Archive

PASSIVE  OPTICAL  NETWORKS  EVOLVE
TOWARD  WAVE  DIVISION  MULTIPLEXING

This part of our study of Passive Optical networks concentrates on future technologies that are still mostly in the R&D stage.  As the title of this paper states, we are looking for PONs beyond the FSAN and E-PON group of technologies, discussed in previous issues, which either have been standardized or are close to that status.

Today, the most promising PON technology still in R&D appears to be Wavelength Division Multiplexing (WDM) PONs (W-PON).  W-PON research focuses on wavelength splitters (which replace the power splitters found in standardized techniques and E-PON).  Because of the large number of possible wavelengths, they do not need to be shared.  Adding a new customer may mean adding a laser with a different wavelength in the central office, but that need have no impact on current subscribers.

LIMITATIONS OF "TRADITIONAL" PON
W-PON can partially overcome issues usually associated with "traditional" PON such as:
--Limitations on the splitting ratio (number of subscribers per OLT port) due to the available power budget.
--Broadcast transmission over PON means both the OLT and ONT must work at the aggregate bit rate.
--Bandwidth sharing limits each subscriber to a fraction of the aggregate bandwidth, and may require contention for bandwidth.
--Security issues arise from delivering all packets to all sites on the fiber tree.
--OA&M issues.
--Network integrity risk: a corrupted signal from one ONT can affect all network users.

These issues, together with desire to work with higher bit rates, stimulated interest in W-PON development.

"IDEAL" W-PON
W-PON can mitigate many of these issues of "traditional" PONs by allocating a different wavelength to each subscriber.  To add a user, the service provider deploys a laser with a new wavelength and a matching wavelength selection filter at the distribution point.  With a "private" wavelength, each subscriber can communicate to the OLT with a different data rate signal and a different protocol.   A subscriber can be added simply by deploying common modules at both the central office and the subscriber's site.  This feature simplifies inventory management and maintenance. 

A Figure at http://www.viewsletter.com/ illustrates this concept.

In the CO, the "head end" equipment combines the features of a Wavelength Division Multiplexer and multiple OLTs.  Downstream from there, a wavelength-multiplexed signal contains wavelengths for all subscribers.  The outside plant contains wavelength splitters (there may be more than one in a path).  Note that a WL splitter can have less insertion loss than a power splitter used in "traditional" PONs.  After a WL Splitter, individual signals (selected by frequency) are directed to the particular subscriber ONU/ONT.  It is also possible to combine WL and power splitters on a path.  That is, one wavelength can support a group of users (the same as a PON) or multiple wavelengths may reach a subscriber (in which case the receiver is equipped with a wavelength filter that admits only that subscriber's signal).

In the upstream direction, each subscriber can transmit on a wavelength pre-defined for his location during W-PON design as shared or not.  Even when shared, the smaller number of users per WL means it's easier to coordinate subscribers transmissions than on "traditional" PONs.  In the upstream direction, a WL Splitter acts as a passive wavelength combiner.  CO equipment demultiplexes wavelengths, among other OLT functions. 

Note that there are different flavors of W-PONs, including, but not limited to:
--Hybrids of "traditional PON" and W-PON using both power and WL splitters;
--Upgrade of existing fiber infrastructure to W-PON by replacing power splitters with WL splitters.

With W-PON development concentrated on R&D, it's too early for standardization activity in this arena.  In spite of this, companies are delivering proprietary equipment (for example, Korean company Novera Optics).  Another approach is coming from the Essex Corp., which announced manufacturing of passive WDM splitters with very narrow spacing between channels.  For example, its WDM can function with a data rate of 1.28 Gbit/s over channels spaced only 3.125 GHz apart. This opens a lot of new opportunities to increase the capacity of W-PON systems.

W-PON ISSUES<
The issue with W-PON is not so much the WL multiplexing in the optical links themselves, but the customer premises equipment.  Deploying W-PON would be a large operational problem if for each subscriber the service provider had to install a different WDM ONT for each laser frequency supported.  That approach would require a supply of ONTs for all possible frequencies, with spares, creating an excessive inventory and high OPEX and CAPEX spending. 

Moreover, most W-PON solutions appear to be based on passive signal splitters, with the lasers in the end-user's equipment functioning at different wavelengths.  Such a model creates the risk of interference in the passive signal splitter when subscribers transmit on adjacent wavelengths unless end points have sophisticated filters.

WHAT TO DO
Three main directions for solutions have been formulated (so far): 1. Fit equipment at one or both ends with tunable lasers.
2. Introduce an optical connecting cable for the ONT that determines the laser frequency.
3. Deploy an ONT containing a modulating mirror rather than a laser.

1. Tunable lasers are more expensive than fixed lasers and have less power, making them less attractive when used with the preferred passive power splitters.  These devices reduce signal strength by at least 15 dB under the best conditions (a ratio of 1:32; -3 dB is half power).  In practice, loss increases to 17 dB or more. Currently, a downward trend for prices of tunable lasers is becoming obvious.  Maturing technology may create an opportunity for them in W-PON.

2. A specially treated fiber cable can determine the laser frequency of the equipment in the home or business.  In this case a cable determines the wavelength, while the laser in the ONT can be the same for all wavelengths, simplifying provisioning and hardware sparing.  If the ONT has problems, a serviceman can simply replace the ONT with standard equipment;  the existing connecting cable will ensure proper wavelength of the light.

This can work because a laser needs two reflecting surfaces to function.  One mirror is only partially reflective and allows part of the light to pass into an optical fiber (to the output).  Mirrors typically are placed at the ends of a laser, creating a "cavity."  This technique replaces the output mirror with a special cable.  The laser has only one mirror, on the back side.

UV-treatment creates a pattern of 'light' and 'dark' rings (of different refractive indices) at regular intervals along the fiber used in the connecting cable.  This pattern works as an optical grating with the properties of a wavelength-selective mirror that transmits (and reflects) only one precise wavelength.  The cable, as the second mirror, determines the wavelength of the laser.  This technology is still in development.

3. Lucent tested a solution that requires no laser in the ONU.  A continuous laser signal sent from the OLT is reflected back by the ONU.  The ONU modulates the returned signal on or off (a "1" and a "0").  The reflected pulses are detected in the OLT's receiver.  Thus the OLT controls the wavelengths in both directions;  the ONUs and ONTs are uniform.  Adjusting the wavelength simplifies to changing a cable.  

Altogether, we see a bright future for W-PON.  This group of technologies should have a place in the general PON infrastructure, particularly if combining E-PON with CWDM.  Analysts are also talking about IP-based PONs--not a surprise, but they are far from ready.