Reflection type all-dielectric polarization beam combining grating for any wave band

Abstract

The invention discloses a reflective all-dielectric polarization beam-combining grating suitable for any wave band. The reflective all-dielectric polarization beam-combining grating is composed of a grating etching layer, a phase matching layer, a periodic film system and a grating substrate in sequence from top to bottom. The invention has the characteristics of easy preparation, wide band and high diffraction efficiency, can effectively solve the problems of low PBS anti-laser damage threshold and low synthesis efficiency in the current polarization beam combination technology, and provides solid component support for the realization of the larger array scale and higher power laser synthesis technology in the future.

Description

Technical Field

[0001] The present invention relates to the field of optical devices, and in particular to a reflective polarization beam combining grating for arbitrary wavelength bands. This grating, through an optimized design of a multilayer dielectric film structure and a trapezoidal groove grating etch layer, achieves efficient polarization beam combining of lasers in arbitrary wavelength bands. It is particularly suitable for combining TE (transverse electric) and TM (transverse magnetic) polarized light in high-power laser systems. Background Art

[0002] High-power laser systems have enormous application value in cutting-edge basic scientific research, intelligent manufacturing, and national defense security. However, due to issues such as nonlinear losses in laser materials, thermal effects, and optical damage, the output power of a single laser module is difficult to meet the actual high-power requirements of high-efficiency laser processing and high-energy laser weapons.

[0003] In order to increase the output power of lasers, the main methods currently used at home and abroad are spatial beam combining, polarization beam combining, and wavelength beam combining to couple multiple lasers into a laser beam for direct output or output through optical fiber coupling.

[0004] Spectral beam-combining (SBC) technology based on diffraction gratings can significantly increase the number of laser modules (that is, increase the output power) while maintaining the overall beam quality, thereby achieving high-power, high-beam-quality output from the laser system. Compared with other beam-combining technologies, SBC technology can achieve the highest "power in the barrel." Given the unique technical advantages of spectral beam-combining technology, high-power laser output powers based on this technology have reached hundreds of kilowatts in recent years and have shined in the field of high-energy laser weapons. In the future, further increasing the output power of single-channel lasers or increasing the array size of beam combining will be the main research directions for achieving higher-power laser output.

[0005] The combination of polarization beam combining and spectral beam combining is expected to significantly increase the array size and output power of beam combining lasers. Currently, polarization beam splitters (PBS) are mainly used to achieve polarization beam combining. However, due to the presence of a transmitted beam in the PBS, its thermal damage and multi-interface Fresnel reflection under high-power laser action limit its power load capacity and synthesis efficiency. In comparison, reflective polarization beam combining gratings based on multilayer dielectric films have higher resistance to laser damage and higher synthesis efficiency due to the absence of a high-energy transmitted beam. However, existing reflective gratings still have the following problems: (1) The working band is limited and it is difficult to cover the entire spectrum from ultraviolet to infrared; (2) The traditional rectangular grating structure is sensitive to the duty ratio and is difficult to prepare; (3) The reflectivity of the multilayer film system is insufficient, resulting in limited diffraction efficiency and bandwidth. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of limited wavelength band, low diffraction efficiency, insufficient laser damage resistance, and complex fabrication in existing polarization beam combining technologies. By doing so, the present invention provides a reflective all-dielectric polarization beam combining grating suitable for any wavelength band. Fabricated on a multilayer dielectric film with high reflectivity, high laser damage threshold, and extremely low absorption, the grating offers unique advantages such as broadband, high diffraction efficiency, and low-temperature rise distortion. It is particularly well-suited for high-power polarization beam combining laser systems, achieving efficient beam combining of 0-level TM light and -1-level TE light.

[0007] The technical solutions of the present invention are as follows:

[0008] A reflective all-dielectric polarization beam combining grating suitable for any wavelength band is characterized in that the grating is composed of a grating etching layer with a trapezoidal groove cross-section, a phase matching layer, a periodic film system and a grating substrate from top to bottom. The periodic film system is composed of a high refractive index material layer and a low refractive index material layer. The grating substrate material and the periodic film system constitute a bottom high reflective layer. The film system of the bottom high reflective layer is S(H2L) m H, where S is the grating substrate with low expansion and high thermal conductivity; H and L represent the optical thickness λ r / 4 high refractive index material layer and low refractive index material layer, λ r is the reference wavelength, m represents the number of film periods, the phase matching layer is composed of a high refractive index material layer, and the grating etching layer with a trapezoidal groove profile is composed of a lower high refractive index material sub-grating layer and an upper low refractive index material top grating layer.

[0009] When the grating is used in the 575 nanometer band, the period Λ of the grating is 390 to 405 nanometers, the etching depth (h1+h2) is 185 to 200 nanometers, the inclination angle φ between the grating side wall and the grating vector direction is 80°, w is the width of the grating top, and the duty ratio w / Λ of the grating top is 0.2 to 0.4.

[0010] When the grating is used in the 1055 nanometer band, the period Λ of the grating is 620 to 650 nanometers, the etching depth (h1+h2) is 465 to 500 nanometers, the inclination angle φ between the grating side wall and the grating vector direction is 75° to 85°, w is the width of the grating top, and the duty ratio w / Λ of the grating top is 0.25 to 0.4.

[0011] When the grating is used in the 3.5-micron wave band, the period Λ of the grating is 2460 to 2480 nanometers, the etching depth (h1+h2) is 962 to 982 nanometers, the inclination angle φ between the grating side wall and the grating vector direction is 78° to 83°, w is the width of the grating top, and the duty ratio w / Λ of the grating top is 0.25 to 0.4.

[0012] The low refractive index material used in the grating etching layer and the periodic film system is SiO2; the high refractive index material used in the grating etching layer and the phase matching layer is HfO2; and the high refractive index material used in the periodic film system is Ta2O5.

[0013] The grating etching layer is composed of an upper layer of low-refractive index material and a lower layer of high-refractive index material, and has a trapezoidal groove structure. The top trapezoidal structure effectively reduces the duty ratio of the grating, improves the duty ratio tolerance, and thus reduces the difficulty of preparation; the phase matching layer is composed of a high-refractive index material and has a rectangular structure, which ensures the high diffraction efficiency and broadband characteristics of the grating; the periodic film system is composed of a high-refractive index material layer and a low-refractive index material layer stacked together. The setting of this periodic film system and the grating substrate can effectively improve the bottom reflectivity.

[0014] Compared with the prior art, the present invention has the following beneficial technical effects:

[0015] 1. The reflective all-dielectric polarization beam combiner grating of the present invention solves the problem that transmissive and reflective components such as polarization beam splitters are subject to thermal damage under high-power laser action and limited power load capacity and synthesis efficiency due to multi-interface Fresnel reflection due to the presence of a transmitted beam. Since the present invention does not have a high-energy transmitted beam, it has a higher resistance to laser damage and higher synthesis efficiency.

[0016] 2. This invention features easy fabrication, broadband, and high diffraction efficiency. The reflective all-dielectric polarization beam combining grating can achieve an average diffraction efficiency exceeding 90% at -1st and 0th orders for incident light in both TE and TM polarization modes, respectively, within any wavelength bandwidth. This invention has significant application value and prospects for increasing the power of sub-beams required in fields such as spectral synthesis and coherent combining, and for promoting the development of high-power laser beam combining technology.

[0017] Therefore, the research and preparation of broadband and high-efficiency reflective all-dielectric polarization beam combining gratings has important application value and prospects for promoting the development of high-power laser beam combining technology, and provides solid component support for the realization of larger array scale and higher power laser synthesis technology in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a cross-sectional view of an embodiment of the reflective all-dielectric polarization beam combining grating for arbitrary wavelength bands of the present invention.

[0019] Figure 2 The relationship between the TM0-level diffraction efficiency of the polarization beam combining grating in Example 1 and the wavelength and duty ratio is as follows:

[0020] Figure 3This is a graph showing the relationship between the diffraction efficiency of the polarization beam combining grating TM0 in Example 1 and the wavelength when the duty ratio is 0.28.

[0021] Figure 4 The polarization beam combining grating TE of Example 1 -1 Relationship between order diffraction efficiency and wavelength and duty ratio

[0022] Figure 5 The polarization beam combining grating TE of Example 1 -1 The relationship between the diffraction efficiency of the order and the wavelength when the duty ratio is 0.28

[0023] Figure 6 The relationship between the TM0-level diffraction efficiency of the polarization beam combining grating in Example 2 and the wavelength and duty ratio is as follows:

[0024] Figure 7 This is a graph showing the relationship between the diffraction efficiency of the polarization beam combining grating TM0 in Example 2 and the wavelength when the duty ratios are 0.31, 0.32, 0.33, and 0.34 respectively.

[0025] Figure 8 This is the polarization beam combining grating TE of embodiment 2 -1 Relationship between order diffraction efficiency and wavelength and duty ratio

[0026] Figure 9 This is the polarization beam combining grating TE of embodiment 2 -1 The relationship between the diffraction efficiency and wavelength when the duty ratio is 0.3, 0.31, 0.32, and 0.33 respectively

[0027] Figure 10 The relationship between the TM0-level diffraction efficiency of the polarization beam combining grating in Example 3 and the wavelength and duty ratio is as follows:

[0028] Figure 11 This is a graph showing the relationship between the diffraction efficiency of the polarization beam combining grating TM0 in Example 3 and the wavelength when the duty ratio is 0.33.

[0029] Figure 12 This is the polarization beam combining grating TE of embodiment 3 -1 Relationship between order diffraction efficiency and wavelength and duty ratio

[0030] Figure 13 This is the polarization beam combining grating TE of embodiment 3 -1 The relationship between the diffraction efficiency of the order and the wavelength when the duty ratio is 0.33

[0031] In the figure: 1-grating substrate, 2-multilayer dielectric reflective film, 3-high refractive index phase matching layer, 4-high refractive index grating layer, 5-low refractive index grating layer, 6-incident area (air), 7-grating groove area (air), 8-coupled output laser, Φ-grating trapezoidal part inclination angle, β-TM light incident angle and coupling output angle, α-TE light incident angle, Λ-grating period, w-grating top width, coupled output laser is TM0 level and TE -1 Level combined laser beam. DETAILED DESCRIPTION

[0032] The following further illustrates the embodiments of the present invention in conjunction with the examples and drawings, but this should not limit the scope of protection of the present invention.

[0033] Please refer to Figure 1 , Figure 1 This is a cross-sectional view of an embodiment of a reflective all-dielectric polarization beam combining grating for any wavelength band of the present invention. As can be seen from the figure, a reflective all-dielectric polarization beam combining grating for any wavelength band is composed of the following four layers from top to bottom.

[0034] The grating etching layer is composed of a top grating layer 5 of a low refractive index material and a sub-grating layer 4 of a high refractive index material with a trapezoidal groove cross-section structure. Grating grooves 7 (air medium) are formed between each trapezoidal groove.

[0035] Phase matching layer 3, a rectangular cross-section layer made of high refractive index material

[0036] The multilayer stack of periodic films consists of alternating layers of high-refractive-index material H and low-refractive-index material L. The film structure is S(H2L)15H. In this embodiment, each period contains two H layers and one L layer (H2L), repeated 15 times. The final layer is H, forming a highly reflective termination layer. S represents the grating substrate 1.

[0037] In this embodiment, the high refractive index materials are: HfO2 (n=2.0, phase matching layer and sub-grating layer), Ta2O5 (n=2.1, periodic film system), and the low refractive index materials are: SiO2 (n=1.45, grating top and periodic film system).

[0038] TE polarized incident light corresponds to the vibration direction of the electric field vector being perpendicular to the incident plane, while TM polarized incident light corresponds to the vibration direction of the magnetic field vector being perpendicular to the incident plane. Figure 1It can be seen that the incident medium above the grating and the medium inside the grating grooves are both air. When the incident light of different polarizations and the same wavelength enters the polarization beam combining grating from the air at different angles that satisfy the grating equation, after being modulated by the grating layer and the multi-layer dielectric film high reflective mirror layer, the two polarized light beams are reflected back to the air layer in the same direction, forming a single light output, thus achieving the spectral combination of the incident laser beams with different polarization directions. The incident angles of the two polarized light beams must satisfy the formula:

[0039] d(sinα n + sinβ)=mλ n (d is the grating period, m is the diffraction order, and m = -1 when the beam is combined).

[0040] The present invention uses rigorous coupled-wave theory [prior art 3: MG Moharam, et al., J. Opt. Soc. Am. A12, 1077-1086 (1995)] to calculate the -1 order diffraction efficiency of a reflective polarization beam combining grating, and concludes that by optimizing parameters such as the grating layer, phase layer thickness, grating period, and sidewall tilt angle of the reflective polarization beam combining grating, high diffraction efficiencies at the 0th order and -1st order for TM polarized and TE polarized incident light, respectively, can be achieved within a larger duty cycle tolerance and a wider wavelength band.

[0041] Preparation process steps of reflective all-dielectric polarization beam combining grating for arbitrary wavelength band

[0042] Step 1: Substrate pretreatment and periodic film deposition

[0043] Polish and clean the fused silica substrate (S) to ensure the surface roughness is less than 1 nm.

[0044] Ta2O5 (H layer) and SiO2 (L layer) films are alternately deposited using ion beam sputtering (IBS) or electron beam evaporation (EBE) technology.

[0045] Step 2: Preparation of phase matching layer and sub-grating layer

[0046] A HfO2 layer (thickness h3) is deposited on the surface of the periodic film system and a rectangular structure is formed by reactive ion etching (RIE).

[0047] A HfO2 sub-grating layer (thickness h2) is deposited on the HfO2 layer as a supporting layer for the trapezoidal structure.

[0048] Step 3: Trapezoidal grating etching

[0049] Photoresist is spin-coated on the surface of the sub-grating layer, and a trapezoidal groove pattern is formed by holographic lithography or electron beam lithography.

[0050] The HfO2 sub-grating layer and the upper SiO2 top grating layer are sequentially etched by dry etching (such as inductively coupled plasma etching, ICP), and the etching depth (h1+h2) and the sidewall tilt angle Φ are controlled.

[0051] The residual photoresist is removed to form air-filled trapezoidal grating trenches.

[0052] Adjust the grating period Λ, etching depth (h1+h2), duty ratio (w / Λ) and sidewall tilt angle Φ according to the target band:

[0053] Example 1: 575nm visible light band

[0054] In such Figure 1 In the grating structure shown, the grating period Λ is 400 nm, the grating layer etching depths h1 and h2 are 51.6 nm and 138.9 nm respectively, the phase matching layer thickness h3 is 315 nm, and the grating material refractive index n L and n H They are 1.45 and 2.0 respectively, and the high reflective layer structure is: S(H2L) 15 H, H, and L represent high-refractive-index hafnium oxide and low-refractive-index silicon oxide materials with optical thicknesses of λr / 4 (λr = 429 nm), respectively. The bottom angle of the trapezoidal grating is Φ = 78°. The incident angle of the TM wave is given by the formula sinα i = sinθ, the incident angle of the TE wave is determined by the formula sinα i = sinθ-λ i / Λ is determined by the grating width, and the duty ratio of the grating is defined as the ratio of the top width w to the grating period Λ, w / Λ. Figure 2 As shown in FIG, when the grating top width ratio is between 0.2 and 0.3, the average diffraction efficiency of the TM0 level of the grating is greater than 90% within a 20-nanometer bandwidth (565 nanometers to 585 nanometers). In particular, when the grating width ratio is 0.28, the average diffraction efficiency of the grating in the 573.7-585 nanometer band is greater than 98%, and the average diffraction efficiency in the 575.7-584.3 nanometer band is greater than 99%. Figure 3 When the grating top width ratio is between 0.2-0.4, as shown in Figure 4 As shown in FIG, the TE-1 average diffraction efficiency of the grating is greater than 90% within a 15 nm bandwidth (565 nm-580 nm), and in particular, when the grating duty ratio is 0.28, the average diffraction efficiency of the grating within the 565 nm-575.5 nm band is greater than 98%, as shown in FIG. Figure 5 shown.

[0055] Example 2: 1055nm near-infrared band

[0056] In such Figure 1In the grating structure shown, the grating period Λ is 630 nm, the grating layer etching depths h1 and h2 are 362 nm and 132 nm respectively, the phase matching layer thickness h3 is 485 nm, and the grating material refractive index n L and n H They are 1.45 and 2.0 respectively, and the high reflective layer structure is: S(H2L) 15 H, H and L represent the optical thickness λ r / 4,(λ r =835 nm) and a high-refractive-index hafnium oxide material and a low-refractive-index silicon oxide material. The bottom angle Φ of the trapezoidal grating portion is 80°. The incident angle of the TM wave is given by the formula sinα i = sinθ, the incident angle of the TE wave is determined by the formula sinα i = sinθ-λ i / Λ is determined by the grating width, and the duty ratio of the grating is defined as the ratio of the top width w to the grating period Λ, w / Λ. Figure 6 As shown in FIG, when the grating top width ratio is between 0.25 and 0.4, the average diffraction efficiency of the TM0 level of the grating is greater than 92% within a 50-nanometer bandwidth (1030 nm-1080 nm). In particular, when the grating width ratio is 0.34, the average diffraction efficiency of the grating in the 1030 nm-1074 nm band is greater than 98%, and the average diffraction efficiency in the 1030 nm-1068 nm band is greater than 99%. Figure 7 When the grating top width ratio is between 0.25-0.4, as shown in Figure 8 As shown in FIG, the TE-1 average diffraction efficiency of the grating is greater than 90% within a 50-nanometer bandwidth (1030 nm-1080 nm), and in particular, when the grating duty ratio is 0.31, the average diffraction efficiency of the grating in the 1049-1068 nm band is greater than 98%, and the average diffraction efficiency in the 1058-1061 nm band is greater than 99%. Figure 9 shown.

[0057] Example 3: 3.5μm mid-infrared band

[0058] In such Figure 1 In the grating structure shown, the grating period Λ is 2474 nm, the grating layer etching depths h1 and h2 are 2 nm and 977 nm respectively, the phase matching layer thickness h3 is 853 nm, the grating material refractive indices nL and nH are 1.45 and 2.0 respectively, and the high reflective layer structure is: S(H2L)15H, where H and L represent a high-refractive-index hafnium oxide material and a low-refractive-index silicon oxide material with an optical thickness of λr / 4 (λr = 5074.6 nm), respectively. The bottom angle Φ of the trapezoidal grating portion is 80°, and the incident angle of the TM wave is given by the formula sinα i = sinθ, the incident angle of the TE wave is determined by the formula sinαi = sinθ-λ i / Λ is determined by the grating width, and the duty ratio of the grating is defined as the ratio of the top width w to the grating period Λ, w / Λ. Figure 10 As shown in FIG, when the grating top width ratio is between 0.25 and 0.4, the average diffraction efficiency of the TM0 level of the grating is greater than 90% within an 86-nanometer bandwidth (3450 nanometers to 3536 nanometers). In particular, when the grating width ratio is 0.33, the average diffraction efficiency of the grating in the 3450-3507 nanometer band is greater than 98%, and the average diffraction efficiency in the 3450-3491 nanometer band is greater than 99%. Figure 11 When the grating top width ratio is between 0.25-0.4, as shown in Figure 12 As shown in FIG. 1 , the TE-1 average diffraction efficiency of the grating is greater than 90% within a 100 nm bandwidth (3450 nm-3550 nm). In particular, when the grating duty ratio is 0.33, the average diffraction efficiency of the grating within the 3450 nm-3530 nm band is greater than 98%, and the average diffraction efficiency within the 3472 nm-3515 nm band is greater than 99%. Figure 13 shown.

[0059] Experiments have shown that the embodiments of the present invention, through the trapezoidal grating structure, all-dielectric material system and parameter optimization, cover visible light to mid-infrared (575nm~3.5μm), and the parameters can be extended to ultraviolet and far infrared; the TE / TM polarization beam combining efficiency in any band is greater than 90%, and in some bands it is greater than 99%. There is no transmission energy loss, and the damage resistance threshold is increased by more than 10 times, making it suitable for megawatt-class laser systems. It has the characteristics of easy preparation, polarization correlation, broadband, and high diffraction efficiency. The present invention has important application value and prospects for improving the sub-beam laser power required in the fields of spectral synthesis, coherent synthesis, and promoting the development of high-power laser beam combining technology.

Claims

1. A reflective all-dielectric polarization beam combining grating suitable for any wavelength band, characterized in that: The grating includes, from top to bottom: The grating etching layer has a trapezoidal groove cross section and is composed of a lower sub-grating layer (4) of a high refractive index material and an upper top grating layer (5) of a low refractive index material; Phase matching layer, a rectangular structure made of high refractive index material; Periodic film system, composed of high refractive index material layer (H) and low refractive index material layer (L) alternately stacked, the film structure is S (H2L) m H, where S is the grating substrate, H and L represent the optical thickness λ r / 4 high refractive index material layer and low refractive index material layer, λ r is the reference wavelength, m represents the number of film periods; The grating substrate is made of low expansion and high thermal conductivity material.

2. The reflective all-dielectric polarization beam combining grating applicable to any wavelength band according to claim 1, characterized in that: The trapezoidal groove profile of the grating etched layer meets the following conditions: The grating top width ratio w / Λ is 0.2 to 0.4, where w is the grating top width and Λ is the grating period; The inclination angle φ between the grating side wall and the grating vector direction is 75° to 85°; The etching depth h1+h2 is adapted according to the target band, where h1 is the thickness of the sub-grating layer and h2 is the thickness of the top grating layer.

3. The reflective all-dielectric polarization beam combining grating applicable to any wavelength band according to claim 2, characterized in that: When the grating is used in different bands, the parameters are set as follows: In the visible light 575 nm band, the grating period Λ is 390-405 nm, the etching depth h1+h2 is 185-200 nm, and the inclination angle φ between the grating sidewall and the grating vector direction is 80°; In the 1055 nm band, the grating period Λ is 620-650 nm, the etching depth h1+h2 is 465-500 nm, and the inclination angle φ between the grating sidewall and the grating vector direction is 75°-85°; In the infrared 3.5 micron band, the grating period Λ is 2460-2480 nanometers, the etching depth h1+h2 is 962-982 nanometers, and the inclination angle φ between the grating side wall and the grating vector direction is 78°-83°.

4. The reflective all-dielectric polarization beam combining grating applicable to any wavelength band according to claim 3, characterized in that: When the grating is used in different wavebands, the thickness h3 of the phase matching layer (3) is optimized according to the target waveband: Visible light 575 nm band: h3 = 315 nm; 1055nm near-infrared band: h3=485nm; 3.5μm mid-infrared band: h3=853nm.

5. The reflective all-dielectric polarization beam combining grating applicable to any wavelength band according to claim 1, characterized in that The low refractive index material used in the grating etching layer and the periodic film system is SiO2; the high refractive index material used in the grating etching layer and the phase matching layer is HfO2; and the high refractive index material used in the periodic film system is Ta2O5.

6. A reflective all-dielectric polarization beam combining grating suitable for any wavelength band according to claim 1, wherein when TE polarized light is incident, the -1 order diffraction efficiency of the grating is greater than 90% within the target wavelength band bandwidth; when TM polarized light is incident, the 0 order diffraction efficiency is greater than 90% within the target wavelength band bandwidth.

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