An integrated optical mode converter

Abstract

The application discloses an integrated optical mode converter, and belongs to the technical field of integrated optical information processing, which comprises a polarization separation rotator and a waveguide surface holographic grating connected by adiabatic tapered waveguides, two adiabatic tapered waveguides are arranged and connected to two output ports of the polarization separation rotator and two input ports of the waveguide surface holographic grating respectively; integrated light is separated into two TE mode lights existing in different waveguides by the polarization separation rotator, and then coupled to the two input ports of the waveguide surface holographic grating through the two adiabatic tapered waveguides respectively, and orbital angular momentum light with different topological charges in free space is generated through the diffraction of the waveguide surface holographic grating; the polarization separation rotator and the waveguide surface holographic grating are combined, the conversion between the polarization mode light of the integrated light and the orbital angular momentum light of the free space light is realized, and an information interaction channel between the two is established.

Description

Technical Field

[0001] This invention relates to an integrated optical mode converter, belonging to the field of integrated optical information processing technology. Background Technology

[0002] In the development of integrated photonic chips based on the SOI platform, the polarization state of waveguide modes has attracted widespread attention. It can both introduce mode dispersion in waveguide transmission and be used to encode information. The large refractive index difference between Si (n=3.476) and SiO2 (n=1.444) makes integrated photonic devices highly sensitive to polarization, which has great potential applications in information processing based on polarization mode encoding. In order to obtain the desired polarization mode when solving the polarization sensitivity problem, polarization splitter rotators (PSRs) have been proposed. In recent years, various types of polarization splitter rotators have emerged, such as asymmetric directional polarization couplers (ADCs), asymmetric Y-shaped polarization splitters, and MMI polarization managers.

[0003] Free-space optical information differs from integrated photonic chips in many ways in information processing. To achieve compatibility between free-space optical and integrated optical information, it is necessary to establish connections and interoperability between them. Summary of the Invention

[0004] The purpose of this invention is to provide an integrated optical mode converter that enables mode conversion between integrated light and free space light.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] This invention provides an integrated optical mode converter, comprising a polarization separation rotator and a waveguide surface holographic grating connected by an adiabatic tapered waveguide. Two adiabatic tapered waveguides are provided, which are respectively connected to two output ports of the polarization separation rotator and two input ports of the waveguide surface holographic grating.

[0007] The integrated light is separated into two TE mode lights in different waveguides by a polarization splitter. The light is then coupled to the two input ports of the waveguide surface holographic grating by two adiabatic tapered waveguides. The light is then diffracted by the waveguide surface holographic grating to generate orbital angle light with different topological charges in free space.

[0008] Furthermore, the polarization separation rotator consists of a main waveguide and a side waveguide;

[0009] The input end of the main waveguide is a rectangular waveguide, and the coupling region is a partially etched tapered waveguide with gradually increasing etch width. The width W of the tapered waveguide is... tThe waveguide gradually decreases from W1 to W3. The end of the coupling region is a partially etched S-bend waveguide with a constant etching width of W3. A partially etched anti-phase cone waveguide is connected before the output end with a gradually decreasing etching width. Finally, the waveguide is restored to a rectangular waveguide at the output end.

[0010] The side waveguide is a rectangular waveguide, but at the output end of the side waveguide it changes into a tapered waveguide and the waveguide width gradually changes to the same width as the main waveguide.

[0011] Furthermore, the two output ports of the polarization separation rotator have the same width.

[0012] Furthermore, the output port of the polarization separation rotator and the input port of the waveguide surface holographic grating are matched and connected through an adiabatic tapered waveguide, which adiabatically transforms the TE mode light of the narrow waveguide into the TE mode light of the wide waveguide.

[0013] Furthermore, the striates used in the waveguide surface holographic grating are formed by the interference of time-reversed TE mode light in the waveguide and orbital angle measurement light with a topological charge of +1 perpendicular to the waveguide surface, with the beam waist of the orbital angle measurement light located on the waveguide surface.

[0014] Furthermore, the integrated optical mode converter is fabricated based on silicon waveguides and features high conversion efficiency and low loss in the wavelength range of 1530nm-1565nm, and can operate in the C-band.

[0015] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0016] This invention provides an integrated optical mode converter that combines a polarization splitter rotator and a waveguide surface holographic grating. The integrated light is split into two TE mode lights existing in different waveguides by the polarization splitter rotator, and then coupled to the two input ports of the waveguide surface holographic grating by two adiabatic tapered waveguides respectively. The diffraction effect of the waveguide surface holographic grating generates orbital angle measurement lights with different topological charges in free space, realizing the conversion between the polarization mode light of the integrated light and the orbital angle measurement light of the free space light, and realizing the mode conversion between the integrated light and the free space light, thereby establishing an information interaction channel between the two, providing a new approach for information encoding conversion between integrated photonic chips and free space light;

[0017] The present invention provides an integrated optical mode converter with a size in the micrometer range, which is compatible with integrated optical devices and has the characteristics of good integrability and scalability. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an integrated optical mode converter provided in an embodiment of the present invention;

[0019] Figure 2 This is a schematic diagram showing the variation of the effective refractive index of TM and TE mode light with waveguide width in the main waveguide and side waveguide provided in the embodiments of the present invention;

[0020] Figure 3 The PCE and ER provided in the embodiments of the present invention t ER r Figure showing the relationship between the etching width of the main waveguide of the polarization separation rotator and its variation.

[0021] Figure 4 The TE mode light transmittance (T) and extinction ratio (ER) of the adiabatic tapered waveguide provided in this embodiment of the invention are related to the tapered waveguide angle (θ). t Relationship diagram between )

[0022] Figure 5 This is a graph showing the relationship between the fidelity and diffraction efficiency of the vortex light generated by holographic grating diffraction on the waveguide surface and the grating size, provided in an embodiment of the present invention.

[0023] Figure 6 This is a graph showing the relationship between the fidelity of the generated orbital angle measurement light and the light wavelength when inputting TM and TE mode light according to an embodiment of the present invention.

[0024] Figure 7 This is a schematic diagram of the mode conversion process of the integrated optical mode converter provided in an embodiment of the present invention;

[0025] Figure 8 This is a field strength and phase diagram of OAM light generated by POAMC from input TM and TE mode light at a wavelength of 1550nm, provided in an embodiment of the present invention. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to illustrate the technical solution of the present invention more clearly, and should not be used to limit the scope of protection of the present invention.

[0027] like Figure 1 As shown in the figure, the integrated optical mode converter (POAMC) provided by the present invention is composed of a polarization separation rotator (PSR) and a waveguide surface holographic grating (WGSHG) organically combined through an adiabatic tapered waveguide.

[0028] The mode converter is made of Si material and is placed on a SiO2 substrate. The thickness of the Si material is 0.22 μm.

[0029] The polarization separation rotator includes a main waveguide and a side waveguide. The main waveguide has a width of 0.6 μm, and the side waveguide has a width of 0.3 μm. The gradient etching depth of the main waveguide is 0.07 μm, and the etching width gradually changes from 0.6 μm to 0.375 μm with a gradient length of 7.5 μm. The two output ports of the polarization separation rotator have the same width of 0.6 μm.

[0030] Two adiabatic tapered waveguides are provided, which are respectively connected to the two output ports of the polarization separation rotator and the two input ports of the waveguide surface holographic grating; the tapered angle of the adiabatic tapered waveguide is 10° and the length is 14.86μm.

[0031] The holographic grating on the waveguide surface has a length and width of 3.2 μm, an etching depth of 0.07 μm, and an effective refractive index of 2.58.

[0032] In the polarization separation rotator section, a tapered etched waveguide is used to shorten the length of the coupling region. The incident integrated light in TM and TE modes is separated into two waveguides (main waveguide and side waveguide) after passing through the polarization separation rotator. The output waveguide width is 0.6 μm, and both are TE mode light. The input TM mode light is coupled to the side waveguide (WG2) and converted into TE mode light with a conversion efficiency of 82%, while the input TE mode light continues to propagate along the main waveguide (WG1).

[0033] The TE mode light output from the two ports of the polarization separator is coupled to the two input ports (port1, port2) of the waveguide surface holographic grating through two adiabatic tapered waveguides. This efficiently and adiabatably transforms the TE mode light output from the two ports of the polarization separator into TE mode light that matches the two input ports of the waveguide surface holographic grating, with a conversion efficiency of 98.7%. At the same time, the TM mode light is filtered out, improving the fidelity and extinction ratio of the vortex light generated by the waveguide surface holographic grating.

[0034] TE mode light passing through two adiabatic conical waveguides is then diffracted by a holographic grating on the waveguide surface to generate orbital angle light (vortex light) with different topological charges (l = -1 and l = +1) in free space; the generated vortex light has high fidelity.

[0035] The present invention provides an integrated optical mode converter. When the input integrated optical wavelength λ = 1550nm, the fidelity of the OAM light generated by the TM and TE mode optical inputs is 0.9 and 0.82, respectively. When the input integrated optical wavelength λ = 1540nm-1590nm, the fidelity of the OAM light generated by the TM and TE mode optical inputs is higher than 0.5, and the operating bandwidth is about 50nm.

[0036] The fringes used in the waveguide surface holographic grating are formed by the interference of time-reversed TE mode light in the waveguide and orbital angle measurement light with a topological charge of +1 perpendicular to the waveguide surface. The beam waist radius of the orbital angle measurement light is 1.5 μm and it is located on the waveguide surface.

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific structural design parameters and the accompanying drawings.

[0038] Regarding the design of PSR:

[0039] Figure 1 This is a schematic diagram of an integrated optical mode converter, with a main waveguide width W1 = 0.6 μm and a height h = 0.22 μm.

[0040] The working principle of PSR is based on the coupling efficiency of the coupled region, and the formula for calculating the coupling efficiency is as follows:

[0041]

[0042] Where γ is the coupling efficiency between WG1 and WG2, Lc is the length of the coupling region, Δβ is the difference in propagation vectors between the two optical waveguides, which can be expressed as Δβ=2π(n1-n2) / λ, where n1 and n2 are the effective refractive indices of WG1 and WG2, respectively; κ is the coupling coefficient, which can be expressed as κ=π(n e -n o ) / λ, where n e and n o Δβ serves as the effective refractive index for the even-symmetric and odd-symmetric modes of the waveguide, respectively. Δβ has a significant impact on the coupling efficiency between waveguides. When Δβ is small, effective coupling can be achieved between WG1 and WG2; conversely, when Δβ is large, the coupling between WG1 and WG2 is very small.

[0043] Figure 2 The diagram illustrates the variation of the effective refractive index of TM and TE mode light with waveguide width in the main waveguide and side waveguide. Based on the coupling efficiency calculation formula, to obtain a smaller Δβ and a more ideal coupling efficiency, the width of the side waveguide WG2 is chosen as W2 = 0.3 μm, h = 0.22 μm, and the gap between the two waveguides is g = 0.1 μm. A tapered etched waveguide structure is designed in the coupling region, with an etching depth h. e =0.07μm, etching length is 7.5μm, etching width W t It changed from 0.6 μm to 0.375 μm.

[0044] Two mode beams, one with TM polarization and the other with TE polarization, enter the PSR structure from the main waveguide (WG1). The wavelength of the incident light is chosen to be λ = 1550 nm. After passing through a 7.5 μm coupling region, the input TM mode light is coupled into WG2 and converted into TE mode light, while the input TE mode light continues to propagate along WG1 without undergoing mode conversion. Simultaneously, an adiabatic tapered waveguide with an angle of 10° and a length of 1.71 μm is connected after the side waveguide. The purpose is to change the width of the side waveguide from 0.3 μm to 0.6 μm, thereby ensuring that the modes at the two output ports of the PSR are identical. Polarization conversion efficiency (PCE) and polarization extinction ratio (ER) are also considered. t ), path extinction ratio (ER) r ) is an important parameter indicator of PSR performance, which can be defined as:

[0045]

[0046]

[0047]

[0048] Among them, P eo P is the TE mode optical power output from the main waveguide or side waveguide. in It is the total input power, P t It is the total output power of the main waveguide or side waveguide, P d It corresponds to P t The total output power of the other waveguide; Figure 3 Based on PCE, ER t ER r The calculation formulas give the PCE and ER of the PSR part. t ER r The graph shows the relationship between the etching width of the main waveguide of the polarization separator and the actual polarization separation rotator. When W3 = 0.375 μm, for TM mode light input, it has PCE = 82% and ER... t =18.1dB, ER r =10.1dB, for TE mode optical input, with PCE=98.5%, ER t =22.7dB, ER r =24.9dB.

[0049] For the design of adiabatic tapered waveguides:

[0050] The two output ports of the PSR are connected to two adiabatic tapered waveguides, the width of which gradually changes from 0.6 μm to 3.2 μm. Their function is to match the output port of the PSR (width of 0.6 μm) with the input port of the WGSHG (width of 3.2 μm), while effectively filtering out the TM mode, improving the diffraction efficiency of the WGSHG and the fidelity of the generated vortex light.

[0051] The angle of the adiabatic tapered waveguide is one of the important factors affecting the transmission of TE mode light. Figure 4 The TE mode transmittance (T) and extinction ratio (ERt) of the adiabatic tapered waveguide are given in relation to the tapered waveguide angle (θ). t The relationship between the two is shown in the diagram. At the same time, the length of the adiabatic tapered waveguide will increase as the angle decreases, thereby increasing the size of the POAMC. Taking all factors into consideration, the angle of the adiabatic tapered waveguide is selected as 10°.

[0052] For the design of waveguide surface holographic gratings:

[0053] The fringes used in the waveguide surface holographic grating are formed by the interference of time-reversed TE mode light in the waveguide and orbital angle measurement light with a topological charge of +1 perpendicular to the waveguide surface. The beam waist radius of the orbital angle measurement light is 1.5±μm and it is located on the waveguide surface.

[0054] When TE mode light is incident on the holographic grating on the waveguide surface, a target OAM beam with l = +1 can be obtained through diffraction. Taking the center of WGSHG as the origin of the coordinate system, the two TE mode lights propagating in different directions (input from port1 and port2 respectively) can be represented as E1 = A1(y, z)e -ikx And E2=A2(y,z)e ikx Where A1(y, z) and A2(y, z) are the amplitudes of the TE mode light in the yz cross section, and k is the wave vector in the x direction; because they propagate in opposite directions, E1 and E2 have opposite phases; when they propagate to the waveguide surface holographic grating, the OAM beam generated by the diffraction effect of the waveguide surface holographic grating can be expressed as GE1 = B1e -ilθ and GE2 = B2e ilθ Where B1 and B2 are the amplitudes of the OAM beam, and θ is the azimuth angle; different input waveguide polarization modes generate diffracted beams with different OAM values; to determine the quality of the generated OAM beam, fidelity is introduced, which can be expressed as:

[0055]

[0056] Among them, E t(x,y,z) and E(x,y,z) are the amplitudes of the target vortex light and the vortex light generated by grating diffraction, respectively, and * indicates conjugate.

[0057] Figure 5 The graphs showing the relationship between the fidelity and diffraction efficiency of vortex light generated by holographic grating diffraction on a waveguide surface and the grating size are presented. It can be seen that the fidelity initially increases with increasing grating size, reaching a certain level when the grating size is 3.2 × 3.2 μm. 2 At a certain point, the fidelity reaches its maximum value of 0.9, and then gradually decreases; simultaneously, the diffraction efficiency of the grating initially increases with the increase of the grating size, and finally tends to a stable value; the grating size is 3.2 × 3.2 μm. 2 The corresponding diffraction efficiency is 0.15; considering both factors, the final size of WGSHG is determined to be 3.2 × 3.2 μm. 2 .

[0058] Finally, under the parameters determined in the above discussion, Figure 8 The field strength and phase of the OAM light generated by POAMC after inputting TM and TE mode light at an incident wavelength of 1550nm are given. For TM / TE mode light input, the fidelity of the generated OAM light is 0.9 / 0.82. Figure 6 The graph shows the relationship between the fidelity of the generated OAM light and the input wavelength when using TM and TE mode light inputs. It can be seen that for TM mode light input, the fidelity is greater than 0.5 in the wavelength range of 1540-1590nm, and for TE mode light input, the fidelity is greater than 0.5 in the wavelength range of 1540-1610nm. Considering both TE and TM modes, the operating bandwidth of POAMC is 50nm (1540-1590nm).

[0059] This invention proposes an on-chip mode converter (POAMC) composed of a PSR and a WGSHG, operating at a wavelength of λ = 1550 nm, for converting integrated optical waveguide polarization mode light (TM, TE) into free-space OAM mode light (l = -1 and l = +1). For TM mode light input, the conversion efficiency is approximately 12.14%, and the extinction ratio is approximately 21.18 dB; for TE mode light input, the conversion efficiency is approximately 14.57%, and the extinction ratio is approximately 23.26 dB. The fidelity of the generated OAM light with l = -1 and l = +1 is 0.9 and 0.82, respectively. The POAMC operates at a bandwidth of 50 nm (1540 nm - 15490 nm). The proposed POAMC can realize mode conversion between integrated photonic chips and free space, providing a new approach for mode encoding conversion in integrated optical information processing. Figure 7 The process of converting integrated light into OAM light was demonstrated.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An integrated optical mode converter, characterized in that, It includes a polarization separation rotator and a waveguide surface holographic grating connected by an adiabatic tapered waveguide. There are two adiabatic tapered waveguides, which are respectively connected to the two output ports of the polarization separation rotator and the two input ports of the waveguide surface holographic grating. The integrated light is separated into two TE mode lights in different waveguides by a polarization splitter, and then coupled to the two input ports of the waveguide surface holographic grating by two adiabatic tapered waveguides respectively. The light is then generated by the diffraction of the waveguide surface holographic grating to generate orbital angle light with different topological charges in free space. The polarization separation rotator consists of a main waveguide and a side waveguide; The input end of the main waveguide is a rectangular waveguide, the coupling region is a partially etched conical waveguide with the etching width gradually increasing, the end of the coupling region is a partially etched S-bend waveguide with the etching width remaining constant, and a partially etched anti-phase conical waveguide is connected in front of the output end with the etching width gradually decreasing. Finally, the output end is restored to a rectangular waveguide. The side waveguide is a rectangular waveguide, but at the output end of the side waveguide it changes into a tapered waveguide and the waveguide width gradually changes to the same width as the main waveguide.

2. The integrated optical mode converter according to claim 1, characterized in that, The two output ports of the polarization separation rotator have the same width.

3. An integrated optical mode converter according to claim 1, characterized in that, The output port of the polarization separation rotator and the input port of the waveguide surface holographic grating are matched and connected through an adiabatic tapered waveguide, which adiabatically transforms the TE mode light of the narrow waveguide into the TE mode light of the wide waveguide.

4. An integrated optical mode converter according to claim 1, characterized in that, The holographic grating on the waveguide surface uses fringes formed by the interference of time-reversed TE mode light in the waveguide and orbital angle measurement light with a topological charge of +1 perpendicular to the waveguide surface, with the beam waist of the orbital angle measurement light located on the waveguide surface.

5. An integrated optical mode converter according to claim 1, characterized in that, The integrated optical mode converter is based on silicon waveguide and features high conversion efficiency and low loss in the wavelength range of 1530nm-1565nm, and can operate in the C-band.

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