JP3854656B2

JP3854656B2 - Light intensity modulator and light wave distance meter

Info

Publication number: JP3854656B2
Application number: JP04522796A
Authority: JP
Inventors: 坂根敏夫; 隆野口
Original assignee: Sumitomo Osaka Cement Co Ltd
Current assignee: Sumitomo Osaka Cement Co Ltd
Priority date: 1996-03-01
Filing date: 1996-03-01
Publication date: 2006-12-06
Anticipated expiration: 2016-03-01
Also published as: JPH09236783A

Description

【０００１】
【発明の属する技術分野】
本発明は、光強度変調器及び光波距離計にかかり、特に、光導波路を伝播する光（以下、導波光と略す）を変調復調することが可能な光強度変調器、及びその光強度変調器を用いてターゲットまでの距離を計測する光波距離計に関する。
【０００２】
【従来の技術】
図５に本発明の基礎になった従来の導波路型の光強度変調器を用いた高分解光波距離計の例を示す（参考文献：吉田他；Proceedings of 11th Meeting on Lightwave Sensing Technology, LST 11-4,あるいは斎藤他；光学、２２巻３号、Ｐ１４２〜（’９３）等）。
【０００３】
この高分解光波距離計では、周波数Ｗで振幅変調したコヒーレントな光をターゲット６に向けて射出し、ターゲット６からの戻り光を周波数（Ｗ−δＷ）で再度変調（復調）する。この戻り光の位相は、ターゲット６までの距離に対応して変化するので、この位相を測定することによりターゲット６までの距離が求められる。この距離を高分解能で求めるためには変調する周波数Ｗを高周波数にする必要があるが、高周波位相の測定は難しいことが知られている。そこで、上述の様に周波数（Ｗ−δＷ）、即ち電気的に位相測定可能な周波数δＷだけ周波数をずらして復調すると、周波数Ｗでの位相情報が周波数δＷの位相情報に含まれるため測定可能となる。
【０００４】
距離測定の分解能δＬは（１）式で与えられる。
δＬ＝（Ｃ／２Ｗ）・δφ ・・・・（１）
ここでＣは光速度、δφは位相測定誤差である。
【０００５】
従って、変調する周波数Ｗが大きい程、また位相測定誤差δφが小さい程、高い距離分解能が得られる。
【０００６】
詳細には、図５の高分解光波距離計は、光源１として、波長１．３μｍの半導体レーザ（ＬＤ）を用いている。光源１から射出されたレーザ光は光ファイバー２により第１の光強度変調器３に導かれ、周波数Ｗで振幅変調される。光強度変調器３から射出された変調光は光ファイバー４の射出側端部に設けられたコリメータ・レンズ５で平行光束にされ、ターゲット６に向けて射出される。
【０００７】
ターゲット６にはコーナーキューブが用いられ、このターゲット６で再帰反射された戻り光はレンズ７で光ファイバー８に集められ、光ファイバー８により第２の光強度変調器９に導かれ、第２の光強度変調器９において周波数（Ｗ−δＷ）の振幅変調を受ける。この様に復調された光は光ファイバー１３によりホトダイオード１４に導かれ、ホトダイオード１４において光電変換される。
【０００８】
ホトダイオード１４から出力される信号は、ロックインアンプ１５へ信号Ｓｉｇとして入力される。なお、周波数Ｗと周波数（Ｗ−δＷ）はそれぞれ発振器１０、１１で作られ、それぞれ光強度変調器３、９に供給されると共に、ミキサー１２にも供給される。これにより、ミキサー１２では差周波数δＷの信号が生成され、この差周波数δＷの信号はロックインアンプ１５へ参照信号ＲＥＦとして入力される。従って、参照信号ＲＥＦの位相を基準として信号Ｓｉｇの位相を測定すれば、ターゲット６までの距離を求めることができる。ターゲット６までの絶対距離は、変調する周波数Ｗを異なる周波数にして再度同様な測定を行ない、両者の測定位相の差から求める事ができる。
【０００９】
この高分解光波距離計では変調する周波数Ｗとして３．５ＧＨｚ、差周波数δＷとして５０ＫＨｚを用いて、距離２５０ｍに於て、約７μｍの精度を得ている。
【００１０】
このような光強度変調器としては、リチュウムニオベイト（ＬｉＮｂＯ₃：以下、ＬＮと略す）導波路を用いたマッハツェンダー型（以下、ＭＺ型という）の光強度変調器が、変調用と復調用に２台用いられている。
【００１１】
この光強度変調器は、詳細には、図６に示すように、ＬＮ基板１８、ＬＮ基板１８上に形成した直線導波路１９、２１、２２、２４及び電極２５、２６から構成されている。直線導波路１９に入射した光はＹ分岐部２０で分岐されて２つの直線導波路２１、２２に５０：５０に２分される。この直線導波路２１、２２上には対となる電極２５、２６がそれぞれ形成されている。電極２５、２６は導波光を超高周波変調するため進行波型電極であり、これら電極２５、２６の始端側には交流を印加する供給電源２７が接続され、終端側には終端抵抗２８が接続されている。
【００１２】
変調用の場合には、電極２５、２６の間に供給電源２７から変調信号を印加すると直線導波路２１、２２を伝播する導波光の位相が相補的に変化する。供給電源２７からの変調信号によって位相差が生じた導波光はＹ分岐部２３によって直線導波路２４へ合波され、両者の位相差に応じた振幅変調を受けた光として導波路２４から射出される（図６の矢印Ａ方向へ向けて射出される）。
【００１３】
また、復調用の場合も同様であるが、導波光の進行方向が逆方向となる。すなわち、図６の例では矢印Ａ方向と逆方向の光が入射される。従って、電極２５、２６の端部であるＹ分岐部２３側に交流を印加する供給電源２７が接続され、終端側であるＹ分岐部２０側に終端抵抗２８が接続される。この電極間に供給電源２７から復調信号を印加して直線導波路２１、２２を変調用の方向と逆方向に伝播する導波光の位相を変化させる。供給電源２７からの変調信号によって位相差が生じた導波光はＹ分岐部２０によって直線導波路１９へ合波され射出される。
【００１４】
このように、従来の光波距離計では、この光強度変調器を２台用いる。
また、２Ｖπ（Ｖπは半波長電圧）を周期とした正弦波状の強度変調特性を示すＭＺ型の光強度変調器は、透過が最大、あるいは最小の場合を動作点とすると、偶数次の高調波変調が得られるため、より高周波数変調ができ、測定精度を向上できる。
【００１５】
このように、超高周波な光強度変調器を２台用いて光を変調及び復調することによって、高精度な光距離測定が可能である。
【００１６】
【発明が解決しようとする課題】
しかしながら、ＬＮ導波路を用いた超高周波な光強度変調器は、主に光通信用として開発されたものであり、信頼性も高く実用化されているが、上述のように光波距離計を構成するには変調用及び復調用の各々に光強度変調器を必要とするため、装置構成が複雑化して大型化すると共に、組み立て調整等の作業が煩雑になる。また、光強度変調器は高コストであり、従来の光波距離計では変調用及び復調用の各々に光強度変調器を用いているために、汎用的な光波距離計を製造する場合の問題となっている。
【００１７】
本発明は、上記事実を考慮して、単純な構成で光導波路を伝播する導波光を変調復調できる光強度変調器、及びその光強度変調器を用いてターゲットまでの距離を計測する光波距離計を得ることが目的である。
【００１８】
【課題を解決するための手段】
上記目的を達成するために請求項１に記載の発明の光強度変調器は、一方に向かって伝播される導波光を分岐すると共に、該導波光と逆方向に向かって伝播される導波光を合波する第１の分岐部と、前記第１の分岐部に連続しかつ導波光を伝播させる複数本の光導波路部と、前記複数本の光導波路部に連続しかつ該光導波路部を伝播した複数の導波光を合波すると共に、該導波光と逆方向に伝播される導波光を分岐して前記複数本の光導波路部の各々へ導く第２の分岐部と、前記光導波路部に設けられると共に、前記第１の分岐部から第２の分岐部に向かう導波光と同一の伝播方向になるように導波光を変調するための変調帯域上限近傍の予め定めた周波数信号が供給されて前記光導波路部を前記第１の分岐部から第２の分岐部に向かって伝播する導波光を変調する第１の進行波電極と、前記第２の分岐部から第１の分岐部に向かう導波光と同一の伝播方向になるように導波光を変調するための変調帯域上限近傍の予め定めた周波数信号が供給されて前記第２の分岐部から第１の分岐部に向かって伝播する導波光を変調する第２の進行波電極とからなる進行波電極と、を備えている。
【００１９】
請求項２に記載の発明の光強度変調器は、一方に向かって伝播される導波光を分岐すると共に、該導波光と逆方向に向かって伝播される導波光を合波する第１の分岐部と、前記第１の分岐部に連続しかつ導波光を伝播させる複数本の光導波路部と、前記複数本の光導波路部に連続しかつ該光導波路部を伝播した複数の導波光を合波すると共に、該導波光と逆方向に伝播される導波光を分岐して前記複数本の光導波路部の各々へ導く第２の分岐部と、前記光導波路部に設けられると共に、前記第１の分岐部から第２の分岐部に向かう導波光と同一の伝播方向になるように導波光を変調するための変調帯域上限近傍の予め定めた周波数信号が供給されて前記光導波路部を前記第１の分岐部から第２の分岐部に向かって伝播する導波光を変調する第１の進行波電極と、前記第２の分岐部から第１の分岐部に向かう導波光と同一の伝播方向になるように導波光を変調するための変調帯域上限近傍の予め定めた周波数信号が供給されて前記第２の分岐部から第１の分岐部に向かって伝播する導波光を変調する第２の進行波電極とからなる進行波電極と、前記光導波路部に設けられると共に、前記変調の動作点を設定するためのバイアス電極と、を備えている。
【００２０】
請求項３に記載の発明の光波距離計は、請求項１に記載の光強度変調器と、前記光強度変調器へ光を入射させる光源と、前記光強度変調器から射出されかつターゲットから戻された光で前記逆方向に伝播される導波光により前記光強度変調器から射出された光を検出する検出手段と、前記第１の進行波電極へ予め定めた周波数信号を供給する第１の信号供給手段と、前記第２の進行波電極へ前記第１の信号供給手段と異なる周波数信号を供給する第２の信号供給手段と、前記検出した光の位相を求め、求めた位相、前記予め定めた周波数信号及び前記異なる周波数信号に基づいてターゲットまでの距離を演算する演算手段と、を備えている。
【００２１】
請求項４に記載の発明の光波距離計は、請求項２に記載の光強度変調器と、前記光強度変調器へ光を入射させる光源と、前記光強度変調器から射出されかつターゲットから戻された光で前記逆方向に伝播される導波光により前記光強度変調器から射出された光を検出する検出手段と、前記第１の進行波電極へ予め定めた周波数信号を供給する第１の信号供給手段と、前記第２の進行波電極へ前記第１の信号供給手段と異なる周波数信号を供給する第２の信号供給手段と、前記バイアス電極へ所定直流電圧を供給するバイアス電圧供給手段と、前記検出した光の位相を求め、求めた位相、前記予め定めた周波数信号及び前記異なる周波数信号に基づいてターゲットまでの距離を演算する演算手段と、を備えている。
【００２２】
請求項５に記載の発明は、請求項３または４に記載の光波距離計において、前記検出手段は、前記光強度変調器へ入射させる光と、前記光強度変調器から射出された光とを分離する分離手段を含むことを特徴としている。
【００２３】
導波光の伝播方向及び伝播速度と、導波光を変調するための電極における信号の進行波の伝播方向及び伝播速度とを一致させる形態である進行波電極では、変調帯域の上限が高くなり、一方、進行波の伝播を考慮しない所謂集中定数回路的な電極形態では、進行波電極と比べて変調帯域の上限が低くなることが知られている。
【００２４】
本発明者等は、ある方向に伝播する導波光に対し進行波電極として機能する電極と、集中定数回路的な電極として機能する電極とを光強度変調器に設け、各々の電極に前記進行波電極の変調帯域の上限近傍に設定した変調周波数を印加すると、この導波光に対する変調は、集中定数回路的な電極として機能する電極による変調が、進行波電極として機能する電極による変調に比べて無視できる、という知見を得た。なお、前記ある方向に伝播する導波光と逆方向に伝播する導波光に対しては、前記進行波電極として機能する電極が集中定数回路的な電極として機能すると共に、前記集中定数回路的な電極として機能する電極が進行波電極として機能することになる。
【００２５】
請求項１、２の発明では、一方に向かって伝播する導波光（以下、射出光という）は、第１の分岐部で分岐され、各々複数本の光導波路部を伝播して第２の分岐部で合波される。また、一方に向かって伝播する導波光と逆方向に伝播する導波光（以下、戻り光という）は、第２の分岐部で分岐され、各々複数本の光導波路部を伝播して第１の分岐部で合波される。この光導波路部上を一方に向かって伝播する導波光は第１の進行波電極で変調され、該導波光と逆方向に伝播する導波光は第２の進行波電極で変調される。
【００２６】
従って、互いに逆方向の導波光を変調する進行波電極により、射出光と戻り光を同一の導波路を用いて変調復調することができる。
【００２７】
請求項１の発明では、単一の光強度変調器上に相互に逆方向に伝播させる１対の進行波電極を用いているので、簡素化された構成の単一の光強度変調器で変調及び復調を行うことができる。また、複数の光強度変調器を用いた装置を簡素化して提供することができる。
【００２８】
ここで、上述のように導波光の伝播方向及び伝播速度と、電極における信号の進行波の伝播方向及び伝播速度とを一致させる形態である進行波電極では、変調帯域の上限が高くなる。一方、進行波の伝播を考慮しない所謂集中定数回路的な電極形態では、進行波電極と比べて変調帯域の上限が低くなる。本発明の光強度変調器では、逆方向に伝播する導波光に対しては進行波電極が逆方向伝播電極として機能し、その変調特性は、集中定数回路的な変調特性となる。
【００２９】
そこで、一方に向かって光導波路を伝播する導波光（射出光）を変調するため、第１の進行波電極に該進行波電極の変調帯域の上限近傍の周波数（例えば、Ｗ）の変調波、逆方向に伝播する導波光（戻り光）に対して第２の進行波電極に第１の進行波電極に印加する周波数とわずかに異なる周波数（例えば、Ｗ−δＷ）の変調波をそれぞれ印加する。これによって、射出光に対して逆方向伝播電極として機能する第２の進行波電極での変調は、第１の進行波電極での変調に比べて無視できる。また、戻り光に対しては、逆方向伝播電極として機能する第１の進行波電極での変調が第２の進行波電極での変調に比べて無視できる。すなわち、同一の導波路を用いて光を変調しているにもかかわらず、射出光と戻り光はそれぞれに対応する異なった変調を受けることになり、複数台の独立した光強度変調器の機能を単一の光強度変調器で実現できることになる。
【００３０】
上記電極により導波光を変調するときは、入出力の関係、すなわち電極に供給する電圧とその電圧によって変調された導波光の強度との関係を安定かつ再現性をよくする必要がある。請求項２の発明では、単一の光強度変調器上に相互に逆方向に伝播させる１対の進行波電極に加え更にバイアス電極を設けているので、光強度変調器において容易に変調の動作点を調整することができる。従って、光強度変調器において導波光を効率よく変調できる。
【００３１】
請求項３、４の光波距離計は、光源により前記光強度変調器へ光を入射させ、ターゲットへ向けて射出させ、ターゲットから戻された光を検出する。光強度変調器では第１の進行波電極へ予め定めた周波数信号を供給し、第２の進行波電極へ異なる周波数信号を供給する。従って、同一の導波路を用いて、射出光と戻り光はそれぞれに対応する異なった変調を受ける。この検出した光の位相を求め、求めた位相、予め定めた周波数信号及び前記異なる周波数信号からターゲットまでの距離を演算する。
【００３２】
また、バイアス電極へ所定直流電圧を供給することによって、光強度変調器において導波光を効率よく変調し、検出した光の位相、予め定めた周波数信号及び異なる周波数信号からターゲットまでの距離を精度よく演算できる。
【００３３】
なお、光強度変調器へ入射させる光と、射出された光とを分離する分離手段を含む検出手段を備えることによって、射出光と戻り光とを分離することができ、効率よく光を利用することができる。
【００３４】
【発明の実施の形態】
以下、図面を参照して本発明の実施の形態の一例を詳細に説明する。
【００３５】
図１に示すように、第１実施の形態の光強度変調器は、上記図６に示した光強度変調器と同様に、Ｚ−カット、Ｙ−伝播のＬＮ基板１８上に形成された直線導波路１９、２１、２２、２４、及びＹ分岐部２０、２３を備え、ＬＮ導波路ＭＺ型光強度変調器を構成している。
【００３６】
本実施の形態では、分岐された直線導波路２１、２２上のＹ分岐部２０側に第１の電極対３７、Ｙ分岐部２３側に第２の電極対３８を設けている。第１の電極対３７は直線導波路２１、２２上のＹ分岐部２０側の部位の各々に対応して設けた電極２９、３０から構成され、第２の電極対３８は直線導波路２１、２２上のＹ分岐部２３側の部位の各々に対応して設けた電極３１、３２から構成されている。
【００３７】
この第１の電極対３７のＹ分岐部２０側の一端３７ａにはＲＦ発生器３３が接続されており、Ｙ分岐部２３側の他端３７ｂに終端抵抗３５が接続されている。同様に、第２の電極対３８のＹ分岐部２３側の一端３８ｂにはＲＦ発生器３４が接続されており、Ｙ分岐部２０側の他端３８ａに終端抵抗３６が接続されている。
【００３８】
ＲＦ発生器３３は、発振器１０、直流電源４９及びバイアスティ５０から構成されている。発振器１０は、直流電源４９が接続されたバイアスティ５０を介して電極対３７に接続される。バイアスティ５０は、発振器１０と電極対３７との間にコンデンサ６４が直列に接続されると共に、コンデンサ６４の一方側に直流電源４９及びコイル６３が接続されている。
【００３９】
発振器１０で発振された変調信号は、バイアスティ５０において直流電源４９の直流電圧が付加され、（例えば変調用で周波数Ｗの）ＲＦ波として電極対３７へ供給される。従って、電極対３７にバイアスティ５０を介して直流電源４９からの直流電圧が付加されることによって変調の動作点を設定できる。
【００４０】
なお、ＲＦ発生器３４は、ＲＦ発生器３３と同一の構成であるため、詳細な説明を省略する。
【００４１】
この第１の電極対３７は、ＲＦ発生器３３からＲＦ波（周波数Ｗ）が印加されたときに、Ｙ分岐部２０側の一端３７ａがＲＦ入力端となり、終端抵抗３５が接続された他端３７ｂが終端となる。一方、第２の電極対３８は、ＲＦ発生器３４からＲＦ波（周波数Ｗ−δＷ）が印加されたときに、Ｙ分岐部２３側の一端３８ｂがＲＦ入力端となり、終端抵抗３６が接続された他端３８ａが終端となる。
【００４２】
従って、Ｙ分岐部２０からＹ分岐部２３へ向かう導波光（射出光）に対しては、第１の電極対３７が順方向伝播の進行波電極となり、第２の電極対３８は終端側からＲＦ入力端側へ向かう逆方向伝播の電極となる。一方、Ｙ分岐部２３からＹ分岐部２０へ向かう導波光（戻り光）に対しては、第２の電極対３８が順方向伝播の進行波電極となり、第１の電極対３７は終端側からＲＦ入力端側へ向かう逆方向伝播の電極となる。
【００４３】
すなわち、第１の電極対３７と第２の電極対３８とは互に逆方向に伝播する光に対しての進行波電極となるように構成されている。従って、直線導波路１９から入射した光、すなわちターゲット６に向けて射出されるべき導波光に対して、第１の電極対３７は進行波電極であるが、第２の電極対３８は逆方向伝播電極となる。一方、直線導波路２４から入射する光、すなわちターゲット６から帰還された導波光に対しては、第１の電極対３７は逆方向伝播電極、第２の電極対３８は進行波電極として機能する。
【００４４】
ここで、一般に、ＬＮ等の強誘電体の電気光学効果を用いた光強度変調器の変調の周波数依存性は、電気光学効果の応答性、強誘電体の材料分散、変調電極構造、電極の損失等に依存するが、主に変調電極構造によって周波数が制限されることが知られている。このため従来より、導波路中を伝播する光の速度と電極に印加したＲＦ波の伝播速度とを一致させると３００ＧＨｚまで変調可能と考えられている。
【００４５】
しかしながら、ＬＮ中での光速とＲＦ波の速度とは約２倍の差異があるため両者の速度を一致させるために様々な工夫がなされているが、現在、変調可能とされる最大帯域は７０ＧＨｚ程度である。
【００４６】
導波光及びＲＦ波の伝播方向及び伝播速度を一致させる形態、所謂進行波電極とすれば変調帯域の上限（カットオフ周波数：以下、ｆｃ₁と略す。）が高くなる。一方、ＲＦ波の伝播を考慮しない所謂集中定数回路的な電極形態では、この回路の抵抗と電極容量とによって変調周波数上限（ｆｃ₂）が決まる。実用的な変調器ではｆｃ₂は１ＧＨｚ近辺であり、ｆｃ₂以上では−６ｄＢ／ｏｃｔで減衰する。また、ｆｃ₁は２０ＧＨｚ近辺にある。図２には変調度の周波数依存特性を示した。図中の曲線３９は進行波電極による特性であり、曲線４０は集中定数電極による特性である。
【００４７】
従って、本実施の形態の光強度変調器では、進行波電極が逆方向伝播の電極として機能するときの変調特性は、ほぼ上述の集中定数回路的な変調特性となる。本発明者等は、変調周波数Ｗ及びＷ−δＷを図２の特性に示すようにｆｃ₁近くに設定すれば、射出光に対して電極対３８での変調は電極対３７での変調に比べて無視でき、また戻り光に対して電極対３７での変調は電極対３８での変調に比べて無視できる、という知見を得た。
【００４８】
これにより、同一の導波路を用いて光を変調しているにもかかわらず、射出光と戻り光はそれぞれに対応する異なった変調を受けることになり、従来２台の独立した変調器の機能を１台の変調器で実現できる。
【００４９】
次に、上記の光強度変調器を用いた光波距離計の一例を図３を参照して説明する。図５と共通する部分には同一符号を付し詳細な説明を省略する。
【００５０】
直線偏光した光を射出する光源１の射出側には光ファイバー４１、入出力分離手段４２、及び光ファイバー４８が順に配置される。光源１からの直線偏光した光は光ファイバー４１を介して入出力分離手段４２へ入射する。この入出力分離手段４２はコリメータ・レンズ（以下、ＣＬと略す）４３、４６、４７、偏向ビームスプリッター（以下、ＰＢＳと略す）４４、及びファラディ素子（以下、ＦＲと略す）４５から構成される。光ファイバー４１からの入射光はＣＬ４３で平行光束にされ、ＰＢＳ４４を透過するときにＰ波の光とされ、ＦＲ４５を透過するときに４５度偏波面が回転され、ＣＬ４６で収束されて光ファイバー４８の一端に結合（入射）する。
【００５１】
光ファイバー４８は偏波面保持ファイバー（以下、ＰＭＦと略す）であり、偏波保持軸とＦＲ４５の透過光の偏光方向とを一致させるためのものである。ＰＭＦ４８の他端には光強度変調器１６が連続し、ＰＭＦ４８からの光は光強度変調器１６の直線導波路１９に結合して射出光となる。
【００５２】
本実施の形態では、上述のように、電極対３７へＲＦ波（周波数Ｗ）を印加するためのＲＦ発生器３３は、発振器１０、直流電源４９及びバイアスティ５０から構成される。また、電極対３７へＲＦ波（周波数Ｗ−δＷ）を印加するためのＲＦ発生器３４も同様に、発振器１１、直流電源５２及びバイアスティ５１から構成される。
【００５３】
上述したように電極対３７により周波数Ｗの振幅変調を受けた光は、直線導波路２４から射出し、光ファイバー６０を介してレンズ６１で平行光束とされてターゲット６へ向けて射出する。レンズ６１からの光はターゲット６で再帰反射し、再帰反射したターゲット６からの戻り光は同一の経路をへて直線導波路２４に入射し、電極対３８にて周波数（Ｗ−δＷ）で復調される。この復調された光は、直線導波路１９からＰＭＦ４８、ＣＬ４６を介してＦＲ４５に光源１からの光を最初に透過させたときの方向と逆方向に入射する。このＦＲ４５を透過するときに４５度の偏波回転がなされＳ波としてＰＢＳ４４に入射する。
【００５４】
ＰＢＳ４４のＳ波の分離（反射）側にはＣＬ４７、光ファイバー５５及びホトダイオード１４と順に配置され、ＰＢＳ４４で分離（反射）されたＳ波の光は、ＣＬ４７及び光ファイバー５５を介してホトダイオード１４へ至り、ホトダイオード１４で光電変換される。このように入出力分離手段４２により射出光と戻り光とが分離される。なお、位相検出に関しては図５と同様であるので説明を省略する。
【００５５】
このように、本実施の形態によれば、ＭＺ型光強度変調器の直線導波路２１、２２上に第１の電極対３７及び第２の電極対３８を設けているので、電極対３７はＲＦ発生器３３からＲＦ波が印加されたときＹ分岐部２０からＹ分岐部２３へ向かう光に対して進行波電極となり、電極対３８はＲＦ発生器３４からＲＦ波が印加されたときにＹ分岐部２３からＹ分岐部２０へ向かう光に対して進行波電極となるように、互に逆方向に伝播する光に対しての進行波電極となる。従って、１つのＭＺ型導波路において、射出光と戻り光に対してそれぞれ別個な変調器が実現できる。
【００５６】
次に、第２実施の形態を説明する。上記第１実施の形態の光強度変調器１６では、電極へ供給するＲＦ波を電気的にバイアス変更し変調の動作点を設定する場合を説明したが、本実施の形態は、光強度変調器に変調の動作点を設定するための電極を有するものである。なお、本実施の形態の構成は上記と略同様のため、以下異なる部分を説明する。
【００５７】
図４に示すように、本実施の形態の光強度変調器１７は、上記図３に示した光強度変調器と同様に、ＬＮ基板１８上に形成された直線導波路１９、２１、２２、２４及びＹ分岐部２０、２３を備えると共に、分岐された直線導波路２１、２２上のＹ分岐部２０側に第１の電極対３７、及びＹ分岐部２３側に第２の電極対３８を設けている。直線導波路２１、２２上であると共に、これら第１の電極対３７、及び第２の電極対３８の間には電極対３７、３８より短い第３の電極対５３が設けられている。この第３の電極対５３には出力電圧が可変の直流電源５４が接続されており、直流電源５４からの直流電圧を第３の電極対５３に印加することによって変調の動作点を設定する。
【００５８】
従って、直線導波路１９からの射出光に対して、第１の電極対３７は進行波電極であり、第２の電極対３８は逆方向伝播電極であり、第３の電極対５３はバイアス電極となる。一方、直線導波路２４からの戻り光に対しては、第１の電極対３７は逆方向伝播電極、第２の電極対３８は進行波電極、そして第３の電極対５３はバイアス電極として機能する。
【００５９】
上記のように、第３の電極対５３を設けた場合には図３に示したバイアスティ５０、５１は不要となる。電極対３７、３８は同一の直線導波路２１、２２上に形成されているため、同じ長さの電極対であれば両者のＶπは等しく、また両者は同じ動作点での動作をすることが望ましいため、第３の電極対によって動作点の設定をするのが好ましい。
【００６０】
このように、本実施の形態では、単一の光強度変調器上に相互に逆方向に伝播させる１対の進行波電極に加え更にバイアス電極を設けているので、光強度変調器において容易に変調の動作点を調整することができる。従って、光強度変調器において導波光を効率よく変調できる。
【００６１】
上記実施の形態では、直線導波路上に略同様の形状の電極を設けた場合について説明したが、図７に示すように、射出光及び戻り光の各々の進行波電極の対となる一方の電極を共通電極７０として構成してもよく、図８に示すように、進行波電極の対となる一方の電極を導波路を跨がるように共通電極７１とすることにより射出光及び戻り光の各々の方向について対称形状となるように構成してもよい。
【００６２】
上記実施の形態は、Ｚ−カット、Ｙ伝播のＬＮ導波路について説明したが、他のカット（例えばＸ−カット、Ｚ伝播）、及び他の強誘電体（例えばＬｉＴａＯ₃等）にも適用が可能である。また、上記の光強度変調器としてはＹ分岐部を有するＭＺ型変調器を用いた場合を説明したが、Ｙ分岐部を有しない結合型分岐部を少なくとも１つ有する、すなわち、上記のＹ分岐部に対応する部位の少なくとも１つを光方向性結合器に設定したＭＺ型変調器への適用も可能である。
【００６３】
【発明の効果】
以上説明したように本発明の光強度変調器によれば、同一の光導波路で射出光と戻り光を変調することができるので、構造を簡略化できる、という効果がある。また、変調の動作点を設定する第３の電極を備えることによって、安定して導波光を変調することができる、という効果がある。
【００６４】
また、本発明の光波距離計によれば、複数の光強度変調器を単一のもので構成できるので、簡素化された構成の光波距離計を提供することができる、という効果がある。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態である光強度変調器の構成を示す線図である。
【図２】周波数と変調度の関係を示す線図である。
【図３】第１の実施の形態の光強度変調器を用いた高分解能光距離計の概略を示すブロック図である。
【図４】第２の実施の形態である光強度変調器の構成を示す線図である。
【図５】従来の高分解能光距離計の概略を示すブロック図である。
【図６】従来の光強度変調器の構成を示す線図である。
【図７】共通電極を有する光強度変調器の構成を示す線図である。
【図８】共通電極を有する光強度変調器の他の構成を示す線図である。
【符号の説明】
１光源
１４ホトダイオード
１５ロックインアンプ
１９、２１、２２、２４直線導波路
２０、２３Ｙ分岐部
３３、３４ＲＦ発生器
３７第１の電極対
３８第２の電極対
４２入出力分離手段
５３第３の電極対[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical intensity modulator and an optical distance meter, and in particular, an optical intensity modulator capable of modulating and demodulating light propagating through an optical waveguide (hereinafter, abbreviated as guided light), and the optical intensity modulator. The present invention relates to a lightwave distance meter that measures the distance to a target using a horn.
[0002]
[Prior art]
Fig. 5 shows an example of a high-resolution optical rangefinder using a conventional waveguide-type light intensity modulator that is the basis of the present invention (reference: Yoshida et al .; Proceedings of 11th Meeting on Lightwave Sensing Technology, LST 11). -4, or Saito et al .; Optics, Vol. 22, No. 3, P142-('93)).
[0003]
In this high-resolution optical rangefinder, coherent light amplitude-modulated at a frequency W is emitted toward the target 6, and return light from the target 6 is modulated (demodulated) again at the frequency (W−δW). Since the phase of the return light changes corresponding to the distance to the target 6, the distance to the target 6 can be obtained by measuring this phase. In order to obtain this distance with high resolution, it is necessary to set the modulation frequency W to a high frequency, but it is known that the measurement of the high-frequency phase is difficult. Therefore, when the frequency is shifted and demodulated by the frequency (W−δW), that is, the frequency δW that can be electrically measured, as described above, the phase information at the frequency W is included in the phase information of the frequency δW, and can be measured. Become.
[0004]
The distance measurement resolution δL is given by equation (1).
δL = (C / 2W) · δφ (1)
Here, C is the speed of light, and δφ is a phase measurement error.
[0005]
Accordingly, the higher the frequency W to be modulated and the smaller the phase measurement error δφ, the higher the distance resolution.
[0006]
Specifically, the high-resolution optical distance meter of FIG. 5 uses a semiconductor laser (LD) having a wavelength of 1.3 μm as the light source 1. The laser light emitted from the light source 1 is guided to the first light intensity modulator 3 by the optical fiber 2 and is amplitude-modulated at the frequency W. The modulated light emitted from the light intensity modulator 3 is converted into a parallel light beam by a collimator lens 5 provided at an emission side end of the optical fiber 4 and emitted toward a target 6.
[0007]
A corner cube is used as the target 6, and the return light retroreflected by the target 6 is collected by the lens 7 onto the optical fiber 8, guided by the optical fiber 8 to the second light intensity modulator 9, and the second light intensity. The modulator 9 receives amplitude modulation of frequency (W−δW). The light demodulated in this way is guided to the photodiode 14 by the optical fiber 13 and is photoelectrically converted in the photodiode 14.
[0008]
A signal output from the photodiode 14 is input to the lock-in amplifier 15 as a signal Sig. The frequency W and the frequency (W−δW) are respectively generated by the oscillators 10 and 11 and supplied to the light intensity modulators 3 and 9 and also to the mixer 12. As a result, the mixer 12 generates a signal having the difference frequency δW, and the signal having the difference frequency δW is input to the lock-in amplifier 15 as the reference signal REF. Therefore, if the phase of the signal Sig is measured with reference to the phase of the reference signal REF, the distance to the target 6 can be obtained. The absolute distance to the target 6 can be obtained from the difference between the measurement phases by performing the same measurement again with the frequency W to be modulated differently.
[0009]
This high-resolution optical distance meter uses 3.5 GHz as the modulation frequency W and 50 KHz as the difference frequency δW, and has an accuracy of about 7 μm at a distance of 250 m.
[0010]
Such light intensity modulators include lithium niobate (LiNbO). _Three 2) Mach-Zehnder type (hereinafter referred to as MZ type) light intensity modulators using waveguides are used for modulation and demodulation.
[0011]
More specifically, the light intensity modulator includes an LN substrate 18, linear waveguides 19, 21, 22, 24 and electrodes 25, 26 formed on the LN substrate 18, as shown in FIG. The light incident on the straight waveguide 19 is branched by the Y branching portion 20 and divided into two straight waveguides 21 and 22 at 50:50. A pair of electrodes 25 and 26 are formed on the linear waveguides 21 and 22, respectively. The electrodes 25 and 26 are traveling wave type electrodes for modulating high-frequency waves of the guided light. A supply power source 27 for applying an alternating current is connected to the start ends of the electrodes 25 and 26, and a termination resistor 28 is connected to the end. Has been.
[0012]
In the case of modulation, when a modulation signal is applied between the electrodes 25 and 26 from the power supply 27, the phase of the guided light propagating through the straight waveguides 21 and 22 changes complementarily. The guided light having a phase difference caused by the modulation signal from the power supply 27 is multiplexed to the straight waveguide 24 by the Y branch portion 23 and emitted from the waveguide 24 as light subjected to amplitude modulation corresponding to the phase difference between the two. (Injected in the direction of arrow A in FIG. 6).
[0013]
The same applies to the case of demodulation, but the traveling direction of the guided light is opposite. That is, in the example of FIG. 6, light in the direction opposite to the arrow A direction is incident. Accordingly, a power supply 27 for applying an alternating current is connected to the Y branch portion 23 side which is an end portion of the electrodes 25 and 26, and a termination resistor 28 is connected to the Y branch portion 20 side which is a termination side. A demodulated signal is applied between the electrodes from the power supply 27 to change the phase of the guided light propagating through the straight waveguides 21 and 22 in the direction opposite to the modulation direction. The guided light having a phase difference caused by the modulation signal from the power supply 27 is combined by the Y branching unit 20 to the straight waveguide 19 and emitted.
[0014]
Thus, the conventional light wave distance meter uses two light intensity modulators.
Further, an MZ type optical intensity modulator having a sinusoidal intensity modulation characteristic with a period of 2Vπ (Vπ is a half-wave voltage) is an even harmonic of an even order when the transmission point is the maximum or minimum. Since modulation is obtained, higher frequency modulation can be performed and measurement accuracy can be improved.
[0015]
In this way, high-precision optical distance measurement is possible by modulating and demodulating light using two ultra-high frequency optical intensity modulators.
[0016]
[Problems to be solved by the invention]
However, an ultra-high frequency light intensity modulator using an LN waveguide has been developed mainly for optical communication, and is highly reliable and put to practical use. For this purpose, a light intensity modulator is required for each of modulation and demodulation, so that the apparatus configuration becomes complicated and large, and the operations such as assembly adjustment become complicated. In addition, the light intensity modulator is expensive, and the conventional light wave distance meter uses a light intensity modulator for each of modulation and demodulation. It has become.
[0017]
In view of the above facts, the present invention provides a light intensity modulator capable of modulating and demodulating guided light propagating through an optical waveguide with a simple configuration, and a lightwave distance meter that measures the distance to a target using the light intensity modulator Is the purpose.
[0018]
[Means for Solving the Problems]
In order to achieve the above object, the light intensity modulator according to the first aspect of the present invention splits the guided light propagating toward one side and the guided light propagating in the opposite direction to the guided light. A first branching section to be multiplexed, a plurality of optical waveguide sections that are continuous with the first branching section and propagate guided light, and are continuous with the plurality of optical waveguide sections and propagate through the optical waveguide section. A plurality of guided light beams, a second branching portion for branching the guided light propagating in the opposite direction to the guided light and leading to each of the plurality of optical waveguide portions, and the optical waveguide portion. Provided, A predetermined frequency signal in the vicinity of the upper limit of the modulation band for modulating the guided light so as to have the same propagation direction as the guided light traveling from the first branch to the second branch is supplied. A first traveling wave electrode that modulates guided light propagating through the optical waveguide section from the first branch section toward the second branch section; A predetermined frequency signal in the vicinity of the upper limit of the modulation band for modulating the guided light to be in the same propagation direction as the guided light from the second branch to the first branch is supplied. A traveling wave electrode including a second traveling wave electrode that modulates guided light propagating from the second branch part toward the first branch part.
[0019]
A light intensity modulator according to a second aspect of the present invention splits guided light propagating toward one side and combines the guided light propagating in a direction opposite to the guided light. A plurality of optical waveguide portions that are continuous with the first branching portion and propagate the guided light, and a plurality of waveguide lights that are continuous with the plurality of optical waveguide portions and propagate through the optical waveguide portion. A second branching section for branching the guided light propagating in the opposite direction to the guided light and leading to each of the plurality of optical waveguide sections, and the optical waveguide section, A predetermined frequency signal in the vicinity of the upper limit of the modulation band for modulating the guided light so as to have the same propagation direction as the guided light traveling from the first branch to the second branch is supplied. A first traveling wave electrode that modulates guided light propagating through the optical waveguide section from the first branch section toward the second branch section; A predetermined frequency signal in the vicinity of the upper limit of the modulation band for modulating the guided light to be in the same propagation direction as the guided light from the second branch to the first branch is supplied. A traveling wave electrode including a second traveling wave electrode that modulates guided light propagating from the second branching portion toward the first branching portion; and an operating point of the modulation provided in the optical waveguide portion And a bias electrode for setting.
[0020]
A lightwave distance meter according to a third aspect of the present invention includes a light intensity modulator according to the first aspect, a light source that makes light incident on the light intensity modulator, a light emitted from the light intensity modulator and returned from the target. Detecting means for detecting the light emitted from the light intensity modulator by the guided light propagating in the reverse direction with the emitted light, and a first frequency signal for supplying a predetermined frequency signal to the first traveling wave electrode A signal supply means, a second signal supply means for supplying a frequency signal different from that of the first signal supply means to the second traveling wave electrode, a phase of the detected light is obtained, Computing means for computing the distance to the target based on the determined frequency signal and the different frequency signal.
[0021]
According to a fourth aspect of the present invention, there is provided an optical rangefinder according to the second aspect of the present invention, a light intensity modulator according to the second aspect, a light source that makes light incident on the light intensity modulator, and a light emitted from the light intensity modulator and returned from the target. Detecting means for detecting the light emitted from the light intensity modulator by the guided light propagating in the reverse direction with the emitted light, and a first frequency signal for supplying a predetermined frequency signal to the first traveling wave electrode Signal supply means; second signal supply means for supplying a frequency signal different from that of the first signal supply means to the second traveling wave electrode; and bias voltage supply means for supplying a predetermined DC voltage to the bias electrode. Calculating means for calculating the phase of the detected light and calculating the distance to the target based on the determined phase, the predetermined frequency signal and the different frequency signal.
[0022]
According to a fifth aspect of the present invention, in the light wave distance meter according to the third or fourth aspect, the detection means includes light incident on the light intensity modulator and light emitted from the light intensity modulator. It is characterized by including separation means for separating.
[0023]
In the traveling wave electrode which is a form in which the propagation direction and propagation speed of the guided light are matched with the propagation direction and propagation speed of the traveling wave of the signal in the electrode for modulating the guided light, the upper limit of the modulation band is increased. It is known that the upper limit of the modulation band is lower in a so-called lumped-constant circuit electrode configuration that does not consider traveling wave propagation as compared to the traveling wave electrode.
[0024]
The present inventors provide an optical intensity modulator with an electrode that functions as a traveling wave electrode for guided light propagating in a certain direction and an electrode that functions as a lumped constant circuit electrode, and the traveling wave is applied to each electrode. When a modulation frequency set near the upper limit of the modulation band of the electrode is applied, the modulation of this guided light is neglected by the modulation of the electrode functioning as a lumped constant circuit electrode compared to the modulation of the electrode functioning as a traveling wave electrode The knowledge that it was possible was acquired. For the guided light propagating in the opposite direction to the guided light propagating in a certain direction, the electrode functioning as the traveling wave electrode functions as a lumped constant circuit electrode and the lumped constant circuit electrode. As a traveling wave electrode, the electrode functioning as will function.
[0025]
In the first and second aspects of the present invention, the guided light propagating toward one side (hereinafter referred to as emission light) is branched at the first branching portion, and propagates through each of the plurality of optical waveguide portions to form the second branching. The parts are combined. In addition, guided light propagating in the opposite direction to the guided light propagating toward one side (hereinafter referred to as return light) is branched at the second branch portion, and propagates through the plurality of optical waveguide portions, respectively. Combined at the branching point. The guided light propagating in one direction on the optical waveguide portion is modulated by the first traveling wave electrode, and the guided light propagating in the opposite direction to the guided light is modulated by the second traveling wave electrode.
[0026]
Therefore, the traveling wave electrode that modulates the waveguide light in the opposite directions can modulate and demodulate the emitted light and the return light using the same waveguide.
[0027]
According to the first aspect of the present invention, since a pair of traveling wave electrodes propagating in directions opposite to each other are used on a single light intensity modulator, modulation is performed with a single light intensity modulator having a simplified configuration. And demodulation can be performed. In addition, a device using a plurality of light intensity modulators can be provided in a simplified manner.
[0028]
Here, as described above, the upper limit of the modulation band is increased in the traveling wave electrode in which the propagation direction and propagation speed of the guided light are matched with the propagation direction and propagation speed of the traveling wave of the signal in the electrode. On the other hand, in the so-called lumped circuit type electrode configuration that does not consider traveling wave propagation, the upper limit of the modulation band is lower than that of the traveling wave electrode. In the light intensity modulator of the present invention, the traveling wave electrode functions as a backward propagation electrode for guided light propagating in the reverse direction, and the modulation characteristic is a modulation characteristic like a lumped constant circuit.
[0029]
Therefore, in order to modulate the guided light (emitted light) propagating through the optical waveguide toward one side, the first traveling wave electrode has a modulated wave having a frequency (for example, W) near the upper limit of the modulation band of the traveling wave electrode, A modulated wave having a frequency (for example, W-δW) slightly different from the frequency applied to the first traveling wave electrode is applied to the second traveling wave electrode with respect to the guided light propagating in the reverse direction (return light). . As a result, the modulation at the second traveling wave electrode that functions as a counter-propagating electrode with respect to the emitted light can be ignored as compared with the modulation at the first traveling wave electrode. For return light, the modulation at the first traveling wave electrode that functions as a counter-propagating electrode is negligible compared to the modulation at the second traveling wave electrode. That is, even though the light is modulated using the same waveguide, the emitted light and the return light are subjected to different modulations corresponding to each, and the functions of a plurality of independent light intensity modulators. Can be realized with a single light intensity modulator.
[0030]
When the guided light is modulated by the electrode, it is necessary to stabilize and improve the relationship between the input and output, that is, the relationship between the voltage supplied to the electrode and the intensity of the guided light modulated by the voltage. In the invention of claim 2, since a bias electrode is provided in addition to a pair of traveling wave electrodes propagating in directions opposite to each other on a single light intensity modulator, the modulation operation can be easily performed in the light intensity modulator. The point can be adjusted. Therefore, the guided light can be efficiently modulated in the light intensity modulator.
[0031]
The lightwave distance meter according to claims 3 and 4 makes light incident on the light intensity modulator by a light source, emits the light toward the target, and detects the light returned from the target. The light intensity modulator supplies a predetermined frequency signal to the first traveling wave electrode and supplies a different frequency signal to the second traveling wave electrode. Therefore, using the same waveguide, the emitted light and the return light are subjected to different modulations corresponding to each. The phase of the detected light is obtained, and the obtained phase, a predetermined frequency signal, and the distance from the different frequency signal to the target are calculated.
[0032]
In addition, by supplying a predetermined DC voltage to the bias electrode, the light intensity modulator efficiently modulates the guided light, and the detected light phase, the predetermined frequency signal, and the distance from the different frequency signal to the target are accurately determined. Can be calculated.
[0033]
In addition, by providing the detection means including the separation means for separating the light incident on the light intensity modulator and the emitted light, the emitted light and the return light can be separated, and the light is used efficiently. be able to.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings.
[0035]
As shown in FIG. 1, the light intensity modulator of the first embodiment is a straight line formed on a Z-cut, Y-propagation LN substrate 18 in the same manner as the light intensity modulator shown in FIG. The waveguides 19, 21, 22, 24, and the Y branch portions 20, 23 are provided to constitute an LN waveguide MZ type light intensity modulator.
[0036]
In the present embodiment, a first electrode pair 37 is provided on the Y branch portion 20 side on the branched straight waveguides 21 and 22, and a second electrode pair 38 is provided on the Y branch portion 23 side. The first electrode pair 37 is composed of electrodes 29 and 30 provided corresponding to the respective portions on the Y branching portion 20 side on the straight waveguides 21 and 22, and the second electrode pair 38 is composed of the straight waveguides 21 and 22, respectively. 22 is composed of electrodes 31 and 32 provided corresponding to each of the portions on the Y branching portion 23 side on 22.
[0037]
An RF generator 33 is connected to one end 37a on the Y branch portion 20 side of the first electrode pair 37, and a termination resistor 35 is connected to the other end 37b on the Y branch portion 23 side. Similarly, an RF generator 34 is connected to one end 38b on the Y branch portion 23 side of the second electrode pair 38, and a termination resistor 36 is connected to the other end 38a on the Y branch portion 20 side.
[0038]
The RF generator 33 includes an oscillator 10, a DC power supply 49, and a bias tee 50. The oscillator 10 is connected to the electrode pair 37 via a bias tee 50 to which a DC power supply 49 is connected. In the bias tee 50, a capacitor 64 is connected in series between the oscillator 10 and the electrode pair 37, and a DC power supply 49 and a coil 63 are connected to one side of the capacitor 64.
[0039]
The modulation signal oscillated by the oscillator 10 is supplied with a DC voltage of the DC power supply 49 at the bias tee 50 and supplied to the electrode pair 37 as an RF wave (for example, for modulation and having a frequency W). Accordingly, the modulation operating point can be set by applying a DC voltage from the DC power source 49 to the electrode pair 37 via the bias tee 50.
[0040]
Since the RF generator 34 has the same configuration as the RF generator 33, detailed description thereof is omitted.
[0041]
When the RF wave (frequency W) is applied from the RF generator 33, the first electrode pair 37 has one end 37a on the Y branch portion 20 side serving as an RF input end and the other end to which the terminating resistor 35 is connected. 37b ends. On the other hand, in the second electrode pair 38, when an RF wave (frequency W-δW) is applied from the RF generator 34, one end 38b on the Y branch 23 side becomes an RF input end, and the termination resistor 36 is connected. The other end 38a ends.
[0042]
Therefore, for the guided light (emitted light) traveling from the Y branching portion 20 to the Y branching portion 23, the first electrode pair 37 becomes a traveling wave electrode propagating in the forward direction, and the second electrode pair 38 is moved from the terminal side. It becomes an electrode for backward propagation toward the RF input end. On the other hand, for the guided light (return light) from the Y branching portion 23 to the Y branching portion 20, the second electrode pair 38 becomes a forward wave traveling wave electrode, and the first electrode pair 37 is connected from the terminal side. It becomes an electrode for backward propagation toward the RF input end.
[0043]
That is, the first electrode pair 37 and the second electrode pair 38 are configured to be traveling wave electrodes for light propagating in opposite directions. Therefore, the first electrode pair 37 is a traveling wave electrode with respect to light incident from the straight waveguide 19, that is, guided light to be emitted toward the target 6, but the second electrode pair 38 is in the reverse direction. Propagation electrode. On the other hand, for light incident from the straight waveguide 24, that is, guided light fed back from the target 6, the first electrode pair 37 functions as a backward propagation electrode, and the second electrode pair 38 functions as a traveling wave electrode. .
[0044]
Here, in general, the frequency dependence of the modulation of the light intensity modulator using the electro-optic effect of the ferroelectric such as LN is the response of the electro-optic effect, the material dispersion of the ferroelectric, the modulation electrode structure, the electrode Although depending on the loss and the like, it is known that the frequency is mainly limited by the modulation electrode structure. For this reason, conventionally, it is considered that modulation up to 300 GHz is possible if the speed of light propagating in the waveguide matches the propagation speed of the RF wave applied to the electrode.
[0045]
However, since the speed of light in the LN and the speed of the RF wave have a difference of about twice, various attempts have been made to match the speeds of the two. Currently, the maximum band that can be modulated is 70 GHz. Degree.
[0046]
If the propagation direction and propagation velocity of the guided light and the RF wave are matched, that is, a so-called traveling wave electrode, the upper limit of the modulation band (cutoff frequency: hereinafter, fc ₁ Abbreviated. ) Becomes higher. On the other hand, in a so-called lumped constant circuit electrode configuration that does not consider RF wave propagation, the modulation frequency upper limit (fc) is determined by the resistance and electrode capacitance of this circuit. ₂ ) Is decided. For practical modulators, fc ₂ Is around 1 GHz and fc ₂ In the above, it attenuates at −6 dB / oct. Also, fc ₁ Is in the vicinity of 20 GHz. FIG. 2 shows the frequency dependence characteristics of the modulation factor. A curve 39 in the figure is a characteristic due to the traveling wave electrode, and a curve 40 is a characteristic due to the lumped constant electrode.
[0047]
Therefore, in the light intensity modulator of the present embodiment, the modulation characteristic when the traveling wave electrode functions as a counter-propagating electrode is substantially the above-described lumped constant circuit modulation characteristic. The inventors have determined that the modulation frequencies W and W−δW are fc as shown in the characteristics of FIG. ₁ If set close, the modulation at the electrode pair 38 can be neglected compared to the modulation at the electrode pair 37 with respect to the emitted light, and the modulation at the electrode pair 37 with respect to the return light is a modulation at the electrode pair 38 We obtained the knowledge that it can be ignored.
[0048]
As a result, even though the light is modulated using the same waveguide, the emitted light and the returned light are subjected to different modulations corresponding to each other. Can be realized by a single modulator.
[0049]
Next, an example of an optical distance meter using the above light intensity modulator will be described with reference to FIG. Portions common to those in FIG. 5 are denoted by the same reference numerals and detailed description thereof is omitted.
[0050]
On the exit side of the light source 1 that emits linearly polarized light, an optical fiber 41, an input / output separation means 42, and an optical fiber 48 are arranged in this order. The linearly polarized light from the light source 1 enters the input / output separation means 42 via the optical fiber 41. The input / output separation means 42 includes collimator lenses (hereinafter abbreviated as CL) 43, 46, 47, a deflecting beam splitter (hereinafter abbreviated as PBS) 44, and a Faraday element (hereinafter abbreviated as FR) 45. . Incident light from the optical fiber 41 is converted into a parallel light beam by the CL 43, converted to P-wave light when passing through the PBS 44, and the polarization plane is rotated by 45 degrees when passing through the FR 45, converged by the CL 46, and converged at one end of the optical fiber 48. To (incident).
[0051]
The optical fiber 48 is a polarization-maintaining fiber (hereinafter abbreviated as “PMF”), and is used for matching the polarization maintaining axis with the polarization direction of the transmitted light through the FR 45. The light intensity modulator 16 is connected to the other end of the PMF 48, and the light from the PMF 48 is coupled to the linear waveguide 19 of the light intensity modulator 16 to become emitted light.
[0052]
In the present embodiment, as described above, the RF generator 33 for applying the RF wave (frequency W) to the electrode pair 37 includes the oscillator 10, the DC power supply 49, and the bias tee 50. Similarly, the RF generator 34 for applying an RF wave (frequency W−δW) to the electrode pair 37 includes an oscillator 11, a DC power supply 52, and a bias tee 51.
[0053]
As described above, the light subjected to the amplitude modulation of the frequency W by the electrode pair 37 is emitted from the linear waveguide 24, converted into a parallel light beam by the lens 61 through the optical fiber 60, and emitted toward the target 6. The light from the lens 61 is retroreflected by the target 6, and the return light from the retroreflected target 6 enters the linear waveguide 24 through the same path, and is demodulated at a frequency (W−δW) by the electrode pair 38. Is done. The demodulated light enters the FR 45 through the straight waveguide 19 through the PMF 48 and CL 46 in the direction opposite to the direction when the light from the light source 1 is first transmitted. When passing through the FR 45, the polarization is rotated by 45 degrees and enters the PBS 44 as an S wave.
[0054]
CL47, optical fiber 55 and photodiode 14 are arranged in this order on the S44 separation (reflection) side of PBS 44, and the S wave light separated (reflected) by PBS44 reaches photodiode 14 via CL47 and optical fiber 55. Photoelectric conversion is performed by the photodiode 14. Thus, the input / output separation means 42 separates the emitted light and the return light. The phase detection is the same as in FIG.
[0055]
As described above, according to the present embodiment, the first electrode pair 37 and the second electrode pair 38 are provided on the linear waveguides 21 and 22 of the MZ type light intensity modulator. When an RF wave is applied from the RF generator 33, a traveling wave electrode is formed with respect to the light traveling from the Y branch portion 20 to the Y branch portion 23, and the electrode pair 38 is Y when the RF wave is applied from the RF generator 34. It becomes a traveling wave electrode for light propagating in opposite directions so that it becomes a traveling wave electrode for light traveling from the branching portion 23 to the Y branching portion 20. Therefore, it is possible to realize separate modulators for the emitted light and the returned light in one MZ type waveguide.
[0056]
Next, a second embodiment will be described. In the light intensity modulator 16 of the first embodiment, the case where the RF wave supplied to the electrode is electrically biased to set the modulation operating point has been described. However, the present embodiment is directed to the light intensity modulator. Have an electrode for setting an operating point of modulation. In addition, since the structure of this Embodiment is as substantially the same as the above, a different part is demonstrated below.
[0057]
As shown in FIG. 4, the light intensity modulator 17 of the present embodiment is similar to the light intensity modulator shown in FIG. 3 described above, and the linear waveguides 19, 21, 22, which are formed on the LN substrate 18. 24 and Y branching portions 20 and 23, and a first electrode pair 37 on the Y branching portion 20 side on the branched straight waveguides 21 and 22, and a second electrode pair 38 on the Y branching portion 23 side. Provided. A third electrode pair 53 that is on the straight waveguides 21 and 22 and shorter than the electrode pairs 37 and 38 is provided between the first electrode pair 37 and the second electrode pair 38. A DC power supply 54 having a variable output voltage is connected to the third electrode pair 53, and a modulation operating point is set by applying a DC voltage from the DC power supply 54 to the third electrode pair 53.
[0058]
Therefore, the first electrode pair 37 is a traveling wave electrode, the second electrode pair 38 is a backward propagation electrode, and the third electrode pair 53 is a bias electrode with respect to the light emitted from the straight waveguide 19. It becomes. On the other hand, for the return light from the straight waveguide 24, the first electrode pair 37 functions as a backward propagation electrode, the second electrode pair 38 functions as a traveling wave electrode, and the third electrode pair 53 functions as a bias electrode. To do.
[0059]
As described above, when the third electrode pair 53 is provided, the bias tees 50 and 51 shown in FIG. 3 are not necessary. Since the electrode pairs 37 and 38 are formed on the same straight waveguides 21 and 22, if the electrode pairs have the same length, both Vπ are equal, and both may operate at the same operating point. Since it is desirable, it is preferable to set the operating point by the third electrode pair.
[0060]
Thus, in this embodiment, since a bias electrode is provided in addition to a pair of traveling wave electrodes that propagate in opposite directions on a single light intensity modulator, the light intensity modulator can be easily used. The operating point of the modulation can be adjusted. Therefore, the guided light can be efficiently modulated in the light intensity modulator.
[0061]
In the above embodiment, the case where the electrodes having substantially the same shape are provided on the straight waveguide has been described. However, as shown in FIG. 7, one of the traveling wave electrodes of the emission light and the return light is paired with each other. The electrode may be configured as the common electrode 70, and as shown in FIG. 8, the emission light and the return light are obtained by using one electrode that is a pair of traveling wave electrodes as the common electrode 71 so as to straddle the waveguide. You may comprise so that it may become a symmetrical shape about each direction.
[0062]
In the above embodiment, the Z-cut and Y-propagating LN waveguides have been described. However, other cuts (for example, X-cut and Z-propagation) and other ferroelectrics (for example, LiTaO) _Three Etc.). Moreover, although the case where the MZ type modulator having the Y branch part is used as the light intensity modulator has been described, it has at least one coupled branch part not having the Y branch part, that is, the above Y branch. The present invention can also be applied to an MZ type modulator in which at least one of the parts corresponding to the part is set as an optical directional coupler.
[0063]
【The invention's effect】
As described above, according to the light intensity modulator of the present invention, since the emitted light and the returned light can be modulated by the same optical waveguide, there is an effect that the structure can be simplified. Further, by providing the third electrode for setting the operating point of modulation, there is an effect that the guided light can be stably modulated.
[0064]
In addition, according to the light wave distance meter of the present invention, since a plurality of light intensity modulators can be configured as a single one, there is an effect that a light wave distance meter having a simplified configuration can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a light intensity modulator according to a first embodiment of the present invention.
FIG. 2 is a diagram showing the relationship between frequency and modulation factor.
FIG. 3 is a block diagram showing an outline of a high-resolution optical distance meter using the light intensity modulator of the first embodiment.
FIG. 4 is a diagram showing a configuration of a light intensity modulator according to a second embodiment.
FIG. 5 is a block diagram showing an outline of a conventional high-resolution optical rangefinder.
FIG. 6 is a diagram showing a configuration of a conventional light intensity modulator.
FIG. 7 is a diagram showing a configuration of a light intensity modulator having a common electrode.
FIG. 8 is a diagram showing another configuration of a light intensity modulator having a common electrode.
[Explanation of symbols]
1 Light source
14 Photodiodes
15 Lock-in amplifier
19, 21, 22, 24 Linear waveguide
20, 23 Y branch
33, 34 RF generator
37 First electrode pair
38 Second electrode pair
42 Input / output separation means
53 Third electrode pair

Claims

A first branching part for branching the guided light propagating toward one side and for multiplexing the guided light propagating in the opposite direction to the guided light;
A plurality of optical waveguide portions that are continuous with the first branching portion and propagate guided light;
The plurality of optical waveguides that combine the plurality of waveguide lights that are continuous with the plurality of optical waveguide sections and propagate through the optical waveguide sections, and branch the waveguide lights that are propagated in the opposite direction to the waveguide light. A second branch that leads to each of the parts;
A predetermined frequency in the vicinity of the upper limit of the modulation band that is provided in the optical waveguide portion and modulates the guided light so as to have the same propagation direction as the guided light traveling from the first branching portion to the second branching portion. A first traveling wave electrode that modulates guided light that is supplied with a signal and propagates through the optical waveguide section from the first branch section toward the second branch section; and from the second branch section to the first A predetermined frequency signal in the vicinity of the upper limit of the modulation band for modulating the guided light so as to have the same propagation direction as that of the guided light directed to the branching portion is supplied to travel from the second branching portion to the first branching portion. A traveling wave electrode comprising a second traveling wave electrode for modulating the propagating guided light,
A light intensity modulator.

A first branching part for branching the guided light propagating toward one side and for multiplexing the guided light propagating in the opposite direction to the guided light;
A plurality of optical waveguide portions that are continuous with the first branching portion and propagate guided light;
The plurality of optical waveguides that combine the plurality of waveguide lights that are continuous with the plurality of optical waveguide sections and propagate through the optical waveguide sections, and branch the waveguide lights that are propagated in the opposite direction to the waveguide light. A second branch that leads to each of the parts;
A predetermined frequency in the vicinity of the upper limit of the modulation band that is provided in the optical waveguide portion and modulates the guided light so as to have the same propagation direction as the guided light traveling from the first branching portion to the second branching portion. A first traveling wave electrode that modulates guided light that is supplied with a signal and propagates through the optical waveguide section from the first branch section toward the second branch section; and from the second branch section to the first A predetermined frequency signal in the vicinity of the upper limit of the modulation band for modulating the guided light so as to have the same propagation direction as that of the guided light directed to the branching portion is supplied to travel from the second branching portion to the first branching portion. A traveling wave electrode comprising a second traveling wave electrode for modulating the propagating guided light,
A bias electrode provided in the optical waveguide section and for setting an operating point of the modulation;
A light intensity modulator.

A light intensity modulator according to claim 1;
A light source for making light incident on the light intensity modulator;
Detecting means for detecting the light emitted from the light intensity modulator by the guided light propagated in the opposite direction by the light emitted from the light intensity modulator and returned from the target;
First signal supply means for supplying a predetermined frequency signal to the first traveling wave electrode;
Second signal supply means for supplying a frequency signal different from the first signal supply means to the second traveling wave electrode;
A calculating means for calculating the phase of the detected light, calculating a distance to the target based on the determined phase, the predetermined frequency signal and the different frequency signal;
Lightwave distance meter equipped with.

A light intensity modulator according to claim 2;
A light source for making light incident on the light intensity modulator;
Detecting means for detecting the light emitted from the light intensity modulator by the guided light propagated in the opposite direction by the light emitted from the light intensity modulator and returned from the target;
First signal supply means for supplying a predetermined frequency signal to the first traveling wave electrode;
Second signal supply means for supplying a frequency signal different from the first signal supply means to the second traveling wave electrode;
Bias voltage supply means for supplying a predetermined DC voltage to the bias electrode;
A calculating means for calculating the phase of the detected light, calculating a distance to the target based on the determined phase, the predetermined frequency signal and the different frequency signal;
Lightwave distance meter equipped with.

5. The light wave distance meter according to claim 3, wherein the detection unit includes a separation unit that separates light incident on the light intensity modulator and light emitted from the light intensity modulator. 6. .