A Polarization Controller Based on Thin-Film Lithium Niobate and Its Applications

Abstract

The present invention relates to a polarization controller based on thin-film lithium niobate and its application, belonging to the technical field of integrated optical chips. It solves the problems in the prior art that the ability to confine light is weak, the size of the lithium niobate bulk waveguide discrete device prepared is large, and it is difficult to be integrated. The polarization controller of the present invention includes a polarization rotation unit and a polarization beam splitting and order reduction unit; a material base layer, a lower cladding layer, a waveguide core layer and an upper cladding layer of the polarization rotation unit; the waveguide core layer is made of thin-film lithium niobate. Based on the existing interferometric fiber optic gyroscope, the polarization controller of the present invention realizes the optical path integration of the fiber optic gyroscope with a thin-film lithium niobate polarizer as the core, and uses a cascaded thin-film lithium niobate polarizer to replace the Y waveguide made of bulk lithium niobate to complete the polarization control function, realizing the miniaturization of the optical gyro measurement device.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated optical chips, and particularly relates to a polarization controller based on thin-film lithium niobate and its application, and more particularly to a polarization controller based on thin-film lithium niobate for fiber optic gyroscopes. Background Art

[0002] Traditional fiber optic gyroscope solutions mainly rely on the development of the optical communication basic industry. Core components such as optical fibers, light sources, couplers, modulators, and photodetectors are all discrete optical devices, thus limiting further cost reduction. At the same time, the devices are connected together by fiber fusion splicing, resulting in a complex production process and difficulty in controlling the product performance consistency. In addition, the coiling of the pigtails of discrete devices needs to consider the influence of the bending radius on the fiber loss and mechanical strength, which to a certain extent limits the design flexibility of the inertial measurement unit structure and increases the difficulty of further miniaturization. The miniaturization and low-cost development of fiber optic gyroscopes have encountered bottlenecks.

[0003] With the rapid development of optoelectronics, microelectronics, and micro-nano processing technologies, integrated photonics chip technology with the advantages of small volume, high performance, and low cost has continuously made breakthroughs. Integrated photonics chips integrate some or all of the active / passive devices with different functions such as light sources, modulators, beam splitters, and detectors on a single chip, and can further integrate the photonics integrated chip with a control circuit or a signal processing circuit chip through system-level packaging integration technology to achieve microsystematization, which is one of the important technical ways for fiber optic gyroscopes to develop towards higher precision, smaller size, and higher integration.

[0004] Lithium niobate combines electro-optic, acousto-optic, and nonlinear photorefractive properties, has a wide transmission window (0.4μm - 5.5μm), and a relatively high refractive index (i.e., when the input wavelength is 1550nm, the refractive index of lithium niobate is approximately 2.2). It is one of the most widely used optoelectronic materials. The Y waveguide based on lithium niobate is the pillar of modern fiber optic communication technology. However, it is limited by the manufacturing process. Generally, a guiding region is formed on the lithium niobate bulk waveguide by titanium diffusion or proton exchange. It has a weak ability to confine light, and the prepared discrete devices of lithium niobate bulk waveguides are large in size and difficult to integrate. Summary of the Invention

[0005] In view of the above problems, the present invention provides a polarization controller based on thin-film lithium niobate and its application, which solves the problems in the prior art of weak light confinement ability, large size of the prepared discrete devices of lithium niobate bulk waveguides, and difficulty in integration.

[0006] The present invention provides a polarization controller based on thin-film lithium niobate, including a polarization rotation unit and a polarization beam splitting and degradation unit; the material base layer, lower cladding layer, waveguide core layer, and upper cladding layer of the polarization rotation unit; and the waveguide core layer is made of thin-film lithium niobate.

[0007] Optionally, the material of the upper cladding is a material with a refractive index less than that of lithium niobate of the waveguide core layer.

[0008] Optionally, the waveguide core layer is a ridge waveguide, including a base portion and a ridge portion.

[0009] Optionally, the ridge height of the ridge portion: the bottom height of the base portion is 2:1.

[0010] Optionally, the polarization rotation unit includes an input waveguide section and a three-section polarization rotator.

[0011] Optionally, the input waveguide section is a transitional linear tapered waveguide.

[0012] Optionally, the three-section polarization rotator includes a first tapered waveguide section, a second tapered waveguide section, and a third tapered waveguide section; the widths of the three tapered waveguide sections increase sequentially.

[0013] Optionally, the polarization beam splitting and reducing unit is connected to the polarization rotation unit; the polarization beam splitting and reducing unit includes a connecting waveguide section, a 3L π / 4 multimode interference section, a first channel, a second channel, a third channel, a 3L π / 2 multimode interference section, and a 3L π / 2 MMI output second section; where L π represents the imaging beat length of the multimode waveguide.

[0014] Optionally, the first channel includes a 3L π / 4 MMI output first section, a phase shifter, and a 3L π / 2 MMI input first section.

[0015] Optionally, the foregoing polarization controller is used for an optical fiber gyroscope.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: Based on the existing interferometric optical fiber gyroscope, the polarization controller of the present invention realizes the optical path integration of the optical fiber gyroscope with a thin-film lithium niobate polarizer as the core, and uses a cascaded thin-film lithium niobate polarizer to replace the Y waveguide made of bulk lithium niobate to complete the polarization control function, realizing the miniaturization of the optical gyro measurement device. Description of the Drawings

[0017] In order to more clearly illustrate the technical solutions of the present application, the following briefly introduces the drawings required for the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 It is a longitudinal sectional view of the polarization rotation unit of the present invention along the transverse direction;

[0019] Figure 2 It is a transformation relation diagram for changing the effective refractive index of the ridge waveguide width mode of the present invention;

[0020] Figure 3 It is a schematic structural diagram of the polarization rotation unit of the polarization controller of the present invention;

[0021] Figure 4 It is a schematic structural diagram of the polarization beam splitting and degradation unit of the polarization controller of the present invention.

[0022] Reference numerals:

[0023] 1. Substrate layer; 2. Lower cladding layer; 3. Waveguide core layer; 4. Upper cladding layer; 5. Input waveguide section; 6. Three-stage polarization rotator; 7. First tapered waveguide section; 8. Second tapered waveguide section; 9. Third tapered waveguide section; 10. Connecting waveguide section; 11. 3L π / 4 multimode interference section (MMI); 12. 3L π / 4 MMI output first section; 13. 3L π / 2 MMI input first section; 14. 3L π / 4 MMI output third section; 15. 3L π / 2 MMI input third section; 16. Phase shifter; 17. Phase shifter; 18. 3L π / 4 MMI output second section; 19. 3L π / 2 MMI input second section; 20. 3L π / 2 multimode interference section (MMI); 21. 3L π / 2 MMI output second section; 22. 3L π / 2 MMI output first section; 301. Base portion; 302. Ridge portion. Detailed implementation manners

[0024] In order to more clearly understand the above objects, features and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. In addition, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0025] A specific embodiment of the present invention, as Figures 1-4 , discloses a polarization controller based on thin-film lithium niobate, including a polarization rotation unit and a polarization beam splitting and degradation unit; the material of the polarization rotation unit includes a substrate layer 1, a lower cladding layer 2, a waveguide core layer 3 and an upper cladding layer 4.

[0026] Among them, referring toFigure 1 , the base layer 1, the lower cladding layer 2, the waveguide core layer 3, and the upper cladding layer 4 are arranged in sequence, and the outside of the upper cladding layer 4 is the outside air; the waveguide core layer 3 is made of thin-film lithium niobate.

[0027] The waveguide core layer 3 of the present invention uses thin-film lithium niobate. The thin-film lithium niobate is peeled off from a bulk lithium niobate crystal by ion slicing, and the obtained polarization controller has a smaller volume and is more suitable for integrated optical devices.

[0028] Furthermore, the material of the upper cladding layer 4 is a material with a refractive index less than that of the lithium niobate in the waveguide core layer 3. Preferably, it is a silica material (i.e., when the light input is 0.85 μm, the refractive index of silica is 1.44).

[0029] Furthermore, the waveguide core layer 3 is a ridge waveguide, including a base portion 301 and a ridge portion 302;

[0030] The ridge portion of the present invention improves the conversion efficiency.

[0031] Furthermore, when determining the thickness of the waveguide core layer 3, the effective refractive index interval of the modes that the waveguide can pass through needs to satisfy:

[0032] NeffTM1 < NeffTE1 ≠ NeffTM0 < NeffTE0 and |NeffTE0 - NeffTE1|? |NeffTE1 - NeffTM0| and |NeffTM1 - NeffTE1|? |NeffTE1 - NeffTM0|;

[0033] Among them, NeffTM1 represents the effective refractive index of TM1 when light is input into the waveguide; NeffTE1 represents the effective refractive index of TE1 when light is input into the waveguide; NeffTM0 represents the effective refractive index of TM0 when light is input into the waveguide; NeffTE0 represents the effective refractive index when light is input, which is called a transverse electric wave; TM represents that there is an electric field component but no magnetic field component in the propagation direction, that is, a transverse magnetic wave; 0 represents the fundamental mode, 1 represents the first-order mode, which is the waveform index and represents the number of standing wave maxima of the electromagnetic field along the wide side of the waveguide.

[0034] Preferably, the total thickness of the waveguide core layer: the ridge height of the ridge portion: the bottom height of the base portion = 3:2:1; furthermore, the total thickness of the waveguide core layer is 0.3 μm, the ridge height of the ridge portion is 0.2 μm, and the bottom height of the base portion is 0.1 μm.

[0035] Furthermore, as Figure 2 shown, the cross-section of the ridge portion 302 is a trapezoid, and the inclination angle between the lower side and the waist is 60° - 80°, preferably 70°.

[0036] The upper ridge width w of the ridge portion is determined in the following manner:

[0037] Changing the dispersion curve of the effective refractive index transformation of the upper ridge width mode, as the width changes, the TM0 mode and the TE1 mode construct an effective refractive index mixing region, as Figure 2 shown by the dashed line in

[0038] According to the effective refractive index mixing region, determine the entrance width w1 of the first tapered waveguide section, the entrance width w2 of the second tapered waveguide section, the entrance width w3 of the third tapered waveguide, and the exit width w4 of the third tapered waveguide as the basis for the geometric structure of the polarization rotation unit of the present invention, as Figure 3 shown.

[0039] Specifically, when determining the width, it is necessary to satisfy: w1 < w2 < w3 < w4, and w2 and w3 are included in the effective refractive index mixing region, while w1 and w4 are outside the effective refractive index mixing region; according to the different upper ridge widths of the four, the actual mode conversion efficiency of the polarization rotation unit is different, thereby affecting the length of the polarization rotation unit. In this example, w1 = 1.954 μm, w2 = 2.064 μm, w3 = 2.177 μm, and w4 = 2.204 μm are taken.

[0040] As Figure 3 shown, the polarization rotation unit, along the optical transmission direction (from left to right), includes an input waveguide section 5 and a three-section polarization rotator 6.

[0041] Specifically, the input waveguide section 5 is a transition linear tapered waveguide that converts a single-mode waveguide into the input end of the polarization rotation unit, gradually changing from a width of 0.8 μm to 2.1 μm, with a length of 30 μm; the three-section polarization rotator 6 includes a first tapered waveguide section 7, a second tapered waveguide section 8, and a third tapered waveguide section 9. The three tapered waveguide sections are sequentially connected from the end of the input waveguide section 5, with different lengths and gradually increasing widths.

[0042] Furthermore, these three tapered waveguide sections are all adiabatically gradually tapered waveguide structures, and the waveguide widths at the entrance and exit ends of the three tapered waveguide sections all cover the effective refractive index mixing region;

[0043] Furthermore, the second tapered waveguide section 8 is the core waveguide section of the polarization rotation unit, showing a parabolic geometric trend, that is, covering the mode mixing center section of the effective refractive index mixing region Hybird (i.e., Figure 2 the region between w2 and w3 of

[0044] Exemplarily, the first tapered waveguide section 7 is a linear waveguide, with a width gradually changing from 1.954 μm (w1) to 2.064 μm (w2), and a length of 40 μm.

[0045] Exemplarily, the second tapered waveguide section 8 is a parabolic waveguide with a width gradually changing from 2.064 μm (w2) to 2.177 μm (w3) and a length of 420 μm;

[0046] Exemplarily, the third tapered waveguide 9 is a linear waveguide with a width gradually changing from 2.177 μm (w3) to 2.204 μm (w4) and a length of 40 μm.

[0047] It can be understood that after the incident light enters the polarization rotation unit, the TE0 component in the incident light remains unchanged in the input waveguide section 5, the first tapered waveguide section 7, the second tapered waveguide section 8, and the third tapered waveguide section 9; the TM0 component in the incident light is converted into the TE1 mode due to the mode mixing phenomenon after passing through the input waveguide section 5, the first tapered waveguide section 7, the second tapered waveguide section 8, and the third tapered waveguide section 9.

[0048] As Figure 4 shown, the polarization controller based on thin-film lithium niobate further includes a polarization beam splitting and order reduction unit, and the polarization beam splitting and order reduction unit is connected to the polarization rotation unit; the polarization beam splitting and order reduction unit includes a connecting waveguide section 10, a 3L π / 4 multimode interference section (MMI) 11, a first channel, a second channel, a third channel, a 3L π / 2 multimode interference section (MMI) 20, and a 3L π / 2 MMI output second section 21; where L π represents the multimode waveguide imaging beat length. Further, where neff0 and neff1 are the effective refractive indices of the TE0 mode and the TE1 mode respectively, which are the output mode and the input mode of the polarization beam splitting and order reduction unit respectively, and can also be applied to other types of order reduction.

[0049] It can be understood that the TE1 input is transmitted and converted through the first channel and the third channel for output; the original TE0 input is transmitted and converted through the second channel for output.

[0050] Further, the first channel includes a 3L π / 4 MMI output first section 12, a phase shifter 16, and a 3L π / 2 MMI input first section 13;

[0051] Further, the second channel includes a 3L π / 4 MMI output second section 18, a 3L π / 2 input second section 19;

[0052] Further, the third channel includes a 3L π / 4 MMI output third section 14, a phase shifter 17, and a 3L π / 2 MMI input third section 15.

[0053] Specifically, the connecting waveguide section 10 is a linear tapered waveguide. The connecting waveguide section 10 is respectively connected to the outlet of the third tapered waveguide section 9 and the 3L π / 4 multimode interference section (MMI) 11; the width of the connecting waveguide section 10 changes from 2.204 μm at the input end to 2.34 μm at the output end, and the length is taken as 15 μm; both the first channel and the third channel are straight waveguides with a width of 0.8 μm and a length of 20 μm; the second channel is also a straight waveguide with a width of 1.2 μm and a length of 20 μm.

[0054] Under the condition that the length of the polarization rotation unit of the present invention is less than 600 μm, the conversion efficiency from TM0 to TE1 can reach higher than 95%, and it does not affect the transmission of the original TE0 mode. Finally, the converted TE0 mode and the original input TE0 mode are output from the same port.

[0055] In use, the TE1 component output via the polarization control device 6 passes through the input waveguide section 10, 3L π / 4 multimode interference section (MMI) 11 and is sequentially input into the first channel and the third channel for propagation. The output is the TE0 mode with a phase difference of π. At the same time, the TE0 component in the incident light remains unaffected and is output from the third channel of the 3L π / 4 multimode interference section (MMI).

[0056] As Figure 4 shown, the TE0 modes with a phase difference of π output from the first channel and the third channel respectively enter the phase shifter 16 and the phase shifter 17, and then become two TE0 modes with a phase difference of π / 2. Then they are respectively directly connected to the first input section 13 of the 3L π / 2MMI and the third input section 15 of the 3L π / 2MMI and are directly input into the 3L π / 2 multimode interference section (MMI) 20. The original TE0 mode output from the second channel is directly connected to the second input section 19 of the 3L π / 2MMI and is input into the 3L π / 2 multimode interference section (MMI) 20.

[0057] The two TE0 modes with a phase difference of π / 2 pass through the 3L π / 2 multimode interference section (MMI) 20 and are combined into a single TE0 mode and output from the second output section 21 of the 3L π / 2MMI; the original input TE0 component is output from the first output section 22 of the 3L π / 2MMI, that is, finally converted into pure TE light.

[0058] The polarization controller of the present invention completes the mode reduction function while performing polarization beam splitting, has a volume less than 1000 microns, and has a reduction conversion rate higher than 86% for a single channel (i.e., E1 conversion to TE0 conversion) under TE1 mode input.

[0059] As described above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A polarization controller based on thin-film lithium niobate, which is used when the optical input is at 0.85 microns, and is characterized in that, It includes a polarization rotation unit and a polarization beam splitting and degradation unit; the material base layer, lower cladding layer, waveguide core layer and upper cladding layer of the polarization rotation unit; the waveguide core layer is made of thin film lithium niobate; The polarization rotation unit includes an input waveguide section and a three-section polarization rotator; the input waveguide section is a transitional linear tapered waveguide; the three-section polarization rotator is composed of a first tapered waveguide section, a second tapered waveguide section and a third tapered waveguide section; the widths of the three tapered waveguide sections increase sequentially; The first tapered waveguide section is a linear waveguide, with the width w1 gradually changing from 1.954 μm to w2 of 2.064 μm, and the length is 40 μm; The second tapered waveguide section is a parabolic waveguide, with the width w2 gradually changing from 2.064 μm to w3 of 2.177 μm, and the length is 420 μm; The third tapered waveguide is a linear waveguide, with the width w3 gradually changing from 2.177 μm to w4 of 2.204 μm, and the length is 40 μm; The waveguide widths at the inlet and outlet ends of the three tapered waveguide sections all cover the effective refractive index mixing region; The polarization beam splitting and reducing unit is connected to the polarization rotation unit; the polarization beam splitting and reducing unit includes a connecting waveguide section, a 3L π / 4 multimode interference section, a first channel, a second channel, a third channel, a 3L π / 2 multimode interference section and a 3L π / 2 MMI output second section; wherein, L π represents the imaging beat length of the multimode waveguide; The first channel includes 3L π / 4 MMI output first segment, phase shifter, and 3L π / 2 MMI input first segment; The second channel includes 3L π / 4MMI outputs the second segment, 3L π / 2 inputs the second segment; The third channel includes 3L π / 4 MMI output third section, phase shifter 17, 3L π / 2 MMI input third section; Both the first channel and the third channel are straight waveguides, with a width of 0.8 μm and a length of 20 μm; the second channel is also a straight waveguide, with a width of 1.2 μm and a length of 20 μm.

2. The polarization controller based on thin-film lithium niobate according to claim 1, characterized in that, The material of the upper cladding layer is a material with a refractive index less than that of lithium niobate in the waveguide core layer.

3. The polarization controller based on thin-film lithium niobate according to claim 1, characterized in that, The waveguide core layer is a ridge waveguide, including a base portion and a ridge portion.

4. The polarization controller based on thin-film lithium niobate according to claim 3, characterized in that The ridge height of the ridge portion: the bottom height of the base portion is 2:

1.

5. Application of a polarization controller based on thin-film lithium niobate, characterized in that, The polarization controller according to any one of claims 1-4 is used in an optical fiber gyroscope.

Images