New technologies for ultradense WDM-PON (udWDM-PON), enabled by coherent techniques and low-cost devices, are developed for an efficient utilization of the optical spectrum, revealing that the “Wavelength-to-the-User” concept can be feasible. In this paper, an udWDM-PON with only 6.25-GHz channel spacing is implemented with conventional DFB lasers, for a splitter-based PON infrastructure with 256 ONUs. The results of the analysis of udWDM access network architecture with respect to their associated complexity, cost, and migration scenarios, exhibit the potential for higher aggregate throughput, higher split ratios, and node consolidation, when compared to competing technologies.

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Technologies for Cost-Effective udWDM-PONs

A technique for direct phase modulating a low-cost distributed feedback laser is presented for user transmitter in ultra dense wavelength division multiplexing passive optical networks. It consists of a simple RC network, which equalizes the phase response. The method is tested with differential phase-shift keying modulation and heterodyne coherent detection at 1.25 and 2.5 Gb/s showing a penalty <;1 dB at BER = 10-3 compared with external phase modulation.

Direct Phase Modulation DFBs for Cost-Effective ONU Transmitter in udWDM PONs

It is predicted that demand in future optical access networks will reach multigigabit/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 per 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-polarization digital) coherent receivers has so far prevented their introduction into access networks. Thus, to exploit the benefits of coherent technology in access networks, low complexity coherent receivers, suitable for implementation in ONUs, are needed. In this paper, the recently proposed low complexity coherent (i.e., polarization-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, of all the configurations considered, 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 implementation of the system), while requiring the lowest optical receiver hardware complexity (in terms of the optical component count). It also exhibits comparable complexity to the currently deployed direct-detection receivers, which typically require over 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 (10 Gb/s) was reported using the Alamouti-coding receiver, which is also the only one, to date, to have been demonstrated in a full system bidirectional transmission.

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

The statistical orthogonal frequency division multiplexing passive optical network (OFDM-PON) concept with multiband optical network units (ONUs) is experimentally tested with two users and an optical line terminal at 2.5/5 Gb/s total effective capacity with binary phase-shift keying (BPSK)/quadrature phase-shift keying (QPSK) modulation. Both downstream and uplink were measured based on intensity modulation and direct detection. The ONUs consisted oflocal nonpreselected wavelength distributed feedback laser sources centrally controlled to reduce overlapping probability. In addition, a radio-frequency mixing stage in the ONUs up/downconverts the user data to/from the OFDM signal, reducing the computational effort. Compared with ONUs processing the whole signal, the multiband approach presents comparable results with almost symmetrical power budgets of around 25 and 20 dB with BPSK and QPSK, respectively, which could increase up to 4.5 dB by allocating a spectral guard interval between the optical carrier and the OFDM data. Furthermore, elastic bandwidth allocation is explored, which is shown to compensate for up to 18 dB differential link loss.

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Experimental demonstration of a statistical OFDM-PON with multiband ONUs and elastic bandwidth allocation [invited]

The art of imparting information onto a light wave by optical signal modulation is fundamental to all forms of optical communication. Among many schemes, direct modulation of laser diodes stands out as a simple, robust, and cost effective method. However, the simultaneous changes in intensity, frequency and phase are a drawback which has prevented its application in the field of secure quantum communication. Here, we propose and experimentally demonstrate a directly phase-modulated light source which overcomes the main disadvantages associated with direct modulation and is suitable for diverse applications such as coherent communications and quantum cryptography. The source separates the tasks of phase preparation and pulse generation between a pair of semiconductor lasers leading to very pure phase states. Moreover, the cavity enhanced electro-optic effect enables the first example of sub-volt halfwave phase modulation at high signal rates. The source is compact, stable and versatile, and we show its potential to become the standard transmitter for future quantum communication networks.

A directly phase-modulated light source

A centralized polarization scrambling and single-PD coherent ONU Rx system is proposed and tested with two Tx and direct-phase modulated DFBs. Polarization independence at the Rx, and a sensitivity at BER=10-3 of ×45dBm with 7.5GHz channel spacing were achieved.

Polarization independent single-PD coherent ONU receiver with centralized scrambling in udWDM-PONs

We propose a heterodyne receiver with a single-photodiode per polarization for ONU receiver in low-cost udWDM-PON. This grants polarization diversity operation showing a penalty of 6dB in sensitivity compared to a balanced detector.

Simplified Polarization Diversity Heterodyne Receiver for 1.25Gb/s Cost-Effective udWDM-PON

A bandwidth-limited distributed feedback laser directly phase modulated with a 5-Gb/s differential quadrature phase shift keying (DQPSK) signal is demonstrated in a statistical ultra-dense wavelength division multiplexing passive optical network. Optical DQPSK modulation was achieved by simply equalizing a four-level waveform. An intradyne receiver based on a 120° optical hybrid with straightforward differential decoding processing was used to detect and recover the data attaining a sensitivity of −38 dBm at $\textrm {BER}=10^{-3}$ . Negligible penalty was observed when adding a second user at 5-GHz channel separation.

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DQPSK Directly Phase Modulated DFB for Flexible Coherent UDWDM-PONs

A 6.25GHz spaced UDWDM PON is experimentally tested with directly-phase modulated lasers, polarization scrambling and homodyne detection. Bitrates from 1.25Gb/s to 5Gb/s per λ are achieved in a 50km fibre-link serving up to 1024 users.

Adolfo Lerin

Papers

Direct Phase Modulation DFBs for Cost-Effective ONU Transmitter in udWDM PONs

Polarization independent single-PD coherent ONU receiver with centralized scrambling in udWDM-PONs

Simplified Polarization Diversity Heterodyne Receiver for 1.25Gb/s Cost-Effective udWDM-PON

DQPSK Directly Phase Modulated DFB for Flexible Coherent UDWDM-PONs

Flexible D(Q)PSK 1.25–5 Gb/s UDWDM-PON with directly modulated DFBs and centralized polarization scrambling