Polarization Scrambling Research Articles

Continuous-variable quantum key distribution under strong channel polarization disturbance

In a commercial fiber-based quantum key distribution (QKD) system, the state of polarization (SOP) of the optical fields is inevitably disturbed by random birefringence of the standard single-mode fiber due to an external complex environment. We analyze theoretically the effect of SOP fluctuations on the continuous-variable (CV) QKD system and experimentally verify its validity. To overcome the influence of the polarization variations, a self-adaptive gradient algorithm is proposed to achieve high-speed polarization controlling under pulsed light with field programmable gate array hardware. For single random polarization scrambling, the polarization extinction ratio achieved is over 30 dB and the average time of polarization control is 827 $\ensuremath{\mu}\mathrm{s}$. In order to eliminate further the effect of the occasional failures of polarization control, we properly filter the raw keys in terms of the relative phase fluctuations between the signal and the local oscillator. By combining this approach with the high-speed polarization control, we demonstrate a CV QKD under a continuous polarization scrambling of 314 rad/s. Our results provide useful references for practical application of fiber-based CV QKD in a field environment.

Microwave frequency transfer over a 112-km urban fiber link based on electronic phase compensation**Project supported by the National Natural Science Foundation of China (Grant Nos. 61825505, 91536217, and 61127901).

We demonstrate the transmission of a microwave frequency signal at 10 GHz over a 112-km urban fiber link based on a novel simple-architecture electronic phase compensation system. The key element of the system is the low noise frequency divider by 4 to differentiate the frequency of the forward signal from that of the backward one, thus suppressing the effect of Brillouin backscattering and parasitic reflection along the link. In terms of overlapping Allan deviation, the frequency transfer instability of 4.2 × 10−15 at 1-s integration time and 1.6 × 10−18 at one-day integration time was achieved. In addition, its sensitivity to the polarization mode dispersion in fiber is analyzed by comparing the results with and without laser polarization scrambling. Generally, with simplicity and robustness, the system can offer great potentials in constructing cascaded frequency transfer system and facilitate the building of fiber-based microwave transfer network.

Phase-coding quantum-key-distribution system based on Sagnac–Mach-Zehnder interferometers

Stability and robustness are important criteria to evaluate the performance of a quantum-key-distribution (QKD) system in real-life applications. However, the inherent birefringence effect of the fiber channel and disturbance caused by the variation of the deployment environment of the QKD system tremendously decrease its performance. To eliminate this adverse impact, we propose a polarization-insensitive phase-coding QKD system based on Sagnac--Mach-Zehnder interferometers. Verified by theoretical analysis and experimental tests, this QKD system is robust against channel polarization disturbance. The robustness and long-term stability of the QKD system is confirmed with a 10-day continuous operation over a 12.6-dB channel, which consists of a 50-km fiber spool and a polarization scrambler (2 rad/s). As a result, an average quantum bit error rate of $0.958%$ and a sustained secure key rate of 3.68 kbps are obtained. Moreover, the secure key rate of the QKD system for a typical channel loss of 10 dB reaches 6.89 kbps, and the achievable maximum transmission distance exceeds 125 km.

Fiber-Optic Silicon Fabry-Perot Interferometric Bolometer: The Influence of Mechanical Vibration and Magnetic Field

Plasma radiation plays an important role in the operation and control of magnetic confinement fusion and is typically measured by bolometers. Here, we study the influence of mechanical vibration and magnetic field, which are typically present in magnetic confinement fusion devices, on the performance of a fiber-optic bolometer (FOB) based on silicon Fabry-Perot interferometer (FPI) demodulated by a scanning diode laser. It is found that both the vibration and magnetic field could cause large noise due to the birefringence of the silicon FPI. To mitigate the birefringence effects, we demonstrated two effective methods: polarization maintaining FOB and polarization scrambling. We show that two methods can be effective in mitigating the effect caused by the sensor birefringence: (1) replacing the regular single-mode fiber in the FOB system with polarization-maintaining fiber and (2) using a polarization scrambler after the laser source. For both methods, no significant increase in the noise was observed in the vibration and magnetic field tests. Our research highlights the importance of the polarization management for the FOB systems toward their practical applications in magnetic-confinement fusion systems.

Polarization scrambling characteristic analysis based on density of polarization states statistics

We report a new method to deeply analyze the scrambling characteristic of polarization scramblers based on density of polarization states (DPS) statistics that makes it possible to describe the DPS distribution in detail on the whole Poincare sphere, thus easy to locate accurately the nonuniform areas of defective polarization scramblers, which cannot be realized by existing methods. We have built a polarization scrambling system to demonstrate the advantages of our method compared with others by experiments and suggested effective evaluation indexes whose validity is well confirmed by applying to a commercial scrambler. Our conclusions are valuable for accurately analyzing and diagnosing the performance of any polarization scrambler, and quality evaluation of polarization controllers or other polarization devices.

Accelerated key generation and distribution using polarization scrambling in optical fiber.

The physical-layer properties of the classical optical fiber channel provide an inherent, unique, random and reciprocal source for secure key generation and distribution (SKGD). However, the key generation rate (KGR) is generally less than kbit/s in the reported SKGD schemes. In this paper, an accelerated SKGD scheme based on active polarization scrambling is proposed in the classical optical fiber channel. A combination of unique birefringence distribution of optical fiber channel and active scrambling of instant state of polarization (SOP) enables a fast and random SOP fluctuation to be securely shared between the legitimate users for accelerated SKGD. The proposed SKGD scheme is experimentally demonstrated over 24-km standard single-mode fiber (SSMF), where a KGR of 200-kbit/s with an error-free operation is achieved after the post-processing procedure. Moreover, the possible fiber-tapping attacks are theoretically and experimentally analyzed for the security robustness of the proposed scheme. The results imply that a faster SKGD scheme could be achieved by incorporating an active polarization scrambling mechanism into the random properties of the fiber channel.

Enhanced stability and sensitivity of slope-assisted Brillouin optical correlation-domain reflectometry using polarization-maintaining fibers

We show that, even when a polarization scrambler is switched off, PMF-based SA-BOCDR can operate with higher stability than that of standard silica-fiber-based systems. This leads to reduced cost and enables the use of the optimized state of polarization for higher sensitivity. After investigation of the strain/temperature dependencies of the Brillouin frequency shift and the Brillouin spectral power in the PMF, we show that the strain/temperature sensitivity of the PMF-based SA-BOCDR is 1.4 times the value of the standard silica-fiber-based configuration; we then demonstrate distributed temperature measurement with higher stability and sensitivity.

Ultrafast broadband circular dichroism in the deep ultraviolet

The measurement of chirality and its temporal evolution are crucial for the understanding of a large range of biological functions and chemical reactions. Steady-state circular dichroism (CD) is a standard analytical tool for measuring chirality in chemistry and biology. Nevertheless, its push into the ultrafast time domain and in the deep-ultraviolet has remained a challenge, with only some isolated reports of subnanosecond CD. Here, we present a broadband time-resolved CD spectrometer in the deep ultraviolet (UV) spectral range with femtosecond time resolution. The setup employs a photoelastic modulator to achieve shot-to-shot polarization switching of a 20 kHz pulse train of broadband femtosecond deep-UV pulses (250–370 nm). The resulting sequence of alternating left- and right-circularly polarized probe pulses is employed in a pump-probe scheme with shot-to-shot dispersive detection and thus allows for the acquisition of broadband CD spectra of ground- and excited-state species. Through polarization scrambling of the probe pulses prior to detection, artifact-free static and transient CD spectra of enantiopure [Ru(bpy)3]2+ are successfully recorded with a sensitivity of <2×10−5 OD (≈0.7 mdeg). Due to its broadband deep-UV detection with unprecedented sensitivity, the measurement of ultrafast chirality changes in biological systems with amino-acid residues and peptides and of DNA oligomers is now feasible.

Polarization division multiplexing pulse coding for eliminating the effect of polarization pulling in Golay-coded BOTDA fiber sensor.

An approach to eliminating the effect of polarization pulling in Golay-coded Brillouin optical time domain analysis (BOTDA) fiber sensor by employing polarization division multiplexing (PDM) pulse coding is presented. Three different schemes, including polarization switching, polarization scrambling, and PDM pulse coding are implemented and compared. Experimental results indicate that sensing stability is enhanced by ~2.5 times using the proposed scheme.

Design rules for optimizing unipolar coded Brillouin optical time-domain analyzers.

The performance of unipolar unicolor coded Brillouin optical time-domain analysis (BOTDA) is evaluated based on both Simplex and Golay codes. Four major detrimental factors that limit the system performance, including decoded-gain trace distortion, coding pulse power non-uniformity, polarization pulling and higher-order non-local effects, are thoroughly investigated. Through theoretical analysis and an experimental validations, solutions and optimal design conditions for unipolar unicolor coded BOTDA are clearly established. First, a logarithmic normalization approach is proposed to resolve the linear accumulated Brillouin amplification without distortion. Then it is found out that Simplex codes are more robust to pulse power non-uniformity compared to Golay codes; whilst the use of a polarization scrambler must be preferred in comparison to a polarization switch to mitigate uncompensated fading induced by polarization pulling in the decoded traces. These optimal conditions enables the sensing performance only limited by higher-order non-local effects. To secure systematic errors below 1.3 MHz on the Brillouin frequency estimation, while simultaneously reaching the maximum signal-to-noise ratio (SNR), a mathematical model is established to trade-off the key parameters in the design, i.e., the single-pulse Brillouin amplification, code length and probe power. It turns out that the optimal SNR performance depends in inverse proportion on the value of maximum single-pulse Brillouin amplification, which is ultimately determined by the spatial resolution. The analysis here presented is expected to serve as a quantitative guideline to design a distortion-free coded BOTDA system operating at maximum SNR.

FBG laser sensor with PS

Owing to unstable multi-wavelength lasing, robust measurement is difficult with a fibre Bragg grating (FBG) laser sensor system that uses FBGs as laser cavity mirrors as well as sensors. A stabilising technique is essential for proper data acquisition. Here, it is demonstrated that polarisation scrambling (PS) stabilises the output of laser sensor system in combination with a spectrometer demodulation. The state of polarisation (SOP) of a fibre-optic laser cavity is changed by a 1 MHz PS. A 5 kHz spectrometer traces the distribution of output laser wavelengths. The rapid changes in SOP make all the FBG wavelengths lase as long as necessary to determine the Bragg wavelength shifts in the sampling windows of a photo-diode array. A laser sensor system with eight FBG sensors is used to demonstrate robust measurement and data collection under environmental perturbations.

Practical gigahertz quantum key distribution robust against channel disturbance.

Quantum key distribution (QKD) provides an attractive solution for secure communication. However, channel disturbance severely limits its application when a QKD system is transferred from the laboratory to the field. Here a high-speed Faraday-Sagnac-Michelson QKD system is proposed that can automatically compensate for the channel polarization disturbance, which largely avoids the intermittency limitations of environment mutation. Over a 50km fiber channel with 30Hz polarization scrambling, the practicality of this phase-coding QKD system was characterized with an interference fringe visibility of 99.35% over 24h and a stable secure key rate of 306k bits/s over seven days without active polarization alignment.

Measuring polarization changes in the human outer retina with polarization-sensitive optical coherence tomography.

Morphological changes in the outer retina such as drusen are established biomarkers to diagnose age-related macular degeneration. However, earlier diagnosis might be possible by taking advantage of more subtle changes that accompany tissues that bear polarization-altering properties. To test this hypothesis, we developed a method based on polarization-sensitive optical coherence tomography with which volumetric data sets of the macula were obtained from 10 young (<25 years) and 10 older (>54 years) subjects. All young subjects and 5 of the older subjects had retardance values induced by the retinal pigment epithelium and Bruch's membrane (RPE-BM) complex that were just above the noise floor measurement (5°-13° at 840 nm). In contrast, elevated retardance, up to 180°, was observed in the other 5 older subjects. Analysis of the degree of polarization uniformity (DOPU) demonstrates that reduced DOPU (<0.4) in the RPE is associated with elevated double pass phase retardation (DPPR) below the RPE-BM complex, suggesting that the observed elevated DPPR in older subjects is the result of increased scattering or polarization scrambling. Collectively, our measurements show that the outer retina can undergo dramatic change in its polarization properties with age, and in some cases still retain its clinically normal appearance.

Demonstration of an optical-coherence converter

Studying the coherence of an optical field is typically compartmentalized with respect to its different optical degrees of freedom (DoFs) -- spatial, temporal, and polarization. Although this traditional approach succeeds when the DoFs are uncoupled, it fails at capturing key features of the field's coherence if the DOFs are indeed correlated -- a situation that arises often. By viewing coherence as a `resource' that can be shared among the DoFs, it becomes possible to convert the entropy associated with the fluctuations in one DoF to another DoF that is initially fluctuation-free. Here, we verify experimentally that coherence can indeed be reversibly exchanged -- without loss of energy -- between polarization and the spatial DoF of a partially coherent field. Starting from a linearly polarized spatially incoherent field -- one that produces no spatial interference fringes -- we obtain a spatially coherent field that is unpolarized. By reallocating the entropy to polarization, the field becomes invariant with regards to the action of a polarization scrambler, thus suggesting a strategy for avoiding the deleterious effects of a randomizing system on a DoF of the optical field.

Fluorescence Imaging Spectrometer (FLORIS) for ESA FLEX Mission

The Fluorescence Explorer (FLEX) mission has been selected as ESA’s 8th Earth Explorer mission. The primary objectives of the mission are to provide global estimates of vegetation fluorescence, actual photosynthetic activity, and vegetation stress. FLEX will fly in tandem formation with Sentinel-3 providing ancillary data for atmospheric characterization and correction, vegetation related spectral indices, and land surface temperature. The purpose of this manuscript is to present its scientific payload, FLORIS, which is a push-broom hyperspectral imager, flying on a medium size platform. FLORIS will measure the vegetation fluorescence in the spectral range between 500 nm and 780 nm at medium spatial resolution (300 m) and over a swath of 150 km. It accommodates an imaging spectrometer with a very high spectral resolution (0.3 nm), to measure the fluorescence spectrum within two oxygen absorption bands (O2A and O2B), and a second spectrometer with lower spectral resolution to derive additional atmospheric and vegetation parameters. A compact opto-mechanical solution is the current instrument baseline. A polarization scrambler is placed in front of a common dioptric telescope serving both spectrometers to minimize the polarization sensitivity. The telescope images the ground scene onto a double slit assembly. The radiation is spectrally dispersed onto the focal planes of the grating spectrometers. Special attention has been given to the mitigation of stray-light which is a key factor to reach good accuracy of the fluorescence measurement. The absolute radiometric calibration is achieved by observing a dedicated Sun illuminated Lambertian diffuser, while the spectral calibration in flight is performed by means of vicarious techniques. The thermal stabilization is achieved by using two passive radiators looking directly to the cold space, counterbalanced by heaters in a closed loop system. The focal planes are based on custom developed CCDs. The opto-mechanical design is robust, stable vs. temperature and easy to align. The optical quality is very good as recently demonstrated by the latest tests of an elegant breadboard. The scientific data products comprise the Top Of Atmosphere (TOA) radiance measurements as well as fluorescence estimates and higher-level products related to the health status of the vegetation addressing a wide range of applications from agriculture to forestry and climate.

Single-shot distributed Brillouin optical time domain analyzer.

We demonstrate a novel single-shot distributed Brillouin optical time domain analyzer (SS-BOTDA). In our method, dual-polarization probe with orthogonal frequency-division multiplexing (OFDM) modulation is used to acquire the distributed Brillouin gain spectra, and coherent detection is used to enhance the signal-to-noise ratio (SNR) drastically. Distributed temperature sensing is demonstrated over a 1.08 km standard single-mode fiber (SSMF) with 20.48 m spatial resolution and 0.59 °C temperature accuracy. Neither frequency scanning, nor polarization scrambling, nor averaging is required in our scheme. All the data are obtained through only one-shot measurement, indicating that the sensing speed is only limited by the length of fiber.

A Parametric Model for Contribution of Superthermal Electrons to Oblique Measurement Electron Cyclotron Spectra Under ITER-Like Conditions

For high-temperature tokamaks like ITER, electron cyclotron emission (ECE) measurements are expected to be affected by many factors like relativistic downshift, harmonics overlap, polarization scrambling, deviation of electron distribution from Maxwellian, etc. Many studies are already reported on the difference between ECE measurements and other measurements like Thomson scattering for existing high-temperature tokamaks like JET, TFTR, D-III-D, etc. As ITER is expected to reach a temperature of around 25 keV with a strong electron-ion coupling and additional heating, the deviation of the ECE radiation temperature from the electron temperature needs to be examined. This paper reports a parametric study on the effect of the presence of small superthermal populations on ECE measurements for ITER. A wide range of parametric space for superthermal parameters is used, assuming a bi-Maxwellian electron distribution, which obeys Kirchhoff law. The computational details and the results of the numerical studies are explained in this paper. Further, an attempt is also made to reconstruct the superthermal contributions from multiple oblique measurements, which is otherwise a difficult task. This reconstruction has been done through numerical calculations for two sets of measurements using detectors placed at same but opposite angles. Then, a scale factor is used to scale the difference between these two measurements to superthermal emission. The detailed procedure and possible physical explanations are presented. The dependence of this scale factor on the superthermal parameters is numerically studied, and a parametric equation is drafted between scale factor and superthermal parameters. The said equation contains two numerical constants, for which the values are numerically obtained from one set of simulations and verified with a number of calculations using different superthermal parameters.

Wide-band polarization controller for Si photonic integrated circuits.

A circuit for the management of any arbitrary polarization state of light is demonstrated on an integrated silicon (Si) photonics platform. This circuit allows us to adapt any polarization into the standard fundamental TE mode of a Si waveguide and, conversely, to control the polarization and set it to any arbitrary polarization state. In addition, the integrated thermal tuning allows kilohertz speed which can be used to perform a polarization scrambler. The circuit was used in a WDM link and successfully used to adapt four channels into a standard Si photonic integrated circuit.

Estimation of Jones matrix, birefringence and entropy using Cloude-Pottier decomposition in polarization-sensitive optical coherence tomography.

Estimation of polarimetric parameters has been a fundamental issue to assess biological tissues that have form birefringence or polarization scrambling in polarization-sensitive optical coherence tomography (PS-OCT). We present a mathematical framework to provide a maximum likelihood estimation of the target covariance matrix and its incoherent target decomposition to estimate a Jones matrix of a dominant scattering mechanism, called Cloude-Pottier decomposition, thereby deriving the phase retardation and the optic axis of the sample. In addition, we introduce entropy that shows the randomness of the polarization property. Underestimation of the entropy at a low sampling number is mitigated by asymptotic quasi maximum likelihood estimator. A bias of the entropy from random noises is corrected to show only the polarization property inherent in the sample. The theory is validated with experimental measurements of a glass plate and waveplates, and applied to the imaging of a healthy human eye anterior segment as an image filter.

Fast and Chaotic Fiber-Based Nonlinear Polarization Scrambler

We report a simple and efficient all-optical polarization scrambler based on the nonlinear interaction in an optical fiber between a signal beam and its backward replica which is generated and amplified by a reflective loop. When the amplification factor exceeds a certain threshold, the system exhibits a chaotic regime in which the evolution of the output polarization state of the signal becomes temporally chaotic and scrambled all over the surface of the Poincare sphere. We numerically derive some design rules for the scrambling performances of our device which are well confirmed by the experimental results. The polarization scrambler has been successfully tested on a 10-Gb/s On/Off Keying Telecom signal, reaching scrambling speeds up to 500-krad/s, as well as in a wavelength division multiplexing configuration. A different configuration based on a following cascade of polarization scramblers is also discussed numerically, which leads to an increase of the scrambling performances.