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In this article, we proposed two schemes for generating a broadband optical frequency comb (OFC) source using dual laser sources. In the first proposed technique, the OFC is generated using a cascaded single-drive Mach-Zehnder modulator (MZM) and dual-drive Mach-Zehnder modulator (DD-MZM). Here, the two CW light sources are modulated by each of the MZM, respectively. The first single-drive MZM generates the subcarriers, while The second DD-MZM carries out intensity modulation on the output spectrum generated by the MZM, ensuring that the power fluctuations of both even and odd sidebands are adjusted to an equal level. The second approach involves replacing the single-drive MZM with a phase modulator (PM). The center frequencies of laser 1 and laser 2 must be carefully set to achieve a non-overlapping or gap-free comb spectrum. The frequency of the RF signal driving the modulators determines the spacing between the laser frequencies. In order to illustrate this aspect, we have utilized three distinct RF signal frequencies: 10 GHz, 16 GHz, and 20 GHz. Furthermore, we have examined the constraints that impact the estimation of the center frequency of laser 2, aiming for a non-overlapping and gap-free comb spectrum. By considering these constraints, we can ensure the successful generation of a uniform and stable comb spectrum with the desired characteristics in our proposed approach. The theoretical and primary analysis of cascaded single-drive MZM and DD-MZM, as well as cascaded PM and DD-MZM, is also investigated, and simulation results confirm the findings. By employing our proposed approach, OFC achieves a significant number of comb lines while exhibiting a power deviation of $\sim $ 2 dB in comparison to single laser configurations.

We integrate a bow-tie antenna on a thin-film lithium niobate (TFLN) phase modulator to form an electromagnetic E-field sensor. The fiber-coupled TFLN chip is probed by a two-tone optical local oscillator generated using an external phase modulator, which after appropriate filtering allows the system to act as a photonic down-converter. The system is compatible with the use of balanced detection, enabling an improved signal-to-noise ratio. We demonstrate the down-conversion of a 28.3 GHz free-space microwave signal to 100 MHz with a sensitivity of 2 mV/m/√Hz. The current sensitivity limitation stems from the relatively high (25 dB) insertion loss of the packaged device, which is not inherent to the design. The technology can be directly scaled to much higher frequencies and is attractive for remote detection of microwave electromagnetic signals.

Modern Active Medical Implantable Devices require communications to transmit information to the outside world or other implantable sub-systems. This can include physiological data, diagnostics, and parameters to optimise the therapeutic protocol. The available options are to use optical, radiofrequency, or ultrasonic communications. However, in all cases, transmission becomes more difficult with deeper transmission through tissue. Challenges include absorption and scattering by tissue, and the need to ensure there are no undesirable heating effects. As such, this paper aims to review research progress in using ultrasound as an alternative for deep tissue communications. We provide an empirical review of the technology and communication protocols that different groups have used, as well as comparing the implications in terms of penetration depth, implant size, and data rate. We conclude that this technique has promise for deeper implants and for intrabody communications between implantable devices (intrabody networks). [ABSTRACT FROM AUTHOR]

The increasing requirement for reliable power demands robust protection with excellent waveform sensors. The presence of nonlinear elements in the modern-day loads brings challenges in sensing the current waveform, which is different from the conventional sinusoids. This research article proposes a novel current waveform sensing philosophy with the help of five numbers of series-connected wire-wound Hall current sensors. Signal conditioning circuits and their mathematical expressions are also discussed here to estimate the current waveform with 9.6% ± 2.4% linearity and 2.1%/A ± 1.3%/A sensitivity. The sensing technology senses the current waveform with a root mean square error (RMSE) value of 0.2. Apart from sensing the typical sinusoidal current waveform, the proposed sensing philosophy also senses the impulse and switching current waveforms with RMSE values 2.7 and 0.65, respectively. These results have a broad scope of improvement by implementing more wire-wound Hall current sensors in series. Digital output features and the optical link between the sensor and receiver offer necessary isolation to the microcontroller against harmful transients and other waveform distortions. The experimental verification and required data analytics confirm the applicability of the sensor with substantial accuracy, sensitivity, and linearity. [ABSTRACT FROM AUTHOR]

Gated InGaAs avalanche photodiodes are often used for synchronous single-photon detection in the near-infrared wavelengths of 1310 and 1550 nm for optical fiber communication. However, one of the main obstacles limiting their application is the difficulty in extracting a weak photon-induced avalanche pulse from background noise. Here, we describe a double-pulse superposition technique to detect the signal, which uses a synchronized pulse to raise the avalanche signal above the discriminating threshold so the avalanche signal can be easily detected. This technique provides a simple idea for a practical gated single-photon detector to sense avalanche signals since the parasitic capacitance signal from an applied external circuit is unavoidable and will always be coupled to the avalanche signal. Under the same measurement conditions, a comparison was made with the conventional capacitive balance technique, and the experimental results showed a high agreement between the effects of the two signal extraction techniques under several nanoseconds scale gating widths, and the double-pulse superposition technique is highly efficient and easy to implement even under complicated background noise. [ABSTRACT FROM AUTHOR]

A fiber-optic hydrophone based on extrinsic Fabry–Perot (F-P) interferometer is proposed. The sensitive element of the hydrophone is an Archimedes spiral metal diaphragm, and the inner surface of the diaphragm and the end face of the optical fiber form the F-P cavity. Due to the grooved surface of the sensitive diaphragm, the hydrophone is minimally affected by hydrostatic pressure. The geometry structure of sensor and finite element method (FEM) modeling is presented. Experiments of underwater acoustic response and sensitivity calibration of the hydrophone are carried out and the experimental phase results are demodulated by the orthogonal phase demodulation method based on birefringent crystals and polarization technology. The experiment results show that the hydrophone has a relatively flat signal-to-noise ratio (SNR) and sensitivity within 1–20 kHz, and the average sensitivity is −172.52 dB (re. 1 rad/ $\mu $ Pa) @ 1–20 kHz. We believe that this type of hydrophone has the potential to be applied in the field of underwater acoustic communication. [ABSTRACT FROM AUTHOR]

As the demand for wireless capacity continues to grow, highly directional wireless communication technologies have the potential to provide massive gains in area spectral efficiency. However, novel challenges arise when considering bidirectional connectivity and multi-cell/multi-user systems with highly directional links. Some of these challenges can be alleviated with the introduction of asymmetric connectivity where the highly directional links are used solely for downlink transmission. As an example, we consider asymmetric links with an optical wireless communication (OWC) downlink and sub-6 GHz RF uplink. More specifically, we consider visible light communication as an instance of OWC, although the presented analysis and validation are applicable to alternative OWC technologies and other simplex downlink transmission technologies. While asymmetric connectivity has been previously demonstrated in scenarios like this, the impact of control-plane asymmetry has not been explored, to our knowledge. In this paper, we first introduce the novel challenges related to local handshaking in wireless networks with control-plane asymmetry. We then develop a theoretical framework for throughput analysis in a network where the sub-6 GHz RF channel is shared between a conventional RF link and an asymmetric RF/OWC link. This analysis is validated via simulation and verified in a testbed system using Mango WARP3 software defined radios and a commercially available RF access point. Finally, we use the derived throughput equations to analyze the impact of various protocol parameters and demonstrate one potential use of the derived equations to evaluate sum throughput in the presence of an unreliable OWC link. [ABSTRACT FROM AUTHOR]

In optical fiber communication, due to the random variation of the environment, the state of polarization (SOP) fluctuates randomly with time leading to distortion and performance degradation. The memory-less SOP fluctuations can be regarded as a two-by-two random unitary matrix. In this paper, for what we believe to be the first time, the capacity of the polarization drift channel under an average power constraint with imperfect channel knowledge is characterized. An achievable information rate (AIR) is derived when imperfect channel knowledge is available and is shown to be highly dependent on the channel estimation technique. It is also shown that a tighter lower bound can be achieved when a unitary estimation of the channel is available. However, the conventional estimation algorithms do not guarantee a unitary channel estimation. Therefore, by considering the unitary constraint of the channel, a data-aided channel estimator based on the Kabsch algorithm is proposed, and its performance is numerically evaluated in terms of AIR. Monte Carlo simulations show that Kabsch outperforms the least-square error algorithm. In particular, with complex, Gaussian inputs and eight pilot symbols per block, Kabsch improves the AIR by 0.20 to 0.30 bits/symbol throughout the range of studied signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

In order to overcome the capacity limitations of current lightwave systems based on the single-mode optical fiber, massively parallel transmission in the spatial domain [space-division multiplexing (SDM)] supported by extended parallelism in the frequency domain (ultrawideband (UWB) systems) must be used. This article reviews key aspects of parallel transmission systems as the only significant capacity scaling option going forward and discusses the various tradeoffs on an architectural level and a hardware integration level. In doing so, this article also serves as an introduction to the more detailed accounts of fiber-optic systems and their future scaling within this Special Issue of the Proceedings of the IEEE. [ABSTRACT FROM AUTHOR]

Optical communication systems form the backbone of today’s communication and information society. Approximately six billion kilometers of optical fiber are installed around the globe today, enough to wrap a string of glass as thin as a human hair around the globe about 150000 times, or 20 times from Earth to Sun and back. Over just a single such strand of optical fiber, cutting-edge optical communication systems are able to carry tens of terabits per second over trans-Pacific distances, taking a mere 50 ms to link North America with South-East Asia. Today’s globally installed base of optical communication transponders is collectively capable to transmit several ten exabits per second of information (1 Ebit = $10^{{18}}$ bits), sending signals over short links between tens of meters and a kilometer within a data center, over tens of kilometers in links connecting datacenters, in cellular wireless fronthaul and backhaul, and in fiber-to-thehome applications, over hundreds of kilometers in metropolitan and regional networks, and over thousands of kilometers in trans-continental and submarine backbones. In short, almost every bit of information we touch or consume today, whether it belongs to an Internet search, to a streamed video, or to a mobile phone call, lives some parts of its life as an infrared photon within a gigantic global fiber-optic communications infrastructure. [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]

Since the 1980s, the single-core optical fiber cable has been the essential data transmission wiring media, with ongoing increases in deployments. Its use case has broadened, with transmission distances ranging from meters to 10000s km, including intrabuilding, fiber-to-the-home (FTTH), terrestrial, and even submarine trunk lines. Currently, optical fiber cable deployment is strongly driven by cloud-based storage, X as a Service (XaaS), and worldwide data center (DC) functionality with the optical interconnection of massive servers on DC campuses. Hence, it is quite natural that the optical fiber cable industry has developed ultrahigh fiber count, high density, and small diameter optical fiber cable, e.g., 6912-fiber-count cable. With high-density cables, massive data capacity can be installed at one time while saving limited space in an underground duct. High-density cable designs have been developed not just by cable structure optimization but through the process of bundling fibers and the use of high-grade optical fibers. Moreover, many enabling technologies have been developed to support the deployment of ultrahigh fiber count cables, such as fiber identification, connectors, mass fusion splicing, and storage systems for fusion splicing points. In this article, state-of-the-art technologies and prospects for ultrahigh-count and ultrahigh-density optical fiber cables and enabling technologies are explained. [ABSTRACT FROM AUTHOR]

Wavelength-division multiplexing (WDM) has historically enabled the increase in the capacity of optical systems by progressively populating the existing optical bandwidth of erbium-doped fiber amplifiers (EDFAs) in the $C$ -band. Nowadays, the number of channels—needed in optical systems—is approaching the maximum capacity of standard $C$ -band EDFAs. As a result, the industry worked on novel approaches, such as the use of multicore fibers, the extension of the available spectrum of the $C$ -band EDFAs, and the development of transmission systems covering $C$ - and $L$ -bands and beyond. In the context of continuous traffic growth, ultrawideband (UWB) WDM transmission systems appear as a promising technology to leverage the bandwidth of already deployed optical fiber infrastructure and sustain the traffic demand for the years to come. Since the pioneering demonstrations of UWB transmission a few years ago, long strides have been taken toward UWB technologies. In this review article, we discuss how the most recent advances in the design and fabrication of enabling devices, such as lasers, amplifiers, optical switches, and modulators, have improved the performance of UWB systems, paving the way to turn research demonstrations into future products. In addition, we also report on the advances in UWB optical fibers, such as the recently introduced nested antiresonant nodeless fibers (NANFs), whose future implementations could potentially provide up to 300-nm-wide bandwidth at less than 0.2 dB/km loss. [ABSTRACT FROM AUTHOR]

Randomly-coupled multi-core fiber (MCF) technology has come to attract lots of attention because of its strong applicability to long-haul transmission systems. Compared with weakly-coupled MCFs with independent cores, it can simultaneously realize higher spatial channel density and ultralow transmission loss using existing ultralow-loss single-mode fiber (SMF) core designs. The strong mode coupling characteristics of randomly-coupled MCFs can provide favorable optical properties, such as suppressed accumulation of modal dispersion (MD), mode-dependent loss (MDL), and nonlinear impairments. This article gives an overview of randomly-coupled MCF technology advancements. First, we describe the classification and design of randomly-coupled MCFs and explain what the randomly-coupled MCFs are and how they are designed. State-of-the-art randomly-coupled MCFs can accommodate four, seven, or 12 cores in a standard 125- $\mu \text{m}$ cladding while achieving ultralow transmission loss and/or small MD, which are very promising for long-haul transmission media. Next, we present the methods to characterize the optical properties of randomly-coupled MCFs and the difference compared to conventional SMF measurements. We also show the low-loss low-MDL connectivity of this type of MCF and the cabling that can suppress MD. A field-deployed randomly-coupled MCF cable testbed is also presented, which confirmed the favorable optical properties of randomly-coupled MCFs after deployment. Then, multi-core amplifier technologies are briefly summarized, and finally, we discuss the performance improvements in the transmissions over randomly-coupled MCFs and suitable application areas. [ABSTRACT FROM AUTHOR]

We propose and test a combined method of suppression of stimulated Brillouin scattering (SBS) in high numerical aperture (NA) optical fibers that makes it possible to use both the application of tensile-compressive stresses and the control of the transverse acoustic refractive index profile. In the experiment on the acoustically tailored Al2O3-GeO2-doped fiber (optically step-index, NA $\sim ~0.2$) with a quasi-four-peak SBS gain spectrum $\sim 0.8$ GHz wide, we used the offset-core technique to create a tensile-compression stress over the whole fiber length, controlled by small-diameter coiling. The stress causes each of the spectral peaks to broaden without overlap. Thus, at the optimal coiling diameter, an 11.3dB net SBS gain suppression has been obtained, where $\sim 5$ dB is due to the tension. Ways to enhance SBS suppression in such fibers up to 18dB and more using the proposed technique are also discussed. [ABSTRACT FROM AUTHOR]

Free space optics (FSO) provides high-capacity optical wireless communications. To maintain an FSO link with a moving node, a beam should be steered toward it. As the FSO link distance gets shorter, it becomes difficult to maintain the link. This is because the beam radius has not largely expanded yet at shorter transmission distances and the received optical power decreases as the receiver moves away from the center of the beam. In this letter, we address an adaptive beam divergence control method to expand the range of the FSO link distance within which the FSO link can be maintained. Theoretical evaluation results show our method expanded the minimum FSO link distance to 5 m in assumed use cases, though it was at least 20 m with a fixed beam divergence. Furthermore, we confirmed the range of the beam divergence angle and the divergence changing interval in our method were feasible. [ABSTRACT FROM AUTHOR]

In this article, architectures for interplanetary communications that feature the use of a data relay are investigated. In the considered "two-leg" architecture, a spacecraft orbiting the Earth, or in orbit at a Lagrange point, receives data from a deep-space probe (leg-1) and relays them toward ground (leg-2). Different wireless technologies for the interplanetary link, namely, radio frequencies above the Ka band and optical frequencies, are considered. Moreover, the cases of transparent and regenerative relaying as well as different different orbital configurations are addressed, offering a thorough analysis of such systems from different viewpoints. Results show that, under certain constraints in terms of pointing accuracy and onboard antenna size, the adoption of a two-leg architecture can achieve the data rates supported by direct space-to-Earth link configurations with remarkably smaller ground station antennas. [ABSTRACT FROM AUTHOR]

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]

Optical intelligent reflecting surface (IRS) is an emerging and low-cost technology that can establish a stable communication route in free space optical (FSO) transmission environment with obstacles. In this work, optical IRS-aided dual-hop mixed FSO and RF system is first proposed for cloud radio access network (C-RAN). Specifically, polar codes are introduced to combat turbulence and building sway induced fading in FSO link, and transmit beamforming (TBF) technique is designed to achieve optimal data-rate for RF link. Supposing that the IRS-aided FSO link is subject to exponentiated Weibull distribution with geometric and misalignment loss, whereas the RF link experiences the Fisher-Snedecor $\mathcal {F}$ composite fading, the analytical closed-form outage probability expression is derived in terms of Meijer G and Lauricella multivariate hypergeometric functions with decode-and-forward strategy. Exact closed-form average bit error rate expressions are obtained when the relay of C-RAN adopts single antenna and multiantenna beamforming techniques. On the basis of moment generating function of end-to-end signal-to-noise ratio, the ergodic capacity is obtained in terms of univariate and multivariate Fox H-functions over independent but not identically distributed Fisher-Snedecor $\mathcal {F}$ fading channels. The correctness of the analytical results is verified through Monte-Carlo simulations. We further provide an asymptotic analysis and discuss the achievable diversity orders. [ABSTRACT FROM AUTHOR]

This article introduces a technique of improving the link performance of the underwater wireless optical communication (UWOC) system in terms of signal-to-noise ratio (SNR) by incorporating a novel ratiometric signal processing (RSP) scheme at the receiver node. In addition, the temporal fluctuations in the attenuation coefficient information (ACI) of the UWOC channel in the nonuniform turbulent regime are analyzed, and the corresponding distribution function is analytically derived. The consistency of the obtained statistical distribution of the ACI parameter is analyzed by simulating the experimental data in a UWOC prototype link within a laboratory-controlled turbulent regime. The experimental outcomes indicate a relative improvement in the SNR by 1.70 to 3.23 dB in the nonuniform turbulent conditions upon incorporating the proposed RSP technique at the receiver node. It is also observed that the mean square error and an average difference of the received images that are processed by incorporating the RSP technique are lower as compared to that of the images without processing at the receiver node in the UWOC prototype link. The approach of implementing the proposed RSP technique is observed to improve the performance of the UWOC link. [ABSTRACT FROM AUTHOR]

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding. [ABSTRACT FROM AUTHOR]

We numerically investigated the behavior of dissipative Kerr solitons (DKSs) in fiber Kerr resonators when random noise fields are suddenly applied to the system. Simulation results based on an iterative computation of an Ikeda-map-like equation system show the first observation of the annihilation and excitation of DKS under the stimulation of the noise with different intensities. Switching among different states of DKSs triggered by high amplitude noise is observed. These results have confirmed the robustness of DKSs against noise perturbations and provide new insights into the DKS dynamics in Kerr resonators, thus opening a new way to achieve different DKS states in fiber Kerr resonators. [ABSTRACT FROM AUTHOR]

We propose a weakly-coupled ultra-flattened chromatic dispersion few-mode photonic crystal fiber (FM-PCF) supporting 10 vector modes (HE11a, HE11b, TM01, HE21a, HE21b, TE01, HE31a, HE31b, EH11a, EH11b) which works over a large range of wavelength across the C+L band. We investigate the chromatic dispersion, minimum effective refractive index difference (minΔneff), confinement loss, bending loss and differential mode delay (DMD) of the proposed fiber under the influences of design parameters by using the finite element method (FEM). The circular holes in the first ring are replaced by elliptical holes to obtain flattened-chromatic dispersion. And an extra small rectangular defected hole is introduced in the fiber core order to achieve ultra-flattened chromatic dispersion and large minΔneff. The simulated results show that the designed weakly-coupled FM-PCF can obtain ultra-flattened (dispersion slope < 1.8 × 10−4 ns/km/nm2) and large minΔneff (> 1.6 × 10−4) in the wavelength range of 1520 nm to 1640 nm. And the proposed weakly-coupled FM-PCF also has ultra-low confinement loss (<10−9dB/km) and good bending resistance. In summary, the proposed weakly-coupled FM-PCF has the potential applications for large-capacity MDM communication systems. [ABSTRACT FROM AUTHOR]

A novel digital signal processing (DSP)-based scheme for physical layer security in coherent optical communication systems is proposed and numerically investigated. The optical layer signal encryption is accomplished by two dispersive elements and one phase modulator (PM) driven by a DSP-generated encryption key, whilst signal decryption uses similar components but with inverted dispersion values and security keys. A critical aspect of the DSP-based physical layer security is that the security keys, driving the PMs to hide/recover the data signals, must be highly unpredictable and noise-like, thus orthogonal frequency division multiplexing (OFDM) signals are employed as they possess these characteristics, they can also be easily generated and cover a suitably wide range of unique keys. Numerical simulations are conducted to determine optimum system parameters for achieving a high level of security, the key parameters requiring optimization are the dispersion of the dispersive elements and the bandwidth of the security keys. Using these determined optimum parameters, in-depth investigations are undertaken of encryption/decryption induced transmission performance penalties, sensitives to various parameter offsets and operation over various transmission distances. To observe any data signal dependencies, various performance metrics are investigated for different combinations of modulation formats (DQPSK and 16QAM) and baud rates (40 Gbaud and 100 Gbaud) for the transmitted data signals. The proposed DSP-based physical layer security scheme is shown to have the potential to achieve, in a low-cost and highly effective manner, a high level of physical layer security with acceptable performance penalties for existing coherent optical communication systems. [ABSTRACT FROM AUTHOR]

A fast polarization tracking and demultiplexing algorithm in polarization-division-multiplexing (PDM) systems is proposed and demonstrated. The scheme utilizes two optimizing parameters in Stokes Space, including the polarization angle and the phase information. In addition to its transparency to modulation formats, the angle-plus-phase algorithm (APA) could provide fast convergence up to >100-Mrad/s polarization tracking speed. Compared to common polarization demultiplexing algorithms such as constant modulus algorithm (CMA) and linear Kalman filter (LKF), experimental results show that the polarization tracking capability of the proposed APA algorithm is significantly improved without sacrificing the transmission performance. Specifically, compared to CMA, the tracking speed can be improved from 9-Mrad/s to 160-Mrad/s for PDM quadrature phase shift keying (PDM-QPSK) signals. Furthermore, compared to CMA and LKF, APA has the best tracking performance, and the tracking speed can be reached to 34-Mrad/s for PDM 16 quadrature amplitude modulation (PDM-16QAM) signal. [ABSTRACT FROM AUTHOR]

In this paper, we model the multi-layer vertical underwater link as a cascaded channel and unify the performance analysis for the underwater optical communication (UWOC) system using generalized Gamma (GG), exponential GG (EGG), exponentiated Weibull (EW), and Gamma-Gamma ($\Gamma \Gamma$) oceanic turbulence models. We derive unified analytical expressions for probability density function (PDF) and cumulative distribution function (CDF) for the signal-to-noise ratios (SNR) considering independent and non-identical (i.ni.d.) turbulent models and zero bore-sight model for pointing errors. We develop performance metrics of the considered UWOC system using outage probability, average bit error rate (BER), and ergodic capacity with asymptotic expressions for outage probability and average BER. We develop the diversity order of the proposed system to provide a better insight into the system performance at a high SNR. We also integrate a terrestrial OWC (TOWC) subjected to the combined effect of generalized Malága atmospheric turbulence, fog-induced random path gain, and pointing errors to communicate with the UWOC link using the fixed-gain amplify-and-forward (AF) relaying. We analyze the performance of the mixed TWOC and multi-layer UWOC system by deriving PDF, CDF, outage probability, and average BER using the bivariate Fox H-function. We use Monte-Carlo simulation results to validate our exact and asymptotic expressions and demonstrate the performance of the considered underwater UWOC system using measurement-based parametric data available for turbulent oceanic channels. [ABSTRACT FROM AUTHOR]

A broadly tunable erbium-doped fiber ring laser is proposed and demonstrated based on the tapered polarization maintaining fiber (PMF) Sagnac interferometer, in which the Sagnac and intermodal interferences are superimposed to generate one localized peak over a wide wavelength range in its transmission spectrum. By heating the tapered PMF, the localized peak may be discretely tuned with the temperature at the step, depending on the intermodal interference. Tunability may become continuous when additional stress is exerted longitudinally on the tapered PMF. The tapered PMF Sagnac interferometer is inserted into the laser cavity to achieve broadly and continuously tunable output. Specifically, by optimizing the PMF length and its tapering section to obtain proper channel intervals of the Sagnac and intermodal interference, the lasing wavelength is continuously tuned over 60 nm covering from 1565.1 nm to 1625.2 nm with sidemode suppression ratio higher than 50 dB and power uniformity better than 1 dB. The tuning responses to temperature and axial stress are both linear with sensitivity of −1.1 nm/°C and −29.4 pm/μϵ. Most importantly, the temperature and axial stress affect the Sagnac and intermodal interferences, respectively, which makes it convenient to set the lasing wavelength beforehand during operation. [ABSTRACT FROM AUTHOR]

This brief presents a power and memory-optimized hardware implementation for the open forward error correction (oFEC) encoder proposed for high-speed fiber optical communications. Instead of storing a large amount of previously encoded data in the memory as suggested in oFEC proposal, we propose an alternative algorithm that requires the knowledge of only a few blocks of preencoded partial parity-check bits with the goal to reduce circuit power consumption. We then present a hardware implementation architecture for both the standard and optimized encoding algorithms and demonstrate that power dissipation and area overhead are reduced multiple times by optimized implementation. [ABSTRACT FROM AUTHOR]

Modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring are essential for elastic optical networks. A method for joint modulation format identification and OSNR monitoring based on ternary neural networks was proposed. Further, a ternary neural network was established, and the constellation images after constant modulus algorithm (CMA) equalization were used as input features. Four commonly used modulation formats were distinguished, including dual-polarization (DP)-QPSK, 8QAM, 16QAM, and 64QAM. A 32G baud simulation system was constructed, and the results were analyzed. First, we investigated the influence of resolution and sample length on the identification accuracy. The values 32 and 7000 were selected for the two parameters, respectively, based on the balance between resource consumption and accuracy. Then, we compared the performance of the binary neural network (BNN), ternary neural network (TNN), and full-precision neural network (FNN). The results indicate that the memory consumption and extraction time of the TNN are similar to those of the BNN, and the accuracy is further improved. Moreover, the robustness of the system was analyzed. The results validate that the proposed method can tolerate fiber nonlinearity (from 500 km to 1500 km). Finally, an experiment was conducted to prove the practicality of the proposed method.

The triple-band (S+C+L) transmission system using only C-band transceivers and all-optical wavelength conversions (AOWCs) without S- and L-band optical transceivers have been proposed. Although an AOWC employs highly nonlinear fibers, it is not applicable to photonic integrated circuits (PICs). For a device with successful wavelength conversion, a silicon-rich nitride (SRN) waveguide provides superior performance, as it is applicable to PIC and facilitates a higher Kerr nonlinearity than conventional stoichiometric silicon nitride waveguides. Thus, in this study, we propose an all-optical simultaneous wavelength conversion method that can be cost-effectively fabricated on a PIC based on four-wave mixing in SRN waveguides. Following an explanation regarding the design of an SRN waveguide for wideband AOWC, the successful operation of a double-stage AOWC of 64-channel 64-Gb/s QPSK signals between the C+L bands and the S-band is demonstrated, which is useful for wideband transmission systems.

Error vector magnitude (EVM) is a metric for assessing the quality of m-ary quadrature amplitude modulation (mQAM) signals. Recently proposed deep learning techniques, e.g., feedforward neural networks (FFNNs) -based EVM estimation scheme leverage fast signal quality monitoring in coherent optical communication systems. Such a scheme estimates EVM from amplitude histograms (AHs) of short signal sequences captured before carrier phase recovery (CPR). In this work, we explore further complexity reduction by proposing a simple linear regression (LR) -based EVM monitoring method. We systematically compare the performance of the proposed method with the FFNN-based scheme and demonstrate its capability to infer EVM from an AH when the modulation format information is known in advance. We perform both simulation and experiment to show that the LR-based EVM estimation method achieves a comparable accuracy as the FFNN-based scheme. The technique can be embedded with modulation format identification modules to provide comprehensive signal information. Therefore, this work paves the way to design a fast-learning scheme with parsimony as a future intelligent OPM enabler.

The demand for information capacity in data center networks has grown exponentially during past decades. Such growth promotes consideration for adoption of coherent transmission technologies in this application space to achieve current and future high data rate requirements at 400G, 800G, and higher. Coherent systems offer higher capacity than direct detection systems using more degrees of freedom to carry information, but with higher system complexity. To meet the cost and power consumption requirements for data center transceivers, coherent-lite technologies have been proposed. Among them, self-homodyne coherent (SHC) is a promising candidate which allows the use of low cost lasers and simplifies digital signal processing (DSP). We study SHC systems and focus on the use of a multicore fiber (MCF) to obtain small channel skew, which allows for further simplification of DSP. We observe and manage stimulated Brillouin scattering (SBS) effects in the local oscillator path. We analyze the optimal power split ratio between the data path and the local oscillator path to maximize data capacity. Moreover, we compare the channel skew and transmission performance between multicore fiber and ribbon fiber. SHC with MCF delivers high data capacity with significant complexity reduction, which may be a promising solution for data center links.

A fast polarization tracking and demultiplexing algorithm in polarization-division-multiplexing (PDM) systems is proposed and demonstrated. The scheme utilizes two optimizing parameters in Stokes Space, including the polarization angle and the phase information. In addition to its transparency to modulation formats, the angle-plus-phase algorithm (APA) could provide fast convergence up to >100-Mrad/s polarization tracking speed. Compared to common polarization demultiplexing algorithms such as constant modulus algorithm (CMA) and linear Kalman filter (LKF), experimental results show that the polarization tracking capability of the proposed APA algorithm is significantly improved without sacrificing the transmission performance. Specifically, compared to CMA, the tracking speed can be improved from 9-Mrad/s to 160-Mrad/s for PDM quadrature phase shift keying (PDM-QPSK) signals. Furthermore, compared to CMA and LKF, APA has the best tracking performance, and the tracking speed can be reached to 34-Mrad/s for PDM 16 quadrature amplitude modulation (PDM-16QAM) signal.

Based on the coupling mode theory and numerical simulation, intramode-group coupling in orbital angular momentum (OAM) fibers with elliptical deformation is analyzed in depth. The elliptical deformation not only introduces a time delay difference between even and odd modes of the same high-order vector mode but also destroys the structure of the high-order vector modes; the structural damage depends on the strength of the elliptical deformation. When the elliptical deformation is relatively small, the structural damage of high-order vector modes can be ignored and coupling can be thought as occurring only between two OAM modes composed of the same high-order vector mode. While the elliptical deformation becomes larger, the structural damage cannot be ignored, coupling appears among four OAM modes in the same OAM mode group, and the coupling between two OAM modes composed of the same high-order vector mode is more serious. The results will pave the way for the establishment of accurate propagation models and the optimization of the OAM fiber design.

High-order modulation formats with high symbol rates are promising candidates for meeting the growing traffic requirements of digital coherent transmission. In using these modulation formats, the impact of transceiver impairment on the signal quality becomes significant. In this paper, we propose an 8×2 widely linear (WL) multiple-input multiple-output (MIMO) equalizer with independent complex filtering on IQ signals to mitigate linear transceiver impairments. Furthermore, we extend the equalizer for digital subcarrier (SC) multiplexing. Accordingly, a 16×4 WL MIMO equalizer is also proposed to mitigate linear transceiver impairments, through application to a pair of SC inputs. Experimental results show that, for transmission of a 58-GBaud polarization-multiplexing 64 quadrature amplitude modulation (PM-64QAM) single-carrier signal, the proposed method effectively compensates for various receiver impairments and also mitigates transmitter (Tx) impairments. Q-penalties of 0.1 dB up to a 10-ps skew, 2.5-dB gain imbalance, 7.2-degree phase error, and –15-dB crosstalk in Tx were achieved for 100-km standard single-mode fiber transmission. Moreover, for a 29-GBaud PM-64QAM dual-SC signal, the proposed 16 × 4 WL MIMO equalizer shows transceiver impairment mitigation performance similar to that in the single-carrier case, even though dual-SC transmission is less tolerant of laser phase noise.

Quantum key distribution (QKD) is an essential branch of quantum communications that facilitates future-proof security. In the near future, large-scale quantum networks will inevitably encounter diverse QKD protocols, ultimately resulting in a multi-protocol quantum network. Motivated by the evolution of advanced QKD protocols, this article introduces a pivotal enabler for multi-protocol quantum networks: software-defined heterogeneous QKD chaining (HQKDC). In contrast to the conventional QKD chain using a single protocol and a single connectivity pattern, a heterogeneous QKD chain is established relying on multiple protocols and diverse connectivity patterns. We elaborate the HQKDC framework by describing the structures of various QKD chains and proposing an architecture of software-defined HQKDC over optical networks. The software defined networking paradigm and optical networks can facilitate the orchestration and deployment of heterogeneous QKD chains, respectively. Furthermore, we present and evaluate the strategies for heterogeneous QKD chain deployment, as well as devise and demonstrate the workflow for heterogeneous QKD chain orchestration. Finally, we discuss open issues toward the practical QKD network and quantum Internet. [ABSTRACT FROM AUTHOR]

Due to the capability of the physics-informed neural network (PINN) to solve complex partial differential equations automatically, it has revolutionized the field of scientific computing. This article studies the applicability of PINN in optical fiber communication and presents five potential solutions for time-domain, frequency-domain, and spatial-domain modeling. By solving the nonlinear Schrödinger equation (NLSE), both forward and backward optical waveform propagation are modeled by PINN; by solving the inverse problem of NLSE, the physical parameters of fiber are identified with limited data, including attenuation, dispersion, and nonlinear coefficient. By solving the stimulated Raman scattering evolution equation, the wideband spectrum evolution is simulated for C+L-band systems. By solving the paraxial Helmholtz equation, the electric field distribution in a fiber is modeled, and five lowest-order fiber modes are studied. In comparison to conventional numerical methods, PINNs achieve the approximated accuracy while having lower computation complexity and less computation time over the same fiber length for a given initial condition. Finally, the challenges faced by PINN are pointed out, and an outlook on future research of the physics-informed approach is concluded. [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]

Underwater optical wireless communications (UOWCs) are promising and potential wireless carriers to envisage underwater phenomenal activities for various applications towards the futuristic 5G and beyond (5GB) wireless systems. The main challenges to deploy underwater applications are the physicochemical properties and strong turbulence channel conditions. In this regard, the end-to-end (E2E) performance analysis of a dual-hop mixed FSO/UVLC system under the intensity modulation/direct detection (IM/DD) technique in consideration of pulse amplitude modulation (PAM) scheme is investigated. Throughout this study, to tackle the issues of moderate-to-strong turbulence channel conditions, this work deploys the Gamma-Gamma (GG) distribution fading model and the links are designed by unifying plane wave models in the corresponding links, respectively. This investigation outperforms higher achievable data rate with minimal delay response and enhance network connectivity in real-time monitoring scenarios as compared with the traditional underwater wireless communication technologies. In more contrast, the probability distribution function (PDF), cumulative distribution function (CDF), and closed-form expression of the system are derived and presented in terms of Meijer-G function as well as Extended Generalized Bivariate Meijer-G Function (EGBMGF). The significant E2E performance metrics are obtained by employing the decode-and-forward (DF) relay protocol in hostile channel conditions. In aggregating this work, we combine the analytical expressions that present an efficient tool to depict the impact of channel parameters on the system. The simulation results are plausible of the system performance metrics as average BER (ABER) and outage probability $(P_{out})$ in the presence of pointing and without pointing error events. Finally, in this work, we use the Monte-Carlo approach for the best fitting curves and validate the numerical expression yields simulation results. [ABSTRACT FROM AUTHOR]

An optical communication link is proposed for high speed point-to-point data transmission. The link incorporates both fiber and optical wireless technologies. Kramer-Kronig coherent detection is used due to its component and implementation simplicity. Spatial multiplexing, via a multi-core fiber followed by a wireless link, and wavelength division multiplexing allow the efficient exploitation of all available degrees of freedom. The link signal-to-noise ratio is estimated when all possible noise sources are taken into account. The impact of link loss and optical amplifier placement on the achievable SNR are studied theoretically. Finally, it is shown that over 1 Tb/s is achievable per wavelength with a 4-core fiber and a simple optical wireless alignment system. An important application of this design would be backhaul and intra-data center links. [ABSTRACT FROM AUTHOR]

Recently, multimode fibers and their application, such as in-vehicle systems where many in-line connectors are inserted in short-range transmissions, have garnered considerable research interest. As such applications require extremely high reliability, simulation technology that considers multimode fiber harnessing under unfavorable conditions, such as various fiber lengths and connector misalignments, has been required. Many calculation methods for modal bandwidth and connector loss have been proposed to date; however, the 3D modal power coupling equation at the connection point has not been derived due to the calculation difficulty by linking the near- and far-field patterns. Herein, we propose a ‘point radiance matrix model’ to compute the complex mode-power conversion at the step-index (SI) fiber connectors. Accordingly, we derived new equations for calculating the power distribution at the receiving fiber according to the fiber connector misalignment parameters, which can be used for link level evaluation from light source to detector. Moreover, the rules governing the power transferred from the emitting fiber to the receiving fiber were investigated under generic alignment conditions, including random combinations of longitudinal, lateral, and tilt misalignments. Applications of the proposed matrix method could contribute toward the computation of mode-power conversion at graded-index (GI) fiber connectors in case the point radiance on the emitting fiber end face is found. [ABSTRACT FROM AUTHOR]

In the frame of ongoing efforts between space agencies to define an on-off-keying-based optical low-Earth-orbit (LEO) direct-to-Earth (DTE) waveform, this paper offers an in-depth analysis of the Variable Data Rate (VDR) technique. VDR, in contrast to the currently adopted Constant Data Rate (CDR) approach, enables the optimization of the average throughput during a LEO pass over the optical ground station (OGS). The analysis addresses both critical link level aspects, such as receiver (time, frame, and amplitude) synchronization, as well as demonstrates the benefits stemming from employing VDR at system level. [ABSTRACT FROM AUTHOR]

We propose a new scheme for polarization-sensitive distributed fiber-optic sensing, called polarization optical correlation-domain reflectometry (POCDR), and experimentally demonstrate its fundamental operations. In this scheme, the states of polarization (SOPs) of reference lights are controlled so that they are uniformly distributed on the Poincaré sphere using a calibration method, which cancels the effect of unknown birefringence in the fibers used in the system. Then the SOP of spontaneous Brillouin scattering in a sensing fiber is estimated using a least-squares method, by measuring the interference powers for the multiple reference lights with different SOPs. As a pilot demonstration, the SOP transitions along a fiber including a three-paddle polarization controller are measured under different conditions. We also measure the SOP transition of a fiber wound on a bobbin and calculate its polarization beat length, which agrees well with the theoretical value. By exploiting its unique nature called random accessibility, high-speed measurement of the roundtrip birefringence between two arbitrary points in a sensing fiber will be potentially achieved. [ABSTRACT FROM AUTHOR]

In a subcarrier multiplexing 125 Gbaud DP-PCS-64QAM system with a net bitrate of 800 Gbit/s, we numerically and experimentally study the performance of transmission-aware carrier phase estimation (CPE) for long-haul systems. The recently proposed transmission-aware dual-reference subcarrier CPE (DRS-CPE) is assessed against a more conventional per-subcarrier CPE approach and a transmission-agnostic joint CPE in 8- and 16-subcarrier transmission. At laser linewidths of $\sim$ 150 kHz, we achieve an experimental signal-to-noise ratio gain over a single-carrier of up to 0.5 dB after 1800 km transmission, enabled by the DRS-CPE. [ABSTRACT FROM AUTHOR]

The unique characteristics of hollow core fibers lead to low non-linearity and reduced sensitivity to environmental influences, making them attractive platform for designing inline fiber optic devices. Among these, an inline fiber polarizer would be most desired for applications such as gyroscopes, interferometric sensors and telecommunication. Hollow-core fiber polarizers, demonstrated to date, suffer from high insertion loss (>5 dB) for the pass polarization, and typically require tens of meters long nested antiresonant fiber to achieve a meaningful polarization extinction ratio (PER) >20 dB. We report a novel approach that makes use of intricate modal loss spectrum within a resonant band of single-ring antiresonant fiber to realize the first high-performance, discrete hollow-core fiber polarizer. The compact device comprising only 6 cm long fiber offers a high PER of >25 dB with <1 dB transmission loss for the pass polarization. In the present form, the polarizing action takes place over 14 nm and 7 nm bandwidth in the 2 μm and 1 μm wavelength range, but the simplicity of design principles makes it straightforward to shift the operating wavelengths to practically any wavelength band. The device can be fusion spliced with a standard antiresonant hollow-core fiber and a large mode area solid-core fiber with splice loss of 0.14 dB and 0.45 dB respectively. Complete integration of the proposed device into an all hollow-fiber setup would lead to a total insertion loss of 1.3 dB including both input and output splicing, which is the lowest for a discrete hollow-core fiber polarizer reported so far. The results encourage the development of other hollow-core fiber components by exploiting the unique characteristics cultivated in a resonant band of an antiresonant fiber. [ABSTRACT FROM AUTHOR]

Tiled diagonal zipper codes (TDZCs) are shown to be a class of competitive forward error correction codes in modern optical fiber communication systems. This paper introduces the multichain tiled diagonal zipper codes (MC-TDZCs) in which multiple TDZC chains are coupled together in a cyclic manner. Firstly, we present the encoding and decoding algorithms of the proposed MC-TDZCs. Secondly, we analyze the graphical structures of the minimum stall patterns of the MC-TDZCs using directed graphs. Thirdly, we derive the multiplicities of these minimum stall patterns. The analytical results show that the size of the minimum-sized stall patterns of the proposed MC-TDZCs is larger than that of the underlying TDZC counterpart. Finally, we present simulation results to show the performance advantages of the proposed MC-TDZCs under the constraint of equal storage size for decoding. Particularly, a rate 0.94 MC-TDZC with four chains shows more than four orders of magnitude improvement in bit error rates compared to its underlying TDZC counterpart. [ABSTRACT FROM AUTHOR]

The rise in demand for underwater wireless communication networks (UWCN) has been driven by the emergence of new applications including unmanned underwater vehicles, deep-sea exploration, maritime and underwater archaeology research, and diver communications. For various applications, underwater network lifetime must be long, since unlike terrestrial sensor networks, it is a major job to change/recharge node batteries in underwater environments. In this paper, we introduce a solution where critical nodes in a UWCN are periodically recharged by small renewable energy sources. Further, a caching mechanism is introduced to relieve critical nodes of heavy workload when their residual energies run low. By leveraging these two techniques of energy replenishment and content-caching, the network lifetime extension challenge is formulated as an optimization problem. The results indicate that our proposed solution offers up to 45% longer UWCN lifetime compared to existing solutions. [ABSTRACT FROM AUTHOR]

Light amplification by stimulated emission of radiation (laser) sources has many advantages for use in high-data-rate optical wireless communications (OWCs). In particular, the low-cost and high-bandwidth properties of laser sources, such as vertical-cavity surface-emitting lasers (VCSELs), make them attractive for future indoor OWCs. In order to be integrated into future indoor networks, such lasers should conform to eye safety regulations determined by the International Electrotechnical Commission (IEC) standards for laser safety. In this article, we provide a detailed study of beam propagation to evaluate the received power of various laser sources, based on which and the maximum permissible exposure (MPE) defined by the IEC 60825-1:2014 Standard, we establish a comprehensive framework for eye safety analyses. This framework allows us to calculate the maximum allowable transmit power, which is crucial in the design of a reliable and safe laser-based wireless communication system. Initially, we consider a single-mode Gaussian beam and calculate the maximum permissible transmit power. Subsequently, we generalize this approach for higher mode beams. It is shown that the $M$ -squared-based approach for analysis of multimode lasers ensures the IEC eye safety limits; however, in some scenarios, it can be too conservative compared to the precise beam decomposition method. Laser safety analyses with consideration of optical elements, such as lens and diffuser, as well as for the VCSEL array, have been also presented. Skin safety, as another significant factor of laser safety, has also been investigated in this article. We have studied the impacts of various parameters, such as wavelength, exposure duration, and the divergence angle of laser sources on the safety analysis by presenting insightful results. [ABSTRACT FROM AUTHOR]

Multimode fibers (MMFs) links using vertical cavity surface-emitting lasers (VCSELs) are still the solution of choice for today’s data center shorter distances thanks to their low cost and robustness. Currently, the market is moving towards 400G-capable systems using multiple lanes at 50 Gbps per lane, based on intensity modulation (IM) and direct detection (DD). A technology switch will probably be required to overcome the 100 Gbps per lane limit that will likely be the bottleneck of future IM-DD MMF-based links. In this manuscript we propose an intra-data center system based on coherent detection and MMF, analyzing experimentally and analytically the performance of polarization multiplexed coherent communication. We show that the advantages enabled by the coherent technology can be fully exploited in an MMF-based link when central launch is ensured. In particular, we study in detail the effect of lateral offsets introduced by the connectors at the interface between two MMF facets, in terms of net optical loss and signal-to-noise ratio (SNR) degradation. A statistical analysis is performed analytically exploiting a large database of OM3 and OM4 fibers modal delays and results are presented for various combinations of fiber length, type and number of MMF-to-MMF connections along the link. We show quantitative results on the maximum acceptable lateral offsets in the MMF connectors, highlighting that in practical conditions average offsets of up to 3 $\mu$ m can be tolerable. [ABSTRACT FROM AUTHOR]