All-optical XOR and XNOR operations at
86.4 Gb/s using a pair of semiconductor optical
amplifier Mach-Zehnder interferometers
I. Kang1*, M. Rasras2, L. Buhl1, M. Dinu1, S. Cabot3, M. Cappuzzo2, L. T. Gomez2, Y. F.
Chen2, S. S. Patel2, N. Dutta3, A. Piccirilli3, J. Jaques3, C. R. Giles1
1
Bell Laboratories, Alcatel-Lucent, 791 Holmdel-Keyport Road, Holmdel, NJ 07733, USA
2
Bell Laboratories, Alcatel-Lucent, 600 Mountain Avenue, Murray Hill, NJ 07974, USA
3
LGS Innovations, 15 Vreeland Road, Florham Park, NJ 07932
* inukkang@alcatel-lucent.com
Abstract: We propose a method for increased-speed all-optical XOR
operation using semiconductor optical amplifiers. We demonstrate XOR
and XNOR operations at 86.4 Gb/s using a pair of photonic-integrated
semiconductor optical amplifier Mach-Zehnder interferometers.
©2009 Optical Society of America
OCIS codes: (060.0060) Fiber optics and optical communications; (070.4340) Nonlinear
optical signal processing; (250.3140) Integrated optoelectronic circuits.
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#117590 - $15.00 USD
(C) 2009 OSA
Received 22 Sep 2009; accepted 28 Sep 2009; published 7 Oct 2009
12 October 2009 / Vol. 17, No. 21 / OPTICS EXPRESS 19062
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1. Introduction
All-optical logic operation is expected to play important roles in future high-speed optical
networks where reducing latency and power consumption is of great interest. For example,
all-optical header processing based on optical logic can mitigate the processing speed
bottlenecks and reduce high power consumption in optical data networking owing to the
limitations in high-speed electronic signal processing [1, 2]. Among the family of Boolean
logic functions, Exclusive-OR (XOR) plays central roles in all-optical bit-pattern matching
[3,4], all-optical label swapping [5], parity checking [6], and pseudo-random bit sequence
generation [7].
All-optical XOR operation of on-off-keyed (OOK) optical signals can be implemented
utilizing cross-phase and gain modulation in a nonlinear optical medium in combination with
an optical interferometer; for example, a Mach-Zehnder interferometer (MZI) incorporating
semiconductor optical amplifiers (SOAs) [4, 5, 8, 9], SOA-based ultrafast nonlinear
interferometers (UNIs) [10–12], and fiber-based nonlinear optical devices [13, 14]. It is well
known that the speed of SOA-based optical signal processing is ultimately limited by the
penalties arising from the nonlinear patterning effects due to the non-instantaneous recovery
dynamics of the cross-phase and gain modulation. The highest bit-rates at which XOR using
SOAs has been demonstrated for pseudo-random OOK signals are 40 Gb/s using the
conventional SOA-MZI configuration [8, 9], and 85 Gb/s using “turbo-switch” in a doubleUNI configuration [12]. In “turbo-switch” configuration, SOAs are cascaded where the SOAs
in the latter stages act as nonlinear temporal filters to reduce the patterning effect. For
example, three SOAs in series are used to achieve 85 Gb/s XOR in [12].
Here, we propose an alternative method of mitigating the patterning effects and achieving
increased-speed all-optical XOR operation that is well suited for large scale integration of
cascadable logic operations. The method utilizes a photonic-integrated SOA-MZI pair each
differentially driven by a data input and its complement. We demonstrate simultaneous XOR
and XNOR operations at 86.4 Gb/s using SOAs having a carrier recovery time of ~20 ps.
2. Principle
The proposed circuit accomplishing simultaneous all-optical XOR and XNOR operations is
shown in Fig. 1. It consists of a pair of SOA-MZIs. Each SOA-MZI is differentially driven by
data and complement. Note that a single SOA-MZI can be used for all-optical XOR as
previously demonstrated in [8, 9]. The arrangement in Fig. 1, in contrast to the conventional
single SOA-MZI XOR set up, ensures that each SOA receives a nearly constant-power stream
of input pulse train, except for the slight temporal shifts due the differential input delay t. This
is illustrated in Fig. 1 using an example of an on-off-keyed (OOK) signal A=[101]. Hence the
patterning effects can be mitigated that arise from the fluctuation in the signal optical powers
when SOAs are directly modulated by OOK signals without the differential set up. For
instance, the most deleterious situation is avoided, where the SOAs receive a long sequence
of 1's followed by a long sequence of 0's, or vice versa.
The working principle of the device shown in Fig. 1 is as follows: the top SOA-MZI
converts the input OOK data streams A and its complement A into a binary phase shift keyed
(BPSK) signal, exp(jπA). The bottom SOA-MZI similarly outputs a BSPK signal exp(jπB).
The conversion of OOK into BPSK was first used for all-optical wavelength conversion of
DPSK signals [15] and its process is detailed in [9, 15]. The next step in XOR operation is the
linear optical interference between these two BPSK signals. The intensity envelopes of the
output signals emerging from the constructive and destructive ports of the 2x2 coupler are
calculated using:
2
exp( jπ A) + exp( jπ B) ~ A ⊕ B
#117590 - $15.00 USD
(C) 2009 OSA
(1)
Received 22 Sep 2009; accepted 28 Sep 2009; published 7 Oct 2009
12 October 2009 / Vol. 17, No. 21 / OPTICS EXPRESS 19063
2
exp( jπ A) − exp( jπ B) ~ A ⊕ B
(2)
The interference yields two OOK signals, A ⊕ B and its complement A ⊕ B , where Å is
Boolean XOR logic, which is equivalent to binary addition modulo two.
Fig. 1. Schematic of the all-optical XOR circuit. CW: continuous wave laser. τ: differential
time delay.
The proposed method enhances XOR operation speed by suppressing the patterning
effects but does require the availability of complementary data input. However, the added
complexity is mitigated by the following factors that are favorable for scalable integration of
multiple optical logic gates: firstly, the circuit provides both XOR and XNOR output data,
which can be used as inputs for the next stages of additional XOR operations. Secondly, in
larger scale integration of multiple logic gates, amplification of signals using SOAs is
required between each steps of operation, for which PSK in comparison to OOK has the
advantage of reduced patterning effects during the SOA amplification. Thus, optical
amplification can be performed at locations where the signal of interest is in a PSK format.
3. Experiment
We implement the circuit outlined in Fig. 1 using a hybrid photonic integrated device
incorporating InGaAsP quantum well SOAs and a silica planar light wave circuits (PLCs).
We show in Fig. 2 a picture of the packaged device. The device consists of three parts: the
passive waveguides making up the input and output interferometer parts are implemented
using 4% index-contrast silica-on-silicon PLCs. The high-index contrast helps in reducing the
device footprint. The optical couplers in the silica PLC are implemented using 1x2 and 2x2
multi-mode interferometers (MMIs). The array of four semiconductor optical amplifiers is a
commercial device from CIP Technologies (Quad-SOA-NL-1550) with 20-25 ps saturated
SOA gain recovery time. The overall size of the device is 2.2 cm x 0.5 cm. The silica PLCs
and SOAs are actively aligned by maximizing the coupled amplified spontaneous emission
(ASE) from the SOAs into the silica PLCs. Using spot size converters on the SOAs and also
on the PLCs, coupling loss between the SOAs and the silica PLC is less than 1 dB per facet.
Fig. 2. A picture of the packaged hybrid XOR device.
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In Fig. 3, we show a schematic of the set up to obtain the OOK data signals using optical
time division multiplexing (OTDM): we start with two commercial 10.8-GHz semiconductor
mode-locked lasers (U2t TMLL 1550) as the pulsed sources producing ~ 2 ps pulses. Pulse
trains from each laser is 4:1 multiplexed via fiber delay lines into two 43.2-GHz pulse trains,
of which one is modulated by a 43.2-Gb/s pseudo-random bit sequence (PRBS) D of length
231-1, using an electroabsorption modulator (OKI OM5642W-30B), and the other is
modulated by the complementary data D using a similar electroabsorption modulator. These
data streams are further multiplexed to obtain 86.4-Gb/s data streams A( A ) and time-delayed
copies B( B ). In practice, complementary data input are not always available and need to be
generated from the data input. One can exploit an all-optical inversion circuit, which can be
implemented using a SOA-MZI, to obtain the complementary data stream. In our
demonstration, such a scenario is simulated by each MLL having a different wavelength.
(1560 nm and 1557 nm). The wavelength of CW is 1550 nm. The power of CW launched to
the XOR circuit is 14 dBm and the power of each data and complementary data input into the
device is ~11 dBm. We estimate that the optical pulse energy coupled to the SOAs is ~12 fJ.
Fig. 3. A layout of the OTDM set up to generate input signals to the XOR circuit. ODL: optical
delay line. MLL: mode-locked laser.
We adjust the hybrid XOR device first by optimizing the BPSK signals from each of the
MZI pair by monitoring the optical spectra and the eye diagrams measured using a 50-GHz
photodiode connected to an 80-GHz sampling module. After BPSK optimization, XOR and
XNOR operations are validated using sampling scope traces. We show an example of XOR
and XNOR operations in Fig. 4, where it is straightforward to verify the correct logic
operations keeping in mind the fact that XOR is equivalent to binary addition modulo two.
We show the signal quality of XOR operation using the eye diagram shown in Fig. 5. The
observation of open eye diagrams for 86.4 Gb/s XOR operation is a substantial improvement
over our previous 80 Gb/s XOR operation using a conventional SOA-MZI configuration [9],
with which we failed to achieve an open eye diagram. The recovery speed of the SOAs used
in [9] is nearly identical to that of the SOAs used for this demonstration, and the result shows
the benefit of reduced patterning effects using the proposed method.
As can be seen in Figs. 4 and 5, the signal quality, more specifically the extinction ratio,
of XNOR is inferior to that of XOR. The causes of the signal degradation are mainly two
fold: firstly, the uneven splitting ratios of the 1x2 MMIs render the MZIs asymmetric, the
result of which is incomplete cancellation of CW component in the BPSK signals. Secondly,
the incomplete extinction of the BPSK signals is further magnified in the constructive
interference process in generating XNOR signal output, while it is canceled in the destructive
interference for generating XOR output.
Aside from correcting the aforementioned device imperfections, we plan to further
optimize the device. It has been reported that the all-optical signal processing using
differential SOA-MZI can greatly benefit from optimization of the differential delays and the
power splitting ratio of the differential input stage [16]. In the current device, the differential
delay is fixed to 3 ps and the nominal splitting ratio is 50%, both of which can be optimized.
#117590 - $15.00 USD
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Received 22 Sep 2009; accepted 28 Sep 2009; published 7 Oct 2009
12 October 2009 / Vol. 17, No. 21 / OPTICS EXPRESS 19065
Obviously, SOAs with faster recovery speed such as reported in [17] will allow faster
XOR/XNOR operation at higher speeds.
Fig. 4. Temporal traces of input data A, B and the XOR and XNOR results.
Fig. 5. Eye diagrams of simultaneous XOR (left) and XNOR (right) operations.
4. Summary
We proposed a novel method of all-optical XOR operation using SOAs with the capability of
suppressing the patterning effects owing to the slow carrier recovery speed of the SOAs. We
fabricated a hybrid photonic integrated device consisting of passive silica PLC and an array of
four SOAs, implementing a pair of SOA-MZIs. We demonstrated a high-quality all-optical
XOR operation at 86.4 Gb/s using the hybrid photonic integrated device.
#117590 - $15.00 USD
(C) 2009 OSA
Received 22 Sep 2009; accepted 28 Sep 2009; published 7 Oct 2009
12 October 2009 / Vol. 17, No. 21 / OPTICS EXPRESS 19066