beam over the Poincaré sphere introduced in our earlier paper [J. Opt. Soc. Am. B28, 100–108 (2011)] has been corrected.

Nonlinear repolarization dynamics in optical fibers: transient polarization attraction

In this work, we present a theoretical and experimental study of the response of a lossless polarizer to a signal beam with a time-varying state of polarization (SOP). By lossless polarizer, we mean a nonlinear conservative medium (e.g., an optical fiber) that is counterpumped by an intense and fully polarized pump beam. Such a medium transforms input uniform or random distributions of the SOP of an intense signal beam into output distributions that are tightly localized around a well-defined SOP. We introduce and characterize an important parameter of a lossless polarizer--its response time. Whenever the fluctuations of the SOP of the input signal beam are slower than its response time, a lossless polarizer provides an efficient repolarization of the beam at its output. Otherwise, if input polarization fluctuations are faster than the response time, the polarizer is not able to repolarize light.

Theory of lossless polarization attraction in telecommunication fibers

In this work, polarization attraction is meant to be the conservative nonlinear effect that transforms any arbitrary input state of polarization (SOP) of an intense optical signal beam fed to a nonlinear medium into approximately one and the same SOP at the output, provided that the medium is driven by a relatively stronger counterpropagating pump beam. Essentially, the combination of the nonlinear medium and the pump beam serves as a lossless polarizer for the signal beam. The degree of polarization of the outcoming signal beam can be close to 100% (90% in our present simulations). With an eye toward the development of such lossless polarizers for fiber optics applications, we theoretically study the polarization attraction effect in the optical fibers that are used in telecommunication links; i.e., randomly birefringent fibers. A generic model for the fiber-based lossless polarizers is derived, and a statistical scheme for the quantification of their performance is proposed.