Optical: systems and elements – Optical modulator – Having particular chemical composition or structure
Reexamination Certificate
2002-09-04
2004-04-13
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Having particular chemical composition or structure
C385S002000, C385S003000
Reexamination Certificate
active
06721085
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on, and claims priority to, Japanese Application No. 2002-062095, filed Mar. 7, 2002, in Japan, and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator and a design method therefor. More particularly, the present invention relates to an optical modulator that utilizes an electro-optic effect of a crystal substrate. The invention also relates to a method of designing an optical modulator which performs electrical-to-optical conversion by modulating a given light beam with an electrical signal.
2. Description of the Related Art
Recent years have seen an increasing use of multimedia applications, with a growing awareness of demands for more advanced optical communications networks that provide higher speeds and larger bandwidths. Optical modulators are one of the key devices for realizing such high-performance optical networks. One type of optical modulator is an external modulator, which performs electrical-to-optical conversion by modulating an incoming light beam with an electrical signal. The modulating signal produces an electric field across an optical waveguide fabricated on a substrate, so that the light beam propagating along the waveguide will be varied in phase as a result of interaction between the light and the electric field being applied to it.
To meet the recent demand for high-speed, high-bandwidth optical communication, technological migration from 10 Gbps-class systems to 40 Gbps-class systems has begun, including the deployment of dense wavelength-division multiplexed (DWDM) optical transmission systems. The new systems require optical modulators to operate four times faster than before. To fulfill this requirement, it is necessary to reduce the drive voltage of modulators since high-speed electronic circuits cannot produce a large voltage swing.
In designing such an external optical modulator as mentioned above, however, we trade off faster operating rates (or wider modulation bandwidths) against lower drive voltages. In general, we can increase the modulation rates if the electric capacitance is small. This would be accomplished by simply cutting the length of the optical waveguide (or actually, reducing the length of a signal electrode that makes a modulating electric field interact with the light beam traveling on the optical waveguide). The reduction of this “interaction length,” however, also reduces the amount of resulting phase displacements, causing a decreased modulation ratio. Contrary to our desire for a lower drive voltage, we now have to increase the drive voltage to yield a sufficient modulation depth. For this reason, there have been difficulties in further improving the performance of conventional optical modulators or reducing the drive voltage for optical modulators.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a high-speed, high-performance optical modulator which operates with a reduced drive voltage without sacrificing its modulation bandwidth.
It is another object of the present invention to provide a method to design a high-speed, high-performance optical modulator which operates with a reduced drive voltage without sacrificing its modulation bandwidth.
To accomplish the objects stated above, according to the present invention, there is provided an optical modulator including an optical waveguide fabricated on a crystal substrate that exhibits an electro-optic effect; a signal electrode placed in the vicinity of the optical waveguide; and ground electrodes formed on both sides of the signal electrode. The characteristic impedance of the signal electrode is set within a range where microwave reflection is limited below a predetermined level. The light beam traveling along the optical waveguide is phase-matched with a microwave signal traveling along the signal electrode. The gap between the signal electrode and ground electrodes is at least 44 &mgr;m, while the interaction length of the signal electrode is at least 41 mm. With such a setup, the light beam can be modulated at a rate of 40 Gbps or higher.
Objects of the present invention are achieved by providing an optical modulator including (a) a ridge; (b) a signal electrode on the ridge, the signal electrode having an interaction length which is at least 41 mm; and (c) a ground electrode. A gap width between the ground electrode and the signal electrode is at least 44 &mgr;m.
Objects of the present invention are also achieved by providing an optical modulator for optically modulating a light, the optical modulator including (a) a substrate; (b) an optical waveguide through which the light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) a ground electrode; and (e) a signal electrode on the ridge. A gap width between the ground electrode and the signal electrode is at least 44 &mgr;m. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A drive signal supplied to the signal electrode causes an electric field to be produced along the optical waveguide as the light travels through the optical waveguide, to optically modulate the light.
Moreover, objects of the present invention are achieved by providing an optical modulator for optically modulating a light, the optical modulator including (a) a substrate; (b) an optical waveguide through which the light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and positioned between the first and second ground electrodes. A gap width between the first ground electrode and the signal electrode, and between the second ground electrode and the signal electrode, is at least 44 &mgr;m. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A drive signal supplied to the signal electrode causes an electric field to be produced along the optical waveguide as the light travels through the optical waveguide, to optically modulate the light.
Objects of the present invention are also achieved by providing an optical modulator for optically modulating a light, the optical modulator including (a) a z-cut LiNbO
3
substrate; (b) an optical waveguide through which the light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and positioned between the first and second ground electrodes. A gap width between the first ground electrode and the signal electrode, and between the second ground electrode and the signal electrode, is at least 44 &mgr;m. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. A buffer layer is between the signal electrode and the ridge. A drive signal supplied to the signal electrode causes an electric field to be produced along the optical waveguide as the light travels through the optical waveguide, to optically modulate the light.
In addition, objects of the present invention are achieved by providing an optical modulator including (a) a crystal substrate that exhibits an electro-optic effect; (b) an optical waveguide through which a light travels; (c) a ridge changing an elevation of the optical waveguide with respect to the substrate; (d) first and second ground electrodes; and (e) a signal electrode on the ridge and between the first and second ground electrodes. A gap width between the signal electrode and the first ground electrode, and between the signal electrode and the second ground electrode, is at least 44 &mgr;m. The signal electrode has an interaction length with respect to the optical waveguide of at least 41 mm. The light traveling through the optical waveguide is phase-matched with a microwave signal traveling through the signal electrode. Characte
Nakazawa Tadao
Sugiyama Masaki
Epps Georgia
Spector David N.
Staas & Halsey , LLP
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