Method for generating ultra-broadband mid-infrared laser in a bulk polar crystal

Abstract

The application relates to the technical field of strong laser nonlinear frequency conversion, in particular to a method for generating ultra-wideband mid-infrared laser in a single piece of polarized crystal. The method uses laser with a wavelength of 800 nm as pump light, laser with a wavelength of 0.95-1.6 microns as signal light, and a difference frequency conversion process occurs in the polarized crystal, thereby generating mid-infrared laser with a wavelength of 1.6-5 microns. In the method, a polarized crystal with a chirped period structure is used, that is, the polarization period of the crystal continuously changes with the position in the crystal. The polarized crystal is a 5% magnesium oxide doped chirped period lithium niobate crystal. The application utilizes quasi-phase matching technology, can use the maximum effective nonlinear coefficient of the chirped period lithium niobate crystal, and does not need to change the crystal angle or the polarization of the laser when the pump light is at different wavelengths. The application overcomes the shortcoming that the wavelength of the current mid-infrared band laser is difficult to expand due to the limitation of the gain medium.

Description

Technical Field

[0001] This invention relates to the field of high-power laser nonlinear frequency conversion technology, and in particular to a method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal. Background Technology

[0002] Mid-infrared lasers (3–5 μm) possess unique photon energy, making them highly valuable for applications in basic research, biomedicine, optical communication, and atmospheric detection. However, traditional laser generation and amplification methods, such as regenerative amplification, are no longer suitable due to limitations in the gain medium of the mid-infrared band. Second-order nonlinear frequency conversion, including second harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG), optical parametric oscillation (OPO), and amplification (OPA) processes, has been widely applied to laser frequency extension. By using difference-frequency conversion, the shorter and more readily available near-infrared laser wavelength can be converted into mid-infrared laser, enabling the achievement of goals that are difficult to achieve with traditional lasers. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for generating ultra-wideband mid-infrared lasers in a single polarized crystal, which can simultaneously generate mid-infrared lasers in the 1.6–5 μm band and has the advantages of being easy to use, having a controllable structure, and being flexible in design. This method overcomes the shortcomings of traditional infrared lasers, such as difficult tuning, large area requirements, and inability to extend into the mid-infrared range.

[0004] The technical solution adopted in this invention is: a method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal. This method is based on quasi-phase matching technology, which utilizes quasi-phase matching technology to perform a difference frequency process in the polarized crystal. The pump light wavelength used in the difference frequency process is 800 nm, and the signal light wavelength is 0.95–1.6 μm. The difference frequency process in the crystal can generate mid-infrared laser with a wavelength of 1.6–5 μm. The specific steps are as follows:

[0005] Step 1: Based on the formula for the refractive index dispersion of pump light and signal light in polarized crystal during the difference frequency process, calculate the phase mismatch generated by mid-infrared laser at each wavelength during the difference frequency process;

[0006] Step 2: Set the polarization crystal structure according to the phase mismatch, determine the specific parameters of the polarization crystal, and determine the specific parameters of the polarization crystal as the initial polarization period, the crystal polarization period, and the crystal chirped polarization period;

[0007] Step 3: According to the requirements of quasi-phase matching technology, the required pump light and signal light are superimposed with e-polarization and incident perpendicularly on the polarized crystal to achieve broadband wavelength conversion. By incidenting signal light of different wavelengths, the corresponding mid-infrared laser generation efficiency of different wavelengths is obtained, and ultra-wideband mid-infrared laser generation is realized in a single crystal.

[0008] A further improvement to the above scheme is that, in step 2, the polarizing crystal is a 5% MgO-doped chirped periodically polarized lithium niobate crystal (CPPLN); the chirped periodically polarized lithium niobate crystal is cuboid in shape, with parallel and polished upper and lower surfaces, a length and width of 20mm*6mm, and a thickness of 1-2mm.

[0009] A further improvement to the above scheme is that the chirped periodically polarized lithium niobate crystal includes a series of domain structures of varying lengths, the length of each domain structure being the polarization period of the crystal, and each of the series of domain structures contains a pair of positive and negative domains of opposite polarization directions and equal lengths.

[0010] That is, Λ = l + +l - ,

[0011] Positive and negative domains represent the signs of the second-order nonlinear coefficients in this region, respectively.

[0012] A further improvement to the above scheme is that the length of the series of domain structures changes along the direction of light propagation according to a continuous chirp variation.

[0013] A further improvement to the above scheme is that the chirped periodically polarized lithium niobate crystal must satisfy the quasi-phase matching condition that requires the femtosecond pulsed laser to be a vertically incident nonlinear crystal, the nonlinear crystal to be cut in the z-axis, and the incident light to be e-polarized.

[0014] A further improvement to the above scheme is that, in step 1, the phase mismatch Δk0 of the difference frequency process is calculated using the Sellmeier equation based on the wavelengths of the pump light and the signal light, and the reciprocal lattice vector G required for the chirped periodically polarized lithium niobate crystal is obtained based on the phase mismatch Δk0. m =Δk0.

[0015] A further improvement to the above scheme is that, in step 2, the initial polarization period Λ0 of the crystal is determined by... We obtain, where G0 is G m The corresponding minimum value.

[0016] A further improvement to the above scheme is that, in step 2, the crystal chirped polarization period D... g Depend on We obtain, where G max For G m The corresponding maximum value.

[0017] A further improvement to the above scheme is that, in step 2, the crystal polarization period Λ(z) is given by the formula... We obtain, where z is the position of the light transmission direction of the crystal.

[0018] A further improvement to the above scheme is that, in step 3, in order to obtain the conversion efficiency of the mid-infrared laser, the following set of nonlinear coupled-wave equations is used for solution:

[0019]

[0020]

[0021]

[0022] Where Ep, Es, and Em are the electric field amplitudes of the pump light, signal light, and mid-infrared light wave, respectively, and c is the speed of light. χ (2) (z) is based on the chirped polarization period Λ=l + +l - The corresponding nonlinear coefficient distribution.

[0023] The beneficial effects of this invention are:

[0024] (1) Based on the principle of quasi-phase matching technology, this invention proposes a method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal based on quasi-phase matching. Based on the chirped periodic polarized crystal structure, it is possible to simultaneously achieve broadband mid-infrared laser output of 1.6 to 5 μm in a monolithic nonlinear crystal.

[0025] (2) This invention utilizes a nonlinear frequency conversion difference frequency generation based on quasi-phase matching technology, using a pump light wavelength of 800 nm and a signal light wavelength of 0.95–1.6 μm, to simultaneously generate a broadband mid-infrared laser output of 1.6–5 μm in a single crystal. Due to the advantages of quasi-phase matching, this method does not require changing the incident laser's incident angle, polarization, or other parameters during use, and has no complex optical path requirements. Simultaneously, the polarization of the output laser remains the same as that of the incident laser. Furthermore, this invention uses a chirped periodically polarized crystal, which, compared to birefringent crystals or fixed-period polarized crystals, can provide more phase mismatch compensation in the same direction, thus giving this invention a significantly larger operating bandwidth.

[0026] (3) The present invention realizes the functions that previously required a complex optical path system in a single crystal. Compared with the past infrared lasers, it can extend to the mid-infrared band and does not require a complex optical path setting or a large area. At the same time, the stability is also significantly improved, which greatly reduces the cost and is conducive to industrialization. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the generation of mid-infrared laser by the chirped periodically polarized lithium niobate crystal of the present invention;

[0028] Figure 2The polarization period required for generating mid-infrared lasers at different wavelengths, as calculated by this invention;

[0029] Figure 3 This invention calculates the phase mismatch corresponding to different wavelengths of mid-infrared laser generation.

[0030] Figure 4 The inverse lattice vector distribution of the chirped periodically polarized lithium niobate crystal designed for this invention and the phase mismatch generated by mid-infrared laser;

[0031] Figure 5 The conversion efficiency of the infrared laser at 5mm in the designed chirped periodically polarized lithium niobate crystal in this invention;

[0032] Figure 6 The conversion efficiency of the infrared laser at 10 mm in the designed chirped periodically polarized lithium niobate crystal in this invention;

[0033] Figure 7 The conversion efficiency of the infrared laser at 15mm in the designed chirped periodically polarized lithium niobate crystal in this invention;

[0034] Figure 8 The conversion efficiency of the infrared laser at 20 mm in the designed chirped periodically polarized lithium niobate crystal is given by the present invention. Detailed Implementation

[0035] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0036] It should be noted that when a component is said to be "fixed to" another component, it can be directly attached to the other component or there may be an intervening component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there may be an intervening component.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0038] like Figures 1 to 8As shown, this invention proposes a method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal based on quasi-phase matching. This method utilizes quasi-phase matching technology to perform a difference frequency process in the polarized crystal. The pump light wavelength used in the difference frequency process is 800 nm, and the signal light wavelength is 0.95–1.6 μm. The difference frequency process in the crystal can generate mid-infrared laser light with a wavelength of 1.6–5 μm. The specific steps are as follows:

[0039] Step 1: Based on the formula for the refractive index dispersion of pump light and signal light in polarized crystal during the difference frequency process, calculate the phase mismatch generated by mid-infrared laser at each wavelength during the difference frequency process;

[0040] Step 2: Set the polarization crystal structure according to the phase mismatch and determine the specific parameters of the polarization crystal, such as the initial polarization period, the crystal polarization period, and the crystal polarization chirp period.

[0041] Step 3: According to the requirements of quasi-phase matching technology, the required pump light and signal light are superimposed with e-polarization and incident perpendicularly on the polarized crystal to achieve broadband wavelength conversion. By incidenting signal light of different wavelengths, the corresponding mid-infrared laser generation efficiency of different wavelengths is obtained, and ultra-wideband mid-infrared laser generation is realized in a single crystal.

[0042] The polarized crystal is a 5% MgO-doped chirped periodically polarized lithium niobate crystal. The chirped periodically polarized lithium niobate crystal is cuboid in shape, with parallel and polished upper and lower surfaces, measuring 20mm x 6mm in length and width, and 1-2mm in thickness. The chirped periodically polarized lithium niobate crystal comprises a series of domain structures of varying lengths. The length of each domain structure is equal to the polarization period Λ of the crystal. Each domain structure contains a pair of positive and negative domains of opposite polarization directions and equal length, i.e., Λ = 1. + +l - Positive and negative domains represent the signs of the second-order nonlinear coefficients in the region, respectively. The length of the series of domain structures changes along the light propagation direction according to a continuous chirp variation. The chirped periodically polarized lithium niobate crystal satisfies the quasi-phase-matching condition requiring the femtosecond pulsed laser to be a perpendicularly incident nonlinear crystal, the nonlinear crystal to be z-axis cut, and the incident light to be e-polarized.

[0043] In the method for generating ultra-wideband mid-infrared laser, step 1 involves calculating the phase mismatch Δk0 of the difference frequency process using the Sellmeier equation based on the wavelengths of the pump light and the signal light. The reciprocal lattice vector G required by the chirped periodically polarized lithium niobate crystal is then obtained based on the phase mismatch Δk0. m =Δk0.

[0044] More specifically, in this embodiment, for the generation of mid-infrared lasers of different wavelengths, the required polarization period and phase mismatch were calculated as follows: Figure 2 and Figure 3 As shown, there is an extreme point in the phase mismatch within the range of 1.6–5 μm. At this point, different wavelengths at both ends can share the same reciprocal lattice vector, thus requiring a relatively small reciprocal lattice vector bandwidth.

[0045] A method for generating ultra-wideband mid-infrared laser, characterized in that, in step 2, the initial polarization period Λ0 of the crystal is determined by... We obtain, where G0 is G m The corresponding minimum value.

[0046] A method for generating ultra-wideband mid-infrared laser, characterized in that, in step 2, the crystal polarization periodicity chirp D... g Depend on We obtain, where G max For G m The corresponding maximum value.

[0047] A method for generating ultra-wideband mid-infrared lasers, characterized in that, in step 2, the crystal polarization period Λ(z) is given by the formula... We obtain, where z is the position of the light transmission direction of the crystal.

[0048] like Figure 4 As shown, the initial polarization period Λ0 = 22.5 μm and the chirp D are set. g =1.5×10 -6 μm -2 With a chirped polarized lithium niobate crystal length L = 20 mm, the polarization period distribution ranges from 22.5 to 20.2 μm. The reciprocal lattice vector distribution of the periodically polarized lithium niobate crystal and the phase mismatch for mid-infrared laser generation can be obtained. It can be seen that the reciprocal lattice vector distribution completely covers the phase mismatch required for mid-infrared laser generation, thus enabling the generation of mid-infrared lasers with a diameter of 1.6–5 μm in a single crystal.

[0049] In step 3 of the method for generating ultra-wideband mid-infrared laser, the following set of nonlinear coupled-wave equations is used to solve for the conversion efficiency of the mid-infrared laser:

[0050]

[0051]

[0052]

[0053] Where Ep, Es, and Em are the electric field amplitudes of the pump light, signal light, and mid-infrared light wave, respectively, and c is the speed of light. χ (2) (z) is based on the chirped polarization period Λ=l + +l -The corresponding nonlinear coefficient distribution.

[0054] The input electric field strength of the 800nm ​​pump light is set to 3*10. 7 V / m corresponds to a power density of 0.12 GW / cm². 2 The center wavelength of the signal light ranges from 0.95 to 1.6 μm, and the input electric field intensity of each single-wavelength signal light is assumed to be 10. 7 V / m corresponds to a power density of 13.26 MW / cm². 2 The nonlinear coefficient of lithium niobate crystal is 27.2 pm / V. Solving the coupled wave equations yields the following result: Figure 5-8 The figure shows the conversion efficiency of mid-infrared laser at different positions in the crystal. It can be seen that the reciprocal lattice band structure provided by the chirped periodically polarized lithium niobate crystal varies with the propagation distance of the laser within the crystal, and the corresponding conversion bandwidth widens with increasing propagation distance. Figure 5 At this point, the laser propagation distance in the crystal is 5 mm, and the effective bandwidth of the crystal is 2.8-4.2 μm. For example... Figure 6 As the laser propagation distance increases to 10 mm, the bandwidth of the mid-infrared laser shows a significant broadening, with the effective bandwidth of the crystal expanding to 2.2-4.8 μm. For example... Figure 7 , 8 As the laser propagation distance increases further, the effective bandwidth extension of the crystal gradually decreases, eventually completely covering the 1.6–5 μm band. This demonstrates that the chirped periodically polarized lithium niobate crystal structure can support the generation of ultra-wideband mid-infrared wavelengths.

[0055] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for generating ultrawideband mid-infrared laser in a monolithic polarized crystal, characterized in that: This method is based on quasi-phase matching technology, which utilizes quasi-phase matching to perform a difference frequency process in a polarized crystal; the pump light wavelength used in the difference frequency process is 800 nm, and the signal light wavelength is... In crystals, difference frequencies can be obtained Mid-infrared laser; the specific steps are as follows: Step 1: Based on the formula for the refractive index dispersion of pump light and signal light in polarized crystal during the difference frequency process, calculate the phase mismatch generated by mid-infrared laser at each wavelength during the difference frequency process; Step 2: Set the polarization crystal structure according to the phase mismatch, determine the specific parameters of the polarization crystal, and determine the specific parameters of the polarization crystal as the initial polarization period, the crystal polarization period, and the crystal chirped polarization period; Step 3: According to the requirements of quasi-phase matching technology, the required pump light and signal light are superimposed with e-polarization and incident perpendicularly on the polarized crystal to achieve broadband wavelength conversion. By incidenting signal light of different wavelengths, the corresponding mid-infrared laser generation efficiency of different wavelengths is obtained, and ultra-wideband mid-infrared laser generation is realized in a single crystal. In step 2, the polarization crystal is a chirped periodically polarized lithium niobate crystal doped with 5% MgO; the chirped periodically polarized lithium niobate crystal is cuboid in shape, with parallel and polished upper and lower surfaces, a length and width of 20mm*6mm, and a thickness of 1~2mm. The chirped periodically polarized lithium niobate crystal includes a series of domain structures of varying lengths, each domain structure having a length equal to the polarization period of the crystal. Each of the series of domain structures of varying lengths contains a pair of positive and negative domains of opposite polarization directions and equal lengths. Right now , For the chirping polarization period, For the positive category, For negative domains; Positive and negative domains represent the signs of the second-order nonlinear coefficients in their respective regions; The quasi-phase-matching condition that a chirped periodically polarized lithium niobate crystal must satisfy requires that the femtosecond pulsed laser be a vertically incident nonlinear crystal, the nonlinear crystal be cut along the z-axis, and the incident light be polarized in the e-direction.

2. The method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal according to claim 1, characterized in that: The lengths of the series of domain structures of varying lengths change along the direction of light propagation according to a continuous chirp variation.

3. The method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal according to claim 1, characterized in that: In step 1, the phase mismatch of the difference frequency process is calculated using the Sellmeier equation based on the wavelengths of the pump light and the signal light. According to the phase mismatch The reciprocal lattice vector required to obtain a chirped periodically polarized lithium niobate crystal .

4. The method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal according to claim 3, characterized in that: In step 2, the initial polarization period of the crystal Depend on Received, among which for The corresponding minimum value.

5. The method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal according to claim 4, characterized in that: In step 2, the crystal chirped polarization period Depend on Received, among which for The corresponding maximum value.

6. The method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal according to claim 5, characterized in that: In step 2, the crystal polarization period From the formula We obtain, where z is the position of the light transmission direction of the crystal.

7. The method for generating ultra-wideband mid-infrared laser in a monolithic polarized crystal according to claim 6, characterized in that: In step 3, to obtain the conversion efficiency of mid-infrared laser, the following set of nonlinear coupled-wave equations is used for solving: Where Ep, Es, and Em are the electric field amplitudes of the pump light, signal light, and mid-infrared light wave, respectively, and c is the speed of light; To determine the chirping polarization period The corresponding nonlinear coefficient distribution.

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