Optical: systems and elements – Holographic system or element – Authentication
Reexamination Certificate
2000-09-15
2001-10-30
Epps, Georgia (Department: 2873)
Optical: systems and elements
Holographic system or element
Authentication
C385S001000, C385S003000
Reexamination Certificate
active
06310700
ABSTRACT:
BACKGROUND OF THE INVENTION
Electro-optic modulators are used in fiberoptic communication systems, in particular, commercial long distance, high speed digital communication systems. Electro-optic modulators convert electrical signals into modulated optical signals. The electrical signals represent the data or voice for transmission over the fiberoptic communication system. The modulated optical signal is the data/voice transmission in an optical waveform.
Techniques for modulating an optical signal include amplitude modulation and phase modulation. Amplitude modulation results in the modulated optical signal or light emitted from the modulator being switched from on to off. A Mach-Zehnder interferometer is an example of a modulator performing amplitude modulation. Phase modulation results in the phase of the optical signal being shifted a certain number of degrees. Phase modulated signals require specialized wave detectors to detect the change in phase, while amplitude modulated signals can be detected by photon detectors that detect the power of the received signal in terms of photons.
The typical structure of an electro-optic modulator includes an electro-optic substrate, such as lithium niobate or III-V semi-conductors, an optical waveguide pattern defined within the substrate, and an electrode structure disposed over the substrate carrying the electrical signals to be converted.
As electrical signals propagate through the electrode structure, an electric field is generated across a length of the waveguide, known as the interaction distance. The application of the electric field across the waveguide for the entire length of the interaction distance affects the refractive index of the electro-optic substrate causing an optical signal (i.e. light) propagating through the waveguide to be modulated.
The electrode structure disposed over the substrate provides a strong electrical-optical interaction. The interaction strength is characterized by a “VL product,” which is the product of a switching voltage (V
Π
) of the electrical signal multiplied by the interaction distance (L) (i.e. V×L). The switching voltage, V
Π
, is the voltage swing required at the modulator electrical input to cause the light being emitted from the modulator either to go from “on” to “off” or to shift its phase a certain amount. This interaction strength is generally a constant, typically 55 volts per millimeter (V-mm) at 1550 nanometers (nm) for lithium niobate.
The present limiting factor of electro-optic modulators is the high drive voltage requirement suitable for transmitting at data rates of 10 gigabits per second (Gbit/s) and above. According to the VL product, the required switching voltage can be lowered if the modulator's interaction distance is increased such that the electric field is applied to the optical signal over a maximum length. However, long modulators require the velocities of the electrical and optical signals to be matched in order to apply the electric field to the optical signal for the entire length of the interaction distance. If not, the electric field would not be applied to the optical signal for the entire length of the interaction distance degrading the modulation.
There are a variety of electrode designs in existence today that achieve the necessary velocity match. All of them make some compromise among velocity, electrical/optical overlap (a measure of modulation efficiency), impedance, microwave loss, and manufacturability. Combined with modulator substrate materials, primarily lithium niobate and III-V semi-conductors, state of the art modulator performance is barely adequate for 10 Gbit/s systems and is inadequate at higher speeds, such as 40 Gbit/s.
A known electrode design for velocity matching is the capacitive loading of a transmission line. The effect of this design is to slow down the velocity of the electrical signal relative to its velocity on the unloaded transmission line, and to lower the impedance below that of the unloaded line. This is an advantage in modulator designs where the electrical velocity on the unloaded line is faster than the velocity of light in the waveguides, such as modulator designs based on III-V semiconductor electro-optic substrates.
However, the slowing effect of this electrode design renders this type of electrode useless for modulators where the velocity of the electrical signal is already slower than the optical velocity, such as modulator designs based on lithium niobate electro-optic substrates.
SUMMARY OF THE INVENTION
According to one embodiment, the electro-optic modulator includes an electro-optic substrate, at least one optical waveguide defined within the substrate having an optical signal propagating at an optical velocity; and an electrode structure having a transmission line and conductive legs. The conductive legs extend the transmission line from the substrate, while an electrical signal propagates at an electrical velocity along the transmission line. According to a further embodiment, the electro-optic modulator may include a dielectric layer disposed between the substrate and the transmission line. The dielectric layer has a dielectric constant lower than the substrate, as in the embodiment where the dielectric layer is made of polyimide.
According to an embodiment, the conductive legs extend the transmission line a distance from a surface of the substrate substantially reducing the strength of an electric field, associated with the electrical signal propagating in the transmission line, in the substrate to maximize the electrical velocity of the electrical signal propagating on the transmission line. The electric field associated with the electrical signal propagating in the transmission line substantially travels within a region having a dielectric constant lower than a dielectric constant of the substrate. The region having a low dielectric constant may be air or a dielectric layer.
The conductive legs provide a capacitance reducing the electrical velocity to match the optical velocity of the optical signal propagating in the waveguide. According to one embodiment, the electro-optic modulator includes a first set of conductive legs opposing a second set of conductive legs along a length of the optical waveguide generating an electric field across the waveguide modulating the optical signal. The conductive legs may be spaced apart from one another.
According to an embodiment, the conductive legs include a low reactance electrical conductor and a loading electrode. Opposing loading electrodes of opposing conductive legs are disposed along a length of the optical waveguide generating an electric field across the waveguide modulating the optical signal. The opposing loading electrodes of the opposing conductive legs generate a capacitance that reduces the electrical velocity on the transmission line to match the optical velocity of the optical signal.
According to one embodiment, the optical waveguide is aligned between opposing loading electrodes of the conductive legs such that the optical signal may be modulated by a horizontal component of the electric field. According to an alternative embodiment, the optical waveguide is aligned adjacent to the loading electrodes such that the optical signal may be modulated by a vertical component of the electric field.
The electro-optic substrate can be modified according to several different embodiments. The electro-optic substrate may be a thin layer disposed over a second substrate having a lower dielectric constant at electrical frequencies. The substrate may be non-planar in which the optical waveguide is ridged. The substrate may be a ferroelectric material, such as lithium niobate, or a III-V semiconductor material.
According to another embodiment, the electro-optic modulator includes an electro-optic substrate, at least one optical waveguide defined within the substrate having an optical signal propagating at an optical velocity, and an electrode structure having a transmission line and conductive legs. The transmission line has a signal electrode an
Epps Georgia
Hamilton Brook Smith & Reynolds P.C.
Massachusetts Institute of Technology
Tra Tuyen
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