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Dear
Group, Let's not forget that it is not just
splitters and the raw fiber we need to worry about at 1600nm, but also the
effects of bends. And that is something that is more field operations
dependent. This is the biggest uncertainty that has made my operator contacts
concerned about using wavelengths longer than 1580nm. I will point out
that the data I’m getting from network operators is very sporadic and
somewhat contradictory. But, if there is doubt, then I think the right
way to go is to assume the worst case. There is another thing to
consider: We may consider proprietary specifications all we want, but in
the end we should be more standards driven. If I look at the G.652-A
fiber recommendation, all it tells me is that the loss (both basic and with
bends) is specified at 1550nm at the longest wavelength. Only G.652-B
specifies the losses at 1625nm. The G.671 standard for couplers specifies
the two traditional windows: 1260~1360 and 1480~1580. So, if we continue
to specify that our ODN is composed of fibers recommended in G.652 and couplers
recommended in G.671 then we really shouldn't go beyond 1580nm. The alternative would be to require
the fiber to conform to G.652-B, and then to require the couplers to conform to
a revised G.671. But that sort of defeats the whole idea of backward
compatibility with existing PONs, which did not have such restrictions. So
that is our problem. I realize that the window is somewhat
narrow, but it seems to be the only safe choice at this point. Sincerely, Frank E. From: Jim Farmer
[mailto:Jim.Farmer@xxxxxxxxxxxxx] Before
submitting a formal comment, we wanted to run this by the reflector for
comment. Regarding the upstream wavelength action reflected below, from
the September meeting in Cost
and power dissipation: The use of a tight wavelength band (e.g., 1577
+/-3 nm) at any wavelength is going to significantly increase the cost of the
laser, both due to the manufacturing tolerance imposed and by the need for
heating the laser. Any DFB laser so far as we know, has a temperature
drift of about 0.1 nm/deg. C. If one were to provide for a normal indoor
temperature range, which can cover 60 degrees, then one will have a 6 nm
wavelength shift due to drift. We have found that there is market demand
for wider temperature range OLTs, which can easily have a wavelength drift of
10 nm over their operating temperature range. Of course, the answer is to
temperature-stabilize the laser, but this requires an added heater/cooler and
control, and the best estimate we can make right now is that it will add 2-4
watts per PON to the OLT power dissipation when operated at room
temperature. Granted, this is small compared with the total power
dissipated by the OLT, but in an age in which we are trying to minimize the
power draw of all equipment, it is going in the wrong direction. And it
will cost money, not to mention that the laser will have to be specified to a
tight wavelength tolerance (or the wavelength tuned by the cooler), adding more
cost. Concerning
the availability of devices, we expect that this application will drive
adequate volume to get the cost down at whatever wavelength we choose.
We’ve been told by one laser manufacturer that the wavelength specified
has little bearing on cost, though of course, tolerance and volume play a big
part in determining the cost. Specification:
We question the statement that fiber and couplers are not fully specified
beyond 1580 nm. Some people are using 1610 nm for OTDRs. A check
with a couple of major manufacturers of fiber and couplers indicates that
specifications are controlled out to 1625 nm. See, for example, http://www.corning.com/WorkArea/showcontent.aspx?id=15535, and http://www.ofsoptics.com/resources/AllWaveFLEX136web.pdf. As
for couplers, I understand that planar couplers are fine over this wavelength
range, though fused biconic couplers may not be as good. Yet
another concern we have with the 1577 nm selection is the filtering when a 1550
nm video carrier is used. The video optical carrier can be as high as
1560 nm. For a 1577 nm downstream data carrier, which can go as low as
1574 nm by specification, the WDM to separate the wavelengths has a 14 nm
transition region. If we use 1590 +/-10 nm for data, the transition
region expands 43%, to 20 nm. This can help reduce ONT cost, as well as
the cost of the WDM at the OLT. For
these reasons, we seek reconsideration of the decision to abandon 1590
nm. We would like to receive comments from the reflector, then we are
planning to submit a formal comment before the deadline. Thanks,
Jim
Farmer, K4BSE |