Finisar's 10 Gig bi-directional DWDM architecture

Finisar has developed a bi-directional 10-gigabit SFP+ module for the metro-access market. The dense wavelength-division multiplexing (DWDM) module is designed to expand capacity at locations where fibre is scarce. And being tunable, the SFP+ also simplifies network planning for the operators.

Finisar demonstrated the module working at the recent ECOC 2017 show held in Gothenburg.

Market applications

Interest is growing in using WDM optics for wireless, metro-access and cable networks that are undergoing upgrades. The interest in WDM at the network edge is due to a need to use fibre resources more efficiently. “We are seeing that globally, more and more dark fibre is being used up,” says Leo Lin, director of product line management at Finisar.

Leo LinGiven the cost of leasing and installing fibre, operators are keen to make the best use of their existing fibre and are willing to pay more for WDM optics.

According to Finisar, leasing a fibre can cost $250-$2,000 per fibre annually while the cost of installing fibre can be $500,000 per 10km. “Using WDM optics, you can get payback in less than a year,” says Lin.

LightCounting Market Research's latest forecast estimates that the global wireless transceiver market for 10 gigabit WDM will be approximately $400 million in 2022.

Finisar’s bi-directional 10-gigabit SFP+ product is also being aimed at two emerging ITU Telecom standards: G.metro and NG-PON2. 

G.Metro and NG-PON2

The G.metro standard supports up to 40 DWDM wavelengths on a 100GHz wavelength grid. Tuneable transponders each at 10 gigabits-per-second (Gbps) are used and have a reach of up to 20km without amplification.

NG-PON2 is a time and wavelength division multiplexing, passive optical network (TWDM-PON) standard. “In addition to TWDM-PON, they want to have a few dedicated point-to-point WDM links, an overlay on top of the PON,” says Lin. 

G.metro uses both the C-band and the L-band: one band is used for the sent wavelengths and the other band for the received wavelengths. In contrast, Finisar’s bi-directional approach sends and receives wavelengths using the C-band only.

“The G.metro standard calls out bi-directional and tuneable optics, and our bi-directional module product can be directly used here,” says Lin. “Since ECOC, we have had quite some support from operators and OEMs that will add our architecture as one of the channel options in both G.metro and NG-PON2.”

Bidi design  

Finisar describes its design as a dual-band bi-directional DWDM approach. To understand the design, it helps to compare it to existing DWDM duplex and single fibre schemes. 

Standard DWDM (A), a hybrid bi-directional scheme that uses 50GHz AWGs (B), and the bi-directional approach (C) using the C- and L-bands being proposed for G.metro and NG-PON2. Finisar's approach is shown in the diagram below. Source Finisar.

With standard DWDM, two fibres are used, each having a multiplexer and demultiplexer pair. The C-band is used with wavelengths sent down one fibre and received on the other (see diagram A). 

The hybrid bi-directional DWDM design (diagram B) sends wavelengths in both directions on one fibre. The hybrid approach is growing in popularity, says Finisar, to address fibre scarcity, for example between a central office and a remote node. For the hybrid scheme, only a single multiplexer-demultiplexer pair is needed. But to fit all the wavelengths on one fibre, a 50GHz channel mux-demux is used rather than a cheaper 100GHz one.

Another bi-directional scheme - one that G.metro and NG-PON2 are promoting - uses 100GHz channels but requires both the C-band and the L-band (diagram C). Here, east-to-west traffic is sent across one band while west-to-east traffic is sent on the other.

“This approach requires cyclic arrayed-waveguide gratings,” says Lin. A cyclic or colourless arrayed-waveguide grating (AWG) can separate or combine wavelengths across multiple bands. But unlike the hybrid bi-directional case, one fibre only connects to each bi-directional transceiver hosting a C-band wavelength in one direction and an L-band one travelling in the opposite direction. Using fewer fibres saves cost and space.

Finisar’s bi-directional design is similar but with one important twist: only the C-band is used.

To do this, two carriers are placed into the single 100GHz channel: one an upstream wavelength and one a downstream one. The result is 40, 10Gbps wavelengths - 80 carriers in total - spread across the C-band (see diagram below).

Finisar's bi-directional architecture uses two carriers per channel spread across the C-band. Source: Finisar

A tuneable filter is used in the module not only to match the channel that the remote module’s tuneable laser will use, but also to select the particular band in a given channel, either the upstream or downstream band. The result is that one bi-directional module can be used for all 40 channels. “One single part number for the far end and the near end,” says Lin.

The technical challenge Finisar faced to make its design work is separating the two closely spaced carriers in a 100GHz channel.

Finisar says that with a 50GHz DWDM system, the wavelength must sit centrally in the channel and that requires a wavelength locker. The two carriers within its 100GHz band are not placed centrally yet Finisar has developed a way to separate the two without needing wavelength-locker technology. 

The tuneable bi-directional approach also simplifies network planning. If an operator wants to add a new wavelength and drop it at an existing node, the node’s optical add-drop multiplexer does not need to be upgraded.

“All operators have different channel plans and customised optical add-drop multiplexers in the field,” says Lin. “In our case, we are even simpler than the duplex. In duplex you need a multiplexer-demultiplexer pair; in our case, any AWG or thin-film filter based design can be used.”

Finisar uses an out-of-band communication channel for the central office module to co-ordinate the channel to be used with a newly inserted remote module. “You can plug in a module on any available port and it establishes a link by itself in under 10 seconds,” says Lin.

Roadmap

Finisar is working to extend the reach of its 10-gigabit bi-directional tuneable SFP+ DWDM architecture to beyond the current 40km to 60km with the use of a bi-directional EDFA.

The current 40km reach is determined by the link budget chosen for the expected use cases with the assumption being that multiple add-drop sites will exist between the central office and the remote end. “The tuneable laser used is the same that is used in our tuneable XFP+, so supporting beyond 80km is not a problem,” says Lin.  

Finisar says it is working on a 25-gigabit bi-directional module that will be available in 2019.

Meanwhile, select customers are evaluating samples of the 10-gigabit bi-directional SFP+ module. General availability is expected by mid-2018.