A Nonlinear Optical Coupling Modulation Method Based on Adiabatic Quantum Driving

Abstract

The present invention discloses a non - linear optical coupling modulation method based on adiabatic shortcuts, which is used to achieve efficient conversion of signal light into output light. It includes: (1) calculation of the anti - diabatic term in frequency conversion based on the adiabatic shortcut theory; (2) design of the modulation function of the coupling parameters in the cascaded wavelength conversion process and the diabatic compensation method; (3) selection and modulation of the laser light source and the non - linear crystal; (4) process of realizing the conversion of signal light into output light. This method is based on the adiabatic shortcut theory and is a scheme for realizing efficient second - order cascaded non - linear optical frequency conversion by modulating the crystal parameters, so as to reduce the limitations on the modulation parameters and the pump light intensity in adiabatic conversion to meet the adiabatic condition. According to the designed coupling parameter modulation function, using the quasi - phase - matching technology, multi - process quasi - phase - matching is realized in the non - linear crystal, thereby obtaining efficient frequency conversion from signal light to output laser.

Description

Technical Field

[0001] The present invention relates to the field of obtaining new light sources, and specifically refers to a non-linear optical coupling modulation method based on adiabatic shortcuts, used to generate laser light sources of different bands required in different fields such as military and environmental detection. Background Art

[0002] Light sources of specific bands play an irreplaceable role in certain fields. However, the optical bands that can directly output laser light through laser working substances are limited. To obtain laser light in some bands, non-linear frequency conversion methods need to be used. To achieve efficient light source frequency conversion and obtain new light sources, the conversion process needs to meet the phase matching condition. Currently, there are mainly two mainstream methods for obtaining new light sources by using non-linear crystals for light source frequency conversion. One is the angular phase matching technology, and the other is the quasi-phase matching technology. The above technical methods for achieving efficient optical-optical non-linear frequency conversion all need to meet specific conditions. The incident angle of the input light and the parameters of the non-linear dielectric material such as temperature and length will all affect the phase matching and seriously reduce the optical-optical conversion efficiency. How to design a suitable non-linear crystal remains a hot topic in the field.

[0003] In 2008, Professor Haim Suchowski gave the adiabatic condition corresponding to achieving nearly complete quantum conversion efficiency theoretically according to the rapid adiabatic passage (RAP) scheme in atomic population transfer. A modulation scheme for non-linear crystals was designed and experimentally verified. This was the first time to introduce the adiabatic evolution theory in the field of atomic physics into the field of non-linear optics, so as to obtain the output light with high-efficiency frequency conversion through the sum-frequency process.

[0004] Obtaining new frequency band wavelengths through a single three-wave mixing process is limited by the wavelengths of the pump light and the signal light, and strict requirements are imposed on the selection of the wavelengths of both. Existing lasers often cannot meet these requirements, and the degree of freedom in the conversion process is not high. In 2012, Gil Porat et al. proposed an adiabatic second-order cascade frequency conversion scheme, which is based on the theory of stimulated Raman adiabatic passage, effectively solved the problem of low degree of freedom in a single three-wave mixing process, and has a high conversion efficiency. And in the following year, a phase-reversed quasi-phase matching technology was designed to modulate non-linear crystals, and mid-infrared lasers were efficiently generated through a cascade difference-frequency process experimentally. The frequency conversion scheme corresponding to the theory of stimulated Raman adiabatic passage requires that the delay parameter satisfies the counter-intuitive coupling order and the coupling parameter satisfies the corresponding adiabatic condition, which has many restrictions on the modulation method of the crystal and the intensity of the pump light.

[0005] Another theory for atomic population transfer - the theory of stimulated Raman adiabatic shortcuts, is a method in which the stimulated Raman technique compensates for the adiabatic driving term, and the frequency connecting the ground state and the excited state is w p1The pump pulse, and the frequency ω linking the excited state and the target state p2 The pulse shape of the Stokes pulse p2 is adiabatically corrected so that even if there is a certain amount of deviation of the original pulse parameters from the adiabatic conversion condition, the same population distribution as that of the final energy state under adiabatic evolution can be achieved, and almost no particles stay in the excited state during the whole process, thus completing the complete population transfer between the ground state and the target state. Summary of the Invention

[0006] The object of the present invention is to provide a non - linear optical coupling modulation method based on adiabatic shortcut for realizing the efficient conversion of signal light into output light. This method is based on the adiabatic shortcut theory and is a scheme for realizing efficient second - order cascaded non - linear optical frequency conversion by modulating crystal parameters, so as to reduce the limitations on the modulation parameters and pump light intensity in adiabatic conversion to meet the adiabatic condition. According to the designed coupling parameter modulation function, using quasi - phase - matching technology, multi - process quasi - phase matching is realized in the non - linear crystal, thereby obtaining efficient frequency conversion from signal light to output laser.

[0007] The above object of the present invention is achieved by the following technical solutions:

[0008] A non - linear optical coupling modulation method based on adiabatic shortcut, characterized in that the method comprises the following steps:

[0009] (1) Calculation of the anti - diabatic term in frequency conversion based on the adiabatic shortcut theory;

[0010] (2) Design of the modulation function of the coupling parameters in the cascaded wavelength conversion process and the diabatic compensation method;

[0011] (3) Selection and modulation of the laser light source and the non - linear crystal;

[0012] (4) Process of realizing the conversion of signal light into output light.

[0013] Based on the adiabatic shortcut theory, the present invention designs a new set of coupling functions, and uses quasi - phase - matching technology to realize the modulation satisfying this coupling function in the non - linear crystal, so that multi - process quasi - phase matching and the required coupling parameter modulation are realized. It can solve the problem that the diabatic process involved in adiabatic second - order cascaded non - linear frequency conversion cannot be efficiently converted, relieve the requirements for crystal modulation parameters, and reduce the requirements for the intensity of the pump light in the efficient frequency conversion process, and realize efficient frequency conversion from signal light to output laser.

[0014] In the present invention, the specific process of the step (1) is as follows:

[0015] Second - order cascaded optical frequency conversion, which includes two three - wave mixing processes, is described by the following kinetic equations

[0016]

[0017] where \(A(z)=[A 1 A 2 A 3 \) T represents the amplitude of each optical field, where \(A 1 represents the signal optical field, \(A 2 represents the intermediate optical field, \(A 3 represents the output optical field, \(i\) represents the imaginary unit, \(z\) represents the propagation distance of light in the nonlinear crystal, and the transfer matrix in the kinetic equation can be expressed as follows

[0018]

[0019] where:

[0020]

[0021]

[0022] is the coupling parameter between optical fields \(j\) and \(l\),

[0023] in the expression of the coupling parameter, the \(\pm\) sign represents different processes, where \(-\) represents the sum - frequency process and \(+\) represents the difference - frequency process, represents the conjugate of the coupling parameter. Here, when \(j\) and \(l\) take different values - 1, 2, 3, they represent the signal optical field, the intermediate optical field, and the output optical field respectively. \(\chi (2) represents the second - order nonlinear coefficient of the crystal, \(A p1 represents the field amplitude of the pump optical field in the first three - wave mixing process, \(A 2 represents the amplitude of the pump optical field in the second three - wave mixing process, \(w\) is the frequency of the light wave, and the phase mismatch is \(\Delta k i =k i \pm k pi -k i+1 + 2\pi / \Lambda i , (\(i = 1,2\)). The \(\pm\) sign in the mismatch represents different processes, where \(+\) represents the sum - frequency process and \(-\) represents the difference - frequency process. \(k j =w j n(w j ) / c\) is the wave vector, \(c\) is the speed of light in vacuum, \(n\) represents the refractive index of light in different bands in the crystal. Among them, \(2\pi / \Lambda i is the reciprocal lattice vector generated by modulating the crystal through the quasi - phase - matching technique, and \(\Lambda i is the crystal modulation period,

[0024] After modulation by the quasi - phase - matching technique, the modulated crystal can completely compensate for the mismatch. At this time, \(\Delta k1 = Δk 2 = 0. In the frequency conversion schemes based on stimulated Raman adiabatic passage (STIRAP) that have been proposed so far, the coupling modulation function between the crystal and the light satisfies the following expression

[0025] where k' 12 represents the coupling intensity modulation law between the signal light and the intermediate light in the frequency conversion scheme based on STIRAP, and k' 32 represents the coupling intensity modulation law between the intermediate light and the output light in the frequency conversion scheme based on STIRAP. s 1 represents the coupling delay in the first three-wave mixing process, and d 1 represents the span of the Gaussian coupling function in the first process. s 2 represents the coupling delay in the second three-wave mixing process, and d 2 represents the span of the Gaussian coupling function in the second process. Under the phase matching assumption, with the initial intermediate light intensity set to zero as the initial condition, the partial differential equation describing the intermediate light is integrated and substituted into the second-order cascade dynamic equation, thereby simplifying the coupled equations of the three optical fields into a dynamic equation system that can be represented by a second-order matrix containing only the signal light and the output light. By

[0026]

[0027] calculating the anti-adiabatic term, M cd is the Hamiltonian representation of the diabatic term, where |n> is the eigenstate of the second-order transformation matrix of the simplified coupled equation system above, <n| represents the left vector of the eigenstate, represents the partial derivative of the distance with respect to the eigenstate, ∑ represents the summation over all eigenstates, i is the imaginary unit, and the Hamiltonian of the anti-adiabatic driving term corresponding to the conversion from the signal light to the output light in the adiabatic conversion process has the following form

[0028]

[0029] The specific expression of the non-diagonal term in is

[0030]

[0031] In the formula, the dot in the superscript of the symbol represents the partial derivative with respect to the distance.

[0032] In the present invention, the specific process of step (2) is as follows:

[0033] According to the adiabatic shortcut theory, that is, after adding an anti-adiabatic drive to the system Hamiltonian, through a rotation transformation to obtain another interaction representation, a modulation function is designed to compensate for the diabatic effect in the frequency conversion process based on the stimulated Raman process. That is, the modulation function based on the adiabatic shortcut is as follows

[0034]

[0035]

[0036] Wherein:

[0037] Φ = arctan(k a / k' 12 )

[0038] represents the variation law of the coupling strength between the signal light optical field and the intermediate light optical field based on the adiabatic shortcut;

[0039] represents the variation law of the coupling strength between the output light optical field and the intermediate light optical field based on the adiabatic shortcut.

[0040] In the present invention, the specific process of the step (3) is as follows:

[0041] Based on the requirements of adiabatic frequency conversion, corresponding to the optical second-order cascaded adiabatic wavelength conversion, it is required that the coupling strength is large enough, that is, it is required that the power of the laser pump light source is large enough to ensure the stable progress of the adiabatic conversion process. Therefore,

[0042] The pump light source 1 selects a nanosecond, picosecond or femtosecond optical pulse light source;

[0043] The pump light source 2 selects a nanosecond, picosecond or femtosecond optical pulse light source;

[0044] The signal light source generally can select a continuous, nanosecond, picosecond or femtosecond optical pulse light source;

[0045] The nonlinear crystal selects LN, KTP, GaAs, etc., and realizes the non-periodic or aperiodic quasi-phase matching structure crystal (hereinafter referred to as the structure crystal) for modulating the above coupling parameters through electric field polarization or domain inversion.

[0046] In the present invention, the specific process of the step (4) is as follows:

[0047] Two pump lights with different wavelengths and a signal light are simultaneously and perpendicularly incident into the modulated nonlinear crystal. The first pump light and the signal light undergo a three-wave mixing process in the crystal, and the variation law of the coupling strength between the two optical fields is The generated intermediate light simultaneously undergoes a second three-wave mixing process with the second pump light, and the variation law of the coupling strength is Thereby generating the final output light, and in this process, the signal light is completely converted into the output light.

[0048] As a preferred embodiment of the present invention:

[0049] Signal light wavelength λ 1 = 1064 nm, and the optical intensity is 100 MW / cm 2 ;

[0050] The wavelength λ of the first pump light p1 = 2800 nm, and the optical intensity is 700 MW / cm 2 ;

[0051] The wavelength λ of the second pump light p2 = 2900 nm, and the optical intensity is 1.6 GW / cm 2 ;

[0052] The intermediate light wavelength λ2 = 1716 nm, and the output light wavelength λ3 = 1078 nm.

[0053] The nonlinear optical coupling modulation method based on adiabatic shortcut of the present invention calculates the diabatic term in the quadratic cascaded frequency adiabatic coupling modulation function of the stimulated Raman adiabatic passage proposed by predecessors, redesigns the coupling function and modulates the parameters of the structural crystal, so that after inputting the corresponding wavelengths, the interaction (coupling) intensity of light in the crystal satisfies the designed modulation function along the transmission direction, thereby realizing efficient quadratic cascaded adiabatic frequency conversion, and the limitations of system parameters (such as delay parameters, pump light intensity, etc.) are broken, and the wavelength band of the output light can almost cover all the wavelength bands that the corresponding crystal can transmit. Different output wavelength bands can be obtained by modulating different crystal parameters and selecting appropriate pump and signal light wavelengths. Brief Description of the Drawings

[0054] The following further describes the present invention in detail with reference to the drawings and specific embodiments.

[0055] Figure 1 It is a schematic diagram of the coupling intensity of the nonlinear optical coupling modulation method based on adiabatic shortcut of the present invention, where the abscissa is the crystal length and the ordinate is the normalized coupling intensity;

[0056] Figure 2 It is the change of the optical intensity of the input light, intermediate light and output light in the crystal length direction in the nonlinear optical coupling modulation method based on adiabatic shortcut of the present invention;

[0057] Figure 3 It is a schematic diagram of the crystal structure of the nonlinear optical coupling modulation method based on adiabatic shortcut of the present invention. Detailed Embodiments

[0058] The nonlinear optical coupling modulation method based on adiabatic shortcut of the present invention realizes efficient conversion from a wavelength of 1064 nm to 1078 nm through a periodically poled lithium niobate crystal. The quadratic cascade schematic diagram is as shown in Figure 1As shown in the figure. First, calculate the coupling function of the modulation crystal, and then process the crystal. When the corresponding laser passes through the crystal, it can automatically meet the conditions required for the adiabatic process and output a laser in a new frequency band.

[0059] The specific steps of this coupling modulation method are as follows:

[0060] Calculate the coupling intensity modulation functions that meet the requirements based on the shortcut to adiabaticity, which are respectively:

[0061]

[0062]

[0063] Among them

[0064] Φ = arctan(k a / k' 12 )

[0065]

[0066]

[0067] Among them, k 12 (z) describes the variation law of the coupling intensity of the first three-wave mixing process in the second-order cascaded wavelength conversion along the propagation direction z. The constant term k 12 represents the magnitude of the coupling parameter of the first conversion process in the unmodulated crystal. k 23 (z) is the coupling intensity of the second three-wave mixing process in the second-order cascaded wavelength conversion along the propagation direction z. k 23 represents the magnitude of the coupling parameter of the second conversion process in the unmodulated crystal. According to the delay parameter and span parameter required for complete conversion in the simulation calculation, determine the coupling delay parameter s 1 = -0.01m, span parameter d 1 = 0.00007m 2 , the coupling delay parameter s 2 = 0.01m, span parameter d 2 = 0.00009m 2 ,

[0068] The modulation of the coupling intensity in the crystal direction is realized by the above functions. The conditions for the input light required for complete conversion are as follows

[0069] The wavelength of the signal light λ 1 = 1064nm, and the light intensity is 100MW / cm 2 ;

[0070] Wavelength λ of the pump light 1 p1 = 2800 nm, and the optical intensity is 700 MW / cm 2 ;

[0071] Wavelength λ of the pump light 2 p2 = 2900 nm, and the optical intensity is 1.6 GW / cm 2 ;

[0072] The wavelength of the intermediate light λ2 = 1716 nm, and the wavelength of the output light λ3 = 1078 nm.

[0073] The phase mismatches corresponding to the two three-wave mixings in the second-order cascaded difference frequency process are respectively:

[0074] Δk 1 = k λ1 - k λ2 - k p1 + 2π / A 1 ,

[0075] Δk 2 = k λ2 - k λ3 - k p2 + 2π / A 2 ,

[0076] k j = n j w j / c.

[0077] Among them, Δk 1 is the phase mismatch of the first difference frequency process during the second-order cascaded wavelength conversion, and Δk 2 is the phase mismatch of the second three-wave mixing process (i.e., the sum frequency process), k j is the wave number, where k p1 is the wave number of the pump light in the first conversion process, k p2 is the wave number of the pump light in the second conversion process, k λ1 is the wave number of the signal light, k λ2 is the wave number of the intermediate light, k λ2 is the wave number of the output light. The terms containing the crystal modulation periods Λ 1 and Λ 2 play a role in compensating for the phase detuning caused by material dispersion. In the formula, the refractive index n j is calculated from the dispersion equation. The set temperature is 100 degrees Celsius, c is the speed of light in vacuum, and w j (j = 1, 2, 3) is the optical field frequency.

[0078] A lithium niobate crystal doped with magnesium oxide with a high laser damage threshold is selected.

[0079] In this embodiment, a lithium niobate (LN) crystal doped with magnesium oxide, which has a high laser damage threshold, is selected as the nonlinear crystal. The direction with a nonlinear coefficient of d33 = 28 pm / V is chosen as the optical axis direction. An aperiodic or non-periodic quasi-phase matching structure for modulating the above coupling parameters is realized by electric field polarization with an electric field strength higher than or slightly lower than 2 kV / mm. An electric field signal with the following spatial form is selected,

[0080] F(z) = sign[-cos(πD 1 ) + cos(2πz / Λ 1 )] × sign[-cos(πD 2 ) + cos(2πz / Λ 2 )]

[0081] where D i = l + / (l + + l - ) is the duty cycle of the crystal modulation of period i, and l + and l - represent the lengths of the positive domain and the negative domain of the crystal, respectively. The first-order approximation term of the Fourier series of the selected modulation function corresponds to an amplitude modulation term with the following form:

[0082] f 1 (z) = (2D 2 - 1)sin(πD 1 )

[0083] f 2 (z) = (2D 1 - 1)sin(πD 2 )

[0084] where Di represents the duty cycle of the crystal modulation, and the optimal duty cycle arrangement along the propagation direction is calculated by a genetic algorithm, so that the two amplitude modulation terms f1(z) and f2(z) respectively satisfy the coupling parameter modulation functions of the first process and the second process designed based on the adiabatic shortcut theory. And Λ 1 = Δk 1 , Λ 2 = Δk 2 .

[0085] Two pump lights with different wavelengths of 2800 nm and 2900 nm respectively and a signal light with a wavelength of 1064 nm are simultaneously incident perpendicularly into an electro-optically modulated lithium niobate crystal. The pump light with a wavelength of 2800 nm and the signal light with a wavelength of 1064 nm first undergo a difference-frequency generation process in the crystal. The variation law of the coupling strength between the two optical fields satisfies the coupling parameter modulation function of the first process designed based on the adiabatic shortcut theory. The generated intermediate light simultaneously undergoes a sum-frequency process with the pump light with a wavelength of 2900 nm, and the variation law of the coupling strength satisfies the coupling parameter modulation function of the second process designed based on the adiabatic shortcut theory, obtaining an output light with a wavelength of 1078 nm. In this process, the intensity of the intermediate light with a wavelength of 1716 nm is always in an extremely low state, and the signal light is finally completely converted into the output light.

[0086] The above embodiments of the present invention do not limit the protection scope of the present invention. The implementation manners of the present invention are not limited thereto. All kinds of modifications, substitutions or changes in other various forms made to the above structure of the present invention according to the above content of the present invention, in accordance with the common general knowledge and customary means in the art, without departing from the above basic technical idea of the present invention, shall fall within the protection scope of the present invention.

Claims

1. A non-linear optical coupling modulation method based on adiabatic shortcut, characterized in that, the method comprises the following steps: Step (1) Calculation of the anti-diabatic term in frequency conversion based on adiabatic shortcut theory; the specific process of this step is as follows: Second-order cascaded optical frequency conversion, including two three-wave mixing processes, is described by the following kinetic equations where A(z) = [A 1 A 2 A 3 T represents the amplitudes of the respective optical fields, where A 1 represents the signal optical field, A 2 represents the intermediate optical field, A 3 represents the output optical field, i represents the imaginary unit, z represents the propagation distance of light in the nonlinear crystal, and the transfer matrix in the kinetic equation can be expressed as follows​ where: k jl is the coupling parameter between optical fields j and l; In the coupling parameter expression The numbers represent different processes, where - represents the sum frequency process and + represents the difference frequency process. represents the conjugate of the coupling parameter. Here, when j and l take different values of -1, 2, and 3, they respectively represent the signal optical field, the intermediate optical field, and the output optical field. χ (2) represents the second-order nonlinear coefficient of the crystal. A p1 represents the field amplitude of the pump optical field in the first three-wave mixing process, A p2 represents the amplitude of the pump optical field in the second three-wave mixing process, w j is the frequency of the light wave, and the phase mismatch is Δk i = k i ±k pi - k i+1 + 2π / Λ i , where the subscript i takes values of 1 and 2, representing the first three-wave mixing process and the second three-wave mixing process respectively; the ± sign in the mismatch represents different processes, where + represents the sum-frequency process and - represents the difference-frequency process, k j = w j n(w j ) / c is the wave vector, k pi refers to the wave vector of the pump light; c is the speed of light in vacuum, n represents the refractive index of light in different bands in the crystal, where 2π / Λ i is the reciprocal lattice vector generated by modulating the crystal through quasi-phase matching technology, Λ i is the crystal modulation period, After modulation by quasi-phase matching technology, the modulated crystal can fully compensate for the detuning, and at this time, there is Δk 1 = Δk 2 = 0. In the frequency conversion scheme based on stimulated Raman adiabatic passage, the coupling modulation function between the crystal and light satisfies the following expression where k' 12 represents the modulation law of the coupling intensity between the signal light and the intermediate light in the frequency conversion scheme based on the stimulated Raman adiabatic passage, and k' 32 represents the modulation law of the coupling intensity between the intermediate light and the output light in the stimulated Raman adiabatic passage, s 1 represents the coupling delay in the first three-wave mixing process, d 1 represents the span of the Gaussian coupling function in the first three-wave mixing process, s 2 represents the coupling delay in the second three-wave mixing process, d 2 represents the span of the Gaussian coupling function in the second three-wave mixing process. Under the phase-matching assumption, with the initial intermediate light intensity set to zero as the initial condition, the partial differential equation describing the intermediate light is integrated and substituted into the kinetic equation of the second-order cascade, thereby simplifying the coupled equations of the three optical fields into a kinetic equation system that can be represented by a second-order matrix containing only the signal light and the output light. Through Calculate the anti-diabatic term, M cd is the Hamiltonian representation of the diabatic term, where |n> is the eigenstate of the second-order transformation matrix of the above simplified coupled equations, <n| represents the left vector of the eigenstate, represents the partial derivative of the distance with respect to the eigenstate, ∑ represents the summation over all eigenstates, i is the imaginary unit, and the Hamiltonian of the anti-diabatic driving term corresponding to the conversion from the signal light to the output light during the adiabatic conversion process has the following form The specific expression of the non-diagonal term in The dot in the superscript of the symbol in the formula represents the partial derivative with respect to distance; Step (2) Design of the modulation function of the coupling parameter in the cascaded wavelength conversion process and the diabatic compensation method; the specific process of this step is as follows: According to the adiabatic shortcut theory, that is, after adding an anti-adiabatic drive to the system Hamiltonian, another interaction representation is obtained through a rotation transformation, and a modulation function that can compensate for the diabatic effect in the frequency conversion process based on the stimulated Raman process is designed. The modulation function based on adiabatic shortcut is as follows, where, Represents the variation law of the coupling strength between the signal light optical field and the intermediate optical field based on adiabatic shortcuts; Represents the variation law of the coupling intensity between the output optical field and the intermediate optical field based on the adiabatic shortcut; Step (3) Selection and modulation of the laser light source and the non-linear crystal; the specific process of this step is as follows: Based on the requirements of adiabatic frequency conversion, corresponding to the optical second-order cascaded adiabatic wavelength conversion, it is required that the coupling strength is large enough, that is, the power of the laser pump light source is large enough to ensure the stable progress of the adiabatic conversion process. The first pump light source selects a nanosecond, picosecond or femtosecond optical pulse light source; The second pump light source selects a nanosecond, picosecond or femtosecond optical pulse light source; The signal light source selects a continuous light source, or a nanosecond, picosecond, femtosecond optical pulse light source; The non-linear crystal selects LN or KTP or GaAs, and a non-periodic or aperiodic quasi-phase matching structure crystal that realizes the above coupling parameter modulation through electric field polarization or domain inversion; Step (4) Process of realizing the conversion of the signal light into the output light, the specific process of this step is as follows: Two pump lights with different wavelengths and a signal light are simultaneously and perpendicularly incident on a modulated nonlinear crystal. The first pump light and the signal light undergo a three-wave mixing process in the crystal, and the variation law of the coupling strength between the two optical fields is The generated intermediate light simultaneously undergoes a second three-wave mixing process with the second pump light, and the variation law of the coupling strength is Thereby, the final output light is generated, and in this process, the signal light is completely converted into the output light.

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