Optical waveguides – Integrated optical circuit
Reexamination Certificate
1999-02-11
2001-12-25
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Integrated optical circuit
C385S002000, C385S003000, C385S004000, C385S008000, C385S045000, C385S129000
Reexamination Certificate
active
06334008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an optical circuit and other optical elements, and more particularly to a wave guide type optical circuit including an optical wave guide formed on a substrate, and other optical elements.
2. Description of the Related Art
Optical communication systems recently put into practice, have a need for larger capacity and higher performances. In order to accomplish such optical communication, it is necessary to develop optically controllable elements operating at higher speed, such as a light modulator and an optical switch. Such optically controllable elements usually include a diffusion type optical wave guide having a substrate which is composed of electro-optic crystal such as LiNbO
3
. (hereinafter, referred to simply as “LN”) or LiTaO
3
(hereinafter, referred to simply as “LT”), and into which titanium, magnesium or proton is doped through a surface and thermally doped.
A plurality of wave guide type optical circuits each including the abovementioned diffusion type optical wave guide can be integrated into a single element by means of conventional photolithography, as described by Okayama et al. in “Shingaku Gihou”, TSSE 94-214, or by Nishimoto et al. in “Optics” Vol. 1, No. 8, pp.521.
As a light modulator making use of quick response of electro-optic crystal, a Mach-Zehnder type high-speed light modulator has been put into practice. As an optical switch, there has been suggested a digital type optical switch where an output wave guide to which a light is coupled is selected among two or three output wave guides diverged in Y-shape, by concurrently applying voltages having different polarities to the output wave guides.
As another method of forming an optical wave guide constituting an optical circuit, Japanese Unexamined Patent Publications Nos. 2-259608, 7-64034, and 8-5854 have suggested a method of forming a ridge type optical wave guide by means of heteto-epitaxial growth where fused flux is used.
Though not for the purpose of forming an optical circuit, Japanese Unexamined Patent Publications Nos. 3-48832, 4-270322, 5-2201, 5-17295, and 5-100271 have suggested methods of partially inverting crystal orientation of LN or inverting domain in LN. In these methods, LN crystal is used to carry out pseudo-phase-matching in a secondary harmonics generating element.
Japanese Unexamined Patent Publication No. 7-5404 has suggested an electro-optic modulator for compensating for mismatching of a phase speed between an optical signal and a harmonics signal. This electro-optic modulator includes an optical wave guide formed on a ferroelectric substrate and coupled to an optical input terminal. A high frequency wave guide is formed on a substrate in order to apply an electric field to a region located adjacent to the optical wave guide and modulate an optical signal. The ferroelectric substrate includes a ferroelectric domain region for compensating for a difference in a phase within a modulation region. The ferroelectric domain region is periodically inversely rotated and normally rotated.
It is quite important to lower an operation voltage as much as possible in order to accomplish a practical light modulator or a practical optical switch both including LN therein. It is necessary to concurrently apply a voltage of about ±50V to two output wave guides in a digital type optical switch. It is advantageous to lower the voltage even to a small degree.
It is necessary in a Mach-Zehnder type optical switch to apply a voltage of about 50V to two phase shifters. Similar to the above-mentioned optical switch, it is advantageous to lower the voltage even to a small degree.
In a high-speed light modulator, a lower operation voltage could accomplish a driver circuit capable of operating at a higher speed. Accordingly, it is desired in a high-speed light modulator to lower an operation voltage as much as possible in order to drive a driver circuit at a higher speed.
SUMMARY OF THE INVENTION
It is an object of the present invention to lower an operation voltage in optical devices making use of electro-optic effect, such as a light modulator and an optical switch.
The present invention provides an optical device including optical wave guides each of which is composed of electro-optic crystal such as LN and LT and is domain-inverted in an electrode, where electric fields having the same orientation are applied to the optical wave guides.
As mentioned later, the present invention may be applied to various optical devices such as an optical circuit, a light modulator, or an optical switch.
In one aspect of the present invention, there is provided an optical circuit including a plurality of optical wave guides each composed of electro-optic crystal and each designed to have a domain orientation different from each other.
There is further provided a light modulator including two optical wave guides each composed of electro-optic crystal and cooperating with each other to constitute a phase shifter, the two optical wave guides being designed to have domain orientations inverted to each other.
There is still further provided an optical switch including two optical wave guides each composed of electro-optic crystal and cooperating with each other to constitute a phase shifter, the two optical wave guides being designed to have domain orientations inverted to each other.
There is yet further provided a digital type optical switch including (a) an input optical wave guide composed of electro-optic crystal, and (b) at least two output optical wave guides connected to the input optical wave guide in Y-shaped fashion. An index of refraction of each of the output optical wave guides is varied by an electric field to thereby select one output optical wave guide through which optical output is obtained, and the output optical wave guides are designed to have domain orientations inverted to each other.
There is still yet further provided a directional coupler type optical switch including a plurality of optical wave guides each composed of electro-optic crystal, optical wave guides located adjacent to each other being designed to have domain orientations inverted to each other.
In another aspect of the present invention, there is provided a method of fabricating an optical circuit including a plurality of optical wave guides each composed of electro-optic crystal and each designed to have a domain orientation different from each other, the method including the step of forming at least a part of the optical circuit by liquid phase epitaxy (LPE).
There is further provided a method of fabricating a light modulator including two optical wave guides each composed of electro-optic crystal and cooperating with each other to constitute a phase shifter, the two optical wave guides being designed to have domain orientations inverted to each other, the method including the step of forming at least a part of the light modulator by liquid phase epitaxy.
There is still further provided a method of fabricating an optical switch including two optical wave guides each composed of electro-optic crystal and cooperating with each other to constitute a phase shifter, the two optical wave guides being designed to have domain orientations inverted to each other, the method including the step of forming at least a part of the optical switch by liquid phase epitaxy.
There is yet further provided a method of fabricating a digital type optical switch including (a) an input optical wave guide composed of electro-optic crystal, and (b) at least two output optical wave guides connected to the input optical wave guide in Y-shaped fashion, an index of refraction of each of the output optical wave guides being varied by an electric field to thereby select one output optical wave guide through which optical output is obtained, the output optical wave guides being designed to have domain orientations inverted to each other, the method comprising the step of forming at least a part of the digital type optical switch by liquid phase epitaxy.
There is still yet furth
McGuireWoods LLP
NEC Corporation
Pak Sung
Sanghavi Hemang
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