Patent Description:
This disclosure relates to the tuning of the relative phases between waveguides in an array, as applicable, for instance, in the field of photonic integrated circuits (PICs).

In wavelength division multiplexed systems, arrayed waveguide gratings (AWGs) are often used as optical multiplexers or demultiplexers. With reference to <FIG>, an AWG <NUM> generally includes an array <NUM> of waveguides <NUM> typically having constant optical path length increments between adjacent waveguides <NUM>, connected between an input free propagation region (FPR) <NUM> and an output FPR <NUM>. When the AWG <NUM> operates as a wavelength demultiplexer, as depicted, input light including multiple wavelengths diffracting out of an input waveguide <NUM> or other input coupler into the input FPR <NUM> propagates through the input FPR <NUM> to illuminate the input ports of the array <NUM> of waveguides <NUM>. After propagating through the array <NUM> of waveguides <NUM> and accumulating different optical phases in different ones of the waveguides <NUM> due to the different respective optical path lengths, light exiting the array <NUM> of waveguides <NUM> at their output ports is refocused in the output FPR <NUM>, whereby light of different wavelengths constructively interferes, and thus refocuses, at different locations. A plurality of output waveguides <NUM> or other output couplers may be placed at the various foci so as to capture light of the respective wavelengths. To operate the AWG <NUM> as a multiplexer, the direction of light propagation through the AWG <NUM>, and thus the roles of the FPRs <NUM>, <NUM>, can be reversed: light of multiple wavelengths is coupled from multiple respective waveguides <NUM> into the FPR <NUM> (which thereby functions as the input to the AWG <NUM>) and dispersed in the FPR <NUM> to illuminate the array <NUM> of waveguides <NUM>, and after propagating through the array <NUM> of waveguides <NUM>, the now mixed-wavelength light from all of the arrayed waveguides <NUM> is refocused in the FPR <NUM> (now functioning as the output of the AWG), from where it exits into the waveguide <NUM>.

When implemented in PICs, AWGs are susceptible to a number of factors that can affect their wavelength response, often resulting in the mismapping of wavelengths to output waveguides. For example, due to fabrication tolerances of PICs, the effective index of the waveguides in the array may not be controlled accurately enough to achieve an intended wavelength response. Fabrication-based deviations from the intended response are especially likely if the waveguide dimensions are small, if the AWG waveguide core is in a deposited layer for which thickness control is poor, or if the refractive index of the waveguide core is dependent upon material growth or deposition conditions. In addition to these problems, the effective index of the waveguides in the array, and thus the wavelength response of the AWG as a whole, varies as a function of temperature. This effect is particularly pronounced where waveguide core materials having a large thermooptic coefficient, such as silicon, are used, and tends to limit the temperature range over which the AWG can be used. One approach to reducing undesirable wavelength shifts of the AWG response due to fluctuations in the ambient temperature, and/or to compensating for fabrication-based deviations from the desired wavelength response, involves actively controlling the temperature of the AWG, which, however, requires a significant amount of power, rendering the PIC less efficient. <CIT> describes systems, apparatuses and methods for providing athermicity and a tunable spectral response for optical filters. Finite impulse response (FIR) filters are commonly implemented in photonic integrated circuits (PICs) to make devices such as wavelength division multiplexing (WDM) devices, asymmetric Mach-Zehnder interferometers (AMZIs) and array waveguide gratings (AWGs). Athermicity of an FIR filter describes maintaining a consistent frequency transmission spectrum as the ambient temperature changes. A tunable spectral response for an FIR filter describes changing the spectrum of an FIR filter based on its application, as well as potentially correcting for fabrication deviations from the design. The authors assert that their system can reduce energy dissipation requirements and control complexity.

Described herein, in various embodiments, are systems, devices, and structures for tuning the relative phases between waveguides in an array by the active, controlled application of heat to the waveguides, for example, for the purpose of tuning the wavelength response of AWGs or other dispersive gratings. In some embodiments, one or more heaters are disposed directly in an AWG in the vicinity of the waveguides (for instance, above, in between, or even partially overlapping with the arrayed waveguides) to impart an incremental heat-induced phase shift between the waveguides. (By "incremental phase shift between the waveguides" is herein meant that the phase shifts between adjacent waveguides are all in the same direction such that the cumulative phase shift changes monotonically across the array. In many but not necessarily all embodiments, the increments between adjacent waveguides are constant across the array. ) In other embodiments, a separate multi-waveguide structure coupled to and focusing light onto the input FPR of an AWG includes one or more heaters to control the relative phases between the waveguides of the separate structure so as to shift a lateral position of the focus, thereby altering the wavelength response of the AWG; such a separate multi-waveguide structure is herein called a "beam sweeper" (or simply "sweeper"). (The "lateral position" herein denotes a position along a direction perpendicular to the general direction of propagation.

Using a beam sweeper at the input of an AWG instead of a heater directly placed in the AWG is beneficial, in particular, in embodiments where the AWG includes a large number of waveguides, rendering the direct heating of the AWG waveguides energy-costly. A separate beam sweeper that has fewer waveguides than the AWG itself may allow for more efficient tuning of the AWG wavelength response. Beam sweepers in accordance herewith may also be used with dispersive gratings other than AWGs, such as, e.g., Echelle gratings or vertical grating couplers (where the beam sweeper may be used to tune the direction of the coupling beam). Furthermore, a beam sweeper as disclosed herein may find application apart from any dispersive gratings, e.g., in optical switching. Heaters used in AWGs or beam sweepers in any of these embodiment may be controlled based on the waveguide temperature as measured with suitable temperature-sensing elements.

In the context of AWGs, the terms "input FPR" and "output FPR" are herein used, for definiteness and to tie them to structure rather than function, with reference to an AWG operating as a demultiplexer. With these designations, operation of an AWG as a multiplexer involves light entering the AWG at the output FPR and exiting the AWG at the input FPR. In this manner, the term "input FPR" is used consistently to refer to the FPR into or out of which multiplexed (mixed-wavelength) light is coupled, and the term "output FPR" is used consistently to refer to the FPR out of or into which demultiplexed light (light of multiple separated wavelengths) is coupled; structurally, an output FPR may be distinguished from the input FPR, e.g., by virtue of the multiple output waveguides (e.g., with reference to <FIG>, waveguides <NUM>) emanating from it. Further, in embodiments including a sweeper and AWG, the sweeper is consistently coupled to the input FPR of the AWG, regardless whether the AWG is used as a demuliplexer (in which case light propagates through the sweeper before entering the AWG) or a multiplexer (in which case light propagates through the AWG before entering the sweeper). Note that, in accordance with some embodiments, the AWG can be bidirectional in that it exhibits a symmetry that allows it to function as either multiplexer or demultiplexer in either direction. In this case, the FPRs on both sides of the array of waveguides function as both input and output FPRs, e.g., by being each connected to both an input waveguide and multiple output waveguides. To tune the wavelength response of such a bidirectional AWG, beam sweepers may be coupled symmetrically to both input/output FPRs.

In accordance with various embodiments, the AWG and associated heaters are implemented as part of a PIC in an SOI substrate including a silicon handle, a buried oxide layer disposed on top of the silicon handle, a silicon device layer disposed on top of the buried oxide layer, and a cladding layer disposed on top of the silicon device layer. For instance, waveguides and FPRs may be formed in the silicon device layer, and heaters and temperature-sensing elements may be embedded in the cladding layer. Alternatively, segments of a heater may be created directly in the silicon device layer, e.g., placed between the waveguides, by doping the silicon to alter is electrical resistance. Various structural features may serve to increase the efficiency of wavelength-response tuning via heat (as may be measured, for example, in terms of the power used to achieve a certain phase shift or, in the case of an AWG, wavelength shift). A back-etched region in the silicon handle in a region underneath a heater eliminates a significant portion of the heat sink that the silicon handle otherwise constitutes, substantially reducing heat dissipation into the silicon handle. Since removal of the silicon handle can result in mechanical stresses on the waveguides, which may, in turn, affect the wavelength response, the width of the back-etched region may be limited to limit the amount of stress, and may further be constant across the waveguides to ensure that any remaining stresses affect all waveguides uniformly (so as to avoid undesirable relative phase shifts). Heating efficiency may also be increased with thermal isolation trenches formed in the silicon device layer and/or the cladding layer surrounding the heated waveguide regions, which can serve to retain the generated heat in those regions and reduce heat dissipation into the substrate at large. Similarly, thermal isolation trenches may be formed within the heated region between heaters (or heater segments) and temperature-sensing elements to ensure that the generated heat is applied primarily to the adjacent waveguides rather than the temperature-sensing elements.

A further approach to increasing the wavelength-tuning efficiency of AWGs as described herein involves the use of bidirectional pairs of heaters. A bidirectional pair of heaters includes first and second heaters that impart phase shifts of opposite signs between the waveguides such that, when light exiting the waveguides is focused down (e.g., by a sweeper onto the input FPR of an AWG, or in the output FPR of an AWG onto output waveguides), the first heater causes a shift in the lateral focus position in a direction opposite to that of a shift in the lateral focus position caused by the second heater. The first and second heaters are herein also referred to as the "forward heater" and "backward heater," respectively. (As will be readily appreciated, at any given time, only one of the heaters of the bidirectional pair is operated, i.e., the forward and backward heaters are not operated simultaneously. ) Using a bidirectional pair of heaters, a given total phase-tuning range (corresponding to the phase difference between the two extreme phases) can be achieved with heaters each of which individually covers only half that range; this, in turn, reduces the peak power consumption associated with the tuning range to about one half, compared with the peak power consumption of a single heater achieving the same range.

In some embodiments, multiple heaters causing phase shifts in the same direction are used in parallel, accumulating the phase shifts induced by the individual heaters (which are subject to limits placed on the power that can be put through a heater) to achieve a greater total phase shift. Using multiple heaters in parallel reduces the drive voltage requirement associated with a given total phase shift, rendering the heaters deployable in systems where large drive voltages are not available and/or compatible with the electronics requirements of many standard photonics packages (such as, e.g., Quad Small Form-factor Pluggables (QSFPs)).

In various embodiments, the heaters are configured to impart a constant incremental phase shift between adjacent waveguides. For example, the heaters may be configured to generate a substantially uniformly heated region (as defined below) so as to impart a uniform phase shift per unit length of heated waveguide, the heated region being shaped and positioned such that heated waveguide portions increase, between adjacent waveguides, by constant length increments. For example, the heated region may be triangular in shape and overlap straight sections of evenly spaced waveguides. Alternatively, the waveguides may be spaced unevenly in the heated region, the spacings being designed based, e.g., on a given temperature distribution achievable with the heater to result in constant phase shifts between adjacent pairs of waveguides. In some embodiments, a heater is implemented by a current-carrying heating filament, e.g., made of platinum or tungsten (or some other suitable metal), that is wound across an area defining the heated region. The heating filament may have a constant width and/or cross-section within that area to simplify modeling of the resulting temperature distribution and/or facilitate more accurate results. As will be readily appreciated by one of ordinary skill in the art, approximating a region to be heated with a heating filament wound across an area defining that region will generally result in some level of variation in the temperature distribution across the heated region; a region is herein considered "substantially uniformly heated" if such variation in temperature is within an acceptable range. In some embodiments, the acceptable range is defined by ± <NUM>% of the average temperature within the region.

The foregoing will be more readily understood from the following detailed description of the drawings.

<FIG> is a top view of a beam sweeper <NUM> coupled to the input FPR <NUM> of an AWG <NUM>. As shown, the beam sweeper <NUM> includes an input FPR <NUM>, an output FPR <NUM> adjoining, and thus optically coupled to, the input FPR <NUM> of the AWG <NUM>, and an array <NUM> of waveguides <NUM> connecting the input and output FPRs <NUM>, <NUM> of the sweeper <NUM>. An input light signal may be provided to the input FPR <NUM> of the sweeper <NUM> via an input waveguide <NUM>. (The designations of the FPRs <NUM>, <NUM> of the sweeper <NUM> as "input FPR <NUM>" and "output FPR <NUM>" are used, consistently with the designations "input FPR <NUM>" and output FPR <NUM>" of the AWG <NUM>, with reference to the AWG <NUM> operating as a demultiplexer. That is, when the AWG <NUM> operates as a demultiplexer, the beam sweeper <NUM> precedes the AWG <NUM>, and light propagates in the sweeper <NUM> from the input FPR <NUM> to the output FPR <NUM>. When the AWG <NUM> operates as a multiplexer, the beam sweeper <NUM> follows the AWG <NUM> in the direction of light propagation, with light entering the beam sweeper <NUM> at the output FPR <NUM> and exiting the sweeper <NUM> at the input FPR <NUM>.

<FIG> is a top view of a system including a bidirectional AWG <NUM> and beam sweepers <NUM> coupled to both FPRs of the AWG <NUM>. Here, the FPRs on either end of the waveguide array <NUM> are structurally and functionally both input and output FPRs <NUM>/<NUM>. To be able to serve as output FPR, each of the FPRs <NUM>/<NUM> is coupled to output waveguides <NUM>. To be able serve as an input FPR, each of the FPRs <NUM>/<NUM> is further coupled to the output FPR <NUM> of a beam sweeper <NUM> that receives its mixed-wavelength input light through an input waveguide <NUM> at input FPR <NUM>. In the illustrated embodiment, the system including the AWG <NUM> and the two beam sweepers <NUM> coupled thereto (as well as the AWG <NUM> itself) is symmetric.

With renewed reference to <FIG>, the waveguides <NUM> and output FPR <NUM> of the sweeper <NUM> may be configured to focus light propagating in the waveguides <NUM> from the input FPR <NUM> to the output FPR <NUM> of the sweeper <NUM> at an interface of the sweeper <NUM> with the AWG <NUM>. For example, as shown in the close-up view of the coupling region between sweeper <NUM> and AWG <NUM> provided in <FIG>, the sweeper waveguides <NUM> may be oriented, in a region immediately preceding the output FPR <NUM>, along rays emanating from a common center point <NUM> at the interface <NUM>. Similarly, the AWG waveguides <NUM> may be oriented, in a region immediately following the input FPR <NUM> of the AWG <NUM>, along rays emanating from the same (or substantially the same) center point <NUM>. (The term "substantially" herein accounts for slight offsets between the center points corresponding to the rays along which the AWG waveguides <NUM> are oriented and the rays along which the sweeper waveguides <NUM> are oriented, respectively, as may arise in practice, e.g., due to fabrication inaccuracies, and generally does not affect the functioning of the device. ) Further, the entrance surface of the output FPR <NUM> of the sweeper <NUM> and the exit surface of the input FPR <NUM> of the AWG <NUM> may each coincide with the circumference of a circle centered at point <NUM>, with the waveguides <NUM>, <NUM> being perpendicular to the respective surface (due to their arrangement on rays emanating from the center point <NUM>).

In the absence of heat applied to the sweeper waveguides <NUM>, light exiting the waveguides <NUM> will be focused at the point <NUM>, and dispersed from point <NUM> into the input FPR <NUM> of the AWG to illuminate the input ports of the AWG waveguides <NUM>. When the heater(s) of the sweeper <NUM> are used to impose a linear phase variation across the array <NUM> of waveguides <NUM>, the focus at the interface <NUM> is translated laterally (side-ways) away from the center point <NUM> (as indicated by arrows <NUM>), resulting in an altered wavelength response of the AWG <NUM>. As the total phase shift applied across the array <NUM> is tuned continuously between one extreme and the other (e.g., between zero and the maximum phase shift for a single heater, or between the maximum phase shifts in either direction for a bidirectional pair of heaters), the focus traces a line along the interface <NUM>.

The output FPR <NUM> of the sweeper <NUM> and the input FPR <NUM> of the AWG <NUM> may be, and generally are when implemented in SOI substrates, contiguous. In the depicted embodiment, the FPRs <NUM>, <NUM> are shaped to form a constriction or "waist" <NUM> at the interface <NUM>. This waist <NUM> can serve as a spatial filter that prevents light of higher diffraction orders from entering the input FPR <NUM> of the AWG <NUM>; when the AWG <NUM> is used as a demultiplexer, this may result in wavelength channels separated onto their respective output waveguides <NUM> with reduced cross-talk. The waist <NUM> also provides a demarcation between the FPRs <NUM>, <NUM>. Note, however, that a waist between the FPRs <NUM>, <NUM> need not be formed in every embodiment. Rather, the contiguous region formed by the FPRs <NUM>, <NUM> may be devoid of a clear visual boundary between the FPRs <NUM>, <NUM> of the sweeper <NUM> and AWG <NUM>. Therefore, the interface <NUM> is herein defined functionally as the focal plane at the output of the sweeper <NUM>, that is, the vertical plane going through the focus at point <NUM> and oriented perpendicular to the general direction of propagation.

Returning again to <FIG>, in various embodiments, the waveguides <NUM> of the beam sweeper <NUM> are equal in length, which avoids wavelength-dependent dispersion to ensure that light of all wavelengths is focused in the same spot (about point <NUM>) at the output of the sweeper <NUM>. (Of course, once a phase gradient is applied across the array <NUM> of waveguides <NUM>, slight wavelength dispersion results. However, this effect is so small as to be negligible. Typically, the maximum heat-induced phase shift between two adjacent waveguides in the sweeper <NUM> amounts to merely a small fraction of the center wavelength. For comparison, the optical path difference between two adjacent waveguides in an AWG may be on the order of tens of wavelengths. ) The depicted configuration of equal-length waveguides is characterized by an array consisting of two equal portions, corresponding to the portions above and below the horizontal line <NUM> through a symmetry center <NUM>, that map onto one another if one portion is rotated by <NUM>° about the symmetry center <NUM>.

When used in conjunction with an AWG <NUM>, the sweeper <NUM> generally includes fewer waveguides than the AWG <NUM> in order to reduce the power requirements associated with tuning the wavelength response of the AWG <NUM> (compared with the use of heaters included directly above the waveguides <NUM> of the AWG <NUM>). On the other hand, the waveguide array <NUM> of the sweeper <NUM> is generally provided with a sufficient number of waveguides <NUM> to limit the insertion loss at the input FPR <NUM> to an acceptable level and achieve a desired image quality of the focus generated at the output of the sweeper <NUM> (that is, at the interface <NUM> between the output FPR <NUM> and the input FPR <NUM> of the AWG <NUM>). With too few waveguides <NUM> in the sweeper <NUM>, a significant portion of the light from the input waveguide <NUM> may not be captured by the waveguide array <NUM>, and/or a significant portion of light at the output may be concentrated in side lobes of the focus rather than the central focus area. Usually, it is desirable to generate a focus that is to good approximation Gaussian. In various embodiments, the array <NUM> in the sweeper <NUM> includes at least three, and typically more (e.g., about <NUM>-<NUM>), waveguides <NUM>.

Referring now to <FIG>, the placement of heaters in the beam sweeper <NUM> is illustrated in a further top view. In general, the beam sweeper <NUM> may include one or more heaters, for example, multiple heaters driven in parallel to obtain a larger overall phase-tuning range at a given drive voltage per heater, or one or more bidirectional pairs of heaters causing phase shifts in mutually opposite directions (indicated by "F" for forward heaters and "B" for backward heaters). In the illustrated embodiment, the sweeper <NUM> includes two bidirectional pairs <NUM>, <NUM> of heaters <NUM>, <NUM>, <NUM>, <NUM>. The heaters are generally placed laterally overlapping (i.e., overlapping in a top view) with the waveguides <NUM>, in the same or a different layer. For instance, in some embodiments, the heaters are placed above the waveguides <NUM> (see also <FIG>), and in other embodiments, the heaters are placed within the layer defining the waveguides <NUM> (and potentially include portions of the waveguides <NUM>). As shown, the heaters <NUM>, <NUM>, <NUM>, <NUM> may be placed above (or within) straight sections of the waveguides <NUM>, which may simplify thermal modeling of the heat-induced phase shifts.

The heaters are configured to cause a constant incremental phase shift between pairs of adjacent waveguides <NUM>. In principle, this can be achieved, for example, by uniformly heating a triangular region overlapping evenly spaced waveguides, with one edge of the triangle being oriented in parallel with the waveguides such that the length of the waveguide portions overlapping with the triangular region increases by a constant increment between adjacent waveguides. In conjunction with a uniform phase shift per unit length of heated waveguide, as results from a uniform temperature across the heated region, this configuration can achieve the desired constant incremental phase shift. In practice, however, it may be difficult to heat the triangular region sufficiently uniformly. In this case, the spacings between the waveguides may be adjusted, based on thermal modeling, to achieve constant phase increments despite the nonuniform temperature distribution. The geometry of the waveguide array <NUM> shown in <FIG> and <FIG> facilitates modifying the waveguide spacings as desired without affecting the overall length of any waveguide, thereby maintaining equal lengths of all of the waveguides <NUM>. For example, to increase the spacing between the top waveguide <NUM> and its adjacent waveguide <NUM> in the straight waveguide region <NUM>, the length of the straight section of the top waveguide <NUM> in the top region <NUM> may be decreased, and the length of the straight section of the top waveguide <NUM> in the bottom region <NUM> increased, by the same distance. The spacing between any other pair of waveguides <NUM> can be adjusted similarly by modifying the length of one of the waveguides by equal but opposite-signed amounts in the top and bottom regions <NUM>, <NUM>.

<FIG> is a top view of an example AWG <NUM> having heaters disposed above straight sections of the waveguides <NUM>, in accordance with various embodiments. Here, the wavelength response of the AWG <NUM> is tuned directly by heat-induced phase shifts imparted on the waveguides <NUM> of the AWG <NUM>, rather than by a separate beam sweeper <NUM> at the input FPR <NUM> of the AWG <NUM>. Various features of heaters included in a beam sweeper <NUM>, as described above, are equally applicable for the heaters in an AWG. For instance, the AWG may generally include two or more heaters, and uses one or more bidirectional pairs of forward and backward heaters and/or multiple heaters driven in parallel. In the illustrated example, the A WG <NUM> includes four bidirectional pairs of heaters <NUM>, <NUM>, <NUM>, <NUM>. As shown, the bidirectional pairs of heaters <NUM>, <NUM>, <NUM>, <NUM> are arranged symmetrically above straight sections of the array <NUM> waveguides <NUM>. Beneficially, heater placement above straight sections allows using identical designs for the forward and backward heaters while achieving maximum phase shifts of equal magnitude in each direction. Note, however, that heaters can generally also be placed above the curved sections of the array <NUM> of waveguides <NUM>.

<FIG> further illustrates the configuration and design of the individual heaters in more detail. As shown, each heater (e.g., forward heater <NUM> of pair <NUM>, indicated by an enclosing dashed line) may include multiple filamentous resistive heating segments <NUM> geometrically arranged in parallel and having suitable lengths to collectively define a heated region of a desired shape. In the illustrated embodiment, for example, the heating segments <NUM> of each heater collectively define a triangular region. Within each individual heater, the heating segments <NUM> are connected in series by metal connections <NUM> so as to effectively form a single heating filament wound across the heated region. A voltage applied between the electrical connection nodes (labeled F+ and F- for the two polarities of the forward heater and B+ and B- for the two polarities of the backward heaters) at opposite ends of the heating filament causes an electrical current that resistively heats the filament. Note, however, that the heating segments <NUM> and the metal connections <NUM> therebetween may differ in their cross-sectional dimensions and material properties and, as a result, their respective electric resistances. For example, low-resistance metal connections <NUM> may be used with higher-resistance heating segment <NUM> such that, as an electric current flows through the filament, heat is generated preferentially in the heating segments <NUM>. The forward heaters may be driven in parallel by connecting their respective positive connection nodes F+ to one another and connecting their respective negative connection nodes F- top one another. Similarly, the backward heaters may be driving in parallel by connecting respective nodes of the same polarities to one another.

In accordance with various embodiments, thermal modeling is used to compute, based on the configuration of the heating filament in a heater, the resulting temperature distribution created by the heater. From this distribution, the phase shifts imparted on the waveguides can, in turn, be computed. In some embodiments, the heating filaments, or at least the heating segments therein, have a constant width and cross-section, which simplifies the modeling. The heater and waveguides collectively may be configured to achieve a desirable constant incremental phase shift between pairs of adjacent waveguides. For example, a triangular heated region of uniform temperature (if achievable) may be used in conjunction with evenly spaced waveguides. For non-uniform temperature distributions, the spacing of the waveguides underneath the heater may be adjusted; for example, if the temperature falls off from a maximum temperature in accordance with an approximately exponential profile, the spacing between waveguides may be increased towards the exponential tail.

Turning now to the implementation of AWGs and beam sweepers as described above in PICs, <FIG> is a cross-section of an example SOI substrate <NUM> having a multi-waveguide structure and associated heating segments and temperature-sensing elements implemented therein, in accordance with various embodiments. The SOI substrate <NUM> includes a silicon handle <NUM>, a buried oxide (BOX) layer <NUM> disposed above the handle <NUM>, a silicon device layer <NUM> on top of the BOX layer <NUM>, and a dielectric cladding layer <NUM> above the silicon device layer <NUM>. A plurality of silicon rib waveguides <NUM> are formed in the silicon device layer <NUM>. The space between the rib waveguides <NUM> (e.g., created by etching) may be filled with dielectric material (e.g., the same material as used for the cladding layer <NUM>). The rib waveguides <NUM> may implement the arrayed waveguides <NUM>, <NUM> of an AWG or beam sweeper as described herein.

In the example embodiment of <FIG>, the heaters are implemented by heating segments <NUM> disposed above the waveguides <NUM> in the cladding layer <NUM>. Alternatively or additionally to being disposed in the layer <NUM> above the waveguides <NUM>, heating segments <NUM> may also be created within the silicon device layer <NUM> (not shown), e.g., by doping the silicon to render it resistive; in this case, the heating segments <NUM> may be placed between the waveguides <NUM> or even include portions of (in other words, at least in part spatially overlap with) the waveguides <NUM>. In either case, the heating segments <NUM> overlap laterally with the waveguides, meaning that their projections into a horizontal plane overlap. Further, as shown, temperature-sensing elements <NUM>, e.g., forming the segments of a resistive thermal device (RTD), may be embedded in the cladding layer <NUM> to directly measure the temperature in the heated region. The temperature-sensing elements <NUM> and heating segments <NUM> may be arranged in an alternating fashion.

Based on the temperature within a heated region as measured by temperature-sensing elements <NUM>, the temperature-dependence of the phase shift imparted by deliberately heating the waveguides (or the temperature-dependence of a resulting optical characteristic such as, in a sweeper, the lateral shift in the focus at the sweeper output) can be calibrated. Alternatively, the phase shift as a function of the voltage or current applied to the heater may be calibrated (obviating, in some embodiments, the need for sensing the temperature of the heated region). To compensate for an undesired phase shift of the AWG <NUM> due to a change in the ambient temperature, the temperature of the waveguides <NUM> of the AWG <NUM> in a region outside the heater may be measured to determine therefrom how much tuning is needed. Accordingly, in various embodiments, temperature-sensing elements are included both within and near the heated region (in the sweeper or a directly heated waveguide section of the AWG, as the case may be) and in a region away from the heater.

With renewed reference to <FIG> and reference further to <FIG>, which provides a top detail view of two heaters disposed above a straight section of waveguides <NUM>, various features directed at increasing the efficiency of thermal phase-tuning are illustrated. One such feature is a back-etched region <NUM> in the silicon handle <NUM>, located directly underneath the heated waveguide region. Silicon removal underneath the heater serves to eliminate heat dissipation into the silicon handle <NUM> within the back-etched region <NUM>, which helps to retain more heat in the heated region of the silicon device layer <NUM>. In accordance with various embodiments, silicon is fully removed within the boundaries of the back-etched region <NUM>, without leaving any silicon "islands," so as to maximize the effect of the back-etch. On the other hand, in order to avoid an unnecessary loss in structural stability of the SOI substrate <NUM>, silicon removal from the handle <NUM> is confined, in accordance with various embodiments, to a region <NUM> substantially coinciding with an area defined by the heater (e.g., not exceeding maximum lateral dimensions of the heater by more than <NUM>%). This is illustrated more clearly in <FIG>. As shown, the back-etched region <NUM> may, for example, be within the outer limits of the heating segments <NUM> in the direction of the waveguides <NUM> and extend only slightly beyond the outer limits of the heating segments <NUM> in the direction perpendicular to the waveguides <NUM> (e.g., such that the dimension of the back-etched region <NUM> measured perpendicularly to the waveguides <NUM> exceeds that of the heater by no more than <NUM>%).

Besides the effect on the overall structural stability of the SOI substrate <NUM>, another concern associated with a back-etched region <NUM> underneath the waveguides <NUM>, <NUM> are mechanical stresses imposed on the waveguides due to the nonuniformity of the substrate. Limiting the width <NUM> of the back-etched region <NUM>, measured in the direction of the waveguides <NUM>, can minimize this problem. In some embodiments, the width <NUM> of the back-etched region <NUM> is less than <NUM>; for comparison, typical dimensions of the waveguide array within an AWG may be on the order of <NUM> x <NUM>. Thus, the back-etched region <NUM> is significantly smaller in area than the waveguide array. In addition, the effect of any remaining mechanical stresses on the phase of the waveguides can be neutralized by ensuring that all waveguides experience the same stress level. This, in turn, may be achieved by using a back-etched region <NUM> that has a constant width <NUM> across the array of waveguides (e.g., that is rectangular in shape) and placing the heater, and thus the back-etched region, in a region of the waveguide array where the waveguides are straight and/or have constant curvature (straight waveguides corresponding to zero curvature), or by otherwise ensuring that the length of waveguide overlapping the back-etched region <NUM> is the same for all waveguides.

In addition to removing the heat sink underneath the heater, the tuning efficiency may be increased by thermally isolating the heated region from surrounding regions in the silicon device layer. For this purpose, various embodiments include thermal isolation channels (or, if open at the top, " trenches") <NUM> in the silicon device layer <NUM> (as shown) and/or the dielectric cladding layer <NUM> (not shown) alongside the waveguides <NUM>, <NUM> on either or both sides of the waveguide array, surrounding the heated portions of the waveguides. Further, in embodiments that include temperature-sensing elements <NUM> adjacent (e.g., arranged alternatingly with) the heating segments <NUM>, thermal isolation trenches <NUM> may be included between adjacent heating and temperature-sensing elements <NUM>, <NUM> such that the heat is applied primarily to the waveguides <NUM> rather than the temperature-sensing elements <NUM>.

Claim 1:
A photonic integrated circuit, PIC, comprising:
a silicon-on-insulator, SOI, substrate comprising a silicon handle, a buried oxide layer disposed on top of the silicon handle, a silicon device layer disposed on top of the buried oxide layer, and a cladding layer disposed on top of the silicon device layer;
an array of waveguides formed at least partially within the silicon device layer, wherein the array of waveguides is connected between an input free propagation region, FPR, and output FPR formed at least partially within the silicon device layer, the input and output FPRs and the array of waveguides together forming an arrayed waveguide grating, AWG, the waveguides varying in optical path length incrementally across the array to thereby cause wavelength dispersion;
one or more pairs of forward and backward heaters disposed at least partially within at least one of the cladding layer or the silicon device layer and laterally overlapping with the array of waveguides, each heater configured to impart an incremental heat-induced phase shift between the waveguides, the phase shift imparted by the forward heater being of an opposite sign than the phase shift imparted by the backward heater; and
one or more back-etched regions formed within the silicon handle underneath respective one or more pairs of forward and backward heaters.