ADTRAN yesterday announced the development of Subscriber Edge Tunable (SET) optical transceivers for use in its upcoming NG-PON2 optical network terminals (ONTs). As Verizon named ADTRAN and a team of Calix and Ericsson finalists for its NG-PON2 contract competition, the development is not a surprise (see "Verizon narrows NG-PON2 choices to Ericsson/Calix and ADTRAN"). Given the fact that the competition is ongoing, it's also not surprising that ADTRAN isn't saying too much about the tunable transceivers' design.
NG-PON2, based on ITU-T specifications, will support up to eight wavelengths of symmetrical 10-Gbps transmissions over the same PON infrastructure; initial systems likely will limit themselves to support of four such wavelengths. Regardless of the number of wavelengths deployed, each ONT will need a way to receive and transmit via the wavelengths assigned to it. The specifications call for the use of tunable transceivers to promote flexibility and a variety of operational benefits, obviating the need to swap out fixed-wavelength transceivers every time the assigned wavelengths change (see "The Bright Future of TWDM-PON and Wavelength Unbundling").
Speaking at the ADTRAN Connect analyst and press event the company held this week at its Huntsville, AL, headquarters, several ADTRAN sources noted the SET tunable optical transceivers must meet a variety of requirements, including rapid tuning speed at an access-friendly price point, high output power, and minimal spectral drift during burst-mode operation. Jared Cress, principal strategist and senior staff scientist at ADTRAN, said during his presentation that the company expects the SET devices to support switching times of less than 25 ms. Cress added the module also should prove to be an order of magnitude less expensive than current DWDM tunable transceivers, which would mean they will cost in the hundreds of dollars.
However, Cress and other speakers at the event were much more willing to describe the techniques they're not using to achieve such performance than to say what they are using. The rejected approaches include:
- Adopting the design of most current tunable DWDM transceivers, as they're costly, complex, and wouldn't readily support burst-mode operation (in which the laser would need to be turned on and off – or, at least, blocked from transmitting – rapidly and repeatedly). They also would have trouble supporting the 29-dB power budgets NG-PON2 systems will require.
- Thermal-based tuning, which Cress said is unappealing again because of the difficulty of accurate tuning in the face of burst-mode requirements as well as operation in environments that aren't temperature controlled, such as on the side of a residence. Adding components to compensate for these problems would compromise optical power, he noted.
Cress and other speakers noted that, in the face of such issues, researchers have investigated alternative approaches, including designs leveraging distributed Bragg reflector lasers, external cavity lasers paired with MEMS-assisted tuning, and photonic integrated circuits. ADTRAN isn't following any of these paths either, at least not wholly so, said the speakers. What they did say is that the design leverages conventional components that are mature and easily sourced, with the implication that the company may be using elements of the alternative approaches just described (and perhaps other approaches they didn't mention) in a novel configuration.
Cress did reveal that the resulting device will be close to the size of an XFP; it will fit into an XFP cage, but likely will be slightly longer than a fully MSA-compliant module. He said he expects them to be ready for field trials early next year.
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