On-chip optical amplifier based on double-clad erbium-doped LiNbO3 waveguide

Abstract

The application discloses a kind of optical amplifiers based on double-clad doped erbium LiNbO3 waveguide, including double-clad doped erbium LiNbO3 optical amplification waveguide and multimode combiner at its both ends. The design of two-way pumping is used, so that 980nm pumping light is injected from the multimode beam splitter at both ends and converges with 1550nm signal light, to produce stimulated radiation to realize the amplification of signal light power. The structure increases the coupling-in waveguide pump energy through cladding, improves the overlap area of waveguide mode field, and significantly improves the pump efficiency. The total pump energy is improved by multimode transmission mode, so as to improve the overall gain and output power. The doped erbium waveguide amplifier of the application has the advantages of high pump light coupling efficiency and high gain, and has wide application prospect in the field of integrated optoelectronics and optical communication.

Description

Technical Field

[0001] This invention relates to the field of integrated optical active devices, specifically an on-chip optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide. Background Technology

[0002] With the development of optical communication and integrated optics, optical communication systems require integrated, high-gain optical amplifiers. Schemes for realizing optical waveguide amplifiers include nonlinear optical amplifiers and semiconductor laser amplifiers. Compared with these schemes, rare-earth ion-doped waveguide amplifiers have advantages such as low noise, polarization insensitivity, and good temperature stability. Ion-doped waveguide amplifiers are considered a promising technology for future broadband optical communication. Erbium-doped waveguide amplifiers (EDWAs) possess characteristics such as low nonlinearity, low noise amplification, and wide gain covering the telecommunications C-band (1530-1560nm). Researchers have studied EDWAs based on acceptor materials such as alumina and silicon nitride. Lithium niobate (LiNbO3, LN) materials have a loss window covering the ultraviolet, visible, and infrared bands, and possess electro-optic, thermo-optic, and piezoelectric tuning characteristics, showing broad application prospects in optical communication and quantum optical fields. LiNbO3 has a higher refractive index than SiO2, enabling the realization of waveguide structures and compatibility with existing electronic chip fabrication and processing technologies, making it a key material for current photonic integrated chips.

[0003] To improve gain, researchers have developed an on-chip optical amplifier for erbium-doped lithium niobate with a double-clad structure in Ta205. However, the scheme still uses single-mode transmission of signal light and pump light in an erbium-doped lithium niobate waveguide, which cannot achieve high-power pumping. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide an on-chip optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide, thereby solving the problems of low pump light coupling efficiency and low total power in current erbium-doped thin-film lithium niobate on-chip optical amplifiers.

[0005] The technical solution of the present invention is: an optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide, the amplifier structure comprising: a multimode beam combiner, a double-clad erbium-doped LiNbO3 waveguide structure, and a multimode beam splitter;

[0006] The signal light is input from port 1, the pump light is input from port 2 on the same side as port 1 and port 4 on a different side, and the amplified signal is output from port 3 on the same side as port 4; the signal light and the forward pump light in the same direction are converged into the double-clad erbium-doped LiNbO3 waveguide structure through a multimode combiner, and the amplified signal light is separated from the pump light through a multimode splitter.

[0007] The double-clad erbium-doped LiNbO3 waveguide structure comprises, from bottom to top, a silicon substrate layer, a SiO2 buried oxide layer, an erbium-doped LiNbO3 core layer, a cladding layer, and an air layer; the waveguide is a multimode waveguide; the structure is a spiral coiled structure; the signal light energy is concentrated in the core layer, the pump light energy is concentrated in the cladding layer, and some pump light leaks into the core layer to provide continuous pumping.

[0008] Furthermore, the multimode beam combiner and multimode beam splitter have the same structure, both consisting of four S-shaped curved waveguides and one multimode interferometer (MMI).

[0009] Furthermore, the four S-shaped curved waveguides of the multimode combiner and multimode splitter are all connected by two tangent circular arcs, with the tangent part located inside the multimode interferometer (MMI).

[0010] Furthermore, the multimode interferometer (MMI) includes an input waveguide, a parallel coupled waveguide, and an output waveguide;

[0011] The input waveguide and output waveguide are bridging waveguides connecting the S-shaped curved waveguide and the parallel coupled waveguide. The input waveguide and output waveguide are each composed of two parallel and identical ridge-shaped LiNbO3 waveguides with a groove in the middle between the two waveguides. The parallel coupled waveguide is a LiNbO3 multimode waveguide with rectangular sides and trapezoidal cross-section. The two ends of the waveguide are connected to the input waveguide and the output waveguide. The LiNbO3 multimode waveguide is covered with cladding material.

[0012] Furthermore, the erbium-doped LiNbO3 core layer consists of a uniform layer and a ridged layer.

[0013] Furthermore, the cladding structure is trapezoidal, and the refractive index of the cladding material is less than that of the LiNbO3 waveguide.

[0014] The advantages of this invention compared to the prior art are:

[0015] (1) The present invention uses on-chip MMI to realize the splitting and combining of pump light and signal light. Compared with the traditional scheme using wavelength division multiplexing devices, it improves the integration of devices and the coupling efficiency of pump light.

[0016] (2) The present invention improves the pump energy coupled into the waveguide by using Ta2O5 cladding, and increases the overlap area of ​​the waveguide mode field, thereby improving the pump efficiency.

[0017] (3) This invention performs multimode transmission of the pump light. Compared to single-mode pumping, the total pump energy is greater, allowing for the pumping of longer gain waveguides and achieving higher gain and signal light output power. Because near-cutoff modes suffer excessive loss during transmission, they are directly lost and cannot provide gain. Assuming the pump energy is evenly distributed across all modes, the more modes there are (increasing the cladding refractive index and making the waveguide tilt angle close to 90°), the smaller the proportion of near-cutoff modes. Therefore, multimode pumping also reduces transmission loss.

[0018] (4) The present invention reduces bending loss and improves pumping efficiency by increasing the waveguide bending radius and optimizing the waveguide cross-sectional shape. Attached Figure Description

[0019] Figure 1 This is a structural diagram of the on-chip optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide implemented in this invention;

[0020] Figure 2 This is a cross-sectional view of the LiNbO3 waveguide with Ta2O5 cladding according to the present invention;

[0021] Figure 3 This is a structural diagram of the multimode combiner of the present invention.

[0022] Figure 4 This is a cross-sectional view of the input and output waveguides of the MMI module in the multimode combiner of the present invention;

[0023] Figure 5 This is a cross-sectional view of the parallel coupled waveguide of the MMI module in the multimode combiner of the present invention;

[0024] Figure 6 This is a schematic diagram of the on-chip optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide implemented in this invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0026] Please see Figures 1-6 This invention includes: a multimode beam combiner, a double-clad erbium-doped LiNbO3 waveguide structure, and a multimode beam splitter. Signal light is input from port 1, and pump light is input from ports 2 and 4. The signal light and forward pump light are converged by the multimode beam combiner and enter the double-clad erbium-doped LiNbO3 waveguide structure. The amplified signal light is separated from the pump light by the multimode beam splitter and output from port 3, thus achieving signal light amplification.

[0027] The present invention will be further described below with reference to specific embodiments.

[0028] Example 1:

[0029] In this embodiment, the multimode combiner and multimode splitter are structured as follows: Figure 3 As shown, it consists of four S-shaped curved waveguides and one multimode interferometer (MMI). The four S-shaped curved waveguides 1-1 to 1-4 of the multimode beam combiner and the four S-shaped curved waveguides 3-1 to 3-4 of the multimode beam splitter are connected by two 5° tangent circular arcs with a radius of r = 1000 μm. The tangent part is located inside the multimode interferometer (MMI).

[0030] In this embodiment, as Figure 4 and Figure 5 As shown, the multimode interferometer (MMI) of the multimode beam combiner can be divided into input waveguide 1-5, parallel coupling waveguide 1-7, and output waveguide 1-6; the multimode interferometer (MMI) of the multimode beam splitter can be divided into input waveguide 3-5, parallel coupling waveguide 3-7, and output waveguide 3-6. The input and output waveguides of the multimode interferometer (MMI) are bridging waveguides connecting the S-shaped curved waveguide and the parallel coupling waveguide. The input and output waveguides are two parallel ridge-shaped LiNbO3 waveguides with a Ta2O5 cladding material on top. The parallel coupling waveguide is a trapezoidal LiNbO3 waveguide with a Ta2O5 cladding material on top. The refractive index of Ta2O5 is... The core spacing between the input and output waveguides is 2.5, ensuring that the mode field distributions in the two parallel ridge waveguide segments of the multimode beamsplitter's output waveguide do not overlap. The parallel coupled waveguide length is l. c =3965μm. The parallel coupled waveguide length is the least common multiple of the optimal coupling length l1 that maximizes the coupling efficiency of the signal light input and output ports and the optimal coupling length l2 that maximizes the coupling efficiency of the pump light input and output ports.

[0031] In this embodiment, the double-clad optical waveguide structure 2-1 comprises, from bottom to top: a SiO2 buried oxide layer, a LiNbO3 waveguide composed of a uniform layer and a ridge waveguide, a cladding, and air as the outermost layer; the waveguide is a multimode waveguide; the structure is a spiral coiled structure; the signal light energy is concentrated in the core layer, the pump light energy is concentrated in the cladding, and some pump light leaks into the core layer to provide continuous pumping. The relevant structural parameters of the LiNbO3 waveguide are as follows: Figure 2 As shown, the following parameters are satisfied: w0 = 1000nm, w1 = 1140nm, w2 = 4800nm, T0 = 90nm, T1 = 120nm, and T2 = 210nm. The cladding material is Ta2O5. The Ta2O5 cladding thickness is 5000nm, and the waveguide tilt angle is 85°. The Ta2O5-clad LiNbO3 optical amplification waveguide has a helical coil structure. The relevant parameters of the helical coil structure are as follows... Figure 1As shown, the following conditions are met: h1=h4=0.01mm, h2=h3=2mm, h5=4cm, l1=13=0.01mm, l2=20mm, the total waveguide length is approximately 13cm, and the minimum bending radius is 1000nm.

[0032] In this embodiment, the on-chip optical amplifier device provided in this embodiment is fabricated using active and passive waveguide monolithic integration technology. The steps are as follows:

[0033] ① REI-doped TFLNOI substrates and undoped TFLNOI substrates were fabricated. The TFLN z-cut thickness was 500 nm, the SiO2 layer thickness was 2 μm, and the Si layer thickness was 500 μm. A 200 nm thick chromium (Cr) film was coated on both doped and undoped TFLNOI substrates as a hard mask material for subsequent chemical mechanical polishing (CMP) processes. Both the undoped and REI-doped TFLN wafers were standard 4-inch wafers of the same size, with a thickness tolerance within 10 nm.

[0034] ② Polishing was performed using an automated precision polishing machine (Logitech DP1) to polish the sidewalls of the two TFLNOI substrates around the optical interface into a smooth and highly perpendicular surface. The polishing suspension consisted of cerium oxide particles with a particle size of ≈0.8μm.

[0035] ③ A laser welding process was performed to permanently fix the TFLNOI substrate onto the quartz support. During laser welding, an ultrafast laser with a pulse duration τp = 300 fs, wavelength λ = 1030 nm, pulse repetition frequency f = 10 MHz, and single-pulse energy Q = 1 μJ was used. A microscope objective with NA = 0.6 was used to focus the laser beam onto the glass / silicon interface. The sample was translated through a 3D platform at a speed of v = 20 mm s⁻¹.

[0036] ④ Optical waveguide patterns are generated by selective direct writing using femtosecond lasers, and the waveguide patterns are etched into the tiled TFLN using CMP technology.

[0037] Preferably, in step ②, a metal cylinder with high flatness and parallelism is designed and fabricated as a clamp to fix the TFLN substrate. The undoped and rei-doped TFLN substrates are clamped together by two glass blocks. In this step, the undoped and rei-doped TFLNOI substrates are flipped and bonded to a polished glass plate with high flatness. Then, the TFLNOI substrates are further placed on a quartz plate and immediately irradiated with ultraviolet light.

[0038] The above embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. An optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide, characterized in that, The amplifier structure includes: a multimode beam combiner, a double-clad erbium-doped LiNbO3 waveguide structure, and a multimode beam splitter; The signal light is input from port 1, the pump light is input from port 2 on the same side as port 1 and port 4 on a different side, and the amplified signal is output from port 3 on the same side as port 4; the signal light and the forward pump light in the same direction are converged into the double-clad erbium-doped LiNbO3 waveguide structure through a multimode combiner, and the amplified signal light is separated from the pump light through a multimode splitter. The double-clad erbium-doped LiNbO3 waveguide structure comprises, from bottom to top, a silicon substrate layer, a SiO2 buried oxide layer, an erbium-doped LiNbO3 core layer, a cladding layer, and an air layer; the waveguide is a multimode waveguide; the structure is a spiral coiled structure; the signal light energy is concentrated in the core layer, the pump light energy is concentrated in the cladding layer, and some pump light leaks into the core layer to provide continuous pumping.

2. The optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide according to claim 1, characterized in that, The multimode beam combiner and multimode beam splitter have the same structure, both consisting of four S-shaped curved waveguides and one multimode interferometer (MMI).

3. An optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide according to claim 2, characterized in that, The four S-shaped curved waveguides of the multimode combiner and multimode splitter are all connected by two tangent circular arcs, with the tangent part located inside the multimode interferometer (MMI).

4. An optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide according to claim 3, characterized in that, A multimode interferometer (MMI) consists of an input waveguide, a parallel coupled waveguide, and an output waveguide. The input waveguide and output waveguide are bridging waveguides connecting the S-shaped curved waveguide and the parallel coupled waveguide. The input waveguide and output waveguide are each composed of two parallel and identical ridge-shaped LiNbO3 waveguides with a groove in the middle between the two waveguides. The parallel coupled waveguide is a LiNbO3 multimode waveguide with rectangular sides and trapezoidal cross-section. The two ends of the waveguide are connected to the input waveguide and the output waveguide. The LiNbO3 multimode waveguide is covered with cladding material.

5. An optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide according to claim 1, characterized in that, The erbium-doped LiNbO3 core consists of a uniform layer and a ridged layer.

6. An optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide according to claim 1, characterized in that, The cladding structure is trapezoidal, and the refractive index of the cladding material is less than that of the LiNbO3 waveguide.

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