Integrated Brillouin narrow linewidth laser and preparation method thereof

Abstract

The invention discloses an integrated Brillouin narrow linewidth laser and a preparation method thereof, and belongs to the field of integrated photonics. The integrated Brillouin narrow linewidth laser is composed of two layers of silicon nitride micro-rings, a silicon oxide spacing layer is arranged between the two layers of silicon nitride micro-rings, the free spectral path of the two layers of silicon nitride micro-rings is designed to be Brillouin frequency shift, and coupling waveguides are designed beside the micro-rings. The invention also provides a corresponding preparation method, which comprises the following steps of: sequentially depositing the first silicon nitride layer, the first silicon oxide layer, the second silicon nitride layer, the second silicon oxide layer and the etching pattern on the substrate, and finally depositing silicon oxide as a protective layer. The TM mode is intensively distributed in the silicon oxide between the two layers of silicon nitride through the constraint of the two layers of silicon nitride micro-rings, the defect that the Brillouin gain of a silicon nitride material is weak is overcome, the mode size is reduced, the Brillouin threshold value is reduced, and high-efficiency output of the Brillouin narrow linewidth laser is achieved.

Description

Technical Field

[0001] The 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 on a silicon-based platform. It has the advantages of small size, low power consumption, and mass production. It is widely used in computing, communication, sensing and other fields. In recent years, it has received a lot of attention and research from the industry and scientific research community. Since silicon materials themselves cannot emit light, there are great difficulties in the realization of silicon-based light sources, especially narrow-linewidth lasers. Narrow-linewidth lasers are widely used in coherent optical communications, optical atomic clocks, high-precision gyroscopes, optical frequency synthesis and other fields due to their low noise characteristics.

[0003] In the existing scheme, heterogeneously integrated III-V materials can be used to couple the output laser to the silicon waveguide. However, the noise of III-V materials is relatively large, and the line width is usually in the MHz level, which is difficult to meet the actual needs. Another method is to couple the output laser to an external cavity. The external cavity has a large mode volume and low loss, which can significantly reduce the line width of the laser. However, the volume of the external cavity of this scheme is huge and it is extremely sensitive to the feedback loop. Using the Brillouin gain of dielectric materials to generate narrow-linewidth lasers is also a feasible solution. However, the Brillouin gain of most materials is weak and it is difficult to achieve effective excitation. The present invention will use a double-layer coupled microcavity to achieve mode volume compression, and at the same time use 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 in view of the problems of non-monolithic integration, large laser linewidth and large chip area in the prior art.

[0005] To achieve the above-mentioned purpose, the technical scheme adopted by the present invention is as follows: 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 on 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, the upper straight waveguide and the upper microring, the lower straight waveguide and the lower microring are respectively at the same height (the planes where they are located are all parallel to the plane where the substrate is located), 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.

[0006] Further, 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 the TM mode.

[0007] Further, the double-layer straight waveguide and the double-layer micro-ring are made of silicon nitride, and the cladding is made of silicon oxide.

[0008] Further, the free spectral range of the double-layer micro-ring is set to be equal to the Brillouin frequency shift.

[0009] In some preferred embodiments, each layer of the straight waveguide is spaced 200 - 1000 nm from each layer of the micro-ring.

[0010] In some preferred embodiments, the thickness of each layer of the straight waveguide is 100 - 1000 nm, and the width is 1 - 10 μm.

[0011] In some preferred embodiments, the width of each layer of the micro-ring is 1 - 10 μm.

[0012] As a second aspect, the present invention also provides a method for manufacturing an integrated Brillouin narrow linewidth laser. 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 the straight waveguide and the micro-ring are etched; after the straight waveguide and the micro-ring are etched, silicon oxide is deposited to cover the trenches and the top of the pattern as a protective layer.

[0013] The beneficial effects of the present invention are as follows: It provides a generation scheme for an integrated and efficient Brillouin narrow linewidth laser. By inputting the laser into a double-layer coupled waveguide, the cladding material is between the upper and lower core layer waveguides, and the cladding thickness is very small. Due to the confinement of the upper and lower core layer waveguides, the TM mode is confined within the narrow middle cladding, thereby effectively confining the mode volume, and by means of the Brillouin gain of the cladding material, the output of a high-efficiency Brillouin narrow linewidth laser is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a schematic diagram of the generation of narrow linewidth laser in the present invention; Figure 2 is a top view of the double-layer micro-ring and the double-layer waveguide; Figure 3 is a cross-sectional view of the waveguide-coupled double-layer micro-ring; Figure 4 is a schematic diagram of the optical field distribution of the double-core layer waveguide and the single-core layer waveguide. Among them, Figure 4 in (a) is a schematic diagram of the optical field distribution of the TM mode of the double-layer core layer waveguide, Figure 4 in (b) is a schematic diagram of the optical field distribution of the TM mode of the single-layer core layer waveguide; Figure 5 is a manufacturing diagram of the integrated Brillouin laser. Among them, Figure 5Among them, (a) is a schematic diagram of a silicon oxide wafer, Figure 5 Among them, (b) is a schematic diagram of depositing the first silicon nitride layer, Figure 5 Among them, (c) is a schematic diagram of depositing the first silicon oxide layer, Figure 5 Among them, (d) is a schematic diagram of depositing the second silicon nitride layer, Figure 5 Among them, (e) is a schematic diagram of depositing the second silicon oxide layer as a hard mask layer, Figure 5 Among them, (f) is a schematic diagram of a lithography step, Figure 5 Among them, (g) is a schematic diagram of an etching and degluing step, Figure 5 Among them, (h) is a schematic diagram of depositing a silicon oxide protective layer, where blue represents the core layer material, gray represents the cladding material, and black represents silicon. Detailed implementation manners

[0015] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0016] It should be noted that, without conflict, the features in the following embodiments and implementation manners can be combined with each other.

[0017] 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 above the substrate; a double-layer straight waveguide (hereinafter referred to as a waveguide) and a double-layer micro-ring (core layer waveguide) embedded in the cladding. The double-layer straight waveguide is used to couple the pump light to the double-layer micro-ring and output the generated laser. The upper straight waveguide and the upper micro-ring, and the lower straight waveguide and the lower micro-ring are respectively at the same height. As Figure 1 Shown is a schematic diagram of laser generation in a narrow linewidth laser provided by the present invention. The black light represents the pump laser, and the red light represents the backward Brillouin laser.

[0018] The double-layer micro-ring generally uses a material with a relatively high refractive index. In the present invention, silicon nitride (Si3N4) is used, which can be replaced by other materials with a refractive index higher than that of the cladding. The cladding uses a material with a relatively low refractive index, usually silicon oxide (SiO2).

[0019] In a single-layer waveguide structure, due to mechanisms such as total internal reflection, the optical field energy is usually concentrated in the core layer of the high refractive index material. However, in the present invention, two high refractive index material waveguides are arranged adjacent to each other with a low refractive index material in between, and the incident light is controlled to be in the 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 needs to be continuous at the interface, which causes the electric field to change, and the optical field distribution changes accordingly, and the optical energy will transfer from the high refractive index material to the low refractive index material.

[0020] Based on the above principle, when the output laser in the waveguide is coupled into the double-layer microcavity, due to the constraints of the upper and lower core layer waveguides, the TM mode will be confined in the narrow cladding material in the middle, and the mode volume is greatly reduced. Since the Brillouin threshold is proportional to the mode volume and inversely proportional to the square of the quality factor of the microcavity, the threshold for generating Brillouin laser is greatly reduced. At the same time, most of the modes are distributed in the silica cladding, and the Brillouin gain of the silica material is stronger than that of silicon nitride, further reducing the generated threshold. When the pump light power exceeds the threshold, Brillouin narrow linewidth laser can be collected in the reverse direction of the pump light.

[0021] By adjusting the thickness and spacing of the double-layer micro-ring and performing simulation analysis, appropriate parameters that can confine the TM mode in the cladding or achieve "strong confinement" can be obtained. In the present invention, the spacing between the upper and lower micro-rings, that is, the cladding thickness, is set to be 10 - 150 nm, and the thickness of the single-layer micro-ring is 100 - 1000 nm.

[0022] To achieve mode matching and efficient excitation, the pump mode and the Brillouin mode are designed to be exactly separated by one free spectral range (FSR), that is, the FSR of the double-layer micro-ring is equal to the Brillouin frequency shift. The FSR is calculated by the speed of light, the effective refractive index of the mode, and the circumference of the micro-ring; the effective refractive index of the mode is determined by the width, thickness of the micro-ring, and the spacing of the double-layer micro-ring; the shape of the micro-ring is not limited and can be circular or racetrack-shaped or others, as long as it meets the length requirement that the Brillouin frequency shift is equal to the FSR.

[0023] As Figure 2 shown is the top view of the double-layer micro-ring and the double-layer waveguide. From the top view angle, the upper waveguide and the lower waveguide, the upper micro-ring and the lower micro-ring are completely overlapped in position. The spacing between each layer of waveguide and each layer of micro-ring is between 200 - 1000 nm, the width of each layer of waveguide is between 1 - 10 μm, the thickness of the single-layer waveguide is between 100 - 1000 nm, the widths and thicknesses between the upper waveguide and the upper micro-ring, and between the lower waveguide and the lower micro-ring are respectively the same and are at the same height plane, but the thicknesses of the upper and lower layers are not necessarily equal, and only need to meet the requirement of confining the optical field in the cladding.

[0024] As Figure 3The figure shows a cross-sectional schematic diagram of a waveguide-coupled double-layer micro-ring in an integrated Brillouin narrow linewidth laser. The left side is the cross-sectional view of the waveguide-micro-ring coupling, and the right side is the cross-sectional view of the micro-ring. The spacing between the upper and lower waveguides is 10 - 150 nm, which can greatly reduce the mode volume.

[0025] Example 1: Set the micro-ring radius to 2712 μm, width to 1 μm, single-layer thickness to 300 nm, the spacing between the double-layer micro-rings to 50 nm, and the spacing between the waveguide and the micro-ring to 800 nm. The evanescent field of the light transmitted in the waveguide can extend into the micro-ring, thus realizing optical coupling. The width and thickness of the single-layer waveguide are the same as those of the single-layer micro-ring. The double-layer micro-ring is made of silicon nitride with low material loss, which can reduce the threshold of Brillouin generation. The cladding is made of silicon oxide with low material loss and large Brillouin gain, enabling efficient Brillouin excitation.

[0026] As Figure 4 shown, a single-layer core layer is set as a control, where the thickness of the single-layer core layer waveguide is the same as the total thickness of the double-layer core layer. The TM-mode optical field distributions of the waveguide of the single-layer core layer and the double-layer core layer waveguide of this embodiment are compared. The results show that the TM-mode volume of the double-layer core layer (1.69×10 -20 m 3 ) is only 42% of that of the single-layer core layer (4.02×10 -20 m 3 ), and the Brillouin threshold is reduced by 58%.

[0027] Example 2: The present invention also provides a preparation method for an integrated Brillouin narrow linewidth laser. As Figure 5 shown, a first silicon nitride layer (lower core layer), a first silicon oxide layer (cladding), 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 pattern (straight waveguide and micro-ring) is prepared through one-time photolithography etching, and finally silicon oxide is deposited to cover the trenches and the top of the pattern as a protective layer.

[0028] It should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications.

Claims

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

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

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

4. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that, Set the free spectral range of the double-layer micro-ring to be equal to the Brillouin frequency shift amount.

5. The integrated Brillouin narrow linewidth laser according to claim 1, characterized in that Each layer of the straight waveguide is 200 - 1000 nm away from each layer of the micro-ring.

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

7. The integrated Brillouin narrow linewidth laser according to claim 1, wherein, The width of each layer of the micro-ring is 1 - 10 μm.

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

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 out the straight waveguide and the micro-ring, depositing silicon oxide to cover the trenches and the top of the pattern as a protective layer.

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