High diffraction efficiency low polarization large incidence angle immersion grating

Abstract

This invention discloses an immersion grating with high diffraction efficiency, low polarization, and large incident angle. Its operating wavelength is 750–770 nm, and its incident angle is 71.4°. It comprises a grating layer and an equivalent dielectric layer. The grating layer is a rectangular grating structure etched periodically onto a substrate. The equivalent dielectric layer is formed by atomic layer deposition (ALD) of Al₂O₃, TiO₂, Al₂O₃, TiO₂, and Al₂O₃ nanofilms of equal thickness stacked sequentially on the surface of the grating structure. The immersion grating designed in this invention causes total internal reflection of incident light, eliminating transmission orders and significantly improving the grating's diffraction efficiency. The equivalent dielectric layer covering the grating surface, constructed using ALD, avoids light leakage issues encountered during metal plating processes, further enhancing the grating's diffraction efficiency and reducing its polarization sensitivity. This design can be applied to spaceborne gas monitoring to improve detection accuracy.

Description

Technical Field

[0001] This invention relates to the field of grating technology, specifically to an immersion grating with high diffraction efficiency, low polarization, and large incident angle. Background Technology

[0002] Grating-type imaging spectrometers possess advantages such as high resolution, high signal-to-noise ratio, and near-linear dispersion, making them one of the important optical payloads for spaceborne greenhouse gas monitoring. With the continuous improvement of spectral resolution and the demand for miniaturization, immersion gratings are increasingly being used in spaceborne greenhouse gas monitoring imaging spectrometers. Compared to traditional gratings, they offer higher resolution for the same size.

[0003] Currently, the large incident angle diffraction gratings used in spaceborne greenhouse gas monitoring imaging spectrometers have low average diffraction efficiency and high polarization sensitivity. For example, the paper "Development of immersed diffraction grating for the TROPOMI-SWIR spectrometer" (Amerongen A, Visser H, Vink R, et al. [C] / / Proceedings of SPIE, 2010, 7826: 78261D.) uses a trapezoidal immersion grating based on silicon with a surface coated with aluminum; the paper "Efficiency, dispersion and straylight performance tests of immersed gratings for high resolution spectroscopy in the near infrared" (Fernandez-Saldivar J, Culfaz F, Angli N, et al. [C] / / International Conference on Space Optics—ICSO 2012. SPIE, 2017, 10564: 720-729.) uses a trapezoidal immersion grating based on fused silica. The large incident angle diffraction gratings reported in the aforementioned literature have an average efficiency of around 60% to 70% in the operating wavelength range, and a polarization sensitivity of less than 10%. In order to further improve the diffraction efficiency and reduce the polarization sensitivity of the grating, it is urgent to design an immersion diffraction grating with high diffraction efficiency and low polarization sensitivity that can be applied to the field of spaceborne gas monitoring. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides an immersion grating suitable for spaceborne gas monitoring, which achieves high diffraction efficiency, low polarization sensitivity, and a large incident angle.

[0005] To achieve the aforementioned objectives, the present invention provides a high-diffraction-efficiency, low-polarization, large-incident-angle immersion grating with an operating wavelength of 750–770 nm and an incident angle of… The angle is 71.4°; the immersion grating includes a grating layer and an equivalent dielectric layer. The grating layer is a rectangular grating structure etched periodically onto a substrate. The substrate material is fused silica optical glass with a refractive index of 1.454 operating in the 750–770 nm wavelength range. The grating period d of the grating structure satisfies the condition: d = a + b + 2 * L, where b is the width of the rectangular grating body, a is the width of the rectangular groove of the equivalent dielectric layer, and L is the thickness of the equivalent dielectric layer. The groove density of the grating structure is 3550 line pairs / mm, the groove depth h is 224–336 nm, and the duty cycle t is... d The equivalent dielectric layer is 0.12–0.26 nm. It is formed by atomic layer deposition on the surface of the grating structure of the grating layer, by sequentially stacking Al2O3 nanofilm and TiO2 nanofilm using Al2O3, TiO2, Al2O3, TiO2, and Al2O3. These nanofilms are stacked alternately for a total of 5 layers, and the thickness of each nanofilm is equal. The refractive index of Al2O3 is 1.67, the refractive index of TiO2 is 2.10, and the thickness L of the equivalent dielectric layer is 25–79 nm.

[0006] The present invention discloses an immersion grating with high diffraction efficiency, low polarization, and large incident angle. The grating structure of its grating layer has a grating period d of 280 nm, a groove depth h of 248 nm, a grating rectangle width b of 39 nm, and a grating duty cycle t. d The value is 0.14. The thickness L of the equivalent dielectric layer is 55 nm.

[0007] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0008] 1. The immersion grating provided by this invention has a -1st order diffraction efficiency of TE polarization and TM polarization of more than 92% and a relative linear polarization degree of less than 1% throughout the entire working bandwidth. At the center wavelength of the working bandwidth, the diffraction efficiency is more than 92.5% and the relative linear polarization degree is less than 1%, which can be applied to spaceborne high spectral resolution small imaging spectrometers.

[0009] 2. The immersion grating provided by the present invention is a total internal reflection grating. When the incident angle of the grating is greater than the total internal reflection angle, the grating has no transmission order, and the light energy is concentrated on the reflection diffraction order, thereby improving the diffraction efficiency of the grating.

[0010] 3. The immersion grating provided by the present invention has an equivalent dielectric layer obtained by stacking thin films on its surface using atomic layer deposition, which overcomes the shortcomings of the existing technology of plating metal on the grating surface, such as plasma resonance absorption that increases the polarization sensitivity of the grating and light leakage caused by manufacturing process. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the immersion grating provided in an embodiment of the present invention;

[0012] Figure 2 This is a diffraction efficiency curve of the immersion grating in the working wavelength range in the embodiment of the invention;

[0013] Figure 3 This is a polarization sensitivity curve of the immersion grating in the working wavelength range in an embodiment of the invention.

[0014] In the figure: 1. Incident light; 2. Air layer; 3. Equivalent dielectric layer; 4. Grating layer; 5. Al2O3 nanofilm; 6. TiO2 nanofilm. Implementation

[0015] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0016] See appendix Figure 1 This is a schematic diagram of the immersion grating provided in this embodiment; it includes a grating layer 4 and an equivalent dielectric layer 3; the grating layer is a rectangular grating structure etched periodically on a prism substrate; the equivalent dielectric layer is disposed on the surface of the grating structure of the grating layer, and the immersion grating operates in the air layer 2 environment; the incident light 1 is at an angle Incident on the immersion grating, h is the groove depth of the grating layer grating structure, b is the rectangular width of the grating layer grating structure, d is the grating period, the corresponding groove density is 1 / d, a is the rectangular groove width of the equivalent dielectric layer, and L is the thickness of the equivalent dielectric layer; the equivalent dielectric layer of the immersion grating is composed of Al2O3 nanofilm 5 and TiO2 nanofilm 6 stacked sequentially, with a total of 5 nanofilms in the equivalent dielectric layer, the materials being: Al2O3, TiO2, Al2O3, TiO2, Al2O3 in sequence.

[0017] Let θ be the angle of the incident light from the grating, h be the groove depth of the grating layer 4, a be the width of the equivalent dielectric layer 3, b be the width of the grating layer 4, d be the period of the grating layer 4, the corresponding groove density is 1 / d, and L be the thickness of the equivalent dielectric layer 3.

[0018] angle of incidence of the grating and the total reflection angle of the material Conditions met: .

[0019] The ratio of the groove depth h to the period d of the grating layer 4 is the groove depth-to-period ratio h. d The following conditions are met: .

[0020] The ratio of the width to the period of grating layer 4 is the duty cycle t. d The following conditions are met: .

[0021] The direction of the incident photoelectric field vector perpendicular to the incident plane is defined as TE polarization, and its diffraction efficiency is η. TE The direction of the incident photoelectric field vector parallel to the incident plane is defined as TM polarization, and its diffraction efficiency is η. TM The polarization sensitivity of grating diffraction is defined as: .

[0022] The immersion grating designed in this invention operates in the wavelength range of 750–770 nm.

[0023] The incident angle of the immersion grating is 71.4°, the grating layer groove density is 3550 line pairs / mm, the grating layer groove depth h is 224–336 nm, the grating layer width b is 34–73 nm, the equivalent dielectric layer thickness L of the immersion grating is 25–79 nm, and the groove depth-to-period ratio h is... d The duty cycle t of the grating is 0.8–1.2. d The thickness ranges from 0.12 to 0.26. The Al2O3 and TiO2 nanofilms have the same thickness, ranging from 5 to 11 nm.

[0024] By adjusting the above-mentioned grating groove parameters, the present invention can obtain an optimal structural parameter scheme.

[0025] The grating groove parameters are selected as follows: the groove density of grating layer 4 is 3550 lp / mm, the period d of grating layer 4 is 280 nm, the groove depth h of grating layer 4 is 248 nm, the width b of grating layer 4 is 39 nm, the thickness L of the equivalent dielectric layer 3 is 55 nm, and the groove depth-to-period ratio h of the grating is... d The duty cycle t of the grating is 0.88. d The value is 0.14. The incident light of the grating is incident at an angle greater than the total internal reflection angle of the grating layer material, which is 71.4°.

[0026] The grating layer 4 is made of fused silica optical glass with a refractive index of approximately 1.454 in the working wavelength range. The equivalent dielectric layer 3 is prepared by alternating stacks of Al2O3 and TiO2 materials. The refractive index of Al2O3 is 1.67, and the refractive index of TiO2 is 2.10, resulting in an equivalent refractive index of approximately 2 for the equivalent dielectric layer. The equivalent dielectric layer immersing the grating is prepared using atomic layer deposition (ALD), which provides excellent conformal properties, allowing the equivalent dielectric layer 3 to better encapsulate the grating layer and achieve high diffraction efficiency and low polarization sensitivity.

[0027] The diffraction efficiency and polarization sensitivity curves of the immersion grating provided in this embodiment are shown in the appendix. Figure 2 and Figure 3 .

[0028] See appendix Figure 2 The diagram shows the diffraction efficiency of the immersion grating provided in this embodiment. Within the operating wavelength range of 750–770 nm, the diffraction efficiency for TE polarization is in the range of 92.5%–94%, and the diffraction efficiency for TM polarization is in the range of 92%–93%, with an overall average efficiency greater than 92%. This demonstrates that the diffraction efficiency of the grating changes steadily throughout the entire operating wavelength range.

[0029] See appendix Figure 3 The figure shows the polarization sensitivity curve of the immersion grating provided in this embodiment. Within the working wavelength range of 750 to 770 nm, the polarization sensitivity is less than 1%, and at a wavelength of approximately 768 nm, the polarization sensitivity is close to 0.

[0030] The immersion grating designed in this invention causes total internal reflection of incident light, eliminating transmission orders and significantly improving the grating's diffraction efficiency. The equivalent dielectric layer covering the grating surface is deposited using atomic layer deposition, which avoids light leakage issues encountered during metal plating processes, further enhancing the grating's diffraction efficiency and reducing its polarization sensitivity. This design can be applied to spaceborne gas monitoring to improve detection accuracy.

Claims

1. A high-diffraction-efficiency, low-polarization, large-incident-angle immersion grating, characterized in that: Its operating wavelength is 750–770 nm, and the incident angle is... The immersion grating is 71.4°; the immersion grating includes a grating layer (4) and an equivalent dielectric layer (3). The grating layer is a rectangular grating structure etched on a substrate in a periodic arrangement. The substrate material is fused silica optical glass with a refractive index of 1.454 in the 750-770nm working band. The grating period d of the grating structure satisfies the condition: d = a + b + 2*L, where b is the width of the rectangular grating body of the grating structure, a is the width of the rectangular groove of the equivalent dielectric layer, L is the thickness of the equivalent dielectric layer, the groove density of the grating structure is 3550 line pairs / mm, the groove depth h is 224-336nm, and the duty cycle t d The equivalent dielectric layer is 0.12 to 0.26 nm. It is formed by atomic layer deposition on the surface of the grating structure of the grating layer, by sequentially stacking Al2O3 nanofilm (5) and TiO2 nanofilm (6) of Al2O3, TiO2, TiO2 and Al2O3. They are stacked alternately for a total of 5 layers. The thickness of each nanofilm is equal. The refractive index of Al2O3 is 1.67 and the refractive index of TiO2 is 2.

10. The thickness L of the equivalent dielectric layer is 25 to 79 nm.

2. The immersion grating with high diffraction efficiency, low polarization, and large incident angle according to claim 1, characterized in that: The grating structure of the grating layer has a grating period d of 280 nm, a groove depth h of 248 nm, a grating rectangle width b of 39 nm, and a grating duty cycle t. d It is 0.

14.

3. The immersion grating with high diffraction efficiency, low polarization, and large incident angle according to claim 1, characterized in that: The thickness L of the equivalent dielectric layer is 55 nm.

Images