ThFF1 (Contributed Oral)
3:30 PM – 3:45 PM
Combinatorial wavelength selective mirror
concatenation for unambiguous subscriber line
monitoring in FTTH/PON with high subscriber counts
Christopher M. Bentz, Stephan Pachnicke and Peter M. Krummrich
TU Dortmund, Chair for High Frequency Technology, Friedrich-Wöhler-Weg 4, 44221 Dortmund,
Germany, Tel. +49-231-755 4412, e-mail: [email protected]
Abstract: Monitoring PONs with up to 1024 subscribers poses challenges due to splitting losses and
drop fiber differentiation problems. We propose an approach using combination of concatenated
wavelength selective mirrors to distinguish between different subscriber lines.
1. Introduction
Passive optical networks (PON) for Fiber to the Home (FTTH) access networks have the potential to provide high
bandwidth to the users. To fulfill economical constraints, high subscriber counts in PONs are pursued [1-3]. Monitoring
can be utilized to detect and locate faults, facilitating repair activities and improving service availability [4]. Lost
connections between the Optical Line Terminal (OLT) and an Optical Network Unit (ONU) can be separated into faults in
the transparent path or e.g. in the ONU. If the ONU is owned by the subscriber, the carrier should be capable of
monitoring the network up to the interconnection point without participation of the ONU. Using the “U-band” for
monitoring, ranging from 1625 nm to 1675 nm (Cf. ITU-T L.66), does not disturb wavelengths deployed for data
transmission and enables continuous network supervision.
Several monitoring techniques have been published in recent years. Promising approaches are based on optical
code division multiplexing- (OCDM) [5,6] and optical time domain reflectometer- (OTDR) techniques [7-9]. OCDM
monitoring uses optical encoders and decoders which create an autocorrelation peak when a coding match occurs. For
high subscriber counts the OCDM approach requires a considerable effort. OTDR monitoring detects Rayleigh
backscattered and discretely reflected radiation from an incident optical impulse.
The dynamic range of modern OTDRs is typically not sufficient to detect a broken drop fiber in case of high
subscriber counts. Every OTDR impulse detected by the OTDR experiences twice as much attenuation as data signals
that pass the PON only one-way. Therefore the monitoring should focus on reflections from network parts behind the
power splitter rather than Rayleigh backscattered radiation, except for the feeder fiber. Wavelength selective mirrors [7]
have been proposed to distinguish between different subscriber lines. We propose mirrors with coarse 1 nm reflection
wavelength intervals to avoid spectral overlapping of different mirrors.
2. Setup of investigated PON structures
ONU
ONU
ONU
1:32
drop fibers
…
1:32
OLT
feeder fiber
inter split fibers
…
1:32
…
As high subscriber counts are desirable in PONs, high splitting factors have to be
utilized. This can be realized by using passive optical power splitters or passive
optical filters. A PON setup based on power splitters is shown in Fig. 1. The
depicted PON configuration poses the challenges of short or no length differences
in drop fibers, high attenuation and a high count of subscribers.
According to [1] signal transmission in the depicted PON structure is rendered
possible on the basis of ONUs using heterodyne reception to increase sensitivity.
Our approach offers an option to provide also monitoring for this PON structure.
ONU
Fig. 1 A passive optical power splitter
PON for 1024 subscribers.
3. Novel monitoring approach
The idea to supervise high subscriber count PONs is to concatenate wavelength selective mirrors that differ in their
reflection wavelength at each drop fiber end. The concatenated wavelength selective mirrors are described by binomial
coefficients to guarantee that every combination is used only once. The binomial coefficients used with the number of
monitoring wavelengths (Nλm) and the mirror concatenation depths (NMirrors) help determining possible maximum
subscriber counts with an unambiguous combination of concatenated mirrors (Nsubscr.) as shown in equation (1).
Nsubscr. =
Nrefl./λm =
(1)
(2)
The more mirrors are concatenated (NMirrors) and the less monitoring wavelengths (Nλm) are used, the more mirrors
reflecting a specific wavelength are built into the PON. Knowing the number of subscribers the PON is designed for
(Ntarget) enables calculating the number of mirrors reflecting an identical wavelength, respectively the number of
reflections per incident monitoring wavelength (Nrefl./λm). The calculation result is rounded up to the following integer
expressed by the Gaussian bracket for a worst-case described by equation (2).
978-1-4244-8938-1/11/$26.00 ©2011 IEEE
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Fig. 2 shows a plot of Nsubscr. (eq. (1)) against Nλm. A plot of Nrefl./λm (eq. (2)) against Nλm is depicted in Fig. 3. For ease
of illustration the plots start at Nλm = 6. Ntarget is 1024 for Fig. 3.
1200
10
3
refl./λ m
10
400
1:32
λm1,…, λm40
200
6 10
OLT
λ-OTDR
N
subscr.
600
2
20
30
Nλ
40
50
m
Fig. 2 Maximum possible Nsubscr. against Nλm.
Cf. Legend in Fig. 3, the blue curve stands for:
1 mirror, green: 2 concatenated mirrors,
red: 3, cyan: 4.
10
λm1 λm2
…
N
800
ONU
…
1 mirror
2 mirrors
3 mirrors
4 mirrors
1000
ONU
λm3 λm4
1
6
10
20
30
Nλ
40
50
m
Fig. 3 Nrefl./λm against Nλm. “Ntarget” is 1024 for
this plot.
Fig. 4 Schematical novel PON monitoring setup based
on 40 monitoring wavelengths (Nλm = 40) and two
concatenated wavelength selective mirrors (NMirrors = 2).
4. Results
ONU 1
…
The way to compare OTDR traces for testing PONs for faults is explained in the
1:8
ONU 8
following example. After the installation of the PON a table is created containing
OLT
1:32
ONU 249
initial OTDR traces for every measurement wavelength λm (trace table). Every
λ1:8
trace contains Nrefl./λm or less reflection peaks in different spatial distances. The OTDR
ONU 256
OTDR performs a cyclic scanning through all λm, comparing the new measured
dB
OTDR traces to ones stored in the trace table. To check a specific subscriber
line for faults, the traces measured at the reflection wavelengths of the mirrors λm1
of this subscriber line are compared at its specific distance. A missing reflection
peak indicates a fiber fault. A reflective fault will reflect all λm. For ease of
illustration this example is based on 40 monitoring wavelengths (Nλm = 40) and
km
two concatenated wavelength selective mirrors (NMirrors = 2) for Ntarget = 256
dB
subscribers as shown in Fig. 4. These input values result in 13 reflections per
incident monitoring wavelength (Nrefl./λm) which can comfortably be distinguished
in the plot in Fig. 5. The different wavelength selective mirrors reflecting λm1 and λm2
λm2 built into ONU 256 can be found in the OTDR traces, indicated by a dark
grey dotted line. It is shown that the transparent path to ONU 256 is working.
km
ONU 1, ONU 8, ONU 249 and nine more ONUs also reflect λm1 but there is no
reflection match in the trace measured at λm2 and at the specific distance from Fig. 5 Schematical OTDR traces for novel PON
monitoring setup according to Fig. 4.
the OTDR. The unambiguousness is based on the distance and the unique
mirror combination.
This new monitoring concept can easily be combined with proposed roundtrip time (RTT) methods [10] to add
additional information from the working ONUs. As the RTT is intrinsically calculated for OFDM transmission systems, this
information can be utilized to improve the interpretation of monitoring results.
…
…
5. Conclusion
A novel monitoring approach for FTTH/PON systems consisting of well-proven OTDR equipment combined with the new
concept of concatenating wavelength selective mirrors is proposed. It is designed to enable monitoring in PONs with high
subscriber counts (e.g. 1024) and can easily be scaled to larger or smaller counts. Wavelength selective mirrors can
potentially be fabricated relatively inexpensive and small enough to fit into connectors, e.g. as dielectric mirrors. The
concatenation of two different wavelength selective mirrors per drop fiber is a good choice resulting in alleviated OTDR
traces with few reflections compared to the high subscriber count.
6. References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
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