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

[0001] POWER-EFFICIENT VARIABLE OPTICAL ATTENUATOR

[0002] FIELD

[0003]

[0001] The present invention relates to the field of integrated photonics, optical signal attenuation, optical signal balancing, coherent photonics, data processing and remote sensing. A variable optical attenuator having one or more suspended structures is disclosed, the suspended structure enhancing the optical signal attenuation.

[0004] BACKGROUND

[0005]

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

[0006]

[0003] Various VOAs have been realized such as mechanical, acousto-optic, microfluidic, and optoelectronics. Traditionally, mechanical VOAs have been largely adopted for optical communications. However, mechanical VOAs are prone to mechanical wear and tear, and involve precision engineering which can be more expensive. Optoelectronic VOAs, on the other hand, are particularly attractive due to its durability, cost-effectiveness and high structural density which is highly compatible with integrated optical devices.

[0007]

[0004] As VOAs need to be continuously supplied, the main figure-of-merit (FOM) of VOAs is power efficiency. Although optoelectronic VOAs have a generally higher power efficiency compared to mechanical VOAs, there is a push for VOAs with an even lower power consumption as the net power consumption can be significantly high considering the volatile nature of most optoelectronic VOAs, i.e., the need to continuously supply electrical signal to maintain attenuation functionality, and its generally large-scale implementation in data communications for signal balancing purposes.

[0008]

[0005] In view of the above and other issues, it would be desirable to provide an improved photonic component that addresses the above shortcomings.SUMMARY

[0009]

[0006] An aspect of the invention provides a variable optical attenuator comprising:

[0010] a carrier injection region having a waveguide core;

[0011] an under-cladding arranged under the carrier injection region;

[0012] a heat isolation region having a heat conductivity which is lower than a heat conductivity of the under-cladding,

[0013] wherein the under-cladding is at least partially surrounded by the heat isolation region.

[0014]

[0007] In an embodiment, the heat isolation region is arranged under and / or at least one side of the under-cladding.

[0015]

[0008] In an embodiment, the variable optical attenuator further comprises:

[0016] at least one first substrate;

[0017] wherein the heat isolation region includes at least one heat isolation material, wherein the at least one first substrate and at least one heat isolation material are adjacently arranged to provide at least one alternating structure arranged under the undercladding.

[0018]

[0009] In an embodiment, the at least one alternating structure includes a plurality of alternating structures which are identical or non-identical.

[0019]

[0010] In an embodiment, the alternating structures are arranged in at least two rows, wherein the variable optical attenuator further comprises a second substrate interposed between the at least two rows.

[0020]

[0011] In an embodiment, the variable optical attenuator further comprises a third substrate arranged under the alternating structures.

[0021]

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

[0022]

[0013] In an embodiment, doping concentrations of the p-doped region and the n-doped region are symmetric or asymmetric.

[0023]

[0014] In an embodiment, the heat isolation material includes at least one of the group consisting of silicon dioxide, silicon nitride, aluminium nitride, polyimide, germanium, polymethyl methacrylate (PMMA), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), liquid, and air.

[0024]

[0015] In an embodiment, the heat isolation region is arranged parallel and / or transverse to an optical propagation direction of the waveguide core.

[0016] In an embodiment, the variable optical attenuator further comprises a substrate block having an air cavity arranged under the under-cladding, the heat isolation region being the air cavity, wherein the under-cladding is supported by the substrate block.

[0025]

[0017] In an embodiment, the variable optical attenuator further comprises: support members,

[0026] wherein the waveguide core includes two opposite end portions and a non-end portion interposed therebetween,

[0027] wherein the heat isolation region includes an air cavity surrounding the waveguide core and the under-cladding,

[0028] wherein the two opposite end portions of the waveguide core are supported by the support members such that the non-end portion of the waveguide core is suspended in the air cavity.

[0029]

[0018] In an embodiment, the carrier injection region includes one of the group consisting of a vertical junction, a PIPIN junction, an epitaxial vertical junction, a wrapped junction, a junction with counter doping at comers, a zig-zag junction, an interleaved junction, a three-terminal junction, and a horizontal junction.

[0030]

[0019] In an embodiment, the variable optical attenuator further comprises:

[0031] an over-cladding, wherein the carrier injection region is interposed between the overcladding and the under-cladding.

[0032]

[0020] Following a more detailed description of various embodiments as below, it will be apparent to those skilled in the art that the present invention provides a significant advance in the technology to provide a power efficient variable optical attenuator with high spectral bandwidth. Particularly significant in this regard is the potential the invention affords for providing enhanced optical attenuation with a broad optical bandwidth and high structural density. This prevents exceedingly high power requirements to operate attenuators in scaled-up optical communication, sensing and / or ranging systems that adopt such attenuators for signal balancing (e.g., in coherent and wavelength-division multiplexing systems). Additional features and advantages of various embodiments will be better understood in view of the detailed description provided below.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034]

[0021] Figure 1 shows a schematic diagram of a variable optical attenuator according to various embodiments;

[0035]

[0022] Figure 2A to 2I shows various examples of carrier injection regions;

[0023] 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;

[0036]

[0024] Figure 4A shows a simplified cross-sectional view of the conventional variable optical attenuator of Figure 3A while Figure 4B shows a simplified cross-sectional view of the variable optical attenuator of Figure 3B;

[0037]

[0025] Figure 5 shows a simplified isometric view of the variable optical attenuator of Figure 4B;

[0038]

[0026] Figure 6 shows a simplified isometric view of a variable optical attenuator according to an embodiment;

[0039]

[0027] Figures 7A and 7B illustrate microscope images of an embodiment of variable optical attenuator with optical waveguide cladding surrounded by alternating air trench and substrate structures;

[0040]

[0028] Figure 8 is an experimentally measured plot of optical attenuation at varying supplied electrical power delivered across variable optical attenuators that correspond to Figures 3A and 3B;

[0041]

[0029] Figures 9A and 9B show experimentally measured resulting optical transmission spectrums at varying supplied electrical power delivered across variable optical attenuators of Figures 3A and 3B;

[0042]

[0030] Figure 10 is an experimentally measured plot of modulation bandwidth of variable optical attenuators that correspond to Figures 3A and 3B; and

[0043]

[0031] Figure 11 shows a simplified cross-sectional view of a variable optical attenuator according to an embodiment.

[0044]

[0032] It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the variable optical attenuator as disclosed here, including, for example, the specific dimensions of the variable optical attenuator, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to help provide clear understanding. In particular, thin features may be thickened, for example, for clarity of illustration.

[0045] DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0046]

[0033] In the following description, various embodiments are described with reference to the drawings, where like reference characters generally refer to the same parts throughout thedifferent views. It will be apparent to those skilled in the art, that is, to those who have knowledge or experience in this area of technology, that many use and design variations are possible for the power-efficient variable optical attenuator disclosed herein. The following described embodiments illustrate the general principles of the invention to provide a powerefficient variable optical attenuator suitable to optically attenuate optical field on an optical waveguide to suit applications for which its power-efficient variable optical attenuator is designed. Other embodiments suitable for other applications will be apparent to those skilled in the art given the benefit of this disclosure.

[0047]

[0034] It should be understood that the articles "a", "an" and "the" as used with regard to a feature or element include a reference to one or more of the features or elements. The term "and / or" includes any and all combinations of one or more of the associated feature or element. The terms "comprising", "including", "having", and any of their related terms, as used in description and claims, are intended to be open-ended and mean that there may be additional features or elements other than the listed ones. Identifiers such as "first", "second", "third", and so on, are used merely as labels, and are not intended to impose numerical requirements on their objects, nor construed in a manner imposing any relative position or time sequence between limitations. The term “to” may include a reference to “configured to”, "adapted to", and "constructed and arranged to" which may be used interchangeably. Furthermore, terms such as “top”, “bottom”, “upper”, “lower”, “under”, “over”, “on” and their related terms used herein are merely for ease of description and may refer to the orientation of the features or elements as shown in the figures. It should be understood that any orientation of the features described herein is within the scope of the invention.

[0048]

[0035] The present invention relates to an integrated photonic device, specifically a powerefficient variable optical attenuator. According to various embodiments as illustrated by Figure 1 , a variable optical attenuator 100 comprises at least a carrier injection region 120 having an optical waveguide core 130; a cladding 140; and a heat isolation region 170.

[0049]

[0036] The optical waveguide core 130 (alternatively referred to as “waveguide core”) is configured to transmit a received optical signal or field through the variable optical attenuator 100. The waveguide core 130 includes a lengthwise direction which corresponds to its optical signal or field propagation direction as illustrated by arrows in Figure 1 , and a breadthwise direction which is transverse to its optical signal or field propagation direction.

[0050]

[0037] The carrier injection region 120 is configured to attenuate the optical signal or field being transmitted through the waveguide core 130. Carrier injection takes place under forward bias in a PN junction 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-dopedregion (negative or n-type semiconductor material). The forward bias reduces the barrier at the junction, thereby allowing carriers to cross the junction.

[0051]

[0038] The cladding 140 is configured to provide a physical gap or separation between the waveguide core 130 and other elements, including the heat isolation region 170. The cladding 140 includes an under-cladding which is arranged under, and in contact with, the carrier injection region 170, including the waveguide core 130. Particularly, the under-cladding is interposed between the carrier injection region (or waveguide core) and the heat isolation region. The cladding 140 may further include an over-cladding which is arranged over, and in contact with, the carrier injection region 120 (or waveguide core 130).

[0052]

[0039] The heat isolation region 170 is configured to isolate or confine heat that is generated by the carrier injection process to enhance optical attenuation. To achieve heat isolation, a heat conductivity of the heat isolation region 170 is lower than a surrounding region of the carrier injection region 120, e.g., the under-cladding. Additionally, to achieve heat isolation, the surrounding region of the carrier injection region 120, e.g., the under-cladding, is at least partially surrounded, e.g., partially, substantially or fully surrounded, by the heat isolation region 170. In other words, at least the surrounding region of the carrier injection region 120, e.g., the waveguide core 130 and the under-cladding 171 , is suspended in the heat isolation region 170.

[0053]

[0040] Figures 3A, 3B, 4A, 4B, 5, and 6 show a non-limiting example of a carrier injection region 120 which is implemented as a PIN junction. The PIN junction comprises a p-doped region 121, an n-doped region 122, and an intrinsic region 123 interposed therebetween and being the waveguide core 130.

[0054]

[0041] Figures 2A to 2I show other non-limiting examples of carrier injection regions 120. Particularly, Figures 2A to 2I show a horizontal junction, a vertical junction, a PIPIN junction, an epitaxial vertical junction, a wrapped junction, a junction with counter doping at corners, a zigzag junction, an interleaved junction, a three-terminal junction, respectively. Each carrier injection region 120 comprises at least a first p-doped region 121 , a first n-doped region 122, and an intrinsic region 123 which is interposed or coupled between the first p-doped region 121 and the first n-doped region 122. In some examples, each carrier injection region may further comprise a second p-doped region 124 which is arranged adjacent to and more highly doped than the first p-doped region 121 , and also a second n-doped region 125 which is arranged adjacent to and more highly doped than the first n-doped region 122.

[0055]

[0042] Figures 3A and 4A show a conventional variable optical attenuator 310 in which a carrier injection region 120 is implemented as a PIN junction. An under-cladding 171 is arranged under the PIN junction. A first substrate 150 is arranged under the under-cladding 171 such that the under-cladding 171 is substantially or fully supported. In other words, thewaveguide core 130 is substantially or fully supported, or non-suspended. An over-cladding 173 is arranged on, and in contact with, the carrier injection region (or waveguide core 130). Electrodes 172 are arranged on, and in contact with, the p-doped region 121 and n-doped region 122. The conventional variable optical attenuator 310 does not provide any heat isolation region.

[0056]

[0043] In contrast to the conventional variable optical attenuator 310, embodiments of variable optical attenuators according to the present invention provide heat isolation region(s) which may be implemented in different ways as described in the following paragraphs.

[0057]

[0044] Figures 3B, 4B and 5 show an embodiment of a variable optical attenuator 320 in accordance with Figure 1. A carrier injection junction 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 and being the waveguide core 130. An under-cladding 171 is arranged under, and in contact with, the PIN junction. An over-cladding 173 is arranged on the carrier injection region (or waveguide core 130). Electrodes 172 are arranged on the p-doped region 121 and n-doped region 122. A first substrate 150 is arranged under, and in contact with, the undercladding 171. Air trenches 160 are provided in the first substrate 150 (see Figure 3B). A combination of the first substrate 150 and the air trenches 160 provide alternating structures which are arranged under, and in contact with, and support the under-cladding 171 (or waveguide core 130). In other words, the waveguide core 130 is suspended, e.g., at least partially suspended in air. Each alternating structure therefore includes a first substrate 150 (or portion thereof) and an air trench 160. The alternating structures are arranged in two rows. In each row, the alternating first substrates 150 and air trenches 160 extend along the optical propagation direction of the waveguide core 130. A second substrate 152 is interposed between the two rows and is arranged under, and in contact with, the under-cladding 171 , particularly below the carrier injection region 120 (or waveguide core 130). The first substrate 150 and the second substrate 152 may be integrally formed.

[0058]

[0045] In the variable optical attenuator 320 of Figures 3B, 4B, 5, the air trenches 160 achieve heat isolation due to the following reasons. Firstly, air has reduced thermal conductivity as compared to other materials, e.g., substrate, as air is a poor conductor of heat as compared to solid materials. By suspending the waveguide core 130 and under-cladding 171 in air (hereinafter “air bridge”), the thermal pathway from the air bridge to the underlying substrate 150 is minimized and is concentrated at the PIN junction to enhance optical attenuation. Secondly, there is enhanced heat accumulation as the air trenches 160 act as an insulating layer. This contributes to better heat isolation, ensuring a variable optical attenuator with increased attenuation efficiency.

[0046] Figure 6 shows an embodiment of a variable optical attenuator 330 which is similar to the variable optical attenuator 320 of Figure 5. However, the variable optical attenuator 330 may further comprise a third substrate 153 arranged under, and in contact with, the alternating structures. In other words, the third substrate 153 is arranged under, and in contact with, at least some of the first substrates 150 and the air trenches 160, and also the second substrate 152. The first substrate 150, the second substrate 152, and the third substrate 153 may be integrally formed.

[0059]

[0047] In the variable optical attenuator 330 of Figure 6, the third substrate 153 provides additional mechanical reliability due to the following reasons. Firstly, the third substrate 153 provides structural integrity as the air bridge could deform or collapse, leading to physical damage as air bridges are delicate structures that rely on mechanical support to maintain their shape and position. Secondly, without adequate support, the air bridge may sag or shift, thus causing electrical shorts or increased parasitic capacitance and inductance. This may degrade the signal integrity and overall performance of the variable optical attenuator. Also, inadequate support may distort optical signal propagation, and degrade overall performance of the variable optical attenuator. Thirdly, the third substrate 153 eases fabrication challenges, as the lack of mechanical support may pose difficult in maintaining precise alignment and spacing required for the air bridge.

[0060]

[0048] Modifications and / or variations may be made to the variable optical attenuators 320, 330 of Figure 3B, 4B, 5, and 6. For example, instead of arranging the alternating structures as two rows, the alternating structures may be arranged as a single row, or three or more rows. For example, doping concentrations of the p-doped region and the n-doped region may be symmetric or asymmetric. For example, the first substrate(s) 150, the second substrate 152, and the third substrate 153 may be formed of the same or different materials. For example, air trenches 160 which provide the heat isolation region 170 may be substituted by at least one heat isolation material having a heat conductivity which is lower than a heat conductivity of the under-cladding 171.

[0061]

[0049] Figures 7A and 7B show microscope images of a variable optical attenuator 320 in which its under-cladding 140 is surrounded by alternating structures comprising first substrates 150 and air trenches 160. Particularly, Figure 7A shows an overview, while Figure 7B shows a close-up view of a portion of Figure 7A which is marked by dash lines.

[0062]

[0050] Figure 8 shows experimentally measured plots of optical attenuation at varying supplied electrical power delivered across the variable optical attenuators 310, 320 of Figures 3A and 3B. In the example of Figure 8, the experimentally measured variable optical attenuator has defined parameters, such as 0.5 pm waveguide width, 250 pm doping length and 2.5 pmdoping distance between the p- and n-doped regions that form the PIN waveguide structure. It can be observed from Figure 8 that attenuation performance of variable optical attenuator 320 is improved as compared to the conventional variable optical attenuator 310.

[0063]

[0051] Figures 9A and 9B show experimentally measured resulting optical transmission spectrums at varying supplied electrical power delivered across the variable optical attenuators 310, 320 of Figures 3A and 3B. Using conventional structures 310, optical attenuation of about 20dB, 10dB and 1 dB can respectively be attained by delivering electrical power across the PIN waveguide at 251.68 mW, 80.22 mW and 2.57 mW. In contrast, using the efficient variable optical attenuator 320, optical attenuation of about 20dB, 10dB and 1dB can respectively be attained by delivering electrical power across the pin waveguide at 85.12 mW, 38.22 mW and 3,47 mW. For 20dB attenuation, the efficient variable optical attenuator 320 is three times more power efficient than conventional variable optical attenuator 310. For 10dB attenuation, the efficient variable optical attenuator 320 is two times more power efficient than conventional variable optical attenuator 310.

[0064]

[0052] Figure 10 shows a graph describing the experimentally measured modulation bandwidth of variable optical attenuators 310, 320 of Figures 3A and 3B. There is negligibly small difference in modulation bandwidth between the two variable optical attenuators 310, 320, which indicate that there is no marked trade-off between the efficient variable optical attenuator 320 and conventional variable optical attenuator 310.

[0065]

[0053] Other configurations of variable optical attenuators 100 may be envisaged in accordance with Figure 1 , as follows.

[0066]

[0054] In some embodiments, the heat isolation region 170 may be arranged under and / or at least one side of the under-cladding. Optionally, the heat isolation region may be arranged over the over-cladding, if any. The heat isolation region may partially, substantially or fully surround the cladding 140, together with the waveguide core 130.

[0067]

[0055] In some embodiments, a plurality of first substrates and a plurality of heat isolation materials may be arranged alternately to provide a plurality of alternating structures to a variable optical attenuator. In particular, each alternating structure includes a first substrate and a heat isolation material adjacently arranged to each other. In some other embodiments, a single first substrate and a single heat isolation material may be arranged alternately to provide a single alternating structure to a variable optical attenuator.

[0068]

[0056] In some embodiments, the alternating structures may be identical with respect to materials, arrangements and dimensions of the first substrate and the heat isolation material. In some other embodiments, the alternating structures may be non-identical with respect to materials, arrangements and / or dimensions of the first substrate and the heat isolation material.

[0057] In some embodiments, the heat isolation material may be air having a heat conductivity of 0.026 W / m-K. In some other embodiments, the heat isolation material may be selected from one of the following non-limiting examples of materials (heat conductivity indicated in parentheses): silicon dioxide (1.4 W / m-K), silicon nitride (30 W / m-K), aluminium 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), liquid or fluid, e.g., oil with lower heat conductivity compared to silicon (148 W / m-K) to fill the air trenches. In yet some other embodiments, the heat isolation material may include a plurality of the above-mentioned heat isolation materials, including air.

[0069]

[0058] In some embodiments, the heat isolation region, e.g., repetition of the alternating structures, is arranged or extends parallel and / or transverse to an optical propagation direction of the waveguide core. The optical propagation direction generally corresponds to a lengthwise direction of the waveguide core.

[0070]

[0059] In some embodiments, e.g., Figure 11 , the variable optical attenuator 340 includes a PIN junction, waveguide core, under-cladding, over-cladding, electrodes similar to the variable optical attenuator 320 of Figure 4B. However, instead of providing heat isolation region(s) in the first substrate 150 as alternating structures as shown in the variable optical attenuator 320 of Figure 3B, the variable optical attenuator 340 of Figure 11 provides a substrate block 150 having an air cavity 160 which is arranged under, and in contact with, the under-cladding. The air cavity 160 therefore provides the heat isolation region 120. The under-cladding 171 is arranged on, and in contact with, the substrate block 150. In other words, the waveguide core 130, as well as the under-cladding 171 , is suspended over the air cavity 160 and supported by the substrate block 150.

[0071]

[0060] In some embodiments, the variable optical attenuator further comprises support members, e.g., substrate members. The waveguide core includes two opposite end portions and a non-end portion interposed therebetween. The two opposite end portions of the waveguide core are supported by the support members such that the non-end portion of the waveguide core is substantially or fully suspended in air cavity. Hence, the air cavity which provides the heat isolation region may substantially or fully surround the under-cladding.

[0072]

[0061] In some embodiments, the variable optical attenuator further comprises an overcladding which is arranged on, and in contact with, the carrier injection region, and therefore the waveguide core. In other words, the the carrier injection region, as well as the waveguide core, is interposed between the over-cladding and the under-cladding.

[0062] In the various embodiments, the variable optical attenuator may be implemented by phase masks fabricated from, for example, any of silicon (Si), polysilicon, silicon nitride (Si3N4) , silicon dioxide, germanium (Ge), lithium niobate (Li3NbO3) a polymer, a lll-V compound (that is, an alloy containing elements from Groups III and V in the periodic table, and a ll-VI compound (that is, an alloy containing elements from Groups II and VI in the Periodic Table).

[0073]

[0063] In the various embodiments, a waveguide coupled to the variable optical attenuator may comprise any of several different types of waveguides. For example, the waveguide can be a total internal reflection-based waveguide (which makes up the overwhelming majority of optical waveguides traditionally used in integrated photonics), a slot waveguide, and surface plasmon polariton waveguide. Alternatively, an in-plane scattering waveguide may be used, such as waveguide formed from photonic crystals (which also use total internal reflection) and metamaterials. A composition of each of the plurality of waveguides may be, for example, at least one of Si, SiOs, BaTiO3, Li3NbO3, InP, a lll-V compound, a ll-VI compound, and a polymer, for example. The waveguide can support any of the optical waveguide modes, for example, Transverse Electric mode and Transverse Magnetic mode.

[0074]

[0064] In the various embodiments, a composition of the under-cladding and overcladding may be SiC>2. A composition of the first, second and / or third substrate (where applicable) may be Si.

[0075]

[0065] From the foregoing disclosure and detailed description of certain embodiments, it will be apparent that various modifications, additions and other alternative embodiments are possible without departing from the true scope and spirit of the invention. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to use the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

AMENDED CLAIMSreceived by the International Bureau on 22 June 2026 (22.06.2026)1. 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; andat least one first substrate,wherein the under-cladding is at least partially surrounded by the heat isolation region;wherein the heat isolation region includes at least one heat isolation material,wherein the at least one first substrate and the at least one heat isolation material are adjacently arranged to provide at least one alternating structure arranged under the undercladding.

2. The variable optical attenuator of claim 1, wherein the heat isolation region is arranged under and / or at least one side of the under-cladding.

3. The variable optical attenuator of claim 1 or 2, wherein the at least one alternating structure includes a plurality of alternating structures which are identical or non-identical.

4. The variable optical attenuator of claim 3, wherein the alternating structures are arranged in at least two rows, wherein the variable optical attenuator further comprises a second substrate interposed between the at least two rows.

5. The variable optical attenuator of claim 4, wherein the variable optical attenuator further comprises a third substrate arranged under the alternating structures.

6. The variable optical attenuator of claim 4 or claim 5, 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 being the waveguide core.

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

8. The variable optical attenuator of any one of claim 1 to claim 3, wherein the heat isolation material includes at least one of the group consisting of silicon dioxide, silicon nitride, aluminium nitride, polyimide, germanium, polymethyl methacrylate (PMMA), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), liquid, and air.

9. The variable optical attenuator of claim 2 or claim 3, wherein the heat isolation region is arranged parallel and / or transverse to an optical propagation direction of the waveguide core.

10. The variable optical attenuator of claim 1 or claim 2, wherein the heat isolation region comprises an air cavity, the at least one heat isolation material includes air in the air cavity, and the under-cladding is supported by the at least one first substrate.

11. The variable optical attenuator of claim 1 or claim 2, further comprising: support members, wherein the waveguide core includes two opposite end portions and a non-end portion interposed therebetween,wherein the heat isolation region comprises an air cavity, the at least one heat isolation material includes air in the air cavity surrounding the waveguide core and the under-cladding,wherein the two opposite end portions of the waveguide core are supported by the support members such that the non-end portion of the waveguide core is suspended in the air cavity.

12. The variable optical attenuator of any one of claims 1 to 3, 8 to 11 , wherein the carrier injection region includes one of the group consisting of a vertical junction, a PIPIN junction, an epitaxial vertical junction, a wrapped junction, a junction with counter doping at corners, a zigzag junction, an interleaved junction, a three-terminal junction, and a horizontal junction.

13. The variable optical attenuator of any one of claims 1 to 12, further comprising:an over-cladding, wherein the carrier injection region is interposed between the overcladding and the under-cladding.

14. A variable optical attenuator comprising:a carrier injection region having a waveguide core;a single under-cladding arranged under and in contact with the carrier injection region;a heat isolation region having a heat conductivity which is lower than a heat conductivity of the single under-cladding, wherein the single under-cladding is at least partially surrounded by the heat isolation region; anda substrate block having an air cavity arranged under and in contact with the single under-cladding, the heat isolation region being the air cavity, wherein the waveguide core and the single under-cladding are suspended over the air cavity and supported by the substrate block.

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