Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2000-09-22
2003-06-17
Font, Frank G. (Department: 2877)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S003000
Reexamination Certificate
active
06580840
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of optical modulation. In particular, the invention relates to methods and apparatus for high-efficiency electro-optic modulation.
BACKGROUND OF THE INVENTION
Optical modulators impress or modulate RF (or microwave) electrical signals onto a light beam in order to generate a modulated optical beam that carries data. Modulators either directly modulate the optical beam as it is generated at the optical source or externally modulate the optical beam after it has been generated. Direct modulation may be accomplished by modulating the drive current of the optical source. Direct modulation may also be accomplished by modulating the optical intensity of light leaving the source with an integrated electro-absorptive modulator.
External modulation can be accomplished by using an external modulator that is separate from the optical source. External modulation is advantageous because it can modulate signals over a very wide bandwidth. External modulators are typically voltage-controlled devices that include a traveling-wave electrode structure, which is positioned in close proximity to the optical waveguide. The electrode structure produces an electric field that overlaps the optical waveguide over a predetermined distance (the interaction length) and causes an electromagnetic interaction, which modulates the optical signal.
Lithium niobate (LN) electro-optic external modulators are increasingly being used to modulate data on optical signals that are being transmitted at very high data rates and over long distances. Lithium niobate modulators are advantageous because they can modulate optical signal over a broad frequency range, they modulate optical signals with controlled, potentially zero, optical frequency shift (frequency “chirp”), and they operate over a broad wavelength range. These features are particularly desirable for Dense Wavelength Division Multiplexing (DWDM) broadband optical communication systems that transmit optical signals with many optical wavelengths through a single optical fiber.
Lithium niobate crystals have an inherent mismatch between the velocity of optical and electrical signals propagating through the crystal, which lowers modulation efficiency. The RF propagation index is significantly higher than the optical refractive index of lithium niobate. That is, the lithium niobate crystal slows the RF signal relative to the optical signal so that it takes the RF signal a longer period of time to travel over the interaction distance. Thus, the RF signal becomes out-of-phase with or “walks off” the accumulated modulation on the optical signal.
This “walk off” lowers the modulation efficiency. Modulators used for transmission at high speeds and over long distances must be efficient to minimize the use of electronic amplifiers and digital drivers. Electronic amplifiers and digital drivers are costly and occupy valuable space in the transmission link. In addition, electronic amplifiers and digital drivers may fail and lower the quality of service and require expensive maintenance in the field.
FIG. 1
illustrates a top view of a prior art electro-optic device
10
that increases modulation efficiency by compensating for the velocity mismatch between the optical and electrical signals propagating through the device by using phase reversal sections that are co-linear with the optical waveguide. The device
10
includes an optical waveguide
12
and RF electrodes
14
that are positioned in zero degree phase sections
16
and phase reversal sections
18
.
The phase reversal sections
18
periodically flip the RF electrodes
14
to either side of the optical waveguide
12
to produce a 180 degree phase shift in the RF signal relative to the accumulated modulation on the optical signal. The RF electrodes
14
are positioned to alternate between the zero degree phase sections
16
and the phase reversal sections
18
. The length of the zero degree phase shift sections
16
is chosen so that the RF signal “walks off” the accumulated modulation on the optical signal approximately 180 degrees before it is flipped 180 degrees in the phase reversal sections
18
.
The prior art electro-optic device
10
of
FIG. 1
has relatively low modulation efficiency per unit length. This is because the phase of the RF signal is modified with co-linear sections that are positioned at intervals of 180 degrees. When the difference in phase between the RF and optical signals approaches 180 degrees, the incremental increase in modulation depth with incremental change in electrode length approaches zero. Therefore, the total length of the device must be significantly increased in order to achieve the required modulation.
FIG. 2
illustrates a top view of a prior art electro-optic device
30
that increases modulation efficiency by compensating for the velocity mismatch between the optical and electrical signals propagating through the device
30
by using co-linear but intermittent interaction sections. The device
30
includes an optical waveguide
32
and RF electrodes
34
that are positioned to alternate between an interaction region
36
and a non-interaction region
38
relative to the optical waveguide
32
.
The length of the interaction region
36
is chosen so that the RF signal “walks off” the modulation on the optical signal by as much as 180 degrees of phase shift before it is routed away from the optical waveguide
32
in a co-linear direction and into the non-interaction region
38
. The length of the non-interaction region
38
is chosen so that the RF signal becomes phase matched with the accumulated modulation on the optical signal at the end of the non-interaction region
38
. The prior art modulator of
FIG. 2
has non-interaction regions of substantial length that re-align the phase of the modulating signal, but introduce RF loss, and occupy device length.
Increasing the length of a lithium niobate electro-optic modulator increases the size of the package. Increasing the size of the modulator package and the power supplies is highly undesirable because the space on transmitter boards and in transmission huts is very limited. Efficiency is also a major consideration, as more powerful electronic drivers require more space on the transmitter card. State-of-the-art DWDM transmission equipment occupies a significant amount of space because the equipment includes electronics for numerous channels. Most transmission huts were designed for much more modest communication systems and are not very spacious. Many transmission huts cannot be expanded for various reasons.
Another disadvantage of prior art electro-optic modulators in
FIGS. 1 and 2
is that these modulators are not suitable for modulating digital signals. This is because these modulators have non-linear phase characteristics as a function of frequency response. Therefore, the digital pulse shapes are not preserved. In addition, the efficiency is concentrated in a set of narrow band regions, which is suitable for a square wave signal, but is unsuitable for digital signals having an arbitrary bit sequence. Other prior art modulators correct the uniformity of efficiency with frequency by using a periodic, Barker-code, phase reversal locations along the modulator length. However, these prior art modulators still have non-linear phase as a function of frequency.
Some prior-art electro-optic modulators uses a buffer layer to achieve velocity matching as described in connection with FIG.
3
. These prior art devices have non-optimized modulation efficiency because they preserve significant modulation beyond the required bandwidth.
Some prior art electro-optic modulators use z-cut lithium niobate. Using z-cut lithium niobate is advantageous because z-cut lithium niobate inherently provides better overlap between optical and RF fields and thus, has an inherently high modulation efficiency as compared with x-cut lithium niobate electro-optic modulators. Z-cut lithium niobate electro-optic modulators, however, experience bias drift effects. Conductive buffer layers an
Horton Timothy U.
Kissa Karl M.
McBrien Gregory J
Font Frank G.
JDS Uniphase Corporation
Mooney Michael P.
Rauschenbach Kurt
Rauschenbach Patent Law Group, LLC
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