An integrated Brillouin narrow linewidth laser and its preparation method

Abstract

The present invention discloses an integrated Brillouin narrow linewidth laser and a preparation method thereof, belonging to the field of integrated photonics. The integrated Brillouin narrow linewidth laser of the present invention is composed of a double-layer silicon nitride microring, with a silicon oxide spacer layer between the double layers. The free spectral range of the double-layer silicon nitride microring is designed to be the Brillouin frequency shift, and a coupling waveguide is designed next to the microring. The present invention also provides a corresponding preparation method, comprising: depositing a first silicon nitride layer, a first silicon oxide layer, a second silicon nitride layer, and a second silicon oxide layer on a substrate in sequence, etching a pattern, and finally depositing silicon oxide as a protective layer. The present invention, through the constraint of the double-layer silicon nitride microring, concentrates the TM mode in the silicon oxide between the two layers of silicon nitride, overcomes the disadvantage of the weak Brillouin gain of the silicon nitride material, reduces the mode volume, reduces the Brillouin threshold, and realizes high-efficiency output of the Brillouin narrow linewidth laser.

Description

Technical Field

[0001] The present invention belongs to the field of integrated photonics, and in particular relates to an integrated Brillouin narrow-linewidth laser and a preparation method thereof. Background Art

[0002] Integrated photonics integrates various functional devices onto silicon-based platforms. With its advantages of small size, low power consumption, and high-volume production, it is widely used in computing, communications, sensing, and other fields, and has garnered significant attention and research from both industry and research in recent years. Because silicon itself cannot emit light, the realization of silicon-based light sources presents significant challenges, especially for narrow-linewidth lasers. Due to their low noise characteristics, narrow-linewidth lasers are widely used in coherent optical communications, optical atomic clocks, high-precision gyroscopes, optical frequency synthesis, and other fields.

[0003] Existing approaches use heterogeneously integrated III-V materials to couple the output laser into a silicon waveguide. However, III-V materials exhibit high noise levels and typically have linewidths in the MHz range, making them difficult to meet practical needs. Another approach is to couple the output laser into an external cavity, which has a large mode volume and low loss, significantly reducing the laser's linewidth. However, this approach requires a large external cavity and is extremely sensitive to feedback loops. Using the Brillouin gain of dielectric materials to generate narrow-linewidth lasers is also a viable approach. However, the Brillouin gain of most materials is weak, making effective excitation difficult. This invention utilizes a double-layer coupled microcavity to achieve mode volume compression, while leveraging the Brillouin gain of the cladding material to achieve efficient, integrated Brillouin narrow-linewidth laser generation. Summary of the Invention

[0004] The purpose of the present invention is to provide an integrated Brillouin narrow linewidth laser and a preparation method thereof to address the problems of non-monolithic integration, large laser linewidth and large chip area in the prior art.

[0005] To achieve the above objectives, the present invention adopts the following technical solutions: As a first aspect, an integrated Brillouin narrow-linewidth laser is provided, comprising the following structure: a pump source for providing pump light; a substrate; a cladding above the substrate; a double-layer straight waveguide and a double-layer microring embedded in the cladding, the double-layer straight waveguide being used to couple the pump light to the double-layer microring and output the generated laser light, the upper straight waveguide and the upper microring, and the lower straight waveguide and the lower microring being respectively located at the same height (the planes on which they are located are parallel to the plane of the substrate), and are all made of a material having a higher refractive index than the cladding;

[0006] The thickness of each layer of microrings is set to 100-1000 nm, and the interval between double-layer microrings is set to 10-150 nm, so as to achieve strong confinement of the TM mode in the interval between the double-layer microrings.

[0007] Furthermore, the integrated Brillouin narrow linewidth laser further includes: a polarization controller for adjusting the polarization state of the pump light to ensure that the incident light is in TM mode.

[0008] Furthermore, the double-layer straight waveguide and the double-layer microring are made of silicon nitride, and the cladding is made of silicon oxide.

[0009] Furthermore, the free spectrum range of the double-layer microring is set to be equal to the Brillouin frequency shift.

[0010] In some preferred embodiments, each layer of straight waveguide is 200-1000 nm away from each layer of microring.

[0011] In some preferred embodiments, each straight waveguide layer has a thickness of 100-1000 nm and a width of 1-10 μm.

[0012] In some preferred embodiments, the width of each layer of microrings is 1-10 μm.

[0013] As a second aspect, the present invention also provides a method for preparing an integrated Brillouin narrow linewidth laser, wherein a first silicon nitride layer, a first silicon oxide layer, a second silicon nitride layer, and a second silicon oxide layer are sequentially deposited on a substrate, and a straight waveguide and a microring are etched; after etching the straight waveguide and the microring, silicon oxide is deposited to cover the groove and the top of the pattern as a protective layer.

[0014] The beneficial effects of the present invention are: providing an integrated, efficient solution for generating a Brillouin narrow linewidth laser, by inputting laser into a double-layer coupled waveguide, wherein a cladding material is located between the upper and lower core waveguides, and the cladding thickness is very small. Due to the constraints of the upper and lower core waveguides, the TM mode is confined to the narrow cladding in the middle, thereby effectively constraining the mode volume, and by utilizing the Brillouin gain of the cladding material, a highly efficient Brillouin narrow linewidth laser output is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of narrow linewidth laser generation in the present invention;

[0016] Figure 2 Top view of the double-layer microring and double-layer waveguide;

[0017] Figure 3 The cross-sectional view of the waveguide-coupled double-layer microring;

[0018] Figure 4 Schematic diagram of the optical field distribution of the double-core waveguide and the single-core waveguide, where: Figure 4 (a) is a schematic diagram of the light field distribution of the TM mode of the double-layer core waveguide. Figure 4 (b) is a schematic diagram of the light field distribution of the TM mode of a single-layer core waveguide;

[0019] Figure 5 This is the preparation diagram of the integrated Brillouin laser, where: Figure 5 (a) is a schematic diagram of a silicon oxide wafer. Figure 5 (b) is a schematic diagram of depositing the first silicon nitride layer. Figure 5 (c) is a schematic diagram of depositing the first silicon oxide layer. Figure 5 (d) is a schematic diagram of depositing the second silicon nitride layer. Figure 5 (e) is a schematic diagram of depositing a second silicon oxide layer as a hard mask layer. Figure 5 (f) is a schematic diagram of the photolithography step. Figure 5 (g) is a schematic diagram of the etching and stripping steps. Figure 5 (h) is a schematic diagram of depositing a silicon oxide protective layer, where blue represents the core material, gray represents the cladding material, and black represents silicon. DETAILED DESCRIPTION

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0021] It should be noted that, unless there is any conflict, the features in the following embodiments and implementations may be combined with each other.

[0022] The integrated Brillouin narrow linewidth laser provided by the present invention includes the following parts: a pump source for providing pump light; a polarization controller for adjusting the polarization state of the pump light to ensure that the light incident on the straight waveguide is in the TM mode; a substrate; a cladding layer above the substrate; a double-layer straight waveguide (hereinafter referred to as waveguide) and a double-layer microring (core waveguide) embedded in the cladding layer, the double-layer straight waveguide being used to couple the pump light to the double-layer microring and output the generated laser light, and the upper straight waveguide and the upper microring, as well as the lower straight waveguide and the lower microring, are respectively at the same height. Figure 1 The figure shows a schematic diagram of laser generation in a narrow linewidth laser provided by the present invention, where the black light represents pump laser and the red light represents reverse Brillouin laser.

[0023] Double-layer microrings are generally made of materials with a higher refractive index. In this invention, silicon nitride (Si3N4) is used, which can be replaced by other materials with a higher refractive index than the cladding. The cladding is made of a material with a lower refractive index, typically silicon oxide (SiO2).

[0024] In a single-layer waveguide structure, due to mechanisms such as total internal reflection, the light field energy is usually concentrated in the core layer of the high-refractive index material. However, the present invention sets two high-refractive index waveguides in close proximity with a low-refractive index material in between, and controls the incident light to be in TM mode. At this time, due to the constraints of the electromagnetic field boundary conditions, the product of the dielectric constant and the electric field component must be continuous at the interface, thereby causing the electric field to change, and the light field distribution to change accordingly, and the light energy will be transferred from the high-refractive index material to the low-refractive index material.

[0025] Based on the above principle, when the output laser from the waveguide is coupled into the double-layer microcavity, the TM mode is confined to the narrow cladding material in the middle due to the constraints of the upper and lower core waveguides, significantly reducing the mode volume. Because the Brillouin threshold is proportional to the mode volume and inversely proportional to the square of the microcavity's quality factor, the threshold for Brillouin laser generation is greatly reduced. At the same time, most of the mode is distributed in the silicon oxide cladding, and the Brillouin gain of silicon oxide material is stronger than that of silicon nitride, further reducing the generation threshold. When the pump light power exceeds the threshold, Brillouin narrow-linewidth laser light can be collected in the opposite direction of the pump light.

[0026] By adjusting the thickness and spacing of the double-layer microrings and performing simulation analysis, we can determine the appropriate parameters for confining the TM mode to the cladding, or achieving "strong confinement." In this invention, the spacing between the upper and lower microrings, i.e., the cladding thickness, is set to 10-150 nm, while the thickness of the single-layer microring is 100-1000 nm.

[0027] To achieve mode matching and efficient excitation, the pump mode and Brillouin mode are designed to be separated by exactly one free spectral range (FSR), ensuring that the FSR of the double-layer microring is equal to the Brillouin frequency shift. The FSR is calculated from the speed of light, the effective refractive index of the mode, and the circumference of the microring. The effective refractive index of the mode is determined by the width and thickness of the microring, as well as the separation between the two microrings. The microring shape is not restricted and can be circular, racetrack, or other shapes, as long as the length meets the requirement that the Brillouin frequency shift is equal to the FSR.

[0028] like Figure 2 Shown is a top view of a double-layer microring and waveguide. From this top-view perspective, the upper and lower waveguide layers, as well as the upper and lower microring layers, completely overlap. The spacing between each waveguide layer and each microring layer is between 200 and 1000 nm, and the width of each waveguide layer is between 1 and 10 μm. The thickness of a single waveguide layer is between 100 and 1000 nm. The width and thickness of the upper waveguide layer and the upper microring layer, as well as the lower waveguide layer and the lower microring layer, are consistent and located at the same height. However, the thickness of the upper and lower layers does not necessarily need to be equal; it only needs to be sufficient to confine the light field within the cladding.

[0029] like Figure 3Shown is a schematic cross-section of a waveguide-coupled double-layer microring in an integrated Brillouin narrow-linewidth laser. The left side shows the cross-section of the waveguide-microring coupling, and the right side shows the cross-section of the microring. The spacing between the upper and lower waveguide layers is 10-150 nm, which significantly reduces the mode volume.

[0030] Example 1:

[0031] The microring has a radius of 2712μm, a width of 1μm, and a single-layer thickness of 300nm. The spacing between the double-layer microrings is 50nm, and the spacing between the waveguide and the microring is 800nm. The evanescent field of light transmitted in the waveguide extends into the microring, thereby achieving optical coupling. The width and thickness of the single-layer waveguide are consistent with those of the single-layer microring. The double-layer microring is made of silicon nitride, which has low material loss and can reduce the Brillouin generation threshold. The cladding is made of silicon oxide, which has low material loss and high Brillouin gain, achieving efficient Brillouin excitation.

[0032] like Figure 4 As shown in the figure, a single-layer core layer is set as a control, where the thickness of the single-layer core waveguide is the same as the total thickness of the double-layer core layer. The TM mode light field distribution of the single-layer core waveguide and the double-layer core waveguide of this embodiment are compared. The results show that the TM mode volume of the double-layer core (1.69×10 -20 m 3 ) is only a single core layer (4.02×10 -20 m 3 ) by 42% and the Brillouin threshold by 58%.

[0033] Example 2:

[0034] The present invention also provides a method for preparing an integrated Brillouin narrow linewidth laser. Figure 5 As shown, a first silicon nitride layer (lower core layer), a first silicon oxide layer (cladding layer), a second silicon nitride layer (upper core layer), and a second silicon oxide layer (hard mask layer) are sequentially deposited on a substrate (silicon oxide wafer). The thickness of each layer can be precisely controlled by the deposition time. A single photolithographic etching step creates the pattern (straight waveguides and microrings). Finally, silicon oxide is deposited to cover the trenches and the top of the pattern, acting as a protective layer.

[0035] It should be noted that 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 be modified and varied in various ways.

Claims

1. An integrated Brillouin narrow linewidth laser, characterized in that: include: A pump source, used to provide pump light; substrate; a cladding layer above a substrate; A double-layer straight waveguide and a double-layer microring are embedded in the cladding, the double-layer straight waveguide is used to couple pump light to the double-layer microring and output the generated laser light, the upper straight waveguide and the upper microring, the lower straight waveguide and the lower microring are respectively at the same height, and are all made of a material with a higher refractive index than the cladding; The thickness of each layer of microrings is set to 100-1000 nm, and the interval between double-layer microrings is set to 10-150 nm, so as to achieve strong confinement of the TM mode in the interval between the double-layer microrings.

2. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that: The integrated Brillouin narrow linewidth laser further includes a polarization controller for adjusting the polarization state of the pump light to ensure that the incident light is in TM mode.

3. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that: The double-layer straight waveguide and the double-layer micro-ring are made of silicon nitride, and the cladding is made of silicon oxide.

4. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that: The free spectral range of the double-layer microring is set to be equal to the Brillouin frequency shift.

5. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that: The distance between each layer of straight waveguide and each layer of microring is 200-1000nm.

6. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that: The thickness of each straight waveguide layer is 100-1000nm and the width is 1-10μm.

7. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that: The width of each layer of microrings is 1-10 μm.

8. A method for preparing the integrated Brillouin narrow linewidth laser according to any one of claims 1 to 7, characterized in that: A first silicon nitride layer, a first silicon oxide layer, a second silicon nitride layer, and a second silicon oxide layer are sequentially deposited on a substrate, and a straight waveguide and a micro-ring are etched.

9. The method for preparing an integrated Brillouin narrow linewidth laser according to claim 8, characterized in that: The preparation method further comprises: after etching the straight waveguide and the micro-ring, depositing silicon oxide to cover the groove and the top of the pattern as a protective layer.

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