Dual-electrode traveling wave optical phase shifters and methods

The invention discloses phase-shifters, modulators, and method that produces a smaller π by means of a field excitation using multiple electrodes. A negative signal is introduced that travels with the positive signal, which enhances the electric field significantly. The field enhancement is provided by the superposition of the fields accumulated from each electrode. A base or substrate material can be made from any compound having linear electro-optic properties, such as lithium niobate, lithium tantalite, potassium lithium niobate, potassium titanyl phosphate or gallium-arsenide. For lithium niobate, there are two possible orientations of electric field, z-cut orientation or x-cut orientation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to concurrently filed, co-pending application U.S. patent application Ser. No. 09/791,956, entitled “Tri-Electrode Traveling Wave Optical Phase Shifters and Methods” by Marc Hill et al., owned by the assignee of this application and incorporated herein by reference.

This application relates to concurrently filed, co-pending application U.S. patent application Ser. No. 09/792,220, entitled “Tri-Electrode Traveling Wave Optical Modulators and Methods” by Marc Hill et al., owned by the assignee of this application and incorporated herein by reference.

This application relates to concurrently filed, co-pending application U.S. patent application Ser. No. 09/792,222, entitled “Dual-Electrode Traveling Wave Optical Modulators and Methods” by Marc Hill et al., owned by the assignee of this application and incorporated herein by reference.

BACKGROUND INFORMATION

1. Field of the Invention

The present invention relates to the field of optical devices, and particularly to light modulators such as traveling-wave modulators, phase shifters, and switches.

2. Description of Related Art

Telecommunication companies seek to increase the amount of information throughput with fatter pipes and at higher speed to meet the demand from the industrial, business, and consumer markets. This in turn requires a light transmitting system to enlarge transmission and receiving capacity drastically. At present, the light transmission speed of 10 Gb/second has already been reduced to practice for commercial use, with the next hurdle set at 40 Gb/second.

Several testings are underway to find a suitable material for use as an optical waveguide in a traveling-wave light modulator that is capable of operating in broad band at high frequency, such material includes lithium niobate (LiNbO), lithium tantalite, potassium lithium niobate, potassium titanyl phosphate or gallium-arsenide. Lithium niobate and lithium tantalite are excellent ferroelectric materials, with large electro-optical coefficients, which can control a light phase proportional to an electrical field strength within an optical waveguide generated by an electrical signal applied to electrical electrodes.

Factors which effect the modulation of a traveling-wave light modulator include velocity mismatch, impedance mismatch, dispersion, electrode power loss, and the electrical field generation. Among them, velocity mismatch, impedance mismatch and dispersion are principally determined by the structure of the traveling-wave light modulator, which can be achieved with careful electrode design. However, the electrical field strength is determined by the applied electrical signal amplitude, the electrode power loss and the electrode structure, and the excitation mode in the electrode structure. For 40 Gb/sec. modulation, one of the major challenges is to reduce the required driving voltage of the modulator, which is generally dictated by high electrode loss and the difficulties of generating high-voltage swing with semiconductors at this speed.

In electrical field generation and the phase modulation, at the input of one electrical waveguide or electrode, a high-speed electrical signal is applied and triggers an electromagnetic wave propagating along the waveguide. The field strength at a certain point along the waveguide is determined by the particular way in which the EM wave was excited for a given input voltage, and the propagation attenuation along the waveguide. The optical index of the optical waveguide is changed linearly by the applied electrical field, and the overall phase change of the optical signal is an integration of all the incremental phase changes along the waveguide and is proportional to the product of the driving voltage and the modulation length. Due to bandwidth considerations, the effective modulation length cannot be increased beyond a limit and hence a driving voltage above a threshold is required to achieve a required optical modulation. For high-speed communications systems transmitting at 10 Gb/sec or higher, the electrode loss is significant and typically leads to a very high required driving voltage.

Given the high loss and the limited voltage swing, it adds more complexity and cost to realize a practical communication system using such a modulator, if not impossible. The under-driven modulator would lead to significant degradation of the modulated light signal and significantly limits its use to many communication systems. Therefore, a light modulator having a lower driving voltage is in demand.

Attempts have been made to reduce a driving voltage. One method has been a two stage electrode design which uses the first stage of the electrode to primarily achieve the maximum overlap of the electrical field and the optical field, and uses the second stage to achieve the phase velocity match the between the electrical and the optical signals. However, it is difficult to realize due to phase matching required of the two stages of the electrode. Further, it just introduces one more freedom to alleviate the constraints for simultaneous phase and field matching. It does not provide an effective means to reduce the driving voltage.

A ridge structure is a modification of a conventional CPW (co-planar waveguide) design, by raising the center electrode conductor above the two grounding planes. It does provide the advantage of lowering the driving voltage. For example, see K. Noguchi et al, “Highly efficient 40-GHz bandwidth Ti: LiNbO optical modulator employing ridge structure”, IEEE Photonics Technology Letters, Vol. 5, No. 1, January 1993. However, it is difficult to realize due to the additional processes and the additional optical signal losses incurred by fabricating the ridge. Moreover, the reduction of the driving voltage is very limited, which is about 20% typically.

A conventional broadband optical communication uses a Mach-Zehnder interferometer to modulate laser signals in a transmitter. An electric field applied to an optical waveguide changes its index of refraction. A signal strip and ground plane (a zero voltage), form an electrical waveguide (EWG), where the induced electric field creates a change in the refractive index of the inlayed optical waveguide (OWG). The index of the material, for example, LiNbO3 or GaAs, depends on the amplitude and direction of the applied electric field.

Lithium-Niobate Mach-Zehnder modulators require a large voltage and length to provide a π phase shift through an active length L. The voltage level required is too large relative to amount provided by ultra-fast electronic transistors. The length of the modulator is limited by the synchronism of the electric and optical propagating waves. For this reason, the length cannot be increased without a regenerative amplification of the signal or a multistage system that requires precise synchronization.

Accordingly, it is desirable to have phase shifters, modulators, and methods that decrease a Vπvalue or shortening of an active length.

SUMMARY OF THE INVENTION

The invention discloses phase-shifters, modulators, and methods that produce a smaller Vπ by means of a field excitation using multiple electrodes. A negative signal is introduced that travels with the positive signal, which enhances the electric field significantly. The field enhancement is provided by the superposition of the fields accumulated from each electrode. A base or substrate material can be made from any compound having linear electro-optic properties, such as lithium niobate, lithium tantalite, potassium lithium niobate, potassium titanyl phosphate or gallium-arsenide. For lithium niobate, there are two possible orientations of the crystal, z-cut or x-cut orientation. Horizontal electrical field is typically used to drive the x-cut crystal, and vertical electrical field is typically used to drive the z-cut crystal.

In a first aspect of the invention, tri-electrode traveling wave optical phase shifters and methods are disclosed. The optical shifters employing a tri-electrode configuration that are driven differentially and allows for a lower voltage to accumulate a phase shift. This type of shifter can be used in a Mach-Zehnder interferometer or a fast optical switch. Phase shifting an optical signal is desired in optical communications, i.e. in modulators or switches.

In a second aspect of the invention, tri-electrode traveling wave optical modulators and methods are disclosed. The optical modulators employing a tri-electrode configuration that are driven differentially and allow for a lower voltage to modulating an optical signal.

In a third aspect of the invention, dual-electrode traveling wave optical phase shifters and methods are disclosed. The optical shifters employing a differentially-driven dual-electrode that allows for a lower voltage to accumulate a phase shift.

In a fourth aspect of the invention, dual-electrode traveling wave optical modulators and methods are disclosed. The optical modulators employing differential strip fields with a dual-electrode that allow for a lower voltage to modulating an optical signal. One of ordinary skill in the art should know that the term differentially driven could mean the application of driving signals that have opposite polarity from one electrode to another electrode, or other similar definitions.

Optionally, a buffer layer is inserted between electrodes and a substrate to improve phase matching between an electrical signal and an optical signal. Advantageously, the present invention employing tri-electrodes or dual-electrodes allow for a better match of phase velocity and allow for a reduced buffer layer thickness that may be used between the optical and electrical waveguide.

Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1is structural diagram illustrating a cross-sectional view of a tri-electrode phase-shifter10with a vertical electric field in a z-cut orientation. A basic structure of the tri-electrode phase-shifter10has three electrodes, a negative electrode S−13, a positive electrode S+14, and a negative electrode S−15, in which signals are applied on and trigger a traveling wave whose phase velocity matches that of an optical waveguide (WG)17. The traveling electrical signal induces a change in the refractive index in the optical waveguide17and hence induces a phase change. The optical waveguide (WG)17, which has a slightly higher refractive index than the surrounding material, is positioned underneath the base of the positive electrode S+14, thereby creating a vertical electric field in the optical waveguide17. The optical waveguide17, for example, is achieved by doping Ti in LiNbO3. An electrical field E18exists between the positive electrode S+14and the negative electrode S−13, and an electric field E19exists between the positive electrode S+14and the negative electrode S−15. The ground electrodes12and16are used to suppress the couplings to parasitic modes at high frequencies. A substrate11can be selected from a material such as like GaAs, KDP, or LiNbO3, which demonstrates a linear electro-optic effect.

Preferably, the optical waveguide (WG)17is placed in a center position underneath the positive electrode S+14. However, one of ordinary skill in the art should recognize that the optical waveguide (WG)17can be shifted to the left or the right of the positive electrode S+14, or align to the left edge or the right edge of the positive electrode S+14. The optical waveguide17can be doped or diffused with a material that has a slightly higher refractive index than the surrounding material. For example, Ti can be diffused into the material LiNbO3 to cause a higher index of refraction that guides a wave.

A negative signal is introduced from the electrode S−13that travels with the positive signal S+14to enhance an electrical field significantly. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. The material11can be made of any compound having linear electro-optic properties. Lithium Niobate has a preferred direction, depending on the direction of a crystal. For the case of Lithium Niobate, there are two possible orientations of the electric field, z-cut or x-cut. Lithium Niobate is an anisotropic material, in which the z-axis possesses the highest electro-optical coefficient.FIG. 1is intended as one illustration of the tri-electrode phase shifter10with the optical waveguide, for example, in z-cut orientation crystal. It is apparent to one of skill in the art that various types of optically active material, such as gallium-arsenide or lithium niobate x-cut, can be practiced without departing from the spirits of the present invention.

FIG. 2is a circuit diagram illustrating a tri-electrode phase-shifter20with a vertical electric field. An amplifier23receives an input22and generates three electrical outputs through a transmission line S124a, a transmission line S224b, and a transmission line S324c. The transmission line S124aextends through the electrode13to a load L1or termination resistor25aand a ground26a. The transmission line S2extends through the electrode14to a load L2or termination resistor25band a ground26b. The transmission line S324cextends through the electrode15to a load L3or termination resistor25cand a ground26c. Between the negative electrode13and the positive electrode14, a traveling electrical wave ω127is created due to the proximity of the S124atransmission line and the S224btransmission line24b. Between the positive electrode14and the negative electrode15, a traveling electrical wave ω228is created from the proximity of the S224btransmission line and the S324ctransmission line. In this embodiment, an optical wave λin21received from, for example, an optical fiber (not shown), travels underneath the electrode S+14, generating an output λout29. The optical signal λin21travels co-spatially with the electrical signal ω127and ω228. Preferably, the traveling wave ω127is identical or substantially similar to the traveling electrical wave ω228. Furthermore, the optical signal λin21travels with the same or substantially the same velocity as the traveling wave ω127and ω228.

The amplifier23matches the impedance of the transmission lines S124a, S224b, and S324c, and matches with the impedance of the loads L125a, L225b, and L325c. In the preferred mode, the amplitudes of the negative electrodes S−13and S−15have the same amount of negative amplitude as the amplitude of the positive electrode S+14. The amount of signal amplitude applied affects the amount of phase shift. The amount of phase shift is linearly proportional to the signal amplitude generated from the amplifier23. For example, if applying 1-volt, a 45° phase shift may result, and if applying 2-volts, a 90° phase shift may result.

A reduction of Vπ×L is caused by the superposition of the field induced by the electrodes, resulting in the enhancement in the electrical field. In one embodiment, the traveling electrical wave ω127modulation is doubled due to the field excitation between the electrodes13and14. However, the modulation can be more than 2 times, or less than 2×, depending on the distance between the electrodes13and14, the height of each electrode13or14, and the thickness of a buffer layer. Preferably, the ω127modulation is symmetrical to the traveling electrical wave ω128modulation. One of ordinary skill in the art, however, should recognize that the traveling electrical wave ω127modulation and the traveling electrical wave ω128modulation can be designed to be asymmetrical.

Optionally, a direct current (DC) bias field can be applied to each of the electrodes S−13, S+14, and S−15, by direct or indirect coupling.

FIG. 3is a circuit diagram illustrating a single arm modulator30with a tri-electrode phase-shifter with a vertical electric field. The single arm or single arm modulator30receives a light signal input kin31and split the light signal λin31into two optical paths, a λ132and a λ233. The λ132travels in an optical waveguide (not shown) that is routed away from the electrode S−13, S+14, and S−15. The λ233travels underneath the electrode S+14. The λ132and λ233are combined to generate a single optical output λout34. The amplifier23receives the input22and generates three electrical outputs through the transmission line S124a, the transmission line S224b, and the transmission line S324c. The transmission line S124aextends through the electrode13to the load L1or termination resistor25aand the ground26a. The transmission line S2extends through the electrode14to the load L2or termination resistor25band the ground26b. The transmission line S324cextends through the electrode15to the load L3or termination resistor25cand the ground26c. Between the negative electrode13and the positive electrode14, a traveling electrical wave ω127is created due to the close proximity of a gap between them. Between the positive electrode14and the negative electrode15, the traveling electrical wave ω228is created due to the close proximity of the gap between them. In this embodiment, an optical wave λin31received from, for example, an optical fiber travels through the electrode S+14, in generating an output rout34. The optical signal λin31travels beneath traveling the electrical signal ω127. Preferably, the traveling wave ω127is identical or substantially similar to the traveling electrical wave ω228.

FIG. 4is a circuit diagram illustrating one embodiment of two optical phase-shifters to form an optical switch, a Mach-Zehnder type interferometer or modulator40, having an upper phase-shifter41and a lower optical phase-shifter30. The light signal input λin31is split into two paths, the λ132and the λ233, which are re-combined to generate a the λout49. An amplifier42receives the input22and generates a first output to an amplifier43, and a second output to the amplifier23. The amplifier43receives then generates three electrical outputs through a transmission line S144a, a transmission line S244b, and a transmission line S344c. The transmission line S144aextends through a first electrode45ato the load L1or termination resistor46aand the ground47a. The transmission line S244bextends through the electrode45bto the load L2or termination resistor46band the ground47b. The transmission line S344cextends through the electrode45cto the load L3or termination resistor46cand the ground47c. Between the positive electrode45band the negative electrode45a, a traveling electrical wave ω148ais created due to the close proximity of a gap between them. Between the negative electrode45cand the positive electrode45b, the traveling electrical wave ω248bis created due to the close proximity of the gap between them.

Preferably for wide band applications, the electrical wave ω127matches or substantially matches the electrical wave ω228. Similarly, electrical wave ω348amatches or substantially matches the electrical wave ω448b. In addition, the light wave λ132matches or substantially matches the light wave λ233. Optionally, the electrodes13,14,15,45a,45b, and45ccan be connected to a voltage supply (not shown), to generate a DC bias field in the optical waveguide.

Advantageously, this embodiment with three electrodes in the present invention allow for a better match of phase velocity and allow for a reduced buffer layer thickness that may be used between the optical and electrical waveguide.

FIG. 5is a structural diagram illustrating a cross-sectional view of a tri-electrode phase-shifter50utilizing a horizontal electric field and with an x-cut orientation. The basic structure of the tri-electrode phase-shifter50has three electrodes, a negative electrode S−53, a positive electrode S+54, and a negative electrode S−55. An optical waveguide (WG)57is positioned in a gap underneath and in between the positive electrode S+54and the positive electrode S−55, thereby being placed in a substantially horizontal electric field59which exists between the positive electrode S+54and the negative electrode S−55.

Preferably, the optical waveguide (WG)57is placed in a center of and underneath a gap between the positive electrode S+54and the negative electrode S−55. However, one of ordinary skill in the art should recognize that the optical waveguide (WG)57can be shifted to toward the left and closer to the positive electrode S+54or toward the right and closer to the negative electrode S−55, or aligned to the right edge of the positive electrode S+54or the left edge the negative electrode S−55. The optical waveguide57can be doped or diffused with a material that has a slightly higher refractive index than the surrounding material. For example, if LiNbO, a Ti that is diffused into the material and that caused a higher index of refraction that guides a wave.

A negative signal is introduced into the electrode S−53that travels with the positive signal S+54to provide significant enhancement of the electrical field. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. The material51can be made of any compound having linear electro-optic properties. Lithium Niobate has a preferred direction, depending on the direction of a crystal.

FIG. 6is a circuit diagram illustrating a tri-electrode phase-shifter60utilizing a horizontal electric field. An amplifier63receives an input62and generates three electrical outputs through a transmission line S164a, a transmission line S264b, and a transmission line S364c. The transmission line S164aextends through the electrode53to a load L1or termination resistor65aand a ground66a. The transmission line S2extends through the electrode54to a load L2or termination resistor65band a ground66b. The transmission line S364cextends through the electrode55to a load L3or termination resistor65cand a ground66c. Between the negative electrode53and the positive electrode54, a traveling electrical wave ω167is created due to the proximity of the S164atransmission line and the S264btransmission line. Between the positive electrode54and the negative electrode55, a traveling electrical wave ω268is created due to their proximity. In this embodiment, an optical wave λin61received from, for example, an optical fiber, travels between the negative electrode S−53and the positive electrode S+54, in generating an output λout69. The optical signal λin61travels co-spatially with the electrical signal ω167and ω268. Preferably, the traveling wave ω167is symmetrical or substantially symmetrical to the traveling electrical wave ω268.

The amplifier63matches the impedance of the transmission lines S164a, S264band S364c, and matches the impedance of the loads L165a, L265b, and L365c. In the preferred mode, the amplitudes of the negative electrodes S−53and S−55have the same amount of amplitude as the amplitude of the positive electrode S+54.

The amount of signal amplitude applied affects the amount of phase shift. The amount of phase shift is linearly proportional to the signal amplitude generated from the amplifier63. For example, if apply 1-volt, it may result in a45degree phase shift, and if apply 2-volt, it may result in a90degree phase shift.

Optionally, a direct current (DC) bias field can be applied to each of the electrodes S−53, S+54, and S−55, by direct or indirect coupling.

A reduction of Vπ×L is caused by the superposition of the field induced by the electrodes, resulting in the enhancement in the electrical field. In one embodiment, the amplitude of the traveling electrical wave ω167is doubled due to the field excitation between the electrodes53and54. However, the increase can be more than 2 times, or less than 2×, depending on the distance between the electrodes53and54, the height and shape of each electrode53or54, and the thickness of a buffer layer. Preferably, the ω167modulation is symmetrical to the traveling electrical wave ω168modulation. One of ordinary skill in the art, however, should recognize that the traveling electrical wave ω167and the traveling electrical wave ω168can be designed to by asymmetrical.

The electrodes of the optical phase-shifter would be driven as inFIG. 5, where a driver amplifier would provide the signal to the three electrodes, the outer two driven with the same polarity and the center with opposite polarity of the outer. The electrical signal propagates from left to right, where the signal is terminated into matched loads.

FIG. 7is a circuit diagram illustrating a single a modulator70with a tri-electrode phase-shifter utilizing a horizontal electric field. The light signal input λin71is split into two optical paths, a λ172and a λ273. The λ172travels in an optical waveguide (not shown) that is routed away from the electrodes S−53, S+54and S−55, while the λ273travels between the electrode S−53and the electrode S+54. λ172and a λ273are combined to generate a single optical output λout741. The amplifier63receives the input62and generates three electrical outputs through the transmission line S164a, the transmission line S264b, and the transmission line S364c. The transmission line S164aextends through the electrode53to the load L1or termination resistor65aand a ground66a. The transmission line S2extends through the electrode54to the load L2or termination resistor65band the ground66b. The transmission line S364cextends through the electrode55to the load L3or termination resistor65cand the ground66c. Between the negative electrode53and the positive electrode54, a traveling electrical wave ω167is created due to their proximity. Between the positive electrode54and the negative electrode55, a traveling electrical wave ω268is created due to their proximity. In this embodiment, an optical wave λin71received from, for example, an optical fiber (not shown), travels between the negative electrode S−53and the positive electrode S+54, in generating an output λout69. The optical signal λin61travels co-spatially with the electrical signal ω167and ω268. Preferably, the traveling wave ω167is symmetrical or substantially symmetrical to the traveling electrical wave ω268.

FIG. 8is a circuit diagram illustrating a first embodiment of two optical phase shifters80in constructing an optical switch, a modulator, or a Mach-Zehnder type interferometer. The two phase-shifters80has an upper phase shifter81and a lower optical phase shifter70. The light signal input λin82is split into two paths, the λ183and the λ284, which are re-combined to generate a the λout85. In this embodiment, the light signal λ182travels between a positive electrode45band a negative electrode45c, while the light signal λ283travels between the positive electrode13and the negative electrode14. The amplifier42receives the input41and generates a first output to an amplifier43, and a second output to the amplifier23. The amplifier43receives then generates three electrical outputs through a transmission line S144a, a transmission line S244b, and a transmission line S344c. The transmission line S144aextends through a first electrode45ato the load L1or termination resistor46aand the ground47a. The transmission line S244bextends through the electrode45bto the load L2or termination resistor46band the ground47b. The transmission line S344cextends through the electrode45cto the load L3or termination resistor46cand the ground47c. Between the positive electrode45band the negative electrode45a, a traveling electrical wave ω148ais created due to the close proximity of a gap between them. Between the negative electrode45cand the positive electrode45b, the traveling electrical wave ω248bis created due to the close proximity of the gap between them.

FIG. 9is a circuit diagram illustrating a second embodiment of a two phase-shifters in constructing an optical switch, a modulator, or a Mach-Zehnder type interferometer. The light signal input λin91is split into two paths, the λ192and the λ293, which are re-combined to generate a the λout94. In this embodiment, the light signal λ182travels between a negative electrode45aand a positive electrode45b, while the light signal λ283travels between the negative electrode14and the positive electrode15.

FIG. 10is a circuit diagram illustrating a third embodiment of two optical phase shifters100in constructing an optical switch, a modulator, or a Mach-Zehnder type interferometer. The light signal input λin101is split into two paths, the λ1102and the λ2103, which are re-combined to generate a λout104. In this embodiment, the light signal λ1102travels between the negative electrode45aand the positive electrode45b, while the light signal λ2103travels between the negative electrode13and the positive15electrode14.

FIG. 11is a circuit diagram illustrating a fourth embodiment of two optical phase-shifters110in constructing an optical switch, a modulator, or a Mach-Zehnder type interferometer. The light signal input λin111is split into two paths, the λ1112and the λ2113, which are re-combined to generate a λout114. In this embodiment, the light signal λ1102travels between the positive electrode44band the negative electrode44c, while the light signal λ2113travels between the positive electrode14and the negative electrode15.

FIG. 12is a structural diagram illustrating a first embodiment of a cross-sectional view of an optical phase-shifter120with a buffer layer utilizing a vertical electric field in the optical waveguide. A buffer layer121is placed between the substrate11, and the ground electrode12, the negative S− electrode13, the positive S+ electrode14, the negative electrode S−15, and the ground electrode16. The width of the buffer layer121extends all the way from the ground electrode12, through the negative S− electrode13, the positive S+ electrode14, the negative electrode S−15, to the ground electrode16. The buffer layer121preferably has a significantly lower dielectric constant than that of the substrate11. The use of the buffer layer121helps to improve the phase matching between an electrical signal and an optical signal, as well as increasing the electrode impedance.

If the substrate11uses lithium niobate, the preferred material for the buffer layer121is silicon oxide, with a thickness of, for example, 1 microns. With the buffer layer121, the design of a phase shifter is significantly simpler due to the more electric field in the air. Optionally, the thickness of the buffer layer121can be reduced to enhance the electrical field.

FIG. 13is a structural diagram illustrating a second embodiment of a cross-sectional view of a tri-electrode optical shifter130with a buffer layer utilizing a vertical electric field. The width of a buffer layer131extends underneath the negative S− electrode13, the positive S+ electrode14, and the negative electrode S−15. The buffer layer131does not extend to underneath of the ground electrode12and the ground electrode16. The buffer layer131preferably has a significantly lower dielectric constant than that of the substrate11.

FIG. 14is a structural diagram illustrating a third embodiment of a cross-sectional view of an optical phase-shifter with a buffer layer utilizing a horizontal field in the optical waveguide. A buffer layer141is placed between the substrate51, and the ground electrode52, the negative S-electrode53, the positive S+ electrode54, the negative electrode S−55, and the ground electrode56. The width of the buffer layer141extends all the way from the ground electrode52, through the negative S− electrode53, the positive S+ electrode54, the negative electrode S−55, to the ground electrode56. The buffer layer141preferably has a significantly lower dielectric constant than that of the substrate51. The optical waveguide57is positioned in a gap underneath and in between the positive S+ electrode54and the negative electrode S−55.

FIG. 15is a structural diagram illustrating a fourth embodiment of a cross-sectional view of a tri-electrode optical phase shifter with a buffer layer utilizing a horizontal electric field. A buffer layer151is placed between the substrate51, and the ground electrode52, the negative S− electrode53, the positive S+ electrode54, the negative electrode S−55, and the ground electrode56. The width of the buffer layer151extends all the way from the ground electrode52, through the negative S− electrode53, the positive S+ electrode54, the negative electrode S−55, to the ground electrode56. The buffer layer141preferably has a significantly lower dielectric constant than that of the substrate51. An optical waveguide152is positioned in a gap underneath and in between the positive S+ electrode54and the negative electrode S−53.

FIG. 16is a process diagram illustrating a first embodiment of a cross-sectional view of an optical modulator160with a tri-electrode utilizing a horizontal electric field in the optical waveguide. The tri-electrode modulator160has three electrodes, a negative electrode S−163, a positive electrode S+164, and a negative electrode S−165. The center electrode, the positive electrode S+, has one polarity, and the outer electrodes, the negative electrode S−163and the negative electrode S−165, have an opposite polarity of the center. One of ordinary skill in the art should recognize that the center electrode could have a negative electrode, while the outer electrodes have positive electrodes. Optical waveguides167and168are shown in a region of large horizontal field E field169aand169b. The optical waveguide (WG)167is positioned in a gap underneath and in between the negative electrode S−163and the positive electrode S+164, thereby being placed in a substantially horizontal field. Similarly, the optical waveguide (WG)168is positioned in a gap underneath and in between the positive electrode S+164and the negative electrode S−165, thereby being placed a substantially horizontal field. An electrical field E169aexists between the positive electrode S+164and the negative electrode S−163, and an electrical field E169bexists between the positive electrode S+164and the negative electrode S−165.

A first negative signal is introduced into the electrode S−163that travels with the positive signal S+164so to significantly enhance the electrical field in the optical waveguide167. A second negative signal is introduced into the electrode S−165that travels with the positive signal S+164so to significantly enhance the electrical field in the optical waveguide168. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. The material161can be made of any compound having linear electro-optic properties. Lithium Niobate has a preferred x-cut orientation.

FIG. 17is a circuit diagram illustrating the first embodiment of an optical modulator170with a tri-electrode utilizing a horizontal electric field. An amplifier23receives an input22and generates three electrical outputs through a transmission line S124a, a transmission line S224b, and a transmission line S324c. The transmission line S124aextends through the electrode13to a load L1or termination resistor25aand a ground26a. The transmission line S224bextends through the electrode54to a load L2or termination resistor25band a ground26b. The transmission line S324cextends through the electrode55to a load L3or termination resistor25cand ground26c. Between the negative electrode53and the positive electrode54, a traveling electrical wave ω157is created due to their proximity. Between the positive electrode54and the negative electrode55, a traveling electrical wave ω258is created due to their proximity. In this embodiment, an optical wave λin171is received from, for example, an optical fiber (not shown). The optical signal λin171splits into two light signals λ1172and λ2173, before re-combination at the output λout174. Preferably, the traveling wave ω157is symmetrical or substantially symmetrical to the traveling electrical wave ω258.

The amplifier23matches the impedance of the transmission lines S124a, S224b, and S324c, and matches the impedance of the loads L125a, L225b, and L325c. In the preferred mode, the amplitudes of the negative electrodes S−53and S−55have the same amount of amplitude as the amplitude of the positive electrode S+54. The amount of signal amplitude applied affects the amount of phase shift. The amount of phase shift is linearly proportional to the signal amplitude generated from the amplifier23. For example, if applying 1-volt, a45degree phase shift can result, and if applying 2-volts, a 90 degree phase shift can result.

A reduction of Vλ×L is caused by the superposition of the field induced by the electrodes, resulting in the enhancement in the electrical field. In one embodiment, the amplitude of the traveling electrical wave ω157is doubled due to the field excitation between the electrodes53and54. However, the increase can be more than 2 times, or less than 2×, depending on the distance between the electrodes53and54, the height of each electrode53or54, and the thickness of a buffer layer. Preferably, the ω157is symmetrical to the traveling electrical wave ω158modulation. One of ordinary skill in the art, however, should recognize that the traveling electrical wave ω157and the traveling electrical wave ω258can be designed to be asymmetrical.

The electrodes of the optical phase-shifter would be driven as inFIG. 17, where a driver amplifier would provide the signal to the three electrodes, the outer two driven with the same polarity and the center with opposite polarity of the outer. The electrical signal propagates from left to right, where the signal is terminated into matched loads.

Optionally, a direct current (DC) bias field can be applied to each of the electrodes S−53, S+54, and S−55, by direct or indirect coupling.

FIG. 18is a process diagram illustrating a first embodiment of a cross-sectional view of an optical modulator180with a tri-electrode utilizing a horizontal electric field. Optical waveguides181and182are shown in a region of large vertical field E field183and184. The optical waveguide (WG)181is positioned directly underneath the positive electrode S+54. Similarly, the optical waveguide (WG)182is positioned directly underneath the negative electrode S−55, thereby creating a vertical field. An electrical field E183exists between the positive electrode S+54and the negative electrode S−53, and an electrical field E184exists between the positive electrode S+54and the negative electrode S−55.

FIG. 19is a circuit diagram illustrating the second embodiment of an optical modulator190with a tri-electrode utilizing a horizontal electric field. In this embodiment, the optical signal λin191splits into two light signals λ1192and λ2193, before re-combination at the output λout194. The λ1192travels underneath the positive electrode54and the λ2193travels underneath the negative electrode55. Preferably, the traveling wave ω127is symmetrical or substantially symmetrical to the traveling electrical wave ω228.

FIG. 20is a process diagram illustrating a phase shifter200employing dual-electrodes with a horizontal electric field in the optical waveguide. The phase shifter200has two electrodes, a first electrode201and a second electrode202, where the first electrode201has an opposite polarity as the second electrode202. An optical waveguide203is placed in a gap underneath and in between the first electrode201and the second electrode202, in generating a horizontal electric field. Ground electrodes204and205are used to suppress the couplings to parasitic modes at high frequencies. A substrate206can be selected from a material such as like GaAs, KDP, or LiNbO3, which demonstrates a linear electro-optic effect.

A negative signal is introduced into the electrode S−201that travels with the positive signal S+202to enhance an electrical field significantly. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. The material206can be made of any compound having linear electro-optic properties.

FIG. 21is a process diagram illustrating a phase shifter210employing dual-electrodes with a horizontal electric field in the optical waveguide with a buffer layer. A buffer layer211is placed between the substrate206, and the ground electrode204, the negative S− electrode201, the positive S+ electrode202and the ground electrode205. The width of the buffer layer211extends all the way from the ground electrode204, through the negative S− electrode201, the positive S+ electrode202, to the ground electrode205. The buffer layer211preferably has a significantly lower dielectric constant than that of the substrate206. The use of the buffer layer211helps to improve the phase matching between an electrical signal and an optical signal, as well as increasing the electrode impedance.

If the substrate206uses lithium niobate, the preferred material for the buffer layer211is silicon oxide, with a thickness of, for example, 1 microns. With the buffer layer211, the design of a phase shifter is significantly simpler due to the more electric field in the air. Optionally, the thickness of the buffer layer211can be reduced to enhance the electrical field.

FIG. 23is a circuit diagram illustrating a single arm modulator230employing dual-electrodes with a horizontal electric field in the optical waveguide. The single end or single arm modulator230receives a light sign input λin231and splits the light signal λin231into two optical paths, a λ1232and a λ2233. The λ1232travels in an optical waveguide that is routed away from the negative electrode S−201and the positive electrode S+202, while the λ2233travels between the negative electrode S−201and the positive electrode S+202. λ1232and a λ2233are combined to generate a single optical output λout234. The amplifier222receives the electrical input221, generates the first output to a transmission line223to the negative electrode201, a loading or termination resistor224, and the ground225, and generates a second output to a transmission line226to the positive electrode202, a loading or termination resistor227, and the ground228. Between the negative electrode201and the positive electrode202, a traveling electrical wave ω1235is created due to their proximity. In this embodiment with dual-electrode traveling wave optical phase-shifter, the distance D electrode width229between the negative electrode201and the positive electrode202is relatively short in distance, preferably less than or equal to 20 microns.

FIG. 24is a circuit diagram illustrating two phase-shifters240connected in parallel to form a MZ modulator utilizing a horizontal electric field in the optical waveguides, having an upper phase-shifter241and the lower optical phase-shifter230. The light signal input λin247is split into two paths, the λ1248aand the λ2248b, which are re-combined to generate a λout249. An amplifier243areceives the input242and generates a first output244ato an amplifier243a, and a second output244bto the amplifier222. The amplifier243bthen generates two electrical outputs through a transmission line S1245a, and a transmission line S2246a. The transmission line S1245aextends through a first electrode245bto the load L1or termination resistor245cand the ground245d. The transmission line S2246aextends through the electrode246bto the load L2or termination resistor246cand the ground246d. Between the negative electrode201and the positive electrode202, a traveling electrical wave ω1235is created due to their proximity.

Preferably for wide band applications, the electrical wave ω1235matches or substantially matches the electrical wave ω2243c. In addition, the light wave λ1248amatches or substantially matches the light wave λ2248b. Optionally, the electrodes245b,246b,201, and202can be connected to a voltage supply (not shown), to generate a DC bias field in the optical waveguides.FIG. 25is a process diagram illustrating a phase shifter250employing dual-electrodes with a vertical electric field. The phase shifter250has two electrodes, a first electrode201and a second electrode202, where the first electrode201has an opposite polarity as the second electrode202. An optical waveguide251is placed directly underneath the second electrode202, thereby being placed in a substantially vertical electric field. Ground electrodes204and205are used to suppress the couplings to parasitic modes at high frequencies. A substrate206can be selected from a material such as like GaAs, KDP, or LiNbO3, which demonstrates a linear electro-optic effect.

A negative signal is introduced into the electrode S−201that travels with the positive signal S+202to enhance an electrical field significantly. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. The material206can be made of any compound having linear electro-optic properties.

FIG. 26is a circuit diagram illustrating a phase shifter260employing dual-electrodes with a vertical electric field with a buffer layer. A buffer layer261is placed between the substrate206, and the ground electrode204, the negative S− electrode201, the positive S+ electrode202and the ground electrode205. The width of the buffer layer261extends all the way from the ground electrode204, through the negative S-electrode201, the positive S+ electrode202, to the ground electrode205. The buffer layer261preferably has a significantly lower dielectric constant than that of the substrate206. The use of the buffer layer261helps to improve the phase matching between an electrical signal and an optical signal, as well as increasing the electrode impedance.

If the substrate206uses lithium niobate, the preferred material for the buffer layer261is silicon oxide, with a thickness of, for example, 1 microns. With the buffer layer261, the design of a phase shifter is significantly simpler due to the more electric field in the air. Optionally, the thickness of the buffer layer261can be reduced to enhance the electrical field.

FIG. 27is a circuit diagram illustrating a phase shifter270employing dual-electrodes with a vertical electric field in the optical waveguide. The amplifier222receives the electrical input221, generates a first output to a transmission line223to the negative electrode201, a loading or termination resistor224, and a ground225, and generates a second output to a transmission line226to the positive electrode202, a loading or termination resistor227, and a ground228. An input light signal λin271travels underneath the positive electrode202in generating an output light signal272. Between the negative electrode201and the positive electrode202, a traveling electrical wave ω1273is created due to their proximity. In this embodiment with dual-electrode traveling wave optical phase-shifter, a distance D electrode width274between the negative electrode201and the positive electrode202is relatively short in distance, preferably less than or equal to 20 microns.

FIG. 28is a circuit diagram illustrating a single arm modulator280employing dual-electrodes with a vertical electric field in the optical waveguide. The single arm modulator280receives a light signal input λin181and splits the light signal λin281into two optical paths, a λ1282and a λ2283. The λ1282travels in an optical waveguide that is routed away from the negative electrode S−201and the positive electrode S+202, while the λ2283travels underneath the positive electrode S+202. λ1282and λ2283are combined to generate a single optical output λout284. The amplifier222receives the electrical input221, generates the first output to a transmission line223to the negative electrode201, a loading or termination resistor224, and the ground225, and generates a second output to a transmission line226to the positive electrode202, a loading or termination resistor227, and the ground228. Between the negative electrode201and the positive electrode202, a traveling electrical wave λ1273is created due to their proximity. In this embodiment with dual-electrode traveling wave optical phase-shifter, the distance D electrode width274between the negative electrode201and the positive electrode202is relatively short in distance, preferably less than or equal to 20 microns.

FIG. 29is a circuit diagram illustrating two phase-shifters290connected in parallel to form a MZ modulator utilizing a vertical electric field, having an upper phase-shifter241and the lower optical phase-shifter230. The light signal input λin291is split into two paths, the λ1292and the λ2293, which are re-combined to generate a λout294. The λ1292light signal travels underneath a positive electrode245b, while the λ2293light signal travels underneath the negative electrode201. The amplifier243areceives the input242and generates a first output244ato an amplifier243a, and a second output244bto the amplifier222. The amplifier243bthen generates two electrical outputs through a transmission line S1245a, and a transmission line S2246a. The transmission line S1245aextends through a first electrode245bto the load L1or termination resistor245cand the ground245d. The transmission line S2246aextends through the electrode246bto the load L2or termination resistor246cand the ground246d. Between the negative electrode201and the positive electrode202, a traveling electrical wave ω1273is created due to the close proximity of a gap between them. Between the negative electrode246band the positive electrode245b, a traveling electrical wave ω2295is created due to their proximity.

Preferably, the light wave λ1292matches or substantially matches the light wave λ2293. Optionally, the electrodes245b,246b,201, and202can be connected to a voltage supply (not shown), to generate a DC bias field in the optical waveguides.

FIG. 30is a structural diagram illustrating a dual-electrode modulator300where two optical waveguides306and307are placed in regions of a vertical electric field. The dual-electrode modulator300has two electrodes, a negative electrode S−303, and a positive electrode S+304. The two electrodes, the negative electrode S−303and the positive electrode S+304, have opposite polarity from one another. It is apparent to one of ordinary skill in the art that the polarity of the two electrodes can be swapped. The optical waveguide (WG)306directly is underneath the negative electrode S−303, thereby experiencing a substantially vertical electric field. Similarly, the optical waveguide (WG)307is directly underneath the positive electrode S+304, thereby experiencing a substantially vertical electric field.

A first negative signal is introduced into the electrode S−303that travels with the positive signal S+304for significant enhancement of the electrical field in the optical waveguides. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. Ground electrodes302and305are used to suppress the couplings to parasitic modes at high frequencies. A substrate301can be selected from a material such as like GaAs, KDP, or LiNbO3, which demonstrates a linear electro-optic effect.

FIG. 31is a circuit diagram illustrating a dual-electrode modulator310driven from an amplifier with two optical waveguides utilizing a vertical electric field. An amplifier312receives an electrical signal input311and generates a first output to a transmission line S1313aand a second output to a transmission line S2314b. The transmission line S1313aextends through the negative electrode303, to a load or termination resistor313band a ground313c. The transmission line S1314aextends through the positive electrode304, to a load or termination resistor314band a ground314c.

The dual-electrode modulator310receives a light signal input λin315and split the light signal λin315into two optical paths, a λ1316aand a λ2316b. The λ1316atravels underneath the negative electrode S−303, while the λ2316btravels underneath the positive electrode S+304, for generating a single optical output λout319. Between the negative electrode303and the positive electrode304, a traveling electrical wave ω1317is created due to the close proximity of a gap between them. In this embodiment with dipole-enhanced traveling wave optical phase-shifter, the distance D electrode width318is relatively short in distance between the negative electrode303and the positive electrode304, preferably less than or equal to 20 microns.

FIG. 32is a process diagram illustrating a ridge structure320employing tri-electrodes utilizing a vertical electric field. A ridge layer321is added above the element11, with an optical wave guide322internal to the ridge layer321and underneath a positive electrode323. The ridge is layer typically built of the same materials as the element11, which has a linear electro-optic coefficient.

FIG. 33is a process diagram illustrating ridge structure330employing double-electrodes with a horizontal electric field. A ridge layer331is added above the layer206, with an optical wave guide332underneath the buffer layer261, as well as in gaps underneath and in between the negative electrode201, and the positive electrode202. The ridge layer is typically built of the same materials as the element11, which has a linear electro-optic coefficient.

FIG. 34is a structural diagram illustrating a dual-electrode modulator340where two optical waveguides346and347are placed in regions of a horizontal electric field. The dual-electrode modulator340has three electrodes, a negative electrode S−341, and a positive electrode S+343and a negative electrode S−342. The three electrodes, the negative electrode S−341and S−342, and the positive electrode S+343, have opposite polarity from one another. It is apparent to one of ordinary skill in the art that the polarity of the three electrodes can be swapped. The optical waveguide (WG)346is placed directly in the ridge348between the negative electrode S−341and the positive electrode S+343in a substantially horizontal electric field. Similarly, the optical waveguide (WG)347is placed directly in the ridge349between the negative electrode S−342and the positive electrode S+343, thereby experiencing a substantially horizontal electric field.

A first negative signal is introduced into the electrode S−341, and a second negative signal is introduced into the electrode S−342, that travels with the positive signal S+343for significant enhancement of the electrical field in the optical waveguides. The field enhancement is provided by the superposition of the fields created by each electrode giving better performance. Ground electrodes344and345are used to suppress the couplings to parasitic modes at high frequencies. A substrate406can be selected from a material such as like GaAs, KDP, or LiNbO3, which demonstrates a linear electro-optic effect. The ridge348and ridge349typically are built of the same material as substrate406.

In all the preceding diagrams,FIGS. 1–34, the electrodes have been labeled either positive or negative in order to indicate that they are driven with opposite polarity modulation signals. Another suitable notation is to use S and {overscore (S)}, where the symbol S has an opposite polarity from {overscore (S)}. In one embodiment, the polarity referred to is that of the modulation component of the signal applied to the electrode, and is not meant to refer to the absolute polarity of field between the electrodes. For example, applying a large DC offset to one of the electrodes could make the absolute polarity of electric field between the electrodes constant, but the polarity of the modulation components of the signals applied to S+ and S− would still be of opposite polarity.

It should be clear to one of ordinary skill in the art that the actual drive waveform applied to the positive electrode may be either positive or negative at a given point in time, and the actual drive waveform applied to the negative electrode will be of opposite polarity. For example,FIG. 35Ais a diagram illustrating one example of a pair of time-varying signals with opposite modulation polarity. At time slice t1, the signal applied to the positive electrode S+ has a higher voltage than the signal applied to the negative electrode S−. At time slice t2, the signal applied to the positive electrode S+ has a lower voltage than the signal applied to the negative electrode S−.

FIG. 35Bis a graphical diagram illustrating electric field lines between the positive electrode S+ and negative electrode S− at time t1. The electric field between the electrodes flows from S+ to S−.FIG. 35Cis a graphical diagram illustrating electric field lines between the positive electrode S+ and negative electrode S− at time t2. The electric field between the electrodes flows from S− to S+.

If a large DC offset voltage were added to the modulation signal applied to S+, then at time slice t1, the signal applied to the positive electrode S+ would have a higher voltage than the signal applied to the negative electrode S−, and at time slice t2, the signal applied to the positive electrode S+ would be reduced by the modulation component of the signal, but would still have a higher absolute voltage than the signal applied to the negative electrode S−. In this case, with a large DC voltage applied to S+, the electric field lines would flow from S+ to S− as shown inFIG. 35B, but the necessary condition of applying opposite polarity modulation signals to S+ and S− would still be satisfied.

The above embodiments are only illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. For example, although the tri-electrodes have been specified as the negative electrode S−13, the positive electrode S+14, and the negative electrode S−15, one of ordinary skill in the art should know that the polarities can be altered, such as having a positive electrode S+13, a negative electrode S−14, and a positive electrode S+15. The concept is to have the electrode13and electrode15having one polarity, and the electrode14having an opposite polarity from the electrodes13and15. Alternatively, the electrode13and the electrode14can have the same polarity but with a different amplitude where the difference in amplitude is equal or substantially similar to the amplitude difference between a positive electrode and a negative electrode. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the appended claims.