Essient
Photonics, Glasgow, UK
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Millimeter-wave
(mm-wave) fiber-optic systems are a promising approach
for
broadband wireless access systems. A tradeoff is involved in exploiting
the
full potential of mm-wave band, with increased transmitter and receiver
hardware
complexity. Owing to that there is a growing interest in the
exploitation
of the enabling photonic technology necessary to support such
applications.
Components based on Resonant Tunneling Diodes (RTDs), due to the high
frequency
response of the resonant tunneling process (higher than 100 GHz), have
been
proposed to be used as electronic and optical devices for mm-wave and
square
wave generation, frequency multiplication, switching, static and
multi-state
memories, light emission, optical switching and modulation, and
photo-detection.
A Resonant Tunneling Diode (RTD) embedded in an optical waveguide has
been
investigated as an alternative to conventional p-i-n electro-absorption
modulators
(EAMs) currently employed in optical communication systems, due to the
monolithic
integration of a high speed electronic amplifier with an optoelectronic
component.
The electronic amplifier relies on the RTD and the optoelectronic
component
is an electro-absorption modulator. In more detail the resonant
tunneling
diode electro-absorption modulator (RTD-EAM) is based on the
integration
of a RTD with an optical ridge waveguide. For wavelengths around 1550
nm,
we employ a unipolar In0.53Ga0.42Al0.05As optical waveguide containing
a
In0.53Ga0.47As/AlAs double-barrier RTD. The new modulator concept based
on
the InGaAlAs material system lattice matched to InP is a promising
route
towards high speed, low radio frequency (rf) power consumption,
optoelectronic
converters (rf-optical and optical-rf), because it can cover the
wavelength
range 1000 to 1600 nm where optical fibers have the lowest loss and
chromatic
dispersion. Contrary to the conventional p-i-n electro-absorption
modulators,
the RTD-EAM when correctly DC biased presents wideband (theory suggests
up
to 100 GHz) gain via negative differential resistance (NDR) requiring
low
driving voltage at high frequency, therefore a small high frequency ac
signal
(<1 V) can induce high speed electrical field switching producing
substantial
modulation of light at photon energy slightly lower than the waveguide
band-gap
energy. Independent RTD-EAM characterization has shown a modulation
depth
of 5 dB for a voltage change of 1 mV. We demonstrated its operation at
long
haul optical communications wavelength (specifically 1550 nm) and
preliminary
results have shown modulation depths up to 28 dB at around 1565 nm.
Modelling
studies indicated a potential RTD-EAM bandwidth higher than 60 GHz with
extinction
ratio larger than 20 dB with driving voltage lower than 0.5 V. Another
key
advantage is that because it has an integrated amplifier it does not
require
an expensive high speed amplifier and can be driven directly from the
usual
digital logic circuits based on CMOS.
The goal of this proposal is to study further the potential
applications
of the RTD-EAMs on the optical communication domain, specifically its
capability
to act as external optical modulators. We propose to take a step
further
and optimize the RTD-EAM characteristics taking into account its
intrinsic
device parameters, the device packaging and the overall system
performance
in order to obtain electrical and optical packaged RTD-EAM prototypes
operating
at the wavelength in the range 1300-1600 nm. The RTD-EAM performance
will
be experimentally tested in communication system test-beds.
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Resonant Tunnelling Diode Electroabsorption Modulator
(RTD-EAM)
Light
source: Tuneable diode laser operating in cw mode; RTD-EAM input: CW light at 1545 nm; rf driving signal;
Photo-detector:
45 GHz newfocus PD
Photodetector output
versus
dc bias for
100 mV RTD-EAM driving
signal
Photodetector output
versus
dc bias for 1
V RTD-EAM
driving signal
Photodetector output versus dc bias for 1 V modulating
signal
The photodetector
output signal increases
by 20 dB when the dc operation point switches from the peak to the valley region for a 100 mV
rf driving signal.
Photodetector output
versus
dc bias with rf voltage modulating signal as a parameter
The photodetector
output signal increases
more than 40 dB at
the valley bias when the amplitude of the rf modulating signal increases from 1 mV to 200 mV.
Photodetector output
versus
dc bias for
7.7 dBm rf modulating signal
26 GHz
Click
to see RTD photo-detection results
Jos� Figueiredo,
2001,
Faculdade de Ci�ncias e Tecnologia, Universidade do Algarve,
Campus
de Gambelas, 8000-117 FARO, Portugal
URL: ;
E-mail: mailto:jmfigueiredo@fc.ul.pt;
Fax: +351 289 819403; Tel:
+351 289 800905