1. A method for realizing quasi-phase matching multi-wavelength frequency doubling conversion is characterized in that a crystal with a quasi-periodic structure is used in the method, and Type-I (e + e → o) quasi-phase matching is utilized to realize frequency doubling conversion of different wavelengths by controlling temperature, and the method comprises the following steps:

step 1: giving a quasiperiodic crystal structure model for realizing multi-wavelength frequency multiplication based on quasi-phase matching;

step 2: initial conditions are given, and specific parameters of a quasi-periodic crystal structure are determined;

and step 3: the quasi-periodic crystal structure realizes high-efficiency multi-wavelength frequency doubling conversion to obtain frequency doubling efficiency;

and 4, step 4: and meanwhile, frequency doubling output of a plurality of wavelengths is realized, and frequency doubling output full coverage of common communication wave bands is realized in a single crystal structure through temperature tuning by utilizing the characteristic that the Type-I (e + e → o) Type quasi-phase matching effect is obviously influenced by temperature.

2. The method according to claim 1, wherein in step 1, the crystal is a 5 mol% magnesium oxide doped lithium niobate crystal, the crystal has a rectangular parallelepiped shape, the upper and lower surfaces are parallel and polished, the crystal is continuously nested with two kinds of unit domains A, B each having wavelengths in the light wave propagation direction, the ABBA sequence is used as a period, and the spontaneous polarization direction of each unit domain is arranged from top to bottom.

3. The method as claimed in claim 2, wherein in step 1, the ratio of a to B is lA: lB-0.864: 2.136.

4. The method of claim 1, wherein in step 2, an initial wavelength and a temperature are given, a coherence length required for realizing Type-I (e + e → o) quasi-phase matching under the condition is determined by using a Sellmeier equation, twice the coherence length is taken as a period length of the quasi-periodic structure, lengths of A, B domains are determined by proportion, and a period number of the structure is determined according to the calculated period length.

5. The method according to claim 4, wherein in step 2, the coherence length lc is obtained by the following formula:

wherein λ is the wavelength of fundamental light, n ω is the refractive index of fundamental light in the crystal, and n2 ω is the refractive index of frequency doubled light in the crystal.

6. The method for realizing quasi-phase matching multi-wavelength frequency doubling conversion according to claim 1, 4 or 5, wherein in the step 2, the total length of the quasi-periodic structure crystal is 9-10 mm.

7. The method according to claim 1, wherein in step 3, the multiplication efficiency η is obtained by the following formula:

in the formula, d33 represents the maximum nonlinear coefficient in the z direction, L represents the total length of the crystal, Δ k (λ) represents the phase mismatch amount, and d (z) represents the polarization direction distribution of a single domain unit, and changes along with the change of the z value; when d (z) is 1, the polarization direction is upward, and when d (z) is-1, the polarization direction is downward;

wherein Δ k (λ) is given by the following equation:

8. the method as claimed in claim 6, wherein in step 3, a relative effective nonlinear coefficient dreff (λ) is further introduced as:

the conversion efficiency is measured by introducing a dB value of dreff (lambda).

Background

Second-order nonlinear optical frequency conversion such as frequency doubling, difference frequency, sum frequency and the like has important application in the fields of spectroscopy, ultrashort pulse frequency doubling, signal processing, optical communication and the like. In order to improve the efficiency of frequency conversion, it is necessary to achieve phase velocity matching of each interaction wavelength, so that the energy of the input light is converted into the energy of the target wavelength. For frequency doubling as an example, the phase matching must be such that the refractive indices of the fundamental and second harmonics are equal. Although the birefringence phase matching technology using the crystal birefringence phenomenon can realize complete phase matching, the requirements on temperature and an incident angle are extremely high, the parameter tunability is poor, and the application in actual production is difficult. In contrast, quasi-phase matching techniques (QPM) have many advantages. The quasi-phase matching technology can greatly improve the efficiency of nonlinear frequency conversion by periodically changing the polarization intensity of a nonlinear material and compensating the phase difference generated by the dispersion effect of the material by modulating the second-order polarizability of a nonlinear medium.

Generally, a periodically poled crystal with a fixed period can only provide a reciprocal lattice vector to realize frequency conversion of a single wavelength, but with the continuous development of quasi-phase matching technology, the concept of multiple quasi-phase matching is proposed. The multiple quasi-phase matching technology is that multiple reciprocal lattice vectors can be provided in one crystal by changing the structure of a polarized crystal, so that simultaneous conversion of multiple wavelengths is realized. For this reason, some solutions have been proposed in recent years. In 1992, m.m.fejer et al formed a bi-periodic QPM structure by superimposing a phase-reversal grating on a uniform QPM grating, which provided two reciprocal lattice vectors, achieving dual wavelength frequency doubling of 1.55 μm and 1.551 μm. Subsequently, Zhao LN and the like propose a metric double-period structure, and through deep theoretical research and calculation, the double-wavelength frequency multiplication realized when the metric ratio is 7 is obtained, so that the conversion efficiency is greatly improved. In 1997, a quasi-periodic structure based on Fibonacci series was proposed, and the Fibonacci sequence is a two-component quasi-periodic structure consisting of two units, A and B. The A unit comprises a positive domain A1 and a negative domain A2, and the B unit also comprises a positive domain B1 and a negative domain B2. The phenanthrenacci sequence specifies: lA-lA 1+ lA2, lB-lB 1+ lB2, and τ -lA/lB. When lA1 ═ lB1 ═ l, lA2 ═ l (1+ η), and lB2 ═ l (1 τ η) were given. The quasi-periodic structure of the Fibonacci sequence can be obtained by recursion formula A → AB, B → A. By adjusting the domain length and the matching stage number of each unit, a predetermined inverted lattice vector can be obtained, thereby flexibly satisfying the multi-level matching process. The quasi-periodic structure is realized by simultaneously realizing the frequency doubling of a plurality of wavelengths in a lithium tantalate crystal, and in the multi-wavelength frequency doubling, the reciprocal lattice vector distribution of different superlattice structures is different, and the corresponding matched wavelengths are also different. The novel chirp structure using the projection method proposed in 2010 combines the advantages of chirp and quasi-periodic structures, can be used for multi-quasi-phase matching to realize multi-wavelength frequency multiplication, and can well control the bandwidth of multi-wavelength. Providing more flexibility in the structural design of the optical superlattice. Gu B Y et al propose a based on annealing algorithm selects the non-periodic structure in the phase reversal region, can provide abundant reciprocal lattice vector, realize the frequency multiplication of multiwavelength at the same time, have realized that the effective nonlinear coefficient of frequency multiplication of five wavelength of 0.972 um, 1.082 um, 1.283 um, 1.364 um and 1.5687um is basically the same as 0.23. In 2019, the structure of the phase reversal domain is determined by a genetic algorithm, which is proposed by Meetei et al, so that the generation of multiple quasi-phase matching second harmonics at communication bands of 1.550 μm, 1.569 μm, 1.588 μm, 1.606 μm and 1.629 μm is realized, and the relative conversion efficiency of the second harmonics reaches 18.65%.

The search of the prior art finds that the technology for generating multi-wavelength frequency multiplication based on quasi-phase matching is mature and has obvious advantages, but the following defects still exist: 1. the frequency doubling output of a plurality of wavelengths is realized by adopting a specific structure, the conversion efficiency is high, but the bandwidth is extremely short, the range of the wave band covered by tuning is narrow, and the whole coverage of the common communication wave bands such as 0.86 μm, 1.06 μm, 1.31 μm, 1.55 μm and the like can not be realized by adopting a crystal structure; 2. the specific structure is adopted to realize the bandwidth frequency multiplication output of a certain waveband, but the increase of the bandwidth sacrifices a large amount of conversion efficiency; these deficiencies are difficult to meet with the requirements of non-linear frequency conversion in real world applications.

The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

Disclosure of Invention

The invention aims to overcome the defects in the prior art and provide a method for realizing quasi-phase matching multi-wavelength frequency doubling conversion.

The technical scheme is as follows: the invention provides a method for realizing quasi-phase matching multi-wavelength frequency doubling conversion, which uses a crystal with a quasi-periodic structure, utilizes Type-I (e + e → o) quasi-phase matching, and realizes frequency doubling conversion of different wavelengths by controlling temperature, and comprises the following steps:

step 1: giving a quasiperiodic crystal structure model for realizing multi-wavelength frequency multiplication based on quasi-phase matching;

step 2: initial conditions are given, and specific parameters of a quasi-periodic crystal structure are determined;

and step 3: the quasi-periodic crystal structure realizes high-efficiency multi-wavelength frequency doubling conversion to obtain frequency doubling efficiency;

and 4, step 4: and meanwhile, frequency doubling output of a plurality of wavelengths is realized, and frequency doubling output full coverage of common communication wave bands is realized in a single crystal structure through temperature tuning by utilizing the characteristic that the Type-I (e + e → o) Type quasi-phase matching effect is obviously influenced by temperature.

Further, in the step 1, the crystal is made of 5 mol% magnesium oxide-doped lithium niobate crystal (5 mol% MgO: LN), the crystal is in a cuboid shape, the upper surface and the lower surface are parallel and are polished, the crystal is continuously nested by two unit domains with length of being A, B respectively in the light wave propagation direction, the ABBA sequence is taken as a period, and the spontaneous polarization direction of each unit domain is arranged from top to bottom in sequence.

Further, in step 1, the ratio of the length of a to the length of B is lA: lB ═ 0.864: 2.136.

Further, in the step 2, an initial wavelength and a temperature are given, a coherence length required for realizing Type-I (e + e → o) quasi-phase matching under the condition is determined by using a Sellmeier equation, twice of the coherence length is taken as a period length of the quasi-periodic structure, lengths of A, B domains are determined by proportion, and the period number of the structure is determined according to the calculated period length.

Further, in step 2, the coherence length lc is obtained by the following formula:

wherein λ is the wavelength of fundamental light, n ω is the refractive index of fundamental light in the crystal, and n2 ω is the refractive index of frequency doubled light in the crystal.

Further, in the step 2, the total length of the quasi-periodic structure crystal is 9-10 mm.

Further, in step 3, the frequency doubling efficiency η is obtained by the following formula:

in the formula, d33 represents the maximum nonlinear coefficient in the z direction, L represents the total length of the crystal, Δ k (λ) represents the phase mismatch amount, and d (z) represents the polarization direction distribution of a single domain unit, and changes along with the change of the z value; when d (z) is 1, the polarization direction is upward, and when d (z) is-1, the polarization direction is downward;

wherein Δ k (λ) is given by the following equation:

further, in step 3, a relative effective nonlinear coefficient dreff (λ) is introduced to be expressed as:

the conversion efficiency is measured by introducing a dB value of dreff (lambda).

Compared with the prior art, the invention has the following beneficial effects:

the invention designs a novel quasi-periodic crystal structure based on the principle of quasi-phase matching technology, and can realize frequency doubling output of a plurality of wavelengths simultaneously; meanwhile, by utilizing the characteristic that the Type-I (e + e → o) quasi-phase matching effect is obviously influenced by temperature and by temperature tuning, the frequency doubling output full coverage of a common communication waveband is realized in a single crystal structure, compared with the traditional bandwidth frequency doubling output, the conversion efficiency of frequency doubling light is obviously improved, and the method has important significance for the development of all-optical communication and all-optical network technologies.

Drawings

Fig. 1 is a schematic diagram of a quasi-periodically poled lithium niobate crystal structure provided by an embodiment of the present invention.

Fig. 2 is a graph of frequency doubling efficiency of a quasi-periodic crystal according to an embodiment of the present invention.

FIG. 3 is a graph showing the relationship between the QPM period length and the time-based frequency wavelength at different temperatures under Type-I (e + e → o) quasi-phase matching according to an embodiment of the present invention.

Fig. 4 is a graph showing the relationship between the frequency doubling efficiency and the fundamental wavelength of the quasi-periodic crystal at different temperatures according to the embodiment of the present invention.

Fig. 5 is a schematic diagram of the tuning of the fundamental frequency peak with temperature covering the wavelength range according to the embodiment of the present invention.

Detailed Description

The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.

The method for realizing quasi-phase matching multi-wavelength frequency doubling conversion provided by the embodiment of the invention uses a quasi-periodic structure crystal, the crystal material is a polarized lithium niobate crystal, the crystal is continuously nested by unit domains with the length of A, B, and the nested ordering of single period is ABBA. The method can simultaneously realize frequency doubling conversion of a plurality of wavelengths under the condition of meeting Type-I (e + e → o) Type quasi-phase matching, and can adjust the wave band for generating frequency doubling light by controlling the temperature, thereby realizing the full coverage of the common communication wave band in a single crystal structure. The specific implementation steps are as follows:

step 1: giving a quasiperiodic lithium niobate crystal structure model for realizing multi-wavelength frequency multiplication based on quasiphase matching:

the crystal material adopted by the quasi-periodic structure is a 5 mol% magnesium oxide-doped lithium niobate crystal (5 mol% MgO: LN), the crystal is in a cuboid shape, the upper surface and the lower surface of the crystal are parallel and are polished, the structure of the crystal is shown in figure 1, the crystal is continuously nested by two unit domains with lengths respectively being A, B in the propagation direction of light waves by taking the ABBA sequence as a period, and the spontaneous polarization direction of each unit domain is sequentially arranged from top to bottom. The long ratio of a to B , lA: lB ═ 0.864:2.136, is the optimal value obtained by the algorithm optimization.

Step 2: given initial conditions, determining specific parameters of the quasi-periodic structure:

given an initial wavelength and temperature, the Sellmeier equation is used to determine the coherence length required to achieve Type-I (e + e → o) quasi-phase matching under this condition. The coherence length lc is given by the following equation:

wherein λ is the wavelength of fundamental light, n ω is the refractive index of fundamental light in the crystal, and n2 ω is the refractive index of frequency doubled light in the crystal.

The refractive index n is obtained by the Sellmeier formula:

where λ is the wavelength of the fundamental light, f (T) is the temperature parameter, and the coefficients ai, bi of o-light and e-light are shown in Table 1 when the crystal is 5% MgO-doped LN.

TABLE 15% MgO-doped LN Crystal Sellmeier equation parameter Table

The temperature parameter f is obtained by the following formula:

f(T)＝(T-24.5)(T+570.82)

where T is temperature in degrees Celsius.

Taking twice the coherence length as the period length of the quasi-periodic structure, determining lengths of A, B domains by proportion, wherein the total length of the quasi-periodic structure crystal is 10mm, and determining the period number of the structure according to the calculated period length.

And step 3: by the structure, high-efficiency multi-wavelength frequency doubling conversion can be realized, and the frequency doubling efficiency eta is obtained by the following formula:

in the formula, d33 represents the maximum nonlinear coefficient in the z direction, L represents the total length of the crystal, Δ k (λ) represents the amount of phase mismatch, and d (z) represents the polarization direction distribution of a single domain unit, which varies with the variation of the z value. When d (z) is 1, the polarization direction is upward, and when d (z) is-1, the polarization direction is downward. Wherein Δ k (λ) is given by the following equation:

introducing a relative effective nonlinear coefficient dreff (lambda) to be expressed as:

in the invention, the dB value of dreff (lambda) is introduced to measure the conversion efficiency.

And 4, step 4: and meanwhile, frequency doubling output of a plurality of wavelengths is realized, and frequency doubling output full coverage of common communication wave bands is realized in a single crystal structure through temperature tuning by utilizing the characteristic that the Type-I (e + e → o) Type quasi-phase matching effect is obviously influenced by temperature.

Calculation example:

the specific parameters are set as follows: according to the quasi-periodic structure, 5% MgO-doped lithium niobate crystals are selected as frequency doubling crystals, Type-I (e + e → o) quasi-phase matching is utilized, the temperature is set to be 62.4 ℃, the period length of the quasi-periodic structure is set to be 6.5 microns, the A-Type domain is 0.936 microns long, the B-Type domain is 2.314 microns long, the number of the periods of the crystals is 1500, and the total length is about 10 mm. As shown in fig. 2, under the above conditions, the quasi-periodic structure can realize dual-wavelength frequency multiplication at the fundamental frequency light wavelength of 1.0688 μm and 1.55 μm, and the dB values of the relative conversion efficiency are-4.07 dB and-4.06 dB, respectively.

As shown in fig. 3, when the period length is fixed, the fundamental light wavelength satisfying the Type-I (e + e → o) Type quasi-phase matching condition is greatly affected by temperature change. Therefore, under the condition of not changing the crystal structure, the frequency doubling conversion of different fundamental frequency wavelengths can be realized by accurately controlling the temperature. Fig. 4 is a graph of fundamental wavelength versus relative conversion efficiency when the above parameters are set unchanged and only the temperature is changed in the quasi-periodic structure according to the present invention. As shown in FIG. 4, the structure can simultaneously realize frequency doubling output of three wavelengths, the relative conversion efficiency of frequency doubling light is more than-10 dB, and the wave peaks of three fundamental waves are reduced along with the increase of temperature. Fig. 5 shows the wavelength ranges covered by three peaks of frequency doubling realized by temperature tuning, and as shown in fig. 5, the quasi-periodic structure of the present invention can realize frequency doubling output with fundamental wavelengths in the ranges of 0.84 μm-1.1 μm and 1.28 μm-1.6 μm, and cover all commonly used communication bands. The temperatures required to achieve frequency doubling of the common communications band using the above parameters of the present invention are given in the table below.

TABLE 2 frequency doubling temperature efficiency table for common communication band

Wavelength of light	0.86μm	1.06μm	1.31μm	1.55μm
					Required temperature	204.4℃	80.4℃	224.6℃	62.4℃
Relative conversion efficiency	-9.56dB	-4.30dB	-4.15dB	-4.06dB

In summary, a method for realizing quasi-phase matching multi-wavelength frequency doubling conversion is provided, which designs a novel quasi-periodic crystal structure based on the principle of the quasi-phase matching technology, and can realize frequency doubling output of multiple wavelengths simultaneously; meanwhile, by utilizing the characteristic that the Type-I (e + e → o) quasi-phase matching effect is obviously influenced by temperature and by temperature tuning, the frequency doubling output full coverage of a common communication waveband is realized in a single crystal structure, compared with the traditional bandwidth frequency doubling output, the conversion efficiency of frequency doubling light is obviously improved, and the method has important significance for the development of all-optical communication and all-optical network technologies.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.