Variable optical attenuator

Abstract

The invention relates to a variable optical attenuator. A variable optical attenuator (100) comprises a planar lightwave circuit comprising a waveguide embedded in a cladding supported on a silicon-based silicon dioxide substrate (176), the lightwave circuit comprising a Mach-Zehnder interferometer, the interferometer comprising an optical waveguide (101) comprising first and second optical components (102, 103), at least two waveguide branches (110a, 110b), the at least two waveguide branches comprise a first and a second waveguide extension (111a, 111b). The first optical assembly is arranged to distribute input light into the at least two waveguide branches, and the second optical assembly is arranged to combine light from the at least two waveguide branches together. The attenuator further comprises first and second heating elements (121a, 121b) arranged to heat the first and second waveguide extensions, the central groove structure (130) comprising one or more grooves formed in the silicon-based silica substrate. A first distance (D1) between the first optical component and the nearest groove edge of the central groove structure is greater than 1000 [mu] m.

Description

Technical Field

[0001] The present invention relates to a variable optical attenuator constructed as a Mach-Zehnder planar lightwave circuit. Background Art

[0002] Variable optical attenuators (VOAs) are key components in optical communication networks. The increasing demand for network capacity drives the performance requirements for VOA components, namely reducing electrical power consumption while maintaining high-quality optical performance characteristics. Variable optical attenuators can be based on the thermo-optical effect in an interferometer configuration (e.g., Mach-Zehnder configuration) to achieve externally controlled attenuation.

[0003] A key issue of variable optical attenuators is the polarization dependent loss (PDL) that may be generated due to stress effects.

[0004] US2005 / 135728A1 discloses a variable optical attenuator constructed as a Mach-Zehnder planar lightwave circuit (PLC). The variable optical attenuator includes a channel waveguide support structure for thermal isolation and stress relief to reduce polarization-dependent loss and power consumption in the device. The power reduction groove includes longitudinal segments, and small stress relief pillars of cladding material are left between these longitudinal segments during the etching process. The waveguide is supported by the main pillar structure and the overall stress relief pillars left after the grooves are removed. The waveguide is surrounded by air on three sides to improve thermal isolation. Compared with the continuous groove design of the prior art, the performance of the invention is significantly improved in PDL and extinction ratio, and is also improved to a lesser extent compared with the case where the power reduction groove is not used at all. In all cases including grooveless and grooved devices, the segmented grooves appear to minimize the stress on the two waveguide arms.

[0005] Although US 2005 / 135728 A1 solves the problem of polarization-dependent loss, improvements such as simplified design, reduced polarization-dependent loss and electrical power consumption are still desired.

[0006] JP2007163825A discloses a waveguide type thermo-optical circuit, which includes a directional coupler, two arm waveguides and a thin film heater. In order to generate an optical path difference in the arm waveguide, the core of the first arm waveguide is formed to have a different width from the core of the second arm waveguide, and the refractive index difference caused by the difference in the core width of the arm waveguide is used to make the optical effective distances in the arm waveguides different from each other. Therefore, the wavelength dependence of the waveguide type thermo-optical circuit on the refractive index difference is small.

[0007] Although JP2007163825A studies and provides a solution to the wavelength dependence of attenuation for a given optical path difference in an arm waveguide, it does not consider the problem of polarization-dependent loss associated with the thermal isolation slot structure. Summary of the invention

[0008] The object of the present invention is to improve a variable optical attenuator, in particular to achieve a reduction in polarization dependent losses and a reduction in electrical power consumption.

[0009] In a first aspect of the present invention, there is provided a variable optical attenuator, comprising a main body portion, the main body portion comprising a silicon layer and a glass layer;

[0010] - an optical waveguide arranged in the glass layer, comprising a first and a second optical component, the at least two waveguide branches comprising a first and a second waveguide extension, wherein the first optical component is arranged to distribute input light into the at least two waveguide branches, and the second optical component is arranged to combine light from the at least two waveguide branches into a light output;

[0011] - first and second heating elements extending along at least a portion of the length of the respective first and second waveguide extensions and arranged to heat the first and second waveguide extensions;

[0012] a central groove structure comprising one or more grooves formed in the glass layer, wherein the central groove structure extends along at least a portion of the length of the waveguide extension between the first and second waveguide extensions from a first position of the central groove structure proximate the first optical component to a second position of the central groove structure proximate the second optical component;

[0013] - wherein a first distance from the first optical component to the first position of the central recess is greater than 1000 μm.

[0014] Advantageously, by designing the variable optical attenuator to have a sufficiently large distance of greater than 1000 μm between the first optical component and the first position of the central recess or groove formed in the glass layer to provide thermal isolation, when the recess is formed by deep etching or the like, less stress is generated in the main body portion or less influence is exerted on the polarization behavior of the first and second optical components.

[0015] The first and second optical components include multimode components arranged to distribute light into single mode waveguides and combine light from the single mode waveguides under interference. Thus, the first and second optical components may be multimode interference components, such as first and second optical couplers.

[0016] Thus, the optical waveguide or waveguide circuit (eg, planar waveguide circuit) comprises an interferometer, such as a Mach-Zehnder interferometer.

[0017] A glass layer may be arranged on top of the silicon layer, which may include different glass layers having different properties.

[0018] The first and second waveguide extensions are arranged to extend the length of at least one of the two waveguide branches to support a thermo-optical effect.The first and second waveguide extensions may be arranged parallel to each other.

[0019] The first distance may be in the range of 1000-3000 μm, such as in the range of 1500-2500 μm. Similarly, the second distance between the second optical component and the second location of the central recess structure may be greater than 1000 μm, such as in the range of 1000-3000 μm, such as in the range of 1500-2500 μm.

[0020] The thermal isolation effect generated by the groove structure is applied along the length portion of the first and second heating elements. An excessively large first distance and / or second distance will limit the length portion, so that the variable optical attenuator will require greater power consumption and higher heater temperature to achieve the same light attenuation. However, high temperature is not desirable and may cause thermal stress. Therefore, the upper limit value of the distance range is limited.

[0021] According to one embodiment, the total length of the central groove structure accounts for a certain percentage of the length of the first and second heating elements, thereby ensuring that when electrical power is applied to the first and second heating elements to generate heat, the thermal isolation effect caused by the groove structure is applied along the relatively long length of the first and second waveguide extensions. It has been found that when the total length of the central groove structure accounts for 100% or less than 100% of the length of the first and second heating elements, such as 100% of the length of the first and second heating elements, such as as low as 80% of the length of the first and second heating elements, such as as low as 60% of the length of the first and second heating elements, such as as low as 40% of the length of the first and second heating elements, the groove structure can provide sufficient thermal isolation.

[0022] The central recess structure may comprise a single recess, ie the central recess structure may be formed as a single hole. Alternatively, the central recess structure may be formed by a plurality of recesses separated by walls of the glass layer.

[0023] The length of the central recess structure in the direction along the waveguide extension may be 2-7 mm. The length of any one of the first heating element and the second heating element in the same direction may be in the range of 2-9 mm, and thus may be longer or shorter than the central recess structure.

[0024] According to one embodiment, the variable optical attenuator further comprises first and second side groove structures, each side groove structure comprising one or more grooves formed in the glass layer, wherein the side groove structures extend along at least a portion of the length of the first and second waveguide extensions and extend along the sides of the first and second waveguide extensions facing away from the central groove structure. The first and second side groove structures may independently have a length of 2-7 mm in a direction along the waveguide extension.

[0025] According to one embodiment, the first and second side groove structures together with the central groove structure form first and second longitudinal protrusions or narrow ridges in the interval between the corresponding first and second side groove structures and the central groove structure, and wherein the first and second waveguide extensions are formed in the corresponding first and second longitudinal protrusions.

[0026] Advantageously, the side groove structure constrains the portion of the substrate containing the first and second waveguide extensions to a narrow ridge, thereby further increasing the thermal isolation effect applied to the first and second waveguide extensions.

[0027] The first and second longitudinal projections may have a width in the range of 10-200 μm in a direction perpendicular to the elongated direction of the first and second longitudinal projections.

[0028] According to one embodiment, the main body portion comprises

[0029] - a silicon-based silicon dioxide substrate comprising a silicon substrate and a furnace-grown silicon dioxide layer, and

[0030] - cladding, where

[0031] - The glass layer comprises a furnace grown silica layer and a cladding layer.

[0032] According to one embodiment, the optical waveguide is embedded in the cladding, such as substantially embedded in the cladding, the first and second heating elements are arranged on top of the cladding, opposite the first and second waveguide extensions, and the one or more grooves of the central groove structure extend through the cladding.

[0033] It will be appreciated that the optical waveguide is substantially embedded in the cladding as the process of forming the waveguide means that a portion of the waveguide may extend into the furnace grown silica layer.

[0034] The recesses of the central recess structure and / or the side recess structures may additionally also extend at least partially through the furnace-grown silicon dioxide layer and possibly into the silicon layer.

[0035] An optical waveguide, i.e. a waveguide circuit, is embedded in a cladding glass layer supported on a silicon-based silica substrate, the silicon-based silica substrate comprising a silicon layer and a furnace-grown silica layer forming part of the glass layer, the optical waveguide circuit comprising a Mach-Zehnder interferometer, wherein first and second heating elements are arranged on top of the cladding, opposite first and second waveguide extensions, wherein one or more grooves of a central groove structure extend through the silicon cladding and at least partially through the silicon layer of the silicon-based silica substrate.

[0036] In addition, the core of the optical waveguide can be formed on top of a silicon-based silica substrate in a glass layer and has a higher refractive index than the surrounding furnace-grown silica and cladding glass. The furnace-grown silica layer and cladding define the boundary between the lower and upper parts of the optical waveguide.

[0037] A second aspect of the present invention relates to the use of the variable optical attenuator according to the first aspect.

[0038] Briefly, the present invention relates to a variable optical attenuator. The attenuator comprises a planar lightwave circuit supported on a silicon-based silica substrate, the planar lightwave circuit comprising an optical waveguide, the optical waveguide comprising a first and a second optical component, comprising at least two waveguide branches of a first and a second waveguide extension. The first optical component is arranged to distribute input light into at least two waveguide branches, and the second optical component is arranged to combine light from at least two waveguide branches together. The attenuator also comprises a first and a second heating element, the heating element being arranged to heat the first and the second waveguide extensions, and the central groove structure comprising one or more grooves formed in the silicon-based silica substrate. The minimum distance between the first optical component and the groove edge of the central groove structure closest to the first optical component is a first distance, the value of which is greater than one of the following values: 1000 μm, 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm or 1800 μm. Meanwhile, a value of the first distance is less than one of the following values: 4000 μm, 3000 μm, 2800 μm, 2700 μm, 2600 μm, 2500 μm, 2400 μm, 2100 μm or 2000 μm.

[0039] In general, the various aspects and embodiments of the present invention may be combined and coupled in any possible manner within the scope of the present invention. These and other aspects, features and / or advantages of the present invention will be apparent and elucidated from the embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the present invention will be described below by way of example only with reference to the accompanying drawings, in which

[0041] Figure 1 shows a top view of a variable optical attenuator,

[0042] Figure 2 Shows Figure 1 AA cross-sectional view,

[0043] Figure 3 shows the measurement results of the phase angle change Δφ per mW of electric power applied to one of the heating elements 121a, 121b for different D values,

[0044] Figure 4 and Figure 5 The relationship between polarization dependent loss and distance D is shown under the condition that the heating element is controlled to achieve a specific light attenuation. DETAILED DESCRIPTION

[0045] Figure 1 A schematic top view sketch of a planar lightwave circuit variable optical attenuator 100 is shown. The variable optical attenuator 100 includes an optical waveguide, such as a waveguide circuit 101, substantially embedded in a cladding supported on a silicon-on-silicon dioxide substrate, the optical waveguide 101 including a Mach-Zehnder interferometer. The optical waveguide 101 is fabricated using silicon-on-silicon dioxide process technology starting from a silicon substrate on which a silicon dioxide layer is grown by high temperature oxidation in a furnace. The core of the waveguide is formed on top of the silicon-on-silicon dioxide substrate in a glass layer having a higher refractive index than the surrounding cladding glass.

[0046] Figure 2 Shows Figure 1 1, which shows that the variable optical attenuator 100 includes a body portion 176, which includes a silicon layer 171 and a glass layer 174. The silicon layer 171 is a part of a silicon-based silica substrate 170, which includes a silicon layer or substrate 171 and a furnace-grown silica layer 172. The waveguide core of the optical waveguide 101 is formed on top of the silicon-based silica substrate 170 in the glass layer, and its refractive index is higher than the surrounding cladding glass 173. The glass layer 174 includes a cladding 173, and the waveguide core is embedded in the cladding, located on top of the furnace-grown silica layer 172.

[0047] Figure 2 It is mainly shown that the waveguide portions 111a, 111b of the optical waveguide 101 have a well-defined boundary at the boundary between the cladding layer 173 and the furnace-grown silica layer 172. In fact, the boundary of the optical waveguide 101 (i.e., at Figure 2 A clear boundary may not be formed in the cross section of the optical waveguide 101 , and an extension of the optical waveguide 101 in the cross section may extend into the furnace-grown silicon dioxide layer 172 .

[0048] The optical waveguide 101 comprises a first optical component 102 arranged to distribute input light into at least two waveguide branches 110a, 110b.

[0049] In this example, the first optical component 102 is a 2x2 multimode interference (MMI) coupler. However, the first optical component 102 can be any optical coupler or splitter that is arranged to distribute input light to at least two waveguide branches 110a, 110b. Therefore, typically the optical component 102 can be an MxN optical coupler or splitter, where M can be 1 or more and N can be 2 or more. Examples of the first optical component include a 3dB directional coupler or a 2x2 MMI coupler that is arranged to receive input light from two input waveguides and couple the input light into two waveguide branches 110a, 110b. In another example, the optical component 102 is a 1x2 Y-type splitter that is arranged to receive light from one input waveguide and distribute the input light to two waveguide branches 110a, 110b.

[0050] Input light is light transmitted into the optical component 102 through one or more input waveguides 190 (here, two input waveguides 190 ).

[0051] The optical waveguide 101 further comprises a second optical component 103, which is arranged to combine light from at least two waveguide branches 110a, 110b into a light output (e.g., one or more light outputs). The second optical component 103 can be any type of the first optical component 102, and can be selected to be the same or different type as the first optical component 102.

[0052] The light output is the light transmitted out of the optical assembly 103 through one or more output waveguides 191 (here two output waveguides 191).

[0053] As shown in the enlarged view of the first optical component 102, the optical component 102 includes a multimode waveguide portion 102a connected to two or more waveguide branches 110a, 110b. The waveguide branches 110a, 110b can be single-mode waveguides. Therefore, light is coupled from the multimode waveguide portion 102a into the waveguide branches 110a, 110b. The second optical component 103 is configured in the same manner, that is, it includes a multimode waveguide portion that is arranged to receive light from two or more waveguide branches 110a, 110b and couple it into one or more output waveguides 191.

[0054] The optical waveguide 101 further comprises at least two waveguide extensions 111a, 111b, for example a first and a second waveguide extension, which are arranged as extensions of the two or more waveguide branches 110a, 110b and connect at least two output waveguide branches of the first optical component 102 with two or more input waveguide branches of the second optical component 103. The waveguide extensions may be arranged as straight waveguides and parallel to each other.

[0055] The variable optical attenuator 100 further comprises first and second heating elements 121a, 121b extending along at least a portion of the length of the respective first and second waveguide extensions 111a, 111b. The heating elements 121a, 121b comprise an electrically conductive material, such as a metal. By applying power to the heating elements, heat is generated due to Joule heating. The first and second heating elements 121a, 121b may be arranged on top of the cladding of the respective first and second waveguide extensions 111a, 111b, for example directly opposite the first and second waveguide extensions 111a, 111b. Thus, the heat from the heating elements results in heating of the waveguide extensions and their surroundings.

[0056] The variable optical attenuator 100 further comprises one or more groove structures (130, 131, 132). The groove structures comprise a central groove structure 130 arranged between the first and second waveguide extensions 111a, 111b. The central groove structure 130 extends from a first position 141 of the central groove facing the first optical component 102 to a second position 142 of the central groove facing the second optical component 103. The length of the central groove structure 130, i.e., the extension between the first and second positions, may be shorter or longer than the length of the first and second waveguide extensions 111a, 111b, and shorter or longer than the length of the first and second heating elements 121a, 121b. Therefore, the length of the central groove structure may be at least a fraction of the length of the first and second waveguide extensions 111a, 111b or the length of the first and second heating elements 121a, 121b in the same direction.

[0057] The central recess structure 130 includes one or more recesses formed in the glass layer 174. Each of the one or more recesses, which may be produced by etching, has an opening on the top surface of the cladding layer extending downward into the silicon-based silicon dioxide substrate, i.e., into the furnace-grown silicon dioxide layer 172.

[0058] The central recess structure 130 may include a single recess, i.e., a hole (e.g., Figure 1 ), or may include multiple grooves separated by non-etched areas. A single groove may also include a protrusion (e.g., a pillar structure) of a non-etched portion of a glass layer or a silicon-based silicon dioxide substrate.

[0059] In addition, if Figure 1As shown, the variable optical attenuator 100 may include first and second side groove structures 131, 132 arranged along the sides of the first and second waveguide extensions 111a, 111b, so that the first and second waveguide extensions 111a, 111b extend between the first side groove structure 131 and the central groove structure 130 and between the second side groove structure 132 and the central groove structure 130, respectively. Therefore, the side groove structures 131, 132 extend along at least a portion of the length of the first and second waveguide extensions 111a, 111b, and extend along the sides of the first and second waveguide extensions 111a, 111b, respectively, away from the central groove structure 130.

[0060] As shown, the lengths of the first and second side groove structures 131, 132 and the central groove structure 130 along the first and second waveguide extensions 111a, 111b may be equal or substantially equal.

[0061] The first and second side groove structures 131 , 132 are configured and formed according to the same possible configurations as the central groove structure 130 .

[0062] When power is applied to the first and second heating elements, the one or more grooves of the central groove structure and the first and second side groove structures provide local thermal isolation. That is, the air-filled grooves reduce heat transfer between the first and second waveguide extensions 111a, 111b and generally reduce heat conduction away from the first and second waveguide extensions 111a, 111b. Reduced heat conduction reduces the electrical power required to raise the temperature of the waveguide extensions 111a, 111b by a specified amount, i.e., it allows the variable optical attenuator to be operated more efficiently through the thermo-optical effect.

[0063] The elongated portion of the glass layer including the first and second waveguide extensions 111a, 111b and extending between the first side groove structure 131 and the central groove structure 130 and between the second side groove structure 132 and the central groove structure 130, respectively, forms the first and second longitudinal protrusions 125a, 125b, i.e., structures formed as ridges or walls between the side groove structures (131, 132) and the central groove structure 130. Therefore, the first and second side groove structures 131, 132 together with the central groove structure 130 form the first and second longitudinal protrusions 125a, 125b in the interval between the respective first and second side groove structures 131, 132 and the central groove structure 130. Therefore, the first and second waveguide extensions 111a, 111b extend in the respective first and second longitudinal protrusions 125a, 125b.

[0064] The width 181 of the first and second longitudinal protrusions may be in the range of 10 - 1000 μm, for example in the range of 10 - 200 μm.

[0065] The length of the central groove structure 130 and the side groove structures (131, 132) may be in the range of 2-7 mm along and parallel to the waveguide extensions 111a, 111b, i.e. along the light propagation direction in the waveguide extensions. The length of the central groove structure 130 and / or the side groove structures (131, 132) corresponds to the distance between the first position and the second position 141, 142.

[0066] Similarly, either of the first and second heating elements may have a length along and parallel to the waveguide extensions 111a, 111b in the range of 2 - 9 mm.

[0067] Figure 1 A first distance D1 is shown from the first optical component 102 to the first location 141 of the central recess 130. The first distance D1 is measured from the location of the optical component 102 where the waveguides 110a, 110b branch off from the multimode portion of the optical component 102, as shown in the enlarged view of the optical component 102.

[0068] The first distance D1 is the shortest distance between the optical component 102 and the central recess 130, ie it is measured along a direction parallel to the waveguide extensions 111a, 111b.

[0069] The first distance D1 is preferably greater than 1000 μm, and may be a distance within any range of 1000-4000 μm, 1100-3000 μm, 1200-2800 μm, 1300-2700 μm, 1400-2600 μm, 1500-2500 μm, 1600-2400 μm, and 1700-2100 μm.

[0070] A second distance D2 between the second optical component 103 and the second position 142 of the central recess 130 , which is similarly defined, may have a distance within the same range as the first distance D1 .

[0071] The distance 182 between the first and second optical components 102, 103 measured between the input of the multimode portion of the first optical component 102 and the output of the multimode portion of the second optical component 103 may be in the range of 5 to 15 mm.

[0072] Figure 2 Shows Figure 1 AA cross-sectional view of the variable optical attenuator in FIG. The silicon-based silica substrate 170 includes a silicon substrate 171 and a furnace-grown silica layer 172. The waveguide core of the optical waveguide 101 (shown here as waveguide extensions 111a, 111b) is formed on top of the silicon-based silica substrate 170 in a glass layer, which has a higher refractive index than the surrounding cladding glass 173.

[0073] The furnace grown silica layer 172 and the cladding layer 173 are not necessarily different layers, but are just arbitrary layers that define the boundary between the lower and upper portions of the optical waveguide 101. The furnace grown silica layer 172 aids in optical confinement and provides thermal isolation.

[0074] Thus, the optical waveguide 101 is embedded in the glass layer 174 , for example embedded in the cladding 173 or substantially embedded in the cladding 173 .

[0075] The first and second heating elements 121a, 121b are arranged on top of the cladding layer 173 and are protected by a passivation layer ( Figure 2 The longitudinal center axis, i.e. the axis co-linear with the geometric center of the cross-sectional area of ​​the heating element, is respectively the waveguide extension 111a, 111b, which may lie in the same vertical plane perpendicular to the plane of the planar top surface 175 of the cladding 173.

[0076] The one or more grooves of the central groove structure 130 and the optional one or more grooves of the side groove structures ( 131 , 132 ) extend in a vertical direction, i.e. perpendicular to the plane of the top surface 175 of the cladding 173 , through the cladding and at least partially through the furnace grown silicon dioxide layer 172 .

[0077] Thus, the groove is formed as a hole, the opening of which is located in the planar top surface 175 of the cladding 173 and the hole of which extends vertically downwards towards the silicon layer 171. The hole extends at least through the cladding 173, i.e. below the boundary between the cladding 173 and the silicon-based silica substrate 170. The hole may also extend downwards to the upper surface of the silicon substrate 171, i.e. to the boundary between the furnace-grown silica layer 172 and the silicon substrate 171, or the hole may even extend into the silicon substrate.

[0078] The grooves of the groove structure can be realized by a deep etching method.

[0079] The interferometer configuration of the variable optical attenuator 100 (e.g., the Mach-Zehnder interferometer shown here) includes two optical components 102, 103, which are connected by at least two waveguide branches 110a, 110b, wherein resistive heaters 121a, 121b are located on top of the waveguide extensions 111a, 111b of the at least two waveguide branches so that heat can be applied independently to the waveguide extensions 111a, 111b of the at least two waveguide branches 110a, 110b. The independent heating causes a difference in the optical path length between the independently heated waveguide branches 110a, 110b, thereby resulting in variable optical attenuation.

[0080] The controllable optical attenuation of the variable optical attenuator 100 is achieved by utilizing the thermo-optical effect when the waveguide extensions 111a and 111b are heated respectively to change the refractive index of the waveguide. The light output from the waveguide extensions 111a and 111b is combined in the multimode part of the second optical component 103, thereby causing variable attenuation due to interference.

[0081] The temperature distribution inside the variable optical attenuator 100 is critical to the performance of the variable optical attenuator, such as electrical power consumption and polarization dependent loss (PDL). The grooves of the central groove structure 130 and the optional side groove structures 131, 132 provide the thermal management required to achieve low electrical power consumption.

[0082] Polarization dependent loss is highly sensitive to stress. All stress contributions caused by mechanical and thermal effects around the first and second optical components 102 , 103 and the first and second waveguide branches 110 a , 110 b must be balanced and / or reduced to achieve low PDL for the variable optical attenuator 100 .

[0083] Figure 3 The measured values ​​of the optical phase angle change Δφ per mW of electrical power applied to one of the electrical heating elements 121a, 121b are shown for different values ​​of D. Here, D1 and D2 are equal, so D=D1=D2. The phase angle φ is defined as the angular wave number multiplied by the optical path length difference between the first and second waveguide branches 110a, 110b. Therefore, as shown, an increase in D will reduce the phase angle change Δφ per mW of applied electrical power because the groove length of the center and side groove structures is reduced (e.g., due to the fixed length 182, which corresponds to the fixed length of the variable optical attenuator 100) and thus shorten the length of the first and second longitudinal protrusions 125a, 125b that provide increased thermal isolation of the first and second waveguide extensions 111a, 111b and between the two.

[0084] Figure 4 The polarization dependent loss PDL (in dB) is shown as a function of the distance D when the heating elements 121 a , 121 b are controlled to achieve a light attenuation of 5 dB. Figure 5 The same polarization dependent losses are shown where the heating elements 121a, 121b are controlled to achieve 10 dB of light attenuation.

[0085] Figure 4 and Figure 5 The results in clearly indicate the range of distances D1, D2, within which low PDL (PDL of about 0.5dB at 10dB attenuation and about 0.3dB at 5dB attenuation) can be achieved while still achieving low power consumption by defining the grooves sufficiently far away from the first and second optical components 102, 103. Figure 4 and Figure 5It shows that D1 and / or D2 should be greater than 1800μm to achieve low PDL. Figure 3 An acceptable level of power dissipation is shown, i.e., Δφ per mW of approximately 0.8° / mW.

[0086] As described above, other ranges of the first and second distances D1, D2 are also feasible and are associated with other configurations of the variable optical attenuator 100. Figure 3-5 As shown, distances D1 and D2 may be in the range of 1100-2400 μm, or even outside these ranges, for example distances D1, D2 greater than 1000 μm may be feasible.

Claims

1. A variable optical attenuator (100), comprising: - a main body portion (176) comprising a silicon layer (171) and a glass layer (174); - an optical waveguide (101) arranged in a glass layer (174), wherein The optical waveguide comprises first and second optical components (102, 103), at least two waveguide branches (110a, 110b) comprising first and second waveguide extensions (111a, 111b), wherein the first optical component (102) is arranged to distribute input light into the at least two waveguide branches, and the second optical component (103) is arranged to combine light from the at least two waveguide branches into a light output; - first and second heating elements (121a, 121b) extending along at least a portion of the length of the respective first and second waveguide extensions (111a, 111b) and arranged to heat the first and second waveguide extensions; a central groove structure (130) comprising one or more grooves formed in the glass layer (174), wherein the central groove structure extends along at least a portion of the length of the waveguide extension between the first and second waveguide extensions from a first position (141) of the central groove structure proximate the first optical component (102) to a second position (142) of the central groove structure proximate the second optical component (103); - wherein a first distance (D1) from the first optical component (102) to the first position (141) of the central groove structure is greater than 1000 μm.

2. The variable optical attenuator according to claim 1, wherein the first distance (D1) is in the range of 1100-3000 μm, such as in the range of 1500-2500 μm.

3. The variable optical attenuator according to any of the preceding claims, wherein a second distance (D2) from the second optical component (103) to the second position (142) of the central recess structure is greater than 1000 μm, such as in the range of 1100-3000 μm, such as in the range of 1500-2500 μm.

4. A variable optical attenuator according to any one of the preceding claims, wherein: The first and second optical components are multi-mode interference components.

5. A variable optical attenuator according to any one of the preceding claims, wherein: The central groove structure (130) includes a single groove.

6. The variable optical attenuator according to any of the preceding claims, wherein the length of the central groove structure is in the range of 2 - 7 mm, depending on the distance between the first and second positions (141, 142) of the groove.

7. The variable optical attenuator according to any one of the preceding claims, wherein the length of any one of the first and second heating elements (121a, 121b) is in the range of 2 - 9 mm.

8. A variable optical attenuator according to any one of the preceding claims, comprising first and second side groove structures (131, 132), each comprising one or more grooves formed in a glass layer (174), wherein: The side groove structure extends along at least a portion of the length of the first and second waveguide extensions (111a, 111b) and along sides of the first and second waveguide extensions facing away from the central groove structure.

9. The variable optical attenuator according to claim 8, wherein the first and second side groove structures (131, 132) together with the central groove structure (130) form first and second longitudinal protrusions (125a, 125b) in the interval between the corresponding first and second side groove structures and the central groove structure, and wherein at least a portion of the first and second waveguide extensions (111a, 111b) are formed in the corresponding first and second longitudinal protrusions.

10. The variable optical attenuator according to claim 9, wherein any one of the first and second longitudinal protrusions (125a, 125b) has a width (181) in the range of 10-200 μm in a direction perpendicular to the elongated direction of the first and second longitudinal protrusions.

11. A variable optical attenuator according to any one of the preceding claims, wherein: The first and second optical components (102, 103) are first and second optical couplers.

12. A variable optical attenuator according to any one of the preceding claims, wherein each of the first and second waveguide extensions (111a, 111b) is arranged to extend the length of at least one of the two waveguide branches (110a, 110b).

13. A variable optical attenuator according to any one of the preceding claims, wherein: The elongation direction of each of the first and second waveguide extensions (111a, 111b) is parallel.

14. The variable optical attenuator according to any one of the preceding claims, wherein the main body portion comprises: - a silicon-based silicon dioxide substrate (170) comprising a silicon substrate (171) and a furnace-grown silicon dioxide layer (172), and - a cladding (173), wherein - The glass layer (174) comprises a furnace grown silicon dioxide layer (172) and a cladding layer (173).

15. The variable optical attenuator according to claim 14, wherein - the optical waveguide (101) is embedded in the cladding (173), for example substantially embedded in the cladding (173), - first and second heating elements (121a, 121b) are arranged on top of the cladding (173), opposite the first and second waveguide extensions (111a, 111b), and - One or more grooves of a central groove structure (130) extend through the cladding.

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