NG-PON2 tunable transceiver work shows signs of progress

Systems houses such as ADTRAN, Calix, Nokia, Huawei, and others report considerable progress on optical line terminals (OLTs) for NG-PON2 applications. However, how and when a key piece of the NG-PON2 puzzle – the tunable transceiver at the customer premises optical network terminal (ONT) – will hit both performance and cost targets remains something of a mystery. At the NG-PON2 Forum workshop held June 11 during the Fiber Connect event in Orlando, FL, three companies – two startups and a spin off from Korea's ETRI – described how they plan to step up.

NG-PON2 combines TDM and WDM transmission (generally referred to as TWDM). As the wavelength(s) assigned to each ONT can change (perhaps in a protection switching scenario), the ONT transceivers must be able to adapt quickly to transmission reconfigurations. At the event, speakers discussed 50 msec as the minimum acceptable switching speed, with 25 and even 10 msec switching times identified as targets. Further complicating matters is the potential use of channel (or wavelength) bonding, which means the transceiver would have to accommodate signals on multiple wavelengths.

While tunable transceivers are widely used in carrier networks, current modules are too expensive for ONT use and don't necessarily meet all of the NG-PON2 performance specifications. Therefore, optical module vendors and the systems houses who would use their products face the daunting task of developing high-performance tunable modules at a price point acceptable to optical access networks. The challenge is particularly important to ADTRAN and Calix (in partnership with Ericsson), who are in the final stages of an RFP runoff for Verizon (see "Verizon narrows NG-PON2 choices to Ericsson/Calix and ADTRAN"). The service provider would like to begin deploying NG-PON2 either late this year or early in 2018 – provided the technology, particularly the tunable transceivers, meets their cost requirements.

ADTRAN asserts it has developed a design to meet Verizon's needs (see "ADTRAN describes NG-PON2 tunable ONT transceivers (sort of)"). However, an ADTRAN speaker at the event provided few details of the approach. Similarly, a speaker from Calix described possible technological pathways towards an acceptable tunable transceiver design, but didn't say which pathway the company preferred.

Representatives from three companies proved more forthcoming. Portugal-based PICadvanced is perhaps the furthest along; company co-founder António Teixeira asserted that his company has already shipped engineering samples of its tunable transceiver and should have both Class 2 and Class 3 optical modules in production by this October. The optical transceiver features a proprietary DFB-based DML transmitter combined with a tunable filter and avalanche photodiode (APD) receiver. The DML offers the high output power, reduced burst mode shift, and high dispersion tolerance the application requires, Teixeira asserted. The receiver design provides high sensitivity and granularity as well, he said.

However, on their own, the transmit and receive approaches wouldn't meet the switching times the network would require because of their reliance on relatively slow thermo-electric coolers (TECs). PICadvanced has developed electro-optical accelerator and electronic and firmware control technology to aid the transmit switching speeds and a tuning mechanism accelerator with proprietary driving technology and hardware and firmware to speed the receiver tuning, Texeira said.

To lower cost and accelerate production, PICadvanced (true to its name) will use a photonic integrated circuit (PIC) strategy for the transceiver design, which will include the company's own approach to creation of a bi-directional optical subassembly (BOSA).

Another young European venture, Denmark's BiFrost Communications, plans to apply analog-based coherent detection technology to NG-PON2 transceiver design. The company, a spin out of the Technical University of Denmark, is working with an approach originally designed to be paired with VCSELs for data center applications. According to CTO Jesper Bevensee Jensen and Vice President, Business Development Steen Clausen, the company's analog coherent detection technology improves receiver sensitivity to the point where transceivers could meet NG-PON2 requirements via off-the-shelf DMLs at 0 dBm versus the +7 dBm that likely would be needed otherwise. (Jensen said the company has demonstrated receiver sensitivity of -34.5 dBm over 44 km of standard singlemode fiber.) The resulting, more simplified transceiver design would not require semiconductor optical amplifiers (SOAs) or optical filters, Jensen said.

The company submitted the analog coherent chips for taping this past May and plans to have limited samples available early this fall. The company would consider offering the chips to module vendors, but also plans to develop its own BOSA and ROSA optics to create its own transceiver. The company plans to target OLT transceivers first, but the technology is applicable to the tunable ONT transceivers as well, the company sources said.

Finally, Ben Hur, senior managing director of Lightron Inc., which spun out of Korea's ETRI research organization in 1998 (and not 2007, as earlier reported), also described his company's work in ONT transceiver and component development. Lightron's work focuses on TEC-controlled DML transmitters and Etalon-filtered APDs on the receive side. The company has designed transmit and receive TOs as well as a four-channel tunable BOSA with an eye toward creating a tunable transceiver in an SFP+ form factor. The company lists such a product, the LWEKS-RLP8, on its website. However, it supports 2.5 Gbps in the upstream as opposed to symmetrical 10-Gbps operation.

With the possible exception of PICadvanced, it appears these module technology vendors won't be ready to provide symmetrical 10-Gbps products until 2018 at the earliest. But they show that the quest for a viable, cost-effective tunable transceiver for NG-PON2 applications may finally reach a successful conclusion in the relatively near future. The NG-PON2 Forum likely will offer a platform for status updates via two upcoming workshops it will co-produce with the Broadband Forum. The first will take place October 23 in Berlin as part of the Broadband World Forum; the second will be held as a workshop of the Broadband Access Summit October 28 in Las Vegas.

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