A high-performance polarizer based on subwavelength grating structure

Abstract

The present invention discloses a high-performance polarizer based on a subwavelength grating structure, which includes a single-mode input waveguide, a tapered input structure, a middle vertical subwavelength grating structure, an upper horizontal subwavelength grating structure, a lower horizontal subwavelength grating structure, a tapered output structure, and a single-mode output waveguide. The output end of the single-mode input waveguide is connected to the input end of the tapered input structure. The output end of the tapered input structure is connected to the input end of the middle vertical subwavelength grating structure. The upper horizontal subwavelength grating structure and the lower horizontal subwavelength grating structure are respectively located on the upper and lower sides of the middle vertical subwavelength grating structure. The output end of the middle vertical subwavelength grating structure is connected to the input end of the tapered output structure. The output end of the tapered output structure is connected to the input end of the single-mode output waveguide. The present invention can achieve high-performance polarization of the TE fundamental mode within an ultra-wide bandwidth, and thus has important practical application values in many fields such as optical communication, optical sensing, and spectroscopy.

Description

Technical Field

[0001] The present invention relates to the field of integrated optoelectronic devices, and particularly to a high-performance polarizer based on a sub-wavelength grating structure. Background Art

[0002] With the continuous development of many applications such as 5G and cloud computing, the current optical communication bandwidth has gradually become difficult to meet the requirements, and the expansion of the optical communication bandwidth is imminent. Among various methods for expanding the bandwidth, the most promising method is to expand the optical communication band used, rather than being limited to the traditional C band (1530 - 1565 nm). Currently, many enterprises and companies, including China Mobile, China Telecom, Huawei, FiberHome Communications, etc., are working in this direction. In the case of expanding the optical communication band, the original optical communication devices that are only applicable to the C band will become inappropriate, and new devices that support ultra-wide bandwidth (covering all optical communication bands) and have high performance are urgently needed.

[0003] Due to the many advantages of the silicon photonics platform, such as high integration, strong optical field confinement, and CMOS compatibility, the research on high-performance devices based on the silicon photonics platform has been a research hotspot in the past decade. Although the high refractive index difference of the silicon photonics platform enables the device to be more compact, it also causes serious waveguide birefringence in the silicon waveguide, making many devices and applications only work under a certain polarization, so that the polarizer has become a key device. A device that filters out a certain polarization of light by a certain means and only retains a specific polarization of light is called a polarizer. In the research of the past decade, researchers have proposed various polarizer structures, including directional couplers, grating reflectors, etc., but their performance is still limited, mainly manifested as limited working bandwidth (difficult to cover all optical communication bands), too large loss (greater than 1 dB), insufficient polarization extinction ratio (only achieving a high extinction ratio in a small range), and the cladding material is mostly air (limited cladding selection). Summary of the Invention

[0004] In order to overcome the deficiencies of the existing structure and to meet the requirements of optical communication for large-bandwidth and high-performance devices, the object of the present invention is to provide a high-performance polarizer based on a sub-wavelength grating structure, with a working bandwidth covering all optical communication bands (a total bandwidth of 415 nm from 1260 - 1675 nm), an insertion loss less than 0.5 dB, and an extinction ratio greater than 20 dB or even 30 dB.

[0005] A high-performance polarizer based on a subwavelength grating structure, comprising a single-mode input waveguide, a tapered input structure, a middle vertical subwavelength grating structure, an upper horizontal subwavelength grating structure, a lower horizontal subwavelength grating structure, a tapered output structure, and a single-mode output waveguide; the output end of the single-mode input waveguide is connected to the input end of the tapered input structure; the output end of the tapered input structure is connected to the input end of the middle vertical subwavelength grating structure; the upper horizontal subwavelength grating structure and the lower horizontal subwavelength grating structure are respectively located on the upper and lower sides of the middle vertical subwavelength grating structure; the output end of the middle vertical subwavelength grating structure is connected to the input end of the tapered output structure; the output end of the tapered output structure is connected to the input end of the single-mode output waveguide.

[0006] The single-mode input waveguide and the single-mode output waveguide only support the transmission of TE and TM fundamental modes.

[0007] The tapered input structure is jointly composed of a tapered waveguide with a certain length and a gradually decreasing width and a subwavelength grating structure with a gradually increasing vertical length, and when the vertical length increases to the longitudinal length of the middle vertical subwavelength grating structure, it no longer changes; the tapered output structure is symmetric with the tapered input structure with respect to the middle vertical subwavelength grating structure, and their structural parameters are the same.

[0008] The horizontal period and silicon duty cycle of the subwavelength grating structure with a gradually increasing vertical length are the same as those of the middle vertical subwavelength grating structure; the middle vertical subwavelength grating structure has the same period, silicon duty cycle, and vertical length.

[0009] The upper horizontal subwavelength grating structure is located above the middle vertical subwavelength grating structure, and there are M subwavelength silicon waveguides from bottom to top. Each adjacent upper subwavelength silicon waveguide is shortened by the same length on both the left and right sides compared to the lower one; the lower horizontal subwavelength grating structure is symmetric with the upper horizontal subwavelength grating structure with respect to the middle vertical subwavelength grating structure, and their structural parameters and functions are the same.

[0010] For the high-performance polarizer based on the subwavelength grating structure, the periods and silicon duty cycles of the middle vertical subwavelength grating structure, the upper horizontal subwavelength grating structure, and the lower horizontal subwavelength grating structure are selected such that the refractive index felt by the TM fundamental mode in the upper horizontal subwavelength grating structure and the lower horizontal subwavelength grating structure is higher than that felt in the middle vertical subwavelength grating structure, so that the TM fundamental mode becomes a leaky mode and is lost; while ensuring the leakage of the TM fundamental mode, the refractive index felt by the TE fundamental mode in the upper horizontal subwavelength grating structure and the lower horizontal subwavelength grating structure is lower than that felt in the middle vertical subwavelength grating structure, ensuring that the TE fundamental mode remains a low-loss bound mode, thus achieving the purpose of TE fundamental mode polarization.

[0011] The described high-performance polarizer is based on a silicon-on-insulator platform, with a silicon substrate, a buried oxide layer of silicon dioxide, and a waveguide layer of silicon.

[0012] The described high-performance polarizer is based on a silicon-on-insulator platform, with a silicon substrate, a buried oxide layer of silicon dioxide, and a waveguide layer of silicon. The covering layer material is optional. The covering layer can be air, silicon dioxide, etc.

[0013] Advantages of the present invention:

[0014] (1) The present invention adopts a silicon-on-insulator platform, which has advantages such as a large refractive index difference, a high integration density, and CMOS compatibility.

[0015] (2) The present invention utilizes a subwavelength grating structure, which has the characteristic that the period is much smaller than the optical wavelength; therefore, the polarizer based on this structure has an ultra-compact structural size, which is beneficial to the needs of large-scale integration.

[0016] (3) Due to the anisotropic characteristic of the subwavelength grating structure being insensitive to wavelength, the polarizer can operate in an extremely large bandwidth range, such as covering all optical communication bands.

[0017] (4) Due to the anisotropic characteristic of the subwavelength grating structure being insensitive to the refractive index of the covering layer, the choice of the covering layer material can be determined according to actual applications during device design; for example, an air covering layer can be used for reducing process steps and for rapid testing needs, and a silicon dioxide covering layer can be used for protecting the device structure and for placing electrode structures. Description of the drawings

[0018] Figure 1 is a schematic structural diagram of a high-performance polarizer based on a subwavelength grating structure.

[0019] Figure 2 is a schematic cross-sectional diagram of the waveguide of a high-performance polarizer based on a subwavelength grating structure.

[0020] Figure 3 is an equivalent refractive index diagram of the subwavelength grating structure.

[0021] Figure 4 are the results of the insertion loss and polarization extinction ratio of a high-performance polarizer based on a subwavelength grating structure (with an air covering layer).

[0022] Figure 5 are the results of the insertion loss and polarization extinction ratio of a high-performance polarizer based on a subwavelength grating structure (with a silicon dioxide covering layer).

[0023] In the figure, there are single-mode input waveguide 1, tapered input structure 2, middle vertical sub-wavelength grating structure 3, upper horizontal sub-wavelength grating structure 4, lower horizontal sub-wavelength grating structure 5, tapered output structure 6, single-mode output waveguide 7, cladding 8, waveguide layer 9, buried oxide layer 10, and substrate 11. Detailed implementation mode

[0024] The present invention will be further described below in conjunction with the accompanying drawings and an implementation example of a high-performance polarizer based on a sub-wavelength grating structure.

[0025] Embodiment 1

[0026] Figure 1 It is a schematic structural diagram of a high-performance polarizer based on a sub-wavelength grating structure. The output end of the single-mode input waveguide 1 is connected to the input end of the tapered input structure 2; the output end of the tapered input structure 2 is connected to the input end of the middle vertical sub-wavelength grating structure 3; the upper horizontal sub-wavelength grating structure 4 and the lower horizontal sub-wavelength grating structure 5 are respectively located on the upper and lower sides of the middle vertical sub-wavelength grating structure 3; the output end of the middle vertical sub-wavelength grating structure 3 is connected to the input end of the tapered output structure 6; the output end of the tapered output structure 6 is connected to the input end of the single-mode output waveguide 7.

[0027] Figure 2 It is a schematic cross-sectional view of the waveguide of a high-performance polarizer based on a sub-wavelength grating structure. The substrate 11 is silicon, the buried oxide layer 10 is silicon dioxide with a thickness of 2 μm, the waveguide layer 9 is silicon with a thickness of 220 nm, and the cladding 8 is air in this embodiment.

[0028] The widths of the single-mode input waveguide 1 and the single-mode output waveguide 7 are 430 nm, so as to ensure that the waveguide only supports the TE fundamental mode and the TM fundamental mode in all optical communication bands, and avoid the losses and crosstalk caused by high-order modes.

[0029] The tapered input structure 2 is jointly composed of a tapered waveguide with a length of 4 μm and a width linearly decreasing from 430 nm to 100 nm and a sub-wavelength grating structure with a vertical length linearly increasing from 450 nm (each sub-wavelength structure increases by 50 nm), and when the vertical length increases to the vertical length of 1 μm of the middle vertical sub-wavelength grating structure 3, it will no longer change; the horizontal period Λ 1 = 200 nm and the silicon duty cycle η 1 = 0.4 are the same as those of the middle vertical sub-wavelength grating structure 3; this tapered input structure 2 can ensure that the TE fundamental mode evolves from the single-mode input waveguide with low loss and low reflection to the optical field modes supported by the middle vertical sub-wavelength grating structure 3, the upper horizontal sub-wavelength grating structure 4, and the lower horizontal sub-wavelength grating structure 5.

[0030] The middle vertical sub-wavelength grating structure 3 has exactly the same period, silicon duty cycle, and vertical length.

[0031] The upper horizontal sub-wavelength grating structure 4 is located above the middle vertical sub-wavelength grating structure 3, and its longitudinal period is Λ 2 = 200 nm, the silicon duty cycle is η 2 = 0.6, and there are M = 23 sub-wavelength silicon waveguides from bottom to top; in order to avoid the reflection and resonance of the optical field, the adjacent upper sub-wavelength silicon waveguide is shortened by the same length of 300 nm on both the left and right sides compared to the lower sub-wavelength silicon waveguide, and the length of the Mth structure is fixed at 200 nm; the lower horizontal sub-wavelength grating structure 4 is symmetric with the upper horizontal sub-wavelength grating structure 5 with respect to the middle vertical sub-wavelength grating structure 3, and their structural parameters and functions are exactly the same.

[0032] Figure 3 It is the equivalent refractive index diagram of the sub-wavelength grating structure. Figure 3 The calculation of the curve in is based on the equivalent refractive index formula of the sub-wavelength grating structure as follows:

[0033] ;

[0034] where n ∥ and n ⊥ are the equivalent refractive indices of the light wave when the polarization directions are parallel and perpendicular to the periodic arrangement direction of the sub-wavelength grating structure respectively; Λ is the period of the sub-wavelength grating structure, η is the silicon duty cycle; n Si is the refractive index of silicon, n clad is the refractive index of the cladding material.

[0035] Since the polarization directions of both the TE fundamental mode and the TM fundamental mode are parallel to the periodic arrangement direction of the middle vertical sub-wavelength grating structure 3, the refractive indices induced by them in this part of the structure are both n ∥m . Since the polarization direction of the TE fundamental mode is perpendicular to the periodic arrangement direction of the upper horizontal sub-wavelength grating structure 3 and the lower horizontal sub-wavelength grating structure 4, while the polarization direction of the TM mode remains parallel, the refractive indices induced by the TE fundamental mode and the TM fundamental mode in the upper horizontal sub-wavelength grating structure 3 and the lower horizontal sub-wavelength grating structure 4 are respectively n ⊥s and n ∥s , and there is n ∥m > n ⊥s andn ∥m < n ∥s Therefore, the TM fundamental mode will become a leaky mode and be lost, while the TE fundamental mode remains a bound mode and maintains low loss, thus achieving the purpose of polarization.

[0036] Figure 4 are the insertion loss and polarization extinction ratio results of a high-performance polarizer based on a subwavelength grating structure (the cladding is air). The horizontal axis is the wavelength, which covers all optical communication bands (a total of 415 nm from 1.26 - 1.675 μm). From Figure 4 It can be seen that in the calculated wavelength bands, the insertion loss is less than 0.34 dB and the extinction ratio is greater than 20 dB. The wavelength bands with an extinction ratio greater than 30 dB exceed 380 nm (1.26 - 1.64 μm).

[0037] Example 2

[0038] The difference between this example and Example 1 is that the cladding becomes silica, so as to reflect the characteristic that the polarizer has selectable cladding materials. Only the fine-tuned parameters relative to Example 1 and the final performance results are briefly described below.

[0039] Figure 2 is a schematic cross-section of the waveguide of a high-performance polarizer based on a subwavelength grating structure. The substrate 11 is silicon, the buried oxide layer 10 is silica with a thickness of 2 μm, the waveguide layer 9 is silicon with a thickness of 220 nm, and the cladding 8 is silica in this example. Obviously, for those skilled in the art, other materials can also be used for the cladding.

[0040] The tapered input structure 2 is composed of a tapered waveguide with a length of 4 μm and a width linearly decreasing from 430 nm to 100 nm and a subwavelength grating structure with a vertical length linearly increasing from 450 nm (each subwavelength structure increases by 50 nm). When the vertical length increases to the vertical length of the middle vertical subwavelength grating structure 3, which is 0.9 μm, it will no longer change.

[0041] The upper horizontal subwavelength grating structure 4 is located above the middle vertical subwavelength grating structure 3, and its longitudinal period is Λ 2 = 200 nm, the silicon duty cycle is η 2 = 0.6, and there are M = 26 subwavelength silicon waveguides from bottom to top.

[0042] Figure 5 are the insertion loss and polarization extinction ratio results of a high-performance polarizer based on a subwavelength grating structure (the cladding is silica). The horizontal axis is the wavelength, which covers all optical communication bands (a total of 415 nm from 1.26 - 1.675 μm). FromFigure 5 It can be seen that in the calculated wavelength bands, there are performances with an insertion loss less than 0.3 dB and an extinction ratio greater than 20 dB. The wavelength bands with an extinction ratio greater than 30 dB exceed 320 nm (1.26 - 1.58 μm).

[0043] The embodiments in the above description can be further combined or replaced. Moreover, the embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design idea of the present invention, various changes and improvements made by those of ordinary skill in the art to the technical solutions of the present invention all fall within the protection scope of the present invention. The protection scope of the present invention is given by the appended claims and any equivalents thereof.

Claims

1. A high-performance polarizer based on a sub-wavelength grating structure, characterized in that: It includes a single-mode input waveguide, a tapered input structure, a middle vertical sub-wavelength grating structure, an upper horizontal sub-wavelength grating structure, a lower horizontal sub-wavelength grating structure, a tapered output structure, and a single-mode output waveguide; the output end of the single-mode input waveguide is connected to the input end of the tapered input structure; the output end of the tapered input structure is connected to the input end of the middle vertical sub-wavelength grating structure; the upper horizontal sub-wavelength grating structure and the lower horizontal sub-wavelength grating structure are respectively located on the upper and lower sides of the middle vertical sub-wavelength grating structure; the output end of the middle vertical sub-wavelength grating structure is connected to the input end of the tapered output structure; the output end of the tapered output structure is connected to the input end of the single-mode output waveguide; The single-mode input waveguide and the single-mode output waveguide only support the transmission of TE and TM fundamental modes; The tapered input structure is jointly composed of a tapered waveguide with a certain length and a gradually decreasing width and a sub-wavelength grating structure with a gradually increasing vertical length, and when the vertical length increases to the longitudinal length of the middle vertical sub-wavelength grating structure, it no longer changes; the tapered output structure is symmetric with the tapered input structure about the middle vertical sub-wavelength grating structure, and their structural parameters are the same; The horizontal period and silicon duty cycle of the sub-wavelength grating structure with a gradually increasing vertical length are the same as those of the middle vertical sub-wavelength grating structure; the middle vertical sub-wavelength grating structure has the same period, silicon duty cycle, and vertical length.

2. The high-performance polarizer based on the sub-wavelength grating structure according to claim 1, wherein: The upper horizontal sub-wavelength grating structure is located above the middle vertical sub-wavelength grating structure, and there are M sub-wavelength silicon waveguides from bottom to top. The adjacent upper sub-wavelength silicon waveguide is shortened by the same length on both the left and right sides compared to the lower one; the lower horizontal sub-wavelength grating structure is symmetric with the upper horizontal sub-wavelength grating structure about the middle vertical sub-wavelength grating structure, and their structural parameters and functions are the same.

3. The high-performance polarizer based on the subwavelength grating structure according to claim 1, characterized in that: Select the period and silicon duty cycle of the middle vertical sub-wavelength grating structure, the upper horizontal sub-wavelength grating structure, and the lower horizontal sub-wavelength grating structure, so that the refractive index felt by the TM fundamental mode in the upper horizontal sub-wavelength grating structure and the lower horizontal sub-wavelength grating structure is higher than that felt in the middle vertical sub-wavelength grating structure, so that the TM fundamental mode becomes a leaky mode and is lost; while ensuring the leakage of the TM fundamental mode, make the refractive index felt by the TE fundamental mode in the upper horizontal sub-wavelength grating structure and the lower horizontal sub-wavelength grating structure lower than that felt in the middle vertical sub-wavelength grating structure, and ensure that the TE fundamental mode is still a low-loss bound mode, so as to achieve the purpose of polarizing the TE fundamental mode.

4. The high-performance polarizer based on the sub-wavelength grating structure according to claim 1, wherein: The high-performance polarizer is based on a silicon-on-insulator platform, with a silicon substrate, a buried oxide layer of silicon dioxide, and a waveguide layer of silicon.

5. The high-performance polarizer based on the subwavelength grating structure according to claim 4, characterized in that: The cladding is air.

6. The high-performance polarizer based on the sub-wavelength grating structure according to claim 4, characterized in that: The cladding is silicon dioxide.

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