Stokes Vector Direct Detection for Linear Complex Optical Channels

We propose a novel detection scheme called Stokes vector direct detection (SV-DD) to realize high electrical spectral efficiency and cost-effective optical communication for short and medium reach. With SV-DD, the signal is modulated in only one polarization and combined with the carrier in the orthogonal polarization for fiber transmission. At reception, the combined signal is detected in Stokes space by three or four photo-detectors. Compared with conventional DD technique, SV-DD is resilient to both chromatic dispersion and signal-to-signal beat noise. Furthermore, SV-DD does not require polarization tracking or narrow band optical filtering for carrier extraction. In this paper, we present for the first time the numerical analysis and experimental demonstration of single-carrier SV-DD. We report 62.5-Gb/s data rate single-carrier SV-DD transmission over 160-km SSMF using 12.5-Gbaud 32-QAM modulation.

Stokes vectors direct detection (SV-DD) is an effective solution for short-reach optical communications.In this paper, we investigate 2D-modulation direct detection systems based on three Stokes vector receivers (SVRs).The influences of three key factors, including the states-of-polarization (SOP), the splitting ratio of the coupler, and the excess loss (EL), are studied in detail.It is shown that the splitting ratio for achieving optimum performance will be changed with SOP and EL conditions.Among these SVRs, the 3 × 3 coupler-based receiver with its optimal splitting ratio shows the best BER performance and stability against the change of SOP.

We propose the Stokes vector direct detection (SV-DD) algorithm to achieve photo-detection nonlinearity cancellation and phase diversity. A 160-Gb/s single-polarization modulated SV-DD signal is successfully received after 160-km SSMF transmission with 7.76-bit/s/Hz electrical spectrum efficiency.

Demodulation of polarization-modulated signals can be done via Stokes-vector direct detection. We analyze the receiver sensitivity for the multi-level polarization-modulation format having the cubic-lattice constellation in the three-dimensional Stokes space.

Broadband Passive Optical Access Networks

With the mass availability of Ethernet in local area networks (LANs) deployed globally and connecting LANs to metropolitan area networks (MANs) and wide area networks (WANs) via the current transport technologies there is a need to resolve bandwidth gap between access networks and transport networks. One of the proposals to overcome the famous last mile issue is to use a passive optical network (PON) for this networking segment. Varieties of access methods are under development to increase the offered potential transport capacity of passive optical networks (PONs). With the emergence of Ethernet as the convergence layer in access networks, a new breed of PON, namely Ethernet PON (EPON) has evolved. Ethernet passive optical network is one of the potential solutions, offering to solve the bottleneck issue of access networks and adaptability of carrying Ethernet frames from subscriber to service provider. In this study we will review Ethernet Passive Optical Networks (EPONs) and then we propose a delay-based provisioning algorithm to be able to offer differentiated services and QoS in EPON. Mathematical analysis and simulation results will be presented also.

DOI: 10.1049/ic:20070284 ISBN: 978-3-8007-3042-1 Location: Berlin, Germany Conference date: 16-20 Sept. 2007 Format: PDF We derive the performance and critical design issues of optical-coding based centralized live PON monitoring technique allowing more than 512 clients. We consider PON geographical distribution models in our performance evaluation. (2 pages) Inspec keywords: condition monitoring; encoding; optical fibre networks Subjects: Optical fibre networks; Codes

Optimization of long-reach TDM/WDM passive optical networks

In this paper, we theoretically analyze and demonstrate that spectral efficiency of a conventional direct detection based optical OFDM system (DDO-OFDM) can be improved significantly using frequency interleaving of adjacent DDO-OFDM channels where OFDM signal band of one channel occupies the spectral gap of other channel and vice versa. We show that, at optimum operating condition, the proposed technique can effectively improve the spectral efficiency of the conventional DDO-OFDM system as much as 50%. We also show that such a frequency interleaved DDO-OFDM system, with a bit rate of 48 Gb/s within 25 GHz bandwidth, achieves sufficient power budget after transmission over 25 km single mode fiber to be used in next-generation time-division-multiplexed passive optical networks (TDM-PON). Moreover, by applying 64- quadrature amplitude modulation (QAM), the system can be further scaled up to 96 Gb/s with a power budget sufficient for 1:16 split TDM-PON.

This paper reviews the future directions of next generation passive optical networks. A discussion on standardized 10 Gb/s passive optical network (PON) systems is first presented. Next, new technologies that facilitate multiple access beyond 10 Gb/s time division multiple access (TDMA)-PONs will be reviewed, with particular focus on the motivation, key technologies, and deployment challenges. The wavelength division multiplexed (WDM) PON will be discussed and in combination with TDMA, the hybrid WDM/TDMA PON will be reviewed in the context of improving system reach, capacity, and user count. Next, discussions on complementary high-speed technologies that provide improved tolerance to system impairments, capacity, and spectral efficiency will be presented. These technologies include digital coherent detection, orthogonal frequency division multiple access (OFDMA), and optical code division multiple access (OCDMA).

The performance of coherent pulse detection (CPD) and multichannel coherent detection (MCCD) based on a single dual-balanced homodyne receiver was experimentally demonstrated using a gratingcoupled hybridly mode-locked semiconductor laser. Compared with direct detection, a high coherent gain of over 10 dB, as well as an SNR improvement of over 5 dB, was obtained in both detection schemes. Our experimental results have confirmed that the coherent detection processes in CPD and MCCD are nearly the same based on a square-root LO power dependence. Nevertheless, the MCCD scheme has shown an advantage in a path-length error over the CPD scheme, revealing 2~3 dB improvement in sensitivities.

Compared with single-pass propagation in traditional free-space optical (FSO) systems, the round trip double-pass propagation through atmospheric turbulence in modulating retroreflector (MRR) FSO systems would experience more turbulence-induced fading that impairs system performance. As the transceiver transmits and receives the same laser beam in the MRR FSO system, the coherent detection (CD) could be inherently formed at the transceiver to improve the system performance by virtue of enhanced receiver sensitivity. However, since the CD is susceptible to optical phase distortion caused by atmospheric turbulence, turbulence-resilient methods based on self-interferometric phase compensation and self-coherent mode-mixing enhancement are considered and first integrated into the coherent MRR FSO system in this work. For the direct detection (DD) and the CD with the two turbulence-resilient methods in the MRR FSO system, we describe the theoretical definition of received signal current, wave-optics simulation for channel modelling and Monte-Carlo simulation for bit error rate (BER) evaluation, but we have not yet conducted the corresponding experiments due to the equipment limitations. The wave-optics simulation results show that the channel fading of the DD and the CD could be better fitted by Lognormal and Rician distributions, respectively. The Monte-Carlo simulation results show that the CD could generally outperform the DD and the BER performance of the CD with self-coherent mode-mixing enhancement is the highest, which achieves a signal-to-noise ratio gain of about 6dB compared with the DD.

The exponential growth of Internet data traffic pushes hard the telecommunication infrastructure for upgrading the transmission data rate. At the optical access level, a passive optical network (PON) is the most popular and lowest cost architecture which enables high speed access for enter prise and private users as well as various backhauling and fronthauling functionalities. Standardized PONs have undergone a number of upgrades in data rates. In order to guarantee a stable return of investment for operators, the co-existence of different generations of PONs on the same fiber infrastructure is ensured via a wavelength division multiplexing (WDM) approach. This leads to a spectrum shortage for future allocations if full co-existence is required. Today's fastest 40 Gb/s TWDM PONs [1] support 10 Gb/s lane rate non-return-to-zero (NRZ). Looking forward, the bandwidth demands continue to rise, thus increasing the per-wavelength bit rate is necessary for a future upgrade beyond 40 Gb/s. For such high data rates, the conventional NRZ format becomes demanding with respect to the required transceiver bandwidth and the acceptable fiber dispersion. As an alternative, advanced modulation formats and efficient signal processing can address the upgrade issue. Recently, research efforts have been focusing on 25 Gb/s or 40 Gb/s per wavelength next generation PONs using either NRZ with chirp management [2], or electrical/optical Duobinary [3–7], PAM-4 [3, 6–8], as well as more complex ODFM [9]. Considering PONs are cost-sensitive applications, intensity modulation and direct detection is highly desirable. Compared to DSP intensive OFDM, electrical/optical Duobinary and PAM-4 with low complex DSPs are potentially costeffective solutions. This paper aims to review and compare a number of implementation options for 40 Gb/s per wavelength high speed PONs using electrical/optical Duobinary and PAM-4. Optical link power budget performance, transceiver cost and power dissipation will be analyzed based on numerical approaches. Experimental demonstrations will also be presented.

An ultra-dense multi-band orthogonal frequency division multiplexing (MB-OFDM) passive optical network (PON) with a dedicated band capacity per user of 2.5 or 10 Gb/s is demonstrated experimentally. A band spacing of 6.25 GHz and a low-cost virtual carrier-assisted direct-detection (DD) scheme are employed. The proposed MB-OFDM PON shows a great potential in the context of next-generation optical access networks due to the low receiver bandwidth required, low-cost implementation, and high granularity, flexibility, and scalability provided. The power budget evaluated from the experimental results shows that the virtual carrier-assisted DD MB-OFDM solution is more suited to short-reach PONs. For a PON reach of 20 km and a band capacity of 2.5 Gb/s, the proposed network supports 128 users. For a band capacity of 10 Gb/s, the network supports 32 users in a PON with reach of 10 km.

Recently, extensive research has been conducted to explore the utilization of machine learning (ML) algorithms in various direct-detected and (self)-coherent short-reach communication applications. These applications encompass a wide range of tasks, including bandwidth request prediction, signal quality monitoring, fault detection, traffic prediction, and digital signal processing (DSP)-based equalization. As a versatile approach, ML demonstrates the ability to address stochastic phenomena in optical systems networks where deterministic methods may fall short. However, when it comes to DSP equalization algorithms such as feed-forward/decision-feedback equalizers (FFEs/DFEs) and Volterra-based nonlinear equalizers, their performance improvements are often marginal, and their complexity is prohibitively high, especially in cost-sensitive short-reach communications scenarios such as passive optical networks (PONs). Time-series ML models offer distinct advantages over frequency-domain models in specific contexts. They excel in capturing temporal dependencies, handling irregular or nonlinear patterns effectively, and accommodating variable time intervals. Within this survey, we outline the application of ML techniques in short-reach communications, specifically emphasizing their utilization in high-bandwidth demanding PONs. We introduce a novel taxonomy for time-series methods employed in ML signal processing, providing a structured classification framework. Our taxonomy categorizes current time-series methods into four distinct groups: traditional methods, Fourier convolution-based methods, transformer-based models, and time-series convolutional networks. Finally, we highlight prospective research directions within this rapidly evolving field and outline specific solutions to mitigate the complexity associated with hardware implementations. We aim to pave the way for more practical and efficient deployment of ML approaches in short-reach optical communication systems by addressing complexity concerns.