Travelling wave electrode structures and methods for optical modulators

Abstract

External modulation of a continuous wave (CW) laser can provide increased modulation bandwidth relative to that of a directly modulated laser for further increasing data transmission rates. Such external modulators can be phase modulators or amplitude modulators or a combination thereof. In most material systems a velocity mismatch between the optical and electrical signals must be compensated for to achieve the desired bandwidth whilst increasing the device length to reduce drive voltage. Within some material systems such as silicon for silicon photonics this requires slowing the optical signal. Accordingly, it would be beneficial to achieve this without increasing the optical loss of the modulator.

Description

TRAVELLING WAVE ELECTRODE STRUCTURES AND METHODS FOR OPTICAL MODULATORSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application 63 / 767,219 filed March 5, 2025; the entire contents of which are incorporated herein by reference.FIELD OF THE INVENTION

[0002] This invention is directed to photonic circuits and more particularly to photonic modulators for optical transmitters.BACKGROUND OF THE INVENTION

[0003] Photonics has become a dominant or evolving technological solution in a wide range of applications from sensing, biomedical sensing, to quantum computing, quantum sensing, and telecommunications. Within communications streaming media, mobile data traffic, and cloud computing continue to fuel an increasing demand for bandwidth using either amplitude modulation, employed in the vast majority of current systems, or amplitude and phase modulation within coherent modulation formats which offer a path to increased capacity going forward.

[0004] External modulation of a continuous wave (CW) laser can provide increased modulation bandwidth relative to that of a directly modulated laser for further increasing data transmission rates. Such external modulators can be phase modulators or amplitude modulators or a combination thereof. However, in most material systems a velocity mismatch between the optical and electrical signals must be compensated for to achieve the desired bandwidth and increased device length to reduce drive voltage. Within silicon photonics this requires slowing the optical signal. Accordingly, it would be beneficial to achieve this without increasing the optical loss of the modulator.

[0005] Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to mitigate limitations in the prior art relating to photonic circuits and more particularly to photonic modulators for optical transmitters.

[0007] In accordance with an embodiment of the invention there is provided an optical circuit comprising an input splitter coupled to an input port splitting optical signals received at the input port to a first waveguide and a second waveguide, an output combiner coupled to an output port and to a distal end of the first waveguide and a distal end of the second waveguide and an electrode structure disposed with respect to the first waveguide and the second waveguide.

[0008] In accordance with an embodiment of the invention there is provided an optical circuit comprising an input port coupled a first waveguide, an output port coupled to a distal end of the first waveguide and an electrode structure disposed with respect to the first waveguide.

[0009] Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

[0011] Figure 1 depicts a cross-section of a thin-film lithium niobate (TFEN) active waveguide as employed within embodiments of the invention;

[0012] Figure 2 depicts a travelling wave (TW) Mach-Zehnder modulator (MZM) according to an embodiment of the invention employing a TFEN structure;

[0013] Figure 3 depicts a TW-MZM according to an embodiment of the invention employing a TFEN structure; and

[0014] Figure 4 depicts a TW-MZM according to an embodiment of the invention employing a TFLN structure.DETAILED DESCRIPTION

[0015] The present invention is directed to photonic circuits and more particularly to photonic modulators for optical transmitters.

[0016] The ensuing description provides representative embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the embodiment(s) will provide those skilled in the art with an enablingdescription for implementing an embodiment or embodiments of the invention. It would be understood by one of skill in the art that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the claims. Accordingly, an embodiment is an example or implementation of the inventions and not the sole implementation. Various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention can also be implemented in a single embodiment or any combination of embodiments.

[0017] Reference in the specification to “one embodiment,” “an embodiment,” “some embodiments” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment, but not necessarily all embodiments, of the inventions. The phraseology and terminology employed herein is not to be constmed as limiting but is for descriptive purposes only. It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not to be constmed as there being only one of that element. It is to be understood that where the specification states that a component feature, stmcture, or characteristic “may,” “might,” “can” or “could” be included, that particular component, feature, stmcture, or characteristic is not required to be included.

[0018] Reference to terms such as “left,” “right,” “top,” “bottom”, “front” and “back” are intended for use in respect to the orientation of the particular feature, stmcture, or element within the figures depicting embodiments of the invention. It would be evident that such directional terminology with respect to the actual use of a device has no specific meaning as the device can be employed in a multiplicity of orientations by the user or users.

[0019] Reference to terms “including,” “comprising,” “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, integers or groups thereof and that the terms are not to be constmed as specifying components, features, steps or integers. Likewise, the phrase “consisting essentially of,” and grammatical variants thereof, when used herein is not to be constmed as excluding additional components, steps, features integers or groups thereof but rather that the additional features, integers, steps, components or groups thereof do not materially alter the basic and novel characteristics of theclaimed composition, device or method. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional elements.

[0020] A “two-dimensional” waveguide, also referred to as a 2D waveguide or a planar waveguide, as used herein may refer to, but is not limited to, an optical waveguide supporting propagation of optical signals within a predetermined wavelength range which does not guide the optical signals laterally relative to the propagation direction of the optical signals.

[0021] A “three-dimensional” waveguide, also referred to as a 3D waveguide, a channel waveguide, or simply waveguide as used herein may refer to, but is not limited to, an optical waveguide supporting propagation of optical signals within a predetermined wavelength range which guides the optical signals laterally relative to the propagation direction of the optical signals.

[0022] A “photonic integrated circuit” (PIC) as used herein may refer to, but is not limited to, the monolithic integration of multiple integrated optics devices into a circuit formed upon a common substrate providing an optical routing and processing functionality. The PIC is fabricated using processing techniques at a wafer level, e.g., CMOS manufacturing flows, MEMS processing flows, etc.

[0023] An “adiabatic coupler” as used herein may refer to, but is not limited to, an optical coupler which adiabatically converts the mode of an input optical waveguide into either the even or odd mode of two or more optical waveguides separated by small gap(s). An adiabatic coupler may therefore be a non-zero gap symmetric directional coupler or a non-zero gap asymmetric directional coupler for example.

[0024] Within embodiments of the invention the inventors may refer to the term “hybridly integrated.” This may, within some embodiments of the invention, refer to, but not be limited to, the “integration” of an optical element onto a substrate by attaching the optical element or another element physically integrated with the optical element to the substrate (platform) such that the optical element is retained in position. Such attachment means may include, but not be limited to, soldering, epoxy, van der Waals forces, electrostatic attachment, magnetic attachment, physical interlocking and friction. Accordingly, in these embodiments of the invention the optical element being hybridly integrated may be viewed as being implemented within a parallel manufacturing process to the other optical element(s) prior to being coassembled. This parallel manufacturing process may employ one or more processes selected from the group comprising, but not limited to, liquid phase epitaxy (LPE), metal organic chemical vapor deposition (MOCVD), organometallic vapour-phase epitaxy (OMVPE),selective area epitaxy, an additive manufacturing process, a non-additive manufacturing process, crystal growth, doping, induced damage, etching, doping and deposition.

[0025] This may, within other embodiments of the invention, refer to, but not be limited to, the “integration” of an optical element onto a substrate using a different manufacturing methodology and / or techniques to those employed in forming other optical components upon the substrate. For example, this may employ employing a LPE process to form the other optical element upon the substate wherein the optical component upon the substrate was formed by MOCVD or vice-versa. Alternatively, both the optical component and other optical component may be formed using the same manufacturing methodology or a combination of manufacturing methodologies. These manufacturing methodologies may employ one or more processes selected from the group comprising, but not limited to, LPE, MOCVD, OMVPE, selective area epitaxy, an additive manufacturing process, a subtractive manufacturing process, a nonadditive manufacturing process, crystal growth, doping, induced damage, etching, doping, and deposition

[0026] Within the embodiments of the invention the inventors refer to the terms “hybridly integrated” and “hybrid integration.” This may, within some embodiments of the invention, refer to, but not be limited to, the “integration” of an optical element onto a substrate by attaching the optical element or another element physically integrated with the optical element to the substrate (platform) such that the optical element is retained in position. Such attachment means may include, but not be limited to, soldering, epoxy, van der Waals forces, electrostatic attachment, magnetic attachment, physical interlocking and friction. Accordingly, in these embodiments of the invention the optical element being hybridly integrated may be viewed as being implemented within a parallel manufacturing process to the other optical element(s) prior to being co-assembled. This parallel manufacturing process may employ one or more processes selected from the group comprising, but not limited to, liquid phase epitaxy (LPE), metal organic chemical vapor deposition (MOCVD), organometallic vapour-phase epitaxy (OMVPE), selective area epitaxy, an additive manufacturing process, a non-additive manufacturing process, crystal growth, doping, induced damage, etching, doping and deposition.

[0027] Within the embodiments of the invention the inventors refer to the terms “monolithically integrated” and “monolithic integration.” This may, within some embodiments of the invention, refer to, but not be limited to, the “integration” of an optical element onto a substrate by directly forming the optical element upon a substrate (platform). The optical element may be one of a series of optical elements formed upon the substrate to form an opticalcomponent or optical circuit. These optical elements may be optical waveguides themselves, be interconnected by optical waveguides, be interconnected by other optical elements formed upon the substrate through a subsequent processing step or stage or interconnected via other optical elements hybridly integrated onto the substrate. Accordingly, these monolithically integrated optical elements, e.g. optical waveguides, in these embodiments of the invention may employ one or more manufacturing processes selected from the group comprising, but not limited to, liquid phase epitaxy (LPE), metal organic chemical vapor deposition (MOCVD), organometallic vapour-phase epitaxy (OMVPE), selective area epitaxy, photolithography, direct writing, ion beam milling, an additive manufacturing process, a non-additive manufacturing process, crystal growth, doping, induced damage, chemical etching, reactive ion etching (RIE), plasma etching, sputter etching, ion beam assisted etching, reactive ion beam etching, lift-off and deposition.

[0028] A “ceramic” as used herein may refer to, but is not limited to, an inorganic, nonmetallic solid material comprising metal, non-metal or metalloid atoms primarily held in ionic and covalent bonds. Such ceramics may be crystalline materials such as oxide, nitride or carbide materials, elements such as carbon or silicon, and non-crystalline. Exemplary ceramics may include high temperature ceramics or high temperature co-fired ceramics such as alumina (A12O3), zirconia (ZrO2), and aluminum nitride (AIN) or a low temperature cofired ceramic (LTCC). A LTCC may be formed from a glass - ceramic combination.

[0029] A “metal” or “alloy” as used herein may refer to, but is not limited to, a material having good electrical and thermal conductivity. Metals are generally malleable, fusible, and ductile. Metals as used herein may refer to elements such as gold, silver, copper, aluminum, iron, etc. whilst an alloy as used herein refers to a combination of metals such as bronze, stainless steel, steel etc.

[0030] A “polymer” as used herein may refer to, but is not limited to, is a large molecule, or macromolecule, composed of many repeated subunits. Such polymers may be natural and synthetic and typically created via polymerization of multiple monomers. Polymers through their large molecular mass may provide unique physical properties, including toughness, viscoelasticity, and a tendency to form glasses and semi-crystalline structures rather than crystals.

[0031] A “glass” as used herein may refer to, but is not limited to, a non-crystalline amorphous solid. A glass may be fused quartz, silica, a soda-lime glass, a borosilicate glass, a lead glass, an aluminosilicate glass for example. A glass may include other inorganic and organic materials including metals, aluminates, phosphates, borates, chalcogenides, fluorides,germanates (glasses based on GeO2), tellurites (glasses based on TeO2), antimonates (glasses based on Sb2O3), arsenates (glasses based on As2O3), titanates (glasses based on TiO2), tantalates (glasses based on Ta2O5), nitrates, carbonates, plastics, and an acrylic.

[0032] Embodiments of the invention may be implemented within one or more semiconductor materials (semiconductors), grown for example through LPE, MOCVD or OMVPE. The one or more semiconductors may include, but are not limited to, group III-V semiconductors, II -VI semiconductors, group IV semiconductors, and group IV-V-VI semiconductors. Examples of group III-V semiconductors may include A1P, AIN, AlGaSb, AlGaAs, AlGalnP, AlGaN, AlGaP, GaSb, GaAsP, GaAs, GaN, GaP, InAlAs, InAlP, InSb, InGaSb, InGaN, GalnAlAs, GalnAlN, GalnAsN, GalnAsP, GalnAs, GalnP, InN, InP, InAs, InAsSb, InGaAsP and AllnN. Examples of group II-VI semiconductors may include ZnSe, HgCdTe, ZnO, ZnS, and CdO. Examples of group IV Semiconductors may include Si, Ge, and strained silicon. A group IV-V-VI semiconductor may be GeSbTe.

[0033] Such semiconductors in many instances allowing monolithic integration of passive optical waveguides with active optical elements such as light emitting diodes, semiconductor optical amplifiers, laser diodes (LDs), distributed feedback LDs, external cavity laser diodes (ECLs), photodetectors (PDs) and avalanche photodetectors (APDs). For example, InGaAsP semiconductors support PDs / LDs / ECLs etc. operating in the conventional infrared telecommunication windows known as S-band (1460-1530 nm), C-band (1530-1565 nm) and L-band (1565-1625 nm).

[0034] Within embodiments of the invention the platform or substrate upon which the semiconductors are grown and processed may itself be a semiconductor, e.g., GaAs or InP, or it may within other embodiments of the invention be another material such as silicon, germanium, a ceramic, a glass and a polymer.

[0035] Within embodiments of the invention the platform or substrate upon which the integration is performed may be a silicon substrate wherein the one or more optical waveguides upon the platform exploit a silicon nitride core with silicon oxide upper and lower cladding, a waveguide structure. Alternatively, the one or more optical waveguides may employ a silicon core with silicon nitride upper and lower claddings. Optionally, the upper cladding may be omitted within other embodiments of the invention.

[0036] Embodiments of the invention may be implemented within one or more silicon-on-insulator (SOI) waveguides by way of example, e.g. air-clad Si-^N^ — SiO2, SiO2— Si-^N^ — SiO2, SiO2— Ge: SiO2— SiO2or Si — SiO2. Embodiments of the invention may be implemented with one or more waveguides such as ion exchanged glass waveguides, ionimplanted glass waveguides, polymer-on-silicon waveguides, doped silicon waveguides and polymeric waveguides. Whilst not necessarily leveraging the benefits of photonic integration embodiments of the invention may also be formed using optical fibers, free-space optics, etc.

[0037] Where passive optical waveguides are employed with hybrid integration of active photonic elements, such as LDs, ECLs, PDs then these may be directly butt-coupled or they may employ intermediate coupling optics, e.g., ball lenses, spherical lenses, graded refractive index (GRIN) lenses etc. for free-space coupling and / or photonic wirebonds etc. Other waveguide structures may be employed including vertical and / or lateral waveguide tapers and forming microball lenses on the ends of the waveguides via laser and / or arc melting of the waveguide tip.

[0038] Embodiments of the invention employing optical waveguides may employ a waveguide core embedded within upper and lower claddings, a so-called buried waveguide, an air clad waveguide (i.e. a core with lower cladding and air elsewhere), a rib waveguide, a diffused waveguide, a ridge or wire waveguide, a strip-loaded waveguide, a slot waveguide, an anti-resonant reflecting optical waveguide (ARROW waveguide), a photonic crystal waveguide, a suspended waveguide, an alternating layer stack geometry, a sub-wavelength grating (SWG) waveguides or an augmented waveguide (e.g. St — SiO2— Polymer). Embodiments of the invention may employ a step index waveguide, a graded index waveguide or a hybrid index waveguide (such as combining inverse-step index and graded index).

[0039] External modulation of a continuous wave (CW) laser can provide increased modulation bandwidth relative to that of a directly modulated laser for further increasing data transmission rates. These external modulators can be phase modulators or amplitude modulators or a combination thereof. Within most material systems there is a velocity mismatch between the optical signals upon which electrical data is to be encoded and the electrical signals within the external modulator which have the data being encoded onto the optical signals. This velocity mismatch requires compensation such that an increased interaction length between the optical and electrical signals can be achieved whilst maintaining the desired modulation bandwidth of the external modulator and whilst reducing the required drive voltage.

[0040] Within material systems such as lithium niobate the optical wave travels faster than the electrical (microwave) signal such that the design of the electrodes for the electrical signal increases the electrical signal velocity, referred to as velocity matching. Such electrode structures being referred to as fast-wave electrodes. Within material systems such as silicon, gallium arsenide (GaAs) or indium phosphide (InP) the electrical signals travels faster than theoptical signal and accordingly one design methodology is to design the electrodes for the electrical signal to decrease the electrical signal velocity. Such electrode structures being referred to a slow-wave electrodes.

[0041] However, whilst silicon, GaAs and InP enable integration of optical and electrical elements the slow-wave electrode designs increase the complexity of the electrode design, as well as the manufacturing processes and the tolerances required, such that manufacturing yields decrease, thereby increasing costs, and performance metric distributions widen.

[0042] However, lithium niobate has one of the highest electro-optic coefficients among optical materials enabling efficient modulation, i.e. low drive voltages, and whilst it does not support monolithic integration it can form part of optoelectronic circuits employing hybrid integration such as semiconductor die upon lithium niobate or lithium niobate integration onto a semiconductor substrate. Implementation of the latter has been demonstrated using thin-film lithium niobate (TFLN) which offers a route to:

[0043] Ultra-High Bandwidth'. TFLN modulators can achieve modulation speeds exceeding 100GHz, supporting data rates well beyond ITb / s.

[0044] Reduced Energy Consumption'. Greater modulation efficiency reduces the power required for high-speed data transmission, leading to significant energy savings. While the overall energy consumption of a transceiver module involves multiple elements (transmitters, receivers, drivers, and DSPs), it has been demonstrated within the prior art that TFLN can operate at CMOS-compatible voltages of 1-1.2V, driven by the CMOS electronics. In contrast, silicon modulators typically require a driver to power the modulator which operate in the 3-4V range.

[0045] Improved Thermal Performance'. Lower power consumption translates to less heat generation, simplifying thermal management of optical elements either integrated within the TFLN circuit or semiconductor circuits, for example, within the substrate the TFLN is integrated upon or hybridly integrated with the TFLN.

[0046] Scalability'. The thin-film nature of TFLN supports improved integration with existing semiconductor processes thereby leveraging the benefits of semiconductor manufacturing, integration of electronics and / or optoelectronic elements etc.

[0047] Accordingly, within the following description with respect to Figures 2 to 4 the inventors have established designs for dual-drive TW-MZMs employing TFLN processes.

[0048] Referring to Figure 1 there is depicted a Cross-Section 100 of a thin-film lithium niobate (TFLN) active waveguide as employed within embodiments of the invention. As depicted a TFLN Layer 130 has been formed and patterned upon a Lower Cladding 120, e.g.silicon dioxide (SiO2), which is disposed upon a Substrate 110, e.g. silicon (Si). An Upper Cladding 140 has then been deposited and patterned atop the TFLN Layer 130 and then Electrodes 150 deposited and patterned. The Upper Cladding 140 may be SiO2 whilst the Electrodes 150 may be formed from gold (Au) for example. An adhesion layer, e.g. chromium (Cr), may be employed to enhance adhesion of the Electrode 150. Optionally, the Electrode 140 may be formed upon the Upper Cladding 140.

[0049] As depicted in Figure 1 the TFLN Layer 130 has been patterned such that it provides a rib-loaded waveguide to confine optical signals within the waveguide. The Electrodes 150 provide for application of a potential across the waveguide and therein adjust the refractive index via the electro-optic effect. Accordingly, the geometry depicted in Figure 1 provides for phase modulation of optical signals within the waveguide and thereby as known in the prior art amplitude modulation such as by embedding the phase modulator in one arm of a Mach-Zehnder modulator (MZM) or a pair of phase modulators within both arms of a MZM. Optionally, the phase modulator, as known in the art, may induce amplitude modulation when employed within other waveguide structures such as ring resonators etc.

[0050] Referring to Figure 2 there is depicted a travelling wave (TW) Mach-Zehnder modulator (MZM) 200 according to an embodiment of the invention employing a TFLN structure. As depicted an input (I / P) is coupled to a 1x2 Splitter 210 wherein the outputs of the 1x2 Splitter 210 are first Waveguide 220 and second Waveguide 230 wherein an input optical signal coupled to the IP is split into the first Waveguide 220 and second Waveguide 230 wherein these optical signals propagate through the TW-MZM 200 to combine in 2x1 Combiner 240 and therein provide an optical signal at the output (O / P) where the optical signals combine according to the accumulated phase difference between the optical signal in the first Waveguide 220 and the optical signal in the second Waveguide 230. Without any applied phase shift the static phase shift may be between 0°, such that the optical signals from the first Waveguide 220 and second Waveguide 230 are combined by the 2x1 Combiner 240 into the O / P, and 180°, such that the optical signals from the first Waveguide 220 and second Waveguide 230 are combined by the 2x1 Combiner 240 to a mode radiated into the substrate that there is no combined signal in the O / P.

[0051] Within TW-MZM 200 an electrically induced phase shift is induced into each of the first Waveguide 220 and second Waveguide 230 by the TW Electrode structure comprising first to third Ground Electrodes 260A to 260C respectively coupled to Ground Pads (G) and the pair of Signal Electrodes 250 and 270 respectively which are connected to Signal + Pad (S+) and Signal - Pad (S-). The S+ Signal Electrode 250 is disposed between first and secondGround Electrodes 260A and 260B respectively to form a coplanar waveguide. The S- Signal Electrode 260 is disposed between second and third Ground Electrodes 260B and 260C respectively to form another coplanar waveguide.

[0052] As depicted the coplanar waveguide comprising the S+ Signal Electrode 250 between first and second Ground Electrodes 260A and 260B has a length LI to induce a differential phase shift A<>abetween the first Waveguide 220 and second Waveguide 230. The other coplanar waveguide comprising the S- Signal Electrode 260 disposed between second and third Ground Electrodes 260B and 260C has a length L2 but only a portion L3 overlaps the first Waveguide 220 and second Waveguide 230 to induce a differential phase shift A<>&between the first Waveguide 220 and second Waveguide 230. As the first and second Waveguides 220 and 230 are routed via am optical cross-over between the portion with the coplanar waveguide of length LI and the portion of the other coplanar waveguide with length L3 then the optical phases add within TW-MZM 200 when the optical and electrical delays within the TW-MZM 200 are matched. The electric field (E-field) direction within the TW-MZM 200 being depicted by the arrows where it would therefore be evident that the optical cross-over ensures the appropriate E-field direction across each section with the S- signal to that with the S+ signal.

[0053] The coplanar waveguide comprising the S+ Signal Electrode 250 between first and second Ground Electrodes 260A and 260B is terminated with first and second Resistors 280A and 280B which are disposed between the S+ Signal Electrode 250 and the first and second Ground Electrodes 260A and 260B respectively. The other coplanar waveguide comprising the S- Signal Electrode 260 disposed between second and third Ground Electrodes 260B and 260C is terminated by third and fourth Resistors 290A and 290B which are disposed between the S-Signal Electrode 260 and the second and third Ground Electrodes 260B and 260C respectively.

[0054] Whilst the design depicted in Figure 2 is 1x1 MZM it would be evident to one of skill in the art that by replacing the 1x2 Splitter 210 and / or 2x1 Combiner 240 with 2x2 optical elements that the 1x1 MZM may be designed as a 1x2 MZM, 2x1 MZM or 2x2 MZM. Such devices can act as modulators and ultra-fast optical switches.

[0055] Now referring to Figure 3 there is depicted a TW-MZM 300 according to an embodiment of the invention employing a TFLN structure. As depicted an input (I / P) is coupled to a 1x2 Splitter 210 wherein the outputs of the 1x2 Splitter 210 are first Waveguide 220 and second Waveguide 230 wherein an input optical signal coupled to the IP is split into the first Waveguide 220 and second Waveguide 230 wherein these optical signals propagate through the TW-MZM 200 to combine in 2x1 Combiner 240 and therein provide an optical signal at the output (O / P) where the optical signals combine according to the accumulated phasedifference between the optical signal in the first Waveguide 220 and the optical signal in the second Waveguide 230.

[0056] Within TW-MZM 300 an electrically induced phase shift is induced into each of the first Waveguide 220 and second Waveguide 230 by the TW Electrode structure comprising first and second Ground Electrodes 360A and 360B respectively coupled to Ground Pads (G) and the pair of Signal Electrodes 350 and 370 respectively which are connected to Signal + Pad (S+) and Signal - Pad (S-). The S+ Signal Electrode 350 and S- Signal Electrode 370 are both disposed between first and second Ground Electrodes 360A and 360B respectively to form a dual strip coplanar waveguide. The S+ Signal Electrode 350 and S- Signal Electrode 370 are terminated by Resistor 380. As depicted the optical and electrical structures fold back upon themselves such that the I / P and O / P are on the same side of the TW-MZM 300.

[0057] According an optical phase shift is induced within the second Waveguide 230 by the electric field between the S+ Signal Electrode 350 and S- Signal Electrode 370 as the second Waveguide 230 runs along the TW-MZM 300 from the 1x2 Splitter 210 to the loop back in the second Waveguide 230. The looped back portion of second Waveguide 230 runs back under the second Ground Electrode 360B such that no additional electrically induced phase shift is incurred. This phase shift being denoted as A< >a.

[0058] Another optical phase shift is induced within the first Waveguide 220 by the electric field between the S+ Signal Electrode 350 and S- Signal Electrode 370 as the first Waveguide 220 runs along the TW-MZM 300 back from the loop back in the first Waveguide 220 to the 2x1 Combiner 240. Prior to the loop back the first Waveguide 220 runs along outside the first Ground Electrode 360A such that no additional electrically induced phase shift is incurred. This phase shift being denoted as A< >&.

[0059] Accordingly, the push-pull electrode scheme depicted in Figure 3 induces a total phase shift of A< >a+ A< >&between the first Waveguide 220 and second Waveguide 230. If the optical and electrical delays are matched within the TW-MZM 200 then these are equal such that the total accumulated phase shift is 2A< >a.

[0060] Whilst the design depicted in Figure 3 is 1x1 MZM it would be evident to one of skill in the art that by replacing the 1x2 Splitter 210 and / or 2x1 Combiner 240 with 2x2 optical elements that the 1x1 MZM may be designed as a 1x2 MZM, 2x1 MZM or 2x2 MZM. Such devices can act as modulators and ultra-fast optical switches.

[0061] Referring to Figure 4 there is depicted a TW-MZM 400 according to an embodiment of the invention employing a TFLN structure. As depicted an input (I / P) is coupled to a 1x2Splitter 210 wherein the outputs of the 1x2 Splitter 210 are first Waveguide 220 and second Waveguide 230 wherein an input optical signal coupled to the IP is split into the first Waveguide 220 and second Waveguide 230 wherein these optical signals propagate through the TW-MZM 200 to combine in 2x1 Combiner 240 and therein provide an optical signal at the output (O / P) where the optical signals combine according to the accumulated phase difference between the optical signal in the first Waveguide 220 and the optical signal in the second Waveguide 230.

[0062] Within TW-MZM 400 an electrically induced phase shift is induced into each of the first Waveguide 220 and second Waveguide 230 by the TW Electrode structure comprising first and second Ground Electrodes 460A and 460B respectively coupled to Ground Pads (G), a pair of first Signal Electrodes 450A and 450B which are connected to Signal + Pad (S+) and second Signal Electrode 470 which is connected to the Signal - Pad (S-). The first and second Ground Electrodes 460A and 460B being coupled to each other at the Signal + Pad (S+).

[0063] The S-Signal Electrode 470 is disposed between the first and second S+ Signal Electrodes 450A and 450B respectively whilst these are all disposed between first and second Ground Electrodes 460A and 460B respectively to form a dual strip coplanar waveguide. The first and second S+ Signal Electrodes 450A and 450B respectively are terminated to S- Signal Electrode 470 by first and second Resistors 490A and 490B respectively.

[0064] According an optical phase shift is induced within the first Waveguide 220 by the electric field between the first S+ Signal Electrode 450A and S- Signal Electrode 470 as the first Waveguide 220 runs along the TW-MZM 400 from the 1x2 Splitter 210 to the 2x1 Combiner 240 between the first S+ Signal Electrode 450A and S- Signal Electrode 470. This phase shift being denoted as A<>a.

[0065] According an optical phase shift is induced within the second Waveguide 230 by the electric field between the second S+ Signal Electrode 450A and S- Signal Electrode 470 as the second Waveguide 230 runs along the TW-MZM 400 from the 1x2 Splitter 210 to the 2x1 Combiner 240 between the first S+ Signal Electrode 450B and S- Signal Electrode 470. This phase shift being denoted as — A<>a.

[0066] Accordingly, the push-pull electrode scheme depicted in Figure 4 induces a total phase shift of A<>a+ A „ between the first Waveguide 220 and second Waveguide 230. Whilst the design depicted in Figure 4 is 1x1 MZM it would be evident to one of skill in the art that by replacing the 1x2 Splitter 210 and / or 2x1 Combiner 240 with 2x2 optical elements that the 1x1MZM may be designed as a 1x2 MZM, 2x1 MZM or 2x2 MZM. Such devices can act as modulators and ultra-fast optical switches.

[0067] Whilst the embodiments of the invention have been described with respect to MZM structures the electrode structures may also be applied to phase modulators.

[0068] Whilst embodiments of the invention have been described with respect to TFLN TW-MZM devices it would be evident that the design principles may be applied to other material systems where the optical wave travels faster than the electrical (microwave) signal within the device.

[0069] Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.

[0070] The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.

[0071] Further, in describing representative embodiments of the present invention, the specification may have presented the method and / or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be constmed as limitations on the claims. In addition, the claims directed to the method and / or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.

Claims

CLAIMSWhat is claimed is:

1. An optical circuit comprising:an input splitter coupled to an input port splitting optical signals received at the input port to a first waveguide and a second waveguide;an output combiner coupled to an output port and to a distal end of the first waveguide and a distal end of the second waveguide; andan electrode structure disposed with respect to the first waveguide and the second waveguide.

2. The optical circuit according to claim 1, whereinthe first waveguide and the second waveguide cross-over one another at a point between the input splitter and the output combiner;the electrode structure comprises:a first signal electrode disposed between a first ground electrode and a second ground electrode;a second signal electrode disposed between the second ground electrode and a third ground electrode;a first pair of resistors terminating the first signal electrode to the first ground electrode and the second ground electrode;a second pair of resistors terminating the second signal electrode to the second ground electrode and the third ground electrode;the first waveguide and second waveguide are disposed between the first signal electrode and the first ground electrode and the second ground electrode respectively prior to the cross-over; andthe first waveguide and second waveguide are disposed between the second signal electrode and the third ground electrode and the second ground electrode respectively after the cross-over.

3. The optical circuit according to claim 1, whereinthe electrode structure comprises:a first ground electrode;a first signal electrode disposed between the first ground electrode and a second ground electrode which loops back upon itself;a second signal electrode disposed between the first ground electrode and the second ground electrode which loops back upon itself and is disposed within the loop of the first signal electrode;the second ground electrode which is disposed within the loop of the second signal electrode;a resistor terminating the first signal electrode to the first ground electrode and the second ground electrode;the input splitter, the output combiner, the resistor and feeds to the first ground electrode, second ground electrode, first signal electrode and second signal electrode are all disposed on a first side of the optical circuit;each of the first waveguide and second waveguide comprise a first portion running from the input splitter to a loop-back region and a second portion running from the loop-back region to the output combiner;the first portion of the first waveguide and the second portion of the second waveguide are positioned such that no phase shift is induced by the electrode structure arising from electrical signals applied to the first signal electrode and second signal electrode; the first portion of the second waveguide has a phase shift induced by an electric field between the first signal electrode and second signal electrode arising from electrical signals applied to the first signal electrode and second signal electrode; andthe second portion of the first waveguide has a phase shift induced by another electric field between the first signal electrode and second signal electrode arising from electrical signals applied to the first signal electrode and second signal electrode.

4. The optical circuit according to claim 1, whereinthe electrode structure comprises:a first ground electrode;a second ground electrode;a pair of first signal electrodes disposed between the first ground electrode and a second ground electrode which are coupled to a common electrical feed at one end; a second signal electrode disposed between the pair of first signal electrodes coupled to another electrical feed;a pair of resistors terminating the second signal electrode to the pair of first signal electrodes at a distal end of the pair of first signal electrodes to the common electrical feed;the first waveguide is disposed between the second signal electrode and a first signal electrode of the pair of first signal electrodes;the second waveguide is disposed between the second signal electrode and the other first signal electrode of the pair of first signal electrodes.

5. An optical circuit comprising:an input port coupled a first waveguide;an output port coupled to a distal end of the first waveguide; andan electrode structure disposed with respect to the first waveguide.

6. The optical circuit according to claim 5, whereinthe first waveguide laterally shifts with respect to the electrode structure at a point between the input port and the output port;the electrode structure comprises:a first signal electrode disposed between a first ground electrode and a second ground electrode;a second signal electrode disposed between the second ground electrode and a third ground electrode;a first pair of resistors terminating the first signal electrode to the first ground electrode and the second ground electrode; anda second pair of resistors terminating the second signal electrode to the second ground electrode and the third ground electrode;the first waveguide is disposed between the first signal electrode and the first ground electrode prior to the point; andthe first waveguide is disposed between the second signal electrode and the third ground electrode after the point.

7. The optical circuit according to claim 5, whereinthe electrode structure comprises:a first ground electrode;a first signal electrode disposed between the first ground electrode and a second ground electrode which loops back upon itself;a second signal electrode disposed between the first ground electrode and the second ground electrode which loops back upon itself and is disposed within the loop of the first signal electrode;the second ground electrode which is disposed within the loop of the second signal electrode;a resistor terminating the first signal electrode to the first ground electrode and the second ground electrode;the input port, the output port, the resistor and feeds to the first ground electrode, second ground electrode, first signal electrode and second signal electrode are all disposed on a first side of the optical circuit;the first waveguide comprises a first portion running from the input splitter to a loop-back region and a second portion running from the loop-back region to the output port; the device has a first configuration and a second configuration;in the first configuration:the first portion of the first waveguide is positioned such that no phase shift is induced by the electrode structure arising from electrical signals applied to the first signal electrode and second signal electrode; andthe second portion of the first waveguide has a phase shift induced by another electric field between the first signal electrode and second signal electrode arising from electrical signals applied to the first signal electrode and second signal electrode; andin the second configuration:the second portion of the first waveguide is positioned such that no phase shift is induced by the electrode structure arising from electrical signals applied to the first signal electrode and second signal electrode; andthe first portion of the first waveguide has a phase shift induced by another electric field between the first signal electrode and second signal electrode arising from electrical signals applied to the first signal electrode and second signal electrode.

8. The optical circuit according to claim 5, whereinthe electrode structure comprises:a first ground electrode;a second ground electrode;a pair of first signal electrodes disposed between the first ground electrode and a second ground electrode which are coupled to a common electrical feed at one end; a second signal electrode disposed between the pair of first signal electrodes coupled to another electrical feed;a pair of resistors terminating the second signal electrode to the pair of first signal electrodes at a distal end of the pair of first signal electrodes to the common electrical feed;the device has a first configuration and a second configuration;in the first configuration the first waveguide is disposed between the second signal electrode and a first signal electrode of the pair of first signal electrodes; andin the second configuration the first waveguide is disposed between the second signal electrode and the other first signal electrode of the pair of first signal electrodes.

Images