Orthogonal Polarization Modes Research Articles

Multifunctional near field focusing transmission metasurface based on polarization sensitivity

AbstractIn this paper, a near field focusing transmission metasurface (MS) based on polarization sensitivity is developed at microwave frequency. The MS unit cell has only three metal layers, which is less than the published dual‐polarized transmission structure. And the phase shifting range can cover 320° for x‐ and y‐polarized waves. The biggest bright spot of the focusing MS made of the proposed units is its changeable focus point by rotating the horn 0°–90° round its central axis: focusing switching between one and two focusing points. In addition, the developed MS has the following advantages: multifunctional, low sidelobe, high focusing efficiency, small size, wideband, and less layer number. The focusing relative bandwidth is 9.7%, and the maximum focusing efficiency is 30% at center frequency 15.5 GHz. The efficiency is higher than that of published results in transmission orthogonal polarization mode. The proposed MS has potential applications in medical treatment, noncontact sensing system.

Compact In-Band Full Duplexing Antenna for Sub-6 GHz 5G Applications

A novel compact in-band full duplexing antenna using a cavity-like structure for sub-6 GHz fifth-generation (5G) applications is proposed. By exploring a shared aperture cavity-like structure with two highly orthogonal modes (quarter-wavelength slot mode and half-mode cavity mode), high isolation between Tx and Rx ports is achieved. Two quarter-wavelength slots are etched on both sides of the cavity, so a shared aperture dual-mode cavity is implemented. When this cavity is fed by a pair of even–odd mode feeding structures, two orthogonal polarization modes are excited at the same frequency band. To validate the proposed concepts, a high isolation duplexing antenna with a compact size of 0.45 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.20 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.04 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is fabricated. A measured isolation greater than 43 dB is achieved over the working band from 3.4 to 3.8 GHz (LTE B42/B43 band) and its peak radiation efficiency is larger than 87%. It shows a compact size, low cost, high isolation, good bandwidth, and efficiency, which is well situated for in-band duplexing systems and 5G indoor application.

Flexible fiber-laser ultrasound sensor for multiscale photoacoustic imaging

Photoacoustic imaging (PAI) is a noninvasive biomedical imaging technology capable of multiscale imaging of biological samples from organs down to cells. Multiscale PAI requires different ultrasound transducers that are flat or focused because the current widely-used piezoelectric transducers are rigid and lack the flexibility to tune their spatial ultrasound responses. Inspired by the rapidly-developing flexible photonics, we exploited the inherent flexibility and low-loss features of optical fibers to develop a flexible fiber-laser ultrasound sensor (FUS) for multiscale PAI. By simply bending the fiber laser from straight to curved geometry, the spatial ultrasound response of the FUS can be tuned for both wide-view optical-resolution photoacoustic microscopy at optical diffraction-limited depth (~1 mm) and photoacoustic computed tomography at optical dissipation-limited depth of several centimeters. A radio-frequency demodulation was employed to get the readout of the beat frequency variation of two orthogonal polarization modes in the FUS output, which ensures low-noise and stable ultrasound detection. Compared to traditional piezoelectrical transducers with fixed ultrasound responses once manufactured, the flexible FUS provides the freedom to design multiscale PAI modalities including wearable microscope, intravascular endoscopy, and portable tomography system, which is attractive to fundamental biological/medical studies and clinical applications.

Design and simulation of polarization-insensitive ring resonator based on subwavelength grating and sandwiched structure

Ring resonator fabricated on a silicon-on-insulator is versatile in optical integration, which can be used to realize filters, modulators and switches. However, silicon-on-insulator is difficult to control the polarization dependence, and thus its application is greatly limited. The polarization dependence of the ring resonator is caused mainly by two factors: the coupling coefficients of the coupling region at the same wavelength for the two orthogonal polarization modes are different, and the birefringence effect of curved waveguide results in the different resonant wavelengths of TE and TM polarization modes. When the coupling region polarization independence and the resonant wavelength polarization independence are simultaneously satisfied, the polarization independence of the ring resonator can be realized. In this paper, a new type of polarization-insensitive ring resonator on a silicon-on-insulator is designed based on subwavelength grating and sandwiched structure. Firstly, by adjusting the duty cycle of the subwavelength grating and the refractive index of SiN&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; in the coupling region, polarization independence of the coupling region is achieved. Secondly, the refractive index of SiN&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; in curved waveguides is designed to make the resonance wavelengths for orthogonal polarization modes equal. Thirdly, the parameters of the coupling region are optimized to reduce the insertion loss. The three-dimensional finite-difference time-domain method is used for simulation. The results show that the radius of the ring is only 10 μm, the 3-dB bandwidth of the device is less than 0.8 nm, and the insertion loss is lower than 0.8 dB. It has potential applications in the future dense wavelength division multiplexing systems.

Beat length measurement study of few-mode polarization-maintaining fiber based on low coherence off-axis digital holography

A method based on low coherence off-axis digital holography is proposed to measure the beat length of few-mode polarization-maintaining fiber (PMF). The real-time holograms are recorded based on cross-correlation in time domain. The energy of polarization modes is extracted by frequency-domain filtering. The mode energy fluctuation curve is constructed from a series of holograms. The mode field distribution and time distribution of polarization modes are reconstructed from the holograms recorded at the peaks of mode energy fluctuation curve. The beat length is calculated by analyzing the delay between orthogonal polarization modes. The low coherence off-axis digital holographic experimental system is established and the beat length of single-mode PMF is achieved. The beat length is obtained at multiple wavelengths. The result shows that beat length is proportional to wavelength. We measure the beat length of few-mode PMF. The beat length of high order mode and fundamental mode are obtained at the same time. Our method has some advantages including simple experimental setup and high precision.

Design of polarization-insensitive 1×2 directional coupler demultiplexer based on sandwiched structure

An ultra-compact 1×2 demultiplexer based on directional coupler (DC) waveguide is proposed to separate the 1310 nm wavelength from 1550 nm wavelength, in which a new Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt;/SiN&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;/Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; sandwiched structure is used to realize polarization insensitivity. Firstly, the new sandwiched structure is designed to be polarization-independent. The coupling lengths of two orthogonal polarization modes at the same wavelength versus the gap between two parallel SiN&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; waveguides &lt;i&gt;g&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt; are calculated with several groups of structure parameters of the demultiplexer. The result shows that the coupling lengths for the two orthogonal polarization modes at the same wavelength can be identical by choosing the proper &lt;i&gt;g&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt;. Then, how to realize the function of wavelength separation is studied. When one wavelength propagates at even multiple of coupling length and the other wavelength propagates at odd multiple of coupling length, and vice versa, the two working wavelengths will output from different output ports, thereby the two wavelengths are successfully separated. Under the premise of satisfying such conditions, a comparison of size and performance among the devices with different groups of structure parameters is given to find the best one. The demultiplexer based on Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt;/SiO&lt;sub&gt;2&lt;/sub&gt; platform has a compact structure, easy integration and good tolerance. Three-dimensional(3D) finite-difference time-domain method is used for simulation, and the results show that the length of the DC waveguide is only 23 μm; the insertion loss and crosstalk are as low as 0.1 dB and–26.23 dB respectively; a broad 3-dB bandwidth of 200 nm is achieved. To demonstrate the transmission characteristics of the demultiplexer, the evolution of the excited fundamental mode in the demultiplexer is also given. The novel demultiplexer is polarization-independent and can work at 1310 nm and 1550 nm wavelengths simultaneously. It has a potential application value in future integrated optical circuits.

Circular polarization in radio pulsar PSR B1451−68: coherent mode transitions and intrabeam interference

ABSTRACT The radio emission of pulsar B1451−68 contains two polarization modes of similar strength, which produce two clear orthogonal polarization angle tracks. When viewed on a Poincaré sphere, the emission is composed of two flux patches that rotate meridionally as a function of pulse longitude and pass through the Stokes V poles, which results in transitions between orthogonal polarization modes (OPMs). Moreover, the ratio of power in the patches is inversed once within the profile window. It is shown that the meridional circularization is caused by a coherent OPM transition (COMT) produced by a varying mode ratio at a fixed quarter-wave phase lag. The COMTs may be ubiquitous and difficult to detect in radio pulsar data, because they can leave no trace in polarized fractions and they are described by equation similar to the rotating vector model. The circularization, which coincides with flux minima at lower frequency, requires that profile components are formed by radiation with an oscillation phase that increases with longitude in steps of 90○ per component. The properties can be understood as an interference pattern involving two pairs of linear orthogonal modes (or two non-orthogonal elliptic waves). The frequency-dependent coherent superposition of coplanar oscillations can produce the minima in the pulse profile, and thereby the illusion of components as separate entities. The orthogonally polarized signal that is left after such negative interference explains the enhancement of polarization degree that is commonly observed in the minima between profile components.

Strong superchiral fields and an ultrasensitive chiral sensor of biomolecules based on a dielectric photonic crystal slab with air holes

Detection and quantification of chiral enantiomers of biomolecules are of considerable importance for biomedical diagnostics and pathogen analyses. However, the chiral signals from natural molecules are typically small and weak; a long measuring time and a large amount of samples are needed to fulfill the detection limit for the present technique. How to improve the detection efficiency for realizing ultrasensitive probing of chiral molecules has become a key problem. Here we propose a scheme to perform ultrasensitive detection of chiral molecules based on degeneracy states of orthogonal polarization modes in the achiral dielectric nanostructure. The degeneracy states are realized by designing dielectric photonic crystal slabs with air holes. Based on such degeneracy states, homogeneous superchiral fields of both handednesses over arbitrarily large areas can be obtained in a wide spectral range, resulting in the enhancement of the circular dichroism signal by more than three orders of magnitude, thus paving the road toward ultrasensitive characterization and quantification of molecular chirality.

Mode changing, subpulse drifting, and nulling in four component conal pulsar PSR J2321+6024

ABSTRACT In this study, we report on a detailed single pulse polarimetric analysis of the radio emission from the pulsar J2321+6024 (B2319+60) observed with the Giant Metrewave Radio Telescope, over wide frequencies ranging between 300 to 500 MHz and widely separated observing sessions. The pulsar profile shows the presence of four distinct conal components and belongs to a small group of pulsars classified as a conal quadrupole profile type. The single pulse sequence reveals the presence of three distinct emission modes, A, B, and ABN showing subpulse drifting. Besides, there were sequences when the pulsar did not show any drifting behaviour suggesting the possibility of a new emission state, which we have termed as mode C. The evolution of the mode changing behaviour was seen during the different observing sessions with different abundance as well as the average duration of the modes seen on each date. The drifting periodicities were 7.8 ± 0.3 P, 4.3 ± 0.4 P, and 3.1 ± 0.2 P in the modes A, B, and ABN, respectively, and showed large phase variations within the mode profile. The pulsar also showed the presence of orthogonal polarization modes, particularly in the leading and trailing components, which has different characteristics for the stronger and weaker pulses. However, no correlation was found between the emission modes and their polarization behaviour, with the estimated emission heights remaining roughly constant throughout. We have used the Partially Screened Gap model to understand the connection between drifting, mode changing, and nulling.

High-performance terahertz vortex beam generator based on square-split-ring metasurfaces

Generating a vortex beam is a fundamental task due to the potential applications in wireless communication. Here we propose a kind of high-performance terahertz vortex beam generator with several topological numbers based on the square-split-ring metasurfaces. Via the Fabry-Perot resonance effect of the structure, the transmittance of an orthogonal polarization mode could be remarkably enhanced. We use a Laguerre-Gaussian (LG) mode for illustration and then simulate the far-field propagation of the LG beam by using phase superposition of focusing lens and vortex phase plates. The theoretical and simulation results agree well, which implies a bright application prospect.

A modified dual-core THz fiber polarization splitter with four subwavelength tubes

We propose a terahertz polarization splitter based on suspended dual-core fiber. Each core of the fiber is formed with two adherent subwavelength polymer tubes, which is helpful for a high modal birefringence. Due to the difference of coupling length between x and y polarizations, the proposed splitter can completely separate two orthogonal polarization modes at a specific splitting length. A 1.184 cm long splitter can be obtained at a center frequency of 1 THz. Meanwhile, the low insertion loss of 0.6 dB, the bandwidth of 0.05 THz and the extinction ratio better than −10 dB can be supplied. Moreover, an efficient design rule for the proposed splitter is demonstrated aiming to high extinction ratio at any desired operation frequency. The proposed terahertz polarization splitter has some potential applications in all-optical networks in THz systems.

Manipulation of parabolic polarization- and group-velocity- locked vector solitons

In this manuscript, we report the simulation about manipulating parabolic polarization-locked and group-velocity-locked vector solitons in a polarization resolved fiber system. Through changing input pulse parameters of orthogonal polarization modes, output temporal pulse shapes and corresponding optical spectra are simulated. For the case of polarization-locked vector soliton, fundamental parabolic vector soliton always appears except when projection angle is changed. While for group-velocity-locked vector soliton, pseudo-high-order parabolic vector solitons can be achieved through changing phase difference and projection angle, and maintain fundamental soltion state although other two pulse parameters (amplitude ratio and time delay) are changed.

Cryogenic electro-optic polarisation conversion in titanium in-diffused lithium niobate waveguides.

Many technologies in quantum photonics require cryogenic conditions to operate. However, the underlying platform behind active components such as switches, modulators and phase shifters must be compatible with these operating conditions. To address this, we demonstrate an electro-optic polarisation converter for 1550 nm light at 0.8 K in titanium in-diffused lithium niobate waveguides. To do so, we exploit the electro-optic properties of lithium niobate to convert between orthogonal polarisation modes with a fiber-to-fiber transmission >43%. We achieve a modulation depth of 23.6±3.3 dB and a conversion voltage-length product of 28.8 V cm. This enables the combination of cryogenic photonics and active components on a single integration platform.

Dual-Polarized Wideband Plate Array Antenna With High Polarization Isolation and Low Cross Polarization for D-Band High-Capacity Wireless Application

This letter presents a dual-polarized wideband plate array antenna in D-band for future fixed high-capacity wireless communication. To achieve the high polarization isolation and wideband operation at the same time, a two-stage multimode cavity is introduced, consisting of a polarization isolation cavity and a cascaded dividing cavity. They are combined to optimize and improve the bandwidth of the dual-polarized antenna array. The TE210 and TE120 degenerate modes are selected to resonate in the cavity. They can be used to generate two orthogonal linear polarization modes with the extremely low cross polarization after cascading four integrated horns with a proper placement. To validate the design, an 8 × 8 element array is fabricated by the diffusion bonding of laminated thin copper plates technology. The gain is more than 24.5 dBi from 130 to 149 GHz. The isolation between two channels is higher than 50 dB and the cross polarization level is below −45 dB over the whole band.

Mitigation of Polarization Effects in Single-mode Fiber Spectrographs

The use of single-mode fibers (SMFs) to illuminate radial velocity (RV) spectrographs shows promise to achieve extremely precise Doppler measurements. Due to their small core diameter, SMFs only propagate a single spatial mode which allows for diffraction-limited optical performance while simultaneously eliminating fiber modal noise. The single spatial mode however consists of two orthogonal polarization modes. In circular core fiber with a non-isotropic refractive index profile or asymmetries in the cross-sectional geometry, the two polarization modes propagate with different relative speeds inducing birefringence. Conditions at a telescope observatory will subject the fiber to mechanical (bending and twisting) and thermal stresses, inducing birefringence that varies in time. The interaction of variable birefringence combined with with polarization sensitive optics, such as diffraction gratings, results in an intensity modulation that causes unwanted Doppler shifts via “polarization noise.” In this paper, we characterize variable fiber birefringence both in the laboratory and at the Large Binocular Telescope using a Stokes parameters. We then combine the measured Stokes vector through a numerical model of a SMF spectrograph to understand the impact of variable polarization on RV precision. We find that polarization noise is a tens of cm s−1 to several m s−1 effect, which is exacerbated by the degree of polarization of the light source and the polarization response of the spectrograph optics. Finally we show experimentally mitigating the RV offset using polarization averaging methods and in-line fiber depolarizers and can reduce a several m s−1 polarization noise to σ ≤10 cm s−1.

Training a Multi-Layer Photonic Spiking Neural Network With Modified Supervised Learning Algorithm Based on Photonic STDP

We propose a framework for hardware architecture and learning algorithm co-design of multi-layer photonic spiking neural network (SNN). The vertical-cavity surface-emitting laser with an embedded saturable absorber (VCSEL-SA) which contains two polarization-resolved modes is employed as a spiking neuron. The connection between two identical polarization modes is considered as the excitatory synapse, whereas the connection between two orthogonal polarization modes is regarded as the inhibitory synapse. The physical model of the photonic spiking neuron is derived based on the combination of spin-flip model and Yamada model. The photonic spike timing dependent plasticity (STDP) is applied to design a hardware-friendly biologically plausible supervised learning algorithm for a multi-layer photonic SNN. Thanks to the polarization mode competition effect in the VCSEL-SA, the proposed neuromorphic network is capable of solving the classical XOR problem. The effect of physical parameters of photonic neuron on the training convergence is also considered. We further extend the multi-layer photonic SNN to realize other logic learning tasks. To the best of our knowledge, such a modified supervised learning algorithm dedicated for a multi-layer photonic SNN has not yet been reported, which is interesting for spiking learning of neuromorphic photonics.

Pushing Purcell enhancement beyond its limits

Purcell-enhanced photon emission into a cavity is at the heart of many schemes for interfacing quantum states of light and matter. We show that the intra-cavity coupling of orthogonal polarisation modes in a birefringent cavity allows for the emitter and photon to be decoupled prior to emission from the cavity mode, enabling photon extraction efficiencies that exceed the, previously considered fundamental, limits of Purcell enhancement. Tailored cavity birefringence is seen to mitigate the tradeoff between stronger emitter-cavity coupling and efficient photon extraction, providing significant advantages over single-mode cavities. We then generalise this approach to show that engineered coupling between states of the emitter can equivalently ‘hide’ the emitter from the photon, ultimately allowing the extraction efficiency to approach its fundamental upper limit. The principles proposed in this work can be applied in multiple ways to any emitter-cavity system, paving the way to surpassing the traditional limitations with technologies that exist today.

Exceptional polarization purity in antiresonant hollow-core optical fibres

High-performance interferometers, gyroscopes, frequency combs, quantum information experiments and optical clocks rely on the transmission of light beams with the highest possible spatial and polarization purity. Free-space propagation in vacuum unlocks the ultimate performance but becomes impractical at even modest length scales. Glass optical fibres offer a more pragmatic alternative, but degrade polarization purity and suffer from detrimental nonlinear effects. Hollow-core fibres have been heralded for years as the ideal compromise between the two, but achieving high modal purity in both spatial and polarization domains has proved elusive thus far. Here, we show that carefully designed, low-nonlinearity hollow-core antiresonant fibres can transmit a single pair of orthogonal polarization modes with cross-coupling on the scale of 10–10 m–1; that is, orders of magnitude lower than any other solution. This free-space-like optical guidance can immediately provide a leap in performance for photonics-enabled sensors and instruments. Carefully designed hollow-core antiresonant fibres support a pair of orthogonal polarization modes with a level of purity and cross-coupling that is orders of magnitude lower than other fibre designs and beyond the fundamental Rayleigh scattering limit of glass core fibres.

Real-time optical spike-timing dependent plasticity in a single VCSEL with dual-polarized pulsed optical injection

We propose and numerically realize an optical spike-timing dependent plasticity (STDP) scheme by using a single vertical-cavity surface-emitting laser (VCSEL). In the scheme, the VCSEL is subjected to an orthogonally-polarized continuous-wave optical injection (OPCWOI) and dual-polarized pulsed optical injections (DPPOI). Based on the widely used spin-flip model, the response spiking dynamics of VCSEL is numerically studied, and then the optical STDP in a single VCSEL is explored. The roles of bias current, the strength of OPCWOI and DPPOI, and the frequency detuning on the optical STDP curve are numerically analyzed. It is found that, by simultaneously utilizing the response spiking dynamics in two orthogonal polarization modes, an optical STDP could be achieved by using a single VCSEL. Furthermore, the weight update of STDP curve can be calculated in real-time. Additionally, the STDP curves can also be controlled by adjusting some controllable parameters. The real-time optical STDP based on a single VCSEL is numerically realized for the first time, which paves the way towards fully VCSELs-based photonic neuromorphic systems with low power consumption.

Artefacts of circumpolar cartography in radio pulsar polarization

ABSTRACT Single-pulse data on radio pulsar polarization are traditionally presented in the form of two-dimensional grey-scale patterns with the pulse longitude and polarization angle (PA) on the horizontal and vertical axis, respectively. Such diagrams reveal several enigmatic polarization effects: (1) bifurcations and loops of PA curve under central pulse components, (2) vertical spread of flux at all PA values, (3) exchange of power content between PA tracks of two orthogonal polarization modes (OPMs), and (4) peripherically flat PA swings that span more than 180 deg. It is shown that all these phenomena result from passage of observed polarization state near the pure-V pole of Poincaré sphere. Much of their complexity results from cartographic transformation from Poincaré sphere to the longitude–PA plane. An odd number of near-pole passages produce apparent replacement of OPM power in the profile wings, although the same amount of flux keeps staying in each modal patch on the Poincaré sphere. The fitting of pulsar PA curves should therefore allow for transitions between the primary (strong) and secondary (weak) PA tracks. The Stokes space (or Poincaré sphere) representation of pulsar polarization data contains crucial polarization information and needs to accompany the traditional viewing if the published figures are to be fully useful for interpretation.