Orthogonal Polarization Directions Research Articles

An intensity modulation and coherent balanced detection intersatellite microwave photonic link using polarization direction control

A simple approach for high loss intersatellite microwave photonic link with intensity modulation and coherent balanced detection is proposed. In the transmitter, the double sideband-suppressed carrier (DSB-SC) modulated optical signal and optical carrier (OC) are combined by employing a polarization combiner to chose and control the signals polarization directions, while in the receiver, they are selected respectively by using a polarization splitter for they have orthogonal polarization directions. The separated DSB-SC signal and OC put into balanced detectors and the coherent detection is realized without a local oscillator (LO). At the output, the fundamental signal is augmented and the third-order distortion is suppressed for the DSB-SC modulation, the second-order distortion is removed for the balanced detection and the noise is reduced for the polarization direction control. The signal to noise and distortion ratio (SNDR) can be optimized by adjusting the power of OC and modulation index. The simulation results show that, a SNDR higher than 30 dB can be obtained for the proposed method, which is in agreement with the theoretical analysis.

Mode locking of Yb:GdYAG ceramic lasers with an isotropic cavity

We report on the passive mode locking of a diode pumped Yb:GdYAG ceramic laser with a near isotropic cavity. It is found that the laser could simultaneously mode lock in the two orthogonal principal polarization directions of the cavity, and the mode locked pulses of the two polarizations have identical features and are temporally perfectly synchronized. However, their pulse energy varies out-of-phase periodically, manifesting the antiphase dynamics of mode locked lasers.

Full-field detection of polarization state based on multiplexing digital holography

Based on digital holography recording system, it is proposed to use both polarization and angular multiplexing techniques to detect a polarization state. Reference beam is divided into two beams with orthogonal polarization directions and equal initial phases. And the beams interfere with the two orthogonal components of object wave respectively, which generates two holograms recorded in one frame. The use of angular multiplexing separates the two orthogonal components of object wave in the Fourier domain by respectively introducing carriers towards different directions. By numerical filtering, inverse Fourier transform and then diffraction calculation, complex amplitude of object wave can be obtained at different distances to the holograms. With the obtained complex amplitude, it is possible to work out both the Stokes parameters and Jones vector, which describe the polarization state of the object wave. The measuring of elliptical polarization state and the characterizing of polarization states at different locations on axis prove the validity of this real-time full-field detection method.

Dynamics of 1.55 μm Wavelength Single-Mode Vertical-Cavity Surface-Emitting Laser Output under External Optical Injection

We review the temporal dynamics of the laser output spectrum and polarization state of 1.55 μm wavelength single-mode (SM) vertical-cavity surface-emitting lasers (VCSELs) induced by external optical beam injection. Injection of an external continuous-wave laser beam to a gain-switched SM VCSEL near the resonance wavelength corresponding to its main polarization-mode output was critical for improvement of its laser pulse generation characteristics, such as pulse timing-jitter reduction, linewidth narrowing, pulse amplitude enhancement, and pulse width shortening. Pulse injection of pulse width shorter than the cavity photon lifetime into the SM VCSEL in the orthogonal polarization direction with respect to its main polarization mode caused temporal delay of the polarization recovery after polarization switching (PS), and its delay was found to be the minimum at an optimized bias current. Polarization-mode bistability was observed even in the laser output of an SM VCSEL of a standard circularly cylindrical shape and used for all-optical flip-flop operations with set and reset injection pulses of very low pulse energy of order of the 3.5~4.5 fJ.

Tunable and wideband microwave photonic phase shifter based on a single-sideband polarization modulator and a polarizer

A novel microwave photonic phase shifter based on a single-sideband (SSB) polarization modulator (PolM) and a polarizer is proposed and demonstrated. In the SSB-PolM, two SSB intensity-modulated signals with a phase difference of π along two orthogonal polarization directions are generated. With the polarizer to combine the two signals, the phase of the optical microwave signal can be tuned from -180 to 180 deg by simply adjusting the polarization direction of the polarizer, whereas the amplitude keeps unchanged. An experiment is carried out. A full-range tunable phase shift in the frequency range of 11-43 GHz is achieved. The flat power response, power independent operation, and high stability of the proposed microwave photonic phase shifter is also confirmed.

Analysis of the characteristics of the polarization splitter based on tellurite glass dual-core photonic crystal fiber

A novel kind of polarization splitter based on tellurite glass dual-core photonic crystal fiber is proposed. The characteristics of the polarization splitter have been studied by the finite element method. By adjusting the ellipticity of the dual-core PCF, a very high value of birefringence can be reached. The high birefringence can increase the D-value of coupling length in the two orthogonal polarization directions, then the polarization-independent splitter can be achieved. Comparing with the SF57 glass dual-core PCF, the tellurite glass dual-core PCF has higher extinction ratio, the extinction ratio can reach −31dB at the wavelength λ=1.55μm. The 0.36-mm-long splitter is proposed to achieve extinction ratio better than −10dB and a bandwidth of 20nm.

Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes

The characteristic striped appearance of zebras has provoked much speculation about its function and why the pattern has evolved, but experimental evidence is scarce. Here, we demonstrate that a zebra-striped horse model attracts far fewer horseflies (tabanids) than either homogeneous black, brown, grey or white equivalents. Such biting flies are prevalent across Africa and have considerable fitness impact on potential mammalian hosts. Besides brightness, one of the likely mechanisms underlying this protection is the polarization of reflected light from the host animal. We show that the attractiveness of striped patterns to tabanids is also reduced if only polarization modulations (parallel stripes with alternating orthogonal directions of polarization) occur in horizontal or vertical homogeneous grey surfaces. Tabanids have been shown to respond strongly to linearly polarized light, and we demonstrate here that the light and dark stripes of a zebra's coat reflect very different polarizations of light in a way that disrupts the attractiveness to tabanids. We show that the attractiveness to tabanids decreases with decreasing stripe width, and that stripes below a certain size are effective in not attracting tabanids. Further, we demonstrate that the stripe widths of zebra coats fall in a range where the striped pattern is most disruptive to tabanids. The striped coat patterns of several other large mammals may also function in reducing exposure to tabanids by similar mechanisms of differential brightness and polarization of reflected light. This work provides an experimentally supported explanation for the underlying mechanism leading to the selective advantage of a black-and-white striped coat pattern.

Vertical-cavity surface-emitting laser with liquid crystal overlay

We perform a theoretical study of the spectral and polarization threshold characteristics of Vertical-Cavity Surface-Emitting Lasers with Liquid Crystal overlay (LC-VCSEL) in three different configurations of the LC cell. Our model predicts the possibility of selecting between two orthogonal directions of linear polarization (LP) of the fundamental mode (x or y LP) by choosing appropriate LC length. It further predicts very strong polarization discrimination with LP mode threshold gain difference as large as several times the threshold gain of the lasing mode. We also numerically demonstrate an active control of light polarization by electro-optically tuning the LC director and show that either polarization switching between x and y LP modes or continuous change of the LP direction would be possible. Finally, we numerically demonstrate that LC-VCSEL would be capable of efficient wavelength tuning.

Response of the Compton polarimeter POLAR to polarized hard X-rays

POLAR is a novel compact space-borne Compton polarimeter conceived for a precise measurement of hard X-ray polarization and optimized for the detection of Gamma-Ray Burst (GRB) photons in the energy range 50–500keV. In December 2009 we have performed a systematic calibration of one modular unit of POLAR at four energy levels (200, 288, 356 and 511keV) with a 100% polarized synchrotron radiation source at the beam line ID15A at ESRF. The detector was displaced several times on the beam line in order to achieve a uniform illumination, which mimics the flux from a GRB placed on the zenith of the experiment. Several rotations of the detector on the beam axis allowed us to test the response of POLAR to several polarization angles. Two different analysis methods to reconstruct the polarization angle of the beam and the modulation factor μ100 are presented; the first relies on the existence of a unpolarized sample, produced by merging two data sets with orthogonal polarization directions, and is less dependent on systematic effects due to asymmetries in the detector; the second is independent from unpolarized measurements, and will likely be used to analyze the polarization of GRB during the flight. Both methods reconstruct the input polarization angle within 2° and produce modulation factors μ100 between 30% and 50% depending on the beam energy. Monte Carlo simulations performed with GEANT4 confirm the experimental results.

Application of Seismic Anisotropy Caused by Fissures in Coal Seams to the Detection of Coal-bed Methane Reservoirs

Coal-bed methane is accumulated in micro-fissures and cracks in coal seams. The coal seam is the source terrace and reservoir bed of the coal-bed methane (Qian et al., 1996). Anisotropy of coal seams is caused by the existence of fissures. Based on the theory of S wave splitting: an S wave will be divided into two S waves with nearly orthogonal polarization directions when passing through anisotropic media, i.e. the fast S wave with its direction of propagation parallel to that of the fissure and slow S wave with the direction of propagation perpendicular to that of the fissure. This paper gives the results of laboratory research and field test on the S wave splitting caused by coal-seam fissures. The results show that it is feasible to detect fissures in coal seams by applying the converted S wave and finally gives the development zone and development direction of these fissures.

Dissipative soliton trapping in normal dispersion-fiber lasers

We report on the dissipative soliton (DS) trapping in a fiber ring laser mode locked with a semiconductor saturable absorber mirror and operated in normal dispersion regime. It was shown that despite the fact that a DS is strongly frequency chirped, two DSs formed along the two orthogonal polarization directions of a birefringent cavity fiber laser can incoherently couple and travel with the same group velocity in the laser. Numerical simulations have well confirmed the experimental observations.

POLARIZED INFRARED EMISSION BY POLYCYCLIC AROMATIC HYDROCARBONS RESULTING FROM ANISOTROPIC ILLUMINATION

We study the polarized infrared emission by Polycyclic Aromatic Hydrocarbons (PAHs), when anisotropically illuminated by UV photons. PAH molecules are modeled as planar disks with in-plane and out-of-plane vibrational dipoles. As first pointed out by Leger (1988), infrared emission features resulting from in-plane and out-of-plane modes should have orthogonal polarization directions. We show analytically how the degree of polarization depends on the viewing geometry and the molecule's internal alignment between principal axis of inertia and angular momentum, which gets worse after photon absorption. Longer wavelength features, emitted after better internal alignment is recovered, should be more strongly polarized. The degree of polarization for uni-directional illumination (e.g., by a star) is larger than for diffuse illumination (e.g., by a disk galaxy), all else being equal. For PAHs in the Cold Neutral Medium, the predicted polarization is probably too small to distinguish from the contribution of linear dichroism by aligned foreground dust. The level of polarization predicted for PAH emission from the Orion Bar is only ~0.06% at 3.3 microns; Sellgren et al. (1988) report a much larger value, 0.86+-0.28%, which suggests that the smallest PAHs may have moderately suprathermal rotation rates. Future observations of (or upper limits on) the degree of polarization for the Orion Bar or for dust above edge-on galaxies (e.g., NGC 891 or M82) may constrain the internal alignment of emitting PAHs, thus providing clues to their rotational dynamics.

UV spectral measurements at moderately high resolution and of OH resonance scattering resolved by polarization during the MANTRA 2002–2004 stratospheric balloon flights

A moderately high-resolution (<0.1 nm) grating spectrometer designed to measure the solar radiation in the spectral range 295–315 nm was flown on the MANTRA stratospheric balloon payloads of 2002 and 2004. The instrument measures both the direct sunlight and the radiation scattered by the atmosphere. The latter can be observed in two orthogonal polarization directions, at 90° from the solar azimuth and at several elevations above the horizon. As the OH molecule is the principal resonant scatterer in this spectral region, this permits the inference of both ozone and OH column amounts as well as limited profile information. This paper describes the instrument and its in-flight characterization, the basic data processing and the influence of several aspects of the flight profile. The direct sun measurements are analyzed both to characterize the spectrometer responsivity to scattered radiation and to estimate the ozone abundance at the flight altitude and above. An example of a high-resolution solar spectrum at 37 km altitude is presented and compared with others in the literature. The measured OH and Rayleigh-scattered spectra are used to derive OH radiation intensity measurements (the OH airglow), which are compared with others in the literature.

Soliton trapping in fiber lasers

We report on the soliton trapping in a fiber ring laser modelocked with a SESAM. It was observed that solitons along the two orthogonal polarization directions of the cavity with fairly large difference in central frequency and energy could be coupled together to form a group velocity locked vector soliton. In particular, due to that each of the coupled solitons forms its own soliton sidebands, two sets of soliton sidebands could be observed on the vector soliton spectrum. Numerical simulations have well confirmed the experimental observations.

Superposition of transverse and longitudinal finite-amplitude waves in a deformed Blatz—Ko Material

In this paper, special Blatz—Ko nonlinear elastic materials are considered, which are characterized by a constitutive constant and a constitutive function. We here deal with the propagation of finite-amplitude waves in such materials subjected to an arbitrary static homogeneous deformation. In a previous paper, it was shown that linearly polarized transverse damped inhomogeneous plane waves may propagate. The orthogonal propagation and polarization directions are arbitrary. The special Blatz—Ko materials are compressible so that homogeneous longitudinal waves may also propagate. Here it is shown that the superposition of a transverse damped inhomogeneous wave and of a longitudinal wave is also a solution, in the case when the propagation direction of the longitudinal wave is orthogonal to the polarization direction of the transverse wave. Also, results are obtained for the energy density and the energy flux of the superposition of these waves.

Polarization-dependent beam switch based on an M-plane GaN∕AlN distributed Bragg reflector

The authors demonstrate a two-color distributed Bragg reflector (DBR) consisting of 20 periods of alternating [11¯00]-oriented (M-plane) AlN and GaN layers grown on LiAlO2 by molecular-beam epitaxy. Due to the birefringent nature of GaN and AlN, the wavelength region of the stop band depends on the polarization state of the incoming light beam (parallel or perpendicular) with respect to the c axis of the wurtzite crystal structure. In the wavelength range, where the transmittance for one polarization direction and the reflectance for the orthogonal polarization direction are both high, the DBR can be used as a beam switch or polarization filter.

A high speed 2×2 electro-optic switch using a polarization modulator

A high speed 2x2 electro-optic switch using a polarization modulator (PolM) is proposed and experimentally demonstrated. In the proposed switch, two linearly polarized input lightwaves with orthogonal polarization directions are sent to the PolM which is connected to a polarization beam splitter (PBS). When a switching signal is applied to the PolM, the polarization directions of the two lightwaves at the output of PolM will exchange. Consequently, the lightwaves at the two output ports of the PBS would be switched, a 2x2 switch is thus realized. An optical switch with a crosstalk lower than -35 dB and a switching time less than 25 ps is experimentally demonstrated. The performance of the switch is also experimentally investigated by studying the bit error rates and eye diagrams. A technique to achieve polarization independent operation is also proposed and discussed.

A compact orthogonal tripolarized multiantenna with low mutual coupling for MIMO channel measurements

AbstractA tripolarized multiantenna made up of three compact orthogonal dipoles is proposed. The three dipoles' phase centers are compact within 1/6 wavelength so that the spatial fading condition of the multi input multi output (MIMO) channel in subwavelength scale can be discriminated in three orthogonal directions of polarization by the multiantenna. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1358–1362, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21658

Polarization filtering by nonpolar M-plane GaN films on LiAlO2

The anisotropic optical properties and polarization filtering are investigated for strained M-plane GaN films as a function of the angle of the in-plane polarization vector. Since the c axis of the underlying wurtzite crystal structure lies in the film plane, its symmetry is reduced in comparison to conventional C-plane films, giving rise to an in-plane polarization anisotropy. This anisotropy is further enhanced by the anisotropic compressive in-plane strain, which occurs due to the lattice mismatch between the GaN film and the substrate LiAlO2. In addition to linear birefringence, the film also exhibits linear dichroism, resulting in a polarization filtering of an incident, linearly polarized light beam after transmission. This filtering manifests itself as a rotation of the polarization vector towards the c axis and can be as large as 80°. We compare the measured polarization rotation with calculated values taking into account the birefringence of the GaN film. In the energy range where the filtering is most effective, the birefringence almost disappears and can therefore be neglected for the determination of the rotation angle. Finally, the rotation, which is determined by the transmittance for the two orthogonal polarization directions, can be very well approximated by the product of the film thickness and the difference of the absorption coefficients for the two polarization directions.

Specific Features of the Kinetics of Atoms in a Three-Dimensional Bichromatic Standing Wave

The three-dimensional kinetics of resonance atoms with the J = 0 → J = 1 quantum transition in the weak field of mutually orthogonal bichromatic standing waves under conditions of manifestation of the effect of straightening of radiation forces is considered. It is demonstrated that the character of particle motion (induced by the straightened radiation forces) significantly depends on the relative phase difference of standing waves. In general, it is vortex in character and can suppress the process of localization of atoms in the small vicinity of straightened radiation force nodes even in the presence of a strong friction force. The target-oriented choice of phase relationships between the field components with mutually orthogonal polarization directions guarantees a solution of the problem of purely optical localization of atoms in the mode of vortex-free motion.