A method for realizing continuous adjustable wavelength interval of quasi-phase matching dual-wavelength frequency doubling

Abstract

The invention discloses a method for realizing quasi-phase matching dual-wavelength frequency doubling with continuous adjustable wavelength interval, belonging to the technical field of optical elements. In the method, a crystal with a nested quasi-periodic structure is used, and a dual-wavelength frequency doubling with a narrow wavelength interval of 1.55 μm is realized by combining two QPM types, Type‑0 (o+o→o) and Type‑I (e+e→o). The crystal material is a polarized lithium niobate crystal, and the crystal structure is respectively composed of a right-angled triangle domain, a right-angled trapezoidal domain, and a lateral nesting. By utilizing the difference in temperature sensitivity of two QPM types of the crystal material, a continuous adjustable wavelength interval with a center wavelength of 1.55 μm is realized. At the same time, it is proved that the continuous adjustable wavelength interval characteristic can be realized at different center fundamental wavelengths by setting different crystal periods. The invention can realize dual-wavelength frequency doubling output with a continuously adjustable wavelength interval under the action of the fundamental wave of the Type‑0 (o+o→o) and Type‑I (e+e→o) QPM combination, which can meet the actual needs in many fields such as biomedicine, spectral analysis, optical communication, etc., and has important practical significance.

Description

Technical Field

[0001] The invention belongs to the technical field of optical elements, and in particular relates to a method for realizing continuous adjustable wavelength interval of quasi-phase matching dual-wavelength frequency doubling. Background Art

[0002] Nonlinear frequency conversion is an important aspect of nonlinear optics. According to the order of nonlinear polarizability, it can be divided into second-order nonlinearity and higher-order nonlinearity. Among them, second-order nonlinearity includes sum frequency, frequency doubling and difference frequency. In order to make nonlinear frequency conversion have higher conversion efficiency, in addition to requiring the material to have an intrinsic second-order nonlinear polarizability, the phase velocity of the interacting light waves must be matched to ensure that the energy of the incident light wave is unidirectionally converted to the converted light wave. For example, in the frequency doubling process, this phase matching condition requires that the refractive index of the fundamental frequency light wave and the frequency doubling light wave must be equal. Although the birefringence phase matching technology using the birefringence phenomenon of crystals can achieve complete phase matching, it has extremely high requirements for temperature and incident angle, and the tunability of parameters is poor, so it is difficult to be applied in actual production. In contrast, quasi-phase matching technology (QPM) has many advantages. Quasi-phase matching technology can greatly improve the efficiency of nonlinear frequency conversion by periodically changing the polarization intensity of nonlinear materials and using the modulation of the second-order polarizability of nonlinear media to compensate for the phase difference caused by the material dispersion effect.

[0003] Usually, a periodic polarized crystal with a fixed period can only provide a reciprocal lattice vector to achieve frequency conversion of a single wavelength, but with the continuous development of quasi-phase matching technology, the concept of multiple quasi-phase matching has been proposed. Multiple quasi-phase matching technology refers to changing the structure of the polarized crystal so that multiple reciprocal lattice vectors can be provided in one crystal at the same time, thereby achieving simultaneous conversion of multiple wavelengths. To this end, some solutions have been proposed in recent years.

[0004] In 2019, TSMeetei et al. proposed a phase-reversal superlattice structure. They analyzed the properties of the multiple SHG spectra generated when the phase-reversal domains were distributed at equal and unequal intervals along the length of the device. By assigning four phase-reversal domains to specific positions of the phase-reversal optical superlattice device, five peak QPM SHGs can be generated. The dependence of the phase-reversal domains and their positions in the phase-reversal optical superlattice device were analyzed to design an ideal multi-wavelength converter. In addition to phase-reversal optical superlattices, quasi-periodic optical superlattices can also achieve multiple quasi-phase matching. Zhu Shining's team at Nanjing University proposed a quasi-periodic optical superlattice structure based on the Fibonacci sequence, which simultaneously achieved multi-wavelength SHG. In order to continuously reduce the orderliness of the domain structure and improve the conversion efficiency of multiple quasi-phase matching, non-periodic optical superlattice (AOS) and non-periodic optical superlattice (NOS) have been proposed successively. Zhang Jiandong et al. used genetic algorithms to propose another AOS structure that can achieve broadband SHG output. In 2004, Chen Xianfeng and others proposed a non-periodic optical superlattice (NOS) structure that no longer limits the unit domain length. From periodic optical superlattice to quasi-periodic structure, and then to non-periodic and non-periodic structures, the periodicity of optical superlattice is constantly weakened, and the reciprocal lattice vectors provided are more abundant. A series of multi-wavelength frequency conversion devices that meet practical needs have been designed and prepared.

[0005] After searching the existing technologies, it is found that the technology of multi-wavelength frequency doubling based on quasi-phase matching is already very mature and has obvious advantages, but it still has the following shortcomings: 1. The use of a specific structure to achieve frequency doubling output of multiple wavelengths, the conversion efficiency distribution of different peaks and the flexible modulation of the peak interval are still challenges for multi-wavelength SHG, which will profoundly affect the application of QPM technology. 2. The use of a specific structure to achieve bandwidth frequency doubling output of a certain band, but the increase in bandwidth sacrifices a lot of conversion efficiency; these shortcomings are difficult to meet the needs of nonlinear frequency conversion in real applications. Summary of the invention

[0006] Purpose of the invention: In order to overcome the deficiencies in the prior art, the present invention provides a method for realizing continuous adjustable wavelength interval of quasi-phase matching dual-wavelength frequency doubling.

[0007] Technical solution: In the first aspect, the present invention provides a method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval. In the method, a crystal with a quasi-periodic structure is used, and the fundamental wave of a Type-0 (o+o→o) and Type-I (e+e→o) QPM combination is used as the incident light. The temperature difference between the two QPM types is utilized to realize dual-wavelength frequency doubling with continuously adjustable wavelength interval by adjusting the temperature. The method comprises the following steps:

[0008] Step 1: Provide a quasi-periodic crystal structure model based on quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval;

[0009] Step 2: Given the initial conditions, determine the specific parameters of the quasi-periodic crystal structure;

[0010] Step 3: This quasi-periodic crystal structure uses the fundamental wave of the Type-0 (o+o→o) and Type-I (e+e→o) QPM combination as the incident light to achieve dual-wavelength frequency-doubled output with a wavelength interval of 1.55 μm as the central wavelength;

[0011] Step 4: Utilizing the difference in temperature sensitivity between Type-0 (o+o→o) and Type-I (e+e→o) QPM types, the quasi-periodic crystal structure achieves continuous adjustable wavelength interval with a central wavelength of 1.55μm by adjusting the temperature, which is used to prove that different central fundamental wavelengths can achieve continuous adjustable wavelength intervals by setting different periods.

[0012] In a further embodiment, the material of the crystal in step 1 is 5 mol% magnesium oxide-doped lithium niobate crystal (5 mol% MgO:LN); the crystal is in the shape of a cuboid, with parallel upper and lower surfaces and both polished, and the crystal is composed of nested right-angled triangle domains and right-angled trapezoidal domains; wherein the length of the right-angled side of the right-angled triangle domain along the length direction of the cuboid is equal to the length of the shorter base of the right-angled trapezoidal domain, and is also equal to half of the length of the longer base of the right-angled trapezoidal domain; the crystal is continuously nested in the order of right-angled triangle positive domain, right-angled trapezoid negative domain, right-angled trapezoid positive domain, and right-angled triangle negative domain, and the spontaneous polarization direction of each unit domain is arranged in sequence from upward to downward.

[0013] In a further embodiment, in step 2, an initial wavelength and temperature are given, and the Sellmeier equation is used to determine the coherence length required to achieve Type-0 (o+o→o) quasi-phase matching under this condition, and twice the coherence length is taken as the period length of the quasi-periodic structure, and the domain lengths of the right-angled triangle and right-angled trapezoidal domains are determined by proportion, and the period number of the structure is determined based on the calculated period length.

[0014] In a further embodiment, in step 2, the total length of the quasi-periodic structure crystal is 10 mm.

[0015] In a further embodiment, in step 2, the calculation formula of the coherence length Lc is:

[0016]

[0017] Where λ is the wavelength of the fundamental frequency light, n ω is the refractive index of fundamental frequency light in the crystal, n 2ω is the refractive index of the frequency-doubled light in the crystal.

[0018] In a further embodiment, in step 3, the period length of the quasi-periodic crystal structure is 16.11 μm.

[0019] In a further embodiment, in step 3, the calculation formula of the frequency doubling efficiency η is:

[0020]

[0021] In the formula, I ω represents the fundamental wave intensity, c represents the speed of light in a vacuum, ε0 represents the dielectric constant in a vacuum, λ represents the fundamental wave wavelength, and n ω and n 2ω Respectively represent the refractive index of the fundamental wave and the second harmonic in the crystal, d 33 represents the maximum nonlinear coefficient in the z direction, L represents the total length of the crystal, Δk(λ) represents the phase mismatch, and d(z) represents the polarization direction distribution of a single domain unit, which changes with the change of z value; when d(z) = 1, the polarization direction is upward, and when d(z) = -1, the polarization direction is downward;

[0022] where Δk(λ) is given by the following formula:

[0023]

[0024] In a further embodiment, in step 3, a relatively effective nonlinear coefficient dreff(λ) is introduced and expressed as:

[0025]

[0026] The conversion efficiency is measured by introducing the normalized value of dreff(λ).

[0027] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0028] Based on the principle of quasi-phase matching technology, the present invention designs a nested quasi-periodic optical superlattice structure, adopts the fundamental wave of Type-0 (o+o→o) and Type-I (e+e→o) QPM combination as the incident light, and utilizes the temperature difference of the two QPM types to achieve dual-wavelength frequency doubling output with continuously adjustable wavelength interval by adjusting the temperature. This dual-wavelength frequency doubling with continuously adjustable wavelength interval can meet the actual needs in many fields such as biomedicine, spectral analysis, optical communication, etc., greatly improves the flexibility and availability of QPM frequency doubling, and has important practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic diagram of the quasi-periodic polarized lithium niobate crystal structure provided by an embodiment of the present invention;

[0030] Figure 2 It is a spectrum diagram of fundamental wave wavelength and second harmonic normalized conversion efficiency in a quasi-periodic structure crystal when the incident position is from point A to point E provided by an embodiment of the present invention;

[0031] Figure 3 The embodiment of the present invention provides the variation rules of the fundamental wave peaks of two types of QPM, Type-0 (o+o→o) and Type-I (e+e→o), when the temperature and cycle change;

[0032] Figure 4 A graph showing the relationship between the wavelength interval and the operating temperature when dual wavelength frequency doubling is performed in four quasi-periodic crystal structures with different periods provided in an embodiment of the present invention, when two QPM modes, Type-0 (o+o→o) and Type-I (e+e→o), are combined. DETAILED DESCRIPTION

[0033] In order to more fully understand the technical content of the present invention, the technical solution of the present invention is further introduced and illustrated in conjunction with specific embodiments below, but is not limited thereto.

[0034] An embodiment of the present invention provides a method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval. In the method, a crystal with a quasi-periodic structure is used, and the crystal material is a polarized lithium niobate crystal. The crystal is continuously nested with right-angled triangle s and right-angled trapezoidal domains, and the arrangement of a single periodic nesting is right-angled triangle positive s, right-angled trapezoidal negative domain, right-angled trapezoidal positive s, and right-angled triangle negative domain. The method uses the fundamental wave of the Type-0 (o+o→o) and Type-I (e+e→o) QPM combination as the incident light, utilizes the temperature difference of the two QPM types, and realizes dual-wavelength frequency doubling output with continuously adjustable wavelength interval by adjusting the temperature.

[0035] The specific implementation steps are as follows:

[0036] Step 1: Provide a quasi-periodic crystal structure model based on quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval;

[0037] The crystal material used in the quasi-periodic structure of the present invention is 5 mol% magnesium oxide-doped lithium niobate crystal (5 mol% MgO:LN). The crystal is in the shape of a rectangular parallelepiped, with parallel upper and lower surfaces and both polished. The structure of the crystal is as follows: Figure 1 As shown, the crystal is composed of two types of unit domains with radium lengths, namely right-angled triangle radium and right-angled trapezoidal domain, which are nested continuously in the order of right-angled triangle positive radium, right-angled trapezoidal negative domain, right-angled trapezoidal positive radium, and right-angled triangle negative domain in the direction of light wave propagation. The spontaneous polarization direction of each unit domain is arranged in sequence from upward to downward. Figure 1The length of the right-angled side of the middle right-angled triangle domain along the z-axis direction is equal to the length of the shorter base of the right-angled trapezoidal domain, and is also equal to half of the length of the longer base of the right-angled trapezoidal domain.

[0038] Step 2: Given the initial conditions, determine the specific parameters of the quasi-periodic structure:

[0039] Given an initial wavelength and temperature, the Sellmeier equation is used to determine the coherence length required to achieve Type-0 (o+o→o) quasi-phase matching under this condition. The coherence length Lc is obtained by the following formula:

[0040]

[0041] Where λ is the wavelength of the fundamental frequency light, n ω is the refractive index of fundamental frequency light in the crystal, n 2ω is the refractive index of the frequency-doubled light in the crystal.

[0042] The refractive index n is obtained from the Sellmeier formula:

[0043]

[0044] Where λ is the wavelength of the fundamental frequency light, f is the temperature parameter, and when the crystal is 5% MgO-doped LN, the coefficients of o-light and e-light a i 、b i As shown in Table 1.

[0045] Table 1 Sellmeier equation parameters for 5% MgO doped LN crystal

[0046] <![CDATA[n e ]]> <![CDATA[n o ]]> <![CDATA[a1]]> 5.756 5.653 <![CDATA[a2]]> 0.0983 0.1185 <![CDATA[a3]]> 0.2020 0.2091 <![CDATA[a4]]> 189.32 89.61 <![CDATA[a5]]> 12.52 10.85 <![CDATA[a6]]> <![CDATA[1.32×10 -2 ]]> <![CDATA[1.97×10 -2 ]]> <![CDATA[b1]]> <![CDATA[2.860×10 -6 ]]> <![CDATA[7.941×10 -7 ]]> <![CDATA[b2]]> <![CDATA[4.700×10 -8 ]]> <![CDATA[3.134×10 -8 ]]> <![CDATA[b3]]> <![CDATA[6.113×10 -8 ]]> <![CDATA[-4.641×10 -9 ]]> <![CDATA[b4]]> <![CDATA[1.516×10 -4 ]]> <![CDATA[-2.188×10 -6 ]]>

[0047] The temperature parameter f is obtained by the following formula:

[0048] f(T)=(T-24.5)(T+570.82)

[0049] Where T is the temperature in degrees Celsius.

[0050] The twice of the coherence length is taken as the period length of the quasi-periodic structure, and the domain lengths of the right triangle and right trapezoidal domains are determined by proportion. The total length of the quasi-periodic structure crystal is 10 mm, and the calculated period length is 16.11 μm.

[0051] Step 3: Through this quasi-periodic crystal structure, a combination of Type-0 (o+o→o) and Type-I (e+e→o) QPM is used to achieve dual-wavelength frequency doubling output, and this structure can achieve high-efficiency multi-wavelength frequency doubling conversion. The frequency doubling efficiency η is obtained by the following formula:

[0052]

[0053] In the formula, I ω represents the fundamental wave intensity, c represents the speed of light in a vacuum, ε0 represents the dielectric constant in a vacuum, λ represents the fundamental wave wavelength, and n ω and n 2ω Respectively represent the refractive index of the fundamental wave and the second harmonic in the crystal, d 33 represents the maximum nonlinear coefficient in the z direction, L represents the total length of the crystal, Δk(λ) represents the phase mismatch, and d(z) represents the polarization direction distribution of a single domain unit, which changes with the change of z value; when d(z) = 1, the polarization direction is upward, and when d(z) = -1, the polarization direction is downward;

[0054] where Δk(λ) is given by the following formula:

[0055]

[0056] A relatively effective nonlinear coefficient dreff(λ) is introduced and expressed as:

[0057]

[0058] In the present invention, the dB value of dreff(λ) is introduced to measure the conversion efficiency.

[0059] Step 4: Use this quasi-periodic crystal structure to realize dual-wavelength frequency doubling of the Type-0 (o+o→o) and Type-I (e+e→o) QPM combination; analyze the effects of temperature and period on the wavelength interval, and use the difference in temperature sensitivity between the two QPM types, Type-0 (o+o→o) and Type-I (e+e→o), to achieve continuous adjustable wavelength interval with a central wavelength of 1.55μm; and prove that this continuous adjustability can be achieved at different central fundamental wavelengths by setting different periods.

[0060] The specific parameters are set as follows: the quasi-periodic structure of the present invention selects 5% MgO-doped lithium niobate crystal as the frequency doubling crystal, the length of the NQOS device is 10 mm (about 625 periods), the period is 16.11 μm, the operating temperature is 180 ° C, and the Type-0 (o+o→o) and Type-I (e+e→o) QPM combinations are used. Figure 1 In the NQOS structure shown, five equally spaced points are selected in sequence along the x-axis as the incident positions of the fundamental wave, and the dual-wavelength frequency doubling characteristics of NQOS are analyzed. Figure 2 (a)-(e) show the spectrum of the fundamental wavelength and the normalized conversion efficiency of the second harmonic when the incident position is from point A to point E. Figure 2As shown in (b)-(d), the fundamental frequency light of the Type-0 (o+o→o) and Type-I (e+e→o) QPM combination is incident at points B, C, and D, achieving dual-wavelength frequency doubling with a narrow wavelength interval of 1.5435μm and 1.55μm for the fundamental waves. The wavelength interval between the two fundamental wave peaks is only 6.5nm.

[0061] Figure 3 In (a), we analyze the effect of temperature change on the wavelength of the two QPM fundamental waves, Type-0 (o+o→o) and Type-I (e+e→o). It is easy to find that by combining these two types of QPM, the fundamental wave can be adjusted at a wavelength interval of 1.55μm by adjusting the operating temperature. The wavelength interval can be adjusted continuously in the range of 0 to 352nm. Figure 3 As shown in (b), we analyzed the variation law of the fundamental wavelength of the two modes when the period length increases continuously at a temperature of 180°C. Obviously, the variation law of the fundamental wavelength of the two QPM types, Type-0 (o+o→o) and Type-I (e+e→o), with the period is roughly the same. Therefore, in the quasi-periodic structure, the change of the wavelength interval of the two fundamental waves due to the influence of temperature is much greater than the change caused by changing the period. This means that in devices with different periods, dual-wavelength frequency doubling with continuously adjustable wavelength interval is common, and the size of the period only determines the peak position of Type-0 (o+o→o) QPM.

[0062] Finally, we further analyzed the adjustment range of wavelength interval by temperature in different periods. Figure 4As shown in the figure, we selected four quasi-periodic devices with different periods and analyzed the relationship between the wavelength interval and the operating temperature when the two QPM mode combinations were combined. In the devices with NQOS periods of 8, 12, 16, and 20 μm (the central fundamental wavelength CFW was 1176 nm, 1356 nm, 1539 nm, and 1716 nm, respectively), when the temperature was adjusted in the range of 20-240 ° C, the wavelength interval of the dual-wavelength frequency doubled experienced the change process of 31.6-0-132.6 nm, 165.7-0-129 nm, 346.1-0-121.7 nm, and 591.8-0-117.2 nm, respectively, that is, the wavelength interval of the dual-wavelength frequency doubled was continuously adjustable in the range of 0-132.6 nm, 0-165.7 nm, 0-346.1 nm, and 0-591.8 nm. It can be seen that within the CFW range of 1.18-1.72μm, in devices with different periods, the wavelength interval based on the dual-wavelength frequency doubling of the two QPM modes will achieve continuous adjustable wavelength interval within a certain range, and the maximum continuously adjustable wavelength interval is related to the size of the CFW (the period of the device). As the CFW increases, the maximum adjustable wavelength interval increases, and the continuously adjustable wavelength interval range also increases.

[0063] In summary, a method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval is provided. Based on the principle of quasi-phase matching technology, a new type of quasi-periodic crystal structure is designed, and the fundamental wave of the Type-0 (o+o→o) and Type-I (e+e→o) QPM combination is used as the incident light. By utilizing the temperature difference of the two QPM types and adjusting the temperature, a dual-wavelength frequency doubling output with continuously adjustable wavelength interval is realized. This dual-wavelength frequency doubling with continuously adjustable wavelength interval can meet the actual needs in many fields such as biomedicine, spectral analysis, and optical communication, greatly improves the flexibility and availability of QPM frequency doubling, and has important practical significance.

[0064] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for realizing continuous adjustable wavelength interval of quasi-phase matching dual-wavelength frequency doubling, characterized in that: The method uses a crystal with a quasi-periodic structure, adopts the fundamental wave of a Type-0 (o+o→o) and Type-I (e+e→o) QPM combination as incident light, and utilizes the temperature difference between the two QPM types to achieve dual-wavelength frequency doubling with continuously adjustable wavelength interval by adjusting the temperature; the method comprises the following steps: Step 1: Provide a quasi-periodic crystal structure model based on quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval; Step 2: Given the initial conditions, determine the specific parameters of the quasi-periodic crystal structure; Step 3: This quasi-periodic crystal structure uses the fundamental wave of the Type-0 (o+o→o) and Type-I (e+e→o) QPM combination as the incident light to achieve dual-wavelength frequency-doubled output with a wavelength interval of 1.55 μm as the central wavelength; Step 4: Utilizing the difference in temperature sensitivity between Type-0 (o+o→o) and Type-I (e+e→o) QPM types, the quasi-periodic crystal structure achieves continuous adjustable wavelength interval with a central wavelength of 1.55μm by adjusting the temperature, which is used to prove that different central fundamental wavelengths can achieve continuous adjustable wavelength intervals by setting different periods.

2. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 1, characterized in that: The material of the crystal in step 1 is 5 mol% magnesium oxide-doped lithium niobate crystal (5 mol% MgO:LN); the crystal is in the shape of a cuboid, with upper and lower surfaces parallel and polished, and the crystal is respectively composed of nested right-angled triangle domains and right-angled trapezoidal domains; wherein the length of the right-angled side of the right-angled triangle domain along the length direction of the cuboid is equal to the length of the shorter base of the right-angled trapezoidal domain, and is also equal to half of the length of the longer base of the right-angled trapezoidal domain; the crystal is continuously nested in the order of right-angled triangle positive domain, right-angled trapezoid negative domain, right-angled trapezoid positive domain, and right-angled triangle negative domain, and the spontaneous polarization direction of each unit domain is arranged from upward to downward.

3. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 1, characterized in that: In step 2, an initial wavelength and temperature are given, and the Sellmeier equation is used to determine the coherence length required to achieve Type-0 (o+o→o) quasi-phase matching under this condition. Twice the coherence length is taken as the period length of the quasi-periodic structure, and the domain lengths of the right triangle and right trapezoidal domains are determined by proportion. The period number of the structure is determined based on the calculated period length.

4. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 1, characterized in that: In step 2, the total length of the quasi-periodic structure crystal is 10 mm.

5. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 3, characterized in that: In step 2, the calculation formula of the coherence length Lc is: Where λ is the wavelength of the fundamental frequency light, n ω is the refractive index of fundamental frequency light in the crystal, n 2ω is the refractive index of the frequency-doubled light in the crystal.

6. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 3, characterized in that: In step 3, the period length of the quasi-periodic crystal structure is 16.11 μm.

7. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 1, characterized in that: In step 3, the calculation formula of the frequency doubling efficiency η is: In the formula, I ω represents the fundamental wave intensity, c represents the speed of light in a vacuum, ε0 represents the dielectric constant in a vacuum, λ represents the fundamental wave wavelength, and n ω and n 2ω Respectively represent the refractive index of the fundamental wave and the second harmonic in the crystal, d 33 represents the maximum nonlinear coefficient in the z direction, L represents the total length of the crystal, Δk(λ) represents the phase mismatch, and d(z) represents the polarization direction distribution of a single domain unit, which changes with the change of z value; when d(z) = 1, the polarization direction is upward, and when d(z) = -1, the polarization direction is downward; where Δk(λ) is given by the following formula:

8. The method for realizing quasi-phase matching dual-wavelength frequency doubling with continuously adjustable wavelength interval according to claim 5, characterized in that: In step 3, a relatively effective nonlinear coefficient dreff(λ) is introduced and expressed as: Where L represents the total length of the crystal, Δk(λ) represents the phase mismatch, and d(z) represents the polarization direction distribution of a single domain unit. The conversion efficiency is measured by introducing the normalized value of dreff(λ).

Images