JP4238600B2 - Demultiplexer and wavelength division multiplexing optical transmission module - Google Patents

Description

【００３２】
によって、チャープ型回折格子１６子で回折した光線がプリズム部２２から出射する角度ξを求めることができる。なお、ｎ_０は外界の屈折率（本実施例では空気であるためｎ_０＝１）、ｎはプリズム部２２の屈折率である。（１）式より、回折格子ブロック１４が温度変化しても出射角度ξが変化しない条件を求めることができ、
【数１０】

【００３３】
と表される。αはプリズム部２２の線膨張係数である。なおこの式は、プリズム部２２への入出射角度φ、ξには依存していないことが分かる。特に、プリズム部２２の入射面に対して光線の入射角度と出射角度を等しくなるように、
【数１１】

【００３４】
とする、つまり、
【数１２】

【００３５】
の場合には、（２）式は、
【数１３】

【００３６】
と表される。さらに、ζ≒０となるリトロー配置においては、
【数１４】

【００３７】
となるようにすれば良いことが分かる。
【００３８】
一般に、物質の屈折率ｎと密度ρとの間には、
【数１５】

【００３９】
の関係があることが知られている。但し、βは局所場補正係数を示し、物質に依存する。一般には、βは０から１／３の間の値を取る。この式を用いると、屈折率の温度変化と線膨張率αとの間には、
【数１６】

【００４０】
の関係があることが示される。従って、（２）式より、
【数１７】

【００４１】
を満たすように回折格子ブロック１４のｎ、β及びγを選べば、出射角度ξは温度に依存しなくなる。特に分光器等に一般に用いられているリトロー配置においては、（６）式より、
【数１８】

【００４２】
つまり、
【数１９】

【００４３】
を満たすｎ及びβを選べばよい。
通常βは０から１／３の範囲の値をとるため、（１０）式よりリトロー配置においては屈折率ｎを１．４１４から１．７３２の範囲に選ぶことが望ましい。また、ポリマー材料については、βとして１／３に近い値を取ることが多くこの場合は屈折率１．４１４付近の値をとることが望ましいといえる。この値に近い屈折率を持つポリマー材料としては、ポリメタクリル酸トリフルオロエチレン（屈折率１．４２）、シリコーンポリマー（屈折率１．４３）、シクロヘキサン系のフッ素化エポキシ（屈折率１．４０５）、フッ素化エポキシアクリレート（屈折率１．４１３）、フッ素化エポキシアクリレート（屈折率１．４１７）グリコール系のフッ素化エポキシ（屈折率１．３８５）、フッ素化エポキシアクリレート（屈折率１．４１６）、フッ素化エポキシアクリレート（屈折率１．４１８）、トリフルオロエチルメタクリレート樹脂（屈折率１．４０）等のフッ素化アクリル樹脂があり、これらの材料を用いて回折格子ブロック１４を形成すれば、リトロー配置において波長分波特性の温度依存性を低減することができる。このように、１．３８５〜１．４３の屈折率材料を用いることができる。１．３μｍ帯や１．５５μｍ帯の光通信波長を用いる場合は、赤外波長においてより損失の少ないフッ素系の材料を用いることが望ましい。
（８）式においては、γ、ｉ、θを適当に選ぶことにより、右辺の分母を１より大きく、或いは小さく設定できる。本実施例に示したように、入射角度ψが出射角度ξよりも大きな場合には、この値は１より小さくなるため、１．４１４よりも屈折率の大きな材料を用いても（８）式を満たすことができる。この場合には、ポリメチルメタクリレート（ＰMMＡ、屈折率１．４９）や、ポリカーボネート（ＰＣ、屈折率１．５９）、フッ素化ポリミド（屈折率１．５５）等のフッ素系のプラスチック、重水素化したプラスチック材料なども用いることができ、成形に適した材料を選ぶことができる。
【００４４】
また逆に、入射角度ψが出射角度ξよりも小さい場合には、低屈折率材料を用いることが望ましく、６フッ化プロピレン／４フッ化エチレン／２フッ化ビニリデン共重合体（屈折率１．３６）、テフロンＡＦ（デュポン社製、屈折率１．２９）、サイトップ（旭硝子社製、屈折率１．３４）を用いると赤外光でも損失を少なくすることができる。この場合のように出射角度を大きくすると回折格子の波長分散を大きくできるため、デマルチプレクサを小さくできるという効果がある。したがって、現実的なポリマーの屈性率として１．２９以上１．４１４以下の材料を用いると、デマルチプレクサの小型化の点で望ましいと言える。
【００４５】
以上説明したように、実施例においてはプリズム部２２を用いることにより、チャープ型回折格子１８の熱伸縮による波長分離特性変化をプリズム部２２の屈折率変化による波長分離特性変化により補償することができる。したがって、温度変化に伴う波長分離特性変化を抑制することができ、いわゆるアサーマル化を実現することができる。
【００４６】
なお、プリズム部２２のない平行平板を用いた回折格子の場合、つまりβ＝０の場合は、
【数２０】

【００４７】
となり、出射角度ξはプリズム部の屈折率ｎに依存しなくなる。したがって、回折格子の熱伸縮による特性変化を補償する効果はない。つまり、回折格子ブロック２２の入出射面は、チャープ型回折格子１６面に対して角度を成す必要があるものである。回折格子ブロック２２の入出射面に対して大きな角度を持って光線が入出射すると表面反射が大きくなり好ましくない。したがって、回折格子ブロック２２の入出射面の角度γは、回折格子ブロック２２の入出射面への入射角度と出射角度の間にあることが望ましい。つまり、入射角度と出射角度の大小により、
【数２１】

【００４８】
または、
【数２２】

【００４９】
を満たすことが望ましい。
【００５０】
ここまで、ベースの熱膨張率が小さく、回折格子の熱伸縮による特性変動が支配的な場合についての温度補償を説明してきたが、ベースの熱膨張率が大きく温度変化に伴って光学部品間の相対位置が変動する場合には、回折格子に加えてベースの熱伸縮も考慮し、デマルチプレクサ全体として温度依存性をなくすようにパラメータを設定すればよい。本実施例の配置においては、ベースの熱伸縮と回折格子の熱伸縮とは互いに補償する方向に動くため、ベースに熱膨張率の大きな材料を用いることによって、屈折率が１．４１４よりも大きな材料を用いた場合でもプリズム部２２により温度依存性を低減できる。したがって、ベース３８にポリマー材料等の熱膨張率の大きな材料を用いた場合にも温度補償できる。
【００５１】
また、回折格子の熱伸縮に伴う集光位置の変動がある程度許容される場合には、（８）式を十分に満たしていなくてもよく、実用的な温度補償の効果を達成できる範囲内でパラメータを選べばよい。
【００５２】
本実施例では、チャープ型回折格子１６として格子溝の断面形状が直角三角形をしたいわゆるエシュレット型を用いた。回折格子の作成には、まずルーリングエンジンを用いてチャープ型回折格子１６を成形するためのマスターを加工した。このマスターを用いて金型を作成し、金型を用いて回折格子ブロック１４を射出成形した。チャープ型回折格子１６の格子定数は、所望の集光性能を有するように格子の場所の関数としてチャープさせた。回折効率を大きくするためには、ブレーズ角を各格子位置での入射角度に合わせて変化させることが望ましい。そこで本実施例では、回折格子を複数の領域に分割し、領域内では一定のブレーズ角度で加工し、領域ごとにブレーズ角を変えることによって回折効率の向上を図った。ブレーズ角度は、領域ごとにＴＥ偏光（電場方向が格子の刻線方向の偏光）とＴＭ偏光（電場方向が刻線に垂直な偏光）の回折効率の差が小さくなるように定めた。そのために、領域の中心部において、入射光線７０がブレーズ面で正反射する方向と出射光線７２の回折方向とを一致させた正反射条件におけるブレーズ角よりも大きくした。本実施例においては、領域の中心での正反射条件から決まるブレーズ角より２〜１０°ブレーズ角を大きくした。この差は、格子定数が小さい程大きくすることが望ましい。
【００５３】
エシュレット型回折格子では、波長λ、格子定数ｄとしたとき、λ／ｄが０．６から１．０、ブレーズ角度を２５°から３５°の範囲に選ぶと、偏光依存性が小さく回折効率を大きくすることができる。この場合、
【数２３】

【００５４】
とする必要がある。したがって、（４）式に従い、プリズム入射面の角度γは、１７°以上３０°以下が望ましい。
さらに、格子定数が小さくなるとＴＥ偏光とＴＭ偏光との回折効率の差が大きくなるため、この差が小さく、且つ回折効率が高くなるように回折次数を選んだ。本実施例においては、格子定数の大きな領域では−１次の回折光を用い、格子定数の小さな領域では−２次の回折光を用いるように途中より不連続に格子定数を２倍にした。このように、格子定数が小さい場合には高次の回折光を用いることが望ましく、おおよそ入射波長程度以下の格子定数で２次以上の回折光を用いることが望ましい。この次数の切替えは、ブレーズ角度の切替え位置で行うことにより、回折格子の分割数を増やすことなく、格子定数を切り替えることができる。本実施例では、領域の分割数は６分割とした。分割数は多くするほど理想的なブレーズ角度分布に近い角度分布とすることができる。領域分割数と損失の関係を検討し、分割数を４以上とすると損失が低下し、９以上ではほぼ損失は一定となることが分かった。したがって、分割数を大きくしても、ある一定の分割数を超えると損失の低下は顕著ではなくなる。しかも、領域を余り多くすると、ブレーズ角を変更する回数も増え、加工精度が落ちて損失が低下する可能性がある。したがって、分割数は４〜９、特に６〜９が望ましい。本実施例では、回折格子溝を平面上に加工しているため、回折格子溝の加工において領域のつなぎ部での平面方向及び垂直方向の位置ずれが生じにくい。
【００５５】
このようにＴＥ偏光とＴＭ偏光の回折効率をほぼ等しくすることにより、入射光線の偏光状態が変化しても検出光量の変動を小さくできる。光ファイバ１０から特定の偏光が入射する場合には、この偏光の回折効率が最大となるようにブレーズ角を定めればよい。或いは、偏光変換素子を用いて光ファイバからの出射光の偏光方向をそろえてもよい。偏光変換素子は、一般に知られているように偏光ビームスプリッタとλ／２板を組み合わせることに構成することができる。この場合、回折効率の高い偏光方向となるように偏光変換素子の向きを定めればよい。特に、ＴＭ偏光は、格子ピッチがおおよそ波長よりも小さな領域で回折効率を高くできるため、格子定数を波長よりも小さくしＴＭ偏光に変換して入射することが望ましい。この場合、回折効率は格子溝の形状にほとんどよらなくなるために、エシュレット格子に限らず、波型形状や、矩形のバイナリ回折格子を用いても、高い回折効率を実現できる。
【００５６】
格子パターンの加工は、本実施例に用いたルーリングエンジンの他にも、例えば感光体に入射位置と出射位置からレーザビームを照射して干渉パターンを露光し、ホログラムを形成して作成してもよい。ホログラム回折格子を用いることにより、収差が少なく出射位置に十分に絞って集光することができる。或いは、３次元ＮＣ加工機を用いても加工してもよく、またさらに、フォトリソグラフィの手法を用いて、Ｓｉ等の基板に連続的にブレーズ化したチャープ回折格子を作成することもできる。
【００５７】
チャープ型回折格子１６及びサブ波長格子２０の加工については、ホットプレスを用いてもよい。
【００５８】
なお、本実施例のように回折格子の位置により入射角度が変化する場合には、光強度の一番強い主光線について温度補償条件を満たすように設定すればよい。また回折格子については、本実施例に用いた平面状のチャープ型回折格子に限らず、格子定数が一定な等間隔平面回折格子や凹面回折格子についても同様に適用できるものである。凹面型回折格子を用いる場合には、光ファイバ１０の入射点及びスラブ導波路３２の入射点を凹面型の回折格子１５のいわゆるローランド円上に配置することにより、各波長の集光点距離の変化を小さくできる。その場合には、スラブ導波路部３０は必要ない。
【００５９】
また、プリズム部２２を光線が透過することにより収差が発生するが、収差を補正するように、回折格子の格子ピッチを補正すればよい。光線追跡計算を用いた自動設計により格子の配置を設計することができる。或いは、非球面非対称なシリンドリカルレンズを用いて補正してもよい。
本実施例では、シリンドリカルレンズ１２として円柱状のものを用いた。リトロー配置に近く配置する場合には、１個のシリンドリカルレンズで入射と出射の光線の集光を兼用しても良い。
【００６０】
本実施例においては、多重化されて伝送されてきた中心波長１２７５.７、１３００．２、１３２４.７、１３４９.２nmの４波長を分離する構成とした。この中心波長に対して、光源の製造・温度・スペクトル分布による波長ばらつきにより、最大±５．７nm変動するものとした。したがって、隣り合うチャネル間の波長スペースは、１３.１nmであり、おおよそ伝送波長の１％に設定した。この波長ばらつきは、光源の作成・特性により決まるものであり、特に厳しい光源の選別を必要としない範囲として必要なものである。本実施例では、この使用波長範囲で十分な特性変動の低減を実現できた。
【００６１】
受光用光導波路２８は、上記のようにスラブ導波路部３０とテーパ導波路部３２で構成される。光検出器に信号光が効率よく結合するように光導波路の厚さは６５μｍとした。テーパ導波路部３２は、紙面内において光入射側から出射側にかけてコアの幅が減少するテーパ形状とし、光検出器アレイに光を絞り込む構造としている。効率よく光検出器アレイまでビームを伝搬するためには、スラブ導波路の開口数ＮＡは大きいほうが望ましく、特にＮＡ０．５以上とすることが望ましい。さらに、薄層のクラッドの外側に金属膜を形成すると、仮に光線がクラッドを透過しても金属膜で反射して光検出器に到るため、検出効率を向上できる。本実施例においては、テーパ導波路部３２の幅は、スラブ導波路３０と接合される側で光検出器アレイのピッチと同じくし、光検出器アレイ６０側で光検出器に効率良く結合できる幅となるようにした。このように、波長分離したビームを光検出器に絞り込むことにより、応答速度の速い受光面積の小さな光検出器を用いることができ、高速な光通信に適用することができるようになる。光検出器アレイは、テーパ導波路部３２からのビームが光検出器に効率よく結合するように位置を調整し、テーパ導波路部３２に接着固定した。
【００６２】
本実施例においては、光ファイバ１０として、コア径６２.５μｍ、開口数０．２７５のグレーテッドインデックス（ＧＩ）型マルチモード光ファイバ（６２．５ＭＭＦ）を用いた。但し、光ファイバを限定するものではなく、シングルモード光ファイバ（ＳＭＦ）やコア径５０μｍのマルチモード光ファイバ（５０ＭＭＦ）、さらにコア径の大きなマルチモード光ファイバを用いることもできる。
【００６３】
本発明の第２の実施形態を図５を用いて説明する。図５は、本実施形態に係わる回折格子ブロックの実施例を示す上面図である。本実施例の回折格子ブロックの機能は、第一の実施形態に示した回折格子ブロック１４と同じであり、ここでは異なる点について示す。まず、チャープ型回折格子１６を有する回折格子基板２４をポリマー材料を用いて射出成形する。チャープ型回折格子１６上に金属膜を成膜し反射膜１８を形成する。別に、サブ波長格子２０を有するプリズム部２２も成形する。この回折格子基板２４とプリズム部２２とを光学接着材１９を用いて接着し、回折格子ブロック１４を形成する。プリズム部２２と回折格子基板２４に同じ材料または、同じ線膨張係数を持つ材料を用いることにより、チャープ型回折格子１６の線膨張係数をプリズム部２２と等しくすることできる。
【００６４】
或いは、プリズム部２２と回折格子基板２４に線膨張係数の異なる材料を用いることにより、チャープ型回折格子１６の線膨張係数を調整し、（２）式を満たすようにすることもできる。
【００６５】
本実施例では、チャープ型回折格子１６を平板状の回折格子基板２４上に形成するため、射出成形を用いて容易に精度良く成形することできる。射出成形以外にも、いわゆる２Ｐ法を用いて、平板の基板上に紫外線硬化樹脂を塗布し、型を押し付けながら紫外線を照射して紫外線硬化樹脂を硬化させてチャープ型回折格子１６を形成してもよい。透明な回折格子基板２４を用いることにより基板側から紫外線を照射して硬化させることもできる。２Ｐ法を用いた場合も紫外線硬化樹脂の層を薄くすることにより、回折格子の線膨張係数をプリズム部２２と等しくでき、チャープ型回折格子１６の熱伸縮による特性変化をプリズム部２２の屈折率変化で補償することができる。
【００６６】
本実施例の回折格子ブロック１４を、第一の実施形態に示したようにベース３８に固定する場合は、ベース３８もポリマーを用い、プリズム部２２をベース３８と一体で形成してもよく、ベース３８と一体成形したプリズム部２２に回折格子基板２４を張り合わせてもよい。プリズム部２２をベース３８と一体とすることで、デマルチプレクサの組み立てが容易となるとともに、回折格子ブロック１４の位置ずれが起きず信頼性が向上する。
【００６７】
本発明の第３の実施形態を図６を用いて説明する。図６は、本実施形態に係わるデマルチプレクサの実施例を示す上面図である。光ファイバ１０より拡がって出射された波長多重光信号は、回折格子ブロック１４と一体となったシリンドリカルレンズ１３ａにより紙面の垂直方向についてほぼ平行光となり、回折格子ブロック１４に入射する。回折格子ブロック１４に入射した光線は、回折格子ブロック１４に形成されたチャープ型回折格子１６により波長毎に異なる角度に回折され、集光される。チャープ型回折格子１６により回折された光線は、シリンドリカルレンズ１３ｂにより集光されて受光用光導波路２８に入射する。受光用光導波路２８のスラブ導波路部３０に入射した光線は、波長ごとにテーパ導波路部３２の入射端に集光され、波長毎に受光部を有する光検出器アレイ３６により検出される。
【００６８】
本実施例では、回折格子ブロック１４とシリンドリカルレンズ１３ａ、１３ｂを一体とし、透明ポリマーを用いて形成した。光ファイバ１０からの入射光線７０の主光線がシリンドリカルレンズ１３ａに略垂直に、チャープ型回折格子１６で回折した出射光線７２の主光線がシリンドリカルレンズ１３ｂに対して略垂直になるようにしている。したがって、チャープ型回折格子１６の線膨張係数αと回折格子ブロック１４の屈折率の温度変化が（６）式を満たすようにすれば、温度変化に対して波長分離特性が変化しないようにできるものである。したがって、（１０）式よりポリマー材料としては、屈折率１．４１４付近のものを用いることが望ましい。
【００６９】
本実施例のように、プリズム部２２を通してチャープ型回折格子１６に光線を入射することにより、光ファイバからのビーム拡がり角は空気中に比べてプリズム部の屈折率分低減する。つまり、屈折率１．４の材料を用いた場合には、ビーム拡がり角は７１％程度に小さくなる。したがって、プリズム部２２を通してビームを照射することは、チャープ型回折格子１６に入射するビームの拡がり角を低減する効果を有している。そのため、回折格子の幅が小さくなり、デマルチプレクサ全体を小型化できる効果がある。
【００７０】
本発明の第４の実施形態を図７を用いて説明する。図７は、本実施形態に係わるデマルチプレクサの実施例を示す上面図である。波長多重化され光ファイバ１０を伝送してきた光信号は、シングルモードの光ファイバ１０より光閉じ込め方向についてシングルモードのスラブ導波路４０に入射し、凹面型の回折格子１５に入射する。回折格子１５により波長毎に異なる角度に回折され分離され、波長ごとに集光される。分離集光された光線は、テーパ導波路部３２に入射し、光検出器アレイ６０により検出される。
【００７１】
スラブ導波路４０のコアには透明ポリマーを用いており、端部に回折格子１５を形成した。回折格子１６面には金、銀等の金属膜を形成し、反射面とした。
本実施形態では、光線はスラブ導波路４０中を伝搬するため、スラブ導波路４０コアの屈折率の温度変化とクラッドに用いた基板の線膨張係数αが（６）式を満たすようにすることで温度依存性を無くすことができる。したがって、第一の実施形態で示したように、コアに低屈折率材料を用いることが望ましく、また、クラッド部の線膨張係数αがコア材と近いことが望ましい。
【００７２】
本実施例のように凹面型の回折格子１５を用いた場合には、光ファイバ１０の入射点及びスラブ導波路３２の入射点を凹面型の回折格子１５のいわゆるローランド円上に配置することにより、各波長の集光点距離の変化を小さくできる。
【００７３】
本発明の第５の実施形態を図８を用いて説明する。図８は、本実施形態の係わるデマルチプレクサの上面図である。光ファイバ１０から拡がって出射された波長多重光信号は、レンズ５０ａで略平行光となり、回折格子ブロック１４のプリズム部２２に入射する。回折格子ブロック１４に入射した光線は、平面型の回折格子１５に入射し、波長ごとに異なった回折角度を持って回折される。回折格子１５で回折されたビームは、レンズ５０ｂで集光され、波長ごとにテーパ導波路３５に入射し、光検出器アレイ２４で検出される。
【００７４】
本実施例では、回折格子１５として平行な直線格子溝を持つ等間隔回折格子を用いた。この回折格子１５に略平行な光線が入射するために、回折格子１５の全範囲で（８）式を満たすことができ、回折格子１５全範囲で温度依存性をなくすことができる。また、本実施例においては、集光点が出射光線７２に対してほぼ垂直に並ぶため、受光用光導波路２８にスラブ導波路部は必要ない。レンズ５０には、屈折率分布を持ったロッドレンズを用いてもよい。その場合は、光ファイバ１０をロッドレンズに固定することもできる。また、リトローに近い配置とし、レンズ５０を１個としてもよい。
【００７５】
本発明の第６の実施形態を図９及び図１０を用いて説明する。図９は、本実施形態の係わるデマルチプレクサの実施例を示す上面図である。光ファイバ１０より拡がって出射された波長多重信号は、シリンドリカルレンズ１２ａにより紙面の垂直方向についてほほ平行光となり、固定枠５２に固定されたチャープ型回折格子１６に入射する。チャープ型回折格子１６により回折し集光された光線は、シリンドリカルレンズ１２ｂにより紙面に垂直方向について集光され、受光用光導波路２８に入射する。スラブ導波路部３０に入射した光線は、チャープ型回折格子１６の集光作用により波長ごとに設けられたテーパ導波路の入射端に集光される。波長ごとに分離集光された光線は、それぞれの波長に対応するテーパ導波路部３２に入射し、波長毎に受光部を有する光検出器アレイ３６により検出される。
【００７６】
図１０を用いて固定枠５２及び回折格子基板２４についてさらに詳細に説明する。チャープ型回折格子１６を有する回折格子基板２４には、くさび状の溝部５７ａ、５７ｂが、固定枠５２側には突起部５６ａ、５６ｂが形成されており、突起部５６ａと溝部５７ａ、突起部５６ｂと溝部５７ｂをかみ合わせるようにし、押さえ５４で押さえつけて回折格子基板２４を固定枠５２に固定している。溝部５７ａ、５７ｂと突起部５６ａ、５６ｂにより、チャープ型回折格子１６は固定枠５２に対して正確に位置が定まる。回折格子基板２４を固定した固定枠５２は、ベース３８に設けられた固定溝３９に固定される。突起部５６が固定溝３９の基準点３７上に来るようにすることで、回折格子基板２４は基準点３７を中心として両側に熱伸縮するようにできる。溝部５７の位置を回折格子の中心（光ファイバからの主光線が当たる位置）からずらしておき、温度変化によるスポット移動方向とは逆方向に回折格子基板２４が移動するようにすることによって、回折格子基板２４の熱伸縮による集光位置の変化を回折格子基板２４の中心位置移動を利用して補償することができる。
【００７７】
回折格子基板２４の熱伸縮による移動により、回折格子基板２４の熱伸縮によるｘ方向についての集光スポット移動を補償するためには、ｘ方向の集光スポット移動率について、
【数２４】

【００７８】
となるように、チャープ型回折格子１６の中心と溝部５７との距離Ｌを定めればよい。本実施例では、回折格子基板２４に線膨張率α＝６×１０^−５／Ｋのポリカーボネートを用い、回折格子基板２４の熱伸縮によるスポット移動率が１μｍ／Ｋであったため、Ｌ＝８．３mmとした。
【００７９】
このように、チャープ型回折格子１６の中心と熱伸縮の中心とをずらすことにより、回折格子の熱伸縮による集光点の位置変化を、チャープ型回折格子１６の熱伸縮に伴う中心位置の移動により補償することができる。この場合、平面型の回折格子は、格子面に略平行に移動して温度変化に伴う特性変化を補償する。回折格子自身の熱伸縮を利用して温度補償を行うため、温度変化への追従性が良く、急激な温度変化に対しても補償することができる。
【００８０】
射出成形を用いて回折格子基板２４を形成することにより、回折格子基板２４成形時に溝部５７も形成することができ、溝部５７の位置を精度良く形成することができる。
【００８１】
本実施例においては、回折格子基板２４の伸縮に伴い、ｚ方向にもスポット位置が移動する。しかし、ｚ方向については、スポット移動に伴う特性変動が小さいためこの移動による特性変化は問題とならなかった。
【００８２】
本実施形態においては、第一の実施形態のようにプリズムを用い、プリズムの屈折率変化による温度補償と組み合わせても良い。
【００８３】
本発明の第７の実施形態を図１１を用いて説明する。図１１は、本実施形態の係わるデマルチプレクサの実施例を示す上面図である。光ファイバ１０より拡がって出射された波長多重信号は、シリンドリカルレンズ１２ａにより紙面の垂直方向についてほほ平行光となり、チャープ型回折格子１６に入射する。チャープ型回折格子１６で回折された光線は紙面内において波長ごとに異なる位置に集光される。チャープ型回折格子１６により回折し集光された光線は、シリンドリカルレンズ１２ｂにより紙面に垂直方向について受光用光導波路２８の入射端面に集光され、受光用光導波路２８に入射する。受光用光導波路２８のスラブ導波路部３０に入射した光線は、紙面に垂直な方向はスラブ導波路に閉じ込められて伝搬し、紙面内についてはチャープ型回折格子１６の集光作用により、波長ごとに設けられたテーパ導波路の入射端に集光される。波長ごとに分離集光された光線は、それぞれの波長に対応するテーパ導波路部３２に入射し、波長毎に受光部を有する光検出器アレイ３６により検出される。
【００８４】
本実施例では、固定部６２ａ、６２ｂ、６２ｃに一端が固定された駆動部６０ａ、６０ｂ、６０ｃの熱伸縮を用いて、回折格子基板２４を移動し、回折格子基板２４の熱伸縮による温度特性変動を補償する。本実施例では、駆動部６０ａにより回折格子基板２４を格子面に平行に移動するとともに、駆動部６０ｂ、６０ｃにより回折格子基板２４を格子面に垂直に平行移動することによって、温度補償を行っている。そのために、ｚ方向のスポット移動も補償することができる。本実施例のように回折格子を２次元的に平行に移動することによりより精度良く温度補償することができる。
【００８５】
本発明の第８の実施形態を図１２及び図１３を用いて説明する。図１２は、本実施形態の係わるデマルチプレクサの上面図である。光ファイバ１０より拡がって出射された波長多重信号は、シリンドリカルレンズ１２ａにより紙面に対して垂直方向についてほほ平行光となり、チャープ型回折格子１６に入射する。チャープ型回折格子１６で回折された光線は紙面内において波長ごとに異なる位置に集光される。チャープ型回折格子１６により回折し集光された光線は、温度変化補償用のプリズム５８を透過し、シリンドリカルレンズ１２ｂにより紙面に垂直方向について受光用光導波路２８の入射端面に集光され、受光用光導波路２８に入射する。受光用光導波路２８のスラブ導波路部３０に入射した光線は、紙面に垂直な方向はスラブ導波路に閉じ込められて伝搬し、紙面内についてはチャープ型回折格子１６の集光作用により、波長ごとに設けられたテーパ導波路の入射端に集光される。波長ごとに分離集光された光線は、それぞれの波長に対応するテーパ導波路部３２に入射し、波長毎に受光部を有する光検出器アレイ３６により検出される。
【００８６】
図１３を用いてプリズム５８による温度補償について説明する。本実施形態においては、チャープ型回折格子１６の熱伸縮による特性変化をプリズム５８の屈折率変化で補償することを特徴としている。プリズム５８の屈折率をＮとし、チャープ型回折格子１６への入出射角度及びプリズム５８への入出射角度を図１３に示したように表すと、回折格子基板２４の伸縮に伴って光ファイバ１０からの主光線が受ける回折角度の変化を、プリズム５８の屈折率の温度変化で補償するための条件は、
【数２５】

【００８７】
の場合には、
【数２６】

【００８８】
とすればよい。（１３）式は、プリズム５８からの主光線の出射角度ψが、回折格子基板２４の伸縮に対してが変動しない条件を示している。回折格子基板２４の線膨張係数αに対して、（１２）式又は（１３）式を満たすように、プリズム５８の材料及び角度γ、さらに回折格子への入射角度ｉを選ぶことにより、温度依存性をなくすことができる。回折格子基板２４とプリズム５８を同じ材質を用いる場合には、（７）式より、
【数２７】

【００８９】
を満たすように、プリズム５８の角度γ、回折格子への入射角度ｉを決めればよい。
【００９０】
プリズム５８を用いると、光ファイバからの主光線の出射方向と受光の主光線の方向とを略平行とすることもでき、部品の配置が容易となるという効果も得られる。特に、
【数２８】

【００９１】
となるように、γを選ぶと光ファイバからの出射方向と受光の方向とを略平行にすることができ、望ましい。本実施例においては、Ｎ＝１．４９、ｉ＝３５°、θ＝１８°としたため、γ＝２８．７°とした。
【００９２】
またさらに、本実施例のように出射ビームの幅が入射側よりも広がるようにプリズム５８の入射面を主光線に対して斜めにすることにより、プリズム５８は各波長の受光スポット間の距離を広げる効果もある。そのため、プリズムを用いない場合よりも波長分散の小さい回折格子を用いて波長分離ができ、例えばチャープ型回折格子１６に格子ピッチの広い回折格子を用いても十分に波長を分離することができるようになる。したがって、回折格子の作製が容易となり、また回折効率を高くすることもできる。
【００９３】
またさらに、本実施例のようにプリズム５８を用いた場合には、受光部の波長ピッチをプリズム５８を回転して微調整することもできる。
【００９４】
ここまで、本発明の波長多重分離光デバイスを波長分離する場合について説明したが、本発明の波長多重分離光デバイスは光検出器を配している側に光源アレイを設ければ光合波デバイスとしても用いることができる。マルチプレクサに用いる場合は、温度補償しているため、光源には温度安定性に優れたものが望ましく、ＤＦＢレーザを用いることが望ましい。また、光ファイバ１０の前に光を絞り込むように形成されたテーパを設けることが望ましい。
【００９５】
また本発明は、波長間隔の狭いいわゆるＤＷＤＭにおけるデマルチプレクサにも用いることができる。温度依存性を小さくしたことにより、デマルチプレクサの温度をコントロールしなくても特性が安定しており、信頼性の高い波長分離が出来る。ＤＷＤＭにおいては波長間隔が狭く特に温度に対する高い特性安定性が求められるため、ガラス基板のように熱膨張係数がポリマーよりも小さな材料を用いた回折格子についても、本発明の効果は大きいものである。
【００９６】
本発明の第９の実施形態を図１４を用いて説明する。
【００９７】
図１４は、本実施形態に係わる波長多重伝送モジュールの実施例を示す構成図である。伝送用光ファイバ７４ａ、７４ｂは、コネクタ９６により光伝送モジュール８０に接続され、それぞれ光ファイバ１０、シングルモード光ファイバ１１と突き合わせられる。受信用の伝送用光ファイバ７４ａを伝送してきた波長多重信号は、伝送用光ファイバ７４ａに突合せられたマルチモードの光ファイバ１０に入射し、本発明のデマルチプレクサ８２で波長ごとに分離されて光検出器アレイ３６で検出される。検出された信号は、受信回路８６により増幅され、波形整形される。さらに、パラレルーシリアル変換回路９０により、波長多重化されて伝送されてきた信号をシリアル信号に変換して出力する。送信する場合には、入力された信号をシリアルーパラレル変換回路９２で信号を分離し、送信回路８８で信号ごとにレーザダイオードアレイ９４の各光源を駆動し、波長の異なる光信号とする。各光信号は、シングルモードの光カプラにより構成される波長合波デバイス８４で多重化し、シングルモード光ファイバ１１に出力する。シングルモード光ファイバ１１を通して、送信用の伝送用光ファイバ７４ｂに結合され、送信される。シングルモード光ファイバ１１及びマルチモードの光ファイバ１０を用いて伝送用光ファイバ７４ａ、７４ｂと結合しているため、伝送用光ファイバ７４ａ、７４ｂにはマルチモードファイバ、シングルモードファイバのどちらでも用いることができる。本実施例では、光ファイバ１０にコア径６２．５μｍのＧＩ型マルチモードファイバを用いているため、伝送用光ファイバ７４ａ、７４ｂとして、シングルモード光ファイバ、コア５０μｍ及び６２．５μｍのマルチモード光ファイバ等、光ファイバ１０と等しいか、又は小さなコア径及び開口数を持つ光ファイバを用いることができる。
【００９８】
光ファイバ１０は伝送用のファイバとしてもよく、その場合には光ファイバ１０が着脱できるようにし、着脱によっても光ファイバ１０が所定の位置に突き合わされるようにしてもよい。
【００９９】
本実施例では、第一の実施形態に示したデマルチプレクサ８２を用いた場合についてしめしたが、本発明のデマルチプレクサを用いることにより、温度変化による特性変動を低減した光伝送モジュールを実現することができるものである。
【０１００】
本実施例では、送信と受信の両方の機能を有する波長多重伝送モジュールについて示したが、受信部のみを有する波長多重受信光モジュールとしても本発明の効果を得られるものである。
【０１０１】
【発明の効果】
上記のごとく、本発明のデマルチプレクサにより、回折格子を有する回折格子ブロック又は、光学部材の温度変化に伴う屈折率変化を用いて回折格子の熱伸縮による特性変動を補償することができるようになり、温度追従性の優れたデマルチプレクサすることができるようになった。
【０１０２】
また、本発明のデマルチプレクサにより、回折格子を平行移動することで回折格子の熱伸縮による特性変動を補償することができるようになり、チャープ型回折格子を用いた場合でも温度変化に追従して補償するデマルチプレクサをできるようになった。
【図面の簡単な説明】
【図１】本発明の第１の実施形態に係わるデマルチプレクサの上面図である。
【図２】本発明の第１の実施形態に係わる回折格子ブロックの側面図である。
【図３】本発明の第１の実施形態に係わる回折格子ブロックに設けたサブ波長格子の斜視図である。
【図４】本発明の第１の実施形態に係わる回折格子ブロックの温度補償機能を説明する図である。
【図５】本発明の第２の実施形態に係わる回折格子ブロックの上面図である。
【図６】本発明の第３の実施形態に係わるデマルチプレクサの上面図である。
【図７】本発明の第４の実施形態に係わるデマルチプレクサの上面図である。
【図８】本発明の第５の実施形態に係わるデマルチプレクサの上面図である。
【図９】本発明の第６の実施形態に係わるデマルチプレクサの上面図である。
【図１０】本発明の第６の実施形態に係わる固定枠及び回折格子基板の斜視図である。
【図１１】本発明の第７の実施形態に係わるデマルチプレクサの上面図である。
【図１２】本発明の第８の実施形態に係わるデマルチプレクサの上面図である。
【図１３】本発明の第８の実施形態に係わる温度補償を説明する図である。
【図１４】本発明の第９の実施形態に係わる波長多重伝送モジュールの構成図である。
【符号の説明】
１０ 光ファイバ
１１ シングルモード光ファイバ
１２ａ、１２ｂ シリンドリカルレンズ
１４ 回折格子ブロック
１５ 回折格子
１６ チャープ型回折格子
１８ 反射膜
１９ 光学接着剤
２０ サブ波長格子
２１ 四角錐
２２ プリズム部
２４ 回折格子基板
２６ 光学ブロック
２８ 受光用光導波路
３０ スラブ導波路部
３２ テーパ導波路部
３６ 光検出器アレイ
３７ 基準点
３８ ベース
３９ 固定溝
４０ スラブ導波路
４２ 下部基板
４４ 上部基板
４６ コア
４８ クラッド
５０ａ、５０ｂ レンズ
５２ 固定枠
５４ 押さえ
５６ａ、５６ｂ 突起部
５７ａ、５７ｂ 溝部
５８ プリズム
６０ａ、６０ｂ、６０ｃ 駆動部
６２ａ、６２ｂ、６２ｃ 固定部
７０ 入射光線
７２ 出射光線
７４ａ、７４ｂ 伝送用光ファイバ
８０ 光伝送装置
８２ 波長多重分離デバイス
８４ 波長合波デバイス
８６ 受信回路
８８ 送信回路
９０ パラレルーシリアル変換回路
９２ シリアルーパラレル変換回路
９４ レーザダイオードアレイ
９６ 光コネクタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical device that separates wavelength-multiplexed optical signals transmitted using an optical fiber for each wavelength, and in particular, a diffraction grating used for wavelength separation, a demultiplexer that separates wavelengths using a diffraction grating, and wavelength-multiplexed light. It relates to a transmission module.
[0002]
[Prior art]
In Patent Document 1, the selected center of the filter itself due to the ambient temperature variation is obtained by rotating the diffraction grating using a temperature compensation mechanism unit arranged in the diffraction grating so as to cancel the shift of the selected center wavelength caused by the temperature rise. An optical filter for suppressing wavelength shift and a diffraction grating used for the optical filter are disclosed.
[0003]
[Patent Document 1]
JP-A-6-331850
[0004]
[Problems to be solved by the invention]
However, in the conventional optical filter, for example, a diffraction grating made of a glass substrate is rotated by a temperature compensation mechanism unit using, for example, an aluminum support. The diffraction grating and the temperature compensation mechanism are different in heat capacity, thermal conductivity, and the like. For this reason, there has been a problem that the speed of the characteristic change of the diffraction grating and the speed of compensation of the characteristic change by the temperature compensation mechanism do not coincide with a sudden temperature change, resulting in characteristic fluctuations.
[0005]
In addition, when the diffraction grating is rotated as in the conventional optical filter to suppress the shift of the selected center wavelength of the filter itself due to the ambient temperature fluctuation, the distance from the outgoing optical fiber varies at the outer periphery of the diffraction grating. Therefore, when a chirped diffraction grating having a condensing characteristic is used as the diffraction grating, there is a problem that the position of the spot condensed by the diffraction grating moves and changes the characteristic.
[0006]
The present invention has been made to solve such a problem, and an object of the present invention is to provide a diffraction grating in which a characteristic change follows and compensates for a sudden temperature change, and a demultiplexer using the diffraction grating. Is to provide.
[0007]
Furthermore, another object of the present invention is to provide a diffraction grating in which a characteristic change follows and compensates for a temperature change even when a chirped diffraction grating is used, and a demultiplexer using the diffraction grating. .
[0008]
[Means for Solving the Problems]
To achieve the above object, the present invention comprises a light incident part, a diffraction grating, and an optical path for transmitting a wavelength-multiplexed light beam from the light incident part to the diffraction grating. Photodetector In the demultiplexer for separating the optical signal by irradiating the diffraction beam with the light beam, the diffraction grating And a prism separated from the light detector and in the optical path between the light detector and the optical detector, the characteristic change due to the temperature change of the diffraction grating is compensated by the refractive index change of the prism A demultiplexer characterized by the above is provided.
[0020]
It is desirable to arrange the prism so that the width of the emitted light beam is wider than the width of the incident light beam. A change in characteristics due to a change in temperature of the diffraction grating can be compensated by using a change in the refractive index of the prism.
[0023]
Furthermore, a wavelength division multiplexing optical module and a wavelength division multiplexing optical transmission module are provided using the demultiplexer of the present invention.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
First, a first embodiment of the present invention will be described with reference to FIGS.
[0025]
FIG. 1 is a top view showing an example according to the present embodiment. The optical signal transmitted after being wavelength-multiplexed is emitted from the optical fiber 10. The incident light beam 70 radiated and emitted from the optical fiber 10 is reduced in the divergence in the direction perpendicular to the paper surface (the engraving direction of the chirped diffraction grating 16) by the cylindrical lens 12a to become almost parallel light, and the diffraction grating block 14 Is incident on the prism portion 22. The light beam incident on the prism unit 22 is diffracted and separated at different angles for each wavelength by the chirped diffraction grating 16 of the diffraction grating block 14. The chirped diffraction grating 16 has a grating interval changed so as to have a condensing function, and the light beams diffracted by the chirped diffraction grating 16 are collected at different positions for each wavelength in the paper. The light beam diffracted and collected by the chirped diffraction grating 16 is collected by the cylindrical lens 12b on the incident end face so as to enter the light receiving optical waveguide 28 in the direction perpendicular to the paper surface. The light receiving optical waveguide 28 has a slab waveguide portion 30 on the incident portion side and a tapered waveguide portion 32 on the subsequent stage. The light beam incident on the slab waveguide portion 30 propagates while being confined in the slab waveguide in the direction perpendicular to the paper surface, and in the paper surface, a tapered waveguide provided for each wavelength by the condensing action of the chirped diffraction grating 16. The light is collected at the incident end of the waveguide 32. The separated and condensed light rays enter the tapered waveguide portion 32 corresponding to each wavelength, and are detected by the photodetector array 36 having a light receiving portion for each wavelength. The diffraction grating block 14, the cylindrical lens 12, and the light receiving optical waveguide 28 are fixed on the base 38.
[0026]
In this example, the diffraction grating block 14 having the chirped diffraction grating 16 was manufactured by injection molding suitable for mass production using a transparent polymer. For this reason, the thermal expansion and contraction occurs as the environmental temperature changes, and the lattice constant (pitch) of the chirped diffraction grating 16 changes. Therefore, in this embodiment, the wavelength separation characteristic fluctuation due to the thermal expansion and contraction of the chirped diffraction grating 16 is compensated by the refractive index change of the prism portion 22.
[0027]
FIG. 2 shows an embodiment of the diffraction grating block 14. The chirped diffraction grating 16 is formed integrally with the prism portion 22. Further, an antireflection structure using the sub-wavelength grating 20 is formed on the light incident / exit surface of the prism portion 22. The diffraction grating block 14 of this example was formed by injection molding using a transparent polymer material, and the chirped diffraction grating 16 and the sub-wavelength grating 20 were integrally molded at the time of injection molding. After molding, a metal film such as gold, silver, or aluminum was vapor-deposited on the grating surface of the chirped diffraction grating 16 to form a reflective film 18. A dielectric multilayer film may be used instead of the metal reflection film 18, or a dielectric film may be formed between the chirped diffraction grating 16 and the metal film to improve the reflectance.
[0028]
FIG. 3 is a perspective view of the sub-wavelength grating 20 provided in the diffraction grating block 14. The sub-wavelength grating 20 has a structure of a wavelength or less, and has an antireflection effect. In this embodiment, a pyramid arrangement structure of quadrangular pyramids 21 arranged two-dimensionally is employed. In order to obtain a high antireflection effect regardless of the incident polarized light, it is desirable that the aspect ratio h / p ≧ 2 for the pyramid pitch p and the pyramid height h. The pitch p is set to satisfy p <λ / 2, where λ is the minimum wavelength among the wavelengths used.
[0029]
In this embodiment, since the reflection is prevented by using a minute structure, an antireflection structure by the sub-wavelength grating 20 can be formed simultaneously with the formation of the diffraction grating. For this reason, it is not necessary to provide a dielectric film on the entrance / exit surface using a vacuum process after the prism is formed, and the diffraction grating block 14 can be easily formed. As the molding material, glass may be used in addition to the transparent polymer, but it is preferable to use a transparent polymer because the transferability of the sub-wavelength grating 20 and the chirped diffraction grating 16 is better. The sub-wavelength grating 20 does not need to be arranged accurately and periodically, and may have a sub-wavelength structure. Further, the shape may be any shape as long as the aspect ratio is a desired value, for example, a conical shape. By providing the antireflection structure in this way, it is possible to reduce the loss of the demultiplexer and to prevent crosstalk due to reflected light.
[0030]
In this embodiment, the antireflection structure is integrally formed, but a film having a sub-wavelength grating 20 that is separately prepared may be attached. Alternatively, an antireflection film made of a dielectric film may be provided without using a subwavelength grating.
[0031]
The temperature compensation function of the diffraction grating block 14 will be described with reference to FIG. As shown in FIG. 4, the angle is determined with the chirped diffraction grating 16 as the reference plane, the angle of the incident surface of the prism portion 22 is γ, the angle of the light ray incident on the prism portion 22 is ψ, and the chirped diffraction When the incident angle to the grating 16 is i and the diffraction angle is θ,
[Equation 9]

[0032]
Thus, the angle ξ at which the light beam diffracted by the chirped diffraction grating 16 exits from the prism portion 22 can be obtained. N ₀ Is the refractive index of the outside world (in this embodiment, since it is air, n ₀ = 1), n is the refractive index of the prism portion 22. From the equation (1), it is possible to obtain a condition that the emission angle ξ does not change even if the temperature of the diffraction grating block 14 changes.
[Expression 10]

[0033]
It is expressed. α is a linear expansion coefficient of the prism portion 22. It can be seen that this equation does not depend on the incident / exit angles φ and ξ to the prism portion 22. In particular, the incident angle and the exit angle of the light beam are made equal to the incident surface of the prism portion 22.
[Expression 11]

[0034]
That is,
[Expression 12]

[0035]
In the case of (2),
[Formula 13]

[0036]
It is expressed. Furthermore, in the Littrow arrangement where ζ≈0,
[Expression 14]

[0037]
It can be seen that
[0038]
In general, between the refractive index n and density ρ of a substance,
[Expression 15]

[0039]
It is known that there is a relationship. However, (beta) shows a local field correction coefficient and is dependent on a substance. In general, β takes a value between 0 and 1/3. Using this equation, between the refractive index temperature change and the linear expansion coefficient α,
[Expression 16]

[0040]
It is shown that there is a relationship. Therefore, from equation (2)
[Expression 17]

[0041]
If n, β, and γ of the diffraction grating block 14 are selected so as to satisfy the above, the emission angle ξ does not depend on the temperature. Especially in the Littrow arrangement generally used for spectroscopes and the like, from the equation (6),
[Formula 18]

[0042]
That means
[Equation 19]

[0043]
What is necessary is just to select n and β that satisfy.
Since β normally takes a value in the range of 0 to 1/3, it is desirable to select the refractive index n in the range of 1.414 to 1.732 in the Littrow arrangement from the equation (10). For polymer materials, β often takes a value close to 1/3. In this case, it can be said that it is desirable to take a value in the vicinity of a refractive index of 1.414. Polymer materials having a refractive index close to this value include polytrifluoroethylene methacrylate (refractive index 1.42), silicone polymer (refractive index 1.43), cyclohexane-based fluorinated epoxy (refractive index 1.405). Fluorinated epoxy acrylate (refractive index 1.413), fluorinated epoxy acrylate (refractive index 1.417) glycol-based fluorinated epoxy (refractive index 1.385), fluorinated epoxy acrylate (refractive index 1.416), There are fluorinated acrylic resins such as fluorinated epoxy acrylate (refractive index 1.418) and trifluoroethyl methacrylate resin (refractive index 1.40). If these materials are used to form the diffraction grating block 14, a Littrow arrangement is used. The temperature dependence of wavelength demultiplexing characteristics can be reduced. Thus, a refractive index material of 1.385 to 1.43 can be used. When using an optical communication wavelength of 1.3 μm band or 1.55 μm band, it is desirable to use a fluorine-based material with less loss at infrared wavelengths.
In equation (8), the right side denominator can be set larger or smaller than 1 by appropriately selecting γ, i, and θ. As shown in the present embodiment, when the incident angle ψ is larger than the outgoing angle ξ, this value is smaller than 1. Therefore, even if a material having a refractive index larger than 1.414 is used, equation (8) Can be met. In this case, fluorinated plastics such as polymethyl methacrylate (PMMA, refractive index 1.49), polycarbonate (PC, refractive index 1.59), fluorinated polyimide (refractive index 1.55), deuterated The plastic material etc. which were made can also be used and the material suitable for shaping | molding can be selected.
[0044]
On the other hand, when the incident angle ψ is smaller than the outgoing angle ξ, it is desirable to use a low refractive index material, which is a propylene hexafluoride / 4 ethylene fluoride / 2 vinylidene fluoride copolymer (refractive index 1.. 36), Teflon AF (manufactured by DuPont, refractive index 1.29) and Cytop (manufactured by Asahi Glass Co., Ltd., refractive index 1.34) can reduce loss even with infrared light. If the output angle is increased as in this case, the wavelength dispersion of the diffraction grating can be increased, so that the demultiplexer can be reduced. Therefore, it can be said that it is desirable to use a material having a refractive index of 1.29 or more and 1.414 or less as a realistic polymer in terms of miniaturization of the demultiplexer.
[0045]
As described above, in the embodiment, by using the prism portion 22, the change in the wavelength separation characteristic due to the thermal expansion and contraction of the chirped diffraction grating 18 is caused by the change in the wavelength separation characteristic due to the change in the refractive index of the prism portion 22. Supplement Can be compensated. Therefore, it is possible to suppress a change in wavelength separation characteristics accompanying a change in temperature, and to realize so-called athermalization.
[0046]
In the case of a diffraction grating using a parallel plate without the prism portion 22, that is, β = 0,
[Expression 20]

[0047]
Thus, the emission angle ξ does not depend on the refractive index n of the prism portion. Therefore, there is no effect of compensating for the characteristic change due to thermal expansion and contraction of the diffraction grating. That is, the incident / exit surface of the diffraction grating block 22 needs to form an angle with respect to the chirped diffraction grating 16 surface. If a light beam enters and exits at a large angle with respect to the incident / exit surface of the diffraction grating block 22, surface reflection increases, which is not preferable. Therefore, it is desirable that the angle γ of the entrance / exit surface of the diffraction grating block 22 is between the incident angle and the exit angle to the entrance / exit surface of the diffraction grating block 22. In other words, depending on the size of the incident angle and the outgoing angle,
[Expression 21]

[0048]
Or
[Expression 22]

[0049]
It is desirable to satisfy.
[0050]
Up to this point, we have explained temperature compensation for cases where the coefficient of thermal expansion of the base is small and the characteristic variation due to thermal expansion and contraction of the diffraction grating is dominant. When the relative position fluctuates, in addition to the diffraction grating, the thermal expansion and contraction of the base is taken into consideration, and the parameters may be set so as to eliminate the temperature dependence of the entire demultiplexer. In the arrangement of this embodiment, the thermal expansion and contraction of the base and the thermal expansion and contraction of the diffraction grating move in a direction to compensate each other. Therefore, by using a material having a high thermal expansion coefficient for the base, the refractive index is larger than 1.414. Even when a material is used, the temperature dependence can be reduced by the prism portion 22. Therefore, even when a material having a high thermal expansion coefficient such as a polymer material is used for the base 38, the temperature can be compensated.
[0051]
Further, in the case where fluctuation of the condensing position due to thermal expansion and contraction of the diffraction grating is allowed to some extent, the expression (8) may not be sufficiently satisfied, and within a range in which a practical temperature compensation effect can be achieved. Select a parameter.
[0052]
In the present embodiment, a so-called Eschlet type in which the cross-sectional shape of the grating grooves is a right triangle is used as the chirped diffraction grating 16. To create the diffraction grating, a master for forming the chirped diffraction grating 16 was first processed using a ruling engine. A mold was created using this master, and the diffraction grating block 14 was injection molded using the mold. The grating constant of the chirped diffraction grating 16 was chirped as a function of the grating location so as to have the desired light collection performance. In order to increase the diffraction efficiency, it is desirable to change the blaze angle in accordance with the incident angle at each grating position. Therefore, in this embodiment, the diffraction grating is divided into a plurality of regions, processed at a constant blaze angle in the region, and the diffraction efficiency is improved by changing the blaze angle for each region. The blaze angle was determined so that the difference in diffraction efficiency between TE-polarized light (polarized light with the electric field direction perpendicular to the grating line) and TM-polarized light (polarized light with the electric field direction perpendicular to the marked line) was small for each region. Therefore, at the central portion of the region, the blaze angle in the specular reflection condition in which the direction in which the incident light beam 70 is specularly reflected on the blaze surface and the diffraction direction of the outgoing light beam 72 are made coincident is set. In this example, the blaze angle was increased by 2 to 10 ° from the blaze angle determined from the specular reflection condition at the center of the region. This difference is desirably increased as the lattice constant is decreased.
[0053]
In the Eschlett type diffraction grating, when the wavelength λ and the grating constant d are selected, if the λ / d is selected from 0.6 to 1.0 and the blaze angle is selected from the range of 25 ° to 35 °, the polarization dependence is small and the diffraction efficiency is reduced. Can be bigger. in this case,
[Expression 23]

[0054]
It is necessary to. Therefore, according to the equation (4), the angle γ of the prism incident surface is desirably 17 ° or more and 30 ° or less.
Furthermore, since the difference in diffraction efficiency between TE-polarized light and TM-polarized light increases as the lattice constant decreases, the diffraction order is selected so that this difference is small and the diffraction efficiency is high. In this example, the lattice constant was doubled discontinuously from the middle so that -1st order diffracted light was used in a region having a large lattice constant, and -2nd order diffracted light was used in a region having a small lattice constant. Thus, when the lattice constant is small, it is desirable to use higher-order diffracted light, and it is desirable to use second-order or higher-order diffracted light with a lattice constant that is approximately equal to or less than the incident wavelength. By switching the order at the blaze angle switching position, the grating constant can be switched without increasing the number of divisions of the diffraction grating. In this embodiment, the number of area divisions is six. The greater the number of divisions, the closer to the ideal blaze angle distribution. The relationship between the number of area divisions and the loss was examined, and it was found that when the number of divisions was 4 or more, the loss decreased, and when 9 or more, the loss was almost constant. Therefore, even if the number of divisions is increased, the loss is not significantly reduced beyond a certain number of divisions. Moreover, if the area is increased too much, the number of times of changing the blaze angle increases, which may reduce the processing accuracy and reduce the loss. Therefore, the number of divisions is preferably 4-9, particularly 6-9. In the present embodiment, since the diffraction grating grooves are processed on a plane, positional deviations in the plane direction and the vertical direction at the connecting portions of the regions hardly occur in the processing of the diffraction grating grooves.
[0055]
Thus, by making the diffraction efficiencies of TE polarized light and TM polarized light substantially equal, even if the polarization state of incident light changes, fluctuations in the detected light quantity can be reduced. When specific polarized light enters from the optical fiber 10, the blaze angle may be determined so that the diffraction efficiency of this polarized light is maximized. Or you may align the polarization direction of the emitted light from an optical fiber using a polarization conversion element. The polarization conversion element can be configured by combining a polarization beam splitter and a λ / 2 plate as is generally known. In this case, the direction of the polarization conversion element may be determined so that the polarization direction has high diffraction efficiency. In particular, TM polarized light can increase the diffraction efficiency in a region where the grating pitch is approximately smaller than the wavelength. Therefore, it is desirable to make the grating constant smaller than the wavelength and convert it into TM polarized light before entering. In this case, since the diffraction efficiency hardly depends on the shape of the grating groove, a high diffraction efficiency can be realized not only with the echelette grating but also with a wave shape or a rectangular binary diffraction grating.
[0056]
In addition to the ruling engine used in the present embodiment, the grating pattern can be processed by, for example, irradiating the photosensitive member with a laser beam from the incident position and the emitting position to expose the interference pattern and forming a hologram. Good. By using the hologram diffraction grating, the light can be condensed while being sufficiently narrowed at the emission position with little aberration. Alternatively, it may be processed by using a three-dimensional NC processing machine, and further, a chirped diffraction grating continuously blazed on a substrate of Si or the like can be created by using a photolithography technique.
[0057]
For the processing of the chirped diffraction grating 16 and the sub-wavelength grating 20, a hot press may be used.
[0058]
In the case where the incident angle changes depending on the position of the diffraction grating as in this embodiment, the principal ray having the strongest light intensity may be set so as to satisfy the temperature compensation condition. The diffraction grating is not limited to the planar chirped diffraction grating used in the present embodiment, but can be similarly applied to an equidistant planar diffraction grating or a concave diffraction grating having a constant lattice constant. When the concave diffraction grating is used, the incident point of the optical fiber 10 and the incident point of the slab waveguide 32 are arranged on a so-called Roland circle of the concave diffraction grating 15 so that the condensing point distance of each wavelength is reduced. Change can be reduced. In that case, the slab waveguide section 30 is not necessary.
[0059]
In addition, aberration occurs when light rays pass through the prism portion 22, but the grating pitch of the diffraction grating may be corrected so as to correct the aberration. The grid layout can be designed by automatic design using ray tracing calculation. Or you may correct | amend using an aspherical cylindrical lens.
In this embodiment, a cylindrical lens 12 is used as the cylindrical lens 12. In the case of the arrangement close to the Littrow arrangement, a single cylindrical lens may be used for condensing incident and outgoing rays.
[0060]
In the present embodiment, the four wavelengths of the center wavelengths 1275.7, 1300.2, 1324.7, and 1349.2 nm that have been transmitted after being multiplexed are separated. With respect to this central wavelength, the maximum fluctuation of ± 5.7 nm was assumed due to wavelength variations due to the manufacture of the light source, temperature, and spectral distribution. Therefore, the wavelength space between adjacent channels is 13.1 nm, and is set to approximately 1% of the transmission wavelength. This wavelength variation is determined by the creation and characteristics of the light source, and is necessary as a range that does not require strict selection of the light source. In this example, it was possible to realize a sufficient reduction in characteristic fluctuation in this wavelength range.
[0061]
The light receiving optical waveguide 28 includes the slab waveguide portion 30 and the tapered waveguide portion 32 as described above. The thickness of the optical waveguide was 65 μm so that the signal light was efficiently coupled to the photodetector. The tapered waveguide portion 32 has a tapered shape in which the width of the core decreases from the light incident side to the light emitting side in the paper surface, and has a structure for narrowing the light to the photodetector array. In order to efficiently propagate the beam to the photodetector array, it is desirable that the numerical aperture NA of the slab waveguide is large, and it is particularly desirable that NA is 0.5 or more. Further, when a metal film is formed outside the thin clad, even if light passes through the clad, it is reflected by the metal film and reaches the photodetector, so that the detection efficiency can be improved. In the present embodiment, the width of the tapered waveguide portion 32 is the same as the pitch of the photodetector array on the side joined to the slab waveguide 30, and can be efficiently coupled to the photodetector on the photodetector array 60 side. It became width. In this manner, by narrowing down the wavelength-separated beam to the photodetector, a photodetector with a fast response speed and a small light receiving area can be used, and can be applied to high-speed optical communication. The position of the photodetector array was adjusted so that the beam from the tapered waveguide portion 32 was efficiently coupled to the photodetector, and the photodetector array was bonded and fixed to the tapered waveguide portion 32.
[0062]
In this example, a graded index (GI) multimode optical fiber (62.5 MMF) having a core diameter of 62.5 μm and a numerical aperture of 0.275 was used as the optical fiber 10. However, the optical fiber is not limited, and a single mode optical fiber (SMF), a multimode optical fiber (50 MMF) having a core diameter of 50 μm, and a multimode optical fiber having a larger core diameter can also be used.
[0063]
A second embodiment of the present invention will be described with reference to FIG. FIG. 5 is a top view showing an example of the diffraction grating block according to the present embodiment. The function of the diffraction grating block of this example is the same as that of the diffraction grating block 14 shown in the first embodiment, and different points will be described here. First, the diffraction grating substrate 24 having the chirped diffraction grating 16 is injection molded using a polymer material. A metal film is formed on the chirped diffraction grating 16 to form a reflective film 18. Separately, the prism portion 22 having the sub-wavelength grating 20 is also formed. The diffraction grating substrate 24 and the prism portion 22 are bonded using an optical adhesive 19 to form the diffraction grating block 14. By using the same material or a material having the same linear expansion coefficient for the prism portion 22 and the diffraction grating substrate 24, the linear expansion coefficient of the chirped diffraction grating 16 can be made equal to that of the prism portion 22.
[0064]
Alternatively, by using materials having different linear expansion coefficients for the prism portion 22 and the diffraction grating substrate 24, the linear expansion coefficient of the chirped diffraction grating 16 can be adjusted to satisfy the expression (2).
[0065]
In this embodiment, since the chirped diffraction grating 16 is formed on the flat diffraction grating substrate 24, it can be easily and accurately formed using injection molding. In addition to injection molding, a so-called 2P method is used to apply a UV curable resin on a flat substrate and irradiate the UV light while pressing the mold to cure the UV curable resin to form a chirped diffraction grating 16. Also good. By using the transparent diffraction grating substrate 24, it can be cured by irradiating ultraviolet rays from the substrate side. Even when the 2P method is used, the linear expansion coefficient of the diffraction grating can be made equal to that of the prism portion 22 by making the layer of the ultraviolet curable resin thin, and the characteristic change due to thermal expansion and contraction of the chirped diffraction grating 16 can be changed. It can be compensated by changes.
[0066]
When the diffraction grating block 14 of this example is fixed to the base 38 as shown in the first embodiment, the base 38 may also be made of a polymer, and the prism portion 22 may be formed integrally with the base 38. The diffraction grating substrate 24 may be bonded to the prism portion 22 formed integrally with the base 38. By integrating the prism portion 22 with the base 38, the assembly of the demultiplexer is facilitated, and the positional deviation of the diffraction grating block 14 does not occur, and the reliability is improved.
[0067]
A third embodiment of the present invention will be described with reference to FIG. FIG. 6 is a top view showing an example of the demultiplexer according to the present embodiment. The wavelength-division multiplexed optical signal that has been emitted from the optical fiber 10 becomes parallel light in the direction perpendicular to the paper surface by the cylindrical lens 13 a integrated with the diffraction grating block 14, and enters the diffraction grating block 14. The light beam incident on the diffraction grating block 14 is diffracted at different angles for each wavelength by the chirped diffraction grating 16 formed on the diffraction grating block 14 and condensed. The light beam diffracted by the chirped diffraction grating 16 is condensed by the cylindrical lens 13 b and enters the light receiving optical waveguide 28. The light beam incident on the slab waveguide portion 30 of the light receiving optical waveguide 28 is collected at the incident end of the tapered waveguide portion 32 for each wavelength and detected by the photodetector array 36 having the light receiving portion for each wavelength.
[0068]
In this embodiment, the diffraction grating block 14 and the cylindrical lenses 13a and 13b are integrated and formed using a transparent polymer. The principal ray of the incident ray 70 from the optical fiber 10 is made substantially perpendicular to the cylindrical lens 13a, and the principal ray of the outgoing ray 72 diffracted by the chirped diffraction grating 16 is made substantially perpendicular to the cylindrical lens 13b. Therefore, if the temperature change of the linear expansion coefficient α of the chirped diffraction grating 16 and the refractive index of the diffraction grating block 14 satisfies the equation (6), the wavelength separation characteristic can be prevented from changing with respect to the temperature change. It is. Therefore, from the formula (10), it is desirable to use a polymer material having a refractive index of around 1.414.
[0069]
As in this embodiment, the light beam is incident on the chirped diffraction grating 16 through the prism portion 22, so that the beam divergence angle from the optical fiber is reduced by the refractive index of the prism portion as compared with air. That is, when a material having a refractive index of 1.4 is used, the beam divergence angle is reduced to about 71%. Therefore, irradiating the beam through the prism portion 22 has an effect of reducing the divergence angle of the beam incident on the chirped diffraction grating 16. As a result, the width of the diffraction grating is reduced, and the entire demultiplexer can be reduced in size.
[0070]
A fourth embodiment of the present invention will be described with reference to FIG. FIG. 7 is a top view showing an example of the demultiplexer according to this embodiment. The optical signal that has been wavelength-multiplexed and transmitted through the optical fiber 10 enters the single-mode slab waveguide 40 in the optical confinement direction from the single-mode optical fiber 10 and enters the concave diffraction grating 15. The diffraction grating 15 diffracts and separates at different angles for each wavelength and collects the light for each wavelength. The separated and collected light enters the tapered waveguide section 32 and is detected by the photodetector array 60.
[0071]
A transparent polymer is used for the core of the slab waveguide 40, and the diffraction grating 15 is formed at the end. A metal film made of gold, silver, or the like was formed on the surface of the diffraction grating 16 as a reflection surface.
In this embodiment, since light rays propagate through the slab waveguide 40, the temperature change of the refractive index of the slab waveguide 40 core and the linear expansion coefficient α of the substrate used for the clad satisfy the equation (6). The temperature dependence can be eliminated. Therefore, as shown in the first embodiment, it is desirable to use a low refractive index material for the core, and it is desirable that the linear expansion coefficient α of the cladding portion is close to that of the core material.
[0072]
When the concave diffraction grating 15 is used as in this embodiment, the incident point of the optical fiber 10 and the incident point of the slab waveguide 32 are arranged on a so-called Roland circle of the concave diffraction grating 15. The change in the focal point distance for each wavelength can be reduced.
[0073]
A fifth embodiment of the present invention will be described with reference to FIG. FIG. 8 is a top view of the demultiplexer according to the present embodiment. The wavelength multiplexed optical signal that is emitted from the optical fiber 10 becomes substantially parallel light through the lens 50 a and enters the prism portion 22 of the diffraction grating block 14. The light beam incident on the diffraction grating block 14 is incident on the planar diffraction grating 15 and is diffracted with a different diffraction angle for each wavelength. The beam diffracted by the diffraction grating 15 is collected by the lens 50 b, enters the tapered waveguide 35 for each wavelength, and is detected by the photodetector array 24.
[0074]
In the present embodiment, an equally spaced diffraction grating having parallel linear grating grooves is used as the diffraction grating 15. Since substantially parallel rays are incident on the diffraction grating 15, the expression (8) can be satisfied over the entire range of the diffraction grating 15, and temperature dependence can be eliminated over the entire range of the diffraction grating 15. In the present embodiment, since the condensing points are arranged substantially perpendicular to the emitted light beam 72, the light receiving optical waveguide 28 does not need a slab waveguide portion. As the lens 50, a rod lens having a refractive index distribution may be used. In that case, the optical fiber 10 can also be fixed to the rod lens. Moreover, it is good also as arrangement | positioning close to a Littrow, and it is good also as one lens 50. FIG.
[0075]
A sixth embodiment of the present invention will be described with reference to FIGS. FIG. 9 is a top view showing an example of the demultiplexer according to the present embodiment. The wavelength multiplexed signal emitted from the optical fiber 10 becomes parallel light in the direction perpendicular to the paper surface by the cylindrical lens 12 a and enters the chirped diffraction grating 16 fixed to the fixed frame 52. The light beam diffracted and collected by the chirped diffraction grating 16 is collected in the direction perpendicular to the paper surface by the cylindrical lens 12 b and enters the light receiving optical waveguide 28. The light beam incident on the slab waveguide portion 30 is condensed on the incident end of the tapered waveguide provided for each wavelength by the condensing action of the chirped diffraction grating 16. Light rays separated and condensed for each wavelength are incident on a tapered waveguide portion 32 corresponding to each wavelength, and detected by a photodetector array 36 having a light receiving portion for each wavelength.
[0076]
The fixed frame 52 and the diffraction grating substrate 24 will be described in more detail with reference to FIG. The diffraction grating substrate 24 having the chirped diffraction grating 16 has wedge-shaped grooves 57a and 57b, and protrusions 56a and 56b are formed on the fixed frame 52 side. The protrusion 56a, the groove 57a, and the protrusion 56b. The diffraction grating substrate 24 is fixed to the fixed frame 52 by being pressed by a presser 54. The chirped diffraction grating 16 is accurately positioned with respect to the fixed frame 52 by the grooves 57a and 57b and the protrusions 56a and 56b. The fixed frame 52 to which the diffraction grating substrate 24 is fixed is fixed to a fixed groove 39 provided in the base 38. By making the protrusion 56 come on the reference point 37 of the fixed groove 39, the diffraction grating substrate 24 can be thermally expanded and contracted on both sides with the reference point 37 as the center. The position of the groove portion 57 is shifted from the center of the diffraction grating (position where the principal ray from the optical fiber hits), and the diffraction grating substrate 24 is moved in the direction opposite to the spot moving direction due to temperature change. A change in the condensing position due to thermal expansion / contraction of the grating substrate 24 can be compensated by using the center position movement of the diffraction grating substrate 24.
[0077]
In order to compensate the movement of the focused spot in the x direction due to the thermal expansion / contraction of the diffraction grating substrate 24 by the movement due to the thermal expansion / contraction of the diffraction grating substrate 24,
[Expression 24]

[0078]
The distance L between the center of the chirped diffraction grating 16 and the groove 57 may be determined so that In this embodiment, the linear expansion coefficient α = 6 × 10 ^-5 Since the spot movement rate due to thermal expansion / contraction of the diffraction grating substrate 24 was 1 μm / K using / K polycarbonate, L = 8.3 mm.
[0079]
In this way, by shifting the center of the chirped diffraction grating 16 and the center of thermal expansion and contraction, the position change of the condensing point due to the thermal expansion and contraction of the diffraction grating is changed, and the center position is moved by the thermal expansion and contraction of the chirped diffraction grating 16. Can compensate. In this case, the planar diffraction grating moves substantially parallel to the grating surface and compensates for characteristic changes accompanying temperature changes. Since temperature compensation is performed by utilizing the thermal expansion and contraction of the diffraction grating itself, it is easy to follow the temperature change and can compensate for a sudden temperature change.
[0080]
By forming the diffraction grating substrate 24 using injection molding, the groove 57 can also be formed when the diffraction grating substrate 24 is formed, and the position of the groove 57 can be formed with high accuracy.
[0081]
In the present embodiment, as the diffraction grating substrate 24 expands and contracts, the spot position also moves in the z direction. However, in the z direction, the characteristic change caused by the spot movement is small, and the characteristic change due to this movement has not been a problem.
[0082]
In the present embodiment, a prism may be used as in the first embodiment, and may be combined with temperature compensation by changing the refractive index of the prism.
[0083]
A seventh embodiment of the present invention will be described with reference to FIG. FIG. 11 is a top view showing an example of the demultiplexer according to the present embodiment. The wavelength multiplexed signal that has been emitted from the optical fiber 10 is converted into substantially parallel light in the direction perpendicular to the paper surface by the cylindrical lens 12 a and enters the chirped diffraction grating 16. The light beams diffracted by the chirped diffraction grating 16 are collected at different positions for each wavelength in the paper. The light beam diffracted and collected by the chirped diffraction grating 16 is condensed by the cylindrical lens 12 b on the incident end face of the light receiving optical waveguide 28 in the direction perpendicular to the paper surface and enters the light receiving optical waveguide 28. The light beam incident on the slab waveguide portion 30 of the light receiving optical waveguide 28 is propagated by being confined in the slab waveguide in the direction perpendicular to the paper surface, and in the paper surface by the condensing action of the chirped diffraction grating 16 for each wavelength. The light is condensed at the incident end of the taper waveguide provided in FIG. Light rays separated and condensed for each wavelength are incident on a tapered waveguide portion 32 corresponding to each wavelength, and detected by a photodetector array 36 having a light receiving portion for each wavelength.
[0084]
In the present embodiment, the diffraction grating substrate 24 is moved using the thermal expansion and contraction of the driving units 60a, 60b, and 60c whose one ends are fixed to the fixing units 62a, 62b, and 62c, and the temperature characteristics due to the thermal expansion and contraction of the diffraction grating substrate 24 Compensate for variations. In this embodiment, the driving unit 60a moves the diffraction grating substrate 24 in parallel to the grating surface, and the driving units 60b and 60c move the diffraction grating substrate 24 in parallel to the grating surface to perform temperature compensation. Yes. Therefore, spot movement in the z direction can also be compensated. The temperature can be compensated more accurately by moving the diffraction grating two-dimensionally in parallel as in this embodiment.
[0085]
An eighth embodiment of the present invention will be described with reference to FIGS. FIG. 12 is a top view of the demultiplexer according to the present embodiment. The wavelength-division multiplexed signal emitted from the optical fiber 10 becomes substantially parallel light in the direction perpendicular to the paper surface by the cylindrical lens 12 a and enters the chirped diffraction grating 16. The light beams diffracted by the chirped diffraction grating 16 are collected at different positions for each wavelength in the paper. The light beam diffracted and collected by the chirped diffraction grating 16 passes through the temperature change compensating prism 58, and is collected by the cylindrical lens 12b on the incident end surface of the light receiving optical waveguide 28 in the direction perpendicular to the paper surface. The light enters the optical waveguide 28. The light beam incident on the slab waveguide portion 30 of the light receiving optical waveguide 28 is propagated by being confined in the slab waveguide in the direction perpendicular to the paper surface, and in the paper surface by the condensing action of the chirped diffraction grating 16 for each wavelength. The light is condensed at the incident end of the taper waveguide provided in FIG. Light rays separated and condensed for each wavelength are incident on a tapered waveguide portion 32 corresponding to each wavelength, and detected by a photodetector array 36 having a light receiving portion for each wavelength.
[0086]
The temperature compensation by the prism 58 will be described with reference to FIG. The present embodiment is characterized in that the characteristic change due to thermal expansion and contraction of the chirped diffraction grating 16 is compensated by the refractive index change of the prism 58. When the refractive index of the prism 58 is N and the incident / exit angle to the chirped diffraction grating 16 and the incident / exit angle to the prism 58 are expressed as shown in FIG. The condition for compensating for the change in the diffraction angle received by the chief ray from the light is the temperature change in the refractive index of the prism 58.
[Expression 25]

[0087]
In Case of,
[Equation 26]

[0088]
And it is sufficient. Expression (13) indicates a condition in which the emission angle ψ of the principal ray from the prism 58 does not vary with the expansion and contraction of the diffraction grating substrate 24. By selecting the material and angle γ of the prism 58 and the incident angle i to the diffraction grating so that the linear expansion coefficient α of the diffraction grating substrate 24 satisfies the expression (12) or (13), the temperature dependence Sex can be lost. When the same material is used for the diffraction grating substrate 24 and the prism 58, from the equation (7),
[Expression 27]

[0089]
The angle γ of the prism 58 and the incident angle i to the diffraction grating may be determined so as to satisfy the above.
[0090]
When the prism 58 is used, the emission direction of the principal ray from the optical fiber and the direction of the principal ray for light reception can be made substantially parallel, and the effect of facilitating the arrangement of components can be obtained. In particular,
[Expression 28]

[0091]
Thus, when γ is selected, the emission direction from the optical fiber and the light reception direction can be made substantially parallel, which is desirable. In this example, N = 1.49, i = 35 °, and θ = 18 °, so that γ = 28.7 °.
[0092]
Furthermore, as in this embodiment, the width of the outgoing beam is smaller than the incident side. Also By making the incident surface of the prism 58 oblique to the principal ray so as to spread, the prism 58 also has an effect of widening the distance between the light receiving spots of each wavelength. Therefore, wavelength separation can be performed using a diffraction grating having a smaller wavelength dispersion than when no prism is used. For example, even if a diffraction grating having a wide grating pitch is used as the chirped diffraction grating 16, the wavelength can be sufficiently separated. become. Therefore, the production of the diffraction grating is facilitated, and the diffraction efficiency can be increased.
[0093]
Furthermore, when the prism 58 is used as in the present embodiment, the wavelength pitch of the light receiving unit can be finely adjusted by rotating the prism 58.
[0094]
Up to this point, the wavelength demultiplexing optical device according to the present invention has been described as being wavelength-separated. However, the wavelength demultiplexing optical device according to the present invention can be used as an optical multiplexing device by providing a light source array on the side where the photodetector is disposed. Can also be used. When used in a multiplexer, since temperature compensation is performed, a light source having excellent temperature stability is desirable, and a DFB laser is desirably used. Further, it is desirable to provide a taper formed so as to narrow the light before the optical fiber 10.
[0095]
The present invention can also be used for a demultiplexer in so-called DWDM with a narrow wavelength interval. By reducing the temperature dependence, the characteristics are stable without controlling the temperature of the demultiplexer, and highly reliable wavelength separation can be achieved. Since DWDM has a narrow wavelength interval and particularly requires high characteristic stability with respect to temperature, the effect of the present invention is great even for a diffraction grating using a material having a thermal expansion coefficient smaller than that of a polymer, such as a glass substrate. .
[0096]
A ninth embodiment of the present invention will be described with reference to FIG.
[0097]
FIG. 14 is a configuration diagram illustrating an example of a wavelength division multiplexing module according to the present embodiment. The transmission optical fibers 74 a and 74 b are connected to the optical transmission module 80 by a connector 96 and are abutted with the optical fiber 10 and the single mode optical fiber 11, respectively. The wavelength multiplexed signal transmitted through the receiving transmission optical fiber 74a is incident on the multimode optical fiber 10 abutted on the transmission optical fiber 74a, and is separated for each wavelength by the demultiplexer 82 of the present invention. It is detected by the detector array 36. The detected signal is amplified by the receiving circuit 86 and shaped in waveform. Further, the parallel-serial conversion circuit 90 converts the wavelength-multiplexed signal transmitted into a serial signal and outputs it. In the case of transmission, the input signal is separated by the serial-parallel conversion circuit 92, and each light source of the laser diode array 94 is driven for each signal by the transmission circuit 88 to obtain an optical signal having a different wavelength. Each optical signal is multiplexed by a wavelength multiplexing device 84 constituted by a single mode optical coupler and output to the single mode optical fiber 11. The single mode optical fiber 11 is coupled to the transmission optical fiber 74b for transmission and transmitted. Since the single mode optical fiber 11 and the multimode optical fiber 10 are used to couple to the transmission optical fibers 74a and 74b, the transmission optical fibers 74a and 74b may be either a multimode fiber or a single mode fiber. Can do. In this embodiment, since a GI type multimode fiber having a core diameter of 62.5 μm is used for the optical fiber 10, a single mode optical fiber, multimode light having a core of 50 μm and 62.5 μm is used as the transmission optical fibers 74 a and 74 b. An optical fiber having a core diameter and a numerical aperture equal to or smaller than those of the optical fiber 10 such as a fiber can be used.
[0098]
The optical fiber 10 may be a transmission fiber. In that case, the optical fiber 10 may be detachable, and the optical fiber 10 may be brought into contact with a predetermined position by detachment.
[0099]
In the present example, the case where the demultiplexer 82 shown in the first embodiment is used has been described. However, by using the demultiplexer of the present invention, an optical transmission module in which characteristic fluctuation due to temperature change is reduced is realized. It is something that can be done.
[0100]
In the present embodiment, the wavelength multiplexing transmission module having both transmission and reception functions has been described. However, the effect of the present invention can also be obtained as a wavelength multiplexing reception optical module having only a receiving section.
[0101]
【The invention's effect】
As described above, the demultiplexer of the present invention makes it possible to compensate for fluctuations in characteristics due to thermal expansion and contraction of a diffraction grating using a diffraction grating block having a diffraction grating or a refractive index change accompanying a temperature change of an optical member. The demultiplexer has excellent temperature followability.
[0102]
In addition, the demultiplexer according to the present invention makes it possible to compensate for fluctuations in characteristics due to thermal expansion and contraction of the diffraction grating by moving the diffraction grating in parallel. Even when a chirped diffraction grating is used, it can follow the temperature change. Added demultiplexer to compensate.
[Brief description of the drawings]
FIG. 1 is a top view of a demultiplexer according to a first embodiment of the present invention.
FIG. 2 is a side view of the diffraction grating block according to the first embodiment of the present invention.
FIG. 3 is a perspective view of a sub-wavelength grating provided in the diffraction grating block according to the first embodiment of the present invention.
FIG. 4 is a diagram illustrating a temperature compensation function of the diffraction grating block according to the first embodiment of the present invention.
FIG. 5 is a top view of a diffraction grating block according to a second embodiment of the present invention.
FIG. 6 is a top view of a demultiplexer according to a third embodiment of the present invention.
FIG. 7 is a top view of a demultiplexer according to a fourth embodiment of the present invention.
FIG. 8 is a top view of a demultiplexer according to a fifth embodiment of the present invention.
FIG. 9 is a top view of a demultiplexer according to a sixth embodiment of the present invention.
FIG. 10 is a perspective view of a fixed frame and a diffraction grating substrate according to a sixth embodiment of the present invention.
FIG. 11 is a top view of a demultiplexer according to a seventh embodiment of the present invention.
FIG. 12 is a top view of a demultiplexer according to an eighth embodiment of the present invention.
FIG. 13 is a diagram for explaining temperature compensation according to an eighth embodiment of the present invention.
FIG. 14 is a configuration diagram of a wavelength division multiplexing transmission module according to a ninth embodiment of the present invention.
[Explanation of symbols]
10 Optical fiber
11 Single mode optical fiber
12a, 12b Cylindrical lens
14 Diffraction grating block
15 Diffraction grating
16 Chirped diffraction grating
18 Reflective film
19 Optical adhesive
20 subwavelength gratings
21 square pyramid
22 Prism section
24 Diffraction grating substrate
26 Optical block
28 Optical waveguide for light reception
30 Slab waveguide section
32 Tapered waveguide section
36 Photodetector array
37 Reference point
38 base
39 Fixed groove
40 Slab waveguide
42 Lower substrate
44 Upper substrate
46 core
48 clad
50a, 50b lens
52 Fixed frame
54 Presser
56a, 56b Projection
57a, 57b Groove
58 prism
60a, 60b, 60c drive unit
62a, 62b, 62c fixed part
70 Incident rays
72 Emission light
74a, 74b Transmission optical fiber
80 Optical transmission equipment
82 Wavelength demultiplexing device
84 Wavelength multiplexing device
86 Receiver circuit
88 Transmitter circuit
90 Parallel-serial conversion circuit
92 Serial-parallel conversion circuit
94 Laser diode array
96 Optical connector

Claims (4)

A light incident part, a diffraction grating, an optical path for transmitting a wavelength-multiplexed light beam from the light incident part to the diffraction grating, and a photodetector;
In a demultiplexer that irradiates the light beam to the diffraction grating and separates an optical signal,
The optical path between the diffraction grating and the photodetector has a prism that is separated from the diffraction grating and the photodetector,
A demultiplexer characterized in that a characteristic change due to a temperature change of the diffraction grating is compensated by a refractive index change of the prism . The demultiplexer according to claim 1 , wherein
A demultiplexer characterized in that a light beam is refracted by the prism so that a light beam emitted from the prism is wider than a light beam incident on the prism. A junction with an optical fiber for signal transmission, a demultiplexer that separates wavelength-multiplexed optical signals, a photodetector array, and amplifies signals from the photodetector array;
In a wavelength multiplex receiving optical module having a receiving circuit for waveform shaping,
The demultiplexer according to claim 1 or 2, wherein the demultiplexer is
A wavelength division multiplexing optical module, wherein the junction and the demultiplexer are connected by a multimode optical fiber. A junction with an optical fiber for signal transmission, a demultiplexer that separates wavelength-multiplexed optical signals, a photodetector array, and amplifies signals from the photodetector array;
In a wavelength division multiplexing optical transmission module having a receiving circuit that shapes a waveform, a light source array, a transmission circuit that drives the light source array, and a wavelength multiplexing device that combines optical signals from the light source array,
The demultiplexer according to claim 1 or 2, wherein the demultiplexer is
The junction and the demultiplexer are connected by a multimode optical fiber,
A wavelength division multiplexing optical transmission module, wherein the junction and the wavelength multiplexing device are connected by a single mode optical fiber.

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