Search

A less explored aspect of L-band microwave emission is the surface roughness effect on the multiangular brightness temperature $(T_{\mathrm {B}}^{\mathrm {p}})$ modeling and retrieval of liquid water content $(\theta _{\mathrm {liq}})$ in frozen soil. This letter investigates this effect based on a data set of multiangular ETH L-band radiometer (ELBARA-III) $T_{\mathrm {B}}^{\mathrm {p}}$ measurements performed in a seasonally frozen Tibetan meadow ecosystem. The surface roughness effect is first investigated via comparing the performance of the HQN model and Shi’s parameterization of the integral equation model in simulating the measured $T_{\mathrm {B}}^{\mathrm {p}}$ signatures. Overestimations are noted for the two model simulations at the vertical polarization in comparison to the ELBARA-III measurements, and Shi’s model with Gaussian autocorrelation function shows better performance especially for the vertical polarization at large incidence angles. The HQN model is then calibrated with the multiangular $T_{\mathrm {B}}^{\mathrm {p}}$ measurements, which leads to $h = 0$ and $Q > 0$ that depends on the polarization. As such, the noncoherent emission contribution to cross-polarization mixing can be accounted for by the HQN model. Furthermore, $\theta _{\mathrm {liq}}$ retrievals are derived from the multiangular $T_{\mathrm {B}}^{\mathrm {p}}$ measurements using the default and optimized HQN model and Shi’s model and compared to the in situ $\theta _{\mathrm {liq}}$ . Unbiased root-mean-square errors of less than 0.025 m3 m−3 are obtained for these retrievals, which are well within the satellite mission requirements. This study demonstrates that the multiangular $T_{\mathrm {B}}^{\mathrm {p}}$ modeling can be enhanced with site-specific calibration of the HQN model that also leads to the improvement of the $\theta _{\mathrm {liq}}$ retrievals, and consideration of polarization mixing is necessary for L-band emission modeling.

The soil moisture active passive (SMAP) mission was designed to acquire L-band radiometer measurements for the estimation of soil moisture (SM) with an average ubRMSD of not more than 0.04 $\text{m}^{3}/\text{m}^{3}$ volumetric accuracy in the top 5 cm for vegetation with a water content of less than 5 kg/ $\text{m}^{2}$ . Single-channel algorithm (SCA) and dual-channel algorithm (DCA) are implemented for the processing of SMAP radiometer data. The SCA using the vertically polarized brightness temperature (SCA-V) has been providing satisfactory SM retrievals. However, the DCA using prelaunch design and algorithm parameters for vertical and horizontal polarization data has a marginal performance. In this article, we show that with the updates of the roughness parameter $h$ and the polarization mixing parameters $Q$ , a modified DCA (MDCA) can achieve improved accuracy over DCA; it also allows for the retrieval of vegetation optical depth (VOD or $\tau$ ). The retrieval performance of MDCA is assessed and compared with SCA-V and DCA using four years (April 1, 2015 to March 31, 2019) of in situ data from core validation sites (CVSs) and sparse networks. The assessment shows that SCA-V still outperforms all the implemented algorithms.

A novel, to the best of our knowledge, and simple heterodyne interferometer that uses spatially separated input beams to minimize the influence of the periodic nonlinearity is constructed. A custom designed polarizing beam displacer is used to split the input beams to parallel outputs with orthogonal polarizations, which provides a balanced path and completely symmetric structure for the interferometer. This novel optical setup suppresses the nonlinearity caused by the frequency and polarization mixing, and the very simple optical structure makes the interferometer less susceptible to environmental turbulence with potential use in many sensor applications. Experiments have confirmed that the interferometer maintains sub-nanometer nonlinearities in the laboratory environment.

The large number of spatial modes supported by standard multimode fibers is a promising platform for boosting the channel capacity of quantum and classical communications by orders of magnitude. However, the practical use of long multimode fibers is severely hampered by modal crosstalk and polarization mixing. To overcome these challenges, we develop and experimentally demonstrate a vectorial time reversal technique, which is accomplished by digitally pre-shaping the wavefront and polarization of the forward-propagating signal beam to be the phase conjugate of an auxiliary, backward-propagating probe beam. Here, we report an average modal fidelity above 80% for 210 Laguerre-Gauss and Hermite-Gauss modes by using vectorial time reversal over an unstabilized 1-km-long fiber. We also propose a practical and scalable spatial-mode-multiplexed quantum communication protocol over long multimode fibers to illustrate potential applications that can be enabled by our technique., Comment: Supplementary Information is included

A homodyne interferometer is one of the most important tools in nanometre measurements. However, its nonlinear error seriously affects measurement accuracy at the sub-nanometre level. As one of the dominant factors that cause nonlinear error in a homodyne interferometer with a quadrature detector system, the imperfection of polarizing beam splitters (PBSs) is investigated in this paper. The nonlinear error caused by the imperfection of PBSs in the detection part can be reduced by adjusting the gains of detectors. Nevertheless, eliminating the nonlinear error caused by the polarization mixing of the PBS in the interferometer part is difficult. In this paper, the nonlinear error caused by the polarization mixing of the PBS in the interferometer part is analyzed, and an optical compensation method is proposed to correct this polarization mixing. Theoretical calculation and simulation analysis show that this method can reduce the effect of inherent polarization mixing on nonlinear error significantly. In comparison with using only gain adjustment, the nonlinear error can be reduced by two orders of magnitude when the proposed method is applied. The nonlinear error can be decreased from approximately 4.5 nm to approximately 0.045 nm using the presented method based on the simulation results.

The differentiator consists of carefully designed 2D photonic crystal (PhC) slab that can transform an image into its second-order derivative. Based on interference between the direct transmission and low quality factor quasi-guided modes, the PhC slab exhibits angular-dependent transmission for P polarization but remains reflective for S polarization, which avoids polarization mixing in the transmission matrix. Fourier imaging was carried out showing a quadratic transfer function for an NA up to 0.315, which allows one to resolve features on the scale of 1.94λ. To showcase practical applications, the nanophotonic differentiator was directly integrated into an optical microscope and onto a camera sensor demonstrating the ease at which it can be vertically integrated into existing imaging systems. Furthermore, we demonstrate a compound bilayer flat optical by integrating the differentiator with a metalens for realizing a compact and monolithic image processing system. In all cases, the use of the nanophotonic differentiator allows for a significant reduction in size compared to traditional systems as one does not need to pass through the Fourier plane for performing complex image processing. This freedom should open new doors for optical analog image processing in applications involving machine vision.

A heterodyne grating interferometer (HGI) with high stability and low nonlinearity is presented. An optical arrangement with Littrow configuration combined with Wollaston prism is proposed for the HGI. The compact and symmetrical design make it insensitive to environmental disturbances. The optimal separate angle of the Wollaston prism is investigated with rigorous coupled-wave analysis (RCWA), and the value of 44° corresponding to the grating pitch of 845 nm is selected. The approximately equal diffraction efficiencies of 76.54% and 76.64% for TE and TM polarizations are respectively obtained, which is beneficial to the signal-to-noise ratio (SNR) of HGI. Additionally, the nonlinear errors including polarization mixing, frequency mixing and polarization-frequency mixing are analyzed. The polarization mixing error is dramatically decreased owing to the high extinction ratio of Wollaston prism. It reveals that the developed HGI has the potential for nanometric displacement measurement.

Optical modulation format identification is critical in the next generation of heterogeneous and reconfigurable optical networks. Here, we present a blind modulation format identification method by applying fast density-peak-based pattern recognition in the autonomous receiver of elastic optical networks. In this paper, we find that the different modulation format types show different energy level features which can be used as a metric to identify these modulation formats in two-dimensional Stokes plane. The proposed method does not require training symbols, and is insensitive to carrier phase noise, frequency offset as well as polarization mixing. The effectiveness is verified via numerical simulations and experiments with PDM-BPSK, PDM-QPSK, PDM-8PSK, PDM-16PSK, PDM-8QAM, and PDM-16QAM. The results show that high identification accuracy can be realized using our proposed method over wide optical signal-to-noise ratio ranges. Meanwhile, we also discuss the influence of the residual chromatic dispersion, polarization mode dispersion, and polarization dependent loss impairments to our proposed method. We believe that the simple and flexible identification method would certainly bring a great convenience to the future optical networks.

A differential interferometric heterodyne encoder with spatially separated input beams was developed to minimize periodic nonlinearities resulting from polarization mixing. The laser beams with different frequencies were delivered by two polarization-maintaining fibers to the encoder head. Under laboratory conditions this encoder demonstrated a system stability of 38 pm (standard deviation) and 100 pm over 30 s and 1 h respectively. In a comparison measurement with a differential heterodyne interferometer, this encoder showed periodic nonlinearities of less than 30pm without any additional correction.

We first experimentally demonstrate a polarization-multiplexed continuous-variable quantum key distribution architecture eliminating the requirement for polarization controlling, where the polarization mixing and phase rotation are simultaneously compensated in a dual reference compensation scheme.

We present the design, bench-top setup, and experimental results of a compact heterodyne interferometer that achieves picometer-level displacement sensitivities in air over frequencies above 100 MHz. The optical configuration with spatially separated beams prevents frequency and polarization mixing, and therefore eliminates periodic errors. The interferometer is designed to maximize common-mode optical laser beam paths to obtain high rejection of environmental disturbances, such as temperature fluctuations and acoustics. The results of our experiments demonstrate the short- and long-term stabilities of the system during stationary and dynamic measurements. In addition, we provide measurements that compare our interferometer prototype with a commercial system, verifying our higher sensitivity of 3 pm, higher thermal stability by a factor of two, and periodic-error-free performance.

Based on the vector diffraction theory, this paper focuses on the focusing pattern of cosh-Gaussian beams with polarization mixing cosine phase modulation and discusses the focused optical intensity under different parameters in detail. The results that show by adjusting the polarization parameter C , a tunable focal shift and a continuous shifting of focus on both directions of the optical axis can be achieved and the component that dominates the intensity distribution of the total field can be changed. When the tunable parameter D is positive, the optical chain with tunable length can be structured by increasing D . When D is negative, there will appear one special optical trap focus mode by which the number of optical traps can be changed by tailoring D . When the phase modulation parameter m increases, the focal pattern will wholly enlarge and stretch along the radial direction, changing from two intensity peaks to six optical chains. By increasing the decentered parameter β , the shape of the light intensity distribution in the focusing region can be remarkably adjusted. Besides, by combining the theories and analyzing the field patterns in the transverse plane, there is an electric field intensity dependence on the azimuthal angle φ . Those novel findings may be helpful in the applications such as optical manipulation, optical focusing, and imaging.

Soil moisture and salinity are crucial parameters of the Earth&rsquo, s ecosystem, how to understand the radiation properties of them is of great significance for remote sensing monitoring. In this study, the application of mixed soil dielectric models (Dobson and generalized refractive mixing dielectric model (GRMDM)) and saline soil dielectric models (Dobson-S, HQR (Qingrong Hu), and WYR (Yueru Wu)) were analyzed to select the optimal models to simulate brightness temperature based on observational data. The brightness temperature of the soil moisture and multilevel salinity was simulated by using the Q-H (parameter of polarization mixing and parameter of characterizing height) model and Holmes parameterization scheme of soil effective temperature. The results show that both the Dobson model and the GRMDM model can well reproduce the real part and imaginary part of the dielectric constant of non-saline soil, and the GRMDM model was better. With the increase of the frequency, the simulation error of the dielectric constant of the saline soil by using the Dobson-S model, HQR model, and WYR model also increased, and the simulation result of the WYR model was better in the L band. The simulated result of the brightness temperature of soil moisture between the observation value and simulation value presented a high correlation both in the horizontal polarization and vertical polarization, with R greater than 0.967 and 0.948, and the root mean square error smaller than 3.998 K and 2.766 K, respectively. Meanwhile, the correlation coefficients of the brightness temperature of the saline soil in the horizontal polarization and vertical polarization were 0.935 and 0.971, and the root mean square errors were 5.808 K and 4.65 K, respectively. The brightness temperature decreased as the soil salinity increased, and the higher the salinity content was, the quicker the brightness temperature decreased. We expect that the experimental results can be used as a reference for algorithm developers to further enhance the accuracy of soil moisture and soil salinity retrievals.

We report the angular distribution of the G and 2D Raman scattering from graphene on glass by detecting back focal plane patterns. The G Raman emission can be described by a superposition of two incoherent orthogonal point dipoles oriented in the graphene plane. Due to double resonant Raman scattering, the 2D emission can be represented by the sum of either three incoherent dipoles oriented 120° with respect to each other, or two orthogonal incoherent ones with a 3:1 weight ratio. Parameter-free calculations of the G and 2D intensities are in excellent agreement with the experimental radiation patterns. We show that the 2D polarization ratio and the 2D/G intensity ratio depend on the numerical aperture of the microscope objective. This is due to the depolarization of the emission and excitation light when graphene is on a dielectric substrate, as well as to tight focusing. The polarization contrast decreases substantially for increasing collection angle, due to polarization mixing caused by the air-dielectric interface. This also influences the intensity ratio I(2D)/I(G), a crucial quantity for estimating the doping in graphene. Our results are thus important for the quantitative analysis of the Raman intensities in confocal microscopy. In addition, they are relevant for understanding the influence of signal enhancing plasmonic antenna structures, which typically modify the sample's radiation pattern.

The localized opical modes in photonic liquid crystals are studied for the certainty at the example of chiral liquid crystals (CLCs). The chosen here model (absence of dielectric interfaces in the studied structures) allows one to get rid off the polarization mixing at the surfaces of the CLC layer and the defect structure (DMS) and to reduce the corresponding equations to the equations for the light of diffracting in the CLC polarization only. The dispersion equations determining connection of the EM and DM frequencies with the CLC layer parameters and other parameters of the DMS are obtained. Analytic expressions for the transmission and reflection coefficients of the DMS are presented and analyzed. As specific cases are considered DMS with an active (i.e. transforming the light intensity or polarization) defect layer and CLC layers with locally anisotropic absorption. It is shown that the active layer (excluding an amplifying one) reduces the DM lifetime (and increase the lasing threshold) in compariso...

We propose and experimentally demonstrate a phase-sensitive optical processor, capable of generating two codirectional and pi-phase-shifted phase-sensitive amplifiers (PSAs) in a single device. Phase-sensitive operation is obtained by polarization mixing a phase-locked signal/idler pair generated in a degenerate dual-pump vector parametric amplifier based on four-wave mixing in a highly nonlinear fiber. We refer to this configuration as a polarization assisted PSA and demonstrate some of the applications that it may find. First, we experimentally demonstrate the regeneration of a binary phase shift keying signal in a system that requires only a very low nonlinear phase shift of 0.35 rad. Second, we decompose a quadrature phase shift keying (QPSK) signal into its in-phase and quadrature components. While application to a QPSK signal is shown in our demonstration, we demonstrate numerically that any complex modulation format signal can be decomposed using this approach. Finally, we use our processor to regenerate QPSK signals in a single nonlinear device

We report the surface modification of Invar film through perpendicularly switching of the polarization direction using the scanning of femtosecond laser to eliminate the micro holes and surface ripples, which are unwillingly generated by LIPSS.

A multimode fiber subjected to random mode and polarization mixing represents a complex photonic system with strong coupling of spatial, temporal, spectral and polarization degrees of freedom. By exploiting such coupling, we demonstrate a full control of the polarization state of light transmitted through a multimode fiber by adjusting the spatial profile of incident field. After applying stress to the fiber to induce strong mode and polarization mixing, we measure the polarization-dependent transmission matrices, and find the transmission eigenchannels. By launching light to specific eigenchannnels, we are able to preserve the polarization state despite strong polarization mixing in the multimode fiber, or to convert all transmitted light to the orthogonal polarization state. In addition, we show that the linearly polarized input light can be changed completely to circularly polarized output. Furthermore, arbitrary polarization states can be realized for individual spatial channels at the output by tailoring the incident wavefront of a single polarization. Such global control is possible only in the presence of strong mode mixing in the fiber. Namely, strong polarization mixing itself is not sufficient to generate arbitrary polarization states at the output. Therefore, the strong entanglement of spatial and polarization degrees of freedom is essential to achieve a complete control of polarization. Such global control of the polarization states of all output channels is more challenging than the local control of the polarization state of a single output channel.

We propose and demonstrate a new phase sensitive scheme based on four-wave mixing followed by a polarizer to achieve an ideal binary step-like phase transfer function at nonlinear phase shifts as low as 0.3 radians by significantly increasing the parametric de-amplification component. Phase-sensitive operation is obtained by polarization mixing the phase-locked and orthogonally-polarized signal and idler, which is generated in a degenerate dual-pump vector parametric amplifier.

Near surface soil moisture is being estimated from space-borne passive microwave observations through inverting a physically-based radiative transfer model (RTM), the land surface microwave emission model (LSMEM) at Princeton University for the past several years. The existing retrieval scheme utilizes only the horizontal ( H ) polarization measurement from a single channel (10.65 GHz). This physically-based approach requires a relatively large number of parameters, and it generally suffers from large biases/errors due to the difficulty in determining the correct parameters. This study characterizes these errors in order to improve the retrieval performance. Through model sensitivity analysis, this study finds that a dual polarization approach (using both horizontal and vertical polarizations) is needed to infer the correct vegetation opacity and correct polarization mixing measured by the space-borne sensor. Revisions are then made to the LSMEM formulations and soil moisture retrieval algorithm by 1) combining two vegetation parameters and one roughness parameter into one effective vegetation optical depth (VOD) value; and 2) providing an additional model equation that estimates the effective VOD from both polarizations and an initial guess of soil moisture value. The new retrieval algorithm is implemented to produce a daily 0.25° gridded soil moisture dataset based on observations from the Advanced Microwave Scanning Radiometer-EOS (AMSR-E). Validations are performed globally against land surface model simulations and at local/point scale against in-situ data within the continental United States. The new retrievals are shown to have good and robust performance over most parts of the world in terms of reproducing the spatial and temporal dynamics of soil moisture.

By shaping the incident wavefront, we effectively control the polarization states of light propagating through a multimode fiber with strong polarization and mode mixing. We can either eliminate depolarization and restore the input polarization state, or convert the output fields into a different polarization state.

An analytic approach to the theory of the optical defect modes in chiral liquid crystals (CLC) for the case of an active defect layer is developed. The analytic study is facilitated by the choice of the problem parameters. Namely, an isotropic absorbing or amplifying layer (with the dielectric susceptibility equal to the average dielectric susceptibility of CLC) sandwiched between two CLC layers is studied. The chosen model allows one to get rid off the polarization mixing and to reduce the corresponding equations to the equations for light of diffracting in CLC polarization only. The dispersion equation determining connection of the defect mode frequency with the isotropic layer gain and thickness is obtained. Analytic expressions for the transmission and reflection coefficients of the defect mode structure (CLC-active defect layer-CLC) are presented and analyzed for absorbing and amplifying defect layers. The effect of anomalously strong light absorption at the defect mode frequency for absorbing defect...

An analytic approach to the theory of the optical defect modes in chiral liquid crystals (CLC) for the case of a birefringent defect layer is developed. The analytic study is facilitated by the choice of the problem parameters. Namely, a birefringent layer (with the averaged dielectric susceptibility equal to the average dielectric susceptibility of CLC) sandwiched between two CLC layers inserted in an isotropic medium with the dielectric constant equal to the average dielectric constant of CLC is studied. The chosen model allows one to get rid off the polarization mixing at the external surfaces of the defect structure (DMS) and to reduce the corresponding equations to the equations for light of diffracting in CLC polarization only. The dispersion equation determining connection of the defect mode frequency with the layer birefringence and other parameters of the defect structure is obtained. Analytic expressions for the transmission and reflection coefficients of the defect mode structure (CLC- birefrin...

An analytic approach to the theory of optical defect modes in chiral liquid crystals (CLCs) is developed. The analytic study is facilitated by the choice of the problem parameters. Specifically, an isotropic layer (with the dielectric susceptibility equal to the average CLC dielectric susceptibility) sandwiched between two CLC layers is studied. The chosen model allows eliminating the polarization mixing and reducing the corresponding equations to the equations for light of diffracting polarization only. The dispersion equation relating the defect mode (DM) frequency to the isotropic layer thickness and an analytic expression for the field distribution in the DM structure are obtained and the corresponding dependences are plotted for some values of the DM structure parameters. Analytic expressions for the transmission and reflection coefficients of the DM structure (CLC-defect layer-CLC) are presented and analyzed for nonabsorbing, absorbing, and amplifying CLCs. The anomalously strong light absorption effect at the DM frequency is revealed. The limit case of infinitely thick CLC layers is considered in detail. It is shown that for distributed feedback lasing in a defect structure, adjusting the lasing frequency to the DM frequency results in a significant decrease in the lasing threshold. The DM dispersion equations are solved numerically for typical values of the relevant parameters. Our approach helps clarify the physics of the optical DMs in CLCs and completely agrees with the corresponding results of the previous numerical investigations.

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.

An analytic approach to the theory of the optical defect modes in chiral liquid crystals (CLC) is developed. The analytic study is facilitated by the choice of the problem parameters. Namely, an isotropic layer (with the dielectric susceptibility equal to the average dielectric susceptibility of CLC) sandwiched between two CLC layers is studied. The chosen model allows one to get rid off the polarization mixing and to reduce the corresponding equations to the equations for light of diffracting in CLC polarization only. Analytic expressions for the transmission and reflection coefficients of the defect mode structure (CLC-layer-CLC) are obtained as well as the dispersion equation determining connection of the defect mode frequency with the isotropic layer thickness. A limiting case of infinitely thick CLC-layers is considered in details. The developed approach helps to clarify the physics of the optical defect modes in CLC and manifests an excellent agreement with the corresponding results of the previous ...

The theory of optical subdivision techniques of dual-frequency laser interferometers is stated. And a novel optical subdivision technique is proposed originally to enhance resolution of a commercial interferometer by adding some corner-cubes. Then the performance of the interferometer is tested. The interferometer resolution of 1.24 nm and the average error of below 2 nm are achieved by using the technique. The most novel of the optical subdivision technique is without λ /4 plates. It is less sensitive to environmental changes, it has prodigious potential to improve resolution farther and it can reduce polarization mixing error.

The growing importance of polarimetric radiometers has led to the need for a detailed theory for Stokes antenna temperatures. In this paper, we provide a full Stokes vector formulation of an antenna temperature that accounts for the entire antenna pattern, which includes polarization mixing in the main-beam and sidelobe effects. To derive the Stokes antenna temperatures, we follow the conventional methods in the Earth remote sensing literature while relying on a coherency algebra approach from radio astronomy. Connections and parallels to the conventional approaches are noted along the way. We also introduce generalizations of beam efficiency and cross polarization for use with polarimetric radiometers. These provide important metrics in the design of future systems.

The hybrid plasmonic-electromagnetic modes-or plasmon polaritons-in 2-D square arrays of noncontacting noble-metal nanoparticles are studied via an exactly solvable model. The approach accounts self-consistently for diffractive losses out of the plane and also for the vector nature of the electromagnetic field, and is thus in essence a 3-D theory of the 2-D nanoparticle array. The resulting polarization mixing between modes leads to a coupling between longitudinal and transverse excitation off special directions in the Brillouin zone. This is manifested as warping of the plasmon-polariton dispersion surfaces. Modes with group and phase velocities of opposite signs are observed.

We use a microscopic theory to study hybrid organic-inorganic microcavities in the strong coupling regime. The optically resonant materials are an anisotropic crystalline organic mono-layer and a semiconductor quantum-well, which are coupled only via the cavity photons. The cavity eigenmodes are the hybrid Frenkel-Wannier-Mott exciton-polaritons. Using the quasi-mode approach, we calculate the hybrid cavity linear optical spectra, i.e. the transmission, reflection, and absorption spectra. In hybrid microcavities the anisotropic organic materials induce a mixing between the TE and TM cavity photon polarizations, and a mixing between the transverse and longitudinal heavy-hole (HH) Wannier-Mott excitons, which is in contrast to the case of ordinary semiconductor microcavities. (c) 2006 Elsevier B.V. All rights reserved.

Periodic nonlinearity which can result in error in nanometer scale has become a main problem limiting the absolute distance measurement accuracy. In order to eliminate this error, a new integrated interferometer with non-polarizing beam splitter is developed. This leads to disappearing of the frequency and/or polarization mixing. Furthermore, a strict requirement on the laser source polarization is highly reduced. By combining retro-reflector and angel prism, reference and measuring beams can be spatially separated, and therefore, their optical paths are not overlapped. So, the main cause of the periodic nonlinearity error, i.e., the frequency and/or polarization mixing and leakage of beam, is eliminated. Experimental results indicate that the periodic phase error is kept within 0.0018°.

Negative values of the Casimir entropy occur quite frequently at low temperatures in arrangements of metallic objects. The physical reason lies either in the dissipative nature of the metals as is the case for the plane-plane geometry or in the geometric form of the objects involved. Examples for the latter are the sphere-plane and the sphere-sphere geometry, where negative Casimir entropies can occur already for perfect metal objects. After appropriately scaling out the size of the objects, negative Casimir entropies of geometric origin are particularly pronounced in the limit of large distances between the objects. We analyze this limit in terms of the different scattering channels and demonstrate how the negativity of the Casimir entropy is related to the polarization mixing arising in the scattering process. If all involved objects have a finite zero-frequency conductivity, the channels involving transverse electric modes are suppressed and the Casimir entropy within the large-distance limit is found to be positive., 11 pages, 8 figures

This paper reviews ways and means used for reduction or elimination of periodic nonlinearity in heterodyne interferometers. The periodic nonlinearity resulting from polarization mixing or frequency mixing in heterodyne interferometers was modeled into one expression, which included the initial polarization state of the laser source, the rotational alignment of the beam splitter along with different transmission coefficients for polarization states and the rotational misalignment of a receiving polarizer. Three compensation techniques, measuring two orthogonal output signals, Lissajous Compensation and Chu-Ray Algorithm, are described and discussed for reduction of periodic nonlinearity. These algorithms needed at least one fringe of motion or a constant velocity sweep to properly correct the motion. And five types of two spatially separated beam interferometer configurations are described and discussed for elimination of periodic nonlinearity to a picometer level. It is concluded that the main disadvantage of these configurations was their complex architecture with unbalanced long beam paths.

Periodic nonlinearity is a systematic error limiting the accuracy of displacement measurements at the nanometer level. It results from many causes such as frequency mixing, polarization mixing, polarization–frequency mixing, and ghost reflections. The purpose of this paper is to study the periodic nonlinearity resulting from ghost reflections, which has not been investigated before. A generalized scheme of interferometer, which is free of frequency and polarization mixings, is used in the study. This ensures that the residual periodic nonlinearity is from the ghost reflections only. In this paper, a general form of periodic nonlinearity and a model including two kinds of ghost reflections, one with the same frequency and the other with two frequencies, are presented. The model is verified by experimental results.

In a combined experimental and model study, we investigated effects of surface topography (relief) on the thermal L-band emission of a sandy soil. To this end, brightness temperatures of two adjacent footprint areas were measured quasi-simultaneously with an L-band radiometer at the observation angle of 55° relative to nadir for one year. One footprint featured a distinct relief in the form of erosion gullies with steep slopes, whereas the surface of the second footprint was smooth. Additionally, hydrometeorological variables, in situ soil moisture and temperature were measured, and digital terrain models of the two scenes were derived from terrestrial laser scanning. A facet model, taking into account the topography of the footprint surfaces as well as the antenna’s directivity, was developed and brightness temperatures of both footprints were simulated based on the hydrometeorological and in situ soil data. We found that brightness temperatures of the footprint with the distinct surface relief were increased at horizontal and decreased at vertical polarization with respect to those of the plane footprint. The simulations showed that this is mainly due to modifications of local (facet) observation angles and due to polarization mixing caused by the pronounced relief. Measurements furthermore revealed that brightness temperatures of both areas respond differently to changing ambient conditions indicating differences in their hydrological properties.

We propose an all-optical phase-preserving scheme for multilevel amplitude regeneration based on coherent polarization mixing for optical star-8QAM/star-16QAM signals with a power ratio of 1:5. A regeneration factor of 4.9 for star 8-QAM is achieved.

We compare theoretically two different concepts of vertical light confinement in two-dimensional (2-D) silicon photonic crystals. Light guidance obtained by variation of the refractive index in an SiO/sub 2//Si/SiO/sub 2/ sandwich structure leads to a complete bandgap for all directions and polarizations with a gap-midgap ratio of about 8.5% and a bandgap for even modes only of about 27%. The complete bandgap is 50% smaller than for 2-D photonic crystals due to the lower confinement of light in the high-index material silicon and polarization mixing. Light guidance obtained by a vertical variation of the porosity, i.e., pore radius, leads under optimum conditions to a bandgap for even modes only, with a gap-midgap ratio of about 10%. The feasibility of such a structure is shown for macroporous silicon where the pore diameter can be varied with depth. In both cases, the optimum slab thickness can be approximated by classical waveguide optics, reducing the parameter space for optimization.

LCOS projectors usually employ tilted beam-dividing coatings that see both bright and dark polarization states. The optics must then be designed to eliminate polarization mixing at these coatings, which ordinarily arises whenever the S/P planes of different rays are not aligned with one another. (This non-parallelism is a consequence of the compound incidence angles with which a beam of finite NA intercepts a tilted coating.) We show how to correct this effect using phase-controlled coatings that exploit the double-pass symmetry of the Plumbicon tri-prism geometry, reducing cross-polarized leakage to ∼1×10 −3 . In lowest order, any polarization ellipticity that the coatings may introduce in the light valve illumination will cancel in the return pass through the optics (while induced rotation will double). The condition that must be satisfied to correct rotation will only constrain the coatings in a single collective degree of freedom (for each color channel); this condition depends on both the orientations and phase shifts of the coatings. Even beyond this first order result, we show that for NA≲ 0.2 the computation involved in calculating beam contrast can be reduced to the equivalent of tracing a single ray (in double-pass), despite the fact that contrast along different rays in the beam will vary as a function of both the skew component and incidence angles of the rays at coatings. The efficiency of this algorithm makes it practical to refine the design of all coatings in the system simultaneously to take higher order terms into account.

The nonlinearity of the interferometer is an essential error in nanoscale measurements influenced by anisotropic gain and nonorthogonality of imperfect polarization components. In this paper, polarization error and the corresponding nonlinearity correction method are studied. The paper is divided into two parts, in the first part, main research focuses on the polarization mixing effect of multi-pass interferometer, besides this, polarization beam splitter and retardation plate are also analyzed, then a final synthetic evaluation is obtained through Jones matrix. In the second part, a harmonic separation method of interferometer signals is researched, the method first decomposes signals into Fourier series, then uses least square fitting to estimate coefficients of main terms of series. In the correction process, the primary phase angle is obtained through coefficients of base series and trigonometric formulas; the finer phase angle is obtained through coefficients of harmonics and Taylor expansion. Experimental results demonstrate that the nonlinearity of homodyne interferometer is significantly reduced in nanometer measurements.

We experimentally demonstrate an efficient binary phase quantizer operating at low pump powers. Phase-sensitive operation is obtained by polarization mixing the phase-locked signal/idler pair in a degenerate dual-pump vector parametric amplifier.

We present both simulation and experimental results showing that circularly polarized light maintains its degree of polarization better than linearly polarized light in scattering environments. This is specifically true in turbid environments like fog and clouds. In contrast to previous studies that propagate single wavelengths through broad particle-size distributions, this work identifies regions where circular polarization persists further than linear by systematically surveying different wavelengths through monodisperse particle diameters. For monodisperse polystyrene microspheres in water, for particle diameters of 0.99 and 1.925 microns and varying optical depths, we show that circular polarization’s ability to persist through multiple scattering events is enhanced by as much as a factor of four, when compared to that of linear polarization. These particle diameters correspond to size parameters found for infrared wavelengths and marine and continental fog particle distributions. The experimental results are compared to Monte Carlo simulations for all scattering environments investigated.

Information about the polarization of light is valuable because it contains information about the light source illuminating an object, the illumination angle, and the object material. However, polarization information strongly depends on the direction of the light source, and it is difficult to use a polarization image with various recognition algorithms outdoors because the angle of the sun varies. We propose an image enhancement method for utilizing polarization information in many such situations where the light source is not fixed. We take two approaches to overcome this problem. First, we compute an image that is the combination of a polarization image and the corresponding brightness image. Because of the angle of the light source, the polarization contains no information about some scenes. Therefore, it is difficult to use only polarization information in any scene for applications such as object detection. However, if we use a combination of a polarization image and a brightness image, the brightness image can complement the lack of scene information. The second approach is finding features that depend less on the direction of the light source. We propose a method for extracting scene features based on a calculation of the reflection model including polarization effects. A polarization camera that has micro-polarizers on each pixel of the image sensor was built and used for capturing images. We discuss examples that demonstrate the improved visibility of objects by applying our proposed method to, e.g., the visibility of lane markers on wet roads.