PHOTONIC INTEGRATED CIRCUIT AND CHARACTERIZATION METHOD

A method for characterizing a photonic integrated circuit comprising includes coupling an evanescent field into a scattering element adjacent to a guiding layer of the photonic integrated circuit. The evanescent field is of an optical mode of light propagating in the guiding layer, and has an in-medium wavelength in the guiding layer. A maximum spatial dimension of the scattering element is less than the in-medium wavelength. The method includes scattering, with the scattering element, the coupled evanescent field as a reference scattered-signal. The method also includes detecting one of the reference scattered-signal and a signal derived therefrom.

BACKGROUND

Photonic integrated circuits include photonic devices and waveguides combined in such a way as to perform functions of a circuit (e.g., for communications, signal processing, or sensing). Testing, assembly, and packaging of photonic integrated circuits requires a series of advanced, often nonstandard, steps to verify the waveguide and device quality. Some of these tests are carried out at the wafer level, within the foundry, to assure that individual layers have the correct thickness and material quality. However, many additional tests must be carried out, first at the (finished) wafer level, and then after the wafer is separated (singulated) into individual die for packaging. The goal of much of this testing is to identify ‘known good die’ – chips that function as expected. Identifying known good die (and discarding inferior chips) is very cost-effective, since the expensive packaging process is not applied to inferior chips.

State of the art polarization testing in photonic integrated circuits is limited, with the primary method using a loop-back waveguide measuring the polarization of the output light. The application of this method is costly, time consuming, and requires specialized equipment as two fibers must be coupled to the chip, with one fiber providing the input light and another measuring the return signal.

SUMMARY

Embodiments disclosed herein include systems and methods for polarization testing of photonic integrated circuits using subwavelength photonic test points that may be read remotely with a suitable combination of a microscope, polarimetric optics, and image sensor. Embodiments described herein make use of small (subwavelength) scatterers designed into the circuit as part of the foundry process that will scatter known amounts of light with a known polarization state into a microscope designed for collecting and imaging light from photonic integrated circuits. The scatterer’s spatial dimension is much less than one wavelength and will therefore scatter light in a manner similar to a dipole antenna, where the polarization of the dipole aligns with the polarization state of the waveguide mode.

In a first aspect, a photonic integrated circuit includes a substrate, a cladding layer, and a guiding layer. The substrate has a substrate top-surface and a substrate refractive index. The cladding layer is on the substrate top-surface and has a cladding top-surface and a cladding refractive index. The guiding is layer located between the substrate top-surface and the cladding top-surface and having (i) a core refractive index exceeding both the substrate refractive index and the cladding refractive index, (ii) a guiding-layer thickness above the substrate top-surface, and (iii) a guiding-layer width. In a direction parallel to the substrate top-surface, the guiding layer supports an optical mode that extends to a decay-range into the cladding layer in the direction. An in-medium wavelength of the optical mode exceeds both the guiding-layer thickness and the guiding-layer width. The scattering element is (i) located between the substrate top-surface and the cladding top-surface and (ii) separated from the guiding layer by a gap-distance that is less than the decay-range, and has (a) a maximum spatial dimension that is less than the in-medium wavelength, and (b) a scattering refractive index that exceeds the cladding refractive index.

In a second aspect, a method for characterizing a photonic integrated circuit comprising includes coupling an evanescent field into a scattering element adjacent to a guiding layer of the photonic integrated circuit. The evanescent field is of an optical mode of light propagating in the guiding layer, and has an in-medium wavelength in the guiding layer. A maximum spatial dimension of the scattering element is less than the in-medium wavelength. The method includes scattering, with the scattering element, the coupled evanescent field as a reference scattered-signal. The method also includes detecting one of the reference scattered-signal and a signal derived therefrom.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To provide quantitative information about light within a photonic integrated circuit (PIC), it is preferable to scatter a small amount of light to a sensor without significantly depleting the amount of light available in the circuit. It is also desirable to scatter the light such that very little chip area is used. For example, while a grating coupler scatters light from a waveguide up toward a camera, it scatters a very large amount of light, is typically relatively large (as large as ten micrometers or more in size), and scatters in a manner that is strongly dependent on the wavelength and polarization of the light. While a grating coupler is potentially useful for sparse probing, it is not well suited for probing many test points simultaneously. Random scattering may also be used, but it is very difficult to measure scattering to characterize a waveguide, e.g., loss of low-loss waveguides, because of the large random variations in scattered intensity along the guide.

In addition to the above-referenced limitations, it is difficult to measure quantitative and precise polarization information about the light propagating within a waveguide. This is because grating couplers sample an extended range along the waveguide and modify polarization in the process. Near-field probes can sample the evanescent field accurately, but modern PIC design requires that the evanescent fields be negligible at the surface of the chip.

Embodiments disclosed herein remedy the aforementioned problems via a true subwavelength probe of the fields in the waveguide that (i) may be fabricated with the waveguide in a single process and (ii) scatters light in a way that can be easily sensed with a microscope equipped with polarization-sensitive electronic imaging. A small scatterer (one that has dimension much smaller than the wavelength of light), when placed in proximity to the waveguide, scatters light in a manner similar to a dipole source whose polarization is aligned with the polarization of the local field at the point of the scatterer. A scatterer defined in a foundry process will thus scatter light into the collection aperture of the microscope in a manner that is deterministic and dependent on the polarization state and phase of the light in the waveguide.

Scattering From Evanescent Fields

A single-mode waveguide guides optical energy as a propagating mode, the electric fields of which extend beyond the physical boundary of the waveguide core. The necessary condition for light guiding in this manner is that the refractive index of the core be greater than that of the surrounding material, the cladding. Well known waveguide geometries include slab waveguides (in which the light is trapped in one dimension within a thin film but may spread out in the other direction), ridge and rib waveguides (in which a thicker region of the core serves to confine light in two dimensions), and a photonic wire (in which a core region is completely surrounded by a cladding region). Embodiments disclosed herein are directed to a waveguide such as (i) silicon surrounded by silicon dioxide and (ii) silicon nitride surrounded by silicon dioxide. The waveguide may be one of a slab waveguide, a rib waveguide, or a photonic wire.

For small enough core sizes, the guide supports only a single mode with fields extending into a lower-index cladding. These fields, known as evanescent fields, typically decay exponentially into the cladding region in a manner that is wavelength and polarization dependent. When there is negligible material absorption and when the boundaries are perfectly smooth, the optical mode propagates along the guide with no loss. However, any deviation from a perfectly smooth boundary perturbs the wave and scatter a portion of the guided energy into the cladding regions.

In accord with teachings described hereinbelow, a very small scattering region placed in the evanescent field scatters light into three dimensions in a manner similar to a dipole source, in which the scatterer has a polarizability tensor that, when multiplied by the local field at the scatterer, radiates a polarized field into the collection aperture of a microscope objective.

Photonic Integrated Circuit Fabrication

Embodiments disclosed herein therefore combine a subwavelength scatterer that is adjacent to a waveguide with a means of remotely detecting the scattered light to obtain precise quantitative information about the polarization of light within the guide. The scatterer may be adjacent to the waveguide in any direction that is perpendicular to the direction of light propagation in the waveguide. By precisely defining the location and size of the scatterer, that quantitative information may include at least one of the power and polarization state of the guided mode. In embodiments, multiple scatterers are located at precise locations along the waveguide, which enables measurement of properties such as waveguide scattering loss, component loss, splitting ratio at couplers, efficiency and efficacy of polarization rotators, and a variety of other waveguide and circuit parameters.

One way to introduce a scatterer into a photonic circuit is to penetrate the upper cladding with a nano-indenter. A nano-indenter is a device usually used to measure the mechanical properties of materials by applying a known force to a diamond anvil that is very sharp (submicron dimensions) at its tip. With a suitable force, it can penetrate into the cladding material and create a small void in the proximity of the optical waveguide.

FIG.1shows an example a nano-indenter applied to a photonics chip, and illustrates how one or more of these scatterers scatters light into the camera.FIG.1includes images110and120, each of which is inverted to better illustrate scattered light and interference fringes. Image110is of light scattering from light guided along a silicon waveguide112whose cladding has been penetrated by a nano-indenter at two points114and115along waveguide122. Image120is a pupil image of waveguide112and points114,115. Image120was formed formed using a Bertrand lens, and shows interference fringes formed from the interference of light scattered by points114and115. A relatively small scatterer placed in proximity to the guide scatters a small fraction of the energy to the camera; however, the power level collected is easily detected above the other light scattered from the surface.

Nano-indenters have the disadvantage that they are difficult to place accurately; such placement is time consuming and requires expensive instrumentation. An alternative is to use a scatterer that is designed into the semiconductor foundry fabrication process, preferably made out of the same materials used in the standard process, done in such a way that the dimensions are a repeatable part of the process, and accomplished in a way that does not require extra layers beyond those normally used in the PIC fabrication. Furthermore, the scatterer should be small (less than the wavelength of light in each dimension) and have a significantly different refractive index when compared to the cladding, which may be formed of silicon dioxide.

In a semiconductor photonics foundry process, waveguide layers may include (i) a lower waveguide-layer (formed from a first material), (ii) a middle waveguide-layer placed just above the lower waveguide (formed from a second material), and (ii) an upper waveguide-layer placed just above the middle waveguide (formed from a third material). The first material may be a semiconductor, such as silicon. Each of the second and third materials may be a dielectric, such as silicon nitride. Embodiments disclosed herein are directed to scatterers formed in the layer just above or just below the primary guiding waveguide. For example, when the primary waveguide is in the lower waveguide-layer and formed in the first material, the scatterer may be formed from the second material and be located in the middle waveguide-layer. When the primary waveguide is the middle waveguide-layer, the scatterer may be either formed of the first material and be part of the lower-waveguide layer, or be formed of the third material of the upper waveguide-layer. Finally, when the primary waveguide is the upper waveguide-layer, the scatterer may be formed from the middle waveguide-layer.

In embodiments, a scattering and detection method employs at least one semiconductor subwavelength scattering element above a waveguide (FIGS.2and3), or beside a waveguide (FIGS.7and8). When the scattering element is beside the waveguide, the scattering element and the waveguide are in the same layer above a substrate. The scattering elements produce an observable bright spot of scattered light compared with the background. The intensity of the bright spot may be approximately twenty times that of light scattered from waveguide imperfections. Placement of multiple scattering elements along the waveguide allows for a quantitative analysis of the amount of light lost along the waveguide.

FIG.2is a schematic cross-sectional view layout of a photonic integrated circuit200, hereinafter PIC200.FIG.3is a plan view of PIC200. The cross-sectional view ofFIG.2is in a cross-sectional plane 2-2′ shown inFIG.3. PIC200includes a substrate210, a cladding layer220, a guiding layer230, and a scattering element250. In embodiments, at least one of: substrate210is formed of silicon, cladding layer220is formed of silicon dioxide, guiding layer230is formed of silicon, and scattering element250is formed of silicon-nitride. PIC200may include additional scattering elements 250(k = 2, 3, ..., N), for example scattering element250(2), where k is a positive integer less than or equal to N. Additional scattering elements250allow simultaneous monitoring of optical power in guiding layer230at multiple locations.

FIG.2denotes distance215between guiding layer230and substrate210in direction A3. In embodiments, distance215is between one micrometer and five micrometers. Distance215may be less than one micrometer, for example, equal to zero, such that guiding layer230is directly on top surface219of substrate210.

Scattering element250has spatial dimensions251,252, and253along axes A1, A2, and A3 respectively, each of which may be between 0.1 micrometers and 0.5 micrometers. In embodiments, the minimum value of dimensions251,252, and253is determined by limits of the lithographic resolution of the photonic integrated circuit fabrication hardware and process used to fabricate scattering element250. In embodiments, dimensions251,252, and253are 200 nm, 200 nm and 220 nm, respectively. Scattering element250may be equivalent to a section of waveguide designed for guiding 1.3-micrometer light, e.g., such that it is formed of silicon nitride, dimension251is between 1.1 and 1.3 micrometers, and dimension253is between 200 nm and 240 nm.

Along axis A3, scattering element250is separated from guiding layer230by a gap-distance245. Herein γ denotes a distance, along axis A3, from guiding layer230at which the intensity of a lowest-order optical mode of guiding layer230decays to ⅟e of its maximum value. In embodiments, gap-distance245is at least one of (i) greater than γ to ensure weak coupling and (ii) less than 6γ to ensure adequate coupling. For example, gap-distance245may be between 0.1 micrometers and 0.5 micrometers.

Cladding layer220has a thickness225, which may be at least two times the free-space wavelength of light propagating in guiding layer230. Such a thickness is sufficient to prevent scattering of guided mode272by dirt or impurities on a top surface of cladding layer220. In embodiments, cladding layer220is between two times and five times the free-space wavelength. In embodiments, the free-space wavelength is between 1.0 micrometers and 2.0 micrometers.

At visible and near-infrared wavelengths, silicon nitride has a higher refractive index than silicon dioxide. Accordingly, when scattering element250and cladding layer220are formed of silicon nitride and silicon dioxide respectively, scattering element250has a higher refractive index than cladding layer220, such that scattering element250supports a propagating guided mode.

In embodiments, at least one of dimension231and dimension233is determined such that guiding layer230supports single-mode low loss propagation. The number of modes in a slab waveguide at a free-space wavelength λ0is the smallest integer M greater than 2(d/λ0)NA, where d is the layer thickness and NA is the waveguide’s numerical aperture. For a slab waveguide,

where n1and n2are the refractive indices of the core (guiding layer230) and the cladding (cladding layer220) respectively. For single-mode operation M = 1. In embodiments, 2(d/λ0)NA < 1 for single mode operation. For two-dimensional confinement, each of dimension231and dimension233of guiding layer230is designed in a manner similar to that of the thickness of an equivalent slab waveguide. That is, both 2(d231/λ0)NA < 1 and 2(d233/λ0)NA < 1, where d231and d233equal dimension231and dimension233respectively.

In an example mode of operation, a guided mode272propagating in guiding layer230evanescently couples with scattering element250, which produces a scattered signal274. Scattering element250may be a dipole scattering element whose dipole moment aligns with the polarization guided mode272. This evanescent coupling allows for measurement of polarization states in guiding layer230. Keeping the scattering element250small, subwavelength for example (where candidate wavelengths include those between 1.0 micrometers and 2.0 micrometers, such as 1.3 micrometers or 1.55 micrometers), scattering element250perturbs guided mode272and produces a sufficiently weak perturbation in the mode with negligible power lost in the guided mode. WhileFIG.2illustrates scattered signal274propagating away from scattering element250in predominantly axis A3, scattered signal may also propagate in additional directions, such as long axis A1.

Measuring Light From Multiple Scatterers

In optics, one may either measure an absolute quantity (guided power, polarization state, etc.) or one may measure differences. For example, one may measure a phase difference between two scattering elements250, which gives information about the optical path in the section of guiding layer230between the two scattering elements250. Alternatively, one may measure the difference in scattered power between two identical scattering elements250placed at different points on, above, or adjacent to guiding layer230. A measurement of the phase difference provides an accurate measure of the effective index of guiding layer230, provided the physical path length between the two scattering elements250is known. A measurement of the phase difference as a function of wavelength provides information about dispersion or other properties of guiding layer230. A measurement of the difference in image irradiance between two scattering elements250provides information about the loss of guiding layer230.

Irradiance measurements may be accurately carried out using direct imaging, since the electronic sensor signal strength is determined by the number of photons per pixel per frame absorbed in a sensor element. High quality microscopes and sensors may provide repeatable power measurements independent of the position of the source in the microscope. Thus, a single image may provide the relative power level from multiple scattering elements250during the time it takes to capture one image frame. Accurate measurements of the phase difference between two scatterers require an interferometric measurement; for that, one may reconfigure the microscope to replace the tube lens (ofFIG.7for example) with either a Bertrand lens (a.k.a. a phase telescope) or a cylindrical lens such that a pupil plane image is projected to the sensor. Any pair of scattering elements250, when imaged in this fashion, behave like a pair of Young’s slits, and produce a periodic fringe pattern in the pupil of the objective. The orientation of the fringe pattern yields the orientation of the scatterers; the period of the fringe pattern yields the physical separation of the scatterers. In this way, PIC200may include multiple scattering elements250, placed as test points, at various positions along guiding layer230within PIC200. Provided each pair of scatterers has a unique separation and orientation, they will interfere to form a uniquely identified fringe pattern in the exit pupil of the objective.

For example,FIG.4includes plots401-404. Plots401and402illustrate respective input states of a guided wave in a silicon waveguide430. Plots403and404illustrate numerically calculated output polarization distributions at the pupil for a 200-nm × 200-nm silicon-nitride scatter above silicon waveguide430. The scattering element and waveguide430are respective examples of scattering element250and guiding layer230. Uniform, orthogonal polarizations in the pupil for TE and TM guided waves allows for accurate polarization state measurements. Plots401and402show intensity of a single TE mode and a single TM mode of silicon waveguide430respectively, with polarization vectors superimposed thereon.

Scatterer Adjacent to Waveguide

FIG.5is a schematic cross-sectional view of a photonic integrated circuit500.FIG.6is a schematic plan view of photonic integrated circuit500. The cross-sectional view ofFIG.5is in a cross-sectional plane 5-5′ shown inFIG.6. Cross-sectional plane 5-5′ is parallel to the A1-A2 plane. Photonic integrated circuit500includes a substrate510, a cladding layer520, a guiding layer530, and a scattering element550. Substrate510has a substrate top-surface519and a substrate refractive index. Cladding layer520is on substrate top-surface519, has a cladding top-surface529, and has a cladding refractive index. Axis A3 is perpendicular to top-surface519.

Photonic integrated circuit200,FIG.2, is an example of photonic integrated circuit500. Substrate210, top surface219, cladding layer220, guiding layer230, and scattering element250, are examples of substrate510, top surface519, cladding layer520, guiding layer530, and scattering element550respectively.

To facilitate fabrication, guiding layer530and scattering element550may be formed of the same material, such as a semiconductor. Cladding layer520may be a dielectric. In embodiments, scattering element550is formed of one of a dielectric, a metal, a semiconductor, and a combination thereof. For example, scattering element550may be formed of silicon nitride or silicon.

Guiding layer530is located between substrate top-surface519and cladding top-surface529. Guiding layer530and scattering element550have respective bottom surfaces535and555, proximate substrate top-surface519. In embodiments, for example when guiding layer530and scattering element550are formed in a same fabrication step, surfaces535and555are coplanar in a plane parallel to substrate top-surface519.

Guiding layer530has (i) a core refractive index exceeding both the substrate refractive index and the cladding refractive index, (ii) a guiding-layer thickness533above the substrate top-surface519, and (iii) a guiding-layer width531in a direction parallel to substrate top-surface519. Guiding layer530supports an optical mode that, along axis A1, extends to a decay-range546into cladding layer520. Decay-range546is, for example, a distance from the guiding layer at which the intensity of the optical mode decays to ⅟e of its maximum value.

Each of guiding-layer thickness533and guiding-layer width531may be less than an in-medium wavelength of the optical mode. In embodiments, the in-medium wavelength is a free-space wavelength between 1.1 micrometers and 1.8 micrometers divided by a refractive index of guiding layer530at the free-space wavelength. For example, the free-space wavelength is either 1.3 micrometers or 1.55 micrometers. In embodiments, at least one of guiding-layer thickness533and guiding-layer width531is between 0.15 micrometers and 0.3 micrometers.

Scattering element550is (i) located between substrate top-surface519and cladding top-surface529, and (ii) separated from guiding layer530by a gap-distance545. Gap-distance545is in a plane parallel to top surface519. To ensure weak coupling, gap-distance545may be greater than decay-range546. To ensure adequate coupling, gap-distance545may be less than six times decay-range546. For example, gap-distance545may be between 0.2 micrometers and 0.5 micrometers. Gap-distance245is an example of gap-distance545.

Scattering element550has (a) a maximum spatial dimension that is less than the in-medium wavelength, and (b) a refractive index n550. In embodiments, refractive index n550is greater than the refractive index of cladding layer520such that total-internal reflection may occur therein. In embodiments, the maximum spatial dimension of scattering element550is along one of axes A1and A2. In embodiments, the maximum spatial dimension is at least one of (i) less than guiding-layer width531, (ii) less than guiding-layer thickness533such that scattering element550produces a sufficiently weak perturbation in the mode with negligible power lost in the guided mode. The maximum spatial dimension may be greater than fifty nanometers to ensure sufficient scattering amplitude for measurement.

In an example mode of operation, guided mode572is a mode of guiding layer530and propagates in guiding layer530through a first plane534(1) to a second plane534(2) that intersects guiding layer530. Scattering element550scatters a fraction of guided mode572as a scattered signal574.

In embodiments, photonic integrated circuit500includes a plurality of additional scattering elements550(2, 3, ..., N), where N is a positive integer greater than or equal to three.FIG.6illustrates scattering elements550(2) and550(3), and in an inset502, a scattering element550(k), where k is a positive integer less than or equal to N. Each scattering element550(k) is separated from a respective additional position552(k) of a plurality of positions along guiding layer530by a respective gap-distance545(k) that is less than decay-range546. Each position552(k) corresponds to a respective propagation distance of the optical mode in guiding layer530with respect to cross-sectional plane 5-5′. Along guiding layer530, a distance between adjacent scattering elements550exceeds guiding-layer width531.

Each of the plurality of scattering elements550may have the same spatial dimensions, which enables comparative measurements, e.g., of optical power or polarization, along guiding layer530. For the same reasons, each of scattering elements550may be oriented at the same angle with respect to guiding layer530.

In embodiments, cladding layer520completely surrounds guiding layer530in a plurality of transverse planes perpendicular to substrate top-surface519, such that guiding layer530and cladding layer520form a photonic wire. For example, in PIC200, cladding layer220completely surrounds guiding layer230in a plurality of planes that are parallel to the A1-A3 plane.

Diagnosing Scattering Defects

The disclosed photonic integrated circuits may include at least one deterministic scatterer, examples of which include scattering elements250and550. Guiding layer230and530may include a defect scatterer213, which may be a fabrication error and/or a random defect. A deterministic scatterer may be relied on to scatter light in the same manner from waveguide to waveguide, circuit to circuit, and chip to chip. However, fabrication errors, such as defect scatterer213, are not deterministic. For example, line edge roughness in a fabrication process may induce excess waveguide scattering, as can errors in mask fabrication or exposure. The deterministic scatterer may, with suitable design, function as a reference source to interfere with the light from unknown scatterers, such as defect scatterer213.

In this scenario, it is assumed that the deterministic scatterer designed into the circuit as a test point produces a stronger signal (e.g. signals274and574) than a defect signal214from a defect scatterer213. The signal, herein a “reference signal,” may be stronger by a factor of ten to one hundred, for example. When viewed in the pupil plane (the so-called Fourier plane), the reference signal will interfere with scattered light from other points along the waveguide. As with deterministic scatterers, the location of defect scatterers213with respect to the reference scatterer (scattering elements250or550) may be determined by a Fourier analysis of the fringes that form in the pupil plane. In this way, introducing a deterministic scatterer may provide quantitative information about unknown and unanticipated sources of loss in photonic integrated circuits200and500. Applications

This section describes example applications 1-7 of the disclosed PIC characterization methods. Each of these applications has three common aspects. The first aspect is a source of light coupled to the waveguide, for example by use of an optical fiber that is attached to an external source, although in some cases there may be a source fabricated within the photonic integrated circuit. The second aspect is one or more waveguides incorporated into a photonic integrated circuit that function as photonic wires to connect components to one another and to allow light from the edge of the PIC to be routed to appropriate devices on the PIC. The third aspect is one or more scattering sites precisely fabricated so as to intersect with the evanescent field of the guide.

Example Application 1: Measuring the Polarization State of Coupled Light

Silicon scatterers placed beside (e.g. in the same layer) as a silicon waveguide scatter light well but show poor correlation with the input polarization states. By contrast, a scatterer placed above or below the waveguide (e.g. a silicon-nitride scatterer placed above a silicon waveguide) display a strong correlation with the input polarization.FIG.4displays the input polarization states of silicon guiding layer430in the upper plots401and402. The numerically calculated output states at the pupil are shown (bottom plots403and404) for the 200-nm × 200-nm silicon-nitride scatterer placed above the waveguide. The strong correlation and polarization uniformity permits an accurate measurement of the polarization state within the waveguide.

Measuring the polarization of these scattered fields may be achieved either by imaging the pupil plane through a Bertrand lens to image the pupil and then introducing polarization elements into the imaging system for polarimetric analysis or by direct imaging of the scattered light through suitable polarimetric elements. The polarization elements may include a birefringent element, such as a quarter-wave plate, in combination with a polarizer, such as a linear polarizer. For direct imaging the birefringent element may also be a space-variant birefringent element such as a vortex waveplate or a stress engineered optic. In those cases, the polarizer would be either a circular polarizer or an anisotropic crystal such as calcite to physically separate orthogonal polarization components.

FIG.7is a schematic of an apparatus700detecting light scattered by a photonic integrated circuit709, which may be used for polarization measurements of example applications 1-7. Photonic integrated circuit709includes a substrate701, a guiding layer703directly on or above substrate701, and a scattering element705, which is either adjacent to or above guiding layer703. Accordingly, each of PICs200and500is an example of photonic integrated circuit709, each of guiding layers230and530is an example of guiding layer703, and each of scattering elements250and550is an example of scattering element705. Photonic integrated circuit709includes a cladding layer, not shown, of which cladding layer220and520are examples. The cladding layer and guiding layer703form a waveguide708of photonic integrated circuit709.

Apparatus700includes a lens710, a sensor760, and in order of increasing distance along an optical path between lens710and sensor760, at least one of a birefringent element720, a tube lens730, a Bertrand lens740, and a polarizer750. Sensor760may include at least one InGaAs photodiode, for example, when input light771includes one or more wavelengths between 0.8 micrometers and 1.7 micrometers. Sensor760may include an array of sensors configured to capture an image. For example, sensor760may include a detector array, such as a photodiode array. Sensor760may be a CCD image sensor or a CMOS image sensor. Lens710may be a positive lens, such as a microscope objective, and may have a focal length between one millimeter and two-hundred millimeters. For suitable resolution, magnification, and depth of focus, lens710may have a numerical aperture between 0.05 and 0.5. In embodiments, Bertrand lens740is a positive lens, located between lens710and sensor760, that focuses an image of the exit pupil of lens770to a light-sensitive surface of sensor760. Birefringent element720may be a quarter-wave plate, and polarizer750may be a linear polarizer.

In an example mode of operation, input light771propagates in an optical fiber702and is coupled to guiding layer703of photonic integrated circuit709as coupled light772. Scattering element705scatters a fraction of coupled light772as a scattered signal774, which apparatus700detects. Specifically, lens710collects scattered signal774and directs it to sensor760via at least one of birefringent element720, tube lens730, Bertrand lens740, and polarizer750. Birefringent element720and a polarizer750enable apparatus700to measure the polarization state of coupled light772. Sensor760outputs a detected signal769.

In embodiments, apparatus700includes a data processor780that is communicatively coupled to sensor760and receives detected signal769. Data processor780includes a processor781, a memory782communicatively coupled thereto, and may also include data acquisition hardware784. Each of processor781, memory782, and data acquisition hardware784may be communicatively coupled, directly or indirectly, e.g., via a bus786. In embodiments, sensor760is communicatively coupled to data acquisition hardware784. Memory782stores non-transitory computer-readable instructions as software783. When executed by processor781, software783causes processor781to implement the functionality of data processor780as described herein. Software783may be, or include, firmware.

Memory782may be transitory and/or non-transitory and may include one or both of volatile memory (e.g., SRAM, DRAM, computational RAM, other volatile memory, or any combination thereof) and non-volatile memory (e.g., FLASH, ROM, magnetic media, optical media, other non-volatile memory, or any combination thereof). Part or all of memory782may be integrated into processor781.

Example Application 2: Measuring Polarization Changes Through Waveguide Segments and Components

FIG.8is a schematic of an apparatus800detecting light scattered by photonic integrated circuit200, which may be used for polarization measurements of example applications 1-3. Apparatus800may also be used to monitor changes in the polarization of light through waveguide segments and components. Apparatus800is an example of apparatus700that does not include Bertrand lens740, such that the shape of the point spread function at the waveguide layer provides unambiguous information about the polarization of the light in the guide.

Apparatus800includes a lens810, sensor760, and in order of increasing distance along an optical path between lens710and sensor760, at least one of birefringent element720, tube lens730, and polarizer750. Lens810is located at distance812from scattering element705. Lens810may be a positive lens, such as a microscope objective, and may have a focal length between one millimeter and two-hundred millimeters.

In an example mode of operation, input light771propagates in optical fiber702and is coupled to guiding layer703of photonic integrated circuit709as coupled light772. Scattering element705scatters a fraction of coupled light772as scattered signal774, which apparatus800detects. Specifically, lens810collects scattered signal774and directs it to sensor760via at least one of birefringent element720, tube lens730, and polarizer750. Birefringent element720and a polarizer750enable apparatus700to measure the polarization state of guided mode272or572. In embodiments, lens810collimates scattered signal774. Each of scattered signal274and574is an example of scattered signal774. Sensor760outputs a detected signal869.

Forming a pupil image of a pair of scatterers forms an interference pattern that may reveal the phase shift along guiding layer703between the pair of scatterers. When the measurement is carried out with a broadband or tunable source, the dispersion and effective index of the guide may be measured. By carrying out such a measurement with a tunable laser source, it is also possible to measure propagation-induced changes in polarization as a function of wavelength.

Example Application 3: Polarization Measurement

A use of embodiments disclosed herein is for polarization measurement along a photonic integrated circuit. Scattering elements250and550may also be used for applications that were previously outlined for silicon scattering element with a silicon waveguide. These applications include: monitoring input coupling for fiber attachment, measuring waveguide loss and dispersion, with calibrating optical backscatter. The silicon-nitride elements introduced above have the same function as they introduce little loss (less than silicon scatters) and still produce a signal at the camera sensor.

Example Application 4: Measuring Waveguide Loss

By monitoring a sequence of scattering elements705along the length of guiding layer703, it is possible to precisely probe the light inside guiding layer703by means of the irradiance on the sensor at the image of scattering elements705. By comparing the strength of the scattered signal at discrete points along guiding layer703, it is possible to make a direct measurement of the propagation losses in a waveguide formed by guiding layer703and the cladding layer of photonic integrated circuit709.

For example,FIG.9is a schematic of an apparatus900detecting light scattered by scattering elements905(1) and905(2) of a photonic integrated circuit909. Photonic integrated circuit909is an example of photonic integrated circuit709, where scattering elements905replaces each scattering element705. Apparatus900is an example of apparatus800that includes tube lens730. Apparatus900captures a scattered signal974, which includes respective scattered signals from scattering elements905(1,2). Lenses810and730image scattering elements905onto sensor760as scattering-element images965(1) and965(2). Comparing the signal strength of scattering-element images965, e.g., their respective intensities, yields an accurate measurement of propagation loss in waveguide708.

Example Application 5: Measuring Polarization Changes Through Waveguide Segments and Components

In embodiments, apparatus900includes birefringent element720and polarizer750. In such embodiments, apparatus900may also be used to monitor changes in the polarization of light through waveguide segments and components. Birefringent element720may be configured such that the shape of the point spread function provides unambiguous information about the polarization of the light in the guide. By carrying out such a measurement with a tunable laser source, it is also possible to measure propagation-induced changes in polarization as a function of wavelength.

Example Application 6: Measuring Dispersion of Waveguide Segments and Components

FIG.10is a schematic of an apparatus1000for characterizing photonic integrated circuit909by forming a pupil image, of scattering elements905, on sensor760. Whereas apparatus900forms a direct image of photonic integrated circuit909, apparatus1000includes a lens940, which forms a pupil image. Lens940may be a Bertrand lens, which forms a two-dimensional pupil image, or a cylindrical lens, which forms a one-dimensional image along a direction between scattering elements905. The pupil image includes an interference pattern1065, from which the phase shift of coupled light772between scattering elements905may be determined. When the measurement is carried out with a broadband or tunable source, the dispersion and effective index of waveguide708can be measured.

Example Application 7: Calibrating Optical Backscatter

FIG.11is a schematic of photonic integrated circuit909optically coupled to an optical backscatter sensor1100, hereinafter sensor1100. Sensor1100may be a reflectometer. Photonic integrated circuit909may be used to calibrate sensor1100, for example, when the following is known with high accuracy (i) a physical separation between scattering elements905along guiding layer703, and (ii) the amount of coupled light772each scattering element905back-scatters into guiding layer703. The back-scattered light, return signal1172inFIG.11, propagates in guiding layer703in a direction opposite that of coupled light772. Sensor1100may be communicatively coupled to data processor780, which processes return signal1172to calibrate sensor1100.

FIG.12is a flowchart illustrating a method1200for characterizing a photonic integrated circuit, such as photonic integrated circuits200and700. Method1200includes at least steps1210,1220, and1230.

Step1210includes coupling an evanescent field into a scattering element adjacent to a guiding layer of the photonic integrated circuit. The evanescent field is of an optical mode of light propagating in the guiding layer and has an in-medium wavelength in the guiding layer. A maximum spatial dimension of the scattering element is less than the in-medium wavelength. In a first example of step1210, an evanescent field of guided mode272is coupled into scattering element250, of PIC200,FIG.7. In a second example of step1210, an evanescent field of guided mode572is coupled into scattering element550of photonic integrated circuit500,FIG.5.

Step1220includes scattering, with the scattering element, the coupled evanescent field as a reference scattered-signal. In an example of step1220, scattering element250scatters the evanescent field of guided mode272as scattered signal274,FIG.2. In a second example of step1220, scattering element550scatters the evanescent field of guided mode572as scattered signal574,FIG.5. Each of scattered signals274and754is an example of scattered signal774,FIG.7.

Step1230includes detecting one of the reference scattered-signal and a signal derived therefrom. In an example of step1230, one of apparatus700and apparatus800detects scattered signal774,FIG.7. When the guiding layer includes a defect that produces a defect scattered-signal, step1230may include at least one of steps1232and1234. Step1232includes forming an interference signal from interference between the reference scattered-signal and the defect scattered-signal. In an example of step1232, scattered signal774and defect signal214interfere to yield an interference signal.

Step1234includes locating the defect from a spatial-frequency domain representation of the interference signal. In an example of step1234, sensor760includes a detector array, and data processor780generates a spatial-frequency domain representation of detected signal769and locates defect scatterer213from the spatial-frequency domain representation. In embodiments, data processor780determines defect scatterer213from a dominant spatial-frequency component of the spatial-frequency domain representation, e.g., a spatial frequency component with the largest amplitude. The angle between defect scatterer213and scattering element550determines the spatial frequency corresponding to this dominant spatial-frequency component, as respective detected signal769includes interference between scattered signal774and defect signal214. The vertex of the angle may be along an optical axis of lens710or lens810, for example, the vertex is in the entrance-pupil plane or exist-pupil plane of lens710or lens810. The vertex location may be located at working distance of lens710or810from scattering element705. The location of defect scatterer213may be determined trigonometrically with the working distance, and the angle determined from the dominant spatial frequency component.

When the scattering element is one of a plurality of scattering elements positioned adjacent to the guiding layer, method1200may include a step1240. Step1240includes repeating the steps of coupling (step1210), scattering (step1220), and detecting (step1230) to yield a respective reference scattered-signal of a plurality of reference scattered-signals and a respective scattered-light amplitude of a plurality of scattered-light amplitudes. In an example of step1240, steps1210,1220, and1230are repeated for each scattering element 250(k ≥ 2) or for each scattering element 550(k ≥ 2).

When method1200includes step1240, step1230may include at least one of steps1236,1238, and1239. Step1236includes forming an interference signal from interference between the reference scattered-signal and the additional reference scattered-signal. In an example of step1236, photonic integrated circuit709includes an additional scattering element705(2) that produces an additional scattered signal774(2), which interferes with scattered signal774to produce an interference signal. Plots403and404ofFIG.4illustrate examples of the interference signal.

Step1238includes determining a phase difference from the interference signal. Step1239includes determining an effective refractive index, e.g. a mode index, of the guiding layer from the phase difference. In example of step1238, data processor780of apparatus700(or800) determines a phase difference from detected signal769(or869). In example of step1239, data processor780determines an effective refractive index of guiding layer703. In embodiments, memory782stores a distance between scattering elements705and705(2), and uses this distance in at least one of steps1238and1239.

Combinations of Features

Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following enumerated examples illustrate some possible, non-limiting combinations

(A1) A photonic integrated circuit includes a substrate, a cladding layer, and a guiding layer. The substrate has a substrate top-surface and a substrate refractive index. The cladding layer is on the substrate top-surface and has a cladding top-surface and a cladding refractive index. The guiding is layer located between the substrate top-surface and the cladding top-surface and having (i) a core refractive index exceeding both the substrate refractive index and the cladding refractive index, (ii) a guiding-layer thickness above the substrate top-surface, and (iii) a guiding-layer width. In a direction parallel to the substrate top-surface, the guiding layer supports an optical mode that extends to a decay-range into the cladding layer in the direction. An in-medium wavelength of the optical mode exceeds both the guiding-layer thickness and the guiding-layer width. The scattering element is (i) located between the substrate top-surface and the cladding top-surface and (ii) separated from the guiding layer by a gap-distance that is less than the decay-range, and has (a) a maximum spatial dimension that is less than the in-medium wavelength, and (b) a scattering refractive index that exceeds the cladding refractive index.

(A2) In embodiments of photonic integrated circuit (A1) the gap-distance is a distance between the scattering element and a position on the guiding layer. Such embodiments further include a plurality of additional scattering elements each separated from a respective additional position of a plurality of positions along the guiding layer by less than the decay-range. Each of the plurality of positions corresponds to a respective propagation distance of the optical mode in the guiding layer from the position. A distance between adjacent scattering elements of the plurality of additional scattering elements exceeds the guiding-layer width. Each of the plurality of additional scattering elements has a same size and shape as the scattering element.

(A3) In embodiments of photonic integrated circuit (A2), each of the plurality of additional scattering elements has, with respect to the guiding layer, a same orientation as the scattering element.

(A4) In embodiments of any one of photonic integrated circuits (A1) - (A3), the maximum spatial dimension is at least one of (i) less than the guiding-layer width and (ii) greater than fifty nanometers.

(A5) In embodiments of any one of photonic integrated circuits (A1) - (A4), the maximum spatial dimension is at least one of (i) less than the guiding-layer thickness and (ii) greater fifty nanometers.

(A6) In embodiments of any one of photonic integrated circuits (A1) - (A5), the cladding layer completely surrounds the guiding layer in a plurality of transverse planes perpendicular to the substrate top-surface, such that the guiding layer and the cladding layer form a photonic wire.

(A7) In embodiments of any one of photonic integrated circuits (A1) - (A6), the gap-distance is at least one of (i) greater than γ and (ii) less than 6γ, where γ is a distance from the guiding layer at which the intensity of the optical mode decays to ⅟e of its maximum value.

(A8) In embodiments of any one of photonic integrated circuits (A1) - (A7), the in-medium wavelength is a free-space wavelength between 1.1 micrometers and 1.8 micrometers divided by a refractive index of the guiding layer at the free-space wavelength.

(A9) In embodiments of any one of photonic integrated circuits (A1) - (A8), the guiding-layer width is between 0.15 micrometers and 0.5 micrometers.

(A10) In embodiments of any one of photonic integrated circuits (A1) - (A9), the guiding-layer thickness is between 0.15 micrometers and 0.3 micrometers.

(A11) In embodiments of any one of photonic integrated circuits (A1) -(A10), the scattering element and the guiding layer is formed of a same material.

(A12) In embodiments of any one of photonic integrated circuits (A1) -(A11), the scattering element is formed of a one of a dielectric, a metal, a semiconductor, and a combination thereof.

(A13) In embodiments of any one of photonic integrated circuits (A1) -(A12), the scattering element is adjacent to the guiding layer in a direction parallel to the substrate top-surface, the gap-distance being along the direction.

(A14) In embodiments of photonic integrated circuit (A13), the scattering element and the guiding layer have respective bottom surfaces, proximate the substrate top-surface, that are coplanar in a plane parallel to the substrate top-surface.

(A15) In embodiments of any one of photonic integrated circuits (A1) -(A14), in a surface-normal direction perpendicular to the substrate top-surface, the guiding layer is between the scattering element and the substrate-top surface, and the gap-distance is along the surface-normal direction.

(B1) A method for characterizing a photonic integrated circuit comprising includes coupling an evanescent field into a scattering element adjacent to a guiding layer of the photonic integrated circuit. The evanescent field is of an optical mode of light propagating in the guiding layer, and has an in-medium wavelength in the guiding layer. A maximum spatial dimension of the scattering element is less than the in-medium wavelength. The method includes scattering, with the scattering element, the coupled evanescent field as a reference scattered-signal. The method also includes detecting one of the reference scattered-signal and a signal derived therefrom.

(B2) In embodiments of method (B1), when the guiding layer includes a defect that produces a defect scattered-signal, said step of detecting includes: forming an interference signal from interference between the reference scattered-signal and the defect scattered-signal; and locating the defect from a spatial-frequency domain representation of the interference signal.

(B3) When the scattering element is one of a plurality of scattering elements positioned adjacent to the guiding layer, embodiments of either one of methods (B2) and (B3) include, for each of the plurality of scattering elements: repeating the steps of coupling, scattering, and detecting to yield a respective reference scattered-signal of a plurality of reference scattered-signals and a respective scattered-light amplitude of a plurality of scattered-light amplitudes.

(B4) When the plurality of reference scattered-signals includes the reference scattered-signal and an additional reference scattered-signal scattered from an additional scattering element of the plurality of scattering elements, embodiments of any one of methods (B1) - (B3) include, as part of the step of detecting: forming an interference signal from interference between the reference scattered-signal and the additional reference scattered-signal; determining a phase difference from the interference signal; and determining an effective refractive index of the guiding layer from the phase difference.

(B5) In embodiments of method (B4), the step of detecting includes determining a polarization of the reference scattered-signal.

(B6) In embodiments of any one of methods (B1) - (B5), the evanescent field is of an optical mode propagating in the photonic integrated circuit.

Changes may be made in the above methods and systems without departing from the scope of the present embodiments. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. Herein, and unless otherwise indicated the phrase “in embodiments” is equivalent to the phrase “in certain embodiments,” and does not refer to all embodiments. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.