F-theta lens and scanning optical system using the same

Abstract: An fϑ lens having a skew correction function for use in a scanning optical system comprises a first lens having a negative power in a main scan plane, a second lens having a positive power in the main scan plane and arranged next to the first lens, a third lens having a toric plane and a positive power in the main scan plane and arranged next to the second lens, and an air lens formed between the first lens and the second lens having a positive power in the main scan plane.

Description: The present invention relates to an fϑ lens used in a scanning optical system, and more particularly to an fϑ lens for forming a fine spot and a laser scanning optical system using the same, according to the preambles of claims 1 and 5, respectively. In a prior art laser Scanning optical system, a laser beam emitted by a laser light source is collimated by a collimater lens, reflected by a deflector such as a polygon mirror, and a laser beam spot is formed on an image plane by a focusing lens system and it is scanned. In such a laser scanning optical system, where the beam is scanned at a constant angular speed such as a polygon mirror, a so-called fϑ lens system having an fϑ characteristic (a theoretical image height is given by fϑ where f is a focal distance of the optical system and ϑ is an incident angle) is used as a focusing lens in order to maintain the constant speed in a main scan direction on the image plane. In general, since the image plane which is scanned by the beam is planar, an image plane distortion on the image plane is compensated in the focusing lens. Further, in order to prevent the vibration (ununiformity in pitch) of the scan lines on the image plane due to skew of a reflection plane of a deflector such as a polygon mirror from a predetermined position, an anamorphic optical system such as toric lens is sometimes used in the focusing lens. An fϑ lens having three lenses in the focusing lens system in order to form a fine laser spot (less than 50 µm by a laser having a wavelength λ = 780 nm) has been proposed in USP 4,674,825. This fϑ lens has as a matter of principle a construction shown in Fig. 1 in which a concave spherical lens 51, a convex spherical lens 52 and a toric lens 53 are arranged in the order from a mirror plane M of a deflector to an image plane I. Data of the scanning lens shown in Fig. 1 is shown below. In the Table 1, Ri is a radius of curvature of the i-th lens surface as counted from the mirror plane M of the deflector, Di is a surface to surface distance from the i-th lens surface to the (i+1)th lens surface, and Ni is a refractive index of a medium behind the i-th lens surface. However, in the above three-lens fϑ lens system, a spot shape is materially degraded and the shape may be distorted to a triangle in a peripheral area of a large image angle (scanning angle) image plane even if the fϑ characteristic, the image plane distortion and the skew correction are met. As a result, an effective spot diameter increases. Thus, in the prior art lens system, a high quality image is not attained in the peripheral area of the image plane. The image plane distortion of the fϑ lens shown in Fig. 1 and the fϑ characteristic are shown in Figs. 2 and 3. A spot shape on the image plane at an image angle of 37 degrees is shown in Fig. 4. As seen from those charts, the fϑ characteristic and the image plane distortion are well compensated in the present example but the spot shapes of 1/e² (spot shapes from the peak intensity to 1/e² intensity) are triangular and good spot shape is not attained. As a result, in this fϑ lens, the quality of the image in the peripheral area of the image plane is degraded. Furthermore, US-Patent 4 674 825 discloses an fϑ lens and a scanning optical system having the features of the preambles of the claims 1 and 5, respectively. It is an object of the present invention to provide an fϑ lens to be used in a compact and high resolution scanning optical system. In order to achieve the above object, in accordance with the present invention, a three-lens fϑ lens including a toric lens is provided according to the claims 1 and 5, respectively. The first lens as viewed from a light incident side is one having a negative power in a main scan plane, a second lens is one having a positive power in the main scan plane, and the third lens is one having a tornic plane and a positive power in the main scan plane. Further, an air lens formed between the first lens and the second lens has a positive power in the main scan plane. This air lens is formed by selecting the respective radii and refractive indices of the first and the second lens as well as the distance between them. As a result, the spot shape in the peripheral area of the image plane is kept in good quality and the scanning optical system which is compact in overall scanning system and yet of high resolution is attained. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a main scan sectional view in a prior art system, Fig. 2 shows an image plane distortion in the prior art system, Fig. 3 shows an fϑ characteristic in the prior art system, Fig. 4 shows a spot shape at an image angle of 37 degrees in the prior art, Fig. 5 shows a main scan sectional view in a first embodiment, Fig. 6 shows a sub-scan sectional view in the first embodiment, Fig. 7 shows a scanning optical system which uses the fϑ lens of the present invention, Fig. 8 shows an image plane distortion in the first embodiment, Fig. 9 shows an fϑ characteristic in the first embodiment, Fig. 10 shows a spot shape at an image angle of 37 degrees in the first embodiment, Fig. 11 shows a main scan sectional view of a second embodiment, Fig. 12 shows an image plane distortion in the second embodiment, Fig. 13 shows an fϑ characteristic in the second embodiment, Fig. 14 shows a spot shape at an image angle of 37 degrees in the second embodiment, Fig. 15 shows a main scan sectional view of a third embodiment, Fig. 16 shows an image plane distortion in the third embodiment, Fig. 17 shows an fϑ characteristic in the third embodiment, and Fig. 18 shows a spot shape at an image angle of 37 degrees in the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 5 and 6 show a first embodiment of the present invention. Fig. 5 shows a sectional view of an fϑ lens of the present invention, taken along a main scan plane (which is formed by a light beam to be scanned and includes an optical axis), and Fig. 6 shows a sectional view of the fϑ lens taken along a sub-scan plane (which is orthogonal to the main scan plane and includes the optical axis). The fϑ lens 10 comprises a concave spherical lens L₁, a convex spherical lens L₂ and a toric lens L₃ arranged in this sequence as viewed from a mirror plane position M of a deflector to an image plane I. A symbol c denotes a cylindrical lens arranged between a light source such as a laser and the mirror plane position M. It has a power only in a sub-scan direction. In Figs. 5 and 6, r₁ and r₂ denote radii of curvature of a first surface and a second surface (counted from M) of the concave spherical lens L₁, r₃ and r₄ denote radii of curvature of a third surface and a fourth surface of the convex spherical lens L₂, r₅₁ and r₆₁ denote radii of curvature, in the main scan sectional plane, of a fifth surface and a sixth surface of the toric lens L₃, and r₅₂ and r₆₂ denote radii of curvature, in the sub-scan sectional plane, of the fifth surface and the sixth surface. Fig. 7 shows a construction of a scanning optical system which uses the fϑ lens of the present invention. It comprises a light source unit 1 including a light source or the light source and a collimeter lens, a linear image focusing system 2 having a cylindrical lens for linearly focusing a light beam emitted from the light source unit 1, and a deflector 3 having a deflecting reflection plane 3a arranged in a vicinity of a position at which the light beam is linearly focused by the linear image focusing system 2. Arranged between the deflector 3 and a medium 7 to be scanned are a spherical concave single lens 4, a spherical convex single lens 5 and a single lens 6 having a toric plane which has a main axis and a sub-axis of different defraction powers in two orthogonal directions. A focused spot is formed on the medium 7 by the combined system of those lenses and the medium 7 is scanned by the spot as the deflector 3 is rotated. The fϑ lens 10 having the skew correction function comprises the spherical concave single lens 4, the spherical convex single lens 5 and the single lens 6 having the toric plane, as described above. The laser beam emitted from the light source is collimated by the collimeter lens 10, linearly directed to the vicinity of the mirror plane M of the deflector through the cylinder lens, deflected thereby and focused onto the image plane I. Thus, the laser beam is focused on the mirror plane M of the deflector by the cylindrical lens in the sub-scan sectional plane, and then focused again by the fϑ lens onto the image plane I. A specific example of the fϑ lens is shown in Table 2, in which r₁ etc. are same as those mentioned above, d₀ is a distance between the mirror plane M and the first surface, d i (i = 1 ~ 5) is a distance between the i-th surface and the (i+1)th surface, d₆ is a distance between the sixth surface and the image plane I, n₀, n₂, n₄ and n₆ are refractive indices of air, and n₁, n₃ and n₅ are refractive indices of the lenses L₁, L₂ and L₃, respectively. As seen from Table 2, an air lens is formed between the lenses L₁ and L₂ by the second surface (radius of curvature r₂) and the third surface (radius of curvature r₃), where r₃ < r₂ < 0 (that is, r₂/r₃ ≦ 1). Accordingly, the air lens has a positive power. The fact that the air lens has the positive power means that a combined power of the surfaces r₂ and r₃ is positive. Power of surface r₂  A = 1 - n₁ r₂ Power of surface r₃  B = n₃ - 1 r₃ Combined power  =  A +  B - e A  B where e = d₂ Accordingly the fact that the equation is positive means that the air lens has a positive power. The image plane distortion and the fϑ characteristic of the fϑ lens 10 are shown in Figs. 8 and 9. It is seen that the image plane distortion and the fϑ characteristic in the main scan plane and the sub-scan plane are good. Fig. 10 shows the result of calculation of a spot shape at an image angle of 37 degrees by using the fϑ lens 10. The spot is elliptic (a little longer on a sub-scan plane) at a contour line of 1/e² , and it is keep of good shape. Fig. 11 shows a sectional view of a second embodiment of the present invention, taken along the main scan plane. An fϑ lens 20 comprises a concave spherical lens L₁, a convex spherical lens L₂ and a toric lens L₃, arranged in this order as viewed from a mirror plane M. In the second embodiment, an air lens between the lenses L₁ and L₂ also has a positive power in the main scan plane. A specific example of the fϑ lens is shown in Table 3. The convention of symbols is same as that of Table 2. The image plane distortion, the fϑ characteristic and the spot shape at the image angle of 37 degrees in the second embodiment are shown in Figs. 12, 13 and 14. It is seen that, in the second embodiment, a similar effect to that of the first embodiment is attained. A third embodiment of the present invention is shown in Fig. 15. An fϑ lens 30 comprises a concave spherical lens L₁, a convex spherical lens L₂ and a toric lens L₃, arranged in this order as viewed from a mirror plane M. A specific example of the fϑ lens 30 is shown in Table 4. The convention of symbols is same as that in Table 2. Again, in the third embodiment, r₂ = r₃ < 0 (that is, r₂/r₃ ≦ 1) is met and the air lens between the lenses L₁ and L₂ has a positive power. The image plane distortion, the fϑ characteristic and the spot shape at the image angle of 37 degrees of the fϑ lens 30 are shown in Figs. 16, 17 and 18, respectively. It is seen that, in the third embodiment, a similar effect to that of the first embodiment is attained. The fϑ lens of the present invention which is used in the scanning optical system and has the skew correction function has the first lens, the second lens and the third lens arranged in this order as viewed from the deflection plane. The first lens has a negative power in the main scan plane, the second lens has a positive power in the main scan plane, the third lens has a toric surface and a positive power is the main scan plane, and the air lens between the first lens and the second lens has a positive power in the main scan plane. Preferably, the first lens is a concave spherical lens, the second lens is a convex spherical lens and the third lens is a toric lens. A more remarkable effect is attained when the following condition is met: -2.2 ≦ f/r₃ ≦ 1.7 where r₂ and r₃ are radii of curvature of the concave spherical lens and the convex spherical lens, respectively, and f is a focal distance of the overall lens system. If f/r₃ deviates from the above range, the image plane distortion in the main scan plane is deteriorated and it is difficult to maintain the spot diameter fine over the entire image plane. More preferably, when the condition r₂/r₃ ≦ 1 is met, more effect is attained. If r₂/r₃ > 1, the power of the air lens is negative and the spot shape is degraded. If r₂/r₃ < 0, the spot shape is degraded. Therefore, 0 ≦ r₂/r₃ is desirable. In accordance with the fϑ lens of the present invention, good spot shape is maintained even at a large image angle area and the scanning optical system which is compact and yet has a high resolution is attained.

Claims: An fϑ lens for skew correction and for use in a scanning optical system, said lens comprising in the following sequence in the light-direction of the scanning beam: a first lens (L₁) having a negative power in the main scan plane; a second lens (L₂) having a positive power in the main scan plane and arranged next to said first lens (L₁); a third lens (L₃) having a toric surface and a positive power in the main scan plane and arranged next to said second lens (L₂); characterized in that the respective radii and refractive indices of said first and said second lens as well as the distance between them are selected such that an air lens is formed between said first lens (L₁) and said second lens (L₂), said air lens having a positive power in the main scan plane. An fϑ lens according to claim 1, wherein said first lens (L₁) is a spherical lens having a negative power, said second lens (L₂) is a spherical lens having a positive power and said third lens (L₃) is a toric lens. An fϑ lens according to claim 2, wherein the condition -2.2 ≦ f/r₃ ≦ 1.7 is met, with r₃ being the radius of curvature of the surface of said second lens (L₂) facing said first lens (L₁) and f being the focal distance of the overall lens system. An fϑ lens according to claim 3, wherein the condition r₂/r₃ ≦ 1 is met, with r₂ being the radius of curvature of the surface of said first lens (L₁) facing said second lens (L₂) and r₃ being the radius of curvature of the surface of said second lens (L₂) facing said first leans (L₁). A scanning optical system for skew correction comprising: a light source unit (1); a first optical system (2) for linearly focusing a light beam emitted from said light source unit (1); a deflector (3) having a deflecting reflection plane (3a) arranged in the vicinity of the linear image formed by said first optical system (2); and a second optical system (10) for focusing the light beam deflected by said deflector (3) onto a predetermined plane; said second optical system (10) having a first lens (4), a second lens (5) and a third lens (6) arranged in this sequence as viewed from said deflector (3), said first lens (4) having a negative power in the main scan plane, said second lens (5) having a positive power in the main scan plane, said third lens (6) having a toric surface and a positive power in the main scan plane said scanning optical system being characterized in that the respective radii and refractive indices of said first (4) and said second lens (5) as well as the distance between them are selected such that an air lens is formed between said first lens (4) and said second (5) lens, said air lens having a positive power in the main scan plane. A scanning optical system according to claim 5, wherein said first lens (4) is a spherical lens having a negative power, said second lens (5) is a spherical lens having a positive power and said third lens (6) is a toric lens. A scanning optical system according to claim 6, wherein a condition -2.2 ≦ f/r₃ ≦ 1.7 is met, with r₃ being the radius of curvature of the surface of said second lens (5) facing said first lens (4) and f being the focal distance of the overall second lens system (10). A scanning optical system according to claim 7, wherein a condition r₂/r₃ ≦ 1 is met, with r₂ being the radius of curvature of the surface of said first lens (4) facing said second lens (5) and r₃ being the radius of curvature of the surface of said second lens (5) facing said first lens (4).

Inventors: MAKINO JUN, MAKINO, JUN
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G02B13
IPC codes: G02B13, G02B26