Nonlinear Dynamics of Resonant Tunneling Optoelectronic Circuits for Wireless/Optical Interfaces

Abstract

We report on experimental and modeling results on the nonlinear dynamics of a resonant-tunneling-diode-based (RTD) optoelectronic circuits that can be used as the basis of a wireless/optical interface for wireless access networks. The RTD-based circuits are optoelectronic integrated circuits that have negative differential resistance and act as optoelectronic voltage-controlled oscillators. These circuits display many of the features of classic nonlinear dynamics, including chaos and synchronization. These highly nonlinear oscillators behaves as injection-locked oscillators that can be synchronized by a small injection signal of either wireless or optical origin, and thus, can transfer phase encoded information from wireless to the optical domain or the optical to the wireless domain.

Full text

Page 1

1436IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 11, NOVEMBER 2009
Nonlinear Dynamics of Resonant Tunneling
Optoelectronic Circuits for
Wireless/Optical Interfaces
Bruno Romeira, Student Member, IEEE, José M. L. Figueiredo, Thomas James Slight, Liquan Wang,
Edward Wasige, Member, IEEE, Charles N. Ironside, Senior Member, IEEE, Anthony E. Kelly, and Richard Green
Abstract—We report on experimental and modeling results
on the nonlinear dynamics of a resonant-tunneling-diode-based
(RTD) optoelectronic circuits that can be used as the basis of
a wireless/optical interface for wireless access networks. The
RTD-based circuits are optoelectronic integrated circuits that
have negative differential resistance and act as optoelectronic
voltage-controlled oscillators. These circuits display many of
the features of classic nonlinear dynamics, including chaos and
synchronization. These highly nonlinear oscillators behaves as
injection-locked oscillators that can be synchronized by a small
injection signal of either wireless or optical origin, and thus, can
transfer phase encoded information from wireless to the optical
domain or the optical to the wireless domain.
Index Terms—Chaos, injection locking,
electronics, microwave oscillators, nonlinear systems, optical
receivers, radio broadcasting, resonant tunneling diodes (RTDs),
semiconductor lasers.
integrated opto-
I. INTRODUCTION
H
users [1], [2]. The radio-over-fiber (RoF) systems are one of
the promising schemes for the future broad-band wireless com-
munication systems such as mobile communications, hotspots,
and suburban areas [1]–[3]. Compared with the conventional
high-frequency wireless or coaxial links, RoF systems show
many advantages such as low-cost, high-performance, huge
bandwidth, and long-distance transmission. Such is the demand
that so-called picocellular access [4], with wireless cells of
few meters range, is being considered as a highly promising
route for delivering high-bandwidth mobile access. Since a
IGH data rates mobile access networks are emerging as
the primary choice for many communication systems
Manuscript received April 08, 2009; revised June 22, 2009. Current version
publishedNovember06,2009.ThisworkwassupportedinpartbytheFundação
paraaCiênciaeaTecnologiaunderGrantSFRH/BD/43433/2008,theFundação
Calouste Gulbenkian, Portugal, and the Research Networks-Treaty of Windsor
Programme 2008/09-U32, Portugal.
B. Romeira and J. M. L. Figueiredo are with the Centro de Electrónica, Opto-
electrónica e Telecomunicações, Universidade do Algarve, 8005-139 Faro, Por-
tugal (e-mail: bmromeira@ualg.pt; jlongras@ualg.pt).
T. J. Slight, L. Wang, E. Wasige, C. N. Ironside, A. E. Kelly, and
R. Green are with the Department of Electronics and Electrical En-
gineering, University of Glasgow, Glasgow G12 8LT, U.K. (e-mail:
tslight@ elec.gla.ac.uk; liquan.wang@elec.gla.ac.uk; ewasige@elec.gla.ac.uk;
ironside@elec.gla.ac.uk; tkelly@elec.gla.ac.uk; rgreen@elec.gla.ac.uk).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JQE.2009.2028084
typical RoF picocellular network will require hundreds of
microwave/photonic interface circuits, high-performance pic-
ocellular systems have a great challenge ahead in order to
integrate microwave and optical functions on the same chip as
a mean of delivering low-cost and highly reliable interfaces.
In recent developments on a hybrid optoelectronic integrated
circuit (OEIC) consisting of a laser diode (LD) driven by a
resonant tunneling diode (RTD) oscillator, the RTD-LD OEIC,
we have demonstrated the circuit is capable of behaving non-
linearly, with RTD-LD optical output emulating RTD nonlinear
characteristics, which gives rise to a variety of additional
optoelectronic operation modes, including optoelectronic
voltage-controlled oscillator (OVCO) [5], injection locking,
period-adding [6], and chaotic carriers generation [7], with
potential applications in optical chaotic communications [8].
(Previous work showed that it is possible to monolithically
integrate an RTD with a LD [9]). In this paper, we discuss
the nonlinear dynamics operation of a new RTD-LD pho-
tonic–microwave circuit where the RTD has incorporated a
photoconductive region, the RTD-LD optoelectronic oscillator,
acting both as microwave–photonic and photonic–microwave
converters. The results also show that when connected to
a patch antenna the new RTD-LD circuits can act as wire-
less/optical (W-O) and optical/wireless (O-W) interfaces. In
the context of this paper what we regard as a wireless signal
is the typical signal used in digital communication systems,
such as global system mobile (GSM), and it is an RF signal in
a frequency range 0.5–10 GHz that can be broadcasted with
antenna around a few meters range, and for this application, it
is usually digitally encoded using phase-shift keying (PSK).
The RTD-LD W-O interface uses synchronization of an os-
cillator to convert RF signals into optical subcarriers. In the
presence of a low-power RF signal emitted by an antenna the
RTD-LD synchronizes to the RF signal, locking the phase of
the laser optical subcarrier to the phase of the RF signal. The
RTD-LD O-W interface uses optical injection locking to syn-
chronize the RTD oscillator to convert optical subcarriers into
RF signals that can be broadcasted by wireless systems. These
synchronization capabilities can be used in digital communi-
cation to translate phase-shift-keyed-modulated RF signals into
optical subcarriers and conversely. Due to the negative differen-
tialresistance(NDR)amplificationeffecttheRTD-LDresponds
both to very low-power injected RF signals and to optical sub-
carriers. This is a novel concept that has the advantage of being
a simple way to integrate microwave and optical functions on
0018-9197/$26.00 © 2009 IEEE
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 2

ROMEIRA et al.: NONLINEAR DYNAMICS OF RESONANT TUNNELING OPTOELECTRONIC CIRCUITS FOR WIRELESS/OPTICAL INTERFACES1437
Fig.1. SchematicoftheRTD-LDW/Ointerfaceformicrowave-to-opticalcon-
version. Also shown is the optical injection port (OW) used to the optical-to-
electrical conversion.
a single OEIC chip, rather than having separate monolithic mi-
crowaveintegratedcircuit(MMIC)chips [10] andoptical chips.
In broad terms, the approach that we discuss in this paper
is similar to the classic injection-locked oscillator (ILO) tech-
niques(see,forexample,[11]–[14])thatareroutinelyemployed
in digital wireless communications systems. However, here, we
apply nonlinear dynamical theory to a unique optoelectronic
ILO that has the potential to be integrated in a single chip. Such
chips represent a highly promising route to producing low-cost,
compact, robust, and reliable microwave/optical interface de-
vices for the next generation of wireless access networks.
The paper is organized as follows. Section II describes the
microwave–photonic interface and the characterization exper-
imental setup. Section III is devoted to the RTD-LD nonlinear
dynamics analysis using the optoelectronic circuit numerical
model that consists of a Liénard’s driven system coupled to
single-mode LD rate equations. In Section IV, the experimental
results are presented, discussed, and compared with the circuit
optoelectronic model predictions. The conclusions are pre-
sented in Section V.
II. MICROWAVE–PHOTONIC OSCILLATOR
RTDs are nanoelectronic devices easily integrated with pho-
tonic components, such as LDs, modulators, and photodetec-
tors, due to their simple structure and small size, enhancing
substantially their modulation/detection performances [9], [15].
The optoelectronic interfaces for wireless access networks dis-
cussed here take advantage of the RTD’s strong nonlinear cur-
rent–voltage
–characteristic that shows wide-bandwidth
NDR at room temperature. Among other effects, the NDR can
give rise to electrical amplification, which in the presence of
a resonant circuit can produce ultrafast signals up to terahertz
(THz) frequencies [16], picosecond switching induced by low-
voltage signals [17], locking to signals with frequency close the
circuit natural frequency or one of its sub/harmonics [18], [19],
and generation of high-dimensional broad-band chaos [20].
The microwave–photonics interface, schematically shown in
Fig. 1, is formed by connecting a patch antenna to a hybrid
circuit consisting of an RTD containing a photoconductive re-
gion [optical waveguide (OW)] and an LD mounted in series
directly onto the surface of a printed circuit board [5], [6]. An
optical fiber is used to couple light to the circuit optical injec-
tion port (the RTD photoconductive region). The RTD-LD cir-
cuit dc bias is supplied via a high-bandwidth bias tee. A shunt
Fig. 2. ?–? characteristics of the individual and integrated components, RTD,
LD, and RTD-LD, respectively. Also included is the plot of the physics-based
analytic voltage-dependent current source function ??? ? used to emulate the
nonlinear ?–? characteristic of the RTD.
capacitor and a shunt resistor are connected in parallel with
the RTD-LD in order to impose a low-frequency cutoff and to
avoid spurious oscillations caused by the dc-bias circuitry, re-
spectively. An Agilent E8257D RF signal generator connected
to a patch antenna was used to generate RF signals. With ap-
propriate antenna configuration and/or light coupling the NDR
amplification effect makes possible to phase-lock the RTD-LD
to both very low-power RF broadcasted signals and to moderate
power optical subcarrier signals without the need of external
amplification.
The RTD was fabricated from InGaAlAs/InP RTD epi-ma-
terial that was first used in the work described in [15]. Briefly,
its structure consists of two 2-nm-thick AlAs barriers separated
by a 6-nm-wide InGaAs quantum well, embedded in a 1- m
-thick InGaAlAs OW core. The RTD-OW devices used have
peak currents around 50 mA and peak-to-valley current ratios
(PVCRs) as high as 3 (see Fig. 2), with valley-to-peak voltage
and peak-to-valley current differences,
,around0.6Vandupto34mA,respec-
tively. The LD was a continuous wave (CW) source fabricated
by Compound Semiconductor Technologies Global Ltd., with a
thresholdcurrent
17 mA atthevoltage
Fig. 2), an efficiency around 0.85 mW/mA, and
sitic capacitance. When connected in series with the RTD-OW,
the LD shifts the RTD current peak region to higher voltage by
about 1.8 V, with a slight reduction of the NDR width, as shown
in Fig. 2.
The RTD-LD oscillator operates as follows. Without external
perturbation, dc biasing the circuit in the NDR region (see
Fig. 2), it operates as an autonomous self-sustaining OVCO
[5], [6], producing electrical relaxation oscillations that are
transferred to the laser optical output. By tuning the RTD-LD
quiescent point across the NDR region from the peak to the
valley, the circuit natural oscillation frequency changes from
around 560 MHz up to 1.0 GHz, as shown in Fig. 3. Moreover,
in the presence of injected signals (optical or RF), the RTD-LD
optoelectronic oscillator undergoes a variety of dynamical
operation modes, including locked and unlocked regimes, de-
pending on the frequency and power of the injected signals. In
and
1.8 V(see
50 pF para-
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 3

1438IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 11, NOVEMBER 2009
Fig. 3. Experimental RTD-LD electrical/optical free-running oscillation fre-
quency as function of dc-bias voltage.
Sections III and IV, we analyze numerically and experimentally
the nonlinear dynamics of RTD-LD optoelectronic oscillator
induced by both RF and optical signals.
III. LIÉNARD’S DRIVEN OSCILLATOR
In a previous publication [5], we showed how the RTD-LD
oscillator can be represented as a Liénard’s oscillator and how
that gives considerable insight into the operation of the circuit.
In this section, we go over the Liénard’s driven oscillator repre-
sentation introduced in[7], withparticular regardto thefeatures
relevant to its operation as a driven optoelectronic ILO.
For purposes of analysis and simulation, the RTD is modeled
as a capacitance in parallel with a voltage dependent current
source
[21]. The modeled RTD –
Fig. 2) is represented by the function
characteristic (see
(1)
with fitting parameters
V,
and
charge and the Boltzmann constant, respectively.
The electrical behavior of the RTD-LD optoelectronic oscil-
lator can be described using the small-signal equivalent lumped
circuit shown in Fig. 4 [7], since the shunt capacitor–resistor
used for dc-bias stabilization acts as a short circuit at the fre-
quencies under consideration. Because the bias operating point
is well above the LD threshold current, the LD is replaced by
its small-signal equivalent circuit, i.e., the drop voltage
( 1.8 V) and the series resistance
of the LD is much larger than the RTD device intrinsic capac-
itance [22], the capacitance
presented in Fig. 4 corresponds
to the RTD capacitance. In Fig. 4,
resistance due to the devices’ (RTD and LD) series resistances
and the 50
impedance of the measuring instruments, and
the inductance due to wire bonding and PCB microstrip trans-
missionline.Fromthetransmissionlineandbondwireslengths,
we estimate an equivalent inductance around 6 nH.
A,V,
V,A,,
. The parametersand are the electric
. Since the capacitance
represents the equivalent
is
Fig. 4. Electrical small-signal equivalent lumped circuit of the RTD-LD opto-
electronic oscillator. The circuit parameters used in the numerical analysis were
? ? ??? ?, ? ? ??? nH, and ? ? ??? pF.
Considering the circuit of Fig. 4, the current
the RTD-LD series can be obtained from Kirchhoff’s rules
(using Faraday’s law). The electrical behavior of the RTD-LD
oscillator driven by a wireless carrier signal is well described
by the following two first-order nonlinear coupled differential
equations:
through
(2)
(3)
where
signaland
(3) correspond to a kind of driven oscillator known as a Liénard
system under external injection.
The optoelectronic dynamics of the circuit is modeled using
single-modeLD rate equations withthe current flowingthrough
the LD corresponding to the dc-bias current plus the oscillatory
current given by the Liénard’s driven system (2), (3). For nu-
merical purposes, we make use of the normalized carrier den-
sity
andthenormalizedphotondensity
where
and
threshold carrier density, to obtain the normalized single-mode
rate equations
is the wireless carrier
isthecarrierphasefunction.Equations(2)and
,
is the
(4)
(5)
where
rameters, and
Table I summarizes the LD physical parameters used in the nu-
merical simulations.
We have numerically analyzed the RTD-LD optoelectronic
nonlinear dynamics over a range of RF signal parameters, in-
cluding frequency and power (or the equivalent voltage ampli-
tude), and circuit dc-bias voltage. A simple way to map the
RTD-LD modes of operation is to obtain the system bifurcation
diagrams.Thebifurcationmapswereconstructedbycalculating
the time series of a given system variable, such as the voltage,
current, or photon density, and plotting the corresponding peak
heightsasafunctionofagivencircuitcontrolparameter,which,
andare two dimensionless pa-
is the LD threshold current.
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 4

ROMEIRA et al.: NONLINEAR DYNAMICS OF RESONANT TUNNELING OPTOELECTRONIC CIRCUITS FOR WIRELESS/OPTICAL INTERFACES 1439
TABLE I
DESCRIPTION OF THE STANDARD LASER RATE EQUATIONS PARAMETERS AND
THE TYPICAL VALUES USED IN THE NUMERICAL SIMULATION
Fig.5. Numericalbifurcationmapshowingmaximaofnormalizedphotonden-
sity with the normalized bias voltage at fixed broadcasted carrier frequency
? ? ? GHz with power corresponding to the equivalent voltage amplitudes
(a) ? ? ???? ? and (b) ?
? ???? ?.
inourcase,werethebroadcastedfrequency,thesignalpower(or
equivalent voltage amplitude) and the circuit dc-bias voltage.
For each of these control parameters, the corresponding bifur-
cation map was obtained. In order to avoid transients, the first
50 periods of the RTD-LD dynamics were neglected.
The influence of dc-bias voltage on the dynamics of the
RTD-LD system is summarized in Fig. 5, which shows photon
density bifurcation diagrams with the dc bias as a control
parameter, for two input power equivalent voltage carrier
amplitude values, (a) 0.15 V and (b) 0.25 V, both at 3 GHz. In
order to compare the results with experimental data, we defined
a dc-bias-voltage-normalized parameter
where
is the dc RTD-LD peak voltage, in this case 2.9 V,
and
is the RTD quiescent voltage. The maps show that under
these operating conditions the system oscillates for dc-bias
voltage from 3.0 to 3.59 V.
For certain injected signal amplitudes, the optoelectronic ex-
tended Liénard system can produce waveforms whose periods
are exactly
times the injected RF carrier period
an integer. A period- sequence corresponds to a mode of oper-
ation where the circuit synchronizes to the injected signal (the
waveforms’period satisfy the relation
indicated in Fig. 5, we can expect the RTD-LD will have two
wide dc-bias voltage regionscorrespondingto
The period reduction from
,
, with
). Forthe conditions
and.
towith the dc bias is
Fig. 6. Numerical bifurcation map showing maxima of normalized photon
density with the normalized broadcasted frequency with signal power corre-
sponding to the equivalent voltage ?
? ???? ?.
mainlyduetotheincreaseofthecircuitnaturalfrequency,inac-
cordance to the experimental behavior demonstrated in Fig. 3.
To the right of the locking window
extended region of nonperiodic behavior with intermediate
very narrow branches of locking regions. From the analysis of
Fig. 5(a) and (b), we can conclude that increasing the injected
signal power enlarges the locking windows (regions where a
generated waveform period satisfies the relation
corresponds to larger tuning range
power also makes high-order -sequence periodicity windows
more discernible, as is the case of the period-7 region in
Fig. 5(b).
A bifurcation map of the photon density with the normalized
carrier frequency
as a control parameter, is shown in Fig. 6,
where
(withand
lator natural frequencies, respectively), for normalized dc bias
and carrier amplitude 0.15 V. Under the operating
conditions of Fig. 6, the system undergoes in a period adding
sequence with locking windows corresponding to
and 4. This nonlinear dynamical behavior is a characteristic of
RTD oscillators [18] and is similar to other NDR oscillator sys-
tems, such as the forced van der Pol oscillator [23]. The bifur-
cation diagrams of Figs. 5 and 6 give clear indications the LD
dynamics is determined by the RTD-induced oscillation charac-
teristics, since the laser output shows the same dynamic states
of the RTD-induced oscillatory current.
there is an
), which
. Increasing injected
being the carrier and oscil-
1, 2, 3,
IV. EXPERIMENTAL RESULTS AND DISCUSSION
In what follows, we present the experimental results of the
operation regimes of the RTD-LD optoelectronic circuit acting
as a microwave-to-photonics interface and include preliminary
results on optical injection locking, showing RTD-OW opera-
tion as an optical-to-microwave interface.
A. Free-Running Relaxation Oscillations
Asmentioned,whendcbiasedintheNDRregionandwithout
external excitation, the RTD-LD optoelectronic circuit operates
as a self-sustained relaxation oscillator undergoing repetitive
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 5

1440 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 11, NOVEMBER 2009
switchingbetweentwodcstablepointsthatarebelowandabove
the NDR region [24], [25], with the laser optical output fol-
lowingthecircuitoscillation,withthecorrespondingRFspectra
of both signals showing frequencies up to the tenth harmonic,
confirming the observed optical pulses widths [5], [6].
The RTD-LD free-running oscillation frequency is deter-
mined to a first approximation by the circuit resonator (tank
circuit, see Fig. 4) frequency
oscillation frequency depends also on the RTD nonlinear
impedance, which is a function of the dc-bias voltage [5]. As
shown in Fig. 3, the circuit oscillation frequency does not vary
monotonically with bias voltage: tuning the dc bias from 2.95
to 3.45 V changes the circuit natural oscillation frequency
from 560 MHz oscillations up to 1.0 GHz (3.35 V), decreasing
slightly below 1 GHz for higher voltage values. The RTD-LD
OVCO capabilities can be used in systems where frequency
reuse is needed, and opens out the possibility of using the
RTD-LD circuits in a wide range of applications, as discussed
in the next sections.
GHz. The
B. Injection Locking
Injection-locking experiments were performed using the cir-
cuit setup shown in Fig. 1. In the presence of an RF broadcasted
signal,theRTD-LDoscillatorphase-lockstotheRFsignalwhen
its frequency is close to free-running frequency, with the LD
output power being modulated by an amplified version of the
RF signal.
The photon density bifurcation map with the frequency of
the RF signal as control parameter (see Fig. 6), presents a series
of frequency bands where the circuit produces waveforms
whose period are exactly
times the injected RF carrier period
. We have experimentally analyzed these frequency locking
regions. Fig. 7 shows the measured RF power spectra of photo-
detected LD optical outputs produced in the frequency band
corresponding to period-1
lation operation around 600 MHz, curve (a), and phase-locked
operation induced by
41 and
the reception patch antenna plane, curve (b) and curve (c),
respectively. (The RF-broadcasted power level on the receiver
antenna plane was measured connecting the patch antenna
directly to the RF analyzer; during the experiments described
here, the transmitter and receiver antennas were 2 m apart.)
The curve (a) of Fig. 7 indicates the LD optical output due
to relaxation oscillation operation produces a broad frequency
line centered at 600 MHz, with a gradual decrease of the spec-
tral power density as we move away from the peak. On the con-
trary, in the presence of a
41 dBm RF signal, the RTD-LD
produces an optical output with a much sharper line spectrum,
curve (b), a clear indication the RTD oscillation is phase-locked
to the RF signal detected by the patch antenna. The locking to
the
26 dBmRF signal occurs withan appreciablenoise reduc-
tion, as shown in curve (c). The comparison of spectrum (c) of
Fig. 7 with the spectrum of the signal produced by the generator
shows the LD optical subcarrier has the same spectral charac-
teristics as the injected RF signal. The
phasenoisereductionbymorethan40dBat10kHzoffset,when
, due to free-running oscil-
26 dBm RF power levels at
26 dBm signal leads to
Fig. 7. Photodetected RTD-LD optical outputs showing phase-locking and
phase noise reduction. (a) Free-running oscillation at around 600 MHz. RF
phase-locking due to injected broadcasted signals with 600 MHz carrier fre-
quency: injected power levels (b) ?41 dBm and (c) ?26 dBm. The resolution
and video bandwidths of spectrum analyzer are 1 kHz.
comparedtothefree-runningoscillationsphasenoise.Thesere-
sults demonstrate that the optical subcarrier has the same spec-
tral quality as the broadcasted RF signal, with significant im-
provement over free-running oscillation. It is worth mentioning
that we have observed frequency-locking operation for power
levels as low as
50 dBm. This is a significant result, if we take
in consideration that the RTD and LD, and the circuits were not
optimized for this operation.
As verified in the numerically simulations (see Section III),
the regions where locking occurs are determined by the fre-
quency of the RF signal and circuit dc-bias voltage. Under
the condition of injection around
locking bandwidth
was measured to be approximately
0.3%
MHz , with the LD optical output not
changing appreciable within this locking range. The injec-
tion-locking operation of the RTD-LD oscillator circuit follows
the behavior observed in other negative-resistance oscillators
in the small-signal limit [25], usually quantified using Adler
equation [26]
26 dBm, the normalized
(6)
where
and
valid in the small-signal limit, i.e., when
Fig. 7, we estimate a cold cavity bandwidth of around 12 MHz.
Since the RTD-LD can operate as an ILO that locks onto the
phase of the injected signal, the information encoded on the
phase of the broadcasted RF signal can be transferred to the op-
tical output subcarrier. The ILO is an example of the synchro-
nization of a nonlinear system to a weak signal. Fig. 8 compares
the RF spectrum of a phase-modulated RF signal with corre-
sponding RF spectrum of the detected RTD-LD optical output,
showing the RF signal information content is transferred to the
optical subcarrier. The phase synchronization presented here
can haveapplications inthe nextgeneration wirelessaccess net-
worksthat employ PSK modulation, thedigital versionof phase
modulation.
is the free-running power,
is the cold cavity bandwidth. Equation (6) is only
is the injected power,
. From
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 6

ROMEIRA et al.: NONLINEAR DYNAMICS OF RESONANT TUNNELING OPTOELECTRONIC CIRCUITS FOR WIRELESS/OPTICAL INTERFACES1441
Fig. 8. (a) RF spectrum of a 600 MHz broadcasted electrical phase-modulated
signal with 1 MHz subcarrier signal and phase shift of 180 degrees. (b) Photo-
detected RF spectrum of optical output phase-locked to free-running oscilla-
tions around 600 MHz and showing 1 MHz sidebands resulting from phase
modulation.
As mentioned earlier (see Fig. 6), the RTD-LD circuit
produces waveforms whose periods are exactly
injected RF carrier period
. We have experimentally con-
firmed locking to broadcasted signals with frequencies around
second, third, and fourth harmonics of the circuit free-running
frequency. The LD optical power followed the dynamic be-
havior presented in Figs. 5 and 6 where the dividing ratio
determined adjusting the dc-bias voltage or the injected signal
frequency. We have also confirmed the nonlinear dynamics
sequence of period-5 and period-4 shown in Fig. 5, induced
by changing the dc bias from 2.95 to 3.45 V. Fig. 9 shows the
corresponding measured RF photodetected power spectra and
waveform of period-4 when a 3 GHz RF signal was injected
(frequency division by 4).
The synchronization sequences of period- observed experi-
mentally are summarized in the Arnold tongues map of Fig. 10.
Arnold tongues correspond to phase-locking regions in param-
eterspacewhereasystemrespondssynchronizingtoanexternal
stimulus. The Arnold tongues reveal the regions of frequency
andRFpowerequivalentamplitudesthatcausesynchronization.
We have observed the dividing ratio
with locking ranges up to tens of megahertz at RF powers close
to the free-running power.
times the
is
increases with frequency
C. Optical Injection Locking
AsmentionedinSectionI,theRTDstructureemployedinthis
workwasdesignedtooperateasalow-voltageelectroabsorption
Fig. 9. (a) Experimental spectra of the photodetected LD optical output when
operating as a free-running oscillator and dc biased at 3.073 V ?? ? ??????
and when frequency locked to fourth harmonic of a injected 3 GHz RF signal.
(b) Corresponding time series of frequency division by 4. Inset in grey color is
schematically represented the time series of the 3 GHz injected carrier signal.
Fig. 10. Experimental Arnold tongues showing frequency-division operation
for dc-bias voltage 3.023 V, with the RF power ?
?3 dBm. Each dividing ratio ? is represented by a tongue limited by dotted
lines, inset schematically in color, corresponds to the RTD-LD locking regions.
ranging from ?19 to
modulator (for more details see [15]), consisting of a ridge op-
tical waveguide with 1- m-thick InGaAlAs core incorporating
a double-barrier quantum well. The optical waveguide is essen-
tially a ridge waveguide single mode in the growth direction for
wavelength around 1550 nm. It allows endfire light coupling
and, at the same time, gives a short path for the photogenerated
carrier to be quickly removed from the absorption region by
the electric field across the waveguide core generated by the
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 7

1442 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 11, NOVEMBER 2009
dc bias that is perpendicular to the light guiding direction. The
InGaAlAs waveguide core acts as a photoconductive region for
light with energy close to the core bandgap energy and provides
light confinement along the double-barrier quantum-well plane
increasing the interaction length between the waveguide deple-
tion region and the optical field (augmenting the light absorp-
tion, and hence, the photogenerated current). The absorption of
a modulated optical signal gives rise to photogenerated current
that can be used to tune, frequency modulate, and injection lock
the RTD self-sustained oscillations.
The RTD-OW acting as a optical-to-microwave converter
works as follows: modulated light incident on the photoconduc-
tive region produces photocharges that reduces the RTD series
resistance, leading to peak and valley voltages red shifts, giving
rise to photodetection with responsivity in the order of few tens
of amperes per watt [27]. Moreover, when biased in the NDR
region,certainmodulatedopticalsignalsinducephotogenerated
currents that are able to control the RTD free-running oscil-
lations, producing electrical signals that emulate the optical
subcarrier. Since the incident light changes the NDR region
profile, the RTD current flow dynamics is then controlled by
the incident optical power, making several nonlinear modes of
operation possible. This RTD-OW when integrated with an LD,
as discussed previously, makes possible the implementation
of an optoelectronic voltage controlled oscillator with both
electrical- and optical injection ports, that adds to the electrical-
and optical output ports of the simple RTD-LD circuit. In such
optoelectronic oscillator, the electrical- and optical-output sig-
nals can be controlled by both electrical- and optical-injected
signals.
We now present preliminary results on optical-to-microwave
conversion due to synchronization of RTD oscillations to an op-
tical subcarrier. The RTD oscillator capture range and the low
noise capabilities are discussed. The light source employed was
a Photonetics Tunics tunable LD with emitting wavelength in
therange1460–1600nm,whichcanbedirectlymodulatedupto
1 GHz. In some of the experiments, the Photonetics laser output
was amplified using an erbium-doped fiber amplifier (EDFA),
and then launched into the RTD waveguide using a standard
lensed single-mode fiber. The variable dc bias was applied via
a bias T (45 MHz–26.5 GHz) connected to a high-frequency
probe to contact the RTD CPW transmission line.
First, we analyzed the RTD-OW response to optical modu-
lated signals as function of the dc bias (see Fig. 11). The RTD
electrical response to the optical injected signal was character-
ized using a high-bandwidth spectrum analyzer and an oscil-
loscope. Since we are also interested in optical-to-microwave
interfaces for wireless networks, the generated electrical sig-
nals were fed directly to a patch antenna making part of the
RTD circuit and broadcasted in a range of a few meters to be
detected by an identical patch antenna connected to the oscil-
loscope or to the RF spectrum analyzer. It is worth mention
that no amplification was used to strengthen the RTD-WO gen-
erated RF signal prior to the broadcasting. Fig. 11(a) presents
the RF injection-locking capture level of light modulated sinu-
soidally at 1 GHz, for both peak and valley voltage polariza-
tions,
and, respectively. The capture level increases up to
10dBatthewavelength1550nm.Fig.11(b)showsRFpowerof
Fig. 11. (a) Photodetected RF power as function of wavelength with dc-bias
voltage as parameter for a CW optical signal modulated at 1 GHz and 500 mV.
(b) Photodetected signal due to 1550 nm CW optical signal modulated at 1 GHz
when the RTD-OW is dc biased in the peak ?? ? ??? ?? and valley ?? ?
??? ?? regions.
photodetectedopticalsignalsofwavelength1550nmmodulated
at 1 GHz for dc bias at the peak and the valley regions showing
approximately 9 dB gain in the transition from the peak to the
valley region (the results were limited by the available laser fre-
quency modulation bandwidth and modulation depth).
We also have observed that when the RTD-OW is operating
in the NDR region the RTD oscillation can be locked to op-
tical subcarrier signals. The RF power used to modulated the
LD at microwave frequencies, giving a certain optical signal
modulation depth, and the optical injected power were varied to
investigate the optical locking phenomena. We confirmed pre-
vious results that the RTD oscillations follows the frequency
and the phase of the photodetected RF subcarrier. A notice-
able locking phenomenon with significant noise reduction ap-
peared at RF modulation powers as low as 100 mV. The light
coupled to the RTD-OW was estimated to be less than 1 mW
(corresponding to less than 10% of the input optical signal),
because of the low overlap between the waveguide modes and
the optical fiber mode. Fig. 12 shows the measured spectrum
of free-running oscillation operation around 600 MHz and the
correspondingspectrumduetoopticalinjection-lockingofaop-
tical signal RF modulated at 600 MHz using a 400 mV ampli-
tude voltage waveform. The noise levels of the synchronized
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 8

ROMEIRA et al.: NONLINEAR DYNAMICS OF RESONANT TUNNELING OPTOELECTRONIC CIRCUITS FOR WIRELESS/OPTICAL INTERFACES1443
Fig. 12. RTD-OW free-running oscillation and locking to a 1530 nm optical
signal modulated by an RF signal with 600 MHz and 400 mV.
signalcompareswiththenoisefiguresofthesignalusedtomod-
ulate the Photonetics LD.
The RTD-OW output power allows direct RF broadcasting
of phase-locked signals connecting an RTD-OW to a patch an-
tenna, without the need of external pre-amplification. Under the
earlier operating conditions, the phase noise of the RTD elec-
trical oscillations locked to theoptical injected signal is reduced
by up to 25 dB at a 100 kHz offset, when compared with the
free-running oscillations. Recent results in a similar experiment
with higher modulation frequencies indicate the RTD-OW can
work as a high-speed optical-to-RF converter, taking advantage
of the optical waveguide design.
D. Chaos Dynamics
The earlier results demonstrate regular oscillation dynamics
and noise reduction properties of the RTD-LD/RTD-OW oscil-
lator induced by injected RF/optical signals. We now present
experimental results of the RTD-LD optoelectronic oscillator
chaoticcapabilities.TheRTD-LDshowsalong-periodbehavior
that includes the generation of nonperiodic waveforms, such as
quasi-periodic signals, and in certain situations, namely tran-
sitions between periodic- and nonperiodic signals, the wave-
forms can evolve into chaotic patterns. The alternation between
regions of regular and chaotic dynamics is known as intermit-
tency [28]. We have observed intermittency near tangent bi-
furcation windows of regular dynamics. In these intermittency
transitions a simple periodic orbit is replaced and turns into a
chaoticorbit,withthestabletrajectoryeitherbecomingunstable
or being destroyed, which gives rise to nonperiodic signals as
predicted numerically (see Figs. 5 and 6).
Fig. 13 presents the spectra of the photodetected LD optical
outputs corresponding to quasi-periodic and chaotic signals due
tointermittencyregionsfoundbetweenthewindowsofperiod-5
and period-4 shown in Fig. 5. In Fig. 13(a), the spectrum of a
quasi-periodic signal with high-frequency content corresponds
to a first region of intermittency after period-5. A slight in-
crease of the bias voltage, maintaining the carrier frequency
and amplitude, moves the system to a second region of inter-
mittency [see Fig. 13(b)], where chaotic signals are produced.
Fig. 13. Spectra of the photodetected LD optical output of unlocked signals
due to an injected wireless signal with 3 GHz and 16 dBm. (a) Quasi-periodic
outputand(b)chaoticoutput.Thecomparisonbetween(a)and(b)showsaclear
increase of the signal background level in the spectrum of the chaotic signal,
superior to 10 dBm in the 500 MHz to 2.3 GHz region.
Fig. 13(b) shows the typically features of a chaotic phenomena:
broad-band spectrum and the rise of the spectrum background
level.
V. CONCLUSION
We have presented numerically and experimentally the
synchronized and desynchronized operations of RTD-LD
based microwave/photonics interfaces. The RTD oscillator
capabilities are used to drive the LD making it possible to
operate as a optoelectronic voltage-controlled oscillator. The
injection-locking and frequency-division regimes of operation
were presented and discussed, showing the locking leads to
a considerable reduction of the oscillator phase noise even
for low-power injected signals, meaning the RTD-LD can act
as an injection-locking oscillator that can be used to transfer
broadcasted RF signal phase information to an optical sub-
carrier. The unsynchronized behavior includes the production
of quasi-periodic and chaotic signals. The results presented
confirm that the RTD-LD optical behavior is determined by the
RTD nonlinear characteristics, and that the circuit optoelec-
tronic model based on the Liénard’s driven system approach
describes the RTD-LD nonlinear dynamics quite well under RF
injection.
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 9

1444 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 11, NOVEMBER 2009
In another experiment using the optical waveguide incorpo-
rating an RTD, optical injection was observed and the photode-
tection characteristics of the RTD-OW device were character-
ized. The RTD-OW is capable to lock to optical subcarrier sig-
nals in telecommunication windows, and due to RTD intrinsic
gain, the photodetected phase-locked signals can be directly
broadcasted, without the need of preamplification, in a range
of a few meters with potential applications as an optical–RF in-
terface for RoF systems.
The small size of the circuit and the low-power broadcast
signal levels needed for RF to optical conversion anticipate the
nonlinear dynamics of RTD waveguide-based circuits can have
applications in the next generation of wireless communications
and wireless access networks as low-cost and high-reliability
microwave–photonic interfaces due to the single-chip-platform
capability,lowpowerconsumption,andintrinsicRTDelectrical
gain.Theunsynchronizedoperationmaybeusedinchaoticdata
encoding schemes in both optical and RoF chaos transmission.
ACKNOWLEDGMENT
The authors would like to thank W. Meredith of Compound
Semiconductor Technologies Global Ltd., for providing the
laser diodes.
REFERENCES
[1] N. J. Gomes, M. Morant, A. Alphones, B. Cabon, J. E. Mitchell, C.
Lethien, M. Csornyei, A. Stohr, and S. Iezekiel, “Radio-over-fiber
transport for the support of wireless broadband services,” J. Opt.
Netw., vol. 8, no. 2, pp. 156–178, Feb. 1, 2009.
[2] J. Ala-Laurila, J. Mikkonen, and J. Rinnemaa, “Wireless LAN access
network architecture for mobile operators,” IEEE Commun. Mag., vol.
39, no. 11, pp. 82–89, Nov. 2001.
[3] P. P. Smith, “Optical radio-a review of a radical new technology for
wirelessaccessinfrastructure,”BTTechnol.J.,vol.21,no.3,pp.22–31,
Aug. 2003.
[4] M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocel-
lular network architectures,” J. Lightw. Technol., vol. 25, no. 11, pp.
3301–3320, Nov. 2007.
[5] T. J. Slight, B. Romeira, L. Wang, J. M. L. Figueiredo, E. Wasige,
and C. N. Ironside, “A Liénard oscillator resonant tunnelling diode-
laser diode hybrid integrated circuit: Model an experiment,” IEEE J.
Quantum Electron., vol. 44, no. 12, pp. 1158–1163, Nov./Dec. 2008.
[6] J. M. L. Figueiredo, B. Romeira, T. J. Slight, L. Wang, E. Wasige,
and C. N. Ironside, “Self-oscillation and period adding from a reso-
nant tunnelling-laser diode circuit,” Electron. Lett., vol. 44, no. 14, pp.
876–878, Jul. 3, 2008.
[7] B. Romeira, J. M. L. Figueiredo, T. J. Slight, L. Wang, E. Wasige,
and C. N. Ironside, “Synchronisation and chaos in a laser diode driven
by a resonant tunnelling diode,” IET Optoelectron., vol. 2, no. 6, pp.
211–215, Dec. 2008.
[8] A. Argyris, D. Syvridis, L. Larger, V. Annovazzi-Lodi, P. Colet, I. Fis-
cher, J. Garcia-Ojalvo, C. R. Mirasso, L. Pesquera, and K. A. Shore,
“Chaos-basedcommunicationsathighbitratesusingcommercialfibre-
optic links,” Nature, vol. 438, no. 7066, pp. 343–346, Nov. 17, 2005.
[9] T. J. Slight and C. N. Ironside, “Investigation into the integration
of a resonant tunnelling diode and an optical communications laser:
Model and experiment,” IEEE J. Quantum Electron., vol. 43, no. 7,
pp. 580–587, Jul./Aug. 2007.
[10] D. H. Jun, S. W. Kim, K. S. Seo, J. H. Jang, and J. I. Song, “A high-
power MMIC VCO utilizing metamorphic HEMT technology,” Mi-
crow. Opt. Technol. Lett., vol. 49, no. 9, pp. 2221–2224, Sep. 2007.
[11] T. H. Lee, The Design of CMOS Radio-Frequency Integrated Cir-
cuits.Cambridge, U.K.: Cambridge Univ. Press, 2004.
[12] A. Pikovsky, M. Rosenblum, and J. Kurths, Synchronization: A Uni-
versal Concept in Nonlinear Sciences.
Univ. Press, 2001.
[13] K. Kurokawa, “Injection locking of microwave solid-state oscillators,”
Proc. IEEE, vol. 61, no. 10, pp. 1386–1410, Oct. 1973.
[14] B. Razavi, “A study of injection locking and pulling in oscillators,” J.
Solid-State Circuits, vol. 39, no. 9, pp. 1415–1424, Sep. 2004.
[15] J. M. L. Figueiredo, C. N. Ironside, and C. R. Stanley, “Electric field
switching in a resonant tunneling diode electroabsorption modulator,”
IEEEJ. QuantumElectron.,vol.37,no.12,pp.1547–1552,Dec.2001.
[16] N. Orihashi, S. Suzuki, and M. Asada, “One THz harmonic oscillation
of resonant tunnelling diodes,” Appl. Phys. Lett., vol. 87, no. 23, p.
233501, Dec. 2005.
[17] J. F. Whitaker, G. A. Mouroua, T. C. L. G. Sollner, and W. D. Goodh,
“Picosecond switching time measurement of a resonant tunneling
diode,” Appl. Phys. Lett., vol. 53, no. 5, pp. 385–387, Aug. 1, 1988.
[18] Y. Kawano, Y. Ohno, S. Kishimoto, K. Maezawa, and T. Mizutani,
“50 GHz frequency divider using resonant tunnelling chaos circuit,”
Electron. Lett., vol. 38, no. 7, pp. 305–306, Mar. 22, 2002.
[19] Y. Kawano, Y. Ohno, S. Kishimoto, K. Maezawa, T. Mizutani, and
K. Sano, “88 GHz dynamic 2:1 frequency divider using resonant tun-
nelling chaos circuit,” Electron. Lett., vol. 39, no. 21, pp. 1546–1548,
Oct. 16, 2003.
[20] K. Maezawa, Y. Kawano, Y. Ohno, S. Kishimoto, and T. Mizutani,
“Direct observation of high-frequency chaos signals from the resonant
tunneling chaos generator,” Jpn. J. Appl. Phys., vol. 43, no. 8A, pp.
5235–5238, Aug. 2004.
[21] J. Schulman, H. Santos, and D. Chow, “Physics-based RTD cur-
rent-voltage equation,” IEEE Electron Device Lett., vol. 17, no. 5, pp.
220–222, May 1996.
[22] N. V. Alkeev, V. E. Lyubchenko, C. N. Ironside, J. M. L. Figueiredo,
and C. R. Stanley, “Super high-frequency characteristics of optical
modulators on the basis of InGaAlAs resonance-tunnel heterostruc-
tures,” J. Commun. Technol. Electron., vol. 45, no. 8, pp. 911–914,
Aug. 2000.
[23] U. Parlitz and W. Lauterborn, “Period-doubling cascades and devil’s
staircases of the driven van der Pol oscillator,” Phys. Rev. A, vol. 36,
no. 3, pp. 1428–1434, Aug. 1, 1987.
[24] E. R. Brown, C. D. Parker, S. Verghese, M. W. Geis, and J. F. Harvey,
“Resonant-tunneling transmission-line relaxation oscillator,” Appl.
Phys. Lett., vol. 70, no. 21, pp. 2787–2789, May 26, 1997.
[25] S.Verghese,C.D.Parker,andE.R.Brown,“Phasenoiseofaresonant-
tunneling relaxation oscillator,” Appl. Phys. Lett., vol. 72, no. 20, pp.
2550–2552, May 18, 1998.
[26] R. Adler, “A study of locking phenomena in oscillators,” Proc. IEEE,
vol. 61, no. 10, pp. 1380–1385, 1973.
[27] T. S. Moise, Y. C. Kao, C. L. Goldsmith, C. L. Schow, and J.
C. Campbell, “High-speed resonant-tunneling photodetectors with
low-switching energy,” IEEE Photon. Technol. Lett., vol. 9, no. 6, pp.
803–805, Jun. 1997.
[28] A. N. Pisarchik and V. J. Pinto-Robledo, “Experimental observation of
two-state on-off intermittency,” Phys. Rev. E, vol. 66, no. 2, p. 027203,
Aug. 2002.
Cambridge, U.K.: Cambridge
Bruno Romeira (S’08) received the Diploma degree
in physics and chemistry from the Universidade do
Algarve, Faro, Portugal, in 2006. He is currently
working toward the Ph.D. degree in optoelectronic
integrated circuits incorporating resonant tunneling
devices from the Centro de Electrónica, Optoelec-
trónica e Telecomunicações (CEOT), Faro, Portugal.
From 2006 to 2008, he was with the CEOT, Faro,
Portugal, where he was engaged in optoelectronic
devices containing low-dimensional quantum struc-
tures.Hiscurrentresearchinterests includenonlinear
dynamics of electronic/optoelectronic circuits and the numerical simulation,
design, and characterization of optoelectronic circuits containing resonant
tunneling diodes for communication systems.
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.

Page 10

ROMEIRA et al.: NONLINEAR DYNAMICS OF RESONANT TUNNELING OPTOELECTRONIC CIRCUITS FOR WIRELESS/OPTICAL INTERFACES1445
José M. L. Figueiredo received the B.Sc. degree
in physics (optics and electronics) in 1991, and the
MSc. degree in optoelectronics and lasers in 1995,
both from the University of Porto, Portugal.
From1995to 1999,hewaswiththe Departmentof
PhysicsattheUniversityofPortoandtheDepartment
of Electronics and Electrical Engineering at the Uni-
versity of Glasgow, U.K., as a Ph.D. student working
ontheoptoelectronicpropertiesofresonanttunneling
diodes,receivingthePh.D.degreeinphysicsfromthe
University of Porto in 2000. He is with the Physics
Department at the University of the Algarve since 1999. His research interests
include the design and characterization of electronic and optoelectronic devices
and circuits incorporating low-dimensional quantum structures.
Thomas James Slight received the B.Eng. degree in physics and electronic
engineering and the Ph.D. degree in electronic engineering from the University
of Glasgow, Glasgow, U.K., in 2002 and 2006, respectively.
He is currently with the Department of Electronics and Electrical En-
gineering, University of Glasgow. His current research interests include
optoelectronic integrated circuits utilizing resonant tunneling diodes and short
wavelength quantum cascade lasers for gas sensing.
Liquan Wang was born in China in 1981. He re-
ceived the M.Sc. degree in electronics and electrical
engineering in 2006 from the University of Glasgow,
Glasgow,U.K.,whereheiscurrentlyworkingtoward
the Ph.D. degree in the reliable design of microwave
and millimeter-wave oscillators using tunneling
diodes and resonant tunneling diodes (RTDs).
His current research interests include under-
standing RTD-driven laser diode circuits and
associated applications.
Edward Wasige (S’97–M’02) received the B.Sc.
(Eng.) degree in electrical engineering from the
University of Nairobi, Nairobi, Kenya, in 1988,
the M.Sc. (Eng.) degree in microelectronic systems
and telecommunications from the University of
Liverpool, Liverpool, U.K., in 1990, and the Dr.-Ing.
degree in electrical engineering from the University
of Kassel, Kassel, Germany, in 1999.
During 1990–1993 and 1999–2001, he was en-
gaged in teaching electronics and communications
engineering courses at Moi University, Eldoret,
Kenya. From May 2001 to August 2002, he was a United Nations Educational,
Scientific and Cultural Organization Postdoctoral Fellow at the Technion-Israel
Institute of Technology, Haifa, Israel. In September 2002, he joined the
University of Glasgow, Glasgow, U.K. His current research interests include
the reliable design of resonant tunneling diode microwave and millimeter-wave
oscillators, and the development of new types of gallium nitride-based hetero-
junction FETs for power electronics and microwave applications.
Charles N. Ironside (M’87–SM’05) has been
in the Department of Electronics and Electrical
Engineering, University of Glasgow, Glasgow,
U.K., since 1984. He has been engaged in a variety
of optoelectronic projects that include, ultrafast
all-optical switching in semiconductor waveguides,
monolithicmode-locked
broad-band semiconductor lasers, quantum-cascade
lasers, and optoelectronic integrated chip (OEIC)
devices, which concentrated on the integration of
resonant tunnelling diodes with electroabsorption
semiconductorlasers,
modulators and semiconductor lasers.
Anthony E. Kelly received the B.Sc., M.Sc., and
Ph.D. degrees from the University of Strathclyde,
Glasgow, U.K.
He was with British Telecom Laboratories and
Corning. He is also a cofounder of Kamelian Ltd.,
and Amphotonix Ltd., Glasgow, U.K. He is cur-
rently with the University of Glasgow, Glasgow.
His current research interests include the use of
semiconductor optical amplifiers for PONs, optical
burst switching, and ultrafast optical switching. He
has authored or coauthored more than 100 journal
and conference papers on a range of optoelectronic devices and systems and
holds a number of patents.
Richard Green received the M.Phys. and Ph.D. degrees in material and op-
tical properties of mid-infrared quantum cascade lasers from the University of
Sheffield, Sheffield, U.K.
For three years, he was a Postdoctoral Fellow at the Scuola Normale
Superiore, Pisa, Italy, studying the dynamical characteristics of terahertz
quantum cascade lasers. Since 2008, he has been with the University of
Glasgow, Glasgow, U.K. His current research interests include time-resolved
measurements of optoelectronic materials and devices.
Authorized licensed use limited to: Universidade do Algarve. Downloaded on November 24, 2009 at 14:52 from IEEE Xplore. Restrictions apply.