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To quickly meet the diverse requirements imposed by emerging new services on optical access networks, we recently proposed the new concept of "flexible access system architecture" (FASA), which features the disaggregation of time-critical applications. This paper introduces our activities on FASA along with related technical trends. As our key ongoing challenge, the modularization of the dynamic bandwidth assignment function in a passive optical network is reviewed, and information on its use cases, approach, and sequences is given. Furthermore, a reference hardware implementation of FASA for mobile service is described.

This paper presents a λ-tunable optical network unit (ONU) burst-mode transmitter (B-Tx) with high output power and low burst-off-level power for high-splitnumber time and wavelength division multiplexing passive optical networks (TWDM-PONs). The proposed ONU B-Tx utilizes a burst-mode booster semiconductor optical amplifier (SOA) with a control technique for reverse bias voltage that creates an optical power absorption effect in the burstoff- level state. This paper introduces the design of the biasvoltage- state control circuit, simulations of control-circuit operation, evaluations of the static optical power-absorption effect, and burst-mode operation performance in the booster SOA operated by reverse bias voltage. High output power of over +7.4 dBm and low burst-off-level power of under −73.4 dBm∕15 GHz are successfully achieved in all upstream channels. Crosstalk penalty measurements confirm a very low penalty of under 0.1 dB, which leads to a 256 splitnumber TWDM-PON.

In-service wavelength tuning at ONUs provides advanced-network functions of dynamic wavelength allocation (DWA) in WDM/TDM-PONs. Based on DWA usage cases, we present a hitless l-tuning sequence and backup wavelength pre-assignment as keys in next-generation multi-service convergence access networks.

We propose and demonstrate an optical path protection that offers FEC switching function for WDM/TDM-PON with ring topology. A system demonstration confirms proposed automatic wavelength and FEC switching enables a 30km difference of redundant optical path in ring access networks.

More than +6 dBm output power OLT/ONU transceivers which comply with the wavelength allocation plan of the next-generation passive optical network stange2 (NG-PON2) standard (ITU-T G.989.2) are successfully demonstrated thanks to a newly developed semiconductor optical amplifier (SOA) — integrated transmitter.

We propose a burst-off-level power reduction technique for a burst-mode booster SOA implemented λ-tunable transmitter. A large power reduction of 8.2 dB and a low crosstalk penalty of

We introduce the world's first field trial of a symmetric-rate 40-Gbit/s λ-tunable WDM/TDM-PON that uses a high-speed λ-selective ONU TRx and optical extender. 40-km reach and 1024-split λ-switching transmission, and system demonstrations are successfully performed.

In this paper, fairness-aware dynamic subcarrier allocation (DSA) in a distance-adaptive modulation orthogonal frequency division multiple access passive optical network (OFDMA-PON) is proposed and experimentally demonstrated for elastic lambda aggregation networks (EXANs). By applying the distance-adaptive modulation function to OFDMA-PONs, the system can change the transmission parameters (modulation level and symbol rate) for each user according to the user's end-to-end optical loss, which will yield additional bits. Meanwhile, by applying our proposed DSA algorithm, which allocates sub-carriers to users based on transmission parameters and traffic demands, the additional bits can be shared by all users in a fair manner regardless of a user's end-to-end optical loss. This paper conducts a real-time demonstration using an optical coherent OFDMA transceiver and a medium-access control board. The experiments confirm that our algorithm can greatly improve fairness in terms of frame loss rate among users compared to fixed sub-carrier allocation.

We propose a high-output power λ-selective transmitter utilizing a simple configuration burst-mode booster SOA with gain peak detuning that achieve pattern effect mitigation. We successfully demonstrate over 40-dB loss budget and 40-km reach WDM/TDM-PON.

We demonstrate an approach to enhance the optical frequency domain reflectometry (OFDR)-optical coherence tomography (OCT) sensitivity using a semiconductor optical amplifier (SOA) and a superstructure grating-distributed Bragg reflector (SSG-DBR) laser. We find that the sensitivity of the OCT images of an extracted canine tooth increases as a function of SOA injection current due to amplification through stimulated emission. At the injection current of 150 mA, the sensitivity of the OCT image is increased to a factor of 22.8 dB when compared to the unamplified OCT. Furthermore, an 18 µm axial resolution of the OCT in dental tissue is achieved using the discrete wavelength-swept SSG-DBR laser with an axial scan rate of 0.25 kHz. The observed results suggest that the optical amplification by SOA can significantly enhance the sensitivity of the OFDR-OCT system with a high-resolution.

We propose a λ-selective burst-mode transceiver with 100-ns wavelength switching for the use in C and L-bands for upstream and downstream signals, respectively. The bidirectional 40-km transmission of symmetric 40-Gbit/s λ-tunable WDM/TDM-PON is demonstrated without a PON repeater.

In wavelength (λ)-tunable wavelength-division-multiplexing/time-division-multiplexing passive optical networks (WDM/TDM-PONs), an optical line terminal (OLT) directs optical network units (ONUs) to change wavelengths to achieve functions such as load balancing or power saving. This demands λ-tuning without any data-frame loss, i.e., hitless λ-tuning. In addition, there is a need to increase the dynamic bandwidth allocation cycle period (DBA-p) to expand scalability as regards the splitting ratio or fiber reach. Unfortunately, with a large DBA-p, more buffer memory is needed in the OLTs or ONUs to cover the tuning delay demanded by the conventional hitless λ-tuning sequence, which requires a message exchange between the OLT and the ONU after λ-tuning. In this paper, we propose a novel hitless λ-tuning sequence to reduce the tuning delay. With the proposed sequence, the completion of λ-tuning is decided by calculation, and the buffers for upstream and downstream transmissions are released according to the calculation, instead of the message exchange. Experiments confirm that the proposed sequence can tune the wavelength of the ONU's transceiver without any data-frame losses for multiple measurements. Each measurement includes 300 λ-tuning operations. Additionally, it is shown that, when the DBA cycle is 4 ms, the tuning delay and buffer memory for upstream transmission can be reduced by 98% and 50%, respectively, while those for downstream transmission can be reduced by 86% and 86%, respectively, compared to the conventional sequence. We also show that the maximum upstream throughput is not degraded.

By reducing optical path length with a light collection system, we achieved the smallest dual-rate triplexer for use in 10G-EPON OLT XFP transceivers, while maintaining performance high enough to meet IEEE specifications.

We offer an isolator-free EA-DFB module by employing forward error correction. We analyze the behavior of a laser under optical feedback and the error occurrence mechanism. We obtain error-free operation at the receiver sensitivity of −31 dBm.

A low-cost and small BOSA achieves excellent optical transmission waveforms with a >40% mask margin in a wide temperature range (−10 to 85 oC) by using impedance matching circuits in a low-cost TO CAN package.

We demonstrate a sensitivity improvement in an optical frequency domain reflectometry-optical coherence tomography (OFDR-OCT) system with a discretely swept light source by incorporating a semiconductor optical amplifier (SOA) in a sample arm. With the system, we achieve a high sensitivity of -134.4 dB when we measure the reflective mirror with an A-line rate of 0.25 kHz. This improves the sensitivity (-125.2 dB) by 9.2 dB compared with a system without the SOA. The OCT system without the SOA shows a signal-to-noise ratio (SNR) of 56 dB when the signal light power is attenuated by about 66 dB, and the SNRs of less than 56 dB are obtained at higher attenuation levels. However, an SOA-incorporated OCT system provides the SNR of 56 dB at the much higher attenuation level of 86 dB. This means that using the SOA offers the large signal light power margin of 20 dB needed to obtain SNR of 56 dB. It is shown that the power margin is qualitatively dependent on the optical gain of the SOA. From an experimental analysis of the noises in the SOA-incorporated system, we found that the sensitivity enhancement is mainly limited by the beat noise between the reference light and the amplified spontaneous emission (ASE) of the SOA. We obtained images that show clear cluster structures of enamel crystals near the dentin-enamel junction of an extracted human tooth with our SOA-incorporated discretely swept OFDR-OCT imaging, revealing the potential to achieve a high-speed OCT system with high sensitivity.

We demonstrate dispersion compensation by using a dispersion shifted fiber (zero dispersion at a wavelength of 1550 nm) in an optical-frequency domain imaging (OFDI) - optical coherence tomography (OCT) system for tooth imaging. In the system, we use a tunable laser diode operating in the 1550 nm wavelength region (1533-1573 nm) as a light source, because we can expect a smaller absorption coefficient for the enamel layer of a tooth than with a 1300 nm light source. This simple and cost-effective method provides an axial resolution of 27 mm in air, which is the theoretically expected value, although the value is 36 mm without compensation. By measuring an extracted human tooth with compensation, we also confirm the realization of greatly improved contrast at the boundary between the enamel and dentin layers. This compensation technique might prove even more effective if we use a light source with a wider wavelength range.

We propose a new method to increase the sensitivity of Optical Coherence Tomography (OCT) beyond the conventional shot noise limit using optical amplifiers. Criterion for effective use of optical amplifiers for OCT is discussed. Enhancement of OCT images is demonstrated with optical frequency domain reflectometry (OFDR) OCT.

We have been developing a unique discretely swept optical frequency domain imaging (OFDI) using superstructured-grating distributed Bragg reflector (SSG-DBR) lasers. To increase resolution, four SSG-DBR lasers are being developed to obtain spectral coverage of 160 nm in total. To increase speed of D-OFDI imaging, simultaneous scanning of multiple sources with a parallel OFDI system and unique transversal scanning D-OFDI have been demonstrated. Introduction of an optical amplifier can increase sensitivity beyond the conventional shot noise limit. A deep 12 mm depth range has been demonstrated with the wavelength interval of 0.05 nm.

This is the second of a two-part paper intended to provide technical insight and rationales behind the recently approved ITU-T G.989.2 Recommendation: the physical media dependent layer specification of the 40-gigabit-capable passive optical networks (NG-PON2). While Part 1 of the paper discusses topics related to the optical link design, Part 2 focuses on wavelength control, technology feasibility, management and control channel design, and potential future standardization directions of such a multi-wavelength PON system. As the NG-PON2 system will continue to evolve, technology extensions are also discussed to provide guidance for future research.

This paper proposes a high output power and high burst extinction ratio wavelength (λ)-tunable optical network unit (ONU) transmitter for long-reach and high-splitting ratio wavelength-division multiplexing and time-division multiplexing passive optical network (WDM/TDM-PON) systems. Our λ-tunable ONU burst-mode transmitter (B-Tx) utilizes a simply configured burst-mode controlled booster semiconductor optical amplifier (SOA) that employs the pattern effect mitigation technique of gain peak detuning. A high output power and burst extinction ratio are successfully achieved at all upstream wavelength channels, over +10.0 dBm and 66.0 dB, respectively. This burst extinction ratio performance satisfies the ONU requirement for a 64-split WDM/TDM-PON with crosstalk penalty of better than under 0.1 dB. Moreover, the pattern effect is greatly mitigated and 40 km transmission penalty of under 0.8 dB is achieved. We successfully demonstrate over a 41 dB loss budget and 40 km reach λ-tunable WDM/TDM-PON without an optical-amplifier-based PON repeater.

This paper describes the demonstration of an energy efficient orthogonal frequency division multiplexing passive optical network using the dynamic signal to noise ratio (SNR) management and adaptive modulation. Controlling the calculation precision and modulation format minimizes the energy consumption of digital signal processor while satisfying the requirements of bit error ratio. We show that the calculation precision and modulation format can be optimized according to the optical received power, and realize a 58.7% effective energy efficiency per bit in an FPGA-based receiver experimentally.

We demonstrate dispersion matching of sample and reference arms in an optical frequency domain reflectometry-optical coherence tomography (OFDR-OCT) system with a discretely swept light source centered at 1550 nm, using a dispersion-shifted fiber (DSF) in the reference arm. By adjusting the optical length of the DSF so that it is equal to that of the free space in the sample arm, we achieve a high resolution of 27.2 μm (in air), which is very close to the theoretically expected value of 26.8 μm when we measure the reflective mirror. This improves the degraded resolution (36.1 μm) in a system using a conventional single-mode fiber when the free-space length in the sample arm was 909 mm. We also demonstrate a clear interface between air and the enamel layer of an extracted human tooth with the discretely swept (DS) OFDR-OCT imaging due to the improved resolution provided by this technique. In addition, we confirmed the enhanced sharpness of the cellular structure in a dispersion matched OCT image of an onion sample. These results show the potential of our DS-OFDR-OCT system for a compact low-cost apparatus with a high axial resolution.

Recently, many telecom carriers are promoting the re-architecture of access networks and COs by utilizing SDN/NFV and OSS. We present our research relevant to the software PON-OLT architecture that we proposed for further flexibility.

We implement our abstraction method for provisioning and controlling, via OpenFlow, the disaggregated PON-OLT that features separation of hardware module and softwarized OLT functions, and demonstrate its operation by utilizing open source controllers ONOS / VOLTHA.

Dynamic SNR management is a promising technique for energy efficient OFDM-PON. We show the first experimental demonstration of the proposed technique. The experiments clarify calculation precision can be reduced according to the optical received power.

This paper describes the first demonstration of an energy efficient OFDM-PON using the dynamic SNR management. The controlling calculation precision and modulation format optimizes the DSP energy consumption while satisfying the BER requirements. We realize a 58.7% effective energy efficiency per bit in an FPGA-based receiver experimentally.