Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2000-05-05
2002-03-12
Healy, Brian (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S001000, C385S003000, C385S027000, C385S028000, C385S040000, C385S130000
Reexamination Certificate
active
06356673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally pertains to traveling wave LiNbO
3
intensity modulators and more specifically to a device in which the reduction of electrode loss due to leaky mode coupling is achieved in a traveling wave LiNbO
3
intensity modulator, without a reduced substrate thickness.
2. Description of the Related Art
Traveling wave LiNbO
3
intensity modulators are of great interest for analog radio frequency (RF), microwave link and digital and analog optical communication applications. Of particular interest is the drive voltage of the modulator as this quantity determines link gain, sensor sensitivity, and drive power requirements for high-speed (40 Gbits) digital links. In velocity matched, traveling wave devices drive voltage is determined by, first, the low frequency voltage-length product, secondly, by velocity and impedance match, and lastly, by electrical losses in the traveling wave electrode structure.
In general these devices are Mach-Zehnder interferometers operated with a push-pull electrode structure, so that fields of opposite polarity operate on each arm of the waveguide. These fields serve to change the index of the electro-optic LiNbO
3
, which in turn alters the phase of the light traveling in each waveguide, and thus allows operation of the interferometer.
For Z-cut LiNbO
3
devices, the electrode structure is typically coplanar waveguide (CPW). CPW is known to be an intrinsically leaky structure in general, and in optical modulator devices. SEE, Rutledge et al., INFRARED AND MILLIMETER WAVES, Vol.10, MILLIMETER COMPONENTS AND TECHNIQUES, Part II, K. J. Burton, Ed., Academic Press, Inc., New York, 1983; and Gopalakrishnan et al., ELECTRICAL LOSS MECHANISMS IN TRAVELING WAVE LiNbO
3
OPTICAL MODULATORS, Electron. Lett., Vol. 28, No. 2, pp. 207-208, 1992 Coupling can occur both between the guided mode and radiation modes in the substrate, and between the guided mode and slab, or substrate, modes in the substrate. Once power is coupled out of the guided mode, it is lost and cannot contribute to optical modulation, thus resulting in an increase in measured drive voltage.
One approach used previously to control coupling of the guided mode to substrate modes was to employ very thin substrates, ~0.25 mm or less. SEE, U.S. Pat. No. 5,416,859, Burns et al., issued May 16, 1995. This has the effect of changing the mode dispersion of the slab, so that the undesirable mode coupling only occurred at higher frequencies, out of the range of interest. This approach was effective, but thin substrates were very fragile and hard to work with, resulting in a low yield of surviving devices. This is particularly so as device length increased to reduce drive voltage.
These leaky mode effects increase with frequency, as the calculated loss coefficient for the guided mode is proportional to the cube of the frequency, f
3
, for coupling to radiation modes, and to the square of the frequency, f
2
, for coupling to substrate modes. In both cases, the loss coefficient is proportional to the square of the overall waveguide width, W
tot
H
=S
H
+2W
H
, as defined in
FIG. 1
b.
This implies that the widest part of the waveguide horn is responsible for the largest part of the leaky mode loss, and that this extra propagation loss can be minimized by keeping the horn structure small. However, for packaged devices the end of the horns must be sufficiently large that they can be contacted to a microwave connector either directly or with wire bonding or some other connection method.
SUMMARY OF THE INVENTION
An object of this invention is to control leaky mode loss without having a thin substrate.
Another object of this invention is to make a device having a large enough horn size that connection can be made to a electronic transmission cable either directly or with wire bonds.
Another object of this invention is to make a device that will operate to 40 GHz and beyond.
These and other objectives are accomplished by a low loss coplanar waveguide horn with low drive voltage LiNbO
3
modulators wherein the electrical transmission of the traveling wave electrode structure, and of the input and output coupling structures, sometimes called the “horns”, which transition the active electrode structure with microwave connectors, is maximized by an appropriate design of the horn structure. The conflicting requirements of the leaky mode loss and maintaining sufficient horn size to allow microwave connection, can be reconciled by an adjustment of the ground plane width, which also effects the magnitude of coupling of the guided mode to substrate and radiation modes. Control of both the maximum horn size and the width of the ground planes in particular modulator electrode structures can provide operation to 40 GHz in LiNbO
3
devices with substrate thicknesses of ~≦1.0 mm, without excess leaky mode loss. The horn structures are ≦3 mm long and the active waveguides are ~4-5 cm long. They are made of Z-cut LiNbO
3
, with electroplated gold CPW electrodes of 10-30 &mgr;m thickness.
REFERENCES:
patent: 5416859 (1995-05-01), Burns et al.
patent: 5422966 (1995-06-01), Gopalakrishnon et al.
patent: 5455876 (1995-10-01), Hopfer et al.
patent: 5502780 (1996-03-01), Madabhushi
patent: 5526448 (1996-06-01), Nagata et al.
patent: 5563965 (1996-10-01), Madabhushi
patent: 5644664 (1997-07-01), Burns et al.
patent: 5787211 (1998-07-01), Gopalakrishnan
patent: 5886807 (1999-03-01), Burns et al.
patent: 6016198 (2000-01-01), Burns et al.
patent: 6052496 (2000-04-01), O'Donnell
patent: 6236772 (2001-05-01), Tavlykaev et al.
Rutledge et al.; Infrared and Millimeter Waves, vol. 10, Millimeter components and Techniques, part II, K. J. Burton, Ed., Academic Press. Inc., New York, pp. 1-90, 1983.
Gopalakrishnan et al., Electrical Loss Mechanism in Traveling LiNBO3Optical Modulators, Electron Lett., vol, 28, No. 2, pp. 207-288, 1992.
Tsuji et al., New Interesting Leakage Behavior on Coplanar Waveguides of Finite and Infinite Widths, vol. 29, No. 12, pp. 2130-2137, Dec. 1991.
Noguchi et al., A BroadBand and T.: LiNBO3Optical Modulator with a Ridge Stucture, J. Lightwave Tech, vol. 13, No. 6, pp. 1164-1168, Jun. 1995.
Burns et al.; BroadBand Reflection Traveling Wave LiNBO3Modulator; IEEE Photonics Tech Lett; vol. 10, No. 6.; pp. 805-806; Jun. 1998.
Healy Brian
The United States of America as represented by the Secretary of
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