A novel modulation format of 50% RZ polarization interleaved multiplexing DQPSK with high CD and polarization crosstalk tolerance in high-speed self-homodyne receiving system

We propose a novel modulation format of 50% RZ polarization interleaved multiplexing DQPSK for high-speed self-homodyne receiving systems. The proposed modulation format enables excellent tolerance of chromatic dispersion and polarization crosstalk. In self-homodyne receiving systems, the proposed format can reduce the cost of adaptive equalization module. It has potential applications in long-reach self-homodyne optical communication systems.     

Optical waveguides structured with photoaligning polymers

We present a novel fast fabrication method for making optical waveguides by using a photoaligning and a liquid-crystal polymer. A difference in the refractive indices in the core and cladding regions is achieved by use of polarized UV light to align the polymer chains differently. A refractive-index difference of Deltan = 0.07 was achieved. The propagation loss in a channel waveguide is 4 dB/cm at a wavelength of 1.55 microm. The fabrication method has only a few processing steps and yields short fabrication times. The waveguide structures offer possibilities for generating devices with new applications related to the polarization of the guided light.     

Polarization controllable spatial filter based on azo-dye-doped liquid-crystal film

This investigation establishes the feasibility of exploiting the surface-assisted photoalignment effect in dye-doped liquid-crystal (DDLC) films as spatial filters with controllable polarization in optical signal processing. The fabrication relies on the fact that the various intensities of the diffracted orders are responsible for various changes of the polarization state induced by the photoaligned DDLC film. Specific spatial orders in Fourier optical signal processing can be filtered by use of an analyzer placed behind the sample to control the polarization state of the diffracted orders. A simulation was performed, and the results agree closely with the experimental data.     

Polarization interferometric stellar coronagraph

A nulling stellar coronagraph based on polarization interferometry is proposed that has achromatic characteristics. The principle and the optical setup of the nulling stellar coronagraph are described. Experimental results with monochromatic and white light show the usefulness of the proposed method. A liquid-crystal device is used as a four-quadrant polarization mask, which is the key element of a polarization interferometric stellar coronagraph.     

Polarization of light by a polymer film containing elongated drops of liquid crystal with inhomogeneous interfacial anchoring

An optico-mechanical model describing the coherent (directed) transmittance and the degree of polarization of forward-transmitted light by a polymer film with elongated liquid-crystal (LC) drops has been developed. This model, based on the Foldy–Twersky and anomalous-diffraction approximations, makes it possible to analyze the optical response of a film under extension as a function of the film thickness, refractive index of the polymer, the sizes and anisometry parameters of liquid-crystal drops, their concentration, internal structure, polydispersity, and orientation of optical axes. The model is verified based on the comparison of numerical and experimental data for the inverse modification of interfacial anchoring by an ion-forming surfactant. The internal drop structure is determined by solving the problem of minimizing the volume free energy density. A comparative analysis of the calculated transmittance and degree of polarization for films with uniform homeotropic and modified inhomogeneous interfacial anchoring is performed. The spectral polarization characteristics of a film with elongated LC drops and single-domain internal structure, formed under mechanical extension with the aid of surfactants, are investigated.     

光纤偏振模色散对信号偏振度的影响

采用一种简化的线路传输模型，详细讨论了在高速光纤通信系统中，线路偏振模色散（PMD ），尤其是二阶PMD、输入信号偏振态等对信号偏振度（DOP）的影响，并指出以信号DOP做 反馈控制信号适合于一阶PMD优化补偿系统，但在较大二阶PMD的影响下，将增加控制算法的 复杂性，使系统可能陷入局部最优解. 关键词： 信号偏振度 偏振模色散 信号偏振态     

全光纤量子通信系统中的高速偏振控制方案

偏振控制在光通信中是至关重要的技术, 关系着通信系统的稳定性和误码率. 本文提出一种基于双向Sagnac环工作方式的全光纤高速偏振控制方案, 通过调节环中一个光纤电光相位调制器的相位差而精确控制光场偏振方向, 并且实现了单个端口输出各种偏振态, 无需后续耦合操作. 相位控制精度为10^-3 rad, 最大消光比可达30 dB, 工作速率可达2 GHz. 由于本方案的精度、调制速度和稳定性都很高, 并采用了器件简单、成本低廉的全光纤光路, 易于集成, 在量子保密通信等光通信领域中有很好的应用前景. 关键词： 光纤偏振控制器 Sagnac环 量子通信     

Measurement of polarization dependence of nonlinear susceptibility responsible for Rayleigh-wing and Brillouin scattering

By using a simple optical geometry based on backward light scattering and employing a Sandercock-type tandem Fabry-Perot interferometer, we measure both the linear and the circular polarization dependences of Rayleigh-wing and Brillouin scattering in a sample of liquid-crystal 4-n-pentyl-4'-cyanobiphenyl. Observed polarization dependences are consistent with the third-order nonlinear susceptibilities, taking into account the traceless symmetric scattering tensor for Rayleigh-wing scattering and the isotropic scattering tensor for Brillouin scattering.     

Interference patterns of scattering light induced by orientational fluctuations in an electric-field-biased nematic liquid-crystal film

A new light-scattering phenomenon from a planar aligned nematic liquid-crystal film is observed and studied. This new phenomenon exhibits ring patterns in the orthogonal polarization. A simple model based on optical interference has been developed, and its predictions agree well with experimental observation.     

Simulation of 2-D lateral light propagation in nematic-liquid-crystal cells with tilted molecules and nonlinear reorientational effect

In the general case the optical tensor of a nematic liquid crystal consists of nine nonzero elements, which makes it difficult to calculate light propagation in a liquid-crystal cell. For a two-dimensional (2-D) problem with TM polarization and a parallel liquid-crystal orientation where the molecules are only tilted and not twisted, the full problem can be calculated by using one magnetic field component, thus reducing the problem to a scalar one. This geometry is used to simulate the self-focusing effect which can lead to the generation of spatial optical solitary waves. This self-focusing occurs due to the optical nonlinear effect of field-induced director reorientation. Due to nondiagonal elements of the optical tensor, however, it is expected that the Poynting vector will deviate from the original propagation direction. Our simulations reveal that, in this case, the deviation will not cause the loss of the soliton-like beam propagation regime, but will rather give rise to a transverse undulating behaviour.     

Tailoring the depth of focus for optical imaging systems using a Fourier transform approach

We show how to tailor the depth of focus for an optical system using pupil functions obtained from a Fourier transform approach. These complex amplitude and phase pupil functions are encoded onto a single liquid-crystal spatial light modulator. Experimental results show excellent agreement with theory and indicate the power of this approach.     

Observation of self-diffraction by gratings in nematic liquid crystals doped with carbon nanotubes

Diffraction gratings were studied in cells of the homogeneously aligned liquid-crystal E7 doped with multiwall carbon nanotubes. These phase gratings were induced by interference modulation of two coherent optical beams, in conjunction with an applied dc field that was perpendicular to the unperturbed director axis. Self-diffraction was observed at all angles of incidence of the writing beams, including normal incidence. A superior nonlinear-index coefficient of 5x10(-2)cm(2)/W was obtained after passage of a 44-mW/cm(2) beam through a film with a grating constant of 18 mum under an external voltage of 15 V. The observed phenomenon depends strongly on the applied dc field, and the memory effect in a nematic film depends strongly on the grating constant.     

Polarization shift keying (POLSK) phase diversity homodyne system with multiplying detection in coherent optical communications

We propose a Polarization Shift Keying (POLSK) phase diversity homo-dyne system with multiplying detection in coherent optical communications. The proposed system uses only one of three Stokes parameters, which is the phase difference between two orthogonal polarizations. Although such systems using only one Stokes parameter are very sensitive to polarization fluctuation, the proposed system becomes insensitive to the linear polarization fluctuation by using the multiplying detection with a combination of in-phase and quadrature optical signal components on two orthogonal polarizations. Furthermore, insensitivity to phase noise and offset frequency and attractiveness due to homodyne type in high-speed transmissions are maintained. The most attractive feature of the proposed system is that neither a polarization controller nor electronic feedback loop for matching the state of polarization (SOP) between transmitted and local oscillator (LO) optical signals is needed. We theoretically analyze the receiver sensitivity and the power penalties of the proposed system for the cases of the elliptical SOP of received optical signals and the imbalance optical 90° hybrid.     

Polarization of light transmitted through a polymer film with nanosized droplets of liquid crystal

A method is developed for analyzing the state of polarization of a plane wave transmitted through a polymer-dispersed liquid-crystal (PDLC) film with nanosized liquid-crystal (LC) droplets. This method is based on the anisotropic-dipole approximation for describing scattering by a separate droplet and on the Foldy-Twersky approximation for describing propagation of light in a film. Equations are obtained that relate the ellipsometric parameters of coherent (direct) light transmitted through a PDLC film to the order parameters that characterize the morphological and structural properties of the film. Elliptic and circular polarizations and the rotation of the plane of polarization of a wave transmitted through a film are investigated under the normal illumination of the PDLC film by a linearly polarized plane wave. The order parameters of the PDLC film are determined as a function of a control field under the transition from a partially ordered structure of optical axes of LC droplets to a homeotropic structure.     

Droplet deformation and alignment for high-efficiency polarization-dependent holographic polymer-dispersed liquid-crystal reflection gratings

Droplet deformation and alignment are achieved in holographic polymer-dispersed liquid-crystal reflection gratings by applying an in situ shear during recording. High diffraction efficiency (99%) is obtained for light polarized parallel to the shear, with nearly zero efficiency for perpendicular polarization, and no increase of incoherent scattering. Permanent polarization dependence is related to stress-induced morphology changes of liquid-crystal droplets that are frozen by polymerization. The system is studied by electron microscopy and modeled by anisotropic coupled-wave and scattering theory. The morphology is consistent with the theory of small deformations of liquid droplets in fluid flow. Diffraction efficiency measurements are in agreement with theory incorporating this morphology as well as concomitant orientation and alignment of liquid-crystal molecules.     

Polarization-selective switching in holographically formed polymer dispersed liquid crystals

We have developed an optical stack of holographically formed polymer dispersed liquid-crystal (H-PDLC) devices that is fully operational with nonpolarized light sources. The device consists of two H-PDLC transmission gratings separated by a passive polarization rotator that can output a diffracted s-polarized, p-polarized, or s- and p-polarized beam simultaneously.     

Depolarization of light in microstructured fibers filled with liquid crystals

In the paper we analyze microstructured optical fibers filled with typical nematic liquid crystals, i.e., 5CB and 6CHBT under influence of external electric field or temperature. We use the modified Mueller matrix method with an additional depolarization matrix to calculate degree of polarization changes of the light propagating in the liquid-crystal infiltrated microstructured optical fibers. Preliminary experimental results of light depolarization measurements during propagation in these microstructured fibers are also presented.     

Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

We have observed extraordinarily large optical nonlinearity in Methyl Red-doped nematic liquid-crystal film. Grating diffraction can be generated with an optical intensity as low as 40 microW/cm(2) , and a refractive-index change coefficient of more than 6 cm(2)/ W is obtained. The effect is attributed to formation of an optically induced dc space-charge field and to the resulting reorientation of the highly birefringent nematic director axis.     

Simulation research of medium-short distance free-space optical communication with optical amplification based on polarization shift keying modulation

Circular polarization shift keying (CPolSK) modulation technique has many advantages such as excellent BER performance and freedom from the alignment of polarization coordinates of the transmitter and the receiver, etc., and it turns out to be a good choice to FSO system. In this paper, a FSO system using CPolSK modulation is studied by simulation; it is found that the communication performance of the system is excellent in most weather condition. Additionally, three ways of optical signal amplification are proposed, and contrastive analysis on performance of corresponding optical amplification systems is carried out by examining SNR、BER and transmission distance with different specific attenuation. The results show that the system with optical amplifier at the transmitter have the optimum performance, and then the system with optical amplifier at the both ends with the same total gain, it is worst for the system with optical amplifier at the receiver. In addition, the safety factor for high emission power induced by optical amplification is also considered in this paper for practical application. The study above may be utilized in the system design for enhancing performance.     

Switchable broadband achromatic half-wave plate with nematic liquid crystals

We describe a broadband achromatic half-wave retarder for normally incident light of arbitrary polarization. The device consists of a stack of one nematic liquid-crystal cell with uniform alignment sandwiched between two twisted nematic layers that have identical twist angles (135 degrees) but different orientations of their surface alignment. As a specific application, for light with linear polarization at 45 degrees to the optic axis of the homogeneous cell the stack functions efficiently as an optical switch. The switch rotates the incident linear polarization by 90 degrees in the absence of an electric field. When sufficiently high voltage is applied to all three layers, the device produces a near-zero effect on the incident polarization. An achromatic response in the spectral range 400-700 nm is achieved for both activated and quiescent states.