TM mode polarizer based on cascaded antisymmetric multi-mode bragg grating structure

Abstract

The application discloses a TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure, which comprises an input waveguide group, a first grating, a connecting waveguide group, a second grating and an output waveguide group which are sequentially connected along a transmission direction, a plurality of columns of period units are arranged in the first grating and the second grating respectively, a displacement difference is arranged between two adjacent columns of period units in a horizontal direction, and the periods of the first grating and the second grating are not equal. The application has the advantages of compact structure, large working bandwidth, low insertion loss in an effective working bandwidth and high extinction ratio. Two antisymmetric multimode Bragg gratings with different periods are cascaded to convert input TE0 mode signal light into reversely transmitted TE1 mode, the reversely transmitted TE1 mode is leaked into a cladding through a tapered waveguide, input TM0 mode signal light is directly transmitted through the waveguide without being affected, and finally, low-insertion-loss, high-extinction-ratio and large-bandwidth TM0 mode light signals are obtained at an output end of a single-mode output waveguide.

Description

Technical Field

[0001] This invention relates to a TM mode polarizer, and more particularly to a TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure. Background Technology

[0002] Photonic integrated circuits (PICs) based on silicon photonics have attracted close attention from researchers in related fields in recent years due to their ability to realize small-size, low-power on-chip ultra-compact photonic integrated circuits. Silicon-based optical waveguide devices, as an important research direction in silicon photonics, have developed rapidly, with various novel functional devices constantly being reported.

[0003] As an important silicon-based optical waveguide device, polarizers are currently designed primarily using structures such as plasmonic waveguides, Bragg gratings, and directional couplers. Existing polarizers generally suffer from high insertion loss, low extinction ratio, and narrow bandwidth. Devices based on Bragg grating structures exhibit high-power reflected light, which reduces the stability of the entire photonic integrated system. Therefore, it is essential to design a small-size, low-insertion-loss, high-extinction-ratio, wide-bandwidth, and easily fabricated polarizer to improve the overall performance of photonic integrated systems. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to propose a TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure, which has a simple structure, compact size, high extinction ratio, low insertion loss, large operating bandwidth, and is easy to fabricate.

[0005] Technical solution: The present invention includes an input waveguide group, a first grating, a connecting waveguide group, a second grating and an output waveguide group connected sequentially along the transmission direction. The first grating and the second grating are respectively provided with multiple columns of periodic units. Adjacent columns of periodic units are provided with a displacement difference in the horizontal direction, and the periods of the first grating and the second grating are not equal.

[0006] The input waveguide group includes a single-mode input waveguide and a tapered input waveguide. The left end of the tapered input waveguide is connected to the single-mode input waveguide, and the right end is connected to the first grating.

[0007] The tapered input waveguide widens along the transmission direction, with the left side supporting only the fundamental mode and the right side supporting higher-order transverse electric modes, thereby cutting off the TE1 mode that is transmitted in reverse in the first antisymmetric multimode Bragg grating.

[0008] The connecting waveguide group includes a first tapered waveguide and a second tapered waveguide. The left end of the first tapered waveguide is connected to a first grating, and the right end is connected to a second tapered waveguide. The right end of the second tapered waveguide is connected to a second grating.

[0009] The second tapered waveguide widens along the transmission direction, supporting only the fundamental mode on the left and higher-order transverse electric modes on the right, thereby cutting off the TE1 mode that is transmitted in reverse in the second antisymmetric multimode Bragg grating.

[0010] The output waveguide group includes a tapered output waveguide and a single-mode output waveguide. The left end of the tapered output waveguide is connected to the second grating, and the right end is connected to the single-mode output waveguide.

[0011] The periodic units in the first grating have the same period and the same duty cycle; the periodic units in the second grating have the same period and the same duty cycle.

[0012] The light output from the single-mode input waveguide is a signal light including TE0 and TM0 modes. When the input TE0 mode satisfies the phase matching condition in the first grating and the second grating respectively, the forward-propagating TE0 mode will be converted into the reverse-propagating TE1 mode, and the reverse-propagating TE1 mode will be completely leaked into the cladding.

[0013] The signal light in the TM0 mode does not meet the phase condition and can be output from the single-mode output waveguide, thereby realizing the polarization function of TE0 mode cutoff and TM0 mode pass-through.

[0014] Both the first grating and the second grating are antisymmetric multimode Bragg gratings.

[0015] Beneficial effects: This invention features a compact structure, a large operating bandwidth, and low insertion loss and high extinction ratio within the effective operating bandwidth. It employs a cascaded antisymmetric multimode Bragg grating with different periods to convert the input TE0 mode signal light into a reverse-transmitted TE1 mode. The reverse-transmitted TE1 mode leaks into the cladding through a tapered waveguide, while the input TM0 mode signal light passes directly through the waveguide unaffected. Ultimately, a TM0 mode optical signal with low insertion loss, high extinction ratio, and large bandwidth is obtained at the output of the single-mode output waveguide. The fabrication process is simple and compatible with existing CMOS processes, requiring only one photolithography and etching step, making it easy to fabricate, integrate, and expand. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of the present invention;

[0017] Figure 2 This is a cross-sectional view of the single-mode input waveguide of the present invention;

[0018] Figure 3 The transmission characteristics of the optical signal when passing through the polarizer are related to the operating wavelength, wherein (a) is the relationship between the transmittance / reflectance of the TEO mode and the operating wavelength, and (b) is the relationship between the transmittance / reflectance of the TMO mode and the operating wavelength.

[0019] Figure 4 The diagram shows the electric field distribution along the transmission direction of the optical signal passing through the polarizer in this invention. (a) shows the electric field distribution of the TE0 mode at a wavelength of 1450 nm; (b) shows the electric field distribution of the TM0 mode at 1450 nm; (c) shows the electric field distribution of the TE0 mode at a wavelength of 1550 nm; (d) shows the electric field distribution of the TM0 mode at 1550 nm; (e) shows the electric field distribution of the TE0 mode at a wavelength of 1650 nm; and (f) shows the electric field distribution of the TM0 mode at 1650 nm. Detailed Implementation

[0020] The invention will now be further described with reference to the accompanying drawings.

[0021] like Figure 1 As shown, the present invention includes a single-mode input waveguide 1, a tapered input waveguide 2, a first antisymmetric multimode Bragg grating 3, a first tapered waveguide 4, a second tapered waveguide 5, a second antisymmetric multimode Bragg grating 6, a tapered output waveguide 7, and a single-mode output waveguide 8. The single-mode input waveguide 1, tapered input waveguide 2, first antisymmetric multimode Bragg grating 3, first tapered waveguide 4, second tapered waveguide 5, second antisymmetric multimode Bragg grating 6, tapered output waveguide 7, and single-mode output waveguide 8 are all disposed on a substrate 9. Figure 2 As shown, the outer layer 10 is wrapped around the outside to make the overall height of the above components the same.

[0022] The right port of the single-mode input waveguide 1 is connected to the left port of the tapered input waveguide 2, and the height of the connection is equal. The right port of the tapered input waveguide 2 is connected to the left port of the first antisymmetric multimode Bragg grating 3, and the height of the connection is equal. The right port of the first antisymmetric multimode Bragg grating 3 is connected to the left port of the first tapered waveguide 4, and the height of the connection is equal. The right port of the first tapered waveguide 4 is connected to the left port of the second tapered waveguide 5, and the height of the connection is equal. The right port of the second tapered waveguide 5 is connected to the left port of the second antisymmetric multimode Bragg grating 6, and the height of the connection is equal. The right port of the second antisymmetric multimode Bragg grating 6 is connected to the left port of the tapered output waveguide 7, and the height of the connection is equal. The right port of the tapered output waveguide 7 is connected to the left port of the single-mode output waveguide 8, and the height of the connection is equal.

[0023] The tapered input waveguide 2 gradually widens from left to right, with the left side supporting only the fundamental mode and the right side supporting higher-order transverse electric modes, thus cutting off the TE1 mode propagating in the reverse direction in the first antisymmetric multimode Bragg grating 3; the second tapered waveguide 5 gradually widens from left to right, with the left side supporting only the fundamental mode and the right side supporting higher-order transverse electric modes, thus cutting off the TE1 mode propagating in the reverse direction in the second antisymmetric multimode Bragg grating 6.

[0024] The first antisymmetric multimode Bragg grating 3 is formed by etching multiple rows of periodic cells with the same period and duty cycle in the center of the multimode waveguide. These periodic cells can be rectangular holes, circular holes, etc. In this embodiment, two rows of rectangular holes are etched within the first antisymmetric multimode Bragg grating 3. Figure 1 As shown. There is a certain gap between the two columns of rectangular holes, which only needs to ensure that the loss of the passing TM0 mode is minimized. The two columns of rectangular holes are displaced by half a cycle in the horizontal direction. This achieves the conversion of the TE0 mode, which meets the phase-matching condition in the first Bragg grating, into the reverse-propagating TE1 mode. The second Bragg grating is also formed by etching two columns of periodic units with the same period and duty cycle in the center of the multimode waveguide. The periodic units can be rectangular holes, circular holes, etc. In this embodiment, the periodic units are rectangular holes, such as... Figure 1 As shown, there is a certain interval between the two columns of rectangular apertures, with a horizontal displacement of half a cycle, which realizes the conversion of the TE0 mode, which meets the phase matching condition in the second Bragg grating, into the reverse-transmission TE1 mode. The first and second Bragg gratings have different periods but the same duty cycle.

[0025] Signal light containing TE0 and TM0 modes enters from the left input end of single-mode input waveguide 1, passes through tapered input waveguide 2, and enters the first antisymmetric multimode Bragg grating 3. When the input TE0 mode satisfies the phase matching condition in the first antisymmetric multimode Bragg grating 3, the forward-propagating TE0 mode will be converted into the reverse-propagating TE1 mode and enter the tapered input waveguide 2. Since the left end of the tapered input waveguide 2 does not support higher-order TE1 modes, the reverse-propagating TE1 mode will completely leak from the tapered input waveguide 2 into the cladding. When the input TE0 mode satisfies the phase matching condition in the second antisymmetric multimode Bragg grating 6, the forward-propagating TE0 mode will be converted into the reverse-propagating TE1 mode and enter the second tapered waveguide 5. Since the left end of the second tapered waveguide 5 does not support higher-order TE1 modes, the reverse-propagating TE1 mode will completely leak from the second tapered waveguide 5 into the cladding. The input TM0 mode does not meet the phase condition, so it can pass through unaffected and output from the right output terminal of the single-mode output waveguide 8, thus realizing the polarization function of TE0 mode cutoff and TM0 mode pass-through. Compared with other TM mode pass-through polarizers, this embodiment has the advantages of simple structure, compact size, low internal reflected light power, low insertion loss, high extinction ratio, large bandwidth, and simple fabrication process.

[0026] The transmission characteristics of the input optical signal in the aforementioned polarizer are as follows: Through the optimized design of the period and structural dimensions of the first antisymmetric multimode Bragg grating 3 and the second antisymmetric multimode Bragg grating 6, the input TE0 mode optical signal with wavelength λ1 satisfies the phase matching condition in the first antisymmetric multimode Bragg grating 3. The input TE0 mode optical signal with wavelength λ2 satisfies the phase-matching condition in the second antisymmetric multimode Bragg grating 6. In the formula, Λ1 and Λ2 are the periods of the first antisymmetric multimode Bragg grating 3 and the second antisymmetric multimode Bragg grating 6, respectively. and These are the effective refractive indices of TE0 mode and TE1 mode in the first antisymmetric multimode Bragg grating 3, respectively. and These are the effective refractive indices of TE0 mode and TE1 mode in the second antisymmetric multimode Bragg grating 6, respectively.

[0027] Figure 3 This paper presents the transmission characteristics of the input TE0 and TM0 modes in the cascaded antisymmetric multimode Bragg grating polarizer of this invention, relating them to wavelength. As shown in the figure, the transmittance of the TE0 mode is below -40 dB in the wavelength range of 1410 nm to 1710 nm. In the wavelength range of 1350 nm to 1750 nm, the transmittance of the TM0 mode is greater than -1.68 dB. The device achieves a TM0 insertion loss of less than 1 dB, an extinction ratio of more than 40 dB, and a bandwidth of 275 nm in the wavelength range of 1435 nm to 1710 nm. The extinction ratio and operating bandwidth are significantly higher than those of currently reported polarizers, demonstrating excellent device performance.

[0028] Figure 4 The figure shows the electric field distribution of TE0 and TM0 modes at different input wavelengths in the device of this invention. As can be seen from the figure, the TE0 mode is mainly cut off by the first antisymmetric multimode Bragg grating and the tapered input waveguide at 1450 nm and 1550 nm, while at 1650 nm, the TE0 mode is mainly cut off by the second antisymmetric multimode Bragg grating and the second tapered waveguide; while TM0 modes at different wavelengths can pass directly through the device. This polarizer has advantages such as small size, low insertion loss, high extinction ratio, large operating bandwidth, and simple fabrication process.

Claims

1. A TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure, characterized in that, The device includes an input waveguide group, a first antisymmetric multimode Bragg grating, a connecting waveguide group, a second antisymmetric multimode Bragg grating, and an output waveguide group connected sequentially along the transmission direction. The first and second antisymmetric multimode Bragg gratings each have two columns of periodic units, which have a displacement difference in the horizontal direction. The periods of the first and second antisymmetric multimode Bragg gratings are not equal. The input waveguide group includes a single-mode input waveguide and a tapered input waveguide. The light output from the single-mode input waveguide is a signal light including TE0 and TM0 modes. When the input TE0 mode satisfies the phase matching condition in the first antisymmetric multimode Bragg grating and the second antisymmetric multimode Bragg grating, the forward-propagating TE0 mode will be converted into the reverse-propagating TE1 mode. The reverse-propagating TE1 mode leaks into the cladding through the tapered waveguide. The signal light of the TM0 mode does not satisfy the phase condition and can be output from the single-mode output waveguide. The connecting waveguide group includes a first tapered waveguide and a second tapered waveguide. The left end of the first tapered waveguide is connected to a first antisymmetric multimode Bragg grating, and the right end is connected to the second tapered waveguide. The right end of the second tapered waveguide is connected to a second antisymmetric multimode Bragg grating. The right side of the second tapered waveguide supports higher-order transverse electric modes, thereby cutting off the TE1 mode that is transmitted in reverse in the second antisymmetric multimode Bragg grating.

2. The TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure according to claim 1, characterized in that, The left end of the tapered input waveguide is connected to the single-mode input waveguide, and the right end is connected to the first antisymmetric multimode Bragg grating.

3. A TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure according to claim 2, characterized in that, The tapered input waveguide widens along the transmission direction, with the left side supporting only the fundamental mode and the right side supporting higher-order transverse electric modes.

4. A TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure according to claim 1, characterized in that, The second tapered waveguide widens along the transmission direction, with the left side supporting only the fundamental mode and the right side supporting higher-order transverse electric modes.

5. A TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure according to claim 1, characterized in that, The output waveguide group includes a tapered output waveguide and a single-mode output waveguide. The left end of the tapered output waveguide is connected to a second antisymmetric multimode Bragg grating, and the right end is connected to the single-mode output waveguide.

6. A TM mode polarizer based on a cascaded antisymmetric multimode Bragg grating structure according to claim 1, characterized in that, The periodic cells in the first antisymmetric multimode Bragg grating have the same period and the same duty cycle; the periodic cells in the second antisymmetric multimode Bragg grating have the same period and the same duty cycle.

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