Coherent receivers offer high data rates and reach for optical access but, due to their complexity, have proved resistant to implementation. Here, recent research in low-complexity coherent PON is reviewed, and promising future research directions are identified.

Recent Progress and Outlook for Coherent PON

This paper provides an overview of transceiver technologies to be used for current and next-generation passive optical networks (PONs). The uninterrupted scaling of PONs to higher bitrates in a cost-effective way to meet future bandwidth demands will drive the need for continuous improvement in PON transceiver technologies. In this paper we try to analyze the requirements needed and the impact on this for next-generation transceiver technologies based on past and present PON transceiver designs.

Transceiver technologies for passive optical networks: past, present, and future [Invited Tutorial]

It is predicted that demand in optical access networks will reach multi-Gb/s per user. However, the limited performance of the direct detection receiver technology currently used in the optical network units at the customers' premises restricts data rates/user. Therefore, the concept of coherent-enabled access networks has attracted attention in recent years, as this technology offers high receiver sensitivity, inherent frequency selectivity, and linear field detection enabling the full compensation of linear channel impairments. However, the complexity of conventional (dual-polarisation digital) coherent receivers has so far prevented their introduction into access networks. Thus, to exploit the benefits of coherent technology in the ONUs, low complexity coherent receivers, suitable for implementation in ONUs, are needed. In this paper, the recently proposed low complexity coherent (i.e., polarisation-independent Alamouti-coding heterodyne) receiver is, for the first time, compared in terms of its minimum receiver sensitivity with five previously reported receiver designs, including a detailed discussion on their advantages and limitations. It is shown that the Alamouti-coding based receiver approach allows the lowest number of photons per bit (PPB) transmitted (with a lower bound of 15.5 PPB in an ideal system simulations) whilst requiring the lowest optical receiver hardware complexity. It also exhibits comparable complexity to the currently deployed direct-detection receivers, which typically require >1000 PPB. Finally, a comparison of experimentally achieved receiver sensitivities and transmission distances using these receivers is presented. The highest spectral efficiency and longest transmission distance at the highest bit rate reported using the Alamouti-coding receiver, which is also the only one, to date, to have been demonstrated in a full system bidirectional transmission.

Comparison of Low Complexity Coherent Receivers for UDWDM-PONs ($\lambda$-to-the-user)

Some of the different aspects of the DSP enabled receivers in access networks will be addressed in this article. A revision of the main topics which may impact DSP design will be discussed through application examples. Modulation formats, KK receivers, and coherent receivers are analyzed at light of the DSP enabled receivers’ impact. Also, relevant are FEC and burst mode which are debated at the light of PON requirements. Simplified analog coherent receivers are presented as a use case to serve as simplification path for non-DSP high performance receivers. On the other end, aiming at discussing current PON limits, an ultra dense wavelength division multiplexing (UDWDM) use case is also presented where several advanced topics like pulse shaping, high density, and simplified DSP operations are considered.

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DSP Enabled Optical Detection Techniques for PON

Coherent technologies, along with digital signal processing (DSP), have revolutionized optical communication systems, significantly increasing the capacity of the fiber channel owing to transmission of advanced modulation formats and mitigation of propagation impairments. However, commercial solutions for high-capacity core networks are too complex and costly, and therefore hardly feasible, for access networks with high terminal density, where cost, power budget, and footprint are the main limiting factors. This article analyzes the key enabling techniques to implement a complexity-reduced coherent transceiver (CoTRX) by exploiting photonic integration, simplified optical modulation, low-cost DFB lasers, consumer electronics, and low-complexity DSP. Bulk optical modulators are replaced by direct amplitude-and-phase modulation of an integrated electro-absorption modulated laser (EML) with a smaller footprint, generating up to 8-ary modulation formats. Hardware-efficient DSP algorithms for the coherent transmitter and receiver, including pulse-shaping for direct phase modulation, differential detection for optical phase recovery, and digital pre-emphasis with enhanced tolerance to quantization noise, are investigated to face the challenges imposed by low-cost photonic and electronic devices, such as strong phase noise, wavelength drifts, severe bandwidth limitation, and low resolution data converters. Through numerical simulations and real-time experiments, the results indicate that this new class of CoTRX enables effective implementation of wavelength-to-the-user PON with dedicated 1.25–20 Gb/s per user, in an ultra-dense 6.25–25 GHz spaced WDM optical grid, with >30 dB loss budget, outperforming the current competing technologies for access networks.

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Coherent Ultra-Dense WDM-PON Enabled by Complexity-Reduced Digital Transceivers

We experimentally demonstrate an innovative ultradense wavelength division multiplexing (UDWDM) passive optical networks (PON) that implements the full $\lambda \text {-to-the-user}$ concept in a filterless distribution network. Key element of the proposed system is a novel class of coherent transceivers, purposely developed with a nonconventional technical approach. Indeed, they are designed and realized to avoid D/A–A/D converter stages and digital signal processing in favor of simple analog processing so that they match system, cost, and power consumption requirements of the access networks without sacrificing the overall performance. These coherent transceivers target different use case scenarios (residential, business, fixed, wireless) still keeping perfect compatibility and co-existence with legacy infrastructures installed to support gray, time division multiplexed PON systems. Moreover, the availability of coherent transceivers of different cost/performance ratios allows for deployments of different quality service grades. In this paper, we report the successful field trial of the proposed systems in a testbed where 14 UDWDM channels (and one legacy E-PON system) are transmitted simultaneously in a dark-fiber network deployed in the city of Pisa (Italy), delivering real-time and/or test traffic. The trial demonstrated filterless operations (each remote node selects individually its own UDWDM channel on a fine $\text{6.25}\text{-}\text {GHz}$ grid), real-time GbE transmissions (by using either fully analog or light digital signal processing), multirate transmission ( $\text{1.25}\; \text{and}\; \text{10}\,\text {Gb/s}$ ), high optical distribution network loss (18–40 dB) as well as a bidirectional channel monitoring system.

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Gregorio Azcarate

Papers

Field-Trial of a High-Budget, Filterless, $\lambda$ -to-the-User, UDWDM-PON Enabled by an Innovative Class of Low-Cost Coherent Transceivers