JP3558847B2 - Multi-beam scanner - Google Patents

Description

なる式におけるＲ，Ｋ，Ａ，Ｂ，Ｃ，Ｄ，．．を与えて特定される曲線形状である。
【０００７】
上記光偏向器側のレンズは「メニスカスレンズ」であるので、中央と周辺部、特に、主走査対応方向における中央部と周辺部との肉厚差を有効に軽減する「均肉化」が可能であり、これをプラスチック等の樹脂で成形加工により作製する際の「ヒケやウネリ」といった変形を有効に防止できる。また、光偏向器側のレンズは、凹面を光偏向器側に向けて配備されるので、主走査対応方向の中央部と周辺部で「入射側レンズ面への、偏向の起点からの距離」の変化が小さく、従って上記中央部と周辺部との「副走査対応方向の横倍率の差」を少なくできる。
【０００８】
上記のように、請求項１記載の走査結像レンズは、主走査断面内での形状は、少なくとも３面が「非円弧形状」であるから、これらの非円弧形状を最適化することにより、主走査方向の像面湾曲や等速化特性を、各ビームに対し良好に補正することが可能となる。
また、光軸に平行で主走査断面に直交する面内での形状（前記ティルト角を与える場合には、ティルト角を０とした状態における形状）において、２面（光偏向器側のレンズの両面）が非円弧形状であり、被走査面側のレンズの少なくとも１面において、副走査断面内の曲率半径を主走査対応方向に変化させるので、主走査方向の像面湾曲や等速化特性用に最適化された上記非円弧形状に応じて、上記曲率半径の変化を最適化することにより、副走査方向の像面湾曲を有効に補正することができる。
【０００９】
走査結像レンズにおける上記被走査面側のレンズの「主走査断面内において非円弧形状を有し、且つ、該レンズ面における副走査断面内の曲率中心を主走査対応方向に連ねた曲率中心線が、主走査断面内において上記非円弧形状とは異なる曲線となるように、副走査断面内における曲率半径を主走査対応方向に変化させたレンズ面」は、これを「光偏向器側のレンズ面」とすることができる（請求項２）。この場合、被走査面側のレンズの、被走査面側のレンズ面は「主走査断面内において円弧形状」を有することができる（請求項３）。
【００１０】
また、走査結像レンズにおける被走査面側のレンズは、主走査断面内における屈折力を負とすることができる（請求項４）。前述の如く、走査結像レンズにおける光偏向器側のレンズは「正メニスカスレンズ」であるので、被走査面側のレンズの「主走査断面内における屈折力」を負とすると、主走査断面内における走査結像レンズの屈折力の組合せが「正・負」の組合せとなるので、走査結像レンズを構成する２枚のレンズを「共にプラスチックレンズ」として構成した場合、温・湿度変化の影響は、各レンズで互いに打ち消すように作用するので、走査結像レンズとしては温・湿度変化の影響を受けにくくなる。
【００１１】
また、被走査面側のレンズの「主走査断面内において非円弧形状を有し、且つ、該レンズ面における副走査断面内の曲率中心を主走査対応方向に連ねた曲率中心線が、主走査断面内において上記非円弧形状とは異なる曲線となるように、副走査断面内における曲率半径を主走査対応方向に変化させたレンズ面」の、上記曲率半径の絶対値が「主走査対応方向において光軸を離れるに従い極大値に向かって滑らか且つ単調に増加し、極大位置を超えたのち、光軸を離れるに従い滑らか且つ単調に減少する」ように定めることができる（請求項５）。
主走査対応方向の座標をηとし、上記曲率半径をｒ（η）とするとき、ｒ（η）において主走査方向の位置誤差：Δηがあると、位置：ηにおける曲率半径の誤差は｛ｄｒ（η）／ｄη｝Δηであり、ｒ（η）が「ηの増加に伴い単調増加する場合」だと｛ｄｒ（η）／ｄη｝が常に一定の符号になるので、｛ｄｒ（η）／ｄη｝Δηが著しく大きくなる可能性があるが、この発明におけるように、ｒ（η）が極大を持てば、｛ｄｒ（η）／ｄη｝の符号が、極大の前後で変化するので｛ｄｒ（η）／ｄη｝Δηは有効に小さくなる。従って、被走査面側のレンズのマルチビーム走査装置の光学系への「組付けの公差に対する許容度」が有効に緩和される。
【００１２】
上記請求項１〜５の任意の１に記載のマルチビーム走査装置において、複数の発光源を持つ光源としては、複数のレーザ発光部をモノリシックにアレイ配列した半導体レーザアレイを好適に用いうるほか、独立した半導体レーザからのビームをビーム合成手段で合成するようにした光源も用いることができる。
【００１３】
各光源からのビームは「別個の、または共通のカップリングレンズ」により以後の光学系にカップリングされる。カップリングレンズによりカップリングされた各ビームは「弱い集束性もしくは弱い発散性のビーム」となるようにしてもよいが、カップリングレンズによりそれぞれ「平行ビーム」とし、各平行ビームを別個のまたは共通の線像結像光学系により偏向反射面近傍に、主走査対応方向に長い線像として結像させるようにできる（請求項６）。このようにすると、カップリングレンズと線像結像光学系の間でビームが平行ビームとなるので、この部分で光学系配置の自由度が大きい。
また、請求項１〜６の任意の１に記載のマルチビーム走査装置において、光偏向器として回転多面鏡を用い、サグの影響を軽減させるために、走査結像レンズの各レンズに、主走査断面内でティルト角を与えることができる（請求項７）。なお、回転多面鏡として、同一形状の複数の回転多面鏡を共通の軸に設け、複数のビームの偏向を回転多面鏡ごとに振り分けるようにしてもよい。
【００１４】
【発明の実施の形態】
図１は、マルチビーム走査装置の実施の１形態を略示している。図１（ａ）に示すように、光源１から放射された複数ビーム（図の繁雑を避けるため１ビームのみを示している）は、カップリングレンズ２によりカップリングされ、シリンダレンズ３を透過し、ミラー４により反射され、光偏向器５により偏向され、走査結像レンズを構成するレンズ６，７により被走査面８上に光スポットとして集光し、被走査面８（実体的には通常は「光導電性の感光体」である）の複数走査線を走査する。
この実施の形態において、光源１は、図１（ｂ）に示すように「２つのレーザ発光部をモノリシックにアレイ配列した半導体レーザアレイ」であり、近接した２つのレーザ発光部からそれぞれレーザビームが独立に放射されるようになっている。放射された各ビームは、共通のカップリングレンズ２により夫れ夫れ「平行ビーム」とされ、アパーチュアＡＰにより「ビーム整形」されたのち、線像結像光学系であるシリンダレンズ３により副走査対応方向に集束され、ミラー４で反射されて光偏向器５である回転多面鏡の偏向反射面５Ａ近傍の位置に「主走査対応方向に長い線像」として結像する。光源１における２つの発光部は、副走査対応方向に配列しているので、上記２つの線像は、互いに副走査対応方向へ分離している。ミラー４は、半導体レーザ１から偏向反射面５に到る光学系のレイアウト次第で省略してもよく、シリンダレンズ３は「凹シリンダミラー」で代替してもよい。
光偏向器５は「回転多面鏡」であり、その回転軸５Ｂは偏向反射面５Ａと離れているから、この形態においては偏向反射面５Ａの回転に伴う偏向反射面５Ａと線像の結像位置のずれ、所謂「サグ」が発生する。
走査結像レンズを透過した２本の偏向ビームは被走査面８に向かって集光し、被走査面８上に副走査方向に分離して形成される光スポットにより、被走査面８の２本の走査線が等速的に走査される。
走査結像レンズを構成する２枚のレンズ６，７のうち、光偏向器５側のレンズ６は「光偏向器の側に凹面を向けた正メニスカスレンズ」で両面が共軸非球面である。
【００１５】
被走査面８側に配備されるレンズ７は、光偏向器５側のレンズ面が、主走査断面内において非円弧形状を有し、且つ、該レンズ面における副走査断面内の曲率中心を主走査対応方向に連ねた曲率中心線が、主走査断面内において上記非円弧形状とは異なる曲線となるように、上記副走査断面内における曲率半径が主走査対応方向に変化している。また、レンズ７の主走査断面内の屈折力は負である。
【００１６】
レンズ７の光偏向器側の面の形状を図２を参照して説明する。
後述するように、レンズ６，７には主走査断面内におけるティルト角が与えられるが、図２に即しての説明においては、ティルト角を０とした状態を想定して説明する。このとき、主走査対応方向および副走査対応方向は何れもレンズ７の光軸に直交する。
図２において、Ｙ軸を主走査対応方向に取る。Ｘ軸はレンズ７の光軸方向であり、Ｘ軸の正の方向（図で右の方向）は被走査面側である。ＸＹ面が「主走査断面」である。また「副走査断面」は、ＸＹ面に直交し、且つ、Ｘ軸に平行な平断面である。
図２（ａ）で、Ｘ（Ｙ）は、主走査断面内におけるレンズ７の当該レンズ面の形状であり「非円弧形状」である。また、ｒ（η）は、主走査対応方向（Ｙ方向）における位置座標：ηにおける「副走査断面内の曲率半径」を表している。曲率半径：ｒ（η）は位置座標：ηに応じて変化し、位置：ηにおける「副走査断面内の曲率中心」を主走査対応方向へ連ねた曲率中心線Ｌは「主走査断面内において非円弧形状：Ｘ（Ｙ）とは異なる曲線」である。
図２（ｂ）は、位置：η＝０と、任意の位置：ηとにおける副走査断面内における曲率半径の差の絶対値：｜ｒ（η）｜−｜ｒ（０）｜が主走査対応方向にどのように変化するかを示している。
レンズ７の、被走査面８側の面は主走査断面内において円弧形状であり、レンズ７は「主走査断面内における屈折力が負」である。
図１に戻ると、レンズ６，７には、サグの影響を有効に軽減するために、主走査断面内においてティルト角：α_６，α_７がそれぞれ与えられている。
【００１７】
従って、図１に示す実施の形態は、複数の発光源から放射される複数のビームのそれぞれを主走査対応方向に長い線像として、副走査対応方向に互いに分離して結像させ、各線像の結像位置近傍に偏向反射面５Ａを持つ光偏向器５により等角速度的に偏向させ、各偏向ビームを同一の走査結像レンズ６，７により被走査面８上に副走査方向に分離した光スポットとして集光せしめて被走査面８の複数走査線の等速的な光走査を行なうマルチビーム光走査装置において、走査結像レンズが２枚のレンズ６，７により構成され、光偏向器５側のレンズ６は、光偏向器側に凹面を向けた正メニスカスレンズで、両面が共軸非球面形状であり、被走査面側のレンズ７は、少なくとも１面が主走査断面内において非円弧形状（図２（ａ）のＸ（Ｙ））を有し、且つ、該レンズ面における副走査断面内の曲率中心を主走査対応方向に連ねた曲率中心線（図２（ａ）のＬ）が、主走査断面内において非円弧形状：Ｘ（Ｙ）とは異なる曲線となるように、副走査断面内における曲率半径を主走査対応方向に変化させたものである（請求項１）。
【００１８】
また、レンズ７の「主走査断面内において非円弧形状を有し、且つ、該レンズ面における副走査断面内の曲率中心を主走査対応方向に連ねた曲率中心線が、主走査断面内において上記非円弧形状とは異なる曲線となるように、副走査断面内における曲率半径を主走査対応方向に変化させたレンズ面」は、光偏向器５側のレンズ面であり（請求項２）、被走査面８側のレンズ面は「主走査断面内において円弧形状」であり（請求項３）、且つ、レンズ７は「主走査断面内における屈折力が負」である（請求項４）。
さらに、レンズ７における光偏向器５側の面における、副走査断面内における曲率半径の上記曲率半径の絶対値：｜ｒ（η）｜は、図２（ｂ）に示すように、主走査対応方向（Ｙ方向）において光軸（Ｘ軸）を離れるに従い、｜ｒ（０）｜から、極大値に向かって滑らか且つ単調に増加し、極大位置を超えたのち、光軸を離れるに従い滑らか且つ単調に減少するように定められている（請求項５）。
【００１９】
また、光源１からの各ビームは、カップリングレンズ２により平行ビームとされ、各平行ビームは線像結像光学系３により偏向反射面５Ａ近傍に、主走査対応方向に長い線像として結像される（請求項６）。そして、光偏向器５は「回転多面鏡」であり、サグの影響を軽減させるために、走査結像レンズの各レンズ６，７が、主走査断面内でティルト角：α_６，α_７を与えられている（請求項７）。
【００２０】
【実施例】
図１に示した実施の形態の実施例を示す。
光源１である半導体レーザアレイは発光波長：６７０ｎｍで発光する２つのレーザ発光部を有し、光偏向器５は偏向反射面数：６、偏向反射面の内接円半径：２５ｍｍの回転多面鏡である。光路折り曲げ用のミラー４の側から光偏向器５に入射する光束の入射方向と（ティルト角：α_６＝α_７≡０としたときの）走査結像光学系のレンズ６，７の光軸とが成す角は６０度である。
【００２１】
光源１から光偏向器５に至る光路上の光学系を「第１群」、光偏向器５から被走査面８に至る光路上の光学系を「第２群」とする。
光源１である半導体レーザアレイのカバーガラス、偏向反射面、各レンズのレンズ面の曲率半径（円弧形状でないものについては近軸曲率半径）を主走査対応方向に関してＲｍ、副走査対応法方向に関してＲｓとし、光軸上の間隔をＤ、材質の屈折率をＮとする。「長さの次元」を有する量は「ｍｍ」単位とする。
【００２２】

【００２３】
図１（ｃ）に示す如く、光源１における２つのレーザ発光部１１，１２は「副走査対応方向に１０μｍ離れ」ており、これら２つのレーザ発光部は共に、カップリングレンズ２の光軸の「副走査対応方向における片側」に位置し、一方のレーザ発光部（第１発光部１１）は上記光軸から５μｍの距離に位置し、他方のレーザ発光部（第２発光部１２）は上記光軸から１５μｍの距離に位置する。
Ｄ＝９１．４２は、シリンドリカルレンズ３の射出側面から光偏向器の偏向反射面（線像の結像位置）に至る距離である。
カップリングレンズ２の射出側面（上記面番号：４）は「共軸非球面」であり、カップリングされた光束は「実質的な平行光束」となる。
該共軸非球面は、前記（１）式において、近軸曲率半径：Ｒ（＝Ｒｍ＝Ｒｓ）、円錐定数：Ｋ、Ｙに関する４次、６次、８次、１０次の非球面係数：Ａ，Ｂ，Ｃ，Ｄが以下の値を持つ。
Ｒ＝−８．４１４，Ｋ＝−０．０２１，Ａ＝ １．２３Ｅ−４，
Ｂ＝ １．３６Ｅ−６，Ｃ＝ １．２４Ｅ−８，Ｄ＝ １．５４Ｅ−１０
なお「Ｅ−４」等は「べき乗」を示す。例えば上記「Ｅ−４」は「１０￣^４」を意味し、この値がその直前の数値に掛かる。
【００２４】
第２群において、「α」は前述の「ティルト角（時計回りを「正」とし、単位は「度」とする）」を表す。
【００２５】

Ｄ＝５２．７１は、偏向反射面からレンズ６の入射側面までの距離である。
【００２６】
レンズ６の両面（上記面番号１，２）は「共軸非球面」、レンズ７の射出側面（上記面番号：４）は「ノーマルトロイダル面」である。
レンズ６の入射側面：
前記（１）式において、近軸曲率半径：Ｒ（＝Ｒｍ＝Ｒｓ）、円錐定数：Ｋ、Ｙに関する４次、６次、８次、１０次の非球面係数：Ａ，Ｂ，Ｃ，Ｄは、以下の値を持つ。
Ｒ＝−３１２．６，Ｋ＝ ２．６６７，Ａ＝ １．７９Ｅ−７，
Ｂ＝−１．０８Ｅ−１２，Ｃ＝−３．１８Ｅ−１４，Ｄ＝ ３．７４Ｅ−１８
レンズ６の射出側面：
前記（１）式において、近軸曲率半径：Ｒ（＝Ｒｍ＝Ｒｓ）、円錐定数：Ｋ、Ｙに関する４次、６次、８次、１０次の非球面係数：Ａ，Ｂ，Ｃ，Ｄは、以下の値を持つ。
Ｒ＝−８２．９５，Ｋ＝ ０．０２，Ａ＝ ２．５０Ｅ−７，
Ｂ＝９．６１Ｅ−１２，Ｃ＝４．５４Ｅ−１５，Ｄ＝−３．０３Ｅ−１８ 。
【００２７】
レンズ７の入射側面（上記面番号３）は、主走査断面内において「非円弧形状」であり、上記ティルト角：α_７＝０の状態で、副走査断面内の曲率：Ｃｓ（Ｙ）は、Ｃｓ（Ｙ）＝｛１／Ｒｓ（０）｝＋Σｂ_ｊ・Ｙ＊＊ｊ（ｊ＝１，２，３，．．）（２）におけるＲｓ（０），ｂ_ｊを与えて特定される。主走査対応方向の位置：Ｙにおける副走査断面内の曲率半径は「１／Ｃｓ（Ｙ）」である。「Ｙ＊＊ｊ」はＹのｊ乗を表している。
【００２８】
上記非円弧形状は前記（１）式で表現され、近軸曲率半径：Ｒ（＝Ｒｍ）、円錐定数：Ｋ、Ｙに関する４次、６次、８次、１０次の非球面係数：Ａ，Ｂ，Ｃ，Ｄは、以下の値を持つ。
Ｒ＝−５００．００，Ｋ＝−７１．７３，Ａ＝ ４．３３Ｅ−８，
Ｂ＝−５．９７Ｅ−１３，Ｃ＝−１．２８Ｅ−１６，Ｄ＝５．７３Ｅ−２１
また、上記（２）式における、Ｒｓ（０），ｂ_ｊは以下の値を持つ。
Ｒｓ（０）（＝Ｒ）＝−４７．８５，ｂ_２＝ １．５９Ｅ−３，
ｂ_４＝−２．３２Ｅ−７，ｂ_６＝ １．６０Ｅ−１１，
ｂ_８＝−５．６１Ｅ−１６，ｂ_１０＝ ２．１８Ｅ−２０，
ｂ_１２＝−１．２５Ｅ−２４
Ｙの奇数次に関する係数は全て０であり、従って、レンズ７の入射側面に関する（２）式はＹ方向に関して光軸対称である。
【００２９】
上記の如く決定された「Ｃｓ（Ｙ）」に基づき、曲率半径の絶対値を求めて見ると、絶対値は、図２（ｂ）に示したように「主走査対応方向において光軸を離れるに従い、極大値に向かって滑らか且つ単調に増加し、極大位置を超えたのち、光軸を離れるに従い滑らか且つ単調に減少する」ように変化する。
レンズ７の射出側面は上述したように「ノーマルトロイダル面」であるから、以上により、走査結像レンズを含め光学系の配置が全て決定されたことになる。
【００３０】
実施例に関する像面湾曲の図と等速化特性を図３および図４に示す。
これら図３，図４に示す像面湾曲の図において、破線は「主走査対応方向の像面湾曲」、実線は「副走査方向の像面湾曲」を示し、等速化特性としてのｆθ特性を破線で、リニアリティを実線で示す。
図３は、カップリングレンズ２の光軸から、副走査対応方向に５μｍ離れた位置に位置する第１発光部１１から放射されたビームに関する図であり、図４は、カップリングレンズ２の光軸から、第１発光部側へ１５μｍ離れた位置に位置する第２発光部１２から放射されたビームに関する図である。
先ず、ｆθ特性とリニアリティにより示される等速化特性は各発光部からのビームに対して極めて良好に補正されており、従って、各ビームによる２つの（副走査方向に分離した）光スポット共、極めて良好な等速走査が実現される。
また、主・副走査方向の像面湾曲は、各発光部からのビームに対して、何れも絶対値で１ｍｍ以下であり極めて良好である。のみならず、主・副走査方向の像面湾曲は、各発光部からのビームに対して「実質的に同一」であるので、２つの走査線を同時に走査する光スポットのスポット径は互いに実質的に同一である。なお、図３，図４において、縦座標における「Ｙ」は主走査対応方向の座標ではなく「光スポットの像高」を表している。像面湾曲や等速化特性は、光スポットの像高のプラス側とマイナス側とで非対称的であるが、これは前述の「サグ」の影響によるものである。前記ティルト角：α_６，α_７を共に０とすると、像面湾曲や等速化特性の非対称性がもっと顕著に現われて、光スポットの像高のプラス側あるいはマイナス側で像面湾曲や等速化特性が劣化するが、上記のようにティルト角を与えたことにより、像面湾曲・等速化特性とも、光スポットの像高のプラス側およびマイナス側で良好に補正されているのである。
なお、上記サグの影響を除去するのに、上記ティルトに代えて、あるいはティルトとともに、レンズ６および／またはレンズ７の光軸を主走査対応方向へ平行移動によりずらす「シフト」を与えてもよい。
【００３１】
また、上には発光部の数が２である場合の実施例を挙げたが、発光部の数を３以上とすることができることは言うまでもない。
【００３２】
【発明の効果】
以上に説明したように、この発明によれば、新規なマルチビーム走査装置を実現できる。この発明のマルチビーム走査装置は、走査結像レンズを構成する２枚のレンズのうち、光偏向器側のレンズはメニスカスレンズであるので、中央と周辺部、特に主走査対応方向における中央部と周辺部との肉厚差を有効に軽減する「均肉化」が可能であり、これをプラスチック等の樹脂で成形加工により作製する際の「ヒケやウネリ」といった変形を有効に防止できるので、製造が容易で歩留まりが良く、マルチビーム走査装置の低コスト化に資するところが大きい。また、上記光偏向器側のレンズは、凹面を光偏向器側に向けて配備されるので、主走査対応方向の中央部と周辺部で入射側レンズ面への偏向の起点からの距離の変化が小さく、従って「副走査対応方向の横倍率の差」を少なくでき、光学系への組付け誤差の許容度が大きく、光学系の組立てを容易にできる。
【００３３】
また、走査結像レンズは、主走査断面内で、少なくとも３面が「非円弧形状」であるから、この非円弧形状を最適化することにより、複数発光部からの各ビームに対する、主走査方向の像面湾曲や等速化特性を良好に補正することが可能となり、副走査断面内での形状において２面が非円弧形状であり、被走査面側のレンズの少なくとも１面において副走査断面内の曲率半径を主走査対応方向に変化させるので、主走査方向の像面湾曲や等速化特性用に最適化された上記非円弧形状に応じて、上記曲率半径の変化を最適化することにより、各ビームに対して副走査方向の像面湾曲を有効に補正することができる。
【００３４】
請求項４記載の発明では、被走査面側のレンズは主走査断面内における屈折力が負で、光偏向器側のレンズは正メニスカスレンズであるので、主走査断面内における走査結像レンズの屈折力の組合せが「正・負」の組合せとなり、走査結像レンズを構成する２枚のレンズを共にプラスチックレンズとして構成した場合、温・湿度変化の影響が各レンズで互いに打ち消すように作用し、走査結像レンズとしては温・湿度変化の影響を受けにくく、マルチビーム走査が環境変化に影響されにくいマルチビーム走査装置を実現できる。
【００３５】
請求項５記載のマルチビーム走査装置では、被走査面側のレンズの「主走査断面内において非円弧形状を有し、且つ、該レンズ面における副走査断面内の曲率中心を主走査対応方向に連ねた曲率中心線が主走査断面内において上記非円弧形状と異なる曲線となるように、副走査断面内における曲率半径を主走査対応方向に変化させたレンズ面」の上記曲率半径の絶対値が「主走査対応方向において光軸を離れるに従い極大値に向かって滑らか且つ単調に増加し、極大位置を超えたのち、光軸を離れるに従い滑らか且つ単調に減少する」ように定められるので、被走査面側のレンズのマルチビーム走査装置への「組付けの公差に対する許容度」が有効に緩和され、マルチビーム走査装置の光学系への組込が容易である。
【００３６】
また、請求項７記載のマルチビーム走査装置は、光偏向器として回転多面鏡を用いる場合のサグの影響を有効に軽減させることができる。
【図面の簡単な説明】
【図１】この発明のマルチビーム走査装置の実施の１形態を説明するための図である。
【図２】マルチビーム走査装置に用いる走査結像レンズの、被走査面側レンズのレンズ面形状を説明するための図である。
【図３】実施例における第１発光部からのビームに関する像面湾曲および等速化特性の図である。
【図４】実施例における第２発光部からのビームに関する像面湾曲および等速化特性の図である。
【符号の説明】
１ 光源（半導体レーザアレイ）
２ カップリングレンズ
３ シリンドリカルレンズ
５ 光偏向器
６，７ 走査結像レンズ
８被走査面[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a multi-beam scanning device.
[0002]
[Prior art]
Each of the plurality of beams emitted from the plurality of light sources is formed as a long line image in the main scanning corresponding direction, separated and formed in the sub scanning corresponding direction, and has a deflecting reflection surface near the image forming position of each line image. The light is deflected at an equal angular velocity by an optical deflector, and each deflected beam is condensed on the surface to be scanned by the same scanning image forming lens as a light spot separated in the sub-scanning direction to form a plurality of scanning lines on the surface to be scanned. A multi-beam scanning device that performs rapid optical scanning is intended to be realized in connection with an "image forming apparatus" such as an optical printer or a digital copying machine.
The “main scanning corresponding direction” refers to a direction corresponding to the main scanning direction on an optical path from the light source to the surface to be scanned, and a direction corresponding to the sub scanning direction on the optical path is referred to as a “sub scanning corresponding direction”. To tell.
The scanning image forming lens has a “conjugate function” that sets the image forming position of each line image and the surface to be scanned to a “geometric optically conjugate relationship” with respect to the sub-scanning corresponding direction, and makes optical scanning uniform. It has a "constant speed function". The conjugate function is a function for correcting “surface tilt” of the deflecting reflection surface in the optical deflector.
[0003]
In order to realize good multi-beam scanning, in addition to the above-mentioned conjugation function and uniform velocity function of the scanning imaging lens, the curvature of field in the main and sub-scanning directions must be well corrected. is necessary. If the curvature of field in the main and sub-scanning directions is not sufficiently corrected, the diameter of the light spot varies with the image height of the light spot, so that the resolution of an image to be written is remarkably reduced, thereby deteriorating the image quality.
In the multi-beam scanning, it is necessary that the curvature of field in the sub-scanning direction is properly corrected for each of the plurality of light spots condensed on the surface to be scanned. Otherwise, the diameters of the light spots in the sub-scanning direction of the plurality of light spots are not uniform, and the resolution of the image to be written is "variable" for each light spot. In order to satisfactorily correct the curvature of field in the sub-scanning direction, the curvature radius in one or more lens surfaces of the scanning imaging lens in a sub-scanning cross section (a flat cross section near the lens surface and orthogonal to the main scanning direction). Is known in accordance with the position in the main scanning corresponding direction (for example, JP-A-6-230308), but the radius of curvature in the sub-scanning cross section is “in the main scanning corresponding direction”. As the distance from the optical axis increases, the radius of curvature increases monotonously. Therefore, the deflection radius is greatly different between the deflection angle of 0 and the maximum deflection angle (corresponding to the end of the effective main scanning area). If there is an "assembly error", the curvature of field in the sub-scanning direction is significantly deteriorated, so that there is a problem that the tolerance of assembly is strict and the workability of assembling the optical system is poor.
[0004]
Further, the scanning imaging lens can be manufactured by “plastic molding”, but in plastic molding, “sink” or “undulation” may occur, and it may be difficult to obtain a lens having a desired shape. Further, the optical performance of a lens formed of plastic is liable to change under the influence of changes in temperature and humidity. In addition, since a rotating polygonal mirror, which is a general optical deflector, does not have a rotating axis of the deflecting / reflecting surface within the deflecting / reflecting surface, the positional relationship between each line image formed in the vicinity of the deflecting / reflecting surface and the deflecting / reflective surface. However, there is a problem of so-called "sag" which varies with the rotation of the rotating polygon mirror.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to realize good optical scanning by each light beam in a multi-beam scanning device. Further, it facilitates the assembly of the optical system in the multi-beam scanning device, and when the scanning imaging lens is formed of plastic, the desired lens can be easily obtained with high yield, thereby enabling the cost reduction of the scanning device. Another object is to make scanning less susceptible to temperature, humidity, and the like, and to effectively remove the effect of sag.
[0006]
[Means for Solving the Problems]
According to the multi-beam scanning apparatus of the present invention, “each of a plurality of beams emitted from a plurality of light-emitting sources is formed as a long line image in the main scanning direction, separated from each other in the sub-scanning direction, and each line image is formed. The light is deflected at an equal angular velocity by an optical deflector having a deflecting reflection surface near the image position, and each deflected beam is condensed as a light spot separated in the sub-scanning direction on the surface to be scanned by the same scanning image forming lens. A multi-beam scanning apparatus that performs constant-speed optical scanning of a plurality of scanning lines on a scanning surface, and has the following features. That is, a scanning imaging lens for condensing each deflection beam as a light spot on the surface to be scanned is constituted by two lenses.
Of the two lenses constituting the scanning imaging lens, the lens on the optical deflector side is a “positive meniscus lens having a concave surface facing the optical deflector side, and both surfaces have a coaxial aspherical shape. The lens on the side "has at least one surface having a non-arcuate shape in the main scanning section, and a curvature center line connecting the centers of curvature in the sub-scanning section on the lens surface in the main scanning corresponding direction is the main scanning direction. The radius of curvature in the sub-scanning cross section is changed in the main scanning corresponding direction so that a curve different from the non-circular shape is obtained in the cross section. "
The “main scanning section” refers to a plane section including the lens optical axis and the direction corresponding to the main scanning in the vicinity of the lens surface of each lens constituting the scanning imaging lens.
The “sub-scan section” refers to a plane section near the lens surface and orthogonal to the main scanning direction.
As will be described later, a tilt angle can be given to the two lenses constituting the scanning image forming lens in the main scanning section in order to reduce the effect of sag. The scanning section is defined when the tilt angle is set to 0 in each lens, that is, in a state before the tilt angle is given.
"Non-circular shape" means that when taking coordinates: X in the direction of the lens optical axis and coordinates: Y in the direction orthogonal to the optical axis, the paraxial radius of curvature is R, the conic constant is K, and the higher order coefficients are A and B. , C, D,. . . As well-known,

R, K, A, B, C, D,. . And the curve shape specified by
[0007]
Since the lens on the optical deflector side is a "meniscus lens", it is possible to "uniform" to effectively reduce the thickness difference between the center and the periphery, especially the center and the periphery in the main scanning direction. In this case, deformation such as “sink” or “undulation” can be effectively prevented when it is manufactured by molding with a resin such as plastic. In addition, since the lens on the light deflector side is provided with the concave surface facing the light deflector side, "the distance from the starting point of deflection to the entrance side lens surface" in the central part and the peripheral part in the main scanning corresponding direction. Is small, so that the "difference in the lateral magnification in the sub-scanning corresponding direction" between the central portion and the peripheral portion can be reduced.
[0008]
As described above, in the scanning imaging lens according to the first aspect, since the shape in the main scanning cross section has at least three surfaces being “non-circular shapes”, by optimizing these non-circular shapes, It is possible to satisfactorily correct the curvature of field in the main scanning direction and the uniformity characteristic for each beam.
Further, in a shape in a plane parallel to the optical axis and orthogonal to the main scanning section (in the case where the tilt angle is given, a shape in a state where the tilt angle is set to 0), two surfaces (the lens of the optical deflector side) Both surfaces have a non-circular shape, and at least one surface of the lens on the scanning surface side changes the radius of curvature in the sub-scanning cross section in the main scanning corresponding direction. By optimizing the change in the radius of curvature in accordance with the non-arc shape optimized for use, the curvature of field in the sub-scanning direction can be effectively corrected.
[0009]
In the scanning imaging lens, the lens on the surface to be scanned has a non-arcuate shape in the main scanning section and a center line of curvature in the main scanning corresponding direction in the sub scanning section on the lens surface. However, the lens surface in which the radius of curvature in the sub-scanning cross section is changed in the main scanning corresponding direction so as to have a curve different from the non-arc shape in the main scanning cross section is referred to as the lens on the optical deflector side. Plane "(claim 2). In this case, the lens surface on the scanned surface side of the lens on the scanned surface side can have “an arc shape in the main scanning section”.
[0010]
Further, the lens on the scanning surface side of the scanning image forming lens can have a negative refractive power in the main scanning section. As described above, since the lens on the optical deflector side in the scanning imaging lens is a “positive meniscus lens”, if the “refractive power in the main scanning section” of the lens on the scanned surface side is negative, Since the combination of the refractive powers of the scanning imaging lens in the above is a combination of "positive / negative", if the two lenses constituting the scanning imaging lens are configured as "plastic lenses", the influence of temperature and humidity changes Act so as to cancel each other out with each lens, so that the scanning imaging lens is less susceptible to changes in temperature and humidity.
[0011]
Further, the lens on the scanning surface side has a non-arcuate shape in the main scanning section, and a curvature center line connecting the center of curvature in the sub-scanning section on the lens surface in the main scanning corresponding direction is a main scanning direction. The absolute value of the radius of curvature of the lens surface whose curvature radius in the sub-scanning cross section is changed in the main scanning corresponding direction so as to be a curve different from the non-arc shape in the cross section is `` in the main scanning corresponding direction ''. It increases smoothly and monotonically toward the maximum value as the optical axis is separated, and decreases smoothly and monotonically as the optical axis is separated after exceeding the maximum position.
When the coordinates in the main scanning direction are η and the radius of curvature is r (η), if there is a position error in the main scanning direction at Δ (η): Δη, the error of the radius of curvature at the position: η is ｛dr (Η) / dη｝ Δη, and if r (η) is “monotonically increasing with the increase of η”, {dr (η) / dη} always has a constant sign, so that {dr (η) Although it is possible that / dη 著 し く Δη may be significantly large, as in the present invention, if r (η) has a maximum, the sign of {dr (η) / dη} changes before and after the maximum, so ｛ dr (η) / dη｝ Δη is effectively reduced. Therefore, the "tolerance to the assembly tolerance" of the lens on the scanning surface side to the optical system of the multi-beam scanning device is effectively relaxed.
[0012]
In the multi-beam scanning apparatus according to any one of claims 1 to 5, as a light source having a plurality of light emitting sources, a semiconductor laser array in which a plurality of laser light emitting units are monolithically arranged can be preferably used. A light source in which beams from independent semiconductor lasers are combined by beam combining means can also be used.
[0013]
The beam from each light source is coupled to subsequent optics by a "separate or common coupling lens". Each beam coupled by the coupling lens may be a “weakly converging or weakly diverging beam”, but each is a “parallel beam” by the coupling lens, and each parallel beam is separated or shared. The linear image forming optical system can form an image as a long linear image in the main scanning direction in the vicinity of the deflecting reflection surface (claim 6). In this case, since the beam becomes a parallel beam between the coupling lens and the line image forming optical system, the degree of freedom in arranging the optical system in this portion is large.
Further, in the multi-beam scanning apparatus according to any one of claims 1 to 6, a rotating polygon mirror is used as an optical deflector, and main scanning is performed on each lens of the scanning image forming lens in order to reduce the influence of sag. A tilt angle can be provided in the cross section (claim 7). As the rotating polygon mirror, a plurality of rotating polygon mirrors having the same shape may be provided on a common axis, and the deflection of a plurality of beams may be distributed to each rotating polygon mirror.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 schematically shows one embodiment of a multi-beam scanning device. As shown in FIG. 1A, a plurality of beams (only one beam is shown for the sake of simplicity) emitted from the light source 1 are coupled by the coupling lens 2 and transmitted through the cylinder lens 3. The light is reflected by the mirror 4, deflected by the optical deflector 5, and condensed as a light spot on the surface 8 to be scanned by the lenses 6 and 7 constituting the scanning image forming lens. Is a "photoconductive photoreceptor").
In this embodiment, as shown in FIG. 1B, the light source 1 is a “semiconductor laser array in which two laser light emitting units are monolithically arranged”, and laser beams are respectively emitted from two adjacent laser light emitting units. It is radiated independently. Each of the emitted beams is converted into a “parallel beam” by a common coupling lens 2, “beam-shaped” by an aperture AP, and then sub-scanned by a cylinder lens 3 which is a line image forming optical system. The light is converged in the corresponding direction, is reflected by the mirror 4, and forms an image as a "long line image in the main scanning corresponding direction" at a position near the deflecting reflection surface 5A of the rotary polygon mirror which is the optical deflector 5. Since the two light emitting units of the light source 1 are arranged in the sub-scanning corresponding direction, the two line images are separated from each other in the sub-scanning corresponding direction. The mirror 4 may be omitted depending on the layout of the optical system from the semiconductor laser 1 to the deflecting / reflecting surface 5, and the cylinder lens 3 may be replaced by a "concave cylinder mirror".
Since the light deflector 5 is a "rotating polygon mirror" and its rotation axis 5B is separated from the deflecting / reflecting surface 5A, in this embodiment, the line image is formed on the deflecting / reflecting surface 5A due to the rotation of the deflecting / reflecting surface 5A. Displacement, so-called "sag" occurs.
The two deflecting beams transmitted through the scanning imaging lens are condensed toward the surface 8 to be scanned, and are separated on the surface 8 to be scanned in the sub-scanning direction. The scanning lines are scanned at a constant speed.
Of the two lenses 6 and 7 constituting the scanning image forming lens, the lens 6 on the optical deflector 5 side is a “positive meniscus lens having a concave surface facing the optical deflector” and both surfaces are coaxial aspherical surfaces. .
[0015]
The lens 7 provided on the surface 8 to be scanned has a lens surface on the side of the optical deflector 5 having a non-circular shape in the main scanning section, and has a center of curvature in the sub-scanning section on the lens surface. The radius of curvature in the sub-scanning section changes in the main scanning corresponding direction such that the center line of curvature connected to the scanning corresponding direction becomes a curve different from the non-circular shape in the main scanning section. The refractive power in the main scanning section of the lens 7 is negative.
[0016]
The shape of the surface of the lens 7 on the optical deflector side will be described with reference to FIG.
As will be described later, a tilt angle in the main scanning section is given to the lenses 6 and 7. In the description with reference to FIG. 2, it is assumed that the tilt angle is 0. At this time, both the main scanning corresponding direction and the sub scanning corresponding direction are orthogonal to the optical axis of the lens 7.
In FIG. 2, the Y axis is taken in the main scanning corresponding direction. The X axis is the direction of the optical axis of the lens 7, and the positive direction (the right direction in the figure) of the X axis is the surface to be scanned. The XY plane is the “main scanning section”. The “sub-scan section” is a plane section orthogonal to the XY plane and parallel to the X axis.
In FIG. 2A, X (Y) is the shape of the lens surface of the lens 7 in the main scanning section, which is a “non-arc shape”. Also, r (η) represents “the radius of curvature in the sub-scan section” at the position coordinate: η in the main scanning corresponding direction (Y direction). The radius of curvature: r (η) changes according to the position coordinates: η, and the curvature center line L connecting the “curvature center in the sub-scan section” in the main scanning corresponding direction at the position: η is “in the main scan section”. Non-arc shape: curve different from X (Y) ".
FIG. 2B shows the absolute value of the difference between the radii of curvature in the sub-scanning section at the position: η = 0 and the arbitrary position: η: | r (η) | − | r (0) | It shows how it changes in the corresponding direction.
The surface of the lens 7 on the surface 8 to be scanned has an arc shape in the main scanning section, and the lens 7 has “negative refractive power in the main scanning section”.
Returning to FIG. 1, in order to effectively reduce the effect of sag, the lenses 6 and 7 have a tilt angle α in the main scanning section. ₆ , Α ₇ Are given respectively.
[0017]
Therefore, in the embodiment shown in FIG. 1, each of the plurality of beams emitted from the plurality of light-emitting sources is formed as a long line image in the main scanning corresponding direction and separated from each other in the sub-scanning corresponding direction. Are deflected at an equal angular velocity by an optical deflector 5 having a deflecting / reflecting surface 5A in the vicinity of the image forming position, and each deflected beam is separated on the surface 8 to be scanned in the sub-scanning direction by the same scanning image forming lenses 6 and 7. In a multi-beam optical scanning apparatus that performs light scanning at a constant speed on a plurality of scanning lines on a surface 8 to be scanned by condensing as a light spot, a scanning image forming lens includes two lenses 6 and 7, and an optical deflector. The lens 6 on the 5 side is a positive meniscus lens having a concave surface facing the optical deflector side, both surfaces of which are coaxial aspherical. The lens 7 on the surface to be scanned has at least one surface which is non-spherical in the main scanning section. The arc shape (X (Y) in FIG. 2 (a)) In addition, a curvature center line (L in FIG. 2A) connecting the centers of curvature in the sub-scanning cross section on the lens surface in the main scanning corresponding direction has a non-arc shape in the main scanning cross section: X (Y) The curvature radius in the sub-scanning cross section is changed in the main scanning corresponding direction so as to have a different curve from the above (Claim 1).
[0018]
Further, the lens 7 has a non-arcuate shape in the main scanning section, and a curvature center line connecting the center of curvature in the sub-scanning section on the lens surface in the main scanning corresponding direction is the same as that in the main scanning section. The “lens surface in which the radius of curvature in the sub-scan section is changed in the main scanning direction so as to have a curve different from the non-arc shape” is the lens surface on the optical deflector 5 side (Claim 2). The lens surface on the scanning surface 8 side is “arc-shaped in the main scanning section” (claim 3), and the lens 7 is “negative in refractive power in the main scanning section” (claim 4).
Further, the absolute value of the radius of curvature of the radius of curvature in the sub-scanning section on the surface of the lens 7 on the side of the optical deflector 5 | r (η) | is, as shown in FIG. In the direction (Y direction), the distance gradually increases monotonously from | r (0) | toward the local maximum as the optical axis (X-axis) is moved away from the optical axis (X direction). It is determined to decrease monotonically (claim 5).
[0019]
Each beam from the light source 1 is converted into a parallel beam by the coupling lens 2, and each parallel beam is imaged by the line image imaging optical system 3 near the deflecting reflection surface 5 </ b> A as a long line image in the main scanning corresponding direction. (Claim 6). The optical deflector 5 is a “rotating polygon mirror”, and in order to reduce the influence of sag, each of the lenses 6 and 7 of the scanning image forming lens has a tilt angle α in the main scanning section. ₆ , Α ₇ (Claim 7).
[0020]
【Example】
2 shows an example of the embodiment shown in FIG.
The semiconductor laser array, which is the light source 1, has two laser light emitting units that emit light at an emission wavelength of 670 nm, and the optical deflector 5 has a rotating polygon mirror with the number of deflecting and reflecting surfaces: 6 and the inscribed circle radius of the deflecting and reflecting surface: 25 mm. It is. The incident direction of the light beam incident on the optical deflector 5 from the side of the optical path bending mirror 4 (tilt angle: α ₆ = Α ₇ The angle formed by the optical axes of the lenses 6 and 7 of the scanning imaging optical system (when ≡0) is 60 degrees.
[0021]
The optical system on the optical path from the light source 1 to the optical deflector 5 is referred to as a “first group”, and the optical system on the optical path from the optical deflector 5 to the surface 8 to be scanned is referred to as a “second group”.
The radius of curvature of the cover glass of the semiconductor laser array which is the light source 1, the deflecting / reflecting surface, and the lens surface of each lens (the paraxial radius of curvature for those not having an arc shape) is Rm for the main scanning corresponding direction and Rs for the sub-scanning corresponding direction. Where D is the distance on the optical axis and N is the refractive index of the material. An amount having a "length dimension" is in "mm" units.
[0022]

[0023]
As shown in FIG. 1C, the two laser light emitting units 11 and 12 in the light source 1 are “10 μm apart in the sub-scanning corresponding direction”, and both of these two laser light emitting units are in the optical axis of the coupling lens 2. One laser light emitting portion (first light emitting portion 11) is located at a distance of 5 μm from the optical axis, and the other laser light emitting portion (second light emitting portion 12) is located on “one side in the sub-scanning corresponding direction”. It is located at a distance of 15 μm from the optical axis.
D = 91.42 is the distance from the exit side surface of the cylindrical lens 3 to the deflecting / reflecting surface of the optical deflector (the position where a line image is formed).
The exit side surface (the surface number: 4) of the coupling lens 2 is a “coaxial aspherical surface”, and the coupled light beam is a “substantial parallel light beam”.
In the formula (1), the coaxial aspheric surface has a paraxial radius of curvature: R (= Rm = Rs) and a conical constant: K, Y, fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients: A, B, C, and D have the following values.
R = -8.414, K = -0.021, A = 1.23E-4,
B = 1.36E-6, C = 1.24E-8, D = 1.54E-10
Note that “E-4” and the like indicate “power”. For example, the above “E-4” becomes “10「 ⁴ , And this value is multiplied by the value immediately before it.
[0024]
In the second group, “α” represents the aforementioned “tilt angle (clockwise is“ positive ”and the unit is“ degree ”)”.
[0025]

D = 52.71 is the distance from the deflecting reflection surface to the incident side surface of the lens 6.
[0026]
Both surfaces (the surface numbers 1 and 2) of the lens 6 are “coaxial aspherical surfaces”, and the exit side surface (the surface number: 4) of the lens 7 is a “normal toroidal surface”.
Incident side of lens 6:
In the above equation (1), paraxial radius of curvature: R (= Rm = Rs), conical constant: fourth, sixth, eighth, and tenth aspherical coefficients relating to K and Y: A, B, C, D Has the following values:
R = -312.6, K = 2.667, A = 1.79E-7,
B = -1.08E-12, C = -3.18E-14, D = 3.74E-18
Exit side of lens 6:
In the above equation (1), paraxial radius of curvature: R (= Rm = Rs), conical constant: fourth, sixth, eighth, and tenth aspherical coefficients relating to K and Y: A, B, C, D Has the following values:
R = -82.95, K = 0.02, A = 2.50E-7,
B = 9.61E-12, C = 4.54E-15, D = -3.03E-18.
[0027]
The incident side surface (surface number 3) of the lens 7 has a “non-arc shape” in the main scanning section, and the tilt angle: α ₇ = 0, the curvature in the sub-scanning section: Cs (Y) is Cs (Y) = {1 / Rs (0)} + {b _j Rs (0), b in Y ** j (j = 1,2,3, ...) (2) _j Given. The radius of curvature in the sub-scan section at the position in the main scanning corresponding direction: Y is “1 / Cs (Y)”. “Y ** j” represents Y raised to the j-th power.
[0028]
The non-circular arc shape is expressed by the above equation (1), and paraxial radius of curvature: R (= Rm), conical constant: K, Y, fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients: A, B, C, and D have the following values.
R = -500.00, K = -71.73, A = 4.33E-8,
B = -5.97E-13, C = -1.28E-16, D = 5.73E-21
Also, Rs (0), b in the above equation (2) _j Has the following values:
Rs (0) (= R) =-47.85, b ₂ = 1.59E-3,
b ₄ = −2.32E-7, b ₆ = 1.60E-11,
b ₈ = -5.61E-16, b ₁₀ = 2.18E-20,
b ₁₂ = -1.25E-24
The coefficients relating to the odd-order Y are all 0, and therefore the expression (2) relating to the incident side surface of the lens 7 is symmetric with respect to the Y direction.
[0029]
When the absolute value of the radius of curvature is obtained based on “Cs (Y)” determined as described above, the absolute value is “away from the optical axis in the main scanning corresponding direction” as shown in FIG. 2B. , Increase smoothly and monotonically toward the maximum value, and after exceeding the maximum position, decrease smoothly and monotonously as the optical axis is separated.
Since the exit side surface of the lens 7 is the “normal toroidal surface” as described above, the arrangement of the optical system including the scanning imaging lens has been determined as described above.
[0030]
FIGS. 3 and 4 show a diagram of field curvature and an equalizing characteristic according to the embodiment.
In FIGS. 3 and 4, the broken lines indicate "field curvature in the main scanning direction" and the solid lines indicate "field curvature in the sub-scanning direction". Is indicated by a broken line, and the linearity is indicated by a solid line.
FIG. 3 is a diagram relating to a beam emitted from the first light emitting unit 11 located at a position 5 μm away from the optical axis of the coupling lens 2 in the sub-scanning corresponding direction, and FIG. FIG. 7 is a diagram relating to a beam emitted from a second light emitting unit 12 located at a position 15 μm away from the axis toward the first light emitting unit.
First, the velocity uniformity characteristic indicated by the fθ characteristic and the linearity is corrected very well with respect to the beam from each light emitting unit. Therefore, two light spots (separated in the sub-scanning direction) by each beam are Very good constant speed scanning is realized.
Further, the curvature of field in the main and sub-scanning directions is 1 mm or less in absolute value with respect to the beam from each light emitting unit, which is extremely good. In addition, since the curvature of field in the main and sub scanning directions is “substantially the same” with respect to the beam from each light emitting unit, the spot diameters of the light spots that scan two scanning lines simultaneously are substantially the same. Are identical. In FIGS. 3 and 4, “Y” on the ordinate indicates “image height of light spot” instead of coordinates in the main scanning corresponding direction. The curvature of field and the uniformity characteristic are asymmetric on the plus side and the minus side of the image height of the light spot, but this is due to the above-mentioned "sag". The tilt angle: α ₆ , Α ₇ When both are set to 0, the asymmetry of the curvature of field and the equalization characteristic appears more remarkably, and the curvature of field and the equalization characteristic deteriorate on the plus side or the minus side of the image height of the light spot. By giving the tilt angle as described above, both the curvature of field and the constant speed characteristics are favorably corrected on the plus side and the minus side of the image height of the light spot.
In order to remove the influence of the sag, a “shift” may be provided in place of or in addition to the tilt, in which the optical axis of the lens 6 and / or the lens 7 is shifted by parallel movement in the main scanning corresponding direction. .
[0031]
Although the embodiment in which the number of light emitting units is two is described above, it is needless to say that the number of light emitting units can be three or more.
[0032]
【The invention's effect】
As described above, according to the present invention, a novel multi-beam scanning device can be realized. In the multi-beam scanning device of the present invention, since the lens on the optical deflector side is a meniscus lens among the two lenses constituting the scanning image forming lens, the center and the peripheral portion, particularly the central portion in the main scanning corresponding direction, It is possible to effectively reduce the difference in wall thickness with the peripheral part, and it is possible to effectively prevent deformation such as sink marks and undulations when manufacturing this by molding with a resin such as plastic. It is easy to manufacture, has good yield, and greatly contributes to cost reduction of the multi-beam scanning device. In addition, since the lens on the optical deflector side is provided with the concave surface facing the optical deflector side, a change in the distance from the starting point of deflection to the incident side lens surface in the central portion and the peripheral portion in the main scanning corresponding direction. Therefore, the "difference in the lateral magnification in the sub-scanning direction" can be reduced, the tolerance of the assembly error to the optical system is large, and the assembly of the optical system can be facilitated.
[0033]
In addition, since at least three surfaces of the scanning imaging lens have a “non-arc shape” in the main-scan section, by optimizing the non-arc shape, the main scanning direction with respect to each beam from the plurality of light-emitting units is reduced. Field curvature and constant velocity characteristics of the lens can be satisfactorily corrected. In the shape in the sub-scanning section, two surfaces are non-circular, and at least one surface of the lens on the surface to be scanned has a sub-scanning section. Since the radius of curvature in the main scanning direction is changed in the main scanning direction, the change in the radius of curvature is optimized in accordance with the non-circular shape optimized for the curvature of field in the main scanning direction and the constant velocity characteristic. Thus, the curvature of field in the sub-scanning direction can be effectively corrected for each beam.
[0034]
According to the fourth aspect of the present invention, the lens on the scanning surface has a negative refractive power in the main scanning section and the lens on the optical deflector is a positive meniscus lens. If the combination of refractive power is a combination of "positive / negative" and the two lenses constituting the scanning image forming lens are both plastic lenses, the effects of temperature and humidity changes act to cancel each other out with each lens. As a scanning imaging lens, it is possible to realize a multi-beam scanning device which is hardly affected by changes in temperature and humidity, and in which multi-beam scanning is hardly affected by environmental changes.
[0035]
In the multi-beam scanning apparatus according to the fifth aspect, the lens on the surface to be scanned has a non-arcuate shape in the main scanning section, and the center of curvature in the sub-scanning section on the lens surface in the main scanning corresponding direction. The absolute value of the radius of curvature of the lens surface whose curvature radius in the sub-scanning cross section is changed in the main scanning corresponding direction so that the continuous curvature center line becomes a curve different from the non-arc shape in the main scanning cross section is Scanning is defined as "smoothly and monotonically increases toward the local maximum value as the optical axis is separated in the main scanning corresponding direction, and decreases smoothly and monotonically as the optical axis is separated after exceeding the local maximum position". The “tolerance to tolerance of assembly” of the lens on the surface side to the multi-beam scanning device is effectively relaxed, and the multi-beam scanning device can be easily incorporated into the optical system.
[0036]
In the multi-beam scanning device according to the seventh aspect, the effect of sag when a rotary polygon mirror is used as an optical deflector can be effectively reduced.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining an embodiment of a multi-beam scanning device according to the present invention.
FIG. 2 is a view for explaining a lens surface shape of a scanning surface side lens of a scanning image forming lens used in a multi-beam scanning device.
FIG. 3 is a diagram illustrating a field curvature and a constant velocity characteristic of a beam from a first light emitting unit in the example.
FIG. 4 is a diagram illustrating a field curvature and a constant velocity characteristic of a beam from a second light emitting unit in the example.
[Explanation of symbols]
1 light source (semiconductor laser array)
2 Coupling lens
3 cylindrical lens
5 Optical deflector
6,7 scanning imaging lens
8 scanned surface

Claims (7)

Each of the plurality of beams emitted from the plurality of light-emitting sources is formed as a long line image in the main scanning corresponding direction, separated from each other in the sub-scanning corresponding direction to form an image, and a deflecting reflective surface is formed near the image forming position of each line image. It is deflected at an equal angular velocity by an optical deflector having the same, and each of the deflected beams is condensed as a light spot separated in the sub-scanning direction on the surface to be scanned by the same scanning image forming lens. In a multi-beam optical scanning device that performs uniform optical scanning,
The scanning imaging lens is
It is composed of two lenses,
The lens on the optical deflector side is a positive meniscus lens with the concave surface facing the optical deflector side, both surfaces are coaxial aspherical,
At least one surface of the lens on the surface to be scanned has a non-arcuate shape in the main scanning section, and a curvature center line connecting the center of curvature in the sub-scanning section on the lens surface in the main scanning corresponding direction. A multi-beam scanning apparatus characterized in that a radius of curvature in the sub-scanning cross section is changed in a main scanning corresponding direction so as to have a curve different from the non-arc shape in the main scanning cross section. The multi-beam scanning device according to claim 1,
In the scanning image forming lens, a center line of curvature of the lens on the scanning surface side having a non-circular shape in the main scanning section and connecting the center of curvature in the sub scanning section on the lens surface in the main scanning corresponding direction. However, the lens surface whose curvature radius in the sub-scanning cross section is changed in the main scanning corresponding direction so as to have a curve different from the non-arc shape in the main scanning cross section is the lens surface on the optical deflector side. A multi-beam scanning device, characterized in that: The multi-beam scanning device according to claim 2,
A scanning imaging lens, wherein a lens surface on a scanning surface side of the scanning imaging lens has a circular arc shape in a main scanning section. The multi-beam scanning device according to claim 1, 2 or 3,
A scanning image forming lens, wherein a lens on the surface to be scanned in the scanning image forming lens has a negative refractive power in a main scanning section. The multi-beam scanning device according to claim 1, 2, 3, or 4,
In the scanning image forming lens, a center line of curvature of the lens on the scanning surface side having a non-circular shape in the main scanning section and connecting the center of curvature in the sub scanning section on the lens surface in the main scanning corresponding direction. However, the absolute value of the radius of curvature of the lens surface obtained by changing the radius of curvature in the sub-scanning cross section in the main scanning corresponding direction so that the curve becomes different from the non-arc shape in the main scanning cross section is the main scanning cross section. It is characterized in that it is determined to increase smoothly and monotonically toward the local maximum value as the optical axis is separated in the corresponding direction, and to decrease smoothly and monotonously as the optical axis is separated after exceeding the local maximum position. Multi-beam scanning device. The multi-beam scanning device according to any one of claims 1 to 5,
A plurality of beams from a plurality of light sources are converted into parallel beams by a coupling lens, and each parallel beam is imaged as a long line image in the main scanning corresponding direction near a deflecting reflection surface by a line image forming optical system. A multi-beam scanning device characterized in that the scanning is performed. The multi-beam scanning device according to any one of claims 1 to 6,
A multi-beam scanning apparatus, wherein the optical deflector is a rotating polygon mirror, and each lens of the scanning imaging lens is provided with a tilt angle in a main scanning section in order to reduce the influence of sag.

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