Optical isolator and fiber coupling device

Abstract

The application discloses an optical isolator and a fiber coupling device. The optical isolator comprises a metasurface linear polarization layer and a metasurface quarter-wave plate layer. The fast-axis direction of the metasurface quarter-wave plate layer has a target included angle with the transmission vibration direction of the metasurface linear polarization layer, and the metasurface quarter-wave plate layer is configured to convert the polarization state of incident light between linear polarization and circular polarization. The metasurface linear polarization layer and the metasurface quarter-wave plate layer are integrated into an integral structure. The linear polarizer and the quarter-wave plate of the optical isolator are integrated into an integral element, thereby eliminating air gaps and additional optical interfaces, reducing insertion loss, and improving forward light transmission efficiency. The optical isolator has the advantages of no magnet, passivity, small volume, light weight, low cost, no magnetic interference, semiconductor integration and the like.

Description

Technical Field

[0001] This application relates to the field of optical isolators, and more particularly to an optical isolator and an optical fiber coupling device. Background Technology

[0002] An optical isolator is a functional device that uses the optical non-reciprocity effect to achieve unidirectional optical transmission. Its core feature is that it allows forward-transmitting light to pass through with low loss, while producing high suppression and attenuation for reverse-transmitting light.

[0003] Traditional optical isolators typically have one of two structures. Structure 1: The optical isolator includes a magnet (permanent magnet or electromagnetic coil), a Faraday rotator, and a polarizer (or birefringent crystal). Its working principle utilizes the magneto-optical Faraday effect, causing a non-reciprocal rotation of approximately 45 degrees in the polarization direction of linearly polarized light passing through the magneto-optical crystal. Combined with two polarizers, this allows forward-propagating light to pass through with low loss, while blocking backward-propagating light, thus achieving optical isolation. Therefore, the magnet is an essential component of traditional optical isolators. However, magnets suffer from large size, high cost, and difficulty in integration, making it difficult for this structure to achieve the advantages of small size, low cost, and high integration. Structure 2: The optical isolator includes a polarizer (or polarizing beam splitter) and a quarter-wave plate. The phase delay of the quarter-wave plate is proportional to its thickness (quartz birefringence Δn≈0.009, for the 1550 nm band, to achieve a π / 2 phase difference, the thickness d≈λ / (4Δn)≈43 μm). It has extremely high requirements for thickness tolerance, requiring precision cutting, grinding and polishing, which makes the processing difficult and the yield low. Furthermore, the quarter-wave plate is difficult to integrate precisely with the linear polarizer.

[0004] Therefore, all traditional optical isolators, regardless of their structure, have many drawbacks, and there is an urgent need to provide a new type of optical isolator. Summary of the Invention

[0005] To address the aforementioned technical problems, this application provides an optical isolator and an optical fiber coupling device, aiming to provide an optical isolator that is small in size, low in cost, and semiconductor-integrable.

[0006] According to one aspect of the embodiments of this application, an optical isolator is disclosed, comprising:

[0007] A metasurface linear polarization layer is configured to filter out linearly polarized light with a target polarization state;

[0008] A metasurface quarter-wave plate layer, the fast axis of which forms a target angle with the transmission direction of the metasurface linear polarization layer, is configured to convert the polarization state of incident light between linear polarization and circular polarization.

[0009] The metasurface linear polarization layer and the metasurface quarter-wave plate layer are an integral structure.

[0010] In some embodiments, the metasurface linear polarization layer and the metasurface quarter-wave plate layer are bonded together.

[0011] In some embodiments, the metasurface linear polarization layer and the metasurface quarter-wave plate layer are fabricated on the same wafer using semiconductor processes.

[0012] In some embodiments, the metasurface linear polarization layer is further configured to reflect, absorb, or deflect linearly polarized incident light in a non-target direction.

[0013] In some embodiments, the metasurface linear polarization layer is a metal wire grid array that transmits linearly polarized light of the target polarization state and reflects or absorbs linearly polarized incident light of a non-target direction.

[0014] In some embodiments, the metasurface linear polarization layer is an array of all-dielectric micro / nano structures that transmits linearly polarized light of the target polarization state and reflects or absorbs linearly polarized incident light of the non-target direction.

[0015] In some embodiments, the metasurface linear polarization layer is a phase gradient deflection type all-dielectric micro / nano structure array, which transmits linearly polarized light of the target polarization state and transmits linearly polarized incident light of the non-target direction.

[0016] A second aspect of this application provides an optical fiber coupling device, comprising:

[0017] A collimating lens is used to collimate incident light.

[0018] The optical isolator as described in any of the above is configured to receive light rays collimated by the collimating lens;

[0019] A focusing lens is configured to receive light emitted from the optical isolator and couple it into an optical fiber.

[0020] In some embodiments, the surface normal of the optical isolator is tilted relative to the optical axis of the outgoing light from the collimating lens.

[0021] In some embodiments, the surface normal of the optical isolator is parallel to the optical axis of the outgoing light from the collimating lens.

[0022] The optical isolator provided in this application includes a metasurface linear polarization layer and a metasurface quarter-wave plate layer. The metasurface linear polarization layer is configured to filter linearly polarized light with a target polarization state; the fast axis direction of the metasurface quarter-wave plate layer has a target angle with the transmission direction of the metasurface linear polarization layer, and the metasurface quarter-wave plate layer is configured to convert the polarization state of the incident light between linear polarization and circular polarization. The metasurface linear polarization layer and the metasurface quarter-wave plate layer are an integrated structure. Compared to existing optical isolators that include a polarizer (or polarizing beam splitter) and a quarter-wave plate, the optical isolator provided in this application can have both its metasurface linear polarization layer and the metasurface quarter-wave plate layer mass-produced using standard wafer-level processes such as semiconductor photolithography, etching, and thin film deposition, achieving integrated manufacturing and miniaturization and cost reduction of the optical isolator—something that cannot be achieved with optical isolators using traditional quartz quarter-wave plates. Compared to existing optical isolators that include magnets (permanent magnets or electromagnetic coils), Faraday rotators, and polarizers (or birefringent crystals), the optical isolator provided in this application relies entirely on the wave nature of light (Fresnel reflection, birefringence, phase delay) during the isolation process. It requires no external magnetic field, electric field, nonlinear optical effects, or active control circuitry, enabling integrated design and offering advantages such as small size, low cost, and high integration—features unattainable by traditional optical isolators that include magnets. Furthermore, the optical isolator provided in this application is a single, integrated component. Compared to optical isolators that separate linear polarizers and quarter-wave plates, the optical isolator provided in this application eliminates air gaps and additional optical interfaces, reducing insertion loss and improving forward transmission efficiency. In other words, the optical isolator provided in this application offers advantages such as being magnet-free, passive, small in size, lightweight, low in cost, free from magnetic interference, and capable of semiconductor integration. Attached Figure Description

[0023] The above and other objectives, features and advantages of this application will become more apparent from a detailed description of exemplary embodiments thereof with reference to the accompanying drawings.

[0024] Figure 1 A schematic diagram of the structure of an optical isolator in one embodiment of this application is shown.

[0025] Figure 2 A schematic diagram of the structure of an optical isolator in one embodiment of this application is shown.

[0026] Figure 3 The diagram shows a schematic representation of the structure of the metasurface linear polarization layer in three different embodiments of this application.

[0027] Figure 4 A schematic diagram of the structure of a metasurface quarter-wave plate layer in one embodiment of this application is shown.

[0028] Figure 5A schematic diagram of the structure of an optical isolator in one embodiment of this application is shown.

[0029] Figure 6 It shows Figure 5 A sectional view.

[0030] Figure 7 A schematic diagram of the structure of an optical isolator in one embodiment of this application is shown.

[0031] Figure 8 It shows Figure 7 A sectional view.

[0032] Figure 9 A schematic diagram of the architecture layout of an optical fiber coupling device in one embodiment of this application is shown.

[0033] Figure 10 A schematic diagram of the architecture layout of an optical fiber coupling device in one embodiment of this application is shown.

[0034] Figure Labels

[0035] 100, Optical isolator; 10, Metasurface linear polarization layer; 20, Metasurface quarter-wave plate layer; 30, First substrate layer; 40, Second substrate layer; 50, Third substrate layer; 60, Bonding layer; 70, Optical adhesive layer; 80, Spacer layer; 90a, First antireflection coating layer; 90b, Second antireflection coating layer; 90c, Third antireflection coating layer; 90d, Fourth antireflection coating layer; 90e, Fifth antireflection coating layer; 90f, Sixth antireflection coating layer; 90g, Seventh antireflection coating layer; 90h, Eighth antireflection coating layer; 200, Collimating lens; 300, Focusing lens; 400, Optical fiber; 1000, Optical fiber coupling device. Detailed Implementation

[0036] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided to make the description of this application more comprehensive and complete, and to fully convey the concept of the exemplary embodiments to those skilled in the art. The drawings are merely illustrative of this application and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted.

[0037] Furthermore, the described features, structures, or characteristics can be combined in any suitable manner in one or more exemplary embodiments. Numerous specific details are provided in the following description to give a full understanding of exemplary embodiments of this application. However, those skilled in the art will recognize that the technical solutions of this application can be practiced with one or more specific details omitted, or other modules, components, etc., can be employed. In other instances, well-known structures, methods, implementations, or operations are not shown or described in detail to avoid obscuring various aspects of this application.

[0038] This application provides an optical isolator 100, which allows only one-way light transmission. The operating wavelength of the optical isolator 100 includes, but is not limited to, the operating wavelength of optical communication. For example, the operating wavelength includes, but is not limited to: (1) band code 850nm, wavelength range 800nm–900nm; (2) band code: O band, wavelength range 1260nm–1360nm; (3) band code: S band, wavelength range 1460nm–1530nm; (4) band code: C band, wavelength range 1530nm–1565nm; (5) band code: L band, wavelength range 1565nm–1625nm; band code: U band, wavelength range 1625nm–1675nm.

[0039] Please see Figure 1 or Figure 2 , Figure 1 and Figure 2 The following are schematic diagrams of the structure of an optical isolator 100 in one embodiment of this application. The optical isolator 100 includes a metasurface linear polarization layer 10 and a metasurface quarter-wave plate layer 20.

[0040] The metasurface linear polarization layer 10 is a layered linear polarizer fabricated using semiconductor technology. The metasurface linear polarization layer 10 is configured to filter linearly polarized light with a target polarization state; that is, it filters incident light of arbitrary polarization to linearly polarized light in the target direction, retaining only the component vibrating along its own transmission axis while filtering out the perpendicular component. The target polarization state of the metasurface linear polarization layer 10 includes, but is not limited to, X-polarization or Y-polarization. For example, when the target polarization state of the metasurface linear polarization layer 10 is X-polarization, it only allows X-polarized light to pass through and blocks Y-polarized light from passing through. Similarly, when the target polarization state of the metasurface linear polarization layer 10 is Y-polarization, it only allows Y-polarized light to pass through and blocks X-polarized light from passing through. It is important to note that when arbitrary polarized light is incident on the metasurface linear polarization layer 10, the component of the arbitrary polarized light in the target polarization direction can still pass through. The metasurface linear polarization layer 10 is configured to filter out linearly polarized light of the target polarization state in the following ways: (1) allowing linearly polarized light of the target polarization state to pass through itself and preventing linearly polarized light of the non-target polarization state from passing through, the blocking methods including but not limited to reflection and absorption. (2) allowing both linearly polarized light of the target polarization state and linearly polarized light of the non-target polarization state to pass through itself, but deflecting the optical path of the linearly polarized light of the non-target polarization state from its original optical path, thereby filtering out linearly polarized light of the target polarization state. For example, both linearly polarized light of the target polarization state and linearly polarized light of the non-target polarization state can pass through the metasurface linear polarization layer 10, but deflecting the linearly polarized light of the non-target polarization state by a certain angle.

[0041] The metasurface quarter-wave plate layer 20 is a quarter-wave plate with a layered structure fabricated based on semiconductor technology. The metasurface quarter-wave plate layer 20 is disposed on the light-emitting side of the metasurface linear polarization layer 10. The fast axis direction of the metasurface quarter-wave plate layer 20 has a target angle with the transmission direction of the metasurface linear polarization layer 10. The "target angle" includes 45° or -45°. The transmission direction of the metasurface linear polarization layer 10 should be understood as follows: (1) When the metasurface linear polarization layer 10 is used to allow linearly polarized light of the target polarization state to pass through itself and to prevent linearly polarized light of the non-target polarization state from passing through, the transmission direction of the metasurface linear polarization layer 10 is the vibration direction of the linearly polarized light that is allowed to pass through; (2) When both linearly polarized light of the target polarization state and linearly polarized light of the non-target polarization state can pass through itself, but the optical path of the linearly polarized light of the non-target polarization state is deviated from the original optical path, the transmission direction of the metasurface linear polarization layer 10 is the vibration direction of the linearly polarized light of the target polarization state.

[0042] The optical path difference between the fast and slow axes of the exit light from the metasurface quarter-wave plate is equal to one-quarter of the operating wavelength. The metasurface quarter-wave plate layer 20 is configured to convert the polarization state of the incident light between linear and circular polarization. Specifically, when linearly polarized light of the target polarization state, filtered by the metasurface linear polarization layer 10, is incident on the metasurface quarter-wave plate layer 20, the metasurface quarter-wave plate layer 20 converts it into circularly polarized light and projects the exit light onto the target region, which includes, but is not limited to, an optical fiber coupling end face or a focusing lens. When a small amount of orthogonally circularly polarized light reflected back through the metasurface quarter-wave plate layer 20 is passed through the metasurface quarter-wave plate layer 20 again, it is converted into linearly polarized light. For example, when X-ray linearly polarized light incident at a target angle passes through the metasurface quarter-wave plate layer 20, the metasurface quarter-wave plate layer 20 converts it into circularly polarized light, and a small amount of orthogonally circularly polarized light reflected back is passed through the metasurface quarter-wave plate layer 20 again, it is converted into Y-ray linearly polarized light. For example, when Y-direction linearly polarized light incident at the target angle passes through the metasurface quarter-wave plate layer 20, the metasurface quarter-wave plate layer 20 converts it into circularly polarized light, and a small amount of orthogonally circularly polarized light reflected back passes through the metasurface quarter-wave plate layer 20 again and is converted into X-direction linearly polarized light.

[0043] The metasurface quarter-wave plate layer 20 is a linear reciprocal element. Linearly polarized light incident in the forward direction is converted into circularly polarized light through it. If the light is incident again in the reverse direction, the conversion law is the same as in the forward direction. However, since the reflected light mainly comes from Fresnel reflection, the reflection process causes the circular polarization direction of the light to reverse, and the circular polarization state of the reflected light is orthogonal to the circular polarization state when it is incident. After the orthogonal circularly polarized light passes through the quarter-wave plate layer (20) again, its two orthogonal components are recombined and will inevitably be converted into linearly polarized light orthogonal to the incident light. This is strictly guaranteed by the mathematical properties of the circular polarization direction reversal and the Jones matrix. Moreover, compared with the quarter-wave plate of quartz material, the phase delay achieved by the metasurface quarter-wave plate layer 20 depends on the thickness. The phase delay of the metasurface quarter-wave plate layer 20 comes from the electromagnetic response of the subwavelength micro-nano structure, which is independent of its own physical thickness and does not depend on the macroscopic thickness cutting accuracy. The thickness of the metasurface quarter-wave plate layer 20 is relatively thin, and the effective thickness is only the structural height, which is usually on the order of hundreds of nanometers to 1 micrometer.

[0044] The working principle of the optical isolator 100 provided in this application is as follows: Taking X-ray linearly polarized light as the target polarization state of the metasurface linear polarization layer 10 as an example, the metasurface linear polarization layer 10 only allows X-ray linearly polarized light to pass through and blocks the transmission of Y-axis linearly polarized light. The metasurface quarter-wave plate layer 20 receives the outgoing light from the metasurface linear polarization layer 10 and converts the incident X-axis linearly polarized light into circularly polarized outgoing light. Furthermore, a small amount of orthogonally circularly polarized light reflected back is converted into Y-axis linearly polarized light when it passes through the metasurface quarter-wave plate layer 20 again. Since the Y-axis linearly polarized light is perpendicular to the transmission direction of the target polarization state of the metasurface linear polarization layer 10, it is completely filtered out, thereby enabling light to pass through the optical isolator 100 in only one direction.

[0045] Compared to existing optical isolators that include polarizers (or polarizing beam splitters) and quarter-wave plates, the optical isolator 100 provided in this application can have its metasurface linear polarization layer 10 and metasurface quarter-wave plate layer 20 mass-produced using standard wafer-level processes such as semiconductor photolithography, etching, and thin film deposition. This allows for integrated manufacturing, miniaturization, and cost reduction of the optical isolator 100, which is impossible for optical isolators 100 that include traditional quartz quarter-wave plates. Compared to existing optical isolators that include magnets (permanent magnets or electromagnetic coils), Faraday rotators, and polarizers (or birefringent crystals), the polarization state conversion during the isolation process of the optical isolator 100 provided in this application relies entirely on the wave nature of light (Fresnel reflection, birefringence, phase delay), requiring no external magnetic field, electric field, nonlinear optical effects, or active control circuitry. This allows for integrated manufacturing and offers advantages such as small size, low cost, and high integration, which is impossible for traditional optical isolators that include magnets. Furthermore, the optical isolator 100 provided in this application is an integrated component. Compared to optical isolators with separate linear polarizers and quarter-wave plates, the optical isolator 100 provided in this application eliminates air gaps and additional optical interfaces, thereby reducing insertion loss and improving forward light transmission efficiency. In other words, the optical isolator 100 provided in this application has advantages such as being magnet-free, passive, small in size, light in weight, low in cost, free from magnetic interference, and capable of semiconductor integration.

[0046] In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are both processed by semiconductor technology, so that the thickness of the optical isolator 100 is on the order of millimeters or sub-millimeters, that is, the thickness of the optical isolator 100 is less than 2 millimeters, so as to significantly reduce the volume of the optical isolator 100.

[0047] In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are bonded together to form an integral optical isolator 100.

[0048] Please see Figure 1In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are directly bonded together to form a bonding layer 60 between them. The thickness of the resulting optical isolator 100 is on the order of millimeters. Specifically, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are fabricated using two wafers respectively. At room temperature or medium temperature (200°C to 400°C), atomic-level close contact is achieved using van der Waals forces and hydrogen bonds. The thickness of the bonding layer 60 can be controlled in the sub-nanometer to nanometer range, introducing almost no additional optical path difference or Fresnel reflection.

[0049] Ultrathin antireflective films or refractive index matching layers can be pre-deposited on both sides of the bonding layer 60 to further reduce interface reflection. The metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are bonded together, eliminating the air gap (approximately 1 mm to several millimeters) between them. This reduces the overall thickness of the optical isolator 100 from several millimeters to the sub-millimeter level, avoiding additional Fresnel reflection at the optical interface and reducing insertion loss and return loss. Furthermore, the precision patterning capability of semiconductor technology ensures that the angular alignment error between the linear polarizer and the quarter-wave plate is at the microradian level, significantly lower than the mechanical assembly error of the separate linear polarizer and the metasurface quarter-wave plate in the prior art.

[0050] Please see Figure 1 In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are optically bonded together to form an optical adhesive layer 70 between them. The thickness of the resulting optical isolator 100 is on the sub-millimeter scale. Specifically, an optical adhesive (such as a UV-curable adhesive) that is transparent to the optical communication band and has a matching refractive index is used to optically bond the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20. The thickness of the optical adhesive layer 70 is less than the micrometer scale.

[0051] The metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are optically bonded together, eliminating the air gap (approximately 1 mm to several millimeters) between them. This reduces the overall thickness of the optical isolator 100 from several millimeters to sub-millimeter levels, avoiding additional Fresnel reflections at the optical interface and reducing insertion loss and return loss. Furthermore, the precision patterning capabilities of semiconductor technology ensure that the angular alignment error between the linear polarizer and the quarter-wave plate is at the micro-radian level, significantly lower than the mechanical alignment error of the separate linear polarizer and the metasurface quarter-wave plate in existing technologies.

[0052] Please see Figure 2In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are fabricated on the same wafer using semiconductor processes, specifically, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are fabricated on the same wafer using semiconductor overlay processes. Specifically, either the linear polarization plate layer or the metasurface quarter-wave plate layer 20 is fabricated on a single wafer (such as a silicon wafer or fused silica wafer) using photolithography and etching processes. Based on either of the fabricated linear polarization plate layer and the metasurface quarter-wave plate layer 20, a spacer layer 80 relatively transparent to the target wavelength is deposited using plasma-enhanced chemical vapor deposition or atomic layer deposition. The upper surface of the spacer layer 80 is planarized using chemical mechanical polishing to ensure flatness; the surface roughness after planarization needs to be controlled at the nanometer level. On the planarized spacer layer 80, the other of the linear polarization plate layer and the metasurface quarter-wave plate layer 20 is again fabricated using photolithography and etching processes.

[0053] Furthermore, regardless of whether the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are bonded or optically bonded, they must undergo surface treatment before bonding. Specifically, the processed metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are subjected to plasma activation treatment, such as oxygen plasma or argon plasma treatment, to generate active groups such as hydroxyl groups on the surface, thereby increasing the surface energy and improving the bonding strength between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20.

[0054] Furthermore, regardless of whether the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are bonded or optically bonded, when they are joined, the angle between the fast axis of the metasurface quarter-wave plate layer 20 and the transmission direction of the metasurface linear polarization layer 10 is 45° or -45°. This ensures that when the target linearly polarized light emitted from the metasurface linear polarization layer 10 is incident on the metasurface quarter-wave plate layer 20, the amplitudes of the electric field components decomposed onto the fast and slow axes are equal. The metasurface quarter-wave plate layer 20 is configured to introduce a phase difference of π / 2 (corresponding to an optical path difference of one-quarter of a wavelength) between its fast and slow axes, thereby converting the linearly polarized light along the transmission axis into circularly polarized light. At the same time, the orthogonal circularly polarized light, which has its rotation reversed after reflection at the interface, passes through the metasurface quarter-wave plate layer 20 again and is converted into linearly polarized light orthogonal to the transmission axis of the metasurface linear polarization layer 10, and is then filtered out by the metasurface linear polarization layer 10.

[0055] When the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are optically bonded together, an overlay alignment process is used to make the angle between the fast axis of the metasurface quarter-wave plate layer 20 and the target polarization state (transmission direction) of the metasurface linear polarization layer 10 45° or -45°.

[0056] In some embodiments, the metasurface linear polarization layer 10 is further configured to reflect or absorb linearly polarized incident light in a non-target direction, so that the polarization state of the incident light is unique.

[0057] Further, please refer to Figure 3 (a) In some embodiments, the metasurface linear polarization layer 10 is a metal wire grid, which is a subwavelength periodic metal micro / nano structure. The metal wire grid is used to transmit linearly polarized light of the target polarization state and to reflect or absorb linearly polarized incident light of non-target direction, so that the polarization state of the incident light is unique.

[0058] The working principle of the metal wire grid is based on the selective response of metal to electromagnetic waves: when the incident electric field vector direction is parallel to the arrangement direction of the metal wire grid, strong surface plasmon resonances or free electron oscillations are excited, and the light is reflected or absorbed; when the electric field vector direction is perpendicular to the arrangement direction of the metal wire grid, the metal exhibits a weak response, and the light is transmitted efficiently. By configuring the period (usually less than half the operating wavelength) and duty cycle of the metal wire grid, the metasurface linear polarization layer 10 can have a high extinction ratio and high transmittance.

[0059] Further, please refer to Figure 3 (b) In some embodiments, the metasurface linear polarization layer 10 is an all-dielectric-based micro / nanostructure array, wherein each micro / nanostructure in the all-dielectric-based micro / nanostructure array includes at least one of the following: nanocylindrical structure, hollow nanopillar structure, nanocircular hole structure, nanoring hole structure, nanosquare pillar structure, nanosquare hole structure, nanosquare ring structure, and nanosquare ring hole structure. The all-dielectric-based micro / nanostructure array is used to transmit linearly polarized light of the target polarization state and to reflect or absorb linearly polarized incident light of non-target direction.

[0060] Specifically, the working principle of the all-dielectric-based micro / nanostructure array is based on Mie resonance or waveguide mode coupling: for a specific polarization direction (e.g., the x-direction), the all-dielectric-based micro / nanostructure array supports a low-loss Mie resonance transmission mode; for orthogonal polarization directions (y-direction), the all-dielectric-based micro / nanostructure array excites a strong reflection or higher-order leakage mode, and the incident light is reflected or coupled into a non-propagation mode for absorption. The gaps between the all-dielectric-based micro / nanostructures are filled with low-refractive-index materials (such as silicon dioxide) to planarize the surfaces, facilitating the subsequent deposition of other optical films; air can also be filled between the all-dielectric-based micro / nanostructures to enhance the Mie resonance effect.

[0061] In some embodiments, the metasurface linear polarization layer 10 is further configured to transmit and deflect non-target polarized linearly polarized incident light, thereby avoiding additional back reflection generated by reflection on the surface of the metasurface linear polarization layer 10 and further improving the system's isolation. See also... Figure 3(c) The metasurface linear polarization layer 10 is a phase gradient deflection type all-dielectric micro / nanostructure array. Each micro / nanostructure in the phase gradient deflection type all-dielectric micro / nanostructure array includes at least one of the following: nanocylindrical structure, hollow nanopillar structure, nanocircular hole structure, nanoring hole structure, nanosquare pillar structure, nanosquare hole structure, nanosquare ring structure, and nanosquare ring hole structure. The phase gradient deflection type all-dielectric micro / nanostructure array is used to transmit linearly polarized light of the target polarization state, and also to deflect linearly polarized incident light in directions other than the target direction. Since the linearly polarized incident light in the non-target direction is deflected rather than reflected or absorbed, it avoids additional back-reflection due to reflection on the surface of the metasurface linear polarization layer 10, further improving the system's isolation.

[0062] Specifically, in this application, linearly polarized light orthogonal to the target polarization state is denoted as orthogonal linearly polarized light, which is the polarization component that needs to be blocked. For an all-dielectric metasurface linear polarizer, the metasurface linear polarizer applies a specific linear phase gradient to the orthogonal linearly polarized light, causing it to deviate from the original propagation optical axis according to the generalized Snell's law after transmission. This results in the orthogonal linearly polarized light passing through the metasurface linear polarizer 10 being deflected and separated from the linearly polarized light of the target polarization state passing through the metasurface linear polarizer 10, without interfering with the orthogonal linearly polarized light passing through the metasurface linear polarizer 10. The angle between the orthogonal linearly polarized light and the optical axis after passing through the metasurface linear polarizer 10 is the deflection angle, which is greater than or equal to 3° and less than or equal to 60°. Further, the deflection angle is greater than or equal to 30° and less than or equal to 60°.

[0063] At this point, the phase gradient deflection type all-dielectric micro / nano structure array has a target linear phase gradient in the orthogonal direction of the target polarization state of the metasurface linear polarization layer 10. The transmission angle of orthogonally linearly polarized light satisfy: ,in, Let λ be the incident angle of orthogonally linearly polarized light, λ be the wavelength, and π be pi. By configuring a linear phase gradient... This allows orthogonally linearly polarized light to have a target deflection angle after passing through the metasurface linear polarization layer 10.

[0064] Please see Figure 4 In some embodiments, the metasurface quarter-wave plate layer 20 is an array of micro / nano structures. The material of the second substrate is transparent to the optical communication band, including but not limited to fused silica, sapphire, and silicon. The micro / nano structures have different equivalent refractive indices in two mutually perpendicular directions to form a birefringence effect, thereby generating a fast axis and a slow axis. The micro / nano structures are anisotropic, and the geometry (major axis, minor axis, height) of the micro / nano structures is configured such that light propagating along the slow axis and the fast axis directions produces a phase difference of π / 2, i.e., a quarter-wavelength phase delay.

[0065] By configuring the orientation angle of anisotropic micro / nanostructures, the equivalent fast axis direction at different locations in a plane can be flexibly controlled using the principle of geometric phase (also known as Pancharatnam-Berry phase). Specifically, for anisotropic micro / nanostructures with an orientation angle of θ, their Jones matrix is ​​equivalent to rotating a standard quarter-wave plate by an angle θ. By precisely arranging the orientation angles of micro / nanostructures at different locations, precise control of the entire wavefront phase distribution can be achieved, enabling uniform fast axis direction, radially varying fast axis direction, and compensation for phase delay deviations caused by manufacturing errors or wavelength shifts.

[0066] Precise control of the entire wavefront phase distribution can make the metasurface quarter-wave plate layer 20 have a uniform fast axis direction, that is, the fast axis of the entire metasurface quarter-wave plate layer 20 is consistent, which can achieve the following effects: (1) Linearly polarized light incident at any position is decomposed into components with the same proportion of fast and slow axes, and all are stably generated into standard circularly polarized light without local elliptically polarized stray light. (2) The beam does not have wavefront phase modulation after passing through the metasurface quarter-wave plate layer 20, and there will be no wavefront aberration caused by phase change, which can improve the efficiency of beam coupling into fiber 400. (3) The phase delay of the entire metasurface quarter-wave plate layer 20 is uniform, no partition calibration is required, there is no local phase error during bonding and assembly, and the device has high consistency. (4) When paired with the metasurface linear polarization layer 10, the device has no spatial difference in insertion loss, polarization dependent loss (PDL) and extinction ratio, which can avoid problems such as inconsistent loss between the center and edge of the spot and local leakage in the reverse direction. The optical isolator 100 has a high degree of isolation as a whole.

[0067] Precise control of the entire wavefront phase distribution can achieve radially varying fast axis directions for special wavefront shaping, achieving the following effects: (1) generating vector polarized beams. (2) regulating vortex phase and shaping special wavefronts.

[0068] The ideal phase difference of a standard quarter-wave plate is π / 2. Phase shifts can occur due to etching size deviations, deviations from the design wavelength, and temperature changes. Precise control of the entire wavefront phase distribution can compensate for phase delay deviations caused by manufacturing errors or wavelength shifts, achieving the following effects: (1) Maintaining a standard phase difference of π / 2 across the entire band. Wavelength fluctuations exist in the communication band. This compensates for phase delay deviations caused by wavelength shifts, ensuring that linearly polarized light can be accurately converted to circularly polarized light across a wide band, without generating elliptically polarized light. (2) Counteracting manufacturing errors in micro / nano structures. Due to etching deviations in the micro / nano structure, the equivalent birefringence can be altered. By precisely controlling the entire wavefront phase distribution, phase errors can be corrected, ensuring that the phase delay of the entire metasurface quarter-wave plate layer is stable at λ / 4.

[0069] The shapes of the micro / nanostructures include, but are not limited to, columnar structures (such as elliptical nanopillars and rectangular nanopillars) or porous structures (such as elliptical nanopores and rectangular nanopores). The metasurface quarter-wave plate layer 20 also includes a filling medium with a low refractive index. The filling medium material includes, but is not limited to, silicon dioxide and magnesium fluoride. When the shape of the micro / nanostructure is columnar, the filling medium fills the gaps between the micro / nanostructures to protect them and facilitate subsequent fabrication of other optical films. When the shape of the micro / nanostructure is porous, the filling medium fills the pores to control the equivalent refractive index, or air is filled.

[0070] Materials for micro / nano structures can include silicon (suitable for near-infrared bands, refractive index n≈3.5), germanium (suitable for mid-infrared bands, refractive index n≈4.0), silicon nitride (suitable for visible to near-infrared, refractive index n≈2.0), titanium dioxide (suitable for visible light), or hydrogenated amorphous silicon (a-Si:H). For the 1310 nm and 1550 nm optical communication bands, silicon is a suitable material for micro / nano structures due to its high refractive index contrast and mature semiconductor processing technology.

[0071] Since the structure of the optical isolator 100 obtained by bonding or optically bonding the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 is slightly different from the structure of the optical isolator 100 obtained by processing the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 on the same wafer using semiconductor technology, this application describes their structures separately.

[0072] Please see Figure 5In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are bonded or optically bonded together to form an optical isolator 100. In this case, the optical isolator 100 further includes a bonding layer 60 or an optical adhesive layer 70, a first substrate layer 30, and a second substrate layer 40, with each layered structure stacked. The bonding layer 60 and the optical adhesive layer 70 are located between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20. The formation of the bonding layer 60 and the optical adhesive layer 70 can be referred to above, and will not be repeated here.

[0073] The first substrate layer 30 is made of a material transparent to the optical communication band, including but not limited to fused silica, sapphire, and silicon. The first substrate layer 30 is located on the side of the metasurface linear polarization layer 10 away from the metasurface quarter-wave plate layer 20. The second substrate layer 40 is made of a material transparent to the optical communication band, including but not limited to fused silica, sapphire, and silicon. The second substrate layer 40 is located on the side of the metasurface quarter-wave plate layer 20 away from the metasurface linear polarization layer 10.

[0074] Please refer to the following: Figure 5 and Figure 6 Furthermore, in some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are bonded or optically bonded together to obtain the optical isolator 100. In this case, the optical isolator 100 further includes a first antireflection film, a second antireflection film, a third antireflection film and a fourth antireflection film.

[0075] The first antireflection coating layer 90a is located between the first substrate layer 30 and the metasurface linear polarization layer 10. The first antireflection coating layer 90a is used to reduce the Fresnel reflectivity of the interface between the first substrate layer 30 and the metasurface linear polarization layer 10 to below 0.1%, thereby reducing insertion loss and return loss. The material of the first antireflection coating layer 90a can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the first antireflection coating layer 90a can refer to the structure of antireflection coatings in the prior art; this application does not limit its specific structure.

[0076] The second antireflection coating layer 90b is located on the side of the first substrate layer 30 facing away from the metasurface linear polarization layer 10. The second antireflection coating layer 90b is used to reduce the Fresnel reflectivity of the interface between the first substrate layer 30 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the second antireflection coating layer 90b can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the second antireflection coating layer 90b can refer to the structure of antireflection films in the prior art; this application does not limit its specific structure.

[0077] The third antireflection coating layer 90c is located between the second substrate layer 40 and the metasurface quarter-wave plate layer 20. The third antireflection coating layer 90c is used to reduce the Fresnel reflectivity of the interface between the second substrate layer 40 and the metasurface quarter-wave plate layer 20 to below 0.1%, thereby reducing insertion loss and return loss. The material of the third antireflection coating layer 90c can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the third antireflection coating layer 90c can refer to the structure of antireflection coatings in the prior art; this application does not limit its specific structure.

[0078] The fourth antireflective coating layer 90d is located on the side of the second substrate layer 40 opposite to the third antireflective coating layer 90c. The fourth antireflective coating layer 90d is used to reduce the Fresnel reflectivity of the interface between the second substrate layer 40 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the fourth antireflective coating layer 90d can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the fourth antireflective coating layer 90d can refer to the structure of antireflective coatings in the prior art; this application does not limit its specific structure.

[0079] Please see Figure 7 In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are fabricated on the same wafer using semiconductor technology, meaning that only one wafer is needed to fabricate the optical isolator 100. In this case, the optical isolator 100 also includes a spacer layer 80 and a third substrate layer 50, with the layered structures stacked together. The spacer layer 80 is located between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20. The formation of the spacer layer 80 can be referred to above and will not be repeated here.

[0080] The third substrate layer 50 is part of the wafer. The material of the third substrate layer 50 is transparent to optical communication wavelengths, including but not limited to fused silica, sapphire, and silicon. The third substrate layer 50 is bonded to either the metasurface linear polarization layer 10 or the metasurface quarter-wave plate layer 20. Exemplarily, the third substrate layer 50 is bonded to the metasurface linear polarization layer 10. Exemplarily, the third substrate layer 50 is bonded to the metasurface quarter-wave plate layer 20. This application does not limit the specific location of the third substrate layer 50. This application illustrates the specific structure of the optical isolator 100 by using the example of the third substrate layer 50 being bonded to the metasurface linear polarization layer 10.

[0081] Please refer to the following: Figure 7 and Figure 8In some embodiments, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are fabricated on the same wafer using semiconductor technology, meaning the optical isolator 100 is fabricated on a single substrate. This further reduces the overall thickness, volume, and weight, and also reduces the optical interface, further reducing Fresnel reflection and scattering, resulting in lower insertion loss. In this case, the optical isolator 100 further includes a fifth antireflection coating layer 90e, a sixth antireflection coating layer 90f, a seventh antireflection coating layer 90g, and an eighth antireflection coating layer 90h.

[0082] The fifth antireflection coating layer 90e is located on the side of the third substrate layer 50 facing away from the metasurface linear polarization layer 10. The fifth antireflection coating layer 90e is used to reduce the Fresnel reflectivity of the interface between the third substrate layer 50 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the fifth antireflection coating layer 90e can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the fifth antireflection coating layer 90e can refer to the structure of antireflection coatings in the prior art; this application does not limit its specific structure.

[0083] The sixth antireflection coating layer 90f is located on the side of the third substrate layer 50 facing the metasurface linear polarization layer 10. The sixth antireflection coating layer 90f is used to reduce the Fresnel reflectivity of the interface between the third substrate layer 50 and the metasurface linear polarization layer 10 to below 0.1%, thereby reducing insertion loss and return loss. The material of the sixth antireflection coating layer 90f can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the sixth antireflection coating layer 90f can refer to the structure of antireflection coatings in the prior art; this application does not limit its specific structure.

[0084] Since spacer layer 80 can be designed as the seventh antireflection coating layer 90g, that is, spacer layer 80 can serve as the seventh antireflection coating layer 90g. By configuring the thickness and refractive index of spacer layer 80, it can play an antireflection role in the target wavelength band. Spacer layer 80 has a target thickness to keep the near-field coupling between metasurface linear polarization layer 10 and metasurface quarter-wave plate layer 20 within an allowable range, while providing sufficient mechanical support. The thickness of spacer layer 80 is from several nanometers to several micrometers. When spacer layer 80 serves as the seventh antireflection coating layer 90g, the seventh antireflection coating layer 90g is used to reduce the Fresnel reflectivity of the interface between metasurface linear polarization layer 10 and metasurface quarter-wave plate layer 20 to below 0.1%, thereby reducing insertion loss and return loss. The material of the seventh antireflection coating layer 90g can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the seventh antireflection coating layer 90g can refer to the structure of antireflection coatings in the prior art, and this application does not limit its specific structure.

[0085] The eighth antireflection coating layer 90h is located on the side of the metasurface quarter-wave plate layer 20 facing away from the metasurface linear polarization layer 10. The eighth antireflection coating layer 90h is used to reduce the Fresnel reflectivity of the interface between the metasurface quarter-wave plate layer 20 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the eighth antireflection coating layer 90h can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the eighth antireflection coating layer 90h can refer to the structure of antireflection coatings in the prior art; this application does not limit its specific structure.

[0086] This application also provides a method for manufacturing an optical isolator 100, please refer to... Figure 1 The processing methods include:

[0087] Step S1: Fabricate a metasurface linear polarization layer 10 based on the first substrate layer 30.

[0088] Specifically, a first composite layered structure is obtained on the first substrate layer 30 by electron beam lithography or deep ultraviolet lithography combined with reactive ion etching or inductively coupled plasma etching. The first composite layered structure includes the first substrate layer 30 and a metasurface linear polarization layer 10, wherein the metasurface linear polarization layer 10 is located on the first substrate layer 30, and the specific structure of the metasurface linear polarization layer 10 can be referred to above.

[0089] The first substrate layer 30 includes, but is not limited to, a fused silica sheet, which is a high-purity fused silica glass precision polished thin sheet, belonging to a type of optical wafer, and its chemical composition is amorphous silicon dioxide.

[0090] Step S2: Fabricate a metasurface quarter-wave plate layer 20 based on the second substrate layer 40.

[0091] Specifically, a second composite layered structure is obtained on the second substrate layer 40 by electron beam lithography or deep ultraviolet lithography combined with reactive ion etching or inductively coupled plasma etching. The second composite layered structure includes the second substrate layer 40 and a metasurface quarter-wave plate layer 20, wherein the metasurface quarter-wave plate layer 20 is located on the second substrate layer 40, and the specific structure of the metasurface quarter-wave plate layer 20 can be referred to above.

[0092] The second substrate layer 40 includes, but is not limited to, a fused silica sheet, which is a high-purity fused silica glass precision polished thin sheet, belonging to a type of optical wafer, and its chemical composition is amorphous silicon dioxide.

[0093] Step S3: First, align the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20, and then bond or optically bond the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 together to obtain the optical isolator 100.

[0094] In some embodiments, aligning the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 in step S3 specifically includes: aligning the effective optical regions of the first composite layered structure metasurface linear polarization layer 10 and the second composite layered structure metasurface quarter-wave plate layer 20 with the aid of an optical microscope or an automatic alignment system, while maintaining the transmission direction of the metasurface linear polarization layer 10 at an angle of 45° or -45° to the fast axis direction of the metasurface quarter-wave plate layer 20.

[0095] The bonding connection between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 includes achieving atomic-level close contact between the two layers at room temperature or intermediate temperature (200°C to 400°C) using van der Waals forces and hydrogen bonds. This atomic-level close contact between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20, achieved using van der Waals forces and hydrogen bonds, forms a bonding layer 60 between them. The thickness of the bonding layer 60 can be controlled in the sub-nanometer to nanometer range, introducing almost no additional optical path difference or Fresnel reflection.

[0096] Step S3, after bonding the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20, also includes annealing the entire optical isolator 100 obtained by bonding.

[0097] Annealing can improve bond strength and eliminate residual stress.

[0098] In some embodiments, step S3 optically bonds the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 together, specifically including: using an optical adhesive (such as a UV-curable adhesive) that is transparent to the optical communication band and has a matching refractive index to optically bond the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 together.

[0099] The metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are optically bonded together, and an optical adhesive layer 70 is formed between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20. The thickness of the optical adhesive layer 70 is less than the micrometer level.

[0100] Please combine Figure 5 In some embodiments, step S1 further includes: processing a first antireflection film layer 90a and a second antireflection film layer 90b based on the first substrate layer 30, wherein the first antireflection film layer 90a is located between the first substrate and the metasurface linear polarization layer 10, and the second antireflection film layer 90b is located on the side of the first substrate layer 30 away from the metasurface quarter-wave plate layer 20.

[0101] The first antireflection coating layer 90a is used to reduce the Fresnel reflectivity of the interface between the first substrate layer 30 and the metasurface linear polarization layer 10 to below 0.1%, thereby reducing insertion loss and return loss. The material of the first antireflection coating layer 90a can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. Since the first antireflection coating layer 90a is located between the first substrate layer 30 and the metasurface linear polarization layer 10, its fabrication requires first fabricating the first antireflection coating layer 90a based on the first substrate layer 30, and then fabricating the metasurface linear polarization layer 10. Specific fabrication methods for the antireflection coating layer can be found in existing technologies; this application does not limit the specific fabrication methods.

[0102] The second antireflection coating layer 90b is used to reduce the Fresnel reflectivity of the interface between the first substrate layer 30 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the second antireflection coating layer 90b can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc.

[0103] Please combine again Figure 5 In some embodiments, step S2 further includes: processing a third antireflection layer 90c and a fourth antireflection layer 90d based on the second substrate layer 40, wherein the third antireflection layer 90c is located between the second substrate layer 40 and the metasurface quarter-wave plate layer 20, and the fourth antireflection layer 90d is located on the side of the second substrate layer 40 away from the metasurface linear polarization layer 10.

[0104] The third antireflection coating layer 90c is used to reduce the Fresnel reflectivity of the interface between the first substrate layer 30 and the metasurface linear polarization layer 10 to below 0.1%, thereby reducing insertion loss and return loss. The material of the third antireflection coating layer 90c can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. Since the third antireflection coating layer 90c is located between the second substrate layer 40 and the metasurface quarter-wave plate layer 20, the third antireflection coating layer 90c needs to be fabricated based on the second substrate layer 40 first, and then the metasurface quarter-wave plate layer 20 needs to be fabricated.

[0105] The fourth antireflection coating layer 90d is used to reduce the Fresnel reflectivity of the interface between the first substrate layer 30 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the fourth antireflection coating layer 90d can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc.

[0106] This application also provides another method for fabricating the optical isolator 100, please refer to... Figure 2 The processing methods include:

[0107] Step S10: Based on the third substrate layer 50, fabricate either the metasurface linear polarization layer 10 or the metasurface quarter-wave plate layer 20 to obtain the third composite layered structure.

[0108] In some embodiments, step S10 specifically includes: processing a metasurface linear polarization layer 10 or a metasurface quarter-wave plate layer 20 based on the third substrate layer 50 through photolithography and etching processes, filling the surface of the metasurface linear polarization layer 10 or the metasurface quarter-wave plate layer 20 with a target refractive index material and flattening the filled surface.

[0109] The first substrate layer 30 includes, but is not limited to, a fused silica sheet, which is a high-purity fused silica glass precision polished thin sheet, belonging to a type of optical wafer, and its chemical composition is amorphous silicon dioxide.

[0110] The target refractive index material has a low refractive index, and the target refractive index material includes, but is not limited to, silicon dioxide.

[0111] In some embodiments, planarizing the fill surface specifically includes: planarizing the upper surface of the spacer layer 80 by chemical mechanical polishing to ensure the flatness of subsequent processing. The surface roughness after planarization is on the nanometer scale.

[0112] Step S20: Deposit spacer layer 80 based on the third composite layered structure to obtain the fourth composite layered structure.

[0113] In some embodiments, depositing a spacer layer 80 based on a third composite layered structure specifically includes: obtaining a spacer layer 80 that is relatively transparent to the target wavelength by plasma-enhanced chemical vapor deposition or atomic layer deposition based on the third composite layered structure.

[0114] Step S30: Based on the fourth composite layered structure, fabricate another of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 to obtain the optical isolator 100.

[0115] In some embodiments, step S30 specifically includes: processing another of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 based on the fourth composite layered structure through an overlay alignment process, and ensuring that the angle between the fast axis of the metasurface quarter-wave plate layer 20 and the target polarization state (transmission direction) of the metasurface linear polarization layer 10 is 45° or -45°.

[0116] In the optical isolator 100 processing method provided in this application, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 are processed on the same wafer using semiconductor technology. That is, the optical isolator 100 is processed on a single substrate, which can further reduce the overall thickness, volume and weight, and also reduce the optical interface, further reducing Fresnel reflection and scattering, and resulting in lower insertion loss.

[0117] Please combine Figure 6In some embodiments, step S10 further includes: fabricating any one of the fifth antireflection layer 90e, the sixth antireflection layer 90f, the metasurface linear polarization layer 10, and the metasurface quarter-wave plate layer 20 based on the third substrate layer 50 to obtain a third composite layered structure; wherein the fifth antireflection layer 90e is located on the side of the third substrate away from the metasurface linear polarization layer 10, and the sixth antireflection layer 90f is located on the side of the third substrate facing the metasurface linear polarization layer 10.

[0118] The fifth antireflection coating layer 90e is located on the side of the third substrate layer 50 facing away from the metasurface linear polarization layer 10. The fifth antireflection coating layer 90e is used to reduce the Fresnel reflectivity of the interface between the third substrate layer 50 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the fifth antireflection coating layer 90e can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific processing method of the antireflection coating layer can refer to existing technologies; this application does not limit the specific processing method.

[0119] The sixth antireflection coating layer 90f is located on the side of the third substrate facing the metasurface linear polarization layer 10. The sixth antireflection coating layer 90f is used to reduce the Fresnel reflectivity of the interface to below 0.1% to reduce insertion loss and return loss. The material of the sixth antireflection coating layer 90f can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc.

[0120] Please combine again Figure 6 In some embodiments, step S20, which involves depositing a spacer layer 80 based on a third composite layered structure, specifically includes depositing a spacer layer 80 based on a third composite layered structure and configuring the refractive index and thickness of the spacer layer 80 to form a seventh antireflection film layer 90g.

[0121] Specifically, the thickness and refractive index of the spacer layer 80 are configured to enhance its anti-reflection effect in the target wavelength band. The spacer layer 80 has a target thickness to ensure that the near-field coupling between the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 is within acceptable limits, while providing sufficient mechanical support. The thickness of the spacer layer 80 ranges from several nanometers to several micrometers.

[0122] When spacer layer 80 serves as the seventh antireflection coating layer 90g, the seventh antireflection coating layer 90g is used to reduce the Fresnel reflectivity of the interface to below 0.1%, thereby reducing insertion loss and return loss. The material of the seventh antireflection coating layer 90g can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the seventh antireflection coating layer 90g can refer to the structure of antireflection coatings in the prior art, and this application does not limit its specific structure.

[0123] In some embodiments, the step S30 of fabricating another of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 based on the fourth composite layered structure specifically includes: sequentially fabricating another of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20, as well as an eighth antireflection film layer 90h, based on the fourth composite layered structure. The eighth antireflection film layer 90h is located on the side of the metasurface quarter-wave plate layer 20 away from the metasurface linear polarization layer 10.

[0124] The eighth antireflection coating layer 90h is used to reduce the Fresnel reflectivity of the interface between the third substrate layer 50 and air to below 0.1%, thereby reducing insertion loss and return loss. The material of the eighth antireflection coating layer 90h can be silicon dioxide, silicon nitride, tantalum pentoxide, or magnesium fluoride, etc. The specific structure of the eighth antireflection coating layer 90h can refer to the structure of antireflection coatings in the prior art; this application does not limit its specific structure.

[0125] Please see Figure 9 and Figure 10 This application also provides an optical fiber coupling device 1000 for coupling light emitted from a light source into an optical fiber 400, the light source including but not limited to a laser.

[0126] The fiber optic coupling device 1000 includes a collimating lens 200, an optical isolator 100, and a focusing lens 300. The collimating lens 200 is used to collimate the incident light. The focusing lens 300 is configured to receive the light collimated by the collimating lens 200 and filter out linearly polarized light with the target polarization state. The focusing lens 300 is configured to receive the light emitted from the optical isolator 100 and couple the emitted light into the end face of the optical fiber 400.

[0127] Please see Figure 9 In some embodiments, the surface normal of the optical isolator 100 is tilted relative to the optical axis of the outgoing light from the collimating lens 200, that is, the surface normal of the optical isolator 100 and the optical axis of the outgoing light from the collimating lens 200 have an angle α.

[0128] When the surface normal of the optical isolator 100 forms an angle α with the optical axis of the outgoing light from the collimating lens 200, according to the law of reflection, the reflected light S from the front surface of the optical isolator 100 will deviate from the original optical axis at an angle of 2α and will not return to the source along the original path. Simultaneously, the reflected light (not shown) from the rear surface (outgoing surface) of the optical isolator 100 is also deflected. Therefore, by appropriately designing the tilt angle, the reflected light from the device body can be completely deviated from the laser receiving aperture.

[0129] Furthermore, in some embodiments, 3°≤α≤45°, so that the spatial placement angle of the optical isolator 100 has a greater degree of freedom.

[0130] Furthermore, in some embodiments, 5°≤α≤15° is used to make the reflected light more dispersed from the incident light, which can maximize the protection that the reflected light will not return to the source along the original path.

[0131] It is important to note that when the optical isolator 100 is tilted, the angle of the incident light relative to the micro / nanostructure of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 is no longer normal incidence (0°), but rather an angle α. In this case, the effective period and equivalent electromagnetic response of the micro / nanostructure will change. To ensure that the designed polarization selectivity and phase delay characteristics are maintained under tilted incidence conditions, the micro / nanostructure can be optimized in the following ways:

[0132] (1) A symmetrical micro / nano structure is adopted. The micro / nano structure of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 adopts a circular nanopillar, square nanopillar or cross-shaped structure. The micro / nano structure of the above shapes has a relatively stable polarization response and phase delay characteristics in a large angle range (±15°) and is not sensitive to changes in the incident angle.

[0133] (2) Optimize the structure height and period. Calculate the transmittance and phase delay under oblique incidence using electromagnetic simulation (such as Finite-Difference Time-Domain, FDTD; or Rigorous Coupled-Wave Analysis, RCWA), and optimize the height, period, and shape of the micro / nano structure of the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 so that the phase delay deviation within the target angle range (such as 5° to 15°) does not exceed π / 10 (i.e., does not exceed 10% of the design value).

[0134] (3) Combination of multiple structural units: Multiple micro-nano structures of different sizes are designed in the same pixel (the smallest functional unit region that completes a single phase modulation target). The angular response is smoothed through the averaging effect, thereby improving the angular tolerance.

[0135] In some embodiments, when the surface normal of the optical isolator 100 has an angle α with the optical axis of the outgoing light from the collimating lens 200, and the metasurface linear polarization layer 10 is a phase gradient deflection type all-dielectric micro / nano structure array, the phase gradient of the metasurface linear polarization layer 10 is configured so that the deflection angle of the orthogonal linear polarized light is α or 2α. The deflected orthogonal linear polarized light and the reflected light from the surface of the optical isolator 100 have similar deflection directions, which facilitates optical path management and stray light absorption.

[0136] In some embodiments, when the surface normal of the optical isolator 100 forms an angle α with the optical axis of the outgoing light from the collimating lens 200, and the metasurface linear polarization layer 10 is a phase gradient deflection type all-dielectric micro / nano structure array, the phase gradient of the metasurface linear polarization layer 10 is configured to deflect the orthogonally linearly polarized light to a specific angle (e.g., 30°), completely deviating it from the main optical path. The optical isolator 100 is tilted only to eliminate its own reflected light; the two functions are complementary. That is, the optical isolator 100 prevents reflected light from returning along the original optical path, while simultaneously deflecting the orthogonally linearly polarized light at a specific angle.

[0137] In some embodiments, when the surface normal of the optical isolator 100 forms an angle α with the optical axis of the outgoing light from the collimating lens 200, the effective optical thickness of the antireflective coating layers (i.e., the first antireflective coating layer 90a, the second antireflective coating layer 90b, the third antireflective coating layer 90c, the fourth antireflective coating layer 90d, the fifth antireflective coating layer 90e, the sixth antireflective coating layer 90f, the seventh antireflective coating layer 90g, and the eighth antireflective coating layer 90h mentioned above) changes from its original thickness (denoted as d) to d·cosα. Therefore, the antireflective coating layers can be optimized using the following method:

[0138] (1) Design a non-quarter-wavelength antireflection coating. For example, use a graded-index antireflection coating to maintain low reflectivity within the included angle α range.

[0139] (2) Adopt a multilayer antireflective coating design. By increasing the number of coating layers (e.g., 3 to 5 layers), the angular bandwidth of the antireflective coating layer is expanded, ensuring that reflection can still be effectively suppressed under tilted conditions.

[0140] Please see Figure 10 In some embodiments, the surface normal of the optical isolator 100 is parallel to the optical axis of the outgoing light from the collimating lens 200 to simplify assembly and calibration.

[0141] To fully illustrate the overall performance of the fiber optic coupling device 1000, three sets of embodiments are provided below to verify its performance.

[0142] Example 1

[0143] In Example 1, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 of the optical isolator 100 of the coupling device 1000 are bonded together, and the parameters of the optical isolator 100 are shown in Table 1-1.

[0144] Table 1-1. Parameters of the optical isolator 100 provided in Example 1

[0145]

[0146] Placement: The optical isolator 100 is perpendicular to the incident light, and the surface normal of the optical isolator 100 is parallel to the optical axis (0° tilt angle).

[0147] Optical path sequence: Metasurface linear polarization layer 10 in front (facing the light source), metasurface quarter-wave plate layer 20 behind (facing fiber 400).

[0148] The performance of the fiber optic coupling device 1000 provided in Example 1 is shown in Table 1-2.

[0149] Table 1-2. Performance of the optical isolator 100 provided in Example 1

[0150]

[0151] Example 2

[0152] In Example 2, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 of the optical isolator 100 of the coupling device 1000 are fabricated on the same wafer using semiconductor technology. The parameters of the optical isolator 100 are shown in Table 2-1.

[0153] Table 2-1. Parameters of the optical isolator 100 provided in Example 2

[0154]

[0155] Placement: The optical isolator 100 is perpendicular to the incident light, and the surface normal of the optical isolator 100 is parallel to the optical axis (0° tilt angle).

[0156] Optical path sequence: Metasurface linear polarization layer 10 in front (facing the light source), metasurface quarter-wave plate layer 20 behind (facing fiber 400).

[0157] The performance of the fiber optic coupling device 1000 provided in Example 2 is shown in Table 2-2.

[0158] Table 2-2. Performance of the optical isolator 100 provided in Example 2

[0159]

[0160] Example 3

[0161] In Example 3, the metasurface linear polarization layer 10 and the metasurface quarter-wave plate layer 20 of the optical isolator 100 of the coupling device 1000 are fabricated on the same wafer using semiconductor technology, and the angle between the surface normal of the optical isolator 100 and the optical axis of the outgoing light of the collimating lens 200 is 8°. The parameters of the optical isolator 100 are shown in Table 3-1.

[0162] Table 3-1. Parameters of the optical isolator 100 provided in Example 3

[0163]

[0164] Placement: The optical isolator 100 is tilted relative to the incident light, and the surface normal of the optical isolator 100 intersects the optical axis (8° tilt angle).

[0165] Optical path sequence: Metasurface linear polarization layer 10 in front (facing the light source), metasurface quarter-wave plate layer 20 behind (facing fiber 400).

[0166] The performance of the fiber optic coupling device 1000 provided in Example 3 is shown in Table 3-2.

[0167] Table 3-2. Performance of the optical isolator 100 provided in Example 3

[0168]

[0169] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this application are indicated by the appended claims.

Claims

1. An optical isolator, characterized in that, include: A metasurface linear polarization layer is configured to filter out linearly polarized light with a target polarization state; A metasurface quarter-wave plate layer, the fast axis of which forms a target angle with the transmission direction of the metasurface linear polarization layer, is configured to convert the polarization state of incident light between linear polarization and circular polarization. The metasurface linear polarization layer and the metasurface quarter-wave plate layer are an integral structure.

2. The optical isolator according to claim 1, characterized in that, The metasurface linear polarization layer and the metasurface quarter-wave plate layer are bonded together.

3. The optical isolator according to claim 1, characterized in that, The metasurface linear polarization layer and the metasurface quarter-wave plate layer are fabricated on the same wafer using semiconductor technology.

4. The optical isolator according to claim 1, characterized in that, The metasurface linear polarization layer is also configured to reflect, absorb, or deflect linearly polarized incident light in non-target directions.

5. The optical isolator according to claim 4, characterized in that, The metasurface linear polarization layer is a metal wire grid array that transmits linearly polarized light of the target polarization state and reflects or absorbs linearly polarized incident light of non-target direction.

6. The optical isolator according to claim 4, characterized in that, The metasurface linear polarization layer is an array of all-dielectric micro / nano structures. The array of all-dielectric micro / nano structures transmits linearly polarized light of the target polarization state and reflects or absorbs linearly polarized incident light of non-target direction.

7. The optical isolator according to claim 4, characterized in that, The metasurface linear polarization layer is a phase gradient deflection type all-dielectric micro / nano structure array. The phase gradient deflection type all-dielectric micro / nano structure array transmits linearly polarized light of the target polarization state and transmits linearly polarized incident light of non-target direction.

8. An optical fiber coupling device, characterized in that, include: A collimating lens is used to collimate incident light. The optical isolator as described in any one of claims 1-7 is configured to receive light rays collimated by the collimating lens; A focusing lens is configured to receive light emitted from the optical isolator and couple it into an optical fiber.

9. The optical fiber coupling device according to claim 8, characterized in that, The surface normal of the optical isolator is tilted relative to the optical axis of the light emitted from the collimating lens.

10. The optical fiber coupling device according to claim 8, characterized in that, The surface normal of the optical isolator is parallel to the optical axis of the light emitted from the collimating lens.

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