Scanning optical apparatus with a diffractive lens for compensating temperature dependent focus changes

Abstract: A scanning optical apparatus includes a light source (1), deflector (5) for deflecting a light beam emitted from the light source, and optical device for directing the light beam emitted from the light source onto a surface to be scanned. The optical device includes a diffracting optical element (48). An aberration fluctuation in a sub-scanning direction resulting from environmental fluctuation of the scanning optical apparatus is corrected by the characteristic of the optical device.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a scanning optical apparatus and an image forming apparatus using the same, and particularly is suitable for an image forming apparatus such as a laser beam printer (LBP) or a digital copying apparatus suited for highly minute printing in which a diffraction optical element is provided in at least one of a first optical system disposed on light source means side relative to a light deflector and a second optical system disposed on a surface to be scanned side, whereby an aberration fluctuation (a change in focus) does not occur even if an environmental fluctuation (particularly a temperature change) occurs to the apparatus. Related Background Art Heretofore, in a scanning optical apparatus used in a laser beam printer or a digital copying apparatus or the like, a light beam light-modulated and emitted from light source means in conformity with an image signal has been periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror, and has been converged into a spot-like shape on a photosensitive recording medium (photosensitive drum) by a scanning optical element (imaging element) having the f characteristic, and has optically scanned the surface thereof to thereby effect image recording. A scanning optical apparatus of this kind in which a diffraction optical element is applied to the scanning optical system thereof is proposed, for example, in Japanese Laid-Open Patent Application No. 3-125111. In this publication, the optical system is comprised of a holographic f lens having power only in a main scanning direction, and a cylindrical lens having power in a sub-scanning direction, and the correction of the f characteristic and surface inclination and the separation of first-order diffracted light and diffracted lights of other orders are effected. Also, U.S. Patent No. 5,486,694 discloses a scanning optical system having a surface inclination correcting effect in which a diffracting surface is provided on a toric lens. However, the cylindrical lens in Japanese Laid-Open Patent Application No. 3-125111, when it is formed of a glass material, is more advantageous in cost than a conventional f lens comprising a combination of a plurality of lenses, but the cylindrical lens of a glass material is still expensive as compared with plastic lenses. On the other hand, if a cylindrical lens is formed of a plastic material to reduce the cost thereof, defocus in the sub-scanning direction is caused by an environmental fluctuation, particularly, a temperature change. To prevent this, the cylindrical lens can be brought close to a scanned surface, but if this done, the cylindrical lens will become large and as the result, the entire apparatus will become bulky, and this leads to the problem that the cost becomes high. U.S. Patent No. 5,486,694 suffers from the problem that no consideration is paid to the deterioration of aberrations caused by a change in the angle of diffraction due to the fluctuation of the wavelength of a laser beam. On the other hand, in a scanning optical element (f lens) in a scanning optical system, the mainstream is one using a plastic lens from the requirements for lower costs and compactness. Fig. 1 of the accompanying drawings is a schematic view of the essential portions of a scanning optical apparatus of this kind according to the prior art. In Fig. 1, a divergent light beam emitted from light source means 81 is made into a substantially parallel light beam by a collimator lens 82, and this light beam (the quantity of light) is limited by a stop 83 and enters a cylindrical lens 84 having predetermined refractive power only in a sub-scanning direction. That part of the parallel light beam having entered the cylindrical lens 84 which is in a main scanning cross-section emerges intactly as a parallel light beam. Also, that part of the parallel light beam which is in a sub-scanning cross-section is converged and imaged substantially as a linear image on the deflecting surface (reflecting surface) 85a of a light deflector 85 comprising a rotatable polygon mirror. Here, the main scanning cross-section refers to a light beam cross-section formed with the lapse of time by a light beam deflected and reflected by the deflecting surface of the light deflector. Also, the sub-scanning cross-section refers to a cross-section containing the optical axis of an f lens and orthogonal to the main scanning cross-section. The light beam deflected by the deflecting surface of the light deflector 85 is directed onto a photosensitive drum surface 87 as a surface to be scanned through a scanning optical element (f lens) 86 having the f characteristic, and the light deflector 85 is rotated in the direction of arrow A, thereby optically scanning the photosensitive drum surface 87 in the direction of arrow B. Thereby, image recording is effected on the photosensitive drum surface 87 which is a recording medium. Each of the collimator lens 82, the stop 83 and the cylindrical lens 84 constitutes an element of a first optical system L 81 , and the f lens 86 constitutes an element of a second optical system L 82 . In the scanning optical apparatus of this kind, it is often the case that as previously described, from the requirements for the lower costs and compactness of the f lens, the f lens is comprised of only a plastic lens of an aspherical shape. The plastic material, however, has the nature that the refractive index thereof changes with the fluctuation of its environment of use (particularly, the fluctuation of temperature) and therefore, in a scanning optical apparatus using an f lens comprising the plastic lens, a change in focus is caused by the environmental fluctuation. Under the existing circumstances, the amount of focus fluctuation is set to such a degree that will not affect the actual image and therefore there arises no problem, but when it is desired to stop down the spot diameter of a condensed beam on the scanned surface for the purpose of more highly minute printing or when an attempt is made to bring an f lens close to the light deflector in an attempt to downsize the f lens for the purpose of lower cost and compactness, there is the problem that the above-mentioned focus fluctuation, particularly, focus fluctuation in the sub-scanning direction, cannot be allowed. Further optical scanners are described in GB 2315563, JP 05-060997, JP 06-324280, US 5,212,501. SUMMARY OF THE INVENTION It is the object of the present invention to provide a compact scanning optical apparatus suited for highly minute printing in which aberration fluctuation (a change in focus) does not occur even if environmental fluctuations (temperature change and wavelength change of light source means) occur to the apparatus, and an image forming apparatus using the same. The scanning optical apparatus of the present invention is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of the essential portions of a scanning optical apparatus according to the prior art. Fig. 2 is a cross-sectional view of the essential portions of Embodiment 1 of the scanning optical apparatus in a main scanning direction. Fig. 3 illustrates the principle of correction of a focus fluctuation in a sub-scanning direction in Embodiment 1 . Fig. 4 shows the curvature of field of Embodiment 1 in the sub-scanning direction. Fig. 5 is a cross-sectional view of the essential portions of Embodiment 2 of the scanning optical apparatus of the present invention in the main scanning direction. Fig. 6 shows the curvature of field of Embodiment 2 of the present invention in the sub-scanning direction. Fig. 7 is a cross-sectional view of the essential portions of Embodiment 3 of the scanning optical apparatus of the present invention in the main scanning direction. Fig. 8 shows the curvature of field of Embodiment 3 of the present invention in the sub-scanning direction. Fig. 9 is a cross-sectional view of the essential portions of Embodiment 4 of the scanning optical apparatus in the main scanning direction. Fig. 10 is a cross-sectional view of the essential portions of Embodiment 4 in the sub-scanning direction. Fig. 11 is a graph showing the temperature wavelength characteristic of a laser diode in Embodiment 4. Fig. 12 is a schematic view of the cross-sectional grating pattern of a diffraction. Fig. 13 is a schematic view of the cross-sectional grating pattern of another example of the diffraction lens in Embodiment 4. Fig. 14 is a schematic view of the essential portions showing the refractive power arrangement of Embodiment 4. Fig. 15 is a schematic view of the essential portions showing the refractive power arrangement of Embodiment 5 of the scanning optical apparatus. Fig. 16 is a cross-sectional view of the essential portions of Embodiment 6 of the scanning optical apparatus in the main scanning direction. Fig. 17 is a cross-sectional view of the essential portions of Embodiment 6 in the sub-scanning direction. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 is a cross-sectional view of essential portions in a main scanning direction (main scanning cross-sectional view) when a scanning optical apparatus according to Embodiment 1 is applied to an image forming apparatus such as a laser beam printer or a digital copying apparatus. In Fig. 2, reference numeral 1 designates a light source means comprising, for example, a semiconductor laser. Reference numeral 2 denotes a collimator lens which converts a divergent light beam emitted from the light source means 1 into a substantially parallel light beam. Reference numeral 3 designates an aperture stop which limits the light beam passing therethrough (the quantity of light) and shapes its beam shape. Reference numeral 4 denotes a diffracting optical element having a diffracting surface and added to that flat surface of a flat plate formed, for example, of a glass material or a plastic material which is adjacent to a light deflector 5 which will be described later. The diffracting optical element 4 has power (diffracting power) in a sub-scanning direction perpendicular to the plane of the drawing sheet of Fig. 2, and causes the light beam passed through the aperture stop 3 to be imaged as a substantially linear image on the deflecting surface 5a of the light deflector 5 in a sub-scanning cross-section. The diffracting optical element 4 in the present embodiment has its grating structure comprising, for example, a binary diffracting optical element comprising a staircase-like diffraction grating by photoetching, or a Fresnel-like diffracting optical element comprising a sawtooth-like diffraction grating by surface cutting. Each of the collimator lens 2, the stop 3 and the diffracting optical element 4 constitutes an element of a first optical system L 1 . Reference numeral 5 designates a light deflector as deflecting means comprising, for example, a polygon mirror (rotatable polygon mirror) and rotated at a constant speed in the direction of arrow A by drive means (not shown) such as a motor. Reference numeral 6 denotes an f lens (scanning optical element) having the f characteristic and comprising a single lens formed of a plastic material, and having its opposite lens surfaces in the main scanning direction comprised of an aspherical shape, and having different refractive powers between the main scanning direction and the sub-scanning direction. The f lens causes the light beam based on image information which has been deflected by the light deflector 5 to be imaged on a photosensitive drum surface 7 which is a recording medium which is a surface to be scanned and corrects the surface inclination of the deflecting surface of the light deflector 5. The f6 lens 6 constitutes an element of a second optical system L 2 . In the present embodiment, the divergent light beam emitted from the semiconductor laser 1 is converted into a substantially parallel light beam by the collimator lens 2, and this light beam (the quantity of light) is limited by the aperture stop 3 and enters the diffracting optical element 4. That part of the parallel light beam having entered the diffracting optical element 4 which is in the main scanning cross-section emerges intactly therefrom. Also, in the sub-scanning cross-section, it converges and is imaged as a substantially linear image (a long linear image in the main scanning direction) on the deflecting surface 5a of the light deflector 5. The light beam deflected by the deflecting surface 5a of the light deflector 5 is directed onto the photosensitive drum surface 7 through the f lens 6, and the light deflector 5 is rotated in the direction of arrow A, thereby optically scanning, the photosensitive drum surface 7 in the direction of arrow B. Thereby, image recording is effected on the photosensitive drum surface 7 which is a recording medium. Here, the principle of correction of a focus fluctuation in the sub-scanning direction in the present embodiment will be described with reference to Fig. 3. Fig. 3 is a cross-sectional view of the essential portions of an optical system in the present embodiment in the sub-scanning direction. In Fig. 2, the same elements as the elements shown in Fig. 2 are given the same reference characters. Here, let it be assumed that for example, the scanning optical system has risen in temperature due to the influence of environmental fluctuations. If the f lens 6 is formed of a plastic material, the focus position in the sub-scanning direction will fluctuate from a point P to a point Q in Fig. 3 to thereby cause out-of-focus because the plastic material has the characteristic that its refractive index is reduced when temperature rises. However, the semiconductor laser 1 which is a light source has the characteristic that its oscillation wavelength usually becomes longer than temperature rises. When the oscillation wavelength becomes longer, the angle of diffraction at which it is diffracted by the diffracting optical element 4 becomes greater and thus, the light beam originally imaged in a linear shape at a point R near the deflecting surface of the light deflector 5 is imaged in a linear shape at a point S nearer to the semiconductor laser 1 than the point R as shown in Fig. 3. Thus, the focus position which has been fluctuated to the point Q by a reduction in the refractive index of the f lens is returned to the point P. On the basis of the principle as described above, the powers of the diffracting optical element 4 and the f lens 6 in the sub-scanning direction are optimally set in the present embodiment, whereby a focus fluctuation (aberration fluctuation) in the sub-scanning direction by the environmental fluctuations of the apparatus (particularly temperature fluctuation and the wavelength fluctuation of the semiconductor laser 1) can be corrected substantially completely. Tables 1 and 2 below show the optical arrangement in the present embodiment and the aspherical coefficients of the f lens 6, and the phase term of the diffracting optical element 4, respectively. used wavelength λ (nm) 780 refractive index of f lens n 1.524 polygon angle of incidence i -65 polygon maximum angle of emergence max 45 diffracting surface - polygon d0 23.913 polygon - f lens d1 20.915 center thickness of f lens dL 7.5 f lens - surface to be scanned Sk 125.879 focal length of f lens f 133.69 polygon 20, tetrahedron First surface Second surface R 49.464 R 160.673 when Y is + when Y is + Ku -1.75325E+01 Ku -1.43487E+02 B4u -9.79528E-06 B4u -1.09382E-05 B6u 5.71251E-09 B6u 4.70016E-09 B8u 7.03030E-14 B8u -2.74757E-12 B10u -1.06320E-15 B10u 1.00200E-15 when Y is - when Y is - K1 -4.20091E+00 K1 -7.79269E+00 B41 -9.79528E-06 B41 -6.44856E-06 B61 5.71251E-09 B61 -1.16920E-09 B81 7.03030E-14 B81 3.12133E-12 B101 -1.06320E-15 B101 -1.20296E-15 r 99.464 r -12.5890 when Y is + when Y is + D2u 0.00000E+00 D2u 1.65236E-03 D4u 0.00000E+00 D4u -4.62943E-06 D6u 0.00000E+00 D6u 1.08336E-08 D8u 0.00000E+00 D8u -1.50624E-11 D10u 0.00000E+00 D10u 8.78607E-15 when Y is - when Y is - D21 0.00000E+00 D21 1.24934E-03 D41 0.00000E+00 D4 2.08457E-07 D61 0.00000E+00 D61 -5.35866E-09 D81 0.00000E+00 D81 9.66153E-12 D101 0.00000E+00 D101 -5.47740E-15 phase term of diffracting element C1 -2.09093E-02 C2 0.00000E+00 C3 0.00000E+00 The diffracting surface 4a of the diffracting optical element 4 in the present embodiment is constructed on the light deflector 5 side, and when the phase function is (y,z) and the point of intersection with the optical axis is the origin and the direction of the optical axis is the x-axis and an axis orthogonal to the optical axis in the main scanning plane is the y-axis and an axis orthogonal to the optical axis in the sub-scanning plane is the z-axis, the phase function is represented by the following expression: ( y,z ) = 2π λ ( C 1 z 2 + C 2 y 2 z 2 + C 3 y 4 z 2 ) C 1 to C 3 : phase polynominal coefficient, λ = 780 nm Also, as regards the lens shape of the f lens 6, when the point of intersection of each lens surface with the optical axis is the origin and the direction of the optical axis is the x-axis and an axis orthogonal to the optical axis in the main scanning plane is the y-axis and an axis orthogonal to the optical axis in the sub-scanning plane is the z-axis, the main scanning cross-sectional shape is represented by the following expression: x= y 2 / R 1+(1-(1+ K ) ( y / R ) 2 ) 1/2 + B 4 y 4 + B 6 y 6 + B 8 y 8 + B 10 y 10 In the foregoing expression, R is the radius of curvature, and K and B 4 - B 10 are aspherial coefficients. When the value of y is positive, the main scanning cross-sectional shape is a lens shape calculated by the use of Ku and B 4 u - B 10 u having a suffix u as the aspherical coefficients, and when the value of y is negative, it is a lens shape calculated by the use of kl and B 4 l - B 10 l having a suffix l as the aspherical coefficients. Also, in the sub-scanning cross-section, the radius of curvature thereof changes continuously and as regards the lens coordinates on the main scanning plane, the radius of curvature r' which is y is represented by the following expression: r' = r(l + D 2 y 2 + D 4 y 4 + D 6 y 6 + D 8 y 8 + D 10 y 10 ) In the foregoing expression, r is the radius of curvature on the optical axis, and D 2 - D 10 are coefficients. When the value of y is positive, the radius of curvature in the sub-scanning cross-section is a radius of curvature r' calculated by the use of D 2 u - D 10 u having a suffix u as the coefficients, and when the value of y is negative, it is a radius of curvature r' calculated by the use of D 2 l - D 10 l having a suffix l as the coefficients. Fig. 4 is an aberration graph showing the curvature of field of the present embodiment in the sub-scanning direction before and after a temperature rise, and the solid line indicates the characteristic at the ordinary temperature (25°C), and the dotted line indicates the characteristic when a temperature rise of 25°C occurred and the temperature has reached 50°C. From this graph, it is seen that there is little or no focus change in the sub-scanning direction before and after the temperature rise. Thus, in the present embodiment, the apparatus is constructed by the use of the diffracting optical element 4 instead of the cylindrical lens (see Fig. 1) heretofore used as an element of the first optical system, and the power of the diffracting optical element 4 in the sub-scanning direction (diffracting power) and the power of the f lens 6 in the sub-scanning direction (refracting power) are appropriately set to thereby effectively correct a focus change in the sub-scanning direction resulting from the environmental fluctuations of the apparatus (temperature fluctuation and the wavelength fluctuation of the semiconductor laser 1). Thereby, a scanning optical system strong against environmental fluctuations and suited for highly minute printing can be obtained compactly and inexpensively. Fig. 5 is a cross-sectional view of essential portions in the main scanning direction when a scanning optical apparatus according to Embodiment 2 of the present invention is applied to an image forming apparatus. In Fig. 5, the same elements as the elements shown in Fig. 2 are given the same reference numerals. The difference of the present embodiment from the aforedescribed Embodiment 1 is that a diffracting optical element is added to that lens surface of the f6 lens constituting the second optical system which is adjacent to the light deflector and the first optical system is comprised of a collimator lens, a stop and a cylindrical lens as in the prior-art construction (see Fig. 1). In the other points, the construction and optical action of the present embodiment are substantially similar to those of the aforedescribed Embodiment 1, whereby a similar effect is obtained. That is, in Fig. 5, reference numeral 46 designates an f lens as the second optical system L 42 . The f lens comprises a single lens of an aspherical shape formed of a plastic material, and a diffracting optical element 48 having power (diffracting power) in the sub-scanning direction is added to that lens surface (the first surface) Ra of the single lens which is adjacent to the light deflector 5, and the diffracting surface 48a of the diffracting optical element 48 is represented by the same expression as that in the aforementioned Embodiment 1. That is, as regards the diffracting surface 48a of the diffracting optical element 48 added to the first surface Ra of the f lens 46, when the phase function is ( y , z ) and the point of intersection of the first surface Ra with the optical axis is the origin and the direction of the optical axis is the x-axis and an axis orthogonal to the optical axis in the main scanning plane is the y-axis and an axis orthogonal to the optical axis in the sub-scanning plane is the z-axis, the phase function is represented by the following expression: ( y , z )= 2π λ ( C 1 z 2 + C 2 y 2 z 2 +C 3 y 4 z 2 ) C 1 - C 3 : phase polynominal coefficients, λ = 780 nm Reference numeral 44 denotes a cylindrical lens constituting an element of the first optical system L 41 and having predetermined refractive power in the sub-scanning direction. Description will now be made of the principle of correction of a focus fluctuation in the sub-scanning direction in the present embodiment. Let it be assumed that when the scanning optical apparatus according to the present embodiment is at an environmental temperature t°C, the refractive index of the f lens 46 is n and the oscillation wavelength of the semiconductor laser 1 is λ. When the temperature dependency coefficient of the refractive index of the f lens 46 is dn/dt and the temperature dependency coefficient of the wavelength of the semiconductor laser 1 is dλ/dt and the wavelength dependency coefficient of the refractive index of the f lens is dn/dλ, the refractive index n' of the f lens 46 and the wavelength λ' of the semiconductor laser 1 when the environmental temperature has changed by Δt are respectively expressed as n'= n + dn dt Δ t + dn d λ · d λ dt Δ t and λ'=λ+ d λ dt Δ t . On the other hand, when the grating spatial frequency at a ray passage point on the diffracting surface 48a is H, the power  B of the diffracting optical element 48 formed on the first surface Ra of the f lens 46 is represented by  B = m λ dH dr r = (y 2 + z 2 ) 1/2 , m is the order of diffraction, and λ is the wavelength. Consequently, assuming that the radius of curvature of the first surface Ra of the f lens 46 on the optical axis of the sub-scanning cross-section is r 1 , the radius of curvature of the second surface Rb thereof on the optical axis of the sub-scanning cross-section is r 2 , and for simplicity, a thin-walled system is assumed and the power of the diffracting optical element 48 and the power by reflaction exist at the same place, the power  t of the f lens 46 in the sub-scanning direction when the environmental temperature is t°C is represented by the following expression:  t = m λ dH dr +( n -1) ( 1 r 1 - 1 r 2 ) On the other hand, the power  t , of the f lens in the sub-scanning direction when the environmental temperature has changed by Δt is represented by the following expression:  ' t = m (λ 1 + d λ dt Δ t ) dH dr +( n + dn dt Δ t + dn d λ Δ t -1)( 1 r 1 - 1 r 2 ) If here, irrespective of the environmental temperature, the power of the f lens 46 in the sub-scanning direction is always kept constant, any focus fluctuation in the sub-scanning direction will not occur in principle. Consequently, assuming that expression (1) and expression (2) are equal to each other, finally m dH dr · d λ dt Δ t +( 1 r 1 - 1 r 2 )( dn dt Δ t + dn d λ · d λ dt Δ t )=0 With the order of diffraction as the first order, the power  B of the diffracting optical element (diffracting portion) 48 of the f lens 46 is  B = λ dH dr and the power  L of the other portion (refracting portion) than the diffracting optical element of the f lens 46 is  L =( n -1)( 1 r 1 - 1 r 2 ) and hence, expression (3) becomes  B λ · d λ dt +  L n -1 ( dn dt + dn d λ · d λ dt )=0 and if the power  B of the diffracting portion of the f lens 46 and the power  L of the refracting portion thereof are decided so as to satisfy this expression, the focus fluctuation by environmental fluctuations (temperature fluctuation and the wavelength fluctuation of the semiconductor laser) can be corrected substantially completely. Here, taking into consideration the values which can be actually assumed by dλ/dt, dn/dt and dn/dλ, when finally the power of the diffracting portion of the f lens 46 in the sub-scanning direction is  B and the power of the refracting portion thereof in the sub-scanning direction is  L , it is desirable to set each power so as to satisfy the following condition: 1.0 ≤  L /  B ≤ 2.6 Conditional expression (5) relates to the ratio between the power of the refracting portion constituting the f lens 46 and the power of the diffracting portion, and if conditional expression (5) is departed from, it will become difficult to correct any focus change in the sub-scanning direction resulting from the environmental fluctuations of the apparatus, and this is not good. In the present embodiment,  L /  B = 1.171 and conditional expression (5) is satisfied. Tables 3 and 4 below show the optical arrangement in the present embodiment and the aspherical coefficient of the f lens 46, and the phase term of the diffracting optical element 48, respectively. used wavelength λ(nm) 780 refractive index of f lens n 1.524 polygon incidence of angle i -65 polygon maximum angle of emergence max 45 polygon - f lens d1 20.915 center thickness of f lens dL 7.5 f lens - surface to be scanned Sk 125.879 focal length of f lens f 133.69 polygon 20, tetrahedron First surface Second surface R 49.464 R 160.673 when Y is + when Y is + Ku -1.75325E+01 Ku -1.43487E+02 B4u -9.79528E-06 B4u -1.09382E-05 B6u 5.71251E-09 B6u 4.70016E-09 B8u 7.03030E-14 B8u -2.74757E-12 B10u -1.06320E-15 B10u 1.00200E-15 when Y is - when Y is - K1 -4.200091E+00 K1 -7.79269E+00 B41 -9.79528E-06 B41 -6.44856E-06 B61 5.71251E-09 B61 -1.16920E-09 B81 7.03030E-14 B81 3.12133E-12 B101 -1.06320E-15 B101 -1.20296E-15 r -51.668 r -14.6234 when Y is + when Y is + D2u 0.00000E+00 D2u 1.24290E-03 D4u 0.00000E+00 D4u -3.76988E-06 D6u 0.00000E+00 D6u 5.30548E-09 D8u 0.00000E+99 D8u -3.17865E-12 D10u 0.00000E+00 D10u 1.24960E-16 when Y is - when Y is - D21 0.00000E+00 D21 1.00192E-03 D41 0.00000E+00 D41 -1.44682E-06 D61 0.00000E+00 D61 -1.70141E-10 D81 0.00000E+00 D81 1.26126E-12 D101 0.00000E+00 D101 -5.27229E-16 phase term of diffracting element C1 -1.17334E-02 C2 7.18464E-06 C3 -2.65115E-14 Fig. 6 is an aberration graph showing the curvature of field of the present embodiment in the sub-scanning direction before and after a temperature rise, and the solid line indicates the characteristic at the ordinary temperature (25°C) and the dotted line indicates the characteristic when a temperature rise of 25°C has occurred and the temperature has reached 50°C. From this graph, it is seen that there is little or no focus change in the sub-scanning direction before and after the temperature rise. Thus, in the present embodiment, the diffracting optical element 48 is added to that lens surface Ra of the f lens 46 which is the second optical system L 42 which is adjacent to the light deflector 5 as described above, and the power (diffracting power) of the diffracting portion of the f lens 46 in the sub-scanning direction and the power (refracting power) of the refracting portion thereof in the sub-scanning direction are appropriately set to thereby well correct a focus change in the sub-scanning direction resulting from the environmental fluctuations of the apparatus (temperature fluctuation and the wavelength fluctuation of the semiconductor laser l). Thereby, a scanning optical system strong against environmental fluctuations and suited for highly minute printing can be obtained compactly and inexpensively. Fig. 7 is a cross-sectional view of essential portions in the main scanning direction when a scanning optical apparatus according to Embodiment 3 of the present invention is applied to an image forming apparatus. In Fig. 7, the same elements as the elements shown in Fig. 5 are given the same reference characters. The difference of the present embodiment from the aforedescribed Embodiment 2 is that a diffracting optical element is also added to one surface of the cylindrical lens constituting an element of the first optical system. In the other points, the construction and optical action of the present embodiment are substantially similar to those of the aforedescribed Embodiment 1, whereby a similar effect is obtained. That is, in Fig. 7, reference numeral 64 designates a cylindrical lens constituting an element of a first optical system L 61 , and that lens surface of this cylindrical lens 64 which is adjacent to the light source means 1 is comprised of a cylindrical surface having predetermined power in the sub-scanning direction, and a diffracting optical element 68 having power (diffracting power) in the sub-scanning direction is added to that lens surface (flat surface) which is adjacent to the light deflector 5. Reference numeral 46 denotes an f lens similar to that in the aforedescribed Embodiment 2. In the present embodiment, both of the principle of correction of the focus change in the sub-scanning direction described in Embodiment 1 and the principle of correction of the focus change in the sub-scanning direction described in Embodiment 2 are utilized to well correct any focus change in the sub-scanning direction resulting from the environmental fluctuations of the apparatus. Thereby, in the present embodiment, the power of the diffracting portion (diffracting optical element) of each of the cylindrical lens 64 and the f lens 46 in the sub-scanning direction can be set small and as the result, the grating pitches of the diffracting optical elements 68 and 48 can be made great, and this is very advantageous to manufacture. Tables 5 and 6 below show the optical arrangement in the present embodiment, the aspherical coefficients of the f lens 46, the shape of the cylindrical lens 64, and the phase terms of the diffracting optical elements 68 and 48. used wavelength λ(nm) 780 refractive index of f lens n 1.524 polygon angle of incidence i -65 polygon maximum angle of emergence max 45 diffracting surface - polygon d0 45 polygon - f lens d1 20.915 center thickness of f lens dL 7.5 f lens - surface to be scanned Sk 125.879 focal length of f lens f 133.69 polygon 20, tetrahedron First surface Second surface R 49.464 R 160.673 when Y is + when Y is + Ku -1.75325E+01 Ku -1.43487E+02 B4u -9.79528E-06 B4u -1.09382E-05 B6u 5.71251E-09 B6u 4.70016E-09 B8u 7.03030E-14 B8u -2.74757E-12 B10u -1.06320E-15 B10u 1.00200E-15 when Y is - when Y is - K1 -4.20091E+00 K1 -7.79269E+00 B41 -9.79528E-06 B41 -6.44856E-06 B61 5.71251E-09 B61 -1.16920E-09 B81 7.03030E-14 B81 3.12133E-12 B101 -1.06320E-15 B101 -1.20296E-15 r -55.977 r -12.4590 when Y is + when Y is + D2u 0.00000E+00 D2u 1.19988E-03 D4u 0.00000E+00 D4u -3.56812E-06 D6u 0.00000E+00 D6u 6.10045E-09 D8u 0.00000E+00 D8u -5.35878E-12 D10u 0.00000E+00 D10u 1.86786E-15 when Y is - when Y is - D21 0.00000E+00 D21 9.37917E-04 D41 0.00000E+00 D41 -7.11651E-07 D61 0.00000E+00 D61 -2.25965E-09 D81 0.00000E+00 D81 5.04127E-12 D101 0.00000E+00 D101 -2.97423E-15 Fig. 8 is an aberration graph showing the curvature of field of the present embodiment in the sub-scanning direction before and after a temperature rise, and the solid line indicates the characteristic at the ordinary temperature (25°C), and the dotted line indicates the characteristic when a temperature rise of 25°C has occurred and the temperature has reached 50°C. From this graph, it is seen that there is little or no focus change in the sub-scanning direction before and after the temperature rise. Thus, in the present embodiment, a diffracting optical element is provided in each of the first optical system L 61 and the second optical system L 42 as described above, whereby the grating pitch of each diffracting optical element can be set to a great value and thus, there can be constructed a scanning optical system which is very easy to manufacture. While in the present embodiment, the diffracting optical element is added to one surface of the cylindrical lens 64, this is not restrictive, but a diffracting optical element may be added to one surface of the collimator lens 2. In the above-described Embodiments 2 and 3, a diffracting optical element is added to at least one surface of the optical element constituting each optical system, whereas this is not restrictive, but the diffracting optical element may be constructed independently in the optical path. According to the present invention described above, a diffracting optical element is provided in the second optical system disposed on the surface to be scanned side as previously described, and any aberration fluctuation (focus change) in the sub-scanning direction resulting from the environmental fluctuations of the apparatus (temperature fluctuation and the wavelength fluctuation of the semiconductor laser) is corrected by a change in the power of the diffracting optical element and a change in the power of the optical system, whereby there can be achieved a compact scanning optical apparatus which is not affected by the environmental fluctuations but is suited for highly minute printing, and an image forming apparatus using the same. Particularly when the diffracting optical element is provided in the second optical system, the power of each element is appropriately set so as to satisfy the aforementioned conditional expression (5), whereby there can be achieved a compact scanning optical apparatus which is not affected by the environmental fluctuations but is suited for highly minute printing, and an image forming apparatus using the same. In the above-described embodiments, description has been made of the fact that in a scanning optical apparatus wherein the light beam emitted from the light source means 1 is directed to the deflecting means 5 through the first optical system L 1 and the light beam deflected by the deflecting means is imaged on the surface 7 to be scanned through the second optical system L 2 to thereby optically scan the surface to be scanned, the diffracting optical element 4 is provided in at least one of the first and second optical systems, thereby correcting the aberration fluctuation of the scanning optical apparatus in the sub-scanning direction caused by the environmental fluctuations. In the embodiments shown below, description will be made of the fact that a diffracting optical element is provided in each of a first optical system disposed on the light source means side with respect to the light deflector and a second optical system disposed on the surface to be scanned side, whereby the degree of freedom of the design of the diffracting optical elements is enhanced and the correction of the optical characteristic can be done singly in each optical system or the correction of the optical characteristic can be done by each diffracting optical element. Fig. 9 is a cross-sectional view of the essential portions (main scanning cross-sectional view) of Embodiment 4 of the scanning optical apparatus in the main scanning direction, and Fig. 10 is a cross-sectional view of the essential portions (sub-scanning cross-sectional view) of Embodiment 4 of the present invention in the sub-scanning direction. In Figs. 9 and 10, reference numeral 1 designates light source means comprising, for example, a semiconductor laser (laser diode). The semiconductor laser 1 in the present embodiment can oscillate light beams (laser beams) of two different wavelengths λ 1 and λ 2 , and has the characteristic that the wavelength of the light beam becomes long when the temperature of the apparatus (environmental temperature) becomes high. Fig. 11 is a graph showing the relation between the used temperature of the semiconductor laser 1 and the wavelength of the light beam emitted therefrom. Reference numeral 2 denotes a collimator lens as a first optical element which converts the divergent light beam emitted from the light source means 1 into a substantially parallel light beam (or a convergent light beam) in the main scanning direction. Reference numeral 3 designates an aperture stop which limits the light beam (the quantity of light) passing therethrough. Reference numeral 14 denotes a first diffracting optical element as a second optical element added to that flat surface of a both-surface plane parallel plate formed, for example, of a plastic material, which is adjacent to the aperture stop 3. The first diffracting optical element 14 has diffracting action (e.g. positive diffracting action, i.e., converging action, or negative diffracting action, i.e., divergent action) only in the sub-scanning direction perpendicular to the plane of the drawing sheet of Fig. 9, and causes the light beam passed through the aperture stop 3 to be imaged substantially as a linear image on the deflecting surface of a light deflector 5 which will be described later in the sub-scanning cross-section. The diffracting surface 14a of the first diffracting optical element 14 in the present embodiment is constructed on the light source means 1 side. Each of the collimator lens 2, the aperture stop 3 and the first diffracting optical element 14 constitutes an element of a first optical system L 11 . Fig. 12 is a cross-sectional view showing the grating pattern of the cross-section of the first diffracting optical element 14. This first diffracting optical element 14 is called a multiphase level and may be made by repeating etching by the use of a photomask, or may be made by cutting. In any case, it is a grating pattern only in the sub-scanning direction and therefore is easy to make. The grating pattern of the cross-section of the first diffracting optical element 14 may be a blazed grating as shown in Fig. 13. Reference numeral 5 designates a light deflector comprising, for example, a polygon mirror (rotatable polygon mirror) as deflecting means, and rotated at a constant speed in the direction of arrow A in Fig. 9 by drive means (not shown) such as a motor. Reference numeral 16 denotes an f lens having the f characteristic which comprises an anamorphic optical element as a third optical element, and which comprises, for example, a single lens formed of a plastic material. The f lens 16 in the present embodiment has a second diffracting optical element 24 having diffracting action (e.g. positive diffracting action, i.e., converging action, or negative diffracting action, i.e., divergent action) only in the main scanning direction added to its lens surface (flat surface) Ra on the light deflector 5 side, and has its lens surface Rb on the surface to be scanned side formed by a toric surface. The f lens 16 causes the light beam deflected by the light deflector 5 which is based on image information to be imaged in a spot-like shape on a photosensitive drum surface 7 which is the surface to be scanned, and corrects the surface inclination of the deflecting surface of the light deflector 5. The f lens constitutes an element of a second optical system L 12 . Fig. 12 shows the grating pattern of the cross-section of a second diffracting optical element 24 added to the lens surface Ra of the f lens 16. This second diffracting optical element 24, like the aforedescribed first diffracting optical element 14, is called a multiphase level, and may be made by repeating etching by the use of a photomask, or may be made by cutting. In any case, it is a grating pattern only in the main scanning direction and therefore is easy to make. The grating pattern of the cross-section of the second diffracting optical element 24 may be a blazed grating as shown in Fig. 13. The divergent light beam emitted from the semiconductor laser 1 in the present embodiment is converted into a substantially parallel light beam in the main scanning direction by the collimator lens 2, and the light beam (the quantity of light) is limited by the aperture stop 3 and enters the first diffracting optical element 14. That part of the light beam having entered the first diffracting optical element 14 which is in the main scanning cross-section emerges intactly therefrom. Also, in the sub-scanning cross-section, the light beam converges and is formed substantially as a linear image (a linear image long in the main scanning direction) on the deflecting surface 5a of the light deflector 5. The light beam deflected by the deflecting surface 5a of the light deflector 5 is directed to the photo-sensitive drum surface 7 through the f lens 16, and the light deflector 5 is rotated in the direction of arrow A to thereby optically scan the photosensitive drum surface 7 in the direction of arrow B. Thereby, image recording is effected on the photosensitive drum surface 7 which is a recording medium. In the present embodiment, as described above, the first diffracting optical element 14 has diffracting action (power) in the sub-scanning direction, and when the wavelength of the light beam from the semiconductor laser 1 changes and shifts, for example, to the long wavelength side, the positive power of the f lens 16 in the sub-scanning direction becomes loose and it is corrected by the optical action of the first diffracting optical element 14 that the focus of the sub-scanning cross-section shifts to the over side. That is, the first diffracting optical element 14 at this time has the action of correcting the focus shifting to the over side to the under side because the positive power thereof in the sub-scanning direction becomes strong. Thereby, the focus change of the sub-scanning cross-section when the wavelength of the light beam as the entire apparatus changes can be corrected well and any increase in the spot diameter on the photosensitive drum surface 7 can be suppressed. Also, in the present embodiment, when the used temperature of the entire apparatus has changed to e.g. the high temperature side, the positive power of the f lens 16 in the sub-scanning direction becoming loose during a high temperature and the focus of the sub-scanning cross-section shifting to the over side are corrected by the optical action of the first diffracting optical element 14 by using the semiconductor laser 1 having the characteristic shown in Fig. 11. That is, the first diffracting optical element 14 at this time has the action of correcting the focus shifting to the over side to the under side because the positive power thereof in the sub-scanning direction becomes strong. Thereby, the focus change of the sub-scanning cross-section when the used temperature of the entire apparatus changes can be corrected well and any increase in the spot diameter on the photosensitive drum surface 7 can be suppressed. As described above, in the present embodiment, the focus change of the sub-scanning cross-section resulting from the environmental fluctuations of the apparatus is corrected by the use of a change in the power (refracting power) of the f lens 16 and the first diffracting optical element 14 in the sub-scanning direction by a change in the wavelength, and a change in the power (refracting power) of the f lens 16 and the first diffracting optical element 14 in the sub-scanning direction by a change in temperature. With regard also to the main scanning cross-section, as described above, the second diffracting optical element 24 having diffracting action (power) only in the main scanning direction is added to that lens surface of the f lens 16 which is adjacent to the light deflector 5 and therefore, the focus change of the main scanning cross-section can also be corrected as well as that of the sub-scanning cross-section, whereby any increase in the spot diameter on the photosensitive drum surface 7 can be suppressed. As described above, in the present embodiment, the focus change of the main scanning cross-section resulting from the environmental fluctuations of the apparatus is corrected by the use of a change in the power (refracting power) of the f lens 16 and the second diffracting optical element 24 in the main scanning direction by a change in wavelength, and a change in the power (refracting power) of the f lens 16 and the second diffracting optical element 24 in the main scanning direction by a change in temperature. Generally, when the wavelength of a light source (semiconductor laser) is changed from λ 1 to λ 2 (λ 1 < λ 2 ), for example, by a temperature change, in the prior-art scanning optical apparatus as shown in Fig. 1 which is comprised of only a refractive lens using no diffracting optical element, the power of the first optical system becomes loose and moreover, the power of the second optical system also becomes loose, and this has led to the problem that a remarkable focus change occurs as the entire scanning optical apparatus. So, in the present embodiment, the above-noted problem is solved by making the light source capable of oscillating light beams (laser beams) of at least two different wavelengths λ 1 and λ 2 , and satisfying the following condition: -4.0 < Δ P 1 Δ P 0 - Δ P 1 < 0.0 where  1 the power of the first or second diffracting optical element for the wavelength λ 1 ΔP 0 the amount of focus change of the entire scanning optical apparatus when the wavelength of the light source means is changed from λ 1 to λ 2 ΔP 1 the focus position difference between a case where the power of the first or second diffracting optical element is calculated by  1 when the wavelength of the light source means is λ 1 and a case where the power of the first or second diffracting optical element is calculated by  1 ·(λ 2 /λ 1 ) when the wavelength of the light source means is λ 2 . That is, the power of the diffracting optical element (the first or second diffracting optical element) which was  1 in the case of the wavelength λ 1 becomes  1 ·(λ 2 /λ 1 ) in the case of the wavelength λ 2 and therefore, in conditional expression (6), ΔP 1 is a focus position difference (an amount of focus change) by only the factor of a change in the power of the diffracting optical element occurring as the result of the wavelength change from λ 1 to λ 2 , and (ΔP 0 -ΔP 1 ) is a focus position difference (an amount of focus change) by the other factor than the factor of the change in the power of the diffracting optical element. When the power  1 of the diffracting optical element is 0, ΔP 1 = 0 and as a matter of course, the focus correcting effect by the diffracting optical element becomes entirely null. At this time, usually ΔP 0 ≠ 0 and therefore, ΔP 1 /(ΔP 0 -ΔP 1 ) = 0, when ΔP 1 /(ΔP 0 -ΔP 1 ) = -1, ΔP 0 = 0 and as the entire scanning optical apparatus, the focus change by the wavelength fluctuation of the semiconductor laser becomes null. Consequently, in the range of -1 < ΔP 1 /(ΔP 0 -ΔP 1 ) < 0, the focus correction effect by the diffracting optical element is in an under-corrected state, and in the range of ΔP 1 /(ΔP 0 -ΔP 1 ) < -1 , the focus correction effect represents an over-corrected state. In the present embodiment, the value of this ΔP 1 /(ΔP 0 -ΔP 1 ) is set so as to satisfy the above-mentioned conditional expression (6). Conditional expression (6) is concerned with the ratio between the focus position difference by only the factor of the change in the power of the diffracting optical element and the focus position difference by the other factor than the factor of the change in the power of the diffracting optical element, and if the upper limit value of conditional expression (6) is exceeded, the focus correcting action by the diffracting optical element will become null or reverse correction will result, and this is not good. Also, if the lower limit value of conditional expression (6) is exceeded, over correction will result, and this is not good. More desirably, the lower limit value of conditional expression (6) may be -2.0. Also, generally, when the used temperature of the scanning optical apparatus changes from T 1 to T 2 (T 1 < T 2 ), there has arisen the problem that in the prior-art scanning optical apparatus, the power of the lens of the optical system becomes loose or the wavelength of the light source (semiconductor laser) changes, whereby as the entire scanning optical apparatus, a remarkable focus change occurs. So, in the present embodiment, the above-noted problem is solved by causing the light source to have the characteristic that its wavelength changes from λ 1 to λ 2 when the used temperature of the scanning optical apparatus is changed from T 1 to T 2 , and satisfying the following condition: -4.0 < Δ P 1T Δ P 0T - Δ P 1T < 0.0 where  1 the power of the first or second diffracting optical element for the wavelength λ 1 ΔP 0T the amount of focus change of the entire scanning optical apparatus when the used temperature of the scanning optical apparatus is changed from T 1 to T 2 ΔP 1T the focus position difference between a case where the power of the first or second diffracting optical element is calculated by  1 when the used temperature of the scanning optical apparatus is T 1 and the wavelength of the light source means is λ 1 and a case where the power of the first or second diffracting optical element is calculated by  1 ·(λ 2 /λ 1 ) when the used temperature of the scanning optical apparatus is T 2 and the wavelength of the light source means is λ 2 . That is, the power of the diffracting optical element (the first or second diffracting optical element) which was  1 in the case of the wavelength λ 1 becomes  1 ·(λ 2 /λ 1 ) in the case of the wavelength λ 2 and therefore, in conditional expression (7), ΔP 1T is the focus position difference (an amount of focus change) by only the factor of a change in the power of the diffracting optical element occurring as the result of the wavelength of the light source changing from λ 1 to λ 2 when the used temperature is changed from T 1 to T 2 , and (ΔP 0T -ΔP 1T ) is the focus position difference (the amount of focus change) by the other factor than the factor of the change in the power of the diffracting optical element. When the power  1 of the diffracting optical element is 0, ΔP 1T = 0 and as a matter of course, the focus correcting effect by the diffracting optical element becomes entirely null. Also at this time, usually ΔP 0T ≠ 0 and therefore, ΔP 1T /(ΔP 0T -ΔP 1T ) = 0. When ΔP 1T /(ΔP 0T -ΔP 1T ) = -1, ΔP 0T = 0 and as the entire scanning optical apparatus, the focus change by a change in the used temperature of the apparatus becomes null. Consequently, in the range of -1 < ΔP 1T /(ΔP 0T -ΔP 1T ) < 0, the focus correcting effect by the diffracting optical element is in an under-corrected state, and in the range of ΔP 1T /(ΔP 0T -ΔP 1T ) < -1 , the focus correcting effect represents an over-corrected state. In the present embodiment, the value of this ΔP 1T /(ΔP 0T -ΔP 1T ) is set so as to satisfy conditional expression (7). Conditional expression (7) is concerned with the ratio between the focus position difference by only the factor of the change in the power of the diffracting optical element and the focus position difference by the other factor than the factor of the change in the power of the diffracting optical element, and if the upper limit value of conditional expression (7) is exceeded, the focus correcting action by the diffracting optical element will become null or reverse correction will result, and this is not good. Also, if the lower limit value of conditional expression (7) is exceeded, over correction will result, and this is not good. More desirably, the lower limit value of conditional expression (2) may be -2.0. Numerical value embodiment 4 according to the present embodiment will be shown below. Numerical value embodiment 4 is such that in the construction of Fig. 10, the collimator lens 2, the first diffracting optical element 14 and the f lens 16 are in the refracting power arrangement of thin lenses as shown in Fig. 14, and the numerical values of only the sub-scanning cross-section are written. Numerical Value Embodiment 4 laser wavelength when the temperature of the scanning optical apparatus is 25°C = 780 nm light source 1 - collimator lens 2 interval e 1 =25 collimator lens 2 - first diffracting optical element 14 interval e 2 =20 first diffracting optical element 14 -polygon mirror 5 interval e 3 =25 polygon mirror 5 - f lens 16 interval e 4 =60 power of collimator lens 2 = 0.04 power of first diffracting optical element 14 = 0.04 power of f lens 16 = 0.025 nd and νd of collimator lens 2 = 1.80518, 25.4 nd and νd of f lens 16 = 1.49171, 57.4 laser wavelength when the temperature of the scanning optical apparatus is 50°C = 786.4 nm nd and νd of f lens 16 when the temperature is 50°C = 1.48921, 57.4 In numerical value embodiment 4, when the temperature of the scanning optical apparatus is T 1 = 25°C, the laser wavelength is λ 1 = 780 nm and the focus of the sub-scanning cross-section is at a position (point Q) distant by 120 (mm) from the f lens 16 toward the surface 7 to be scanned. When the temperature of the scanning optical apparatus is T 2 = 50°C, the laser wavelength becomes λ 2 = 786.4 nm and the power of the collimator lens 2 weakens to 1/25.0127 = 0.039980 due to color dispersion, and the power of the first diffracting optical element 14 strengthens to 0.04 × (786.4/780.4) = 0.040328, and the power of the f lens 16 weakens to 1/40.2187 = 0.024864 due to nd becoming as small as 1.48921 by a temperature change and due to color dispersion, and the amount of focus change ΔP 0T of the entire apparatus when the temperature thereof is changed from 25°C to 50°C is ΔP 0T = 1.208. The focus position difference (the amount of focus change) ΔP 1T between a case where the power of the first diffracting optical element 14 is calculated by 0.04 when the temperature is 25°C and the wavelength λ 1 is 780 nm and a case where the power of the first diffracting optical element 14 is calculated by 0.040328 when the temperature is 50°C and the wavelength λ 2 is 786.4 nm is ΔP 1T = -0.806, and ΔP 1T /(ΔP 0T -ΔP 1T ) = -0.40. This shows that good focus correction is effected by a change in the power of the first diffracting optical element caused by the wavelength having been changed when the temperature has been changed, and satisfies the above-mentioned conditional expression (7). Fig. 15 is a schematic view showing the refracting power arrangement of Embodiment 5 of the scanning optical apparatus in the sub-scanning cross-section. In Fig. 15, the same elements as the elements shown in Fig. 14 are given the same reference numerals. The difference of the present embodiment from the aforedescribed Embodiment 4 is that the second optical element is of a hybrid construction of a diffracting element portion (first diffracting optical element) 71 having diffracting action and a refracting lens portion (non-diffracting element lens portion) 72 having refracting action. In the other points, the construction and optical action of the present embodiment are substantially similar to those of Embodiment 4. That is, in Fig. 15, reference numeral 74 designates a second optical element which is of a hybrid construction of the diffracting element portion (first diffracting optical element) 71 having diffracting action only in the sub-scanning direction and the refracting lens portion 72 having positive refracting power in the sub-scanning direction. By also constructing the second optical element 74 as described above, an effect similar to that of the aforedescribed Embodiment 4 can be obtained in the present embodiment. Numerical value embodiment 5 according to Embodiment 5 will be shown below. In numerical value embodiment 5, the numerical values only in the sub-scanning cross-section are written. Numerical Value Embodiment 5 laser reference wavelength = 780 nm wavelength change = 6.4 nm light source 1 - collimator lens 2 interval e 1 =25 collimator lens 2 - diffracting element portion 71 interval e 2 =20 diffracting element portion 71 -refracting lens portion 72 interval e 3 =0 refracting lens portion 72 -polygon mirror 5 interval e 4 =25 polygon mirror 5 - f lens 16 interval e 5 =60 power of collimator lens 2 = 0.04 power of diffracting element portion 71 = 0.01 power of refracting lens portion 72 = 0.03 power of f lens 16 = 0.025 nd and νd of collimator lens 2 = 1.80518, 25.4 nd and νd of refracting lens portion 72 = 1.51633, 64.2 nd and νd of f lens 16 = 1.49171, 57.4 In numerical value embodiment 5, when the laser wavelength is λ 1 = 780 nm, the focus of the sub-scanning cross-section is at a position (point Q) distant by 120 (mm) from the f lens 16 toward the surface 7 to be scanned. When the laser wavelength changes to λ 2 = 786.4 nm, the power of each lens assumes the following value due to color dispersion: power of collimator lens 2 = 0.039980 power of refracting lens portion 72 = 0.029992 power of f lens 16 = 0.024993 Also, the power of the diffracting element portion 71 is 0.01 × (786.4/780) = 0.010082. As the result, the amount of focus change ΔP 0 of the entire apparatus when the wavelength changes from λ 1 to λ 2 is ΔP 0 = -0.031. The focus position difference (the amount of focus change) ΔP 1 between a case where the power of the diffracting element portion 71 is calculated by 0.01 when the wavelength is λ 1 and a case where the power of the diffracting element portion 71 is calculated by 0.010082 when the wavelength is λ 2 is ΔP 1 = -0.204, and ΔP 1 /(ΔP 0 -ΔP 1 ) = -1.20. This shows that when the wavelength changes, good focus correction is effected by a change in the power of the diffracting element portion 71, and satisfies the above-mentioned conditional expression (6). In the above-described numerical value embodiments 4 and 5, the examples of numerical values in the sub-scanning cross-section have been shown, but if the diffracting action (power) of the second diffracting optical element 24 in the main scanning direction is appropriately set so as to satisfy the aforementioned conditional expressions (6) and (7) with respect also to the main scanning cross-section, focus correction can be effected well for environmental fluctuations such as temperature change and a change in the wavelength of the light source. Fig. 16 is a cross-sectional view (main scanning cross-sectional view) of the essential portions of Embodiment 6 of the scanning optical apparatus of the present invention in the main scanning direction, and Fig. 17 is a cross-sectional view (sub-scanning cross-sectional view) of the essential portions of Embodiment 6 in the sub-scanning direction. In Figs. 16 and 17, the same elements as the elements shown in Figs. 9 and 10 are given the same reference characters. The differences of the present embodiment from the aforedescribed Embodiment 4 are that the second optical element is comprised of a cylindrical lens, that a diffracting optical element (first diffracting optical element) is added to one surface of the cylindrical lens and the second optical system is comprised of an f lens, a plane mirror and a cylindrical mirror and that a diffracting optical element (second diffracting optical element) is added to that lens surface of the f lens which is adjacent to the surface to be scanned. In the other points, the construction and optical action of the present embodiment are substantially similar to those of the aforedescribed Embodiment 4, whereby a similar effect is obtained. That is, in Figs. 16 and 17, reference numeral 95 designates a cylindrical lens (referred to also as a diffraction lens) comprising an anamorphic optical element as the second optical element, and a first diffracting element portion (first diffracting optical element) 94 having diffracting action only in the sub-scanning direction is added to that lens surface of the cylindrical lens 95 which is adjacent to the collimator lens 2, and that lens surface of the cylindrical lens 95 which is adjacent to the light deflector 5 is comprised of a cylinder refracting lens portion 93 having positive power in the sub-scanning direction. Reference numeral 96 denotes an f lens having the f characteristic which comprises an anamorphic optical element as a third optical element, and which comprises, for example, a single lens formed of a plastic material. That lens surface of the f lens in the present embodiment which is adjacent to the light deflector 5 is of an aspherical shape, and that lens surface of the f lens which is adjacent to the surface 7 to be scanned is comprised of a refracting lens portion 97 made into a rotation-symmetrical spherical surface having positive power, and a second diffracting element portion (second diffracting optical element) 98 of a rotation-symmetrical grating pattern having diffracting action only in the main scanning direction on the rotation-symmetrical spherical surface. Reference numeral 8 designates a plane mirror, and reference numeral 9 denotes a cylindrical mirror having positive refracting power in the sub-scanning direction. In the present embodiment, a divergent light beam emitted from the semiconductor laser 1 is converted into a substantially parallel light beam in the main scanning direction by the collimator lens 2, and this light beam (the quantity of light) is limited by the aperture stop 3 and enters the cylindrical lens (first diffracting optical element 94) 95. That part of the light beam having entered the cylindrical lens 95 which is in the main scanning cross-section emerges intactly therefrom. Also, in the sub-scanning cross-section, the light beam converges and is formed substantially as a linear image (a linear image long in the main scanning direction) on the deflecting surface 5a of the light deflector 5. The light beam deflected by the deflecting surface 5a of the light deflector 5 is directed onto the photosensitive drum surface 7 by the f lens 96 via the plane mirror 8 and the cylindrical mirror 9, and optically scans the photosensitive drum surface 7 in the direction of arrow B by the light deflector 5 being rotated in the direction of arrow A. Thereby, image recording is effected on the photosensitive drum surface 7 which is a recording medium. In the present embodiment, the f lens 96, the plane mirror 8 and the cylindrical mirror 9 together constitute an inclination correcting system for the light deflector 5, and the positive power in the sub-scanning direction is stronger than the positive power in the main scanning direction. Here, when the focus correction by a wavelength change is to be optimally effected, the positive power of the diffracting element portion in the sub-scanning direction need be made stronger because the positive power is stronger in the sub-scanning direction than in the main scanning direction. However, when the grating pattern of the second diffracting element portion 98 is made rotation-symmetrical to thereby optimize the focus correction in the main scanning direction, the focus correction in the sub-scanning direction becomes under-correction. So, in the present embodiment, the under-correction is compensated for by the first diffracting element portion 94 having diffracting action only in the sub-scanning direction added to that lens surface of the cylindrical lens 95 which is adjacent to the collimator lens 2, whereby as a whole, the focuses in the main scanning direction and the sub-scanning direction are corrected well. In the present embodiment, as described above, the second diffracting element portion 98 of a rotation-symmetrical grating pattern having positive power is added to that lens surface of the f lens which is adjacent to the surface 7 to be scanned, whereby the first diffracting element portion 94 can be weakened in positive power and thus, is of a shape easy to manufacture. While in the present embodiment, the first diffracting element portion (diffracting optical element) is added to one surface of the cylindrical lens, this is not restrictive, but the first diffracting element portion may be added to one surface of the collimator lens. Also, while in the present embodiment, the first and second diffracting element portions are added to the surfaces of the optical elements, this is not restrictive, but they may be constructed independently in the optical path. As described above, in each of the embodiments, the diffracting optical element is provided in each of the first optical system disposed on the light source side with respect to the light deflector and the second optical system disposed on the surface to be scanned side, whereby the optical characteristic can be singly corrected in each optical system, or the correction of the optical characteristic can be dividedly effected by each diffracting optical element and thus, more highly accurate recording of image information can be accomplished. Also, the fact that the correction of the optical characteristic can be dividedly effected means that the grating pattern can be made simpler and realized as compared with a case where only one diffracting optical element is used and therefore, the manufacture of the diffracting optical elements can be made easy. Also, at least one of the first diffracting optical element and the second diffracting optical element is designed to have diffracting action in one of the main scanning direction and the sub-scanning direction, whereby at least one diffracting optical element assumes a straight belt-like grating pattern only in the main scanning direction or the sub-scanning direction, and when for example, a diffracting optical element is to be manufactured by molding, the manufacture becomes possible by only the rectilinear movement of a cutting tool particularly in cutting a mold and making a grating pattern, whereby the manufacture can be made easy. Also, one of the first diffracting optical element and the second diffracting optical element is designed to have diffracting action only in the main scanning direction and the other diffracting optical element is designed to have diffracting action only in the sub-scanning direction, whereby the grating patterns of both of the diffracting optical elements can be made into a straight belt-like shape, whereby the manufacture can be made easy, and yet the optical characteristics can be independently controlled in the main scanning direction and the sub-scanning direction. In the present embodiment, although the manufacture becomes more or less difficult, the first and second diffracting optical elements may be designed to have diffracting action in both of the main scanning direction and the sub-scanning direction, whereby a similar effect is also obtained. As previously described, there can be achieved a scanning optical apparatus in which the diffracting optical element is provided in each of the first optical system disposed on the light source means side with respect to the light deflector and the second optical system disposed on the surface to be scanned side, whereby the degree of freedom of the design of the diffracting optical elements is enhanced, and the correction of the optical characteristic can be singly effected in each optical system or the correction of the optical characteristic can be dividedly effected by each diffracting optical element, and more highly accurate recording of image information can be effected and further, the correction of the optical characteristic can be effected well by the diffracting optical element easy to manufacture with the grating pattern of the diffracting optical element made simpler. A scanning optical apparatus includes a light source, deflector for deflecting a light beam emitted from the light source, and optical device for directing the light beam emitted from the light source onto a surface to be scanned. The optical device includes a diffracting optical element. An aberration fluctuation in a sub-scanning direction resulting from environmental fluctuation of the scanning optical apparatus is corrected by the characteristic of the optical device.

Claims: A scanning optical apparatus comprising: a semi-conductor laser (1) for emitting a light beam; a deflecting means (5) for deflecting the light beam emitted from said light source means (1); a first optical system (L 41 ) for directing the light beam emitted from said light source means (1) to said deflecting means (5); a second optical system (L 42 ) for directing the light beam deflected by said deflecting means (5) onto a surface (7) to be scanned and for imaging the light beam deflected by said deflecting means (5) on said surface (7) at a focus position, wherein said second optical system (L 42 ) includes a refracting portion with plastic material and a diffracting portion (48), characterized in that a power of said refracting portion in a sub-scanning direction of the scanning optical apparatus and a power of said diffracting portion (48) in the sub-scanning direction vary in accordance with a temperature fluctuation of the temperature of the scanning optical apparatus, and in that the condition 1.0 ≤ L/B ≤ 2.6 is satisfied, wherein B is the power of said diffracting portion (48) in the sub-scanning direction and L is the power of said refracting portion in the sub-scanning direction, so that a fluctuation of the focus position in the sub-scanning direction, resulting from said temperature fluctuation, is substantially completely corrected due to the adequate power ratio  L / B . A scanning optical apparatus according to claim 1, wherein a variation of said power of said refracting portion in the sub-scanning direction which is caused by variation of the refractive index of said refractive portion in accordance with said temperature fluctuation and is caused by a wavelength fluctuation of said light source means (1) in accordance with said temperature fluctuation, is corrected by a variation of said power of said diffracting portion (48) in the sub-scanning direction which is caused by the wavelength fluctuation of said light source means (1) in accordance with said temperature fluctuation. A scanning optical apparatus according to claim 1 or 2, wherein said refracting portion of said second optical system (L 42 ) includes a plastic lens. A scanning optical apparatus according to claim 1, wherein said refracting portion of said second optical system (L 42 ) includes a plastic lens, and wherein a focus fluctuation in the sub-scanning direction in accordance with said temperature fluctuation of said plastic lens is corrected by a variation of said power of said diffracting portion (48) in the sub-scanning direction in accordance with a wavelength fluctuation of said light source means (1) which is caused by said temperature fluctuation. A scanning optical apparatus according to claim 4, wherein said focus fluctuation in the sub-scanning direction in accordance with said temperature fluctuation of said plastic lens is caused by a power variation in the sub-scanning direction by a variation of the refractive index of said plastic lens which is caused by said temperature fluctuation and which is caused by said wavelength fluctuation of said light source means (1) in accordance with said temperature fluctuation. A scanning optical apparatus according to claim 4, wherein said diffracting portion is formed by an optical element (48) having a staircase-like grating structure. A scanning optical apparatus according to claim 4, wherein said diffracting portion is formed by an optical element (48) having a Fresnel-like continuous grating structure. A scanning optical apparatus according to claim 4, wherein said first optical system (L 41 ) has a function for imaging the light beam emitted from said light source means (1) onto a deflection surface of said deflecting means (5) as a line image elongated in a main-scanning direction. A scanning optical apparatus according to claim 4, wherein a focus state variation in a main-scanning direction of the scanning optical apparatus in accordance with said temperature fluctuation of said refracting portion is corrected by a variation of the power of said diffracting portion (48) in accordance with the wavelength fluctuation of said light source means (1) which is caused by said temperature fluctuation. An image forming apparatus comprising the scanning optical apparatus according to any of claims 1 to 9 and a photo sensitive drum surface (7) which constitutes said surface to be scanned.

Inventors: FUJIBAYASHI KAZUO, HOSHI KOJI, ISHIBE YOSHIHIRO, FUJIBAYASHI, KAZUO, HOSHI, KOJI, ISHIBE, YOSHIHIRO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: G02B26
IPC codes: G02B26