Polarization Scrambling Research Articles

Polarization scrambling in Brillouin optical correlation-domain reflectometry using polymer fibers

We evaluate whether the measurement stability of Brillouin optical correlation-domain reflectometry (BOCDR) using polymer optical fibers (POFs) can be enhanced by polarization scrambling. In this study, two major factors that affect the signal-to-noise ratio in BOCDR, specifically, the spatial resolution and incident power, are varied, and their influences on distributed measurements with polarization scrambling are experimentally investigated. We thus confirm that in POF-based BOCDR, unlike in BOCDR using standard silica glass fibers, polarization scrambling is an effective means of enhancing the measurement stability only when the spatial resolution is sufficiently low or when the incident power is sufficiently high.

Remote ambient methane monitoring using fiber-optically coupled optical sensors

A tunable diode laser absorption spectroscopy system, employing a 2f wavelength modulation spectroscopy measurement scheme, was developed for remote monitoring of ambient methane fluctuations by way of fiber-optically connected all-optical sensors. Detection of fugitive methane emissions demands a measurement precision less than 2.0 ppmv and a lower detection limit less than ~1.7 ppmv methane in air. To determine the optimum base system configuration, the influence of the laser driving signal frequency and amplitude was characterized to strike a balance between measurement precision and system sensitivity. In addition, relative to the basic system configuration, a 50 and 96 % reduction in measurement deviation was achieved by way of polarization scrambling and thermal stabilization of critical optical components, respectively. For methane concentrations between 2.0 and 50.0 ppmv in air, the laboratory-based system achieved a measured precision of 1.36 ppmv and a lower detection limit of 1.56 ppmv using a 6.0-m single-mode optical fiber and an averaging time of 1 s. The long-term system stability and system performance were analyzed using datasets acquired 4 and 12 months after the initial system calibration, yielding a difference in measured precision within the uncertainties of the calibration gas mixture. Finally, it was determined that fiber length between individual remote optical sensors can lead to a varying measurement bias, which implies that length-specific calibrations for each remote optical sensor may be required for a field implementation.

Simple Method for the Elimination of Polarization Noise in BOTDA Using Balanced Detection and Orthogonal Probe Sidebands

Polarization noise arises in Brillouin optical time-domain analysis due to the strong polarization sensitivity of stimulated Brillouin scattering. To avoid this noise, it turns out to be indispensable to perform some kind of polarization scrambling, either in the pump pulse, the probe signal or both. This is usually achieved using polarization scrambling/switching systems, which, being mechanical, tend to be not as robust as it would be desirable. In this paper, we propose a completely passive system, with no moving parts, to eliminate polarization noise in a BOTDA. It is based on the use of passive depolarization of the pump pulse together with balanced detection among orthogonally polarized Stokes and anti-Stokes bands of the probe signal. The setup requires no alignment readjustment over time and provides a performance similar to a conventional BOTDA using scrambler.

Intensity noise-driven nonlinear fiber polarization scrambler.

We propose and analyze a novel all-optical fiber polarization scrambler based on the transfer (via the Kerr effect) of the intensity fluctuations of an incoherent pump beam into polarization fluctuations of a frequency-shifted signal beam, copropagating in a randomly birefringent telecom fiber. Optimal signal polarization scrambling results whenever the input signal and pump beams have nearly orthogonal states of polarization. The nonlinear polarization scrambler may operate on either cw or high-bit-rate pulsed signals.

PMD mitigation through interleaving LDPC codes with polarization scramblers

The combination of forward error correction (FEC) and distributed fast polarization scramblers (D-FPSs) is approved an effective method to mitigate polarization mode dispersion (PMD) in high-speed optical fiber communication system. The low-density parity-check (LDPC) codes are newly introduced into the PMD mitigation scheme with D-FPSs in this article as one of the promising FEC codes to achieve better performance. The scrambling speed of FPS for LDPC (2040, 1903) codes system is discussed, and the reasonable speed 10MHz is obtained from the simulation results. For easy application in practical large scale integrated (LSI) circuit, the number of iterations in decoding LDPC codes is also investigated. The PMD tolerance and cut-off optical signal-to-noise ratio (OSNR) of LDPC codes are compared with Reed–Solomon (RS) codes in different conditions. In the simulation, the interleaving LDPC codes bring incremental performance of error correction, and the PMD tolerance is 10ps at OSNR=11.4dB. The results show the meaning of the work is that LDPC codes are a substitute for traditional RS codes with D-FPSs and all of the executable code files are open for researchers who have practical LSI platform for PMD mitigation.

Visibility properties of the interferometric optical fiber sensors using polarization scrambling

An interferometric optical fiber sensor relies on the coherent mixing of the optical signals, which is strongly polarization dependent. Random fluctuations in the input state of polarization and the polarization properties of the optical fiber sensor can result in the variation of the visibility and signal fading. Polarization scrambling is an important method to eliminate the input-polarization-induced fading. In this paper, the principles of the polarization scrambling are introduced. The influences of the perturbation to the input fiber and interferometer on the visibility are analyzed in two cases and the visibility properties in an interferometric optical fiber sensor are theoretically and experimentally demonstrated.

Sweep-free brillouin optical time domain analysis using two individual laser sources

We propose a sweep-free measurement technique for distributed Brillouin frequency shift in optical fibers applicable to distributed strain/temperature monitoring systems, using two sets of pump-probe lights generated by two individual laser sources. This technique utilizes the Brillouin gain ratio measured at both the upper and lower side slopes of two different Brillouin spectra induced by two pump lights with different wavelengths. The proposed method offers high-speed measurement due to the combination of the frequency-sweep-free operation and the polarization-insensitive measurement scheme without polarization scramblers. A measurement accuracy of 17.3 μɛ with a spatial resolution of 5m over 2.6 km sensing length and a capability of kHz sampling rate are experimentally demonstrated.

Polarization scramblers with plasmonic meander-type metamaterials

Due to plasmonic excitations, metallic meander structures exhibit an extraordinarily high transmission within a well-defined pass band. Within this frequency range, they behave like almost ideal linear polarizers, can induce large phase retardation between s- and p-polarized light and show a high polarization conversion efficiency. Due to these properties, meander structures can interact very effectively with polarized light. In this report, we suggest a novel polarization scrambler design using spatially distributed metallic meander structures with random angular orientations. The whole device has an optical response averaged over all pixel orientations within the incident beam diameter. We characterize the depolarizing properties of the suggested polarization scrambler with the Mueller matrix and investigate both single layer and stacked meander structures at different frequencies. The presented polarization scrambler can be flexibly designed to work at any wavelength in the visible range with a bandwidth of up to 100 THz. With our preliminary design, we achieve depolarization rates larger than 50% for arbitrarily polarized monochromatic and narrow-band light. Circularly polarized light could be depolarized by up to 95% at 600 THz.

Polarization sensitive optical coherence tomography of melanin provides intrinsic contrast based on depolarization

Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT. In addition to imaging based on tissue reflectivity, PS-OCT also enables depth-resolved mapping of sample polarization properties such as phase-retardation, birefringent axis orientation, Stokes vectors, and degree of polarization uniformity (DOPU). In this study, PS-OCT was used to investigate the polarization properties of melanin. In-vitro measurements in samples with varying melanin concentrations revealed polarization scrambling, i.e. depolarization of backscattered light. Polarization scrambling in the PS-OCT images was more pronounced for higher melanin concentrations and correlated with the concentration of the melanin granules in the phantoms. Moreover, in-vivo PS-OCT was performed in the retinas of normal subjects and individuals with albinism. Unlike in the normal eye, polarization scrambling in the retinal pigment epithelium (RPE) was less pronounced or even not observable in PS-OCT images of albinos. These results indicate that the depolarizing appearance of pigmented structures like, for instance, the RPE is likely to be caused by the melanin granules contained in these cells.

PMD mitigation through interleaving LDPC codes with polarization scramblers

The combination of forward error correction (FEC) and distributed fast polarization scramblers (D-FPSs) is approved as an effective method to mitigate polarization mode dispersion (PMD) in high-speed optical fiber communication system. The low-density parity-check (LDPC) codes are newly introduced into the PMD mitigation scheme with D-FPSs in this paper as one of the promising FEC codes to achieve better performance. The scrambling speed of FPS for LDPC (2040, 1903) codes system is discussed, and the reasonable speed 10MHz is obtained from the simulation results. For easy application in practical large scale integrated (LSI) circuit, the number of iterations in decoding LDPC codes is also investigated. The PMD tolerance and cut-off optical signal-to-noise ratio (OSNR) of LDPC codes are compared with Reed–Solomon (RS) codes in different conditions. In the simulation, the interleaving LDPC codes brings incremental performance of error correction, and the PMD tolerance is 10ps at OSNR=11.4dB. The results show that the meaning of the work is that LDPC codes are a substitute for traditional RS codes with D-FPSs and all of the executable code files are open for researchers who have practical LSI platform for PMD mitigation.

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.

Haze and Polarization Scrambling of Nonlinearly Scattered Light From Antiglare Si Nanorod Surface

The scattering angle, the Haze ratio, and the field polarization factor of the frequency-doubled Nd:YAG laser nonlinearly scattered from the semiconductor nanorod surface are investigated. Both the scattering angle and the reflected Haze ratio of the laser beam reflected from Si nanorod surface present a nonlinearly increasing trend with nanorod length. As the nanorod lengthens from 190 to 2760 nm, the scattering angle broadens from 2.5° to 25° and from 2.5° to 24°. Concurrently, the reflected Haze ratio increases from 5% to 22% and 3% to 21% under TE and TM-mode incidences, respectively. A significant polarization scrambling transfers the linearly polarization into an elliptical polarization, and the field polarization factor increases from 0.54 to 0.86 and from 0.42 to 0.84 under TE- and TM-mode incidences with an enlarging nanorod length from 190 to 2760 nm. The reflected Haze ratio is linearly proportional with the broadened scattering angle. However, the field polarization factor shows a gradually saturating effect at larger scattering angle. The theoretical calculation is used to quantitatively analyze the polarization scrambling effect, which demonstrates that the scattered wave nonlinearly depends on the surface corrugation and can be expressed by the second-order function of the surface corrugation strength. The strong correlations of the TE-mode scattering with the nanorod length and the TM-mode scattering with the laser incident angle are observed. The polarization of the reflected laser beam eventually transfers from a linear to an elliptical one with a nanorod length exceeding over 1 μm.

PMD Mitigation by Various Distribution Patterns of Fast Polarization Scramblers with Forward Error Correction

The joint distributed fast polarization scramblers (D-FPSs) and forward error correction (FEC) is one of methods used to mitigate polarization mode dispersion (PMD). The distribution pattern of FPSs, which indicates how the FPSs are deployed along the fiber link, is a significant factor for this PMD mitigation scheme. The performance of the bit error rate (BER) for the case with or without FPSs is investigated by simulations. Five different distribution patterns of FPSs along the fiber link are designed in this paper, and the performance for each pattern is depicted by a novel representation that is known as the “Ring Chart.” The results show that the property of PMD mitigation varies with different distribution patterns of FPSs.

A novel scheme for achieving quasi-uniform rate polarization scrambling at 752 krad/s

We propose and demonstrate a novel scheme for obtaining quasi-uniform rate polarization scrambling at up to 752 krad/s in fiber optic systems by using cascaded multiple fiber squeezers with each one placed in a certain orientation. Additionally, this polarization scrambler is compatible with both single-polarization and polarization-multiplexing systems. We also show that scrambled SOP with this scheme uniformly covers the whole Poincare Sphere and that the scrambling rates are mostly concentrated towards the high end of the rate distribution histogram. Such a scrambling scheme is advantageous for the deterministic characterization of performances for modern fiber optic transceivers, especially those deploying coherent detection techniques, against rapid polarization variations.

PMD mitigation by LDPC codes with polarization scramblers

Low-density parity-check (LDPC) codes combined with polarization scramblers are used to mitigate polarization mode dispersion (PMD) in 40-Gb/s optical fiber system. The simulations are performed to compare the correction performance of LDPC codes and Reed–Solomon (RS) code used in this PMD mitigation scheme. Results show that LDPC codes can achieve a better performance than the RS code with the same redundancy. The scrambling speed of polarization scramblers for LDPC codes system is also discussed, and the optimal speed of 10 MHz is obtained.

A technique for tunable microwave/millimeter-wave generation based on a polarization-stable dual-wavelength polarization maintaining fiber Bragg grating laser

A technique for tunable microwave/millimeter-wave generation based on a polarization-stable dual-wavelength polarization maintaining fiber Bragg grating (PMFBG) laser is proposed in which the frequency selecting of PMFBG is used to produce two polarization-stable lasing signals, the polarization scrambler is adopted to ensure the consistency of the orthogonal polarization's power, and then the beating frequency in high-speed photodetector is used to generate the microwave/millimeter-wave. Lateral strain loading on the PMFBG allows the frequency of the microwave/millimeter to be controlled. In the experiment, a scheme for tunable microwave/millimeter-wave generation based on a polarization-stable dual-wavelength PMFBG laser is produced, and the microwave signals of 20.407 GHz and 22.050 GHz are generated by different axial pulls loading on PMFBG. In the simulation, the millimeter-wave of 60 GHz is generated and transmission performance of the millimeter-wave in radio-over-fiber downlink is analyzed. The results show that the eye diagrams demodulated in the mobile station are excellent when the optical carrier which is modulated as the millimeter-wave sub-carrier transmits over 80 kilometers from center station to base station. The excellent performance of the system is verified.

PMD mitigation by LDPC codes with polarization scramblers

PMD mitigation by LDPC codes with polarization scramblers

Effects of Harmonic Overlap and Polarization Scrambling on Electron Cyclotron Emission from ITER Plasma

The electron cyclotron emission measurable from the outboard side of ITER plasmas is estimated. The effects of harmonic overlap and polarization scrambling are reviewed with the aim of assessing the impact of any polarization change that might occur in the collected radiation before the O and X polarizations are separated. It is confirmed that any polarization scrambling occurring during the reflection at the wall would not alter the measured intensities of lower harmonics of either the O or X mode but would affect only the higher harmonics, which are optically thin. For the second-harmonic X mode, the observed intensity in the 300- to 400-GHz range is considerably lower than that of the O polarization. Hence, this frequency range may be particularly vulnerable to any O-to-X polarization change occurring prior to their separation into different transmission channels. It is shown that if the electron temperature Te near the core is to be measured to within 10% accuracy, the above polarization fidelity should also be preserved to within 10% or better. It is suggested that this requirement may have impact on the location of the polarization splitter unit. Further analysis is required to evaluate the error in the calculation of Te profiles from the measured Trad values arising due to uncertainties introduced by any polarization conversion.

Simultaneous Polarization-Insensitive Wavelength Conversion of 80-Gb/s RZ-DQPSK Signal and 40-Gb/s RZ-OOK Signal in a Ti:PPLN Waveguide

Simultaneous all-optical wavelength conversion of an 80-Gb/s return-to-zero differential quadrature phase shift keying (RZ-DQPSK) data signal and a 40-Gb/s return-to-zero ON--OFF keying (RZ-OOK) data signal using a Ti:PPLN waveguide in a polarization-diversity loop configuration are demonstrated. The wavelength conversion is fully transparent to polarization, data rate, modulation format, and modulation level. The conversion efficiency for the signals was -21 dB, which includes 9.2-dB passive losses in the whole Ti:PPLN subsystem. Error-free performance for both wavelength-converted signals was achieved for polarization-scrambled input data signals without additional penalty caused by the polarization scrambling.

Polarisation-insensitive parametric tunable dispersion compensator

Polarisation-insensitive operations of a parametric tunable dispersion compensator based on a polarisation diversity scheme using a polarisation-maintaining highly nonlinear fibre are demonstrated. The polarisation-dependent power penalty is less than 0.5 dB for 43 Gbit/s non-return-to-zero on-off-keying signals with polarisation scrambling.