Ultra-wideband low-oscillation low-dispersion mirror based on back coating and its implementation method

Abstract

An ultra-wideband low-oscillation low-dispersion mirror based on back coating and its implementation method, the film system structure is air (A) / anti-reflection film (AR) / substrate (S) / dispersion compensation film layer / air (A) in sequence, wherein the anti-reflection film is composed of alternating high-refractive index and low-refractive index materials, and is plated on the front surface of the substrate; the dispersion compensation film layer is also composed of alternating high-refractive index and low-refractive index materials, and its low-refractive index material matches the substrate material; by matching the positive dispersion of the substrate with the negative dispersion of the dispersion compensation film layer, the total dispersion of the entire element approaches zero. Suitable materials are selected according to the requirements of specific bands, and white light interferometer is used for performance evaluation. During the test, light is incident from the anti-reflection film, penetrates the substrate, and is emitted after reflection from the dispersion compensation layer to comprehensively evaluate the dispersion performance of the mirror. The present invention ensures that ultrashort pulses can be transmitted while maintaining high reflectivity, while avoiding pulse broadening, thereby achieving efficient and stable transmission of ultrashort pulses.

Description

Technical Field

[0001] The invention belongs to the field of ultrafast laser thin films, and in particular is an ultra-wideband low-oscillation low-dispersion mirror based on back coating and a realization method thereof. Background Art

[0002] With the advancement of ultrafast laser systems, the bandwidth and energy of the next generation of lasers based on optical parametric chirped pulse amplification technology have been significantly improved. Low-dispersion mirrors are optical components used to transmit laser energy in ultrafast laser systems. Their reflection efficiency, bandwidth and damage threshold are of great significance to the output of ultrafast laser pulses by the entire system.

[0003] Low-dispersion mirrors under traditional optimization design strategies, such as Bragg reflectors, are limited by the ratio between high-refractive-index and low-refractive-index materials. Their reflection bandwidth and dispersion bandwidth are both facing bottlenecks, making it difficult to adapt to the transmission requirements of ultrashort pulses with shorter pulse widths. Although chirped-structure broadband low-dispersion mirrors (such as CN112666641A) have expanded the reflection and dispersion bandwidths to a certain extent, when the spectral width exceeds 300nm, its group delay dispersion curve will inevitably produce periodic oscillations, leading to pulse broadening. Although metal reflectors (such as silver mirrors, aluminum mirrors, and gold mirrors) can achieve zero-dispersion transmission over a wide spectral range, they cause significant energy loss during ultrashort pulse transmission due to their low reflectivity, and they do not have an advantage in damage threshold compared to dielectric mirrors.

[0004] Therefore, a new method that can efficiently transmit ultrashort pulses over a wide spectral range is urgently needed. Summary of the invention

[0005] In view of the above-mentioned deficiencies of the prior art, the present invention proposes an ultra-wideband low-oscillation low-dispersion mirror based on back coating, whose film system structure is A / AR / S / R / A, wherein A represents air, AR represents anti-reflection film, S represents substrate, and R represents dispersion compensation film layer. The present invention reduces the design pressure of the dispersion compensation film layer by rationally utilizing the positive dispersion introduced by the substrate and the anti-reflection film (anti-reflection film), so that it can provide stable dispersion compensation in a wider bandwidth.

[0006] The technical problem solved by the present invention is that a method for realizing an ultra-wideband low-oscillation low-dispersion mirror based on back coating is proposed. The ultra-wideband low-oscillation low-dispersion mirror based on back coating is composed of an anti-reflection film, a substrate and a dispersion compensation film layer. The anti-reflection film on the front surface of the substrate is used to offset part of the reflection of the substrate, and together with the substrate, improves the impedance matching between the dispersion compensation film layer on the rear surface of the substrate and the incident medium. The dispersion compensation film layer on the rear surface of the substrate provides negative dispersion to compensate for the positive dispersion of the anti-reflection film and the substrate, and at the same time provides high reflectivity, ultimately achieving zero dispersion introduction, low dispersion oscillation, and low energy loss of the entire element within the working band. High-efficiency transmission.

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

[0008] On the one hand, the present invention provides an ultra-wideband low-oscillation low-dispersion mirror based on back coating, which is characterized in that the film system structure is A / AR / S / R / A, wherein A represents air, AR represents an anti-reflection film layer, S represents a substrate, and R represents a dispersion compensation film layer;

[0009] The anti-reflection film is composed of alternating high-refractive index materials and low-refractive index materials, and is plated on the front surface S1 of the substrate to eliminate partial reflection of the front surface of the substrate;

[0010] The substrate has a refractive index between the equivalent refractive index of the anti-reflection film and the equivalent refractive index of the dispersion compensation film layer, and the material selection is consistent with the low refractive index material in the dispersion compensation film layer to achieve impedance matching and reduce group delay dispersion oscillation;

[0011] The dispersion compensation film layer is composed of a high refractive index material and a low refractive index material that is consistent with the substrate material, and is plated on the rear surface S2 of the substrate to provide negative dispersion to offset the positive dispersion of the anti-reflection film layer and the substrate, and provide high reflectivity;

[0012] Among them, by effectively utilizing the positive dispersion of the substrate and the negative dispersion of the dispersion compensation film layer, the total dispersion introduced by the entire element within the working band is close to zero, thereby achieving efficient transmission with low dispersion oscillation and low energy loss.

[0013] The reflection dispersion satisfies the following conditions:

[0014]

[0015] In the formula is the double-pass transmission dispersion of the substrate, is the group velocity dispersion of the substrate, is the wavelength, c is the speed of light, n is the refractive index of the film material, d is the physical thickness of the substrate material, is the refractive index of air, is the incident angle, is the refractive index of the substrate material, is the transmission dispersion of the anti-reflection film, It is the reflection dispersion of the dispersion compensation film layer.

[0016] The physical thickness of the substrate is 0.01-20 mm; the material type of the substrate is SiO2, CaF2, MgF2, and sapphire.

[0017] The high refractive index material of the anti-reflection film is Nb2O5, Ta2O5, HfO2, Al2O3, TiO2, ZrO2, Si, and the low refractive index material is SiO2, MgF2, CaF2.

[0018] The high refractive index film layer material of the dispersion compensation film layer is Nb2O5, Ta2O5, HfO2, Al2O3, TiO2, ZrO2, Si; the low refractive index film layer material is the same as the material selected for the substrate.

[0019] The preparation method is ion beam sputtering, magnetron sputtering and electron beam evaporation.

[0020] On the other hand, the present invention also provides a method for realizing an ultra-wideband low-oscillation low-dispersion mirror based on back coating, which is characterized in that it includes the following steps:

[0021] Step 1: According to the actual application band of the ultra-wideband low-oscillation low-dispersion mirror based on back coating, select high-refractive index material and low-refractive index material. The low-refractive index material of the dispersion compensation film layer is consistent with the material of the substrate.

[0022] Step 2: According to the actual application band of the ultra-wideband low-oscillation low-dispersion mirror based on back coating, select the substrate material and the physical thickness of the substrate, and calculate the double-pass transmission dispersion of the substrate.

[0023] Step 3: According to the actual application band of the ultra-wideband low-oscillation low-dispersion mirror based on back coating, the target values ​​of required transmittance and transmission dispersion are given, and the anti-reflection film structure that meets the transmittance and transmission dispersion requirements is designed through the optimization algorithm.

[0024] Step 4: Multiply the transmission dispersion of the anti-reflection film structure obtained in step 3 by 2 to obtain the double-pass transmission dispersion, add it to the double-pass transmission dispersion obtained in step 2 and then take the inverse to obtain the reflection dispersion target value of the dispersion compensation film layer.

[0025] Step 5: According to the actual application band of the ultra-wideband low-oscillation low-dispersion mirror based on back coating, the reflection dispersion target value of the dispersion compensation film layer obtained in step 4 is used as the design target, and the required reflectivity target value is given. The dispersion compensation film layer structure that meets the reflectivity and reflection dispersion requirements is designed through the optimization algorithm.

[0026] Step 6: The anti-reflection film and the dispersion compensation film layer are respectively coated on the S1 surface and the S2 surface of the substrate, and finally an ultra-wideband low-oscillation low-dispersion mirror structure based on back coating is obtained.

[0027] Step 7: Use white light interferometer to characterize the dispersion performance of the prepared ultra-wideband low oscillation low dispersion mirror based on back coating. The light first passes through the anti-reflection film, then passes through the substrate, and finally is reflected back by the back chirped film layer and received by the white light interferometer, thereby measuring the dispersion introduced by the entire ultra-wideband low oscillation low dispersion mirror based on back coating.

[0028] Compared with the prior art, the technical effect of the present invention is

[0029] 1. It is possible to obtain a dispersion curve with low dispersion oscillation and near-zero dispersion in a wide spectral range (greater than 300nm). At the same time, the overall reflectivity is higher than 99%. This overcomes the problem that low-dispersion mirrors designed by traditional methods cannot provide high reflectivity and near-zero dispersion in a broadband range at the same time.

[0030] 2. It can achieve efficient transmission of ultrashort pulses, especially short-cycle pulses. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the ultra-wideband low-oscillation and low-dispersion mirror based on back coating of the present invention.

[0032] Figure 2 Schematic diagram of the anti-reflection film structure of Example 1.

[0033] Figure 3 Schematic diagram of spectral transmittance and group delay dispersion of the anti-reflection film of Example 1.

[0034] Figure 4 Schematic diagram of the dispersion compensation film layer structure of Example 1.

[0035] Figure 5 Schematic diagram of spectral reflectivity and group delay dispersion of the dispersion compensation film layer of Example 1.

[0036] Figure 6 Schematic diagram of the overall reflectivity and overall group delay dispersion design results of the ultra-wideband low oscillation and low dispersion mirror based on back coating in Example 1

[0037] Figure 7 This is a schematic diagram of the preparation results of Example 1. DETAILED DESCRIPTION

[0038] 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.

[0039] See also Figure 1 , Figure 1This is a schematic diagram of an ultra-wideband low-oscillation low-dispersion mirror based on back coating of the present invention, which is designed to achieve efficient transmission of ultra-short pulses while ensuring high reflectivity and low-dispersion oscillation. As shown in the figure, its structure from top to bottom includes an anti-reflection film 1, a substrate 2, and a dispersion compensation film layer 3. By rationally utilizing the positive dispersion of the substrate and the anti-reflection film, and the negative dispersion of the dispersion compensation film layer, the total dispersion introduced by the entire element is ultimately close to zero.

[0040] Anti-reflection film 1: It is composed of high refractive index materials and low refractive index materials alternately, and is plated on the front surface of the substrate, i.e., the S1 surface, to eliminate partial reflection on the front surface of the substrate and improve the transmittance. In this embodiment, in order to improve the threshold, the high refractive index material used in the anti-reflection film is Ta2O5.

[0041] Substrate 2: Fused quartz (SiO2) is selected as the substrate material, and its refractive index is between the equivalent refractive index of the anti-reflection film (anti-reflection film) and the equivalent refractive index of the dispersion compensation film layer. This choice helps to improve the impedance matching between the entire film layer and the incident medium and reduce group delay dispersion oscillation.

[0042] Dispersion compensation film layer 3: It is also composed of alternating high refractive index materials and low refractive index materials, but the low refractive index material is consistent with the substrate material (SiO2), and is plated on the rear surface of the substrate, that is, the S2 surface. This layer provides negative dispersion to offset the positive dispersion of the anti-reflection film and the substrate, and achieves high reflectivity. In this embodiment, Nb2O5 is selected as the high refractive index material of the dispersion compensation film layer.

[0043] This three-layer structure helps to improve the impedance matching between the film layer as a whole and the incident medium to reduce group delay dispersion oscillation. The appropriate material is selected according to the band for preparation, and the performance is characterized using a white light interferometer. The light is incident from the anti-reflection film, passes through the substrate to the dispersion compensation layer, and then reflects out to test the overall dispersion performance of the mirror. The present invention can ensure that ultrashort pulses are transmitted at high reflectivity without causing pulse broadening, thereby achieving efficient transmission of ultrashort pulses.

[0044] The index of the ultra-wideband low oscillation and low dispersion mirror based on back coating required by Example 1 is: the group delay dispersion value is less than 0±25fs 2 @650-1100 nm, reflectivity greater than 99%, incident angle 5 degrees, P polarization. This mirror is used for the transmission of ultrashort pulses.

[0045] The design steps are as follows:

[0046] 1) Determine the design band and material: According to the application requirements, the design band of the ultra-wideband low-oscillation low-dispersion mirror based on back coating is determined to be 650-1100 nm, with a wide bandwidth. Nb2O5 and SiO2 with a large high-low refractive index ratio are selected as the high-refractive index material and low-refractive index material of the dispersion compensation film layer. In order to increase the threshold, Ta2O5 is used as the high-refractive index material used in the anti-reflection film, and SiO2 is used as the low-refractive index material. The refractive index parameters of these three thin film materials can be obtained by the Cauchy formula Determine, as shown in Table 1. To complete the impedance matching process, the material selection of the substrate should be consistent with the low refractive index material of the dispersion compensation film layer, so the substrate is selected as fused silica (SiO2).

[0047]

[0048] 2) Calculate substrate dispersion: The substrate material is quartz with a thickness of 1 mm. Based on the material type and physical thickness of the substrate and the incident angle, the two-way dispersion of the substrate is calculated according to the following formula. According to the material refractive index database, the refractive index parameters of fused silica (SiO2) can be obtained from the Sellmeier formula Determine, as shown in Table 2.

[0049]

[0050] 3) Design of anti-reflection film: Set the transmittance design target of the anti-reflection film to 100% and the transmission dispersion target to 0 fs 2 Select the Needle optimization algorithm to optimize the film system and obtain an anti-reflection film that meets the target value requirements. The film system structure is as follows Figure 2 As shown, the transmittance and transmission dispersion are Figure 3 shown.

[0051] 4) Design the dispersion compensation film layer: multiply the transmission dispersion curve value of the anti-reflection film by 2, add it to the double-pass dispersion value of the substrate calculated in step 2, and obtain the dispersion target value of the dispersion compensation film layer according to the following formula: :

[0052] Then, with this target value As the optimization target of reflection dispersion, the reflectivity target is set to 100%, and the Needle optimization algorithm is selected to optimize the film system to obtain a dispersion compensation film layer that meets the target value requirements. The final film system structure of the dispersion compensation film layer is as follows: Figure 4 As shown, the reflectivity and reflection dispersion are Figure 5 shown.

[0053] Combination and preparation: The design results are combined into Figure 1The overall reflectivity and overall group delay dispersion of the ultra-wideband low-oscillation low-dispersion mirror based on back coating are obtained by combining the structures, such as Figure 6 As shown, the final group delay dispersion value is less than 0±10fs 2 @650-1100 nm, reflectivity greater than 99.5%, incident angle of 5 degrees, P polarization design results. The designed film system was prepared by ion beam sputtering method and deposited on both sides of the substrate.

[0054] The dispersion performance of the ultra-wideband low-oscillation low-dispersion mirror based on back coating is characterized by white light interferometry. The light first passes through the anti-reflection film layer, then passes through the substrate, and finally is reflected back by the dispersion compensation film layer and received by the white light interferometer, thereby achieving the dispersion measurement introduced by the entire ultra-wideband low-oscillation low-dispersion mirror based on back coating. The spectral performance characterization can be completed using a spectrophotometer.

[0055] The low-dispersion mirror proposed in the present invention can achieve stable dispersion compensation in a wide spectral range and reduce group delay dispersion oscillation. By rationally utilizing the positive dispersion introduced by the substrate and anti-reflection film, and the negative dispersion of the dispersion compensation film layer, the final dispersion of the entire element is close to zero. It achieves a balance between high reflectivity and low dispersion oscillation, and is suitable for efficient transmission of ultrashort pulses.

[0056] 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. An ultra-wideband low-oscillation low-dispersion mirror based on back coating, characterized in that: The film structure is A / AR / S / R / A, where A represents air, AR represents anti-reflection film, S represents substrate, and R represents dispersion compensation film. The anti-reflection film is composed of alternating high-refractive index materials and low-refractive index materials, and is coated on the front surface (S1) of the substrate to eliminate partial reflection on the front surface of the substrate; The substrate has a refractive index between the equivalent refractive index of the anti-reflection film and the equivalent refractive index of the dispersion compensation film layer, and the material selection is consistent with the low refractive index material in the dispersion compensation film layer to achieve impedance matching and reduce group delay dispersion oscillation; The dispersion compensation film layer is composed of a high refractive index material and a low refractive index material that is consistent with the substrate material, and is coated on the rear surface (S2) of the substrate to provide negative dispersion to offset the positive dispersion of the anti-reflection film layer and the substrate, and provide high reflectivity; The ultra-wideband refers to a wide spectral range greater than 300nm, and the high reflectivity refers to a reflectivity greater than 99%; Among them, by effectively utilizing the positive dispersion of the substrate and the negative dispersion of the dispersion compensation film layer, the total dispersion introduced by the entire element within the working band is close to zero, thereby achieving efficient transmission with low dispersion oscillation and low energy loss; Double-pass transmission dispersion of the substrate , the group velocity dispersion of the substrate , the transmission dispersion of the anti-reflection film , the reflection dispersion of the dispersion compensation film layer The following conditions must be met: In the formula, is the wavelength, c is the speed of light, n is the refractive index of the film material, d is the physical thickness of the substrate material, is the refractive index of air, is the refractive index of the substrate material, is the incident angle on the air side.

2. The ultra-wideband low-oscillation low-dispersion mirror based on back coating according to claim 1, characterized in that: The high refractive index material of the anti-reflection film is Nb2O5, Ta2O5, HfO2, Al2O3, TiO2, ZrO2 or Si, and the low refractive index material is SiO2, MgF2 or CaF2; the high refractive index material of the dispersion compensation film layer is Nb2O5, Ta2O5, HfO2, Al2O3, TiO2, ZrO2 or Si, and the low refractive index material is the same as the material selected for the substrate, and the material of the substrate is SiO2, CaF2, MgF2 or sapphire.

3. A method for realizing an ultra-wideband low-oscillation low-dispersion mirror based on back coating, characterized in that: The steps include: Step 1: Determine the design band and material: Determine the design band according to the actual application requirements, and select the high refractive index material and low refractive index material of the dispersion compensation film layer, the low refractive index material of the dispersion compensation film layer is consistent with the material of the substrate; and the high refractive index material and low refractive index material of the anti-reflection film; the high refractive index material of the dispersion compensation film layer is Nb2O5, Ta2O5, HfO2, Al2O3, TiO2, ZrO2 or Si; Step 2: According to the refractive index of the substrate and physical thickness , calculate the two-pass transmission dispersion of the substrate , the formula is as follows: In the formula, is the group velocity dispersion of the basis, , is the refractive index of air, is the incident angle, is the wavelength, c is the speed of light, and n is the refractive index of the film material; Step 3: Design the anti-reflection film: According to the actual application band of the ultra-wideband low-oscillation low-dispersion mirror based on back coating, the target values ​​of required transmittance and transmission dispersion are given, and the anti-reflection film structure that meets the transmittance and transmission dispersion requirements is designed through the optimization algorithm; Step 4: Calculate the target value of the reflection dispersion of the dispersion compensation film layer: multiply the transmission dispersion of the anti-reflection film structure obtained in step 3 by 2, add it to the double-pass transmission dispersion obtained in step 2, and then take the inverse to obtain the target value of the reflection dispersion of the dispersion compensation film layer; Step 5: Designing a dispersion compensation film structure to meet the reflectivity and reflection dispersion requirements: Taking the reflection dispersion target value of the dispersion compensation film obtained in step 4 as the design target, and given the required reflectivity target value, the dispersion compensation film structure that meets the reflectivity and reflection dispersion requirements is designed through an optimization algorithm; Step 6: coating the anti-reflection film and the dispersion compensation film layer on the front surface and the back surface of the substrate respectively, and finally obtaining an ultra-wideband low-oscillation low-dispersion mirror structure based on back coating; The ultra-wideband refers to a wide spectral range greater than 300 nm.

4. The method for realizing an ultra-wideband low oscillation and low dispersion mirror based on back coating according to claim 3, characterized in that: It also includes step 7: using a white light interferometer to characterize the dispersion performance of the ultra-wideband low-oscillation low-dispersion mirror based on back coating, and evaluating the overall dispersion performance of the component by the light incident from the anti-reflection film, passing through the substrate, and reflected from the dispersion compensation layer.

5. The method for realizing an ultra-wideband low oscillation and low dispersion mirror based on back coating according to claim 3, characterized in that: In the step 6, ion beam sputtering, magnetron sputtering or electron beam evaporation is used.

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