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The near‐infrared (NIR) light polarization metrology is crucial in a broad area of applications, such as optical sensing, imaging, and communications. The conventional measurement of NIR light polarization requires bulky polarization optics by the spatial or temporal splitting of the input light beam with expensive infrared detectors. Here, an intelligent fiber‐integrated NIR polarimeter based on upconversion nanoparticles (UCNPs) is demonstrated. Er‐doped UCNPs are synthesized in core–shell structures for bright upconversion luminescence (UCL), and the tapered multimode fiber (MMF) is designed for random Rayleigh‐distributed speckles. It is demonstrated for the first time that spectra‐polarization‐dependent upconverted speckle patterns captured by a cost‐effective visible image sensor can be accurately decoded via a deep neural network (ResNet50). The experimental results show that ResNet50 has a recognition accuracy of over 99.5% for the full‐Stokes parameters of NIR light polarization, along with 100% accuracy for the light wavelength ranging from 1535 nm to 1565 nm. The developed intelligent optical sensors provide a new design paradigm for fiber‐integrated multi‐dimensional NIR light field detection and can be applied in fiber‐optic sensors and communications networks. [ABSTRACT FROM AUTHOR]

Forthcoming capacity scaling requirements of optical networks and advances in optical fiber communications beyond the omnipresent single-mode fiber operating over the conventional band introduces new opportunities and challenges for exploiting the expanded spectral and spatial resources available for fiber-optic network designs. The spectral and spatial degrees of freedom introduce utilization tradeoffs that can be leveraged under different applications. After briefly reviewing the supporting network building elements (emerging fiber types, multiplexers, optical switches, and amplifiers), we consider networking scenarios such as intra/inter datacenter, terrestrial, undersea, and wireless 5G/6G hauling networks and identify the best utilization plan and key attributes that can be harnessed by the offered spectral and spatial degrees of freedom for each scenario and how optical switching can be employed therein for traffic management. Networks supporting these scenarios are experiencing tremendous traffic growth pushing their existing implementations to their physical capacity-distance limits. New network builds must offer significant capacity multipliers in an efficient and cost-effective manner, requiring new switching solutions for supporting the unabated traffic growths foreseen in the years to come. We discuss how space-division multiplexed networks can help address this challenge in key communication scenarios. [ABSTRACT FROM AUTHOR]

Mode-division multiplexing (MDM) using few-mode fibers (FMFs) appears as a promising technology to increase fiber capacity by a few orders of magnitude and sustain the traffic demand for the decades to come. The potential of MDM lies in its ability to exploit multiple modes within a single optical fiber strand. Since 2011 and the first promising demonstrations, impressive progress has been made. In this article, we will show how the most recent advances in design and fabrication have improved the performance of FMFs and MDM systems, and allowed to turn research demonstrations into practical deployments. [ABSTRACT FROM AUTHOR]

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For enhancing channel capacity and spectrum efficiency, vortex beam carrying orbital angular momentum (OAM) has been ardently investigated in optical communication. However, the crosstalk among OAM modes induced by mode coupling has seriously hindered the development of vortex beam communication (OBC). A novel concept of mitigating crosstalk within an OAM-mode group for a short-haul optical fiber communication (OFC) is proposed by reversing the transmission matrix. To support more OAM modes propagation, a step-index OAM fiber with ring-shaped profile is also designed and fabricated, which can support 6 vector/OAM modes propagation. A proof-of-concept experiment is performed for verifying the feasibility of the proposed concept, where a quadrature-phase-shift-keying (QPSK) signal is utilized as an excitation signal. Bit-error-ratio (BER) and constellation diagram (CD) are employed to evaluate the performance of the proposed system. The captured intensity profiles and interference diagrams demonstrate that the propagated four OAM beams within the 2rd OAM-mode group are successfully received. The measured BER and CD illustrate that the crosstalk within the mode group can be significantly mitigated. Further, although the data speed or fiber length is increased, the system performance using the proposed concept is still superior to the case without increasing fiber length or data speed when the proposed concept is missed. Therefore, the proposed concept is effective for mitigating the crosstalk and can be then used for a short-haul OAM-based OFC. [ABSTRACT FROM AUTHOR]

We investigate the potential of adaptive blind equalizers based on variational inference for carrier recovery in optical communications. These equalizers are based on a low-complexity approximation of maximum likelihood channel estimation. We generalize the concept of variational autoencoder (VAE) equalizers to higher order modulation formats encompassing probabilistic constellation shaping (PCS), ubiquitous in optical communications, oversampling at the receiver, and dual-polarization transmission. Besides black-box equalizers based on convolutional neural networks, we propose a model-based equalizer based on a linear butterfly filter and train the filter coefficients using the variational inference paradigm. As a byproduct, the VAE also provides a reliable channel estimation. We analyze the VAE in terms of performance and flexibility over a classical additive white Gaussian noise (AWGN) channel with inter-symbol interference (ISI) and over a dispersive linear optical dual-polarization channel. We show that it can extend the application range of blind adaptive equalizers by outperforming the state-of-the-art constant-modulus algorithm (CMA) for PCS for both fixed but also time-varying channels. The evaluation is accompanied with a hyperparameter analysis. [ABSTRACT FROM AUTHOR]

A low frequency-noise ring random fiber laser (RFL) based on random Bragg grating array (RBGA) feedback is proposed. A dual-cavity fiber Bragg grating Fabry-Perot (FBG-FP) filter, which matches the wavelength of RBGA, is specially designed. Due to the narrower bandwidth, flatter response and steeper rolloff, the dual-cavity FBG-FP filter can significantly suppress the competition-induced frequency jitter and undesired residual random lasing modes, and contribute to establish stable lasing and further reduce frequency noise of RFL. The frequency noise of the proposed RFL is ∼3 Hz/√Hz above 1 kHz, and the frequency jitter is ∼3.11 × 10−11 over 10 s. The 3-dB linewidth is ∼490 Hz with a ∼40 dB of optical signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

In ultra-long-haul optical communication systems, polarization-dependent gain from single-mode semiconductor optical amplifiers or mode-dependent gain from multi-mode erbium-doped fiber amplifiers causes a channel’s achievable information rate (AIR) using linear minimum-mean-square error (MMSE) detection to become substantially lower than the capacity of optimal maximum likelihood detection owing to loss of orthogonality between modes. However, such capacity loss can be mitigated using techniques that retain reasonable complexity, such as successive interference cancellation (SIC). In wireless systems, multicarrier transmission with many subcarriers transforms the frequency-selective channel into a set of uncoupled memoryless subchannels, and it is possible to obtain good performance using SIC on each subchannel independently. Multicarrier transmission in optical fibers, however, is susceptible to performance loss caused by Kerr nonlinearity. Therefore, we study methods for applying SIC on frequency-selective optical fiber channels using single-carrier transmission. We compare the theoretical performance of MMSE and SIC equalizers on frequency-selective optical fiber channels operating at 64 Gbaud with transmission distances up to 15,000 km and evaluate the penalties of SIC implementation. We consider single-mode fiber links with random artificial polarization-mode dispersion inserted along the link to enhance frequency diversity, as well as multi-core fiber links supporting 14 spatial and polarization modes. We show that SIC can increase outage capacity by 19%, but the AIR increase is reduced by almost half if error correction coding is not performed across modes. We confirm that SIC can adapt to fast channel perturbations with only a 3% decrease in AIR. [ABSTRACT FROM AUTHOR]

Integrated polarization stabilizers along with corresponding stabilizing algorithms have been investigated for years to tackle the polarization fluctuation issue in polarization dependent systems such as photonics integrated circuits and coherent optical communication systems. Here, we propose and demonstrate a novel integrated polarization stabilizer which is composed of an in-line polarimeter and two cascaded Mach-Zehnder interferometers-based polarization converter, with a footprint of 2.8 mm × 0.8 mm. Benefitting from the integrated in-line polarimeter and two controlling algorithms, namely feed-forward tracking algorithm and feedback tracking algorithm, the stabilizer can track the incoming light with arbitrary state of polarization in one-time manipulation. The whole process consumes about 30 μs, which is approaching the limitation of the response time of the adopted thermal phase shifters, making the proposed device the fastest one with thermal phase shifters on silicon photonics platform. Meanwhile, the incoming light is endlessly tracked in real time and 99% converted to horizontal linear polarized light. [ABSTRACT FROM AUTHOR]

• We synthesized dielectric material particles by copolymerizing (-)-MnMA and MMA with benzoyl peroxide (BPO). Combining with the co-extrusion method, the dielectric chiral fiber is drawn by several meters for the first time, which has a more simplified fiber fabrication process and higher fiber drawing speed with good continuity and consistency comparing with structural chiral fiber. • Some important optical performances of dielectric chiral fiber (DCF) were tested. The results of optical activity of dielectric materials, optical power loss, circular polarization maintaining characteristic demonstrated that DCF can be used as a novel concept circular polarization maintaining fiber. Besides, the measurement of refractive index data fills in a gap in refractive index of DCF, which can not only supply the basis of analyzing the distribution of optical field and modes for transmission, but also designing DCF with different optical characteristics according to the requirements in different applications. • We designed a dual-core photonic crystal fiber with optimized parameters as a beam splitter which can separate different polarization states of light. The polarized beam split effect demonstrated that there is a potential of DCF for polarization division multiplexing in theory. Compared with linearly polarized light, circularly polarized light can hardly cause the cross coupling effect during transmission. Therefore, it is important to study chiral fibers for optical fiber communication and optical fiber sensing. However, the requirements in structural chiral fiber fabrication raise its cost. In the paper, we demonstrated the entire fiber fabrication method of dielectric chiral fiber. This is the first fabrication of long dielectric chiral fiber. We also measured key optical properties of the chiral material. The value of fabricating dielectric chiral fiber is demonstrated by its physical implementation and good performance in circular polarization maintaining characteristic. The fiber fabrication method is expected to provide a solution for low cost. Moreover, the dielectric chiral fiber supplies the potential application of dielectric chiral fiber for optical fiber communication and optical fiber sensing. [ABSTRACT FROM AUTHOR]

A transparentOptical-subTHz-Optical link providing record-high single line rates of 240 Gbit/s and 192 Gbit/s on a single optical carrier over distances from 5 to 115 m is demonstrated. Besides a direct mapping of the optical to a 230 GHz subTHz-carrier frequency by means of a uni-traveling carrier (UTC) photodiode, we demonstrate direct conversion of data from the subTHz domain back to the optical domain by a plasmonic modulator. It is shown that the subTHz-to-optical upconversion can even be performed at good quality without any electrical amplifiers. Finally, at the receiver, the local oscillator is employed to directly map the optical signal back to the electrical baseband within a coherent receiver. [ABSTRACT FROM AUTHOR]

We experimentally demonstrated a long-haul wavelength-division multiplexed (WDM) and space-division multiplexed (SDM) transmission of 32-Gbaud polarization-division multiplexed quadrature phase-shift keying signals over 52-km coupled-4-core fiber spans with amplification by coupled-4-core erbium-doped fiber amplifiers (EDFAs). Error-free performance after forward error correction was achieved up to 2700 km. From converged multi-input multi-output filter coefficients, we estimated the mode dependent loss (MDL) that occurred in this experiment. The MDL was frequency-dependent within a WDM channel and the root-mean square MDL was 1.0 dB per loop. To assess the effect of MDL on SDM transmission performance, we also estimated the outage probability through Monte-Carlo simulations with a Gaussian noise model. The estimated outage probability under MDL showed similar performance to that obtained in the transmission experiment. This suggests that the MDL of coupled-multi-core EDFAs was one of the major limiting factors of the transmission performance and that the rms MDL per span should be below 0.4 dB for further long-haul transmission distance over 6000 km. [ABSTRACT FROM AUTHOR]

The key idea of this work is to treat the symbol- pattern-dependent inter-symbol-interference (ISI) that is avoided in conventional single-symbol modulation systems as useful symbol correlation, incorporate the feature of the symbol correlation into both adaptive equalization and sequence detection and thus, upgrade the system to a multi-symbol-oriented pattern-dependent equalization and detection regime. Specifically, we propose an adaptive equalization scheme sharing a pattern-dependent look- up-table with a maximum likelihood sequence estimation process to facilitate tolerance and mitigation on symbol-pattern-dependent ISI. The proposed scheme can be applied to various transmission systems with linear or nonlinear pattern-dependent ISI, such as bandwidth constrained systems with severe linear ISI, or high-power long-distance transmission systems with fiber nonlinearity induced self-phase modulation. In exemplary coherent transmission systems with digital narrow filtering before digital to analog conversion at transmitter, we experimentally demonstrate effective demodulation and detection of a polarization-division-multiplexing quaternary- phase-shift-keying signal with a constrained 3-dB transmitter bandwidth down to 1/8 of thesignal baud rate. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a selectively microfluidic-infiltrated method for the group birefringence regula-tion in a microstructured optical fiber. Meanwhile, an Optical Frequency Domain Reflectometry (OFDR) with an equivalent resolution of 0.6 Hz or 10−6 RIU is utilized to characterize its polarization separation and group birefringence change. By selectively infiltrating a fluid with a high thermal-optic coefficient into twin air holes nearby the silica fiber core, tunable group birefringence is observed and measured by polarization separation in beat frequency domain. Moreover, due to the co-effects of the filled and unfilled fiber segments, the twin critical zeros of the overall group birefringence are observed at 16.5° and 43.0°, respectively. The unique position exchange of LP01x and LP01y presents at the twin critical zeros, which is investigated via the OFDR technique as well. Therefore, by combining the unfilled fiber segment with negative group birefringence and filled fiber segment with positive group birefringence, dual zeros in group birefringence are achieved with position exchange of the LP01x and LP01y peaks in beat frequency domain. Our demonstration provides potentials in flexible regulation and real-time observation of the group birefringence in microstructured optical fiber. [ABSTRACT FROM AUTHOR]

A technique to suppress both the second and third order nonlinear distortions in a long-haul microwave photonic link (MPL) is presented. The MPL consists of a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), a polarizer, a length of fiber, an optical bandpass filter (OBPF), and a balanced photodetector (BPD). The DP-DPMZM has two sub-DPMZMs. For a two-tone signal at f1 and f2, the second order harmonic distortions (SHD) and the second order intermodulation distortions (IMD2) at f2+f1 are suppressed by biasing one sub-DPMZM to operate with single-sideband suppressed carrier (SSB-SC) modulation. The IMD2 at f2–f1 is suppressed by balanced detection at the BPD. The third order intermodulation distortions (IMD3) are suppressed by adjusting the state of polarization of the light wave into the polarizer. The spurious free dynamic range (SFDR) in a multi-octave band is free from fiber dispersion effect due to the bias setting of the DP-DPMZM. As a result, a high multi-octave SFDR can be obtained irrespective of the transmission distance. The proposed MPL is analyzed theoretically and is verified experimentally. The measured multi-octave SFDR is 95.9 dB·Hz1/2 and 92.1 dB·Hz1/2 when a long fiber link at 10 km and 20 km is used for signal transmission, respectively, which is 16.1 dB and 17.3 dB higher compared to a conventional Mach-Zehnder modulator (MZM) based MPL operating under the same condition. [ABSTRACT FROM AUTHOR]

Integrated microwave photonics (MWP) is a fast growing area where high frequency microwave signals are processed in the optical domain, merging key advantages of both microwave photonics and photonic integrated circuits (PICs) technologies including low-loss, reconfigurability, advanced functionality, enhanced stability, and reduced footprint. Plenty of functionalities have been demonstrated in integrated MWP, especially based on spectral shaping technique, where the phase and amplitude of the optical spectrum is precisely tailored by PICs. However, on-chip linearization is lagging behind and has not been investigated deeply. It is crucial and urgent to study on-chip linearization methods, which will lead to advanced integrated MWP systems with large spurious-free dynamic range (SFDR). In this paper, we present two novel techniques for on-chip linearization of microwave photonic links. The first technique is based on line-by-line complex spectral shaping using a series of ring resonators. The second technique relies on spatial separation to achieve parallel spectral shaping in two complementary spatial channels. Both methods are demonstrated in low-loss programmable silicon nitride circuits that can already host a number of advanced functionalities. Our results point to the great potential of integrating advanced functionalities and linearization in the same integrated platform. [ABSTRACT FROM AUTHOR]

We investigate the random intensity noises that occur in forward-pumped distributed Raman amplifier systems. First, we show pump-to-signal intensity noise transfer characteristics, which strongly depend on the group velocities of pump light and signal light in optical fiber used as a gain medium. When signal light is in the C-band, dispersion shifted fiber (DSF) transfers much larger noise compared with standard single mode fiber. Next, we discuss the origin of the noise induced in pump light. We define the concept of “synthesized polarization” and show that fluctuation in the state of synthesized polarization (SOSP) can induce a large gain instability even if the relative intensity noise (RIN) of each pump laser source is negligible experimentally. Next, we propose a novel optical depolarizer for pump light. It can simultaneously depolarize pump light generated by multiple laser sources. Moreover, it can manage optical phases precisely to suppress the gain instability induced by the fluctuation in SOSP. Finally, we present a measurement of the Q factor of a 16-QAM signal (32 Gbaud) after 35-km transmission, with and without a forward-pumped Raman amplifier. Two pump laser sources with RINs of –134 to –126 dB/Hz were depolarized by the proposed depolarizer, and the generated on-off gain was 7 dB. Though amplified signal light was in the C-band and the gain medium was DSF, 16-QAM transmission specifications were successfully improved, and acceptable noise was observed for 5 hours operation. [ABSTRACT FROM AUTHOR]

An undersampling free-space optical (FSO) communication system based on compressive sensing (CS) and in-fiber grating is proposed. Our proposal drastically mitigates the data scale unfavorable issue and reduces the great amount of required data. Also, our proposal employs low-bandwidth and low-cost photodetectors (PDs) and analog-to-digital converters (ADCs) and decreases the cost of data detection. Besides, the data transmitted in the free space is encrypted, which enhances the security of the system. The in-fiber grating, which has the advantage of highly efficient thanks to its inherent compatibility with optical fiber links, plays the roles of light emitter, in-fiber polarizer, and light diffraction device, simultaneously. A sinusoidal radio frequency (RF) signal with a frequency of 1 GHz is utilized for a 2.4 m data transmission. And each PD with a bandwidth of 300 MHz is employed for detecting the transmitted signal on each channel. The proposal obtains data compression ratios of 25%, 15%, and 5% both in the temporal domain and frequency domain. Besides, a two-tone signal and a random RF signal are applied in the proposed FSO transmission for data transmission. Our proposal not only has the merits of secure, compact, stable, and highly efficient, but also has great potential in extremely secure situations, such as secured indoor and under water data communication. [ABSTRACT FROM AUTHOR]

We investigate polarization insensitive fiber optical parametric amplifiers (FOPAs) employing a balanced polarization diversity loop with at least two unidirectional gain fibers. We describe and compare three variants of looped polarization insensitive FOPAs optimized for noise figure, mitigation of nonlinear impairments and their trade-off, respectively. The test scenario consists of amplifying, by up to 14 dB, a set of 21 × 50 GHz-spaced channels including a 35 GBaud PDM-QPSK signal, and evaluating a power of nonlinear crosstalk, noise figure and amplified signal BER for each variant. For the first time we demonstrate a polarization insensitive FOPA amplifying WDM signals with a noise figure as low as 5.8 dB, and a polarization insensitive FOPA with output WDM signal power of 23 dBm. The testing results let us identify likely application scenarios for each looped FOPA variant. We justify potential implementation of polarization-insensitive FOPAs in future optical communication systems by arguing its ability to deliver low noise figure <6 dB for output signal power as high as 29 dBm and to enable polarization insensitive gain for the most prominent single-polarization FOPA achievements realizing ultrawide high gain. [ABSTRACT FROM AUTHOR]

International standardization bodies (IEEE and ITU-T) working on the evolution of transmission technologies are still considering traditional direct detection solutions for the most relevant short-reach optical link applications, that are Passive Optical Networks (PON) and intra-data center interconnects. Anyway, future jumps towards even higher bit rates per wavelength will require a complete paradigm shift, moving towards coherent technologies. In this paper, we thus study both analytically and experimentally the scaling laws of unamplified coherent transmission in the short-reach communications ecosystems. We believe that, given the extremely tight techno-economic constraints, such a revolutionary transition towards coherent in short-reach first requires a very detailed study of its intrinsic capabilities in largely extending the limitation currently imposed by direct detection systems. To this end, this paper focuses on the ultimate physical layer limitations of unamplified coherent systems in terms of bit rate and power budget. The main parameters of our performance estimation model are extracted through fitting with a set of experimental characterizations and later used as the starting point of a scaling laws study regarding local oscillator power, modulator-induced attenuation, bit rate, and maximum achievable power budget. The analytically predicted performance is then verified through transmission experiments, including a demonstration on a 37-km installed metropolitan dark fiber in the city of Turin. Our findings show that coherent detection without optical pre-amplification and using PM-QPSK can tolerate optical power budget (OPB) well above, for instance, the 29 dB imposed by the current PON standards even at extremely high raw bit rates up to 800 $Gbps$. PM-16QAM, on the other hand, can provide up to 190 $Gbps$ at 29 dB OPB only if combined with soft FEC algorithms. Even higher bit rates are also shown for the less demanding power budget needed in intra-data centers links. [ABSTRACT FROM AUTHOR]

We investigate the complexity and performance of recurrent neural network (RNN) models as post-processing units for the compensation of fibre nonlinearities in digital coherent systems carrying polarization multiplexed 16-QAM and 32-QAM signals. We evaluate three bi-directional RNN models, namely the bi-LSTM, bi-GRU and bi-Vanilla-RNN and show that all of them are promising nonlinearity compensators especially in dispersion unmanaged systems. Αs far as inference is concerned, οur simulations show that the three models provide similar compensation performance, therefore, in real-life systems, the simplest scheme based on Vanilla-RNN units should be preferred. We compare bi-Vanilla-RNN in its many-to-many form with Volterra nonlinear equalizers and exhibit its superiority both in terms of performance and complexity, thus highlighting that RNN processing is a very promising pathway for the upgrade of long-haul optical communication systems utilizing coherent detection. [ABSTRACT FROM AUTHOR]

This article presents a detailed survey on both asynchronous and synchronous Optical Code-Division Multiple-Access (OCDMA) systems based on 1-D, 2-D, and 3-D codes, distinctively. Accordingly, different coding schemes along with their multiplexing capacity, correlation features, advantages and drawbacks are addressed and benchmarked. Additionally, different detection techniques such as direct and coherent detection and their capability to handle Multiple Access-Interference are also discussed. Furthermore, the main OCDMA systems from the last two decades are summarized considering their coding schemes, performance, device components, and applications. Therefore, a new comparative study in terms of spectral efficiency and bit error rate for 1-D, 2-D, and 3-D asynchronous and synchronous OCDMA systems is carried out. Numerical results shown that the 3-D MD code stands out among the codes with superior performance as well as support to higher data rates and number of users. [ABSTRACT FROM AUTHOR]

In this tutorial, we discuss the evolution of the technology deployed for optical interconnects and the trade-offs in the design of low complexity, low power DSP and implementation for direct detect and coherent, pluggable optical modules for data center applications. The design trade-offs include the choice of modulation format, baud rate, optical link design, forward error correction, signal shaping and dispersion compensation. [ABSTRACT FROM AUTHOR]

Vector modes (VMs) with spatially nonhomogeneous polarization patterns, combining polarization and spatial-mode degrees of freedom together, are regarded as the eigenmodes of optical fiber. Such eigenmodes have potential to be used to implement the high capacity spatial division multiplexing (SDM)/mode division multiplexing (MDM) based optical fiber communication systems. Here, we respectively demonstrate 2 VMs of the same order (l = +2, EH11o and EH11e modes) and 2 VMs of different orders (l = 0, HE11o mode and l = +2, EH11e mode) multiplexing transmissions over 5-km few-mode fiber (FMF). The mode crosstalk after FMF transmission has been minimized to prepare high capacity signal transmission without multiple-input-multiple-output (MIMO) digital signal processing (DSP). To increase the transmission capacity and decrease the system cost, the Kramers-Kronig (KK) receiver has been employed and the single-wavelength two sets of eigenmodes multiplexed 360 Gbit/s and 400 Gbit/s signal transmissions over 5-km FMF link are realized, respectively. Then, to further burst the transmission capacity, 2 vector-mode-division-multiplexing channels (HE11o and EH11e modes) in combination of 5 wavelength-division-multiplexing (WDM) channels with a total of 10 parallel channels have been demonstrated. A total data rate of 1.12 Tbit/s with 28 GBaud 16-state quadrature amplitude modulation (QAM) signal has been realized over the 5-km FMF. In all data-carrying experimental demonstrations, the bit-error rate (BER) performance of each VM channel is below the 7% hard decision forward error correction threshold (FEC) at BER of 3.8 × 10−3. The experimental results highlight that the VMDM-based optical transmission technology could find enormous potential in huge-capacity short-reach optical interconnects. [ABSTRACT FROM AUTHOR]

Equalization enhanced phase noise (EEPN) occurs due to the interplay between laser phase noise and electronic dispersion compensation (EDC) module. It degrades significantly the performance of uncompensated long-haul coherent optical fiber communication systems. In this work, a general expression accounting for EEPN is presented based on Gaussian noise model to evaluate the performance of multi-channel optical communication systems using EDC and digital nonlinearity compensation (NLC). The nonlinear interaction between the signal and the EEPN is analyzed. Numerical simulations are carried out in nonlinear Nyquist-spaced wavelength division multiplexing (WDM) coherent transmission systems. Significant performance degradation due to EEPN in the cases of EDC and NLC are observed, with and without the consideration of transceiver (TRx) noise. The validation of the analytical approach has been done via split-step Fourier simulations. The maximum transmission distance and the laser linewidth tolerance are also estimated to provide important insights into the impact of EEPN. [ABSTRACT FROM AUTHOR]

Polarization variations in optical fibers are complex and would severely affect the performances of polarization-sensitive signal distribution systems. Owing to advances in experimental techniques and theoretical tools, we observe the effect of polarization variations in the optical fibers and we demonstrate that polarization mode dispersion (PMD) dominates the free-running fiber noise. Ultrahigh correlation, over 99%, is found between the phase fluctuation induced by polarization variation and the temperature fluctuation impacted on the optical fibers. By means of a polarization scrambling technique, the polarization variations in the optical fiber are averaged and the phase perturbations are minimized. It provides a promising solution for suppressing the random disturbances of polarization in the fiber-based optical frequency transfer system and quantum key distribution (QKD) system. [ABSTRACT FROM AUTHOR]

Huffman-coded sphere shaping (HCSS) is an algorithm for finite-length probabilistic constellation shaping, which provides nearly optimal energy efficiency at low implementation complexity. In this paper, we experimentally study the nonlinear performance of HCSS employing dual-polarization 64-ary quadrature amplitude modulation (DP-64QAM) in an extended-reach single-span link comprising 200 km of standard single-mode fiber (SSMF). We investigate the effects of shaping sequence length, dimensionality of symbol mapping, and shaping rate. We determine that the naïve approach of Maxwell–Boltzmann distribution matching — which is optimal in the additive white Gaussian noise channel — provides a maximum achievable information rate (AIR) gain of 0.18 bits/4D-symbol with respect to uniform signaling at optimum launch power in the infinite length regime. Conversely, HCSS can achieve a gain of 0.37 bits/4D-symbol over uniform signaling using amplitude sequence length of 32, which may be implemented without multiplications, using integer comparison and addition operations only. Coded system performance, with a net data rate of approximately 425 Gb/s for both shaped and uniform inputs, is also analyzed. [ABSTRACT FROM AUTHOR]

We propose a new four-dimensional orthant-symmetric 128-ary modulation format (4D-OS128) with a spectral efficiency of 7 bit/4D-sym. The proposed format fills the gap between polarization-multiplexed 8- and 16-ary quadrature-amplitude modulation (PM-8QAM and PM-16QAM). Numerical simulations show that 4D-OS128 outperforms two well-studied 4D geometrically-shaped modulation formats: 128SP-16QAM and 7b4D-2A8PSK by up to 0.65 dB for bit-interleaved coded modulation at the same spectral efficiency. These gains are experimentally demonstrated in a 11×233 Gbit/s wavelength division multiplexing (WDM) transmission system operating at 5.95 bit/4D-sym over 6000 and 9000 km for both EDFA-only and hybrid amplification scenarios, respectively. A reach increase of 15% is achieved with respect to 128-ary set-partitioning 16QAM. Furthermore, the proposed 4D-OS128 is also compared to D4 lattice-based constellation, 16QAM and probabilistically-shaped 16QAM with finite blocklength via simulation. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a dual-polarization (DP) transceiver imbalance monitoring scheme, which is applicable for the coherent system using square QAM signals. The scheme can separate/monitor various kinds of transceiver imbalances and has technical advantages of multi-dimension, wide monitoring range, and modulation-format-transparency. In the proposed scheme, a polarization scrambler is configured between transmitter (Tx) and receiver (Rx). After coherent detection, the proposed digital signal processing (DSP) modules are implemented to monitor transceiver imbalance based on the received signals. To simplify the DSP modules, the implementation sequence of DSP modules is in reverse order compared to the order in which the sequence of impairments is introduced. Firstly, Godard timing error detection (TED) and Gram–Schmidt orthogonalization procedure (GSOP) is used to monitor and compensate Rx imbalances. After compensating the Rx imbalances, Tx imbalance monitoring is implemented. Complex/real maximum likelihood independent component analysis (ML-ICA) and Godard-TED are used to monitor Tx imbalance. Finally, the amplitude ratios of the final output signal and Tx in-phase/quadrature (IQ) amplitude imbalance are fed back to mitigate the interaction of Tx imbalances on the signal power and improve monitoring accuracy. The transceiver IQ and x-polarization/y-polarization (XY) imbalances are separated for the first time by using the frequency offset (FO) naturally existing in the system and the continuous polarization rotation induced by a polarization scrambler. The separation method combined with the reverse algorithm design and amplitude ratio feedback mechanism makes the proposed scheme achieve the most multi-dimensional imbalance monitoring by far. The simulation and experimental results verify that the proposed scheme is modulation-format-transparent and can separate/monitor multi-dimensional transceiver imbalances within a wide range. [ABSTRACT FROM AUTHOR]

This paper proposes a novel approach and apparatus along with its calibration method to use the digital predistortion (DPD) technique in radio-over fiber (RoF) cloud RAN (C-RAN) transmitters that encompass physically spaced central baseband unit (C-BBU) and remote radio head (RRH). The proposed approach requires a calibrated observation path (COP) connecting the RRH to the BBU to mitigate any hardware impairments and distortion using a broadband post-distortion technique. This feedback (FB) link provides on-demand tracking of the change in the downlink (DL) optoelectronic component’s behavior and sends a copy of the radio frequency (RF) output signal to the C-BBU for DPD identification and processing. The proposed technique is experimentally validated using 40 MHz, and 100 MHz LTE-A as DL data signals, while the post-compensation model is built by a 200 MHz LTE-A signal. [ABSTRACT FROM AUTHOR]

In this paper, an all-optical quadrature phase shift keying (QPSK) phase regeneration scheme using vector phase sensitive amplification (PSA) is proposed, analyzed, and simulated. Compared with the conventional scalar PSA scheme, the vector superposition of harmonics makes it easy to obtain high gain extinction ratio (GER) with low pump powers. A subsequent phase-preserving amplitude regenerating device is also introduced to achieve full-quadrature regeneration of signal without phase conjugation and frequency conversion. In addition to the theoretical analysis of the implementation principle, a series of simulation results' analysis, including comparison with traditional scheme and regeneration performance assessment, is also included. The results show that undesired harmonics are greatly reduced by nearly 40 dB, and high GERs are available independent of input power. What is more, various levels of phase and amplitude noise can be effectively compressed for 10-Gbd QPSK signal. The regeneration system also reduces the signal bit error rate and gets 2-dB receiver optical signal noise ratio (OSNR) improvements at 13-dB OSNR input, which has potential applications in enhancing transmission distance in the future optical communication network.

Nonlinearity compensation is considered as a key enabler to increase channel transmission rates in the installed optical communication systems. Recently, data-driven approaches – motivated by modern machine learning techniques – have been proposed for optical communications in place of traditional model-based counterparts. In particular, the application of neural networks (NN) allows improving the performance of complex modern fiber-optic systems without relying on any a priori knowledge of their specific parameters. In this work, we introduce a novel design of complex-valued NN for optical systems and examine its performance in standard single mode fiber (SSMF) and large effective-area fiber (LEAF) links operating in relatively high nonlinear regime. First, we present a methodology to design a new type of NN based on the assumption that the channel model is more accurate in the nonlinear regime. Second, we implement a Bayesian optimizer to jointly adapt the size of the NN and its number of input taps depending on the different fiber properties and total length. Finally, the proposed NN is numerically and experimentally validated showing an improvement of 1.7 dB in the linear regime, 2.04 dB at the optimal optical power and 2.61 at the max available power on Q-factor when transmitting a WDM 30 × 200G DP-16QAM signal over a 612 km SSMF legacy link. The results highlight that the NN is able to mitigate not only part of the nonlinear impairments caused by optical fiber propagation but also imperfections resulting from using low-cost legacy transceiver components, such as digital-to-analog converter (DAC) and Mach-Zehnder modulator. [ABSTRACT FROM AUTHOR]

This paper proposes a full-duplex Radio-over-Fiber-over-Space (RoFoS) transceiver’s architecture with mixerless frequency conversions. The polarization splitting of the optical carrier into two orthogonal polarizations provides away to carry both downlink (DL) and uplink (UL) optical signals in one optical channel. In this work, the UL and DL signals are chosen to provide proof-of-implementation for the 5G fronthaul networks. In the baseband unit (BBU), the DL signal at the intermediate frequency (IF) 2 GHz is sent through RoFoS to the remote radio head (RRH) and broadcasted at 19 GHz, without the need for any mixing stage. In the UL path, at the RRH, the UL signal with an interferer (I) and added noise are received at 4 GHz then optically modulated and sent through the same optical channel to the BBU to be compensated for any distortion or hardware impairments. Digital compensation polynomial based models are used to reduce the distortion generated from the optical, electrical, and electro-optical components in the RoFoS transceiver. The error vector magnitude (EVM) and the signal to noise ratio (SNR) are calculated and compared to the 5G/NR standards test requirements. [ABSTRACT FROM AUTHOR]

Nonlinear distortion and chromatic dispersion are two long-standing limiting factors of radio-over-fiber (ROF) links. Here, we proposed an ROF link with both the two problems mitigated simultaneously. Furthermore, a tunable optical carrier-to-sideband ratio (CSR) can also be implemented in this simple system to improve the gain and the noise figure (NF) of the link. All these three factors are optimized in a simple ROF link which shows increased performance of the system. The scheme is based on a single dual-polarization Mach–Zehnder modulator (DPol-MZM). One bias voltage of the Dpol-MZM is employed to mitigate the nonlinear distortion. The polarization direction and the phase relationship of the modulated polarization multiplexed signal are used to adjust the CSR and suppress the dispersion, respectively, by adjusting a polarization controller (PC). In the experiment, 3–30-GHz RF signal is transmitted without power fading. A two-tone signal with the frequencies of 12.5 and 12.505 GHz is employed to verify the optimization of three main parameters of the ROF link [link gain, NF, and spurious-free dynamic range (SFDR)] where the link gain, NF, and SFDR are improved by 11.9, 5.8, and 14.5 dB, respectively, compared with conventional quadrature biased link based on one Mach–Zehnder modulator for 50-km single-mode fiber (SMF) transmission. Meanwhile, 100-Mbaud 16 quadrature amplitude modulation (QAM) signals centered at 12.5 and 14.7 GHz are used to verify the transmission performance. After dispersion compensation, nonlinear optimization, and CSR optimization, error vector magnitude (EVM) is improved from 17.03% to 2.97%. [ABSTRACT FROM AUTHOR]

Using the concept of a "THz-wireless fiber extender," we can combine the flexibility of wireless networks with the high capacity of fiber optic communication. The availability of a large, contiguous bandwidth in the frequency band around 300 GHz creates the opportunity to seamlessly interconnect coherent THz-wireless and fiber optic transceiver frontends using a transparent, analog baseband interface. In this article, we discuss this concept in more detail and report on the recent demonstration of a real-time, short-range THz-wireless fiber extender with 100 Gb/s net capacity. This combined fiber optic/THz-wireless transmission system is operated by a high-speed fiber optic real-time modem, which is capable of compensating the channel impairments of both the optical and THz-wireless links. In addition, we discuss the potential of THz-wireless links to achieve long-range transmission distances by reporting on the operation of a 500-m-long line-of-sight THz-wireless outdoor link in Berlin, Germany. We analyze the effect of weather conditions on the transmission performance and determine the maximum physical layer net data rate of the system by means of various modulation formats and symbol rates. Finally, we summarize all of our recent high-capacity experiments using THz-wireless transmission, including a field trial with a 1-km-long link, and compare our results to theoretical limits and achieved data rates in the laboratory. [ABSTRACT FROM AUTHOR]

Pulse amplitude modulation (PAM) is a widely employed digital modulation format. PAM formats are generally classified as bipolar PAM (BPAM) if using both positive and negative amplitude levels, and unipolar PAM (UPAM) if using only non-negative amplitude levels. While BPAM formats are in principle more energy efficient, they are not compatible with conventional direct detection (DD) schemes, so that UPAM formats are usually preferred for short-reach optical communications. In this work, we propose a novel DD scheme that employs oversampling at two samples per symbol to extract both the amplitude and phase information from the received optical signal, enabling the detection of BPAM signals. The proposed scheme uses a single photodetector, has no special requirements in terms of bandwidth, and requires only some minimal additional processing compared to a conventional scheme. A theoretical analysis, confirmed by numerical simulations, shows that, in the presence of optical amplifier noise, the proposed BPAM/DD transmission technique provides large optical signal-to-noise ratio  gains compared to a more conventional UPAM/DD transmission and a good tolerance to group velocity dispersion and non-ideal filtering. [ABSTRACT FROM AUTHOR]

An analog photonic link (APL) based on a dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) is proposed and experimentally demonstrated. The proposed APL can suppress the nonlinearity of modulator, compensate the power fading induced by dispersion (PFID), and improve the power efficiency of link, leading to large dynamic range. The PFID and the nonlinearity are suppressed by adjusting the phase of optical carrier (OC) and polarization controller (PC), respectively. In addition, by optimizing the optical carrier to sideband ratio (OCSR) in the DP-DPMZM, the output RF power and the power efficiency of link can be significantly improved while the total optical power received by the photodetector (PD) remains constant. An experiments are performed to demonstrate the feasibility of the APL. The results show that an spurious free dynamic range (SFDR) of 103.2 dB·Hz4/5 for 13.73 GHz is achieved after compensating PFID and nonlinearity, which is 32 dB higher than that of a conventional double sideband (DSB) assisted APL. After OCSR optimizing, the output RF power achieves 13-dB improvement, and the SFDR is further raised to 114.1 dB·Hz4/5. [ABSTRACT FROM AUTHOR]

Polarization topological charge (PTC) of a vector beam can be defined as the repetition number of polarization state change along the azimuthal axis, with its sign standing for the rotating direction of the polarization. In this paper, we present a design for PTC sorting of vector beams with polarization conjugated-phase component (PCC). The components, made of birefringent crystals, can induce conjugated spatial phase modulations for the orthogonal polarizations. Our simulations show, using a pair of PCC with specific design in a 4-f optical system, the system focuses vector beam modes on the locations representing the PTC of corresponding mode, and sorts PTC with separation efficiency as high as 77%. Our designs have the potential applications in modal characterization, communication, encryption, and so on.

Optical communications networks include basic components such as EDFAs and WSSs that present a polarization gain imbalance. The gain imbalance at the component output depends on the incident state-of-polarization. This well-known phenomenon called Polarization Dependent Loss (PDL) is revisited in this article and systematically analyzed within the framework of communications theory and practical achievable information rates. The rate impairment induced by PDL is discussed on an optical channel with distributed PDL and on a simplified equivalent lumped PDL channel. We have previously designed the New Spatially Balanced (NSB) signaling, an optimal 4D modulation based on this simplified model. We explore in this article its performance through numerical simulations and experimentally over a transmission in a recirculating loop with in-line PDL. Experimental measurements confirm an accurate channel modeling and significant information rate gains brought by NSB that results in SNR gains of the order of 0.4 dB at a practical transmission outage. [ABSTRACT FROM AUTHOR]

Optical performance monitoring (OPM) plays an essential role in the intelligent optical fiber communication networks, which perform dynamic network planning and service deployment based on the knowledge of the real-time conditions of the optical layer. Common parameters estimated by OPM include optical power, optical signal-to-noise ratio, chromatic dispersion, polarization mode dispersion, and so on. To be cost-effective and integrated, a universal OPM for multiple parameters realized on a single hardware platform is highly desirable. Although the sources of distinct impairments are independent, the induced distortions on the signal are mixed together, thus challenging the realization of universal OPM. In this article, the existing OPM methods are reviewed with a focus on those based on spectral analysis. A viable solution of universal OPM based on a double-homodyne scheme with all low-cost components is presented. [ABSTRACT FROM AUTHOR]

Coherent optical links improve spectral efficiency over their intensity-modulation direct-detect (IM-DD) counterparts using advanced modulation schemes such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), and by utilizing dual polarization (DP) of light. This increase in spectral efficiency also leads to stringent requirements for the link components. The transmitter (TX) must simultaneously achieve high bandwidth (BW), linearity, output swing, and reliability. In this article, we present a silicon-photonic Mach–Zehnder modulator-based optical TX on an Silicon-on-Insulator (SOI) process, and linear, high-swing SiGe drivers on 130-nm BiCMOS process. The output stage of the driver uses a voltage breakdown enhancement technique to ensure the reliability of the TX. A resistor-based capacitor splitting technique is introduced, and aided by other methods such as zero-peaking and degeneration, the targeted gain, BW, swing, and linearity for the driver are realized. The driver achieves a differential swing of 6 V peak-to-peak, a total harmonic distortion (THD) of 3.6%, and more than 40 GHz of electrical BW. Co-designed and co-packaged with the silicon-photonic modulators, the TX achieves 272 Gb/s/wavelength with DP-16 QAM at 6-V peak-to-peak driver swing and exceeds 0.5 Tb/s/wavelength data rates with DP-16 QAM at 2.4-V peak-to-peak driver swing. The low-cost, compact, and all-Si/SiGe design matches the required optical SNR performance of LiNbO3 modulators with III–V drivers at 34 Gbaud. [ABSTRACT FROM AUTHOR]

As a fundamental issue, determining the proportions of spatial modes in multimode fibers has been continuously discussed over the past few years. People have successfully measured the components of spatial modes with different orders but have rarely addressed the measurement of degenerated modes under these spatial modes. In this article, we propose an interference method to measure the proportions of these degenerated modes. This method is based on the discussion of coherent superposition of degenerated modes in multimode fibers. In the experiment, we randomly generate spatial states in two-mode fibers and simulate these phenomena according to the proposed method, which agrees well with the experiment. This measuring method improves the accuracy of mode measurement from mode groups to degenerated modes and may provide a unique perspective for the field of multimode fibers. [ABSTRACT FROM AUTHOR]

We propose a novel implementation of a complex analog equalization filter for the compensation of frequency-dependent variations in coherent optical links. The analog compensation filter can be used in coherent-lite optical communication links where digital signal processing (DSP) is removed to limit the complexity and power consumption. In these links, the filter can compensate for electrical bandwidth limitations and distortion introduced by chromatic dispersion in the fiber. The complex filter is implemented by combining four distributed analog finite-impulse response (FIR) filters to obtain the necessary response. The filter delays are implemented using active delay cell structures to create a compact solution. The analog filter is implemented in a 55-nm BiCMOS technology and consumes 185-mW core power for five complex filter taps. Performance is evaluated using the S-parameter measurements, noise and linearity measurements, and real-time system experiments using 112-Gb/s 16-QAM-modulated signals. [ABSTRACT FROM AUTHOR]

Polarization-modulated optical signals can be demodulated via direct detection of Stokes vectors. In this article, we develop a quantum theory of noise in the Stokes vector receiver. Examining the conditions that enable simultaneous measurement of three elements of the Stokes vector, we demonstrate that an optical pre-amplifier in front of the Stokes vector receiver can reduce the power penalty associated with simultaneous measurements, down to the quantum limit of 3 dB. Then, the 3-D Gaussian noise model for the shot noise and optical pre-amplifier noise is discussed. Subsequently, based on the noise analyses, we derive bit error rate formulae for the cubic-lattice polarization modulation format and evaluate the sensitivity of the Stokes vector receiver. [ABSTRACT FROM AUTHOR]

This article presents simultaneous data and power transmission systems using a double-clad fiber (DCF). In future radio-over-fiber (RoF) networks, a large number of remote antenna units (RAUs) will be required to provide various kinds of mobile communication services. Power-over-fiber (PWoF), which delivers electrical power to drive the RAUs in optical fibers, is an attractive technique that offers cost-effective installation, operation, and maintenance of RAUs, and achieves the power savings across the entire RoF networks. In particular, the use of double-clad fibers (DCFs), which consist of a single-mode (SM) core and an inner cladding that surrounds the SM core, are useful for much higher power transmission than conventional PWoF techniques. Along the DCF link, optical data signals are transmitted into the SM core, whereas high-power feed light for optical powering is transmitted into the inner cladding, which has a core area that is approximately 240 times larger core area than that of conventional SM cores. In this article, we experimentally demonstrate a PWoF feed with up to 150-W of power using a 1-km DCF. To show the feasibility of the PWoF system, we investigate the bend performance and temperature characteristics of the DCF link. We also evaluate data and power transmission performance under the 150-W PWoF feed in the DCF link. [ABSTRACT FROM AUTHOR]

Motivated by the recent interest in single-mode semiconductor optical amplifiers and multimode erbium-doped fiber amplifiers, we present a unified, comprehensive treatment of the effect of polarization- and mode-dependent gain (PDG and MDG) on the capacity of ultra-long-haul transmission systems. We study the problem using simulations of a multisection model, including the effects of PDG or MDG and polarization mode dispersion (PMD) or modal dispersion. We also analytically derive exact expressions for the capacity distribution of PDG-impaired single-mode systems. In agreement with previous work, we find that PDG and MDG cause fluctuations in capacity, which reduces average capacity and may cause outage. We show that the multimode systems studied, with at least $D = 14$ spatial/polarization modes, have sufficient modal diversity and frequency diversity to strongly suppress capacity fluctuations and reduce outage probability so that the outage capacity approaches the average capacity. We show that single-mode systems, by contrast, inherently provide low modal and frequency diversity, making them more prone to outage. To alleviate this problem, frequency diversity can be increased by artificially inserting PMD. Finally, we quantify the PDG/MDG requirements of optical amplifiers to ensure that the average capacity is close (within a 1-2 dB effective SNR loss) to the theoretical optimum. We show that these PDG/MDG requirements are stringent, especially considering the minimum-mean-square error linear equalizers implemented in typical multiple-input multiple-output receivers. [ABSTRACT FROM AUTHOR]

As coherent reception technology continues to move downstream the optical telecommunication infrastructure, the complexity of the involved transceiver technology can quickly introduce a techno-economic roadblock. Under this umbrella, we experimentally demonstrate a conceptually simple, single-polarization and analogue coherent homodyne receiver that builds on no more than an optically locked externally modulated laser. We evaluate this coherent homodyne receiver in the context of analogue radio-over-fiber transmission – a demanding application setting, where a small degradation in signal integrity is leading to large reception penalties. We conduct a continuous-mode characterization of the locking methodology, which enables homodyne detection and hence the transparent translation of electrical signals from the optical to the electrical domain during the coherent reception process. Furthermore, the locking dynamics are being investigated for packet-level reception at a 1 MHz frame rate and two time division multiplexed channels, which are sourced by two optical emitters with free-running laser sources. The radio-over-fiber transmission performance is evaluated for 64-ary quadrature amplitude modulated, orthogonal frequency division multiplexed radio with a short guard interval of 2.7 μs between the packet radio signals. A data rate of 0.5 Gb/s over 100 MHz radio bandwidth is obtained at an optical loss budget of >35 dB between transmitter and receiver, without resorting to digital signal processing resources for the purpose of signal recovery. Moreover, a small ∼0.3% penalty in error vector magnitude between continuous- and packet-mode confirms the compatibility of the analogue coherent receiver in networks with fast locking requirements. [ABSTRACT FROM AUTHOR]

Graphene and hBN as two-dimensional materials with excellent optical properties can be combined with other materials. In this paper, graphene and hBN is sequentially deposited on the surface of the silicon-core fiber to serve as transverse magnetic (TM) or transverse electric (TE) mode polarizer alternatively depending on the input wavelength and the diameter of the silicon-core. The Gr/hBN stack structure on the surface silicon-core microfiber is used not only for enlarging the wavelength bandwidth, such as from 350 nm (TM) and 670 nm (TE) for double Gr/hBN stack to 300 nm (TM) and 550 nm (TE) for single Gr/hBN stack, but also enhancing the light-graphene interaction to multiply the polarization extinction ratio, as compared with that of monolayer graphene. Small radius silicon core could provide large light-graphene interaction for flat extinction ratio with different thickness of the hBN. [ABSTRACT FROM AUTHOR]