Polarization-independent reflective dielectric grating based on refractive index-controlled thin film

Abstract

The present invention relates to a polarization-independent reflective dielectric grating based on a refractive index regulating film, belonging to the field of optics, wherein the grating comprises a substrate 1 and a grating layer 4, and a reflective film 2 and an oxide dielectric film connecting layer 3 are sequentially plated on the substrate 1, and a grating groove structure is etched on the grating layer; the reflective film 2 is a refractive index regulating multilayer dielectric film, and the refractive index regulating multilayer dielectric film 2 comprises a high refractive index material film 2‑1 and a low refractive index material film 2‑2. The central wavelength of the polarization-independent reflective dielectric grating based on the refractive index regulating film provided by the present invention is 1064 nanometers, and when the incident angle is the ‑1st Littrow angle, the ‑1st order diffraction efficiency in the TE and TM polarization directions can be simultaneously higher than 99% in the range of 1050-1080 nanometers, and the highest diffraction efficiency in the band is higher than 99.5%, and high-efficiency diffraction of polarization-independent incident light can be achieved.

Description

Technical Field

[0001] The invention relates to the field of optics, and in particular to a high-line-density polarization-independent reflective dielectric grating based on a refractive index-controlled film and with a central wavelength in a near-infrared band. Background Art

[0002] In current high-power laser systems, the output power of a single-channel laser is limited by factors such as nonlinear effects and thermal damage, making it difficult to achieve high-power laser output. The use of spectral synthesis and beam combining technology can effectively increase the power density per unit area, and is one of the effective technical approaches to increase the output power of laser systems. Among them, the commonly used technical path to achieve high-efficiency spectral synthesis is to use the dispersion characteristics of reflective dielectric gratings to achieve common aperture synthesis of sub-beams of different wavelengths under certain incident conditions. However, for this type of method, the light emitted by the single-channel laser to be combined is non-polarized light, which contains both TE polarization components and TM polarization components. Therefore, to achieve high-efficiency spectral synthesis, the reflective dielectric grating needs to achieve high diffraction efficiency for both polarization components. In addition, in chirped pulse amplification technology, a reflective grating with a wide spectrum and high diffraction efficiency is also required, and the grating also needs to achieve high diffraction efficiency for both TE polarization components and TM polarization components.

[0003] Reflective dielectric film gratings used in spectrum synthesis and chirped pulse amplification generally need to have a relatively high line density. Under this condition, the period of the reflective dielectric film grating is sub-wavelength, which makes the grating diffraction characteristics show a strong polarization correlation. It is very difficult to design a polarization-independent grating with high diffraction efficiency within a certain bandwidth. Moreover, the diffraction characteristics of sub-wavelength periodic gratings cannot be calculated using the scalar grating diffraction equation. Using the rigorous coupled wave analysis method, the diffraction characteristics of sub-wavelength periodic gratings can be accurately calculated, and combined with the genetic optimization algorithm, the grating structure can be optimized. Summary of the invention

[0004] The present invention provides an all-dielectric polarization-independent reflective dielectric grating for a central wavelength of 1064nm. The grating has a line density of 1300 lines / mm, and the -1 order diffraction efficiency of the grating for incident light in TE and TM polarization modes under the Littrow angle incidence condition is higher than 99% within a bandwidth range of 1050-1080nm. The polarization-independent grating structure proposed by the present invention has the characteristics of high line density and high diffraction efficiency, and has important application value in the fields of spectrum synthesis and chirped pulse amplification.

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

[0006] A polarization-independent reflective dielectric grating based on a refractive index control film, characterized in that the grating comprises a substrate 1 and a grating layer 4, and a reflective film 2 and an oxide dielectric film connecting layer 3 are sequentially plated on the substrate 1, and a grating groove structure is etched on the grating layer; the reflective film 2 is a refractive index control multilayer dielectric film, and the refractive index control multilayer dielectric film 2 comprises a high refractive index material film 2-1 and a low refractive index material film 2-2;

[0007] The low refractive index coating material of the low refractive index material film 2-2 is SiO2, the high refractive index material of the high refractive index material film 2-1 is Ta2O5 or HfO2, and the material of the oxide dielectric film connecting layer 3 is SiO2.

[0008] The substrate 1 is a fused quartz substrate, and the grating layer 4 is a refractive index-controlled multilayer dielectric film grating layer.

[0009] The refractive index regulating multilayer dielectric film 2 and the grating layer 4 are prepared by ion beam sputtering of binary spliced ​​targets, and the material system is SiO2:Ta2O5 or SiO2:HfO2.

[0010] During the film coating process, the spatial position between the ion source and the target material is adjusted to achieve the mixing ratio control of high and low refractive index materials in the film layer.

[0011] Furthermore, the refractive index control trend of the refractive index control multilayer dielectric film 2 increases with the grating etching depth.

[0012] The groove structure of the grating layer 4 is a rectangular groove or a trapezoidal groove, the grating period is 1300 lines / mm, the etching depth is less than 1um, and the duty cycle ranges from 0.3 to 0.7.

[0013] The grating period is no greater than 769 nm.

[0014] The beneficial technical effects of the present invention are as follows:

[0015] 1. The grating structure provided by the present invention is based on the refractive index control technology to optimize the grating film layer and the groove structure, so that the incident light in the TE and TM polarization states under the Littrow angle incidence condition, the average diffraction efficiency of the -1 order is higher than 99% within the bandwidth range of 1050~1080nm, and the highest diffraction reaches up to 99.8%.

[0016] 2. The present invention can be prepared by combining holographic exposure with reactive ion etching. The preparation method is simple and efficient and has important application prospects in the field of high-power lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1It is a schematic diagram of the polarization-independent reflective medium film grating structure based on the refractive index regulating film provided by the present invention.

[0018] Figure 2 3 is a curve showing the change of the refractive index of the grating groove film layer with the etching depth in the embodiment of the present invention.

[0019] Figure 3 It is the -1 order diffraction efficiency curve of the polarization-independent grating based on the refractive index control film in the embodiment of the present invention in the 050~1080nm band.

[0020] The meanings of the reference numerals in the accompanying drawings are: 1 fused quartz substrate, 2 refractive index control multilayer dielectric film, 2-1 high refractive index material film, 2-2 low refractive index material film, 3 oxide film connecting layer, 4 grating layer. DETAILED DESCRIPTION

[0021] In order to enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention is clearly and completely described below in conjunction with the accompanying drawings of the present invention. Based on the embodiments in this application, other similar embodiments obtained by ordinary technicians in this field without making any creative work should all fall within the scope of protection of this application.

[0022] like Figure 1 As shown, an all-dielectric polarization-independent reflection grating with a central wavelength of 1064nm is used. The grating film layer and the groove structure are optimized based on the refractive index control technology, so that the -1 order diffraction efficiency of the incident light in the TE and TM polarization states is higher than 99% in the bandwidth range of 1050~1080nm under the Littrow angle incidence condition.

[0023] The grating includes a fused quartz substrate 1 and a grating layer 4. A reflective film 2 and an oxide dielectric film connecting layer 3 are sequentially plated on the fused quartz substrate 1, and a grating groove structure is etched on the grating layer 4. The reflective film 2 is a refractive index regulating multilayer dielectric film, which includes a high refractive index material film 2-1 and a low refractive index material film 2-2 from bottom to top. The grating layer 4 is a refractive index regulating multilayer dielectric film grating layer.

[0024] In the grating structure, the low refractive index coating material of the low refractive index material film 2-2 is SiO2, the high refractive index material of the high refractive index material film 2-1 is Ta2O5 or HfO2. The material of the oxide dielectric film connection layer 3 is SiO2.

[0025] The refractive index control multilayer dielectric film 2 and the grating layer 4 are prepared by the method of ion beam sputtering binary splicing target material, and the material system is SiO2:Ta2O5 or SiO2:HfO2. During the film coating process, the spatial position between the ion source and the target material is adjusted to achieve the control of the mixing ratio of high and low refractive index materials in the film layer. According to the Lorentz-Lorentz model, the corresponding relationship between the mixing ratio of high and low refractive index materials and the refractive index of the film layer is established, so that the refractive index of the film layer can be accurately controlled by adjusting the mixing ratio of high and low refractive index materials. Furthermore, the refractive index control trend of the refractive index control multilayer dielectric film 2 increases with the grating etching depth, and the specific refractive index of each layer and the corresponding thickness are determined by the specific optimization algorithm used. The groove structure of the grating layer is a rectangular groove or a trapezoidal groove, the grating period is 1300 lines / mm, the etching depth is less than 1um, and the duty cycle range is 0.3~0.7.

[0026] After the initial grating structure is determined, the grating structure is optimized using a rigorous coupled wave method combined with a genetic optimization algorithm. Figure 1 The trapezoidal groove structure in the grating is optimized for the optimization goal of -1 diffraction efficiency of TE mode and TM mode in the 1030~1080nm band to be higher than 99%. The specific parameters of the optimized grating structure are:

[0027] The refractive index of the fused silica substrate 1 is 1.41;

[0028] The material of the high refractive index material film 2-1 in the high-low refractive index periodic reflective film 2 is Ta2O5, with a refractive index of 1.91 and a thickness of 133.2nm;

[0029] The material of the low refractive index material film 2-2 is SiO2, the refractive index is 1.39, and the thickness is 219.6nm;

[0030] The material of the oxide thin film connection layer 3 is SiO2, with a refractive index of 1.39 and a thickness of 121 nm;

[0031] The grating groove etching depth of the trapezoidal grating layer 4 is d=633nm, and the top duty ratio of the groove is f1 =0.35, the bottom duty cycle is f2= 0.66, period P= 769nm, the refractive index of the grating layer changes with the etching depth as shown in Figure 3 shown.

[0032] Based on the polarization-independent reflective dielectric grating in the 1050-1080 wavelength band and an incident angle of 43 degrees, the -1 order diffraction efficiency curves of the TE polarization mode and the TM polarization mode are as follows: Figure 3It can be seen that the average diffraction efficiency of the polarization-independent reflective dielectric grating provided by the present invention exceeds 99% in the 1050-1080 wavelength band, and the highest diffraction efficiency reaches 99.8%.

Claims

1. A polarization-independent reflective dielectric grating based on a refractive index control film, characterized in that: The grating comprises a substrate (1) and a grating layer (4), and a reflective film (2) and an oxide dielectric film connecting layer (3) are sequentially plated on the substrate (1), and a grating groove structure is etched on the grating layer; the reflective film (2) is a refractive index regulating multilayer dielectric film, and the refractive index regulating multilayer dielectric film (2) comprises a high refractive index material film (2-1) and a low refractive index material film (2-2); The low refractive index coating material of the low refractive index material film (2-2) is SiO2, the high refractive index material of the high refractive index material film (2-1) is Ta2O5 or HfO2, and the material of the oxide dielectric film connecting layer (3) is SiO2; The grating layer (4) is a refractive index-controlled multi-layer dielectric film grating layer; The refractive index regulating multilayer dielectric film (2) and the grating layer (4) are prepared by a method of ion beam sputtering a binary spliced ​​target material, and during the film layer plating process, the spatial position between the ion source and the target material is adjusted to achieve control of the mixing ratio of high and low refractive index materials in the film layer; The corresponding relationship between the mixing ratio of high and low refractive index materials and the refractive index of the film layer is established according to the Lorentz-Lorentz model, so that the refractive index of the film layer can be precisely controlled by adjusting the mixing ratio of high and low refractive index materials; Furthermore, the refractive index regulation trend of the refractive index regulation multilayer dielectric film (2) increases with the grating etching depth.

2. The polarization-independent reflective dielectric grating based on a refractive index control film according to claim 1, characterized in that: The substrate (1) is a fused quartz substrate.

3. The polarization-independent reflective dielectric grating based on a refractive index control film according to claim 1, characterized in that: The groove structure of the grating layer (4) is a rectangular groove or a trapezoidal groove, the grating period is 1300 lines / mm, the etching depth is less than 1um, and the duty cycle ranges from 0.3 to 0.

7.

4. The polarization-independent reflective dielectric grating based on a refractive index control film according to claim 3, characterized in that: The grating period is no greater than 769 nm.

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