NeoPhotonics' PIC transceiver tackles PON business case

Gazettabyte completes its summary of optical announcements at ECOC, held in Amsterdam. In the third and final part, NeoPhotonics’ GPON multiport transceiver is detailed.

Part 3: NeoPhotonics

“Anything that can be done to get high utilisation of your equipment, which represents your up-front investment, helps the business case"

Chris Pfistner, NeoPhotonics  

NeoPhotonics has announced a Gigabit passive optical network (GPON) transceiver designed to tackle the high up-front costs operators face when deploying optical access.

The GPON optical line terminal (OLT) transceiver has a split ratio of 1:128 - a passive optical network (PON) supporting 128 end points - yet matches the optical link budget associated with smaller split ratios. The transceiver, housed in an extended SFP module, has four fibre outputs, each supporting a conventional GPON OLT. The transceiver also uses a mode-coupling receiver implemented using optical integration.

According to NeoPhotonics, carriers struggle with the business case for PON given the relatively low take-up rates by subscribers, at least initially. “Anything that can be done to get high utilisation of your equipment, which represents your up-front investment, helps the business case,” says Chris Pfistner, vice president of product marketing at NeoPhotonics. “With a device like this, you can now cover four times the area you would normally cover.”

The GPON OLT transceiver, the first of a family, has been tested by operator BT that has described the technology as promising.

Reach and split ratio

The GPON transceiver supports up to 128 end points yet meets the GPON Class B+ 28dB link budget optical transceiver specification.

The optical link budget can be traded to either maximise the PON’s distance, limited due to the loss per fibre-km, or to support higher split ratios. However, a larger split ratio increases the insertion loss due to the extra optical splitter stages the signal passes through. Each 1:2 splitter introduces a 3.5dB loss, eroding the overall optical link budget and hence the PON’s reach.

GPON was specified with a Class B 20dB and Class C 30dB link budget. However once PON deployments started a 28dB Class B+ was created to match the practical requirements of operators. For Verizon, for example, a reach of 10-11km covers 95% of its single family units, says NeoPhotonics.

Operators wanting to increase the split ratio to 1:64 need an extra 4dB. This has led to the 32dB link budget Class C+. For shorter runs, in such cases as China, the Class C+ is used for a 1:128 split ratio. “They [operators] are willing to give up distance to cover an extra 1-by-2 split,” says Pfistner.

NeoPhotonics supports the 1:128 split ratio without suffering such loss by introducing two techniques: the mode-coupling receiver (MCR) and boosting the OLT transceiver's transmitter power.

A key issue dictating a PON performance is the sensitivity of the OLT's burst mode receiver. The upstream fibres are fed straight onto the NeoPhotonics’ MCR, eliminating the need for a 4x1 combiner (inverse splitter) and a resulting 6dB signal loss.

The GPON OLT transceiver showing the transmit and the mode-coupling receiver. Source: NeoPhotonics

The MCR is not a new concept, says Pfistner, and can be implemented straightforwardly using bulk optics. But such an implementation is relatively large. Instead, NeoPhotonics has implemented the MCR as a photonic integrated circuit (PIC) fitting the design within an extended SFP form factor.

“The PIC draws on our long experience of planar lightwave circuit technology, and [Santur’s] indium phosphide array technology, to do fairly sophisticated devices,” says Pfistner. NeoPhotonics acquired Santur in 2011.

The resulting GPON transceiver module fits within an SFP slot but it is some 1.5-2cm longer than a standard OLT SFP. Most PON line cards support four or eight OLT ports. Pfistner says a 1:4 ratio is the sweet spot for initial rollouts but higher ratios are possible.

On the transmit side, the distributed feedback (DFB)  laser also goes through a 1:4 stage which introduces a 6dB loss. The laser transmit power is suitably boosted to counter the 6dB loss.

Operators

BT has trialled the optical performance of a transceiver prototype. “BT confirmed that the four outputs each represents a Class B+ GPON OLT output,” says Pfistner.  Some half a dozen operators have expressed an interest in the transceiver, ranging from making a request to working with samples. 

China is one market where such a design is less relevant at present. That is because China is encouraging through subsidies the rollout of PON OLTs even if the take-up rate is low.  Pfistner, quoting an FTTH Council finding, says that there is a 5% penetration typically per year: “Verizon has been deploying PON for six years and has about a 30% penetration.” 

Meanwhile, an operator only beginning PON deployments will first typically go after the neighbourhoods where a high take-up rate is likely and only then will it roll out PON in the remaining areas.

After five years, a 25% uptake is achieved, assuming this 5% uptake a year. At a 4x higher split ratio, that is the same bandwidth per user as a standard OLT in a quarter of the area, says NeoPhotonics. 

“One big concern that we hear from operators is: Now I'm sharing the [PON OLT] bandwidth with 4x more users,” says Pfistner. “That is true if you believe you will get to the maximum number of users in a short period, but that is hardly ever the case.”

And although the 1:128 split ratio optical transceiver accounts for a small part of the carrier’s PON costs, the saving the MCR transceiver introduces is at the line card level. "That means at some point you are going to save shelves and racks [of equipment],” says Pfistner.

Roadmap

The next development is to introduce an MCR transceiver that meets the 32dB Class C+ specification. “A lot of carriers are about to make the switch from B+ to C+ in the GPON world,” says Pfistner. There will also be more work to reduce the size of the MCR PIC and hence the size of the overall pluggable form factor.

Beyond that, NeoPhotonics says a greater than 4-port split is possible to change the economics of 10 Gigabit PON, for GPON and Ethernet PON. “There are no deployments right now because the economics are not there,” he adds.

“The standards effort is focussed on the 'Olympic thought': higher bandwidth, faster, further reach, mode-coupling receiver (MCR) whereas the carriers focus is: How do I lower the up-front investment to enter the FTTH market?” says Pfistner. 

Further reading:

GPON SFP Transceiver with PIC based Mode-Coupled Receiver, Derek Nesset, David Piehler, Kristan Farrow, Neil Parkin,  ECOC Technical Digest 2012 paper.

Lightwave: Mode coupling receiver increases PON split ratios, click here

Ovum: Lowering optical transmission cost at ECOC 2012, click here

Summary Gazettabyte stories from ECOC 2012, click here