A device for high-resolution imaging in a wide spectrum

Abstract

The present invention discloses a high-resolution imaging device for a wide spectral band. The imaging device adopts a pupil filter. The structure of the pupil filter is a parallel flat glass. The substrate material is a transmission optical material and is placed at the pupil surface of the optical system. Different film layers are coated on the front and back surfaces of the flat glass. The present invention is based on mature coating work, does not involve phase changes, has a simple structure, is easy to manufacture, has low cost, and does not require micro-nano processing technology. The filter has universality and is suitable for super-resolution imaging of different spectral bands.

Description

Technical Field

[0001] The present invention relates to a high-resolution imaging device for a wide spectrum band, which can effectively improve the acquisition capability of high-frequency information of an optical system and is suitable for imaging in a wide spectrum band and a large field of view. Background Art

[0002] Pupil filtering technology is the core branch of super-resolution imaging technology. As early as 1952, Toraldo first introduced the concept of limited field of view super-diffraction limit resolution into the field of optical imaging. Due to the objective size limitation of the physical aperture, any optical system can be regarded as a low-pass filter that transmits limited bandwidth spatial frequency information. The pupil filter modulates the transfer function of the optical system in the frequency domain by changing the amplitude or phase of the light field in the pupil plane of the optical system, thereby effectively improving the focused light field and imaging performance in the spatial domain. This is the basic working principle of the pupil filter. Depending on the modulation object, pupil filters are divided into pure amplitude type, pure phase type and complex amplitude type. Pupil filtering can achieve a variety of modulation effects. It is currently mainly used in super-resolution imaging to compress the main lobe of the focused light spot and form a super-resolution focused diffraction light spot. It is widely used in confocal scanning microscopy. In addition, pupil filtering can also be used in laser confinement fusion, micromachining, high-density storage, extended depth of focus microscopes, fluorescence microscopy and retinal imaging.

[0003] At present, most of the research on pupil filters is focused on coherent and partially coherent imaging systems illuminated by monochromatic light, and only the degree of compression of the main spot is concerned, and the influence of the transfer function has not been considered. The research on pure phase-type pupil filters is the most, and various types of phase-type pupil filters have been designed and analyzed. The prior art is to use phase-type pupil filters to achieve super-resolution. However, the super-resolution imaging achieved by pupil filtering is a point-to-point imaging, and is accompanied by an increase in the intensity of the side lobes, which is contrary to the expectations of large-field super-resolution imaging. In addition, phase-type super-resolution diffraction elements are all made using binary optical processes, which have a strong dependence on wavelength and are not suitable for wide-band imaging systems. In order to solve this technical problem, the industry has proposed a technical concept based on concave imaging and proposed a large-field, local super-resolution imaging system. The system scans the image area of ​​interest point by point, and reconstructs the local super-resolution image and merges it with the low-resolution image of the large field of view, and finally achieves large-field, local super-resolution imaging. However, this method has low imaging efficiency and sacrifices time resolution to improve spatial resolution, so its application is also limited. Summary of the invention

[0004] The technical problem solved by the present invention is: a device for high-resolution imaging in a wide spectral band is proposed, which can achieve the effect of contrast enhancement at high frequencies in the wide spectral band, and overcome the limitation that conventional pupil filters cannot be used for wide-field-of-view wide-spectral-band imaging.

[0005] The technical solution of the present invention is: a high-resolution imaging device for a wide spectrum, wherein the imaging device adopts a pupil filter, the structure of the pupil filter is a parallel flat glass, the substrate material is a transmissive optical material, and is placed at the pupil surface position of the optical system; different film layers are coated on the front and back surfaces of the flat glass.

[0006] The aperture size and shape of the flat glass are consistent with the pupil of the optical system.

[0007] The front surface of the parallel plate is plated with a bandpass filter film.

[0008] The bandpass filter film on the front surface divides the surface into a number of concentric ring bands, with the geometric center of the plate as the center of the circle, and different transmission ring bands are arranged in sequence along the radius outward, and the spectral width of each ring band gradually widens from the inside to the outside along the radius direction; the outermost ring is the full spectral width of the working spectrum of the optical system, and the spectral increase is on the order of ten nanometers.

[0009] The transmittance of each ring zone is greater than 93%.

[0010] The transmittance of each ring zone increases from the inside to the outside along the radial direction, showing a discrete gradient distribution with low transmittance in the center and high transmittance at the edge.

[0011] The transition area between each annular filter film is less than 0.25mm; the steepness of the filter film is less than 28nm.

[0012] The rear surface of the parallel plate is coated with an anti-reflection film to ensure that the transmittance is greater than 95% within the working spectrum range.

[0013] After coating the front and rear surfaces of the parallel plate, in the design of the film layer, in addition to ensuring the film layer spectral bandwidth and transmittance requirements, the total thickness of the film layer must also be constrained to ensure that the difference in the maximum total optical thickness of each ring zone film layer is less than 30nm.

[0014] The working spectrum is ultraviolet light, visible light or infrared light.

[0015] The advantages of the present invention compared with the prior art are:

[0016] (1) The present invention provides a spectrum and amplitude regulated annular filter, which realizes modulation of the spectrum and transmittance of different annular bands at the pupil through special film design to achieve super-resolution imaging. The annular pupil filter divides the surface into a number of concentric annular bands, and the spectral width of each annular band gradually widens from the inside to the outside. The spectrum increase is about tens of nanometers, which is specifically set according to the super-resolution characteristic requirements.

[0017] (2) The present invention provides a spectrum and amplitude regulated annular filter, which can achieve contrast enhancement in a wide spectrum and a large field of view at high frequencies. It overcomes the limitation that conventional pupil filters based on micro-nano processing technology are only applicable to imaging in small fields of view and narrow-band spectrum. The filter is based on mature coating work, does not involve phase changes, has a simple structure, is easy to manufacture, is low-cost, and does not require micro-nano processing technology. The filter is universal and suitable for super-resolution imaging in different spectrum bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of the structure of a visible light optical system used in the present invention;

[0019] Figure 2 It is a schematic diagram of the structure of the multi-ring partition pupil filter of the present invention;

[0020] Figure 3 It is a schematic diagram of the film characteristics of the filter of the present invention;

[0021] Figure 4 Schematic diagram of the comparison of the optical MTF of the camera before and after the pupil filter of the present invention is added. DETAILED DESCRIPTION

[0022] According to Abbe imaging theory, the optical system can be equivalent to a pupil plane, and the modulation of the object light wave can be expressed by a generalized pupil function. The pupil filter modulates the transfer function of the optical system in the frequency domain by changing the amplitude or phase of the light field in the pupil plane of the optical system, thereby achieving effective improvement of the focused light field and imaging performance in the spatial domain. This is the basic working principle of the pupil filter.

[0023] The general expression of the multi-ring partition pupil function P is:

[0024]

[0025] Number of rings n, radius of rings σ i , spectral bandwidth and transmittance of each ring band as an adjustment amount.

[0026] The structure of the pupil filter of the present invention is a parallel flat glass, the substrate material can be a transmission optical material, and it is placed at the pupil surface position of the optical system to realize ultra-high resolution imaging.

[0027] The aperture size and shape of the flat glass are consistent with the pupil (entrance pupil, aperture stop, exit pupil) of the optical system; and different film layers are coated on the front and back surfaces of the flat glass.

[0028] The front surface of the parallel plate is coated with a special filter film, which divides the surface into a number of concentric rings. With the center origin as the center of the circle, different transmission rings are sequentially arranged along the radius outward, and the bandpass spectrum width of each ring gradually widens from the inside to the outside along the radius, and the outermost ring is the full spectrum width of the working spectrum of the optical system; the transmittance of each ring can be uniform, all greater than 93%, or can be sequentially increased from the inside to the outside along the radius, presenting a discrete gradient distribution with low center transmittance and high edge transmittance. The transition zone of each ring is less than 0.25mm; the steepness is less than 28nm; the band spectrum characteristics and amplitude transmittance characteristics of the ring can be achieved by coating the bandpass filter film process.

[0029] The rear surface of the parallel plate is coated with an anti-reflection film, and the transmittance is required to be greater than 95% within the working spectrum range.

[0030] In addition, when designing the film layers on the front and back surfaces, the maximum total optical thickness difference of each ring is guaranteed to be less than 30nm, reducing the impact of the introduced discrete phase difference on the image quality.

[0031] The working spectrum can be ultraviolet light, visible light or infrared light.

[0032] The present invention will be further described in detail below with reference to the accompanying drawings.

[0033] Take a coaxial three-mirror optical system visible light optical system as an example, Figure 1 As shown. The main parameters of the lens composed of primary and secondary three mirrors are aperture: 444mm; focal length: 4600mm, field of view: 0.3×0.12°, and designed average transfer function 0.176@119lp / mm (close to the diffraction limit). There is a real exit pupil of the system after the three mirrors, with an aperture of 43mm and a working spectrum of 0.45~0.7um.

[0034] According to the optical focal length, aperture, spectrum, exit pupil size and other parameters, the pupil filter parameters are designed, such as Figure 2 As shown. The pupil filter uses a parallel plate made of fused quartz material, with an aperture of 47mm, r0=7.5mm, r1=10.75mm, and r2=14mm. Different bandpass films are plated in different areas. The coating requirements are as follows: Area A: normalized 0~r0 circular domain, blackened, no coating required; Area B: normalized r0~r1 circular domain, bandpass filter film, bandpass spectrum range 0.45~0.53um; Area C: normalized r1~r2 annular domain, bandpass filter film, bandpass spectrum range 0.45~0.61um; Area D: normalized r2~1 annular domain, bandpass filter film, bandpass spectrum range 0.45~0.7um; normalized according to the aperture diameter of 43mm, r0=7.5mm, r1=10.75mm, r2=14mm before normalization.

[0035] In particular, in terms of coating design and process, we have conducted a variety of process improvement studies to achieve a transition zone of each ring band of less than 0.25mm and an angular drift of less than 5nm. At the same time, considering the influence of wavefront aberration caused by the thickness difference of the film layer in different regions, we added the constraint of optical path in the film layer design stage to control the difference of the maximum optical thickness nd of each ring band to less than 15nm. The measured spectrum curve of the film layer is shown below Figure 3 .

[0036] After adding the pupil filter, the camera's optical transfer function in the high frequency band is significantly improved from 0.176 at 119 line pairs / mm before adjustment to 0.21. It can be seen that this type of pupil filtering has a significant effect on improving the medium and high frequency spatial frequencies, and can be applied to space cameras to achieve small-aperture high-resolution imaging capabilities.

[0037] Contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art. Although the structural drawings describe the specific implementation of the present invention, it is clear to those skilled in the art that several modifications, substitutions and improvements can be made without departing from the invention, and these are also considered to be within the scope of protection of the present invention.

Claims

1. A device for high-resolution imaging in a wide spectral range, characterized in that: The imaging device adopts a pupil filter, the structure of which is a parallel flat glass, the substrate material is a transmissive optical material, and is placed at the pupil plane of the optical system; the front and back surfaces of the flat glass are coated with different film layers; The front surface of the parallel plate is plated with a bandpass filter film; The bandpass filter film on the front surface divides the surface into a number of concentric rings, with the geometric center of the plate as the center, and different transmission rings are arranged in sequence along the radius outward, and the spectral width of each ring gradually widens from the inside to the outside along the radius direction; the outermost ring is the full spectral width of the working spectrum of the optical system, and the spectral increase is in the order of ten nanometers; The transmittance of each ring zone increases from the inside to the outside along the radial direction, showing a discrete gradient distribution with low transmittance in the center and high transmittance at the edge. The transition area between the annular filter films is less than 0.25mm; the steepness of the filter films is less than 28nm; The rear surface of the parallel plate is coated with an anti-reflection film to ensure that the transmittance is greater than 95% within the working spectrum range.

2. A device for wide-band high-resolution imaging according to claim 1, characterized in that: The aperture size and shape of the flat glass are consistent with the pupil of the optical system.

3. A device for wide-band high-resolution imaging according to claim 1, characterized in that: The transmittance of each ring zone is greater than 93%.

4. The device for wide-band high-resolution imaging according to claim 1, characterized in that: After the front and rear surfaces of the parallel plate are coated, it is ensured that the difference in the maximum total optical thickness of each ring zone film layer is less than 30nm.

5. A device for wide-band high-resolution imaging according to any one of claims 1 to 4, characterized in that: The working spectrum is ultraviolet light, visible light or infrared light.

Images