Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
1999-05-11
2002-11-19
Font, Frank G. (Department: 2877)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S003000
Reexamination Certificate
active
06483953
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of optical modulation and, in particular, to methods and apparatus for high-speed external optical modulations.
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 is typically accomplished by modulating the drive current of the optical source. An integrated electro-absorptive modulator can modulate the optical intensity of light leaving the source as well.
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 minimal 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.
Modulators used for transmission at high speeds and over long distances must be efficient to avoid the use of expensive electronic amplifiers and digital drivers. In addition, modulators need to be compact in order to minimize the required space on the transmitter card.
Lithium niobate crystals have an inherent mismatch between the velocity of optical and electrical signals propagating through the crystal which impacts 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 optical signal. Consequently, the modulation becomes inefficient. The longer the interaction distance, the greater the inefficiency. Using a buffer layer can minimize velocity walk-off, however, the required interaction length is long.
FIG. 1
illustrates a top view of a prior art electro-optic device
10
that compensates 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 in 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 optical signal. The RF electrodes
14
are positioned to alternate between the zero degree phase shift sections
16
and the 180 degree phase shift sections
18
. The length of the zero degree phase shift sections
16
is chosen so that the RF signal “walks off” the optical signal approximately 180 degrees before it is flipped 180 degrees in the phase reversal sections
18
.
FIG. 2
illustrates a top view of a prior art electro-optic device
30
that compensates 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 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 optical signal at the end of the non-interaction region
38
.
One disadvantage of prior art electro-optic devices that compensate for the velocity mismatch between the optical and electrical signals propagating through the device is that they have 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 increased in order to achieve the required modulation. Increasing the length of a lithium niobate device increases the size of the package containing it, which is undesirable, because of the limited space on the transmitter board. State-of-the-art DWDM systems have stringent space requirements due to their high channel count. In addition, more expensive and larger power supplies must be used because higher drive voltages are required.
SUMMARY OF THE INVENTION
It is therefore a principal object of this invention to provide an electro-optic device that includes a compensation network that modifies at least one of the phase or the amplitude of the electrical signal relative to the phase or amplitude of the accumulated modulation on the optical signal without introducing significant loss or decreasing the modulation efficiency. It is another principle object for such a compensation network to compensate for velocity mismatch between the electrical signal and the optical signal. It is another principle object for such a compensation network to compensate for the effects of external perturbations in the substrate of the modulator, such as the effects of temperature on a lithium niobate substrate. It is another principle object for such a compensation network to be removably attached to the device to facilitate modifying the frequency response of the device. It is yet another principle object of the present invention to construct a modulator with such a compensation network that is used in conjunction with prior art broadband modulator to form a combined modulator that is capable of producing bandwidth extension of the broadband modulator into the narrow band modulator region.
A principal discovery of the present invention is that an electro-optic device can be constructed with a compensation network that temporarily directs the electrical signal in a path that is in a non-co-linear direction relative to the direction of propagation of the optical signal and that such a compensation network has numerous advantages over the prior art. For example, such a compensation network can modify the phase of the electrical signal relative to the optical signal in order to minimize the effects of velocity mismatch, while introducing very low loss. Such a compensation network can also compensate for the effects of external perturbations on the electro-optic device. In one embodiment of the invention, such a compensation network is used to construct a modulator that provides more efficient modulation per unit length of electrode.
Kissa Karl M.
McBrien Gregory J.
Wooten Ed
Font Frank G.
JDS Uniphase Corporation
Mooney Michael P.
Rauschenbach Kurt
Rauschenbach Patent Law Group
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