Wide-temperature frequency-doubling conversion method based on quasi-phase matching

Abstract

The present invention provides a wide-temperature frequency doubling conversion method based on quasi-phase matching, comprising a periodically polarized crystal, in which the phase mismatch in the nonlinear optical crystal is approximately linearly related to the crystal temperature, and the second-order partial derivative of the phase mismatch with respect to temperature is much greater than the first-order partial derivative. By reasonably designing the cutting angle and polarization period of the nonlinear optical crystal, the quasi-phase matching condition is met and the first-order partial derivative of the phase mismatch with respect to temperature is zero, so that the frequency conversion efficiency is basically not affected by the change in crystal temperature. By changing the cutting angle of the traditional polarized crystal, the nonlinear optical frequency conversion maintains a higher conversion efficiency in a wider temperature range, and the temperature adaptability and stability of the nonlinear optical frequency conversion crystal device are improved. The present invention does not require temperature control equipment, so that the structure of the nonlinear optical frequency conversion crystal device is simpler and more compact, while reducing device energy consumption, reducing device cost, and improving device performance.

Description

Technical Field

[0001] The invention relates to the technical field of nonlinear optical frequency conversion crystal devices, and in particular to a wide-temperature frequency doubling conversion method based on polarized crystals. Background Art

[0002] Quasi-phase matching technology periodically changes the polarization direction of nonlinear optical crystals so that the laser participating in nonlinear optical frequency conversion meets the quasi-phase matching conditions and can achieve non-critical phase matching to avoid spatial walk-off. It can conveniently utilize the maximum effective nonlinear coefficient of nonlinear optical crystals, so it is widely used in nonlinear optical frequency conversion. However, like birefringence phase matching, quasi-phase matching is also affected by crystal temperature changes, and its temperature peak half width is usually small, resulting in poor adaptability to ambient temperature.

[0003] When the PPKTP crystal with a conventional cutting angle is used to double the wavelength of a 1064nm laser, the temperature peak half width is low, resulting in poor temperature stability of the doubled laser. When the ambient temperature slightly deviates from the matching temperature, the doubled laser power drops sharply. This feature restricts its application in environments with changing temperatures. The current solution is to perform constant temperature control on the crystal, which will increase system energy consumption and complexity. Summary of the invention

[0004] Compared with the existing frequency conversion technology, the wide-temperature frequency doubling conversion method based on polarized crystals of the present invention does not require a temperature control system, and has the advantages of simple and compact structure, low energy consumption, low cost, etc.

[0005] Based on the wide temperature frequency doubling conversion method of quasi-phase matching, in order to achieve quasi-phase matching, the polarization period Λ of the nonlinear crystal is usually the coherence length l c Twice, taking the sum frequency as an example

[0006]

[0007] where k i (i=1,2,3) is the laser wave vector involved in the sum frequency. In quasi-phase matching, the phase mismatch factor

[0008]

[0009] Under the small signal approximation condition, the conversion efficiency η is

[0010]

[0011] Among them, d eff is the effective nonlinear coefficient, L is the crystal length; when Δk=0, the conversion efficiency is the highest, and when ΔkL=0.886π, the conversion efficiency is half of the maximum value; expand Δk(T) according to the Taylor series

[0012]

[0013] When the first-order term in the expanded form of formula (4) is zero, that is, The temperature half-width can be calculated from the quadratic term

[0014]

[0015] The phase mismatch in nonlinear optical crystals is approximately linearly related to the crystal temperature. The first-order derivative of the phase mismatch with respect to temperature is much larger than the second-order derivative. Therefore, T″ BW >>T′ BW ; By rationally designing the cutting angle and polarization period of the nonlinear optical crystal, Δk(T0) = 0 and That is, the quasi-phase matching condition is met and the first-order partial derivative of the phase mismatch with respect to temperature is zero, so that the frequency conversion efficiency is basically not affected by the change of crystal temperature;

[0016] Where T0 is the crystal temperature when the quasi-phase matching condition is met, ΔT = T-T0, and

[0017]

[0018] where n i (i=1,2,3) is the refractive index of the laser participating in the sum frequency in the crystal; since Δk(T0)=0, take the first-order term of the above expansion and set Δk(T)L=0.886π; the sum frequency temperature half width is obtained at this time:

[0019]

[0020] The invention comprises a wide temperature frequency doubling device based on quasi-phase matching, comprising a periodically polarized crystal cut at a certain angle so that the first-order derivative of the phase mismatch with respect to temperature is zero, thereby greatly improving the temperature width of the device.

[0021] The present invention provides a wide-temperature frequency-doubling conversion method based on quasi-phase matching. By changing the cutting angle of the traditional polarized crystal, the nonlinear optical frequency conversion maintains a high conversion efficiency in a wider temperature range, and improves the temperature adaptability and stability of the nonlinear optical frequency conversion crystal device. The device does not require a temperature control device, making the nonlinear optical frequency conversion crystal device structure simpler and more compact, while reducing device energy consumption, reducing device cost, and improving device performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1This is a schematic diagram of the structure of a wide-temperature frequency doubling device based on quasi-phase matching; wherein: A represents the fundamental frequency light λ1=1064nm, Z represents the optical axis Z axis and polarization direction of the nonlinear optical crystal, f represents the polarization direction of the laser fast light in the nonlinear optical crystal, and Λ represents the polarization period of the nonlinear optical crystal.

[0023] Figure 2 This is a curve of the conversion efficiency changing with crystal temperature when the wavelength of the doubled frequency wave in the PPKTP crystal is 1064nm laser.

[0024] Figure 3 This is a curve of the conversion efficiency changing with crystal temperature when the wavelength of the doubled frequency wave in the PPMgLN crystal is 1064nm laser. DETAILED DESCRIPTION

[0025] Quasi-phase matching technology periodically changes the polarization direction of nonlinear optical crystals so that the laser participating in nonlinear optical frequency conversion meets the quasi-phase matching conditions and can achieve non-critical phase matching to avoid spatial walk-off. It can conveniently utilize the maximum effective nonlinear coefficient of nonlinear optical crystals, so it is widely used in nonlinear optical frequency conversion. However, like birefringence phase matching, quasi-phase matching is also affected by crystal temperature changes, and its temperature peak half width is usually small. For example, when using a 1cm long polarized potassium titanyl phosphate (PPKTP) crystal to double the wavelength of a laser with a wavelength of 1064nm, the temperature peak half width is only 4.7℃, resulting in poor adaptability to ambient temperature.

[0026] To achieve quasi-phase matching, the polarization period Λ of the nonlinear crystal is usually equal to the coherence length l c twice as much,

[0027]

[0028] where k i (i=1,2,3) is the participating laser wave vector, in quasi-phase matching, the phase mismatch factor

[0029]

[0030] Under the small signal approximation condition, the conversion efficiency η

[0031]

[0032] Among them, d eff is the effective nonlinear coefficient, and L is the crystal length. When Δk = 0, the conversion efficiency is the highest, and when ΔkL = 0.886π, the conversion efficiency is half of the maximum value. Expand Δk(T) according to the Taylor series

[0033]

[0034] When the first-order term in the expanded form of formula (4) is zero, that is, The temperature half-width can be calculated from the quadratic term

[0035]

[0036] Where T0 is the crystal temperature when the quasi-phase matching condition is met, ΔT = T-T0, and

[0037]

[0038] where n i (i=1,2,3) is the refractive index of the laser participating in the sum frequency in the crystal. Since Δk(T)=0, take the first-order term of the above expansion and set Δk(T)L=0.886π. The sum frequency temperature half width can be obtained at this time:

[0039]

[0040] When the PPKTP crystal is used to double the wavelength of a 1064nm laser, the PPKTP crystal is usually cut at an angle of (90°, 0°). At this time, the first-order derivative of the phase mismatch with respect to temperature is 1.191 cm -1 ℃ -1 , so its temperature peak half width is only 4.7℃. It can be seen that when the PPKTP crystal with a traditional cutting angle is used to double the wavelength of a laser of 1064nm, the temperature peak half width is low, resulting in poor temperature stability of the doubled laser. When the ambient temperature slightly deviates from the matching temperature, the doubled laser power drops sharply. This feature restricts its application in temperature-changing environments. The current solution is to control the crystal temperature, but this will increase system energy consumption and complexity.

[0041] The following uses the 1064 nm laser in the PPKTP crystal as an example to illustrate the device design principle. Type II phase-matched frequency doubling can be used to solve the cutting angle and polarization period of the crystal that meets the critical quasi-phase matching conditions in a wide temperature range. Assume that the crystal cutting angle is Temperature is T, phase mismatch

[0042]

[0043] in are the refractive indices of the fast light and slow light of the fundamental frequency, respectively. When the temperature is T0 = 293K, the cutting angle can be obtained according to the Sellmeier equation and the thermo-optical dispersion equation of the KTP crystal: The refractive index of the fundamental frequency light and the doubled frequency light is calculated, and then the polarization period that satisfies the quasi-phase matching condition at this angle is obtained.

[0044]

[0045] Due to the thermal expansion and contraction effect, the polarization period of the crystal at temperature T is

[0046] Λ(T)=Λ0·(1+α(T-T0)) (10)

[0047] Where α is the linear thermal expansion coefficient. The phase mismatch with respect to the cutting angle can be obtained from the Sellmeier equation of KTP crystal, the thermo-optical dispersion equation and equation (10). And the function of crystal temperature, the partial derivative of the function with respect to temperature is calculated. When the first-order partial derivative is 0, that is When the cutting angle of the corresponding crystal is obtained At the same time, the effective nonlinear coefficient of the nonlinear crystal at this angle is calculated. The results are as follows: Figure 1 As shown in the figure, it can be seen that In the cutting direction, The angle range is 62°-90°, corresponding to The angle range is 90°-56.8°. The crystal with zero first-order partial derivative is periodically polarized to meet the quasi-phase matching condition, thereby achieving frequency doubling in a wide temperature range.

[0048] Example 1

[0049] The nonlinear optical crystal is PPKTP crystal, the crystal cutting angle is (90°, 56.8°), and the crystal size is 3×1×10mm 3 The crystal polarization period is 170.74μm, and the light-transmitting surfaces of the crystal are coated with anti-reflection films for the 1064nm and 532nm optical bands. Using type II phase matching, the polarization direction of the fundamental frequency light in the input plane of the PPKTP crystal is at an angle of 45° with the z-axis of the PPKTP crystal.

[0050] After determining the PPKTP crystal parameters, the phase mismatch of the crystal at different temperatures T is calculated and substituted into formula (3) to obtain the curve of the frequency doubling normalized conversion efficiency changing with temperature, as shown in Figure 2 As shown in the figure, when the temperature is 20℃, the normalized conversion efficiency reaches the maximum value of 100%, and when the temperature is 140℃, the conversion efficiency is the lowest, which is 95.8%. When the temperature changes between -100-140℃, the frequency doubling conversion efficiency is very stable, and the maximum deviation is less than 5%.

[0051] Example 2

[0052] The nonlinear optical crystal is 5 mol% MgO doped LiNbO3 crystal (PPMgLN), and the crystal cutting angle is (θ = 18.6°, ), the polarization period is Λ=5.6488μm, and the crystal size is 3×1×10mm 3 , the phase matching type is eo-o. The light-transmitting surfaces of the crystal are coated with anti-reflection films of the 1064nm and 532nm optical bands, and type II phase matching is adopted. After determining the parameters of the PPMgLN crystal, the phase mismatch of the crystal at different crystal temperatures T is calculated and substituted into formula (3) to obtain the normalized frequency doubling conversion efficiency curve with crystal temperature, as shown in Figure 3 As shown in the figure, it can be seen that in the temperature range of -150-100℃, the normalized conversion efficiency is higher than 96%, and when the temperature range is -130-84℃, the conversion efficiency is higher than 99%, the change is stable, and the maximum deviation is less than 1%. The simulation results show that the temperature stability of the frequency doubling in this crystal is excellent.

Claims

1. A wide temperature frequency doubling conversion method based on quasi-phase matching, characterized in that To achieve quasi-phase matching, the polarization period Λ of the nonlinear crystal is equal to the coherence length l c twice as much, where k i (i=1,2,3) is the laser wave vector involved in the sum frequency. In quasi-phase matching, the phase mismatch factor Under the small signal approximation condition, the conversion efficiency η is Among them, d eff is the effective nonlinear coefficient, L is the crystal length; when Δk=0, the conversion efficiency is the highest, and when ΔkL=0.886π, the conversion efficiency is half of the maximum value; Δk(T) is expanded by Taylor series at the crystal temperature T0 Where T is the crystal temperature, Δk(T) and Δk(T0) are the phase mismatch when the crystal temperature is T and T0 respectively, ΔT = T-T0, when the first-order term in the expanded formula (4) is zero, that is, The temperature half-width is calculated from the quadratic term The phase mismatch in nonlinear optical crystals is approximately linearly related to the crystal temperature. The second-order derivative of the phase mismatch with respect to temperature is much larger than the first-order derivative. Therefore, T″ BW >>T′ BW ; Combining the crystal thermo-optical dispersion equation and the Sellmeier equation, let Δk(T0) = 0 and The cutting angle and polarization period of the nonlinear optical crystal that satisfy the quasi-phase matching condition and the first-order derivative of the phase mismatch with respect to temperature is zero can be solved; Where T0 is the crystal temperature when the quasi-phase matching condition is met, ΔT = T-T0, and Where n i (i=1,2,3) is the refractive index of the participating laser in the crystal; since Δk(T0)=0, take the first-order term of the above expansion and set Δk(T)L=0.886π; the temperature half width at this time is obtained:

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