JP4508458B2 - Optical scanning device and image forming apparatus using the same - Google Patents

Description

【００７１】
（但し、Ｒは曲率半径、Ｋ、Ｂ4 、Ｂ6 、Ｂ8 、Ｂ10は非球面係数）
副走査方向（光軸を含み主走査方向に対して直交する方向）と対応する子線方向が、
【００７２】
【数２】

【００７３】
ここで
ｒ’＝ｒ0 （１＋D2Y²＋D4Y⁴＋D6Y⁶＋D8Y⁸＋D10Y¹⁰）
（但し、ｒ0 は光軸上の子線曲率半径、D2、D4、D6、D8、D10 は非球面係数）
（イ）回折面．．．主走査方向が１０次まで、副走査方向が主走査方向の位置により異なる２次の位相関数で表される回折面
φ＝2πｍ/λ[ｂ2 Ｙ² ＋ｂ4 Ｙ⁴ ＋ｂ6 Ｙ⁶ ＋ｂ8 Ｙ⁸ ＋ｂ10 Ｙ¹⁰
＋（ｄ0 ＋ｄ1 Ｙ＋ｄ2 Ｙ² ＋ｄ3 Ｙ³ ＋ｄ4 Ｙ⁴ ＋ｄ5 Ｙ⁵ ＋ｄ6 Ｙ⁶ ）Ｚ² ]
（但し、φは位相関数、ｍは回折次数、λは使用波長（408ｎｍ）、Ｙはレンズ光軸からの高さ、b2,b4,b6,b8,b10、d0,d1,d2,d3,d4,d5,d6は位相係数、実施形態１〜４では＋１次回折光を使用）なる式で表わされる。
【００７４】
本実施形態では走査光学手段６を構成する屈折面と回折面のパワー配置を最適化することにより、主走査方向はレーザ光源の波長変動により生じる結像位置の変化を、副走査方向は装置の環境変動（温度，湿度，気圧の変化、以下は環境変動として温度変化を例にとり説明するが、他の環境変動も同様である。）により生じるピントの変化を共に補償できるようにしている。
【００７５】
即ち、本実施形態において主走査方向は正の分散をもつトーリックレンズ（屈折素子）６１と、負の分散をもつ複合光学素子（回折素子）６２とにより走査光学手段６の倍率色収差を補償している（倍率色補償）。
【００７６】
そのパワー配置はトーリックレンズ６１の屈折面６１ａ，６１ｂと複合光学素子６２の屈折面６２ａの主走査方向の合成パワーをΦr、トーリックレンズ６１の材質のアッべ数をνr、複合光学素子６２の回折面６２ｂの主走査方向のパワーをΦd、回折素子７１のアッべ数をνdとしたとき、
Φr／νr＋Φd／νd＝０ ・・・（式１）
を満足させることが望ましい。
【００７７】
但し，アッべ数νrは、
νr＝（ｎ−１）／Δｎ＝（ｎ−１）／（ｄｎ/ｄλ） ・・・（式２）
である。
【００７８】
上記関係式（式２）と回折素子７１の分散特性は波長に対して線形に変化することを利用すると、（式１）は以下のように変形できる。
【００７９】
（Φｒ／Φｄ）×（ｄｎ／ｄλ）＝−（ｎ−１）／λ ・・（式３）
レーザ光源の発振波長λの範囲を３８０〜５００ｎｍ、一般的な光学材料の材質の屈折率ｎの範囲を１．４５〜１．９０とすると、
−（１．４５−１）／３８０＝−１．１８×１０^−３
−（１．９０−１）／３８０＝−２．３６８×１０^−３
−（１．４５−１）／５００＝−０．９×１０^−３
−（１．９０−１）／５００＝−１．８×１０^−３
従って（式３）のとりうる範囲は、
−２．３７×１０^−３ (/nm)＜（Φｒ／Φｄ）×（ｄｎ／ｄλ）＜−０．９×１０^−３ (/nm)
・・・（式４）
となる。即ち、本実施形態では屈折面の主走査方向の合成パワーをΦｒ、回折面の主走査方向の合成パワーをΦｄ、光源手段の発振波長に対する屈折面を形成する光学素子の材質の屈折率の変化率をｄｎ／ｄλとしたとき、上記（式４）なる条件を満足させるようにしている。ここで合成パワーとは屈折面が１つのときも含み、また回折面が１つのときも含む。
【００８０】
また、回折格子のパワーが強すぎると、格子ピッチが狭くなり、総論体数が増加し、加工難易度が高くなる。それゆえ、回折格子のパワーはある範囲内に抑えることが望ましく、より良好な倍率色収差補正をする為には（式４）を
−２．２５×１０^−３ (/nm)＜（Φｒ／Φｄ）×（ｄｎ／ｄλ）＜−１．３０×１０^−３ (/nm)
・・・（式４′）
を満足させるのが良い。
【００８１】
尚、複合光学素子６２の回折格子７１を除いた面がパワーを持つときは、そのパワーは屈折面のパワーとして取扱う。
【００８２】
またｄｎ/ｄλは上記の如く半導体レーザー１の発振波長λに対する屈折面を形成する光学素子（本実施形態ではトーリックレンズ６１又は複合光学素子６２）の材質の屈折率ｎの変化率である。
【００８３】
条件式（式４）は走査光学手段６を構成するトーリックレンズ６１と複合光学素子６２との屈折面と、該複合光学素子６２の回折面との主走査方向のパワー比と、レンズ材質の分散特性に関するものであり、条件式（式４）を外れると半導体レーザー１の波長変動による被走査面８上での主走査方向の結像位置変化（倍率色収差）を補正することが難しくなってくるので良くない。
【００８４】
一方、副走査方向では装置の環境変動（特に昇温）による走査光学手段６の屈折率変化を、同じく環境変動による半導体レーザー１の波長変化で補償し、ピント移動を相殺している（温度補償）。
【００８７】
ここで昇温により副走査方向のピント変化を抑えるためには、以下の式を満足する必要がある。
【００８８】
ｄψr（ｄｎ/ｄｔ）+ｄψr（ｄλ/ｄｔ）+ｄψd（ｄλ/ｄｔ）＝0・・(式８)
これより、
【００８９】
【数３】

【００９０】
となる。
【００９１】
一般的な光学材料の各特性値と温度に対する窒化ガリウム系の青紫色の半導体レーザーの発振波長の変化率（波長温度特性）（ｄλ/ｄｔ＝０．０４〜０．０６ｎｍ／℃）が赤外レーザーの波長温度特性より変化が少ないことを加味すると、
１５．０(℃/nm)＜ψr／[ψd×（ｄλ/ｄｔ）]＜４５．０(℃/nm) ・‥（式１０）
とするのが良い。
【００９２】
但し、
ψr：トーリックレンズの屈折面と複合光学素子の屈折面との副走査方向
の合成パワー
ψd：複合光学素子の回折面の副走査方向のパワー
である。
【００９３】
また、回折格子のパワーが強すぎると、格子ピッチが狭くなり、総論体数が増加し、加工難易度が高くなる。それゆえ、回折格子のパワーはある範囲内に抑えることが望ましく、より良好な温度補償をする為には（式１０）を
１８．０(℃/nm)＜ψr／[ψd×（ｄλ/ｄｔ）]＜３８．０(℃/nm) ・・・（式１０′）
を満足させるのが良い。
【００９４】
ここでｄλ/ｄｔは温度ｔに対する半導体レーザー１の発振波長λの変化率である。
【００９５】
条件式（式１０）は走査光学手段６を構成するトーリックレンズ６１と複合光学素子６２との屈折面と、該複合光学素子６２の回折面との副走査方向のパワー比と、レーザ光源の温度特性に関するものであり、条件式（式１０）を外れると光走査装置の環境変動（温度変化）に伴なう副走査方向のピント変化を補正することが難しくなってくるので良くない。
【００９６】
表−１に本実施形態における光学配置とトーリックレンズ６１の非球面係数及び複合光学素子６２の非球面係数と位相項を示す。
【００９７】
【表１】

【００９８】
本実施形態におけるトーリックレンズ６１と複合光学素子６２との屈折面と、該複合光学素子６２の回折面とのパワー比は各々、
主走査方向：（Φr／Φd）×（ｄｎ/ｄλ）＝−１．７３Ｅ−３
副走査方向：ψr／[ψd×（ｄλ/ｄｔ）] ＝３３．０
であり、これは条件式（式４）及び条件式（式１０）を各々満たしている。
【００９９】
尚、「Ｅ−ｘ」は「×１０^-x」を表す。
【０１００】
図４は本実施形態における半導体レーザー１の波長変化による主走査方向の結像位置変化を示す説明図であり、半導体レーザー１の初期波長と波長差１．５ｎｍ及び５ｎｍ与えた場合の結像位置との差を示している。
【０１０１】
図５は本実施形態における環境変動前後の近軸収差（主走査方向及び副走査方向の像面湾曲）を示した説明図であり、実線は環境変動前の特性（設計値）であり、破線は光走査装置が＋２５℃昇温したときの特性（実効値）である。
【０１０２】
一般に複数の光走査装置から複数の感光ドラムに各色毎の画像情報を記録し、カラー画像を形成する装置では、各色間のレジストレーションずれや各色間の画像濃度ムラを視覚上目立たなくするためには、半導体レーザーの波長変動による主走査方向の結像位置ずれを１０μｍ以下、装置の環境変動（温度変動）による主走査方向及び副走査方向のピントずれを±１．０ｍｍ以下とすることが必要である。
【０１０３】
図４より本実施形態では＋５ｎｍの波長差による結像位置ずれが２μｍであり、全く問題ないレベルに抑えられていることが分かる。
【０１０４】
同様に図５より＋２５℃昇温時の副走査方向のピント移動量が０．１ｍｍであり、主走査方向と共に視覚上問題のないレベルに抑えられていることが分かる。
【０１０５】
尚、ここでは環境変動に対して昇温時の挙動を中心に説明したが、例えば温度低下等の他の環境変動においても上記と同様な効果を得ることができる。
【０１０６】
また本実施形態ではこれらの環境変動の補償をすべてプラスチック製の光学素子で行っており、これにより成形による製造コストの低減や、非球面を用いた高画角収差補正による短光路長化等を同時に達成することができる。
【０１０７】
このように本実施形態では上述の如く光源手段として短波長レーザーを用い、従来の約半分のスポット径を達成することで階調表現に優れた高解像度の画像形成装置を可能とし、走査光学手段６としてプラスチック製のトーリックレンズ６１と複合光学素子６２とを用い、従来、短波長レーザーをプラスチックレンズで使用した場合、補正が難しかった倍率色収差を補正可能としている。また環境変動によるピント移動をも補償し、各色間の濃度ムラの少ないカラー画像形成装置を容易な構成で、かつ安価に実現している。
【０１０８】
［実施形態２］
図６は本発明の実施形態２の光走査装置の主走査方向の要部断面図である。同図において図３に示した要素と同一要素には同符番を付している。
【０１０９】
本実施形態において前述の実施形態１と異なる点は光源手段として複数の発光点（発光部）を有するモノリシックなマルチビームレーザー５１を使用し、実施形態１と同様に高精度に光源手段の波長差による主走査方向の結像位置ずれ（倍率色収差）を補償したことである。その他の構成及び光学的作用は実施形態１と略同様であり、これにより同様な効果を得ている。
【０１１０】
即ち、同図において５１は光源手段であり、複数の発光点（本実施形態では２つだが、３つ以上でも良い。）を有するモノリシックなマルチビームレーザー（マルチ半導体レーザ光源）より成り、各々独立に光変調された２つの光ビーム（図中では１本のみ図示）を出射している。
【０１１１】
４６はｆθ特性を有する光学素子としての走査光学手段であり、屈折素子６３と回折素子６４とを有している。屈折素子６３は主走査方向と副走査方向とで互いに異なるパワーを有する単一のプラスチック製のトーリックレンズより成り、該トーリックレンズ６３の主走査断面内の両レンズ面６３ａ，６３ｂは共に非球面形状より成っている。
【０１１２】
回折素子６４は主走査方向と副走査方向とで互いに異なるパワーを有する長尺の複合光学素子より成り、該複合光学素子６４は入射側の面６４ａが主走査方向と副走査方向とで互いに異なるパワーを有する非球面、出射側の面６４ｂが平面上に回折格子７２を付加した回折面から成っている。
【０１１３】
本実施形態においてマルチビームレーザー５１から出射された２本の光ビームは前述の実施形態１と同様の経路を通り、被走査面８上を副走査方向に一定量隔離した状態で同時に走査している。
【０１１４】
表−２に本実施形態における光学配置とトーリックレンズ６３の非球面係数及び複合光学素子６４の非球面係数と位相項を示す。
【０１１５】
【表２】

【０１１６】
本実施形態における光走査装置の光源手段としては前述の実施形態１と同様に窒化ガリウム系の青紫色の半導体レーザー５１を使用しており、その発振波長は４０８ｎｍである。前述した通り、レーザ光源の波長を短波長化することで、従来の約半分のスポット径を達成している。これにより更なる高解像化を図り、特に階調表現に優れた画像を形成することが可能となる。
【０１１７】
本実施形態においても前述の実施形態１と同様に走査光学手段４６を構成する屈折面と回折面のパワー配置を適切に設定することにより、主走査方向はマルチビームレーザー５１の波長変動により生じる結像位置の変化を、副走査方向は装置の環境変動（特に昇温）により生じるピント変化を共に補償できるようにしており、更に本実施形態では複数の発光点間の波長差により生じる感光ドラム面８上における主走査方向の走査間隔の変化（ジッター）も補償できるようにしている。
【０１１８】
本実施形態におけるトーリックレンズ６３と複合光学素子６４との屈折面と、該複合光学素子６４の回折面とのパワー比は各々、
主走査方向：（Φr／Φd）×（ｄｎ/ｄλ）＝−２．２０E−３
副走査方向：ψr／[ψd×（ｄλ/ｄｔ）] ＝２４．３
であり、これは条件式（式４）及び条件式（式１０）を各々満たしている。
【０１１９】
図７は本実施形態におけるマルチビームレーザー５１の波長変化による主走査方向の結像位置変化を示す説明図であり、マルチビームレーザー５１の初期波長と波長差１．５ｎｍ及び５ｎｍ与えた場合の結像位置との差を示している。
【０１２０】
図８は本実施形態における環境変動前後の近軸収差（主走査方向及び副走査方向の像面湾曲）を示した説明図であり、実線は環境変動前の特性（設計値）であり、破線は光走査装置が＋２５℃昇温したときの特性（実効値）である。
【０１２１】
図７より本実施形態では＋５ｎｍの波長差による結像位置ずれが２μｍであり、これは例えば解像度６００ｄｐｉのプリンターの場合、約１／２０画素相当以内の画素（位置）ずれに抑えられていることが分かる。
【０１２２】
同様に図８より＋２５℃昇温時の副走査方向のピント移動量が０．１ｍｍであり、主走査方向と共に視覚上問題のないレベルに抑えられていることが分かる。
【０１２３】
尚、ここでは環境変動に対して昇温時の挙動を中心に説明したが、例えば温度低下等の他の環境変動においても上記と同様な効果を得ることができる。
【０１２４】
また本実施形態ではこれらの環境変動の補償をすべてプラスチック製の光学素子で行っており、これにより成形による製造コストの低減や、非球面を用いた高画角収差補正による短光路長化等を同時に達成することができる。
【０１２５】
また本実施形態の特徴として倍率色収差（主走査方向の結像位置変化）を高精度に補正しているため、マルチビームレーザー５１の複数の発光点間の波長差によるジッターの低減も挙げられる。
【０１２６】
このように本実施形態では上述の如く光源手段として短波長レーザーを用い、従来の約半分のスポット径を達成することで階調表現に優れた高解像度の画像形成装置を可能とし、走査光学手段４６としてプラスチック製のトーリックレンズ６３と複合光学素子６４とを用い、従来、短波長レーザーをプラスチックレンズで使用した場合、補正が難しかった倍率色収差を補正可能としている。またマルチビームレーザー５１の使用時のジッターを抑え、更に環境変動によるピント移動をも補償し、各色間の濃度ムラの少ないカラー画像形成装置を容易な構成で、かつ安価に実現している。
【０１２７】
尚、本実施形態では光源手段としてモノリシックなマルチビームレーザー５１を用いたが、これに限らず，例えば単一の光ビームを出射するレーザ光源を複数設け，該複数のレーザ光源から出射した複数の光ビームをビーム合成手段により同一方向の光路に合成する構成にしても良い。
【０１２８】
［実施形態３］
図９は本発明の実施形態３の光走査装置の主走査方向の要部断面図である。同図において図６に示した要素と同一要素には同符番を付している。
【０１２９】
本実施形態において前述の実施形態２と異なる点は光源手段としてのモノリシックなマルチビームレーザー５１の発振波長を４４０nmとした点である。その他の構成及び光学的作用は実施形態２と略同様であり、これにより同様な効果を得ている。
【０１３０】
表−３に本実施形態における光学配置とトーリックレンズ６３の非球面係数及び複合光学素子６４の非球面係数と位相項を示す。（但し、位相関数で使用される波長は４４０nmである。）
【０１３１】
【表３】

【０１３２】
本実施形態においても前述の実施形態２と同様に走査光学手段４６を構成する屈折面と回折面のパワー配置を適切に設定することにより、主走査方向はマルチビームレーザー５１の波長変動により生じる結像位置の変化を、副走査方向は装置の環境変動（特に昇温）により生じるピント変化を共に補償できるようにしており、更に本実施形態でも複数の発光点間の波長差により生じる感光ドラム面８上における主走査方向の走査間隔の変化（ジッター）も補償できるようにしている。
【０１３３】
本実施形態におけるトーリックレンズ６３と複合光学素子６４との屈折面と、該複合光学素子６４の回折面とのパワー比は各々、
主走査方向：（Φr／Φd）×（ｄｎ/ｄλ）＝−１．８２E−３
副走査方向：ψr／[ψd×（ｄλ/ｄｔ）] ＝２４．４
であり、これは条件式（式４）及び条件式（式１０）を各々満たしている。
【０１３４】
図１０は本実施形態におけるマルチビームレーザー５１の波長変化による主走査方向の結像位置変化を示す説明図であり、マルチビームレーザー５１の初期波長と波長差１．５ｎｍ及び５ｎｍ与えた場合の結像位置との差を示している。
【０１３５】
図１１は本実施形態における環境変動前後の近軸収差（主走査方向及び副走査方向の像面湾曲）を示した説明図であり、実線は環境変動前の特性（設計値）であり、破線は光走査装置が＋２５℃昇温したときの特性（実効値）である。
【０１３６】
図１０より本実施形態では＋５ｎｍの波長差による結像位置ずれが２μｍであり、これは例えば解像度６００ｄｐｉのプリンターの場合、約１／２０画素相当以内の画素（位置）ずれに抑えられていることが分かる。
【０１３７】
同様に図１１より＋２５℃昇温時の副走査方向のピント移動量が０．１ｍｍであり、主走査方向と共に視覚上問題のないレベルに抑えられていることが分かる。
【０１３８】
尚、ここでは環境変動に対して昇温時の挙動を中心に説明したが、例えば温度低下等の他の環境変動においても上記と同様な効果を得ることができる。
【０１３９】
また本実施形態ではこれらの環境変動の補償をすべてプラスチック製の光学素子で行っており、これにより成形による製造コストの低減や、非球面を用いた高画角収差補正による短光路長化等を同時に達成することができる。
【０１４０】
このように本実施形態では上述の如く光源手段として短波長レーザーを用い、従来より微小のスポット径を達成することで階調表現に優れた高解像度の画像形成装置を可能とし、走査光学手段４６としてプラスチック製のトーリックレンズ６３と複合光学素子６４とを用い、従来、短波長レーザーをプラスチックレンズで使用した場合、補正が難しかった倍率色収差を補正可能としている。またマルチビームレーザー５１の使用時のジッターを抑え、更に環境変動によるピント移動をも補償し、各色間の濃度ムラの少ないカラー画像形成装置を容易な構成で、かつ安価に実現している。
【０１４１】
［実施形態４］
図１２は本発明の実施形態４の光走査装置の主走査方向の要部断面図である。同図において図６に示した要素と同一要素には同符番を付している。
【０１４２】
本実施形態において前述の実施形態２と異なる点は光源手段としてのモノリシックなマルチビームレーザーの発振波長を４８０nmとした点である。その他の構成及び光学的作用は実施形態２と略同様であり、これにより同様な効果を得ている。
【０１４３】
表−４に本実施形態における光学配置とトーリックレンズ６３の非球面係数及び複合光学素子６４の非球面係数と位相項を示す。（但し、位相関数で使用される波長は４８０nmである。）
【０１４４】
【表４】

【０１４５】
本実施形態においても前述の実施形態２と同様に走査光学手段４６を構成する屈折面と回折面のパワー配置を適切に設定することにより、主走査方向はマルチビームレーザー５１の波長変動により生じる結像位置の変化を、副走査方向は装置の環境変動（特に昇温）により生じるピント変化を共に補償できるようにしており、更に本実施形態でも複数の発光点間の波長差により生じる感光ドラム面８上における主走査方向の走査間隔の変化（ジッター）も補償できるようにしている。
【０１４６】
本実施形態におけるトーリックレンズ６３と複合光学素子６４との屈折面と、該複合光学素子６４の回折面とのパワー比は各々、
主走査方向：（Φr／Φd）×（ｄｎ/ｄλ）＝−１．６８E−３
副走査方向：ψr／[ψd×（ｄλ/ｄｔ）] ＝２２．９
であり、これは条件式（式４）及び条件式（式１０）を各々満たしている。
【０１４７】
図１３は本実施形態におけるマルチビームレーザー５１の波長変化による主走査方向の結像位置変化を示す説明図であり、マルチビームレーザー５１の初期波長と波長差１．５ｎｍ及び５ｎｍ与えた場合の結像位置との差を示している。
【０１４８】
図１４は本実施形態における環境変動前後の近軸収差（主走査方向及び副走査方向の像面湾曲）を示した説明図であり、実線は環境変動前の特性（設計値）であり、破線は光走査装置が＋２５℃昇温したときの特性（実効値）である。
【０１４９】
図１３より本実施形態では＋５ｎｍの波長差による結像位置ずれが２μｍであり、これは例えば解像度６００ｄｐｉのプリンターの場合、約１／２０画素相当以内の画素（位置）ずれに抑えられていることが分かる。
【０１５０】
同様に図１４より＋２５℃昇温時の副走査方向のピント移動量が０．１ｍｍであり、主走査方向と共に視覚上問題のないレベルに抑えられていることが分かる。
【０１５１】
尚、ここでは環境変動に対して昇温時の挙動を中心に説明したが、例えば温度低下等の他の環境変動においても上記と同様な効果を得ることができる。
【０１５２】
また本実施形態ではこれらの環境変動の補償をすべてプラスチック製の光学素子で行っており、これにより成形による製造コストの低減や、非球面を用いた高画角収差補正による短光路長化等を同時に達成することができる。
【０１５３】
このように本実施形態では上述の如く光源手段として短波長レーザーを用い、従来より微小のスポット径を達成することで階調表現に優れた高解像度の画像形成装置を可能とし、走査光学手段４６としてプラスチック製のトーリックレンズ６３と複合光学素子６４とを用い、従来、短波長レーザーをプラスチックレンズで使用した場合、補正が難しかった倍率色収差を補正可能としている。またマルチビームレーザー５１の使用時のジッターを抑え、更に環境変動によるピント移動をも補償し、各色間の濃度ムラの少ないカラー画像形成装置を容易な構成で、かつ安価に実現している。
【０１５４】
尚、各実施形態では走査光学手段を単一の屈折素子（トーリックレンズ）と単一の回折素子（複合光学素子）の２枚の光学素子より構成したが、これに限らず、例えば屈折面と回折面を含む単一の光学素子，もしくは屈折面と回折面を含む３枚以上の光学素子より構成しても良い。
【０１５５】
また各実施形態では上記条件式（式４）及び条件式（式１０）の双方を各々満たしているが、一方の条件式のみであっても良い。
【０１５６】
また各実施形態では装置の環境変動による被走査面上での副走査方向のピント位置変化を屈折面と回折面とのパワー変化と光源手段の波長変化とにより補償したが、もちろん主走査方向のピント位置変化も屈折面と回折面とのパワー変化及び光源手段の波長変化により補償されている。
【０１５７】
［画像形成装置］
図１５は、本発明の画像形成装置の実施形態を示す副走査方向の要部断面図である。図において、符号１０４は画像形成装置を示す。この画像形成装置１０４には、パーソナルコンピュータ等の外部機器１１７からコードデータDｃが入力する。このコードデータDｃは、装置内のプリンタコントローラ１１１によって、画像データ（ドットデータ）Dｉに変換される。この画像データDｉは、実施形態１〜４に示した構成を有する光走査ユニット１００に入力される。そして、この光走査ユニット１００からは、画像データDｉに応じて変調された光ビーム１０３が出射され、この光ビーム１０３によって感光ドラム１０１の感光面が主走査方向に走査される。
【０１５８】
静電潜像担持体（感光体）たる感光ドラム１０１は、モータ１１５によって時計廻りに回転させられる。そして、この回転に伴って、感光ドラム１０１の感光面が光ビーム１０３に対して、主走査方向と直交する副走査方向に移動する。感光ドラム１０１の上方には、感光ドラム１０１の表面を一様に帯電せしめる帯電ローラ１０２が表面に当接するように設けられている。そして、帯電ローラ１０２によって帯電された感光ドラム１０１の表面に、前記光走査ユニット１００によって走査される光ビーム１０３が照射されるようになっている。
【０１５９】
先に説明したように、光ビーム１０３は、画像データDｉに基づいて変調されており、この光ビーム１０３を照射することによって感光ドラム１０１の表面に静電潜像を形成せしめる。この静電潜像は、上記光ビーム１０３の照射位置よりもさらに感光ドラム１０１の回転方向の下流側で感光ドラム１０１に当接するように配設された現像器１０７によってトナー像として現像される。
【０１６０】
現像器１０７によって現像されたトナー像は、感光ドラム１０１の下方で、感光ドラム１０１に対向するように配設された転写ローラ（転写器）１０８によって被転写材たる用紙１１２上に転写される。用紙１１２は感光ドラム１０１の前方（図１５において右側）の用紙カセット１０９内に収納されているが、手差しでも給紙が可能である。用紙カセット１０９端部には、給紙ローラ１１０が配設されており、用紙カセット１０９内の用紙１１２を搬送路へ送り込む。
【０１６１】
以上のようにして、未定着トナー像を転写された用紙１１２はさらに感光ドラム１０１後方（図１５において左側）の定着器へと搬送される。定着器は内部に定着ヒータ（図示せず）を有する定着ローラ１１３とこの定着ローラ１１３に圧接するように配設された加圧ローラ１１４とで構成されており、転写部から搬送されてきた用紙１１２を定着ローラ１１３と加圧ローラ１１４の圧接部にて加圧しながら加熱することにより用紙１１２上の未定着トナー像を定着せしめる。更に定着ローラ１１３の後方には排紙ローラ１１６が配設されており、定着された用紙１１２を画像形成装置の外に排出せしめる。
【０１６２】
図１５においては図示していないが、プリントコントローラ１１１は、先に説明したデータの変換だけでなく、モータ１１５を始め画像形成装置内の各部や、後述する光走査ユニット内のポリゴンモータなどの制御を行う。
【０１６３】
［カラー画像形成装置］
図１６は本発明の実施態様のカラー画像形成装置の要部概略図である。本実施形態は、光走査装置を４個並べ各々並行して像担持体である感光ドラム面上に画像情報を記録するタンデムタイプのカラー画像形成装置である。図１６において、６０はカラー画像形成装置、１１，１２，１３，１４は各々実施形態１〜４に示したいずれかの構成を有する光走査装置、２１，２２，２３，２４は各々像担持体としての感光ドラム、３１，３２，３３，３４は各々現像器、５１は搬送ベルトである。
【０１６４】
図１６において、カラー画像形成装置６０には、パーソナルコンピュータ等の外部機器５２からＲ（レッド）、Ｇ（グリーン）、Ｂ（ブルー）の各色信号が入力する。これらの色信号は、装置内のプリンタコントローラ５３によって、Ｃ（シアン），Ｍ（マゼンタ），Ｙ（イエロー）、Ｂ（ブラック）の各画像データ（ドットデータ）に変換される。これらの画像データは、それぞれ光走査装置１１，１２，１３，１４に入力される。そして、これらの光走査装置からは、各画像データに応じて変調された光ビーム４１，４２，４３，４４が出射され、これらの光ビームによって感光ドラム２１，２２，２３，２４の感光面が主走査方向に走査される。
【０１６５】
本実施態様におけるカラー画像形成装置は光走査装置（１１，１２，１３，１４）を４個並べ、各々がＣ（シアン），Ｍ（マゼンタ），Ｙ（イエロー）、Ｂ（ブラック）の各色に対応し、各々平行して感光ドラム２１，２２，２３，２４面上に画像信号（画像情報）を記録し、カラー画像を高速に印字するものである。
【０１６６】
本実施態様におけるカラー画像形成装置は上述の如く４つの光走査装置１１，１２，１３，１４により各々の画像データに基づいた光ビームを用いて各色の潜像を各々対応する感光ドラム２１，２２，２３，２４面上に形成している。その後、記録材に多重転写して１枚のフルカラー画像を形成している。
【０１６７】
前記外部機器５２としては、例えばＣＣＤセンサを備えたカラー画像読取装置が用いられても良い。この場合には、このカラー画像読取装置と、カラー画像形成装置６０とで、カラーデジタル複写機が構成される。
【０１６８】
【発明の効果】
本発明によれば前述の如く５００ｎｍ以下の短波長光源を用いた光走査装置及びそれを用いた画像形成装置において、走査光学手段を構成する少なくとも１つの屈折面と少なくとも１つの回折面の主走査方向又は／及び副走査方向のパワー配置を最適化することにより、波長変動による主走査方向の結像位置ずれ（倍率色収差）、又は／及び環境変動（温度変化等）に伴なう副走査方向のピントズレ（特に副走査方向の像面湾曲移動）を抑え、簡易な構成で高精細な印字を容易に得ることができる光走査装置及びそれを用いた画像形成装置を達成することができる。
【図面の簡単な説明】
【図１】 本発明の実施形態１のカラー画像形成装置の要部概略図。
【図２】 図１に示した光走査装置とそれに対応する像担持体を示した要部概略図。
【図３】 図２に示した走査光学手段の主走査方向の要部断面図。
【図４】 本発明の実施形態１における波長変化による主走査方向の結像位置変化を示す図。
【図５】 本発明の実施形態１における波長変化による像面湾曲の変化を示す図。
【図６】 本発明の実施形態２の走査光学手段の主走査方向の要部断面図。
【図７】 本発明の実施形態２における波長変化による主走査方向の結像位置変化を示す図。
【図８】 本発明の実施形態２における波長変化による像面湾曲の変化を示す図。
【図９】 本発明の実施形態３の走査光学手段の主走査方向の要部断面図。
【図１０】 本発明の実施形態３における波長変化による主走査方向の結像位置変化を示す図。
【図１１】 本発明の実施形態３における波長変化による像面湾曲の変化を示す図。
【図１２】 本発明の実施形態４の走査光学手段の主走査方向の要部断面図。
【図１３】 本発明の実施形態４における波長変化による主走査方向の結像位置変化を示す図。
【図１４】 本発明の実施形態４における波長変化による像面湾曲の変化を示す図。
【図１５】 本発明の画像形成装置の実施形態を示す副走査断面図
【図１６】 本発明の実施態様のカラー画像形成装置の要部概略図
【図１７】 従来のマルチビーム光走査装置を説明する図
【図１８】 従来のカラー画像形成装置の要部概略図
【図１９】 従来の光走査装置の主走査方向の要部断面図
【図２０】 従来の走査光学手段における波長変化による主走査方向の結像位置変化を示す図
【図２１】 光学樹脂の屈折率波長特性を示す図
【符号の説明】
１、５１‥‥光源手段
２‥‥コリメータレンズ
３‥‥絞り
４‥‥シリンドリカルレンズ
５‥‥偏向手段
６，４６‥‥走査光学手段
６１，６３‥‥屈折素子
６２，６４‥‥回折素子（複合光学素子）
７１，７２‥‥回折格子
８‥‥被走査面
１１、１２、１３、１４‥‥光走査装置
２１、２２、２３、２４‥‥像担持体（感光ドラム）
３１、３２、３３、３４‥‥現像器
４１‥‥搬送ベルト
５１‥‥マルチビームレーザー
５２‥‥外部機器
５３‥‥プリンタコントローラ
６０‥‥カラー画像形成装置
１００ 光走査装置
１０１ 感光ドラム
１０２ 帯電ローラ
１０３ 光ビーム
１０４ 画像形成装置
１０７ 現像装置
１０８ 転写ローラ
１０９ 用紙カセット
１１０ 給紙ローラ
１１１ プリンタコントローラ
１１２ 転写材（用紙）
１１３ 定着ローラ
１１４ 加圧ローラ
１１５ モータ
１１６ 排紙ローラ
１１７ 外部機器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical scanning device and an image forming apparatus using the optical scanning device, and more particularly to an image forming position shift (magnification chromatic aberration) in the main scanning direction due to wavelength variation or / and a sub scanning direction due to environmental variation (temperature change). For example, it is suitable for devices such as a laser beam printer (LBP) and a digital copying machine that suppress the focus shift.
[0002]
[Prior art]
In general, this type of optical scanning apparatus reflects and deflects a light beam (light beam) emitted from a laser light source by a polygon mirror, which is an optical deflector, and applies light onto a surface to be scanned by scanning optical means (scanning optical system) having fθ characteristics. An image is formed as a spot.
[0003]
A semiconductor laser or the like is frequently used as a laser light source, and a divergent light beam emitted from the semiconductor laser is converted into a substantially parallel light beam by a collimator lens, and the outer shape of the light beam is limited by an aperture. The light beam whose outer shape is limited is reflected and deflected by a polygon mirror rotating at a constant angular velocity and is incident on the scanning optical means. The scanning optical means has an fθ characteristic that causes a light beam reflected and deflected at a constant angular velocity to be scanned at a constant speed on a surface to be scanned arranged at a predetermined interval, and forms a minute light spot over the entire scanning area. There is a need for good correction of field curvature.
[0004]
In addition, since the polygon mirror has processing errors on the mirror surface (deflection surface), vibration of the rotating shaft, etc., many scanning optical means have a scanning position shift in a direction perpendicular to the main scanning section, that is, in the sub-scanning section. A tilt correction function is provided to correct the above. For this reason, the scanning optical means is an anamorphic lens system having different imaging characteristics in the main scanning section (main scanning direction) and the sub-scanning section (sub-scanning direction).
[0005]
Conventionally, the scanning optical means is processed to have a toric surface and a cylindrical surface with a glass material, and an antireflection film is applied to this type of glass lens by vapor deposition or the like. On the other hand, since processing of glass lenses is difficult and costly, in recent years, plastic lenses that are low in cost and capable of correcting aberrations in a free shape are frequently used.
[0006]
Furthermore, various multi-beam optical scanning devices that simultaneously form a plurality of scanning lines with a plurality of light beams emitted from a plurality of laser light sources have been proposed in response to a demand for higher speed.
[0007]
FIG. 17 is a schematic view of the main part of this type of conventional multi-beam optical scanning apparatus. In the figure, each light beam emitted from the laser light sources 81 and 82 is made into a substantially parallel light beam by the corresponding collimator lenses 83 and 84, and then combined into an optical path in the same direction by the combining optical element 85. The combined two light beams form a long line image in the main scanning direction in the vicinity of the deflection surface 87a of the polygon mirror 87 by the cylindrical lens 86 having a predetermined refractive power only in the sub-scanning direction, and then have a scanning optical characteristic having fθ characteristics. Light spots are formed at different positions in the sub-scanning direction on the surface of the photosensitive drum 89 by means (scanning optical system) 88. Thus, since two scanning lines can be formed by one optical scanning, the speed can be remarkably increased as compared with the conventional optical scanning device.
[0008]
Further, as a laser light source for a multi-beam optical scanning device, there are a monolithic multi-beam laser having a large number of light emitting points, in addition to those using a plurality of separated light sources. When this monolithic multi-beam laser is used, since a synthetic optical element is not required, simplification of the optical system and optical adjustment is also achieved.
[0009]
In addition, a semiconductor laser used as a conventional laser light source is an infrared laser (for example, 780 nm) or a visible laser (for example, 675 nm), but a short wavelength laser having an oscillation wavelength of 500 nm or less is used because of a demand for higher resolution. Development of an optical scanning device capable of obtaining a minute spot shape is in progress.
[0010]
The advantage of using a short wavelength laser is that a spot diameter that is about half that of the prior art can be achieved while the exit F number of the scanning optical means is kept at the same level as before. In order to reduce the spot diameter to half that of the prior art using an infrared laser, the brightness of the scanning optical means must be approximately doubled. The depth of focus is proportional to the oscillation wavelength of the laser light source to be used, and is proportional to the square of the emission F number of the scanning optical means. Therefore, when trying to obtain the same spot diameter, the depth of focus of the infrared laser is shorter than that of the short wavelength laser. Will be about 1/2 or less.
[0011]
[Problems to be solved by the invention]
FIG. 18 is a schematic view of a main part of a conventional color image forming apparatus. In the figure, a plurality of optical scanning devices 211 to 214 are used simultaneously, and image information for each color is recorded on the surfaces of different photosensitive drums 221 to 224 to form color images. In such a color image forming apparatus, an image is formed by superimposing a plurality of scanning lines. In particular, it is important to reduce scanning line deviation between colors and image density unevenness between colors. For this reason, optical scanning devices
(1) Compensation for image position change (spot position deviation) in the main scanning direction due to wavelength fluctuation of a light beam emitted from a semiconductor laser as a laser light source; (2) Environment fluctuation such as temperature rise. Accordingly, the focus change in the sub-scanning direction, which has a particularly large influence, is compensated (the focus change in the main scanning direction is originally small and thus does not cause a problem).
As well as optical performance within one optical scanning device, the light source wavelength of the optical scanning device (the wavelength of the light beam emitted from the laser light source) and the usage environment (especially the environmental temperature) fluctuate. In addition, it is necessary to have a configuration that does not cause registration deviation or image unevenness between colors even if there are differences in the light source wavelengths and use environments of a plurality of optical scanning devices.
[0012]
Further, even in a monochromatic optical scanning device using a multi-beam laser light source, a laser light source is used to solve jitter (variation in scanning line interval in the main scanning direction on the photosensitive drum surface) due to a wavelength difference between the plurality of laser light sources. In order to minimize the wavelength difference between them, measures such as selecting a laser light source have been taken.
[0013]
In order to correct jitter (magnification chromatic aberration) due to a wavelength difference between a plurality of laser light sources by a scanning optical means, a plurality of lenses having different dispersion characteristics are required. Compared with scanning optical means that does not correct lateral chromatic aberration, this generally increases the number of sheets and increases the complexity of the optical system and increases the cost. In addition, there is a limit to the wavelength selection of the laser light source, and it is difficult to make the wavelengths completely coincide with each other, and the cost for wavelength selection is also a problem. In addition, when the semiconductor laser rises, the image quality deteriorates due to wavelength fluctuations called mode hopping. Therefore, even if it is not a color image forming apparatus or an optical scanning apparatus using a multi-beam laser light source, it is necessary to suppress jitter due to wavelength fluctuation as much as possible in order to improve the stability of image quality.
[0014]
Further, in the case of a high-resolution optical scanning device in which the wavelength of the laser light source is shortened, there is a problem that the dispersion of the optical material is larger than when an infrared laser is used.
[0015]
FIG. 19 is a cross-sectional view of a main part of a general optical scanning apparatus using two plastic lenses as scanning optical means. In the figure, a light beam emitted from a laser light source 91 is converted into a substantially parallel light beam by a collimator lens 92, and then a deflection surface of a polygon mirror 95 by a cylinder lens 94 having a predetermined refractive power only in the sub-scanning direction. (Reflection surface) Image is formed once in the sub-scanning direction in the vicinity of 95a. The light beam reflected and deflected by the polygon mirror 95 is scanned at a constant speed by two refracting lenses 961 and 962 having fθ characteristics, and is formed into a minute spot on the scanned surface 98.
[0016]
When such an optical scanning device is used, the lateral chromatic aberration is calculated for an infrared laser (780 nm) that has been used as a conventional laser light source and a short wavelength laser (408 nm) that is used for a high-resolution optical scanning device. 20 graphs.
[0017]
The graph in the figure is obtained by plotting the image forming position in the main scanning direction at the reference wavelength minus the image forming position in the main scanning direction at a wavelength difference of 1.5 nm, plotted for each image height ( For example, the difference between the imaging position of 781.5 nm and the imaging position of 780 nm). In this optical system using two plastic lenses of the same material, it is basically impossible to correct lateral chromatic aberration. Until now, the oscillation wavelength of the laser light source was long, and the dispersion characteristics of the material were relatively insignificant. Therefore, even if the chromatic aberration of magnification was not corrected by the scanning optical means, jitter was reduced by measures such as laser light source selection. It was done.
[0018]
However, when the same optical system as this is used with a short wavelength laser, the dispersion characteristic of the material is deteriorated by 4 to 8 times (see FIG. 21), so that lateral chromatic aberration is generated about 20 μm at the peripheral portion of the image (see FIG. 20). ). This is about half of one dot in an image forming apparatus with a resolution of 600 dpi. Therefore, it is assumed that an optical scanning device using a short wavelength laser of 500 nm or less corrects lateral chromatic aberration.
[0019]
However, as described above, even if a plurality of lenses having different dispersion characteristics are used for the scanning optical means, the optical design itself becomes difficult because of the very large dispersion. Furthermore, since there are few materials with different dispersion characteristics in plastic lenses, it is difficult to correct lateral chromatic aberration only with plastic lenses.
[0020]
In addition, as shown in FIG. 21, a general optical resin has a refractive index that varies with changes in environmental temperature. FIG. 21 is a graph in which the wavelength characteristics of the refractive index of ZEONEX E48R manufactured by Nippon Zeon Co., Ltd. are plotted in a 25 ° C. environment and a 40 ° C. environment. As can be seen from this graph, the refractive index of a general optical resin decreases as the environmental temperature increases. The amount is −8.996102E−5 / ° C. at a wavelength of 780 nm and −8.000785E−5 / ° C. at a wavelength of 408 nm, which are two orders of magnitude larger than those of optical glass. E-5 is 10^-FiveRepresents.
[0021]
For this reason, when the environmental temperature rises, there is a problem that the focus surface is farther away from the photosensitive drum surface than when a glass lens is used, and the spot diameter is enlarged on the photosensitive drum surface.
[0022]
The present invention relates to an optical scanning apparatus using a short-wavelength light source of 500 nm or less and an image forming apparatus using the same, and an imaging position deviation (magnification chromatic aberration) in the main scanning direction due to wavelength fluctuation or / and environmental fluctuation (temperature change, etc.) And an image forming apparatus using the optical scanning device that can easily obtain high-definition printing with a simple configuration.
[0023]
[Means for Solving the Problems]
  An optical scanning device according to a first aspect of the invention comprises a deflecting means for deflecting a light beam emitted from a light source means, and a plastic scanning optical means for forming an image of the light beam deflected by the deflecting means on a surface to be scanned. In an optical scanning device having
  The oscillation wavelength of the light source means is in the range of 380 nm to 500 nm,
  The scanning optical means has at least one refracting surface and at least one diffractive surface, the combined power of the refracting surface in the main scanning direction is Φr, the combined power of the diffractive surface in the main scanning direction is Φd, and the light source means When the rate of change of the refractive index of the material of the optical element that forms the refractive surface with respect to the oscillation wavelength is dn / dλ,
-2.37 × 10^-3 (/ nm)<(Φr / Φd) × (dn / dλ) <− 0.9 × 10^-3 (/ nm)
It is characterized by satisfying the following conditions.
However, the optical elements forming the refractive surface have the same refractive index and the same Abbe number.
[0024]
  The invention of claim 2 is the invention of claim 1, wherein the scanning optical means is
-2.25 × 10^-3 (/ nm)<(Φr / Φd) × (dn / dλ) <− 1.30 × 10^-3 (/ nm)
It is characterized by satisfying the following conditions.
[0025]
  A third aspect of the invention is characterized in that, in the first or second aspect of the invention, the light source means comprises a monolithic multi-beam laser having a plurality of light emitting points.
[0026]
  According to a fourth aspect of the present invention, there is provided an image forming apparatus comprising: the optical scanning device according to any one of the first to third aspects; a photoconductor disposed on the surface to be scanned; and light scanned by the optical scanning device. A developing unit that develops the electrostatic latent image formed on the photosensitive member by a beam as a toner image, a transfer unit that transfers the developed toner image to a transfer material, and a fixing of the transferred toner image to the transfer material And a fixing device.
[0027]
  An image forming apparatus according to a fifth aspect of the present invention converts the code data input from the optical scanning device according to any one of the first to third aspects and an external device into an image signal and inputs the image signal to the optical scanning device. And a printer controller.
[0057]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
FIG. 1 is a schematic view of a main part of a color image forming apparatus according to Embodiment 1 of the present invention.
[0058]
In the figure, 11, 12, 13, and 14 are optical scanning devices, 21, 22, 23, and 24 are photosensitive drums as image carriers, 31, 32, 33, and 34 are developing units, and 41 is a conveyor belt. is there. As will be described later, the color image forming apparatus according to the present embodiment performs the above-described optical scanning in which the variation in the imaging position due to the wavelength variation of the light beam (light beam) emitted from the laser light source and the aberration variation due to the environmental variation (temperature change) are suppressed to be small. Four devices (11, 12, 13, and 14) are arranged, each corresponding to each color of C (cyan), M (magenta), Y (yellow), and B (black). Image signals (image information) are recorded on surfaces 22, 23, and 24, and color images are printed at high speed.
[0059]
Next, a method for satisfactorily correcting a change in image forming position due to a wavelength variation of a laser light source and a change in aberration accompanying an environmental variation, which are features of the present invention, and its optical element will be described.
[0060]
FIG. 2 is a schematic diagram of a main part showing one optical scanning device and an image carrier corresponding thereto, and FIG. 3 is a main part cross-sectional view (main scanning cross-sectional view) in the main scanning direction of the optical system shown in FIG. is there.
[0061]
2 and 3, reference numeral 1 denotes a light source means, which is composed of, for example, a gallium nitride blue-violet semiconductor laser (laser light source) having an oscillation wavelength of 408 nm. A collimator lens 2 converts a divergent light beam emitted from the laser light source 1 into a substantially parallel light beam. Reference numeral 3 denotes an aperture stop, which limits a passing light beam (light quantity). Reference numeral 4 denotes a cylindrical lens (cylinder lens) having a predetermined power (refractive power) only in the sub-scanning direction. A substantially linear image is formed on the deflection surface (reflection surface) 5a.
[0062]
Reference numeral 5 denotes an optical deflector composed of, for example, a polygon mirror (rotating polygonal mirror) as a deflecting means, and is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor.
[0063]
Reference numeral 6 denotes scanning optical means (scanning optical system) having an fθ characteristic, and includes a refractive element (refractive lens) 61 and a diffractive element (diffractive lens) 62. The refractive element 61 is made of a plastic toric lens having different powers in the main scanning direction and the sub-scanning direction, and both lens surfaces 61a and 61b in the main scanning section of the toric lens 61 are both aspherical. Yes.
[0064]
The diffractive element 62 is composed of a long composite optical element having different powers in the main scanning direction and the sub-scanning direction. The composite optical element 62 has a non-existent surface 62a having a predetermined power only in the main scanning direction. The spherical surface (the sub-scanning direction is a flat surface), and the emission-side surface 62b are formed of a diffraction surface with a diffraction grating 71 added on the flat surface. Here, as the grating shape of the diffraction grating 71, for example, a Fresnel-like grating shape composed of a sawtooth diffraction grating by surface cutting or a step-like diffraction grating shape by photoetching is suitable. The composite optical element 62 in the present embodiment is made of plastic manufactured by injection molding, but the same effect can be obtained even if a diffraction grating is manufactured by a replica on a glass substrate.
[0065]
In the present embodiment, a toric lens 61 is disposed on the polygon mirror 5 side from a midpoint between the rotation axis of the polygon mirror 5 and the surface to be scanned 8, and a composite optical element 62 is disposed on the surface to be scanned 8. Both of these optical elements have different powers in the main scanning direction and the sub-scanning direction as described above, and form an image of the deflected light beam from the polygon mirror 5 on the surface to be scanned 8 and the polygon mirror in the sub-scan section. The tilt of the deflection surface is corrected. Reference numeral 8 denotes a photosensitive drum surface which is a surface to be scanned.
[0066]
As described above, the color image forming apparatus according to this embodiment uses the light beams based on the respective modulation signals by the four optical scanning devices 11, 12, 13, and 14, and the photosensitive drums 21, 22, and 23 corresponding to the latent images, respectively. , 24 on the surface. For example, latent images of C (cyan), M (magenta), Y (yellow), and B (black) are formed on the corresponding photosensitive drums 21, 22, 23, and 24, and then multiple-transferred onto a recording material. One full color image is formed.
[0067]
As described above, the gallium nitride blue-violet semiconductor laser is used as the laser light source of the optical scanning device in this embodiment, and its oscillation wavelength is 408 nm. As described above, by reducing the wavelength of the laser light source, the spot diameter is about half that of the prior art. As a result, it is possible to further increase the resolution and form an image particularly excellent in gradation expression.
[0068]
In this embodiment, a blue-violet semiconductor laser having a laser light source with an oscillation wavelength of 408 nm is used, but as long as the laser light source has an oscillation wavelength in the range of 380 nm to 500 nm as in the embodiments described later.
[0069]
The shapes of the refracting surface and the diffractive surface of the toric lens 61 and the composite optical element 62 constituting the scanning optical means 6 of the optical scanning device in the present embodiment are respectively
(A) Refractive surface. . . Aspherical shape that can be expressed by a function up to the 10th order in the main scanning direction, with the intersection point with the optical axis as the origin, the optical axis direction as the x axis, the axis orthogonal to the optical axis in the main scanning section, and the sub scanning section When the axis perpendicular to the optical axis is the z axis, the bus direction corresponding to the main scanning direction is
[0070]
[Expression 1]

[0071]
(Where R is the radius of curvature, and K, B4, B6, B8 and B10 are aspheric coefficients)
The sub-scanning direction (the direction including the optical axis and orthogonal to the main scanning direction) and the sub-line direction are
[0072]
[Expression 2]

[0073]
here
r '= r0 (1 + D2Y²+ D4Y^Four+ D6Y⁶+ D8Y⁸+ D10Y^Ten)
(Where r0 is the radius of curvature on the optical axis and D2, D4, D6, D8, and D10 are aspheric coefficients)
(A) Diffraction surface. . . Diffraction surface expressed by secondary phase function with main scanning direction up to 10th order and sub-scanning direction depending on position in main scanning direction
φ = 2πm / λ [b2 Y² + B4 Y^Four + B6 Y⁶ + B8 Y⁸ + B10 Y^Ten
+ (D0 + d1 Y + d2 Y² + D3 Y^Three + D4 Y^Four + D5 Y^Five + D6 Y⁶ Z²]
Where φ is the phase function, m is the diffraction order, λ is the wavelength used (408 nm), Y is the height from the lens optical axis, b2, b4, b6, b8, b10, d0, d1, d2, d3, d4 , d5, d6 are expressed by the equation: phase coefficient, and in the first to fourth embodiments, + 1st order diffracted light is used.
[0074]
In this embodiment, by optimizing the power arrangement of the refracting surface and the diffractive surface constituting the scanning optical means 6, the main scanning direction changes the imaging position caused by the wavelength variation of the laser light source, and the sub-scanning direction Both focus changes caused by environmental changes (changes in temperature, humidity, and atmospheric pressure, the following will be described by taking temperature changes as an example of environmental changes, but the same applies to other environmental fluctuations) can be compensated together.
[0075]
That is, in this embodiment, the chromatic aberration of magnification of the scanning optical means 6 is compensated by the toric lens (refractive element) 61 having positive dispersion in the main scanning direction and the composite optical element (diffractive element) 62 having negative dispersion. Yes (color magnification compensation).
[0076]
The power arrangement is such that the combined power in the main scanning direction of the refractive surfaces 61 a and 61 b of the toric lens 61 and the refractive surface 62 a of the composite optical element 62 is Φr, the Abbe number of the material of the toric lens 61 is νr, and the diffraction of the composite optical element 62 When the power in the main scanning direction of the surface 62b is Φd and the Abbe number of the diffraction element 71 is νd,
Φr / νr + Φd / νd = 0 (Expression 1)
It is desirable to satisfy
[0077]
However, the Abbe number νr is
νr = (n−1) / Δn = (n−1) / (dn / dλ) (Expression 2)
It is.
[0078]
Using the above relational expression (Expression 2) and the fact that the dispersion characteristic of the diffraction element 71 changes linearly with respect to the wavelength, (Expression 1) can be modified as follows.
[0079]
    (Φr / Φd) × (dn / dλ) = − (n−1) / λ (Expression 3)
  When the range of the oscillation wavelength λ of the laser light source is 380 to 500 nm and the range of the refractive index n of a general optical material is 1.45 to 1.90,
    − (1.45-1) /380=−1.18×10^-3
    − (1.90-1) /380=−2.368×10^-3
    − (1.45-1) /500=−0.9×10^-3
    − (1.90−1) /500=−1.8×10^-3
Therefore, the possible range of (Equation 3) is
  -2.37 × 10^-3 (/ nm)<(Φr / Φd) × (dn / dλ) <− 0.9 × 10^-3 (/ nm)
                                                          ... (Formula 4)
It becomes. That is, in this embodiment, the combined power of the refracting surface in the main scanning direction is Φr, the combined power of the diffractive surface in the main scanning direction is Φd, and the refractive index change of the material of the optical element that forms the refracting surface with respect to the oscillation wavelength of the light source means. When the rate is dn / dλ, the above condition (Equation 4) is satisfied. Here, the combined power includes the case where there is one refracting surface and also includes the case where there is one diffractive surface.
[0080]
  On the other hand, if the power of the diffraction grating is too strong, the grating pitch becomes narrow, the number of generalized bodies increases, and the difficulty of processing increases. Therefore, it is desirable to keep the power of the diffraction grating within a certain range, and in order to perform better lateral chromatic aberration correction,
  -2.25 × 10^-3 (/ nm)<(Φr / Φd) × (dn / dλ) <− 1.30 × 10^-3 (/ nm)
                                                                ... (Formula 4 ')
It is good to satisfy.
[0081]
When the surface of the composite optical element 62 excluding the diffraction grating 71 has power, the power is handled as the power of the refractive surface.
[0082]
Further, dn / dλ is the rate of change of the refractive index n of the material of the optical element (in this embodiment, the toric lens 61 or the composite optical element 62) that forms the refractive surface with respect to the oscillation wavelength λ of the semiconductor laser 1 as described above.
[0083]
Conditional expression (Formula 4) is the power ratio in the main scanning direction between the refractive surface of the toric lens 61 and the composite optical element 62 constituting the scanning optical means 6 and the diffraction surface of the composite optical element 62, and the dispersion of the lens material. If the conditional expression (Expression 4) is not satisfied, it becomes difficult to correct the change in the imaging position (magnification chromatic aberration) in the main scanning direction on the surface to be scanned 8 due to the wavelength variation of the semiconductor laser 1. So not good.
[0084]
On the other hand, in the sub-scanning direction, the change in the refractive index of the scanning optical means 6 due to the environmental change (particularly the temperature rise) of the apparatus is compensated by the change in the wavelength of the semiconductor laser 1 caused by the environmental change, thereby canceling the focus movement (temperature compensation). ).
[0087]
Here, in order to suppress the focus change in the sub-scanning direction by increasing the temperature, it is necessary to satisfy the following expression.
[0088]
dψr (dn / dt) + dψr (dλ / dt) + dψd (dλ / dt) = 0 (Equation 8)
Than this,
[0089]
[Equation 3]

[0090]
It becomes.
[0091]
The characteristic value of a general optical material and the rate of change of the oscillation wavelength (wavelength temperature characteristic) (dλ / dt = 0.04 to 0.06 nm / ° C.) of the gallium nitride-based blue-violet semiconductor laser with respect to temperature are infrared. Considering that there is less change than the wavelength temperature characteristics of the laser,
15.0 (° C / nm) <ψr / [ψd × (dλ / dt)] <45.0 (° C / nm) (Equation 10)
It is good to do.
[0092]
However,
ψr: Sub-scanning direction between the refractive surface of the toric lens and the refractive surface of the composite optical element
The combined power of
ψd: power in the sub-scanning direction of the diffraction surface of the composite optical element
It is.
[0093]
On the other hand, if the power of the diffraction grating is too strong, the grating pitch becomes narrow, the number of generalized bodies increases, and the difficulty of processing increases. Therefore, it is desirable to keep the power of the diffraction grating within a certain range, and for better temperature compensation, (Equation 10)
18.0 (° C./nm)<ψr/[ψd×(dλ/dt)]<38.0 (° C./nm) (Formula 10 ′)
It is good to satisfy.
[0094]
Here, dλ / dt is a rate of change of the oscillation wavelength λ of the semiconductor laser 1 with respect to the temperature t.
[0095]
Conditional expression (Expression 10) is the power ratio in the sub-scanning direction between the refractive surface of the toric lens 61 and the composite optical element 62 constituting the scanning optical means 6 and the diffraction surface of the composite optical element 62, and the temperature of the laser light source. This is related to the characteristics, and if the conditional expression (Expression 10) is not satisfied, it is difficult to correct the focus change in the sub-scanning direction due to the environmental change (temperature change) of the optical scanning device.
[0096]
Table 1 shows the optical arrangement, the aspheric coefficient of the toric lens 61, the aspheric coefficient of the composite optical element 62, and the phase term in this embodiment.
[0097]
[Table 1]

[0098]
The power ratio between the refractive surface of the toric lens 61 and the composite optical element 62 and the diffraction surface of the composite optical element 62 in this embodiment is
Main scanning direction: (Φr / Φd) × (dn / dλ) = − 1.73E-3
Sub-scanning direction: φr / [φd × (dλ / dt)] = 33.0
This satisfies the conditional expression (Expression 4) and the conditional expression (Expression 10), respectively.
[0099]
“Ex” is “× 10”.^-x".
[0100]
FIG. 4 is an explanatory diagram showing changes in the imaging position in the main scanning direction due to the wavelength change of the semiconductor laser 1 in this embodiment, and the imaging positions when the wavelength differences 1.5 nm and 5 nm from the initial wavelength of the semiconductor laser 1 are given. The difference is shown.
[0101]
FIG. 5 is an explanatory diagram showing paraxial aberration (field curvature in the main scanning direction and sub-scanning direction) before and after environmental change in the present embodiment, the solid line is the characteristic (design value) before environmental change, and the broken line Is a characteristic (effective value) when the temperature of the optical scanning device is raised by + 25 ° C.
[0102]
In general, an apparatus that records image information for each color from a plurality of optical scanning devices to a plurality of photosensitive drums to form a color image in order to make the registration shift between the colors and the image density unevenness between the colors inconspicuously. Requires that the imaging position shift in the main scanning direction due to wavelength fluctuations of the semiconductor laser be 10 μm or less, and that the focus shift in the main scanning direction and sub-scanning direction be ± 1.0 mm or less due to environmental variations (temperature fluctuations) of the apparatus. It is.
[0103]
As can be seen from FIG. 4, in this embodiment, the image formation position shift due to the wavelength difference of +5 nm is 2 μm, which is suppressed to a level with no problem.
[0104]
Similarly, it can be seen from FIG. 5 that the amount of focus movement in the sub-scanning direction when the temperature is raised by + 25 ° C. is 0.1 mm, which is suppressed to a level that causes no visual problems in the main scanning direction.
[0105]
In addition, although the explanation has been made here centering on the behavior at the time of temperature rise with respect to the environmental variation, for example, the same effect as described above can be obtained also in other environmental variation such as a temperature decrease.
[0106]
In this embodiment, all of these environmental fluctuations are compensated by a plastic optical element, thereby reducing the manufacturing cost by molding, shortening the optical path length by correcting high angle of view aberration using an aspherical surface, etc. Can be achieved at the same time.
[0107]
As described above, in this embodiment, a short wavelength laser is used as the light source unit as described above, and a high-resolution image forming apparatus excellent in gradation expression can be realized by achieving a spot diameter about half that of the conventional one. 6, a plastic toric lens 61 and a composite optical element 62 are used, and when a short wavelength laser is used as a plastic lens, it is possible to correct lateral chromatic aberration, which has been difficult to correct. In addition, a focus image movement due to environmental fluctuations is also compensated, and a color image forming apparatus with little density unevenness between colors is realized with a simple configuration and at a low cost.
[0108]
[Embodiment 2]
FIG. 6 is a cross-sectional view of the main part in the main scanning direction of the optical scanning device according to the second embodiment of the present invention. In the figure, the same elements as those shown in FIG.
[0109]
This embodiment is different from the first embodiment described above in that a monolithic multi-beam laser 51 having a plurality of light emitting points (light emitting portions) is used as the light source means, and the wavelength difference of the light source means with high accuracy as in the first embodiment. This compensates for the imaging position shift (magnification chromatic aberration) in the main scanning direction. Other configurations and optical actions are substantially the same as those of the first embodiment, and the same effects are obtained.
[0110]
That is, in the figure, 51 is a light source means, which is composed of a monolithic multi-beam laser (multi-semiconductor laser light source) having a plurality of light emitting points (two in this embodiment, but three or more). Two light beams (only one is shown in the figure) are modulated.
[0111]
Reference numeral 46 denotes scanning optical means as an optical element having an fθ characteristic, and includes a refraction element 63 and a diffraction element 64. The refractive element 63 is composed of a single plastic toric lens having different powers in the main scanning direction and the sub-scanning direction, and both lens surfaces 63a and 63b in the main scanning section of the toric lens 63 are both aspherical. Is made up of.
[0112]
The diffractive element 64 is composed of a long composite optical element having different powers in the main scanning direction and the sub-scanning direction, and the composite optical element 64 has an incident-side surface 64a different in the main scanning direction and the sub-scanning direction. An aspherical surface having power and an exit-side surface 64b are formed of a diffraction surface with a diffraction grating 72 added on a plane.
[0113]
In the present embodiment, the two light beams emitted from the multi-beam laser 51 pass through the same path as in the above-described first embodiment, and simultaneously scan in a state where the scanning surface 8 is separated by a certain amount in the sub-scanning direction. Yes.
[0114]
Table 2 shows the optical arrangement, the aspheric coefficient of the toric lens 63, the aspheric coefficient of the composite optical element 64, and the phase term in this embodiment.
[0115]
[Table 2]

[0116]
As the light source means of the optical scanning device in this embodiment, a gallium nitride blue-violet semiconductor laser 51 is used as in the first embodiment, and its oscillation wavelength is 408 nm. As described above, by reducing the wavelength of the laser light source, the spot diameter is about half that of the prior art. As a result, it is possible to further increase the resolution and form an image particularly excellent in gradation expression.
[0117]
Also in the present embodiment, the main scanning direction is caused by the wavelength variation of the multi-beam laser 51 by appropriately setting the power arrangement of the refracting surface and the diffractive surface constituting the scanning optical means 46 as in the first embodiment. The change in the image position can be compensated for both the focus change caused by the environmental fluctuation (particularly the temperature rise) of the apparatus in the sub-scanning direction. Further, in this embodiment, the photosensitive drum surface caused by the wavelength difference between the plurality of light emitting points. 8 can also compensate for a change (jitter) in the scanning interval in the main scanning direction.
[0118]
The power ratio between the refractive surface of the toric lens 63 and the composite optical element 64 and the diffraction surface of the composite optical element 64 in this embodiment is
Main scanning direction: (Φr / Φd) × (dn / dλ) =-2.20E-3
Sub-scanning direction: φr / [φd × (dλ / dt)] = 24.3
This satisfies the conditional expression (Expression 4) and the conditional expression (Expression 10), respectively.
[0119]
FIG. 7 is an explanatory diagram showing changes in the imaging position in the main scanning direction due to changes in the wavelength of the multi-beam laser 51 in the present embodiment, and the results when the wavelength differences 1.5 nm and 5 nm from the initial wavelength of the multi-beam laser 51 are given. The difference from the image position is shown.
[0120]
FIG. 8 is an explanatory diagram showing paraxial aberration (field curvature in the main scanning direction and sub-scanning direction) before and after environmental change in the present embodiment, the solid line is the characteristic (design value) before environmental change, and the broken line Is a characteristic (effective value) when the temperature of the optical scanning device is raised by + 25 ° C.
[0121]
From FIG. 7, in the present embodiment, the imaging position shift due to the wavelength difference of +5 nm is 2 μm, and this is suppressed to a pixel (position) shift within about 1/20 pixel in the case of a printer with a resolution of 600 dpi, for example. I understand.
[0122]
Similarly, it can be seen from FIG. 8 that the amount of focus movement in the sub-scanning direction when the temperature is raised by + 25 ° C. is 0.1 mm, and is suppressed to a level that causes no visual problems with the main scanning direction.
[0123]
In addition, although the explanation has been made here centering on the behavior at the time of temperature rise with respect to the environmental variation, for example, the same effect as described above can be obtained also in other environmental variation such as a temperature decrease.
[0124]
In this embodiment, all of these environmental fluctuations are compensated by a plastic optical element, thereby reducing the manufacturing cost by molding, shortening the optical path length by correcting high angle of view aberration using an aspherical surface, etc. Can be achieved at the same time.
[0125]
Further, since the chromatic aberration of magnification (change in imaging position in the main scanning direction) is corrected with high accuracy as a feature of the present embodiment, the reduction of jitter due to the wavelength difference between a plurality of emission points of the multi-beam laser 51 can also be mentioned.
[0126]
As described above, in this embodiment, a short wavelength laser is used as the light source unit as described above, and a high-resolution image forming apparatus excellent in gradation expression can be realized by achieving a spot diameter about half that of the conventional one. When a toric lens 63 made of plastic and a composite optical element 64 are used as 46 and a short wavelength laser is conventionally used as a plastic lens, it is possible to correct lateral chromatic aberration that was difficult to correct. In addition, a color image forming apparatus that suppresses jitter when using the multi-beam laser 51, compensates for focus shift due to environmental fluctuations, and has little density unevenness between colors can be realized with an easy configuration and at low cost.
[0127]
In this embodiment, the monolithic multi-beam laser 51 is used as the light source means. However, the present invention is not limited to this. For example, a plurality of laser light sources that emit a single light beam are provided, and a plurality of laser light sources emitted from the plurality of laser light sources are provided. A configuration may be adopted in which the light beams are combined into an optical path in the same direction by beam combining means.
[0128]
[Embodiment 3]
FIG. 9 is a cross-sectional view of a main part in the main scanning direction of the optical scanning device according to the third embodiment of the present invention. In the figure, the same elements as those shown in FIG.
[0129]
This embodiment is different from the above-described second embodiment in that the oscillation wavelength of the monolithic multi-beam laser 51 as the light source means is set to 440 nm. Other configurations and optical functions are substantially the same as those of the second embodiment, and the same effects are obtained.
[0130]
Table 3 shows the optical arrangement, the aspheric coefficient of the toric lens 63, and the aspheric coefficient and phase term of the composite optical element 64 in this embodiment. (However, the wavelength used in the phase function is 440 nm.)
[0131]
[Table 3]

[0132]
Also in the present embodiment, the main scanning direction is caused by the wavelength variation of the multi-beam laser 51 by appropriately setting the power arrangement of the refracting surface and the diffractive surface constituting the scanning optical means 46 as in the second embodiment. The change in the image position can be compensated for both the focus change caused by the environmental fluctuation (particularly the temperature rise) of the apparatus in the sub-scanning direction. Further, in this embodiment, the photosensitive drum surface caused by the wavelength difference between the plurality of light emitting points. 8 can also compensate for a change (jitter) in the scanning interval in the main scanning direction.
[0133]
The power ratio between the refractive surface of the toric lens 63 and the composite optical element 64 and the diffraction surface of the composite optical element 64 in this embodiment is
Main scanning direction: (Φr / Φd) × (dn / dλ) = − 1.82E−3
Sub-scanning direction: [psi] r / [[psi] d * (d [lambda] / dt)] = 24.4
This satisfies the conditional expression (Expression 4) and the conditional expression (Expression 10), respectively.
[0134]
FIG. 10 is an explanatory diagram showing a change in the imaging position in the main scanning direction due to a wavelength change of the multi-beam laser 51 in this embodiment. The results are obtained when wavelength differences of 1.5 nm and 5 nm from the initial wavelength of the multi-beam laser 51 are given. The difference from the image position is shown.
[0135]
FIG. 11 is an explanatory diagram showing paraxial aberration (field curvature in the main scanning direction and sub-scanning direction) before and after environmental change in the present embodiment, the solid line is the characteristic (design value) before environmental change, and the broken line Is a characteristic (effective value) when the temperature of the optical scanning device is raised by + 25 ° C.
[0136]
As shown in FIG. 10, in this embodiment, the imaging position shift due to the wavelength difference of +5 nm is 2 μm. For example, in the case of a printer with a resolution of 600 dpi, the pixel (position) shift within about 1/20 pixel is suppressed. I understand.
[0137]
Similarly, it can be seen from FIG. 11 that the amount of focus movement in the sub-scanning direction when the temperature rises by + 25 ° C. is 0.1 mm, and is suppressed to a level that causes no visual problem with the main scanning direction.
[0138]
In addition, although the explanation has been made here centering on the behavior at the time of temperature rise with respect to the environmental variation, for example, the same effect as described above can be obtained also in other environmental variation such as a temperature decrease.
[0139]
In this embodiment, all of these environmental fluctuations are compensated by a plastic optical element, thereby reducing the manufacturing cost by molding, shortening the optical path length by correcting high angle of view aberration using an aspherical surface, etc. Can be achieved at the same time.
[0140]
Thus, in the present embodiment, as described above, a short wavelength laser is used as the light source means, and a finer spot diameter is achieved than before, thereby enabling a high-resolution image forming apparatus excellent in gradation expression, and the scanning optical means 46. As described above, when a plastic toric lens 63 and a composite optical element 64 are used and a short wavelength laser is conventionally used as a plastic lens, it is possible to correct lateral chromatic aberration, which is difficult to correct. In addition, a color image forming apparatus that suppresses jitter when using the multi-beam laser 51, compensates for focus shift due to environmental fluctuations, and has little density unevenness between colors can be realized with an easy configuration and at low cost.
[0141]
[Embodiment 4]
FIG. 12 is a cross-sectional view of a main part in the main scanning direction of the optical scanning device according to the fourth embodiment of the present invention. In the figure, the same elements as those shown in FIG.
[0142]
This embodiment is different from the second embodiment described above in that the oscillation wavelength of a monolithic multi-beam laser as a light source means is set to 480 nm. Other configurations and optical functions are substantially the same as those of the second embodiment, and the same effects are obtained.
[0143]
Table 4 shows the optical arrangement, the aspheric coefficient of the toric lens 63, and the aspheric coefficient and phase term of the composite optical element 64 in this embodiment. (However, the wavelength used in the phase function is 480 nm.)
[0144]
[Table 4]

[0145]
Also in the present embodiment, the main scanning direction is caused by the wavelength variation of the multi-beam laser 51 by appropriately setting the power arrangement of the refracting surface and the diffractive surface constituting the scanning optical means 46 as in the second embodiment. The change in the image position can be compensated for both the focus change caused by the environmental fluctuation (particularly the temperature rise) of the apparatus in the sub-scanning direction. Further, in this embodiment, the photosensitive drum surface caused by the wavelength difference between the plurality of light emitting points. 8 can also compensate for a change (jitter) in the scanning interval in the main scanning direction.
[0146]
The power ratio between the refractive surface of the toric lens 63 and the composite optical element 64 and the diffraction surface of the composite optical element 64 in this embodiment is
Main scanning direction: (Φr / Φd) × (dn / dλ) = − 1.68E−3
Sub-scanning direction: [psi] r / [[psi] d * (d [lambda] / dt)] = 22.9
This satisfies the conditional expression (Expression 4) and the conditional expression (Expression 10), respectively.
[0147]
FIG. 13 is an explanatory diagram showing a change in the imaging position in the main scanning direction due to a wavelength change of the multi-beam laser 51 in this embodiment, and the results when the wavelength differences 1.5 nm and 5 nm from the initial wavelength of the multi-beam laser 51 are given. The difference from the image position is shown.
[0148]
FIG. 14 is an explanatory diagram showing paraxial aberration (field curvature in the main scanning direction and sub-scanning direction) before and after environmental change in the present embodiment, the solid line is the characteristic (design value) before environmental change, and the broken line Is a characteristic (effective value) when the temperature of the optical scanning device is raised by + 25 ° C.
[0149]
From FIG. 13, in the present embodiment, the imaging position shift due to the wavelength difference of +5 nm is 2 μm. For example, in the case of a printer with a resolution of 600 dpi, the pixel (position) shift within about 1/20 pixel is suppressed. I understand.
[0150]
Similarly, it can be seen from FIG. 14 that the amount of focus movement in the sub-scanning direction when the temperature is raised by + 25 ° C. is 0.1 mm, and is suppressed to a level that causes no visual problem with the main scanning direction.
[0151]
In addition, although the explanation has been made here centering on the behavior at the time of temperature rise with respect to the environmental variation, for example, the same effect as described above can be obtained also in other environmental variation such as a temperature decrease.
[0152]
In this embodiment, all of these environmental fluctuations are compensated by a plastic optical element, thereby reducing the manufacturing cost by molding, shortening the optical path length by correcting high angle of view aberration using an aspherical surface, etc. Can be achieved at the same time.
[0153]
Thus, in the present embodiment, as described above, a short wavelength laser is used as the light source means, and a finer spot diameter is achieved than before, thereby enabling a high-resolution image forming apparatus excellent in gradation expression, and the scanning optical means 46. As described above, when a plastic toric lens 63 and a composite optical element 64 are used and a short wavelength laser is conventionally used as a plastic lens, it is possible to correct lateral chromatic aberration, which is difficult to correct. In addition, a color image forming apparatus that suppresses jitter when using the multi-beam laser 51, compensates for focus shift due to environmental fluctuations, and has little density unevenness between colors can be realized with an easy configuration and at low cost.
[0154]
In each embodiment, the scanning optical means is composed of two optical elements, a single refractive element (toric lens) and a single diffractive element (composite optical element). A single optical element including a diffractive surface or three or more optical elements including a refractive surface and a diffractive surface may be used.
[0155]
In each embodiment, both the conditional expression (Expression 4) and the conditional expression (Expression 10) are satisfied, but only one conditional expression may be used.
[0156]
In each embodiment, the change in the focus position in the sub-scanning direction on the surface to be scanned due to the environmental variation of the apparatus is compensated by the power change between the refracting surface and the diffractive surface and the wavelength change of the light source means. The focus position change is also compensated by the power change between the refractive surface and the diffractive surface and the wavelength change of the light source means.
[0157]
[Image forming apparatus]
FIG. 15 is a cross-sectional view of the main part in the sub-scanning direction showing the embodiment of the image forming apparatus of the present invention. In the figure, reference numeral 104 denotes an image forming apparatus. Code data Dc is input to the image forming apparatus 104 from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) Di by a printer controller 111 in the apparatus. The image data Di is input to the optical scanning unit 100 having the configuration shown in the first to fourth embodiments. The light scanning unit 100 emits a light beam 103 modulated according to the image data Di, and the light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.
[0158]
The photosensitive drum 101 serving as an electrostatic latent image carrier (photoconductor) is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103. Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to contact the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100.
[0159]
As described above, the light beam 103 is modulated based on the image data Di, and by irradiating the light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101. This electrostatic latent image is developed as a toner image by a developing device 107 disposed so as to abut on the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103.
[0160]
  The toner image developed by the developing unit 107 is transferred below the photosensitive drum 101 so as to face the photosensitive drum 101.(Transfer device)Then, the image is transferred onto the paper 112 as a transfer material. The paper 112 is stored in the paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 15), but can be fed manually. A paper feed roller 110 is provided at the end of the paper cassette 109, and feeds the paper 112 in the paper cassette 109 into the transport path.
[0161]
As described above, the sheet 112 on which the unfixed toner image has been transferred is further conveyed to a fixing device behind the photosensitive drum 101 (left side in FIG. 15). The fixing device includes a fixing roller 113 having a fixing heater (not shown) therein, and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113, and the sheet conveyed from the transfer unit. The unfixed toner image on the paper 112 is fixed by heating 112 while applying pressure at the pressure contact portion between the fixing roller 113 and the pressure roller 114. Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and the fixed paper 112 is discharged out of the image forming apparatus.
[0162]
Although not shown in FIG. 15, the print controller 111 controls not only the data conversion described above, but also controls each part in the image forming apparatus including the motor 115 and a polygon motor in the optical scanning unit described later. I do.
[0163]
[Color image forming apparatus]
FIG. 16 is a schematic view of a main part of a color image forming apparatus according to an embodiment of the present invention. This embodiment is a tandem type color image forming apparatus in which four optical scanning devices are arranged side by side and image information is recorded on a photosensitive drum surface as an image carrier in parallel. In FIG. 16, 60 is a color image forming apparatus, 11, 12, 13, and 14 are optical scanning devices each having one of the configurations shown in the first to fourth embodiments, 21, 22, 23, and 24 are image carriers. The photosensitive drums 31, 32, 33 and 34 are developing units, and 51 is a conveyor belt.
[0164]
In FIG. 16, R (red), G (green), and B (blue) color signals are input to the color image forming apparatus 60 from an external device 52 such as a personal computer. These color signals are converted into C (cyan), M (magenta), Y (yellow), and B (black) image data (dot data) by a printer controller 53 in the apparatus. These image data are input to the optical scanning devices 11, 12, 13, and 14, respectively. From these optical scanning devices, light beams 41, 42, 43, and 44 modulated according to each image data are emitted, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are caused by these light beams. Scanned in the main scanning direction.
[0165]
The color image forming apparatus according to this embodiment includes four optical scanning devices (11, 12, 13, and 14) arranged in each color of C (cyan), M (magenta), Y (yellow), and B (black). Correspondingly, image signals (image information) are recorded on the photosensitive drums 21, 22, 23, and 24 in parallel, and a color image is printed at high speed.
[0166]
As described above, the color image forming apparatus in this embodiment uses the light beams based on the respective image data by the four optical scanning devices 11, 12, 13, and 14, and the photosensitive drums 21 and 22 respectively corresponding the latent images of the respective colors. , 23, 24 on the surface. Thereafter, a single full color image is formed by multiple transfer onto a recording material.
[0167]
As the external device 52, for example, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.
[0168]
【The invention's effect】
According to the present invention, as described above, in the optical scanning apparatus using a short wavelength light source of 500 nm or less and the image forming apparatus using the same, main scanning of at least one refracting surface and at least one diffractive surface constituting scanning optical means. By optimizing the power arrangement in the direction and / or sub-scanning direction, the sub-scanning direction due to image position displacement (magnification chromatic aberration) in the main scanning direction due to wavelength fluctuation or / and environmental fluctuation (temperature change, etc.) Therefore, it is possible to achieve an optical scanning device and an image forming apparatus using the optical scanning device that can suppress high-definition printing with a simple configuration.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a main part of a color image forming apparatus according to a first embodiment of the present invention.
2 is a main part schematic diagram showing the optical scanning device shown in FIG. 1 and an image carrier corresponding thereto. FIG.
3 is a cross-sectional view of the main part in the main scanning direction of the scanning optical means shown in FIG.
FIG. 4 is a diagram showing a change in imaging position in the main scanning direction due to a wavelength change in the first embodiment of the present invention.
FIG. 5 is a diagram showing a change in field curvature due to a wavelength change in the first embodiment of the present invention.
FIG. 6 is a cross-sectional view of the main part in the main scanning direction of the scanning optical means according to the second embodiment of the present invention.
FIG. 7 is a diagram showing a change in imaging position in the main scanning direction due to a wavelength change in Embodiment 2 of the present invention.
FIG. 8 is a diagram showing a change in field curvature due to a wavelength change in Embodiment 2 of the present invention.
FIG. 9 is a cross-sectional view of the main part in the main scanning direction of the scanning optical means according to the third embodiment of the present invention.
FIG. 10 is a diagram showing a change in imaging position in the main scanning direction due to a wavelength change in Embodiment 3 of the present invention.
FIG. 11 is a diagram showing a change in field curvature due to a wavelength change in Embodiment 3 of the present invention.
FIG. 12 is a cross-sectional view of the main part in the main scanning direction of the scanning optical means according to the fourth embodiment of the present invention.
FIG. 13 is a diagram showing a change in imaging position in the main scanning direction due to a wavelength change in Embodiment 4 of the present invention.
FIG. 14 is a diagram showing a change in field curvature due to a wavelength change in Embodiment 4 of the present invention.
FIG. 15 is a sub-scan sectional view showing an embodiment of an image forming apparatus according to the invention.
FIG. 16 is a schematic diagram of a main part of a color image forming apparatus according to an embodiment of the present invention.
FIG. 17 is a diagram for explaining a conventional multi-beam optical scanning device;
FIG. 18 is a schematic diagram of a main part of a conventional color image forming apparatus.
FIG. 19 is a cross-sectional view of the main part of a conventional optical scanning device in the main scanning direction.
FIG. 20 is a diagram showing an imaging position change in the main scanning direction due to a wavelength change in the conventional scanning optical means.
FIG. 21 is a graph showing the refractive index wavelength characteristic of an optical resin.
[Explanation of symbols]
1, 51 ... Light source means
2. Collimator lens
3 ... Aperture
4. Cylindrical lens
5. Deflection means
6, 46 Scanning optical means
61, 63 ... Refraction element
62, 64 ... Diffraction element (Composite optical element)
71, 72 ... Diffraction grating
8 ... Scanned surface
11, 12, 13, 14... Optical scanning device
21, 22, 23, 24 ... Image carrier (photosensitive drum)
31, 32, 33, 34 ... Developer
41 ... Conveyor belt
51 ... Multi-beam laser
52 ... External equipment
53... Printer controller
60 Color image forming apparatus
100 Optical scanning device
101 Photosensitive drum
102 Charging roller
103 Light beam
104 Image forming apparatus
107 Developing device
108 Transfer roller
109 Paper cassette
110 Paper feed roller
111 Printer controller
112 Transfer material (paper)
113 Fixing roller
114 Pressure roller
115 motor
116 Paper discharge roller
117 External equipment

Claims (5)

In an optical scanning apparatus comprising: a deflecting unit that deflects a light beam emitted from a light source unit; and a plastic scanning optical unit that images the light beam deflected by the deflecting unit on a surface to be scanned.
The oscillation wavelength of the light source means is in the range of 380 nm to 500 nm,
The scanning optical means has at least one refracting surface and at least one diffractive surface, the combined power of the refracting surface in the main scanning direction is Φr, the combined power of the diffractive surface in the main scanning direction is Φd, and the light source means When the rate of change of the refractive index of the material of the optical element that forms the refractive surface with respect to the oscillation wavelength is dn / dλ,
-2.37 × 10 ⁻³ ( /nm)<(Φr/Φd)×(dn/dλ)<−0.9×10 ⁻³ (/ nm)
An optical scanning device characterized by satisfying the following conditions.
However, the optical elements forming the refractive surface have the same refractive index and the same Abbe number. The scanning optical means includes
−2.25 × 10 ⁻³ ( /nm)<(Φr/Φd)×(dn/dλ)<−1.30×10 ⁻³ (/ nm)
The optical scanning device according to claim 1, wherein the following condition is satisfied.   3. The optical scanning device according to claim 1, wherein the light source means includes a monolithic multi-beam laser having a plurality of light emitting points.   The optical scanning device according to any one of claims 1 to 3, a photosensitive member disposed on the surface to be scanned, and electrostatic formed on the photosensitive member by a light beam scanned by the optical scanning device. A developing device that develops a latent image as a toner image, a transfer device that transfers the developed toner image onto a transfer material, and a fixing device that fixes the transferred toner image onto the transfer material. Image forming apparatus.   The optical scanning device according to claim 1, and a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device. An image forming apparatus.

Images