Directional coupling type on-chip polarizing structure based on lithium niobate film

Abstract

The invention belongs to the field of on-chip lithium niobate film optical waveguide devices, and provides a directional coupling type on-chip polarization structure based on a lithium niobate film, which comprises a silicon substrate, a silicon dioxide substrate, a lithium niobate rectangular waveguide and a silicon dioxide buffer layer which are sequentially stacked from bottom to top. The lithium niobate rectangular waveguide comprises a main waveguide and a polarizing waveguide. According to the coupling type on-chip polarization structure, for light beams input into the optical waveguide, the TM mode can be transmitted, the TE mode can be cut off, the coupling type on-chip polarization structure can serve as the input end of a lithium niobate film optical waveguide electro-optical modulator or an electric field sensor, and waveguide single-mode single-polarization transmission is achieved.

Description

Technical Field

[0001] The invention belongs to the field of on-chip lithium niobate thin film optical waveguide devices, and in particular relates to a directional coupling on-chip polarizing structure based on the lithium niobate thin film. Background Art

[0002] With the development of electronic information, the demand for electromagnetic environment measurement is increasing. The traditional antenna plus spectrum analyzer measurement method can no longer meet the sensing and measurement needs of small space, wide bandwidth, and low disturbance. The MZ interferometer electric field sensor based on lithium niobate optical waveguide has received widespread attention due to its small size, wide bandwidth, low disturbance, and accurate measurement.

[0003] The traditional lithium niobate optical waveguide MZ interferometer electric field sensor uses a proton exchange process to etch a waveguide structure on the lithium niobate body material. Its advantages are low attenuation and low cost. In addition, since the proton exchange optical waveguide can naturally form single polarization transmission, no external polarizer is required. However, the optical waveguide made by proton exchange has a low refractive index difference (about 0.01), poor waveguide beam optical capacity, and a wide waveguide line width (single-mode width of about 6μm), which leads to a wide electrode spacing on the waveguide, low electrode gain, and poor sensor sensitivity.

[0004] With the maturity and commercialization of lithium niobate thin film technology, the realization of ridge-type and rectangular lithium niobate waveguides has become possible. Compared with the optical waveguide formed by proton exchange, the core layer and substrate of the lithium niobate thin film optical waveguide can form a refractive index difference of about 0.7, which greatly improves the light beaming ability of the optical waveguide and reduces the waveguide width to less than 1μm. At the same time, it can greatly reduce the electrode spacing and improve the electrode gain and sensitivity. In addition, due to the improvement of electrode gain, the half-wave electric field of the sensor can be greatly reduced, thereby achieving a predetermined modulation depth with a shorter waveguide structure and expanding the working bandwidth of the sensor.

[0005] However, since polarization modes in two directions can be transmitted in the lithium niobate thin film optical waveguide, the equivalent refractive indexes of the two polarization modes are different, so the corresponding linear working points are different, which will lead to reduced sensor working efficiency. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention proposes a directional coupling on-chip polarization structure based on lithium niobate film. For the light beam in the input optical waveguide, the TM mode can be transmitted, the TE mode will be cut off, and the isolation is 20dB. The coupled on-chip polarization structure can be connected to the lithium niobate film optical waveguide electro-optical modulator or electric field sensor as an input end to achieve single polarization input to eliminate the influence of polarization on the sensor.

[0007] The technical solution of the present invention is as follows:

[0008] A directional coupling on-chip polarization structure based on lithium niobate film, comprising a silicon substrate, a silicon dioxide substrate, a lithium niobate rectangular waveguide and a silicon dioxide buffer layer stacked in sequence from bottom to top;

[0009] The lithium niobate rectangular waveguide comprises a main waveguide and a polarizing waveguide.

[0010] Preferably, the polarizing waveguide includes a straight waveguide in a coupling section, an S-shaped waveguide in a curved section, and a straight waveguide in an output section.

[0011] Preferably, the width of the main waveguide is smaller than the width of the polarizing waveguide.

[0012] Preferably, the distance between the main waveguide and the polarizing waveguide is 0.5 μm.

[0013] Preferably, the width of the main waveguide is 1 μm, the width of the polarizing waveguide is 2.1 μm, and the floating range does not exceed 1 dB.

[0014] Preferably, the thickness of the silicon substrate is 0.5 mm, the thickness of the silicon dioxide substrate is 4.7 μm, the thickness of the lithium niobate rectangular waveguide is 0.4 μm, the thickness of the silicon dioxide buffer layer is 1 μm, and the floating range is 1 dB.

[0015] Preferably, the coupled on-chip polarizing structure can transmit the TM component in the input light and isolate the TE component with an isolation degree of 20 dB, thereby realizing waveguide single-mode single-polarization transmission.

[0016] The invention discloses an application of a directional coupling on-chip polarizing structure based on a lithium niobate film as the input end of a lithium niobate film optical waveguide electro-optical modulator or an electric field sensor.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention proposes a directional coupling on-chip polarizing structure which can be used as the input end of a lithium niobate thin film optical waveguide electro-optical modulator or an electric field sensor. The structure can realize single polarization transmission in a single-mode waveguide and has a simple structure and is easy to process. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. The features and advantages of the present invention can be more clearly understood by referring to the drawings. The drawings are schematic and should not be understood as limiting the present invention in any way. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1It is a cross-sectional schematic diagram of the directional coupling on-chip polarizing structure based on lithium niobate film of the present invention.

[0021] Figure 2 It is a schematic diagram of the lithium niobate rectangular waveguide structure in the present invention.

[0022] Figure 3 This is a polarization effect diagram of the directional coupling on-chip polarization structure based on lithium niobate film of the present invention.

[0023] 1-silicon substrate, 2-silicon dioxide substrate, 3-polarizing waveguide, 4-main waveguide, 5-silicon dioxide buffer layer, 6-coupling section straight waveguide, 7-bent section S-type waveguide, 8-lead section straight waveguide. DETAILED DESCRIPTION

[0024] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0025] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0026] The working principle of the coupled on-chip polarization structure of the present invention is as follows:

[0027] When there is a guided mode with an equivalent propagation constant k in two optical waveguides with similar distances, this mode can be transmitted from one waveguide to the other. That is, if the equivalent propagation constant of the guided mode in the first waveguide is: k, k 11 ,k 12 ,……, the propagation constant of the guided mode in the second waveguide is: k,k 21 ,k 22 ,……, then mode k can be coupled from one waveguide to another waveguide.

[0028] According to the above principle, there are two polarization modes k in a single-mode optical waveguide: 1 ,k 2 , there is a mode in the designed multimode waveguide equal to k 1 , then when the multimode waveguide approaches the single-mode waveguide, k 1 The mode will be coupled from the mode waveguide to the multimode waveguide. Design the coupling section length, when the coupling efficiency reaches the maximum, use the S-type waveguide structure to couple the k 1 The mode is guided to a direction away from the single-mode waveguide so that it cannot be recoupled back to the single-mode waveguide, thereby achieving single polarization transmission in the single-mode waveguide.

[0029] The above principle is verified by a specific example below.

[0030] Example 1

[0031] The present invention provides a directional coupling on-chip polarization structure based on lithium niobate film, such as Figure 1 As shown, the coupled on-chip polarization structure includes the following structures from bottom to top:

[0032] 0.5mm thick silicon substrate, 4.7μm thick silicon dioxide substrate, 0.4μm lithium niobate rectangular waveguide and 1μm silicon dioxide buffer layer.

[0033] like Figure 2 As shown, the lithium niobate rectangular waveguide providing polarization includes a 1μm wide main waveguide and a 2.1μm wide polarization waveguide. The polarization waveguide consists of three sections, namely a coupling section straight waveguide, a curved section S-type waveguide and an extraction section straight waveguide. First, the coupling section straight waveguide is parallel to the main waveguide and is 0.5μm apart; then, the curved section S-type waveguide is separated from the main waveguide to remove the polarization mode to be eliminated from the main waveguide; finally, it is extracted through the extraction section straight waveguide parallel to the main waveguide.

[0034] Figure 3 The directional coupling on-chip polarization structure based on lithium niobate film of the present invention is demonstrated. When a Gaussian beam is input into the main waveguide, when the input Gaussian beam is in TE mode, the TE light will be coupled into the multimode optical waveguide and stripped out of the main waveguide optical path (see Figure 3 (a)); When the input light is in TM mode, it will be transmitted along the main waveguide to the back-end structure (see Figure 3 (b)). It can be seen that the directional coupling on-chip polarizing structure based on lithium niobate film of the present invention realizes the polarization selection function.

[0035] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0037] In the present invention, the terms "first", "second", "third", "fourth" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance. The term "plurality" refers to two or more, unless otherwise clearly defined.

[0038] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A directional coupling on-chip polarization structure based on lithium niobate film, characterized in that: It includes a silicon substrate, a silicon dioxide substrate, a lithium niobate rectangular waveguide and a silicon dioxide buffer layer which are stacked in sequence from bottom to top; The lithium niobate rectangular waveguide comprises a main waveguide and a polarizing waveguide.

2. The directional coupling on-chip polarization structure based on lithium niobate film according to claim 1, characterized in that: The polarizing waveguide comprises a coupling section straight waveguide, a curved section S-shaped waveguide and a lead-out section straight waveguide.

3. The directional coupling on-chip polarization structure based on lithium niobate film according to claim 1, characterized in that: The width of the main waveguide is smaller than the width of the polarizing waveguide.

4. The directional coupling on-chip polarization structure based on lithium niobate film according to claim 1, characterized in that: The distance between the main waveguide and the polarizing waveguide is 0.5 μm.

5. The directional coupling on-chip polarization structure based on lithium niobate film according to claim 1, characterized in that: The width of the main waveguide is 1 μm, the width of the polarizing waveguide is 2.1 μm, and the floating range does not exceed 1 dB.

6. The directional coupling on-chip polarization structure based on lithium niobate film according to claim 1, characterized in that: The thickness of the silicon substrate is 0.5 mm, the thickness of the silicon dioxide substrate is 4.7 μm, the thickness of the lithium niobate rectangular waveguide is 0.4 μm, and the thickness of the silicon dioxide buffer layer is 1 μm.

7. The directional coupling on-chip polarization structure based on lithium niobate film according to claim 1, characterized in that: The coupled on-chip polarizing structure can transmit the TM component in the input light and isolate the TE component with an isolation degree of 20 dB, thereby realizing waveguide single-mode single-polarization transmission.

8. An application of a directional coupling on-chip polarization structure based on lithium niobate film as claimed in any one of claims 1 to 7, characterized in that: As the input end of lithium niobate thin film optical waveguide electro-optical modulator or electric field sensor.

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