Photonic circuit and method of manufacturing thereof

Abstract

A photonic circuit (200) is manufactured as follows., A strip of silicium carbide (107) is added on top of an optically functional layer (104) supported by a substrate (105). The optically functional layer (104) is between 100 nm and 500 nm thick. Prior to the adding of the strip of silicium carbide (107), a protective layer (106) may be deposited on the optically functional layer (104).

Description

[0001] Photonic circuit and method of manufacturing thereof.

[0002] FIELD OF THE INVENTION

[0003] An aspect of the invention relates to a method of manufacturing a photonic circuit. The manufacturing of the photonic circuit may involve a material, such as, for example, lithium niobate, barium titanate, or another material having a refractive index that varies with a magnitude of an electric field. Other aspects of the invention relate to a photonic circuit and a photonic system.

[0004] BACKGROUND ART

[0005] A photonic circuit generally comprises an optically functional layer on top of which other elements may be added to form functional components for photonic signal processing. For example, a waveguide may be formed by adding on top of the optically functional layer a strip of material having an appropriate refractive index and thickness relative to that of the optically functional layer. The strip of material may be added using, for example, a photolithographic technique.

[0006] The optically functional layer of a photonic circuit may comprise a material having a refractive index that varies with a magnitude of an electric field. This allows the photonic circuit to actively process photonic signals similar to an electrical integrated circuit that processes electrical signals, but at significantly higher speeds. This makes the photonic circuit particularly suitable for applications that require fast processing, such as, for example quantum computing and artificial intelligence.

[0007] For an efficient processing of photonic signals, it is desirable that the optically functional layer has a relatively high Pockels coefficient. The higher the Pockels coefficient is, the greater the extent to which the refractive index varies with the electric field. Certain oxide-based crystals have a relatively high Pockels coefficient. These include, for example, lithium niobate (LiNbCh), barium titanate (BaTiCh), potassium niobate (KNbCh), and materials designated by the acronym KTN. The optically functional layer may thus comprise at least one of the aforementioned materials.

[0008] However, materials that have a relatively high Pockels coefficient are generally difficult to process. To begin with, these materials are difficult to etch. This etching difficulty may result in significant sidewall roughness, which adversely affects optical signal processing due to, for example, scattering. Moreover, the materials are susceptible to degradation when processed, due to, for example, an undesired chemical reaction causing a change in chemical composition or surface roughness, or both. This may reduce the Pockels coefficient and adversely affect other optical properties. All this makes manufacturing of photonic circuits complex and therefore costly. Low-cost, mass production of photonic circuits has not yet been achieved.

[0009] There is another factor that stands in the way of low-cost, mass production of photonic circuits. Materials that have a relatively high Pockels coefficient are generally not accepted in semiconductor manufacturing plants. This is because these materials may contaminate equipment used for manufacturing semiconductor circuits, in particular those of the CMOS type. Unless decontaminated, this may render this equipment no longer suitable for manufacturing semiconductor circuits. In particular, lithium and barium may have detrimental effects on semiconductor manufacturing equipment.

[0010] Patent publication WO2022248037A1 describes a strip-loaded optical waveguide that comprises a slab portion and a strip portion. The strip portion is disposed over or on the slab portion to define a light confinement region in the slab and strip portions. A refractive index of the strip portion is lower than a refractive index of the slab portion. The strip portion is made out of a polymer material.

[0011] SUMMARY OF THE INVENTION

[0012] There is a need for a photonic circuit manufacturing technique that offers an improvement in at least one of the following aspects: manufacturing cost, manufacturing yield, and manufacturing quality, in particular in terms of photonic circuit performance.

[0013] An aspect of the invention relates to a method of manufacturing a photonic circuit. The method comprises: adding a strip of silicon carbide on top of an optically functional layer supported by a substrate, whereby the optically functional layer is between 100 nm and 500 nm thick.

[0014] A further aspect of the invention relates to a photonic circuit obtained by the method cited hereinbefore, wherein the optically functional layer and the strip of silicon carbide on top thereon jointly form a waveguide for a photonic signal having a wavelength in vacuum. Yet a further aspect of the invention relates to a photonic system comprising an optical coupler arranged between a waveguide in a photonic circuit as defined hereinbefore and a discrete waveguide, the optical coupler comprising a cylindrical lens or a prismatic lens.

[0015] In each of these aspects, there is no need for etching the optically functional layer to form a waveguide. A waveguide is formed by the strip of silicon carbide being added to the optically functional layer. The waveguide formed in this manner has relatively good optical properties. One factor that accounts for this is that silicon carbide has a band gap that is relatively large compared to photon energy in light propagating through the waveguide. This prevents so-called dangling bonds from causing scattering and absorption in the waveguide.

[0016] Light propagating through the waveguide will primarily propagate through the optically functional layer rather than through the strip of silicon carbide, in particular if this strip is a relatively thin. This further contributes to achieving relatively low propagation loss. Moreover, the waveguide will typically support a single mode only for light propagation. The strip of silicon carbide being relatively thin, the strip has relatively short sidewalls, which prevents scattering due to sidewall roughness. In addition, since most of the light is concentrated in the optically functional layer, the light may efficiently be modulated by an electric field. That is, a high modulation efficiency may be achieved: a relatively weak electrical field is sufficient to modulate the light to a desired degree.

[0017] Since the strip of silicon carbide may be relatively thin, this facilitates application of photoresist for masking and pattering of other structures on the photonic circuit. These other structures may thus be accurately defined, allowing a relatively high yield and a relatively good performance of the photonic circuit. Each of the aforementioned factors thus offers an improvement in at least one of the following aspects: manufacturing cost, manufacturing yield, and manufacturing quality, in particular in terms of photonic circuit performance.

[0018] In an embodiment, the strip of silicon carbide is less than 250 nm thick.

[0019] In an embodiment, a protective layer is deposited on the optically functional layer prior to adding the strip of silicon carbide on top of the optically functional layer.

[0020] In an embodiment, the protective layer has a refractive index that is lower than that of the optically functional layer.

[0021] In an embodiment, the protective layer is less than 50 nm thick.

[0022] In an embodiment, the protective layer is thicker than 3 nm. In an embodiment, the protective layer comprises at least one of the following materials: silicon nitride, aluminum oxide, nickel oxide, and chromium oxide.

[0023] In an embodiment, after adding the strip of silicon carbide, the protective layer is removed in areas adjacent to the strip of silicon carbide.

[0024] In an embodiment, the optically functional layer has a refractive index that varies with a magnitude of an electric field.

[0025] In an embodiment, a polarization density in the optically functional layer varies non linearly with the magnitude of the electric field.

[0026] In an embodiment, the optically functional layer comprises at least one of the following materials: lithium niobate, barium titanate, potassium niobate, and materials designated by the acronym KTN.

[0027] In an embodiment, adding the strip of silicon carbide on top of the optically functional layer comprises: adding a layer of silicon carbide on top of the optically functional layer; and etching at least a portion of the silicon carbide layer so as to form the strip of silicon carbide.

[0028] In an embodiment, the strip of silicon carbide is less wide than twice the wavelength in vacuum of the photonic signal.

[0029] In an embodiment, the strip of silicon carbide is wider than one tenth of the wavelength in vacuum of the photonic signal.

[0030] In an embodiment, the strip of silicon carbide has a cross-sectional area that is less than — X2, being the wavelength in vacuum divided by the modal index of the photonic signal.

[0031] In an embodiment, the optically functional layer is less thick than a third of the wavelength in vacuum of the photonic signal.

[0032] In an embodiment, the optically functional layer is thicker than one twentieth of the wavelength in vacuum of the photonic signal.

[0033] In an embodiment, the cylindrical lens is comprised in the photonic circuit.

[0034] In an embodiment, the cylindrical lens is fixedly coupled to an end of the discrete waveguide.

[0035] In an embodiment, the discrete waveguide is an optical fiber. For the purpose of illustration, some embodiments of the invention are described in detail with reference to accompanying drawings. In this description, additional features will be presented, some of which are defined in the dependent claims, and advantages will be apparent.

[0036] BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. l is a flow chart diagram of a method of manufacturing a photonic circuit.

[0038] FIG. 2 is a schematic top view of a part of a first exemplary photonic circuit.

[0039] FIG. 3 is a schematic cross-sectional diagram of the part of the first exemplary photonic circuit.

[0040] FIG. 4 is a schematic top view of a part of a second exemplary photonic circuit.

[0041] FIG.5 is a schematic cross-sectional diagram of the part of the second exemplary photonic circuit.

[0042] FIG. 6 is a schematic block diagram of a photonic system.

[0043] DESCRIPTION OF SOME EMBODIMENTS

[0044] FIG. 1 schematically illustrates an exemplary method 100 of manufacturing a photonic circuit. FIG. 1 provides a flow chart diagram of this method 100, which comprises several steps 101-103. The method 100 is applied to an optically functional layer 104 supported by a substrate 105. The optically functional layer 104 may have a refractive index that varies with a magnitude of an electric field. Furthermore, a polarization density in the optically functional layer 104 may vary non linearly with the magnitude of the electric field. This allows the photonic circuit to actively process photonic signals similar to an electrical integrated circuit that processes electrical signals, but at significantly higher speeds. This makes the photonic circuit particularly suitable for applications that require fast processing, such as, for example quantum computing and artificial intelligence.

[0045] For an efficient processing of photonic signals, it is desirable that the optically functional layer 104 has a relatively high Pockels coefficient. The higher the Pockels coefficient is, the greater the extent to which the refractive index varies with the electric field. Certain oxide-based crystals have a relatively high Pockels coefficient. These include, for example, lithium niobate (LiNbCh), barium titanate (BaTiCh), potassium niobate (KNbCh), lithium iodate (LiIO3), monopotassium phosphate (KH2PO4, KDP), lithium triborate (LBO), P-barium borate (BBO), gallium selenide (GaSe), potassium titanyl phosphate (KTP), ammonium dihydrogen phosphate (ADP), and materials designated by the acronym KTN. The optically functional layer 104 may thus comprise at least one of the aforementioned materials.

[0046] The optically functional layer 104 may be relatively thin. Specifically, the optically functional layer 104 may be thinner than a nominal wavelength in vacuum of photonic signals to be processed by the photonic circuit to be manufactured. For example, the optically functional layer 104 may be between 100 nm and 500 nm thick. More specifically, the optically functional layer 104 may be between 200 nm and 300 nm thick and, even more specifically, may be between 250 nm and 300 nm thick. A thickness, or rather thinness, within any of these indicated ranges may provide advantages that will be discussed hereinafter.

[0047] In a first step 101, a protective layer 106 is deposited on the optically functional layer 104. The protective layer 106 may have a refractive index that is lower than that of the optically functional layer 104. The protective layer 106 may comprise, for example, silicon nitride. Alternatively, or additionally, the protective layer 106 may comprise aluminum oxide, chromium oxide, or nickel oxide, or any combination of these materials.

[0048] The protective layer 106 may be relatively thin, at least thinner than the optically functional layer 104. The protective layer 106 may be, for example, less than 50 nm thick. Notwithstanding, the protective layer 106 may be thicker than 3 nm. A thickness, or rather thinness, within these boundaries may provide advantages that will be discussed hereinafter. Satisfactory experimental results have been obtained with a protective layer 106 of about 5 to 30 nm thick.

[0049] The protective layer 106 may protect the optically functional layer 104 in subsequent manufacturing steps, some of which will be discussed hereinafter. There is a risk of the optically functional layer 104 being altered in a subsequent manufacturing step. This, in turn, may adversely affect one or more desired optical properties of the optically functional layer 104 and thereby degrade performance of the photonic circuit that is manufactured.

[0050] The protective layer 106 may also have a sealing function preventing material comprised in the optically functional layer 104 from contaminating an environment in which at least a part of the manufacturing method 100 is carried out. For example, in case the optically functional layer 104 comprises lithium niobate, this material may contaminate equipment that is used for manufacturing semiconductor circuits, in particular those of the CMOS type. Unless decontaminated, this may render this equipment no longer suitable for manufacturing semiconductor circuits. Thus, the protective layer 106 favors manufacturing photonic circuits, at least partially, in manufacturing sites that have been set up for manufacturing electrical circuits and should continue to do so.

[0051] In a second step 102, a strip of silicon carbide 107 is added on top of the optically functional layer 104. The strip of silicon carbide 107 is relatively thin, although this strip may be thicker than the protective layer 106. The strip of silicon carbide 107 may be less than 250 nm thick. This feature will be in greater detail discussed hereinafter, as well as its advantages.

[0052] In this embodiment, the protective layer 106 constitutes an intermediate between the optically functional layer 104 and the strip of silicon carbide 107. This protects the optically functional layer 104 against potentially damaging effects associated with the addition of the strip of silicon carbide 107. For example, the addition of the strip of silicon carbide 107 generally involves deposition of silicon carbide using a mixture of silane and methane gases. Hydrogen is released during this process. The optically functional layer 104 is then exposed to hydrogen in a relatively strong concentration. This may affect the optically functional layer 104 through chemical reaction, in particular if the optically functional layer 104 is oxide-based. The chemical reaction of hydrogen with oxide in the optically functional layer 104, which produces water, may significantly affect desired optical properties of this layer. The protective layer 106 constitutes a shield preventing this, or at least, significantly mitigating this degradation of the optically functional layer 104.

[0053] Silicon nitride is a suitable choice of material for the protective layer 106, although other materials may also provide satisfactory results. Depositing a layer of silicon nitride on the optically functional layer 104 may also involve use of silane gas in addition to nitrogen. However, the silane gas may be used in a relatively low concentration, whereas nitrogen is used in a relatively strong concentration. The optically functional layer 104 will therefore be exposed to hydrogen in a significantly lower concentration than in the process of depositing silicon carbide discussed hereinbefore. Moreover, an impermeable layer of silicon nitride is quickly formed on the optically functional layer 104. This is because a layer of silicon nitride is already substantially free of pinholes beyond a thickness of about 3 nm only. Indeed, it was observed that depositing a layer of silicon nitride on the optically functional layer 104 had no significant adverse effect on this layer.

[0054] The strip of silicon carbide 107 may be added using, for example, a photolithographic technique. First, a layer of silicon carbide is added on top of the protective layer 106. Subsequently, a layer of lithographic material is added on top of the layer of silicon carbide. The lithographic material is processed so as to form a lithographic mask defining a topology of strips and other geometric forms. Then, the layer of silicon carbide is etched. The silicon carbide is thereby removed in exposed areas, which are not covered by the lithographic mask. Conversely, the silicon carbide will remain on top of the protective layer 106 and thus on the optically functional layer 104 in areas covered by the lithographic mask.

[0055] In a third step 103, which is optional, at least some areas of the protective layer 106 that are not covered by silicon carbide are removed. The protective layer 106 will remain present between the optically functional layer 104 and the strip of silicon carbide 107. However, the protective layer 106 will no longer be present in areas adjacent to the strip of silicon carbide 107. This selective removal of the protective layer 106 may provide advantages that will be discussed hereinafter.

[0056] The method 100 of manufacturing the photonic circuit may comprise further processing steps. These further processing steps may include, for example, any of the following ones. The strip of silicon carbide 107 may undergo machining, in particular its edges. Electrodes may be deposited on the photonic circuit. In order to further protect the optically functional layer 104, as well as other parts, a relatively thick additional protective layer may be deposited on the photonic circuit. Further materials may be implemented in the photonic circuit to make active devices in the photonic circuit, such as, for example, photonic signal sources, which may be in the form of lasers.

[0057] The third step 103, which comprises the selective removal of the protective layer 106, may be carried out in a final stage of the method 100 of manufacturing the photonic circuit. Accordingly, the protective layer 106 may thus remain intact in the further processing steps that the method 100 may comprise, some of which were indicated hereinbefore. The protective layer 106 may thus prevent the optically functional layer 104 from being adversely altered during these further processing steps.

[0058] FIGS. 2 and 3 schematically illustrate a part of a first exemplary photonic circuit 200. FIG. 2 provides a schematic top view of the part of the first exemplary photonic circuit 200. FIG. 3 provides a schematic cross-sectional diagram of the part of the first exemplary photonic circuit 200 along a cut line indicated in FIG. 2. The first exemplary photonic circuit 200 may be obtained by the method 100 described hereinbefore with reference to FIG. 1, wherein the third step 103 has been omitted.

[0059] FIGS. 4 and 5 schematically illustrates a part of a second exemplary photonic circuit 400. FIG. 4 provides a schematic top view of the part of the second exemplary photonic circuit 400. FIG. 5 provides a schematic cross-sectional diagram of the part of the second exemplary photonic circuit 400 along a cut line indicated in FIG. 4. The second exemplary photonic circuit 400 may be obtained by the method 100 described hereinbefore with reference to FIG. 1, wherein the third step 103 has been carried out. The first exemplary photonic circuits 200 and the second exemplary photonic circuit 400 illustrated in FIGS. 2 and 3 and in FIGS. 4 and 5, respectively, will be jointly referred to hereinafter as the exemplary photonic circuits 200, 400 for ease of reading. Like elements are denoted by like references.

[0060] The exemplary photonic circuits 200, 400 each comprise an optically functional layer 104 supported by a substrate 105. The optically functional layer 104 may have features corresponding to those described hereinbefore with regard to the method 100 of manufacturing illustrated in FIG. 1. The substrate 105 may be a composite of a basic substrate 301 and a layer of material 302 having a refractive index lower than that of the optically functional layer 104. The basic substrate 301 may be any suitable material providing satisfactory structural support.

[0061] The exemplary photonic circuits 200, 400 each comprise a strip of silicon carbide 107 on top of the optically functional layer 104. The exemplary photonic circuits 200, 400 further comprise a protective layer 106 that constitutes an intermediate between the optically functional layer 104 and the strip of silicon carbide 107. The strip of silicon carbide 107 and the protective layer 106 may have features corresponding to those described hereinbefore with regard to the method 100 of manufacturing illustrated in FIG. 1.

[0062] In the first exemplary photonic circuit 200, which is illustrated in FIGS. 2 and 3, the protective layer 106 extends over the optically functional layer 104 beyond an area covered by the strip of silicon carbide 107. In the second exemplary photonic circuit 400, which is illustrated in FIGS. 4 and 5, the protective layer 106 is present in the area covered by the strip of silicon carbide 107 but is not present in areas adjacent to the strip of silicon carbide 107. Advantages of this latter feature will be discussed hereinafter. In the exemplary photonic circuits 200, 400, the strip of silicon carbide 107 on top of the optically functional layer 104 defines a waveguide 303 within the photonic circuits. The waveguide 303 is essentially comprised of a section in the optically functional layer 104 located between the strip of silicon carbide 107 and the substrate 105. This section forms a channel in the optically functional layer 104 as can be understood from FIGS. 3 and 5.

[0063] The waveguide 303 generally serves to transfer a photonic signal from one photonic device to another photonic device within the photonic circuit. In practice, the photonic signal will be attenuated to some extent and otherwise adversely affected while propagating through the waveguide 303. That is, there is propagation loss in the waveguide 303. The propagation loss should generally be as low as possible, at least below a critical level. It is further generally desirable that the waveguide 303 is single mode, that is, supports only one propagation mode. This helps eliminate unwanted effects such as modal interference.

[0064] The waveguide 303 may comprise a bend 201 as illustrated in FIGS. 2 and 4. The propagation loss in the bend 201 is generally higher than that in a straight-line section of the waveguide 303. The propagation loss in the bend 201 may be referred to as bend loss. The sharper the bend 201 is, the higher the bend loss is. That is, the smaller the radius of the bend 201 is, the higher the bend loss is. Bend loss is of particular concern in photonic circuits. Photonic circuits generally need to be compact. This may make that waveguides need to include relatively sharp bends. These bends may have a radius in the order of micrometers.

[0065] In the exemplary photonic circuits 200, 400, the propagation loss in the waveguide 303 may be relatively low. This makes that these photonic circuits may achieve satisfactory performance while being relatively compact and relatively easy to manufacture as discussed hereinbefore. One factor that accounts for this is the optically functional layer 104 being shielded by the protective layer 106. This prevents degradation of the optically functional layer 104, which entails a higher propagation loss and a decrease of the Pockels coefficient of this layer. The protective layer 106 thus prevents this.

[0066] Propagation loss, in particular bend loss, may be somewhat lower in the second exemplary photonic circuit 400 illustrated in FIGS. 4 and 5 than in the first exemplary photonic circuit 200 illustrated in FIGS. 2 and 3. That is, removing the protective layer 106 in areas adjacent to the strip of silicon carbide 107 may somewhat reduce propagation loss, in particular bend loss. Thus, it may be advantageous to carry out the third step 103 of the method 100 of manufacturing a photonic circuit described hereinbefore with reference to FIG. 1.

[0067] Features that further contribute to achieving low propagation loss and that provide additional advantages are discussed in what follows.

[0068] The strip of silicon carbide 107 may be less wide than the wavelength in vacuum of the photonic signal propagating through the waveguide 303. Namely, in case the strip is wider than this wavelength, the waveguide 303 may be multimode, supporting multiple propagation modes. As an initial design step, the strip of silicon carbide 107 may have a width that is slightly less that the aforementioned wavelength. As a further design step, this width may be somewhat reduced to manipulate a shape of the single mode to improve efficiency of coupling the photonic signal into or out of the waveguide 303. If the width reduced too much, there is no waveguide 303 anymore. That is, a too narrow strip of silicon carbide 107 on top of the optically functional layer 104 does not define a waveguide 303 therein.

[0069] For example, let it be assumed that the wavelength in vacuum of the photonic signal is 1550 nm. The strip of silicon carbide 107 may then be between 800 nm and 1400 nm wide. More specifically, the strip may be between 1000 nm and 1300 nm wide. Satisfactory experimental results have been obtained with the strip of silicon carbide 107 being about 1200 nm wide, whereby the wavelength in vacuum of the photonic signal was 1550 nm.

[0070] The strip of silicon carbide 107 may be between 20 nm and 250 nm thick. More specifically, the strip of silicon carbide 107 may be between 50 nm and 150 nm thick. Satisfactory experimental results have been obtained with the strip of silicon carbide 107 being about 80 nm thick. In general, there is a proportional relation between the thickness of the strip of silicon carbide 107 and that of the optically functional layer 104. A specific application for which the photonic circuit is intended may also play a role with regard to the thickness of the optically functional layer 104 and that of the strip of silicon carbide 107. As a rule of thumb, the strip of silicon carbide 107 preferably has a cross- sectional area that is less than / 7, , being the wavelength in vacuum divided by the modal index of the photonic signal. If the cross-sectional area is too small, less than / 7, light in the section in the optically functional layer 104 that is covered by the strip of silicon carbide 107 may not be properly confined within this section, in particular in lateral direction. This may make that this section does not constitute a waveguide 303, at least not properly. Three tables are presented illustrating that the propagation loss depends on both the width and thickness of the silicon carbide strip. Each table comprise several columns representing various widths, and several rows representing various thicknesses of the strip of silicon carbide 107. Each table indicates propagation losses for these various widths and thicknesses. Specifically, a cell indicates a propagation loss for a width corresponding to the column to which the cell belongs, and a thickness corresponding to the row to which the cell belongs. The propagation losses are expressed in units of dB / m. The propagation losses have been determined by computer simulation using a program dedicated to optical analysis. In this computer simulation, lithium niobate was selected as the material constituting the optically functional layer 104.

[0071] Table 1, which is presented here, indicates propagation losses of a straight waveguide. In this table, as well as in the other two tables, “NG” indicates that no waveguide is formed, and “MM” indicates that the waveguide is multimode. Table 2, which is presented here, indicates propagation losses of a bent waveguide with 100 pm bending radius.

[0072] Table 3, which is presented here, indicates propagation losses of a bent waveguide with 200 pm bending radius.

[0073] 5 In general, the propagation loss, in particular the bend loss, may also vary as a function of a ratio between the width of the strip of silicon carbide 107 and the thickness of the optically functional layer 104. An optimum may be found for this ratio where the propagation loss is minimum.

[0074] Concerning the thickness of the optically functional layer 104, there may be 0 a critical maximum thickness and a critical minimum thickness. If the optically functional layer 104 is too thick, thicker than the critical maximum thickness, light in the section covered by the strip of silicon carbidel07 will insufficiently interact with this strip 107. This may make that this section does not constitute a waveguide 303, at least not properly. If the optically functional layer 104 is too thin, thinner than the critical minimum thickness, light in the section covered by the strip of silicon carbide 107 will not be properly confined within this section, in particular in vertical direction. There will be too much propagation loss, in particular too much bend loss. The critical maximum thickness may be, for example, about 500 nm. The critical minimum thickness may be about 100 nm.

[0075] As a rule of thumb, satisfactory performance is obtained if the thickness of the optically functional layer 104 is about the wavelength of the photonic signal in vacuum divided by 4, or somewhat less. Performance aspects, such as, for example, efficient modulation of the photonic signal, low propagation loss, and the single mode having a particular shape, may vary as a function of the thickness of the optically functional layer 104. These performance aspects may have different degrees of importance in different photonic circuit applications. An optimum thickness of the optically functional layer 104 may thus depend on a specific application for which the photonic circuit is intended.

[0076] The protective layer 106 affects to a certain extent optical properties of the waveguide 303, including propagation loss. This is because light in the waveguide 303 is not entirely concentrated in the optically functional layer 104. Light extends into the protective layer 106 interacting with the strip of silicon carbide 107. In case the protective layer 106 is too thick, exceeding a critical maximum thickness, this interaction is insufficient making that no waveguide 303 is constituted, at least not properly. The maximum thickness may be about 50 nm.

[0077] There may be an optimum thickness for the protective layer 106 due to the following two opposite effects. One the one hand, propagation loss may be reduced by a stronger interaction of light in the optically functional layer 104 with the strip of silicon carbide 107. The thinner the protective layer 106 is, the stronger the aforementioned interaction is. On the other hand, propagation loss may be reduced by making that a smaller portion of the photonic signal effectively propagates through the strip of silicon carbide 107. This is because propagation loss in the strip of silicon carbide 107 is generally higher than in the optically functional layer 104. The thicker the protective layer 106 is, the smaller the portion of the photonic signal propagating through the strip of silicon carbide 107 is. However, it is generally preferable to reduce the thickness of the strip of silicon carbide 107 as much as possible to reduce the portion of the photonic signal propagating through this strip 107, thereby reducing propagation loss.

[0078] In any case, the protective layer 106 is preferably sufficiently thick to adequately protect the optically functional layer 104 as discussed hereinbefore with regard to the method 100 illustrated in FIG. 1. In particular, it is desirable that the protective layer 106 is substantially free of pinholes. As discussed hereinbefore, a thickness of about 3 nm is already sufficient for a layer of silicon nitride to be is substantially free of pinholes.

[0079] FIG. 6 schematically illustrates a photonic system 600. FIG. 6 provides a schematic block diagram of the photonic system 600. The photonic system 600 comprises a photonic circuit 601 and a discrete waveguide 602, which is external to the photonic circuit 601. The photonic circuit 601 comprises an integrated waveguide 603. The photonic circuit 601 may correspond to, for example, the first exemplary photonic circuit 200 illustrated in FIGS. 2 and 3 or the second exemplary photonic circuit 400 illustrated in FIGS. 4 and 5. The discrete waveguide 602 may be in the form of, for example, an optical fiber 602. The discrete waveguide 602 will be referred to hereinafter as the optical fiber 602 for ease of reading. The optical fiber 602 may serve to apply a photonic signal to the photonic circuit 601, or to read out a photonic signal from the photonic circuit 601, or both.

[0080] A photonic signal propagating through the optical fiber 602 will generally have a circular optical mode. However, a photonic signal propagating through the integrated waveguide 603 within the photonic circuit 601 will generally have an elliptical optical mode, which may be strongly elliptically shaped. This is because the optically functional layer 104 in the photonic circuit 601 is relatively thin as discussed hereinbefore. There may be relatively little overlap between the circular optical mode in the optical fiber 602 and the elliptical optical mode in the integrated waveguide 603 within the photonic circuit 601. This lack of overlap is a potential source of loss in transferring a photonic signal from the optical fiber 602 to the integrated waveguide 603, and vice versa. This loss will be referred to hereinafter as coupling loss.

[0081] The photonic system 600 comprises an optical coupler 604 arranged between the integrated waveguide 603 in the photonic circuit 601 and the optical fiber 602. The optical coupler 604 comprises a cylindrical lens 605. The cylindrical lens 605 converts a circular optical mode into an elliptical optical mode, and vice versa. In a manner of speaking, seen from the optical fiber 602 toward the integrated waveguide 603, the cylindrical lens 605 provides optical scaling with orientation-dependent factors. A scaling factor in a plane perpendicular to the optically functional layer 104 differs from a scaling factor in a plane parallel to the optically functional layer 104.

[0082] For coupling into the photonic circuit 601, the cylindrical lens 605 converts the circular optical mode in the optical fiber 602 so that there is significant overlap with the elliptical mode in the integrated waveguide 603 in the photonic circuit 601. Conversely, for coupling out of the photonic circuit 601, the cylindrical lens 605 may convert the elliptical mode in the integrated waveguide 603 in the photonic circuit 601 so that there is significant overlap with the circular optical mode in the optical fiber 602. Accordingly, the cylindrical lens 605 may significantly mitigate the coupling loss.

[0083] The cylindrical lens 605 may be a monolithic optical element or an assembly of various optical elements. The cylindrical lens 605 may comprise one or more materials, such as, for example, glass, in particular, flint glass, quartz, borosilicate glass, or fused silica, a transparent polymer, such as, for example, polycarbonate, cyclic olefin copolymer, or hardened optical glue. The cylindrical lens 605 may have dimensions between, for example, about 5 pm and several millimeters.

[0084] The cylindrical lens 605 may be fixedly coupled to an end of the optical fiber 602. In another embodiment, the cylindrical lens 605 may be a separate component, distinct from the optical fiber 602 and the photonic circuit 601. FIG. 6 may be regarded as a schematic representation of such embodiments. In yet another embodiment, the cylindrical lens 605 may be comprised in the photonic circuit 601.

[0085] NOTES

[0086] The embodiments described hereinbefore with reference to the drawings are presented by way of illustration. The invention may be implemented in numerous different ways. In order to illustrate this, some alternatives are briefly indicated.

[0087] The invention may be applied in numerous types of products or methods related to photonic circuits. In the presented embodiments, the substrate is in the form of a composite substrate having two layers. In other embodiments, the substrate may be monolithic or may comprise more than two layers. The term substate should be interpreted broadly. This term encompasses any type of suitable support for an optically functional layer.

[0088] There are numerous different ways of adding a strip of silicon carbide on top of an optically functional layer in a method in accordance with the invention. In the embodiments presented hereinbefore, the strip is added using photolithographic technique. In other embodiments, the strip may be added using a deposition technique.

[0089] The term electrical field should be interpreted broadly. This term encompasses an electromagnetic field, a magnetic field, as well as other types of energy fields capable of inducing refractive index variations in an optically functional layer.

[0090] The term cylindrical lens should be interpreted broadly. This term encompasses any type of optical device capable of converting a circular optical mode into an elliptical optical mode, and vice versa. For example, the cylindrical lens may be in the form of include a tapered fiber or a grating structure, such as a hologram. These and other embodiments of what is referred to as cylindrical lens are capable of inducing an effect of direction-dependent optical scaling, which may be magnification or reduction.

[0091] The remarks made hereinbefore demonstrate that the embodiments described with reference to the drawings illustrate the invention, rather than limit the invention. The invention can be implemented in numerous alternative ways that are within the scope of the appended claims. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. Any reference sign in a claim should not be construed as limiting the claim. The verb “comprise” in a claim does not exclude the presence of other elements or other steps than those listed in the claim. The same applies to similar verbs such as “include” and “contain”. The mention of an element in singular in a claim pertaining to a product, does not exclude that the product may comprise a plurality of such elements. Likewise, the mention of a step in singular in a claim pertaining to a method does not exclude that the method may comprise a plurality of such steps. The mere fact that respective dependent claims define respective additional features, does not exclude combinations of additional features other than those reflected in the claims.

[0092] Beyond the invention claimed, it is contemplated that a waveguide-forming strip may be made of a material other than silicon carbide. An alternative material also having a relatively high refractive index may be used, such as, for example, hydrogenated silicon carbide, mono-crystalline silicon, amorphous silicon, hydrogenated amorphous silicon, amorphous Germanium, aluminum-oxide (AI2O3), or Indium phosphide. Accordingly, beyond the invention claimed, it is contemplated that a protective layer may be used to protect an optically functional layer to which a waveguide-forming strip other than silicon carbide is added. Beyond the invention claimed, it is contemplated that a cylindrical lens may be used as an optical interface between any type of waveguide in a photonic circuit that has an elliptical optical mode and a discrete waveguide, which is external to the photonic circuit. Such use of a cylindrical lens is not limited to a waveguide formed by a strip of silicon carbide on top of an optically functional layer. A waveguide formed by strips made of materials other than silicon carbide may also have an elliptical optical mode, in particular if the optically functional layer is relatively thin.

Claims

CLAIMS:

1. A method of manufacturing a photonic circuit (200, 400), the method comprising: adding a strip of silicon carbide on top of an optically functional layer supported by a substrate, wherein the optically functional layer is between 100 nm and 500 nm thick and wherein the strip of silicon carbide is less than 50 nm away from the optically functional layer.

2. A method of manufacturing a photonic circuit according to claim 1, wherein the strip of silicon carbide (107) is less than 250 nm thick.

3. A method of manufacturing a photonic circuit according to any of claims 1 and 2, comprising prior to adding the strip of silicon carbide (107) on top of the optically functional layer (104): depositing a protective layer (106) on the optically functional layer.

4. A method of manufacturing a photonic circuit according to claim 3, wherein the protective layer (106) has a refractive index that is lower than that of the optically functional layer (104).

5. A method of manufacturing a photonic circuit according to claim 4, wherein the protective layer (106) is less than 50 nm thick.

6. A method of manufacturing a photonic circuit according to claim 5, wherein the protective layer (106) is thicker than 3 nm.

7. A method of manufacturing a photonic circuit according to any of claims 3 to 6, wherein the protective layer (106) comprises at least one of the following materials: silicon nitride, aluminum oxide, nickel oxide, and chromium oxide.

8. A method of manufacturing a photonic circuit according to any of claims 3 to 7, wherein after adding the strip of silicon carbide (107), the protective layer (106) is removed in areas adjacent to the strip of silicon carbide.

9. A method of manufacturing a photonic circuit according to any of claims 1 to 8, wherein the optically functional layer (104) has a refractive index that varies with a magnitude of an electric field.10 A method of manufacturing a photonic circuit according to claim 9, wherein a polarization density in the optically functional layer (104) varies non linearly with the magnitude of the electric field.

11. A method of manufacturing a photonic circuit according to claim 10, wherein the optically functional layer (104) comprises at least one of the following materials: lithium niobate, barium titanate, potassium niobate, and materials designated by the acronym KTN.

12. A method of manufacturing a photonic circuit according to any of claims 1 to 11, wherein adding the strip of silicon carbide (107) on top of the optically functional layer (104) comprises: adding a layer of silicon carbide on top of the optically functional layer; and etching at least a portion of the silicon carbide layer to form the strip of silicon carbide.

13. A photonic circuit (200, 400) obtained by a method according to any of claims 1 to 12, wherein the optically functional layer (104) and the strip of silicon carbide (107) on top thereon jointly form a waveguide (303) for a photonic signal having a wavelength in vacuum.

14. A photonic circuit according to claim 13, wherein , the strip of silicon carbide (107) is less wide than the wavelength in vacuum of the photonic signal.

15. A photonic circuit according to claim 14, wherein the strip of silicon carbide (107) is wider than one tenth of the wavelength in vacuum of the photonic signal.

16. A photonic circuit according to any of claims 13 to 15, wherein , the strip ofX silicon carbide (107) has a cross-sectional area that is less than —2, being the wavelength in vacuum divided by the modal index of the photonic signal.

17. A photonic circuit according to claim any of claims 13 to 16, wherein the optically functional layer (104) is less thick than a third of the wavelength in vacuum of the photonic signal.

18. A photonic circuit according to claim any of claims 13 to 17, wherein the optically functional layer (104) is thicker than one twentieth of the wavelength in vacuum of the photonic signal.

19. A photonic system comprising: a photonic circuit (601) according to any of claims 13 to 18; a discrete waveguide (602), which is external to the photonic circuit; and an optical coupler (604) arranged between the waveguide (603) in the photonic circuit and the discrete waveguide, wherein the optical coupler comprises a cylindrical lens (605).

20. A photonic system according to claim 19, wherein the cylindrical lens (605) is comprised in the photonic circuit (601).

21. A photonic system according to claim 19, wherein the cylindrical lens (605) is fixedly coupled to an end of the discrete waveguide (602).

22. A photonic system according to any of claims 19 to 21, wherein the discrete waveguide (602) is an optical fiber.

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