Apodization grating coupler based on thin film lithium niobate platform

Abstract

An apodization grating coupler based on a thin film lithium niobate platform comprises a silicon substrate, a buried silicon oxide layer, a thin film lithium niobate layer and a silicon oxide cladding which are sequentially arranged from bottom to top, and an end face coupler, a thin film lithium niobate waveguide, a light beam expander and an apodization grating are sequentially arranged in the thin film lithium niobate layer. The end face coupler couples light of the conical optical fiber into the thin film lithium niobate waveguide, and after the light is input into the apodization grating through the light beam expander, large-beam-waist Gaussian light beams are coupled through diffraction of the apodization grating. On the basis of the large-size apodization grating, through a compact plane photon structure, the 700 nm * 300 nm lithium niobate high-refractive-index single-mode waveguide can be coupled to a near-surface normal Gaussian beam with a beam waist of about 110 m and good collimation, the diffraction angle is 10 degrees, centimeter-level propagation is achieved in a free space, and the loss is 5.6 dB.

Description

Technical Field

[0001] This invention relates to a technology in the field of integrated photonics, specifically an apodization grating coupler based on a thin-film lithium niobate platform for coupling waveguide light to a beam pattern in free space. Background Technology

[0002] The integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can drive scientific advancements and enable new miniaturized hybrid devices and technologies. Optical interactions with small evanescent volumes limit the performance of applications such as gas spectroscopy, and there is a general need for the ability to access light fields photonically within large free-space volumes. However, conventional inverted taper and grating couplers cannot be directly scaled to produce wide, high-quality collimated beams, requiring additional bulky collimating optics and expensive alignment to propagate low-loss, diffraction-free beams several millimeters in free space. Summary of the Invention

[0003] This invention addresses the shortcomings of existing technologies, which can only achieve coupling between gratings and optical fibers, involve complex processes and extremely low coupling efficiency, and require optical fiber coupling to propagate freely in free space. It proposes an apodization grating coupler based on a thin-film lithium niobate platform. Based on a large-size apodization grating and a compact planar photonic structure, it can couple a 700nm×300nm high-refractive-index lithium niobate single-mode waveguide to a well-collimated near-surface normal Gaussian beam with a beam waist of approximately 110µm, achieving a diffraction angle of [missing information]. With a range of 10°, centimeter-level propagation is achieved in free space with a loss of 5.6dB.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to an apodization grating coupler based on a thin-film lithium niobate platform, comprising: a silicon substrate, a buried silicon oxide layer, a thin-film lithium niobate layer, and a silicon oxide cladding layer arranged sequentially from bottom to top, wherein: an end-face coupler, a thin-film lithium niobate waveguide, a beam expander, and an apodization grating are sequentially arranged in the thin-film lithium niobate layer; the end-face coupler couples the light from the tapered optical fiber into the thin-film lithium niobate waveguide; after passing through the beam expander and inputting into the apodization grating, a large-waisted Gaussian beam is coupled out through diffraction by the apodization grating.

[0006] The beam expander is preferably implemented as a tapered waveguide, and more preferably has a length of 2 mm.

[0007] The dimensional parameters of the apodization grating are obtained as follows: The period and duty cycle of the apodization grating can be determined based on the target Gaussian beam power distribution and by calculating the effective refractive index of the etched and unetched waveguides. This allows for the creation of a device capable of converting waveguide light into spatial light that can propagate in free space. Specifically, the duty cycle satisfies: Prague Conditions Where: FF is the duty cycle, z is the propagation direction, G(z) is the Gaussian beam intensity distribution with unit total power, d is the fitted value related to the etching depth, and n0 and n e ηa represents the effective refractive index of the unetched teeth and etched grooves of the waveguide grating cross section, respectively, and n1 represents the refractive index of the top dielectric. The diffraction angle, λ is the wavelength.

[0008] The external dimensions of the apodization grating are preferably 300μm×300μm.

[0009] This invention relates to an application of the aforementioned apodization grating coupler for frequency stabilization of an on-chip laser.

[0010] The application specifically includes: converting the fiber-coupled 852nm cooling and re-pumping beam into a millimeter-wide non-divergent free-space cooling and re-pumping beam using the aforementioned apodization grating coupler, and then directly sending it to a cesium atom gas cell for atomic cooling, thereby achieving frequency stabilization of the on-chip laser.

[0011] Technical effect

[0012] Compared with existing technologies, this invention, based on a large-size apodization grating and a compact planar photonic structure, enables the coupling of a 700nm×300nm lithium niobate high-refractive-index single-mode waveguide to a well-collimated near-surface normal Gaussian beam with a beam waist of approximately 110µm at an operating wavelength of 852nm, achieving a diffraction angle of [missing information]. With a range of 10°, centimeter-level propagation is achieved in free space with a loss of 5.6dB. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of the present invention;

[0014] Figure 2 The inset is a 2D apodization grating simulation power spectrum diagram of this invention, and the inset is a 3D grating planar power diagram.

[0015] Figure 3 This is a loss diagram of the apodization grating of the present invention;

[0016] In the figure: 1. Silicon substrate, 2. Buried silicon oxide layer, 3. Thin film lithium niobate layer, 4. Silicon oxide cladding, 5. Beam expander, 6. Apodization grating, 7. Tapered optical fiber, 8. Thin film lithium niobate waveguide, 9. End coupler. Detailed Implementation

[0017] like Figure 1As shown, this embodiment relates to an apodization grating coupler based on a thin-film lithium niobate platform, fabricated on a TFLN platform. It includes: a silicon substrate 1, a buried silicon oxide layer 2, a thin-film lithium niobate layer 3, and a silicon oxide cladding layer 4 arranged sequentially from bottom to top. The thin-film lithium niobate layer 3 is provided with an end-face coupler 9, a thin-film lithium niobate waveguide 8, a beam expander 5, and an apodization grating 6 in sequence. The end-face coupler 9 couples the light from the tapered fiber 7 into the thin-film lithium niobate waveguide 8. After passing through the beam expander 5 and inputting into the apodization grating 6, the light is diffracted and coupled into a Gaussian beam with a large beam waist through the apodization grating 6.

[0018] The etching depth of the apodization grating is 70 nm.

[0019] The refractive index of the buried silicon oxide layer 2 is n=1.445.

[0020] The thickness of the buried silicon oxide layer 2 is 2 μm.

[0021] The refractive index of the thin-film lithium niobate layer 3 is n=2.2.

[0022] The thickness of the thin-film lithium niobate layer 3 is 300 nm.

[0023] The refractive index of the silicon oxide cladding 4 is n=1.445.

[0024] The thickness of the silicon oxide cladding 4 is 1 μm.

[0025] This embodiment relates to the implementation method of the above-mentioned large-scale beam external coupler based on a large-size apodization grating, including:

[0026] Step 1: Simulate a uniform grating coupler to obtain the exponential attenuation power of the output beam. .

[0027] Step 2: According to the formula The inverse solution yields d = 0.0092.

[0028] Step 3: The normalized Gaussian power function is Set the target waist w0=100μm.

[0029] Step 4: According to Solve for the duty cycle FF(z).

[0030] Step 5: Based on the Prague conditions: Determine the period Where: n0 = 2.069394, n e =1.997747, n1=1.445, It is 852nm.

[0031] Step 6: The end-face coupler couples the light in the tapered fiber to the thin-film lithium niobate waveguide. The light in the waveguide is then fed into the apodization grating via a beam expander. The apodization grating then diffracts and couples the light into a Gaussian beam with a large beam waist.

[0032] Compared with the prior art, the present invention changes the duty cycle through steps 1-5. and cycle The fabricated apodization grating can couple out a near-Gaussian beam with a beam waist of nearly 110µm, realizing the conversion of light from on-chip waveguide to free space. It can directly emit light waves with a diameter of hundreds of micrometers from the chip, and can propagate for several millimeters in free space with low loss and no diffraction without additional bulky collimating optics and expensive alignment. At the same time, it is compatible with microelectronic CMOS technology and the fabrication process is relatively simple.

[0033] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. An apodization grating coupler based on a thin-film lithium niobate platform, characterized in that, include: The structure consists of a silicon substrate, a buried silicon oxide layer, a thin-film lithium niobate layer, and a silicon oxide cladding layer arranged sequentially from bottom to top. The thin-film lithium niobate layer contains an end-face coupler, a thin-film lithium niobate waveguide, a beam expander, and an apodization grating. The end-face coupler couples the light from the tapered optical fiber into the thin-film lithium niobate waveguide. After passing through the beam expander and being input into the apodization grating, the light is diffracted and coupled into a Gaussian beam with a large beam waist through the apodization grating.

2. The apodization grating coupler based on a thin-film lithium niobate platform according to claim 1, characterized in that, The dimensional parameters of the apodization grating are obtained as follows: The period and duty cycle of the apodization grating can be determined based on the target Gaussian beam power distribution and by calculating the effective refractive index of the etched and unetched waveguides. This results in a device capable of converting waveguide light into spatial light that can propagate in free space. Specifically, the duty cycle satisfies: Prague Conditions Where: FF is the duty cycle, z is the propagation direction, G(z) is the Gaussian beam intensity distribution with unit total power, d is the fitted value related to the etching depth, and n0 and n e ηa represents the effective refractive index of the unetched teeth and etched grooves of the waveguide grating cross section, respectively, and n1 represents the refractive index of the top dielectric. The diffraction angle, λ is the wavelength.

3. The apodization grating coupler based on a thin-film lithium niobate platform according to claim 1 or 2, characterized in that, The apodization grating has an external dimension of 300μm × 300μm; the apodization grating has an etching depth of 70nm; the buried silicon oxide layer 2 has a refractive index n = 1.445; the buried silicon oxide layer 2 has a thickness of 2μm; the thin-film lithium niobate layer 3 has a refractive index n = 2.2; the thin-film lithium niobate layer 3 has a thickness of 300nm; the silicon oxide cladding layer 4 has a refractive index n = 1.445; the silicon oxide cladding layer 4 has a thickness of 1μm.

4. A method for implementing the apodization grating coupler according to any one of claims 1-3, characterized in that, include: Step 1: Simulate a uniform grating coupler to obtain the exponential attenuation power of the output beam. ; Step 2: According to the formula The inverse solution yields d = 0.0092; Step 3: The normalized Gaussian power function is Set the target waist w0 = 100 μm; Step 4: According to Solve for the duty cycle FF(z); Step 5: Based on the Prague conditions: Determine the period Where: n0 = 2.069394, n e =1.997747, n1=1.445, It is 852nm; Step 6: The end-face coupler couples the light in the tapered fiber to the thin-film lithium niobate waveguide. The light in the waveguide is then fed into the apodization grating via a beam expander. The apodization grating then diffracts and couples the light into a Gaussian beam with a large beam waist.

5. An application of an apodization grating coupler based on any one of claims 1-3 or the method of claim 4, characterized in that, Its application to frequency stabilization of on-chip lasers specifically includes: converting the fiber-coupled 852nm cooling and re-pumping beam into a millimeter-wide non-divergent free-space cooling and re-pumping beam through the aforementioned apodization grating coupler, and then directly sending it to the cesium atom gas chamber for atomic cooling, thereby achieving frequency stabilization of the on-chip laser.

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