Change In Polarization State Research Articles

A Quantum Key as the Fiber Optic Security Sensor

The paper describes the methodology of identifying an interference in the optical ber. The paper presents the technology widely known as quantum key distribution. The quantum key distribution is based on the technique of constant comparison of quantum characteristics of the input light source and its characteristics at the end of the ber optic. Methodology of presented work includes the evaluation of the functional objectives through the constructive assumptions for the laboratory models development. This paper presents the model of a system based on the comparison of polarization states of light quanta using two asymmetric Mach Zender interferometers as transmitting and receiving systems to enable compensation of polarization state changes at the input and output of the ber optics sensing cable. Continuous monitoring of the state of the reference signal, the speci c changes of natural or ambient e ects on the ber will attempt to identify interference in the optical waveguide as a change of the polarization of the quantum states of the light. The authors indicate the possibility of using such ber optic sensor as a security sensor to protect the extensive critical infrastructure facilities. In this article the future research conception of using compressed sensing algorithms for data compression in quantum key distribution systems is presented.

Propagation-induced changes in the instantaneous polarization state, phase, and carrier–envelope phase of few-cycle pulsed beams

We describe the temporal evolution of the electric field of few-cycle optical pulses with arbitrary, time-varying polarization states by means of the instantaneous polarization ellipse and phase, whose physical meanings for few-cycle pulses are clarified. A physically meaningful definition of carrier–envelope phase (CEP) for arbitrarily polarized pulses is introduced. This description is used to study the changes in the temporal evolution of the electric field of a few-cycle pulsed beam. Propagation is found to result in significant changes in the polarization state, phase, and CEP. Approximate analytical formulas for these effects are provided.

A stable polarization switching laser from a bidirectional passively mode-locked thulium-doped fiber oscillator

We report on a novel polarization switching laser from a bidirectional passively mode-locked thulium(Tm)-doped fiber oscillator, which was characterized by the periodical change of polarization state of every pulse. The switching laser was created by combing two orthogonally stable vector solitons, which were found to be wave-breaking-free pulses in the all-anomalous-dispersion regime. The measured repetition rates of switching laser and the corresponding vector solitons were 49.596 MHz, 24.798 MHz, and 24.798MHz. By controlling wave plates, either of the polarized pulse trains can be switched on or off. To our knowledge, this is the first report of polarization switching laser with vector solitons in Tm fiber oscillators.

Simulation of the polarization effects induced by the bilayer absorber alternating phase-shift mask in conical diffraction

Hyper numerical aperture (NA) and off-axis illumination enable extension of ArF lithography for 45 nm technology node and beyond. Also, rigorous electromagnetic field modeling is taken into account for the optical and topographical properties of the mask. A rigorous three-dimensional mask model for bilayer absorber alternating phase shift mask (AltPSM) with the rigorous coupled-wave analysis (RCWA) is established. First, the harmonic waves are expanded based on the least common multiple of the periods in order to model the diffraction of multiple grating layers with different periods. Second, Lalanne’s formulation is used to improve the convergence of RCWA for multiple grating layers in conical diffraction. Third, the enhanced transmittance matrix approach is also extended to conical diffraction to avoid the numerical instability. Given the chromium oxide/chromium AltPSM, the change of polarization state as a function of mask and incident light properties is investigated. When the linewidth is below 30 nm, the mask acts as a transverse magnetic field polarizer, which is not preferred in terms of image quality, so the mask-induced polarization effects must be considered in the hyper NA lithography.

Estimation of flow rate and direction of medium with low scattering coefficient via linear polarization measurement

Many biological systems involve flow of various types of liquids. Estimation of the rate and direction of the flow is highly important for a large number of bio-medical related applications. The working point discussed in this paper involves a medium with a relatively low scattering coefficient such that the polarized light going through it is not depolarized, but rather modifies its polarization state instead. An experimental validation is presented, which shows that illuminating flow in a medium with linear polarized light, and measuring the change in polarization state at the output of the medium, can correlate highly with the direction and flow rate through that medium. By inspecting the spatial shape change in the spot of light, and the change of polarization state in relation to different integration times as well, the direction and flow rate can be fully extracted.

On experimental characterization of polarization fluctuation dynamics in random optical beams

Polarization correlation functions that characterize the rate of change of the instantaneous polarization state of an arbitrary fluctuating electromagnetic beam were recently introduced. In this work, we describe techniques that enable the measurement of these functions leading to the determination of the so-called polarization time of a random field.

Polarization dependence of Brillouin linewidth and peak frequency due to fiber inhomogeneity in single mode fiber and its impact on distributed fiber Brillouin sensing

The dependence of Brillouin linewidth and peak frequency on lightwave state of polarization (SOP) due to fiber inhomogeneity in single mode fiber (SMF) is investigated by using Brillouin optical time domain analysis (BOTDA) system. Theoretical analysis shows fiber inhomogeneity leads to fiber birefringence and sound velocity variation, both of which can cause the broadening and asymmetry of the Brillouin gain spectrum (BGS) and thus contribute to the variation of Brillouin linewidth and peak frequency with lightwave SOP. Due to fiber inhomogeneity both in lateral profile and longitudinal direction, the measured BGS is the superposition of several spectrum components with different peak frequencies within the interaction length. When pump or probe SOP changes, both the peak Brillouin gain and the overlapping area of the optical and acoustic mode profile that determine the peak efficiency of each spectrum component vary within the interaction length, which further changes the linewidth and peak frequency of the superimposed BGS. The SOP dependence of Brillouin linewidth and peak frequency was experimentally demonstrated and quantified by measuring the spectrum asymmetric factor and fitting obtained effective peak frequency respectively via BOTDA system on standard step-index SMF-28 fiber. Experimental results show that on this fiber the Brillouin spectrum asymmetric factor and effective peak frequency vary in the range of 2% and 0.06MHz respectively over distance with orthogonal probe input SOPs. Experimental results also show that in distributed fiber Brillouin sensing, polarization scrambler (PS) can be used to reduce the SOP dependence of Brillouin linewidth and peak frequency caused by fiber inhomogeneity in lateral profile, however it maintains the effects caused by fiber inhomogeneity in longitudinal direction. In the case of non-ideal polarization scrambling using practical PS, the fluctuation of effective Brillouin peak frequency caused by fiber inhomogeneity provides another limit of sensing frequency resolution of distributed fiber Brillouin sensor.

SOP and PMD monitoring on deployed aerial optical fibres

State of polarization (SOP) and polarization mode dispersion (PMD) are stochastic in nature due to changes in the properties of the optical fibres and its positions because of both intrinsic and extrinsic perturbations. With 184 PMD values obtained by use of the FTB-5700 single-ended dispersion analyzer, the predicted theoretical Gaussian fit was obtained with a mean of 0.47ps and standard deviation of 0.08ps. This small standard deviation was justification for its accuracy in measuring PMD of aerial optical fibres. A comparison is also made on the accuracy of the GINTY and the FTB-5700 on measurement of PMD of aerial optical fibres with results showing that the latter is more accurate compared to the former. The time scale over which to compensate PMD in aerial optical fibres was determined and it is slightly higher than 400s; the decorrelation time obtained for SOPs on a particular windy and hot day. This is because the changes of the PMD vector are known to be slower than SOP changes.

A Transflective Nano-Wire Grid Polarizer Based Fiber-Optic Sensor

A transflective nano-wire grid polarizer is fabricated on a single mode fiber tip by focused ion beam machining. In contrast to conventional absorptive in-line polarizers, the wire grids reflect TE-mode, while transmitting TM-mode light so that no light power is discarded. A reflection contrast of 13.7 dB and a transmission contrast of 4.9 dB are achieved in the 1,550 nm telecom band using a 200-nm wire grid fiber polarizer. With the help of an optic circulator, the polarization states of both the transmissive and reflective lights in the fiber may be monitored simultaneously. A kind of robust fiber optic sensor is thus proposed that could withstand light power variations. To verify the idea, a fiber pressure sensor with the sensitivity of 0.24 rad/N is demonstrated. The corresponding stress-optic coefficient of the fiber is measured. In addition to pressure sensing, this technology could be applied in detecting any polarization state change induced by magnetic fields, electric currents and so on.

Study on Polarization Distribution of Polarization Encoder in Oblique Incidence

Principle of polarization encoding based on electro-optic effect of LiNbO3 crystal is analyzed, effact on polarization encoding optical field effects is studied when the incident light direction changes. Theoretical calculations show that, with the angle between the direction of incident laser beam and the main axis of LiNbO3 crystal increases, rotation angle of polarization ellipse remain unchanged,but ellipticity changes at any other position except in the center of the encoder,at the top and bottom of encoder,polarization state changes from circularly polarized to elliptically polarized,at other position of the encoder, polarization states are still elliptically polarized light but with different ellipticity angle. Experiment measurement results are in accord with ones of the calculation.

Polarization and pressure effects in caesium 6S–8S two-photon spectroscopy

This work analyses the effects of polarization and pressure in caesium 6S–8S two-photon spectroscopy. The linewidth was broadened and the frequency was shifted by a change of polarization states. The frequency shift and the linewidth broadening of the caesium 6S–8S two-photon transition were measured as a function of laser power using one single-frequency Ti:sapphire ring cavity laser, two caesium cells and two quarter-wave plates to ensure polarization states of light, and we showed that the linewidth cannot be evaluated just by fitting data to a Lorentzian shape. As determined by fitting the data to a Voigt profile, the natural linewidth is independent of the polarization states of the pump beams, the laser power and the pressure. Caesium 6S–8S two-photon transitions pumped by a circularly polarized beam have narrower linewidths and smaller shifts than those pumped by a linearly polarized beam. The light shift obtained by pumping with the circularly polarized beam is −6.75(57) Hz (mW mm−2)−1, and that obtained by pumping with a linearly polarized beam is −7.25(45) Hz (mW mm−2)−1. These results agree closely with theoretical calculations. The pressure shift is −588(387) Hz mPa−1. This work shows how to evaluate two-photon transitions with a Voigt profile, and then helps us to understand two-photon transitions with different polarization states, and improve the signal quality obtained when they are used as frequency markers.

Polarization change induced by a galvanometric optical scanner

We study the optical properties of a two-axis galvanometric optical scanner constituted by a pair of rotating planar mirrors, focusing our attention on the transformation induced on the polarization state of the input beam. We obtain the matrix that defines the transformation of the propagation direction of the beam and the Jones matrix that defines the transformation of the polarization state. Both matrices are expressed in terms of the rotation angles of two mirrors. Finally, we calculate the parameters of the general rotation in the Poincaré sphere that describes the change in the polarization state for each mutual orientation of the mirrors.

Statistical characterization of the output state of polarization in deployed single-mode fibers

Changes in the optical fiber properties due to both intrinsic and extrinsic variations result in polarization mode dispersion and state of polarization (SOP) becoming stochastic in nature. The statistics for first-order PMD and the second-order PMD approach the Maxwellian and Foschini et al. [IEEE Photonics Technol. Lett.12, 293 (2000)] distributions, respectively. In this Letter, we investigate a theoretical statistical distribution that corresponds to output SOP variations. The SOP variations can either be with wavelength (for buried fiber) or with time (for aerial fiber). Our results show that the statistics of the relative SOP changes approach the distribution proposed by Foschini et al..

Fiber-Optic Pressure Sensor Based on Tunable Liquid Crystal Technology

We propose a photoelastic pressure sensor using a liquid crystal (LC) tunable fiber polarizer. The polarizer contains an etched single-mode fiber sealed in an LC cell. A low-index LC is used as a tunable birefringent cladding of the fiber. Any electric-field-induced LC director reorientation may thus affect the fiber's light-guiding characteristics. Different polarization states experience different LC cladding indices. The transmittance contrast between the x- and y-polarized modes reaches 31.9 dB, which is sensitive enough to monitor any polarization state change in the fiber. A pressure sensor with the sensitivity of 0.25 rad/N is thus demonstrated. The corresponding stress-optic coefficient of the fiber is measured at 1550-nm telecomm band. The advantages and applications of the LC tuning and sensing technologies are discussed.

Polarization-Multiplexed Optical Wireless Transmission With Coherent Detection

In this paper, we propose, analyze, and experimentally verify polarization-multiplexed optical wireless transmission with coherent detection as a means of improving performance and increasing the per-wavelength data rate in the presence of atmospheric turbulence. It is first shown that in terrestrial last-mile applications, the polarization state changes of the optical wireless signal are governed by optical source properties both in the absence and in the presence of mild to moderately severe turbulence. Under these conditions, it is shown that the polarization evolution is reduced to a polarization state rotation wherein the cross-polarization interference can be equalized with well-known blind algorithms. We also experimentally verify the analytical findings, demonstrating 112 Gb/s POLMUX-QPSK transmission with coherent detection in a non-fading free-space channel, and numerically extend the experimental results to turbulent settings by invoking the lognormal model for turbulence-induced fading. Performance gains of 5-14 dB along with a doubling of the data rate through polarization multiplexing are exhibited compared to legacy single-polarization intensity modulation/direct detection (IM/DD) optical wireless systems. The presented analytical and experimental results indicate that the proposed approach is promising for notably increasing the data rate in last-mile free-space optical transmission.

INFLUENCE OF POLARIZATION MODE DISPERSION ON THE EFFECT OF CROSS-PHASE MODULATION IN INTENSITY MODULATION-DIRECT DETECTION WDM TRANSMISSION SYSTEM

Cross-phase modulation (XPM) changes the state-of-polarization (SOP) of the channels through nonlinear polarization rotation and induces nonlinear time dependent phase shift for polarization components that leads to amplitude modulation of the propagating waves in a wavelength division multiplexing (WDM) system. Due to the presence of birefringence, the angle between the SOP changes randomly and as a result polarization mode dispersion (PMD) causes XPM modulation amplitude fluctuation random in the perturbed channel. In this paper we analytically determine the probability density function of the random angle between the SOP of pump and probe, and evaluate the impact of polarization mode dispersion on XPM in terms of bit error rate, channel spacing etc for a two channel intensity modulation-direct detection WDM system at 10 Gb/s. It is found that the XPM induced crosstalk is polarization independent for channel spacing greater than 3 nm or PMD coefficient larger than 2 ps/√km. We also investigate the dependence of SOP variance on PMD coefficient and channel spacing.

Intensity-dependent change in polarization state of light in normal incidence on an isotropic nonlinear Kerr medium

It is shown that all optical polarization states of light except plane and circular polarization states undergo an intensity-dependent change in normal incidence of light in an isotropic nonlinear Kerr medium. This effect should be detectable and we propose an experiment for detecting nonlinear susceptibility involved in that part of nonlinear polarization, which depends on the polarization state of light also.

Condition for Gaussian Schell-model beam to maintain the state of polarization on the propagation in free space

In the free space optical communication system with circle polarization shift keying (CPolSK) modulation, the changes of polarization state of light beam have significant influence on the system performance. Keeping the state of polarization (SOP) unchanged on propagation can reduce the bit error rate. Based on the unified theory of coherence and polarization, we derive the sufficient condition for Gaussian Schell-model (GSM) beam to keep the SOP unchanged. We found that when the three spectral correlation widths (delta(xx), delta(yy) and delta(xy)) equal to each other and sigma(x) = sigma(y), the GSM beam maintains the SOP on propagation. This conclusion can be helpful for the design of the transmitter in the CPolSK system.

Focusing of vectorial fields by a slab of indefinite media

We present a fully vectorial description of the focusing of electromagnetic beams by a slab of indefinite media, in which waves can exhibit dispersion surfaces in the form of an ellipsoid, a double-sheeted hyperboloid, or a single-sheeted hyperboloid. Firstly, by means of the angular spectrum representation for electromagnetic beams, we analyze the transmission properties of a paraxial beam through a slab of indefinite media with the different dispersion relations, respectively. We found that a paraxial beam undergoes focusing in both x and y axes for the slab with an ellipsoid dispersion relation, and undergoes focusing or dispersion in different axes for the slab exhibiting a dispersion surface of a single-sheeted hyperboloid; while the beam cannot transmit in the slab with a double-sheeted hyperboloid dispersion relation. The inherent physics underlying these phenomena is that the effective refractive indices of indefinite media with different kinds of dispersion relations have different sign combinations. Secondly, the paraxial scheme is generalized to describe the propagation of slightly nonparaxial beams in the indefinite media slabs and the nonparaxial correction to the paraxial solution is derived. It is demonstrated that the contribution of nonparaxial correction to the longitudinal component is greater than the corresponding contribution to the transverse component. Importantly, we find that, due to the coupling effect of vectorial field components in indefinite media, the beam suffers a slight change of polarization state.

Towards skin polarization characterization using polarimetric technique

Measurement of optical properties of skin is an expanding and growing field of research. Recent studies have shown that the biological tissue, especially skin, changes the polarization state of the incident light. Using this property will enable the study of abnormalities and diseases that alter not only the light intensity but also its polarization state. In this paper we report an experimental study for measuring changes of polarization state of the light scattered from a phantom similar to a sample model of scattering skin. Using the notation of Stokes vector for the polarized light and Mueller matrix for the sample with its polarization properties, we have shown that some elements of the matrix were particularly sensitive to the changes of the polarization-altering physical properties of the scatterers within the phantom.