JP3680891B2 - Optical scanning device - Google Patents

Description

【００４３】
第２整形レンズ２及び第３走査レンズ１６の非球面を表す式は、
ｚ_i ＝（ｙ² ／ｒ_i ）／［１＋｛１−（Ｋ_i ＋１）（ｙ／ｒ_i ）² ｝^1/2 ］＋Ａ_i ｙ⁴ ＋Ｂ_i ｙ⁶ ＋Ｃ_i ｙ⁸
であり、その非球面係数を次の表−２に示す。
【００４４】

【００４５】
この具体例において、第３走査レンズ１６の入射面Ｓ₂₅は、ｒ_25y ＝−１４７５．３９３７８の円弧をｒ_25x ＝３７．９５６７５で回転させて形成されるトーリック面である。なお、第２走査レンズ１５、第３走査レンズ１６を通過するときのように、光路が屈折されるときは、光軸は主光線と同じように屈折されるものとし、表−１、表−２のパラメータの基準となる光軸は、常に走査中心を走査するビームの主光線に一致するものとする。
【００４６】
また、回転多面鏡４の面数は１２、その内接円直径は３８．６４ｍｍであり、回転多面鏡４の第１反射面５、第２反射面６への光ビームの副走査方向の入射角は何れも６°であり、第１伝達ミラー１０、第２伝達ミラー１３への光ビームの副走査方向の入射角は何れも３°である。また、第２走査レンズ１５の射出面Ｓ₂₄は副走査断面において１３°傾いており、第３走査レンズ１６の入射面Ｓ₂₅は副走査断面において８．７５０３８７°傾いており、第３走査レンズ１６の射出面Ｓ₂₆は副走査断面において２．８７５３７４°傾いている。これらの傾き角の向きについては、図２、図４参照。
【００４７】
また、第１整形レンズ入射面Ｓ₃ に一致して、主走査方向０．７１５４ｍｍ、副走査方向１．０５２６ｍｍの矩形のアパーチャが配置されている。そして、副走査方向において、発光点１と回転多面鏡４の第１反射面５は幾何光学的共役関係から外れている。ただし、回転多面鏡４の第１反射面５、第２反射面６、被走査面１７の３面は、何れも互いに共役関係にあるため、回転多面鏡４の面倒れ補正が行われている。したがって、発光点１と被走査面１７は共役関係から外れているともいえる。しかしながら、回折の影響により、光ビームが最小となる位置は幾何光学的結像点からずれた位置にあり、光ビームが略最小となる位置に被走査面１７が配置されている。
【００４８】
なお、上記具体例の伝達光学系２２の主走査方向の光学倍率βは８．２４、副走査方向の光学倍率β_t は１．１２、走査光学系２３の副走査方向の光学倍率β_s は０．４０６である。
【００４９】
ここで、第２走査レンズ１５は、前記したように、副走査方向にのみ屈折作用を有するプリズムである。このプリズムの作用について説明する。回転多面鏡４の反射面６で反射され偏向された光ビームは円錐状の軌跡を描き、第２走査レンズ１５のプリズムを配置しない場合、第３走査レンズ１６の長尺レンズ上で湾曲したビーム軌跡となってしまう。このプリズム１６は、図９に模式的に示すように、円錐状の光ビームａの軌跡を第３走査レンズ１６の入射面上で直線状のビーム軌跡Ａに変換する作用を有している。
【００５０】
図１０は、上記の具体例の第３走査レンズ１６の入射面におけるビーム軌跡を示した図であり、そのビーム軌跡を実線で示す。なお、図のＹ方向が主走査方向、Ｘ方向が副走査方向を示す。比較のために、上記具体例の光学系の回転多面鏡４の第２反射面６から第３走査レンズ１６までの距離は変えずに、第２走査レンズ１５のみを取り除いた場合の、第３走査レンズ１６の入射面におけるビームの軌跡を破線で示す。図１０より、第２走査レンズ１５のプリズム作用によりビームの軌跡を直線状に補正する作用があることが分かる。
【００５１】
図１１は、第３走査レンズ１６の副走査断面を主走査方向の数か所（５か所）の位置で示したもので、断面形状の設計値に対する測定値の誤差を示したものである。図中、Ｘ、Ｙ、Ｚはそれぞれ副走査方向、主走査方向、光軸方向とする。図１１のように、第３走査レンズ１６のような鞍型トーリック面を持つレンズの形状誤差は、主走査方向の位置によらず略同じ様子を示すが、副走査方向に周期的に変化する特徴がある。上記のように、第２走査レンズ１５のプリズム作用により、第３走査レンズ１６上のビーム軌跡は直線Ａとなり、ビームは主走査方向の位置に係わらず点Ｂ₁ 〜Ｂ₅ の常に形状誤差が凸の部分に入射する。主走査方向の何れの位置においても、第３走査レンズ１６の形状誤差が凸の部分に光ビームが入射すると、副走査方向の結像位置は設計された位置より手前にずれるが、走査領域全体にわたって常に同一量だけ手前にずれるため、第３走査レンズ１６の位置を調整する等、光学系の調整をすれば補正することが可能であり、このような調整により像面湾曲は生じない。
【００５２】
さて、ここで、回転多面鏡４の回転軸４１に対する反射面の位置ずれについて検討する。回転多面鏡４の回転軸４１に対する反射面５、６の位置ずれの原因として、次の２点があげられる。１つは、回転多面鏡４の中心軸４１とモータの回転軸の偏心であり、もう１つは、回転多面鏡４の各反射面を切削する際の製造誤差によるものである。
【００５３】
まず、前者について説明する。図１２に示すように、回転多面鏡偏向装置２５はモータ２６の回転軸に取り付けられた回転多面鏡４から構成されているが、図１３に模式的に示すように、回転多面鏡４の中心軸４１’とモータ２６の回転軸２７の中心２８とが製造誤差によりδだけ偏心しており、回転軸２７に対する回転多面鏡４の各反射面の位置は、図１４に示すように、１回転１周期で正弦的に変動する。前記実施例の回転多面鏡４の面数は１２面であり、回転多面鏡４の中心軸４１’とモータ２６の回転軸２７のずれは、通常、数十μｍ程度存在する。
【００５４】
後者については、回転多面鏡４の各反射面の位置ずれに規則性はなく、ランダムに発生する。ただし、前者の位置ずれに比べて後者の位置ずれは小さく、前者の原因による位置ずれが支配的である。
【００５５】
ここで、回転多面鏡４の反射面の位置ずれによる走査線への影響について説明する。回転多面鏡４の第２反射面６へ１度だけ入射するときのその反射面６の位置ずれを考えると、図１５に副走査方向の光路を示すように、回転多面鏡４の反射面６に副走査方向の入射角αで入射するとき（斜め入射）、回転多面鏡４の回転軸４１に対する反射面６の位置ずれがδであれば、反射されたビームの位置ずれｄは、
ｄ＝２αδ
となる。ただし、αは小さいため、ｔａｎα≒αと近似している。走査光学系２３の副走査方向の倍率をβ_s とすると、被走査面１７におけるビームの副走査方向への位置ずれｅは、
ｅ＝２αδβ_s
となる。
【００５６】
そこで、本発明においては、前記実施例のように、回転多面鏡４の２つの反射面５、６へ２度入射するようにし、かつ、その第１反射面５と第２反射面６を互いに平行で互いに１８０°の角度をなして対向するように配置して、回転多面鏡４の回転軸４１の偏心に基づく走査線の副走査方向への位置ずれをなくすようにしたものである。その原理を図１６に基づいて説明する。図１６に本発明に基づく光走査装置の回転多面鏡４近傍の副走査方向の主光線の光路を示す。図１６のように、光源からの光ビームは、まず、回転多面鏡４の第１反射面５に１度目の入射をし、ここで１度目の偏向がなされた光ビームは伝達光学系２２によって伝達され、第１反射面５と対向する回転多面鏡４の第２反射面６に２度目の入射をして２度目の偏向がなされ、その偏向ビームは走査光学系２３により被走査面１７上に結像され走査される。そして、第１反射面５、第２反射面６に入射する光ビームは、何れも副走査方向に角度を持って入射するように設定されている。このような配置であると、図１６から明らかなように、回転多面鏡４の回転軸４１が偏心している場合、第１反射面５と第２反射面６のずれ方向δは同じ方向となり、光路は図の実線から破線に変化する。第１反射面５と第２反射面６が図示のように左側にずれている場合、第１反射面５での反射直後の光路は下側にずれるが、伝達光学系２２の結像作用により、第２反射面６の直前では光路は逆に上側にずれる。しかし、第２反射面６には上方から斜め入射するため、反射後の主光線は、位置ずれのない場合（実線）の反射点と略同じ位置を通ることになる。第２反射面６で反射さたビームの角度は異なるが、第２反射面６と被走査面１７は走査光学系２３により共役関係にあるため、被走査面１７上では位置ずれのない場合と略同じ位置になる。この作用は、図１６と図１５を比較すれば明らかである。本発明の上記原理は、回転多面鏡４の回転軸４１が偏心していても、第１反射面５と第２反射面６のずれ方向が同じ方向となるから、光ビームのずれを両反射面５、６で相互に相殺する配置となっていると言うことができる。
【００５７】
以上は定性的な説明であったが、以下に定量的な説明を行う。図１７に示すように、第１反射面５への入射角をα₁ 、第２反射面６への入射角をα₂ 、伝達光学系２２の副走査方向の倍率をβ_t とすると、第１反射面５と第２反射面６の回転多面鏡４の回転軸４１の偏心に基づく位置ずれがδである場合、第１反射面５で反射された光ビームの位置ずれｄ₁ は、
ｄ₁ ＝２α₁ δ
となる。ただし、α₁ は小さいため、ｔａｎα₁ ≒α₁ と近似している。α₁ の単位はラジアン。この位置ずれｄ₁ は、伝達光学系２２により第２反射面６上では、２α₁ δβ_t となる。また、第２反射面６の位置ずれにより発生する第２反射面６で反射された光ビームの位置ずれｄ₂ は、
ｄ₂ ＝２α₂ δ
となる。ただし、α₂ は小さいため、ｔａｎα₂ ≒α₂ と近似している。α₂ の単位はラジアン。第１反射面５、第２反射面６の両面の位置ずれを考慮すると、第２反射面６で反射された光ビームの位置ずれｄは、

となる。走査光学系２３の副走査方向の倍率をβ_s とすると、被走査面１７における光ビームの位置ずれｅは、
ｅ＝２δβ_s ｜α₁ β_t −α₂ ｜
となる。したがって、
β_t ＝α₂ ／α₁ ・・・（１）
とすれば、回転多面鏡４の回転軸４１の偏心に基づく反射面５、６の位置ずれによる走査線の副走査方向への位置ずれは補正され、走査線の位置は一定となる。
【００５８】
ここで、（１）式とは別に、ｅの許容量は、被走査面１７上での副走査方向の走査線の間隔ｐの１／４であり、この値を越えると、被走査面１７における露光量のむらが問題となる。したがって、
２δβ_s ｜α₁ β_t −α₂ ｜≦ｐ／４
すなわち、
δβ_s ｜α₁ β_t −α₂ ｜／ｐ≦１／８ ・・・（２）
の条件を満たすことが必要である。
【００５９】
前記の具体的な数値例においては、β_t ＝１．１２、β_s ＝０．４０６、α₁ ＝α₂ ＝０．１０５ラジアン（＝６°）、ｐ＝０．０４２３ｍｍ（６００ｄｐｉ相当）であり、δの最大値は０．０３ｍｍであるので、
δβ_s ｜α₁ β_t −α₂ ｜／ｐ＝０．００３６
となり、（２）式を満足している。この場合の走査線の位置ずれを図１８示す。この図のＹ方向は主走査方向、Ｘ方向は副走査方向を示す。第１反射面５だけの位置ずれによる走査線の位置ずれを破線で示し、第２反射面６だけの位置ずれによる走査線の位置ずれを一点鎖線で示し、第１反射面５、第２反射面６の両面の位置ずれによる走査線の位置ずれを実線で示す。本発明により第１反射面５、第２反射面６の位置ずれによる影響が相殺されて、走査線の位置ずれが良好に補正されていることが分かる。
【００６０】
以上、本発明の光走査装置を実施例に基づいて説明してきたが、本発明はこれらに限定されず、種々の変形が可能である。
【００６１】
【発明の効果】
以上の説明から明らかなように、本発明の光走査装置によれば、回転多面鏡への２度入射の光走査装置において、第１反射面と第２反射面を回転軸を挟んで互いに平行で互いに１８０°の角度をなして対向する反射面に設定し、両反射面へ斜め入射をさせ、第１反射面、第２反射面、被走査面を略共役関係にしているので、光ビームのずれを両反射面で相互に相殺する配置となっており、回転多面鏡の回転軸の偏心に基づく各反射面の位置ずれによる走査線の副走査方向への位置ずれは補正され、走査線の位置は一定となり、画像にむらが発生せず正確で良好な画像を再現することができる。
【図面の簡単な説明】
【図１】本発明の光走査装置の１実施例の構成を示す平面図である。
【図２】図１の光走査装置の側面図である。
【図３】図１の光走査装置の主要部の斜視図である。
【図４】図１の光走査装置の主要部の側面図である。
【図５】図１の光走査装置の整形光学系の主走査方向と副走査方向の光路図である。
【図６】図１の光走査装置の伝達光学系の主走査方向と副走査方向の光路図である。
【図７】図１の光走査装置の走査光学系の主走査方向と副走査方向の光路図である。
【図８】伝達光学系の作用を説明するための主走査面の断面展開図である。
【図９】屈折プリズムの補正作用を説明するための図である。
【図１０】本発明の１つの具体例の第３走査レンズの入射面におけるビーム軌跡を示した図である。
【図１１】本発明の１つの具体例において像面湾曲が発生しない理由を説明するための図である。
【図１２】本発明の光走査装置において用いる回転多面鏡偏向装置の１例を示す斜視図である。
【図１３】回転多面鏡の中心軸とモータの回転軸の中心が製造誤差により偏心する様子を模式的に示す図である。
【図１４】回転軸に対する回転多面鏡の各反射面の位置が１回転１周期で正弦的に変動する様子を示す図である。
【図１５】回転多面鏡へ１度だけ入射するときのビームの位置ずれを示す副走査方向の光路図である。
【図１６】本発明の光走査装置においてビームの位置ずれが起こらないことを示す副走査方向の光路図である。
【図１７】本発明の光走査装置における各パラメータを示す図である。
【図１８】本発明の１つの具体例における走査線の位置ずれ示す図である。
【符号の説明】
１…半導体レーザー（光源）
２…第１整形レンズ
３…第２整形レンズ
４…回転多面鏡
５…回転多面鏡の第１反射面
６…回転多面鏡の第２反射面
７…第１伝達レンズ
８…第２伝達レンズ
９…第３伝達レンズ
１０…第１伝達ミラー
１１…第４伝達レンズ
１２…第５伝達レンズ
１３…第２伝達ミラー
１４…第１走査レンズ
１５…第２走査レンズ（プリズム）
１６…第３走査レンズ（長尺レンズ）
１７…被走査面
２１…整形光学系
２２…伝達光学系
２３…走査光学系
２４…主走査方向正屈折力伝達レンズ群
２５…回転多面鏡偏向装置
２６…モータ
２７…モータの回転軸
２８…モータの回転軸の中心
４１…回転多面鏡の回転軸
４１’…回転多面鏡の中心軸[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical scanning device used in a laser beam printer or the like, and in particular, light that causes a light beam to enter a rotary polygon mirror twice in order with an angle with respect to a scanning plane perpendicular to the rotation axis of the rotary polygon mirror. The present invention relates to an optical scanning device in which a scanning line position variation based on a positional deviation of a reflecting surface with respect to a rotation axis of a rotary polygon mirror is prevented.
[0002]
[Prior art]
In an image recording apparatus such as a laser beam printer and an optical scanning apparatus used in various image reading and measuring apparatuses, a rotating polygon mirror has been often used as a deflector for deflecting and scanning a light beam.
[0003]
In these apparatuses, two-dimensional scanning is performed by repeatedly scanning a light beam on a scanning surface on a straight line or a curve, and relatively moving a scanning medium positioned on the scanning surface in a direction substantially orthogonal to the scanning direction. I do. The scanning direction by the former optical scanning device is the main scanning direction, and the relative movement direction of the latter scanning medium is the sub-scanning direction.
[0004]
In recent years, higher speed optical scanning devices have been required for the above-described devices in order to improve resolution and processing speed. In an optical scanning device using a rotating polygon mirror for deflecting a light beam, in order to increase the scanning speed (scanning frequency),
(1) Increase the rotational speed of the rotary polygon mirror.
(2) Increase the number of surfaces of the rotating polygon mirror.
There are two possible methods.
[0005]
In order to increase the rotational speed of the rotary polygon mirror, a bearing capable of high-speed rotation is required. However, with the most frequently used ball bearings, the upper limit is about 20,000 revolutions per minute. If an air bearing is used, it can be used at a rotational speed of 30000 revolutions per minute or more, but the apparatus that can be used is limited because the bearing is expensive. In particular, it cannot be used for an inexpensive laser beam printer for general consumers.
[0006]
On the other hand, when the number of surfaces of the rotating polygon mirror is increased, the rotation angle per one reflecting surface is decreased. Also, if the size of each reflecting surface is to be secured above a certain level, the diameter of the rotating polygon mirror will increase.
[0007]
In an optical scanning device, a light beam is often formed on a surface to be scanned, but when a laser beam is scanned, a rotating polygon mirror is used in order to form an image on a small spot depending on the divergence angle of the light beam. The reflecting surface needs to have a certain size in the main scanning direction. However, when the number of surfaces of the rotary polygon mirror is increased, the scanning angle of the light beam is also reduced because the rotation angle on one reflecting surface is small. If the scanning angle of the light beam is small, the focal length of the scanning optical system becomes long to obtain a predetermined scanning width, and the optical path length from the rotary polygon mirror to the surface to be scanned also increases. For this reason, the diameter of the light beam in the main scanning direction on the reflecting surface of the rotating polygon mirror also increases, the reflecting surface becomes larger than when the number of surfaces is small, and the size of the rotating polygon mirror further increases. .
[0008]
In other words, the size of the rotating polygon mirror (the size of the inscribed cylinder) is inconsistent with the increase in the number of surfaces of the rotating polygon mirror because the required size of the reflecting surface becomes larger than when the number of surfaces is small. If the size is determined, the upper limit of the number of faces is determined. For example, in an optical scanning device used for a laser beam printer, when the required scanning width is 350 mm, the wavelength is 780 nm, the radius of the inscribed cylinder of the rotary polygon mirror is 25 mm, and the spot diameter in the main scanning direction on the scanned surface is 50 μm or less, The upper limit of the number of faces is generally seven.
[0009]
Therefore, increasing the diameter of the rotating polygon mirror to increase the number of surfaces increases the weight and inertial moment of inertia of the rotating polygon mirror, and increases the air resistance (windage loss) associated with the rotation. Limited to low.
[0010]
Thus, since there are upper limits on the number of surfaces and the number of rotations of the rotary polygon mirror, various optical scanning devices have been devised in order to obtain a scanning speed exceeding the upper limit.
[0011]
For example, in the technique described in JP-A-51-100742, a semiconductor laser array is used as a light source, and the scanning surface is simultaneously scanned with a plurality of laser beams to improve the scanning speed. According to this method, the scanning speed can be increased by the number of lasers integrated in the element without increasing the rotational speed of the rotary polygon mirror.
[0012]
On the other hand, in Japanese Patent Application Laid-Open No. 51-32340, a light beam emitted from a light source is incident on a rotating polygon mirror in a main scanning direction with a very small diameter, and the deflected light beam is transmitted through a transmission optical system. A method of re-entering the rotating polygon mirror is disclosed. That is, the light beam is incident twice on the rotating polygon mirror.
[0013]
In the latter method, the diameter of the light beam in the main scanning direction when the light beam is first incident on the rotary polygon mirror is made extremely small compared to the case where the light beam is incident on the second time. The transmission optical system is configured so that the incident light beam follows the center point in the main scanning direction of the rotating reflecting surface.
[0014]
With this configuration, when the light beam first enters the rotating polygon mirror, the diameter of the light beam can be made extremely small, so that the scanning can be performed up to the full dividing angle of the rotating polygon mirror. When the light beam deflected by the first reflecting surface enters the rotary polygon mirror for the second time via the transmission optical system, the diameter of the light beam is necessary to obtain a predetermined spot on the surface to be scanned. Although it is enlarged to a large size, the size of the light beam can be set regardless of the rotation angle of the rotary polygon mirror in order to follow the rotation of the reflecting surface.
[0015]
On the other hand, in an optical scanning device, a device that makes a light beam incident and deflects at an angle with respect to a scanning surface perpendicular to the rotation axis of the rotary polygon mirror is known, for example, in JP-A-1-169422.
[0016]
In this specification, the incidence on the reflecting surfaces of the rotating polygonal mirrors twice in order as described above will be referred to as “double incidence”. Further, the incidence of the light beam with an angle on the scanning surface of the rotary polygon mirror is referred to as “oblique incidence”.
[0017]
[Problems to be solved by the invention]
By the way, when deflecting by performing such oblique incidence on the rotating polygon mirror, the position of the scanning line on the surface to be scanned is changed in the direction of the scanning line because the position of the reflecting surface is shifted with respect to the rotation axis of the rotating polygon mirror. This causes a problem in that the image changes in the sub-scanning direction at right angles to the image, causing unevenness in the image and making it difficult to reproduce an accurate and good image.
[0018]
The present invention has been made in view of such a point of the prior art, and its object is to prevent the position variation of the scanning line caused by the displacement of each reflecting surface based on the eccentricity of the rotating shaft of the rotary polygon mirror 2. It is an object to provide a high-speed optical scanning apparatus that is obliquely incident and obliquely incident.
[0019]
[Means for Solving the Problems]
An optical scanning device of the present invention that achieves the above object includes a light source that generates a light beam, a rotating polygon mirror having a plurality of reflecting surfaces that reflect and deflect the light beam from the light source, and a first reflection of the rotating polygon mirror. A transmission optical system for transmitting and incident the light beam reflected and deflected by the surface to the second reflecting surface of the rotating polygon mirror, and the light beam reflected and deflected by the second reflecting surface of the rotating polygon mirror on the surface to be scanned In an optical scanning device comprising a scanning optical system for forming and scanning a beam spot,
The rotating polygon mirror has a plurality of pairs of reflecting surfaces that are parallel to each other across an axis of rotation and that are opposed to each other at an angle of 180 °, and the first reflecting surface and the second reflecting surface are rotated. It is set on the reflecting surfaces that are parallel to each other across the rotation axis of the polygon mirror and are opposed to each other at an angle of 180 °,
The transmission optical system has an even number of reflecting surfaces in an optical path from the first reflecting surface to the second reflecting surface,
The light beam from the light source is incident on the first reflecting surface with an angle in the sub-scanning direction, and the light beam transmitted by the transfer optical system is incident on the second reflecting surface with an angle in the sub-scanning direction. It is arranged to be, and
The transmission optical system has a substantially conjugate relationship between the first reflecting surface and the second reflecting surface in the sub-scanning direction, and the scanning optical system connects the second reflecting surface and the scanned surface in the sub-scanning direction. It is characterized by a substantially conjugate relationship.
[0020]
In this case, the incident angles in the sub-scanning direction of the light beam on the first reflecting surface and the second reflecting surface are α ₁ and α ₂ , the magnification in the sub-scanning direction of the transmission optical system is β _t , and the sub-scanning of the scanning optical system is performed. The magnification in the direction is β _s , the maximum value of the positional deviation of the first reflecting surface and the second reflecting surface with respect to the rotation axis of the rotary polygon mirror is δ, and the interval between scanning lines in the sub-scanning direction on the scanned surface is p. When
δβ _s | α ₁ β _t −α ₂ | / p ≦ 1/8 (2)
It is desirable to satisfy.
[0021]
Alternatively, when the incident angles of the light beam on the first reflecting surface and the second reflecting surface in the sub-scanning direction are α ₁ and α ₂ , respectively, and the magnification in the sub-scanning direction of the transmission optical system is β _t ,
β _t = α ₂ / α ₁ (1)
It is desirable to satisfy.
[0022]
Further, the optical axis of the optical system for causing the light beam from the light source to enter the first reflecting surface, the optical axis of the transmission optical system, and the optical axis of the scanning optical system are within the common sub-scanning plane including the rotation axis of the rotary polygon mirror. It can be arranged.
[0023]
In the present invention, in the optical scanning device that is incident twice on the rotary polygon mirror, the first reflection surface and the second reflection surface are parallel to each other across the rotation axis and are opposed to each other at an angle of 180 °. Since the first reflection surface, the second reflection surface, and the surface to be scanned are in a substantially conjugate relationship, the light beam shift is offset between the two reflection surfaces. The displacement of the scanning lines in the sub-scanning direction due to the displacement of each reflecting surface based on the eccentricity of the rotation axis of the rotating polygon mirror is corrected, the position of the scanning lines is constant, and there is no unevenness in the image. Can reproduce a good image.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
The optical scanning device of the present invention will be described below in detail with reference to the drawings.
First, an embodiment of the optical scanning device of the present invention will be described. FIG. 1 is a plan view showing the configuration of the optical scanning device of this embodiment, FIG. 2 is a side view thereof, FIG. 3 is a perspective view of the main part thereof, and FIG. 4 is a side view of the main part thereof. Hereinafter, in the present invention, at an arbitrary position of the optical system, a surface that includes the optical axis of the optical system at that position and is parallel to the rotation axis 41 of the rotary polygon mirror 4 that is a deflector is defined as a sub-scanning surface, and the optical axis A plane that is perpendicular to the sub-scanning plane is defined as a main scanning plane. Further, a direction perpendicular to the optical axis in the main scanning plane is defined as a main scanning direction, and a direction perpendicular to the optical axis in the sub scanning plane is defined as a sub scanning direction.
[0025]
The light beam emitted from the semiconductor laser 1 as the light source is shaped by passing through the first shaping lens 2 and the second shaping lens 3, and enters the first reflecting surface 5 of the rotary polygon mirror 4 as the deflector. A second deflection is made. At this time, the light beam is incident on the first reflecting surface 5 at an angle with respect to a plane perpendicular to the rotation axis 41 of the rotary polygon mirror 4, so that the incident light beam and the reflected light beam do not interfere with each other. The light beam reflected by the first reflecting surface 5 passes through the first transmission lens 7, the second transmission lens 8, and the third transmission lens 9 and is reflected by the first transmission mirror 10. 5 is transmitted through the transmission lens 12 and is reflected by the second transmission mirror 13, and is incident again on the second reflection surface 6 of the rotary polygon mirror 4 to be deflected a second time. Also at this time, since the light beam is incident on the second reflecting surface 6 at an angle with respect to the plane perpendicular to the rotation axis 41 of the rotary polygon mirror 4, the incident light beam and the reflected light beam do not interfere with each other. .
[0026]
The light beam reflected by the second reflecting surface 6 is imaged and scanned as a light beam spot on the scanned surface 17 by the first scanning lens 14, the second scanning lens 15 and the third scanning lens 16. The number of surfaces of the rotary polygon mirror 4 is 12 (even numbers). The third scanning lens 16 is eccentric in the sub-scanning direction, and the direction is the direction of the arrow in FIG. The reason why the third scanning lens 16 is decentered in this way is that the light beam reflected and deflected by the second reflecting surface 6 of the rotary polygon mirror 4 draws a conical locus, and the coordinate system of the cross section of the light beam is deflected. The rotation depends on the angle and the shape of the imaging spot on the surface to be scanned 17 is collapsed, but the collapse of the third scanning lens 16 can prevent the collapse.
[0027]
By the way, the optical system between the semiconductor laser 1 and the first reflecting surface 5 is the shaping optical system 21, the optical system between the first reflecting surface 5 and the second reflecting surface 6 is the transmission optical system 22, and the second reflecting surface 6. If the optical system from the scanning surface 17 to the scanned surface 17 is referred to as a scanning optical system 23, the first reflecting surface 5 and the second reflecting surface 6 of the rotary polygon mirror 4 are opposed to each other with the rotating shaft 41 therebetween and parallel to each other. The optical axis of the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 is a reflecting surface, and is disposed in a common sub-scanning plane including the rotation axis 41. Therefore, this optical scanning device has a symmetric configuration with respect to the sub-scanning surface while being obliquely incident at 2 degrees. With such an arrangement, the optical axes of the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 are arranged on a straight line when viewed on the main scanning surface, so that the structural reference plane in the main scanning direction is 1. It is possible to arrange each element constituting the optical system with high accuracy on the surface. Further, since the optical axis of the transmission optical system 22 partially overlaps with the optical axes of the shaping optical system 21 and the scanning optical system 23 when viewed from the main scanning plane, it can be arranged in a small space, and the installation area of the optical scanning device is reduced. The device can be miniaturized. With such an arrangement, as will be described in detail later, it is possible to prevent the position variation of the scanning line in the sub-scanning direction based on the eccentricity of the rotating shaft 41 of the rotary polygon mirror 4.
[0028]
FIG. 5 shows an optical path diagram (a) in the main scanning direction and an optical path diagram (b) in the sub-scanning direction of the shaping optical system 21. The light beam emitted from the semiconductor laser 1 having the cover glass is converted into a parallel light beam by the first shaping lens 2 constituting the aspherical collimator lens. The second shaping lens 2 is a positive cylindrical lens having a positive refractive power only in the sub-scanning direction. Therefore, the light beam that has passed through the second shaping lens 2 enters the first reflecting surface 5 as a parallel light beam on the main scanning surface, and forms an image (converges) near the first reflecting surface 5 on the sub-scanning surface. .
[0029]
FIG. 6 shows an optical path diagram (a) in the main scanning direction and an optical path diagram (b) in the sub-scanning direction of the transmission optical system 22. The first transmission lens 7, the second transmission lens 8, and the third transmission lens 9 are all cylindrical lenses having refractive power only in the main scanning direction, and the first transmission lens 7 and the second transmission lens 8 are positive cylindrical lenses, The third transmission lens 9 is a negative cylindrical lens, and these three lenses constitute a main scanning direction positive refractive power transmission lens group 24. The fourth transmission lens 11 is a positive cylindrical lens having a positive refractive power only in the sub-scanning direction, and the fifth transmission lens 12 is a spherical lens having a positive refractive power. With these functions, the light beam reflected by the first reflecting surface 5 is once imaged by the main scanning direction positive refractive power transmission lens group 24 on the main scanning surface. The image-side focal point of the transmission lens group 24 and the object-side focal point of the fifth transmission lens 12 coincide with each other to constitute an afocal optical system on the main scanning surface. Therefore, the light beam is converted again into a parallel light beam by the fifth transfer lens 12 and is incident on the second reflecting surface 6. On the sub-scanning surface, the first reflecting surface 5 and the second reflecting surface 6 are conjugated with each other due to the combined positive refractive power of the fourth transmission lens 11 and the fifth transmission lens 12, and the vicinity of the first reflecting surface 5. Is converged again in the vicinity of the second reflecting surface 6.
[0030]
FIG. 7 shows an optical path diagram (a) in the main scanning direction and an optical path diagram (b) in the sub-scanning direction of the scanning optical system 23. The first scanning lens 14 is a spherical lens having a positive refractive power. The second scanning lens 15 is a prism having a refractive action only in the sub-scanning direction, and the third scanning lens 16 is a long lens made of resin and long in the main scanning direction. The incident surface of the third scanning lens 16 has a concave shape with a large curvature radius in the main scanning direction, and has a convex shape with a small curvature radius in the sub-scanning direction. This is a surface formed by rotating around an axis parallel to the main scanning direction and located closer to the scanned surface 17 than the incident surface. Such a surface is also called a saddle type toric surface. The exit surface of the third scanning lens 16 has a convex non-arc shape with a large curvature radius in the main scanning direction, and the cross-sectional shape in the sub-scanning direction is a straight line and has no refractive power. The scanning optical system 23 having such a configuration forms an image of the convergence point in the vicinity of the second reflecting surface 6 on the scanned surface 17 in a conjugate relationship between the second reflecting surface 6 and the scanned surface 17 on the sub-scanning surface. . On the main scanning surface, a parallel light beam reflected from the second reflecting surface 6 is imaged on the scanned surface 17.
[0031]
Next, the operation of the transmission optical system 22 will be described. FIG. 8 is a cross-sectional development view of the main scanning surface of the transmission optical system 22. The main scanning direction positive refractive power transmission lens group 24 composed of the first transmission lens 7, the second transmission lens 8, and the third transmission lens 9 is simplified and shown as a single lens. The fourth transfer lens 11 is not shown because it has no refractive power in the main scanning direction. FIGS. 8A and 8B show the state of the light beam when the rotary polygon mirror 4 rotates. By the way, as shown in FIGS. 1 to 4 and the like, the optical path of the transmission optical system 22 is reflected twice by the transmission mirrors 10 and 13. That is, it is reflected even times. In FIG. 8, since these even number of reflections are developed, the rotation directions of the first reflecting surface 5 and the second reflecting surface 6 are the same as shown in FIG. 8B.
[0032]
The diameter of the parallel light beam incident on the first reflecting surface 5 is w _i . Since transfer optical system 22 in the main scanning plane constitute an afocal optical system, the light beam incident on the second reflecting surface 6 is also parallel, the diameter of the light beam is w _o. Assuming that the focal length of the transfer lens group 24 is f ₁ and the focal length of the fifth transfer lens 12 is f ₂ , the value of the ratio of the diameter of the light beam by dividing w _o by w _i is f ₂ divided by f ₁ . Is equal to
[0033]
As shown in FIG. 8B, when the rotary polygon mirror 4 rotates by the angle θ ₁ , the light beam is deflected by the angle 2θ _{1 on} the first reflecting surface 5. The deflected light beam passes through the transmission lens group 24 and the fifth transmission lens 12 and is deflected by an angle θ ₂ . This light beam intersects the optical axis at point Q. On the second reflecting surface 6, the distance between the deflected light beam and the optical axis is d, the rotary polygon mirror 4 is rotated by an angle theta _1, so that also the second reflecting surface 6 moves by the same distance d Is set to a proper positional relationship. Therefore, the amount of movement of the light beam matches the amount of movement of the second reflecting surface 6, and the light beam does not protrude from the second reflecting surface 6.
[0034]
At this time, the deflected light beam is deflected to the side where the incident angle increases by an angle θ _{2 with} respect to the second reflecting surface 6, and thus the scanning angle θ _{s of} the light beam reflected by the second reflecting surface 6. Is expressed as θ _s = 2θ ₁ + θ ₂ .
[0035]
Since the transmission optical system 22 of the present embodiment is an afocal optical system on the main scanning plane, the optical magnification β is a value obtained by dividing the focal length f ₂ by the focal length f ₁ . It is also equal to the ratio of diameters w _o / w _i . Further, since the deflection angle of the light beam transmitted through the transmission optical system 22 changes from the angle 2θ ₁ to the angle θ ₂ , the optical magnification β can also be expressed as 2θ ₁ / θ ₂ . Therefore, the optical magnification β is expressed by the following equation.
[0036]
β = w _o / w _i = f ₂ / f ₁ = 2θ ₁ / θ ₂
In this embodiment, the optical magnification β is 1 <β <20.
[0037]
The optical scanning device in which the light beam is deflected twice by the rotary polygon mirror 4 in this embodiment can increase the scanning speed as compared with the conventional optical scanning device in which the light beam is deflected only once. This will be described next.
[0038]
In a conventional optical scanning device that deflects only once, the reflecting surface moves when the rotating polygon mirror rotates, so that the light incident on the rotating polygon mirror always enters the same reflecting surface in one scan. The size of the reflecting surface must be larger than the size of the beam in the main scanning direction. Therefore, the number of reflecting surfaces of the rotary polygon mirror cannot be increased too much.
[0039]
In this embodiment, a light beam parallel to the first reflecting surface 5 is incident on the main scanning surface. Since β> 1, the diameter w _i of the light beam on the first reflecting surface 5 in the main scanning direction is smaller than the diameter w _o of the light beam on the second reflecting surface 6 in the main scanning direction. Therefore, even if the size of the first reflecting surface 5 is smaller than that of the conventional optical scanning device, the entire light beam can always be put on the same reflecting surface in one scanning. The smaller the w _i is, the smaller the size of the first reflecting surface 5 can be made. In the second deflection, the amount of movement of the light beam when the rotary polygon mirror 4 rotates matches the amount of movement of the second reflecting surface 6, so that the size of the second reflecting surface 6 in the main scanning direction is It is only necessary to have at least the same size as the incident light beam.
[0040]
Therefore, in comparison with the conventional optical scanning device that deflects only once, in the optical scanning device that deflects twice according to the present embodiment, the light beam on the second reflecting surface 6 has a diameter w _{o in} the main scanning direction. Thus, by reducing the diameter w _i of the light beam on the first reflecting surface 5 in the main scanning direction, the reflecting surface of the rotary polygon mirror 4 can be reduced, so that the number of reflecting surfaces can be increased. And the scanning speed can be increased accordingly.
[0041]
Specific numerical examples of the thus configured optical scanning device are shown in Table-1. In Table 1, the cylindrical surface and the toric surface have the radius of curvature in the sub-scanning direction and the main scanning direction as r _ix and r _iy (i indicates the surface number from the light source 1 to the scanned surface 17). In addition, for a surface that is an aspherical surface, the radius of curvature indicates a value on the optical axis. The unit of length is mm.
[0042]

[0043]
The expressions representing the aspheric surfaces of the second shaping lens 2 and the third scanning lens 16 are
z _i = (y ² / r _i ) / [1+ {1− (K _i +1) (y / r _i ) ² } ^1/2 ] + A _i y ⁴ + B _i y ⁶ + C _i y ⁸
The aspheric coefficient is shown in the following Table-2.
[0044]

[0045]
In this specific example, the incident surface S ₂₅ of the third scanning lens 16 is a toric surface formed by rotating an arc of r _25y = −1475.39378 at r _25x = 37.95675. In addition, when the optical path is refracted as when passing through the second scanning lens 15 and the third scanning lens 16, the optical axis is refracted in the same manner as the principal ray. It is assumed that the optical axis serving as the reference for the parameter 2 always coincides with the principal ray of the beam that scans the scanning center.
[0046]
The number of surfaces of the rotating polygon mirror 4 is 12, and the inscribed circle diameter is 38.64 mm, and the light beam is incident on the first reflecting surface 5 and the second reflecting surface 6 of the rotating polygon mirror 4 in the sub-scanning direction. The angles are both 6 °, and the incident angles of the light beams on the first transmission mirror 10 and the second transmission mirror 13 in the sub-scanning direction are both 3 °. Also, the exit surface S ₂₄ of the second scanning lens 15 is inclined 13 ° in the sub-scanning cross section, the incident surface S ₂₅ of the third scanning lens 16 is inclined 8.750387 ° in the sub-scanning cross section, the third scanning lens Sixteen exit surfaces S ₂₆ are inclined by 2.875374 ° in the sub-scan section. Refer to FIG. 2 and FIG. 4 for the directions of these inclination angles.
[0047]
In addition, a rectangular aperture having a main scanning direction of 0.7154 mm and a sub-scanning direction of 1.0526 mm is arranged so as to coincide with the first shaping lens incident surface S ₃ . In the sub-scanning direction, the light emitting point 1 and the first reflecting surface 5 of the rotary polygon mirror 4 are out of the geometric optical conjugate relationship. However, since the three surfaces of the first and second reflecting surfaces 5 and 6 and the scanned surface 17 of the rotating polygon mirror 4 are all in a conjugate relationship, the surface tilt correction of the rotating polygon mirror 4 is performed. . Therefore, it can be said that the light emitting point 1 and the scanned surface 17 are out of the conjugate relationship. However, due to the influence of diffraction, the position where the light beam is minimized is shifted from the geometric optical imaging point, and the scanned surface 17 is disposed at a position where the light beam is substantially minimized.
[0048]
In the transmission optical system 22 of the above specific example, the optical magnification β in the main scanning direction is 8.24, the optical magnification β _t in the sub scanning direction is 1.12, and the optical magnification β _{s in} the sub scanning direction of the scanning optical system 23 is 0.406.
[0049]
Here, as described above, the second scanning lens 15 is a prism having a refractive action only in the sub-scanning direction. The operation of this prism will be described. The light beam reflected and deflected by the reflecting surface 6 of the rotating polygon mirror 4 draws a conical locus, and when the prism of the second scanning lens 15 is not disposed, the beam is curved on the long lens of the third scanning lens 16. It becomes a trajectory. As schematically shown in FIG. 9, the prism 16 has a function of converting the locus of the conical light beam a into a linear beam locus A on the incident surface of the third scanning lens 16.
[0050]
FIG. 10 is a diagram showing a beam locus on the incident surface of the third scanning lens 16 of the above specific example, and the beam locus is shown by a solid line. In the figure, the Y direction indicates the main scanning direction, and the X direction indicates the sub scanning direction. For comparison, a third case where only the second scanning lens 15 is removed without changing the distance from the second reflecting surface 6 to the third scanning lens 16 of the rotary polygon mirror 4 of the optical system of the above specific example. A beam trajectory on the incident surface of the scanning lens 16 is indicated by a broken line. From FIG. 10, it can be seen that there is an effect of correcting the locus of the beam linearly by the prism action of the second scanning lens 15.
[0051]
FIG. 11 shows the sub-scanning cross section of the third scanning lens 16 at several positions (5 places) in the main scanning direction, and shows the error of the measured value with respect to the design value of the cross-sectional shape. . In the figure, X, Y, and Z are the sub-scanning direction, the main scanning direction, and the optical axis direction, respectively. As shown in FIG. 11, the shape error of a lens having a saddle-shaped toric surface such as the third scanning lens 16 is substantially the same regardless of the position in the main scanning direction, but periodically changes in the sub-scanning direction. There are features. As described above, due to the prism action of the second scanning lens 15, the beam locus on the third scanning lens 16 becomes a straight line A, and the beam always has a shape error at points B _{1 to} B ₅ regardless of the position in the main scanning direction. Incident on the convex part. At any position in the main scanning direction, when the light beam is incident on the convex portion where the shape error of the third scanning lens 16 is incident, the imaging position in the sub-scanning direction is shifted to the near side from the designed position. Therefore, it can be corrected by adjusting the optical system, such as adjusting the position of the third scanning lens 16, and such adjustment does not cause curvature of field.
[0052]
Now, the positional deviation of the reflecting surface with respect to the rotation axis 41 of the rotary polygon mirror 4 will be examined. The following two points can be cited as causes of the displacement of the reflecting surfaces 5 and 6 with respect to the rotation axis 41 of the rotary polygon mirror 4. One is the eccentricity of the central axis 41 of the rotary polygon mirror 4 and the rotation axis of the motor, and the other is due to a manufacturing error when cutting each reflecting surface of the rotary polygon mirror 4.
[0053]
First, the former will be described. As shown in FIG. 12, the rotary polygon mirror deflecting device 25 is composed of the rotary polygon mirror 4 attached to the rotation shaft of the motor 26. As schematically shown in FIG. 13, the center of the rotary polygon mirror 4 is provided. The axis 41 'and the center 28 of the rotating shaft 27 of the motor 26 are eccentric by δ due to manufacturing errors, and the position of each reflecting surface of the rotary polygon mirror 4 with respect to the rotating shaft 27 is 1 rotation per rotation as shown in FIG. It fluctuates sinusoidally with the period. The number of surfaces of the rotary polygon mirror 4 of the above embodiment is 12, and the deviation between the central axis 41 ′ of the rotary polygon mirror 4 and the rotation shaft 27 of the motor 26 is usually about several tens of μm.
[0054]
Regarding the latter, the positional deviation of each reflecting surface of the rotary polygon mirror 4 is not regular and occurs randomly. However, the latter displacement is smaller than the former displacement, and the displacement caused by the former is dominant.
[0055]
Here, the influence on the scanning line due to the positional deviation of the reflecting surface of the rotary polygon mirror 4 will be described. Considering the positional deviation of the reflecting surface 6 when entering the second reflecting surface 6 of the rotating polygon mirror 4 only once, the reflecting surface 6 of the rotating polygon mirror 4 is shown in FIG. Is incident at an incident angle α in the sub-scanning direction (oblique incidence), if the positional deviation of the reflecting surface 6 with respect to the rotation axis 41 of the rotary polygon mirror 4 is δ, the positional deviation d of the reflected beam is
d = 2αδ
It becomes. However, since α is small, it is approximated as tan α≈α. When the magnification of the scanning optical system 23 in the sub-scanning direction is β _s , the positional deviation e of the beam on the scanned surface 17 in the sub-scanning direction is
e = 2αδβ _s
It becomes.
[0056]
Therefore, in the present invention, as in the above-described embodiment, the light is incident on the two reflecting surfaces 5 and 6 of the rotary polygon mirror 4 twice, and the first reflecting surface 5 and the second reflecting surface 6 are mutually connected. They are arranged so as to be parallel and facing each other at an angle of 180 °, so that the positional deviation of the scanning lines in the sub-scanning direction due to the eccentricity of the rotating shaft 41 of the rotary polygon mirror 4 is eliminated. The principle will be described with reference to FIG. FIG. 16 shows the optical path of the principal ray in the sub-scanning direction near the rotary polygon mirror 4 of the optical scanning device according to the present invention. As shown in FIG. 16, the light beam from the light source first enters the first reflecting surface 5 of the rotary polygon mirror 4 for the first time, and the light beam deflected for the first time is transmitted by the transmission optical system 22. The transmitted light is incident on the second reflecting surface 6 of the rotary polygon mirror 4 facing the first reflecting surface 5 for the second time and deflected for the second time. The deflected beam is applied to the scanned surface 17 by the scanning optical system 23. Is imaged and scanned. The light beams incident on the first reflecting surface 5 and the second reflecting surface 6 are both set to enter with an angle in the sub-scanning direction. With this arrangement, as is apparent from FIG. 16, when the rotation axis 41 of the rotary polygon mirror 4 is decentered, the displacement direction δ of the first reflecting surface 5 and the second reflecting surface 6 is the same direction, The optical path changes from a solid line to a broken line in the figure. When the first reflecting surface 5 and the second reflecting surface 6 are shifted to the left as shown in the drawing, the optical path immediately after the reflection on the first reflecting surface 5 is shifted downward, but due to the imaging action of the transfer optical system 22. The optical path is shifted to the upper side immediately before the second reflecting surface 6. However, since the second reflective surface 6 is obliquely incident from above, the principal ray after reflection passes through substantially the same position as the reflection point when there is no positional shift (solid line). Although the angle of the beam reflected by the second reflecting surface 6 is different, the second reflecting surface 6 and the scanned surface 17 are in a conjugate relationship by the scanning optical system 23, and therefore there is no positional deviation on the scanned surface 17. It becomes almost the same position. This effect is clear when FIG. 16 and FIG. 15 are compared. According to the above principle of the present invention, even if the rotary shaft 41 of the rotary polygon mirror 4 is decentered, the shift directions of the first reflecting surface 5 and the second reflecting surface 6 are the same, so that the shift of the light beam is made to the both reflecting surfaces. 5 and 6 can be said to be mutually offset arrangements.
[0057]
The above is a qualitative explanation, but a quantitative explanation is given below. As shown in FIG. 17, when the incident angle to the first reflecting surface 5 is α ₁ , the incident angle to the second reflecting surface 6 is α ₂ , and the magnification of the transmission optical system 22 in the sub-scanning direction is β _t , When the positional deviation based on the eccentricity of the rotating shaft 41 of the rotary polygon mirror 4 between the first reflecting surface 5 and the second reflecting surface 6 is δ, the positional deviation d ₁ of the light beam reflected by the first reflecting surface 5 is
d ₁ = 2α ₁ δ
It becomes. However, since α ₁ is small, it is approximated as tan α ₁ ≈α ₁ . The unit of α ₁ is radians. This positional deviation d ₁ becomes 2α ₁ δβ _t on the second reflecting surface 6 by the transmission optical system 22. Further, the positional deviation d ₂ of the light beam reflected by the second reflective surface 6 caused by the positional deviation of the second reflective surface 6 is:
d ₂ = 2α ₂ δ
It becomes. However, since α ₂ is small, it is approximated as tan α ₂ ≈α ₂ . The unit of α ₂ is radians. Considering the positional deviation of both the first reflective surface 5 and the second reflective surface 6, the positional deviation d of the light beam reflected by the second reflective surface 6 is:

It becomes. If the magnification of the scanning optical system 23 in the sub-scanning direction is β _s , the positional deviation e of the light beam on the scanned surface 17 is
e = 2δβ _s | α ₁ β _t −α ₂ |
It becomes. Therefore,
β _t = α ₂ / α ₁ (1)
If so, the positional deviation of the scanning lines in the sub-scanning direction due to the positional deviation of the reflecting surfaces 5 and 6 based on the eccentricity of the rotating shaft 41 of the rotary polygon mirror 4 is corrected, and the position of the scanning lines becomes constant.
[0058]
Here, apart from the equation (1), the allowable amount of e is ¼ of the scanning line interval p in the sub-scanning direction on the scanned surface 17, and if this value is exceeded, the scanned surface 17 The unevenness of the exposure amount becomes a problem. Therefore,
2δβ _s | α ₁ β _t −α ₂ | ≦ p / 4
That is,
δβ _s | α ₁ β _t −α ₂ | / p ≦ 1/8 (2)
It is necessary to satisfy the following conditions.
[0059]
In the specific numerical example, β _t = 1.12, β _s = 0.406, α ₁ = α ₂ = 0.105 radians (= 6 °), p = 0.0423 mm (equivalent to 600 dpi). Yes, the maximum value of δ is 0.03 mm,
δβ _s | α ₁ β _t −α ₂ | /p=0.0036
Thus, the expression (2) is satisfied. FIG. 18 shows the positional deviation of the scanning line in this case. In this figure, the Y direction indicates the main scanning direction, and the X direction indicates the sub scanning direction. The positional deviation of the scanning line due to the positional deviation of only the first reflective surface 5 is indicated by a broken line, the positional deviation of the scanning line due to the positional deviation of only the second reflective surface 6 is indicated by a one-dot chain line, and the first reflective surface 5 and the second reflective surface are indicated. The positional deviation of the scanning line due to the positional deviation of both surfaces of the surface 6 is indicated by a solid line. According to the present invention, it can be seen that the influence of the displacement of the first reflecting surface 5 and the second reflecting surface 6 is offset, and the displacement of the scanning line is corrected well.
[0060]
The optical scanning device according to the present invention has been described based on the embodiments. However, the present invention is not limited to these, and various modifications can be made.
[0061]
【The invention's effect】
As is apparent from the above description, according to the optical scanning device of the present invention, in the optical scanning device that is incident twice on the rotary polygon mirror, the first reflection surface and the second reflection surface are parallel to each other with the rotation axis therebetween. Since the first reflection surface, the second reflection surface, and the surface to be scanned are in a substantially conjugate relationship, the reflection surfaces are set to be opposite to each other at an angle of 180 °, and are incident obliquely on both reflection surfaces. The offset is offset between the two reflecting surfaces, and the positional deviation in the sub-scanning direction of the scanning line due to the positional deviation of each reflecting surface based on the eccentricity of the rotational axis of the rotary polygon mirror is corrected. The position is constant, and an accurate and good image can be reproduced without causing unevenness in the image.
[Brief description of the drawings]
FIG. 1 is a plan view showing a configuration of one embodiment of an optical scanning device of the present invention.
FIG. 2 is a side view of the optical scanning device of FIG.
3 is a perspective view of a main part of the optical scanning device of FIG. 1. FIG.
4 is a side view of a main part of the optical scanning device in FIG. 1. FIG.
5 is an optical path diagram in a main scanning direction and a sub-scanning direction of a shaping optical system of the optical scanning device in FIG. 1. FIG.
6 is an optical path diagram in a main scanning direction and a sub-scanning direction of a transmission optical system of the optical scanning device in FIG. 1. FIG.
7 is an optical path diagram in the main scanning direction and the sub-scanning direction of the scanning optical system of the optical scanning device in FIG. 1. FIG.
FIG. 8 is a developed sectional view of the main scanning plane for explaining the operation of the transmission optical system.
FIG. 9 is a diagram for explaining the correcting action of the refraction prism.
FIG. 10 is a diagram showing a beam locus on an incident surface of a third scanning lens according to one specific example of the present invention.
FIG. 11 is a diagram for explaining the reason why field curvature does not occur in one specific example of the present invention.
FIG. 12 is a perspective view showing an example of a rotary polygon mirror deflecting device used in the optical scanning device of the present invention.
FIG. 13 is a diagram schematically showing a state in which the center axis of the rotary polygon mirror and the center of the rotation axis of the motor are decentered due to a manufacturing error.
FIG. 14 is a diagram showing a state in which the position of each reflecting surface of the rotary polygon mirror with respect to the rotation axis fluctuates sinusoidally in one rotation and one cycle.
FIG. 15 is an optical path diagram in the sub-scanning direction showing a positional deviation of a beam when entering the rotating polygon mirror only once.
FIG. 16 is an optical path diagram in the sub-scanning direction showing that there is no beam misalignment in the optical scanning device of the present invention.
FIG. 17 is a diagram showing parameters in the optical scanning device of the present invention.
FIG. 18 is a diagram showing a positional deviation of a scanning line in one specific example of the present invention.
[Explanation of symbols]
1. Semiconductor laser (light source)
DESCRIPTION OF SYMBOLS 2 ... 1st shaping lens 3 ... 2nd shaping lens 4 ... Rotary polygon mirror 5 ... 1st reflective surface 6 of a rotary polygon mirror ... 2nd reflective surface 7 of a rotary polygon mirror ... 1st transmission lens 8 ... 2nd transmission lens 9 3rd transmission lens 10 ... 1st transmission mirror 11 ... 4th transmission lens 12 ... 5th transmission lens 13 ... 2nd transmission mirror 14 ... 1st scanning lens 15 ... 2nd scanning lens (prism)
16 ... Third scanning lens (long lens)
DESCRIPTION OF SYMBOLS 17 ... Scanned surface 21 ... Shaping optical system 22 ... Transmission optical system 23 ... Scanning optical system 24 ... Main scanning direction positive refractive power transmission lens group 25 ... Rotating polygon mirror deflecting device 26 ... Motor 27 ... Motor rotating shaft 28 ... Motor Rotational axis center 41 of the rotating polygon mirror 41 ′ Rotating polygon mirror central axis

Claims (4)

A light source for generating a light beam, a rotating polygon mirror having a plurality of reflecting surfaces for reflecting and deflecting the light beam from the light source, and a light beam reflected and deflected by a first reflecting surface of the rotating polygon mirror for the rotating polygon mirror A transmission optical system for transmitting and incident on the second reflecting surface, and a scanning optical system for scanning the light beam reflected and deflected by the second reflecting surface of the rotary polygon mirror by forming a beam spot on the surface to be scanned. In the optical scanning device provided,
The rotating polygon mirror has a plurality of pairs of reflecting surfaces that are parallel to each other across an axis of rotation and that are opposed to each other at an angle of 180 °, and the first reflecting surface and the second reflecting surface are rotated. It is set on the reflecting surfaces that are parallel to each other across the rotation axis of the polygon mirror and are opposed to each other at an angle of 180 °,
The transmission optical system has an even number of reflecting surfaces in an optical path from the first reflecting surface to the second reflecting surface,
The light beam from the light source is incident on the first reflecting surface with an angle in the sub-scanning direction, and the light beam transmitted by the transfer optical system is incident on the second reflecting surface with an angle in the sub-scanning direction. It is arranged to be, and
The transmission optical system has a substantially conjugate relationship between the first reflecting surface and the second reflecting surface in the sub-scanning direction, and the scanning optical system connects the second reflecting surface and the scanned surface in the sub-scanning direction. An optical scanning device having a substantially conjugate relationship. The incident angles of the light beams on the first reflecting surface and the second reflecting surface in the sub-scanning direction are α ₁ and α ₂ , the sub-scanning magnification of the transmission optical system is β _t , and the sub-scanning of the scanning optical system The magnification in the direction is β _s , the maximum value of the positional deviation of the first reflecting surface and the second reflecting surface with respect to the rotation axis of the rotary polygon mirror is δ, and the scanning line interval in the sub-scanning direction on the scanned surface is When p is
δβ _s | α ₁ β _t −α ₂ | / p ≦ 1/8 (2)
The optical scanning device according to claim 1, wherein: When the incident angles of the light beam on the first reflecting surface and the second reflecting surface in the sub-scanning direction are α ₁ and α ₂ , respectively, and the magnification in the sub-scanning direction of the transmission optical system is β _t ,
β _t = α ₂ / α ₁ (1)
The optical scanning device according to claim 2, wherein:   The optical axis of the optical system that causes the light beam from the light source to enter the first reflecting surface, the optical axis of the transmission optical system, and the optical axis of the scanning optical system include a common sub-axis including the rotation axis of the rotary polygon mirror. 4. The optical scanning device according to claim 1, wherein the optical scanning device is disposed within a scanning plane.

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