Silicon-based electro-optic (EO) modulator is an indispensable building block for integrated lightwave circuits. In this
paper, we report an EO modulator that incorporates a heterojunction bipolar transistor (HBT) with Ge composition
graded base. The emitter is n-type doped silicon with a doping concentration of 1021/cm3. The width of the emitter strip
is 0.2μm and the thickness of the emitter layer is 0.16μm. The base has a thickness of 40nm with varying Ge
composition from zero at the emitter-base junction side to 20% at the base-collector junction side. Raised extrinsic base
is incorporated for base contact. The intrinsic base is p-type doped with a concentration of 4×1019/cm3. The HBT is
biased at VCE = 0.5 V whereas VBE is switched between -1.0V and 1.0V. The carrier distribution at "ON" state of the EO
modulator and the transient analysis are performed by MEDCI simulation. The changes of the refractive indices of the
HBT are computed from the carrier density in all regions, and then the refractive index map is imported into an optical
mode solver (RSoft BeamProp). The HBT EO modulator that supports only one optical mode is ideal, but a trade-off
between modal property and device speed is observed. For current design, we achieved a π-phase modulation length of
less than 600μm, and a switching delay less than 62ps.
In this paper, we compare the power consumption of a "line-of-sight" free space optical (FSO) link and a radio frequency
(RF) data link. We investigate a 2.5 Gbps line-of-sight FSO interconnection, which consists of a vertical cavity surface
emitting laser (VCSEL) driver (MAX3795), a VCSEL laser diode (LD), a PIN photodiode (PD), a transimpedance
amplifier (MAX3864), and a limiting amplifier (MAX3746). It is shown that the total power consumption is about 370.6
mW in simple NRZ on-off keying (OOK) modulation format. Different lens configurations are discussed in terms of the
integration/setup efforts and the beam controlling effects. A 250 Kbps commercial radio frequency (RF) link comprised
in Tmote sky module (Moteiv Corporation) is explored to compare with the FSO link. The average power supplied to the
radio transceiver is about 50.76 mw. The estimated energy consumption for the aforementioned RF link is 2.03×10-4
mJ/bit, while the end-to-end FSO consumes 1.48×10-7 mJ/bit. This paper provides design outlines from the aspect of the
power consumption of FSO and RF wireless communication technologies for distributed sensor network applications.
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