Patent Description:
Integrated photonic systems such as, e.g., PIC-implemented optical transceivers, often include one or more photonic (or, synonymously, optical) switches to selectively change, for a given wavelength of an optical signal, the signal power at an output port between a high-transmission, low-attenuation state (herein also simply "transmission state" or "on state") and a low-transmission, high-attenuation state (herein also "attenuation state" or "off") state"). The photonic switch may be implemented, for instance, using an MZI that includes, in one or both of its interferometer arms, an electronically controlled phase tuner adjustable to cause constructive or destructive interference at the output port. Photonic switches are typically maintained in one switch state (e.g., the attenuation state) over the majority of their operational life, and transition to the other state (e.g., the transmission state) only for brief periods. To minimize power consumption, it is therefore desirable to "align" the passive, unpowered operational mode of the switch with the predominant switch state. Further, optical transceivers and other photonic systems often use optical filters implemented by asymmetric MZIs (AMZIs) to align the wavelength of a tunable laser to a desired operational wavelength. The AMZI has an optical path-length difference between its interferometer arms that is designed to achieve maximum transmission (minimum loss) at the operational wavelength, thereby serving as a wavelength reference. In some cases, an AMZI including a phase tuner acts both as a switch and a filter, switch between "on" and "off" states and also providing a wavelength reference in the "on" state.

In practice, fabrication variations often result in misalignment of optical switches and filters, requiring the device to be actively powered over the duration of its operational life for high accuracy in maintaining high or low transmission at the operational wavelength. The need for continuous active powering increases control complexity and power consumption. This problem becomes significant for PICs with large numbers of switches or filters, such as, e.g., optical transceiver PICs with high lane counts, especially those with built-in self-test functionality, which may include five or more tunable switches per lane.

Document <CIT> discloses a technology aiming at improving output light intensity and at improving an extinction ratio, when a mechanism is used, where transmitted light is theoretically attenuated and a transmission loss is generated in a part applied as a phase controller. This document discloses an optical functional element of a Mach-Zehnder type interferometer including an input optical waveguide, a branching section, a first optical path, a second optical path <NUM>, an optical modulation section disposed in the second optical path, a multiplexing section connected to the first optical path and the second optical path, and output waveguides for receiving multiplexed light from the multiplexing section. The light propagation loss in the second optical path is made larger than that in the first optical path resulting from the optical modulation section, the branching section has a fixed non-equivalent branch ratio, and light intensity incident into the second optical path is made higher than light intensity incident into the first optical path.

Document <CIT> discloses various configurations of optical switches. Various embodiments aim at reducing or at entirely eliminating crosstalk using a coupler that has a power-splitting ratio that compensates for amplitude imbalance caused by phase modulator attenuation. Some embodiments implement a plurality of phase modulators and couplers as part of a dilated switch network, aiming at increasing overall bandwidth and further at reducing potential for crosstalk.

Document <CIT> discloses in one embodiment, an optical directional coupler including an input terminal configured to receive an input optical signal and a first coupler optically coupled to the input terminal, where the first coupler has a first coupling length, and where the first coupler is configured to couple a first portion of the input optical signal to a first optical leg and a second optical portion of the input optical signal to a second optical leg. The optical directional coupler also includes the first optical leg, where the first optical leg is configured to phase shift the first portion of the optical signal to produce a first phase shift signal and the second optical leg, where the second optical leg is configured to phase shift the second portion of the optical signal to produce a second phase shift signal, and where the first phase shift signal has a phase difference relative to the second phase shift signal. Additionally, the optical directional coupler includes a second coupler configured to receive the first phase shift signal and the second phase shift signal and to output a first output optical signal to a first output, where the second coupler has a second coupling length, where a crosstalk of the input optical signal to a second output is below -<NUM> dB for both transverse electrical polarized light and transverse magnetic polarized over a wavelength range spanning <NUM>.

Document <CIT> discloses an external modulation being accomplished in a dual waveguide device wherein substantially identical input optical beams are supplied to the waveguides and wherein each waveguide through its electrode is subject to individual, mutually exclusive control. Modulation signals are applied to each waveguide via its separate electrode. Control signals are applied to each waveguide for adjusting the modulation chirp parameter to a desired fixed, non-zero value. Typically, the desired value of the chirp parameter is one which provides the lowest fiber dispersion penalty for the system. Modulated lightwave signals emerging from the waveguides are combined to form a single output signal suitable for transmission over an optical fiber. In one embodiment, Mach-Zehnder interferometer having separately controllable waveguides has its input coupled to a CW laser. Both III-V semiconductor and Ti:LiNbO<NUM> Mach-Zehnder interferometers have been utilized as external modulators in accordance with the principles of this document.

Document <CIT> discloses Mach-Zehnder interferometers comprising heater elements configured to have projections in the plane of optical waveguides positioned such that two adjacent sections of one optical waveguide arms are heated by a common heater element. The heater and at least a substantial section of the heated waveguide segments can be curved. Configurations of an optical waveguide arm can comprise an outer curved heated section, an inner curved heated section, and a loopback waveguide section connecting the outer curved heated section and the inner curved heated section, with average radius of curvature selected to form an open accessible space. Appropriate configurations of the two optical waveguide arms provide for nested configurations of the arms that provide for a compact structure for the interferometer.

In the following description of embodiments of the disclosed subject matter, reference is made to the accompanying drawings.

The invention is defined in the appended independent claim. Optional embodiments of the invention are described in the dependent claims. Described herein are various approaches to lowering power consumption in MZI-based integrated photonic switches or filters over the lifetime of the device by reducing misalignment between the unpowered operational mode of the switch or filter and the predominant switch state and/or the operational wavelength, and/or by enabling low-power compensation for any such misalignment. One approach aims at reducing the degree of phase misalignment due to refractive-index changes resulting from fabrication variations in the waveguide width of the interferometer arms by increasing the waveguide width, thereby decreasing the sensitivity of the refractive index to width variations. In this approach, wider waveguides are generally accompanied by a greater waveguide bend radius to maintain single-mode operation. Another approach addresses strain-induced refractive-index or path-length changes by matching non-waveguide layers, including, in particular, any metal structures, on both sides of a symmetry axis of the MZI. To achieve symmetry, the PIC may, in a region surrounding the MZI, include "dummy" structures, that is, structures that do not have any function, but serve merely to match functional components (e.g., a heater implementing the phase tuner) on the other side of the symmetry axis. In yet another approach, applicable to switches (with or without filter functionality), the MZI is equipped with phase tuners in both interferometer arms to allow for active phase compensation for any misalignment resulting from fabrication variations. Electronic driver circuitry that controls and supplies power to the phase tuners includes a buck converter that provides a lower drive power for small adjustments in the predominant switch state, in which the switch is intended to be operated passively, and a higher drive power for operating in the active operational mode of the switch. In the description that follows, the operational mode in which the switch is operated passively except for any slight adjustments to compensate for phase misalignment (or "nominally passively"), is, for ease of reference, referred to as the "unbiased" mode, and the operational mode in which the switch is actively powered regardless of any misalignment is also referred to as the "biased" mode.

The following detailed description of the drawings further illustrates both the problem caused by fabrication variations in optical switches and filters as well as various example embodiments addressing this problem. While the discussion focuses on switches, its application to filters will be readily apparent to those of ordinary skill in the art.

<FIG> is a conceptual diagram of an optical transceiver <NUM> including an MZI-based photonic switch <NUM> in accordance with various embodiments, illustrating the switch <NUM> in the context of an example use case. The optical transceiver <NUM> serves, e.g., within an optical communication system, to send and receive data imparted on optical signals. It includes an optical transmitter <NUM> with a laser to generate an optical carrier signal and an electronically controlled (electro-optic or electro-absorption) modulator that imparts an electronic input signal onto the laser light in the form of a phase and/or amplitude modulation to thereby generate the optical signal carrying the data to be transmitted. Optionally, the optical transceiver <NUM> may include multiple lasers emitting at multiple respective wavelengths, along with multiple associated modulators, whose outputs can be multiplexed into a multichannel optical signal. Via an output port <NUM> of the transceiver <NUM>, the optical signal may be coupled, e.g., to an outgoing optical fiber. The optical transceiver <NUM> further includes an optical receiver <NUM>, e.g., implemented with a photodetector, that receives and converts a modulated optical input signal into an electronic output signal, from which data can then be extracted.

In the context of the optical transceiver <NUM>, the photonic switch <NUM> may serve to direct the output of the optical transmitter <NUM> selectively either to the transmitter output port <NUM> during the normal operational mode (also "mission mode") or via an optical "loopback" path <NUM> to the optical receiver <NUM> in self-test mode. The self-test mode may be used, e.g., during wafer-level testing of a PIC implementing the optical transceiver <NUM> or during system-level testing of the complete transceiver package upon integration of the PIC with the associated electronics, to test the function of the optical receiver <NUM> using the built-in light source provided by the transmitter <NUM>, or, conversely, to test the function of the optical transmitter <NUM> using the built-in receiver <NUM>. In self-test mode, a functioning link from the transmitter <NUM> via the loopback path <NUM> to the receiver <NUM> with high received optical power and/or error-free data transmission indicates that both transmitter <NUM> and receiver <NUM> are working properly. (If the link is not working, subtests can be used to identify the source of the problem, e.g., employing monitor photodiodes after the laser, modulator, and through the loopback path. ) Compared with the use of external light sources for receiver testing or external receivers for transmitter testing, this self-test functionality provides time and cost savings. Since the self-test mode is used only at the start of life of the optical transceiver <NUM>, but disabled for the majority of its life, the switch <NUM> is desirably configured to be operated passively when coupling light into the output port <NUM> during mission mode, and to be actively powered only during self-test mode.

Turning now to the structure of the MZI-based photonic switch <NUM>, the switch may generally be implemented, along with the optical transmitter <NUM> and the optical receiver <NUM>, as part of a PIC in a semiconductor-on-insulator substrate, such as, e.g., a silicon-on insulator (SOI) substrate including a silicon device layer on top of a buried oxide (BOX) or other dielectric layer. While the description of various example embodiments references, specifically, silicon implementations, it is to be understood that other semiconductor material platforms may also be used. Optionally, the SOI (or other) substrate may include a cladding layer disposed on top of the device layer, which may embed various components of the switch <NUM> (e.g., the phase tuners discussed below) and the PIC at large.

The switch <NUM> generally includes an optical input coupler <NUM>, two interferometer arms <NUM>, <NUM>, and an optical output coupler <NUM> collectively forming an MZI. As shown, the MZI may be geometrically symmetric, with interferometer arms <NUM>, <NUM> being equal in length. Alternatively, the MZI may be asymmetric, having a path-length difference between the interferometer arms <NUM>, <NUM> that causes the output intensity to vary periodically with the wavelength of the light, such that the MZI switch simultaneously serves as an optical filter. In either case, the MZI includes, in at least one of the interferometer arms <NUM>, a phase tuner <NUM> that allows adjusting the relative phase between the signals interfering at the output coupler <NUM>. The interferometer arms <NUM>, <NUM> may be implemented by optical waveguides (e.g., rib waveguides) formed in the (e.g., silicon) device layer of the PIC substrate. The phase tuner <NUM> may generally be any device adapted to change the refractive index in the waveguide <NUM> in a controllable manner, e.g., electro-optically or thermo-optically. For example, in some embodiments, a thermal phase tuner implemented by a controllable-power resistive heater placed adjacent (e.g., above or next to) the waveguide <NUM>, is used. In other embodiments, the phase tuner <NUM> includes a PN or PiN junction formed in the waveguide, e.g., having a U shape formed by a two-dimensional doping concentration profile in the waveguide cross section.

The input and output couplers <NUM>, <NUM> may be implemented, for instance, by rectangular multi-mode interferometers (MMIs) each configured as a 2x2 couplers with two input ports and two output ports, as shown. At the input coupler <NUM>, one of the input ports (e.g., port <NUM>) may receive the optical signal from the optical transmitter <NUM>; the other input port (e.g., port <NUM>) may remain unused. At the output coupler <NUM>, one of the output ports (e.g., port <NUM>) may serve to provide the optical signal to the transmitter output port <NUM>, whereas the other output port (e.g., port <NUM>) couples the optical signal into the loopback path <NUM>. The phase tuner <NUM> can be operated to switch the output signal between these two output ports <NUM>, <NUM>. In some embodiments, the switch, when including an AMZI, doubles as a multiplexer, combining two optical signals of different wavelengths received at the two input ports <NUM>, <NUM> into a multiplexed output signal at the output ports <NUM> (in unbiased mode), <NUM> (in biased mode).

Note that, even if only one of the two input ports <NUM>, <NUM> is utilized, implementing the input coupler <NUM> in a symmetric MZI as a 2x2 coupler is beneficial in that it allows achieving the "on" state at port <NUM> and the "off state" at port <NUM> at zero power (whereas, with a symmetric MZI having a 1x2 input coupler and a 2x2 output coupler, π/<NUM> or 3π/<NUM> tuning would be used in this state). In an AMZI-based optical switch/filter, on the other hand, a 1x2 input coupler may be used in conjunction with a 2x2 output coupler, as the phase offset associated with the input coupler can, in this case, be compensated for by the path difference in the AMZI arms. In general, alternatively to MMIs, other types of couplers, such as directional couplers, evanescent waveguide couplers, or waveguide Y-junctions (where 1x2 couplers are adequate) may also be used for the input and output coupler <NUM>, <NUM>.

MZI-based photonic switches <NUM>, although described above with reference to an example use for switching between mission and test modes, can also serve various other purposes. For example, photonic switches are often used as variable optical attenuators in the receiver path of an optical transceiver, where they are ordinarily kept in a high-transmission state, but occasionally set to low transmission to attenuate high input optical power to prevent damage to the receiver. In another example, photonic switches are used in the loopback path <NUM> in the low-transmission state during mission mode to prevent transmitter light from reaching the receiver <NUM>; only during self-test is the loopback path <NUM> enabled and the loopback switch is changed to a high-transmission state. As these examples illustrate, the predominant switch state (for which the switch is configured to operate in the unbiased mode) may correspond to the transmission state or the attenuation state of the switch, depending on the particular application. Note also that, if a switch with two output ports is used, high transmission at one port corresponds to low transmission at the other port. Further, in the loopback path <NUM>, as well as in other applications where the light is either transmitted or attenuated, but not switched between ports, the input coupler need not include two input ports, and the output coupler need not include two output ports. Rather, a single input port at the input coupler and a single output port at the output coupler may suffice, allowing for the use of, e.g., waveguide Y-junctions or 1x2 MMI couplers. The same holds for AMZIs implementing optical filters, which may have a single output port transmitting light only at a specified wavelength within a filter period.

<FIG> are graphs of the optical transmission of an MZI-based photonic switch <NUM> as a function of the relative phase shift between the interferometer arms <NUM>, <NUM> for an example target design as well as for example deviations from the target design due to fabrication variations. The phase shift is given in radians along the abscissa, and the relative transmission is indicated on a logarithmic scale (in decibel) along the ordinate. The depicted graph is for an output port of the photonic switch that is intended to be in attenuation state in the predominant operational mode of the switch, such as, e.g., the port <NUM> at the loopback path <NUM> of switch <NUM> in the example of <FIG>. For such a port, the transmission curve <NUM> for the target design, which corresponds to passive switch operation, exhibits a transmission minimum <NUM> at a zero phase shift between the interferometer arms <NUM>, <NUM>; at this minimum <NUM>, the optical signal input to the switch is attenuated to about -<NUM> dB (which is about <NUM>% of the input intensity). The residual transmitted light is a consequence of a power imbalance of about <NUM>% between the interferometer arms <NUM>, <NUM> (with power coupling of <NUM>% and <NUM>%, rather than the nominal <NUM>%) that is typical of optical couplers in practice.

Fabrication variations can cause the transmission minimum to shift to positive or negative phase offsets. If the transmission minimum shifts to a relative phase shift of ±π/<NUM>, as indicated by curves <NUM>, <NUM>, the attenuation at phase shift of zero will be only about -<NUM> dB (which is about <NUM>% of the input intensity). In other words, the amount of light that is still transmitted in the attenuation (or "off') state has increased by a factor of about <NUM> as a result of the fabrication variation. Such poor blocking of light in the "off' state may not be tolerable. For example, with a switch <NUM> used, as shown in <FIG>, to switch an optical transceiver <NUM> between self-test and mission modes, designed to transmit all light to the transmitter output port <NUM> at zero relative phase shift and operate in the "off' state for the loopback port <NUM>, residual transmission of <NUM>% of the optical power into the loopback path <NUM> might interfere with the operation of, or even cause damage to, the optical receiver <NUM>. To avoid this problem, the optical switch <NUM> may need to be actively powered even in the "off' state. The following drawings illustrate various approaches to lower the transmission in the "off" state sufficiently to avoid a need for active powering, or at least lower the power requirements associated with the "off' state. As will be appreciated by those of ordinary skill in the art given the benefit of this specification, the same principles can be straightforwardly applied to maximize the transmission in the "on" state of a photonic switch or a filter. Indeed, in a switch with two output ports, minimizing transmission in one port inherently maximizes transmission in the other port.

<FIG> is a graph of the effective refractive index of an example rib waveguide as a function of width. Rib waveguides, which often serve as the interferometer arms <NUM>, <NUM> of the MZI-bases switch <NUM>, are created by partially etching the (silicon) device layer to form a waveguide with rectangular cross section on top of a wider (silicon) slab. The effective refractive index experienced by a fundamental optical mode in a rib waveguide depends, in addition to the bulk refractive index of the waveguide, on the precise waveguide geometry and the refractive index of the surrounding cladding. For example, for a silicon-dioxide-clad silicon rib waveguide having a thickness of about <NUM>, the refractive index of the fundamental mode at a wavelength of <NUM> is around <NUM>, but varies slightly as a function of rib width. As shown in <FIG>, for waveguide widths ranging from <NUM> to <NUM>, the effective refractive index may vary between <NUM> and <NUM>. The sensitivity of the effective refractive index to variations is greater for smaller widths. Thus, a fabrication variation of, for instance, ± <NUM> in waveguide width will change the effective refractive index at a width of <NUM> by only about <NUM>, whereas the same variation at a waveguide width of <NUM> changes the refractive index by about <NUM>. A wider waveguide can tolerate greater width variation between the interferometer arms <NUM>, <NUM> without creating an excessive refractive index difference, and thus phase shift, between them. In some embodiments, this relationship between waveguide width and sensitivity to fabrication variation is exploited to reduce phase misalignment due to variations in waveguide width by designing the switch with wider waveguides.

<FIG> are schematic top views of example MZI-based optical switches or filters <NUM>, <NUM>, contrasting a conventional waveguide configuration (switch <NUM>) with a more fabrication-tolerant waveguide configuration (switch <NUM>) in accordance with one or more embodiments. Both switches <NUM>, <NUM> are generally similar to the photonic switch <NUM> depicted in <FIG> in that they each include two geometrically symmetric waveguides <NUM>, <NUM> and <NUM>, <NUM>, respectively, coupled between an optical input coupler <NUM> and an optical output coupler <NUM>. The two switches <NUM>, <NUM> differ, however, in the dimensions of their waveguides <NUM>, <NUM> and <NUM>, <NUM>. The waveguides <NUM>, <NUM>, <NUM>, <NUM> may all be rib waveguides. In the conventional configuration, rib widths, e.g., on the order of <NUM> are often used; this small width serves to accommodate a small bending radius to rapidly separate the two waveguide arms <NUM>, <NUM> of the switch for thermal isolation between the two sides. As shown, the waveguides <NUM>, <NUM> of the fabrication-tolerant switch <NUM> are significantly wider, e.g., by a factor of at least <NUM>, than the waveguides <NUM>, <NUM> in the conventional switch <NUM>. In some embodiments, the waveguide width is increased to greater than <NUM>. This increased width achieves lower sensitivity to fabrication variations, as described above. However, the optical waveguide width is generally limited, in the embodiments contemplated herein, by the desire to guide only the fundamental optical mode; once the waveguide width exceeds a certain limit, additional spatial optical modes are launched, resulting in interference between modes, which reduces signal quality and, thus, adds a penalty to optical data transmission.

This undesirable effect of the increased waveguide width can be counteracted by simultaneously increasing the bend radius along the waveguides. In various embodiments, therefore, the smallest bend radius of the waveguides <NUM>, <NUM> in the fabrication-tolerant switch <NUM> is increased (to radius R<NUM>), relative to the smallest bend radius along the waveguides <NUM>, <NUM> of the conventional switch <NUM> (radius R<NUM>), to allow for the greater waveguide width while maintaining single-mode operation. In some embodiments, the smallest bend radius R<NUM> is at least <NUM>. To keep, despite the increase in bend radius, the length of the MZI (corresponding to the distance between the input and output couplers <NUM>, <NUM>) the same, the two waveguides <NUM>, <NUM> are brought closer to each other, and the bend angles that they undergo in the curved waveguide sections <NUM> that connect the parallel, straight center sections <NUM> to the input and output coupler <NUM>, <NUM>, respectively, are decreased.

To illustrate: in the depicted switches <NUM>, <NUM>, each of the curved waveguide sections (<NUM> in <FIG>) connecting a straight center waveguide section (<NUM> in <FIG>) to either the input coupler <NUM> or the output coupler <NUM> has an approximate S-shape rotationally symmetric about an inflection point. In the example conventional configuration shown in <FIG>, each waveguide <NUM>, <NUM> incurs a <NUM>° bend angle <NUM> between the output of the input coupler <NUM> and the inflection point <NUM> (drawn in only for waveguide <NUM>), and then another <NUM>° bend angle in the other direction from the inflection point <NUM> to the straight waveguide section, where the waveguides <NUM>, <NUM> are again parallel. Similarly, the waveguides <NUM>, <NUM> each incur two <NUM>° bend angles in opposite directions between the straight waveguide sections and the input to the output coupler <NUM>. In the fabrication-tolerant configuration shown in <FIG>, by contrast, the bend angle <NUM> from the input coupler to the inflection points <NUM>, from the inflection point <NUM> to the straight waveguide section <NUM>, from the straight waveguide section <NUM> to the inflection point <NUM>, and from the inflection point <NUM> to the output coupler <NUM> are each only about <NUM>°. In some embodiments, the bend angle is kept even lower, e.g., to less than <NUM>°. As a result of the decreased bend angle, the separation distance d<NUM> between the waveguide <NUM>, <NUM> has been significantly reduced. For example, whereas the separation distance d<NUM> between the waveguide <NUM>, <NUM> in the conventional switch <NUM> may be between <NUM> and <NUM> or even greater, the separation distance d<NUM> between the wider waveguide <NUM>, <NUM> with increased bend radius may be reduced to less than <NUM>, incidentally to changing the bend radius.

The specific values of various angles and dimensions mentioned above are provided to illustrate by way of example, but not limitation, how a given MZI-based switch design can be modified to achieve greater fabrication tolerance via greater wavelength widths while maintaining single-mode operation and avoiding an undesirable increase in the footprint of the switch. Those of ordinary skill in the art will know how to apply the general principles to any given MZI-based switch configuration taken as a starting point. It is noted that bringing the two waveguide arms of the MZI too close together can lower the thermal tuning efficiency of a heater-based phase tuner <NUM> because the closer the waveguides, the greater will be the effect of dissipating heat on the non-heated waveguide. With thermo-optic phase tuners, therefore, the proposed approach to increasing fabrication tolerance is counterintuitive, and presents a tradeoff between thermal tuning efficiency and increased fabrication tolerance. However, the improved fabrication tolerances reduce the mission-mode power consumption, whereas the closer MZI arm spacing increase self-test power consumption, which is far more power-tolerant. In various embodiments, for a given waveguide path as characterized by the distance between input and output couplers <NUM>, <NUM>, the waveguide separation (which is kept at least large enough to achieve acceptable tuning efficiency), and the bend angles, the waveguide width is chosen to be as large as possible consistent with single-mode waveguiding at the operational wavelength, or at least greater than <NUM>%, preferably greater than <NUM>%, of that maximum width for single-mode operation. In various example embodiments, the switch or filter has a total length from input to output of less than <NUM>, and a separation between the straight waveguide sections of the interferometer arms between about <NUM> and about <NUM>. The smallest bend radius of the arms may be between <NUM> and <NUM>, and the waveguide widths may be between <NUM> and <NUM>.

Apart from variability in waveguide width, different stresses acting on the waveguide arms <NUM>, <NUM> of the MZI due to non-uniformity in the surrounding structures can change the refractive indices and/or path lengths of the waveguides <NUM>, <NUM>, causing or contributing to phase misalignment during passive switch operation. In various embodiments, this problem is addressed by designing a portion of the PIC surrounding the MZI-based switch to be symmetric about a geometric symmetry axis of the switch.

<FIG> are schematic top views of example MZI-based optical switches and the surrounding non-waveguiding layers, contrasting an asymmetric conventional configuration <NUM> with a more fabrication-tolerant symmetric configuration <NUM> in accordance with one or more embodiments. In addition to the silicon structures of the MZI-based switch, which include the input and output couplers <NUM>, <NUM> and the waveguides <NUM>, <NUM>, <FIG> also illustrate various metal structures associated with the MZI. The metal structures include a heating element <NUM> disposed above one of the waveguide arms <NUM>, along with one or more layers of redistribution metal <NUM> and vertical metal vias <NUM> embedded in the cladding as well as metal bumps <NUM> (for solder bumps) collectively providing the metal connections for applying a current through the heating element <NUM>. If, in lieu of a heater, another type of electronically controlled phase tuner (e.g., an electrically driven PN junction) is used, the MZ may include similar metal structures. In addition, metal structures or other embedded structures (e.g., III-V layers) associated with devices other than the MZI may extend, in some cases, into a region circumscribing the MZI. All these structures can subject the waveguides <NUM>, <NUM> to strain, thereby affecting their relative phase in the passive operational mode. To balance out such strain effects, any structures that may affect the optical path length of one waveguide are duplicated for the other waveguide, rendering a region surrounding the MZI structurally symmetric.

In more detail, with reference to <FIG>, a symmetry region <NUM> centered about a geometric axis <NUM> of the MZI that extends between the waveguides <NUM>, <NUM> and through the input coupler <NUM> and the output coupler <NUM> is defined. The symmetry region <NUM> may, for instance, be chosen to be rectangular in shape, and is generally sized to fully contain the MZI including input and output couplers <NUM>, <NUM> as well as the structural components of the phase tuner <NUM>. Inside the symmetry region <NUM>, the device layer and cladding layer of the PIC, including any embedded structures, are made symmetric about the axis <NUM>. For example, mirroring the heating element <NUM> and associated metal connections in one waveguide arm <NUM>, a second heating element <NUM>, along with all redistribution metal <NUM>, vertical metal vias <NUM>, and metal bumps <NUM> is added to the other waveguide arm <NUM>, regardless whether that second heater is used or not. In some embodiments, both heaters are used for phase adjustments between the two waveguide arms <NUM>, <NUM> (e.g., as discussed below with reference to <FIG> and <FIG>), but, in other embodiments, only one of the heaters is ultimately used. In general, any functional device component or structure on one side of the axis <NUM> may be mirrored by a structurally substantially identical, but non-functional "dummy" component.

In PICs, regions that are not patterned for device structures are often patterned with a "dummy" silicon or metal fill that has no device function, but serves to achieve a desired density requirement, e.g., to render the silicon density across the silicon device layer or across any metal layers as homogenous as possible. The fill pattern may, for instance, include a regular array of silicon islands sized and spaced to match the average density of areas containing the waveguides or other silicon device structures. In various embodiments, the symmetry region <NUM> is free of any such dummy fill to avoid introducing asymmetries. To elaborate, dummy fill is usually added by the foundry after device design in any allowed regions. The resulting fill is not fully symmetric around every device, but merely "quasi-symmetric" due to dummy fill spacing. Consider, for instance, a switch that is about <NUM> long and about <NUM> wide, and a dummy fill with feature sizes between <NUM> and <NUM>. The dummy fill is subject to the constraints that the metal dummy fill does not overlap with any metal traces and that the silicon dummy fill does not overlap with the waveguide. Depending on how the dummy fill pattern is positioned relative to the switch, these constraints can cause more dummy fill (e.g., one extra row of fill features) to be placed on one side of the switch than on the other, resulting in a different phase due to local stress differences from the dummy fill. This issue is avoided, in accordance with various embodiments, by excluding the defined symmetry region from dummy fill patterning during device manufacture, that is, treating the boundary of the symmetry region as a "keepout" outline.

The embodiments of <FIG> and <FIG> may be used alone or in combination to reduce phase misalignment between the passive, unbiased operational mode and the predominant switch state as may result from fabrication-induced waveguide width variations and/or stresses on the waveguide interferometer arms of the MZI. As will be appreciated by those of ordinary skill in the art, these approaches are equally applicable to (pure) optical filters, where they can help reduce phase misalignment between the transmission peak during passive operation of the filter and the desired operational wavelength (e.g., of a laser to be aligned with the filter).

For photonic switches that operate predominantly in one state, which by design is generally chosen to coincide with the unbiased operational mode, another approach to reducing or eliminating phase misalignment is to actively adjust the phase in the unbiased operational mode to phase-align the two interferometer arms, but in a power-efficient manner. Phase-aligning the interferometer arms is herein understood to adjust the phase difference between the interferometer arms to zero for a symmetric MZI, and to tune the phase difference between the interferometer arms to the nominal, desired value for an AMZI. Using resistive heating for phase adjustments, the phase in the heated waveguide arm can be tuned in only one direction. With PN-junction-based phase tuners, the junction can, in principle, be forward or reverse-biased, but since it is much more efficient in forward bias, standard operation is to use the switch in forward bias only, thereby limiting phase tuning to one direction. To allow compensating for any phase misalignment in either direction, various embodiments therefore utilize (e.g., heater-based or PN-junction-based) phase tuners in both waveguide interferometer arms. Active phase alignment may be used in conjunction with the fabrication-tolerant waveguide and layer designs described above to further reduce power requirements.

<FIG> is a schematic top view of an example MZI-based optical switch <NUM> with phase tuners <NUM>, <NUM> in both interferometer arms <NUM>, <NUM>, in accordance with one or more embodiments. For definiteness, the tuner <NUM> in one arm <NUM> is herein called the "upper phase tuner (Rtuner, upper)", and the tuner <NUM> in the other arm <NUM> is referred to as the "lower phase tuner (Rtuner, lower). " The phase tuners <NUM>, <NUM>, which are both implemented by resistive heaters in the depicted example, change the relative phase between the two interferometer arms <NUM>, <NUM> in mutually opposite directions, enabling compensation for any fabrication-induced phase misalignment. For example, for a target switch design having a transmission minimum at a phase shift of zero, as indicated by curve <NUM> in <FIG>, if fabrication variations have resulted in a shift of the transmission minimum to a negative relative phase, corresponding to curve <NUM>, the upper phase tuner <NUM> may be used to shift the curve back to the target design; conversely, if fabrication variations have caused the transmission minimum to shift to a positive relative phase, corresponding to curve <NUM>, the lower phase tuner <NUM> may be used to shift it back to zero. A photonic switch with a pair of phase tuners, thus, allows compensating actively for any deviations from the target design. To minimize the power requirements associated with such active tuning in the predominant switch state, electronic circuitry associated with the phase tuners <NUM>, <NUM> is configured, in accordance with various embodiments, to supply low power for the small adjustments in the unbiased (nominally passive) switch mode while providing higher power for operating the photonic switch <NUM> in the biased mode.

<FIG> is a circuit diagram of electronic driver circuitry <NUM> for adjusting the power to the phase tuners <NUM>, <NUM> of the photonic switch <NUM> of <FIG>, in accordance with one or more embodiments. The electronic driver circuitry <NUM> includes a first driver <NUM> associated with the upper phase tuner <NUM>, a second driver <NUM> associated with the lower phase tuner <NUM>, and a microcontroller <NUM> to control the first and second drivers <NUM>, <NUM>. The first and second drivers <NUM>, <NUM> may each include a transistor <NUM> in series with the resistive heater implementing the respective phase tuner <NUM> or <NUM>, and a digital-to-analog converter (DAC) <NUM> that, based on a control signal from the microcontroller <NUM>, adjusts the control voltage applied at the gate (also "gate voltage") of the respective transistor <NUM> to thereby controllably tune the current through the phase tuner <NUM> or <NUM>. The transistors <NUM> may be, e.g., field-effect transistors (FETs). In some embodiments, the control voltage is tuned along a continuum of voltages similar to a low-dropout linear voltage regulator. In other embodiments, the DACs <NUM> are pulse-width modulated DACs (PWM DACs) that vary the duty cycle of a pulse train according to the input digital code, thereby varying the duty cycle of the phase tuners <NUM>, <NUM> and, thus, the average current through the phase tuners <NUM>, <NUM>. In various embodiments, the PWM time period (that is, the time before the signal repeats) is faster than the thermal response time of the heaters implementing the phase tuners <NUM>, <NUM> (e.g., <NUM> ns PWM time period and <NUM> thermal time constant), such that a constant PWM duty cycle results in a constant temperature on the heater and the PWM duty cycle is not visible on the heater, but is instead the average of the PWM signal. The pulsed output of the PWM DACs <NUM> ensures that the FET transistors <NUM> are fully ON or OFF and not in-between, which results in lower power dissipation on the FET transistors <NUM>. However, a PWM DAC can typically not operate at <<NUM>% of its full range. Therefore, with a phase tuner <NUM>, <NUM> driven by a fixed voltage, phase alignments involving <<NUM>% adjustments are accomplished by tuning the phase tuner to 2π plus the desired phase adjustment to be within the operating range of the PWM DAC, wasting 2π worth of tuning power, which would be saved if the phase tuner <NUM>, <NUM> could operate at a lower power setting.

The electronic circuitry <NUM> further includes a DC-DC buck converter <NUM> that converts a fixed input voltage Vin to a tunable output voltage Vrail at a voltage rail <NUM> to which the upper and lower phase tuners <NUM>, <NUM> are connected in parallel. Buck converters are well-known to those of ordinary skill in electronics. The buck converter <NUM> is responsive to control signals from the microcontroller <NUM>, and serves to adjust the output voltage Vrail between a low drive voltage and high drive voltage supplied to the phase tuners <NUM>, <NUM>.

In the predominant switch state (corresponding, e.g., to the attenuation state), in which the switch <NUM> is intended to be operated passively, the buck converter <NUM> sets the output voltage Vrail to a lower drive voltage, such that the drivers <NUM>, <NUM> can, via the control voltages or duty cycle applied at the transistors <NUM>, effect fine adjustments to the (average) current through, and thus heat generated by, the phase tuners <NUM>, <NUM>, and thus to the relative phase shift between the interferometer arms <NUM>, <NUM>. Such fine adjustments may be based on measurements of the optical power received at one or both output ports of the MZI. The photonic switch may, for instance, include taps and monitor photodiodes at the output ports, and provide the photodiode output signals to the microcontroller <NUM> for use as feedback to drive the phase tuners <NUM>, <NUM> via the drivers <NUM>, <NUM>. For example, the drivers <NUM>, <NUM> may be controlled to adjust the phases at the phase tuners <NUM>, <NUM> until the optical power detected at the port that is intended to be "off' is substantially zero.

To operate the switch in the biased mode (e.g., to enable switching to transmission state), the buck converter <NUM> sets the output voltage Vrail to a higher drive voltage for the phase tuners <NUM>, <NUM>. In some embodiments, the lower drive voltage for phase alignment is about <NUM> V, whereas the higher drive voltage for the biased operational mode of the switch is about <NUM> V. Since the power dissipation of a thermal phase tuner is proportional to the square of the voltage, this fivefold change in drive power corresponds to a <NUM>-fold change in power dissipation at the max DAC setting (i.e., the full range setting) for a PWM DAC. In this manner, by switching between a lower drive power in the unbiased operational mode and a higher driver power in the biased operational mode, the buck converter <NUM> reduces the power consumed to compensate for any phase misalignment in the unbiased operational mode. In addition, for drivers <NUM>, <NUM> with a given dynamic range of the control voltage, the buck converter <NUM> effectively increases the dynamic range for tuning the phase, allowing fine phase adjustments in the unbiased state while also facilitating sufficient phase adjustments to switch between transmission and attenuation.

Although the inventive subject matter has been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader scope of the inventive subject matter as defined by the attached claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claim 1:
An integrated optical switch (<NUM>) or filter comprising:
a semiconductor-on-insulator substrate comprising a semiconductor device layer;
a Mach-Zehnder interferometer formed in the semiconductor device layer, the Mach-Zehnder interferometer comprising an input coupler (<NUM>), an output coupler (<NUM>), and two waveguide arms connected between the input coupler (<NUM>) and the output coupler (<NUM>); and
at least a first phase tuner (<NUM>) associated with one of the waveguide arms,
wherein bend radii of the two waveguide arms are greater than <NUM> along entire lengths of the waveguide arms,
and wherein a distance (d<NUM>) between the straight center sections (<NUM>) does not exceed <NUM>,
wherein the two waveguide arms comprise parallel straight center sections (<NUM>), first curved sections (<NUM>) connecting the center sections (<NUM>) to the input coupler (<NUM>), and second curved sections (<NUM>) connecting the center sections (<NUM>) to the output coupler (<NUM>), each of the first curved sections (<NUM>) and second curved sections (<NUM>) including an inflection point (<NUM>, <NUM> ), characterized in that: the bend angles (<NUM>) of the first curved sections (<NUM>) between the input coupler (<NUM>) and the inflection points (<NUM>) and between the inflection points (<NUM>) and the straight center sections (<NUM>) and bend angles of the second curved sections (<NUM>) between the straight center sections (<NUM>) and the inflection points (<NUM>) and between the inflection points (<NUM>) and the output coupler (<NUM>) each do not exceed <NUM>°.