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IEICE TRANS. ELECTRON., VOL.E82–C, NO.2 FEBRUARY 1999
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INVITED PAPER
Joint Special Issue on Photonics in Switching: Systems and Devices
Wavelength Converter Technology∗
Kristian E. STUBKJAER†a) , Allan KLOCH† , Peter Bukhave HANSEN† ,
Henrik N. POULSEN† , David WOLFSON† , Kim Stokholm JEPSEN† ,
Anders Thomas CLAUSEN† , Emmanuel LIMAL† ,
and Alvaro BUXENS† , Nonmembers
SUMMARY Wavelength conversion is important since it ensures full flexibility of the WDM network layer. Progress in optical wavelength converter technology is reviewed with emphasis
on all-optical wavelength converter types based on semiconductor
optical amplifiers.
key words: wavelength converter, cross connect, WDM, optical
network, optical processing
1.
Introduction
Wavelength division multiplexing (WDM) is considered
one of the most feasible ways to upgrade point-to-point
transmission links and to meet the huge demand for
transmission capacity. Next step in the rapid evolution is the introduction of WDM networks for efficient
transport of information. As the network technology
evolves we want the possibility of wavelength conversion or translation within the network or at its interfaces. The motivation is to achieve an optical layer
providing dynamic transport reconfiguration, high level
restoration and efficient use of the available fiber bandwidth the same way as the synchronous digital hierarchy (SDH) layer provides efficiency to the transport
layer by time division multiplexing, time slot grooming
and high speed protection.
The aim of this paper is to briefly review the current state of optical wavelength conversion technology
with emphasis on all-optical wavelength converters. Although WDM networks have the attractive feature that
different signal formats can be transported in different
wavelength layers, our focus will be on binary signal formats that are by far dominant for telecommunication
traffic.
The paper will start by giving a short motivation
for wavelength converters in WDM networks in Sect. 2.
The various types of wavelength converters are examined in Sect. 3, while Sect. 4 gives examples of new development areas for wavelength converters. Finally, a
conclusion is given in Sect. 5.
Manuscript received October 6, 1998.
The authors are with COM Center, Bldg. 349, Techn.
Univ. of Denmark, DK-2800 Lyngby, Denmark.
a) E-mail: [email protected]
∗
This paper is also published in IEICE Trans. Commun.,
Vol.E82-B, No.2, pp.390–400, February 1999.
†
2.
Wavelength Converters in WDM Networks
Several field trials with WDM networks that feature
wavelength switching and routing have been reported,
e.g. [1]–[3]. Although the work on WDM network architectures is in the initial phase, it seems that for full
flexibility it is very attractive to be able to translate
the channel wavelengths in an easy way [4], [5]. This
is motivated by the resulting capacity increase, easier failure recovery, and reduced network complexity,
e.g. [4]–[13]. The converters make it possible to assign
wavelengths on a link by link or on a sub-network basis thereby relaxing the requirements to the wavelength
precision throughout the whole network [4]. Moreover,
wavelength conversion eases the recovery from link or
node failures by allowing for local rather than global
reconfigurations in the network. Networks will also become more scaleable by use of converters and a slight
capacity increase will be achieved.
Wavelength converters will be placed in the network nodes where they can be operated with either
a fixed input wavelength and a variable output wavelength or a variable input and a fixed output wavelength. Only special cases will require variable in- and
output wavelengths at the same time. A converter with
a fixed output wavelength is simpler than one with a
variable, but the complexity of the wavelength converter is traded for that of other parts of the optical
cross connects [14]. The converters will under all circumstances add complexity to the hardware. Therefore
it is still an open issue if converters should be used in,
e.g., all cross connects or at network segment interfaces.
The first trials with wavelength converters in
switch blocks have just begun. A fine example is the optical cross connects made by the ACTS project OPEN.
It has 4 ports with 4 wavelengths per port and has recently been tested in a field trial between Norway and
Denmark [1]. Another impressive optical cross connect
has been demonstrated by NTT. It is a 16 × 16 switch
with 8 wavelengths employing tuneable wavelength converters at the input [15].
Even more advanced switch blocks with wavelength converters are investigated for optical packet
switching. In this type of switch block the convert-
STUBKJAER et al: WAVELENGTH CONVERTER TECHNOLOGY
339
ers can be used to address space in optical delay-line
buffers [16]–[18] or to address output ports in the switch
[18], [19]. Examples of packet switch block demonstrators with wavelength converters are reported in [20],
[21].
So, network solutions with wavelength conversion
are emerging. Besides, it should be noted that wavelength conversion is already used in commercial WDM
point-to-point systems in transponders that ensure correct channel wavelengths irrespective of the transmission equipment used in the tributary links.
The requirements to the converters will clearly be
application dependent, but desired features include:
– Transparency to the signal format (typically IMNRZ)
– Bit rate capabilities of at least 10 Gbit/s
– High dynamic range (because signal levels in the
switch blocks will be path dependent)
– Signal reshaping (because other optical network elements can corrupt the signals)
– Cascadability of a few stages of converters (because
multihop connections will be needed in most networks)
– Low electrical power consumption
– Operation at moderate optical power levels
– Easy implementation (to enable low price and reliability)
In the following we review the progress in wavelength converters towards these goals.
3.
Wavelength Converter Types
The converters fall into four groups: 1) Opto-electronic
converters; 2) Laser converters; 3) Coherent converters
(four wave mixing and difference frequency generation);
and 4) Converters based on optically controlled optical
gates.
Opto-electronic conversion is a straightforward solution that is already used in WDM point to point
links as mentioned above. Of the last three converter
types, which are all-optical, those based on optically
controlled gates are currently the most promising. We
will therefore put most emphasis on the description of
these.
3.1 Opto-Electronic Converters
The opto-electronic (O/E/O) converter is conceptually
simple since it consists of a detector followed by amplification or regeneration and transmitter stages as
shown in Fig. 1. The approach has the big advantage
that it can be implemented with “off the shelf” technology. So implementation is relatively simple. The optoelectronic converters have low optical power requirements and potentially a large input power dynamic
Fig. 1
Schematic of opto-electronic converter.
range and they are polarization independent. Depending on the use of simple electrical amplifiers or electronic regenerators they will even reshape the signal.
Demonstrators with such converters have given impressive results. As an example a 4-channel 2.5 Gbit/s
network experiment with opto-electronic converters in
three nodes has been reported [22]. Reference [15]
reports an 8-channel 16×16 wavelength translating
switch block with line rates of 2.5 Gbit/s. The O/E/O
converters in this experiment are built into compact
boards.
Recently, very compact units made directly for the
purpose of wavelength conversion have been reported
[23]. The compact, reconfigurable 2.5 Gbit/s O/E/O
converter uses an integrated pin/HBT receiver for driving the modulator section of an opto-electronic integrated array laser containing up to 12 selective channels
spaced 100 GHz apart.
Most opto-electronic converters reported till now
operate at 2.5 Gbit/s. For wavelength conversion in
high speed networks operating at 10 Gbit/s and in the
future possibly at 100 Gbit/s, the power consumption
of the opto-electronic converter will be high and bandwidth limitations of electronic circuitry may be encountered. Therefore, all-optical converters are highly interesting.
3.2 Laser Converters
All-optical wavelength conversion can be performed in
a very simple way by optical control of single frequency
lasers as shown in Fig. 2. The input signal (λi ) to be
converted is launched into the laser where it causes gain
saturation that controls the oscillation of the laser. The
result can be either IM or CPFSK output formats depending on the operation of the laser [24]–[26]. The
lasing wavelength (λc ) is either fixed or electronically
tuneable depending on the system requirements.
With steep input-output transfer functions (see
Fig. 2) the IM output format mode can achieve fine signal waveforms that even allow for cascading of a few
converter stages [27]–[29]. The IM output mode is,
however, associated with chirp [24], [30] that will limit
transmission on non dispersion shifted optical fibers.
The problems related to chirp could be overcome by
operating the laser well above threshold in the CPFSK
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Fig. 2 Principle of wavelength converters based on semiconductor lasers together with schematic of output versus input
power characteristic for laser converter.
output mode. In this case a frequency discriminator/filter is, however, needed at the output to obtain
an IM signal format [24], [31].
Note, that the laser converter basically consists of
a single component, so it is conceptually simple. Its
optical input power requirements are 0–10 dBm, but
unless a special waveguide design is implemented for
the gain section of the laser, it is polarization dependent. Moreover, the maximum bit rate, determined by
the laser’s resonance frequency, is limited to around
10 Gbit/s [26]. Because of the speed limitation, laser
converters are less interesting.
Work on laser converters exhibiting optical bistability for regeneration has also been pursued, e.g. [32]–
[34]. The bistable element does, however, limit the
conversion speed to the order of 1 Gbit/s, making the
bistable converters unattractive.
3.3 Coherent Converters
Converters that rely on four wave mixing (FWM) have
been extensively investigated using both optical fibers,
e.g., [35] and semiconductor optical amplifiers (SOAs),
e.g., [36]–[39], as nonlinear elements. The scheme is
inherently fast for both fiber and semiconductor nonlinear elements. The converters can handle all signal
modulation formats in contrast to other types of converters that are more suited for IM input signals. The
transparency to signal format also implies that many
WDM channels can be converted simultaneously [40].
The conversion efficiency is normally low (typically
around −20 dB), so optical power levels of ∼10 dBm
have to be used for the pump of SOA converters while
10–20 dBm is needed for fiber based converters. Because of the low conversion efficiencies the signal-tonoise ratio for the converted signals needs attention,
especially if converters have to be cascaded. Experiments using SOA converters with very long cavities
Fig. 3 Schematic of conversion in nonlinear element using
four-wave mixing and difference frequency generation.
have, however, resulted in conversion efficiencies approaching 0 dB [41] thereby making FWM more attractive.
A serious drawback for the FWM converter is the
dependency of the output wavelength on both the pump
(λp ) and the input signal (λi ) wavelengths, so the pump
must be tuneable even for converters with fixed output
wavelength. Moreover, two pumps will be needed to
ensure polarization insensitive operation [40], [42].
Because of the relatively complex pumping scheme,
FWM converters will probably only be used for conversion of ultra fast signals at bit rates above 100 Gbit/s.
Another potentially important application for FWM
converters is mid-span spectral inversion for dispersion
compensation. Here the optical phase conjugation of
the converted signal is used to advantage. For midspan spectral inversion a fixed pump wavelength can be
used, so the set-up is simpler than that of a converter.
Impressive results have been reported using 2 mm long
SOAs [43]. For these S/N ratios of 45 dB in 0.1 nm
bandwidth were achieved and a 40 Gbit/s signal successfully transmitted over 406 km of NDS fiber. Similarly, an impressive mid-span spectral inversion experiment relying on four wave mixing in highly nonlinear
dispersion shifted fiber has been reported in [44]. Here
simultaneous conversion of five 40 Gbit/s channels together with transmission over 105 km of standard single
mode fiber is achieved.
Wavelength conversion based on difference frequency generation (see Fig. 3) in periodically loaded
waveguide structures of LiNbO3 [45] or AlGaAs [46]
have also been reported. One advantage is polarization insensitivity of the scheme, otherwise it features
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Fig. 5 Bit error rate for 40 Gbit/s signals: 1554 nm (input)
and converted signals at 1548 nm using a single and double SOA
converter [56].
Fig. 4 Schematic of wavelength converter based on optically
controlled gate and the dependency of the converted signal on
the transfer characteristic of the gate.
almost the same advantages and disadvantages as four
wave mixing. Conversion efficiencies are in the −17 dB
range therefore requiring relatively high optical power
levels.
3.4 Converters Based on Optically Controlled Gates
Converters made from optically controlled gates appear
at the moment to be the most promising all-optical converter types. As illustrated in Fig. 4, the principle is to
let the input power at λi control the gating of CW light
at λc . Thereby the data are converted from λi to λc .
The CW light originates from a light source with either a fixed or a tunable output wavelength depending
on the application of the converter. Clearly, the transfer function of the gate should be as steep as possible
and depending on its positive or negative slope the converted signal will be in-phase or inverted relative to the
input.
3.4.1 XGM Gate
A simple optical gate is realized by a semiconductor
optical amplifier (SOA). Its gain saturation due to an
optical input signal is controlling the gain and thereby
the state of the gate [47], [48]. The resulting converter,
also called a cross gain modulated (XGM) converter, is
extremely simple to assemble. It is polarization insensitive because of polarization independent SOA gain and
is very power efficient. It has, however, a number of
shortcomings: The signal is inverted relative to the input signal (negative slope) and the extinction ratio for
the converted signal may degrade going from shorter to
longer wavelength [49]. Moreover, the converted signal
has a relatively large frequency chirp. Despite the nonideal properties, the converter has been used with fine
results in a number of switch block experiments, e.g.
[19], [50], and it remains attractive for many applications because of its simplicity. The shortcomings may
also be eliminated if an interferometric converter (see
next section) follows the XGM converter.
The conversion speed is determined by the carrier
dynamics that are governed by the relative slow interband carrier recombinations [51]. The detailed analyses
in [52] and [53] show that gain saturation plays a significant role for obtaining the high bit rate. Simple
guidelines for achieving high bit rate conversion is to
operate the SOAs with:
1) Large current injection
2) High optical power levels
Moreover, the SOA waveguides should have:
3) Large optical confinement factors
4) Large differential gain
Since the allowed injection current is limited the conversion speed can also be increased using longer cavity
lengths. Direct coupling of two SOAs is an approach to
a longer cavity [54].
For 1.2 mm long specially designed SOAs [55]
40 Gbit/s conversion has been achieved as shown in
Fig. 5 [56]. The penalties are only ∼4 dB for a single SOA converter and ∼2 dB for two directly coupled
SOAs used as a converter unit. Recently impressive
100 Gbit/s wavelength conversion has been achieved by
XGM conversion in a 2 mm long SOA followed by a
grating for FM to AM conversion of the chirped output
to extend the conversion bandwidth [57].
Obviously, the SOA converters add spontaneous
emission noise to the converted signals. Still, with
noise figures of 8–10 dB (including 2–4 dB input coupling loss), it is possible to cascade a few SOA-XGM
converters as shown in, e.g., [58].
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MZI converter. Therefore it works very similarly. It
should be noted, however, that the MZI allows the
CW-light and the input signal to be launched counterdirectionally, whereas, for the MI, a part of the original
signal will always pass through the converter and be
transmitted along with the converted signal. This prohibits the input and output wavelengths to be identical
due to interference crosstalk.
NOLM-converters:
Nonlinear loop mirror configurations with SOAs are
also reported [67], [68]. The converter is inherently balanced, but only RZ signals can be handled and the
signal bit-rate is determined by the placement of the
SOA in the loop. NOLM-converters with fiber as the
nonlinear element are also reported, e.g. [69].
Fig. 6 Wavelength conversion principle using MZI and MI
structures with SOAs.
3.4.2 XPM Gates
Better wavelength converters are obtained by integration of two or more SOAs into interferometric waveguide configurations (Fig. 6). In these converters, the
optical input signal controls the phase difference between the interferometer arms through the relation between the carrier density and the refractive index in the
SOAs (cross phase modulation, XPM); thereby a CW
light is modulated [59].
For stable operation the XPM converters must be
integrated. An impressive activity on opto-electronic
integration of interferometric converters has taken place
resulting in rapid progress towards compact and efficient monolithically integrated converters [60]–[67].
Several approaches are taken:
MZI-converters:
The Mach-Zehnder structures have been developed into
structures with a separate waveguide for coupling the
input signal into only one of the SOAs as shown in
Fig. 6 (a). The idea is that the input signal depletes the
carrier concentration in only one of the SOAs thereby
creating the needed phase difference between the two
interferometer arms in a very efficient way [63]–[66].
It should be noted that the work on Mach-Zehnder
interferometric converters started with two-port devices
[60], [61].
MI-converters:
Michelson interferometric converters have a simpler
structure since they offer direct access for the input
signal to the SOAs [62], cf. Fig. 6 (b). The high optical power levels due to the direct coupling will enable a high modulation bandwidth. The MI converter
has a reflective facet making it a folded version of the
Different approaches are also taken regarding integration of the SOAs into the inter-ferometer:
All-active: Both splitters and gain sections have the
same material composition. The gains in the different parts of the structure are determined by the
bias current to the electrodes. This approach has
the advantage of being simple and very compact
[63]. Initially concern was raised that the pumped
in- and output couplers would contribute significantly to the noise level. Experiments show that
this is not so.
Active-passive: The active SOA sections are coupled to
passive waveguides leading to a simpler electrode
structure and to more well defined gain sections
[64]–[66].
Hybrid active-passive: Hybrid integration of SOAs on
PLCs has also been reported [70]. The approach
is interesting since waveguide technology in Si is
relatively mature.
More work is still needed to determine which approach
should be taken. It should be noted that almost polarization independent operation can be achieved for
correct design of the interferometers.
The interferometric converters have the advantage
of very steep transfer functions (see Fig. 7) enabling
extinction ratio regeneration of the converted signal.
As also seen, the conversion can take place on either
the positive or the negative slope of the interferometer curve, where clearly the positive slope is the most
attractive since the converted signal is non-inverted.
Only small input signals are needed to introduce a π
phase difference between the interferometer arms, so
very efficient conversion is obtained almost independent
of wavelength. Because of the small modulation associated with the π phase shift, the frequency chirp of the
output signal will also be small compared to, e.g., the
XGM converter [62], [72], [73]. Moreover, for conversion
on the positive slope the resulting chirp compensates
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Fig. 7 Examples of static characteristics of active-passive MZI
converter for two different bias currents the SOA in one of the
interferometer arms [71].
Fig. 9 Schemes for enhancement of the input power dynamic
range of interferometric wavelength converters [84].
Fig. 8 BER performance of 40 Gbit/s wavelength conversion
with all-active MI-converter. Results are after conversion ("")
and back-to-back (!|) [80].
against the influence of fiber dispersion.
Besides signal waveform and spectral reshaping,
the interferometric converters have excellent noise properties with optical signal-to-noise ratios for the converted signals as high as 30 dB (measured in 1 nm spectral bandwidth). Moreover, the noise is redistributed
due to the transfer function [74], [75]. As a result, the
noise is accumulating less rapidly than for a chain of
optical amplifiers. This allows for cascading of several
converters [76]. The regenerative properties of the interferometric converters are important for construction
of all-optical cross connects in which other components
(e.g., amplifiers and gates) may degrade the signal quality.
The interferometric converters will typically have
< 1 dB penalty for bit rates of 10 Gbit/s as has been
reported by many groups, e.g., [62], [77]–[79]. Like the
XGM converters, the interferometric XPM converters
can also achieve 40 Gbit/s as seen in Fig. 8 [80]. The
same guidelines as for XGM converters apply for achieving high bit rate. Especially coupling losses should be
kept small to enable high optical power levels. Therefore the MI structure with direct coupling of the input
signal has achieved the highest conversion bandwidth so
far. The MI converter used in the 40 Gbit/s experiment
is realized using multiquantum-well based layer structures with a 10 well tensile strained InGaAs/InGaAsP
waveguide core. It consists of an active 3 dB coupler followed by two 600 µm long amplifier sections, resulting
in a total device length of only 1.3 mm [77]. It should
be noted, however, that differential switching schemes
(e.g. [81], [82]) can be used for penalty free conversion of
RZ-signals at 40 Gbit/s with potential for much higher
bit rates [83].
Progress toward development of practical interferometric converters has been achieved with the realization of packaged all-active MZI structures [1]. These
packaged converters have been tested in field trials. A
practical obstacle for the interferometric converters is
their small input power dynamic range of typically 2–
3 dB. This can, however, be significantly improved by
simple compensation schemes as shown in Fig. 9 [84].
In 10 Gbit/s experiments it was shown that Scheme A
with simple power monitoring and electronic feed forward gives an 8 dB dynamic range, while the use of an
EDFA preamplifier operated in saturation, as shown in
Scheme B, can give a 40 dB dynamic range. It may be
a disadvantage that the EDFA has a response time in
the millisecond range. Use of a SOA preamplifier with
electronic gain control as shown in Scheme C is much
faster and can give a 28 dB dynamic range.
Another solution for a high input power dynamic
range is the combination of a XGM and a MI-XPM
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converter as discussed in [85]. The scheme has the additional advantage of a relatively fast conversion bandwidth combined with possibility for conversion to the
same wavelength.
Other very interesting advances in the field of interferometric converters include the integration of optical preamplifiers on the same wafer [86]. Recently MZI
converters with a bi-modal waveguide structure have
also been reported [87]. They allow the input signal
and the probe signal to co-propagate in the structure
using different spatial modes. Therefore, conversion to
the same wavelength is possible without use of counterpropagation, which normally results in a smaller conversion bandwidth.
complicated set-ups for polarization insensitive operation. A very interesting application is, however, midspan spectral inversion for dispersion compensation.
Interferometric converters have shown excellent
properties with respect to bit rate capability and signal
quality for the converted signal. Importantly work on
packaging of these relatively complicated devices has
been reported and research on “second generation” devices is under way. Attention should also be given to
XGM and XAM based converters that may be very interesting for a number of applications.
Moreover, the work on wavelength converters
seems to lead to exciting new system blocks such as
all-optical 3-R regenerators.
3.4.3 XAM Gate
Acknowledgement
Briefly it should also be noted that very promising results have been achieved for cross absorption modulation in electro-absorption modulators [88]. It has been
shown that the technique works up to at least 40 Gbit/s,
that extinction ratio enhancement may be achieved and
that 30 nm operation bandwidths are possible.
4.
All-Optical 3-R Regenerators
The interferometric (XPM) wavelength converters have
2-R regenerating capabilities. It is, however, very exciting that XGM and XPM wavelength converters combined with optical clocks lead to 3-R regenerative capabilities [89]–[91]. A fine example of the construction
of such a regenerator is given in [90], where an XGM
converter is used in the first stage to sample the input
signal with extracted clock pulses and to equalize input power fluctuations. In the second stage an XPM
converter follows. It regenerates the extinction ratio resulting in complete regeneration. The scheme has been
demonstrated for cascading a number of 140 km links in
a 10 Gbit/s loop experiment thereby allowing for transmission over more than 200,000 km of fiber [92].
5.
Conclusions
Progress in the field of optical wavelength conversion
has been assessed. The development and test of different converter technologies is still in progress, so final
conclusions can not be made. It appears, however, that
at bit rates of 2.5 Gbit/s the O/E/O converters will remain unchallenged because of their fine properties and
well known technology.
At 10 Gbit/s and above all-optical conversion
schemes may become competitive as they feature high
bit-rate operation. This is important especially for cascading of many converters. Coherent converter types
will probably be restricted in use to extremely high
bit rates since they require tunable pumps and more
Many of the reported results are generated within
the ACTS projects KEOPS, OPEN and HIGHWAY.
We are grateful to our colleagues within the projects
for their help. In particular we should mention P.
Doussiere, M. Schilling, and C. Janz of Alcatel, S.
Bouchoule of CNET and R. Hess and H. Melchior of
ETH. We also thank N. Vodjdani and F. Ratovelomanana of Thomson CSF, our colleague R.J. Pedersen
as well as our old colleagues B. Mikkelsen (now with
Lucent Technologies), M. Vaa (now with Tyco), and C.
Joergensen and S.L. Danielsen (now with L.M. Ericsson).
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Kristian E. Stubkjaer
was born in
1953 and hold M.Sc. and Ph.D. degrees.
After research experience at Tokyo Institute of Technology, Japan, and IBM T.J.
Watoson Research Center, United States,
he became an associated professor at the
Technical University of Denmark in 1983.
His research is in the field of active components for optical systems. From 1985
to 1990 he was head of the Electromagnetics Institute. He served as chairman
of the Electrotechnical Committee under the Danish Technical
Research Council (Danish Ministry for Research) from 1991 to
1997. In 1998 he was appointed director for the new center at
the Technical University of Denmark for Communications, Optics
and Materials.
Allan Kloch
was born in 1971 and
became M.Sc. in 1996. He is now a Ph.D.
student at the Center for Communications, Optics and Materials at the Technical University of Denmark. His fields of
interests are components for signal processing in WDM networks and supervision of optical networks.
Peter Bukhave Hansen
was born in
1971 and received the M.Sc.E.E. degree in
1996. He is currently a Ph.D. student at
Center for Communications, Optics and
Materials at Technical University of Denmark, where optical packet switching and
WDM networks are his special areas of
interest.
Henrik N. Poulsen
was born in
1969 in Copenhagen. In 1995 he received
his M.Sc.E.E. from the Technical University of Denmark and is now working towards the Ph.D. degree at the Center for
Communications, Optics and Materials at
the Technical University of Denmark. His
field of interest is high bit rate signal
processing in semiconductor devices, in
particular all-optical demultiplexing techniques and their applications for telecommunication.
David Wolfson
was born in 1973 and
received the M.Sc.E.E. degree in 1997. He
is currently a Ph.D. student at Center for
Communications, Optics and Materials at
Technical University of Denmark, where
optical regeneration and all-optical signal
processing are his areas of interest.
Kim Stokholm Jepsen
was born in
Copenhagen in 1963 and holds the M.Sc.
and Ph.D. degrees from the Technical
University of Denmark. His research has
mainly been in the fields of OTDM networks and high-speed optical signal processing using semiconductor optical amplifier based devices.
Anders Thomas Clausen
received
his M.Sc. E.E. degree in 1997. After
graduation he became a research associate at the Department of Electromagnetics Systems and now at the Center for
Communications, Optics and Materials.
His research are been in the fields of alloptical regeneration, all optical clock recovery and OTDM systems.
Emmanuel Limal
was born in
Fontainebleau, France, in 1970. He received his M.Sc. from the Ecole Spéciale
des Travaux Publics et de l’Industrie,
Paris, in 1993.
In 1995, he finished an International Master Programme
in Physics and Engineering Physics
from Chalmers University of Technology,
Göteborg. Since then he was employed
as a research associate at the Center
for Broadband Telecommunications at the
Department of Electromagnetics Systems, Technical University
of Denmark, where he is currently working toward the Ph.D.
degree in optical networks. His field of interest is protection
against network’s link or node disruption, network scalability,
cross-connect architectures and network management.
Alvaro Buxens
was born in Bilbao
in 1971 and holds a M.Sc. degree from the
Telecommunications Ingennering School,
University of the Basque Country. He
completed his studies at the Electromagnetics Institute at the Technical University of Denmark in October 1997 were
he continued working as a research associate. His research is in the field of optical networks based in OTDM and diagnosis methods for optical communication
systems. He is currently with the COM-center at the Technical
University of Denmark.