CN216791399U - Immersed imaging spectrometer based on aberration-eliminating holographic grating - Google Patents
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Abstract
The utility model relates to an immersion type imaging spectrometer based on an aberration-eliminating holographic grating. The optical fiber holographic imaging system sequentially comprises an incident slit, a high-refractive-index medium, a spherical reflector, an aberration-eliminating convex holographic grating and an imaging sensor according to the incident direction of light; the optical path of the optical system is immersed in a high-refractive-index medium, the spherical reflector and the aberration-eliminating convex holographic grating are coaxial, the inner surface of the spherical reflector is a spherical surface bent towards the incident direction of light rays, and the incident slit and the imaging sensor are positioned on the same side of the inner surface of the spherical reflector; the length direction of the incident slit is parallel to the scribing direction of the aberration-eliminating convex holographic grating and is vertical to the optical axis. The imaging spectrum system has the characteristics of wide field of view, high resolution and low astigmatism, well controls the full-wave-band full-field aberration, has high light energy utilization rate and low spectral bending and color distortion, meets the application requirements of remote sensing detection with wide field of view, high resolution and compact volume, is easy to install and adjust, and has wide application prospect.
Description
Technical Field
The utility model belongs to the technical field of spectral imaging, and particularly relates to an immersion type imaging spectrometer based on an aberration-eliminating holographic grating.
Background
The hyperspectral imaging technology can acquire image information and high-resolution spectral information of a target scene, and substance identification and component analysis can be performed through acquired spectral data, so that the hyperspectral imaging technology has wide application requirements in the fields of forestry resource investigation, fire early warning, water resource pollution, mineral exploration and the like. The convex holographic grating imaging spectrum system is based on an Offner concentric structure, has good symmetry, good optical performance and simple and compact structure, and meets the application requirements of light and small size.
One of the core parameters of an imaging spectrometer is the entrance slit length, the longer the slit, the larger the field of view of the system. With the increasing demand for wide ground coverage in various application fields, the development of the imaging spectrometer spectroscopic system in the direction of a long slit is required. However, astigmatism is the most dominant geometrical aberration when the entrance slit is long, since the perfect symmetry of the Offner structure is destroyed by the diffractive splitting of the convex holographic grating. In order to realize the balance of astigmatism caused by asymmetry of the system, the technical means mainly adopted is to divide the main mirror into two parts to optimize the curvature radius of the main mirror, increase the off-axis amount of the two reflecting mirrors and increase the inclination of the convex grating so as to eliminate the astigmatism. In addition, the introduction of the meniscus lens or the free-form surface into the Offner structure can also greatly improve the imaging condition of the system, but increases the difficulty and cost of the development of the optical system, makes the processing of the optical element and the adjustment and spectral calibration of the optical system more difficult, and adds additional load to the system.
In the prior art, document "astigmatism elimination design of variable-pitch convex grating imaging spectrum system" (optical precision engineering, 2020, vol.28, No. 10, zhao mei hong) reports an aberration elimination convex grating imaging spectrum system based on a variable-pitch convex grating, wherein the F number of the aberration elimination convex grating imaging spectrum system is 2.7, the slit length is 10mm, the spectral resolution is 3.3nm, the system length is 100mm, the size is large, and the recording optical path of the aberration elimination convex grating is complicated.
Disclosure of Invention
Aiming at the defects in the prior art, the utility model provides the immersion type imaging spectrometer which is light, small, simple and compact in structure, long in entrance slit and high in image quality and based on the aberration-eliminating holographic grating.
In order to realize the purpose of the utility model of the utility model, the adopted technical scheme is to provide an immersion type imaging spectrometer based on the aberration-eliminating holographic grating, and an optical system of the immersion type imaging spectrometer sequentially comprises an incident slit, a high refractive index medium, a spherical reflector, an aberration-eliminating convex holographic grating and an imaging sensor according to the incident direction of light; the optical path of the optical system is immersed in a high-refractive-index medium, and the refractive index n of the high-refractive-index medium is more than or equal to 1.2 and less than or equal to 1.7; the spherical reflector and the aberration-eliminating convex holographic grating are coaxial, the inner surface of the spherical reflector is a spherical surface bending to the incident direction of light, and the incident slit and the imaging sensor are positioned on the same side of the inner surface of the spherical reflector; the length direction of the incident slit is parallel to the reticle direction of the aberration-eliminating convex holographic grating and is perpendicular to the optical axis.
The utility model provides an immersion type imaging spectrometer based on an aberration-eliminating holographic grating, wherein the value range of the F number of an optical system is more than or equal to 1.5 and less than or equal to 5; the ratio of the length S of the entrance slit to the total length L of the system is more than or equal to 0.15 and less than or equal to 0.4.
The utility model provides an immersion type imaging spectrometer based on an aberration-eliminating holographic grating, wherein the ratio K of the curvature radius of a spherical reflector to the curvature radius of an aberration-eliminating convex holographic grating is more than or equal to 1.8 and less than or equal to 2.2; the grating groove of the aberration-eliminating convex holographic grating is in a curve shape with variable intervals.
The utility model provides an immersion type imaging spectrometer based on an aberration-eliminating holographic grating, wherein an optical system is immersed in a high-refractive-index medium, light rays obtained by a long incident slit pass through the high-refractive-index medium and are reflected by a partial reflecting surface in a spherical reflector, light beams are converged and incident on the aberration-eliminating convex holographic grating, the geometric aberration of a holographic aberration compensation system is utilized, divergent light beams split by the grating are reflected by the other partial reflecting surface of the spherical reflector again and are converged on an imaging sensor, and images with wide field of view, high image quality and high spectral resolution are obtained. The utility model utilizes the inherent geometric aberration, especially astigmatism, of the holographic aberration compensation system and adopts an immersion type light path, thereby realizing a long entrance slit under the limitation of compact volume and meeting the application requirements of wide field of view and high spectral resolution.
The principle of the utility model is as follows: the aberration-eliminating convex holographic grating not only has a dispersion effect, but also can compensate geometric aberration introduced by using a structure by utilizing self holographic aberration. In the conventional Offner-type spectral imaging system, since the diffraction beam splitting of the grating destroys the perfect symmetry of the Offner structure, there is large astigmatism, and it is difficult to realize a long entrance slit. The aberration-eliminating holographic convex surface grating is applied to the Offner imaging spectrum system, the geometric aberration of the holographic aberration compensation system of the aberration-eliminating holographic convex surface grating is utilized, the recording light path is simple, and meanwhile, the immersed light path is adopted, so that the system volume is further reduced, and the light and small imaging spectrum system, the simple and compact structure and the high-image-quality imaging are realized at the same time.
The aberration-eliminating holographic convex grating is manufactured by using spherical waves to perform interference exposure on the grating, so that different points on the surface of a grating substrate are exposed at different interference angles to form a curved-groove variable-pitch grating, aberration coefficients are optimized through an optical path function theory, minimum values are optimized and solved by taking the aberration coefficients as objective functions, reasonable recording parameters are further selected, and the holographic aberration is used for compensating inherent geometric aberration, particularly astigmatism, in the offner structure.
The whole system is immersed in a high-refractive-index medium, light rays obtained by the long incidence slit are reflected by the spherical reflector after passing through the high-refractive-index immersion medium, light beams converge and enter the aberration-eliminating convex holographic grating, the inherent geometric aberration of the system is compensated by utilizing the holographic aberration of the spherical reflector, divergent light beams split by the grating are converged on the imaging sensor after being reflected by the spherical reflector again, and an image with low astigmatism, wide field of view and high spectral resolution is obtained. The utility model corrects astigmatism by utilizing holographic aberration and adopts an immersion type light path, thereby realizing the realization of a long entrance slit in a compact volume and meeting the application requirements of wide field of view and high spectral resolution.
Compared with the prior art, the utility model has the beneficial effects that:
1. compared with most of the existing convex grating imaging spectrometers, the imaging performance of the spectrometer can be effectively improved while the advantages of simple and compact original Offner optical structure, easiness in adjustment, light and small realization and the like are kept.
2. The convex holographic grating is a core part of an Offner imaging spectrometer light splitting system, and has the advantages of no ghost line and little stray light compared with a mechanical engraving grating.
3. When the convex holographic grating is designed, an optimization method taking each aberration coefficient as a target function is established by utilizing an optical path function theory, the positions of two beams of recording spherical wave sources are analyzed and calculated, and the convex holographic grating with the aberration correction capability can be designed. The utility model compensates the off-axis aberration of the system by utilizing the holographic aberration of the convex grating, thereby reducing the influence of the off-axis quantity on the image quality, being simple and effective, not increasing the extra load and the adjusting difficulty of the system, and better realizing the high imaging performance of the optical system and the light and small size of the system.
Drawings
FIG. 1 is a schematic diagram of an optical system of an imaging spectrometer according to an embodiment of the present invention;
FIG. 2 is a recording optical path of an aberration-eliminating holographic grating of an imaging spectrometer provided by an embodiment of the utility model;
FIG. 3 is a graph of a full field full band transfer function curve MTF of the imaging spectrometer provided by the embodiment of the present invention;
FIG. 4 is a plot of the RMS spot radius for the full operating band for the full field of view provided by an embodiment of the present invention.
In the figure, 1. an entrance slit; 2. a high refractive index medium; 3. a spherical reflector; 4. an aberration-eliminating convex holographic grating; 5. an imaging sensor.
Detailed Description
The technical scheme of the utility model is further explained by combining the drawings and the embodiment.
Example 1
Referring to fig. 1, it is a schematic structural diagram of an optical system of an immersion imaging spectrometer based on an aberration-eliminating holographic grating according to this embodiment. According to the incident direction of light, the optical elements of the optical system are an incident slit 1, a high-refractive-index medium 2, a spherical reflector 3, an aberration-eliminating convex holographic grating 4 and an imaging sensor 5 in sequence; the optical path of the optical system is immersed in a high-refractive-index medium, and the refractive index n of the high-refractive-index medium is more than or equal to 1.2 and less than or equal to 1.7; the spherical reflector and the aberration-eliminating convex holographic grating are coaxial, the inner surface of the spherical reflector is a spherical surface bending to the incident direction of light, and the incident slit and the imaging sensor are positioned on the same side of the inner surface of the spherical reflector; the length direction of the incident slit is the same as the reticle direction of the aberration-eliminating convex holographic grating and is perpendicular to the optical axis.
In the embodiment, the value range of the F number of the optical system is that F is more than or equal to 1.5 and less than or equal to 5; the total length L of the optical system is more than or equal to 30mm and less than or equal to 50 mm; the ratio of the length S of the entrance slit to the total length L of the system is more than or equal to 0.15 and less than or equal to 0.4; the curvature radius ratio K of the spherical reflector and the aberration-eliminating convex holographic grating is more than or equal to 1.8 and less than or equal to 2.2.
The specific parameters of the imaging spectrometer provided in this embodiment satisfy the conditions in table 1.
TABLE 1
In this example, each optical element (surface) satisfies the conditions shown in table 2.
Table 2:
in the imaging spectrometer provided by this embodiment, the optical system is immersed in a high refractive index medium, light obtained by the long entrance slit passes through the high refractive index medium and is reflected by a partial reflecting surface in the spherical mirror, the light beam is converged and incident on the aberration-eliminating convex holographic grating, the geometric aberration of the holographic aberration compensation system is utilized, and the divergent light beam split by the grating is reflected by another partial reflecting surface of the spherical mirror again and then converged on the imaging sensor, so as to obtain an image with a wide field of view, high image quality and high spectral resolution. The utility model utilizes the inherent geometric aberration, especially astigmatism, of the holographic aberration compensation system and adopts an immersion type light path, thereby realizing a long entrance slit under the limitation of compact volume and meeting the application requirements of wide field of view and high spectral resolution.
Referring to FIG. 2, it is a recording optical path diagram of the aberration-eliminating holographic grating of the imaging spectrometer provided in this embodiment, in the figure, the intersection point of the grating and the optical axis is O, R0The curvature radius of the grating is defined by the recording point C, DδAndγrecording the angleδTo be positive, record the angleγIs negative. The method adopts a lensless Fourier transform holographic method, uses two divergent coherent beams to record the convex grating, and the position of the point light source is positioned on the different side of the spherical center of the grating substrate and is asymmetric about the normal of the vertex of the grating substrate.
The effective grating constant of the aberration-eliminating convex holographic grating is as follows:
wherein,λ 0for recording the wavelength, the present embodiment employs a light source with a wavelength of 413 nm; angle between recording point and optical axisδ=3.12°，γ=-9.34°。
The expression F for the optical path function of the system is expressed as:
and performing power series expansion on y and z:
the aberration coefficients have a well-defined physical meaning, F02Represents astigmatism, F30Represents meridional coma, F12Representing sagittal coma, higher order aberration terms have less effect on imaging quality.
Wherein:
wherein,λis the wavelength used by the imaging spectrometer, m is the diffraction order, rCAnd rDTo record arm length, R0Is the radius of curvature of the grating in the offner structure.
And (3) taking each aberration coefficient as an objective function, and optimizing and solving a minimum value:
where m is the number of optimized aberration terms, n is the number of wavelengths sampled,
the optimized weights for the different wavelengths are,
corresponding to the optimized weight of the aberration coefficient.
Referring to fig. 3, it is a graph of the full-band transfer function MTF of the imaging spectrometer provided in this embodiment; in the figure, (a), (b) and (c) are all field transfer function MTF curves of the imaging spectrometer provided by the embodiment at the image plane corresponding to the wavelengths of 0.4 μm, 0.6 μm and 0.8 μm respectively. As can be seen from FIG. 3, the optical transfer functions of the full field of view of the working waveband from 0.4 μm to 0.8 μm under 120lp/mm are all greater than 0.5, close to the diffraction limit, and the curve is smooth and compact, which indicates that the system has clear and uniform imaging and has good imaging quality in the full waveband and the full field of view.
Referring to fig. 4, it is a graph of RMS spot radius in the full field of view full operating band of the imaging spectrometer provided in this embodiment. Curve (a) is the RMS radius of the full field full operating band and curve (b) is the RMS radius of the full operating band at the diffraction limit. As can be seen from FIG. 4, in the full-field full-operating band, the RMS root-mean-square spot radius of the system is less than 1.5 μm and less than the diffraction limit RMS radius, and the energy is concentrated, thereby meeting the use requirements.
The result proves that the immersion type imaging spectrometer based on the aberration-eliminating holographic grating provided by the utility model has the advantages that on the premise of ensuring the compact structure, the working F number can reach 3, the slit length can reach 8mm, the sampling frequency is 120lp/mm, the optical transfer functions of the full working waveband and the full field of view are both more than 0.5, the astigmatism, the spectral curvature and the color distortion are extremely low, and the requirements of the spectral imaging system on wide field of view, miniaturization and high resolution are met.
Claims (4)
1. An immersed imaging spectrometer based on an aberration-eliminating holographic grating is characterized in that: the optical system of the optical system sequentially comprises an incident slit (1), a high-refractive-index medium (2), a spherical reflector (3), an aberration-eliminating convex holographic grating (4) and an imaging sensor (5) according to the incident direction of light rays; the optical path of the optical system is immersed in a high-refractive-index medium, and the refractive index n of the high-refractive-index medium is more than or equal to 1.2 and less than or equal to 1.7; the spherical reflector (3) and the aberration-eliminating convex holographic grating (4) are coaxial, the inner surface of the spherical reflector is a spherical surface bending to the incident direction of light, and the incident slit (1) and the imaging sensor (5) are positioned on the same side of the inner surface of the spherical reflector (3); the length direction of the incident slit (1) is parallel to the line direction of the aberration-eliminating convex holographic grating (4) and is vertical to the optical axis.
2. The immersion imaging spectrometer based on the aberration-eliminating holographic grating as claimed in claim 1, wherein: the value range of the F number of the optical system is that F is more than or equal to 1.5 and less than or equal to 5.
3. The immersion imaging spectrometer based on the aberration-eliminating holographic grating as claimed in claim 1, wherein: the ratio of the length S of the entrance slit to the total length L of the system is more than or equal to 0.15 and less than or equal to 0.4.
4. The immersion imaging spectrometer based on the aberration-eliminating holographic grating as claimed in claim 1, wherein: the ratio K of the curvature radius of the spherical reflector to the curvature radius of the aberration-eliminating convex holographic grating is more than or equal to 1.8 and less than or equal to 2.2.
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