Orthogonal Polarization Modes Research Articles

Polarization mode hybridization and conversion in phononic wire waveguides

Phononic wire waveguides of subwavelength cross sections support two orthogonal polarization modes: the out-of-plane motion dominated Rayleigh-like and the in-plane motion dominated Love-like modes, analogous to transverse-electric and transverse-magnetic modes in photonic waveguides. Due to the anisotropic elasticity of the substrate material, the polarization states of phonons propagating along certain crystallographic orientations can strongly hybridize. Here, we experimentally investigate the orientation-dependent mode hybridization in phononic wire waveguides patterned from GaN-on-sapphire thin films. Such mode hybridization allows efficient actuation of piezoelectrically inactive Love-like modes using common interdigital electrodes designed for Rayleigh-like modes and further enables on-chip polarization conversion between guided transverse modes. Both are important for on-chip implementation of complex phononic circuits.

Temperature compensated fiber optic anemometer based on graphene-coated elliptical core micro-fiber Bragg grating.

The ambient temperature fluctuation is a long-standing challenge for the hot wire anemometer due to the strong cross-talk. Here, a graphene-coated elliptical core micro-fiber Bragg grating has been proposed for the detection of flow rate with the temperature compensation. With the strong interaction between the graphene layers and the heating light, the graphene coat on the surface of the microfiber can be heated, acting as a heater. The flow rate can be measured through the different responses of two orthogonal polarization modes to the refractive index change of the graphene layer induced by the heat transfer. More importantly, due to the identical response of two polarization modes to the ambient temperature, the fiber-optic anemometer could compensate the temperature cross-talk effectively. The experimental results show that the sensitivity of the 0.42 nm/(m/s) for the fiber anemometer can be achieved, and the temperature standard variation is only 0.084 nm with the range from 20°C to 50°C. The proposed fiber-optic anemometer is very attractive in the fields of various industries for the temperature self-compensation detection of gas flow.

Four-channels reservoir computing based on polarization dynamics in mutually coupled VCSELs system.

A novel Four-channels reservoir computing (RC) based on polarization dynamics in mutually coupled vertical cavity surface emitting lasers (MDC-VCSELs) is proposed and demonstrated numerically. Here, the four channels are realized in two orthogonal polarization modes (x-polarization and y-polarization modes) of two VCSELs for the first time. A chaotic time series prediction task is employed to quantitatively evaluated the prediction performance of the proposed system. It is found that the Four-channels RC based on MDC-VCSELs can produce comparable prediction performance with One-channel RC, and it is possible to increase four times information processing rate by using the Four-channels RC. Besides, the effects of injection current, external injection strength, frequency detuning, coupling strength, as well as internal parameters on the prediction performance of the Four-channels RC based on MDC-VCSELs are carefully examined. Moreover, the influences of sampled period of input signal and the number of virtual nodes are also considered. The proposed Four-channels RC based on MDC-VCSELs is valuable for further enhancing the information processing rate of RC-based neuromorphic photonic systems.

Two-mode Dicke model from nondegenerate polarization modes

We realize a non-degenerate two-mode Dicke model with competing interactions in a Bose-Einstein condensate (BEC) coupled to two orthogonal polarization modes of a single optical cavity. The BEC is coupled to the cavity modes via the scalar and vectorial part of the atomic polarizability. We can independently change these couplings and determine their effect on a self-organization phase transition. Measuring the phases of the system, we characterize a crossover from a single-mode to a two-mode Dicke model. This work provides perspectives for the realization of coupled phases of spin and density.

Coexistence and Interactions between Nonlinear States with Different Polarizations in a Monochromatically Driven Passive Kerr Resonator.

We report on experimental observations of coexistence and interactions between nonlinear states with different polarizations in a passive Kerr resonator driven at a single carrier frequency. Using a fiber ring resonator with adjustable birefringence, we partially overlap nonlinear resonances of two orthogonal polarization modes, achieving coexistence between different nonlinear states by locking the driving laser frequency at various points within the overlap region. In particular, we observe coexistence between temporal cavity solitons and modulation instability patterns, as well as coexistence between two nonidentical cavity solitons with different polarizations. We also observe interactions between the distinctly polarized cavity solitons, as well as spontaneous excitation and annihilation of solitons by a near-orthogonally polarized unstable modulation instability pattern. By demonstrating that a single frequency driving field can support coexistence between differentially polarized solitons and complex modulation instability patterns, our work sheds light on the rich dissipative dynamics of multimode Kerr resonators. Our findings could also be of relevance to the generation of multiplexed microresonator frequency combs.

Radio pulsar polarization as a coherent sum of orthogonal proper mode waves

ABSTRACT Radio pulsar polarization exhibits a number of complex phenomena that are classified into the realm of ‘beyond the rotating vector model’ (RVM). It is shown that these effects can be understood in geometrical terms, as a result of coherent and quasi-coherent addition of elliptically polarized natural mode waves. The coherent summation implies that the observed tracks of polarization angle (PA) do not always correspond to the natural propagation mode (NPM) waves. Instead, they are statistical average of coherent sum of the NPM waves, and can be observed at any (and frequency-dependent) distance from the natural modes. Therefore, the observed tracks of PA can wander arbitrarily far from the RVM, and may be non-orthogonal. For equal amplitudes of the NPM waves two pairs of orthogonal polarization modes (OPMs), displaced by 45°, can be observed, depending on the width of lag distribution. Observed pulsar polarization mainly results from two independent effects: the change of mode amplitude ratio and the change of phase lag. In the core region both effects are superposed on each other, which can produce so complex behaviour as observed in the cores of PSR B1933+16, B1237+25, and J0437−4715. Change of the phase lag with frequency ν is mostly responsible for the observed strong evolution of these features with ν. The coherent addition of orthogonal natural waves is a useful interpretive tool for the observed radio pulsar polarization.

Side-Polished DBR Fiber Laser with Enhanced Sensitivity for Axial Force and Refractive Index Measurement

A highly sensitive optical fiber sensor for the measurement of axial force and refractive index (RI) is presented. The optical sensor consists of a distributed Bragg reflector (DBR) based fiber laser, the laser cavity of the DBR is polished to D-shape by the wheel fiber polishing system. Birefringence has been introduced with side polishing of the fiber, and then, it is utilized for sensing applications. The working principle of the device is based on the principle of the change in beat frequency of the two orthogonal polarization modes of the DBR fiber laser with external perturbations. In this work, it has been shown that the dual polarization beat frequency can be used for RI-based measurement applications along with the measurement of the axial force. The sensitivity of the DBR fiber laser sensor with change in axial force is enhanced significantly with side polishing. Such a device is robust, simple in fabrication and most importantly low-cost interrogation techniques can be used for the detection purpose. A resolution of 5.6014 × 10–8 N has been achieved in case of recognition of axial force and 1.9259 × 10–5 RIU is the observable detection limit for the RI based measurement.

Simultaneous measurement of bending and temperature based etched PMFBG FP filter

An etched polarization maintaining fiber Bragg grating (PMFBG) Fabry-Perot filter for simultaneous measurement of bending and temperature is proposed and experimentally demonstrated. A 4 mm region between two PMFBGs is etched. As bending or temperature is applied to the sensor, the resonance wavelength of the two orthogonal polarization modes shift. Meanwhile, the wavelength interval between the resonance wavelength of two orthogonal polarization modes changes proportionally to the temperature, while it remains the same as the bend angle increases. Due to different responses of the filter to bending and temperature change, the device can be used for the simultaneous measurement of bending and temperature.

Ultrahigh-contrast cross-polarized entangled photon pairs from a strongly birefringent photonic-crystal fiber

Four-wave mixing with cross-polarized pump and cross-polarized sidebands is identified as a powerful resource of quantum entanglement, enabling creation of efficient fiber-optic sources of entangled photon pairs. Generation of cross-polarized photon pairs is demonstrated by coupling the laser pump into orthogonal polarization modes of a highly birefringent, highly nonlinear, anomalously dispersive photonic-crystal fiber. With the pump wavelength set to lie deeply within the anomalous dispersion region of the fiber, the frequency offset of the photon pairs relative to the pump exceeds 45 THz, enhancing the contrast of photon pairs against the Raman noise to yield photon pairs with a coincidence-to-accidental ratio as high as 250: 1.

Generation of inhomogeneously polarized vector vortex modes in few mode optical fiber

We have demonstrated the generation of inhomogeneously polarized vector-vortex modes in a step index, few mode fiber other than well-known zeroth order vector modes such as radial, azimuthal and hybridly polarized modes. These vector-vortex modes are generated by the combination of orthogonal linearly polarized fiber modes with diagonal and anti-diagonal polarization vector. The generation of these modes is controlled by the coupling conditions and state of polarization at the input fiber end. A single half-wave plate is used at the input end to switch between modes of orthogonal polarization for both linearly polarized modes and inhomogeneously polarized vector-vortex modes. All the generated modes are analysed using Stokes formalism by obtaining spatial polarization maps and ellipse orientation maps. The obtained experimental results are found to be in good agreement with the simulated results.

Coherent interaction of orthogonal polarization modes in a photonic crystal nanofiber cavity.

We show that coherent interaction between two sets of multiple resonances leads to exotic resonant effects, such as Fano-type resonances, optical analogue of electro-magnetically induced transparency, and avoided crossing between modes, under different coupling regimes. We experimentally demonstrate such resonant effects in a photonic crystal nanofiber cavity using two sets of cavity modes with orthogonal polarizations. The interaction between the modes arises due to intra-cavity polarization mixing. The observed line shapes are reproduced using a multiple-mode interaction model. Such spectral characteristics may further enhance the capabilities of the nanofiber cavity as a fiber-in-line platform for nanophotonics and quantum photonics applications.

Panda Type Few-Mode Fiber Capable of Both Mode Profile and Polarization Maintenance

We report the design, fabrication, and characterization of a panda type few-mode fiber (panda-FMF) capable of both mode profile and polarization maintenance, due to the use of two applied stress rods into the conventional FMF. The panda-FMF can support six spatial modes including LP01, LP11a, LP11b, LP21a , LP21b, and LP02, and each possesses two orthogonal linear polarization modes. According to the characterization results of panda-FMF, we theoretically identify that the effective refractive index difference between either two spatial or orthogonal linear polarization modes is more than 1 × 10−4 over the C-band. Moreover, the experimental results verify that both mode profile and polarization of high-order radially asymmetric modes can be perfectly maintained even under severe twist rate of 1.57 rad/m.

Spin polarization modulation for high-speed vertical-cavity surface-emitting lasers

The spin polarization modulation in birefringent vertical-cavity surface-emitting lasers (VCSELs) is investigated theoretically and experimentally for obtaining tailored polarization modulation characteristics suitable for high-speed data communications. A spin-flip rate equation analysis reveals that the spin polarization modulation response of VCSELs is flattened by shortening the electron spin relaxation time and its 3-dB bandwidth is broadened to a range determined by a frequency split between two orthogonal polarization modes. Optical modulations of spin polarized electrons in commercially-available InAlGaAs quantum well VCSELs demonstrate a wide 3-dB bandwidth of 23 GHz determined by the frequency split, which indicates that the spin relaxation time in the InAlGaAs quantum well VCSEL is around 20 ps at room temperature and suitable for use in high-speed data communications at a telecom wavelength of 1.55 μm.

Linear Transformation of the Polarization Modes in Coiled Optical Spun Fibers II. Resonant Transformation

The resonance linear transformation of the integral mutually orthogonal polarization modes has been calculated for the typical parameters of coiled optical spun fibers for four cases. In the first three cases, the spatial periods (lengths) of the polarization beats of the unperturbed linear birefringence caused by the winding of the spun fiber to the coil, unperturbed linear birefringence and circular birefringence associated with the twisting of the spun fiber pairwise coincide. It is shown that, in all three cases, pronounced resonant phenomena occur, which consist in periodic changes in the ellipticity of the intrinsic polarization modes of spun fibers. In this case, the polarization of integral eigenmodes changes from linear to right-circular, then to linear, then to left-circular, then again to linear. In all three cases, there are some qualitative differences. It is shown that, at certain values of the length, the spun fiber loses strong birefringence. The case is also considered when all three indicated lengths of polarization beats are equal to each other. It is shown that in this case the evolution of the integral polarization modes differs markedly from the three previous cases. The evolution of the state of the radiation polarization on the length of spun fibers with its representation on the Poincare sphere is calculated. An optomechanical analogy of the resonant linear transformation of orthogonal polarization modes in spun fibers is presented.

Influence of the Polarization Mode Dispersion on Propagation of Ultrashort Optical Pulses in the Spun Light Guides with Very Weak Linear Birefringence and Random Inhomogeneities

Although the spun light guides were developed in early 1980s, their quantity production has started only a few years ago. At present, the spun light guides with both strong and weak natural (unperturbed) linear birefringence are produced. The former are used for creating sensors of various physical parameters, mainly, current and magnetic field. The latter are very promising for use in fiber communication lines, since they have a considerable advantage over the conventional telecommunication light guides. In this work, we consider the influence of the polarization mode dispersion due to random inhomogeneities in the single-mode fiber light guides (SMFLGs) on propagation of ultrashort optical pulses in the fiber communication lines, which are based on the SMFLGs with very weak linear birefringence and strong regular twisting (such SMFLGs are called telecommunication spun light guides). The dependences of evolution of the envelope function of the ultrashort optical pulses and their spectra on the length of a spun light guide with very weak linear birefringence and random inhomogeneities are obtained by the numerical-simulation method. It is shown that an increase in the pulse width is proportional to the SMFLG length rather than occurs as per the diffuse law, as it is the case in the absence of torsion. It is established that the pulse widening to a certain (rather long) length with increasing length of the spun light guide occurs much slower than that in the untwisted light guides and faster for very long lengths. It is also shown that the relative intensity of radiation which is pumped from one orthogonal polarization mode to another over the depolarization length of nonmonochromatic radiation in the telecommunication spun light guides is small compared with unity, whereas an opposite situation is observed in the conventional telecommunication light guides.

Light polarization measurements in tests of macrorealism

According to the world view of macrorealism, the properties of a given system exist prior to and independent of measurement, which is incompatible with quantum mechanics. Leggett and Garg put forward a practical criterion capable of identifying violations of macrorealism, and so far experiments performed on microscopic and mesoscopic systems have always ruled out in favor of quantum mechanics. However, a macrorealist can always assign the cause of such violations to the perturbation that measurements effect on such small systems, and hence a definitive test would require using non-invasive measurements, preferably on macroscopic objects, where such measurements seem more plausible. However, the generation of truly macroscopic quantum superposition states capable of violating macrorealism remains a big challenge. In this work we propose a setup that makes use of measurements on the polarization of light, a property which has been extensively manipulated both in classical and quantum contexts, hence establishing the perfect link between the microscopic and macroscopic worlds. In particular, we use Leggett-Garg inequalities and the criterion of no-signaling in time to study the macrorealistic character of light polarization for different kinds of measurements, in particular with different degrees of coarse-graining. Our proposal is non-invasive for coherent input states by construction. We show for states with well defined photon number in two orthogonal polarization modes, that there always exists a way of making the measurement sufficiently coarse-grained so that a violation of macrorealism becomes arbitrarily small, while sufficiently sharp measurements can always lead to a significant violation.

Key Distribution Based on Phase Fluctuation Between Polarization Modes in Optical Channel

This letter proposes and experimentally demonstrates an approach for generating and sharing a secret key based on the random phase fluctuation between orthogonal polarization modes (OPMs). A delay interferometer (DI) is placed to extract the phase differences inside the optical channel, where reconfigurable lengths of polarization-maintaining fiber are applied as the main source of fluctuation. It is proven that the output signals from the DI can generate random and highly correlated waveforms that can be extracted as the key and shared between the two communication parties. Due to the environment-related randomness and channel-dependent uniqueness of phase fluctuation in OPMs, the security enhancement of the proposed scheme is evaluated with the capability against various tapping attacks. As a proof of concept, the key generation rate of 220 bits/s with bit-error rate of 5% is successfully demonstrated over 25-km standard single-mode fiber. Moreover, the proposed scheme has the advantage of low cost, compatible with current optical fiber networks, and suitable for long-distance transmission, since optical amplifiers can be applied.

Enhanced cooperativity for quantum-nondemolition-measurement–induced spin squeezing of atoms coupled to a nanophotonic waveguide

We study the enhancement of cooperativity in the atom-light interface near a nanophotonic waveguide for application to quantum nondemolition (QND) measurement of atomic spins. Here the cooperativity per atom is determined by the ratio between the measurement strength and the decoherence rate. Counterintuitively, we find that by placing the atoms at an azimuthal position where the guided probe mode has the lowest intensity, we increase the cooperativity. This arises because the QND measurement strength depends on the interference between the probe and scattered light guided into an orthogonal polarization mode, while the decoherence rate depends on the local intensity of the probe. Thus, by proper choice of geometry, the ratio of good to bad scattering can be strongly enhanced for highly anisotropic modes. We apply this to study spin squeezing resulting from QND measurement of spin projection noise via the Faraday effect in two nanophotonic geometries, a cylindrical nanofiber and a square waveguide. We find that, with about 2500 atoms and using realistic experimental parameters, $ \sim 6.3 $ and $ \sim 13 $ dB of squeezing can be achieved on the nanofiber and square waveguide, respectively.

Linear Transformation of the Polarization Modes in Coiled Optical Spun-Fibers with Strong Unperturbed Linear Birefringence. I. Nonresonant Transformation

A linear transformation of orthogonal polarization modes in coiled optical spun-fibers with strong unperturbed linear birefringence, which causes the emergence of the dependences of the integrated elliptical birefringence and the ellipticity and azimuth of the major axis of the ellipse, as well as the polarization state of radiation (PSR), on the length of optical fiber has been considered. Optical spun-fibers are subjected to a strong mechanical twisting, which is frozen into the structure of the optical fiber upon cooling, in the process of being drawn out from the workpiece. Since the values of the local polarization parameters of coiled spunwaveguides vary according to a rather complex law, the calculations were carried out by numerical modeling of the parameters of the Jones matrices. Since the rotation speed of the axes of the birefringence is constant on a relatively short segment of a coiled optical spun-fiber in the accompanying torsion (helical) coordinate system, the so-called “Ginzburg helical polarization modes” (GHPMs)—two mutually orthogonal ellipses with the opposite directions of traversal, the axis of which rotate relative to the fixed coordinate system uniformly and unidirectionally—are approximately the local normal polarization modes of such optical fiber. It has been shown that, despite the fact that the unperturbed linear birefringence of the spun-fibers significantly exceeds the linear birefringence, which is caused by the winding on a coil, the integral birefringence of an extended segment of such a fiber coincides in order of magnitude with the linear birefringence, which is caused by the winding on the coil, and the integral polarization modes tend asymptotically to circular ones. It has been also shown that the values of the circular birefringence of twisted single-mode fibers, which were calculated in a nonrotating and torsion helical coordinate systems, differ significantly. It has been shown that the polarization phenomena occur in the process of linear transformation of local polarization modes, which lead to small quasi-harmonic oscillations of the birefringence integral parameters of the optical spun-fibers, which depend on their length, and the period of these oscillations is approximately equal to half of the effective period of polarization beating.

Fiber Birefringence Measurement of Single-Mode Fiber by a Polarimetric Fiber Ring Laser

A novel and low-cost fiber birefringence measurement scheme is proposed and studied by a polarimetric fiber ring laser interrogated by the beat frequency technique. For the fiber ring laser, modes splitting occur between two orthogonal polarization modes because of intrinsic fiber birefringence. As a result, two types of beat frequency signals (i.e., longitudinal mode beat frequency and polarization mode beat frequency signals) are generated simultaneously. The principle of measurement of fiber birefringence by the fiber ring laser has been analyzed theoretically in detail. Two alternative expressions for fiber birefringence change are obtained, which is very convenient for users to choose different order longitudinal and polarization mode beat frequency signals. The variation of fiber birefringence caused by applied strain has been calculated in terms of beat frequency shift. To validate the proposed sensing system, the strain-induced fiber birefringence change in commercial Erbium-doped fiber is investigated and obtained by measuring frequency shift of two types of beat frequency signals. The experimental results show that the fiber birefringence experiences a linear decrease when the applied strain is increased. A strain response coefficient of –1.756 × 10–11/μe is obtained. The proposed sensing scheme has obvious advantages of simple structure, low cost, and high sensitivity and it will provide great convenience for measuring fiber birefringence for short-length single mode fiber.