Three-layer all-dielectric rectangular grating for realizing-2-level broadband high efficiency
Technical Field
The invention relates to a broadband high-efficiency grating, in particular to a three-layer all-dielectric rectangular grating for realizing-2-level broadband high efficiency.
Background
In the field of femtosecond laser, a light splitting device such as a prism cannot be used due to the introduction of material dispersion. The grating can separate the spectrum and compensate the dispersion, has the advantages of compensating the dispersion, compact structure, high efficiency, easy manufacture and the like, and is the most important optical device in the field of femtosecond laser. And the all-dielectric rectangular grating has a high damage threshold compared with a gold-plated grating, so that the all-dielectric grating has an important practical prospect.
The all-dielectric rectangular grating is simple to manufacture and can be processed by conventional processes such as one-time exposure, development, etching and the like of a holographic interference field. To obtain a high-efficiency wide-spectrum diffraction grating structure, Thomas w.mossberg et al propose a negative first-order high-efficiency rectangular grating structure based on three layers of media [ patent No.: US20090116790 a1 discloses obtaining high efficiency polarization independent diffraction characteristics in the 1550 communication band. As is known, the resolving power of a grating can be expressed as R λ/Δ λ m × N, where Δ λ is the minimum resolvable wavelength interval, N is the number of grating lines in the aperture, and m is the diffraction order. In general lithography, the size of the grating is difficult to be made large. The larger the diffraction order m is, the higher the resolving power of the grating is, and the traditional grating is optimally designed aiming at the-1 st diffraction efficiency. At present, no one designs a-2-level high-efficiency diffraction grating aiming at a transmission type multilayer medium rectangular grating.
Disclosure of Invention
The invention aims to provide a three-layer all-dielectric rectangular grating structure for realizing a-2-level high-efficiency diffraction characteristic. The grating can realize transmission-2 order diffraction efficiency close to 1 for TE polarized light. The grating has the characteristics of simple structure, rectangular structure, small depth-to-width ratio, easy processing and large manufacturing tolerance, is easy to realize under the processing of a microelectronic deep etching process combined with the current optical holographic recording technology or electron beam direct writing technology, has mature process and low manufacturing cost, can be produced in batches and has important application prospect.
The first technical solution of the invention is as follows:
a three-layer all-dielectric rectangular grating for realizing-2-level broadband high efficiency comprises fused quartz as an incident area medium and three-layer all-dielectric rectangular grating with grating ridges respectively comprising a first layer of aluminum oxide, a second layer of titanium dioxide and a third layer of fused quartz, and is characterized in that the refractive index of the middle layer of the three-layer rectangular grating is the largest, the refractive index of each layer of material is between the refractive indexes of two adjacent materials, and femtosecond laser adopts a secondary Bragg angle thetain=arcsin(λ/n1d) Incident of wherein thetainRepresenting the angle of incidence, n1And if the refractive index of the incident area, d represents the grating period, and b represents the width of the grating ridge, the duty ratio is f ═ b/d, and h1, h2 and h3 are the layer thicknesses of alumina, titania and silica in the three-layer all-dielectric rectangular grating structure respectively.
For 1500 nm waveIn the period, the duty ratio of the three-layer all-dielectric rectangular grating is 0.38-0.42, the period is 1553-1728 nanometers, the grating ridge is divided into three layers, and the first layer is alumina (nr)11.66), the second layer is titanium dioxide (nr)22.28) and the third layer is fused silica (nr)31.44), the corresponding depths are h respectively1Is 151.6-466.9 nm, h2862.4-962.1 nm, h3And 273.8-406.6 nm.
The duty cycle of the grating is 0.4, the period is 1.6246 microns, and the depth h1Is 289.1 nm, h2Is 904.9 nm, h3At 338.8 nm.
When the grating is used in 800 nm waveband, the duty cycle of the grating is 0.4076-0.4494, the period is 883.9-924.2 nm, the first layer is alumina (nr)11.67), the second layer is titanium dioxide (nr)22.095) and the third layer was fused silica (nr)31.453) corresponding to a depth of h, respectively1113.9-257.9 nm, h2447.9-534.1 nm, h3Is 156.8-216 nm.
The duty ratio of the grating is 0.4263, the period is 0.8647 microns, and the depth h1Is 184.2 nm, h2Is 506 nm, h3At 186.1 nm.
The invention has the following technical effects:
1. when used in the 1500 nm band:
when the grating duty cycle is 0.4, the period is 1.6246 microns, and the depth h1Is 289.1 nm, h2Is 904.9 nm, h3At 338.8 nm, a femtosecond laser with a wavelength of 1500 nm at a secondary Bragg angle θin=arcsin (λ/n1d) (wherein n is11.44 for the incident area index) of the grating, the diffraction efficiency of the transmitted negative second order is 99.58%, and the diffraction efficiency of 95% or more can be achieved in the incident wavelength range of 1.454 to 1.531 μm. If the incident angle is in the range of 37.32-43.3 when the incident wavelength is 1.5 microns, diffraction efficiency of more than 95% can be achieved.
2. When used in 800 nm band, the duty cycle of the grating0.4263, period 0.8647 μm, depth h1Is 184.2 nm, h2Is 506 nm, h3At 186.1 nm, a femtosecond laser with a wavelength of 800 nm at a secondary Bragg angle thetain=arcsin(λ/n1d) (wherein n is11.453 for the incident area refractive index) was incident on the grating, the diffraction efficiency of the transmitted negative second order was 97.82%, and the diffraction efficiency of 95% or more was achieved in the incident wavelength range of 771.5-813.9 nm. If the incident angle is in the range of 37.89 to 42.82 degrees with the incident wavelength being 0.8 microns, diffraction efficiencies of more than 95% can be achieved.
3. The invention can realize high-efficiency negative two-stage diffraction, has the characteristics of simple structure, rectangular structure, small depth-to-width ratio, easy processing and large manufacturing tolerance, is easy to realize under the processing of the micro-electronic deep etching process combined with the current optical holographic recording technology or electron beam direct writing technology, has mature process and low manufacturing cost, can be produced in batches and has important application prospect.
Drawings
FIG. 1 is a schematic diagram of a three-layer all-dielectric rectangular grating structure with negative second-order high efficiency.
FIG. 2 is a-2 diffraction efficiency diagram of a 1.5 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 1.5 microns, the duty ratio is 0.4, the depth of the grating region is 0.289 micron of an alumina layer, 0.905 micron of a titanium dioxide layer and 0.339 micron of a silicon dioxide layer) under different period values.
FIG. 3 is a-2 diffraction efficiency diagram of a 1.5 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 1.5 microns, the period is 1.625 microns, the depth of the grating region is 0.289 micron of an alumina layer, 0.905 micron of a titanium dioxide layer and 0.339 micron of a silicon dioxide layer) under different duty ratios.
Fig. 4 is a-2-order diffraction efficiency diagram of a 1.5-micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 1.5 microns, the period is 1.625 microns, the duty ratio is 0.4, and the depths of grating regions are respectively 0.905 micron of a titanium dioxide layer and 0.339 micron of a silicon dioxide layer) under different depths of an aluminum oxide layer.
FIG. 5 is a-2 diffraction efficiency diagram of a 1.5 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 1.5 microns, the period is 1.625 microns, the duty ratio is 0.4, the depths of grating regions are 0.289 microns of an aluminum oxide layer and 0.339 microns of a silicon dioxide layer respectively) of the invention under different depths of titanium dioxide layers.
FIG. 6 is a-2 diffraction efficiency diagram of a 1.5 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 1.5 microns, the period is 1.625 microns, the duty ratio is 0.4, and the depths of grating regions are 0.289 microns for an alumina layer and 0.905 microns for a titanium dioxide layer, respectively) under different depths of a silicon dioxide layer.
FIG. 7 shows the secondary Bragg angle θ of a 1.5-micron waveband three-layer all-dielectric rectangular grating (wherein the period is 1.625 microns, the duty ratio is 0.4, and the depths of the grating regions are respectively 0.289 micron for an alumina layer, 0.905 micron for a titania layer and 0.339 micron for a silica layer) of the inventionin=arcsin(λ0/n1d)=39.88°(λ0Center wavelength 1.5 microns) of the incident light at different incident wavelengths.
Fig. 8 is a-2-order diffraction efficiency diagram of a 1.5-micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 1.5 microns, the period is 1.625 microns, the duty ratio is 0.4, and the depths of grating regions are respectively 0.289 microns of an alumina layer, 0.905 microns of a titanium dioxide layer and 0.339 microns of a silicon dioxide layer) under different incident angles.
FIG. 9 is a graph of diffraction efficiency of the 0.8 micron waveband three-layer all-dielectric rectangular grating of the present invention (wherein, the incident wavelength is 0.8 micron, the duty cycle is 0.4263, the depth of the grating region is 0.1842 microns, the depth of the titanium dioxide layer is 0.506 micron, and the depth of the silicon dioxide layer is 0.1861 microns) in the order of-2 at different values of the period.
Fig. 10 is a-2 order diffraction efficiency graph of a 0.8 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 0.8 micron, the period is 0.8647 microns, the depth of the grating region is 0.1842 microns, the depth of the titanium dioxide layer is 0.506 micron, and the depth of the silicon dioxide layer is 0.1861 microns) under different duty cycles.
Fig. 11 is a-2 order diffraction efficiency graph of a 0.8 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 0.8 micron, the period is 0.8647 microns, the duty ratio is 0.4263, the depths of the grating regions are respectively 0.506 micron of a titanium dioxide layer and 0.1861 microns) under different depths of an aluminum oxide layer.
Fig. 12 is a-2 order diffraction efficiency graph of a 0.8 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 0.8 micron, the period is 0.8647 microns, the duty ratio is 0.4263, the depths of the grating regions are 0.1842 microns and the depth of the silicon dioxide layer is 0.1861 microns, respectively) of the invention under different depths of titanium dioxide layers.
Fig. 13 is a-2 order diffraction efficiency graph of a 0.8 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 0.8 micron, the period is 0.8647 microns, the duty ratio is 0.4263, the depths of the grating regions are 0.1842 microns, and the depth of the titanium dioxide layer is 0.506 micron) of the invention under different depths of silicon dioxide layers.
FIG. 14 is a diagram of-2 diffraction efficiency at different incident wavelengths of a 0.8 micron waveband three-layer all-dielectric rectangular grating (wherein the grating period is 0.8647 microns, the duty ratio is 0.4263, the grating region depth is 0.1842 microns, the titanium dioxide is 0.506 microns, and the silicon dioxide layer is 0.1861 microns) of the present invention.
FIG. 15 is a-2 diffraction efficiency diagram of a 0.8 micron waveband three-layer all-dielectric rectangular grating (wherein, the incident wavelength is 0.8 micron, the grating period is 864.7 nm, the duty ratio is 0.4263, the depths of the grating regions are 0.1842 microns, titanium dioxide is 0.506 micron, and the silicon dioxide layer is 0.1861 microns, respectively) under different incident angles;
wherein 1 is fused silica (refractive index n at wavelength of 1.5 μm)1Refractive index n at wavelength 0.8 μm ═ 1.4411.453) and 2 is alumina (refractive index nr at a wavelength of 1.5 μm1Refractive index nr at wavelength 0.8 μm of 1.6611.67) and 3 is titanium dioxide (refractive index nr at a wavelength of 1.5 μm2Refractive index nr at wavelength 0.8 μm of 2.2822.095) and 4 is fused silica (refractive index nr at a wavelength of 1.5 μm)3Refractive index nr at wavelength 0.8 μm of 1.4431.453), 5 represents incident light, and 6 represents-2 order diffracted light.
Detailed Description
The present invention will be further described with reference to the following examples and drawings, but the scope of the present invention should not be limited thereto.
The first embodiment is as follows:
referring to fig. 1, fig. 1 is a schematic diagram of a negative two-level high-efficiency three-layer all-dielectric rectangular grating of 1.5 μm wavelength band according to the present invention, in which: thetainRepresentative of the angle of incidence, θ-2,θ0Diffraction angles of-2 and 0 orders, respectively, h1、h2And h3The thicknesses of the alumina, the titanium dioxide and the silicon dioxide in the three layers of all-dielectric rectangular grating structures are respectively. b represents the width of the grating ridge and d represents the grating period (duty cycle f ═ b/d). Regions 1,2,3,4,7 are all homogeneous media. The grating vector K is positioned in the incident plane, the TE polarized light is vertical to the incident plane corresponding to the vibration direction of the electric field vector, and the TE polarized light is at a certain angle thetain=arcsin(λ/n1d) (defined as a second order Bragg angle) is incident on the grating region, and the diffraction angles of the diffraction orders of-2 order and 0 order are symmetrically distributed on the left and right sides of the-1 order-2=θ0=arcsin(λ/n0d) Where λ represents the incident wavelength, n0D is the period of the grating, which can realize transmission-2-order diffraction efficiency close to 1 under broadband high tolerance to incident light of TE polarization. As can be seen from the figure, the grating structure comprises three layers of dielectric materials with different refractive indexes. And the refractive index of each layer of material is between the two layers of the upper and lower interfaces. The refractive index matching is realized, the interface reflection energy is prevented from being too strong, and the working bandwidth is increased. The duty cycle of the grating was 0.4 with a period of 1.6246 microns. Wherein the grating structure has three grating regions, and the first layer of alumina (nr) is used as the material11.66), the second layer is titanium dioxide (nr)22.28) and the third layer is fused silica (nr)31.44). Depth is respectively h1Is 289.1 nm, h2Is 904.9 nm, h3At 338.8 nm.
The invention adopts a strict coupled wave theory algorithm to carry out optimization design, and carries out the following calculation analysis aiming at the high tolerance characteristic: fig. 2,3,4, 5 and 6 show control variables for the period, duty cycle f, h1, h2, h3, respectively, and the effect of their tolerance on the-2 order diffraction efficiency values was studied. With a diffraction efficiency of 95% as a standard, the tolerance range is: period (1553-1728 nm), duty ratio f (0.38-0.42), h1(151.6-466.9 nm), h2(862.4-962.1 nm), and h3(273.8-406.6 nm). It can be seen that for each parameter variable, diffraction efficiency outputs above 95% can be achieved within a large tolerance range. With good high tolerance characteristics.
The broadband characteristics thereof were investigated. From fig. 7 and 8, when the duty cycle of the grating is 0.4, the period is 1.6246 microns. The grating structure described therein has three grating regions in depth, and the material used in this patent is a first layer of aluminum oxide (nr)11.66), the second layer is titanium dioxide (nr)22.28) and the third layer is fused silica (nr)31.44). Depth is respectively h1Is 289.1 nm, h2Is 904.9 nm, h3At 338.8 nm, when the incident angle is the second Bragg angle, the grating structure can realize diffraction efficiency of more than 95% in the range of 1454 nm to 1531 nm of incident wavelength. And when the incident wavelength is 1.5 microns, the diffraction efficiency output of more than 95% is realized within the range of 37.32-43.3 degrees of the incident angle, and the broadband optical fiber has good broadband characteristics.
The second embodiment is as follows:
because the wavelengths are different and the refractive indexes corresponding to the materials are different, the three-layer structure is re-optimized at the lower part, the grating structure still adopts the structure shown in figure 1, the structure parameters are re-optimized, when the duty ratio of the grating is 0.4263, the period is 0.8647 microns, and the depth h is1Is 184.2 nm, h2Is 506 nm, h3The effect is optimal when the particle size is 186.1 nanometers. Fig. 9, 10, 11, 12 and 13 show the control variables for the duty cycle, duty f, h1, h2, h3, respectively, and the effect of their tolerance on the-2 diffraction efficiency values was investigated. It can be seen that for each parameter variable, diffraction efficiency outputs above 95% can be achieved within a large tolerance range.The diffraction efficiency of the structure is more than 95% as a standard, and tolerance intervals are respectively period (833.9-924.2 nanometers), duty ratio (0.4076-0.4494), h1(113.9-257.9 nanometers), h2(477.9-534.1 nanometers) and h3(156.8-216 nanometers), so that the structure has good high tolerance characteristics.
The broadband characteristics of the grating were studied, and from fig. 14 and 15, when the duty cycle of the grating was 0.4263, the period was 0.8647 μm. Wherein the grating structure is divided into three parts in depth in the grating region, the material used in this embodiment is a first layer of aluminum oxide (nr)11.67), the second layer is titanium dioxide (nr)22.095) and the third layer was fused silica (nr)31.453). Depth is respectively h1Is 184.2 nm, h2Is 506 nm, h3When the incident angle is the second Bragg angle incidence at 186.1 nanometers, the grating structure can realize the diffraction efficiency of more than 95 percent within the range of the incident wavelength of 772.7 nanometers to 813.1 nanometers. And when the incident wavelength is 0.8 micron, the diffraction efficiency output of more than 95% is realized within the range of 37.89-42.82 degrees of the incident angle, and the broadband optical fiber has good broadband characteristics.
The three-layer all-dielectric rectangular grating can effectively inhibit reflection between interfaces as a-2-level high-efficiency grating, concentrates main energy on the-2 level, has simple structure and mature process, can be produced in large batch at low cost by combining a holographic grating recording technology with a microelectronic deep etching process, and has large manufacturing tolerance and wide bandwidth. The dispersion element is a high-quality and stable dispersion element, and has important application prospect in the fields of femtosecond laser, dispersion compensation, wavelength division multiplexing and the like.