CN111240012B - Light beam near-field shaping method based on guided mode resonance sub-wavelength grating coding - Google Patents
Light beam near-field shaping method based on guided mode resonance sub-wavelength grating coding Download PDFInfo
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Abstract
The invention relates to a light beam near field shaping method based on guided mode resonance sub-wavelength grating coding, belonging to the technical field of light beam near field shaping, wherein a grating coding device is arranged in a transmission light path of a light beam in an off-axis manner, the grating coding device is coded according to the expected output of a shaped light beam, the light beam is transmitted by the grating coding device and then split into a 0-order diffraction light beam and a-1-order diffraction light beam, the light beam is linearly polarized light, the 0-order diffraction light beam is filtered by an optical filtering system to remove high-frequency components, and the near field shaping of the light beam is realized. Near-field shaping of a linearly polarized beam is achieved.
Description
Technical Field
The invention belongs to the technical field of near-field shaping of light beams, and particularly relates to a light beam near-field shaping method based on guided mode resonance sub-wavelength grating coding.
Background
In the field of high-energy and high-power laser research and related applications, the near-field intensity distribution of a light beam generally has higher requirements, and accordingly, the actual requirement of near-field shaping of the light beam derives. For example, in order to accurately measure the laser damage threshold of a large-aperture optical element in a related device, a test beam with a relatively uniform light intensity distribution needs to be provided; in order to ensure the safe and stable operation of a high-energy and high-power laser device, the modulation degree of the near-field intensity distribution of a light beam needs to be strictly controlled so as to avoid the damage of an optical element caused by small-scale self-focusing; gains corresponding to an amplification light path module in a laser system are usually in spatial non-uniform distribution, and the near field of a light beam is often required to be shaped in order to finally obtain expected near field output. In addition, in a high-energy and high-power laser device, under a high-flux operation condition, in order to avoid further deterioration of a local damage point existing in a preceding stage optical element and avoid adverse effects on downstream optical path transmission, a light beam needs to be locally shielded at a corresponding position in an optical path, and the local damage point is still in the nature of beam near-field shaping.
Optical devices for near-field beam shaping mainly include spatial light modulators, digital micromirrors, binary optical elements, soft-edge diaphragms, wedge arrays, birefringent lenses, etc. (references: High-large-threshold beam shaping using binary phase plates, Optics Letters, vol.34, 2330-2332, 2009; High-quality near-field beam shaped in a High-power laser base on SLM adaptive beam shaping system, Optics Express, vol.23, 681-689, 2015). The space light modulator, digital micro-mirror and other programmable devices can realize various optical shaping effects flexibly in principle, but the pairThe damage threshold is low, and the application of the laser near-field shaping device in high-energy and high-power laser is limited; soft-edge diaphragms, wedge arrays, birefringent lenses, etc. are generally only suitable for specific near-field distributions; the binary optical element prepared by the optical medium corresponds to dozens of J/cm2The damage threshold of (2) is very suitable for near-field shaping of laser under high-flux conditions.
Disclosure of Invention
The inventors found in long-term practice that: the guided mode resonance sub-wavelength grating coding device (called as a grating coding device for short) has a high damage threshold equivalent to that of a binary optical element, but the two devices are different from each other in the beam shaping principle. The binary optical element realizes the distribution of light beam energy in different diffraction orders based on the scalar diffraction optical principle, and the etching line width corresponding to the traditional binary optical element is usually in the order of 10 mu m. The grating coding device realizes the distribution of light beam energy in different diffraction orders based on the polarization beam splitting effect of the guided mode resonance sub-wavelength grating, and the etching line width corresponding to the device is only in the wavelength order. Because the minimum etching line width corresponding to the grating coding device is only in the sub-wavelength level, namely compared with a binary optical element, the grating coding device can realize a finer near-field shaping effect of the light beam in a space domain. Therefore, in order to solve the above problems, a method for near-field shaping of a light beam based on guided-mode resonance sub-wavelength grating encoding is proposed.
In order to achieve the purpose, the invention provides the following technical scheme:
a light beam near-field shaping method based on guided mode resonance sub-wavelength grating coding comprises the following steps:
s1: a grating coding device is arranged in a transmission light path of the light beam in an off-axis mode, the grating coding device is coded according to the expected output of the shaped light beam, the light beam is split into a 0-order diffraction light beam and a-1-order diffraction light beam after being transmitted by the grating coding device, wherein the 0-order diffraction light beam is the expected output of the shaped light beam, and the light beam is linearly polarized light;
s2: and the high-frequency component of the 0-order diffraction light beam is filtered by an optical filtering system, so that the near-field shaping of the light beam is realized.
Preferably, the grating coding device comprises a substrate and a plurality of gratings positioned on the substrate, the gratings are rectangular gratings, and the ridge areas of the gratings and the substrate are made of the same dielectric material.
Preferably, in step S1, to ensure that there are only two diffraction orders of 0-1 order after the light beam is transmitted through the grating encoder, the incident angle of the light beam incident on the grating
The requirements are as follows:
Preferably, under different duty cycles f, two guided modes always exist for the TE polarization component in the optical beam, and then:
wherein k is0＝2π/λ，nb、ngRespectively representing the refractive index of the grating ridge and the groove region,
the corresponding mode is a guided mode, and the equivalent refractive index corresponding to the existing two guided modes is recorded as
Preferably, the accumulated phase difference of the two guided modes in the TE polarization component after propagating through the grating satisfies the following formula:
Preferably, the method for encoding the grating encoding device by adopting an area encoding mode comprises the following steps:
(1) the spatial light intensity distribution of the light beam and the expected output of the light beam are respectively recorded as Iin (x, y) and Iout (x, y), and for the effective area of Iin (x, y) >0, the 0-order diffraction light beam transmittance distribution design target of the grating coding device is as follows: itar (x, y) is normalized to Iout (x, y)/Iin (x, y), and is denoted as Inorm (x, y);
(2) dividing a raster coding device into M multiplied by N sub-regions, wherein the area of the raster coding device codes a design target S (i, j), i is more than or equal to 1 and less than or equal to M, and j is more than or equal to 1 and less than or equal to N, then S (i, j)/(delta x delta y) is 1-mean [ Inorm (x, y) ], (i-1) x delta x is less than or equal to i multiplied by delta x, (j-1) x delta y is less than or equal to j multiplied by delta y, and delta x and delta y respectively represent the sizes of the sub-regions, and a mean () function represents the averaging operation.
Preferably, in step S1, the encoding of the grating encoder device by the grating etching angle encoding method, the exit of the-1 st order diffracted beam along a certain cone, includes the following steps:
(1) setting the grating direction, grating strip direction and grating normal direction as x, y and z axes, respectively, the incident direction of light beam and x-z plane included angle is phi, and the incident direction of light beam and z axis included angle is
Angle between electric field component of light beam and incident surface
Using strict couplingCalculating the diffraction efficiency corresponding to 0-order diffraction beams under different encoding angle phi values by using wave theory, wherein phi belongs to [0 DEG, 90 DEG ]]Establishing a normalized mapping curve eta norm (phi) between the encoding angle and the diffraction efficiency, wherein eta is more than or equal to 0 and less than or equal to 1;
(2) the spatial light intensity distribution of the light beam and the expected output of the light beam are respectively recorded as Iin (x, y) and Iout (x, y), and for the effective area of Iin (x, y) >0, the 0-order diffraction light beam transmittance distribution design target of the grating coding device is as follows: itar (x, y) is normalized to Iout (x, y)/Iin (x, y), and is denoted as Inorm (x, y);
(3) dividing a grating coding device into M multiplied by N sub-regions, wherein an angle coding design target phi (i, j) of the grating coding device is equal to or larger than 1 and equal to or smaller than i and equal to or larger than 1 and equal to or smaller than N, and eta norm [ phi (i, j) ], wherein eta norm [ Inorm (x, y) ], (i-1) x delta x is equal to or smaller than i multiplied by delta x, and (j-1) x delta y is equal to or smaller than j multiplied by delta y, wherein the delta x and the delta y respectively represent the widths of the sub-regions along the x direction and the y direction, and a mean () function represents the averaging operation.
Preferably, the optical filter system is a 4f filter system.
The invention has the beneficial effects that:
for linearly polarized light beams, a grating coding device is arranged in an optical path, the characteristics of high diffraction efficiency, sensitivity to the polarization state of the light beams and sensitivity to the incident angle of the light beams of the guided mode resonance sub-wavelength grating are fully utilized, the energy distribution proportion of the light field on different diffraction orders is regulated and controlled in a space domain, the grating coding device can adopt a grating etching angle coding or grating etching area coding mode, an optical filtering system is connected subsequently, only the low-frequency component of 0-order diffraction light beams is reserved, and the near-field shaping of the light beams is realized.
Drawings
FIG. 1 is a schematic view of the overall structure of the present invention;
FIG. 2 is a schematic diagram of a structure of a grating encoder device;
FIG. 3 is a diagram showing the diffraction characteristics of a light beam in the case of cone diffraction;
FIG. 4(a) is a diagram showing the light intensity distribution of a light beam, and FIG. 4(b) is a diagram showing the light intensity distribution of a desired output;
FIG. 5 is a diagram illustrating a mapping curve between diffraction efficiencies corresponding to encoding angles and 0-order diffraction beams;
FIG. 6 is a schematic diagram of the angle coding design result of a grating coding device;
FIG. 7 is a diagram illustrating the near-field shaping effect of the light beam finally realized in the second embodiment;
FIG. 8 is a diagram illustrating the relative error between the near field of the optical beam and the near field of the optical beam desired to be output in accordance with the second final implementation of the embodiment;
FIG. 9(a) is a diagram of the light intensity distribution of a light beam, and FIG. 9(b) is a diagram of a binary coded near field of a desired output light beam;
FIG. 10 is a schematic diagram of the area code design result of a grating code device;
FIG. 11 is a diagram illustrating the near-field shaping effect of the light beam finally achieved in the third embodiment;
fig. 12 is a schematic diagram of an error between the near-field shaping effect of the optical beam and the near-field of the optical beam expected to be output finally achieved by the third embodiment.
In the drawings: 1-light beam, 2-grating encoder, 3-1 order diffraction light beam, 4-0 order diffraction light beam, 5-lens I, 6-filter aperture, 7-lens II, 8-output shaping light beam, 9-substrate, and 10-grating.
Detailed Description
In order to make the technical solutions of the present invention better understood, the following description of the technical solutions of the present invention with reference to the accompanying drawings of the present invention is made clearly and completely, and other similar embodiments obtained by a person of ordinary skill in the art without any creative effort based on the embodiments in the present application shall fall within the protection scope of the present application. In addition, directional terms such as "upper", "lower", "left", "right", etc. in the following embodiments are directions with reference to the drawings only, and thus, the directional terms are used for illustrating the present invention and not for limiting the present invention.
The first embodiment is as follows:
referring to fig. 1, a near-field shaping method for a light beam based on guided-mode resonance sub-wavelength grating coding includes the following steps:
s1: the grating coding device 2 is arranged in a transmission light path of a light beam in an off-axis mode, the grating coding device 2 is coded according to expected output of a shaped light beam, the grating coding device 2 can be prepared on optical media such as fused quartz, the fused quartz has a high damage threshold value and a small thermal expansion coefficient, and can stably work in a high-energy and high-power laser environment, so that the designed grating coding device 2 is suitable for a high-energy and high-power laser environment, a light beam 1 is transmitted by the grating coding device 2 and then split into a 0-level diffraction light beam 4 and a-1-level diffraction light beam 3, wherein the 0-level diffraction light beam 4 is expected output of the shaped light beam, and the light beam 1 is linearly polarized light.
S2: the 0-order diffracted light beam 4 is subjected to high-frequency component filtering by an optical filtering system to obtain an output shaped light beam 8, so that near-field shaping of the light beam 1 is realized.
The fundamental principle of guided mode resonance sub-wavelength grating (reference: Investigation of the Polarization-dependent dispersion of deep direct dispersion of vertical dispersion of.
Referring to fig. 2, the grating encoder 2 includes a substrate 9 and a plurality of gratings 10 on the substrate 9, in order to obtain a higher damage threshold and reduce the processing difficulty, the grating ridge region and the substrate 9 are made of the same dielectric material, and the gratings 10 are rectangular gratings. To ensure that only two diffraction orders of 0 and-1 order exist after the light beam is transmitted through the grating encoder 2, the incident direction of the light beam is in the x-z plane shown in FIG. 2, and the incident angle
Satisfies the following conditions:
namely, it is LitThe rrow angle of incidence, λ is the beam wavelength, where d ═ b + g, d denotes the grating period, b denotes the individual grating ridge area width, g denotes the individual grating groove area width, and 0.5 λ<d<1.5 λ, while the corresponding transmission directions of the 0 order diffracted beam and the-1 order diffracted beam are symmetric about the z-axis.
For deep-etched guided-mode resonance sub-wavelength gratings, the diffraction propagation behavior of a light beam in the grating region has certain similarity with that of a waveguide. When light beams enter the interface between air and the grating area at a certain angle, a guided mode supported by the grating structure is excited, and a certain phase difference is accumulated after the guided mode is transmitted through the grating area. When the guided mode further propagates to the interface between the grating region and the substrate, energy will be coupled between the guided mode and each diffraction order, i.e. the diffraction efficiency of each diffraction order is determined by the interference effect between the guided modes. By optimally designing the structural parameters of the grating, the diffraction efficiency of each order of the light beam under different polarization states is regulated and controlled, and a foundation is laid for the design of a grating coding device.
As can be seen from the analysis by the mode method, under the Litrrow incident angle condition, for the TE polarization component in the light beam (as shown in fig. 2, the electric field vibration direction thereof is parallel to the y axis), under the condition of different duty ratios f (duty ratio f is b/d), two guided modes always exist for the TE polarization component in the light beam, and then:
wherein k is0＝2π/λ，nb、ngRespectively representing the refractive index of the grating ridge and the groove region,
the corresponding mode is a guided mode, and the equivalent refractive index corresponding to the existing two guided modes is recorded as
For the TM polarization component in the beam (with the electric field vibration in the x-z plane and perpendicular to the wave vector direction, as shown in fig. 2), under different duty cycles (the fraction of the dielectric material portion that occupies the entire grating period), there are typically one or two guided modes, then:
by optimally designing parameters such as grating period d, grating groove depth h, duty ratio f and the like, when the phase difference accumulated after two guided modes in the TE polarization component are propagated by the grating meets the formula:
In this embodiment, a light beam is incident on a grating along an Litrrow angle, the light beam is in a linear polarization state, and a corresponding electric field component is perpendicular to an incident plane formed by an incident direction of the light beam and a normal direction of an encoding device, and when the above conditions are satisfied, the grating encoding device is encoded by using a grating etching angle encoding mode, and a-1-order diffracted light beam is not corresponding to a uniform fixed exit direction but exits along a certain conical surface, including the following steps:
(1) setting the grating direction, grating strip direction and grating normal direction as x, y and z axes, respectively, the incident direction of light beam and x-z plane included angle is phi, and the incident direction of light beam and z axis included angle is
Electric field component and incident surface of light beamIncluded angle
Calculating the diffraction efficiency corresponding to the 0-order diffraction beam under different encoding angle phi values by adopting a strict coupled wave theory (reference document: Normal vector method for converting and using the RCWA for converting and transforming, Journal of the Optical Society of America A, Vol.24, 2880-2890, 2007)]Establishing a normalized mapping curve eta norm (phi) between the coding angle and the diffraction efficiency, wherein eta is more than or equal to 0 and less than or equal to 1.
(2) The spatial light intensity distribution of the light beam and the expected output of the light beam are respectively recorded as Iin (x, y) and Iout (x, y), and for the effective area of Iin (x, y) >0, the 0-order diffraction light beam transmittance distribution design target of the grating coding device is as follows: itar (x, y) is normalized to Iout (x, y)/Iin (x, y), and is denoted as Inorm (x, y).
(3) Dividing a grating coding device into M multiplied by N sub-regions, wherein an angle coding design target phi (i, j) of the grating coding device is equal to or larger than 1 and equal to or smaller than i and equal to or larger than 1 and equal to or smaller than N, and eta norm [ phi (i, j) ], wherein eta norm [ Inorm (x, y) ], (i-1) x delta x is equal to or smaller than i multiplied by delta x, and (j-1) x delta y is equal to or smaller than j multiplied by delta y, wherein the delta x and the delta y respectively represent the widths of the sub-regions along the x direction and the y direction, and a mean () function represents the averaging operation.
(4) The grating coding device is divided into M multiplied by N sub-regions, and the selection reference of the region sizes delta x and delta y is determined according to the shaping requirement and the processing technology condition. The reticle direction of the grating in each sub-region is an independent variable, that is, the azimuth angle phi shown in fig. 3 can be coded according to the region, an angle coding design target phi (i, j) of a grating coding device is set, i is greater than or equal to 1 and less than or equal to M, j is greater than or equal to 1 and less than or equal to N, and eta norm [ phi (i, j) ], i-1 × Δ x is less than or equal to i × Δ x, (j-1) × Δ y is less than or equal to j × Δ y, and a mean () function represents the averaging operation.
In other embodiments, when the grating coding device is coded in a grating etching area coding mode, the method comprises the following steps:
(1) and optimally designing parameters such as grating period d, grating groove depth h, duty ratio f and the like, so that when the light beam is incident along the Litrrow angle, the 0-order diffraction efficiency of the TE polarization component is 0.
(2) The spatial light intensity distribution of the light beam and the expected output of the light beam are respectively recorded as Iin (x, y) and Iout (x, y), and for the effective area of Iin (x, y) >0, the 0-order diffraction light beam transmittance distribution design target of the grating coding device is as follows: itar (x, y) is normalized to Iout (x, y)/Iin (x, y), and is denoted as Inorm (x, y).
(3) The grating coding device is divided into M multiplied by N sub-regions, and the selection reference of the region sizes delta x and delta y is determined according to the shaping requirement and the processing technology condition. The etching area of the grating in each sub-area is an independent variable, the area of the grating coding device codes a design target S (i, j), i is greater than or equal to 1 and less than or equal to M, j is greater than or equal to 1 and less than or equal to N, then S (i, j)/(Δ x × Δ y) is 1-mean [ Inorm (x, y) ], (i-1) x Δ x is less than or equal to i × Δ x, (j-1) x Δ y is less than or equal to j × Δ y, and the mean () function represents the averaging operation.
Example two:
parts of this embodiment that are the same as those of the first embodiment are not described again, except that:
the light beam is 75X 75mm2The light beam is a 14-order super-Gaussian light beam with the aperture of 70 x 70mm2(the aperture cut-off point of the beam is 10% of the light intensity point), the softening factor is 10%, and the wavelength is 1053 nm. The substrate material selected by the grating coding device is fused quartz.
And coding the grating coding device by adopting a grating etching angle coding mode, and shaping the near field of the light beam. Under the condition of plane diffraction, the optimal design obtains the optimal design result of a group of grating structures: d-597.8 nm, h-1313 nm, and f-0.5, the above parameters are defined as shown in fig. 3. By adopting a strict coupled wave theory, a mapping curve between the encoding angle and the diffraction efficiency corresponding to the 0-order diffracted beam is calculated under the condition of cone diffraction, as shown in fig. 5, the abscissa represents the encoding angle, and the ordinate represents the diffraction efficiency corresponding to the 0-order diffracted beam. Wherein, when the coding angle is 0 degrees, the diffraction efficiency corresponding to the 0-order diffraction light beam is 0.06 percent, and when the coding angle is 34.8 degrees, the diffraction efficiency corresponding to the 0-order diffraction light beam is 89.4 percent.
The encoding angle range is limited to [0 degrees, 34.8 degrees ], the encoding angle interval is 0.2 degrees, and a normalized mapping curve eta norm (phi) between the encoding angle and the diffraction efficiency can be established to obtain an angle encoding pattern of the grating encoding device, as shown in fig. 6. In fig. 6, the abscissa represents the long side of the grating code device, the ordinate represents the short side of the grating code device, the dimensions are mm, and the dimension of the angle code design result is degree.
And an optical filtering system is connected subsequently, only the low-frequency component of the 0-order diffracted beam is reserved, and the near-field shaping of the beam is realized. The focal lengths of the first lens and the second lens selected in the 4f filtering system are 1200mm, the aperture of the light transmission is 120mm, the small filtering hole is located on the frequency spectrum surface of the 4f system, the aperture size is 30 times of the diffraction limit, and the light beam near-field shaping effect is finally achieved, as shown in fig. 7, the relative error between the small filtering hole and the light beam near field expected to be output is shown in fig. 8, the relative error PV value and the relative error RMS value are 1.32% and 0.16% respectively, and therefore the ideal near-field shaping effect is achieved.
Example three:
parts of this embodiment that are the same as those of the first embodiment are not described again, except that:
the light beam is 75X 75mm2The expected output light beam is a binary coded light beam with the wavelength of 1053 nm. The substrate material selected by the grating coding device is fused quartz.
And coding the grating coding device by adopting a grating etching area coding mode, and shaping the near field of the light beam. Under the condition of plane diffraction, the optimal design obtains the optimal design result of a group of grating structures: d-597.8 nm, h-1313 nm, and f-0.5, the above parameters are defined as shown in fig. 2. The linearly polarized light beam is limited to be TE wave, if the light beam is not TE wave, the polarization state conversion is needed before shaping, or the TE wave is analyzed and polarized at the output end of the light beam. As shown in fig. 10, the abscissa represents the long side of the grating encoder, the ordinate represents the short side of the grating encoder, the dimensions are mm, and the area code design value represents the area ratio of the grating etching region in the corresponding sub-region.
And an optical filtering system is connected subsequently, only the low-frequency component of the 0-order diffracted beam is reserved, and the near-field shaping of the beam is realized. The focal lengths of the first lens and the second lens selected in the 4f filtering system are 1200mm, the clear aperture is 120mm, the small filtering hole is located on the frequency spectrum surface of the 4f system, the aperture size is 80 times of the diffraction limit, the finally realized near-field shaping effect of the light beam is shown in fig. 11, the corresponding signal-to-noise ratio is about 14.8dB, and the ideal near-field shaping effect is obtained on the whole. The error between the finally realized near-field shaping effect of the light beam and the near-field of the light beam desired to be output, as shown in fig. 12, is concentrated at the position of the abrupt change of the intensity of the near-field image of the desired output light beam due to the fact that the high-frequency components of the light field in the above region are relatively more and are shielded by the optical filtering system, and the shaped image is distorted in the corresponding region. And under the condition that the condition allows, the aperture of the filter aperture is further increased, and the near-field shaping effect of the light beam can be improved.
The present invention has been described in detail, and it should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
Claims (8)
1. A light beam near-field shaping method based on guided mode resonance sub-wavelength grating coding is characterized by comprising the following steps:
s1: a grating coding device is arranged in a transmission light path of the light beam in an off-axis mode, the grating coding device is coded according to the expected output of the shaped light beam, the light beam is split into a 0-order diffraction light beam and a-1-order diffraction light beam after being transmitted by the grating coding device, wherein the 0-order diffraction light beam is the expected output of the shaped light beam, and the light beam is linearly polarized light;
s2: and the high-frequency component of the 0-order diffraction light beam is filtered by an optical filtering system, so that the near-field shaping of the light beam is realized.
2. The near-field beam shaping method according to claim 1, wherein the grating encoder comprises a substrate and a plurality of gratings on the substrate, the gratings are rectangular gratings, and the ridge regions of the gratings and the substrate are made of the same dielectric material.
3. A near-field beam shaping method as claimed in claim 2, wherein in step S1, to ensure that there are only two diffraction orders of 0-1 order after the light beam is transmitted through the grating encoder, the incident angle of the light beam to the grating is determined
The requirements are as follows:
4. A near-field optical beam shaping method as claimed in claim 3, wherein two guided modes always exist for the TE polarization component in the optical beam under different duty cycles f, then:
wherein k is0＝2π/λ，nb、ngRespectively representing the refractive index of the grating ridge and the groove region,
the corresponding mode is a guide mode, and the equivalent fold corresponding to the two existing guide modesRefractive index is recorded
5. The near-field beam shaping method according to claim 4, wherein the accumulated phase difference of the two guided modes in the TE polarization component after propagating through the grating satisfies the following formula:
6. A near-field beam shaping method according to claim 5, wherein in step S1, the method for encoding the grating encoding device by using an area encoding method includes the following steps:
(1) the spatial light intensity distribution of the light beam and the expected output of the light beam are respectively recorded as Iin (x, y) and Iout (x, y), and for the effective area of Iin (x, y) >0, the 0-order diffraction light beam transmittance distribution design target of the grating coding device is as follows: itar (x, y) is normalized to Iout (x, y)/Iin (x, y), and is denoted as Inorm (x, y);
(2) dividing a raster coding device into M multiplied by N sub-regions, wherein the area of the raster coding device codes a design target S (i, j), i is more than or equal to 1 and less than or equal to M, and j is more than or equal to 1 and less than or equal to N, then S (i, j)/(delta x delta y) is 1-mean [ Inorm (x, y) ], (i-1) x delta x is less than or equal to i multiplied by delta x, (j-1) x delta y is less than or equal to j multiplied by delta y, and delta x and delta y respectively represent the sizes of the sub-regions, and a mean () function represents the averaging operation.
7. A near-field beam shaping method according to claim 5, wherein in step S1, the grating encoding device is encoded by a grating etching angle encoding method, and the-1 st order diffracted beam exits along a certain cone surface, comprising the following steps:
(1) setting the grating direction and lightThe grating strip direction and the grating normal direction are respectively an x axis, a y axis and a z axis, the included angle between the light beam incidence direction and the x-z plane is phi, and the included angle between the light beam incidence direction and the z axis is
Angle between electric field component of light beam and incident surface
Calculating the diffraction efficiency corresponding to 0-order diffracted beams under different encoding angle phi values by adopting a strict coupled wave theory, wherein phi belongs to [0 DEG, 90 DEG °]Establishing a normalized mapping curve eta norm (phi) between the encoding angle and the diffraction efficiency, wherein eta is more than or equal to 0 and less than or equal to 1;
(2) the spatial light intensity distribution of the light beam and the expected output of the light beam are respectively recorded as Iin (x, y) and Iout (x, y), and for the effective area of Iin (x, y) >0, the 0-order diffraction light beam transmittance distribution design target of the grating coding device is as follows: itar (x, y) is normalized to Iout (x, y)/Iin (x, y), and is denoted as Inorm (x, y);
(3) dividing a grating coding device into M multiplied by N sub-regions, wherein an angle coding design target phi (i, j) of the grating coding device is equal to or larger than 1 and equal to or smaller than i and equal to or larger than 1 and equal to or smaller than N, and eta norm [ phi (i, j) ], wherein eta norm [ Inorm (x, y) ], (i-1) x delta x is equal to or smaller than i multiplied by delta x, and (j-1) x delta y is equal to or smaller than j multiplied by delta y, wherein the delta x and the delta y respectively represent the widths of the sub-regions along the x direction and the y direction, and a mean () function represents the averaging operation.
8. The near-field beam shaping method as claimed in any one of claims 1 to 7, wherein the optical filter system is a 4f filter system.
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