Patent Description:
Optical modulators incorporating Mach-Zehnder interferometer structures can be used to impose RF modulation on an optical signal, such as light from a laser source. The RF modulation can encode information within the optical signal for transport to remote locations through an optical communications network. The encoded information can include, for example, data signals, voice signals, video signals and/or other communication information.

The Mach-Zehnder interferometer structure splits the optical signal into two arms that interfere when recombined. The RF signal is superimposed onto the optical signal using the RF transmission line through alteration of the index of refraction of the optical waveguide by an electromagnetic RF signal. High fidelity transfer of the RF signal into modulation of the optical signal becomes more challenging as the frequency of the RF signal increases to accommodate higher bandwidth in the optical signal.

<CIT> discloses a Mach-Zehnder optical modulator. In the embodiment of <FIG> of this document, a common ground plane is configured in a flip-chip configuration positioned on a surface of a chip but with a distributed bridging structure that bridges the ground plane with lateral ground conductors via conductive bumps.

According to the invention, there is proposed an optical modulator comprising the features of claim <NUM>. Individual embodiments of the invention are the subject matter of the dependent claims.

In a first aspect, the invention pertains to an optical modulator comprising a submount, electrical conduction pathways designed to carry RF signals, and an optical chip comprising a substrate and two semiconductor optical waveguides with electrical conductive elements along at least a portion of the optical waveguide surface. In some embodiment, the optical chip is attached to the submount with the optical chip substrate oriented away from the submount and the two semiconductor optical waveguides oriented toward the submount. The submount can comprise a conducting plane offset from the conductive RF electrodes of the optical chip. Generally, the optical chip further comprises two conductive RF electrodes adjacent respectively to corresponding optical waveguides and additional conductive elements connecting one of the conductive RF electrodes to the corresponding conductive element along a surface of the corresponding optical waveguide, and the electrical conduction pathways electrically connect to the conductive RF electrodes of the optical chip.

In a further aspect, the invention pertains to an optical modulator comprising a Mach-Zehnder interferometer and a pair of RF electrodes interfaced with the Mach-Zehnder interferometer, the Mach-Zehnder interferometer comprising an optical splitter connected to an optical input waveguide, two optical waveguide arms optically connected to the optical splitter and an optical combiner optically connected to the two optical waveguides and to an output waveguide. The optical waveguides can comprise a semiconductor optical material, and an electrical contact can be located on portions of the optical waveguide arms surface. Each RF electrode of the pair can comprise transmission line electrodes connected by additional electrodes to electrical contacts on respective optical waveguides. In some embodiments, a ground plane is spaced away in a distinct plane from the transmission line electrodes. The optical modulator can be used in a method for modulating an optical telecommunication signal, in which the method comprises exposing laser light split between the two optical waveguides of the Mach-Zehnder interferometer to separate RF electric fields transmitted along RF transmission lines; and recombining the light from the two optical waveguides to form a modulated optical signal. Furthermore, the method can further comprise embodiments of optical modulators with four Mach-Zehnder interferometers, delivering separate RF electric fields to each individual Mach-Zehnder interferometer; and multiplexing the optical signal by combining the resulting optical signal in orthogonal phase states and polarization states.

In another aspect, the invention pertains to a method of forming an optical modulator in which the method comprises bonding an inverted optical chip to a submount. The inverted optical chip can comprise a Mach-Zehnder interferometer with optical waveguides and two RF electrodes associated with distinct arms of the Mach-Zehnder interferometer. The submount can comprise electrical contacts aligned with electrical contact points along the adjacent surface of the waveguide structure. The submount can comprise a ground plane spaced away from the mounting surface of the submount.

An optical modulator can be formed with desirable high frequency performance using a mounting structure or submount with a conduction plane and an optical chip with optical waveguides for the arms of a Mach-Zehnder interferometer that can be positioned with RF electrodes on the optical waveguides. The conduction plane provides a convenient electrical ground plane to support the RF traveling wave that modulates an adjacent optical signal transmitted through the optical waveguides, generally semiconductor waveguides, but potentially other types of electro-optic waveguides. The resulting configuration effectively confines the electric field primarily within the structure providing for low cross talk with neighboring structures. Generally, the resulting mounted structure forms a RF transmission line. The direct mounting of the optical chip onto the submount provides for a convenient RF electrode configuration that introduces manufacturing efficiencies consistent with integration with associated optical components. Specifically, a ball or bump joining process can be used to form the electrical connections between aligned conduction elements such that heating flows a deposit of conductive metal, such as gold or silver, associated with one of the conductive elements to form the conductive bond with little or no manually-produced connections. The modulators described herein are designed to provide appropriate broadband signal modulation at RF frequencies extending to greater than <NUM>, which are desirable for state of the art optical telecommunication systems as well as future anticipated systems. At high RF frequencies, the RF electrode configurations herein can be appropriately designed to provide for appropriate alignment of RF transmission speeds with the optical transmission through the semiconductor or other electro-optic waveguides.

Optical modulators can be used to introduce a modulation to a continuous wave optical transmission to encode the optical transmission with a desired data signal. To provide the modulation of the optical transmission, a radio frequency (RF) transmission line is placed in close proximity to an optical waveguide. The wavelength of the optical signal can be selected for incorporation into an optical telecommunication network, such as operating with C-Band from <NUM> to <NUM> or S-Band from <NUM> to <NUM>, or L-band from <NUM> to <NUM>.

The modulators described herein have a Mach-Zehnder interferometer (MZI) that comprises two optical couplers/splitters with two optical waveguides connecting the couplers/splitters that form arms of the MZI. The MZI arms each interface with electrodes of an RF transmitter. The electrodes generally are traveling-wave electrodes that consist of at least two electrodes that form a transmission line (one for each arm), for example, oriented approximately parallel to the optical waveguides. A series of electrode extensions or conductive elements connected to the transmission line electrodes are positioned more proximal to the optical waveguide. Specifically, these conductive elements directly adjacent to the optical waveguides are connected through bridge conductors to the transmission line electrodes. Electric fields from the transmission line electrodes and connected elements, as well as the ground, interact with the properties of optical waveguide material, and therefore the optical signals in the arms of the MZI. Generally, the two respective traveling RF electrodes are driven in an anti-phase (opposite voltages relative to ground) sense relative to each other. Due to the optical modulation, the interference of the optical signals when recombined from the MZI arms constructively or destructively interfere as a function of the modulation so that the transmitted optical signal from the MZI optical circuit is modulated based on the RF signals, and deconvolution of the modulated optical signal at a receiver can extract out the encoded information from the transmitted optical signal.

The speed of the optical signal through the waveguides depends on the index of refraction of the optical waveguide, which is generally a semiconductor or high-index dielectric. Similarly, the speed of the RF wave is dependent on the effective RF index of refraction for the RF transmission line electrodes in combination with a ground electrode and surrounding medium. Typically, the speed of the optical signal in the optical waveguide is noticeably different than the speed of the electrical signal in a simple corresponding transmission line. If the speeds of the optical signal and the electrical signal are sufficiently different from each other to result in a walk-off on the order of the modulation wavelength, such as on the order of a centimeter, the signal encoded by the modulation can be distorted, washed out, or lost. Adjusting the speed of optical propagation in the optical waveguide by an appreciable amount is generally impractical. Therefore, efforts have been made to design the structure of the RF transmission lines to better match the RF transmission speed to the optical propagation speed. As RF frequencies increase to allow for a higher bandwidth, the RF wavelengths shrink and the matching of the RF transmission to the optical propagation is correspondingly evaluated using tighter tolerances.

The design of the RF electrodes as well as a ground electrode influences the RF transmission. In particular, the electrode design influences power consumption, modulation efficiency and speed of RF transmission. At the same time that the RF transmission speed should appropriately match the optical propagation, it is desirable to have a low power consumption and low cross talk between other modulators mounted nearby while maintaining a desirable degree of signal modulation. An additional constraint is that the electrical impedance of the modulator should be matched to the output impedance of the RF source, including, but not limited to, an external generator or amplifier, or a co-packaged driver. The modulator designs described herein involve the location of the modulator on a submount, i.e., a mount structure that provides matching degree of freedom to match electrical impedance, also enabling high performance while providing suitable commercial processing. With the RF electrode facing toward the ground plane associated with the submount, the RF mode is confined in the submount without significant sensitivity to the airgap between the semiconductor chip and the submount.

With respect to the modulator structures herein, the mount provides a lateral displacement, i.e., non-coplanar relationship, between (a) the modulating electrodes placed on or adjacent the surface of the semiconductor based optical waveguide and connected to adjacent transmission lines and (b) a ground supported in a spaced away configuration laterally displaced from the plane of the modulating electrodes. The effective RF index of refraction seen by the RF signal depends on the structure. The optical waveguide structure is mounted in an inverted configuration relative to the supporting structure in the sense that the modulating electrodes are generally placed on "top" of the semiconductor optical waveguides relative to a substrate on which the optical waveguides are formed. Generally, the semiconductor optical waveguide is formed with a p-doped semiconductor layer adjacent the modulating RF electrodes, so the structure described herein can be referred to as a 'p-down' structure, although any reference to top or up or the like necessarily has some reference to a supposed orientation of the composite structure since any of the composite structures can be moved around in free space to reorient the structures. While the discussion herein focuses on current commercial processing approaches that are convenient for the structures described, the structures themselves can in principle be formed using direct build up of the desired structures as commercial processing evolves and may make such approaches competitive with the separate formation and mounting processes connecting an optical chip with a mounting structure.

In contrast, the use of coplanar grounding electrodes suitable for high frequency modulation are described in <CIT>(hereinafter the `<NUM> patent), entitled "Electrical Waveguide Transmission Device for Use With a Mach-Zehnder Optical Modulator,". The `<NUM> patent has a design intended to remove a ground electrode between the transmission line modulator electrodes while maintaining an effective optical modulator. Another RF optical modulator design with coplanar positioned ground conductors is presented in <CIT>(hereinafter the `<NUM> patent), entitled "Mach-Zehnder Optical Modulator Using a Balanced Coplanar Stripline With Lateral Ground Planes,". The `<NUM> patent asserts to achieve lower waveguide capacitance with reduced power consumption and reduced cross talk using "a balanced coplanar stripline with lateral ground planes. " In contrast with these applications, the present modulators are augmented by coupling to a submount in an inverted configuration relative to the submount, and having a ground plane is positioned vertically displaced within the submount. In some embodiments, no lateral ground plane is present. The vertically displaced ground plane is distinct from a non-grounded, i.e., floating, semiconductor conduction plane linking the semiconductor optical waveguides, and a floating semiconductor conduction plane is generally inherent in the present system on the opposite side of the semiconductor optical waveguides away from the electrically conductive ground plane.

An embodiment of the RF optical modulator as described herein is shown in <FIG>. Referring to <FIG>, an optical modulator structure <NUM> is shown with an optical chip <NUM> mounted on a submount <NUM>. An exploded or separated view is shown in <FIG> with optical chip <NUM> rotated relative to submount <NUM> to show the mated structures. <FIG> shown an expanded view of a portion of optical chip <NUM>. Referring to <FIG>, optical chip <NUM> has substrate <NUM>, two optical waveguides <NUM>, <NUM>, two transmission line RF electrodes <NUM>, <NUM> and respective bridge electrodes <NUM>, <NUM>. The transmission line RF electrodes can be, in some embodiments, from about <NUM> microns to about <NUM> microns wide, respective transmission line RF electrodes <NUM>, <NUM> can be spaced apart from each other, in some embodiments, from about <NUM> microns to <NUM> microns apart. Respective transmission line RF electrodes <NUM>, <NUM> terminate respectively with bond pads <NUM>, <NUM> and <NUM>, <NUM>. Submount <NUM> comprises substrate <NUM>, conduction stripes <NUM>, <NUM>, <NUM>, <NUM> connected respectfully with bond pads <NUM>, <NUM>, <NUM>, <NUM>, and conductive plane <NUM>. Substrate <NUM> can comprise a ceramic material, such as Aluminum Nitride, Alumina, or others; and/or a polymer material, such as polycarbonate or PET or the like. Electrically conductive elements of submount <NUM> can be formed with metal films, such as copper, silver, gold, alloys thereof or the like. Bond pads <NUM>, <NUM> are configured for respective attachment to bond pads <NUM>, <NUM>, and bond pads <NUM>, <NUM> are configured for respective attachment to bond pads <NUM>, <NUM>. Conductive plane <NUM> generally can be grounded to provide a ground for transmission line RF electrodes <NUM>, <NUM>. Collectively, transmission line RF electrodes <NUM>, <NUM> and conductive plane <NUM> (ground) form a RF transmission line in which the electric and magnetic fields extends through the optical waveguides.

Sectional views of optical modulator <NUM> are shown in <FIG>. The cross section of <FIG> is positioned to show the bridge electrodes, which are not shown in <FIG>. The figure shows the spatial relationship of transmission line RF electrodes <NUM>, <NUM> with conductive plane <NUM> that functions as a ground plane. In general, the airgap between transmission line RF electrodes <NUM>, <NUM> and the top of the submount can be on the order of <NUM> of microns, and the thickness of the submount can be on the order of <NUM> microns. An alternative embodiment is shown in <FIG>. In this embodiment, substrate <NUM> of the submount comprises a conductive plane <NUM> within the substrate structure rather than on a surface, as shown in <FIG>. With such an alternative embodiment, the distance from the conductive plane to the transmission line can be designed to achieve desired RF performance without fixing the thickness of the submount. Also, the conductive plane can be on the top surface of the submount across an airgap from the RF transmission line, although it is generally desirable for the conductive plane to be a further distance from the RF line to provide for managed RF mode confinement and control of impedance in the RF line.

Generally, for these MZI based optical modulators, the optical waveguides can be semiconductor based materials. A basic exemplified structure of a semiconductor optical waveguide is shown in <FIG>, with both arms of the MZI depicted. Referring to <FIG>, optical structure <NUM> comprises first optical waveguide <NUM>, second optical waveguide <NUM>, semiconductor support <NUM> and base support layer <NUM>. First optical waveguide <NUM> and second optical waveguide <NUM> generally each comprise a p-i-n diode structure or more generally a c-n-c (conductive-nonconductive-conductive) structure, although additional layers and/or sublayers within the depicted layers can be included to provide desired performance. In the present context of semiconductor materials, as would be recognized by a person of ordinary skill in the art, a non-conductive material would not be absolutely non-conductive, but it would have a very significant electrical resistance, and conductive materials would not necessarily be as conductive as a metal, but in a relative sense the conductive materials would generally have a conductivity at least a factor of ten greater conductivity than the non-conductive regions and in many embodiments at least a factor of a hundred greater conductivity and in further embodiments at least a factor of <NUM> greater conductivity. Specifically, in the depicted embodiments first optical waveguide <NUM> can comprise electrical contact <NUM> (connected to a bridge electrode), doped (such as p-doped) or conducting layer <NUM>, intrinsic or non-conducting layer <NUM> and doped (such as n-doped) or conducting layer <NUM>, and second optical waveguide <NUM> can comprise electrical contact <NUM>, doped (such as p-doped) or conducting layer <NUM>, intrinsic layer <NUM> and doped (such as n-doped) or conducting layer <NUM>. First optical waveguide <NUM> and second optical waveguide <NUM> can be spaced sufficiently to reduce any optical interactions to a desired level, and a person of ordinary skill in the art can evaluate appropriate distances based on the material in the optical waveguides. Semiconductor support <NUM> generally can comprise a doped or conducting semiconductor layer, and base support layer <NUM> can comprise an intrinsic or non-conducting semiconductor layer. Electrical contacts <NUM>, <NUM> generally comprise metal layers, such as silver, gold, platinum or copper, and as indicated in <FIG> and shown more explicitly in reference to <FIG> below, the electrical contacts can only be associated with portions of the optical waveguides. For the other portions of the waveguide, the structure can be the same without the electrical contact layer. Suitable semiconductors or other optical-waveguide substrates include, for example, InP, GaAs, LiNbO<NUM>, Si or other suitable material. Suitable dopants to generate conducting semiconductors generally include Si, S or P as an n dopant and Zn or B as a p dopant as well as other suitable dopant elements. A substrate can support the optical waveguide structure, such as a silicon support. Base support layer <NUM> can have a thickness of <NUM> to <NUM> of microns to provide a desirable level of support. The remaining layers are generally on the order of <NUM> nanometers (nm) to <NUM> microns. Also, there can be additional semiconductor layers or sublayers within a specific optical waveguide design. InP based modulators offer the possibility of a compact modulator format, but low impedance in conventional designs can limit the interaction lengths between electrical and optical waves. An approach to address these issues are described in <NPL>,. Because the position of the submount ground plane influences the impedance of the RF transmission line mode, the modulator designs herein offer an effective solution to these concerns with a convenient processing approach suitable for commercial manufacturing.

Sequentially expanded views of a fragment of an optical chip are shown in <FIG>. In these views, the two arms of an MZI are shown with two corresponding transmission line RF electrodes with corresponding bridges to the electrical contacts located on the optical waveguides. Referring to <FIG>, optical chip <NUM> comprises a substrate <NUM>, RF electrodes <NUM>, <NUM>, optical waveguides <NUM>, <NUM>, and two sets of electrode bridges <NUM>, <NUM>, which are generally formed from appropriately patterned metal films. Referring to the further expanded view in <FIG>, the connection of electrical bridges <NUM>, <NUM> to electrical contacts <NUM>, <NUM> associated with respective optical waveguides <NUM>, <NUM>. Electrical contacts <NUM>, <NUM> cover selected portions of optical waveguides <NUM>, <NUM>. Electrical contacts correspond with electrical contacts <NUM>, <NUM> in the structure of <FIG>. Referring to the further expanded view of <FIG>, a doped or conducting substrate <NUM> and base substrate <NUM> can be seen in association with optical waveguides <NUM>, <NUM>. Doped or conducting substrate <NUM> and base substrate <NUM> correspond respectively with semiconductor support <NUM> and base support layer <NUM> in the structure of <FIG>.

The larger integration of the components of a modulator <NUM> are shown for an example embodiment in <FIG> with the optical chip substrate and any overlapping elements being transparent to show the interface of the optical chip with the submount in an assembled configuration. As a result of this depiction, the optical chip and submount are not readily distinguishable in this view, although they are distinguishable in other figures. Referring to <FIG>, optical waveguides <NUM>, <NUM> connect with couplers/splitters <NUM>, <NUM> at their ends. An input waveguide <NUM> of optical coupler/splitter <NUM> is optically connected to an optical connector <NUM> at the edge of modulator <NUM> to provide for attachment to an optical fiber or an optical waveguide of another device, such as a laser structure. In some embodiments, input waveguide <NUM> of coupler/splitter <NUM> can interface with additional integrated optical elements on the optical chip as an alternative to directly interfacing with an optical connector. Optical coupler/splitter <NUM> interfaces with an output waveguide <NUM>.

Transmission line RF electrodes <NUM>, <NUM> connect respectively with optical waveguides <NUM>, <NUM> through bridge electrodes <NUM>, <NUM>. Transmission line RF electrode <NUM> connects with conductive pads <NUM>, <NUM>, and conductive pads <NUM>, <NUM> are in electrical contact with, respectively, conductive elements <NUM>, <NUM> that terminate at conductive pads <NUM>, <NUM>. Transmission line RF electrode <NUM> connects with conductive pads <NUM>, <NUM>, and conductive pads <NUM>, <NUM> are in electrical contact with, respectively, conductive elements <NUM>, <NUM> that terminate at conductive pads <NUM>, <NUM>. Conductive elements <NUM>, <NUM>, <NUM>, <NUM> are supported by the submount. Conductive pads <NUM>, <NUM>, <NUM>, <NUM> connect electrodes along the optical waveguides on the optical chip with electrodes supported on the submount. The connection pads associated with conductive elements <NUM>, <NUM>, <NUM>, <NUM> can be wired or otherwise electrically connected to the RF generator associated with providing the modulation signals and a suitable impedance matched RF termination network.

For use in present state of the art coherent optical communication, it can be desirable to group <NUM> MZI based modulator elements in association with each other, although smaller numbers of MZI based modulators such as one, two or three, can be integrated in a single optical chip for transmission of coherent or non-coherent communication signals based on further integrated structures. As used in the art, coherent refers to optical communication signals that carry information in both the amplitude and phase of the optical field and its polarization. For these signals, it is typical that two orthogonal polarization states of the optical signal are independently modulated, each according to the two degrees of optical phase and amplitude, and then recombined for transmission, which together can be referred to as polarization multiplexing (PM). Applying different schemes of electrical modulation to the embedded MZIs can generate various standard forms of coherent optical constellations to encode the data transmission. For instance, if the optical output for each polarization is at a constant amplitude and modulated to one of four optical phases separated by <NUM>-degrees, this is conventionally referred to as "quadrature phase-shift keying" (QPSK). When used this way, the four-MZI configuration described provides "PM-QPSK" coherent optical transmission. This provides four binary 'bits' of information in each transmitted optical symbol (two polarizations times two bits - any of four possible phases - per polarization). Alternatively, it is possible to vary both the phase and amplitude of each transmitted polarization through modulation. This is conventionally called "nQAM" (n quadrature-amplitude modulation, where 'n' is the number of phase-amplitude states in the allowed palette). Typically desired modulation patterns include 16QAM (<NUM> bits per polarization) and 64QAM (<NUM> bits per polarization). The <NUM>-MZI configuration group described herein is generally optimal for supporting transmission of any of the existing coherent transmission formats (e.g. PM-QPSK, PM-16QAM) or emerging formats (e.g. PM-64QAM, or other). Although not a widely-adopted terminology at the time of the present application, all these and other suitable coherent formats can be gathered under the readily-recognized term 'PM-IQ' (polarization multiplexed; in-phase/quadrature amplitudes). The <NUM>-MZI embodiments described as example herein may therefore be referred to as PM-IQ modulators. Of course, larger groups of MZI based modulators can be packaged together, optionally in multiples of <NUM>, to for instance to provide PM-IQ modulation of multiple independent optical wavelengths. To support a more specific discussion of packaging, for convenience, the immediately following discussion focuses on modulator structures with <NUM> MZI based modulator elements.

For assembly, the optical chip is formed using appropriate patterning and layer build up processing, such as CVD and photolithography. For the application of metal electrodes, sputtering, other chemical or physical vapor deposition approaches, conductive paste that can be cured for metal film formation, or the like can be used. The optical components can be formed, for example, on a smooth InP or silicon wafer and then diced to form the individual components. The metal electrodes are added along with the bridge electrodes and electrical contacts on the optical waveguide through bridge electrodes supported on appropriate support structure, as part of the RF transmission line and provide connection points to the submount. The submount can be a printed circuit board, flex-circuit, ceramic, metal layer stack or other similar structure that provides a desired structure with an insulating substrate and designed electrical connections. If a commercial printed circuit board, flex-circuit, ceramic or other appropriate structure has an appropriate thickness, a conductive layer can be placed along its surface distal to the optical chip as a ground plane. Alternatively a submount can be constructed with an appropriately elsewhere positioned conductive plane as a ground, such as within the submount, as described above, or along the proximal surface across an air gap from the optical chip. The optical chip can be inverted and placed onto the submount. The submount can comprise insulating support elements to support at least portions of the optical chip in a spaced away relationship from the surface of the submount. Support elements can be, for example, glass posts, ceramic columns, or the like. The electrical connections with the optical chip electrodes can be made by wire bonding, but in some embodiments appropriate assembly can be performed using mated bonding pads on the submount so that positioning of the optical chip with the submount aligns the bonding pads on each that can then be connected, such as with reflow of solder. In some embodiments, conductive metal bonding balls or pads can have appropriate dimensions to support the optical chip on the submount, which can then replace insulating posts or the like. Since wire bonding balls would be placed at suitable locations, there is no concern that they are conductive with no corresponding insulating structures between the elements. Other suitable processing approaches can be used.

Referring to <FIG>, a fragmentary view of a modulator <NUM> is shown with an optical chip <NUM> supported on a submount <NUM>. Optical chip <NUM> has <NUM> MZI structures <NUM>, <NUM>, <NUM>, <NUM> mounted on the chip to modulate a coherent optical signal that results following the combining of the <NUM> separately modulated polarizations and amplitude-phase states from each MZI modulator component. Glass posts or wirebond balls <NUM> support optical chip <NUM> mounted on submount <NUM>. As shown in <FIG>, pairs of optical waveguides <NUM>, <NUM>, <NUM>, <NUM> associated with each MZI structure <NUM>, <NUM>, <NUM>, <NUM>, respectively, pass through modulation zones between transmission line RF electrodes connected to electrical contacts on the optical waveguides and continue on both ends of the modulation zones. As suggested in <FIG>, the two arms of each pair are directed on both ends to an optical combiner/splitter, and also then the four optical signals from the <NUM> MZI elements can be combined with an optical combiner to form a modulated output optical signal for transition. Each transmission line RF electrode is connected to a bond pad at or near each end, which electrically connects to electrodes <NUM> on submount <NUM>. In this embodiment, a conduction plane <NUM> is shown located on the lower surface of submount <NUM>, although locations within submount <NUM> can be equally effective. Submount <NUM> directly or indirectly provides electrical connection with an RF generator that provides desired modulation for data transmission. Referring to <FIG>, an alternative view of modulator <NUM> is shown with portions of the optical chip removed.

A schematic layout of a modulator as described herein is shown in <FIG> as a portion of an optical telecommunications transmitter. A laser <NUM>, such as a semiconductor diode laser or laser array, directs light output to a light channel <NUM>, such as a waveguide or optical fiber, which interfaces with modulator <NUM>. Through appropriate optical elements, modulator <NUM> ultimately interfaces with an optical fiber <NUM> that is part of an optical telecommunications network for transmission generally to a location remote from modulator <NUM>. Modulator <NUM> generally is also electrically connected to an RF generator <NUM> through appropriate electrical connections <NUM> that have an electrical termination <NUM>. Modulator <NUM> generally can have the various designs described herein to provide desirable modulator function. The connections of modulator <NUM> with the input and output components noted in <FIG> can use planar components, free space components or combinations thereof along with appropriate connectors, such as those known in the art to connect planar lightwave circuit to each other and/or optical fibers to planar lightwave circuits.

Referring to <FIG>, the layout is shown of a coherent optical chip <NUM> for a corresponding modulator. Coherent optical chip <NUM> comprises an input waveguide <NUM> connected to an optical splitter <NUM>, which is connected by split waveguides <NUM>, <NUM> to a first coupled pair of modulated Mach-Zehnder interferometers (MZI) <NUM> and a second coupled pair of modulated MZI <NUM>. The coupled pairs of modulated Mach-Zehnder interferometers can be referred to as I-Q pairs based on terminology in the coherent optical telecommunications art. First coupled pair of modulated MZI <NUM> comprises optical splitter <NUM>, first MZI <NUM>, second MZI <NUM>, input split waveguides <NUM>, <NUM> connecting optical splitter <NUM> with first MZI <NUM> and second MZI <NUM>, optical coupler <NUM>, output split waveguides <NUM>, <NUM> connecting optical coupler <NUM> with first MZI <NUM> and second MZI <NUM>, and X output waveguide <NUM>. First MZI <NUM> comprises optical splitter <NUM>, first MZI arm <NUM>, second MZI arm <NUM> and optical coupler <NUM>. RF electrodes <NUM>, <NUM> interface respectively with first MZI arm <NUM> and second MZI arm <NUM> with appropriate electrical connections as described herein. Similarly, second MZI <NUM> comprises optical splitter <NUM>, first MZI arm <NUM>, second MZI arm <NUM> and optical coupler <NUM>. RF electrodes <NUM>, <NUM> interface respectively with first MZI arm <NUM> and second MZI arm <NUM> with appropriate electrical connections as described herein. Second coupled pair of modulated MZI <NUM> comprises optical splitter <NUM>, third MZI <NUM>, fourth MZI <NUM>, input split waveguides <NUM>, <NUM> connecting optical splitter <NUM> with third MZI <NUM> and fourth MZI <NUM>, optical coupler <NUM>, output split waveguides <NUM>, <NUM> connecting optical coupler <NUM> with third MZI <NUM> and fourth MZI <NUM>, and Y output waveguide <NUM>. Third MZI <NUM> and fourth MZI <NUM> comprise similar structure as first MZI <NUM> and second MZI <NUM> which is not labeled in the drawing to simplify the drawing. While optical chip <NUM> is described as a single planar optical structure, in some embodiments, the functions described for optical chip <NUM> can be divided in three or more planar optical structures <NUM>, <NUM>, <NUM> divided by the dashed lines in <FIG> that have suitable optical coupling connecting them. Optical structure <NUM> can be an optical chip with semiconductor waveguides forming the MZI while optical structures <NUM>, <NUM> can be, for example, optical chips comprising silica glass waveguides forming splitters and combiners and associated optical components.

To introduce the polarization dependence, a polarizer and polarization beam combiner can be used based on the X output signal and the Y output signal. Referring to <FIG>, X output waveguide <NUM> interfaces with a <NUM> degree polarizer <NUM>, such as a waveplate, that rotates the polarization for this signal. Polarized waveguide <NUM> from polarizer <NUM> is directed to polarization beam combiner <NUM> that is also connected to Y output waveguide <NUM>. Polarization beam combiner <NUM> directs output to polarization multiplexed output waveguide <NUM>. The structure shown in <FIG> performs multiplexing of the optical signal by combining the resulting optical signal in orthogonal phase states and polarization states. Polarization beam combiners are described, for example, in <CIT>, entitled "Polarization Beam Combiner/Splitter, Polarization Beam Combining/Splitting Structure, Light Mixer, Optical Modulator Module, and Method for Manufacturing Polarization Beam Combiner/Splitter,".

Claim 1:
An optical modulator (<NUM>, <NUM>) comprising a submount (<NUM>, <NUM>), electrical conduction pathways designed to carry RF signals, and an optical chip (<NUM>, <NUM>, <NUM>, <NUM>) comprising a substrate (<NUM>, <NUM>) and two semiconductor optical waveguides (<NUM>, <NUM>, <NUM>, <NUM>) with electrical conductive elements along at least a portion of the optical waveguide surface, wherein the optical chip is attached to the submount with the optical chip substrate oriented away from the submount and the two semiconductor optical waveguides oriented toward the submount, wherein the submount comprises a conducting plane (<NUM>, <NUM>, <NUM>) offset in a distinct plane from the conductive RF electrodes of the optical chip, wherein the optical chip further comprises two conductive RF electrodes (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) adjacent respectively to corresponding optical waveguides and additional conductive elements (<NUM>, <NUM>, <NUM>, <NUM>) connecting one of the conductive RF electrodes to the corresponding conductive element along a surface of the corresponding semiconductor waveguide, wherein the electrical conduction pathways electrically connect to the conductive RF electrodes of the optical chip, and wherein no lateral ground plane associated with the RF electrodes (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is present in the optical chip.