CN112327454A - Short-focal-length two-piece free-form surface f-theta objective lens - Google Patents
Short-focal-length two-piece free-form surface f-theta objective lens Download PDFInfo
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- CN112327454A CN112327454A CN202011311079.1A CN202011311079A CN112327454A CN 112327454 A CN112327454 A CN 112327454A CN 202011311079 A CN202011311079 A CN 202011311079A CN 112327454 A CN112327454 A CN 112327454A
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B13/00—Optical objectives specially designed for the purposes specified below
- G02B13/0005—Optical objectives specially designed for the purposes specified below having F-Theta characteristic
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B13/00—Optical objectives specially designed for the purposes specified below
- G02B13/18—Optical objectives specially designed for the purposes specified below with lenses having one or more non-spherical faces, e.g. for reducing geometrical aberration
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
- G02B26/10—Scanning systems
Abstract
The invention relates to a short-focal-length two-piece free-form surface f-theta objective lens, which comprises a first lens and a second lens, wherein the first lens is provided with a first optical surface and a second optical surface, and the second lens is provided with a third optical surface and a fourth optical surface; the first optical surface and the third optical surface are incident light surfaces, and the surface types of the two optical surfaces are aspheric surfaces; the second optical surface and the fourth optical surface are emergent light surfaces, the surface types of the two optical surfaces are free-form surfaces as a whole, the two optical surfaces are symmetrical aspheric surfaces in the sub-scanning direction, and the two optical surfaces are asymmetrical free-form surfaces in the main scanning direction; the laser is converged by the first lens and further converged by the second lens. The emergent light surfaces of the two lenses adopt free-form surfaces, so that the requirements of high resolution, large working surface and miniaturization of a scanning system are met.
Description
Technical Field
The invention belongs to the technical field of laser scanning devices, and particularly relates to a short-focal-length two-piece free-form surface f-theta objective lens.
Background
Laser technology was introduced in the early 60 s and was put into practical use in the early 70 s. Semiconductor lasers have been produced in the end of the 70 s as semiconductor technology has matured. Meanwhile, due to the maturity and mass production of the processing technology of the free-form curved surface, the laser printing technology enters the field of practical application.
The present invention is directed to the design of the overall optical portion of a laser printer. The F-theta lens is a core component of a laser printing system, and essentially the F-theta lens makes the image height linear proportional to the scan angle by introducing barrel distortion. With the increase of the working area, the key to design the F-theta lens is to obtain high-quality flat-field image points in a large image plane.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a short-focal-length two-piece type free-form surface f-theta objective lens which can meet the requirements of high resolution, large working surface and miniaturization of a scanning system and is used for a laser scanning device of a rotary polygon mirror.
The above object of the present invention is achieved by the following technical solutions:
a short-focal-length two-piece free-form surface f-theta objective lens is characterized in that: comprises a first lens and a second lens, wherein the first lens is provided with a first optical surface and a second optical surface, and the second lens is provided with a third optical surface and a fourth optical surface; the first optical surface and the third optical surface are incident light surfaces, and the surface types of the two optical surfaces are aspheric surfaces; the second optical surface and the fourth optical surface are emergent light surfaces, the surface types of the two optical surfaces are free-form surfaces as a whole, the two optical surfaces are symmetrical aspheric surfaces in the sub-scanning direction, and the two optical surfaces are asymmetrical free-form surfaces in the main scanning direction; the laser is converged by the first lens and further converged by the second lens.
Further: the asymmetric free-form surface design of the first lens and the second lens meets the following formula requirements:
the equation expression in the sub-scanning direction is:
wherein z is the vector height in the z-axis direction, k is the coefficient of the quadratic term, c is the surface curvature, r is the radial distance from the optical axis z, r is2＝x2+y2，αiThe aspheric surface deformation coefficient is a high-order term;
in the main scanning direction, the equation expression is:
in the formula, kx，kyCoefficients of quadratic terms in x-and y-directions, respectively, cx，cyIs the curvature of the surface in the x, y directions, alpha1，α2，α3… …, aspheric coefficient in x-direction, beta1，β2，β3… …, the aspheric coefficients in the y-direction.
Further: the combined focal length of the two lenses is 104.88mm, and the full scanning angle is 120 degrees.
Further: the positioning distance between the first optical surface and the fourth optical surface is 29.36mm, and the positioning distance between the second optical surface and the third optical surface is 19.83 mm.
The invention has the advantages and positive effects that:
1. the free-form surface design is adopted when the objective lens is designed, and the free-form surface is characterized by having obvious effect on correcting the off-axis aberration, and the f-theta lens has barrel-shaped distortion to enable the angle of a view field to be in linear relation with the scanning distance, so that the free-form surface is more suitable to use.
2. The scanning light rays of Gaussian beams reflected by a pentamirror can be subjected to distortion correction, namely, the relation of time-angular velocity is converted into the relation of time-distance, and the beam radius of the scanning light rays in the X direction and the Y direction passes through each angle of an f-theta lens to generate uniform light spots on an imaging surface so as to provide resolution meeting requirements; and the light-gathering distance can be shortened, so that the volume of the whole device is reduced, and the requirements of high resolution, large working surface and miniaturization are met.
3. The two f-theta lenses can correct the emergence angle from 0-degree scanning light to 120-degree scanning light, so that the two light spots of the two scanning lights at the same time interval on the imaging surface have the same distance and the light spots have uniform size, and a high-quality flat-field image point can be obtained.
Drawings
FIG. 1 is a diagram of a short focal length two-piece f-theta objective lens of the present invention;
FIG. 2 is a diagram of a short focal length laser scanning system of the present invention;
FIG. 3 is a position identifying diagram of a short focal length two-piece f-theta objective lens of the present invention;
FIG. 4 is a diagram of the diffuse speckle pattern of a short focal length two-piece f-theta objective lens according to the present invention;
FIG. 5 is a schematic diagram of the edge (scan maximum angle position) and center position diffuse spot size of the short-focal-length two-piece f-theta objective lens of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the following embodiments, which are illustrative only and not limiting, and the scope of the present invention is not limited thereby.
In order to facilitate understanding of the design of the present invention, the design principle and the design process of the present invention are described first: the invention aims to design a short-focal-length two-piece f-theta objective lens to meet the requirements of high resolution, large working surface and miniaturization.
1. High resolution
In order to improve the resolution of the system, the objective lens needs to have diffraction-limited focusing performance, and according to the rayleigh judgment theory, the diffraction limit airy spot radius δ of the circular aperture scanning system is as follows:
δ＝1.22λ×F<50μm (1)
F＝f′/D (2)
wherein, δ is the minimum spot radius of the scanning light on the selenium drum, λ is the working wavelength of the system, F is the aperture of the system, F' is the focal length of the objective lens, and D is the entrance pupil diameter of the objective lens. From equation (1), it can be seen that in order to obtain a scanning objective lens with high resolution, the value of F should be minimized for a given operating wavelength. High resolution can be achieved by either decreasing the f-theta objective focal length or increasing the entrance pupil diameter according to equation (2).
2. Large working surface and miniaturization
The working surface of the invention is 216mm, in order to meet the requirement of miniaturization, the scanning angle is increased as much as possible in the design process, the scanning half angle of the design is 60 degrees, and the invention belongs to an ultra-wide angle f-theta objective lens. The design difficulty of wide-angle f-theta objectives is that linear distortion is difficult to correct. The design utilizes the free curved surfaces of the second optical surface and the fourth optical surface to introduce barrel distortion to correct linear distortion, and finally realizes the design of the ultra-wide-angle f-theta objective lens.
3. In order to meet the requirements of high resolution, large working surface and miniaturization, the following researches are carried out:
the invention requires that the angle of the field of view is 120 degrees, the printing range is 216mm, the focal length is 104.88mm according to the F-theta formula, and the printing width values corresponding to different field angles can be calculated by the formula
H＝Fθ (3)
H is half width, F is focal length, theta is half view field angle (radian), and the image plane is equally divided into 60 view fields for control optimization according to calculation. The final diffuse spot is shown in figure 1, in order to correct the off-axis aberration caused by a large field of view, a free-form surface design is adopted when the objective lens is designed, and the free-form surface is characterized in that the effect of correcting the off-axis aberration is obvious, and the f-theta lens has barrel-shaped distortion to enable the angle of the field of view to be in linear relation with the scanning distance, so that the free-form surface is more suitable to be used.
The working surface of the system is a plane, and the focal powers of the lens 1 and the lens 2 of the two-piece f-theta objective lens are calculated according to the flat field condition and the focal power distribution formula
The refractive indices n1, n2, and the lens spacing d should satisfy the relationship shown below:
φ1n1+φ2n2＝0 (1)
φ1+φ2-dφ1φ2＝0 (2)
this requires a separation of the positive and negative optical powers of the optical system. The lens designed by the invention has the positioning distance between the first optical surface and the fourth optical surface of 29.36mm, and the positioning distance between the second optical surface and the third optical surface of 19.83 mm.
And the expression of the aspherical rise is shown as follows:
wherein z is the vector height in the z-axis direction, k is the coefficient of the quadratic term, c is the surface curvature, and r is the radial distance r from the optical axis z2＝x2+y2，αiThe aspheric surface deformation coefficient is a high-order term;
in order to obtain high resolution, the entrance pupil diameter is increased as much as possible, and the surface area of the polygon mirror is fully utilized, i.e. a 2.2 x 0.6mm long-strip light spot is formed on the surface of the polygon mirror, so that a relatively large astigmatism is introduced into the f-theta objective lens, and cannot be corrected only by using an aspheric surface. The system adopts the asymmetric surface of the free-form surface to correct astigmatism, and the formula is as follows:
in the formula, kx，kyCoefficients of quadratic terms in x-and y-directions, respectively, cx，cyIs the curvature of the surface in the x, y directions, alpha1，α2，α3… …, aspheric coefficient in x-direction, beta1，β2，β3… …, the aspheric coefficients in the y-direction. The surface type controls rise by x and y directions respectively, and diopters in the x and y directions are different, so that the rotation symmetry constraint of the aspheric surface is broken through.
Based on the design principle and the design process, the invention has the innovation points that: comprises a first lens with a first optical surface and a second optical surface, a second lens with a third optical surface and a fourth optical surface; the first optical surface and the third optical surface are incident light surfaces, and the surface types of the two optical surfaces are aspheric surfaces; the second optical surface and the fourth optical surface are emergent light surfaces, the surface types of the two optical surfaces are free-form surfaces, the two optical surfaces are asymmetric free-form surfaces in the main scanning direction (y), and the two optical surfaces are symmetric aspheric surfaces in the sub-scanning direction (X); the laser is converged by the first lens and further converged by the second lens, so as to meet the requirements of high resolution, large working surface and miniaturization.
The invention is described in detail below with reference to specific embodiments and with reference to the following figures:
FIG. 1 is a diagram of an optical path of a short-focus two-piece f-theta objective Lens, which includes a first Lens01 having a first optical surface s1 and a second optical surface s2, and a Lens02 having a third optical surface s3 and a fourth optical surface s4, and is suitable for a laser scanning device.
Fig. 2 is a diagram of a short-focus laser scanning system device, which mainly includes a laser light source 1, a collimator lens 2, a cylindrical surface shaping lens 3, a rotary polygon mirror 4, and an f-theta objective lens 5. The light emitted by the laser source 1 is collimated and emitted by the collimating lens 2, then passes through the cylindrical shaping mirror, and is projected on the rotary multi-surface reflector 4, and 2.2 x 0.6mm long-strip-shaped light spots are formed. The polygon mirror rotates about a central rotation axis, and reflects a light beam into a scanning beam, the scanning beam being in a sub-scanning direction in the X direction (the short side direction of the lens is the sub-scanning direction), and the scanning beam being in a main scanning direction in the y direction (the long side direction of the lens is the main scanning direction). The scanning light finally converges on the selenium drum to form a small scanning spot through the f-theta objective lens 5.
The scanning light rays of Gaussian beams reflected by a pentamirror can be subjected to distortion correction, namely, the relation of time-angular velocity is converted into the relation of time-distance, and the beam radius of the scanning light rays in the X direction and the Y direction passes through each angle of an f-theta lens to generate uniform light spots on an imaging surface so as to provide resolution meeting requirements; and the light-gathering distance can be shortened to reduce the volume of the whole device.
In order to achieve the above purpose, the two pieces of f-theta of the design adopt free-form surface lenses, and the surface form equation of the free-form surfaces of the two lenses is as follows:
the main scanning direction (Y) equation is:
the sub-scanning direction (x) equation is:
in order to maintain the equal scanning speed of the scanning light on the target object, and maintain the equal distance between the two light spots in the same two time intervals, the two f-theta lenses of the invention can correct the emergence angle from the scanning light of 0 degree to the scanning light of 120 degrees, so that the distance between the two light spots on the imaging surface of the two scanning lights in the same time interval is equal, and the light spots are uniform in size, so as to obtain the best analysis effect, as shown in fig. 4 and 5.
The uniformity of the light spot formed on the imaging surface after the scanning light passes through the f-theta can be represented by the ratio of the minimum light spot diameter to the maximum light spot diameter of the scanning light on the imaging surface, namely the following formula is satisfied:
although the embodiments of the present invention and the accompanying drawings are disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims, and therefore the scope of the invention is not limited to the disclosure of the embodiments and the accompanying drawings.
Claims (4)
1. A short-focal-length two-piece free-form surface f-theta objective lens is characterized in that: comprises a first lens and a second lens, wherein the first lens is provided with a first optical surface and a second optical surface, and the second lens is provided with a third optical surface and a fourth optical surface; the first optical surface and the third optical surface are incident light surfaces, and the surface types of the two optical surfaces are aspheric surfaces; the second optical surface and the fourth optical surface are emergent light surfaces, the surface types of the two optical surfaces are free-form surfaces as a whole, the two optical surfaces are symmetrical aspheric surfaces in the sub-scanning direction, and the two optical surfaces are asymmetrical free-form surfaces in the main scanning direction; the laser is converged by the first lens and further converged by the second lens.
2. The short-focal-length two-piece freeform f-theta objective lens of claim 1, wherein: the asymmetric free-form surface design of the first lens and the second lens meets the following formula requirements:
the equation expression in the sub-scanning direction is:
wherein z is the vector height in the z-axis direction, k is the coefficient of the quadratic term, c is the surface curvature, r is the radial distance from the optical axis z, r is2＝x2+y2，αiThe aspheric surface deformation coefficient is a high-order term;
in the main scanning direction, the equation expression is:
in the formula, kx，kyCoefficients of quadratic terms in x-and y-directions, respectively, cx，cyIs the curvature of the surface in the x, y directions, alpha1，α2，α3… …, aspheric coefficient in x-direction, beta1，β2，β3… …, the aspheric coefficients in the y-direction.
3. The short-focal-length two-piece freeform f-theta objective lens of claim 1, wherein: the combined focal length of the two lenses is 104.88mm, and the full scanning angle is 120 degrees.
4. The short-focal-length two-piece freeform f-theta objective lens of claim 1, wherein: the positioning distance between the first optical surface and the fourth optical surface is 29.36mm, and the positioning distance between the second optical surface and the third optical surface is 19.83 mm.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202011311079.1A CN112327454A (en) | 2020-11-20 | 2020-11-20 | Short-focal-length two-piece free-form surface f-theta objective lens |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202011311079.1A CN112327454A (en) | 2020-11-20 | 2020-11-20 | Short-focal-length two-piece free-form surface f-theta objective lens |
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| CN112327454A true CN112327454A (en) | 2021-02-05 |
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| CN202011311079.1A Pending CN112327454A (en) | 2020-11-20 | 2020-11-20 | Short-focal-length two-piece free-form surface f-theta objective lens |
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- 2020-11-20 CN CN202011311079.1A patent/CN112327454A/en active Pending
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Application publication date: 20210205 |