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

Abstract

The invention discloses an optical amplifier based on a double-cladding erbium-doped LiNbO3 waveguide. The optical amplifier comprises a double-cladding erbium-doped LiNbO3 optical amplification waveguide and multimode beam combiners at the two ends of the double-cladding erbium-doped LiNbO3 optical amplification waveguide. A bidirectional pumping design is adopted, so that 980nm pumping light is injected from multimode beam splitters at two ends and is converged with 1550nm signal light, and stimulated radiation is generated to amplify the power of the signal light. According to the structure, the pumping energy coupled into the waveguide is increased through the cladding, the overlapping area of a waveguide mode field is increased, and the pumping efficiency is remarkably improved. And the total pumping energy is improved in a multimode transmission mode, so that the overall gain and the output power are improved. The erbium-doped waveguide amplifier has the advantages of high pump light coupling efficiency and high gain, and has wide application prospects in the field of integrated photoelectronics and the field of optical communication.

Description

Technical Field

[0001] The invention relates to the field of integrated optical active devices, in particular to an on-chip optical amplifier based on a double-cladding erbium-doped LiNbO3 waveguide. Background Art

[0002] With the development of optical communication and integrated optics, optical communication systems require integrated, high-gain optical amplifiers. The solutions for realizing optical waveguide amplifiers include nonlinear optical amplifiers, semiconductor laser amplifiers, etc. Compared with the above solutions, rare earth ion-doped waveguide amplifiers have the advantages of low noise, polarization insensitivity, and good temperature stability. Ion-doped waveguide amplifiers have been regarded as a promising technology for future broadband optical communications. Erbium-doped waveguide amplifiers (EDWAs) have the characteristics of low nonlinearity, low noise amplification, and wide gain covering the telecommunication c-band (1530-1560nm). Researchers have studied EDWAs based on acceptor materials such as alumina and silicon nitride. The loss window of lithium niobate (LiNbO3, LN) material covers the ultraviolet band, visible band to infrared band, and has electro-optical, thermo-optical and piezoelectric tuning characteristics. It has broad application prospects in the field of optical communication and photonic quantum fields. The refractive index of LiNbO3 is larger than that of SiO2 material, which can realize waveguide structure and is compatible with existing electronic chip preparation and processing technology. It is the key material for current photonic integrated chips.

[0003] In order to improve the gain, researchers have developed a Ta205 double-clad structure erbium-doped thin film lithium niobate on-chip optical amplifier to improve the gain. However, the solution still uses single-mode transmission of signal light and pump light in the erbium-doped thin film lithium niobate waveguide, which cannot achieve high-power pumping. Summary of the invention

[0004] The technical problem solved by the present 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 to solve the problems of low pump light coupling efficiency and low total power in the current erbium-doped thin film lithium niobate on-chip optical amplifier.

[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 multi-mode combiner, a double-clad erbium-doped LiNbO3 waveguide structure, and a multi-mode 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 beam splitter;

[0007] The double-clad erbium-doped LiNbO3 waveguide structure includes, 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 spirally coiled structure; the signal light energy is concentrated in the core layer, the pump light energy is concentrated in the cladding layer, and part of the pump light leaks into the core layer to provide continuous pumping.

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

[0009] Furthermore, the four S-shaped curved waveguides of the multi-mode combiner and the multi-mode splitter are all connected by two tangent arcs, and the tangent part is located inside the multi-mode interferometer MMI.

[0010] Further, the multi-mode interferometer MMI comprises an input waveguide and a parallel coupling waveguide and an output waveguide;

[0011] The input waveguide and the output waveguide are bridge waveguides connecting the S-shaped curved waveguide and the parallel coupled waveguide; the input waveguide and the output waveguide are respectively composed of two parallel and identically structured ridge-shaped LiNbO3 waveguides, with a groove in the middle of the two waveguides; the parallel coupled waveguide is a LiNbO3 multimode waveguide with a rectangular side and a trapezoidal cross section, with both ends of the waveguide connected to the input waveguide and the output waveguide, and a cladding material is located above the LiNbO3 multimode waveguide.

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

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

[0014] The advantages of the present invention compared with the prior art are:

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

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

[0017] (3) The present invention performs multi-mode transmission on the pump light. Compared with the single-mode pumping case, the total pump energy is greater and can pump a longer gain waveguide to achieve higher gain and signal light output power. Because the mode loss near the cutoff is too large, it is directly lost during the transmission process and cannot provide gain. Assuming that the pump energy is evenly distributed in each mode, the more modes there are (increasing the cladding refractive index and the waveguide tilt angle close to 90°), the smaller the proportion of the mode near the cutoff. Therefore, the multi-mode pumping method 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a structural diagram of an on-chip optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide implemented by the present invention;

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

[0021] Figure 3 It is a structural diagram of the multi-mode combiner of the present invention.

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

[0023] Figure 5 is a cross-sectional view of the parallel coupled waveguides of the MMI module in the multi-mode combiner of the present invention;

[0024] Figure 6 This is a schematic diagram of an on-chip optical amplifier based on a double-clad erbium-doped LiNbO3 waveguide implemented by the present invention. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0026] See also Figure 1-Figure 6 The present invention includes: a multimode combiner, a double-clad erbium-doped LiNbO3 waveguide structure, and a multimode beam splitter. The signal light is input from port 1, and the pump light is input from port 2 and port 4. The signal light and the forward pump light are converged into the double-clad erbium-doped LiNbO3 waveguide structure through the multimode combiner, and the amplified signal light is separated from the pump light through the multimode beam splitter and output from port 3 to achieve amplification of the signal light.

[0027] The present invention is further described below in conjunction with specific embodiments.

[0028] Embodiment 1:

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

[0030] In this embodiment, if Figure 4 and Figure 5 As shown, the multimode interferometer (MMI) of the multimode combiner can be divided into input waveguides 1-5, parallel coupling waveguides 1-7 and output waveguides 1-6; the multimode interferometer (MMI) of the multimode beam splitter can be divided into input waveguides 3-5, parallel coupling waveguides 3-7 and output waveguides 3-6. The input and output waveguides of the multimode interferometer (MMI) are bridge waveguides connecting the S-shaped bending waveguide and the parallel coupling waveguide. The input waveguide and the output waveguide are two parallel ridge-shaped LiNbO3 waveguides with Ta2O5 cladding material on top. The parallel coupling waveguide is a trapezoidal LiNbO3 waveguide with Ta2O5 cladding material on top. The refractive index of Ta2O5 is The core spacing between the input waveguide and the output waveguide is 2.5, so that the mode field distribution in the two parallel ridge waveguides of the multimode beam splitter output waveguide does not overlap. c =3965 μm. The parallel coupling waveguide length is the least common multiple of the optimal coupling length l1 that makes the signal light input and output port coupling efficiency the highest and the optimal coupling length l2 that makes the pump light input and output port coupling efficiency the highest.

[0031] In this embodiment, the double-clad optical waveguide structure 2-1 includes, 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 on the outermost side; the waveguide is a multimode waveguide; the structure is a spirally coiled structure; the signal light energy is concentrated in the core layer, the pump light energy is concentrated in the cladding, and part of the 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, w0 = 1000nm, w1 = 1140nm, w2 = 4800nm, T0 = 90nm, T1 = 120nm, and T2 = 210nm are satisfied. The cladding material is Ta2O5. The thickness of the Ta2O5 cladding is 5000nm, and the waveguide inclination angle is 85°. The LiNbO3 optical amplifier waveguide with Ta2O5 cladding has a spiral winding structure. The relevant parameters of the spiral winding structure are as follows Figure 1As shown, it satisfies: h1=h4=0.01mm, h2=h3=2mm, h5=4cm, l1=13=0.01mm, l2=20mm, the total length of the waveguide is about 13cm, and the minimum bending radius is 1000nm.

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

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

[0034] ② The sidewalls of the two TFLNOI substrates around the optical interface were polished into smooth and highly vertical surfaces using an automatic precision grinding and polishing machine (Logitech DP1), wherein the polishing suspension consisted of cerium oxide with a particle size of ≈0.8 μm.

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

[0036] ④ Generate optical waveguide patterns by selective direct writing of femtosecond lasers, and etch the waveguide patterns into the tiled TFLN using CMP process

[0037] Preferably, in step ②, a metal cylinder with high flatness and parallelism is designed and processed as a fixture for fixing the TFLN substrate, and the undoped and doped TFLN substrates are clamped together by two glass blocks. In this step, the undoped and Rei-doped TFLN substrates are flipped and bonded on a polished glass plate with high flatness, and then the TFLN substrates are further placed on a quartz plate and immediately irradiated with ultraviolet light.

[0038] The above embodiments are used to illustrate the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection 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 multi-mode beam combiner, a double-clad erbium-doped LiNbO3 waveguide structure, and a multi-mode 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 beam splitter; The double-clad erbium-doped LiNbO3 waveguide structure includes, 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 spirally coiled structure; the signal light energy is concentrated in the core layer, the pump light energy is concentrated in the cladding layer, and part of the pump light leaks into the core layer to provide continuous pumping.

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

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

4. The optical amplifier based on double-clad erbium-doped LiNbO3 waveguide according to claim 3, characterized in that: The multi-mode interferometer MMI comprises an input waveguide, a parallel coupling waveguide and an output waveguide; The input waveguide and the output waveguide are bridge waveguides connecting the S-shaped curved waveguide and the parallel coupled waveguide; the input waveguide and the output waveguide are respectively composed of two parallel and identically structured ridge-shaped LiNbO3 waveguides, with a groove in the middle of the two waveguides; the parallel coupled waveguide is a LiNbO3 multimode waveguide with a rectangular side and a trapezoidal cross section, with both ends of the waveguide connected to the input waveguide and the output waveguide, and a cladding material is located above the LiNbO3 multimode waveguide.

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

6. The optical amplifier based on 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 smaller than that of the LiNbO3 waveguide.

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