| 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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