ICTON 2010
We.P.16
Multiple-Bit All-Optical Logic Based on Cross-Gain Modulation
in a Semiconductor Optical Amplifier
Asier Villafranca1, Ignacio Garcés2, Miguel Cabezón1, Juan José Martínez1, David Izquierdo2, José Pozo3
1
TOYBA Lab. Photonic Technologies Group, University of Zaragoza, Walqa PT, 22197 Cuarte, Huesca, Spain
Tel: +34 974 215481, Fax: +34 974 215482, e-mail: asier.villafranca@unizar.es
2
Photonic Technologies Group, CPS University of Zaragoza, 50018 Zaragoza, Spain
3
COBRA Research Institute, Eindhoven University of Technology, 5612 AZ Eindhoven, Netherlands
ABSTRACT
In this paper we have evaluated the non-linear response of a semiconductor optical amplifier (SOA) in terms of
Cross Gain Modulation effect (XGM). The different ranges of optical power that make the SOA work as a logic
gate have been determined, as well as the optimum relation between inputs to obtain the best response. Taking
into account the SOA response, a schematic model for an all-optical 4-input NOR gate using a single SOA has
been proposed. Our experimental set-up has been tested with four streams of 10 Gbps RZ data to verify the
proper and stable operation of the NOR gate under diverse power levels for each data input. The logic output has
been evaluated for different inputs using an optical eye diagram to verify its quality; in addition, the possibility
of using the NOR output as the input to other logical semiconductor structures, which would allow to chain
multiple gates, has been demonstrated. The concatenation of NOR gates allows to build more complex all-optical
devices which will permit the design of advanced mechanisms in the field of transparent networks, such as
comparators or look-up tables for switching nodes, without the necessity of optical-electrical conversion.
Keywords: Logic Gate, All-Optical Data Processing, SOA, XGM.
1. INTRODUCTION
In the field of optical networks the inclusion of additional information (labels with destination and
monitorization data) in the transmission stream has been considered through the last years as a mean to cope
with the increasing capacity demands of optical networks. The traditional solution to process this information is
converting all the optical information to the electric domain in order to utilize traditional processing, and then to
transform again in optical format. This has important drawbacks: complexity (need of regeneration, buffering),
can’t be done in real time, has limitations in bitrate and elevated cost.
All-optical processing of the labels has been present in the literature [1]; however, many of these proposals
introduce the label information separated from the user information: in time, placed as header with a significant
guard band; in frequency using different wavelengths or in codification with different modulations schemes.
However, these proposals add additional complexity in the networks structures; so the possibility of having real
time optical logic in the bit stream will simplify the processing, not only for the label but for the entire bit
stream. This real time processing will make possible to build all-optical network devices that can perform
different tasks traditionally made by electrical processing, such as comparators, look up tables (LUT), error
correction, etc [2].
Is possible to take advantage of the different non-linearities present inside the SOAs (Four Wave Mixing [3,4],
Cross Gain Modulation [3,5], Cross Phase Modulation [6,7]) in order to achieve different logic functions.
Combining these non-linear effects allows to build diverse models of basic logic gates made using SOAs in
various configurations [8]. In our design we propose a NOR gate with several bit stream inputs (up to four in the
experiments) based in the XGM effect in a single SOA, which presents a simpler structure than other logic gates
solutions that need more amplifiers and more complex configurations.
2. XGM BEHAVIOUR EVALUATION
Figure 1. Basic XGM co-propagating setup.
Cross Gain Modulations is one of the non-linear effects that can arise inside a semiconductor optical cavity (as
a SOA). It takes place when a high power signal (called Pump) is injected into the SOA, depleting most of the
978-1-4244-7797-5/10/$26.00 ©2010 IEEE
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carriers present in the active region when it is amplified, if we simultaneously inject a lower power signal
(Probe) in the SOA it will be attenuated due to the absorption of the carriers. In the case where pump and probe
are binary signals modulated in amplitude, the XGM causes that the output probe signal can be interpreted as the
logic function (Probe)AND(NOT(Pump)), this response has been extensively referenced in the literature. In our
proposal we have chose a co-propagating setup similar to the depicted in Figure 1 [9,10]; although it requires
different wavelengths for each signal, its non dependence of circulators simplifies the design and allows the
possibility of translating the setup to an integrated solution.
A basic XGM characterization was performed in the scheme of Fig. 1, modulating both Probe and Pump
signals with a 10 GHz clock, the equivalent to a all one RZ 10 Gbps amplitude modulation, RZ is chosen
because its robustness against bursts and its overall better behaviour. Additionally, bit synchronization must be
secured between both input signals (Pump and Probe) for the experiment. The resulting Probe output power for
different input probes can be seen in Fig. 2.
Figure 2. Output Probe power as a function of input Pump and Probe powers.
Output probe power depends on the Pump but also a minimum Probe must be present in order to have an
output signal. Fig. 2 shows reduction of the output power as the Pump increases, as expected, the minimum
values are found for the maximum of Pump and the minimum of Probe, but this reduction only considers the
proper input values for a “0” output. In order to have also an appropriate “1” output value, Probe signal must
also have high enough optical power. Also, the highest values of Pump can not be used to avoid the suppression
of all pulses in the output, even the ones that should be a “1”. Considering all those restrictions, the optimum
range of power for the logic gate is in the marked zone of the graph.
3. NOR PROPOSAL AND EXPERIMENTAL SETUP
Figure 3. Experimental setup of 4-bit NOR.
Previous XGM characterization showed a wide range of possible Pump power that can be used and still obtain
a working logic gate, taking advantage of this fact, we propose using a sum of different optical signals as the
Pump. Key factor lies in that all the possible Pump power of the sum must have the same effect over the Prove
signal, so using the characterization we can determine the optimal working zone for our experiments.
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Specifically we choose a sum of four different data signals (A, B, C D) for the Pump, and an all-one signal for
the probe, this way the function (Probe)AND(NOT(Pump)) transforms into NOT(A + B + C + D) which is the
equivalent to NOR(A, B, C, D), this way we have a 4-input NOR gate using only one SOA.
Fig. 3 presents the experimental scheme for the 4-input NOR. Five tunable laser sources (TLSs) at different
wavelengths are used as the signals, four for the Pump and one for the Probe. Pump signals are coupled and
injected into two cascade Mach Zehnder Modulators (MZM) in order to obtain a 10 Gbps RZ 27-long PRBS
modulation while Probe TLS is modulated only with the clock to obtain an 10 Gbps RZ all-one signal. Pump
signals are amplified and separated through a WDM demultiplexer, after that each signal is delayed a different
number of bits in order to obtain diverse bit patterns. Later, all signals (Pumps and Probe) are coupled via
a WDM multiplexer into the SOA, finally the outcoming Probe is filtered in order to obtain only the logic gate
response and also to improve the overall quality of the final signal.
Figure 4. Traces of the signals at different points of the experiment.
Fig. 4 illustrates the behaviour of the NOR gate at different phases of the experiment, the synchronization of
the four Pump patterns is a key factor in order to obtain the desired sum; it can be seen how the total Pump
presents four different levels of “1” and that all of them creates a similar “0” level in the output. It is important
in order to obtain similar levels of “0” and maintain the quality of the output signal that the highest and the
lowest peak in total Pump signal generate the same response of the SOA. Best behaviour is obtained when the
Pump pulses arrive at the SOA slightly earlier than the Probe ones, as can be seen in the Fig. 4, in order to start
depleting the carriers in the SOA.
4. RESULTS
-4
(a)
x 10
8
7
7
6
6
5
5
Power (w)
Power (w)
8
4
3
2
2
0
0
1
Time (s)
1.5
-1
0
2
(b)
3
1
0.5
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1
-1
0
x 10
0.5
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Time (s)
-10
x 10
1.5
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-10
x 10
Figure 5. Eye diagram of the output signal a) High Power Setup b) Low Power Setup.
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Proper behaviour of the NOR logic gate (“0” output when one or more inputs are “1” and “1” output when all
inputs are “0”) has already been verified in Fig. 4. In order to evaluate the quality of the NOR output the PRBS
order of the Pump signals has been change to 231-long PRBS to evaluate a wider group of input conditions; also,
two different power setups were used: the optimum case (high power setup) with 0.6 mW Probe peak power and
2.5 mW peak power for each Pump signal; and the low power setup (10 dB lower), with noteworthy lower
powers: 0.25 mW peak power for each Pump and 0.065 mW peak power for the Probe.
Results for both configurations can be seen in Fig. 5: high power setup ensures a clear output eye with a peak
power of 0.65 mW, which is slightly higher than the used Probe power but clearly below the Pump. On the other
hand, although low power setup has a lower quality eye (but still quite acceptable) its peak output power is
around 0.35 mW, which is enough to act as both Pump and Probe inputs. This is a key factor to consider because
it will allow to chain more optical logic gates, since the logic output can act as a Pump signal for other SOA
based logic gates.
5. CONCLUSIONS
We have build an all-optical 4-bit NOR logic gate capable of working with 10 Gbps RZ modulated data streams
based on the XGM effect of a single SOA; the proper behaviour of the gate, as well as the quality of the output
signal has been evaluated an measured. Furthermore, it has been proved that when we work with the appropriate
power balance, the output of the logic gate suits the requirements to become the input for another gate, leading to
the possibility to build more complex schemes. Additionally, being able to perform the logical operation using
a single SOA implies a significant reduction in cost and complexity over the typical proposals of all-optical
logic. Both characteristics of the proposal (concatenation and simplicity) are vital for the future development of
integrated all-optical devices capable of perform the tasks in an optical network that currently are done via
electrical processing, such as: comparators, look up tables, forward error correction or label readers, etc; all of
them an important highlight in the way to achieve an all-optical transparent network.
ACKNOWLEDGEMENTS
This work was supported by the funds of DGA and IAF for research at Walqa Technology
Park, by the Spanish “Comisión Interministerial de Ciencia y Tecnología” through project CICYT-TEC200613907-C04-03/MIC and by the University of Zaragoza through project UZ2009-TEC-03.
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