Optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide and its manufacturing method

Abstract

The present invention discloses an optical amplifier based on a rare earth element-doped lithium niobate waveguide on lithium tantalate and a method for making the same, relating to the field of optical communication technology. The optical amplifier comprises a lithium tantalate substrate layer, a rare earth element-doped thin film lithium niobate device layer, and an upper cladding layer stacked sequentially from bottom to top. The rare earth element-doped thin film lithium niobate device layer is provided with a ridge waveguide, the height and width of the ridge waveguide are both at the micron level, and both end faces of the ridge waveguide are coupled with optical fibers. Compared to the currently reported lithium niobate thin film nanowaveguide optical amplifiers, the present invention has a larger mode volume and a longer effective waveguide length, a greater overall gain effect of the device, and a high saturated output power. The ridge waveguide has excellent mode matching with the lens fiber / high numerical aperture fiber, effectively solving the problems of low coupling efficiency, high device insertion loss, low output power and low saturation power in similar nanowaveguide optical amplifiers, significantly improving performance and reducing production costs, and possessing greater practical application potential.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to an optical amplifier based on a rare earth element-doped lithium niobate waveguide on lithium tantalate and a manufacturing method thereof. Background Art

[0002] Optical amplifiers are a key component in long-haul optical communications. The invention of the first erbium-doped fiber amplifier (EDFA) in 1980 provided a crucial means for amplifying optical signals, making long-haul fiber-optic communications a reality and significantly advancing the development of modern optical communications. Erbium ions, a common rare earth element ion, possess a multi-level structure. They can achieve efficient population inversion under the action of pump light (at a wavelength of 980 nanometers or 1480 nanometers) and stimulated emission under the influence of signal light, thereby providing stable, low-noise, broadband gain in the traditional fiber-optic communication band (C-band). Consequently, EDFAs play a crucial role in optical communications and laser applications. With the continuous advancement of fiber-optic communications technology, higher requirements have been placed on the performance, size, and integration of optical amplifiers, necessitating the continuous exploration of new materials and technologies to meet the demands of scientific and technological development.

[0003] Currently, common optical amplifiers include erbium-doped fiber amplifiers (EDFAs), ytterbium-doped fiber amplifiers (YDFAs), semiconductor optical amplifiers (SOAs), fiber Raman amplifiers (RFAs), fiber Brillouin amplifiers (BFAs), and optical parametric amplifiers (OPAs). Erbium-doped fiber amplifiers offer advantages such as high gain, low noise, wide bandwidth, and mature technology. However, they require fiber lengths of 1-100 meters, resulting in large device sizes and difficulty in miniaturization and integration. Furthermore, fiber is significantly affected by temperature inhomogeneities. Semiconductor optical amplifiers offer low power consumption and ease of integration, but they suffer from high noise figures, poor stability, and inability to operate continuously for extended periods. Fiber Raman amplifiers, fiber Brillouin amplifiers, and optical parametric amplifiers all operate based on nonlinear optical principles to achieve optical amplification. However, they place high demands on the coherence of the pump light, limiting their potential for miniaturization and integration.

[0004] With the development of integrated optoelectronics technology, referring to the development path of EDFA, on-chip integrated rare earth element doped waveguide amplifiers are considered to be an effective way to solve the bottleneck of chip-level gain and laser generation. In recent decades, people have studied a large number of host materials and waveguide configurations, and have successfully developed high-performance on-chip optical amplifiers and lasers on platforms such as erbium-doped silicon nitride, erbium-doped lithium niobate, and thulium-doped aluminum oxide. In 2022, the Swiss Federal Institute of Technology in Lausanne produced a high-performance erbium-doped silicon nitride waveguide amplifier that can achieve 30dB of on-chip gain in the low-power C band, basically reaching the level of commercial EDFA; in 2023, a German research team jointly prepared a thulium-doped aluminum oxide waveguide amplifier with a mode spot size of 30 square microns, achieving an on-chip output of 1W in the NIR-IIc band.

[0005] Compared with other material platforms, lithium niobate has become one of the most popular materials due to its wide transparency window, high nonlinear properties, electro-optic / acousto-optic effects and piezoelectric properties. In recent years, the emergence of lithium niobate on insulator (LNOI) or thin film lithium niobate (TFLN) has promoted the rapid development of lithium niobate-based integrated optics and applications. Compared with traditional bulk lithium niobate, the thin film lithium niobate structure inherits the excellent physical properties of bulk lithium niobate, and can also use lithography, etching and other technologies to prepare various complex waveguide shapes, thereby localizing the light field intensity at the micro-nano scale. This not only greatly reduces the size of the device, but also enables high integration with other photonic devices.

[0006] Currently, research on waveguide optical amplifiers based on the LNOI platform primarily focuses on nanowaveguide optical amplifiers made from erbium-doped nanothin-film lithium niobate (Er:TFLN). However, the large mode mismatch between the nanowaveguide and standard single-mode optical fiber leads to high coupling losses, affecting the input of pump light and the output of signal light, resulting in high overall insertion loss of the device and limiting its application range. Although the on-chip gain is currently large, the actual fiber-to-fiber net gain is unsatisfactory. In contrast, micron-scale waveguide optical amplifiers based on the Er:LNOI platform can effectively reduce coupling losses and expand the absorption and emission cross sections. However, the inability to introduce waveguide bending limits the device size and effective gain length. As described in Chinese Patent Publication No. CN118033814A, further lengthening the micron-scale doped lithium niobate-on-insulator waveguide presents certain difficulties and is insurmountable.

[0007] Therefore, those skilled in the art are committed to developing a waveguide optical amplifier with low optical fiber insertion loss and high net gain. Summary of the Invention

[0008] In light of the aforementioned shortcomings of the existing technology, the present invention addresses the technical problem of developing a fiber-compatible waveguide optical amplifier with high net gain. This approach addresses the limitations of existing micron-thin-film lithium niobate erbium-doped amplifiers in miniaturization by proposing a novel approach to achieve a helical single-mode waveguide structure. This approach allows for device miniaturization, resulting in a practical device with anticipated future industrialization.

[0009] To achieve the above objectives, the present invention provides an optical amplifier based on a rare earth element-doped lithium niobate waveguide on lithium tantalate, comprising a lithium tantalate substrate layer, a rare earth element-doped thin film lithium niobate device layer, and an upper cladding layer stacked in sequence from bottom to top. The rare earth element-doped thin film lithium niobate device layer is provided with a ridge waveguide, the height and width of the ridge waveguide are both in the micron order, and optical fibers are coupled to both end faces of the ridge waveguide.

[0010] In a preferred embodiment of the present invention, the concentration of the rare earth element in the rare earth element-doped thin film lithium niobate device layer is greater than 0.1 mol%.

[0011] Preferably, the rare earth element is selected from any one or both of erbium and ytterbium.

[0012] In a preferred embodiment of the present invention, the material of the upper cladding layer is selected from either silicon dioxide or tantalum pentoxide.

[0013] In a preferred embodiment of the present invention, the rare earth element-doped thin film lithium niobate device layer and the lithium tantalate substrate layer have the same cutting direction, which is X-cut or Z-cut.

[0014] In a preferred embodiment of the present invention, the thickness of the lithium tantalate substrate layer is 0.3-1 mm; the thickness of the rare earth element-doped thin film lithium niobate device layer is 2-10 μm; the thickness of the upper cladding layer is 0.5-1 μm; the length of the ridge waveguide is 10 cm-1 m, and the width is 2-10 μm.

[0015] In a preferred embodiment of the present invention, the optical fiber is a lens fiber or a fused high-value aperture fiber, and the core size of the optical fiber is slightly smaller than or equal to the size of the ridge waveguide, and the output light spot size of the optical fiber matches the mode size in the ridge waveguide.

[0016] In a preferred embodiment of the present invention, the ridge waveguide is configured as a spiral structure, and the spiral bending radius is greater than or equal to 1 mm.

[0017] The present invention also discloses a method for preparing an optical amplifier based on a rare earth element-doped lithium niobate waveguide on lithium tantalate, comprising the following steps:

[0018] Step 1: doping lithium niobate with a predetermined concentration of rare earth elements; bonding a rare earth element-doped thin-film lithium niobate device layer to a lithium tantalate substrate layer using a crystal bonding technique, and then performing chemical mechanical polishing to reduce the rare earth element-doped thin-film lithium niobate device layer to a predetermined thickness;

[0019] Step 2: sputtering metallic chromium onto the lithium niobate thin film layer by magnetron sputtering, then defining a spiral waveguide pattern on the chromium layer with photoresist by ultraviolet lithography, then transferring the waveguide pattern to the chromium layer and then to the lithium niobate thin film layer by dry etching, forming a ridge waveguide structure on the lithium niobate thin film layer, then polishing the waveguide sidewalls with a chemical mechanical polisher to reduce roughness, and finally removing the residual photoresist and metallic chromium on the surface by wet etching;

[0020] Step 3: depositing an upper cladding layer on the rare earth element-doped thin film lithium niobate device layer in step 2 by PECVD;

[0021] Step 4: Polish the two end faces of the ridge waveguide, then perform input and output optical coupling through optical fibers, and finally package the device to obtain an optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide.

[0022] In a preferred embodiment of the present invention, in step 2, the dry etching depth is greater than half the thickness of the lithium niobate thin film.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention provides an optical amplifier based on a rare-earth-doped lithium niobate waveguide on lithium tantalate. The lithium tantalate crystal and lithium niobate crystal have similar lattices, allowing for bonding. Furthermore, the refractive index of the lithium tantalate is only slightly lower than that of the lithium niobate, reducing the refractive index contrast of the lithium niobate waveguide and causing the micronized lithium niobate waveguide to degenerate into a single-mode waveguide. Compared to currently reported lithium niobate thin-film waveguide optical amplifiers, this design utilizes the single-mode characteristics of the lithium niobate micronized ridge waveguide on lithium tantalate to fabricate a spiral long-distance waveguide. This design utilizes doping (with rare earth ions) to achieve optical amplification, extending the distance, resulting in a larger mode volume and effective waveguide length, greater overall device gain, and higher saturated output power. The ridge waveguide exhibits excellent mode matching with lensed fiber / high numerical aperture fiber, effectively resolving the challenges of low coupling efficiency, high device insertion loss, and low output and saturation power found in similar nanowaveguide optical amplifiers. This significantly improves performance and reduces manufacturing costs, demonstrating its potential for practical applications.

[0025] This invention dopes a lithium niobate waveguide based on lithium tantalate with rare earth elements. This combines the excellent gain effect of rare earth elements with the integrated low-loss waveguide based on the thin-film lithium niobate platform, achieving high net gain per unit length and thus realizing a highly efficient on-chip optical amplifier. The lithium tantalate substrate suppresses the generation of high-order modes in the lithium niobate waveguide. Combined with a suitable waveguide bending structure, an on-chip helical waveguide structure is achieved, significantly increasing the effective waveguide length per unit length of the chip.

[0026] The present invention uses rare earth element lithium niobate doped on micron-sized lithium tantalate with a thickness of 2 to 10 microns, which increases the size of the light spot, increases the gain area, has higher gain, is directly compatible with optical fibers, and can be integrated with lithium niobate devices on micron-sized lithium tantalate.

[0027] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic structural diagram of an optical amplifier according to embodiment 1 of the present invention;

[0029] Figure 2 This is a schematic structural diagram of the optical amplifier according to embodiment 1 of the present invention. DETAILED DESCRIPTION

[0030] The following describes several preferred embodiments of the present invention with reference to the accompanying drawings to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms of embodiments, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0031] In the drawings, components with identical structures are denoted by the same reference numerals, and components with similar structures or functions are denoted by similar reference numerals. The size and thickness of each component shown in the drawings are arbitrary and are not limited by the present invention. For clarity, the thickness of components in some places in the drawings is appropriately exaggerated.

[0032] Example 1

[0033] like Figure 1As shown, in an embodiment of the present invention, the present invention provides an optical amplifier based on a straight waveguide of lithium niobate doped with rare earth elements on lithium tantalate, comprising a lithium tantalate substrate layer 1, a rare earth element-doped thin film lithium niobate device layer 2, and a silicon dioxide or tantalum pentoxide upper cladding layer 3, wherein the silicon dioxide or tantalum pentoxide upper cladding layer 3, the rare earth element-doped thin film lithium niobate device layer 2 and the lithium tantalate substrate layer 1 are stacked in sequence to form a three-layer structure, thereby achieving good optical confinement, and a ridge waveguide is provided in the rare earth element-doped thin film lithium niobate device layer 2, and the height and width of the ridge waveguide are both in the micron level.

[0034] In this invention, the ridge waveguide has a height and width of 5 microns and a length of 10 centimeters. Light propagates in the fundamental mode within the ridge waveguide. The core size 4-1 of the coupling optical fiber 4 is slightly smaller than or equal to that of the ridge waveguide 2. The coupling optical fiber 4 can be manufactured as a lensed fiber or fused with a section of high-numerical-aperture fiber to reduce the spot size. The mode size within the ridge waveguide matches the output spot size 4-2 of the coupling optical fiber 4 to minimize coupling loss. In the optical communication band, coupling loss is typically below 1.5 dB.

[0035] In addition, in the present invention, the thickness of the silicon dioxide or tantalum pentoxide upper cladding layer is 0.5 microns. The introduction of this upper cladding layer can form an appropriate refractive index gradient and form a protective layer at the same time.

[0036] The optical amplifier based on a rare-earth-element-doped lithium niobate waveguide on lithium tantalate, proposed in this invention, exhibits a larger mode volume and effective waveguide length than currently reported lithium niobate thin-film waveguide optical amplifiers, resulting in greater overall device gain and higher saturated output power. The rare-earth-element-doped lithium niobate ridge waveguide, with a waveguide width and height of 5 microns, exhibits excellent mode matching with lensed fibers and high-numerical-aperture fibers. This effectively addresses the challenges of low coupling efficiency, high device insertion loss, and low output and saturation powers found in similar nanowaveguide optical amplifiers, significantly improving performance and reducing manufacturing costs, demonstrating its potential for practical applications.

[0037] Example 2

[0038] like Figure 2 As shown, in an embodiment of the present invention, another optical amplifier based on a rare earth element-doped lithium niobate micron spiral waveguide on lithium tantalate is provided. The optical amplifier comprises a lithium tantalate substrate layer 1, a rare earth element-doped thin-film lithium niobate device layer 2, and a silicon dioxide or tantalum pentoxide upper cladding layer 3. The silicon dioxide or tantalum pentoxide upper cladding layer 3, the rare earth element-doped thin-film lithium niobate device layer 2, and the lithium tantalate substrate layer 1 are stacked in sequence to form a three-layer structure, achieving excellent optical confinement. A ridge waveguide is provided on the rare earth element-doped thin-film lithium niobate device layer 2, with the height and width of the ridge waveguide both on the micrometer scale.

[0039] In this embodiment, the silicon dioxide or tantalum pentoxide upper cladding layer 3 is 1 micron thick, the rare earth element-doped thin-film lithium niobate device layer 2 is 3 microns thick, and the lithium tantalate substrate layer 1 is between 0.3 and 1 mm thick. The ridge waveguide is 3 microns high and 3 microns wide, and is curved to form a spiral with a bend radius greater than or equal to 1 mm. Light in the curved ridge waveguide still propagates in the fundamental mode, without introducing higher-order modes. This allows for a shorter and smaller waveguide chip, resulting in a smaller packaged waveguide optical amplifier device.

[0040] The tangent direction of the rare earth element doped thin film lithium niobate device layer 2 is X-cut or Z-cut, and the tangent direction of the lithium tantalate substrate layer 1 is X-cut or Z-cut. Generally, the tangent directions of the two are the same.

[0041] In the embodiment of the present invention, the length of the ridge waveguide 2 is 10 cm to 1 m, and in order to reduce the area of ​​the optical amplifier, a curved portion can be introduced into the waveguide to realize a spiral waveguide. In this case, the length of the waveguide is 20 cm.

[0042] Preferably, the focusing spot diameter of the lens fiber used for coupling in the 1550 nm band is 3 μm, which matches the ridge waveguide mode with a height of 3 μm and a width of 3 μm.

[0043] Example 3

[0044] A method for preparing an optical amplifier based on a straight waveguide of lithium niobate doped with rare earth elements on lithium tantalate comprises the following steps:

[0045] Step 1: doping lithium niobate with a predetermined concentration of rare earth elements; bonding the rare earth element-doped thin-film lithium niobate device layer 2 to the lithium tantalate substrate layer 1 through crystal bonding technology, and then performing chemical mechanical polishing to reduce the rare earth element-doped thin-film lithium niobate device layer to a predetermined thickness;

[0046] In this step, a predetermined concentration of rare earth elements is doped into the lithium niobate, and the predetermined concentration is preferably greater than 0.1 mol%;

[0047] For the optical amplifier prepared with rare earth element-doped lithium niobate waveguide on lithium tantalate, the thickness of the lithium tantalate substrate layer is 0.3-1 mm; the thickness of the rare earth element-doped thin film lithium niobate device layer is 2-10 microns; the thickness of the upper cladding layer is 0.5-1 micron; the length of the ridge waveguide is 10 cm-1 meter, and the width is 2-10 microns.

[0048] For example, in Example 1, the height of the ridge waveguide is 5 microns, the width is 5 microns, and the length is 10 centimeters; in Example 2, the height of the ridge waveguide is 3 microns, the width is 3 microns, and the length is 20 centimeters.

[0049] Step 2: Sputter metallic chromium onto the lithium niobate thin film layer by magnetron sputtering, define the spiral waveguide pattern on the chromium layer with photoresist by ultraviolet lithography, and then use a dry etching process to transfer the waveguide pattern to the chromium layer in sequence, and then transfer it to the lithium niobate thin film layer, prepare a ridge waveguide structure in the lithium niobate thin film layer, and then polish the waveguide sidewalls with a chemical mechanical polisher to reduce the roughness. Finally, remove the residual photoresist and metallic chromium on the surface by wet etching.

[0050] In this step, the dry etching depth is greater than half the thickness of the lithium niobate thin film.

[0051] Step 3: depositing a layer of silicon dioxide or tantalum pentoxide upper cladding on the optical amplifier of lithium niobate waveguide doped with rare earth elements on lithium tantalate by PECVD or the like;

[0052] In this step, the predetermined thickness of the deposited upper cladding layer is greater than or equal to 0.5 micrometers. For example, the thickness of the upper cladding layer in Example 1 is 0.5 micrometers, and the thickness of the upper cladding layer in Example 2 is 1 micrometer.

[0053] Step 4: Polish the two end faces of the ridge waveguide; couple the input and output light through a lens fiber or a high numerical aperture fiber; and finally, through device packaging, realize a high-efficiency optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide with small size, high amplification efficiency, and high output power.

[0054] The core of the lens fiber is slightly smaller than or equal to the size of the ridge waveguide, and the focused light spot of the lens fiber matches the mode of the ridge waveguide.

[0055] This invention dopes a lithium niobate waveguide based on lithium tantalate with rare earth elements. This combines the excellent gain effect of rare earth elements with the integrated low-loss waveguide based on the thin-film lithium niobate platform, achieving high net gain per unit length and thus realizing a highly efficient on-chip optical amplifier. The lithium tantalate substrate suppresses the generation of high-order modes in the lithium niobate waveguide. Combined with a suitable waveguide bending structure, an on-chip helical waveguide structure is achieved, significantly increasing the effective waveguide length per unit length of the chip.

[0056] The present invention uses rare earth element lithium niobate doped on micron-sized lithium tantalate with a thickness of 2 to 10 microns, which increases the size of the light spot, increases the gain area, has higher gain, is directly compatible with optical fibers, and can be integrated with lithium niobate devices on micron-sized lithium tantalate.

[0057] In an embodiment of the present invention, the signal light spectrum used for amplification is in the range of 1525-1570 nanometers. The pump light used is generated by a 980-nanometer laser diode. After passing through a fiber polarization controller, it is combined by a 980-nanometer / 1550-nanometer wavelength division multiplexer. Using end-face coupling, it is coupled into a lithium niobate ridge waveguide at the input end through the end face of a single-mode lensed fiber, i.e., a unidirectional pumping mode. Using a bidirectional pumping mode, the output signal light and the input pump light can be split at the output end through another single-mode lensed fiber and another 980-nanometer / 1550-nanometer wavelength division multiplexer, i.e., a bidirectional pumping mode.

[0058] The preferred embodiments of the present invention have been described in detail above. It should be understood that numerous modifications and variations based on the concepts of the present invention are possible without inventive effort by those skilled in the art. Therefore, any technical solution that can be derived by one skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. An optical amplifier based on a rare earth element-doped lithium niobate waveguide on lithium tantalate, characterized in that: The device comprises a lithium tantalate substrate layer, a rare earth element-doped thin film lithium niobate device layer, and an upper cladding layer stacked sequentially from bottom to top. The rare earth element-doped thin film lithium niobate device layer is provided with a ridge waveguide. The height and width of the ridge waveguide are both in the micron level, and both end faces of the ridge waveguide are coupled to optical fibers. The rare earth element-doped thin film lithium niobate device layer, the rare earth element concentration is greater than 0.1% mol; The rare earth element is selected from any one or two of erbium and ytterbium; The optical fiber is a lensed optical fiber, and the core size of the optical fiber is smaller than or equal to the size of the ridge waveguide, and the output spot size of the optical fiber matches the mode size in the ridge waveguide; The ridge waveguide is configured as a spiral structure, and the spiral bending radius is greater than or equal to 1 mm.

2. The optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide according to claim 1, characterized in that: The material of the upper cladding layer is selected from any one of silicon dioxide and tantalum pentoxide.

3. The optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide according to claim 1, characterized in that: The rare earth element doped thin film lithium niobate device layer and the lithium tantalate substrate layer have the same cutting direction, which is X-cut or Z-cut.

4. The optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide according to claim 1, characterized in that: The thickness of the lithium tantalate substrate layer is 0.3-1 mm; the thickness of the rare earth element-doped thin film lithium niobate device layer is 2-10 μm; the thickness of the upper cladding layer is 0.5-1 μm; the length of the ridge waveguide is 10 cm-1 m, and the width is 2-10 μm.

5. A method for preparing an optical amplifier based on a rare earth element-doped lithium niobate waveguide on lithium tantalate as claimed in claim 1, characterized in that: The steps include: Step 1: doping lithium niobate with a predetermined concentration of rare earth elements; bonding a rare earth element-doped thin-film lithium niobate device layer to a lithium tantalate substrate layer using a crystal bonding technique, and then performing chemical mechanical polishing to reduce the rare earth element-doped thin-film lithium niobate device layer to a predetermined thickness; Step 2: sputtering metallic chromium onto the lithium niobate thin film layer by magnetron sputtering, then defining a spiral waveguide pattern on the chromium layer with photoresist by ultraviolet lithography, then transferring the waveguide pattern to the chromium layer and then to the lithium niobate thin film layer by dry etching, forming a ridge waveguide structure on the lithium niobate thin film layer, then polishing the waveguide sidewalls with a chemical mechanical polisher to reduce roughness, and finally removing the residual photoresist and metallic chromium on the surface by wet etching; Step 3: depositing an upper cladding layer on the rare earth element-doped thin film lithium niobate device layer in step 2 by PECVD; Step 4: Polish the two end faces of the ridge waveguide, then perform input and output optical coupling through optical fibers, and finally package the device to obtain an optical amplifier based on lithium tantalate-doped rare earth element lithium niobate waveguide.

6. The preparation method according to claim 5, characterized in that In step 2, the dry etching depth is greater than half the thickness of the lithium niobate thin film.

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