Power-efficient variable optical attenuator

Abstract

The present disclosure relates to a variable optical attenuator comprising: a carrier injection region having a waveguide core; an under-cladding arranged under the carrier injection region; a heat isolation region having a heat conductivity which is lower than a heat conductivity of the under-cladding, wherein the under-cladding is at least partially surrounded by the heat isolation region. Accordingly, by suspending the waveguide core and its under-cladding in the heat isolation region, optical attenuation is enhanced.

Description

Technical Field

[0001] This invention relates to the fields of integrated photonics, optical signal attenuation, optical signal balancing, coherent photonics, data processing, and telemetry. It discloses a variable optical attenuator with one or more suspension structures that improves optical signal attenuation. Prior Technology

[0002] Variable optical attenuators (VOAs) are used to attenuate optical signals to the desired amplitude for optical signal balancing and modulation. In optical transceivers, VOAs achieve signal control and balancing. For example, in dense wavelength division multiplexing (DWDM) systems, signal loss due to long-distance transmission and component losses can be compensated by adjusting the optical power level.

[0003] Various types of variable optical attenuators have been realized, such as mechanical, acousto-optic, microfluidic, and optoelectronic types. Traditionally, mechanical variable optical attenuators have been widely used in optical communications. However, mechanical variable optical attenuators are susceptible to mechanical wear and involve potentially more expensive precision engineering. On the other hand, optoelectronic variable optical attenuators are particularly attractive due to their durability, cost-effectiveness, and high structural density, and they are highly compatible with integrated optical devices.

[0004] Since variable optical attenuators require a continuous supply, their primary performance indicator is power efficiency. While optoelectronic variable optical attenuators typically offer higher power efficiency than mechanical ones, the volatile nature of most optoelectronic attenuators—requiring a continuous electrical signal supply to maintain attenuation—and their widespread implementation in data communications for signal balancing purposes, can result in very high net power consumption. Therefore, there is a demand for variable optical attenuators with lower power consumption.

[0005] Based on the above and other issues, it is ideal to provide an improved photonic element that overcomes the aforementioned shortcomings. Summary of the Invention

[0006] One embodiment of the present invention provides a variable optical attenuator, comprising: a carrier injection region having a waveguide core; a lower cladding layer disposed below the carrier injection region; and a thermal isolation region having a thermal conductivity lower than that of the lower cladding layer, wherein the lower cladding layer is at least partially surrounded by the thermal isolation region.

[0007] In one embodiment, the thermal insulation zone is disposed below the lower covering layer and / or at least on one side.

[0008] In one embodiment, the variable optical attenuator further includes: at least one first substrate; wherein the thermal isolation region includes at least one thermal insulation material, wherein the at least one first substrate and the at least one thermal insulation material are arranged adjacently to provide at least one alternating structure disposed below the lower cladding layer.

[0009] In one embodiment, at least one alternating structure comprises multiple identical or different alternating structures.

[0010] In one embodiment, the alternating structure is configured as at least two columns, wherein the variable optical attenuator further includes a second substrate inserted between the at least two columns.

[0011] In one embodiment, the variable optical attenuator further includes a third substrate disposed below the alternating structure.

[0012] In one embodiment, the carrier injection region includes a PIN junction having a p-doped region, an n-doped region, and an intrinsic region interposed therebetween that serves as the waveguide core.

[0013] In one embodiment, the doping concentration of the p-doped region and the doping concentration of the n-doped region are either symmetrical or asymmetrical.

[0014] In one embodiment, the thermal insulation material comprises at least one of the group consisting of silicon dioxide, silicon nitride, aluminum nitride, polyimide, germanium, polymethyl methacrylate (PMMA), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), liquid, and air.

[0015] In one embodiment, the thermal isolation region is configured to be parallel to and / or transverse to the light propagation direction of the waveguide core.

[0016] In one embodiment, the variable optical attenuator further includes a substrate block having an air cavity disposed below the lower cladding layer, the thermally insulating region being the air cavity, wherein the lower cladding layer is supported by the substrate block.

[0017] In one embodiment, the variable optical attenuator further includes: a support member, wherein the waveguide core includes two opposite ends and a non-end inserted therebetween, wherein the thermal isolation region includes an air cavity surrounding the waveguide core and the lower cladding layer, wherein the two opposite ends of the waveguide core are supported by the support member such that the non-end of the waveguide core is suspended in the air cavity.

[0018] In one embodiment, the carrier injection region includes one of the following groups: vertical junction, PIPIN junction, epitaxial vertical junction, cladding junction, junction with anti-doped corner, serrated junction, staggered junction, three-terminal junction, and horizontal junction.

[0019] In one embodiment, the variable optical attenuator further includes: an upper cladding layer, wherein a carrier injection region is interposed between the upper cladding layer and the lower cladding layer.

[0020] Following a more detailed description of various embodiments, those skilled in the art will understand that the present invention provides a significant technological advancement in providing highly efficient power-variable optical attenuators with high spectral bandwidth. Particularly important in this respect is the invention's potential to provide enhanced optical attenuation with wide optical bandwidth and high structure density. This prevents excessively high power requirements when operating attenuators in extended optical communication, sensing, and / or ranging systems employing such attenuators for signal balancing (e.g., in coherent and wavelength-splitting multiplexing systems). Additional features and advantages of the various embodiments will be better understood in the detailed description provided below. Simple Explanation of the Diagram

[0021] Figure 1 shows a schematic diagram of a variable optical attenuator according to various embodiments. Figures 2A to 2I show various examples of carrier injection regions. Figure 3A shows a schematic diagram of a conventional variable optical attenuator, while Figure 3B shows a schematic diagram of a variable optical attenuator according to an embodiment of the present invention. Figure 4A shows a simplified cross-sectional view of the conventional variable optical attenuator in Figure 3A, while Figure 4B shows a simplified cross-sectional view of the variable optical attenuator in Figure 3B. Figure 5 shows a simplified isometric view of the variable optical attenuator in Figure 4B. Figure 6 shows a simplified isometric view of a variable optical attenuator according to one embodiment. Figures 7A and 7B show microscope images of an embodiment of a variable optical attenuator with an optical waveguide cladding surrounded by alternating air trenches and a substrate structure. Figure 8 shows experimental measurements of optical attenuation when different supply electrical powers are applied to the variable optical attenuators corresponding to Figures 3A and 3B. Figures 9A and 9B show the experimentally measured optical transmission spectra when different supply electrical powers are applied to the variable optical attenuators in Figures 3A and 3B. Figure 10 is an experimental measurement diagram of the modulated bandwidth of the variable optical attenuator corresponding to Figures 3A and 3B. Figure 11 shows a simplified cross-sectional view of a variable optical attenuator according to one embodiment. It should be understood that the drawings are not necessarily drawn to scale, but rather present a simplified representation of various features illustrating the basic principles of the invention. Specific design features of the variable optical attenuator disclosed herein, including, for example, the specific dimensions of the variable optical attenuator, will be determined in part by the specific intended application and usage environment. Specific features of the illustrated embodiments are enlarged or distorted relative to other features to aid in clarity. In particular, thin features, for example, may be thickened for clarity of illustration. Implementation

[0022] In the following description, various embodiments are illustrated with reference to drawings, wherein the same reference numerals generally denote the same parts in different views. It will be apparent to those skilled in the art, i.e., those with knowledge or experience in this art, that the high-efficiency power variable optical attenuator disclosed herein may have many uses and design variations. The embodiments described below illustrate the general principles of the invention to provide a high-efficiency power variable optical attenuator suitable for optically attenuating the optical field on an optical waveguide, adaptable to applications for which the high-efficiency power variable optical attenuator is designed. Other embodiments suitable for other applications will be apparent to those skilled in the art upon gaining the benefits of this disclosure.

[0023] It should be understood that the articles “a” and “described” used with respect to features or elements include references to one or more features or elements. The term “and / or” includes any and all combinations of the associated features or elements. The terms “comprising,” “including,” “having,” and related terms used in the specification and claims are intended to be open-ended, meaning that additional features or elements may exist in addition to those listed. Identifiers such as “first,” “second,” and “third,” etc., are used merely as labels and are not intended to impose numerical requirements on their objects, nor should they be interpreted in a manner that imposes any relative position or chronological order on limitations. The term “to” may include references to “configured as,” “suitable for,” and “constructed and configured in,” which are used interchangeably. Furthermore, the terms “above,” “below,” “upper,” “lower,” “below,” “above,” “on top,” and “above,” and their related terms, used herein, are for convenience of description only and may refer to the orientation of features or elements shown in the figures. It should be understood that any orientation of the features described herein is within the scope of the invention.

[0024] This invention relates to an integrated photonic device, and more particularly to a high-efficiency power-variable optical attenuator. According to various embodiments shown in FIG1, the variable optical attenuator 100 includes at least one carrier injection region 120, the carrier injection region 120 having an optical waveguide core 130, a cladding layer 140, and a thermally isolated region 170.

[0025] The optical waveguide core 130 (also referred to as the "waveguide core") is configured to transmit the received optical signal or field through the variable optical attenuator 100. The waveguide core 130 includes a longitudinal direction corresponding to the direction of optical signal or field propagation indicated by the arrow in FIG1, and a width direction perpendicular to the direction of optical signal or field propagation.

[0026] The carrier injection region 120 is configured to attenuate optical signals or fields transmitted through the waveguide core 130. Carrier injection occurs under a forward bias at a PN or PIN junction, where carriers are injected across the junction, i.e., between the p-doped region (positive or p-type semiconductor material) and the n-doped region (negative or n-type semiconductor material). The forward bias lowers the barrier at the junction, thereby allowing carriers to cross the junction.

[0027] The cladding layer 140 is configured to provide a physical gap or separation between the waveguide core 130 and other components, including the thermal isolation region 170. The cladding layer 140 includes a lower cladding layer disposed below and in contact with the carrier injection region 120 (including the waveguide core 130). Specifically, the lower cladding layer is interposed between the carrier injection region (or waveguide core) and the thermal isolation region. The cladding layer 140 may also include an upper cladding layer disposed above and in contact with the carrier injection region 120 (or waveguide core 130).

[0028] Thermal isolation region 170 is configured to isolate or limit the heat generated by the carrier injection process to enhance optical attenuation. To achieve thermal insulation, the thermal isolation region 170 has a lower thermal conductivity than the surrounding region of the carrier injection region 120, such as the lower cladding layer. Furthermore, to achieve thermal insulation, the surrounding region of the carrier injection region 120, such as the lower cladding layer, is at least partially surrounded by thermal isolation region 170, for example, partially, mostly, or completely surrounded. In other words, at least the surrounding region of the carrier injection region 120, such as the waveguide core 130 and the lower cladding layer 171, is suspended within thermal isolation region 170.

[0029] Figures 3A, 3B, 4A, 4B, 5, and 6 show a non-limiting example of a carrier injection region 120 implemented as a PIN junction. The PIN junction includes a p-doped region 121, an n-doped region 122, and an intrinsic region 123 interposed therebetween and serving as a waveguide core 130.

[0030] Figures 2A to 2I show other non-limiting examples of the carrier implantation region 120. Specifically, Figures 2A to 2I show horizontal junctions, vertical junctions, PIPIN junctions, epitaxial vertical junctions, cladding junctions, counter-doping junctions, sawtooth junctions, staggered junctions, and three-terminal junctions, respectively. Each carrier implantation region 120 includes at least a first p-doped region 121, a first n-doped region 122, and an intrinsic region 123 interposed or coupled between the first p-doped region 121 and the first n-doped region 122. In some examples, each carrier implantation region may also include a second p-doped region 124 disposed near and more heavily doped than the first p-doped region 121, and a second n-doped region 125 disposed near and more heavily doped than the first n-doped region 122.

[0031] Figures 3A and 4A show a conventional variable optical attenuator 310, in which the carrier injection region 120 is implemented as a PIN junction. A lower cladding layer 171 is disposed below the PIN junction. A first substrate 150 is disposed below the lower cladding layer 171, such that the lower cladding layer 171 is substantially or completely supported. In other words, the waveguide core 130 is substantially or completely supported, or in a non-suspended state. An upper cladding layer 173 is disposed above and in contact with the carrier injection region (or waveguide core 130). An electrode 172 is disposed above and in contact with the p-doped region 121 and the n-doped region 122. The conventional variable optical attenuator 310 does not have any thermal isolation region.

[0032] Compared to a conventional variable optical attenuator 310, embodiments of the variable optical attenuator according to the present invention provide thermal isolation regions, which can be implemented in different ways as described in the following paragraphs.

[0033] Figures 3B, 4B, and 5 illustrate one embodiment of the variable optical attenuator 320 according to Figure 1. The carrier injection region 120 is implemented as a PIN junction, having a p-doped region 121, an n-doped region 122, and an intrinsic region 123 interposed therebetween, serving as a waveguide core 130. A lower cladding layer 171 is disposed below and in contact with the PIN junction. An upper cladding layer 173 is disposed above the carrier injection region (or waveguide core 130). An electrode 172 is disposed above the p-doped region 121 and the n-doped region 122. A first substrate 150 is disposed below and in contact with the lower cladding layer 171. An air trench 160 is disposed in the first substrate 150 (see Figure 3B). The combination of the first substrate 150 and the air trench 160 provides alternating structures disposed below, in contact with, and support the lower cladding layer 171 (or waveguide core 130). In other words, the waveguide core 130 is suspended, for example, at least partially suspended in air. Each alternating structure thus includes a first substrate 150 (or a portion thereof) and an air trench 160. The alternating structures are configured in two rows. In each row, the alternating first substrate 150 and air trench 160 extend along the light propagation direction of the waveguide core 130. A second substrate 152 is interposed between the two rows and is disposed below and in contact with the lower cladding layer 171, particularly below the carrier injection region 120 (or the waveguide core 130). The first substrate 150 and the second substrate 152 may be integrally formed.

[0034] In the variable optical attenuator 320 of Figures 3B, 4B, and 5, the air trench 160 provides thermal insulation for the following reasons. First, air has a lower thermal conductivity compared to other materials such as substrates because it is a poor conductor of heat compared to solid materials. By suspending the waveguide core 130 and the lower cladding layer 171 in the air (hereinafter referred to as the "air bridge"), the thermal path from the air bridge to the underlying first substrate 150 is minimized and concentrated at the PIN junction to enhance optical attenuation. Furthermore, heat accumulation is enhanced because the air trench 160 acts as an insulating layer. This contributes to better thermal insulation, ensuring that the variable optical attenuator has increased attenuation efficiency.

[0035] Figure 6 shows an embodiment of a variable optical attenuator, similar to the variable optical attenuator 320 of Figure 5. However, the variable optical attenuator may further include a third substrate 153 disposed below and in contact with the alternating structure. In other words, the third substrate 153 is disposed below and in contact with at least some of the first substrates 150 and air trenches 160, and also in contact with the second substrate 152. The first substrates 150, second substrates 152, and third substrate 153 may be integrally formed.

[0036] In the variable optical attenuator of Figure 6, the third substrate 153 provides additional mechanical reliability for the following reasons. First, the third substrate 153 provides structural integrity because the air bridge can deform or collapse, resulting in physical damage, as the air bridge is a delicate structure that relies on mechanical supports to maintain its shape and position. Furthermore, insufficient support for the air bridge can cause it to sag or shift, leading to electrical short circuits or increased parasitic capacitance and inductance. This can reduce signal integrity and the overall performance of the variable optical attenuator. In addition, insufficient support can distort optical signal propagation and degrade the overall performance of the variable optical attenuator. Third, the third substrate 153 alleviates manufacturing challenges because a lack of mechanical support can make it difficult to maintain the precise alignment and spacing required for the air bridge.

[0037] The variable optical attenuators of Figures 3B, 4B, 5, and 6 can be modified and / or changed. For example, the alternating structure can be configured as a single row or three or more rows instead of two rows. For example, the doping concentration of the p-doped region and the doping concentration of the n-doped region can be symmetrical or asymmetrical. For example, the first substrate 150, the second substrate 152, and the third substrate 153 can be formed of the same or different materials. For example, the air trench 160 providing the thermal isolation region 170 can be replaced by at least one thermally insulating material with a thermal conductivity lower than that of the lower cladding layer 171.

[0038] Figures 7A and 7B show microscope images of the variable light attenuator 320, whose cladding layer 140 is surrounded by an alternating structure including a first substrate 150 and air trenches 160. Specifically, Figure 7A shows an overall view, while Figure 7B shows a close-up view of a portion marked by dashed lines in Figure 7A.

[0039] Figure 8 shows experimental measurements of optical attenuation when different supply electrical powers are applied to the variable optical attenuators 310 and 320 of Figures 3A and 3B. In the example in Figure 8, the experimentally measured variable optical attenuators have defined parameters, such as a waveguide width of 0.5 µm, a doping length of 250 µm, and a doping distance of 2.5 µm between the p-doped and n-doped regions forming the PIN waveguide structure. As can be observed from Figure 8, the attenuation performance of the variable optical attenuator 320 is improved compared to the conventional variable optical attenuator 310.

[0040] Figures 9A and 9B show the experimentally measured optical transmission spectra when different supply electrical powers are applied to the variable optical attenuators 310 and 320 of Figures 3A and 3B. When using the conventional variable optical attenuator 310, approximately 20 dB, 10 dB, and 1 dB of optical attenuation are obtained by delivering electrical power of 251.68 mW, 80.22 mW, and 2.57 mW, respectively, to the PIN waveguide. In contrast, when using the high-efficiency variable optical attenuator 320, approximately 20 dB, 10 dB, and 1 dB of optical attenuation are obtained by delivering electrical power of 85.12 mW, 38.22 mW, and 3.47 mW, respectively, to the PIN waveguide. For an attenuation of 20 dB, the high-efficiency variable optical attenuator 320 is three times more power efficient than the conventional variable optical attenuator 310. For an attenuation of 10 dB, the high-efficiency variable optical attenuator 320 is twice as power efficient as the conventional variable optical attenuator 310.

[0041] Figure 10 shows a graph illustrating the experimentally measured modulation bandwidth of the variable optical attenuators 310 and 320 of Figures 3A and 3B. The difference in modulation bandwidth between the two variable optical attenuators 310 and 320 is small and negligible, indicating that there is no significant trade-off between the efficient variable optical attenuator 320 and the conventional variable optical attenuator 310.

[0042] Other configurations of the variable optical attenuator 100 can be envisioned based on Figure 1, as described below.

[0043] In some embodiments, the thermal isolation region 170 may be disposed below and / or at least on one side of the lower cladding layer. Alternatively, if there is an upper cladding layer, the thermal isolation region may be disposed above the upper cladding layer. The thermal isolation region may partially, substantially, or completely surround the cladding layer 140 and the waveguide core 130.

[0044] In some embodiments, a plurality of first substrates and a plurality of thermal insulation materials may be alternately configured to provide a plurality of alternating structures for a variable optical attenuator. Specifically, each alternating structure includes adjacently configured first substrates and thermal insulation materials. In some other embodiments, a single first substrate and a single thermal insulation material may be alternately arranged to provide a single alternating structure for a variable optical attenuator.

[0045] In some embodiments, the multiple alternating structures may be identical in material, configuration, and size of the first substrate and the thermal insulation material. In some other embodiments, the multiple alternating structures may differ in material, configuration, and / or size of the first substrate and the thermal insulation material.

[0046] In some embodiments, the insulation material may be air having a thermal conductivity of 0.026 W / m·K. In some other embodiments, the insulation material may be selected from a non-limiting list of materials (indicated in parentheses by thermal conductivity): silicon dioxide (1.4 W / m·K), silicon nitride (30 W / m·K), aluminum nitride (30 W / m·K), polyimide (0.12 W / m·K), germanium (60 W / m·K), polymethyl methacrylate (PMMA) (0.19 W / m·K), polyethylene (PE) (0.33 W / m·K), polypropylene (PP) (0.22 W / m·K), polytetrafluoroethylene (PTFE) (0.25 W / m·K), liquids, or fluids such as oil having a lower thermal conductivity compared to silicon (148 W / m·K) to fill the air channels. In still other embodiments, the insulation material may include multiple of the above-mentioned insulation materials, including air.

[0047] In some embodiments, the thermal isolation region, such as a repeating alternating structure, is configured or extends parallel to and / or transverse to the light propagation direction of the waveguide core. The light propagation direction typically corresponds to the longitudinal direction of the waveguide core.

[0048] In some embodiments, such as FIG. 11, the variable optical attenuator 340 includes a PIN junction, a waveguide core, a lower cladding layer, an upper cladding layer, and electrodes, similar to the variable optical attenuator 320 of FIG. 4B. However, unlike the variable optical attenuator 320 of FIG. 3B, which shows a thermally insulating region in an alternating structure on the first substrate 150, the variable optical attenuator 340 of FIG. 11 has a substrate block 150 with an air cavity 160 disposed below and in contact with the lower cladding layer. The air cavity 160 thus provides a thermally insulating region. The lower cladding layer 171 is disposed above and in contact with the substrate block 150. In other words, the waveguide core 130 and the lower cladding layer 171 are suspended above the air cavity 160 and supported by the substrate block 150.

[0049] In some embodiments, the variable optical attenuator further includes a support member, such as a substrate member. The waveguide core includes two opposing ends and a non-end inserted therebetween. The two opposing ends of the waveguide core are supported by the support member so that the non-end of the waveguide core is substantially or completely suspended in an air cavity. Thus, the air cavity providing the thermal isolation zone can substantially or completely surround the lower cladding layer.

[0050] In some embodiments, the variable optical attenuator further includes an upper cladding layer disposed above and in contact with the carrier injection region, and thus also in contact with the waveguide core. In other words, the carrier injection region and the waveguide core are interposed between the upper and lower cladding layers.

[0051] In various embodiments, the variable optical attenuator can be implemented using phase masks, which can be made of, for example, any of the following materials: silicon (Si), polycrystalline silicon, silicon nitride (Si3N4), silicon dioxide, germanium (Ge), lithium niobate (Li3NbO3), polymers, group III-V compounds (i.e., alloys containing elements of groups III and V in the periodic table), and group II-VI compounds (i.e., alloys containing elements of groups II and VI in the periodic table).

[0052] In various embodiments, the waveguide coupled to the variable optical attenuator can comprise any of several different types of waveguides. For example, the waveguide can be a waveguide based on total internal reflection (which constitutes the vast majority of optical waveguides conventionally used in integrated photonics), a slot waveguide, and a surface plasmon polariton waveguide. Alternatively, planar scattering waveguides, such as those formed from photonic crystals (which also utilize total internal reflection) and metamaterials, can be used. Each of the multiple waveguides can be composed of at least one of, for example, Si, SiO2, BaTiO3, Li3NbO3, InP, group III-V compounds, group II-VI compounds, and polymers. The waveguide can support any optical waveguide mode, such as a transverse electric mode and a transverse magnetic mode.

[0053] In various embodiments, the lower and upper cladding layers may be composed of SiO2. The first, second, and / or third substrates (if applicable) may be composed of Si.

[0054] It will be apparent from the foregoing disclosure and detailed description of specific embodiments that various modifications, additions, and other alternative embodiments may be made without departing from the true scope and spirit of the invention. The discussed embodiments have been selected and described to provide the best illustration of the principles of the invention and its practical application, thereby enabling those skilled in the art to use the invention in various embodiments and make various modifications suitable for a particular intended use. All such modifications and variations are within the scope of the invention, defined by the appended claims and interpreted to the fullest extent that they are fairly, legally, and impartially entitled to.

[0055] 100, 320, 340: Variable optical attenuator

[0056] 120: Carrier injection region

[0057] 121: p-doped region, first p-doped region

[0058] 122: n-doped region, first n-doped region

[0059] 123: Essential Zone

[0060] 124: Second p-doped region

[0061] 125: Second n-doped region

[0062] 130: Optical waveguide core, waveguide core

[0063] 140: Covering layer

[0064] 150: First substrate, substrate block

[0065] 152: Second substrate

[0066] 153: Third substrate

[0067] 160: Air groove, air cavity

[0068] 170: Thermal isolation zone

[0069] 171: Lower cladding layer

[0070] 172: Electrode

[0071] 173: Upper coating layer

[0072] 310: Variable optical attenuator, traditional variable optical attenuator

Claims

1. A variable optical attenuator, comprising: The carrier injection region contains a waveguide core; A lower coating layer is disposed below the carrier injection region; A thermally insulating region having a lower thermal conductivity than the lower covering layer, wherein the lower covering layer is at least partially surrounded by the thermally insulating region; and at least one first substrate; wherein the thermal insulation region includes at least one thermal insulation material, wherein the at least one first substrate and the at least one thermal insulation material are arranged adjacently to provide at least one alternating structure, the alternating structure being disposed below the lower covering layer.

2. The variable optical attenuator as claimed in claim 1, wherein the thermal isolation region is disposed below and / or at least on one side of the lower cladding layer.

3. The variable optical attenuator as claimed in claim 1, wherein the at least one alternating structure comprises a plurality of alternating structures, the plurality of alternating structures being identical or different.

4. The variable optical attenuator as claimed in claim 3, wherein the alternating structure is configured in at least two columns, and wherein the variable optical attenuator further includes a second substrate inserted between the at least two columns.

5. The variable optical attenuator as claimed in claim 4, wherein the variable optical attenuator further includes a third substrate disposed below the alternating structure.

6. The variable optical attenuator as claimed in claim 4, wherein the carrier injection region includes a PIN junction having a p-doped region, an n-doped region, and an intrinsic region interposed therebetween and serving as the waveguide core.

7. The variable optical attenuator as claimed in claim 6, wherein the doping concentration of the p-doped region and the doping concentration of the n-doped region are symmetric or asymmetric.

8. The variable light attenuator as claimed in claim 1, wherein the at least one insulating material comprises at least one of the group consisting of silicon dioxide, silicon nitride, aluminum nitride, polyimide, germanium, polymethyl methacrylate (PMMA), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), liquid, and air.

9. The variable optical attenuator as claimed in claim 2, wherein the thermal isolation region is configured to be parallel to and / or transverse to the optical propagation direction of the waveguide core.

10. The variable optical attenuator as claimed in claim 1 or claim 2, wherein the at least one thermal insulation material comprises air in at least one air cavity, wherein the lower cladding layer is supported by the at least one first substrate.

11. The variable optical attenuator as claimed in claim 1 or claim 2, further comprising a support member, wherein the waveguide core includes two opposing ends and a non-end inserted therebetween, wherein the at least one thermal insulation material comprises air in at least one air cavity surrounding the waveguide core and the lower cladding layer, wherein the two opposing ends of the waveguide core are supported by the support member such that the non-end of the waveguide core is suspended in the at least one air cavity.

12. The variable optical attenuator as claimed in claim 1 or claim 2, wherein the carrier injection region comprises one of the group consisting of a vertical junction, a PIPIN junction, an epitaxial vertical junction, a cladding junction, a corner dedoped junction, a sawtooth junction, an interleaved junction, a three-terminal junction, and a horizontal junction.

13. The variable optical attenuator as described in claim 1 or claim 2, further comprising: An upper coating layer, wherein the carrier injection region is interposed between the upper coating layer and the lower coating layer.

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