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Optoelectronic
Modulators for Optical Interconnections As computer technology advances,
the information processing power of silicon CMOS chips is always
increasing. Yet, the information
transmission properties of the wires that connect these chips to each other
are limited by fundamental physical factors.
In order for the performance of the total system to continue to
improve, this interconnection problem must be solved. Long distance
telecommunications networks currently use fiber optics to transmit
information. Many groups have
investigated using optical links instead of electrical wires to connect these
silicon chips to each other inside smaller systems like computers and
internet routers. Of course, in order
to utilize electronic chips with an optical interconnection, some device must
be used to convert the signals from the electrical domain into the optical
domain and vice versa. We have focused our
research in the area of semiconductor optoelectronic modulators to perform
this function. An ideal modulator
would exhibit the following features: ·
High contrast ratio o
For high quality optical signal integrity ·
Low voltage drive o
For compatibility with the low voltages of future CMOS chips. ·
Wide wavelength range o
For optical systems that use wavelength-division multiplexing (which
allows a higher aggregate data transfer rate) or that use uncooled
laser sources off-chip (which are less expensive than fixed-wavelength cooled
lasers). ·
Surface-normal optical access o
For fabrication in two-dimensional arrays (which also permits a higher
aggregate data rate). ·
Centered on a wavelength range in the telecommunications C-band at l~1.55mm. o
For compatibility with existing fiber-based telecom networks. ·
Simple fabrication and packaging features such as tolerance to
misalignments of the device with respect to the input and output optical
beams. o
For simpler, less expensive and more practical complete optical
interconnection systems. We have developed a new
architecture called the quasi-waveguide angled-facet electroabsorption
modulator (QWAFEM). The QWAFEM is a p-i-n diode surrounded
by two flat mirrors etched into the semiconductor substrate at equal angles,
steeper than 45° with respect to the x-axis, as shown below. The input laser beam passes through the
transparent substrate and reflects off one angled mirror before entering the p-i-n diode
at a large incident angle. The
absorption of the light is modulated by the voltage applied across the diode
which typically contains multiple quantum wells (MQW). The beam then reflects off the
semiconductor-air interface, passes back through the diode, reflects off the
second angled mirror and exits through the substrate.
This design achieves all
of the points laid out above, including the misalignment tolerance, as
demonstrated in the animation below.
QWAFEM devices were grown
via MOCVD in the InGaAsP/InP material system in
collaboration with David Bour at Agilent Labs. The
fabrication was carried out in the Ginzton Microfab, Stanford Nanofabrication Facility and NNIN at
UCSB (with help from Ning Cao). The key step is the formation of the angled
mirrors. We use a two-step wet
chemical etching process based on selective etching of the crystallographic
planes of InP (see P. Bonsch
et al. in J.Electrochem.Soc., v.145, p.1273-6
(1998).) The first step uses HBr to etch V-grooves which define the (111)A plane surface at 54.7 degrees from the (100)
plane. The second step greatly reduces
the roughness of this surface to mirror-quality. The scanning electron
micrograph of some finished devices is shown below.
The contrast ratio and
misalignment tolerance of fabricated QWAFEMs were
experimentally measured and are shown here.
(Above-Left) Contrast ratio peaks at 3.5 dB and exceeds 3
dB for a wavelength range of 10 nm for only 1 V drive. (Above-Right) Misalignment tolerance
measured by displacing the device relative to the input beam and measuring
the reflectivity of the device at each point. We are currently investigating modifications to this
design in order to improve its contrast ratio for a given voltage drive
(around 1V). |
