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Coherent multicarrier lightwave technology for flexible capacity
networks (invited)
Citation for published version (APA):
Khoe, G. D. (1994). Coherent multicarrier lightwave technology for flexible capacity networks (invited). IEEE
Communications Magazine, 32(3), 22-41.
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Published: 01/01/1994
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Download date: 08. Jul. 2021Coherent Multicarrier Lightwave
Technology for Flexible Capacity
Networks
Highly flexible and survivable networks can be built by allocating optical carriers
of heterodyne systems.
Giok-Djan Khoe
mportant advantages offered by optical het- ticarriers, tunability, and selectivity. Then specific
erodyne systems are the selectivity and tun- application areas that may benefit from flexible
ability of the receivers. Heterodyne multicarrier allocation schemes will be discussed.
receivers can be seen as an almost ideal opti- Examples will be taken from the RACE Phase I1
cal filter with very attractive features. project R2065, coherent optical systems imple-
The oDtical filtering function, which cor- mented for business traffic routing and access
responds to the‘narrow passband of the receiver, can (COBRA). Next, trends and progressin heterodyne
be tuned rapidly to any desired optical carrier systems in general and related key components
within a wide frequency range [l].A tuning range will be summarized, and then examples of ongo-
of more than 7 nm and a passband of around 1 ing field trials in Europe will be discussed. Final-
GHz can be realized simultaneously. ly, the coherent multicarrier technology will be
The optical transmitter carrier frequency of compared briefly with direct detection multiwave-
heterodyne systems can be varied as well. Hetero- length technology.
dyne systemsthus appear tobeavery attractive tech-
nology for the realizationof networks in which optical
multicarriers can be allocated in a flexibleway. Such Basic Principles and
networks can serve as survivable routes with a Capabilities
flexible capacity in the core network and in multi-
he principle of optical heterodyne detection is
ple-accesssystems for business users. Another advan-
tage is that the attractive features just described T shown in Fig. la. An incoming optical signal
is converted into a radio frequency signal of inter-
are not restricted to the passband of a certain
optical amplifier. Heterodyne systems can be mediate frequency (IF). Alocaloscillator (LO) laser,
used over the entirewavelength range offered by the together with an optical coupler and a photodiode,
single-mode optical fiber, from 1250 to 1600 nm. is used for the conversion process. The IF signal
Another technical advantage offered by het- isdetectedusingmethods derivedfromconventional
erodyne detection is the superior receiver sensi- radio techniques. In principle, optical systemsbased
tivities and related transmission spans. That on heterodyne detection use techniques and prin-
advantage has been partly eroded since the intro- ciples similar to those developed for radio systems.
duction of optical fiber amplifiers.An optical receiv- System details depend on modulation formats
er operating without the heterodyne method but such as amplitude shift keying (ASK),frequencyshift
equippedwith an opticalfiber preamplifier and some kkeying (FSK), and differential phase shift keying
other special measures [2] has a sensitivity similar (DPSK). Bit rates ranging from 155Mb/s to 2.5 Gb/s
to that achieved with a heterodyne receiver. have been used in field trials.
Recent work in Europe concentrates on the Figure l b [3] illustrates how two optical fre-
exploitation of the earlier described flexibility quenciesfs andfL0 of equal amplitudesare converted
features. The work is being pursued within the frame- into a frequency IF, which is the envelope of the
work of the E u r o p e a n Community program, “mixed” frequency fM. T h e frequency values
GIOK-DJANK€€OEis Research andTechnologyDevelopment in Advanced given as an example in Fig. l a indicate why the
with Philips Research Communications Technologies in Europe (RACE), optical heterodyne receiver acts as an almost perfect
Laboratories, Eindhoven, and directed toward the implementation of multi- optical filter. Using well-known electrical filtering
The Netherland, and the carrier systems in field trials. techniques, it is easy to realize, for example, a
Eindhoven Universiryof The basic features of heterodyne systems will bandpass of a few hundred megahertz around the IF
Technology. be reviewed, especially in view of the use of mul- frequency of 1 GHz. A pure optical domain filter
22 0163-6804/94/$04.000 1994 IEEE IEEE Communications Magazine March 1994
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.with comparable properties would mean that a
passband of a few hundred megahertz would
have to be made around an optical signal carrier
of about 200 THz. Today, LO lasers can be tuned
over a range of 7 nm, and the heterodyne receiv-
er thus acts as an optical filter that combines high
selectivity with wide tunability.
Using heterodyne techniques, it is possible to
exploit the enormous bandwidth potential of the
optical fiber in an efficient way. Single-mode
fibersused today can be used in the wavelength region
rangingfrom about 1250to 1600nm.That range rep-
resents a bandwidth of around 50,000 GHz. If the
signal carrier frequencies of the heterodyne systems
are separated by 10 GHz from each other, 5000
carriers can be accommodated simultaneously in
that wavelength region.
The minimum optical channel spacing between
carriers of a heterodyne system is dependent on
t h e value of I F a n d t h e minimum electrical
domain channel spacing. A multicarrier system
for N x 155 Mb/s will require less channel spacing
than a system accommodating N x 2.5 Gbis.
Connection Examples
Tunable transmitter lasers and LO lasers are
available today, and any pair of signal and LO
frequencies can be formed within the tuning
range of those lasers to create a desired IF signal.
Possible combinations are illustrated in Fig. 2.
For a simple link, it is common to use a transmit-
ter laser and a LO laser with fixed frequencies. In
a distribution system, each heterodyne receiver
can be tuned to any of the incoming signals. In a
multi-access system, each of the transmitters and
receivers can be tuned to each other to establish
a communication channel. By properly allocating
the pairs of signal and LO frequencies, the chan-
nels will not disturb each other, owing to the per-
fect filtering properties of the receivers. It has
been demonstrated recently that a tunable diode
laser can be used to switch and lock a receiver to Figure 1. Principles of an optical heterodyne receiver. (a) An incoming opti-
anincoming signalwithin 0.5 ps [4]. Suchspeedswill cal signal with a frequency fs is optically “mixed”with a signal from an LO,
open the way towards applications in the area of data signal fL0. The photodiode retrieves the envelope of the mixed signal f M and
packet switching as well. creates an intermediate electrical I F signal. Optical frequencies of 200 THz
Full duplex or even full multiplex connections and 200,001 THz will result in an IF signal of I GHz when processed in this
can be established without the need to duplicate way. An electrical filter can be placed around the I F of I GHz to separate it
transmitters and receivers. Part of the optical sig- from other unwanted “mixingproducts.” Data that w’as modulated on the
nal of the LO can be modulated and sent back incoming optical carrier can be detected from the IF using “conventional”
into the same fiber link, as shown in Fig. 3. The radio techniques. (b) The mixed optical signal can hr constructed by simply
heterodyne receiver can thus operate as a single laser adding the signals fs and fLOpoint by point. When fS and fLO have equal
transceiver (transmitter-receiver) [S-71. Connec- amplitudes, the value of the frequency fM is half of the sum of the two rnixed
tions between heterodyne transceivers can there- frequencies. The envelope of the mixed signal will show up at the electrical
fore offer full duplex or even full multiplex output of the photodiode.
channels [6].
Incorporation of heterodyne techniques is there-
fore attractive in those systemareas where high capac- use of an optical network for the high bit rates on the
ity, high flexibility, and multi-access capabilities lines. T h e role of optics in the present SDH-
are required. We point out in the next section based network is restricted only to transmission,
that the heterodyne technique does not conflict with while the flexibility provided by switching and
other techniques. O n the contrary, it is a very multiplexing functions is performed by electrical
attractive method to complement time domain means. SDH supports flexible transportation
and other optical domain methods. capabilities offered by ATM techniques [Y, 101.
A hierarchical principle was introduced within
Combination with Other Technologies the ATM networks to simplify the management
The evolution in telecommunication networks is of signal flow and transfer. The layers within such
strongly influenced by the synchronous digital a hierarchy are called ATM “channels” and ATM
hierarchy (SDH), a standard (8, 91 that deals with “paths,” as indicated in Fig. 4 [Y, 1 1 , 121. The
aspects of time multiplexing of present and future ATM path can be considered as a “pipe” sup-
communication channels and that assumes the porting a cluster of ATM channels.
IEEE Communications Magazine March 1994 23
r
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.-
The cross-
connect is
one of the
areas in the
telecommu-
nication
network
where the
flexibility
offered by
W Figure 2. Basic capabilities of heterodyne systems. (a) Point-to-point connections can be served by imple-
heterodyne menting one transmitter and one receiver. The capacig of such point-to-point connections can be
increased by implementing coherent multicam'er (CMC) systems where a number of parallel optical cam'-
techniques ers are used simultaneously. Paired transmitters and receivers are operated simultaneously, each at a differ-
ent optical cam'erfrequency. (b)A distribution system can be realized by means of a CMC system when
can be the receivers are tunable. (c) When both transmitters and receivers are tunable, a multi-access system is
created around an optical star coupler.
used to
enhance the
functionality.
W Figure 3. Heterodyne receivers that can be operated as transmitters at the same time, using just one laser.
This single-laser transceiver can be used to establish full duplex connections: (a) The LO laser is modulat-
ed with data which is carried by a microwave cam'er subcam'er (SC) [72-741. Part of the optical signal of
the modulated LO is coupled back into the fiber link. If both sides of the fiber link are connected to a
transceiver, collision of the data will occur in the electrical domain. However, ifthe values of SCl and SC2
are different and chosen properly, the detection part of the electrical receiver can separate the microwave
carriers from each other easily. More than two transceivers can communicate with each other in a full mul-
tiplex mode if more than two different subcam'ers are used. (b) Part of the unmodulated optical signal of
the LO laser is coupled into an optical modulator and subsequently coupled back into the fiber link. This
option can only o f e r a full duplex connection.
24 IEEE Communications Magazine March 1994
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.Following the same concept, we propose to
introduce further multiplex hierarchies using the
optical domain techniques [12, 131, as shown in
Fig. 4. Each optical carrier can be used to trans-
port, using heterodyne technology,a cluster of ATM
paths at a standard bit rate of, for example, 2.5
Gbis. Clustersof optical carriers can be grouped into
a wavelength-division multiplex (WDM) layer.
The WDM option separates the optical domain into
wavelength bands with a width of a few nanome-
ters each, using optical domain filter techniques [14,
151. Each of these wavelength bands can accom-
modate many narrowly spaced optical carriers,
supporting a multicarrier heterodyne system.
The optical fiber and cable can be considered
as further hierarchies in the telecommunications
network. Although optical fibers will not be phys-
ically moved around to provide flexible connections, W Figure 4. Hierarchical representationof multiplexing technologies in the
the functionality as such may be realized by intro- telecommunication networks. The lowest level is represented by TDM tech-
ducing optical space switches. niques. ATM is considered to be an important method. In the A T M hierarchy,
It is clear that opticaldomain techniques can com- ATM channels are clustered into ATMpaths. The optical cam‘ers of hetero-
plement the existing(electrical) time domain meth- dyne systems can be seen as a next layer which can support a cluster of A T M
ods in a very attractive way. Using heterodyne paths up to a standard bit rate oj for example, 2.5 Gbis. A cluster of these nar-
techniques, WDM, and optical space switching, rowly spaced heterodyne carriers can be grouped into a next layer, separated
telecommunications networks can gradually from each other by WDM techniques. According to the WDM option, the opti-
evolve towards broadband networks with huge capac- cal domain is divided into bands of a few nanometers each by means of opti-
ity and high flexibility. cal domain filtering techniques. The fiber and cable can be seen as further
Illustration of the Flexibility Aspect levels in the proposed hierarchy. Fibers and cables can accommodate jlexibili-
ly functions if provided with optical space switches at the proper locations.
T h e crossconnect is one of the areas in the
telecommunications network where the flexibility
offered by heterodyne techniques can be used to the optical carriers in a particular fiber connected
enhance the functionality. Optical systems oper- to the RENO can also be operated with lower bit
ating without heterodyne receivers, called “direct rates of, for example, 155 Mb/s.
detection” systems, are commercially available Details of the RENO will be discussed fur-
today. Those direct detection systems operate at ther. Management of the existing SDH crosscon-
the SDH level of 2.5 Gb/s. We will therefore use nect has to be upgraded to accommodate proper
a crossconnect with 2.5-Gbis fiber outputs and allocation of the optical carriers in each link.
inputs as a basis for our example. Within the A different application area is illustrated in
crossconnect itself, the 2.5-Gbis signals will be Fig. 5c. In thiscase, businessusers are connectedvia
broken down into smaller units with lower bit single-modefibers to an optical star coupler. Assum-
rates to allow routing of the different information ing a bit rate of 155 Mb/s in this case, it is possi-
flows. Advanced crossconnects will commonly ble to incorporate 100carrierdfiber simultaneously.
serve as a node with optical fiber connections The business customer premises network (BCPN)
operated at a fixed transmission bit rate of 2.5 system thus offers interconnectionbetween lOOpairs
Gbis each. One possibility for increased through- of business users, representing a total system
put of the node is to incorporate a bit rate of capacityof 15.5Gb/s. Collisionsdonot occur because
10 Gbis, the next SDH level, in each fiber link. separate optical carriers are allocated to each
Another possibility is to introduce a heterodyne connection. Obviously, network management is
multicarrier system, operating at 2.5 Gbisicarrier, required to ensure proper allocation and inter-
as an overlay to the existing electrical 2.5-Gb/s connection procedures. More detailed exampleswill
SDH node. A node with the heterodyne multicar- be discussed later.
rier overlay will be called a reconfigurable node
(RENO). Multicarrier Networks
The RENOcanoffera throughputofNx2.5Gbis Interconnected RENOs and BCPNs provide
per fiber. Figures 5a and 5b illustrate the recon- business users with a highly flexible and surviv-
figurability of such a node. In Fig. 5a, the capaci- able network. As discussed above, the RENOs are
ty of fiber links interconnecting the RENOs are interconnected by optical fibers using, for exam-
varied as needed by traffic requirement in a par- ple, 2.5-Gb/s multicarrier transmission.Connections
ticular period of time. Special events may cause a between a particular RENO and BCPNs can be
temporary need to increase the capacity of a link made via other fibers using multicarriers with a
between two nodes. In another time slot, the lower bit rate of 155Mb/s.As mentioned earlier, the
increase in capacity may be required between coherent multicarrier technique is compatible
two other nodes. Failure in one of the links con- with ATM. One of the BCPNfield trials, as planned
necting two nodes is another reason for reconfig- by the RACE project COBRA, combines coher-
uring the connections, as shown in Fig. 5b. Using ent multicarrier techniques with ATM. Other
present-state heterodyne equipment, more than 16 field trials illustrate that the coherent multicarri-
carriersof 2.5 Gb/seach can beusedsimultaneously. er technique can be used to upgrade a WDM-
The throughput of each link can therefore be based BCPN and to overlay an existing electrical
varied from 2.5 to 40 Gbis as necessary. Obviously, SDH crossconnect.
IEEE Communications Magazine March 1994 25
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.Technology Progress
rogress and the state of the art will be dis-
P cussed in this section in order to provide an
overview of the tools available today for the real-
ization of flexible multicarrier systems. In the
next section, we will proceed to a more detailed
description of laboratory demonstrators and field
trial plans in the area of RENOs and multiple-access
business systems.
At the beginning of heterodyne multicarrier
system experimentation, it was uncertain whether
or not semiconductor lasers suitable for the
development of engineered and reliable systems
would ever become available.Over the past fewyears,
remarkable progress has been achieved in the devel-
opment of dedicated lasers for the 1550-nm wave-
length region. This progress is related to the
achievement of high optical output powers, a
considerable decrease of laser linewidths, high
stability and reliability, and, last but not least,
increased tuning ranges.
Avery important feature of lasers for heterodyne
multicarrier systems is their long-term stability. It
may be stated that experience gained by many
laboratories has indicated that laser long-term
frequency stability is much better than originally
expected. Bellcore, for example, has measured
the absolute frequency of 11 lasers, used in a 16-
caniersystem,overaperiodof 16months [16].Results
show that it is possible, by controlling only the
laser package current and temperature, to stabi-
lize transmitter laser frequencies to within about
200 MHz over a period of more than six months
without usingoptical spectrometersor absolute fre-
quency reference for feedback. Remeasurement
of six lasers after a 16-month period showed achange
of less than 375 MHz for any of the six lasers.
Another important achievement in the area of
lasers is the possibility to order transmitters as
well as tunable LO lasers from different manufac-
turers with accurate specifications set in advance.
That result has been achieved in one of the RACE
projects,carriedout in the period 1988-1992[17-191.
Three manufacturers - Plessey (now GMMT),
Siemens, and Philips - successfully made fully
compatible transmitters and tunable LO lasers,
which were incorporated in an engineered 10-
channel heterodyne broadcast system.
It is encouraging to see that many groups have
contributed to the progress, as illustrated in Fig. 6.
The developments are thus supported on a broad
basis,worldwide. Many laboratories have also devel-
oped engineered heterodyne multicarrier sys-
Figure 5.System application examples with the incorporation of aflexible tems, and several field trials have been carried
carrier allocation. (a)A RENO is basically a crossconnect, operating at, for out [13, 201.
example, a bit rate of 2.5 Gbls, which is complemented with an optical overlay
of multicamer heterodyne systems. Today, I6 hkterodyne camers operating at Transmitters and Receivers
2.5 Gbls each can be realized, thus providing a flexible throughput of the node O n e of the most common modulation formats
rangingfrom 2.5 Gbls to 40 Gbls for each fiber link, as needed. Special major used in optical heterodyne systemsis frequency shift
events may create a need to temporarily increase the throughput of a particular keying (FSK). Modulation of the transmitter
link. (b) Another important reason for reconfiguring the network is the failure laser frequency can be obtained simply by modu-
of a particular fiber link, caused, for example, by a broken cable. By allocating lating the current through the semiconductor
more cam’ers to the other links, rerouting of the communication path can be laser. O n e of the requirements for a directly
accommodated easily. (c) Business users can communicate via fibers connect- modulated laser is that its frequency spectrum
ed to an optical star coupler. Using equipment available today, 100 channels (linewidth) be sufficiently narrow under all mod-
oJ for example, 155 Mbls each can be implemented. The total system capacity ulation conditions for the intended system. Prin-
is thus 15.5 Gbls. Collisions among different connections do not occur if the cipal parameters for a direct frequency-modulated
optical cam’ers are allocated in aproper way. The configuration is thus a mul- (FM) transmitter laser are therefore linc xidth,
tiple-access system with high capacity and flexibility. FM response, and output power. Linewidth and
26 IEEE Communications Magazine March 1994
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.W Figure 6. Diagrams illustratingprogress in heterodyne transmitters and receivers. It can be seen that many groups worldwide contribut-
ed to the progress. (a) Semiconductor lasers incorporated in heterodyne systems must have a narrow spectrum (linewidth). The results
shown in the diagram demonstrate that narrow linewidths of less than 1 MHz can be maintained even at high output powers of more
than 50 m W. (b) The narrow linewidth must be maintained for tunable semiconductor LO lasers as well. Linewidths below 20 MHz
have been demonstrated for tuning ranges of around 7 nm. (e) Heterodyne receivers have been built for field trials using data rates up to
2.5 Gbis, showing a performance close to predicted limits.
power results obtained for semiconductor lasers are channel spacing. A key element in the receiver is the
shown in Fig. 6a [21-291, T h e FM response is LO laser, and the important parameters of this device
commonlyadequate for the applications under con- a r e the tuning range (preferably continuous),
sideration; Fujitsu reported a response range of linewidth, and output power [30]. Continuous
100 kHz t o 10 GHz while t h e linewidth of the tuning is clearly preferred, because in that case it
same laser was 0.49 MHz at 38 mW ex-facet out- is possible to tune the heterodyne receiver to any
put power [21]. incoming signal frequency within the given range.
The performance of a heterodyne multicarrier Considerably higher tuning speeds can be obtained
receiver can be characterized principally in terms by adjustinglaser current or currents (electrical tun-
of tuning range, sensitivity, and minimum optical ing) than by adjusting laser temperature (thermal
IEEE Communications Magazine March 1994 27
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.dyne system operating at a bit rate of N x 2.5 Gb/s
was proposed as a future candidate for system capac-
ity upgrade [66].
An engineered multicarrier demonstrator was
built within the framework of the E u r o p e a n
Community RACE projects [17-191. That experi-
ment deals with a 10-channel TV distribution sys-
tem. The main purpose of the test was to demonstrate
that three different manufacturers are able to
produce fullycompatible engineered equipment and
key components according to interface specifica-
tions set in advance. The demonstrator was oper-
ated continuously for more than ayear and was open
to visitors. Transmitter and receiver modules
Table 1.Field trials and major demonstrators. made by the three different manufacturerswere fully
interchangeable due t o the detailed interface
specificationsagreed to in advance. Equipment taken
from that demonstratorwillbeimplementedin afol-
low-up RACE project, “COBRA.” We will dis-
cussdetailsofthose fieldtrial plansin thenext section.
A further multicarrier test was recentlyconducted
by NTT. A shelf-mounted laboratory setup for
the demonstration of a 128-channelheterodyne sys-
tem was described [67]. The system incorporates
16~622Mb/schannelsand112~156Mb/schannels.
The results just discussed illustrate that het-
Note: Goals 1995 are optional and depend on support from the European erodyne techniques have reached the level of
Community industrial manufacturing. Until now, field trials have
Table 2. COBRA project:field trials and demo plans. been conducted within the scope of long trans-
mission spans and high bit rates. In the next sec-
tion, we will discuss field trial plans aimed towards
tuning). Most relevant papers therefore quote the exploitation of system flexibility offered by
electrical tuning rather than the total range pos- heterodyne techniques.
sible by electrical and thermal tuning. Continu-
ous electrical tuning range and linewidth range
are plotted in Fig. 6b [31-361. It can be seen that
RACE Project COBRA
a continuous electrical tuning range of 7 nm and O B R A is the acronym of R A C E project
a linewidth smaller than 20 MHz is possible.
Progress in heterodyne receivers can be illus-
C R2065, and stands also for strategies towards
a future broadband fiber optic network in Europe.
trated by meansof plottingreceiver sensitivity against Participants a r e BBC, H H I , I M E C , GMMT,
data rate, as shown in Fig. 6c [37-551. The shot GPT, Philips, Siemens, the Dutch postal, tele-
noise limit of a particular receiver configuration graph and telephone commission (PTT), and the
employing FSKis given for comparison. Results indi- Portuguese PTT. It is intended to achieve this
cate that, at present, 2.5-Gbls heterodyne receivers goal by the possible introduction of heterodyne mul-
can be made with a performance close to theoret- ticarrier technologies into all segments of the net-
ical predictions. work, namely the core, the access, and the customer
premises network. Obviously, it is not possible to
Engineering and Field Trials build a field trial covering the network as a whole.
Heterodyne transmission technology has left the Key areas are therefore selected and demonstrators
stage of experimental workon the optical bench and and field trials will be built to show the feasibility
reached a stage where engineered systems and of the proposed technique in those network segments
field trials come to the fore. These results mean ~71.
that all key parts can be manufactured by many The core network of COBRA is based on the
suppliers,lasersrequired for the systemscan be made R E N O concept. T h e R E N O experiment will
on specifications set in advance, and the relevant show that it is possible to upgrade existing 2.5-
electro-optical devices can be made and pack- Gb/s crossconnects to a version that offers a flexi-
aged to allow continuous system operation extend- ble throughput of N x 2.5 Gb/s as needed by
ing over long periods. A review of field trials and traffic demand or for survivabilityrequirements.
engineered demonstrators that have been operat- For the BCPN, three different types of busi-
edoverextendedtimeperiodsisgiveninTable1[17- ness users were selected for the experiments. The
19,56-651. BBC studios represent an environment where
Most experiments listed in Table 1 deal with multiple-access communication using very high
point-to-point links. Optical amplifiers (OAs) bit rates is necessary. Very high bit rates a r e
have been incorporated in some of the tests. essential here because of the need to use uncom-
Those amplifiers, in combination with the superi- pressed format for TV and high-definition TV
or sensitivity of the heterodyne receiver, allow for (HDTV) signals. The system consists basically of
coverage of longer repeater spans. The one-year a multicarrier multiple-accesssystem using a bit rate
field experiment conducted by Nippon Tele- of 2.5 Gb/s/optical carrier. A system incorporat-
phone and Telegraph (NTT) has recently been ing the WDM technique (Fig. 4)is being incorpo-
successfully concluded. A multicarrier hetero- rated at present, and the introduction of heterodyne
28 IEEE Communications Magazine March 1994
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.multicarrier systems in this environment will be a
good example for the hierarchical view presented
in Fig. 4. The Dutch PTT is currently operating a
field trial for business subscribers, using direct detec-
tion optical techniques. Incorporation of a multi-
carrier heterodyne system in that environment
will show how this technique can complement
existing techniques. A typical feature of this sys-
tem is the use ofvariable-bit-rate services, and in this
case, heterodyne techniques and ATM [lo, 111
will be combined in a field experiment for the
first time. The Portugese PTT (CET) will test the
multicarrier systemin anenvironment inwhichcon-
tinuous-bit-rate services are typical. The setup
will resemble a video conferencing configuration,
connecting professional users who need to dis-
cuss high-quality video information.
T h e access network is n o t covered by the
COBRA project but this particular area has been
covered in a previous RACE project, “CMC”
[17-19, 71, 1021. Engineered equipment taken
from that projectwill be implemented in the COBRA
field trial setups. An overview of the demonstra-
tor plans is presented in Table 2.
The RENO Demonstrator
The basic outline of a RENO is shown in Fig. 7a.
As discussed before, the core of the RENO may
be an existing electrical crossconnect (ECC) with
optical fiber connections to other crossconnects.
Each fiber of the existing crossconnect may sup- Figure 7. The general configuration of’u RENO and the particular configura-
port one connection at 2.5 Gbls. Within the cross- tion to be used in the RENO demonstrator of the European Community
connect, electrical switching a n d routing is RACE project COBRA. ( U ) Functional bloch diagram of a full-size RENO
performed at much lower bit rates, and data isdemul- The block in the center of the figure is an existing ECC Without the hetero-
tiplexed and multiplexed at the o u t p u t s a n d dyne equipment overlay, such ECC nodec may sen>r in an optical network
inputs of the crossconnect to and from a level of operating at a standard bit rate of 2 5 Gbls. Each LCC node thus has optical
2.5 Gbls. fiber inputs and outputc, each supporting one 2.5 Ghls connection. The trans-
According to the RENO concept, all 2.5-Gbis mitters and receiver3 used for the ECC are common, commercially available
inputs and outputs of the ECC are connected to a direct detection typey. In the RENO configuration, the outputs ofthe ECC are
pool oftunable heterodyne receivers (CRs) and tun- connected to a pool of heterodyne CRs and CTs. The optical inputs of the
able heterodyne transmitters ( a s ) . Anopticalspace receivers and optical oiitputs of the transmitter5 are routed by OSSes to the
switch (OSS) module is inserted between the het- optical fibers. Each fiber cun thii, be allocated any number of optical carriery
erodyne units and the optical fibers. Using the between 1 and N. (b) The RENO demonttratorplurined by the RACEproject
OSS modules [69, 701, it is possible t o connect COBRA. The demonrtrator will contentrute on the heterodyne overlay only; a
the outputs of N heterodyne transmitters t o just full-we RENO is beyond the stope of the project The RENO demonstrator
one of the M output fibers, ‘iich, in that case, will have f o ~ ijiber
r inpiits and outputs, and the maximum number of caners
will accommodate a multical :r transmission of to be allocated per fiber is eight. The nzuimiim throiighput per fiber is there-
Nx2.5 Gbls. Note that the opticalswitchesmayswitch fore 20 Gbls. The pool of heterodyne receivers and tranv”ers consists of 16
rather slowly, since they arc not used to switching modules. It is msumed that a worct-case situation will never occur on all fiber
data packets in this case. The same option is pre- links ~imultaneously.
sent at the input side of the RENO; one of the M
input fibers carrying an N x 2.5-Gbls multicarrier sig-
nal can be coupled by the space switch t o N het- require a large amount of effort and is therefore
erodyne receivers. The node management has the beyond the scope of the R A C E project. T h e
important task of controlling the optical space switch RACE demonstrator will therefore focus only on
and allocating the optical carriers for the heterodyne the novel features of the heterodyne multicarrier
receivers and transmitters. overlay. A management part capable of control-
We note here that the pool of heterodyne ling the optical space switch and the allocation of
modules will not contain M x N receivers and optical carriers will be included. It is clear from
transmitters. It is assumed that a worst-case situa- the considerations given earlier that the overlay
tion requiring a maximum capacity will never can be used to complement an existing electrical
occur o n all fiber links simultaneously. T h e crossconnect. The configuration of the RENO
dynamic allocation of optical frequency carriers demonstrator is outlined in Fig. 7b.
opens the way towards the realization of very- The R E N O will consist of a pool of 16 2.5-
high-capacity connections as needed, without the Gbis coherent transmitters and receivers. and
necessity of constructing a network built for eight of the 16 tunable receivers can be allocated
worst-case situations on all links simultaneously. to each of the four input fibers. It is therefore assumed
A full-sizedemonstrator, which also includes elec- that a maximum throughput of 20 Gbis may be
trical switching, routing, and networking at lower required on a fiber at a particular time, but never
levels than the optical transmission bit rate, will simultancously on all fibers. At the receiver side, one
IEEE CommunicationsM a g u i n c March 1994 29
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Among
to eight optical power dividers and a nonblocking
OSS will be used to realize the proper allocation
of the tunable receivers. At the transmitter side,
Basic functions of network management will
be provided: optical carrier management, call
control, securityfunctions,andmaintenance. Inorder
all 16 coherent transmitters are connected to a 16 to have full isolation between traffic and signal-
technica1 x4switchwithbroadcastingcapability[68].Anadvan-
tage of this transmitter configuration is that dis-
ing, a separate communication medium, the overlay
network, is incorporated for signaling between
tribution is possible. The total demonstrator will the LEX and the subscribers. T h e overlay net-
solutions, contain the RENO just described and four small- work, which operates at 1300 nm, is separated by
er-size satellite RENOs. That configuration will WDM components from the heterodyne carriers,
heterodyne be sufficient to show the functionality of a net- which operate in the 1550-nm wavelength region.
work. The optical carriers will be in the 1550-nm An ATM switch will be present in the LEX for
multicam'er wavelength region.
As mentioned earlier, the RENO can be used
the routing of traffic on the various heterodyne
carriers. By combining ATM, WDM, and hetero-
to overlay an existing electrical 2.5-Gb/s S D H dyne carriers, the flexibility of the network can be
technology crossconnect. The RENO demonstrator will be pro- optimally exploited.
vided with SDH-compatible interfaces, and a The combination of all the aspects just dis-
may be con- field test is included as an option in the COBRApro-
ject for 1995.
cussed must result in a realistic and complete
approach towards a flexible high-capacity net-
work, suitablefor fulfilling all the broadband demands
sidered very The Business System Demonstrators of future business subscribers. The demonstrator
As discussed in a preceding section, the BCPN will be compatible with existing standards for
promising, as systems consist of fiber links connected t o an ATM and 155-Mb/s SDH.
optical star network. Optical star networks are
it combines available today, and different configurations can The BBC Demonstrator -The BBC is a typical
be constructed using a few basic modules [75-801. broadcaster that requires a highly flexible BCPN
The purpose of these demonstrators is to show with multigigabit capacity for use within its studio
high that heterodyne multicarrier techniques can be centers for making television programs. A passive
incorporated to upgrade existing systems and that star optical network combined with either WDM
sensitivity, the technique can be used to complement other
methods. Two examples will be discussed in the
and/or multicarrier heterodyne techniques poten-
tially offer many advantages with the advent of
digital video processing and HDTV. Note that
high capacity, next section.
uncompressed digital video signals are required.
The Dutch PTT Demonstrator - The demand The BBC is planning to conduct a field trial exper-
and high for broadband connections will arise initially in iment with WDM, and detailed performance tests
BCPNs. That need is among others, related to the will be carried out to discover whether or not the
j-laibility. development of all kinds of (interactive) video ser-
vices and data communication. High-quality video
multicarrier heterodyne hardware will work in
the WDM network. Heterodyne multicarriers will
is required in medical environments where moving be incorporated in a spare 4-nm WDM channelwhich
pictures must be discussed or analyzed in detail. has a central wavelength of 1552 nm. Heterodyne
Another environment where high-quality video is transmitters and receiverswill be interconnectedvia
required is the industrialfactory environment. Indus- an optical star coupler, as shown in Fig. 8b. The
trial processes require accurate monitoring and the heterodyne transmitters and receivers will be
qualityof thevideo material must allow detailed anal- used to interconnect H D T V cameras, HDTV
ysis at a later stage, if required. High-quality monitors, and professionalvideotape recorders. The
video pictures must remain uncompressed to transmission bit rate on each heterodyne carrier will
allow processing without loss of information. be 2.5 Gb/s, and the experiment will start with
Among many technical solutions, heterodyne three heterodyne carriers.
multicarrier technology may be considered very The long-term goal of the demonstrator will
promising, as it combines high sensitivity, rely on:
high capacity, and high flexibility. These advan- The use of a passive optical star network.
tages can be exploited in the design of a net- All signal routing carried out by optical tuning
work capable of fulfilling all future demands of of the receivers.
business customers. The feasibility of multicar- The assignment of one optical carrier per signal.
rier heterodyne transmission in BCPNs and the The need for several hundreds, possibly several
testing in field environments a r e t h e main thousands of carriers eventually.
goals of the 155-Mb/s PTT demonstrator. ATM The use of components with common specifica-
will be used to multiplex audio, video, and data. tions.
T h e use of ATM will allow a variety of multi- The BBC demonstrator is thus aimed at show-
media applications. ing the increased capacity and routing flexibility
In Fig. 8a, the scheme of the PTT demonstra- of heterodyne multicarrier networks over other types
tor is shown. The topology inside the BCPN is of optical networks for transporting studio-quali-
based on a passive optical star. Five heterodyne ty TV and HDTV signals (> 1 Gb/s).
transceivers at various locations are able to com-
municate at several carriers. Moreover, connections Economic Aspects
with the outside world are possible. These con- The evaluation of economic aspects of heterodyne
nections will be realized over the public network multicarrier systems is an important part of the work
via the PTT local exchange (LEX). In the BCPN- within the COBRA project. In order to allow a
LEX link, an OA is introduced to compensate for comparison between heterodyne multicarrier sys-
the fiber attenuation and the tree-splitting losses tems and other types of systems, one must select
in the LEX. a particular functionality. In the analysis, key
30 IEEE Communications Magazine March 1994
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.functionalities of the demonstrators will be simu-
lated using diffcrent technologics. Cost comparisons
will sometimes rely on estimations because spe-
cific performance targets may not exist in prac-
ticc at thc present timc. Examples are crossconnects
witha throughput of IOGb/sifibcr. Asdiscussedear-
lier, that throughput can be achicvcd by using bit
rates of IO Gbis or by using a four-channcl multi-
carrier system with a bit rate of 2.5 Gblsichannel.
First results of the evaluation already indicate
that heterodyne multicarrier systems may be an
attractive solution in many areas. Detailswill be pro-
vided in future reports.
Currently, a worldwide effort in optical and opto-
electronic integration is underway. Clearly, the main
aim is to rcducc costs in the long run, in much
the same way as intcgration has worked for elec-
tronic circuits. Ovcr the last couplc of years, sev-
eral companies havc demonstrated [81-971 the
integration of essential parts of the hctcrodyne
receivers, in eithcr a monolithic or hybrid way.
Whether and how soon this will lead to cost
rcduction is very difficult to estimate, since we
are at the beginning ofthe learning curve in this field.
An achievement that also contributes to cost
reduction is the availabilityof mature modeling tools
for heterodyne systems and relatedcomponents [98-
1011.Experience with the usc ofthese modeling tools
in the RACE projects has dcmonstrated that
these models are accurate and that thcy can sig-
nificantly reduce the number of hardware tests
I I
required in thc course of the design of systems Figure 8. Examples of the BCPN system triu1.Y of'tlie European Community
and components. RACE project COBRA. ( U ) Outline of the Dutch PTT system trial. The BCPN
consists of heterodyne transceivers (CTCs) which ure connected to U passive
Comparison with Direct Detection optical star. Connections to the outside world are reulized over the public net-
Methods work via a PTT LEX. ATM will be used to miihipli~.xdifferent types of senices,
A detailed comparison between optical hetero- and a variety of multimedia applications will he possible. Heterodyne optical
dyne systems and optically preamplificd direct carrim operating at 155 Mbis in the 1550-tini wuidength region will be sepu-
detection systemsindicates that the two schemescan rated b-v WDM devices from an optical direct drtection overlay operuting at
havc equivalent performance [2). In that case, m u c h lower bit rates in the 1300-nnz wavelength rexion. The overlay network
however, the preamplified direct detection receiv- will be used for network nianagement and cotitrolpurpo.ses. An O A is includ-
er has to be provided with ii polarization filter ed in the BCPN-LEX link to compensatefor thc~jihcrattenuation and tree-
and means to handle the polarization state of the splitting losses in the LEX. The .system concept is htiseri on full-scule operation
incoming optical signal. Both schemes therefore with the incorporation of more than 100 hetcwdjnr trunsceivers. (b) Outline
contain similar functional components and mod- o f the RBC system trial. The heterodyne midticurrier .system test will be con-
ules. One important difference is that, currently, ducted in an existing WDM network infrastructure. Heterodyne multicarriers,
optical amplifiers based on Erbium-doped silica with m bit rate of2.5 Ghls each, will be incopirutrd in ( I spare 4-nm WDM
fibers (EDFAs) have a limited band of about 30 channel with U central wavelength of 1552 nni. Tlic system will be used to
nm in the 1550-nm wavelength region, while het- interconnect HDTV monitors, HDTV camerus, and .stirdio-quality VCRs.
erodyne receivers with similar performance are
not hampered by restrictions imposed by such
special components. In addition, the EDFA gain [102]. However, it is difficult to realize an optical
within this band of 30 nm may show nonunifor- tunable filter that can combine all these proper-
mities [103, 1041, leading to the need to compen- ties in one device. It is much easier to combine selec-
sate the attenuation budget of separate WDM tivity, large tuning range and high tuning speed in
channels within that band. Heterodyne receivers heterodyne rcccivers. Semiconductor lasers can
can be realized in any wavelength range covered be tunedveryrapidly and tuning ranges of more than
by available semiconductor lasers. This is one of 7nm (continuous) and 30nm (not continuous) havc
the reasons why the operator of the TV studio been reported by many. The filteringfunction in het-
mentioned earlier is considering upgrading a WDM- erodyne receivers is performed by electrical
bascd BCPN with heterodyne multicarrier tech- means in the IF domain. It is therefore much eas-
niques. ier to realize filterswith a sharp profile and adequate
An important difference in the case of multi- far-off blocking properties. A multiwavelength trans-
carrier o r multiwavelength transmission is the port network concept employing both WDM and
performance of the devicesused for tuning and select- heterodyne techniques is being studied in the RACE
ing. In WDM systems, channel selection can be Phase I1 project R202X MWTN [ 1031.The resultsof
performed by means of a tunable optical filter. A that project will also contribute to the under-
survey of the existingtunable optical filters illustrates standing of how different multicarrier or multi-
that such devices can be made to have a high wavclength techniques can be implemented in
selectivity,a large tuning range, or a fast tuningspeed the most efficient way.
IEEE CommunicationsMagaLine March 19'94 31
~
Authorized licensed use limited to: Eindhoven University of Technology. Downloaded on June 24,2010 at 09:41:46 UTC from IEEE Xplore. Restrictions apply.-
Cun-entlv,a
Conclusions
e have seen a steady progress in hetero-
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[781 T Hermes. "New Reflective Type Star Couplers for Full Duplex nieurfrom the EindhovenUniversityofTechnology. Eindhoven,The Nether-
Channels," Elect Lett. vol 27, pp 2136-2138. Nov. 1991 lands, i n 1971 and received t h e l E E E Fellow Award i n 1991. He
1791 F J. Westphal e t al.. "Coherent Multi Channel Experiment for worked at the F 0 . M Institute of Plasma Physics, Rijnhuizen. The
Bidirectional Communication at the Same Frequency Using an Netherlands, on laser diagnostics of plasmas from 1971 t o 1973. In
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vol 1, pp. 401-404. paper WeA9-3. Sept. 27-0ct 1. 1992. he pioneered in single-modefiber systemsand devices SinceAugust 1983,
1801 K . Kato et a / , "Packaging of Large-scale Integrated-Optic N x N he has also served as part-time Professor at the Eindhoven University
Star Couplers." lEEE Phot. Tech. Lett., vol. 4, pp 348-351, Mar. 1993. of Technology At the University, he i s currently presenting a fourth^
[ 8 l I J Hankey et al.. "A Balanced Polarisation Diversity Receiver Using year course on Lightwave Technology. Within the framework of the
Hybrid Assembly Techniques for Coherent Multichannel Systems,'' European Committee RACE program, he participated in several audit
Elect. Lett.. vol. 27, pp. 1935.1937. Oct 1991. and evaluation panels. He i s currently Project Leader of the RACE pro
1821 H Rodler et al., "High Performance Balanced Heterodyne Front ject COBRA. which deals with the field tests of heterodyne multicarrier
End Using Special Fibre Coupling Scheme," Elect. Lett., vol. 27. systemsin broadband optical networks Hehasauthoredand coauthored
no. 3, pp 249-251, Jan 1991. more than 60 papers, invited papers and books, and he holds 38 Unit-
1831 J G. Bauer e t a / , "Monolithic Dual PIN-FET Combination for ed States patents and more than 100 patents in other countries
IEEE Communications Magazine March 1994 33
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