Design method of ridge waveguide for eliminating stray light in nonlinear frequency change and obtained ridge waveguide

Abstract

Stray light generated by nonlinear frequency change may compete for energy resources of a nonlinear process with a target frequency signal, weaken a main signal, and trigger secondary frequency, harmonic waves or other frequency mixing components. The invention discloses a ridge waveguide design method for eliminating non-linear frequency change stray light, and the method achieves the continuous sine modulation in a waveguide through designing a waveguide structure in a sine curve region domain polarization direction, inhibits stray light, facilitates the concentration of pumping energy for the generation of a target frequency, improves the conversion efficiency, and achieves the purpose of eliminating non-linear frequency change stray light. And the long-term working stability and repeatability of the system can be improved.

Description

Technical Field

[0001] The present invention belongs to the field of optical design, and in particular relates to a ridge waveguide design method for eliminating stray light caused by nonlinear frequency variation and the obtained ridge waveguide. Background Art

[0002] Quasi-phase matching (QPM) is an important technique for nonlinear optical frequency conversion. By constructing periodic structures within a nonlinear medium, it effectively achieves nonlinear frequency conversion. Compared to phase matching, QPM is more likely to achieve greater nonlinear effects. QPM has been widely used in the field of nonlinear optics.

[0003] Quasi-phase-matched crystals include ferroelectric crystals such as lithium niobate, lithium tantalate, and potassium titanyl phosphate. These ferroelectric crystals possess ferroelectric domains that can be reversed by applying an electric field. Quasi-phase matching exploits this characteristic of ferroelectric crystals by periodically reversing the domain orientation, causing the nonlinear polarizability along the direction of light propagation to alternate between +1, -1, +1, and -1, thereby compensating for the phase mismatch in nonlinear frequency conversion. However, this ±1 periodic variation has multiple reciprocal lattice vectors in Fourier space. During nonlinear frequency conversion, stray light from the frequency conversion caused by these multiple stray reciprocal lattice vectors is often observed. This stray light constitutes noise, significantly affecting the results of the nonlinear frequency conversion.

[0004] Therefore, a method for eliminating stray light during the nonlinear frequency conversion process is urgently needed. Summary of the Invention

[0005] Purpose of the invention: To solve the problem of eliminating stray light in the nonlinear frequency conversion process in the prior art, the present invention proposes a quasi-phase-matched ridge waveguide design method and the resulting ridge waveguide, which can weaken or even eliminate stray reciprocal lattice vectors.

[0006] Technical solution:

[0007] The present invention proposes a ridge waveguide design method for eliminating nonlinear frequency-varying stray light, comprising:

[0008] Setting the waveguide parameters of the second-order nonlinear process, including the wavelength of the fundamental light and the wavelength of the frequency-doubled light; setting the standard ridge waveguide material and geometric parameters, including the height and width of the raised portion of the standard ridge waveguide;

[0009] Calculating the effective refractive index of the fundamental wave light and the frequency-doubled light in the waveguide by a finite element algorithm to obtain a phase matching condition, and calculating the polarization period based on the effective refractive index and the phase matching condition;

[0010] Designing a sinusoidal curve based on the polarization period, making a pattern template of the sinusoidal curve, and preparing a metal electrode corresponding to the pattern template on a standard ridge waveguide;

[0011] The ridge waveguide is subjected to room temperature electric field polarization, one side of the curve is polarized to a positive domain, and the other side is polarized to a negative domain. The metal electrode is removed to obtain a polarized ridge waveguide.

[0012] Furthermore, the standard ridge waveguide material is a ferroelectric material, including thin film lithium niobate.

[0013] Furthermore, the finite element algorithm includes:

[0014] Establish a two-dimensional structural model of the waveguide cross section in the simulation software, clarify the material distribution and structural dimensions, and set the material properties at different frequencies;

[0015] Set up an electromagnetic field eigenvalue problem and solve the propagation function β at the frequencies corresponding to the two wavelengths:

[0016]

[0017] The effective refractive index n1 of the fundamental light and the effective refractive index n2 of the frequency-doubled light are obtained, and the phase matching condition Δk is further calculated:

[0018] Δk=n2-2n1

[0019] Furthermore, the calculated polarization period Λ is:

[0020]

[0021] Where λ1 represents the wavelength of the fundamental light.

[0022] Furthermore, the designed sinusoidal curve includes:

[0023]

[0024] Where x and y are the coordinates of the points on the sine curve, and W is the width or height of the protruding part of the standard ridge waveguide.

[0025] Furthermore, the pattern template is a metal photolithography template made by electron beam etching, and the metal photolithography template is engraved with the sinusoidal curve pattern.

[0026] Furthermore, the step of preparing a metal electrode corresponding to the pattern template on the standard ridge waveguide includes:

[0027] When W is the width of the standard ridge waveguide, align the pattern template with the upper surface of the standard ridge waveguide; when W is the height of the standard ridge waveguide, align the pattern template with the side surface of the standard ridge waveguide;

[0028] After alignment, metal electrodes were fabricated by photolithography.

[0029] Furthermore, the ridge waveguide is subjected to room temperature electric field polarization, and the effective nonlinear coefficient d at any point (x, y) on the curve is eff (x) is expressed as:

[0030]

[0031] Where Sign(y) is the sign function that describes the distribution of positive and negative domains, E is the electric field strength, and d xx is the second-order polarization coefficient tensor, E(y) is the distribution function of the light field in the Y direction;

[0032] Get d eff (x) = sin(x)d xx , indicating that the nonlinear coefficient is periodically modulated after polarization.

[0033] A ridge waveguide obtained by the aforementioned ridge waveguide design method for eliminating nonlinear frequency variation stray light, the ridge waveguide comprising a substrate and a raised portion, the upper surface or side surface of the raised portion being engraved with a sinusoidal curve pattern, the sinusoidal curve extending in the same direction as the waveguide extending direction, and the sinusoidal curve being tangent to the boundary edge between the upper surface and the side surface of the raised portion.

[0034] Furthermore, the ridge waveguide is bounded by the sinusoidal curve, with one side polarized as a positive domain and the other side polarized as a negative domain.

[0035] Beneficial effects:

[0036] The stray light generated by nonlinear frequency changes may compete with the target frequency signal for the energy resources of the nonlinear process, weaken the main signal, and induce sub-frequency, harmonics or other mixing components. The present invention realizes continuous sinusoidal modulation in the waveguide by designing a waveguide structure that distinguishes the polarization direction of the domain along a sine curve, suppresses stray light, helps to concentrate pump energy for the generation of the target frequency, improves conversion efficiency, and helps to improve the long-term working stability and repeatability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The three-dimensional structure of the ridge waveguide of the present invention;

[0038] Figure 2 is a top view of the ridge waveguide of the present invention;

[0039] Figure 3 is a curve diagram of the boundary between the positive domain and the negative domain of the ridge waveguide of the present invention;

[0040] Figure 4 It is a side view of another implementation of the present invention. DETAILED DESCRIPTION

[0041] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0042] The three-dimensional view of the ridge waveguide in this embodiment and the relationship between the coordinate axes are as follows: Figure 1 As shown, the ridge waveguide includes a substrate and a raised portion. The material of the ridge waveguide is thin film lithium niobate. The Z axis is the ridge waveguide raised direction, the Y axis is the waveguide width direction, and the X axis is the waveguide extension direction. The basic structure of the present invention is a sinusoidally polarized ridge waveguide, such as Figure 2 The figure shows a top view of the ridge waveguide, in which the light-colored area of the raised portion is the positive domain, the dark-colored area of the raised portion is the negative domain, and the domain boundary between the positive and negative domains is a sine (or cosine) curve.

[0043] The amplitude of the sinusoidal curve at the boundary between the positive domain and the negative domain is equal to half the width W of the waveguide protrusion. The extension direction of the sinusoidal curve is the same as the extension direction of the waveguide. The sinusoidal curve is tangent to the boundary edge between the upper surface and the side of the protrusion. Figure 3 As shown, the expression of the curve is:

[0044]

[0045] y is also the distance of the curve from the Z axis. At the same time, the distance of the curve from the horizontal line where W is located is:

[0046]

[0047] set up Figure 3 The part below the middle curve is the positive domain, and the part above is the negative domain. At any point ( Figure 3 The effective nonlinear coefficient of the dotted light wavefront E(y) is the integral along the Y axis at this point, expressed as:

[0048]

[0049] Among them, d eff (x) is the effective nonlinear coefficient at x, E is the electric field intensity, d xx is the second-order polarizability tensor, such as d xx =d 33 d xx =d 31 E(y) is the distribution function of the light field in the Y direction.

[0050] Where Sign(y) is the sign function, according to the distribution of positive and negative domains mentioned above:

[0051]

[0052] Simplifying the wavefront E(y) into a plane wave E, the effective nonlinear coefficient can be simplified as:

[0053]

[0054] Comparing the left and right terms, we can get:

[0055] d eff (x) = sin(x)d xx

[0056] Due to the sinusoidal polarization, the effective nonlinear coefficient d xx A periodic modulation is generated along x. Unlike quasi-phase matching, this modulation is continuous sinusoidal. According to the Fourier transform of the sine function, this sinusoidal modulation has only ±1 order reciprocal lattice vectors, and the spurious reciprocal lattice vectors are eliminated.

[0057] Below is a Figure 1 The specific example of the ridge waveguide in the form of a ridge waveguide is used to further illustrate the process of the design method of the present invention.

[0058] First, the waveguide parameters are designed according to the required second-order nonlinear process. The second-order nonlinear process set in this embodiment is the frequency doubling process of the 1550nm fundamental wave light, and the wavelength of the frequency doubling light is 775nm.

[0059] A ridge waveguide with a width of W = 1.6 μm was fabricated using a 0.28 μm-thick thin-film lithium niobate waveguide through micromachining. A finite element method was used to create a two-dimensional structural model of the waveguide cross section in simulation software. The material distribution and structural dimensions were determined, and the material properties at different frequencies were set. By setting up an electromagnetic field eigenvalue problem, the propagation function β was solved at the frequencies corresponding to the two wavelengths:

[0060]

[0061] The effective refractive index n1 of the fundamental light and the effective refractive index n2 of the frequency-doubled light are obtained, and the phase matching condition Δk is further calculated:

[0062] Δk=n2-2n1

[0063] Then the required polarization period is calculated. The specific calculation method refers to the literature "Red and green light generation in TPPLNOIridge optical waveguide with 1550nm picosecond laser" (Yan, Congliao, et al. Optics Communications 529(2023):129023.), which is:

[0064]

[0065] The calculated polarization period Λ in this embodiment is 4.4 μm. According to the aforementioned sinusoidal curve expression, after substituting specific parameters, the sinusoidal curve is expressed as:

[0066]

[0067] According to the above curve, a pattern template is made. A photolithography template is made by electron beam etching, where half of the template is covered by metal and the other half is transparent. The edge between the metal and transparent is designed. The metal edge of the template is aligned with the upper surface of the standard ridge waveguide using a microscope. Then, metal electrodes are prepared by photolithography and polarized by room temperature electric field. The polarized ridge waveguide has Figure 1 The shape shown in the figure is experimentally verified to show that after coupling in the fundamental wave light with a wavelength of 1550nm, the doubled frequency light is only 775nm, and no other stray light is generated.

[0068] In addition, another implementation of the present invention is as follows Figure 4 As shown in Figure 1, a sinusoidal polarization pattern is distributed on the side of the raised portion of the ridge waveguide. This pattern template is aligned with the side of the standard ridge waveguide in the previous article to form the metal electrode. This matching method uses the same principle as the previous one and can also achieve the effect of eliminating nonlinear frequency variation stray light.

Claims

1. A ridge waveguide design method for eliminating nonlinear frequency variation stray light, characterized in that: include: Setting the waveguide parameters of the second-order nonlinear process, including the wavelength of the fundamental light and the wavelength of the frequency-doubled light; Setting the material and geometric parameters of the standard ridge waveguide, wherein the geometric parameters include the height and width of the raised portion of the standard ridge waveguide; Calculating the effective refractive index of the fundamental wave light and the frequency-doubled light in the waveguide by a finite element algorithm to obtain a phase matching condition, and calculating the polarization period based on the effective refractive index and the phase matching condition; Designing a sinusoidal curve based on the polarization period, making a pattern template of the sinusoidal curve, and preparing a metal electrode corresponding to the pattern template on a standard ridge waveguide; The ridge waveguide is subjected to room temperature electric field polarization, so that one side of the sinusoidal curve is polarized to a positive domain and the other side is polarized to a negative domain, and the metal electrode is removed to obtain a polarized ridge waveguide.

2. The ridge waveguide design method according to claim 1, wherein: The standard ridge waveguide material is a ferroelectric material, including thin film lithium niobate.

3. The ridge waveguide design method according to claim 2, wherein: The finite element algorithm includes: Establish a two-dimensional structural model of the waveguide cross section in the simulation software, clarify the material distribution and structural dimensions, and set the material properties at different frequencies; Set up an electromagnetic field eigenvalue problem and solve the propagation function β at the frequencies corresponding to the two wavelengths: The effective refractive index n1 of the fundamental light and the effective refractive index n2 of the frequency-doubled light are obtained, and the phase matching condition Δk is further calculated: Δk=n2-2n1.

4. The ridge waveguide design method according to claim 3, wherein: The calculated polarization period Λ is: Where λ1 represents the wavelength of the fundamental light.

5. The ridge waveguide design method according to claim 4, wherein: The designed sine curve includes: Where x and y are the coordinates of the points on the sine curve, and W is the width or height of the protruding part of the standard ridge waveguide.

6. The ridge waveguide design method according to claim 5, wherein: The pattern template is a metal photolithography template made by electron beam etching, and the metal photolithography template is engraved with the sinusoidal curve pattern.

7. The ridge waveguide design method according to claim 6, wherein: The method of preparing a metal electrode corresponding to a pattern template on a standard ridge waveguide comprises: When W is the width of the standard ridge waveguide, align the pattern template with the upper surface of the standard ridge waveguide; when W is the height of the standard ridge waveguide, align the pattern template with the side surface of the standard ridge waveguide; After alignment, metal electrodes were fabricated by photolithography.

8. The ridge waveguide design method according to claim 7, wherein: The ridge waveguide is subjected to room temperature electric field polarization, and the effective nonlinear coefficient d at any point (x, y) on the curve is eff (x) is expressed as: Where Sign(y) is the sign function that describes the distribution of positive and negative domains, E is the electric field strength, and d xx is the second-order polarization coefficient tensor, E(y) is the distribution function of the light field in the Y direction; Get d eff (x) = sin(x)d xx , indicating that the nonlinear coefficient is periodically modulated after polarization.

9. A ridge waveguide obtained by the ridge waveguide design method for eliminating nonlinear frequency variation stray light according to claim 1, characterized in that: The ridge waveguide includes a substrate and a raised portion, wherein a sinusoidal curve pattern is engraved on the upper surface or side surface of the raised portion. The extension direction of the sinusoidal curve is the same as the extension direction of the waveguide, and the sinusoidal curve is tangent to the boundary edge between the upper surface and the side surface of the raised portion.

10. The ridge waveguide according to claim 9, wherein: The ridge waveguide is bounded by the sinusoidal curve, with one side polarized as a positive domain and the other side polarized as a negative domain.

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