1. 644 ELECTRONICS LETTERS 24th April 2014 Vol.50 No.9
instability if the relative
launch polarisations of the
new wavelengths are not
sufficiently scrambled and
the signals travel over a high-
quality, low-polarisation mode
dispersion (PMD) fibre. Today it
is hard to buy fibre that is not low
PMD.
I’ve proposed two techniques to neutralise
Raman crosstalk and enable a 16-fold increase in
new-service wavelengths. The first is to modify the
electrical (RF) spectrum of the interfering wave-
lengths through a fairly weak, (RF) frequency-spe-
cific, physical-layer (PHY)-filter placed between
the digital ones and zeros and the optical transmit-
ter. In previous work, non-standard line-coding
shapes the interfering spectra. In this work, the
PHY-filter requires no new coding standards or sili-
con at either transmitter or subscriber receivers.
Secondly, polarisation interleaving new launch
wavelengths reduces worst-case RF and DC Raman
crosstalk by 3-dB and stabilises the subscriber-
received 1490-nm power in low-PMD fibre links.
What are the potential applications?
This work is relevant whenever one wants to
exploit the unused optical spectrum of in-use fibre
plants for new multi-wavelength services. Most pho-
tonic integrated circuit or silicon photonics based
multi-wavelength transmitter designs launch all sig-
nals with identical polarisation. This Letter shows
that polarisation interleaving designs may be
required in certain cases. The PHY-filter may be
applicable in multi-wavelength networks supporting
mixed modulation formats, such as when baseband-
modulated lightwaves co-propagate with subcarrier-
multiplexed or discrete multi-tone modulated
lightwaves. I hope the impact of my work is that the
nonlinear Raman effect is kept in mind when adding
new wavelength bands to in-use fibre plants. Com-
pared to other nonlinear optical wave-mixing pro-
cesses, nonlinear Raman is quite robust and
powerful.
What challenges remain and how do you
think your work will be used?
Implementing the PHY-filter into a real-world
product, such as a 10-Gb/s pluggable optical trans-
ceiver module as a real, physical RF-filter would be
challenging. I have pointed out that the PHY-filter
function could also be realised in the digital domain
with a low-cost, modest-speed (few-hundred MHz)
digital-to-analog converter. I think that engineering
that signal processing and timing correctly is the
remaining challenge. In all areas of optical fibre
communications, there is constant demand to do
more with less for less cost. It’s my hope that my
Letter contributes to a tool-kit for doing just that.
Dr David Piehler, Chief Sci-
entist at NeoPhotonics, on
the work behind the paper
‘On minimising nonlinear
Raman crosstalk in future
network overlays upon legacy
passive optical networks’,
Page 687.
Tell us a little bit about your field.
I’ve spent most of my career developing optical
fibre communications technologies for the access
portion of the network which involves providing
bandwidth in end-user sized portions over single-
mode fibre ranging in length from a few to 50 km. In
the early 2000s my group of Verizon FiOS (USA)
was the first to model, measure and publish a strong
nonlinear optical interaction between a wavelength
carrying digital Ethernet data and another carrying
analogue and digital RF video services. Today, I
spend most of my time applying lessons learned in
developing must-be-low-cost optical access technol-
ogies to the emerging world of must-be-low-cost
100-Gb/s single-mode fibre interconnects for data-
centres, however I’ve yet to escape some sort of
involvement with nonlinear Raman crosstalk.
What are the crosstalk issues in legacy
and future networks?
Telecommunication carriers have been bringing
high-speed Fibre-To-The-Home (FTTH) via point-
to-multipoint, single-fibre, passive optical networks
(PONs). A 1490-nm laser broadcasts a continuous
data-stream to all homes. Each home communicates
upstream with 1310-nm laser in orchestrated, unique
time-slots. Some carriers also provide broadcast
video services through a one-way 1550-nm cable-
TV-like signal. The (now-legacy) PON optical
equipment and network design rules were created to
minimise interference between these different sig-
nals at different wavelengths.A big part of the FTTH
investment rationale is that the unused optical spec-
trum in the new fibre plant is available for future
advanced services. It’s quite important that as new
wavelengths and services are added, they don’t inter-
fere with the existing wavelengths or vice-versa.
What have you reported in your Letter?
I’ve shown how the nonlinear Raman crosstalk
limits both the number and launch power of new
wavelengths that a carrier can add to the legacy
PON. In my example, RF nonlinear Raman crosstalk
shows up as noise or snow on low-number analogue
video channels carried by the 1550-nm wavelength.
DC nonlinear Raman crosstalk shows up as an
additional optical loss between the central office and
the home at 1490 nm. Interestingly, the DC Raman
crosstalk-impaired 1490-nm signal at the
subscriber’s receiver can have multi-dB power
interview
“In all areas of optical fibre communications,
there is constant demand to do more
with less for less cost”
David Piehler
t
qu
dispe
is hard
PMD
’,
doi: 10.1049/el.2014.1284
reprinted from Electronics Letters
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