Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2004-03-05
2004-12-28
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C359S246000, C359S248000, C438S718000
Reexamination Certificate
active
06836350
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of optoelectronic devices, and specifically relates to the coupling of driving signals to semiconductor devices for modulating optical signals.
BACKGROUND OF THE INVENTION
Semiconductor modulators for optical signals are extensively used in various applications, particularly in the field of telecommunications. One type of semiconductor modulator is an electroabsorption modulator (EAM). In order to operate an EAM, a variable voltage (RF signal) is provided across the terminals of the EAM. The voltage dependency of the absorption of the EAM at a selected operating wavelength results in a modulated optical signal. In practice it is often desirable for a DC bias voltage to be applied to the EAM as well. Adjustment of the DC bias voltage may allow tuning of the EAM performance and/or tuning of the operating wavelength of the EAM.
FIG. 1A
illustrates a prior art EAM circuit employing DC coupled drive electronics represented by an RF source. Drive electronics
100
are represented as voltage source
102
coupled across a resistor
104
. Drive electronics
100
are coupled to hybrid
Integrated circuit (HIC) assembly, or packaging,
106
. HIC
106
is shown as a transmission line
108
and an inductor
110
to represent the connection between the transmission line
108
and EAM
112
. EAM
112
is represented as resistor
114
and diode
116
in series, with a voltage controlled current source
118
representing the photocurrent, and pad capacitance
120
in parallel. Termination
122
includes a resistor
126
, with an inductor
124
representing the connection to the EAM
112
. The EAM has an n-type semiconductor side and a p-type semiconductor side. The n-side of EAM
112
is connected to a source of reference potential
136
(e.g. ground), while the p-side is coupled to the drive electronics through HIC
106
. Any DC offset voltage provided to EAM
112
must be supplied by drive electronics
100
. The need to provide the DC offset voltage may strain the drive electronics and lead to early component failure.
An alternative prior art circuit is shown with reference to FIG.
1
B. In this circuit the n-side of the EAM is also connected to ground
136
. This circuit employs a bias tee circuit
128
to connect drive electronics
100
to EAM
112
. A DC bias to EAM
112
may be provided through bias tee circuit
128
by DC voltage supply
134
. The use of bias tee circuit
128
in the circuit of
FIG. 1B
permits the DC offset voltage to be set with precision, and, compared to the prior art circuit shown in
FIG. 1A
, has less load on drive electronics
100
. However, a suitable bias tee for high speed applications is relatively large, generally much larger than the EAM package itself, and adds significantly to the cost of a package incorporating drive electronics, the EAM, and other related electronics.
SUMMARY OF THE INVENTION
One embodiment of the present invention is drive circuitry to provide a DC bias voltage and a high frequency modulation current to an electroabsorption modulator (EAM), which includes a first semiconductor type contact and an second semiconductor type contact. The drive circuitry includes a high frequency modulation current source, a coupling capacitor, and a first DC lead. The first modulation lead of the high frequency modulation current source is electrically coupled to the first semiconductor type contact of the EAM and the second modulation lead of the high frequency modulation current source is electrically coupled to an AC ground. The coupling capacitor includes a EAM-side capacitor electrode which is electrically coupled to the second semiconductor type contact of the EAM, a non-EAM-side capacitor electrode which is electrically coupled to the AC ground, and a dielectric layer which is disposed between the EAM-side capacitor electrode and the non-EAM-side capacitor electrode. The first DC lead is electrically coupled to the EAM-side capacitor electrode and configured to be coupled to a first DC potential.
Another embodiment of the present invention is a monolithic EAM and coupling capacitor. The monolithic EAM and coupling capacitor include a substrate with a top surface. A non-EAM-side capacitor electrode is coupled to the top surface of the substrate, a capacitor dielectric layer is coupled to the non-EAM-side capacitor electrode and an EAM-side capacitor electrode is coupled to the capacitor dielectric layer to form the coupling capacitor. An EAM base layer is formed of a first type semiconductor material. This EAM base layer is electrically coupled to the EAM-side capacitor electrode. An EAM waveguide, which includes an electroabsorption portion, is formed on the EAM base layer. An EAM second type semiconductor layer is formed on the EAM waveguide and an EAM electrode is electrically coupled to the EAM second type semiconductor layer.
A further embodiment of the present invention is an alternative monolithic EAM and coupling capacitor. The alternative monolithic EAM and coupling capacitor includes a substrate formed of a first type semiconductor material with a top surface and a bottom surface. An EAM-side capacitor electrode is coupled to the bottom surface of the substrate, a capacitor dielectric layer is coupled to the EAM-side capacitor electrode and a non-EAM-side capacitor electrode is coupled to the capacitor dielectric layer to form the coupling capacitor. An EAM waveguide, which includes an electroabsorption portion, is formed on the top surface of the substrate. An EAM second type semiconductor layer is formed on the EAM waveguide and an EAM electrode is electrically coupled to the EAM second type semiconductor layer.
Yet another embodiment of the present invention is a method of manufacturing a monolithic EAM and coupling capacitor. A substrate formed of a first type semiconductor material with a top surface and a bottom surface is provided. An EAM waveguide layer, which includes an electroabsorption portion, is formed on the top surface of the substrate. An EAM second type semiconductor layer in formed on the EAM waveguide. The EAM second type semiconductor layer and the EAM waveguide layer are etched to form an EAM second type semiconductor region and an EAM waveguide. An EAM electrode is formed on the EAM second type semiconductor region. An EAM-side capacitor electrode is formed on the substrate. A capacitor dielectric layer, which is electrically coupled to the EAM-side capacitor electrode, is formed and a non-EAM-side capacitor electrode is formed on the capacitor dielectric layer.
A still further embodiment of the present invention is an additional monolithic EAM and coupling capacitor. The additional monolithic EAM and coupling capacitor includes a substrate, including a first type semiconductor material portion having a top surface. An EAM electrode is electrically coupled to the first type semiconductor material portion of the substrate. An EAM waveguide is formed on the top surface of the first type semiconductor material portion of the substrate and includes an electroabsorption portion. An EAM second type semiconductor layer is formed on the EAM waveguide. An EAM-side capacitor electrode is electrically coupled to the EAM second type semiconductor layer, a capacitor dielectric layer is formed on the EAM-side capacitor electrode, and a non-EAM-side capacitor electrode formed on the capacitor dielectric layer.
An additional embodiment of the present invention is an alternative method of manufacturing a monolithic EAM and coupling capacitor. A substrate including a first type semiconductor material portion having a top surface is provided. An EAM waveguide layer, which includes an electroabsorption portion, is formed on the top surface of the first type semiconductor material portion of the substrate. An EAM second type semiconductor layer is formed on the EAM waveguide layer. The EAM second type semiconductor layer and the EAM waveguide layer are etched to form an EAM second type semiconductor region and an EAM waveguide. An EAM electrode is formed on the first type semicondu
Foulk Helga
Maile Keith
Redinger Scott
Singh Prashant
Stronczer John
Epps Georgia
RatnerPrestia
T-Networks, Inc.
Thompson Timothy
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