Determination of high-purity polarization state of X-rays

We report on the measurement of the highest purity of polarization of X-rays to date. The measurements are performed by combining a brilliant undulator source with an X-ray polarimeter. The polarimeter is composed of a polarizer and an analyzer, each based on four reflections at channel-cut crystals with a Bragg angle very close to 45°. Experiments were performed at three different X-ray energies, using different Bragg reflections: Si(400) at 6457.0 eV, Si(444) at 11,183.8 eV, and Si(800) at 12,914.0 eV. At 6 keV a polarization purity of 1.5 × 10^− 9 is achieved. This is an improvement by more than two orders of magnitude as compared to previously reported values. The polarization purity decreases slightly for shorter X-ray wavelengths. The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super-strong laser fields.     

Spin polarization and structure of molecular oxygen adsorbed on Fe(1 1 0)

We investigated the structural and magnetic properties of Fe(1 1 0) for molecular oxygen adsorption at room temperature. For the c(2 × 2) and c(3 × 1) superstructures, spin-polarized secondary electron emission (SPSEE) induced by protons and electrons reveals a nearly unchanged polarization compared to clean Fe(1 1 0). An appreciable decrease in polarization is found for the disordered layer of oxygen at a coverage Θ ≈ 1. This decrease is more pronounced for the spin polarization determined by electron capture (EC) to excited atomic He levels after grazing scattering. From a comparison of data obtained by proton-induced SPSEE and spin-polarized EC we conclude that the polarization at the vacuum boundary vanishes for an oxygen coverage Θ ≈ 1 while the polarization in the underlying Fe substrate layers is hardly changed.     

200-Gb/s wavelength conversion using a delayed-interference all-optical semiconductor gate assisted by nonlinear polarization rotation

The pattern-induced intensity fluctuation (PIF) of output signals with a bit-rate above 160 Gb/s has been one of the major issues regarding all-optical semiconductor gates. We have demonstrated that the nonlinear polarization rotation (NPR) in the semiconductor optical amplifier (SOA) plays a significant role in the high-frequency operation of a delayed-interference signal-wavelength converter (DISC). We did this using a cross-correlation system whose temporal resolution is 1.5 ps which was developed to monitor our 200-Gb/s, 4992-bit-long binary-patterned waveforms. When we experimentally optimized the NPR effect inside our DISC specially for our 200-Gb/s wavelength conversion, the PIF was significantly improved (from 5.0 to 1.5, for example). Our systematically measured dependence of the PIF on the polarization settings was qualitatively explained with the new gate model that we developed earlier in this work.     

Extraction of weak crack signals based on sparse code shrinkage combined with wavelet packet filtering

Early crack signals in critical infrastructure components of major equipment are hardly to be extracted due to its low signal noise ratio (SNR). A de-noising method combined wavelet packet (WP) technology with sparse code shrinkage (SCS) is proposed in this study. Firstly, WP reconstruction technology is used to reserve the crack signal with a specified frequency range. That is, the signal is decomposed by Meyer wavelet into five layers, and the signal with the frequency range from 187.5 kHz to 609.375 kHz is reserved. Then SCS method removes noise within the specified frequency range. Namely, the probability density function (PDF) of the signal independent coefficients is estimated via the generalized Gaussian model (GGM) in the independent component analysis (ICA) space. The nonlinear de-noising is finished by utilizing maximum a posteriori (MAP) estimate. The results obtained by the combined method are compared with those generated by the SCS method and the WP de-noising method. It demonstrates that the combined method is the best one among the three methods in extracting weak signals. Its output SNR is −2.38 dB and the correlation coefficient (CC) is 0.54 when the input SNR is −20 dB. They are higher than those obtained by the SCS method (SNR −4.46 dB and CC 0.51). The WP method is the worst (SNR −3.54 dB and CC −0.003). Therefore, the combined method is quite suitable for weak signal extraction.     

A novel polarization splitter based on dual-core hybrid photonic crystal fibers

Highly birefringent dual-core photonic crystal fibers (PCFs) can be used as a polarization splitter because the orthogonal polarization modes with dissimilar coupling lengths are easily separated from each other. Different from the traditional methods achieving high birefringence, a new highly birefringent hybrid PCF that guides light by both index guiding and bandgap guiding is proposed. Firstly, a novel polarization splitter based on this kind of dual-core hybrid PCF is designed. The transmission modes, coupling lengths for the two orthogonal polarizations and performance of the proposed polarization splitter are investigated and numerically analyzed. The results demonstrate that it is possible to obtain a 4.72-mm-long polarization splitter. The splitting ratio is better than −20 dB in a large wavelength range of 1.53-1.72 μm. Its bandwidth is about 190 nm.     

Macroscopic polarization and thermal conductivity of GaN

We theoretically investigated the effect of macroscopic polarization (sum of spontaneous and piezoelectric polarization) on the thermal conductivity of wurtzite GaN. Macroscopic polarization contributes to the effective elastic constant of the GaN and thus modifies the phonon group velocity. We used the revised phonon velocity to estimate the Debye frequency and temperature. Different phonon scattering rates were calculated as functions of the phonon frequency. The thermal conductivity of GaN was estimated using revised parameters such as the phonon velocity and phonon relaxation rate. The revised thermal conductivity at room temperature increased from 250 to 279 W m⁻¹ K⁻¹ due to macroscopic polarization. The method we developed can be used for thermal budget calculations for GaN optoelectronic devices.     

High carrier suppression double sideband modulation using polarization state rotation filter and optical external modulator

We propose a high carrier suppression double sideband modulation technique using a Mach-Zehnder modulator (MZM) and an integrated polarization state rotation filter (PSRF), which is designed to improve the carrier suppression ratio. With the functions of MZM and PSRF, about 30 dB carrier suppression ratio relative to first-order sidebands is demonstrated. Moreover, we demonstrate the optical generation of microwave/millimeter-wave signals by beating the carrier suppressed double sideband (DSB-SC) lightwave signals. The experimental results show that the improvement of carrier suppression ratio with PSRF can effectively cancel the modulating RF frequency component. A tunable and high purity microwave signal, which is limited by the bandwidth of MZM and photodetector (PD), is obtained, and it does not suffer from obvious phase noise degradation with 25 km transmission.     

Frequency modulated and polarization maintaining fiber laser with narrow linewidth

A novel configuration of fiber laser with frequency modulation is presented. Frequency modulation, stable polarization state and narrow linewidth are realized by using the waveguide phase modulator, polarization maintaining devices and saturable absorber. It is shown that the laser output reaches 23 mW, linewidth is less than 1 kHz, polarization extinction ratio is higher than 20 dB and maximum value of frequency deviation can reach 7.5 MHz.     

Experimental demonstration of polarization multiplexing for simultaneously providing broadband wireless and wired access

A fiber-based wavelength-division-multiplexing (WDM) network utilizing polarization multiplexing (PolMUX) is proposed to simultaneously provide broadband wireless and wired services. In such a dual-service access network, the wireless and wired services are separately delivered in two orthogonal states of polarization with well independence in a single WDM channel. The impact of several polarization-dependent interferences becomes insignificant due to the relatively short transmission distance in access networks. The feasibility of PolMUX is experimentally demonstrated with a power penalty at BER = 10^-9 of about 0.5 dB and 1 dB for 2.5 Gb/s wired and wireless downstream services, respectively. The proposed system is compatible with the current reported techniques in either WDM passive optical networks (WDM-PON) or radio-over-fiber (ROF) systems.     

Highly birefringent photonic crystal fiber polarization splitter made of soft glass

A soft glass dual core polarization splitter based on highly birefringent photonic crystal fiber (PCF) is proposed and the full vector finite element method (FEM) is employed to analyze the impacts of structural parameters on birefringence and the coupling length, and simulation results show that high birefringence on the order of 10⁻² can be obtained at 1.55 μm, moreover, hole size, hole pitch and elliptic ratio all affect birefringence and the coupling length. Based on these results, the PCF's structure is optimized to realize a polarization splitter of 282 μm whose largest extinction ratio is around −45.42 dB at 1.55 μm. Meanwhile, the bandwidth at the extinction ratio of −10 dB is about 90 nm, and around 32 nm at −20 dB.     

Switchable dual-wavelength erbium-doped fiber-ring laser based on one polarization maintaining fiber Bragg grating in a Sagnac loop interferometer

A switchable erbium-doped fiber-ring laser providing dual-wavelength outputs with orthogonal polarizations when operating at room temperature is proposed. One polarization-maintaining fiber Bragg grating (PMFBG) in a Sagnac loop interferometer is used as the wavelength-selective filter. Due to the enhancement of the polarization hole burning (PHB) by the PMFBG, the laser can be designed to operate in stable dual-wavelength or wavelength-switching modes with a wavelength spacing of 0.336 nm at room temperature by adjusting a polarization controller (PC). The optical signal-to-noise ratio (OSNR) is over 42 dB. The amplitude variation over 90 min is less than 0.6 dB for both wavelengths.     

Design and implementation of polarization filter for quantum states discriminator in optical quantum communication

In optical quantum communication, quantum state measurement is necessary. This paper proposes a new technique for realization of polarization filter based on planar lightwave circuit (PLC). This filter is used for quantum state discriminator in quantum communication and also as a Bell-state analyzer in quantum repeater. Electro-optics interferometer has been used in design and implementation of polarization filter. We use lithium niobate as a wafer material and Ti:LiNbO₃ for waveguide. Two directional couplers have been used in this device. The length and spacing of these directional couplers have been designed so that each polarization is routed in specific output. The proposed device has one input and two outputs. If polarization of the input photon is vertical, then this photon will appear in output 1, otherwise if the input photon has horizontal polarization, it appears in output 2. For vertical polarization input, the power overlaps integral (POI) shows that isolation between two outputs is 14.96 dB. As to horizontal polarization input, the isolation between two outputs is 13.8 dB. The designed polarization filter has length of 33 mm and width of 60 μm. This device is very suitable for use in integrated optics.     

Vortex solitons with inhomogeneous polarization in nonlocal self-focusing nonlinear media

Both azimuthally and radially polarized vortex solitons are investigated to be able to exist in highly nonlocal nonlinear media. We get exactly analytical solutions of azimuthally polarized vortex solitons with only polarization singularities and radially polarized vortex solitons with both phase singularities and polarization singularities. Both azimuthally and radially polarized vortex solitons can exist in nonlocal self-focusing nonlinear media with proper modulation of the beam power and the degree of nonlocality. Contrary to those of radially polarized counterparts in local Kerr media, the topological charge can be any integer. When the topological charge m ≠ 0, both phase singularities and polarization singularities work. When m = 0, the polarization singularities work. Azimuthally polarized vortex solitons with polarization singularities corresponds to the linearly polarized vortex solitons with single charge. Our results show that polarization singularities work the same way as phase singularities in some sense.     

Effect of gamma irradiation on the light polarization variation of PMMA polymer

Analyzing the effect of ionized gamma irradiation on the optical parameters of materials is a solution for finding newer techniques in the field of detector and dosimeter systems. A PMMA (polymethylmethacrylate) polymer was radiated from a ⁶⁰Co source with a power of 1800 C and a constant dose rate of 1.44 kGy/h in three steps of 5, 25 and 61.2 kGy. The ionized gamma irradiation affected the refractive index of polymer and therefore it changes the polarization of the incident light. The difference in the polarization phase shift of the polymer depended on the dose it had been irradiated with.     

Characterization of structural irregularities in highly birefringent photonic crystal fiber using torsional acoustic polarization coupling

We demonstrate a novel method to characterize structural irregularities in a highly birefringent photonic crystal fiber (HB PCF) with high accuracy based on polarization coupling by torsional acoustic wave. Birefringence variation induced by the irregularities of air-hole structure in the fiber cross-section is analyzed via the transmission spectra of the acousto-optic coupling between two orthogonal polarization modes propagating along the fiber. The estimated maximum birefringence variation of two sections in the same batch of the HB PCF is 3 × 10^− 6 and 10^− 5, respectively, from nominal birefringence value of 4.86 × 10^− 4. The comparison between the experimental results and the numerical simulation resulted in the estimation of few tens of nm variation in the air-hole structure in the PCF.     

All-optical clock recovery of 20 Gbit/s NRZ-DPSK signals using polarization-maintaining fiber loop mirror filter and semiconductor optical amplifier fiber ring laser

All-optical clock recovery (CR) from 20 Gbit/s nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signals are demonstrated experimentally by using a polarization-maintaining fiber loop mirror filter (PMF-LMF) and a semiconductor optical amplifier (SOA) fiber ring laser. Only by adjusting polarization controller (PC), NRZ-DPSK signals were conveniently and fast converted to pseudo return-to-zero (PRZ) signal via PMF-LMF. Then the PRZ signals are injected into the SOA fiber laser for CR. The recovered clock signals is with the extinction ratio (ER) of 10 dB and the root-mean-square (RMS) timing jitter of 750 fs in 2³¹ − 1 long pseudorandom binary sequence (PRBS) NRZ-DPSK signals measurement. Moreover, the broad wavelength tunability of recovered clock stemmed from the use of SOAs as modulator and the gain medium are shown too.     

Spin polarization at the NiMnSb/MgO(1 0 0) interface

The half-metallic NiMnSb alloy was tested as a possible fully polarized electrode in a magnetic tunnel junction. For this purpose, epitaxial NiMnSb(0 0 1) and NiMnSb/MgO(0 0 1) systems were grown by molecular beam epitaxy. Spin-polarized photoemission experiments using the synchrotron radiation at the ESRF were performed on both the uncovered NiMnSb(0 0 1) surface and on the NiMnSb/MgO(0 0 1) interface. Starting from a NiMnSb(0 0 1) uncovered surface presenting at room temperature a 40% polarization at the Fermi level measured in the whole Brillouin zone, this polarization was observed to dramatically decrease when a MgO(0 0 1) epitaxial barrier was grown on top of it. On the other hand, the X-ray absorption experiments clearly show some oxidization of Mn after growing the MgO layer. This behavior was confirmed by testing the electronic and chemical properties of a unique atomic plane of Mn grown on a Fe buffer layer, before and after growing a MgO layer on top of it.     

Oxygen pressure dependent interfacial polarization of perovskite oxide multilayers on Si substrates

The SrTiO₃/La_0.9Sr_0.1MnO₃ (STO/LSMO) multilayers were fabricated on n-Si (1 0 0) substrates using a computer-controlled laser molecular-beam epitaxy (MBE) technique at 1 × 10⁻² Pa and 2 × 10⁻⁴ Pa oxygen partial pressures, respectively. The dielectric properties of the multilayers (MLs) were investigated. The differences of the dielectric properties of the two samples were explained by an impedance analysis technique, which indicates that the interfacial polarization has a close relationship with the oxygen pressure.     

The effect of the excitation and of the temperature on the photoluminescence circular polarization of AlInAs/AlGaAs quantum dots

In this paper, we present a study of photoluminescence (PL) from AlInAs/AlGaAs quantum dots (QDs) structures grown by molecular beam epitaxy. Specifically, we describe the effects of the temperature and of the excitation density on the photoluminescence circular polarization. We have found that the circular polarization degree depends on temperature. On the other hand, the study of the excitation density dependent circular polarization PL degree shows that the last increases in the case of the sample of weak dot density. However, in the case of large dot density, it is almost constant in the excitation density range from 0.116 W cm⁻² to 9 W cm⁻².