Optical waveguide circuit

Abstract

An optical waveguide circuit (100) according to an example of the present disclosure is provided with: a substrate (101); a lower cladding layer (102) provided on the substrate (101); a waveguide core (103) provided on the lower cladding layer (102); an upper cladding layer (104) provided so as to cover the waveguide core (103) and the lower cladding layer (102); a groove (105) provided along at least one of both sides of the waveguide core (103) in the width direction; a thermal expansion material (106) provided inside the groove (105) and having a linear expansion coefficient different from those of the cladding layers (102, 104); and at least one heater element (107-1, 107-2) for heating the thermal expansion material (106).

Description

optical waveguide circuit

[0001] The present disclosure relates to optical waveguide circuits.

[0002] Optical communication networks are rapidly developing against the backdrop of the explosive growth in data communications, typified by the Internet. In particular, wavelength division multiplexing (WDM) technology, which multiplexes and transmits a large number of wavelength signals in a single optical fiber, is important as one of the means to realize large-capacity optical communications.

[0003] For example, when WDM technology is applied to transmission over a distance of 100 km or more, one or more optical amplifiers are placed along the optical fiber transmission path.

[0004] The wavelength dependence of the gain spectrum of an optical amplifier affects the optical signal-to-noise ratio (OSNR). In order to flatten the gain spectrum as much as possible, for example, a gain equalizer configured by connecting Mach-Zehnder interferometers in multiple stages has been proposed (for example, Non-Patent Document 1).

[0005] In a gain equalizer with such a configuration, optical signals are discarded from the unconnected output ports of each Mach-Zehnder interferometer, resulting in increased optical loss. For this reason, gain equalizers with a configuration called a "lattice filter type optical circuit" have been studied in recent years (for example, Non-Patent Document 2).

[0006] K. Suzuki, T. Kitoh, S. Suzuki, Y. Inoue, Y. Hibino, T. Shibata, A. Mori, and M. Shimizu, “PLC-based dynamic gain equalizer consisting of integrated Mach-Zehnder interferometers with C- and L-band equalizing range,” Electronics Letters, Vol. 38, No. 18, pp. 1030 (2002).TR Schlipf, MW Street, J. Pandavenes, R. McBride, and DRS Cumming, “Design and Analysis of a Control System for an Optical Delay-Line Circuit Used as Reconfigurable Gain Equalizer,” Journal of Lightwave Technology, Vol. 21, No. 9, pp. 1944 (2003).

[0007] A lattice filter type optical circuit may include an optical waveguide circuit including a polarization rotation structure in order to reduce polarization dependent loss (PDL). However, since it is difficult to change or adjust the amount of polarization rotation after the physical structure is determined, there is room for improvement.

[0008] An exemplary object of the present disclosure is to provide an improved optical waveguide circuit that allows for control of the amount of polarization rotation.

[0009] Therefore, an optical waveguide circuit according to one aspect of the present disclosure includes a substrate, a lower clad layer provided on the substrate, a waveguide core provided on the lower clad layer, an upper clad layer provided so as to cover the waveguide core and the lower clad layer, a groove provided along at least one of both sides in the width direction of the waveguide core, a thermal expansion material provided inside the groove and having a linear expansion coefficient different from that of the lower clad layer and the upper clad layer, and at least one heater element for heating the thermal expansion material.

[0010] FIG. 1 is a schematic cross-sectional view showing an example of an optical waveguide circuit according to a first embodiment. FIG. 1 is a schematic cross-sectional view showing another example of the structure illustrated in FIG. 1. FIG. 1 is a schematic cross-sectional view showing another example of the structure illustrated in FIG. 1. FIG. 1 is a schematic cross-sectional view showing another example (double groove structure) of the structure illustrated in FIG. 1. FIG. 5A is a schematic top view of an optical waveguide circuit according to a second embodiment. FIG. 5B is a cross-sectional view of a part of an optical waveguide circuit cut along the y-z plane indicated by X1-X1' in FIG. 5A. FIG. 5C is a schematic cross-sectional view showing an example of an optical waveguide circuit having a polarization rotation structure.

[0011] Hereinafter, embodiments will be described in detail with reference to the drawings. However, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims. Furthermore, more detailed descriptions than necessary may be omitted. For example, detailed descriptions of well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0012] Furthermore, in the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. The drawings are schematic, and the dimensional relationships or ratios of elements may differ from reality. The drawings may also include portions in which the dimensional relationships or ratios differ. When numerical values ​​are used in the following description, they are merely examples, and other numerical values ​​may be used in addition or instead.

[0013] <Summary> There are several methods for realizing various optical functional circuits used in optical communication networks. One example is a method called PLC (Planar Lightwave Circuit), which forms a quartz waveguide on a planar substrate. This method is widely used because it combines multifunctionality, mass production, and low cost.

[0014] Optical circuits using silica-based glass waveguides can realize low-loss optical waveguides because they share the same materials as the optical fibers used in optical communications. Furthermore, because the waveguides are formed on planar substrates, it is easy to combine various functional elements, and complex optical circuits can be fabricated with good reproducibility. Optical circuits such as wavelength multiplexing / demultiplexing elements or optical switches fabricated using these technologies are important elements in building optical networks.

[0015] Furthermore, the gain equalizer using the lattice filter type optical circuit described above is configured, for example, with N (N is an integer of 2 or more) directional couplers and (N-1) arm waveguides, each of which includes two waveguides sandwiched between each directional coupler.

[0016] By applying heat to one of the two waveguides that make up the arm waveguide, a phase shifter that utilizes a change in refractive index due to the thermo-optic effect can control the phase of light propagating through the waveguide. This control makes it possible to adjust the phase difference between the optical signals propagating through the two waveguides that make up the arm waveguide. This makes it possible to adjust the optical interference state in the subsequent directional coupler, and control the transmission spectrum for each wavelength.

[0017] In a gain equalizer configured using such a lattice filter type optical circuit, if the interference state differs for each polarization direction of light propagating within the optical circuit, polarization dependency may occur in the transmission spectrum finally output from the lattice filter type optical circuit.

[0018] The polarization dependence of the transmission spectrum appears as polarization dependent loss (PDL) when viewed as the performance of an optical device. In a lattice filter optical circuit with many stages, the PDL generated in one arm waveguide can be amplified with each subsequent stage. Therefore, the PDL of the entire lattice filter optical circuit can increase as the number of stages increases.

[0019] PDL can occur when birefringence exists in an optical waveguide and there is a difference in the effective refractive index depending on the polarization of the propagating optical signal. effis defined by the following equation (1).

[0020]

[0021] In formula (1), n y represents the effective refractive index in the y-axis direction, and n x represents the effective refractive index in the x-axis direction. y >n x In the case of eff is a positive value, and conversely, n y <n x In the case of eff will be negative.

[0022] Birefringence Δn eff In an optical waveguide where ρ is not zero, two mutually orthogonal polarization modes propagate: a TM mode having an electric field component perpendicular to the substrate surface, and a TE mode having an electric field component horizontal to the substrate surface.

[0023] Here, in the i-th (i=1, 2, ..., N-1) arm waveguide of the (N-1)-stage lattice filter type optical circuit configured by (N-1) arm waveguides as described above, the phase difference Θi occurring between two waveguides is expressed by the following equation (2):

[0024]

[0025] In equation (2), λ is the wavelength of light, n eff is the effective refractive index of the optical waveguide, ΔL i is the difference in length between the two optical waveguides that make up the i-th arm waveguide, φ i represents the phase difference applied between the optical waveguides by controlling the phase shifter in the i-th arm waveguide.

[0026] When birefringence exists in the optical waveguide and the effective refractive index of the TE mode differs from that of the TM mode, the phase difference expressed by equation (2) differs depending on the polarization mode, and therefore the interference state in the downstream directional coupler differs depending on the polarization mode, resulting in PDL.

[0027] On the other hand, in an optical waveguide interferometer, polarization dependence can also occur due to asymmetric rotation of polarization between interference waveguides. For example, the birefringence Δn eff Even if the difference in the interference state due to birefringence is minimized by minimizing the absolute value of , PDL due to polarization rotation can still occur. Note that polarization rotation in optical waveguides can be suppressed by finite birefringence, for example, in what is known as polarization-maintaining fiber.

[0028] In order to minimize the PDL that can occur in a gain equalizer using a lattice filter optical circuit, the following circuit configuration has been considered. For example, a gain equalizer using a lattice filter optical circuit is constructed using two lattice filter optical circuits of the same design and a folded connection structure that optically connects the two lattice filter optical circuits. The folded connection structure has the function of rotating the polarization direction of the propagating light by about 90 degrees.

[0029] By using a folded connection structure, for example, light that was in TE (or TM) mode when passing through a first lattice filter type optical circuit undergoes polarization rotation of approximately 90° in the folded connection structure and passes through a second lattice filter type optical circuit as TM (or TE) mode.

[0030] Therefore, the PDL generated in the first lattice filter type optical circuit is cancelled out by the PDL generated in the second lattice filter type optical circuit, thereby realizing a gain equalizer with suppressed PDL as a whole.

[0031] As an example of a structure that provides the function of rotating the polarization direction of light propagating through an optical waveguide, a structure in which a groove 604 is provided along a waveguide (core) 603 near one side of the core 603, as shown in an optical waveguide circuit 600 in Fig. 6, in Fig. 6, 601 represents a substrate, 602 represents a cladding layer provided on the substrate 601, and 605 represents an air gap inside the groove 604.

[0032] In an optical waveguide circuit using a quartz-based material, even if the structural birefringence due to the structure of the optical waveguide is zero, birefringence due to stress can occur depending on the difference in the linear expansion coefficient of the material. For example, this is because the linear expansion coefficients of the substrate material, cladding material, and core material are different from one another, and compressive stress in a direction parallel to the substrate surface can act on the core 603.

[0033] When the cladding layer 602 and / or core 603 of the optical waveguide circuit 600 is formed using, for example, a flame deposition method, a high-temperature heating process is included in the fabrication of the optical waveguide circuit, and therefore, when the temperature returns to room temperature, a large compressive stress may act on the core 603. The mechanism by which compressive stress is imparted to the optical waveguide circuit 600 is as follows. For example, when a silica glass film is formed on the substrate 601 during a high-temperature heating process, the stress is zero in the heated state. When the substrate 601 is then cooled to room temperature, the degree of contraction differs greatly due to the difference in the linear expansion coefficients of the substrate 601 and the silica glass film, and therefore a large compressive stress is applied to the silica glass film.

[0034] For example, in the core 603 of the silica-based optical waveguide circuit 600, birefringence can occur with a principal axis in a direction perpendicular or horizontal to the substrate surface.

[0035] When a groove 604 is provided in a portion of the cladding layer 602 on one side of a core 603 in an optical waveguide circuit 600 in which birefringence may occur, asymmetric stress is applied in a direction horizontal to the substrate surface centered on the core 603, and the principal axis of birefringence tilts in response to this stress.

[0036] Here, the way in which stress is applied depends on the distance between the core 603 and the groove 604. Therefore, by adjusting this distance, it is possible to change the degree of inclination of the principal axis of birefringence.

[0037] When linearly polarized light passes through a birefringent object with a tilted principal axis, the angle θ between the polarization direction of the input light and the principal axis of the birefringent object, and the birefringence Δn eff , and / or the polarization state of the light propagating through the birefringent object changes depending on the propagation distance L of the light in the birefringent object.

[0038] For example, the angle θ and the birefringence Δn eff and the propagation distance L satisfies the following formula (3), a linearly polarized wave whose polarization direction is rotated by "2θ" with respect to the input linearly polarized wave is output.

[0039]

[0040] In equation (3), k represents the wave number of the input light. By utilizing this characteristic, the angle θ and the birefringence Δn eff , and the propagation distance L, it is possible to impart a desired amount of polarization rotation to the input linearly polarized wave.

[0041] Therefore, in an existing lattice filter type optical circuit, PDL can be reduced by adopting a circuit configuration including the above-described polarization rotation structure.

[0042] In this way, in the optical waveguide circuit 600 including the polarization rotation structure, the desired amount of polarization rotation can be obtained by adjusting parameters such as the distance between the core 603 and the groove 604 and / or the length of the groove 604 in the direction of light propagation in the core 603.

[0043] However, once the polarization rotation structure including the grooves 604 is determined (in other words, fabricated), the amount of polarization rotation is fixed, which may limit the applications of the optical waveguide circuit 600.

[0044] Furthermore, when a lattice filter type optical circuit having a polarization rotation structure is actually used, the lattice filter type optical circuit can be adhesively fixed onto a mount that is common to a power supply PCB (Printed Circuit Board), for example.

[0045] This fixation allows for a stable supply of power to the thermo-optic phase shifter mounted on the lattice filter optical circuit. The mount is usually made of a metal material such as stainless steel. When fixing the lattice filter optical circuit to the mount, substantially the entire bottom surface of the lattice filter optical circuit is firmly fixed to the mount to ensure long-term reliability.

[0046] However, when a lattice filter optical circuit is fixed to a mount, its characteristics can change significantly due to changes in the environmental temperature. For example, in a lattice filter optical circuit made of a silicon substrate and a quartz glass layer, which have different thermal expansion coefficients, significant changes in warping and / or thermal expansion can occur depending on changes in the environmental temperature.

[0047] Here, when the entire bottom surface of the lattice filter type optical circuit is adhesively fixed to the mount, warping and / or thermal expansion of the lattice filter type optical circuit is suppressed, and as a reaction to this, the stress applied to the optical waveguide (core) can change significantly.

[0048] As described above, in the polarization rotation structure of the lattice filter type optical circuit, by forming a groove 604 in the cladding layer 602 on one side of the core 603, it is possible to appropriately adjust the stress applied to the core 603 and control the amount of rotation of the polarization direction of light propagating through the core 603 to about 90°.

[0049] In other words, the polarization rotation function is affected by changes in stress applied to the core 603, making it difficult to maintain or guarantee an appropriate amount of polarization rotation. For example, if the mount to which the lattice filter optical circuit is adhesively fixed causes changes in stress in response to changes in environmental temperature, the PDL reduction effect may be reduced. In other words, the optical characteristics of the lattice filter optical circuit may be degraded.

[0050] As described above, in a polarization rotation structure for reducing PDL, it is difficult to change or adjust the amount of polarization rotation after the structure is determined, which may limit the applications of the polarization rotation structure and may also result in the failure to obtain the desired polarization rotation characteristics due to changes in the environmental temperature, etc.

[0051] Therefore, several embodiments relating to an improved optical waveguide circuit that can control the amount of polarization rotation depending on the application and / or the environment of use will be described below.

[0052] 1 is a schematic cross-sectional view showing an example of an optical waveguide circuit 100 as a PLC according to a first embodiment. In Fig. 1, the x-axis direction corresponds to the width direction of the optical waveguide circuit 100, and the y-axis direction corresponds to the height (or thickness) direction of the optical waveguide circuit 100. The z-axis direction is the length direction of the optical waveguide circuit 100 and corresponds to the propagation direction of light propagating through a core (hereinafter also referred to as a "waveguide core") 103.

[0053] As illustrated in FIG. 1 , the optical waveguide circuit 100 includes, for example, a substrate 101, a lower (under) clad layer 102 provided on the substrate 101, a rib-shaped core 103 provided on the lower clad layer 102, and an upper (over) clad layer 104 provided to cover the core 103.

[0054] The substrate 101 may be, for example, a silicon (Si) substrate, and the cladding layers 102 and 104 and the core 103 may be made of a quartz-based material. The lower cladding layer 102 and the upper cladding layer 104 may have the same composition or different compositions.

[0055] When the lower cladding layer 102 and the upper cladding layer 104 have the same composition, the lower cladding layer 102 and the upper cladding layer 104 may be understood to form a single cladding layer, such as the cladding layer 602 illustrated in FIG. 6, with the core 103 embedded within this cladding layer.

[0056] Furthermore, a groove 105 extending in the z-axis direction along the core 103 is provided on one of both sides of the core 103 in the x-axis direction. The groove 105 is only required to have a shape that can impart a stress distribution to the core 103, as will be described later, and the shape and / or depth of the groove 105 are not particularly limited.

[0057] As a non-limiting example, the shape of the groove 105 may be rectangular when viewed in cross section along the xy plane in Figure 1, and the depth of the groove 105 may be a depth that reaches from the surface of the upper cladding layer 104 to the bottom surface of the lower cladding layer 102.

[0058] Other examples of the shape of the groove 105 include shapes in which the width narrows continuously or intermittently according to the depth in the y-axis direction, such as a V-shape, a wedge shape, or a step shape, when viewed in cross section in the x-z plane. Grooves 105 of such shapes can reduce the amount of thermal expansion material 306 filled inside the groove 105 compared to rectangular grooves 105, which can contribute to reducing the cost of the optical waveguide circuit 100, for example.

[0059] The thermal expansion material 306 filled inside the groove 105 is a material that can expand or contract when heated. A heater structure 107 that can apply heat to the thermal expansion material 106 is provided on the surface of the upper clad layer 104 near the groove 105.

[0060] 1, heater elements 107-1 and 107-2 are provided on either side of a groove 105 on the surface of the upper cladding layer 104. In this example, the heater elements 107-1 and 107-2 form a heater structure 107.

[0061] The material constituting the optical waveguide circuit 100 may be silicon, a semiconductor, a ferroelectric, SiOx, SiON, SiN, etc. For example, it is sufficient that there is a difference in linear expansion coefficient between any two of the lower cladding layer 102, the upper cladding layer 104, and the substrate 101, and that the upper cladding layer 104 and / or the lower cladding layer 102 is formed at a temperature different from the temperature of the device's operating environment.

[0062] Regarding the difference in the linear expansion coefficients, the linear expansion coefficient TCE (substrate) of the substrate 101, the linear expansion coefficient TCE (UC) of the lower cladding layer 102, the linear expansion coefficient TCE (OC) of the upper cladding layer 104, and the linear expansion coefficient TCE (material) of the thermal expansion material 106 may have any of the following relationships:

[0063] TCE (substrate) > TCE (UC), or TCE (substrate) > TCE (OC), or TCE (OC) > TCE (UC), or TCE (material) > TCE (UC), or TCE (material) > TCE (OC). It may be understood that the above relationships are equivalent in other embodiments described later.

[0064] Any film-forming method may be used as long as it can form a uniform and smooth layer, and for example, flame deposition, chemical vapor deposition (CVD), sputtering, or the like may be used.

[0065] In this embodiment, a manufacturing method will be described using as an example a silica-based optical waveguide circuit 100. In manufacturing a silica-based optical waveguide circuit 100 by a typical flame deposition method, soot glass that will become the lower cladding layer 102 is deposited on a silicon substrate 101 by flame deposition, and the soot-like soot glass is converted into transparent glass by holding the substrate in a high-temperature atmosphere.

[0066] Next, a glass layer that will become the core 103 is similarly formed on the lower cladding layer 102, and a desired pattern of the core 103 is formed by techniques such as photolithography and reactive ion etching. Finally, a glass layer that will become the upper cladding layer 104 is formed on the core 103 and the lower cladding layer 102 by flame deposition, thereby completing the optical waveguide circuit 100.

[0067] Generally, the transparentizing temperature of glass is 1000°C or higher, so in the process of transparentizing each layer, the silicon substrate 101 expands at high temperatures (1000°C or higher), and the stress is released in this state, turning it into transparent glass, which is then rapidly cooled, forming a glass film.

[0068] The silicon substrate 101 expands at high temperatures of 1000°C or higher, and contracts as it cools. On the other hand, the cladding layers 102 and 104, which are glass layers, have a smaller linear expansion than silicon, and are therefore subjected to a compressive force while being restrained by the silicon substrate 101.

[0069] Therefore, the glass layer is subjected to compressive stress from the silicon substrate 101. This compressive stress is prominent in a direction parallel to the substrate surface (xz plane), and therefore, the core 103 is also subjected to compressive stress in a direction parallel to the substrate surface.

[0070] In contrast, in the direction perpendicular to the substrate surface (y-axis direction), the glass layer is not constrained by the silicon substrate 101, and stress is applied to the side walls of the core 103 due to the difference between the linear expansion coefficient of the upper cladding layer 104 and the linear expansion coefficient of the core 103.

[0071] Since both the upper cladding layer 104 and the core 103 are primarily composed of quartz, the difference in their linear expansion coefficients is smaller than that at the interface between the silicon substrate and the glass layer, and the core 103 is subjected to relatively small stress in the direction perpendicular to the substrate surface.

[0072] In this way, the internal stress acting on the core 103 differs between the direction parallel to the substrate surface and the direction perpendicular to the substrate surface. This difference in direction-dependent internal stress is the cause of birefringence in the optical waveguide circuit 100, and is one of the causes of the polarization dependence of the various optical devices described above.

[0073] The polarization dependence is in accordance with a physical phenomenon called the photoelastic effect, and is expressed, for example, as refractive index distributions in the x-axis and y-axis directions by the following equations (4.1) and (4.2), respectively.

[0074] n x (x, y) = C1σ x (x, y) + C2(σ y (x, y) + σ z (x, y) + n x0 (x, y) (4.1)

[0075] n y (x, y) = C1σ y (x, y) + C2(σ x (x, y) + σ z (x, y)) + n y0 (x, y) (4.2)

[0076] In equations (4.1) and (4.2), σ x (x,y), σ y (x,y), σ z (x, y) represent the stress distribution in the x-axis, y-axis, and z-axis directions, respectively, and C1 and C2 represent the photoelastic coefficients determined by the materials of the lower cladding layer 102 and the upper cladding layer 104. The z-axis represents the light propagation direction, and n x0 (x, y), ny0 (x, y) represent the refractive index distribution in the x-axis direction and the y-axis direction, respectively, before the application of stress. The difference between equations (4.1) and (4.2) corresponds to the birefringence after the application of stress.

[0077] 1, a groove 105 is provided along the core 103 in the cladding layers 104 and 102 on one side of the core 103. The groove 105 is formed, for example, after the upper cladding layer 104 is formed.

[0078] By forming the grooves 105, internal stress in a direction parallel to the substrate surface is released on the side where the grooves 105 exist. Since the deeper the grooves 105, the more likely it is that internal stress will be released, in this example, the grooves 105 are formed to a depth that reaches from the top surface of the upper cladding layer 104 to the bottom surface of the lower cladding layer 102. In other words, the cladding material on the substrate 101 is removed in the areas of the grooves 105.

[0079] On the other hand, a relatively large stress is localized at a position in the lower cladding layer 102 near the boundary between the sidewall of the groove 105 and the silicon substrate 101. When viewed from the position of the core 103, the localized stress corresponds to a position diagonally downward on the substrate 101 side in the direction in which the groove 105 exists, and therefore, stress exists that is asymmetric with respect to the light propagation direction (z-axis direction). Therefore, the stress birefringence caused by the above-mentioned photoelastic effect also has an asymmetric distribution with respect to the light propagation direction (z-axis direction).

[0080] Such an asymmetric distribution has the effect of rotating the birefringence axis of the optical waveguide circuit 100. For example, in existing optical waveguide circuits, the major axes of birefringence are parallel and perpendicular to the substrate surface, whereas by providing a groove 105 near one side of the core 103 and making the stress distribution asymmetric with respect to the light propagation direction, the major axis of birefringence can be tilted. Because the stress distribution depends on the distance between the waveguide core 103 and the groove 105, adjusting this distance can change the degree of tilt of the major axis of birefringence.

[0081] When linearly polarized light passes through a birefringent object with a tilted principal axis, the angle θ between the polarization direction of the input light and the principal axis of the birefringent object, and the birefringence Δn effThe polarization state of light propagating through the birefringent object changes depending on the propagation distance L of the light through the birefringent object.

[0082] For example, when these relationships satisfy the formula (3), a linearly polarized wave whose polarization direction is rotated by 2θ with respect to the input linearly polarized wave is output.

[0083]

[0084] Here, k is the wave number of the input light. By utilizing this characteristic, the angle θ and the birefringence Δn eff , and the propagation distance L can be individually adjusted to provide a desired amount of polarization rotation for an input linearly polarized wave.

[0085] Here, the formation position of the groove 105 may be, for example, a position where a stress distribution is obtained such that the major axis of the refractive index ellipse with respect to the light propagation direction (z-axis direction) coincides with a desired angle with respect to the refractive index distribution of the core 103 determined by the stress distribution applied to the core 103. For example, if it is desired to rotate the polarization direction by 2θ=90°, the formation position of the groove 105 is determined so that θ=45°.

[0086] Furthermore, the length of the groove 105 in the z-axis direction, which determines the propagation distance L, can be determined based on the relationship, for example, "difference between the minor axis and major axis of the refractive index ellipse" × "length of the groove 105" = "amount of phase rotation." Here, the difference between the minor axis and major axis of the refractive index ellipse can be determined, for example, according to the difference in stress between the horizontal direction and the vertical direction of the substrate surface applied during the fabrication of the optical waveguide (before the formation of the groove 105), and therefore depends on the fabrication process of the optical waveguide. Meanwhile, the length of the groove 105 in the z-axis direction can be adjusted during the formation of the groove 105 after the fabrication of the optical waveguide. Therefore, a desired amount of phase rotation can be obtained by appropriately setting the length of the groove 105 in the z-axis direction.

[0087] Furthermore, in this embodiment, a groove 105 provided on one side of the side surface of the waveguide core 103 is filled with a material having a large thermal expansion coefficient, and a heater structure 107 is provided on the surface of the upper clad layer 104 near the groove 105. By heating the thermal expansion material 106 with the heater structure 107, the expansion or contraction of the thermal expansion material 106 can be controlled.

[0088] For example, if the thermal expansion material 106 is controlled to a contracted state and does not apply stress to the cladding layers 102 and 104, the stress distribution applied to the core 103 will be the same as when the thermal expansion material 106 is not present inside the groove 105, and an amount of polarization rotation corresponding to the tilt of the principal axis of birefringence due to the asymmetric stress distribution can be obtained.

[0089] In contrast, when the thermal expansion material 106 is expanded, the thermal expansion material 106 applies stress to the cladding layers 102 and 104, causing a change in the stress distribution applied to the core 103. In response to this change in stress distribution, the degree of tilt of the principal axis of birefringence of the core 103 also changes, and the amount of rotation of the polarization direction of light propagating through the core 103 can be changed.

[0090] For example, if the same stress as that applied to the opposite side of the core 103 (the side where the groove 105 is not formed) is applied by the expansion of the thermal expansion material 106, the tilt of the principal axis of birefringence can be returned to zero. Therefore, it is also possible to make the optical waveguide circuit 100 function as a circuit in which polarization rotation does not occur.

[0091] As described above, the optical waveguide circuit 100 according to this embodiment has the function of rotating the polarization direction of input light, and the amount of polarization rotation can be controlled by designing an appropriate optical waveguide structure. In addition, even after the optical waveguide structure is determined, the amount of polarization rotation can be adjusted or controlled by driving the heater structure 107.

[0092] The thermal expansion material 106 is, for example, a material different from the material constituting the optical waveguide circuit 100, and any type of material may be used as long as it has a linear expansion coefficient greater than those of the upper clad layer 104 and the lower clad layer 102. For example, a resin material or a metal material may be used as the thermal expansion material 106.

[0093] 1, the thermal expansion material 106 may be filled so as to fill the groove 105, or may be provided as a layer 206 of the thermal expansion material 106 by vapor deposition or sputtering on the sidewalls and bottom surface of the groove 105, as shown in the optical waveguide circuit 200 of FIG. 2. For convenience, the layer 206 of the thermal expansion material 106 may be abbreviated as the thermal expansion material layer 206.

[0094] In the example of Figure 1, the expansion or contraction of the thermally expansive material 106 imparts a global stress to the upper cladding layer 104 and the lower cladding layer 102. In the example of Figure 2, the expansion or contraction of the thermally expansive material layer 206 imparts a localized stress to the interfaces between the thermally expansive material layer 206 and the cladding layers 102 and 104.

[0095] Therefore, in the example of Fig. 2, the generated stress may be lower than in the example of Fig. 1, but more precise stress control may be possible than in the example of Fig. 1. Therefore, it may be possible to control the degree of inclination of the principal axis of birefringence of the core 103 with higher precision than in the example of Fig. 1.

[0096] 2, similarly to the example of Fig. 1, the heater structure 107 may be formed on the upper surface of the upper cladding layer 104 in the vicinity of the groove 105. For example, two heater elements 107-1 and 107-2 may be provided on either side of the groove 105, or only one heater element 107-1 or 107-2 may be provided on either side of the groove 105.

[0097] In other words, the heater structure 107 only needs to be positioned at a position where stress caused by heat directly transmitted to the core 103 during heating occurs asymmetrically in the width direction (x-axis direction) around the core 103.

[0098] For example, on the upper surface of the upper cladding layer 104, the heater elements 107-1 and 107-2 can be arranged such that their centers in the width direction (x-axis direction) are shifted from the center in the width direction of the core 103. With such an arrangement, it is possible to efficiently generate asymmetric stress in the width direction (x-axis direction) around the core 103.

[0099] 3 , the heater structure 307 may be provided on the upper surface of the upper clad layer 104 as a lid to cover the thermal expansion material 306 after the thermal expansion material 306 is filled inside the groove 105. The thermal expansion material 306 may be, for example, the same material as the thermal expansion material 106 described above.

[0100] In the example of Figure 3, when the thermal expansion material 306 expands due to temperature control of the heater structure 307, the heater structure 307 presses down the thermal expansion material 306, thereby preventing or suppressing the thermal expansion material 306 from escaping toward the opening of the groove 105.

[0101] Therefore, it is possible to suppress a decrease in the stress that the thermal expansion material 306 applies to the cladding layers 102 and 104 in response to its expansion, thereby enabling efficient stress application. In other words, the heater structure 307 illustrated in Fig. 3 has both the function of heating the thermal expansion material 306 and the function of acting as a lid that prevents or suppresses the thermal expansion material 306 from escaping from the outlet of the groove 105.

[0102] <Double Groove Structure> Although FIGS. 1 to 3 illustrate a configuration in which the groove 105 is provided on one side of the core 103, grooves 105-1 and 105-2 extending in the z-axis direction along the core 103 may be provided on both sides of the core 103, as illustrated in the optical waveguide circuit 400 of FIG. 4, for example.

[0103] Each of the grooves 105-1 and 105-2 is not particularly limited in shape and / or depth as long as it is capable of imparting a stress distribution to the core 103, similar to the groove 105 illustrated in FIGS.

[0104] 4, and has a depth that extends from the surface of the upper cladding layer 104 to the top surface of the lower cladding layer 102. However, the grooves 105-1 and 105-2 may have different shapes and / or depths.

[0105] One or both of the grooves 105-1 and 105-2 are filled with a thermal expansion material 406 that expands or contracts when heated, similar to the thermal expansion material 106 described above. In addition, heater structures capable of applying heat to the thermal expansion material 406 are provided near each of the grooves 105-1 and 105-2.

[0106] 4, heater structures 407-1 and 407-2 may be provided corresponding to grooves 105-1 and 105-2, respectively. Heater structure 407-1 may be configured to include, for example, two heater elements 471-1 and 471-2 provided on either side of one groove 105-1, similar to the configuration illustrated in FIG. 1. Similarly, heater structure 407-2 may be configured to include two heater elements 472-1 and 472-2 provided on either side of the other groove 105-2, for example.

[0107] Note that one or both of heater structures 407-1 and 407-2 may have a structure in which one heater element is provided only on either side of groove 105-1 or groove 105-2. Hereinafter, grooves 105-1 and 105-2 may be abbreviated as "groove 105" when they are not distinguished from each other, and similarly, heater structures 407-1 and 407-2 may be abbreviated as "heater structure 407" when they are not distinguished from each other.

[0108] 2, the thermal expansion material 406 may be provided as a layer on the sidewalls and bottom surface of one or both of the grooves 105-1 and 105-2 by evaporation or sputtering.

[0109] Furthermore, when the thermal expansion material 406 is filled in the grooves 105, a heater structure 407 may be provided on the upper surface of the upper clad layer 104 as a lid covering the thermal expansion material 406, similar to the example of FIG.

[0110] In other words, in the optical waveguide circuit 400 illustrated in FIG. 4, the manner in which the thermal expansion material 406 is formed for each groove 105 and the manner in which the heater structure 407 is formed can be applied by combining the manners illustrated in FIG. 2 or FIG. 3 individually for each groove 105.

[0111] The materials constituting the optical waveguide circuit 400 illustrated in FIG. 4, the thermal expansion material 406, and the method of forming them may be the same as or similar to the materials and the method of forming them already described with reference to FIG.

[0112] 4, at the stage where the grooves 105 are formed on both sides of the core 103, the horizontal internal stress caused by the difference in the linear expansion coefficient between the silicon substrate 101 and the glass layers (cladding layers 102 and 104) is released on both sides of the core 103. Therefore, the stress is applied symmetrically in the width direction (x-axis direction) around the light propagation direction (z-axis).

[0113] In contrast, the expansion or compression of the thermal expansion material 406 filled inside each of the two grooves 105 can be individually controlled by the corresponding heater structure 407, thereby generating an asymmetry in stress on the core 103.

[0114] In the example of Fig. 4, the stresses from both sides of the core 103 can be controlled individually, and therefore it is possible to precisely control the balance of the stresses applied from both sides of the core 103. Therefore, it is possible to more precisely control the tilt of the principal axis of birefringence of the core 103 compared to the single-groove structure exemplified in Figs. 1 to 3. In other words, it is possible to more precisely control the amount of polarization rotation of light propagating inside the core 103 compared to the example of Fig. 1.

[0115] Second Embodiment Next, a second embodiment will be described with reference to Fig. 5A and Fig. 5B. Fig. 5A is a schematic top view of an optical waveguide circuit 500 according to the second embodiment, and Fig. 5B is a cross-sectional view of a part of the optical waveguide circuit 500 taken along the y-z plane indicated by X1-X1' in Fig. 5A.

[0116] As shown in FIG. 5A, the optical waveguide circuit 500 includes, for example, an even number of polarization-dependent optical circuits (illustratively, a first polarization-dependent optical circuit 51 and a second polarization-dependent optical circuit 52) ​​and a folded connection structure 53 that connects the polarization-dependent optical circuits 51 and the polarization-dependent optical circuits 52 to each other.

[0117] The polarization-dependent optical circuits 51 and 52 may have any configuration as long as they each have one or more input ports and one or more output ports, but here we will explain the case of a lattice filter type optical circuit as a non-limiting example.

[0118] The first lattice filter type optical circuit 51 includes an input / output waveguide 511, optical directional couplers 513-1 to 513-N, arm waveguides 514-1 to 514-(N-1), and an input / output waveguide 512. Note that N is an integer equal to or greater than 2, and Fig. 5A illustrates, as a non-limiting example, the case where N = 6.

[0119] Each of the arm waveguides 514-1 to 514-(N-1) is made up of two waveguides, and one or both of the two waveguides is provided with a phase shifter 515-1 to 515-(N-1).

[0120] Light input from the input / output waveguide 511 passes through the optical directional couplers 513 - 1 to 513 -N and the arm waveguides 514 - 1 to 514 -(N−1) in this order, and is output from the input / output waveguide 512 .

[0121] The second lattice filter type optical circuit 52 has, for example, the same design as the first lattice filter type optical circuit 51, and similarly includes an input / output waveguide 521, optical directional couplers 523-1 to 523-N, arm waveguides 524-1 to 524-(N-1), phase shifters 525-1 to 525-(N-1), and an input / output waveguide 522.

[0122] The phase shifters 515-1 to 515-(N-1) and 525-1 to 525-(N-1) have the function of controlling the phase of light passing through them. The "phase shifter" may be any phase shifter as long as it can control the phase of the light passing through it, but an example of such a phase shifter is a thermo-optic phase shifter that uses heat generated by a heater and the thermo-optic effect.

[0123] The input / output waveguide 512 of the first lattice filter type optical circuit 51 and the input / output waveguide 522 of the second lattice filter type optical circuit 52 are connected by a folded connection structure 53. With this structure, light that has propagated through the first lattice filter type optical circuit 51 and is output from the input / output waveguide 512 is input to the second lattice filter type optical circuit 52 via the input / output waveguide 522.

[0124] 1 to 4 in the first embodiment may be applied to the folded connection structure 53. In the following, an example in which the structure illustrated in FIG. 5B is applied as the folded connection structure 53 will be described.

[0125] 5B is equivalent to the structure illustrated in FIG. 1 and includes, for example, a substrate 501, a lower cladding layer 502, a core 503, an upper cladding layer 504, and a groove 505. A thermal expansion material 506 is filled inside the groove 505, and a heater structure 507 (e.g., heater elements 507-1 and 507-2) is provided near the groove 505. As described in the first embodiment, the polarization direction of light propagating through the core 503 of the folded connection structure 53 is rotated by controlling the expansion or contraction of the thermal expansion material 506 filled in the groove 505.

[0126] As illustrated in FIG. 5A, an input / output waveguide 512 of the first lattice filter type optical circuit 51 and an input / output waveguide 522 of the second lattice filter type optical circuit 52 are connected by a core 503, and a groove 505 is provided on one side of the core 503 along the core 503.

[0127] 5A is input to a first lattice filter type optical circuit 51 via an input / output waveguide 511. The light input to the lattice filter type optical circuit 51 is attenuated according to the transmittance for each wavelength (hereinafter referred to as the transmission spectrum) determined by the phase modulation amount of each of the phase shifters 515-1 to 515-(N-1), and then output from an input / output waveguide 512.

[0128] At this time, the transmission spectrum differs depending on the polarization direction of the light due to polarization dependent loss (PDL) occurring in the first lattice filter type optical circuit 51, and therefore the spectrum of the light output from the input / output waveguide 512 has polarization dependency.

[0129] The output light from the input / output waveguide 512 propagates through the core 503 of the folded connection structure 53 and is then input to the input / output waveguide 522. At this time, the folded connection structure 53 rotates the polarization direction of the propagating light by approximately 90°, so that the light is input to the input / output waveguide 522 in a state where the TE mode and the TM mode are converted into each other in the core 503.

[0130] The light input to the second lattice filter type optical circuit 52 via the input / output waveguide 522 is attenuated according to the transmission spectrum determined by the phase modulation amount of each of the phase shifters 525-1 to 525-(N-1), and then output from the input / output waveguide 521.

[0131] In this way, the light input through the input / output waveguide 512 is transmitted in turn through the first lattice-filter type optical circuit 51 and the second lattice-filter type optical circuit 52. The light that was in TE mode when it transmitted through the first lattice-filter type optical circuit 51 is subjected to polarization rotation of about 90° in the return connection structure 53, and is transmitted through the second lattice-filter type optical circuit 52 as light in TM mode.

[0132] If the first lattice filter type optical circuit 51 and the second lattice filter type optical circuit 52 have the same design and the same phase modulation conditions, the PDL generated in the previous stage can be canceled out by the PDL generated in the subsequent stage, and therefore, a lattice filter type optical waveguide circuit 500 in which PDL is suppressed (e.g., minimized) as a whole can be realized.

[0133] Therefore, it is desirable that the phase modulation conditions in the phase shifters 515-1 to 515-(N-1) of the first lattice filter type optical circuit 51 and the phase modulation conditions in the phase shifters 525-1 to 525-(N-1) of the second lattice filter type optical circuit 52 are substantially the same.

[0134] In other words, it is desirable that the transmission spectrum of the first lattice-filter type optical circuit 51 and the transmission spectrum of the second lattice-filter type optical circuit 52 substantially match. Note that "substantially match" includes, for example, "perfect match" and the fact that a degree of mismatch is acceptable so that the remaining PDL that cannot be completely canceled out falls within a characteristically acceptable range.

[0135] When the optical waveguide circuit 500 described above is actually used, the optical waveguide circuit 500 may be adhesively fixed onto a mount. When the entire bottom surface of the optical waveguide circuit 500 is adhesively fixed to a mount, warping or thermal expansion of the optical waveguide circuit 500 may occur due to changes in the environmental temperature, and, for example, the stress applied to the core 603 may change significantly.

[0136] In this case, there is a risk that the amount of polarization rotation in the folded connection structure 53 may deviate from 90°. However, even in such a case, the amount of polarization rotation can be maintained at approximately 90° by adjusting the expansion or contraction of the thermal expansion material 506 with the heater structure 507 to control the stress applied to the core 503. Therefore, the effect of suppressing PDL that may occur in the optical waveguide circuit 500 can be maintained (or guaranteed).

[0137] As described above, according to the second embodiment, it is possible to provide the optical waveguide circuit 500 that can achieve good optical characteristics such as low PDL, independent of environmental changes (for example, temperature changes).

[0138] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0139] The present disclosure is useful, for example, in optical circuits that implement various optical functions used in optical communication networks.

[0140] 100, 200, 300, 400 Optical waveguide circuit 101 Substrate 102 Lower cladding layer 103 Core 104 Upper cladding layer 105, 105-1, 105-2, 505 Groove 106, 206, 306, 406, 506 Thermal expansion material 107, 307, 407-1, 407-2 Heater structure 107-1, 107-2, 471-1, 471-2 Heater element 500 Optical waveguide circuit 51 First polarization dependent circuit (lattice filter type optical circuit) 52 Second polarization dependent circuit (lattice filter type optical circuit) 53 Folded connection structure 511, 512, 521, 522 Input / output waveguide 513-1 to 513-N, 523-1 to 523-N: Optical directional couplers 514-1 to 514-(N-1), 524-1 to 524-(N-1): Arm waveguides 515-1 to 515-(N-1), 525-1 to 525-(N-1): Phase shifters

Claims

1. An optical waveguide circuit comprising: a substrate; a lower clad layer provided on the substrate; a waveguide core provided on the lower clad layer; an upper clad layer provided so as to cover the waveguide core and the lower clad layer; a groove provided along at least one of both sides in the width direction of the waveguide core; a thermal expansion material provided inside the groove and having a linear expansion coefficient different from that of the lower clad layer and the upper clad layer; and at least one heater element for heating the thermal expansion material.

2. The optical waveguide circuit according to claim 1, wherein said groove has a depth that extends from the top surface of said upper cladding layer to the bottom surface of said lower cladding layer.

3. The optical waveguide circuit according to claim 1 or 2, wherein the thermal expansion material fills the interior of the groove or is provided as a layer covering the bottom surface and side walls inside the groove.

4. An optical waveguide circuit according to claim 1 or 2, wherein the heater element is provided on the surface of the upper cladding layer or on the thermal expansion material at a position where the center of the heater element in the width direction is shifted from the center of the waveguide core in the width direction.

Images