A Novel Monolithically Integrated Mach-Zehnder Wavelength Converter
Using Cross Modulation in Electro-Absorber
Y. Du (1), T. Tekin (1), R. G. Broeke (1), N. Chubun (1), C. Ji (1), J. Cao (1) and S. J. B. Yoo (1)
K. Y. Liou (2), J. R. Lothian (2), S. Vatanapradit (2), S. N. G. Chu (2), B. Patel (2),
W. S. Hobson (2), D. V. Tishinin (2), and W. T. Tsang (2)
1: Department of Electrical and Computer Engineering, University of California, Davis, California 95616, USA.
2: Multiplex, Inc., 5000 Hadley Road, South Plainfield, New Jersey 07080, USA.
Abstract We present a novel monolithically integrated all-optical MZI switch for wavelength conversion consisting
of MQW based electro-absorbers. The device has the potential of providing low noise and high-speed wavelength
All-optical switch is one of the key components in
ultra fast optical communications. Mach-Zehnder
interferometer (MZI)  type of all-optical switches
have been frequently used for wavelength conversion
 in Wavelength Division Multiplexing (WDM)
applications, for demultiplexing  in Optical Time
Division Multiplexing (OTDM) systems, for
thresholding detection  in Optical Code Division
Multiple Access (OCDMA) networks, for synchronous
modulation  and for all-optical 3R regeneration .
Conventional all-optical MZI switches are based on
semiconductor optical amplifiers (SOAs), where the
cross-phase modulation (XPM) of the SOAs provides
differential phase shift required for the interferometric
switching. While the monolithically integrated MZI
combines high contrast performance and required
stability , the inclusion of SOAs in the two
interfering arms adds noise at the output. Further, the
carrier recovery time within the SOA is approximately
~100 psec even at high current injections. To enable
the fast switching within the SOA based MZI, even
higher current density levels in long SOAs, higher
optical injection power levels, or differential input
signal mode MZI operation becomes necessary.
Power consumption beyond 1 W and complicated
input signal configuration with low differential output is
quite common in high speed (>10 Gb/s) SOA-MZI
G r o u n d P a dG r o u n d P a d
U p p e r E AU p p e r E A
L o w e r E A L o w e r E A
P h a s e
m o d u la to r sm o d u la to r s
P h a s e
Figure 1. Photograph of the monolithically integrated
MZI wavelength converter based on Electro-
Absorbers and Phase Shifters
New EA based all-optical MZI
We propose and demonstrate a new class of all-
optical monolithically integrated MZI based on biased
electro-absorbers (EA) . The carrier sweep-out time
under the reverse bias condition typically in the tens
of picoseconds determines the fast switching time of
the EA based MZI. The passive nature of this novel
switch also eliminates the SOA induced noise within
the MZI. The combined effects of reduced noise and
faster response time indicate favourable high-speed
operation with increased
requiring high injection current into SOAs. Fig. 1
depicts the photograph of such an integrated EA-MZI
integration involving MOCVD growth, RIE dry etching,
and MOCVD lateral regrowth of Fe-doped InP. As
Fig. 1 indicates, the EA-MZI consists of four multi-
mode interference (MMI) based 3dB couplers, two
EAs (MQW) of unequal lengths and two phase
shifters (passive waveguide) of equal lengths.
The two EA sections in the EA-MZI are purposely
designed to have unequal lengths to achieve
balanced absorption strength when one of the EAs
(shorter EA) is reverse biased. At this balanced
amplitude condition, one of the phase modulators
receive adjusted injection currents to achieve
destructive interference at the output. The light
injection into one of the EAs (shorter EA) will induce
cross modulation  in phase and amplitude to break
the balanced destructive interference condition at the
output. Hence, the device operates as a ‘non-
inverting’ mode switch (or wavelength converter).
Similarly, ‘inverting’ mode operation can be achieved
by applying the bias on the longer EA and achieving
balanced destructive interference when desired level
optical input is present.
Experimental results and discussion
Current injection in the phase shifter sections of the
EA-MZI will indicate the
interferometric switch under various conditions. We
observed this while injecting a CW probe signal at
1550nm through the centre probe input port of the
MZI. Fig. 2 shows the measurement of the probe
signal transmission (shown in relative to the lowest
power throughput) while varying the bias current on
the upper arm phase shifter. We repeat two sets of
measurements with (solid line) and without (dashed
sensitivity of the
line) injecting of a 1540 nm CW pump signal into the
upper EA at 1540nm. The injected optical power for
both probe and pump signals are 3dBm and 12dBm,
before the estimated coupling loss of 9 dB. The
lengths of the monolithically integrated EAs in the
upper and lower branches of the EA-MZI and the
length of the phase shifter sections are 200um, 50um,
and 600um, respectively. The reverse bias voltages
for upper and lower EAs are 1V and 12V.
0 1020 3040
phase current [mA]
Figure 2 Transmission of the probe signal in absence
and in presence of injected optical pump signal.
Fig. 2 indicates that the destructive interference
occurs at approximately 35 mA current injection into
the phase section when no pump signal is present,
but that the pump signal injection significantly
modifies the EA-MZI transmission response through
amplitude and phase modulation of the EA section.
At the given 35 mA level phase current injection and
the pump signal present, the on-off ratio of 13dB can
Fig. 3 shows all-optical switching experiment results
obtained under the 35 mA phase current injection.
Fig. 3(a) is the static switching result showing the
transmission of the probe light in dB scale
(normalized against the lowest throughput) against
the input pump light power in dBm. Approximately
18dB extinction ratio is obtained for 10 dB variations
in the input power, indicating the potential optical
signal regeneration capability of the EA-MZI. Fig.
3(b) shows the electric waveform at the receiver after
wavelength conversion of 2.5 Gb/s signal modulated
with 2^7-1 pseudo-random-bit-sequence.
input power [dBm]
8 1012 1416 18
Figure 3. (a) MZI output power as a function of the
input pump power; (b) Electrical waveform after
We proposed a novel monolithically integrated EA-
MZI wavelength conversion device via cross
modulation effects in a reverse biased EA. We
demonstrate wavelength conversion ability of this
device at 2.5 Gb/s. The realized EA-MZI structure
combines the added cross phase modulation and the
cross-absorption modulation effects to achieve very
high contrast switching compared to the cross-
absorption modulation EA wavelength conversion.
Compared to SOA based MZI devices, the EA-MZI
devices suffer from no ASE noise from the SOA and
expect to achieve higher speed operation. Dynamic
characterizations and all optical switching at higher bit
rate with this novel device are currently in progress.
This work was supported in part by DARPA/SPAWAR
under agreement number N66001-02-1-8937, and by
DARPA/ARO under agreement number W911NF-04-
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