Design Method of Ultraviolet Broadband Highly Reflective Dispersive Mirror

Abstract

The structure of the ultraviolet broadband high-reflection dispersion mirror consists of a substrate layer, an ultraviolet high-reflection metal layer, a Gires-Tournois-like cavity, a chirped dispersion layer, and a surface anti-reflection layer from bottom to top. The basic expression of the dispersion mirror structure is: #imgabs0#S represents the substrate, C represents the ultraviolet high-reflection metal layer, G represents the Gires-Tournois-like cavity, #imgabs1# represents the chirped dispersion layer (a j k ), #imgabs2# represents the surface anti-reflection layer, A represents air, and H and L represent a high-refractive-index material film layer and a low-refractive-index material film layer with an optical thickness of #imgabs3#, respectively. While utilizing an ultraviolet highly reflective metal layer as a reflective layer, the present invention increases reflectivity and simultaneously performs dispersion compensation by constructing a Gires-Tournois-like cavity and a chirped dispersion layer on the metal film. The thickness ratio of the H layer and the L layer is reduced in the initial structure, making the low-refractive-index layer a dominant factor in the film design. By macro-controlling the distribution ratio of the film material, the absorption loss of the laser during oscillation in the Gires-Tournois-like cavity is reduced, ultimately designing an ultraviolet broadband highly reflective dispersion mirror based on low-absorption materials.​

Description

Technical Field

[0001] The invention belongs to the field of ultrafast laser films, and in particular relates to an ultraviolet broadband high-reflection dispersion mirror in an ultraviolet ultrafast laser. The invention is an optical element used for dispersion control of an ultraviolet ultrafast laser system. Background Art

[0002] In recent decades, femtosecond lasers have become an important tool for studying microscopic phenomena at the atomic and molecular scale, demonstrating tremendous potential for applications in time-resolved spectroscopy, ultra-wideband optical communications, and micro- and nanofabrication. A fundamental limitation in generating ultrashort pulses in the femtosecond range is pulse broadening caused by material dispersion and other nonlinear effects in the system. This leads to reduced pulse peak energy and degraded beam quality. Dispersion control and compensation of the entire system, external to the laser, is crucial for stable, high-quality ultrafast laser pulse output. Commonly used dispersion compensation components include prisms, gratings, and dispersive mirrors. Prisms and gratings are prone to introducing high-order dispersion, and these components require high angular accuracy when correcting for dispersion, making them difficult to tune. In contrast, dispersive mirrors provide highly tunable dispersion over a wide range with extremely low loss within ultra-wide or ultra-narrow bandwidths. Systems using dispersive mirrors as dispersion control components can be more compact, facilitating integration. In recent years, film design software for dispersive mirrors has been continuously optimized, and fabrication processes have matured. Currently, dispersive mirrors can provide dispersion control over a wide range of up to 15,000 fs. 2 The dispersion can reach one frequency doubling in bandwidth, thus compensating the dispersion of the system.

[0003] As the laser pulse width is further compressed, the central wavelength of the laser shifts toward the ultraviolet band. Currently reported types of ultraviolet lasers primarily include solid-state ultraviolet lasers and gas ultraviolet lasers, with excimer lasers being the predominant type of gas ultraviolet lasers. Ultraviolet lasers offer distinct advantages in laser processing: their short wavelength enables processing of smaller components; they directly destroy the chemical bonds of the material during processing, resulting in a minimal heat-affected zone for "cold" material processing; and most materials effectively absorb ultraviolet light, enabling the processing of materials that infrared and visible light lasers cannot. Ultraviolet lasers also boast compact structure, high average power, ease of maintenance, simple operation, low cost, and high productivity, finding widespread application in laser processing fields such as bioengineering, material preparation, all-optical optical device fabrication, integrated circuit boards, and the semiconductor industry.

[0004] The stable operation of ultraviolet lasers also requires dispersion compensation. The purpose of this invention is to design a low-dispersion, high-reflection, broadband dispersive mirror that matches current ultraviolet lasers, which is of great significance for improving the beam quality output by ultraviolet lasers.

[0005] Compared with the infrared broadband dispersion mirror which also uses metal and dielectric as film materials, the present invention uses the following in the initial design: The general Gires-Tournois cavity and The chirped dispersion layer is a general-purpose layer that replaces the conventional design with an ordinary HL layer as the basic film structure. In the initial design, the proportion of the low-refractive index layer thickness in the entire film structure is artificially increased significantly, which reduces the absorption of the film layer macroscopically. In the optimization process, a low-absorption film layer structure with a high LH layer thickness ratio is selected to replace the conventional random optimization design that can provide the same dispersion compensation and the same reflectivity, so that the electric field peak exists in the low-refractive index low-absorption film layer, further reducing the absorption of the film system and improving the thin film threshold. In the initial design and optimization design, the low-absorption and low-refractive index material film layer always plays a dominant role in the proportion of the film layer thickness (a p <1)(b q >1)(a j k ), thus reducing the overall absorption coefficient of the film structure. This multi-layer dielectric film design, primarily based on low-refractive-index, low-absorption materials, when combined with a UV-high-reflective metal film, further enhances the excellent reflective properties of the UV-high-reflective metal, overcoming the shortcomings of conventional films, such as severe UV absorption and low reflectivity. Summary of the Invention

[0006] The present invention increases the thickness of the low refractive index and low absorption layer and reduces the thickness of the high refractive index and high absorption layer during the initial structural design. The general Gires-Tournois cavity and The chirped dispersion layer is a general-purpose layer that replaces the conventional design with the ordinary HL layer as the basic film structure. The structure is similar to the Gires-Tournois cavity structure and the chirped dispersion film layer. During the optimization process, the low absorption and low refractive index material layer always plays a dominant role in the film thickness ratio (a j k By adjusting the period number and cavity thickness, increasing the ratio of the L-layer to the H-layer thickness, and designing the electric field peak within the L-layer and the low-absorption material layer, the film structure's absorption of UV light is reduced. By rationally adjusting the parameters of the initial structure, UV broadband high-reflectivity dispersive mirrors with varying dispersion, reflectivity, and bandwidth can be designed. Compared to traditional extracavity dispersion compensation methods, the UV ultrafast laser system using this invention can achieve a more compact structure.

[0007] The technical solution provided by the present invention is as follows:

[0008] In one aspect, the present invention provides an ultraviolet broadband high-reflection dispersion mirror structure, which is characterized by comprising, from bottom to top, a substrate layer, an ultraviolet high-reflection metal layer, a Gires-Tournois-like cavity, a chirped dispersion layer, and a surface anti-reflection layer. The basic expression of the dispersion mirror structure is: ​​

[0009]

[0010] S represents the substrate, C represents the ultraviolet high-reflective metal material film layer, and G represents the Gires-Tournois cavity. The general structure is: in and is the cavity wall, n x The value is between 1 and 20, and n y The value range is 1-10. p and b q is the cavity system number, a p The value range is between 0 and 1, b q The value range is between 1 and 3, m z is the cavity thickness coefficient, with a value between 1 and 5; is the general structure of the chirped dispersion layer, in particular, it emphasizes the requirement of a j k , both values range from 0 to 3, m g The value range is between 1-8; It is the surface anti-reflection layer, and the value range of n4 is between 1-5. represents the surface antireflection layer, A represents air, H and L represent the optical thickness respectively. high refractive index material film layer and low refractive index material film layer.

[0011] By adjusting n x and n y The value of n can adjust the thickness of the reflective layer at both ends of the cavity as the cavity wall. x The value is between 1 and 20, and n y The value ranges from 1 to 10. p and b q is the cavity system number, a p The value range is between 0 and 1, b q The value range is between 1 and 3. z The value ranges from 1 to 5, controlling the cavity length. These parameters jointly determine the performance of the Gires-Tournois-like cavity structure, controlling the cavity structure's reflection and dispersion compensation capabilities from multiple angles, and also giving the Gires-Tournois-like cavity structure better directivity and good plasticity during optimization and adjustment.

[0012] is the general formula of the chirped dispersion compensation layer, (a j k ),a j and a k ​​The values for both are between 0 and 3. The thickness coefficient of the L layer is set to be greater than that of the H layer. By increasing the thickness of the low-absorption material, the thickness of the high-absorption material is reduced. By rationally selecting the thickness ratio of the H and L layers, the peak of the electric field is introduced into the low-refractive index layer through design, so that almost all of the electric field peak exists within the low-absorption, low-refractive index film layer, significantly reducing the total absorption of the film layer and raising the threshold. It is the surface anti-reflection layer, and the value range of n4 is between 1-5.

[0013] The substrate material is quartz glass, any one of JGS1, BK7 and CaF2.

[0014] The UV high reflectivity metal reflective layer can be made of Al or Cu, and the appropriate thickness is selected according to the reflectivity requirements. Within a certain range, the larger the value, the higher the reflectivity of the film.

[0015] The general structure of the Gires-Tournois cavity is: By adjusting n x and n y The value of n can adjust the thickness of the reflective layer at both ends of the cavity as the cavity wall, x The value is between 1-20, and n y The value ranges from 1 to 10. p and b q is the cavity system number, a o The value range is between 0 and 1, b q The value range is between 1 and 3. z The value ranges from 1 to 5, controlling the cavity length. By appropriately reducing the ratio of the H and L layer thicknesses within the cavity, while ensuring that the design dispersion and reflectivity requirements are met, the thickness of the high-refractive-index, highly absorptive film layer within the cavity is reduced. This reduces absorption and loss during laser oscillation within the cavity, further improving the reflective performance of the Gires-Tournois-like cavity structure. These parameters collectively determine the performance of the Gires-Tournois-like cavity structure, controlling the cavity structure's reflection and dispersion compensation capabilities from multiple angles. This also gives the Gires-Tournois-like cavity structure improved directivity and flexibility during optimization and adjustment.

[0016] The general structure of the dispersion chirped layer is: Chirp coefficient a j and a k The value is between 0-3, and (a j k ​), making the coefficient of the L layer greater than that of the H layer, by increasing the thickness of the low-absorption material and reducing the thickness of the high-absorption material. By rationally selecting the thickness ratio of the H layer and the L layer, the peak of the electric field is introduced into the low-refractive index layer through design, so that almost all the electric field peak exists in the low-absorption low-refractive index material film layer, greatly reducing the total absorption of the film layer and improving the threshold. g The value range is between 1-8, which is the number of cycles.

[0017] On the other hand, the present invention also provides a method for designing an ultraviolet broadband high-reflection dispersion mirror structure, comprising the following steps:

[0018] 1. According to the design requirements of the dispersion mirror, including the dispersion, reflectivity, bandwidth of the laser working band and the transmittance of the pump band, select the appropriate high and low refractive index materials. The high refractive index materials include oxide materials such as ZrO2, Al2O3, HfO2, etc. The low refractive index materials generally include SiO2, MgF2, YF3, AlF3, LaF3. H 、n L It is obtained by inversion from the actual coating experiment.

[0019] 2. Ultraviolet broadband high-reflection dispersion mirror structure, characterized by comprising, from bottom to top, a substrate layer, an ultraviolet high-reflection metal layer, a Gires-Tournois-like cavity, a chirped dispersion layer, and a surface anti-reflection layer. The basic expression of the dispersion mirror structure is:

[0020]

[0021] S represents the substrate, C represents the ultraviolet high reflective metal material film layer, and G represents the Gires-Tournois cavity structure. The general structure is: represents the chirped dispersion layer (a j k ), represents the surface antireflection layer, A represents air, H and L represent the optical thickness respectively. high refractive index material film layer and low refractive index material film layer.

[0022] Chirped dispersion layer general formula, (a j k ), both of which have a value range of 1-3; It is the surface anti-reflection layer, and the value range of n4 is between 1-5.

[0023] ​​3. After preliminarily selecting the initial structural parameters of the dispersion mirror, the corresponding optimization targets are set according to the requirements of the designed dispersion mirror, including the group delay dispersion value, reflectivity, transmittance and the wavelength range of the dispersion mirror working area. TFCalc and Essential MacLeod film optimization software are used to optimize the film system through simplex parameters, optimac parameters and needle algorithm multiple times to obtain the final optimization result based on this initial structural parameters.

[0024] 4. Observe whether the final result meets the required index requirements of the ultraviolet broadband high reflectivity dispersion mirror. If the group delay dispersion requirements of the required dispersion mirror are not met, increase the thickness and number of low refractive index layers, the period number m of the chirped layer, and the chirped layer period. g , and adjust the Gires-Tournois cavity structure coefficient a p , b q , m z , adjust the cavity shape and cavity thickness, repeat step 3 until the dispersion mirror requirements are finally met; if the reflectivity requirements of the required dispersion mirror working band are not met, change the thickness of the metal reflective layer and the thickness parameter n of the Gires-Tournois cavity wall. x To improve the reflectivity, appropriately reduce the ratio of the thickness of the L layer to the thickness of the H layer in the cavity structure of the chirped film system to improve the reflectivity of the chirped layer, and repeat step 3 until the dispersion mirror requirements are finally met; if the threshold requirements of the dispersion mirror are not met, adjust the thickness of the low refractive index layer and the position of the cavity structure so that the electric field peak exists as much as possible in the low refractive index layer and the low absorption layer, and repeat step 3 until the dispersion mirror requirements are finally met.

[0025] 5. Finally, the ultraviolet broadband high-reflection dispersion mirror structure is obtained.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. Use The structural design is similar to the Gires-Tournois cavity structure, which increases the thickness ratio of the low refractive index and low absorption film layer, reduces the thickness ratio of the high refractive index and high absorption layer, reduces the absorption of the laser when it oscillates in the cavity, and makes the laser loss smaller. j Ha k L, (a j k )’s basic structure replaces the ordinary quarter-wavelength HL layer to construct the chirped dispersion compensation structure, which reduces the absorption of the film layer macroscopically. In the design, the proportion of the thickness of the low-refractive index material film layer is artificially increased, while meeting the requirements of dispersion and bandwidth, and introducing the electric field peak into the film layer where the low-refractive index and low-absorption material is located.

[0028] ​2. A multilayer dielectric structure mainly composed of low-absorption and low-refractive index film materials will be combined with a characteristic metal with strong reflectivity in the ultraviolet band. Because the outer dielectric multilayer structure mainly composed of low-absorption and low-refractive index materials has low absorption of ultraviolet light, the high reflective characteristics of the metal layer in the ultraviolet band can be more efficiently utilized. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic structural diagram of the ultraviolet broadband high-reflection dispersion mirror of the present invention.

[0030] In the figure: 1-base layer, 2-ultraviolet high-reflection metal layer, 3-periodic Gires-Tournois cavity structure, 4-chirped dispersion layer, 5-surface anti-reflection layer.

[0031] Figure 2 This is a film structure diagram of Example 1 of the ultraviolet broadband high-reflection dispersion mirror structure of the present invention.

[0032] Figure 3 This is the final film structure of Example 1.

[0033] Figure 4 This is the reflection spectrum curve of Example 1.

[0034] Figure 5 This is a group delay dispersion curve diagram of Example 1. DETAILED DESCRIPTION

[0035] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0036] Figure 1 This is a schematic diagram of the structure of the ultraviolet broadband high-reflection dispersion mirror of the present invention. As shown in the figure, from bottom to top, it includes 1-base layer, 2-ultraviolet high-reflection metal layer, 3-periodic Gires-Tournois cavity structure, 4-chirped dispersion layer, and 5-surface anti-reflection layer.

[0037] The dispersion mirror index required by Example 1 is: Group delay dispersion value -20s 2 @290-360nm, reflectivity 80±2%, reflectivity required to be >90% at the center wavelength of 325nm.

[0038] The design steps are as follows:

[0039] 1. According to the requirements of group delay dispersion and bandwidth, the dispersion is relatively large and the bandwidth is wide, so the high refractive index material Al2O3 with a higher refractive index is selected, and the low refractive index material is SiO2. The refractive index parameters of the high and low refractive index materials are determined by the Cauchy formula Determine, as shown in Table 1.

[0040] <![CDATA[A0]]> <![CDATA[A1]]> <![CDATA[A2 <!-- 4 -->]]> <![CDATA[SiO2]]> 1.45198 1.1899598e-2 -3.628906e-4 <![CDATA[Al2O3]]> 1.62639 7.841e-3 -1.50425e-5

[0041] Table 12. Based on the requirements of the dispersion mirror and the initial structure

[0042]

[0043] (G represents a Gires-Tournois cavity, and its general structure is

[0044] Select n0=6, a0=0.5, a1=0.6, a2=0.7, b0=2, b1=2.1, b2=2.2, n1=1, n2=2, n3=2, a3=0.7, a4=1, a5=0.8, a6=1.1, a7=0.9, a8=1.2, m1=1,m 2-3 =2,m5=1,n4=2,m 4-6 =3

[0045] S / C(HL) 6 (0.5HL)(H2L) 1 (HL) 1 (0.6HL)(H2.1L) 2 (HL) 2 (0.7HL)(H2.2L) 2 (HL) 2

[0046] (0.7HL) 3 (0.8H1.1L) 3 (0.9H1.2L) 3 (HL) 2 / A

[0047] Where S represents the substrate material and A represents the incident medium air. Figure 2 shown.

[0048] 3. Based on

[0049] S / C(HL) 6 (0.5HL)(H2L) 1 (HL) 1 (0.6HL)(H2.1L) 2 (HL) 2 (0.7HL)(H2.2L) 2 (HL) 2

[0050] (0.7HL) 3 (0.8H1.1L) 3 (0.9H1.2L) 3 (HL) 2 / A

[0051] The initial design uses a reference wavelength of 325 nm, a design band of 290-360 nm, an incident angle of 10 degrees, P polarization, and an optimized target group delay dispersion (GDD) of -20 fs. 2 , set the reflectivity optimization target value to 100%, and optimize the film thickness through the variable scale algorithm (simplex parameters and optimacparameters) to obtain the final film structure as shown below Figure 3 As shown, the reflectivity curve is Figure 4 As shown, the group delay dispersion curve is as follows Figure 5 shown.

[0052] 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 principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. Ultraviolet broadband high reflective dispersion mirror structure, characterized in that: From bottom to top, they are substrate, ultraviolet high-reflection metal layer, Gires-Tournois-like cavity, chirped dispersion layer, and surface anti-reflection layer. The expression of the dispersion mirror structure is: In the formula, S represents the substrate, C represents the ultraviolet high reflective metal layer, and G represents the Gires-Tournois cavity structure. represents the chirped dispersion layer, where a j k , a j and a k is the dispersion control coefficient, m g , v is the number of periods of the chirp dispersion layer controlled by the chirp coefficient, represents the surface antireflection layer, n4 controls the thickness of the antireflection layer, A represents air, H and L represent the optical thickness respectively. A high-refractive-index material film layer and a low-refractive-index material film layer, and the thickness coefficient of the low-refractive-index material film layer L is greater than the thickness coefficient of the high-refractive-index material film layer H;​ The general structural formula of the Gires-Tournois cavity is: in, and is the cavity wall, n x is the wall thickness coefficient, ranging from 1 to 20, n y is the wall thickness coefficient, which ranges from 1 to 10. x and n y The size of the cavity wall changes accordingly; (a p HL) and (Hb q L) is the cavity, a p and b q is the cavity system number, a p The value range is between 0 and 1, b q The value range is between 1 and 3, m z is the cavity thickness coefficient, with a value between 1 and 5; The general structural formula of the chirped dispersion layer is: a j k The value range is between 0 and 3, m g The value range is between 1-8. By reducing the thickness ratio of the H layer and the L layer, the peak of the electric field is introduced into the low refractive index layer, so that almost all the electric field peaks exist in the low-absorption and low-refractive index material film layer, thereby reducing the total absorption of the film layer and increasing the threshold.​ 2. The ultraviolet broadband high-reflection dispersion mirror structure according to claim 1, characterized in that: The low refractive index material is SiO2, MgF2, YF 3, AlF3 or LaF3.

3. The ultraviolet broadband high-reflection dispersion mirror structure according to claim 1, characterized in that: The high refractive index material is ZrO2, Al2O3 or HfO2.

4. The ultraviolet broadband high-reflection dispersion mirror structure according to claim 1, wherein: The value range of n4 is between 1 and 5.

5. The ultraviolet broadband high-reflection dispersion mirror structure according to claim 1, characterized in that: The substrate is SiO2, JGS1, BK7 or CaF2.

6. The ultraviolet broadband high-reflection dispersion mirror structure according to claim 1, characterized in that: The ultraviolet high reflective metal layer is Al, Cu or Rh, and the appropriate thickness is selected according to the reflectivity requirements.

7. A design method for an ultraviolet broadband high-reflection dispersion mirror structure, characterized in that: The steps are as follows: 1) Select high and low refractive index materials based on the design requirements of the dispersion mirror, including the dispersion, reflectivity, bandwidth of the laser operating band and the transmittance of the pump band; 2) Design a UV broadband high-reflection dispersion mirror structure, which consists of a substrate layer, a UV high-reflection metal layer, a Gires-Tournois-like cavity, a chirped dispersion layer, and a surface anti-reflection layer. The basic expression of the dispersion mirror structure is: S represents the substrate, C represents the ultraviolet high reflective metal material film layer, and G represents the Gires-Tournois cavity structure. The general structure is: represents the chirped dispersion layer (a j k ), represents the surface antireflection layer, A represents air, H and L represent the optical thickness respectively. a high refractive index material film layer and a low refractive index material film layer; Chirped dispersion layer general formula, (a j k ), both of which have a value range of 1-3; It is the surface antireflection layer, and the value range of n4 is between 1-5;​​ 3) Based on the requirements of the designed dispersive mirror, corresponding optimization targets are set, including group delay dispersion value, reflectivity, transmittance, and the wavelength range of the dispersive mirror operating area. TFCalc and EssentialMacLeod film optimization software are used to optimize the film system through simplexparameters, optimacparameters, and needle algorithm multiple times to obtain the final optimized result based on this initial structural parameter. 4) Observe whether the required index requirements of the ultraviolet broadband high reflectivity dispersion mirror are met. If the group delay dispersion requirements of the required dispersion mirror are not met, increase the thickness and number of low refractive index layers, the period number m of the chirped layer, and the chirped layer period. g , and adjust the Gires-Tournois cavity structure coefficient a p , b q , m z , adjust the cavity shape and cavity thickness, repeat step 3) until the dispersion mirror requirements are finally met; if the reflectivity requirements of the required dispersion mirror working band are not met, change the thickness of the metal reflective layer and the thickness parameter n of the Gires-Tournois cavity wall to obtain the desired dispersion mirror. x To improve the reflectivity, appropriately reduce the ratio of the thickness of the L layer to the thickness of the H layer in the cavity structure of the chirped film system to increase the reflectivity of the chirped layer, and repeat step 3) until the dispersion mirror requirements are finally met; if the dispersion mirror threshold requirements are not met, adjust the thickness of the low refractive index layer and the position of the cavity structure so that the electric field peak is as much as possible in the low refractive index layer and the low absorption layer, and repeat step 3) until the dispersion mirror requirements are finally met; 5) Finally, an ultraviolet broadband high-reflection dispersion mirror structure is obtained.

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