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LASER POWER SHARING IN
THE SUBSCRIBER LOOP
Indexing terms: Telecommunications, Integrated optics
An experimental system that demonstrates the sharing of
laser power among 64 and 128 subscribers is reported. In this
system only two laser transmitters are employed at the
'remote electronics' site while 600 Mbit/s signals are transmitted up- and downstream between subscribers and the remote
electronics sites using T i : LiNbO 3 external modulators.
Introduction: Optical fibre telecommunication systems are key
elements for supporting future broadband digital services for
residential and business customers. The communication link
between the subscriber's premises and the first switching node,
'remote electronics', is provided mostly on a dedicated basis.
Therefore, the cost of that link represents a major consideration for realisation of the local access network. In particular,
the cost and reliability of light sources, specifically lasers, are
critical for successful deployment of optical fibre systems in
the subscriber loop. A network architecture has been proposed where very few lasers are centrally used while their
output power is shared among several users in a local community. 1 Modulation of the optical power in this design is
obtained through external modulators. The implementation of
laser sharing systems depends critically on whether or not
external modulator technologies—LiNbO 3 , semiconductor
multiple quantum well structure 2 or other technologies—yield
significantly less expensive and more reliable devices than the
semiconductor laser technology.
In this letter we report a demonstration of a local access
network with laser power sharing among 64 and 128 subscribers.
Local access network description: The local access network
architecture, shown in Fig. 1, is based on the double-star
approach and assumes 600 Mbit/s remote electronics (RE)remote electronics
subscriber (downstream) data rates and either 600 Mbit/s or
150 Mbit/s upstream data rates. 3 Upstream and downstream
signals are transmitted unidirectionally on two single-mode
fibres for each subscriber.
As shown in the Figure, two laser transmitters are operating
in a CW mode in the RE site. Their output power is shared
among N subscribers using two l x A f power splitters and
single-mode fibre links. One laser, at wavelength Al5 distributes CW power to each of the subscribers through one of the
splitters. The CW optical power is modulated by the subscriber data, through an external modulator, and sent upstream over a return fibre to the RE. At the RE, N receivers,
each allocated for a subscriber, receive the upstream signals
and convert them as input signals for the local access switch.
The downstream electronic signals from the switch modulate
the CW output power of the second laser (A2). This modulation is done through N external modulators, each of which is
associated with a subscriber and located at each output of the
second 1 x N splitter.
The CW power at At and the downstream signals at A2 are
combined on each of the subscriber down-link fibres through
a two-channel WDM multiplexer at the RE and separated by
a WDM demultiplexer at the subscriber site.
Experiment: Fig. 1 of the local access network architecture
can also be used for description of the experimental system.
The experiment was performed twice to simulate both 64 and
128 subscriber systems. Two commercial laser transmitters
with multilongitudinal-mode BH semiconductor lasers were
used in a CW mode. Their centre wavelengths were Ax cz
1538 nm and X2 ^ 1281 nm. Two power splitters were each
made by cascading 1 x 16 and 1 x 4 couplers for 64 users, or
1 x 16 and 1 x 8 couplers for 128 users. These splitters are
commercial fused-fibre directional couplers set for 3 dB coupling at the appropriate wavelengths. The 1300/1500 nm WDM
multiplexer and demultiplexer are also made by the fused-fibre
directional coupler technology and exhibit better than 17-5 dB
crosstalk isolation. Fibre pigtailed T i : LiNbO 3 MachZehnder devices were used as external modulators. Modula-
fibre links
subscri bers
r.sub.1
WD M
1 xN
splitter
•
i
M
WDM
A
downstream ,
I I I II
CW
laser
R
.
( A 2 ) +CW, ( A , )
1
1
1
•
WDM
•—
r
M,
CW
laser
1 x N
splitter
downstream N
(A 2 ) +CW ( A , )
sub.N
RN
1 k
•
•
•
WDM
L
MN
—'
M
-^ |
!
1
from local
access switch
upstream
to local
access switch
„
R
N
(A )
1
N
upstream, (A , )
•—
R
,
Fig. 1 Subscriber loop network architecture with laser power sharing among N subscribers
M = external modulator, R = receiver
ELECTRONICS LETTERS 27th August 1987 Vol. 23 No. 18
943
systems can support up to 64 subscribers (potentially 128) in a
local community, each subscriber receives 600 Mbit/s signal
and transmits either 150 Mbit/s or 600 Mbit/s. It is clear that
successful realisation of this system in the subscriber loop is
critically dependent on the availability of external modulators
that are significantly less expensive and more reliable than
semiconductor laser transmitters.
tion voltages and extinction ratios were 10 V and 24 dB for the
1-5/im, and 7 V and 12 dB for the 1-3/xm modulator. Each
modulator was preceded by a manual polarisation controller.
Two single-mode fibres, 2-25 km in length, were used as the up
and down transmission links. Transimpedance PINFET receivers were used both at the RE and subscriber's sites. The
entire system is connectorised using biconic connectors.
Table 1 UPSTREAM LINK (1-5 fim) POWER
BUDGET
Launched power
OdBm(peak)
Insertion losses*
1 x 16 splitter
1 x 4 splitter (64 users)
WDM MUX/DEMUX pair
4-5 km fibre (round trip)
Polarisation controller
Mach-Zehnder intensity modulator
Modulation
Total loss
12-5 dB
6-4 dB
0-9 dB
1-5 dB
0-3 dB
8-5 dB
30 dB
331 dB
Receiver sensitivity 150Mbit/s
at BER = 10" 9 (average power)
System margin
-38-8dBm
5-7 dB
Receiver sensitivity 600 Mbit/s
System margin
-33-6dBm
0-5 dB
* Each of the components includes a biconic connector
Fig. 2 Eye diagrams at BER = 10 ~9 for N = 64
Experimental results: The power budget and insertion losses
measured for the upstream (either 150 Mbit/s or 600 Mbit/s)
and downstream (600 Mbit/s) links, simulating a system with
64 subscribers, are shown in Tables 1 and 2, respectively.
For 128 users, the additional insertion losses for both links
were ^3-3dB, resulting in insufficient received power for
BER = 10" 9 . Note also that average losses are tabulated for
the splitters; the worst case was 3-2dB more lossy. On the
other hand, receivers with >4dB better sensitivity are commercially available while state-of-the-art direct detection receivers have <—40dBm sensitivity at 600Mbit/s. In addition,
nonoptimised fibre-modulator coupling and waveguide
bending cause the relatively high insertion losses of the
LiNbO 3 Mach-Zehnder modulators. These losses are
expected to be reduced to ^ 3 dB level. It is also expected that
lasers with higher launched power (>10dBm) will be available in the near future. Consequently, laser sharing among up
to 128 subscribers can be implemented with sufficient system
margin.
Eye diagrams of the received signals at the 64 user system
are shown in Fig. 2 with 600 Mbit/s downstream data and
both 600 Mbit/s and 150 Mbit/s upstream data.
Conclusions: A local access network system has been demonstrated experimentally. The network is based on the doublestar architecture, on laser power sharing and extensive use of
external modulation. The architecture does not require optical
sources at the subscriber site. It has been shown that such
Table 2 DOWNSTREAM LINK (1 3/zm) POWER
BUDGET
Launched power
MOdBm(peak)
Insertion losses*
1 x 1 6 splitter
1 x 4 splitter (64 users)
Polarisation controller
Mach-Zehnder intensity modulator
WDM MUX/DEMUX pair
2-25 km fibre
Modulation
Total loss
12-6 dB
6-4 dB
0 2dB
7-5 dB
10dB
1-2 dB
30 dB
31 9 dB
Receiver sensitivity 600 Mbit/s
System margin
Acknowledgments: The authors wish to thank J. Johnson for
attaching fibre pigtails on the modulators, L. Curtis and R.
Spicer for providing biconic connectors in the system components, and C. A. Brackett for helpful discussions.
H. KOBRINSKI
S. S. CHENG
16th July 1987
Bell Communications Research
435 South Street
Morristown, NJ 07960, USA
References
1
CHENG, s. s.: 'Novel systems architecture for broadband distribution in the local access and transport areas'. Technical digest of
conference on optical fiber communications, San Diego, 1985, p. 8
2
WOOD, T. H., CARR, E. C , KASPER, B. L., LINKE, R. A., BURRUS, C. A.,
and WALKER, K. L. : 'Bidirectional fibre-optical transmission using a
multiple-quantum-well (MQW) modulator/detector', Electron.
Lett., 1986, 22, pp. 528-529
3 LINNELL, L. R.: 'A wide-band local access system using emergingtechnology components', IEEE J. Sel. Areas Commun., 1986,
SAC-4, pp. 612-618
FIELD PATTERNS OF TM 0 MODES IN A
SHIELDED DIELECTRIC RESONATOR
Indexing terms: Resonators, Dielectrics
-32-8dBm
20 dB
* Each of the components includes a biconic connector
944
a 600 Mbit/s downstream on k2 — 1-3/zm
b 600 Mbit/s upstream on At = 1-5 nm
c 150 Mbit/s upstream on Xt = 1-5/mi
The resonant modes in a cylindrical dielectric resonator
located within a cylindrical shielding cavity are studied by
means of the finite integral technique. The field patterns of
five lowest transverse-magnetic modes are presented, and the
controversies of the mode designations are discussed.
Transverse magnetic modes in dielectric resonators (DRs) with
no variation in azimuthal direction have been studied by Verplanken and VanBladel1 and by Guillon et al.2 The lowest
resonant mode of this kind is usually denoted, in Cohn's notation,3 as TM01(5. The discretised field plots of this mode have
ELECTRONICS LETTERS 27th August 1987 Vol. 23
No. 18
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