Broadband low nonlinear optical effect chirped mirror and its design method

Abstract

A broadband low nonlinear optical effect chirped mirror comprises, from bottom to top, a substrate, a broadband chirped film layer and a low nonlinear optical effect chirped film layer. Based on the initially designed chirped film system, the present invention calculates the distribution of the refractive index increment Δn in the film when a femtosecond pulse is incident, and designs a broadband chirped film layer and a low nonlinear optical effect chirped film layer according to the distribution result, thereby reducing the value of the refractive index increment Δn in the film. Compared with the traditional broadband chirped mirror, the broadband low nonlinear optical effect chirped mirror can suppress the third-order nonlinear optical effect and reduce the influence of the change of optical thickness on the spectral reflectivity R and group delay dispersion GDD. This design method can maintain good linear optical properties under the condition that the chirped mirror has a wide spectral bandwidth, and well meets the design requirements of chirped mirrors with high reflectivity, wide bandwidth and low nonlinear optical effect in ultrafast laser systems and extracavity post-compression systems.

Description

Technical Field

[0001] The present invention belongs to the field of ultrafast laser films, and in particular is an optical film element capable of suppressing nonlinear optical effects and used for pulse compression in an ultra-strong ultra-short laser system and an extra-cavity post-compression system. Background Art

[0002] With the continuous development of ultra-intense ultrashort pulse laser technology, the pulse width can be compressed to a few femtoseconds, and the peak power will reach the order of hundreds of petawatts. Chirped mirrors are one of the commonly used optical elements for dispersion compensation in ultra-intense ultrashort pulse laser systems and extracavity post-compression systems. Their dispersion compensation performance directly restricts the quality of the ultrashort pulses finally produced. The development of ultra-intense ultrashort laser technology has put forward new requirements for optical films: wider working bandwidth and more accurate dispersion compensation. Chirped mirrors provide negative group delay dispersion within the reflection bandwidth to achieve accurate compensation for the positive dispersion introduced by the front-stage stretcher. In the hundred-petawatt laser device currently under construction, the spectral width of the pulse is required to reach 200nm or even more than 300nm, and under the incidence condition of high pulse peak power density pulses, the chirped mirror, which is considered to be a linear optical element, will also show nonlinear optical effects, which will seriously affect the pulse quality after dispersion compensation. Therefore, designing and preparing chirped mirrors that can suppress nonlinear optical effects is a research focus of high-power ultrashort pulse lasers.

[0003] High refractive index materials, such as TiO2 and Nb2O5, have a higher nonlinear refractive index n2 than low refractive index material SiO2. When high peak power density pulses interact with thin films, obvious nonlinear optical effects will be generated. Using high refractive index materials with smaller nonlinear refractive index n2, such as HfO2 and Al2O3, is conducive to suppressing the generation of nonlinear optical effects in chirped mirrors. However, since the linear refractive index n0 of HfO2 and Al2O3 itself is relatively low, directly replacing all high refractive index film layers with HfO2 and Al2O3 will seriously limit the working bandwidth of the chirped mirror. Therefore, designing a broadband chirped mirror with low nonlinear optical effect has become an urgent problem to be solved. Traditional broadband chirped mirrors are made of alternating high and low refractive index materials, and the high refractive index materials are mostly TiO2, Nb2O5 and Ta2O5. The nonlinear optical effect caused by the incidence of high peak power density pulses is not considered, which restricts the application of chirped mirrors in ultra-strong ultra-short laser systems and extra-cavity post-compression systems. Therefore, we hope to design a broadband chirped mirror with low nonlinear optical effect to achieve good pulses. Summary of the invention

[0004] The technical problem solved by the present invention is to propose a design method for a broadband low nonlinear effect chirped mirror. By calculating the distribution Δn(r, t) of the refractive index increment in the film when a high peak power density pulse is incident, a broadband chirped film layer and a low nonlinear optical effect chirped film layer are designed, thereby suppressing the nonlinear optical effects in the chirped mirror, such as the third-order nonlinear optical effect, such as the change in the optical thickness of the film layer causing the change in the spectral reflectivity R and the group delay dispersion GDD. The designed broadband low nonlinear chirped mirror has a better linear optical response, which is of great significance to the development of ultra-strong ultra-short laser technology and extra-cavity post-compression technology.

[0005] The technical solution provided by the present invention is as follows:

[0006] A chirped mirror with broadband low nonlinear optical effect, characterized in that it comprises, from bottom to top, a substrate, a broadband chirped film layer and a low nonlinear optical effect chirped film layer;

[0007] The broadband chirped film layer is formed by alternating high refractive index film layer material A and low refractive index film layer material B, wherein the linear refractive index of the high refractive index film layer material A is n 0A , the nonlinear refractive index is n 2A The linear refractive index of the low refractive index film material B is n 0B , the nonlinear refractive index is n 2B ;

[0008] The low nonlinear optical effect chirped film layer is formed by alternating high refractive index film layer material C and low refractive index film layer material B, wherein the linear refractive index of the high refractive index film layer material C is n 0C , the nonlinear refractive index is n 2C , and n 2C <n 2A .

[0009] Furthermore, the high refractive index film layer material A is Nb2O5, Ta2O5, TiO2, ZrO2, Si, fluoride or sulfide, the low refractive index film layer material B is SiO2 or MgF2, and the high refractive index film layer material C is HfO2 or Al2O3.

[0010] Furthermore, the broadband chirped film layer is used to provide a wide spectral bandwidth of the chirped mirror, and the low nonlinear optical effect chirped film layer is used to reduce the size of the refractive index increment Δn, thereby suppressing the third-order nonlinear optical effect and reducing the influence of the refractive index increment Δn on the spectral reflectivity R and group delay dispersion GDD of the chirped mirror.

[0011] A design method for a broadband low nonlinear optical effect chirped mirror is characterized in that it comprises the following steps:

[0012] Step 1: According to the requirements of the chirped mirror design, including dispersion, reflectivity and spectral bandwidth, select the materials to be used: select high refractive index and wide spectral bandwidth materials such as Nb2O5, Ta2O5, TiO2, ZrO2, Si, fluoride or sulfide as high refractive index film material A; select low refractive index and low nonlinear refractive index materials such as SiO2 or MgF2 as low refractive index film material B; select high refractive index and low nonlinear refractive index materials such as HfO2 or Al2O3 as high refractive index film material C;

[0013] Step 2: Obtaining an initial chirped film system: According to the requirements of the chirped mirror design, select the optimization target values, including the working wavelength, the incident angle, the spectral reflectivity and the group delay dispersion, and use the optimization algorithm in the film system design software to perform multiple optimizations to obtain the initial chirped film system composed of the alternating high refractive index film layer material A and the low refractive index film layer material B;

[0014] Step 3: According to the incident pulse intensity I(λ), the refractive index increment Δn generated by the photoelectric field of all wavelengths acting on the initial chirped film system is calculated to obtain a three-dimensional distribution Δn(r,t) of the chirped mirror depth coordinate r, the action time t, and the refractive index increment Δn;

[0015] Step 4: Δn(r,t)>>n in the high refractive index film material A 2B The film layer material of I is replaced with the high refractive index film layer material C, so that the low refractive index film layer material B and the high refractive index film layer material C alternately form a low nonlinear optical effect chirped film layer, wherein I is the incident pulse peak power density; the remaining unreplaced high refractive index film layer material A and the low refractive index film layer material B alternately form a broadband chirped film layer; and the structure is optimized in the film system design software again to obtain a preliminary design result;

[0016] Step 5: According to the incident pulse intensity I(λ), calculate Δn(r, t) again for the preliminary design results obtained in step 4, and observe whether the size of Δn(r, t) is reduced. If not, increase the number of replacements of the high refractive index film layer material C until the size of the refractive index increment Δn(r, t) is reduced as a whole in the film layer, and finally a broadband low nonlinear optical effect chirped mirror is obtained.

[0017] Furthermore, in step 3, the refractive index increment Δn generated by the electric field of all wavelengths acting on the initial chirped film system is calculated according to the incident pulse intensity I(λ), and a three-dimensional distribution Δn(r, t) of the chirped mirror depth coordinate r, the action time t, and the refractive index increment Δn is obtained, and the formula is as follows:

[0018]

[0019] In the formula, I(λi ) is the incident pulse intensity at the i-th wavelength λ, Δn(λ i ,r,t) is the incident pulse intensity I(λ i ) is the distribution of the refractive index increment Δn of the chirped mirror at the depth coordinate r and the action time t under the action of the chirped mirror. n2 is the corresponding nonlinear refractive index coefficient of the high refractive index film material A, the low refractive index film material B and the high refractive index film material C, E(λ i ,r,t) is the incident pulse intensity I(λ i ) is the electric field intensity distribution of the chirped mirror at depth coordinate r and action time t under the action of air, and η0 is the air impedance.

[0020] Compared with the prior art, the technical effect of the present invention is

[0021] 1. Design low nonlinear optical effect film layer by calculating the time distribution of the refractive index increment in the film when high peak power density laser is incident. The low nonlinear optical effect film layer can be adjusted according to different design requirements to obtain a broadband low nonlinear effect dispersion mirror that meets the requirements.

[0022] 2. Ensure that the chirped mirror can provide a wide spectral bandwidth while suppressing nonlinear optical effects, achieving high reflectivity and precise dispersion compensation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The figure is a schematic diagram of the film structure of a broadband low nonlinear optical effect chirped mirror of the present invention.

[0024] Figure 2 Schematic diagram of the distribution of the number of film layers in the initial film system structure of Example 1.

[0025] Figure 3 Schematic diagram of the spectral reflectivity and group delay dispersion of the initial film structure of Example 1.

[0026] Figure 4 FIG. 1 is a schematic diagram of the incident pulse spectrum I(λ) of Example 1. FIG.

[0027] Figure 5 Schematic diagram of the three-dimensional distribution Δn(r, t) of the initial film structure of Example 1.

[0028] Figure 6 Schematic diagram of the initial design three-dimensional distribution Δn(r, t) of the broadband low nonlinear optical effect chirped mirror of Example 1.

[0029] Figure 7 Schematic diagram of film layer distribution of the final design result of the broadband low nonlinear optical effect chirped mirror of Example 1.

[0030] Figure 8Schematic diagram of spectral reflectivity and group delay dispersion of the final design result of the broadband low nonlinear optical effect chirped mirror of Example 1. DETAILED DESCRIPTION

[0031] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the protection scope of the present invention shall not be limited thereto.

[0032] See also Figure 1 , Figure 1 This is a schematic diagram of the initial structure of the present invention, as shown in the figure, from bottom to top, it includes a substrate 1, a broadband chirped film layer 2, and a low nonlinear optical effect film layer 3. The broadband chirped film layer 2 is made of alternating high and low refractive index materials, located on the substrate 1, and its high refractive index film layer material is A, and its low refractive index film layer material is B; the low nonlinear optical effect chirped film layer is made of alternating high and low refractive index materials, located on the broadband chirped film layer, and its high refractive index film layer material is C, and its low refractive index film layer material is B.

[0033] The design index of the broadband low nonlinear effect chirped mirror required in Example 1 is: the group delay dispersion value is -50fs 2 ±100fs 2 @700-1100nm, the spectral reflectivity is greater than 99%, and it has low nonlinear optical effect. This broadband low nonlinear effect chirped mirror is used for paired dispersion compensation of ultra-strong ultra-short laser systems and extracavity compression systems.

[0034] The design steps are as follows:

[0035] 1) According to the requirements of reflectivity, group delay dispersion, spectral bandwidth and low nonlinear optical effect, the actual spectral bandwidth is wide, the reflectivity requirement is high, and the refractive index increment is small. Therefore, the high refractive index film material A of the broadband chirped film layer is selected as Nb2O5, the high refractive index film material C of the low nonlinear optical effect film layer is selected as HfO2, and the low refractive index film material B is SiO2. The values ​​of the nonlinear refractive index n2 of Nb2O5, HfO2 and SiO2 are 2.14e-15, 5.8e-16 and 3.67e-16 respectively, in units of cm 2 / W. The incident pulse intensity I(λ) is Figure 4 Given that the peak power of the incident pulse is 19.58 TW / cm 2 The refractive index parameters of high and low refractive index materials are given by the Cauchy formula Determine, as shown in Table 1.

[0036] <![CDATA[A0]]> <![CDATA[A1]]> <![CDATA[A2]]> <![CDATA[SiO2]]> 1.461870 8.7865400e-6 3.5344300e-4 <![CDATA[Nb2O5]]> 2.133340 0.031252 2.3943400e-3 <![CDATA[HfO2]]> 1.912093 1.1209308e-2 3.3906601e-4

[0037] Table 1

[0038] 2) According to the target requirements of the designed chirped mirror, different optimization methods in the film design software are used to design an initial broadband chirped mirror composed of alternating Nb2O5 and SiO2, as shown below: Figure 2 The initial film structure shown in Figure 3 Shown are the spectral reflectivity and group delay dispersion of the initial structure.

[0039] 3) According to the incident pulse spectrum I(λ), Figure 4 As shown, according to the following formula

[0040] E(λ i ,r,t)=A(r)T(λ i ,r,t)

[0041]

[0042]

[0043]

[0044]

[0045]

[0046] Calculate the electric field for all wavelengths acting on Figure 2 The refractive index increment Δn generated after the initial film structure is obtained, and the three-dimensional distribution Δn(r, t) of the chirped mirror depth coordinate r, action time t, and refractive index increment Δn is obtained. The results are shown in Figure 5 In the above formula, E(λ i ,r,t) is the incident pulse intensity I(λ i ) under the action of the chirped mirror at depth coordinate r and action time t, E0 is the peak electric field intensity, A(r) is the amplitude modulation term, T(i i ,r,t) is the time electric field evolution term at different wavelengths, ω0 is the spot waist radius, x0 is the Rayleigh range, ω is the angular frequency, k is the wave number, t0 is the delay time of pulse incidence, Δt is the pulse width of the incident femtosecond pulse, n2 is the nonlinear refractive index of the material, and η0 is the air impedance.

[0047] 4) According to Figure 5 As a result, Δn(r,t) is much larger than n 2B The high refractive index Nb2O5 film layer with I≈0.01 is replaced by HfO2, thereby forming a broadband chirped film layer and a low nonlinear optical effect chirped film layer, respectively, forming the initial design of a broadband low nonlinear optical effect chirped mirror.

[0048] 5) Combination Figure 4The incident pulse spectrum I(λ) is used to calculate the initial design of the broadband low nonlinear optical effect chirped mirror obtained in 4), and the three-dimensional distribution Δn(r, t) of the chirped mirror depth coordinate r, action time t, and refractive index increment Δn is calculated. The results are as follows Figure 6 shown.

[0049] 6) Observation Figure 6 The calculated results show that the peak value of the change is attenuated from 0.06 to 0.015, and the overall Δn(r, t) is reduced. The design is considered to be completed. The final chirped mirror is as follows Figure 7 Its spectral reflectivity and group delay dispersion are shown in Figure 8 shown.

[0050] 7) If there are still some layers whose refractive index increment Δn(r,t) is much greater than n 2B If the design is not satisfactory, the number of HfO2-substituted Nb2O5 film layers is increased, and the optimization is performed again in the film system design software to obtain the initial design of the broadband low nonlinear chirped mirror and calculate Δn(r, t) until the design result is met.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A chirped mirror with broadband low nonlinear optical effect, characterized in that: From bottom to top, they are a substrate (1), a broadband chirped film layer (2), and a low nonlinear optical effect chirped film layer (3); The broadband chirped film layer (2) is formed by alternating high refractive index film layer material A and low refractive index film layer material B, wherein the linear refractive index of the high refractive index film layer material A is n 0A , the nonlinear refractive index is n 2A The linear refractive index of the low refractive index film material B is n 0B , the nonlinear refractive index is n 2B ; The low nonlinear optical effect chirped film layer (3) is formed by alternating high refractive index film layer material C and low refractive index film layer material B, wherein the linear refractive index of the high refractive index film layer material C is n 0C , the nonlinear refractive index is n 2C , and n 2C <n 2A ; The broadband chirped film layer (2) is used to provide a wide spectral bandwidth of the chirped mirror, and the low nonlinear optical effect chirped film layer (3) is used to reduce the size of the refractive index increment Δn, thereby suppressing the third-order nonlinear optical effect and reducing the influence of the refractive index increment Δn on the spectral reflectivity R and group delay dispersion GDD of the chirped mirror.

2. The broadband low nonlinear optical effect chirped mirror according to claim 1, characterized in that: The high refractive index film layer material A is Nb2O5, Ta2O5, TiO2, ZrO2, Si, fluoride or sulfide, the low refractive index film layer material B is SiO2 or MgF2, and the high refractive index film layer material C is HfO2 or Al2O3.

3. A design method for a broadband low nonlinear optical effect chirped mirror, characterized in that: The steps include: Step 1: According to the requirements of the chirped mirror design, including dispersion, reflectivity and spectral bandwidth, select the materials to be used: select high refractive index and wide spectral bandwidth materials such as Nb2O5, Ta2O5, TiO2, ZrO2, Si, fluoride or sulfide as high refractive index film material A; select low refractive index and low nonlinear refractive index materials such as SiO2 or MgF2 as low refractive index film material B; select high refractive index and low nonlinear refractive index materials such as HfO2 or Al2O3 as high refractive index film material C; Step 2: Obtaining an initial chirped film system: According to the requirements of the chirped mirror design, select the optimization target values, including the working wavelength, the incident angle, the spectral reflectivity and the group delay dispersion, and use the optimization algorithm in the film system design software to perform multiple optimizations to obtain the initial chirped film system composed of the alternating high refractive index film layer material A and the low refractive index film layer material B; Step 3: According to the incident pulse intensity I(λ), the refractive index increment Δn generated by the photoelectric field of all wavelengths acting on the initial chirped film system is calculated to obtain a three-dimensional distribution Δn(r,t) of the chirped mirror depth coordinate r, the action time t, and the refractive index increment Δn; Step 4: Δn(r,t)>>n in the high refractive index film material A 2B The film layer material of I is replaced with the high refractive index film layer material C, so that the low refractive index film layer material B and the high refractive index film layer material C alternately form a low nonlinear optical effect chirped film layer (3), wherein I is the incident pulse peak power density; the remaining unreplaced high refractive index film layer material A and the low refractive index film layer material B alternately form a broadband chirped film layer (2); and the structure is optimized in the film system design software again to obtain a preliminary design result; Step 5: According to the incident pulse intensity I(λ), calculate Δn(r, t) again for the preliminary design results obtained in step 4, and observe whether the size of Δn(r, t) is reduced. If not, increase the number of replacements of the high refractive index film layer material C until the size of the refractive index increment Δn(r, t) is reduced as a whole in the film layer, and finally a broadband low nonlinear optical effect chirped mirror is obtained.

4. The design method of a broadband low nonlinear optical effect chirped mirror according to claim 3, characterized in that: The step 3 calculates the refractive index increment Δn generated by the electric field of all wavelengths acting on the initial chirped film system according to the incident pulse intensity I(λ), and obtains the three-dimensional distribution Δn(r, t) of the chirped mirror depth coordinate r, the action time t, and the refractive index increment Δn, and the formula is as follows: In the formula, I(λ i ) is the incident pulse intensity at the i-th wavelength λ, Δn(λ i ,r,t) is the incident pulse intensity I(λ i ) is the distribution of the refractive index increment Δn of the chirped mirror at the depth coordinate r and the action time t under the action of the chirped mirror, n2 is the corresponding nonlinear refractive index coefficient of the high refractive index film material A, the low refractive index film material B and the high refractive index film material C, E(λ i ,r,t) is the incident pulse intensity I(λ i ) is the electric field intensity distribution of the chirped mirror at depth coordinate r and action time t under the action of air, and η0 is the air impedance.

Images