Polarization-multiplexed Research Articles

Mode Coupling in Mode Division Multiplexing techniques for futuristic high speed optical networks and exploring optical fiber parameters to control mode coupling

Fiber optic communications are inevitable to achieve higher data rates of modern telecom networks. After utilization of Wavelength division multiplexing, higher order modulations and polarization multiplexing, mode division multiplexing is a new dimension to achieve higher transmission capacity for optical fiber communication links. Different spatial distributions of optical energy along cross sectional area of optical fiber allows simultaneous transmission of data by considering each mode as an independent channel. During such simultaneous transmissions, possibility of mixing of signals amongst modes causes signal degradations and acts as limiting factor for bandwidth – distance product of the link. This effect of mode coupling has been explored in this article by presenting its mathematical formulations. A simulation has been performed to study the impact of fiber constructional parameters on mode coupling using optical wavelengths used for telecommunication systems. The observations help to develop fibers for reduced mode coupling for particular group of modes and operating wavelengths. This article paves the way forward for study of mode coupling in micro and macro bending conditions for forthcoming research endeavours.

Single-short-cavity dual-comb fiber laser with over 120 kHz repetition rate difference based on polarization multiplexing.

An all-fiber single-short-cavity dual-comb laser with a high repetition rate of up to 500 MHz and a high repetition rate difference of over 120 kHz was demonstrated. The laser setup exploits high birefringence of a polarization-maintaining gain fiber to generate asynchronous combs based on the polarization-multiplexing method. By adopting short-linear-cavity and all-birefringent configuration, a repetition rate difference several orders of magnitude larger than that of a previous work was achieved. The soliton dual-comb showed good mutual coherence and stability, which reveals the potential to enhance the acquisition rate and accuracy of dual-comb measurement systems.

Analysis of System Capacity and Spectral Efficiency of Fixed-Grid Network

In this article, the performance of a fixed grid network is examined for various modulation formats to estimate the system's capacity and spectral efficiency. The optical In-phase Quadrature Modulator (IQM) structure is used to build a fixed grid network modulation, and the homodyne detection approach is used for the receiver. Data multiplexing is accomplished using the Polarization Division Multiplexed (PDM) technology. 100 Gbps, 150 Gbps, and 200 Gbps data rates are transmitted under these circumstances utilizing various modulation formats. Various pre-processing and signal recovery steps are explained by using modern digital signal processing systems. The achieved spectrum efficiencies for PM-QPSK, PM-8 QAM, and PM-16 QAM, respectively, were 2, 3, and 4 (bits/s)/Hz. Different modulation like PM-QPSK, PM-8-QAM, and PM-16-QAM each has system capacities of 8-9, 12-13.5, and 16-18 Tbps and it reaches transmission distances of 3000, 1300, and 700 kilometers with acceptable Bit Error Rate (BER≤ 2× 10-3 ) respectively. Peak optical power for received signal detection and full width at half maximum is noted for the different modulations under a fixed grind network.

Joint timing recovery and adaptive equalization based on training sequences for PM-16QAM faster-than-Nyquist WDM systems.

Since the timing error detectors sensitivity (TEDS) of the timing recovery algorithm is close to zero under the singularity condition of azimuth θ about ±π/4 and differential group delay (DGD) about n × 1/2T (n is an odd number, T is the symbol period), it makes the squared Gardner phase detector (SGPD) timing recovery algorithm fail to achieve timing synchronization. What's worse, in the faster-than-Nyquist wavelength division multiplexing (FTN-WDM) systems, the tight filtering introduces inter-symbol-interference (ISI) so severe that the convergence cost of the SGPD timing recovery algorithm is extremely large even under the non-singularity condition. This paper proposes a joint timing recovery and adaptive equalization scheme for FTN coherent systems based on training sequences that could calculate channel matrix and indicate polarization characteristics, thereby avoiding the influence of azimuth on adaptive equalization and polarization demultiplexing (AEPD) embedded in the timing recovery feedback loop. Since embedded AEPD could compensate for most of DGD, the TEDS could be restored and timing synchronization could be achieved under the above adverse conditions. Thanks to the innovative scheme, which equalizes ISI and DGD during the feedback process of the loop, the convergence cost of timing recovery could be reduced with similar computational complexity compared with the conventional one. The simulation results of 128 GBaud polarization multiplexing (PM) 16-quadrature amplitude modulation (QAM) FTN-WDM transmission systems demonstrate that the proposed scheme could stably achieve timing synchronization under the singularity condition. And compared with the conventional scheme, the convergence cost is reduced by at least 42% @ 0.9 acceleration factor. In addition, 40 GBaud PM-16QAM FTN experiment results show that the proposed scheme could not only achieve timing synchronization stably but also exhibit an optical signal-to-noise ratio tolerance gain of 0.8 dB compared with the conventional scheme under 800 km transmission.

Multi-Aperture Coherent Digital Combining Based on Complex-Valued MIMO 2N × 2 Adaptive Equalizer for FSO Communication

Collecting light using multiple small apertures offers a lot of advantages over doing so using a single large aperture, including the effective mitigation of scintillation under strong turbulence, low implementation cost, and ease of scalability. However, the tricky of a multi-aperture system is to combine the multiple branches signals with static skew mismatch and with time-varying characteristics of gain, phase and state of polarization, especially for coherent digital combining of polarization multiplexing (PM) signals. In this paper, a complex-valued multiple-input multiple-output (MIMO) 2N×2 adaptive equalizer is designed to combine multi-aperture signals, and the tap coefficient of each branch is calculated based on the constant modulus algorithm (CMA). This technique relaxes hardware constraints on the time alignment of multi-aperture branch since the skew can be compensated electronically thanks to the proposed blind digital combining algorithm. In addition, laser linewidth within evaluation range smaller than 10 MHz has no effect on the performance of 2N×2 adaptive equalizer. Moreover, the feasibility of the proposed adaptive equalizer for moderate and strong turbulence suppression is verified by a 10-Gbps data rate PM-QPSK modulation offline two- and four-aperture simulated turbulence experiment.

Sagnac Loop Based Broadband Self-Interference Cancellation and Down-Conversion for Distributed Antenna Systems

Focusing on distributed antenna systems, we propose a photonics-assisted self-interference cancellation and down-conversion method based on a Sagnac loop structure. Two inversely cascaded modulators form a Sagnac loop through polarization multiplexing. To support in-band full duplex (IBFD) communication, a double parallel Mach–Zehnder modulator (DPMZM) operated in a clockwise direction is used as a self-interference canceller (SIC) to achieve the phase conversion in the optical domain. Another MZM modulator working in the counterclockwise direction is used to modulate the local oscillation (LO) signal for frequency down-conversion. The adjustment of bias voltage is analyzed to optimize the conversion efficiency of the system. The power fading effect generated by fiber dispersion is compensated by a polarization-sensitive phase modulator (PS-PM). The experimental results show that the average cancellation depth is 56.4 dB at a single frequency in the operating frequency range of 6–22 GHz, and 29.1 dB and 23.7 dB for 20 MHz and 100 MHz bandwidth signals, respectively. In addition, the error vector map is improved from 16.50% to 6.26% after dispersion compensation. The self-interference cancellation, down-conversion and dispersion compensation are integrated in a microwave photonics link. Two cascaded modulators are equivalent to a parallel structure by the Sagnac loop, simultaneously ensuring high LO isolation and conversion efficiency. The proposed method provides an alternative for IBFD-based distributed antenna communication applications.

Polarization Multiplexing Based UOWC Systems Under Bubble Turbulence

High-speed and turbulence-tolerant transmission links are required to meet the growing demand for underwater optical wireless communication (UOWC). However, most previous works were implemented in statically simulated underwater channels with uniform temperature and salinity gradients irrespective of bubble motion. This paper presents an experimental demonstration of a polarization multiplexing (PolMux) based UOWC system considering bubble-induced turbulence. A theoretical transmission model is built and experiments are carried out to investigate the impact of bubble turbulence on PolMux transmission. The general rules of bubble impairments are summarized by manipulating different bubble flow rates, positions and bubble-effected region thicknesses. Moreover, to mitigate these effects, subchannel pairwise coding is implemented to address the subchannel signal-to-noise-ratio (SNR) imbalance issue, and subcarrier pairwise coding is utilized to overcome the subcarrier SNR imbalance, reaching a sum data rate of 8.58 Gbps. This work evaluates the performance of PolMux-UOWC links in the presence of air bubbles and demonstrates the feasibility of pairwise coding to achieve high-speed transmission in UOWC systems with bubble turbulence.

Optical Transport Architecture for the Fronthaul Segment in a 5G NR Network based on Polarization and Wavelength Multiplexing

This paper presents the design of an optical transport architecture for the fronthaul segment in a 5G New Radio (NR) network based on both polarization and wavelength multiplexing using Polarization Splitters (PS) and an Arrayed Waveguide Grating (AWG), respectively. The proposed architecture is assessed by the transport of Orthogonal Frequency Division Multiplexing (OFDM) services, which allows the definition of different numerologies and service profiles; those are key aspects in the framework of 5G RN. The proposed design shows a flexible and scalable management of numerologies per polarized light component, featuring a suitable response in terms of Bit Error Rate (BER) and Error Vector Magnitude (EVM) measurements for 15 kHz, 30 kHz, and 60kHz subcarrier spacing of an OFDM based on 4QAM modulation format.

Low-complexity algorithms for coherent optical systems with transceiver IQ imbalance.

The next-generation coherent optical communication relies on higher symbol transmission rates with higher modulation cardinality. The imbalance between the in-phase (I) and the quadrature (Q) arms in such systems leads to deteriorated performance if not corrected. In this work, we propose a reference constellation adaptation-based single-tap, novel carrier phase recovery algorithm, tolerant to the transmitter IQ imbalance and a pilot-free geometric parameter extraction-based receiver IQ imbalance correction method for MQAM signals. The proposed scheme is numerically investigated through extensive simulations for 32 GBaud polarization multiplexed (PM)-16QAM, and 64QAM modulation formats and experimentally verified for 32 GBaud PM-16QAM signal. The proposed pilot-free scheme is applicable to any modulation format and offers lower computational complexity than traditional algorithms.

A polarization-dependent near-infrared metamaterial absorber based on the hybrid halide perovskite in two orthogonal directions

A wide-angle and polarization-sensitive metamaterial absorber is highly desired for near-infrared optical applications, such as stealth coatings, photovoltaic materials, and detectors. Although current near-infrared metamaterial absorbers achieve high absorption rates, they are typically either polarization-independent or only sensitive to one direction. Herein, a (CH3NH3)PbBr3-based perfect absorber with high absorption in both orthogonal directions and polarization sensitivity has been designed. The maximum absorption in the transverse magnetic (TM)-polarized light exceeds 99.6% at 800–1000 nm. The maximum absorption in the transverse electric (TE)-polarized light exceeds 98.9% at 1500–1800 nm, and the peak absorption is more than 84.3% at 1800–2200 nm using finite-difference time-domain methods. By modifying various structural parameters, the absorption of the absorber could be separately shifted in a wide wavelength region. In addition, the designed absorber maintains good performance at large angles of incidence. This structure can be applied to other hybrid perovskite materials by simply changing the structural parameters. These results present a huge potential for applications based on metamaterial absorbers, such as infrared polarization imaging, selective spectral detection, near-infrared sensing, and polarization multiplexing.

Nonlinear dynamics of the dual-loop optoelectronic oscillator based on polarization multiplexing

This paper focuses on the temporal dynamics of the dual-loop optoelectronic oscillator (OEO) based on polarization multiplexing. The delay-differential equation (DDE) for the dual-loop OEO system is presented for the first time. After employing microwave envelope method to simplify the DDE, the envelope signal equation describing the system dynamics is obtained. Stability analysis of the static solution of the envelope signal equation is also performed, and the gain thresholds for the system to exhibit various dynamic behaviours are calculated. Moreover, the proposed nonlinear dynamics approach reveals the stability of microwave envelope in detail as a function of the feedback gain and coupling coefficient, which indicates that there is not monotonously incremental relationship between envelope amplitude of the ultrapure microwave and the feedback gain. Finally, the experimental measurements confirm the results obtained from the theoretical and numerical analysis.

Exact Noise Statistics in Equalized Polarization Multiplexing Systems in the Presence of Polarization Dependent Loss

In polarization multiplexing systems, signal and noise have different correlation matrices in the presence of polarization dependent loss (PDL). Therefore, the noise will be correlated after the signal equalization process, and the statistical characteristics of the noise correlation matrix is vital for the system capacity evaluation. In this paper, a stochastic differential equation (SDE) for the evolution of the noise correlation matrix after equalization is derived and the statistics of the correlation matrix elements are obtained by the related ordinary differential equations (ODEs). A threshold of the link gain, which equals half of the local PDL vector variance, exists. If the gain is above the threshold, one may ensure the average accumulated noise power not to grow exponentially. Monte Carlo simulations are conducted to verify the proposed theory and an excellent agreement is observed.

Spectrally Efficient PDM-Twin-SSB Direct-Detection THz System Without Active Polarization Control

This Letter proposed a spectrally efficient polarization division multiplexed (PDM) twin single-sideband (twin-SSB) transmission scheme for direct-detection THz communication systems. The optical carrier of the twin-SSB is added at the optical THz converter, rather than being generated at the optical transmitter as in traditional SSB scheme. Hence the direct detection of each polarization branch of the PDM-twin-SSB signals at the THz receiver can be achieved, without active polarization control. Moreover, the SSB field recovery enabled by Kramers–Kronig algorithm can effectively eliminate the signal-to-signal beating interferences and contributes to better polarization de-multiplexing. The feasibility of the proposed scheme is verified with 5.75-GBd 16-quadrature amplitude modulation PDM-twin-SSB signals transmission over the 300-GHz direct-detection THz communication system by simulation. This scheme can not only significantly improve the spectral efficiency of direct-detection THz communication systems, but also effectively enhance the system’s operability and robustness.

Physical layer encryption for coherent PDM system based on polarization perturbations using a digital optical polarization scrambler.

In this paper, a security enhanced physical layer encryption scheme is proposed for coherent optical polarization division multiplexing (PDM) systems. The concept of a digital optical polarization scrambler (DOPS) is introduced to apply high speed rotation of state of polarization (RSOP) to the transmitted signal, which enables encryption based on polarization perturbations and offers superior flexibility in polarization management. By utilizing different combinations of digital polarization device matrices and adjusting their key parameters, four encryption modes are designed. The proposed encryption scheme is successfully implemented in a PDM-QPSK system at the data rate of 32 Gbps. Experimental results demonstrate that authorized users can successfully decrypt the received signal, while the eavesdroppers cannot derive useful information with a bit error rate (BER) at approximately 0.5. To enhance system security, a 5-D chaotic system is introduced with superior properties of high sensitivity to initial values and improved uniform distribution, which guarantees the large entropy and further the system's security. This scheme can effectively prevent against brute attacks with the expanded key space of 1060.

Spatial and Polarization Division Multiplexing Harnessing On‐Chip Optical Beam Forming

AbstractOn‐chip spatial and polarization multiplexing has emerged as a powerful strategy to boost the data transmission capacity of integrated optical transceivers. State‐of‐the‐art multiplexers require accurate control of the relative phase or the spatial distribution among different guided optical modes, seriously compromising the optical transmission bandwidth and performance of the devices. To overcome this limitation, a new approach based on the coupling between guided modes in integrated waveguides and optical beams free‐propagating on the chip plane is proposed. The engineering of the evanescent coupling between the guided modes and free‐propagating beams allows spatial and polarization multiplexing with state‐of‐the‐art performance. A two‐polarization multiplexed link and a three‐mode multiplexed link using standard 220‐nm‐thick silicon‐on‐insulator technology have been developed. The two‐polarization link shows a measured −35 dB crosstalk bandwidth of 180 nm, while the three‐mode link exhibits a −20 dB crosstalk bandwidth of 195 nm. These links are used to demonstrate error‐free operation (bit‐error‐rate <10−9) in multiplexing and demultiplexing of two and three non‐return‐to‐zero signals at 40 Gbps each, with power penalties below 0.08 and 1.5 dB for the two‐polarization and three‐mode links, respectively. The approach demonstrated for two polarizations and three modes is transferable to future implementation of more complex multiplexing schemes.

Fast and robust calibration method of liquid-crystal spatial light modulator based on polarization multiplexing.

We propose a fast and robust method for calibrating Spatial Light Modulators (SLMs) based on polarization phase-shifting interferometry. Our method effectively calibrates the SLM by addressing both the static aberration and nonlinear phase response, utilizing specially designed gray images loaded sequentially onto the SLM. Notably, we introduce a novel kinoform that effectively eliminates the influence of tilt phase shift between two shots of the polarization camera. This results in a highly accurate phase aberration map and phase modulation curve with exceptional stability, making it an ideal method to calibrate the SLM with exceptional efficiency and precision in real applications.

Design of OMCG based high capacity hybrid passive optical network (PON) and FSO system employing three SOPs/λ

Abstract This paper presents a cost effective optical multi-carrier generation (OMCG) based 96 Gbps hybrid passive optical network (PON) and free space optical (FSO) communication system. Polarization division multiplexing (PDM) using three different state of polarizations (SOPs) such as 0°, 45° and 90° has been realized and incorporated in the system for tripling the capacity of the proposed hybrid single mode fiber (SMF) and FSO system. Non-return-to-zero (NRZ) modulation format is considered for the cost comparison analysis between conventional PON system, existing PON + FSO system and proposed NRZ modulated three SOPs enabled hybrid PON + FSO system. Results revealed that proposed system provide 24 % cost reduction than conventional PONs and 0.64 % cost reduction as compared to exiting hybrid SMF + FSO PON. Further, effects of different modulation formats such as return-to-zero (RZ), AND compressed spectrum RZ (CSRZ) are investigated in terms of log bit error rate (BER) at different SMF link lengths and fixed FSO range. Our demonstrated hybrid SMF + FSO system can successfully cover the 25 km SMF distance and 3.4 km FSO link range using NRZ showing BER 10−20, RZ 10−6 and CSRZ 10−21.

Polarization Detection of Terahertz Waves using All‐Silicon Metasurfaces with Tightly Focusing Behavior

AbstractThe manipulation of polarization states is reflected in the tailoring of light–matter interactions and has great applications in fundamental science. Nevertheless, the conventional polarization‐separated detection behavior in the terahertz (THz) band is very challenging when applied to visualize the incident polarization state since its measurement requires sophisticated instrumentation. Here, the feasibility of its reconstruction of the full‐Stokes parameter matrix in the THz band is explored by establishing an all‐silicon decoupled metasurface based on the polarization multiplexing encoding technique. The pixelated focal spots gathered in the target plane allow us to employ more elaborate methods to extract the characteristic parameters of the incident polarization states. The resolvability of the THz polarization detection behavior with a single focal spot is further optimized benefiting from the longitudinal polarization component (Ez) generated by the tightly focused beam in the propagation direction. The capability of the Ez‐component in determining the key parameters that compose the polarization ellipse is evaluated by predefining the random incident polarization on a standard Poincaré sphere. Thus, the proposed scheme offers significant advantages in future THz communications, providing opportunities for ultra‐compact, high‐resolution full‐Stokes polarization imaging and multidimensional information processing.

An Analytical Model for Coherent Transmission Performance Estimation After Generic Jones Matrices

In this paper, we propose the extension of a previously presented analytical model for the estimation of the signal-to-noise ratio (SNR) at the output of an adaptive equalizer in polarization multiplexed (PM)-QAM coherent optical systems when transmission is modeled as a generic 2x2 frequency dependent transfer function matrix. We present the model and then we statistically test its accuracy in two possible application environments. Our findings show a remarkable agreement between time-domain simulations and analytical results, with average SNR discrepancies of the order of 0.1 dB. We believe our model can find important applications in next generation physical layer aware network planning tools that need to take into account polarization dependent loss/gain and strong filtering, for instance in ultra high baud rate coherent systems.

4096-QAM OFDM THz-Over-Fiber MIMO Transmission Using Delta-Sigma Modulation

A terahertz (THz)-over-fiber multiple-input multiple-output (MIMO) transmission of 4096-ary quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) supported by 46-GBaud quadrature phase shift keying (QPSK) signal at 320 GHz is experimentally demonstrated in this letter. By employing the polarization division multiplexing (PDM) technique and 1-bit delta-sigma modulation (DSM) quantization, we successfully implement 4.6-GHz 4096-QAM OFDM signal transmission over 20 km of standard single-mode fiber (SSMF) and a 3-m MIMO wireless link, and satisfy the soft-decision forward error correction (SD-FEC) threshold of 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> with 15% overhead, which corresponds to a net rate of 84.4 Gbps for THz-over-fiber transmission. To the best of our knowledge, fusing OFDM and PDM, this is the highest rate achievable for DSM applications in THz communication. This experimental demonstration can provide a promising solution for high-order QAM signals in a fiber-wireless seamless network in future 6G networks.