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).

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Next-Generation Broadband Access Networks and Technologies

Wavelength-division multiplexing (WDM) technologies are expected to play a key role in realizing the next generation scalable and flexible passive optical networks (PONs). One candidate is WDM-PON, in which each optical network unit (ONU) uses a different wavelength, i.e., a unique wavelength, in each direction to communicate with the optical line terminal. Another candidate is WDM/time-division multiplexing (TDM)-PON; it combines WDM with TDM technology. This paper reviews recent state-of-the-art research on the enabling technologies needed to realize future WDM-PON and WDM/TDM-PON systems, and discusses future directions toward practical PON systems.

Enabling Technologies for Future Scalable and Flexible WDM-PON and WDM/TDM-PON Systems

The relative merits of coherent-enabled optical access network architectures are explored, with a focus on achievable capacity, reach and split ratio. We review the progress in implementing the particular case of the ultra dense wavelength division multiplexed (UDWDM) passive optical network (PON), and discuss some challenges and solutions encountered. The applicability of digital signal processing (DSP) to coherent receivers in PONs is shown through the design and implementation of parallelized, low-complexity application-specific digital filters. In this work, we focus on mitigating the impact of local oscillator laser (LO) relative intensity noise (RIN) on receiver sensitivity, and propose an algorithm which compensates for this impairment. This phenomenon is investigated theoretically and then experimentally by evaluating the sensitivity of a coherent receiver incorporating different tunable light sources; a low-RIN external cavity laser (ECL) and a monolithically integrated digital supermode distributed Bragg reflector (DS-DBR) laser. It is shown that the RIN of the signal laser does not significantly contribute to the degradation of the receiver sensitivity. Finally, a 10 Gbit/s coherent PON is demonstrated using a DS-DBR laser as the LO laser. It is found that a receiver sensitivity of -38.8 dBm is achievable assuming the use of hard-decision forward error correction.

Digital Coherent Receivers for Long-Reach Optical Access Networks

We propose a simple self-polarization-stabilization technique for the wavelength-division-multiplexed passive optical network implemented with reflective semiconductor optical amplifiers (RSOAs) and self-homodyne coherent receivers. By placing a 45° Faraday rotator in front of the RSOA in the optical network unit, the state-of-polarization of the upstream signal becomes orthogonal to that of the linearly polarized seed light at the input of the coherent receiver regardless of the birefringence in the transmission link. Thus, we can achieve the polarization stability of the upstream signal at the input of the coherent receiver. We first implement a self-homodyne receiver by using the proposed self-polarization-stabilization technique and measure its sensitivity by using 2.5-Gb/s binary phase-shift keying signals in the laboratory. The result shows an excellent receiver sensitivity of -46.4 dBm. We also confirm the efficacy of the proposed technique in the transmission experiment over 68-km long link partially composed of installed (buried and aerial) fibers. No significant degradation in the receiver sensitivity is observed during the 10-h experiment despite the large polarization fluctuations occurred in these installed fibers.

Long-Reach Coherent WDM PON Employing Self-Polarization-Stabilization Technique

Optical transmitter apparatus 10 comprising a reflective optical amplifier 12, a driver 14, an optical splitter 16, polarisation compensation apparatus 18 and an optical router 20. The reflective optical amplifier is arranged to receive an optical seed signal. The driver is arranged to generate a drive signal arranged to cause the reflective optical amplifier to amplify the optical seed signal to form an optical signal. The optical splitter is arranged to receive the optical signal and to split off a part of the optical signal to form a further optical signal. The polarisation compensation apparatus is arranged to receive the further optical signal and to rotate a polarisation of the further optical signal by a pre-determined amount, to form a further optical seed signal. The optical router is arranged to receive the further optical seed signal and to direct the further optical seed signal to the reflective optical amplifier for amplification thereby.

Passive optical networks

We demonstrate a long-reach wavelength-division-multiplexed passive optical network (WDM PON) operating at the symmetric rate of 10.3 Gb/s. For the cost-effectiveness, we realize the upstream transmission by utilizing directly-modulated TO-can packaged reflective semiconductor optical amplifiers (RSOAs) and digital coherent receivers. In addition, to overcome the limited modulation bandwidth of this TO-can packaged RSOA (~2.2 GHz) and operate it at 10.3 Gb/s, we utilize the quadrature phase shift keying (QPSK) format and the electronic phase equalization technique. The result shows that we can extend the maximum reach of the 10.3-Gb/s RSOA-based WDM PON to ~80 km without using any remote amplifiers.

Long-reach 10-Gb/s RSOA-based WDM PON employing QPSK signal and coherent receiver

We demonstrate a simple self-polarization-stabilization technique for the long-reach RSOA-based WDM PON implemented with self-homodyne receivers. The effectiveness of this technique is confirmed in 2.5-Gb/s transmission experiments over 68-km long link partially composed of installed fibers.

Self-polarization-stabilization technique for long-reach coherent WDM PON

We demonstrate 10-Gb/s, 80-km-reach RSOA-based WDM PON by utilizing the digital coherent detection and electronic phase equalization techniques. The upstream signal is obtained by directly modulating a TO-can packaged RSOA with 10-Gb/s QPSK signal.

10-Gb/s, 80-km reach RSOA-based WDM PON employing QPSK signal and self-homodyne receiver

We have experimentally investigated the reflection tolerance of upstream signals by comparing Manchester-encoded downstream signals with NRZ ones in a RSOA-based fiber loop back system for a WDM PON The results showed that the reflection tolerance of the upstream signals strongly dependeds on the downstream signal modulation formats, and that the Manchester format was more tolerant than the NRZ one against the reflection of the upstream signal

Enhanced reflection tolerance of upstream signal in a RSOA-based WDM PON by using Manchester coding

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