JP2005006138A

JP2005006138A - Reflection type optical communication system utilizing polarized wave, and station and user terminal used therefor

Info

Publication number: JP2005006138A
Application number: JP2003168750A
Authority: JP
Inventors: Toshio Ito; 敏夫伊藤; Rieko Satou; 里江子佐藤; Yasuhiro Suzuki; 安弘鈴木; Hiroshi Toba; 弘鳥羽
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2003-06-13
Filing date: 2003-06-13
Publication date: 2005-01-06

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reflection type optical communication system utilizing polarized waves, and a station and a user terminal used therefor, capable of carrying out transmission and reception of optical signals with high quality by suppressing the effect of the Rayleigh scattering caused between the uplink and downlink optical signals. <P>SOLUTION: The station K transmits a continuous light Pc and a downlink optical signal Pd to the user terminal U via an optical fiber 104. In this case, the polarization direction of the continuous light Pc is differentiated from that of the downlink optical signal Pd. A downlink optical signal receiver 107 of the user terminal U receives the downlink optical signal Pd. Further, an optical limiter amplifier 114 decreases the extinction ratio of the downlink optical signal Pd. A reflection type optical modulator 106 modulates the continuous light Pc to generate an active uplink optical signal Pu1 and modulates the downlink optical signal Pd the extinction ratio of which is decreased to generate a standby uplink optical signal Pu2. The two optical signals Pu1, Pu2 are transmitted to the station K wherein an uplink optical signal receiver 108 receives the signals. Even if the polarization direction of either of the uplink optical signals Pu1, Pu2 is coincident with that of the down link optical signals Pc, Pd and the waveform is disturbed by the Rayleigh scattering, the polarization direction of the other of the uplink optical signals Pu1, Pu2 is not coincident with the polarization direction of the downlink optical signals Pc, Pd, thereby enabling excellent signal transmission. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は光ファイバを伝搬する高速光通信、光交換、光情報処理等の光伝送システムに適用され、特に１本の光ファイバを上り・下りの双方向で使用する反射型の光通信システムに適用して有効な技術である。
【０００２】
【従来の技術】
ＩＴ技術の発展に伴う伝送容量の増加に伴い、光ファイバの伝送帯域を有効に利用することが重要になっている。伝送帯域を有効に利用するシステムとして、反射型光通信システムがある。
【０００３】
本願発明者は、効率よく上り光信号を送ることができると共に、上り光信号の符合誤り率の改善をはかることができる反射型光通信システムを開発した。
【０００４】
ここで本願発明者が最近開発した反射型光通信システムについて説明する。
【０００５】
［第１の例］
図６は本願発明者が最近開発した反射型光通信システムの第１の例を示す。同図に示すように、局Ｋとユーザ端末Ｕとは１本の光ファイバ１０４により接続されている。局Ｋには、連続光源１０１と下り光信号送信器１０２と光カプラ１０３と上り光信号受信器１０８が備えられている。ユーザ端末Ｕには、光カプラ１０５と反射型光変調器１０６と下り光信号受信器１０７と光リミッタアンプ１１４が備えられている。なお、反射型光変調器１０６は、例えば反射型光増幅器を用いて構成されており、「上り光信号送信器」として機能するものである。
【０００６】
光リミッタアンプ１１４は、光カプラ１０５と反射型光変調器１０６との間に接続されている。この光リミッタアンプ１１４は、通過する光の入力光強度が弱いときには利得が大きくなり、通過する光の入力光強度が強いときには利得が小さくなる半導体光増幅器で構成している。このような半導体光増幅器としては、非特許文献１等に記載されている半導体光増幅器がある。
【０００７】
このようなリミッタ特性（増幅特性）を有する光リミッタアンプ１１４を用いることによって、光リミッタアンプ１１４を通過する光信号は、光強度の弱い「０」の部分の利得が大きく、「１」の部分の利得が小さくなる。このため、光リミッタアンプ１１４を通過した光信号は、「１」部分と「０」部分とのレベル差が小さくなって平坦になってくる。このような光リミッタアンプ１１４を用いることが本システムのポイントの１つとなっている。
【０００８】
局Ｋ側では、連続光源１０１で発生した連続光Ｐｃと、下り光信号送信器１０２で発生した下り光信号Ｐｄとを、時分割方式で送出している。しかも、下り光信号Ｐｄの消光比を３〜５ｄＢと、あえて悪い値に設定している。このように、下り光信号Ｐｄの消光比を悪い値に設定していることが、本システムのポイントの１つとなっている。
【０００９】
ユーザ端末Ｕ側の反射型光変調器１０６では、連続光Ｐｃを受光すると、この連続光Ｐｃを第１の電気信号で変調して正規上り光信号Ｐｕ１を生成し、この正規上り光信号Ｐｕ１を送り返す。また反射型光変調器１０６は、光リミッタアンプ１１４を通過して消光比が１〜３ｄＢと小さくなっている下り光信号Ｐｄを受光すると、この下り光信号Ｐｄを第２の電気信号で変調して予備上り光信号Ｐｕ２を生成し、この予備上り光信号Ｐｕ２を送り返す。
【００１０】
この反射型光通信システムでは、局Ｋ側におかれた連続光源１０１から連続光Ｐｃを送り、ユーザ端末Ｕはこの連続光Ｐｃを変調のうえで正規上り光信号Ｐｕ１として送り返す構成になっている。このため連続光Ｐｃは連続光源１０１→光カプラ１０３→光ファイバ１０４→光カプラ１０５→光リミッタアンプ１１４→反射型光変調器１０６と進んだところで反射型光変調器１０６によって第１の電気信号により変調されて正規上り光信号Ｐｕ１となり、反射型光変調器１０６の端面で反射される。連続光Ｐｃを変調してなる正規上り光信号Ｐｕ１は、光リミッタアンプ１１４→光カプラ１０５→光ファイバ１０４→光カプラ１０３→上り光信号受信器１０８と戻ってくる。
【００１１】
また、下り光信号Ｐｄは、下り光信号送信器１０２→光カプラ１０３→光ファイバ１０４→光カプラ１０５→下り光信号受信器１０７と進み、ユーザ端末Ｕの下り光信号受信器１０７によって受信される。ビットレートは上り光信号Ｐｕ，下り光信号Ｐｄとも１００Ｍｂｉｔ／ｓ〜１Ｇｂｉｔ／ｓ程度である。
【００１２】
ここで大事なのは、前述したように、下り光信号送信器１０２から送出した下り光信号Ｐｄの消光比を、３〜５ｄＢ程度と悪い値に設定していることである。但し、下り光信号Ｐｄは伝送距離が短いため伝搬損失の影響が小さく、下り光信号受信器１０７にて受信した下り光信号Ｐｄの消光比が悪くても、下り光信号Ｐｄの信号状態判定をする際に問題が生じることはない。
【００１３】
また下り光信号Ｐｄは下り光信号受信器１０７で受信される他、光カプラ１０５にて分岐され、光リミッタアンプ１１４を通して反射型光変調器１０６に送られる。送られてきた下り光信号Ｐｄは、消光比が３〜５ｄＢと悪くなっているため、光リミッタアンプ１１４を通過した後の下り光信号Ｐｄは、光強度の弱い「０」の部分の利得が大きくなるとともに、「１」の部分の利得が小さくなり、その消光比は１〜２ｄＢ程度と更に小さくなる。このように消光比が極めて小さくなった下り光信号Ｐｄは、反射型光変調器１０６において、第２の電気信号で変調され、予備上り光信号Ｐｕ２となる。この予備上り光信号Ｐｕ２は、光リミッタアンプ１１４→光カプラ１０５→光ファイバ１０４→光カプラ１０３→上り光信号受信器１０８と戻ってくる。
【００１４】
このように、連続光Ｐｃを変調した正規上り光信号Ｐｕ１のみならず、消光比を小さくした下り光信号Ｐｄを変調した予備上り光信号Ｐｕ２をも、上り光信号として送り返すことができる。つまり、従来では下り光信号Ｐｄを送り返してはいたが、上り光信号として送り返すことはしていなかったが、本システムでは、この下り光信号Ｐｄをも上り光信号として利用することができるようになったため、効率よく信号（上り光信号）を送ることができるようになった。
【００１５】
この反射型光通信システムの波形を図７に示す。
図７（ａ）は下り（局→ユーザ端末）の波形であり、下り光信号Ｐｄと連続光Ｐｃが時分割で送信されている。ここで大事なのは下り光信号Ｐｄの消光比が３〜５ｄＢ程度と悪い値に設定していることである。下り光信号Ｐｄは伝送距離が短いために伝搬損失の影響が小さく、消光比が悪くとも問題は生じない。
【００１６】
図７（ｂ）は光リミッタアンプ１１４を通過して反射型光変調器１０６に向かう連続光Ｐｃ及び下り光信号Ｐｄの波形である。前述したように、光リミッタアンプ１１４は入力光強度が弱いときには利得が大きく、入力光強度が強いときには利得の小さな半導体増幅器であるため、光強度の弱い「０」の部分の利得が大きく、「１」の部分の利得が小さくなる。この結果、光リミットアンプ１１４を通過した後の下り光信号Ｐｄの消光比は、１〜２ｄＢ程度と小さくなる。
【００１７】
図７（ｃ）は、上り光信号の波形である。ここで重要なことは連続光部分を変調してなる正規上り光信号Ｐｕ１だけでなく、下り光信号部分を変調してなる予備上り光信号Ｐｕ２をも、上り光信号とすることができることである。つまり、下り光信号部分も新たに変調が可能な点である。これは、従来では使用していなかった部分（下り光信号部分）も、上り光信号として使用することになるため、効率よく上り光信号を送ることができることを意味する。
【００１８】
［第２の例］
次に本願発明者が最近開発した第２の例を説明する。第２の例では、図６のシステム構成を用いるが、ユーザ端末Ｕ側において、連続光部分を変調した正規上り光信号Ｐｕ１と、下り光信号部分を変調した予備上り光信号Ｐｕ２に同一の信号を振り付ける。つまり正規上り光信号Ｐｕ１を生成するために連続光Ｐｃを変調する第１の電気信号の信号内容（信号情報）と、予備上り光信号Ｐｕ２を生成するために下り光信号Ｐｄ（消光比が小さくなっている下り光信号Ｐｄ）を変調する第２の電気信号の信号内容（信号情報）を等しくしている。これにより、反射型光変調器１０６から送出される正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２との信号内容が等しくなる。
【００１９】
局Ｋ側では、この２つの信号（正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２）を受信し、両者を比較する。ここで予備上り光信号Ｐｕ２と正規上り光信号Ｐｕ１が一致する場合には、信号は正しいと判定することができる。両者が一致しない場合には誤り訂正を試みるか、ユーザ端末Ｕ側に再送を要求する。
この方式により、上り信号の符号誤り率を改善することができる。
【００２０】
［第３の例］
次に本願発明者が最近開発した第３の例を、図８を参照しつつ説明する。図８において、１０９は下り光信号送信器兼連続光源、１１０は光カプラ、１１１は光ファイバ、１１２は下り光信号受信器兼反射型光変調器、１１３は上り光信号受信器である。
【００２１】
図８において、局Ｋ側の下り光信号送信器兼連続光源１０９は、連続光Ｐｃと、消光比を３〜５ｄＢ程度に下げた下り光信号Ｐｄとを時分割で送信可能である。ユーザ端末Ｕ側の下り光信号受信器兼反射型光変調器１１２は、下り光信号Ｐｄの受信と、連続光Ｐｃの変調と下り光信号Ｐｄの変調を時分割で行うことができ、変調して生成した正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２を時分割して送出することができる。
【００２２】
下り光信号送信器兼連続光源１０９は、例えば直接変調型のＤＦＢレーザであり、連続光Ｐｃを送信するときにはＤＣ電流を、下り信号を送信するときには変調電流を注入する。
【００２３】
また下り光信号受信器兼反射型光変調１１２は、例えば非特許文献２に示す反射型の半導体増幅器を用い、受信時には逆バイアスを印加して受光電流をモニタし、送信時には正バイアスの変調電流を注入する。
【００２４】
さらには下り光信号受信器兼反射型光変調器１１２にかける正バイアスの変調電流を最適に設計することで、非特許文献３に示すように、下り光信号受信器兼反射型光変調器１１２に、光リミッタアンプとしての機能も持たせることが可能である。つまり、下り光信号受信器兼反射型光変調器１１２を、下り光信号受信器兼反射型光変調器兼光リミッタアンプとして機能させることができる。
【００２５】
図８の構成となっている反射型光通信システムにおいても、図７に示すように、連続光Ｐｃを第１の電気信号で変調して正規上り光信号Ｐｕ１を生成し、この正規上り光信号Ｐｕ１を送り返し、また消光比を小さくした下り光信号Ｐｄを第２の電気信号で変調して予備上り光信号Ｐｕ２を生成し、この予備上り光信号Ｐｕ２を送り返すことで、上り光信号の符号誤り率の改善をはかることができる。
【００２６】
【非特許文献１】
Ｙ．Ｓｈｉｂａｔａ，Ｙ．Ｙａｍａｄａ，Ｋ．Ｈａｂａｒａ，ａｎｄＮ．Ｙｏｓｈｉｍｏｔｏ， ”Ｓｅｍｉｃｏｎｄｕｃｔｏｒｌａｓｅｒｄｉｏｄｅｏｐｔｉｃａｌａｍｐｌｉｆｉｅｒｓ／ｇａｔｅｓｉｎｐｈｏｔｏｎｉｃｐａｃｋｅｔｓｗｉｔｃｈｉｎｇ”，ＪｏｕｒｎａｌｏｆＬｉｇｈｔｗａｖｅＴｅｃｈｎｏｌｏｇｙ，ｖｏｌ．１６，ｎｏ．１２，ｐｐ．２２２８−２２３５，１９９８
【非特許文献２】
Ｍ．Ｏｋａｙａｓｕ，Ｙ．Ｓｕｚｕｋｉ，Ｎ．Ｙｏｓｈｉｍｏｔｏ，Ｒ．Ｉｇａ，Ｈ．Ｓｕｇｉｕｒａ，ａｎｄＪ．Ｙｏｓｈｉｄａ， ”Ｂｉ−ｄｉｒｅｃｔｉｏｎａｌｔｒａｎｓｍｉｓｓｉｏｎｅｘｐｅｒｉｍｅｎｔｕｓｉｎｇａｓｅｍｉｃｏｎｄｕｃｔｏｒｏｐｔｉｃａｌａｍｐｌｉｆｉｅｒｔｒａｎｓｃｅｉｖｅｒｍｕｄｕｌｅｆｏｒＷＤＭ−ＰＯＮａｎｄＷＤＭ−ＬＡＮａｐｐｌｉｃａｔｉｏｎｓ”，ＢｒｏａｄｂａｎｄＡｃｃｅｓｓａｎｄＴｅｃｈｎｏｌｏｇｙ，Ｄ．Ｗ．ＦａｕｌｋｅｒａｎｄＡ．Ｌ．Ｈａｒｍｅｒ（Ｅｄｓ．），ＩＯＳＰｒｅｓｓ，１９９９，ｐｐ．２７８−２８３，ＩＯＳＰｒｅｓｓ，ＩＳＢＮ４２７４９０２９６ＸＣ３０５５、オーム社
【非特許文献３】
Ｈ．ＴａｋｅｓｕｅａｎｄＴ．Ｓｕｇｉｅ， ”ＤａｔａｒｅｗｒｉｔｅｏｆｗａｖｅｌｅｎｇｔｈｃｈａｎｎｅｌｕｓｉｎｇｓａｔｕｒａｔｅｄＳＯＡｍｏｄｕｌａｔｏｒｆｏｒＷＤＭｍｅｔｒｏ／ａｃｃｅｓｓｎｅｔｗｏｒｋｓｗｉｔｈｃｅｎｔｒａｌｉｚｅｄｌｉｇｈｔｓｏｕｒｃｅｓ”，２８ｔｈＥｕｒｏｐｅａｎＣｏｎｆｅｒｅｎｃｅｏｎＯｐｔｉｃａｌ，ＥＣＯＣ２００２，Ｓｅｐｔｅｍｂｅｒ８−１２，２００２，Ｃｏｐｅｎｈａｇｅｎ，Ｄｅｎｍａｒｋ
【００２７】
【発明が解決しようとする課題】
しかしながら、図６〜図８を基に説明した、本願発明者が最近開発した反射型光通信システムには、さらに改良すべき点があった。
【００２８】
すなわち、両方向を伝送する光信号が光ファイバ１０４，１１１で引き起こす干渉である。一般に双方（上りと下り）の光信号の偏光方向が異なる場合には、図９に示すように、きれいな、すなわち光信号の０レベル，１レベルともに変動しないような波形を受信することができる。通常、受信をするためには０レベルと１レベルのほぼ中間点に閾値３０１を設け、閾値３０１を超えれば１、閾値３０１を下回っていれば０と判定する。
【００２９】
しかしながら双方（上りと下り）の光信号の偏光方向が一致した場合には、いずれか一方の光信号の一部が光ファイバのレイリー散乱によって反射され、その反射光と他方の光信号が干渉することで受信波形が乱れるようになる。この干渉の影響は特に１レベルに顕著で、図１０に示すように、１レベルが太くなるように見える。その結果、閾値４０１によりレベル検出を行う際に、１レベルが０と判定されやすくなる問題があった。
【００３０】
したがって先に説明した、図６，図８に示す反射型光通信のシステムでは局Ｋからユーザ端末Ｕに向かう下り光信号Ｐｄ、連続光Ｐｃの偏光方向が同一で、かつ、ユーザ端末Ｕから局Ｋに向かう予備上り光信号Ｐｕ２、正規上り光信号Ｐｕ１の偏光方向が同一であるので、上り光信号のどちらか一方が干渉により乱れてしまうと、他方も乱れてしまう欠点があった。
【００３１】
このように、反射型光通信システムにおいては、光ファイバを伝搬する上りと下りの光信号の偏光状態が一致したときに干渉が起こり、受信波形が劣化する問題があった。
【００３２】
本発明は、上記従来技術に鑑み、受信波形の劣化を防止して良質な光信号の送受信が可能な反射型偏波利用光通信システムならびにこれに用いる局及びユーザ端末を提供することを目的とする。
【００３３】
【課題を解決するための手段】
上記課題を解決する本発明の反射型偏波利用光通信システムの構成は、局とユーザ端末とが光ファイバで接続され、
前記局は、連続光と下り光信号を時分割方式で前記ユーザ端末に送出すると共に、前記ユーザ端末から送出された上り光信号を受信し、
前記ユーザ端末は、前記局から送出された下り光信号を受信すると共に、受信した連続光を変調して生成してなる上り光信号を前記局に送り返す反射型光通信システムにおいて、
前記局では、偏光方向を異ならせて前記連続光と前記下り光信号を前記ユーザ端末に送出し、
前記ユーザ端末では、受信した連続光を第１の電気信号により変調して生成してなる正規上り光信号を前記局に送り返すのみならず、受信した下り光信号の消光比を減少させ消光比が減少した下り光信号を第２の電気信号により変調して生成してなる予備上り光信号を前記局に送り返すことを特徴とする。
このとき、前記ユーザ端末で用いる第１の電気信号と第２の電気信号の信号内容が同一であり、
前記局は、受信した正規上り光信号と予備上り光信号の信号内容が一致した時に信号内容が正しいと判定したり、
前記局は、消光比が３〜５ｄＢとなっている下り光信号を送出することを特徴とする。
【００３４】
また本発明の反射型偏波利用光通信システムに用いる局は、連続光を送出する連続光源と、下り光信号を送出する下り光信号送信器と、上り光信号である正規上り光信号と予備上り光信号を受信する上り光信号受信器とを有しており、
しかも前記連続光源から送出する連続光と前記下り光信号送信器から送出する下り光信号の偏光方向を異ならせていることを特徴とする。
このとき、この局は、連続光を送出する連続光源と、下り光信号を送出する下り光信号送信器と、上り光信号である正規上り光信号と予備上り光信号を受信する上り光信号受信器と、偏波コントローラとを有しており、
しかも前記連続光源から送出する連続光と前記下り光信号送信器から送出する下り光信号の偏光方向を異ならせており、
更に、前記偏波コントローラは、前記連続光源から送出された連続光と前記下り光信号送信器から送出された下り光信号との偏光方向を制御してから局外に送出したり、
または、上り光信号受信器で受信した正規上り光信号と予備上り光信号の信号内容が一致するまで、前記偏波コントローラにより連続光と下り光信号との偏光方向を制御することを特徴とする。
【００３５】
また本発明の反射型偏波利用光通信システムに用いる局は、下り光信号と連続光を時分割方式で送出する下り光信号送信器兼連続光源と、上り光信号である正規上り光信号と予備上り光信号を受信する上り光信号受信器と、偏波光スイッチとを有しており、
前記偏波光スイッチは、下り光信号送信器兼連続光源から送出された下り光信号と連続光の偏光方向を異ならせるように偏光方向を切り換えることを特徴とする。
このとき、この局は、下り光信号と連続光を時分割方式で送出する下り光信号送信器兼連続光源と、上り光信号である正規上り光信号と予備上り光信号を受信する上り光信号受信器と、偏波光スイッチと、偏波コントローラとを有しており、
前記偏波光スイッチは、下り光信号送信器兼連続光源から送出された下り光信号と連続光の偏光方向を異ならせるように偏光方向を切り換え、
更に、前記偏波コントローラは、前記偏波光スイッチから送出された連続光と下り光信号との偏光方向を制御してから局外に送出したり、
また上り光信号受信器で受信した正規上り光信号と予備上り光信号の信号内容が一致するまで、前記偏波コントローラにより連続光と下り光信号との偏光方向を制御することを特徴とする。
【００３６】
また本発明の反射型偏波利用光通信システムに用いるユーザ端末は、下り光信号を受信する下り光信号受信器と、受信した下り光信号の消光比を減少させる光リミッタアンプと、受信した連続光を第１の電気信号により変調して正規上り光信号を生成して前記局に送り返すと共に前記光リミッタアンプを通過したことにより消光比が減少した下り光信号を第２の電気信号により変調して予備上り光信号を生成して前記局に送り返す反射型光変調器とを有することを特徴とする。
このとき、このユーザ端末は、下り光信号を受信すると共に、受信した連続光を第１の電気信号により変調して正規上り光信号を生成して前記局に送り返すと共に受信した下り光信号を第２の電気信号により変調して予備上り光信号を生成して前記局に送り返す下り光信号受信器兼反射型光変調器を有していたり、
このユーザ端末は、下り光信号を受信すると共に、受信した下り光信号の消光比を減少させ、更に受信した連続光を第１の電気信号により変調して正規上り光信号を生成して前記局に送り返すと共に消光比を減少させた下り光信号を第２の電気信号により変調して予備上り光信号を生成して前記局に送り返す下り光信号受信器兼反射型光変調器兼光リミッタアンプを有することを特徴とする。
【００３７】
【発明の実施の形態】
以下に本発明の実施の形態を図面に基づき詳細に説明する。本実施の形態に係る反射型偏波利用光通信システムは、図６や図８に示した、先に開発した反射型光通信システムを改良したものであるので、先に開発した反射型光通信システムと同一機能を果たす部分には、同一符合を付し、重複する説明は省略する。
【００３８】
［第１の実施の形態］
図１は本発明の第１の実施の形態に係る反射型偏波利用光通信システムを示す。第１の実施の形態は、図６に示す反射型光通信システムを改良したものである。第１の実施の形態では、局Ｋに偏波保持光カプラ１００１を備え、連続光源１０１と偏波保持光カプラ１００１を偏波保持光ファイバＦ１により接続し、下り光信号送信器１０２と偏波保持光カプラ１００１を偏波保持光ファイバＦ２により接続している。連続光源１０１から出力される連続光Ｐｃと、下り光信号送信器１０２から出力される下り光信号Ｐｄは、時分割方式により送出されるようになっており、しかも、本実施の形態では、連続光Ｐｃと下り光信号Ｐｄの偏光方向をずらしている（例えば偏光方向を３０〜４５度ずらしている）。このように、連続光Ｐｃと下り光信号Ｐｄの偏光方向をずらしている（偏光方向を異ならせている）ことが、本システムの最大のポイントになっている。なお、他の部分の構成は図６に示すものと同様になっている。
【００３９】
連続光源１０１から出力された連続光Ｐｃは、消光比が１〜３ｄＢと小さくなっており、この連続光Ｐｃは、偏波保持光ファイバＦ１→偏波保持光カプラ１００１→光カプラ１０３→光ファイバ１０４→光カプラ１０５→光リミッタアンプ１１４→反射型光変調器１０６と進んだところで反射型光変調器１０６によって第１の電気信号により変調されて正規上り光信号Ｐｕ１となり、反射型光変調器１０６の端面で反射される。連続光Ｐｃを変調してなる正規上り光信号Ｐｕ１は、光リミッタアンプ１１４→光カプラ１０５→光ファイバ１０４→光カプラ１０３→上り光信号受信器１０８と戻ってくる。
【００４０】
また、下り光信号Ｐｄは、下り光信号送信器１０２→偏波保持光ファイバＦ２→偏波保持光カプラ１００１→光カプラ１０３→光ファイバ１０４→光カプラ１０５→下り光信号受信器１０７と進み、ユーザ端末Ｕの下り光信号受信器１０７によって受信される。
【００４１】
また下り光信号Ｐｄは下り光信号受信器１０７で受信される他、光カプラ１０５にて分岐され、光リミッタアンプ１１４を通して反射型光変調器１０６に送られる。送られてきた下り光信号Ｐｄは、消光比が３〜５ｄＢと悪くなっているため、光リミッタアンプ１１４を通過した後の下り光信号Ｐｄは、光強度の弱い「０」の部分の利得が大きくなるとともに、「１」の部分の利得が小さくなり、その消光比は１〜２ｄＢ程度と更に小さくなる。このように消光比が極めて小さくなった下り光信号Ｐｄは、反射型光変調器１０６において、第２の電気信号で変調され、予備上り光信号Ｐｕ２となる。この予備上り光信号Ｐｕ２は、光リミッタアンプ１１４→光カプラ１０５→光ファイバ１０４→光カプラ１０３→上り光信号受信器１０８と戻ってくる。
【００４２】
このように、連続光Ｐｃを変調した正規上り光信号Ｐｕ１のみならず、消光比を小さくした下り光信号Ｐｄを変調した予備上り光信号Ｐｕ２をも、上り光信号として送り返すことができる。つまり、本システムでは、この下り光信号Ｐｄをも上り光信号として利用することができるようになったため、効率よく信号（上り光信号）を送ることができるようになった。
【００４３】
この場合、ビットレートは上り光信号Ｐｕ１，Ｐｕ２、下り光信号Ｐｄとも１００Ｍｂｉｔ／ｓ〜１Ｇｂｉｔ／ｓ程度である。
【００４４】
また、ユーザ端末Ｕ側において、連続光部分を変調した正規上り光信号Ｐｕ１と、下り光信号部分を変調した予備上り光信号Ｐｕ２に同一の信号を振り付けるようにしてもよい。つまり正規上り光信号Ｐｕ１を生成するために連続光Ｐｃを変調する第１の電気信号の信号内容（信号情報）と、予備上り光信号Ｐｕ２を生成するために下り光信号Ｐｄ（消光比が小さくなっている下り光信号Ｐｄ）を変調する第２の電気信号の信号内容（信号情報）を等しくしてもよい。このようにすると、反射型光変調器１０６から送出される正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２との信号内容が等しくなる。
【００４５】
このとき局Ｋ側の上り光信号受信器１０８では、この２つの信号（正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２）を受信し、両者を比較する。ここで予備上り光信号Ｐｕ２と正規上り光信号Ｐｕ１が一致する場合には、信号は正しいと判定することができる。両者が一致しない場合には誤り訂正を試みるか、ユーザ端末Ｕ側に再送を要求する。
この方式により、上り信号の符号誤り率を改善することができる。
【００４６】
前述したように本実施の形態では、連続光Ｐｃと下り光信号Ｐｄの偏光方向を、例えば３０〜４５度ずらしているため、連続光Ｐｃを変調してなる正規上り光信号Ｐｕ１と、消光比を小さくした下り光信号Ｐｄを変調してなる予備上り光信号Ｐｕ２との偏光方向も、同様にずれることになる。
【００４７】
このようにすることで、上り光信号の一方（例えば予備上り光信号Ｐｕ２）の偏光方向が、下りの連続光Ｐｃまたは下り光信号Ｐｄの偏光方向と完全に一致し、波形が乱れた場合でも、上り光信号の他方（例えば正規上り光信号Ｐｕ１）の偏光方向は、下りの連続光Ｐｃまたは下り光信号Ｐｄの偏光方向からずれることになる。その結果、レイリー散乱による波形の劣化を防ぐことが可能になる。
【００４８】
なお図１に示す反射型光通信システムにおける信号波形は、図７（ａ）（ｂ）（ｃ）に示すものと同じである。
【００４９】
［第２の実施の形態］
図２は本発明の第２の実施の形態に係る反射型偏波利用光通信システムを示す。第２の実施の形態は、図８に示す反射型光通信システムを改良したものである。第２の実施の形態では、局Ｋにおいて、下り光信号送信器兼連続光源１０９と光カプラ１１０との間に偏波光スイッチ１００２を備えている。この偏波光スイッチ１００２としては、参考文献１に示す、偏波光スイッチを用いる。なお、他の部分の構成は図８に示すものと同様になっている。
【００５０】
参考文献１：Ｒ．Ｃ．ＡｌｆｅｒｎｅｓｓａｎｄＬ．Ｌ．Ｂｕｈｌ， ”Ｗａｖｅｇｕｉｄｅｅｌｅｃｔｒｏ−ｏｐｔｉｃｐｏｌａｒｉｚａｔｉｏｎｔｒａｓｆｏｒｍｅｒ”，ＡｐｐｌｉｅｄＰｈｙｓｉｃＬｅｔｔｅｒｓ，ｖｏｌ．３８，ｐｐ．６６５，１９８１またはＭ．Ａ．ＳａｎｔｏｒｏａｎｄＣ．Ｄ．Ｐｏｏｌｅ， ”Ｐｏｌａｒｉｚａｔｉｏｎｓｃｒａｍｂｌｉｎｇｕｓｉｎｇａｓｈｏｒｔｐｉｅｃｅｏｆｈｉｇｈ−ｂｉｒｅｆｒｉｎｇｅｎｃｅｏｐｔｉｃａｌｆｉｂｅｒａｎｄａｍｕｌｔｉｆｒｅｑｕｅｎｃｙｌａｓｅｒｄｉｏｄｅ”，Ｊｏｕｒｎａｌｏｆｌｉｇｈｔｗａｖｅｔｅｃｈｎｏｌｏｇｙ，ｖｏｌ．１２，ｎｏ．２，ｐｐ．２８８−２９３，１９９４
【００５１】
局Ｋ側の下り光信号送信器兼連続光源１０９は、連続光Ｐｃと、消光比を３〜５ｄＢ程度に下げた下り光信号Ｐｄとを時分割で送信可能である。ユーザ端末Ｕ側の下り光信号受信器兼反射型光変調器１１２は、下り光信号Ｐｄの受信と、連続光Ｐｃの変調と下り光信号Ｐｄの変調を時分割で行うことができ、変調して生成した正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２を時分割して送出することができる。
【００５２】
また下り光信号受信器兼反射型光変調器１１２は、反射型の半導体増幅器を用い、受信時には逆バイアスを印加して受光電流をモニタし、送信時には正バイアスの変調電流を注入する。さらには下り光信号受信器兼反射型光変調器１１２にかける正バイアスの変調電流を最適に設計することで、下り光信号受信器兼反射型光変調器１１２に、光リミッタアンプとしての機能も持たせることが可能である。つまり、下り光信号受信器兼反射型光変調器１１２を、下り光信号受信器兼反射型光変調器兼光リミッタアンプとして機能させることができる。
【００５３】
偏波光スイッチ１００２は、下り光信号送信器兼連続光源１０９から時分割方式で出力される連続光Ｐｃが通過する際の偏光方向と、下り光信号送信器兼連続光源１０９から時分割方式で出力される下り光信号Ｐｄが通過する際の偏光方向を切り換える。このため、偏波光スイッチ１００２を通過してきた、連続光Ｐｃと下り光信号Ｐｄとの偏光方向が３０〜４０度ずれる。
【００５４】
このように局Ｋから送出される連続光Ｐｃと下り光信号Ｐｄとの偏光方向が３０〜４０度ずれているため、図１に示す第１の実施の形態と同様に、上り光信号の一方（例えば予備上り光信号Ｐｕ２）の偏光方向が、下りの連続光Ｐｃまたは下り光信号Ｐｄの偏光方向と完全に一致し、波形が乱れた場合でも、上り光信号の他方（例えば正規上り光信号Ｐｕ１）の偏光方向は、下りの連続光Ｐｃまたは下り光信号Ｐｄの偏光方向からずれることになる。その結果、レイリー散乱による波形の劣化を防ぐことが可能になる。
【００５５】
［第３の実施の形態］
図３は本発明の第３の実施の形態に係る反射型偏波利用光通信システムを示す。第３の実施の形態は、図１に示す第１の実施の形態に、更に偏波コントローラ１００３を備えたものである。偏波コントローラ１００３は、偏波保持光カプラ１００１と光カプラ１０３との間に備えられている。連続光Ｐｃと下り光信号Ｐｄは偏波コントローラ１００３を通過してから局外に向かって送出されるようになっており、しかも、偏波コントローラ１００３は、連続光Ｐｃと下り光信号Ｐｄの偏波状態（偏光方向）を制御することができる。
【００５６】
本実施の形態では、ユーザ端末Ｕ側において、連続光部分を変調した正規上り光信号Ｐｕ１と、下り光信号部分を変調した予備上り光信号Ｐｕ２に同一の信号を振り付けるようにしている。そして、局Ｋ側の上り光信号受信器１０８では、この２つの信号（正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２）を受信し、両者を比較する。ここで予備上り光信号Ｐｕ２と正規上り光信号Ｐｕ１が一致する場合には、信号は正しいと判定することができる。両者が一致しない場合には、両者の信号状態が一致するまで、偏波コントローラ１００３により、連続光Ｐｃと下り光信号Ｐｄの偏波状態（偏光方向）を制御していく。このようにすることにより、誤りのない良質な光信号の送受信が可能になる。
【００５７】
［第４の実施の形態］
図４は本発明の第４の実施の形態に係る反射型偏波利用光通信システムを示す。第４の実施の形態は、図２に示す第２の実施の形態に、更に偏波コントローラ１００４を備えたものである。偏波コントローラ１００４は、偏波光スイッチ１００２と光カプラ１１０との間に備えられている。連続光Ｐｃと下り光信号Ｐｄは偏波コントローラ１００４を通過してから局外に向かって送出されるようになっており、しかも、偏波コントローラ１００４は、連続光Ｐｃと下り光信号Ｐｄの偏波状態（偏光方向）を制御することができる。
【００５８】
本実施の形態では、ユーザ端末Ｕ側において、連続光部分を変調した正規上り光信号Ｐｕ１と、下り光信号部分を変調した予備上り光信号Ｐｕ２に同一の信号を振り付けるようにしている。そして、局Ｋ側の上り光信号受信器１０８では、この２つの信号（正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２）を受信し、両者を比較する。ここで予備上り光信号Ｐｕ２と正規上り光信号Ｐｕ１が一致する場合には、信号は正しいと判定することができる。両者が一致しない場合には、両者の信号状態が一致するまで、偏波コントローラ１００４により、連続光Ｐｃと下り光信号Ｐｄの偏波状態（偏光方向）を制御していく。このようにすることにより、誤りのない良質な光信号の送受信が可能になる。
【００５９】
［変形例］
なお、図３及び図４に示す実施の形態では、偏波コントローラ１００３，１００４を設けたが、偏波コントローラ１００３，１００４の代わりに、光遅延線を設け、正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２の受信結果が一致するまで、光遅延線により下り光信号の位相制御をして、連続光Ｐｃと下り光信号Ｐｄの偏波状態（偏光方向）を制御していくようにしてもよい。
【００６０】
また、偏波コントローラ１００３，１００４を設ける代わりに、図３における下り光信号送信器１０２や、図４における下り光信号送信器兼連続光源１０９の電気信号入力部分に電気遅延回路を設け、その遅延量を制御することでも同様な効果を得ることができる。
【００６１】
ここで参考のために、偏波コントローラ１００３または偏波コントローラ１００４を制御する制御回路について、図５を基に説明する。この制御回路を使用するときには、正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２の前に、それぞれプリアンブルパターン信号（１０〜１６ビットの固定パターン信号）を付与しておく。なお、正規上り光信号Ｐｕ１に付与するプリアンブルパターン信号と、予備上り光信号Ｐｕ２に付与するプリアンブルパターン信号は、同じパターン信号である。また、この例では、正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２に同一の信号を振り付けているものとする。
【００６２】
図５に示すように、光カプラ１０３（または１１０）にて分岐した正規上り光信号Ｐｕ１と予備上り光信号Ｐｕ２は光ファイバ５０１により伝送され、上り光信号受信器１０８（または１１３）内に備えられた受光器５０２にて受光される。
受光器５０２は、正規上り光信号Ｐｕ１を電気信号に変換した正規上り信号Ｅｕ１（プリアンブルパターン信号に相当する電気信号も含む）を出力すると共に、予備上り光信号Ｐｕ２を電気信号に変換した予備上り信号Ｅｕ２（プリアンブルパターン信号に相当する電気信号も含む）を出力する。
【００６３】
プリアンブル検出回路５０３は、電気信号となっているプリアンブルパターン信号を検出する。高速スイッチ５０４は、プリアンブル検出回路５０３がプリアンブルパターン信号を検出する毎にスイッチングを行う。このため、正規上り信号Ｅｕ１と予備上り信号Ｅｕ２のうちの一方の信号がラインＬ１に出力され、他方の信号がラインＬ２に出力される。
【００６４】
プリアンブル検出回路５０５では、ラインＬ１を通る上り信号に付与されているプリアンブルパターン信号を検出し、プリアンブル検出回路５０６ではラインＬ２を通る上り信号に付与されているプリアンブルパターンを検出する。一致比較回路５０７は、プリアンブル検出回路５０５，５０６の検出結果をＡＮＤ演算し、パワーモニタ５０８はＡＮＤ演算結果の平均出力パワーを検出する。位相回路調節回路５０９は、平均出力パワーが最大となるように、ラインＬ１に介装された位相回路５１０の位相状態を調整する。このような位相調整をすることにより、一致比較回路５１１には、位相状態が一致した正規上り信号Ｅｕ１と予備上り信号Ｅｕ２が入力される。
【００６５】
一致比較回路５１１では、位相状態が一致した正規上り信号Ｅｕ１と予備上り信号Ｅｕ２をＡＮＤ演算し、パワーモニタ５１２では、このＡＮＤ演算結果の平均出力パワーを検出する。偏波コントローラ調節回路５１３は、この平均出力パワーが最大となるように、偏波コントローラ１００３（または１００４）の偏波状態（偏光方向）を調節する。
【００６６】
かくして、偏波コントローラ１００３（１００４）による、連続光Ｐｃと下り光信号Ｐｄに対する偏光方向の制御を調整することにより、誤りのない良好な光信号の送受信が可能になる。
【００６７】
また、ラインＬ１側から受信結果（位相状態及び信号情報が一致している正規上り信号Ｅｕ１と予備上り信号Ｅｕ２のうちの一方の信号）を出力する。
【００６８】
【発明の効果】
以上説明したように、本発明の反射型偏波利用光通信システムならびにこれに用いる局及びユーザ端末では、下り光信号の消光比を減少させ消光比が減少した下り光信号を第２の電気信号で変調して予備上り光信号を生成し、この予備上り光信号を、ユーザ端末から局に送り返すようにした。このため、連続光を第１の電気信号で変調してなる正規上り光信号と、消光比が減少した下り光信号を第２の電気信号で変調してなる予備上り光信号を、上り光信号として使用することができるので、効率よく上り光信号を送ることができる。
しかも、連続光と下り光信号の偏光方向を異ならせているため、上りと下りの光信号の干渉であるレイリー散乱により、上り光信号（正規上り光信号と予備上り光信号）の一方の波形が劣化しても、他方の上り光信号の波形劣化を防ぐことができるため、良質な光信号の送受信が可能となった。
【００６９】
更に、第１の電気信号と第２の電気信号の信号状態を同じにし、ユーザ端末において、正規上り光信号と予備上り光信号とを比較することにより、上り光信号の符号誤り率の改善をはかることができる。
また、正規上り光信号と予備上り光信号の信号内容が一致するまで、連続光と下り光信号の偏光方向の制御をすることにより、更に良質な光信号の送受信が可能となった。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態に係る反射型偏波利用光通信システムを示す構成図。
【図２】本発明の第２の実施の形態に係る反射型偏波利用光通信システムを示す構成図。
【図３】本発明の第３の実施の形態に係る反射型偏波利用光通信システムを示す構成図。
【図４】本発明の第４の実施の形態に係る反射型偏波利用光通信システムを示す構成図。
【図５】偏波コントローラを制御する制御回路の一例を示す回路図。
【図６】本願発明者が最近開発した反射型光通信システムを示す構成図。
【図７】反射型光通信システムにおける光信号の信号波形を示す特性図。
【図８】本願発明者が最近開発した反射型光通信システムの他の例を示す構成図。
【図９】反射型光通信システムにおいて、レイリー散乱による波形劣化がない場合の信号波形図。
【図１０】反射型光通信システムにおいて、レイリー散乱による波形劣化が生じた場合の信号波形図。
【符合の説明】
１０１連続光源
１０２下り光信号送信器
１０３光カプラ
１０４光ファイバ
１０５光カプラ
１０６反射型光変調器
１０７下り光信号受信器
１０８上り光信号受信器
１０９下り光信号送信器兼連続光源
１１０光カプラ
１１１光ファイバ
１１２下り光信号受信器兼反射型光変調器
１１３上り光信号受信器
１００１偏波保持光カプラ
１００２偏波光スイッチ
１００３，１００４偏波コントローラ
Ｋ局
Ｕユーザ端末
Ｐｃ連続光
Ｐｄ下り光信号
Ｐｕ上り光信号
Ｐｕ１正規上り光信号
Ｐｕ２予備上り光信号
Ｅｕ１正規の上り信号
Ｅｕ２予備上り信号[0001]
BACKGROUND OF THE INVENTION
The present invention is applied to an optical transmission system such as high-speed optical communication propagating through an optical fiber, optical switching, and optical information processing, and more particularly to a reflection-type optical communication system that uses one optical fiber in both upstream and downstream directions. It is an effective technology to apply.
[0002]
[Prior art]
As the transmission capacity increases with the development of IT technology, it is important to effectively use the transmission band of the optical fiber. There is a reflection type optical communication system as a system that effectively uses a transmission band.
[0003]
The inventor of the present application has developed a reflection type optical communication system that can efficiently transmit an upstream optical signal and can improve the code error rate of the upstream optical signal.
[0004]
Here, a reflection type optical communication system recently developed by the present inventor will be described.
[0005]
[First example]
FIG. 6 shows a first example of a reflective optical communication system recently developed by the present inventors. As shown in the figure, the station K and the user terminal U are connected by a single optical fiber 104. The station K includes a continuous light source 101, a downstream optical signal transmitter 102, an optical coupler 103, and an upstream optical signal receiver 108. The user terminal U includes an optical coupler 105, a reflective optical modulator 106, a downstream optical signal receiver 107, and an optical limiter amplifier 114. The reflection type optical modulator 106 is configured using, for example, a reflection type optical amplifier, and functions as an “upstream optical signal transmitter”.
[0006]
The optical limiter amplifier 114 is connected between the optical coupler 105 and the reflective optical modulator 106. The optical limiter amplifier 114 is formed of a semiconductor optical amplifier that has a large gain when the input light intensity of the passing light is low and has a small gain when the input light intensity of the passing light is strong. As such a semiconductor optical amplifier, there is a semiconductor optical amplifier described in Non-Patent Document 1 or the like.
[0007]
By using the optical limiter amplifier 114 having such a limiter characteristic (amplification characteristic), the optical signal passing through the optical limiter amplifier 114 has a large gain at the “0” portion where the light intensity is weak, and the “1” portion. The gain becomes smaller. For this reason, the optical signal that has passed through the optical limiter amplifier 114 becomes flat as the level difference between the “1” portion and the “0” portion becomes small. The use of such an optical limiter amplifier 114 is one of the points of this system.
[0008]
On the station K side, the continuous light Pc generated by the continuous light source 101 and the downstream optical signal Pd generated by the downstream optical signal transmitter 102 are transmitted in a time division manner. Moreover, the extinction ratio of the downstream optical signal Pd is set to a bad value of 3 to 5 dB. Thus, setting the extinction ratio of the downstream optical signal Pd to a bad value is one of the points of this system.
[0009]
When the reflection type optical modulator 106 on the user terminal U side receives the continuous light Pc, the continuous light Pc is modulated with the first electric signal to generate the normal upstream optical signal Pu1, and the normal upstream optical signal Pu1 is generated. Send back. Further, when the reflection type optical modulator 106 receives the downstream optical signal Pd having passed through the optical limiter amplifier 114 and has an extinction ratio as small as 1 to 3 dB, it modulates the downstream optical signal Pd with the second electrical signal. Thus, the backup upstream optical signal Pu2 is generated, and this backup upstream optical signal Pu2 is sent back.
[0010]
In this reflection type optical communication system, continuous light Pc is sent from a continuous light source 101 placed on the station K side, and the user terminal U is configured to send back this modulated continuous light Pc as a regular upstream optical signal Pu1. . For this reason, the continuous light Pc is generated by the first electrical signal by the reflection type light modulator 106 when the continuous light source 101 → the optical coupler 103 → the optical fiber 104 → the optical coupler 105 → the optical limiter amplifier 114 → the reflection type optical modulator 106. Modulated to be a regular upstream optical signal Pu 1 and reflected by the end face of the reflective optical modulator 106. The regular upstream optical signal Pu1 obtained by modulating the continuous light Pc returns as the optical limiter amplifier 114 → the optical coupler 105 → the optical fiber 104 → the optical coupler 103 → the upstream optical signal receiver 108.
[0011]
The downstream optical signal Pd proceeds in the downstream optical signal transmitter 102 → the optical coupler 103 → the optical fiber 104 → the optical coupler 105 → the downstream optical signal receiver 107 and is received by the downstream optical signal receiver 107 of the user terminal U. . The bit rate of both the upstream optical signal Pu and downstream optical signal Pd is about 100 Mbit / s to 1 Gbit / s.
[0012]
What is important here is that, as described above, the extinction ratio of the downstream optical signal Pd transmitted from the downstream optical signal transmitter 102 is set to a bad value of about 3 to 5 dB. However, since the downstream optical signal Pd has a short transmission distance, the influence of the propagation loss is small. Even if the downstream optical signal Pd received by the downstream optical signal receiver 107 has a low extinction ratio, the signal state determination of the downstream optical signal Pd is performed. There will be no problems in doing so.
[0013]
The downstream optical signal Pd is received by the downstream optical signal receiver 107, branched by the optical coupler 105, and sent to the reflective optical modulator 106 through the optical limiter amplifier 114. Since the downstream optical signal Pd transmitted has a bad extinction ratio of 3 to 5 dB, the downstream optical signal Pd after passing through the optical limiter amplifier 114 has a gain of “0” where the light intensity is weak. As the value increases, the gain of the portion “1” decreases, and the extinction ratio further decreases to about 1 to 2 dB. The downstream optical signal Pd having such an extremely low extinction ratio is modulated by the second electrical signal in the reflection type optical modulator 106 to become the preliminary upstream optical signal Pu2. This spare upstream optical signal Pu2 returns as follows: optical limiter amplifier 114 → optical coupler 105 → optical fiber 104 → optical coupler 103 → upstream optical signal receiver 108.
[0014]
As described above, not only the regular upstream optical signal Pu1 obtained by modulating the continuous light Pc but also the backup upstream optical signal Pu2 obtained by modulating the downstream optical signal Pd having a reduced extinction ratio can be sent back as the upstream optical signal. That is, although the downstream optical signal Pd has been sent back in the past, it has not been sent back as an upstream optical signal. However, in this system, the downstream optical signal Pd can also be used as an upstream optical signal. Therefore, the signal (upstream optical signal) can be sent efficiently.
[0015]
The waveform of this reflective optical communication system is shown in FIG.
FIG. 7A shows the waveform of the downlink (station → user terminal), and the downlink optical signal Pd and the continuous light Pc are transmitted in a time division manner. What is important here is that the extinction ratio of the downstream optical signal Pd is set to a bad value of about 3 to 5 dB. Since the downstream optical signal Pd has a short transmission distance, the influence of the propagation loss is small, and no problem occurs even if the extinction ratio is low.
[0016]
FIG. 7B shows the waveforms of the continuous light Pc and the downstream optical signal Pd that pass through the optical limiter amplifier 114 and travel toward the reflective optical modulator 106. As described above, since the optical limiter amplifier 114 is a semiconductor amplifier having a large gain when the input light intensity is low and a small gain when the input light intensity is strong, the gain of the “0” portion having a low light intensity is large. The gain of the “1” portion is reduced. As a result, the extinction ratio of the downstream optical signal Pd after passing through the optical limit amplifier 114 is as small as about 1 to 2 dB.
[0017]
FIG. 7C shows the waveform of the upstream optical signal. What is important here is that not only the regular upstream optical signal Pu1 obtained by modulating the continuous optical part but also the backup upstream optical signal Pu2 obtained by modulating the downstream optical signal part can be used as the upstream optical signal. . That is, the downstream optical signal portion can be newly modulated. This means that a portion that has not been used in the past (downlink optical signal portion) is also used as an upstream optical signal, so that the upstream optical signal can be efficiently transmitted.
[0018]
[Second example]
Next, a second example recently developed by the present inventor will be described. In the second example, the system configuration of FIG. 6 is used, but on the user terminal U side, the same signal as the normal upstream optical signal Pu1 that modulates the continuous optical portion and the backup upstream optical signal Pu2 that modulates the downstream optical signal portion. Choreograph. That is, the signal content (signal information) of the first electric signal that modulates the continuous light Pc to generate the normal upstream optical signal Pu1, and the downstream optical signal Pd (the extinction ratio is small) to generate the backup upstream optical signal Pu2. The signal content (signal information) of the second electric signal for modulating the downstream optical signal Pd) is made equal. As a result, the signal contents of the regular upstream optical signal Pu1 and the backup upstream optical signal Pu2 transmitted from the reflective optical modulator 106 become equal.
[0019]
On the station K side, these two signals (regular upstream optical signal Pu1 and backup upstream optical signal Pu2) are received and compared. Here, if the backup upstream optical signal Pu2 and the regular upstream optical signal Pu1 match, it can be determined that the signal is correct. If the two do not match, an error correction is attempted, or retransmission is requested to the user terminal U side.
This scheme can improve the code error rate of the uplink signal.
[0020]
[Third example]
Next, a third example recently developed by the present inventor will be described with reference to FIG. In FIG. 8, 109 is a downstream optical signal transmitter / continuous light source, 110 is an optical coupler, 111 is an optical fiber, 112 is a downstream optical signal receiver / reflection type optical modulator, and 113 is an upstream optical signal receiver.
[0021]
In FIG. 8, the downstream optical signal transmitter / continuous light source 109 on the station K side can transmit the continuous light Pc and the downstream optical signal Pd with the extinction ratio lowered to about 3 to 5 dB in a time division manner. The downstream optical signal receiver / reflective optical modulator 112 on the user terminal U side can perform time-division reception of downstream optical signal Pd, modulation of continuous light Pc, and downstream optical signal Pd. The normal upstream optical signal Pu1 and the backup upstream optical signal Pu2 generated in this manner can be transmitted in a time-sharing manner.
[0022]
The downstream optical signal transmitter / continuous light source 109 is, for example, a direct modulation type DFB laser, and injects a DC current when transmitting the continuous light Pc, and a modulation current when transmitting the downstream signal.
[0023]
The downstream optical signal receiver / reflective optical modulator 112 uses, for example, a reflective semiconductor amplifier shown in Non-Patent Document 2, applies a reverse bias during reception to monitor the received light current, and transmits a positive bias modulation current during transmission. Inject.
[0024]
Furthermore, by optimizing the positive bias modulation current applied to the downstream optical signal receiver / reflective optical modulator 112, as shown in Non-Patent Document 3, the downstream optical signal receiver / reflective optical modulator 112 is designed. It is also possible to provide a function as an optical limiter amplifier. That is, the downstream optical signal receiver / reflection optical modulator 112 can function as a downstream optical signal receiver / reflection optical modulator / optical limiter amplifier.
[0025]
Also in the reflection type optical communication system configured as shown in FIG. 8, as shown in FIG. 7, the normal upstream optical signal Pu1 is generated by modulating the continuous light Pc with the first electrical signal. By sending back Pu1 and modulating the downstream optical signal Pd with a reduced extinction ratio with the second electrical signal to generate the backup upstream optical signal Pu2, and sending back the backup upstream optical signal Pu2, the code error of the upstream optical signal The rate can be improved.
[0026]
[Non-Patent Document 1]
Y. Shibata, Y. et al. Yamada, K .; Habara, and N.H. Yoshimoto, “Semiconductor laser diode optical amplifiers / gates in photonic packet switching”, Journal of Lightwave Technology. 16, no. 12, pp. 2228-2235, 1998
[Non-Patent Document 2]
M.M. Okayasu, Y. et al. Suzuki, N .; Yoshimoto, R.A. Iga, H .; Sugiura, and J.A. Yosida, “Bi-directional transmission experiential using a semi-conductor, amplifier transducer mud for WDM-PON and WDM-LAN applications.” W. Faulker and A.M. L. Harmer (Eds.), IOS Press, 1999, pp. 278-283, IOS Press, ISBN 4 274 90296 X C3055, Ohm Corporation [Non-Patent Document 3]
H. Takesue and T. Sugie, "Data rewrite of wavelength channel using saturated SOA modulator for WDM metro / access networks with centralized light sources", 28th European Conference on Optical, ECOC2002, September 8-12, 2002, Copenhagen, Denmark
[0027]
[Problems to be solved by the invention]
However, the reflection type optical communication system recently developed by the present inventor described based on FIGS. 6 to 8 has a further point to be improved.
[0028]
That is, the interference caused in the optical fibers 104 and 111 by the optical signal transmitted in both directions. In general, when the polarization directions of both (upstream and downstream) optical signals are different, as shown in FIG. 9, it is possible to receive a clean waveform, that is, a waveform in which neither the 0 level nor the 1 level of the optical signal fluctuates. Usually, in order to receive data, a threshold value 301 is provided at an approximately halfway point between the 0 level and the 1 level.
[0029]
However, when the polarization directions of both (upstream and downstream) optical signals coincide, a part of either optical signal is reflected by Rayleigh scattering of the optical fiber, and the reflected light interferes with the other optical signal. As a result, the received waveform is disturbed. The influence of this interference is particularly noticeable at level 1, and as shown in FIG. 10, it appears that level 1 becomes thicker. As a result, when level detection is performed using the threshold 401, there is a problem that one level is easily determined to be zero.
[0030]
Accordingly, in the reflection type optical communication system shown in FIGS. 6 and 8, the polarization direction of the downstream optical signal Pd and the continuous light Pc from the station K to the user terminal U are the same, and the user terminal U to the station Since the polarization directions of the spare upstream optical signal Pu2 and the regular upstream optical signal Pu1 heading for K are the same, there is a drawback that if one of the upstream optical signals is disturbed by interference, the other is also disturbed.
[0031]
As described above, in the reflection type optical communication system, there is a problem that interference occurs when the polarization states of the upstream and downstream optical signals propagating through the optical fiber coincide with each other, and the received waveform deteriorates.
[0032]
The present invention has been made in view of the above prior art, and an object of the present invention is to provide a reflection polarization optical communication system capable of transmitting and receiving a high-quality optical signal by preventing deterioration of a received waveform, and a station and a user terminal used therefor To do.
[0033]
[Means for Solving the Problems]
The configuration of the reflection polarization polarization optical communication system of the present invention that solves the above problem is that the station and the user terminal are connected by an optical fiber,
The station transmits continuous light and downstream optical signals to the user terminal in a time division manner, and receives upstream optical signals transmitted from the user terminal,
In the reflective optical communication system, the user terminal receives a downstream optical signal transmitted from the station, and sends back an upstream optical signal generated by modulating the received continuous light to the station.
The station transmits the continuous light and the downstream optical signal to the user terminal with different polarization directions,
In the user terminal, not only the regular upstream optical signal generated by modulating the received continuous light with the first electric signal is sent back to the station, but also the extinction ratio of the received downstream optical signal is decreased to reduce the extinction ratio. A spare upstream optical signal generated by modulating the reduced downstream optical signal with the second electrical signal is sent back to the station.
At this time, the signal contents of the first electric signal and the second electric signal used in the user terminal are the same,
The station determines that the signal content is correct when the signal content of the received regular upstream optical signal and the backup upstream optical signal match,
The station transmits a downstream optical signal having an extinction ratio of 3 to 5 dB.
[0034]
The station used in the reflection polarization optical communication system of the present invention includes a continuous light source that transmits continuous light, a downstream optical signal transmitter that transmits downstream optical signals, a regular upstream optical signal that is an upstream optical signal, and a standby optical signal. An upstream optical signal receiver for receiving the upstream optical signal,
In addition, the polarization directions of the continuous light transmitted from the continuous light source and the downstream optical signal transmitted from the downstream optical signal transmitter are different.
At this time, this station receives a continuous light source that transmits continuous light, a downstream optical signal transmitter that transmits downstream optical signals, and an upstream optical signal reception that receives regular upstream optical signals and backup upstream optical signals that are upstream optical signals. And a polarization controller,
Moreover, the polarization direction of the continuous light transmitted from the continuous light source and the downstream optical signal transmitted from the downstream optical signal transmitter are different,
Further, the polarization controller controls the polarization direction of the continuous light transmitted from the continuous light source and the downstream optical signal transmitted from the downstream optical signal transmitter, and then transmits it outside the station.
Alternatively, the polarization direction of the continuous light and the downstream optical signal is controlled by the polarization controller until the signal contents of the regular upstream optical signal and the backup upstream optical signal received by the upstream optical signal receiver match. .
[0035]
The station used in the reflective polarization-based optical communication system of the present invention includes a downstream optical signal transmitter / continuous light source that transmits a downstream optical signal and continuous light in a time division manner, and a regular upstream optical signal that is an upstream optical signal. An upstream optical signal receiver that receives the preliminary upstream optical signal, and a polarization optical switch;
The polarization optical switch switches the polarization direction so that the polarization direction of the downstream optical signal transmitted from the downstream optical signal transmitter / continuous light source is different from that of the continuous light.
At this time, this station transmits a downstream optical signal and continuous light in a time division manner and a downstream optical signal transmitter / continuous light source, and an upstream optical signal that receives an upstream optical signal, a regular upstream optical signal and a backup upstream optical signal. A receiver, a polarization optical switch, and a polarization controller;
The polarization optical switch switches the polarization direction so that the downstream light signal transmitted from the downstream optical signal transmitter and continuous light source is different from the polarization direction of the continuous light,
Further, the polarization controller controls the polarization direction of the continuous light and the downstream optical signal sent from the polarization optical switch and then sends it out of the station.
Further, the polarization direction of the continuous light and the downstream optical signal is controlled by the polarization controller until the signal contents of the regular upstream optical signal and the backup upstream optical signal received by the upstream optical signal receiver match.
[0036]
Further, the user terminal used in the reflective polarization-based optical communication system of the present invention includes a downstream optical signal receiver that receives a downstream optical signal, an optical limiter amplifier that decreases the extinction ratio of the received downstream optical signal, and the received continuous The optical signal is modulated by the first electrical signal to generate a normal upstream optical signal, which is sent back to the station, and the downstream optical signal whose extinction ratio is reduced by passing through the optical limiter amplifier is modulated by the second electrical signal. And a reflection type optical modulator that generates a spare upstream optical signal and sends it back to the station.
At this time, the user terminal receives the downstream optical signal, modulates the received continuous light with the first electrical signal, generates a normal upstream optical signal, sends it back to the station, and receives the received downstream optical signal. A downstream optical signal receiver and a reflective optical modulator that modulates the electrical signal of 2 to generate a preliminary upstream optical signal and send it back to the station;
The user terminal receives the downstream optical signal, reduces the extinction ratio of the received downstream optical signal, further modulates the received continuous light with the first electric signal to generate a normal upstream optical signal, and And a downstream optical signal receiver / reflective optical modulator / optical limiter amplifier, which generates a backup upstream optical signal by modulating the downstream optical signal having a reduced extinction ratio with a second electrical signal and returns it to the station. It is characterized by that.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the drawings. Since the reflection type polarization optical communication system according to the present embodiment is an improvement of the reflection type optical communication system previously developed as shown in FIG. 6 and FIG. Portions that perform the same functions as the system are given the same reference numerals, and redundant descriptions are omitted.
[0038]
[First Embodiment]
FIG. 1 shows a reflection-type polarization optical communication system according to a first embodiment of the present invention. In the first embodiment, the reflection type optical communication system shown in FIG. 6 is improved. In the first embodiment, a polarization maintaining optical coupler 1001 is provided in the station K, the continuous light source 101 and the polarization maintaining optical coupler 1001 are connected by the polarization maintaining optical fiber F1, and the downstream optical signal transmitter 102 and the polarization are connected. The holding optical coupler 1001 is connected by a polarization holding optical fiber F2. The continuous light Pc output from the continuous light source 101 and the downstream optical signal Pd output from the downstream optical signal transmitter 102 are transmitted by a time division method, and in this embodiment, continuous light Pc is transmitted continuously. The polarization directions of the light Pc and the downstream optical signal Pd are shifted (for example, the polarization direction is shifted by 30 to 45 degrees). Thus, shifting the polarization directions of the continuous light Pc and the downstream light signal Pd (changing the polarization directions) is the greatest point of this system. The configuration of other parts is the same as that shown in FIG.
[0039]
The continuous light Pc output from the continuous light source 101 has a small extinction ratio of 1 to 3 dB. This continuous light Pc is a polarization maintaining optical fiber F1 → a polarization maintaining optical coupler 1001 → an optical coupler 103 → an optical fiber. 104 → Optical coupler 105 → Optical limiter amplifier 114 → Reflective optical modulator 106 In the process, the reflective optical modulator 106 modulates with the first electric signal to become the normal upstream optical signal Pu1, and the reflective optical modulator 106 Reflected at the end face of The regular upstream optical signal Pu1 obtained by modulating the continuous light Pc returns as the optical limiter amplifier 114 → the optical coupler 105 → the optical fiber 104 → the optical coupler 103 → the upstream optical signal receiver 108.
[0040]
Further, the downstream optical signal Pd proceeds in the order of downstream optical signal transmitter 102 → polarization maintaining optical fiber F2 → polarization maintaining optical coupler 1001 → optical coupler 103 → optical fiber 104 → optical coupler 105 → downstream optical signal receiver 107, Received by the downstream optical signal receiver 107 of the user terminal U.
[0041]
The downstream optical signal Pd is received by the downstream optical signal receiver 107, branched by the optical coupler 105, and sent to the reflective optical modulator 106 through the optical limiter amplifier 114. Since the downstream optical signal Pd transmitted has a bad extinction ratio of 3 to 5 dB, the downstream optical signal Pd after passing through the optical limiter amplifier 114 has a gain of “0” where the light intensity is weak. As the value increases, the gain of the portion “1” decreases, and the extinction ratio further decreases to about 1 to 2 dB. The downstream optical signal Pd having such an extremely low extinction ratio is modulated by the second electrical signal in the reflection type optical modulator 106 to become the preliminary upstream optical signal Pu2. This spare upstream optical signal Pu2 returns as follows: optical limiter amplifier 114 → optical coupler 105 → optical fiber 104 → optical coupler 103 → upstream optical signal receiver 108.
[0042]
As described above, not only the regular upstream optical signal Pu1 obtained by modulating the continuous light Pc but also the backup upstream optical signal Pu2 obtained by modulating the downstream optical signal Pd having a reduced extinction ratio can be sent back as the upstream optical signal. That is, in this system, since this downstream optical signal Pd can be used as an upstream optical signal, a signal (upstream optical signal) can be efficiently transmitted.
[0043]
In this case, the bit rate is about 100 Mbit / s to 1 Gbit / s for both the upstream optical signals Pu1 and Pu2 and the downstream optical signal Pd.
[0044]
Further, on the user terminal U side, the same signal may be assigned to the normal upstream optical signal Pu1 that modulates the continuous optical portion and the backup upstream optical signal Pu2 that modulates the downstream optical signal portion. That is, the signal content (signal information) of the first electric signal that modulates the continuous light Pc to generate the normal upstream optical signal Pu1, and the downstream optical signal Pd (the extinction ratio is small) to generate the backup upstream optical signal Pu2. The signal content (signal information) of the second electric signal that modulates the downstream optical signal Pd) may be equalized. In this way, the signal contents of the regular upstream optical signal Pu1 and the backup upstream optical signal Pu2 transmitted from the reflective optical modulator 106 are equal.
[0045]
At this time, the upstream optical signal receiver 108 on the station K side receives these two signals (regular upstream optical signal Pu1 and backup upstream optical signal Pu2) and compares them. Here, if the backup upstream optical signal Pu2 and the regular upstream optical signal Pu1 match, it can be determined that the signal is correct. If the two do not match, an error correction is attempted, or retransmission is requested to the user terminal U side.
This scheme can improve the code error rate of the uplink signal.
[0046]
As described above, in the present embodiment, the polarization directions of the continuous light Pc and the downstream light signal Pd are shifted by, for example, 30 to 45 degrees, so that the normal upstream light signal Pu1 obtained by modulating the continuous light Pc and the extinction ratio Similarly, the polarization direction of the backup upstream optical signal Pu2 obtained by modulating the downstream optical signal Pd having a smaller value is also shifted.
[0047]
In this way, even when the polarization direction of one of the upstream optical signals (for example, the backup upstream optical signal Pu2) completely coincides with the polarization direction of the downstream continuous light Pc or the downstream optical signal Pd, and the waveform is disturbed. The polarization direction of the other upstream optical signal (for example, the regular upstream optical signal Pu1) is deviated from the polarization direction of the downstream continuous light Pc or the downstream optical signal Pd. As a result, it is possible to prevent waveform degradation due to Rayleigh scattering.
[0048]
The signal waveforms in the reflective optical communication system shown in FIG. 1 are the same as those shown in FIGS. 7 (a), (b), and (c).
[0049]
[Second Embodiment]
FIG. 2 shows a reflection-type polarized light optical communication system according to the second embodiment of the present invention. In the second embodiment, the reflection type optical communication system shown in FIG. 8 is improved. In the second embodiment, the station K includes the polarization optical switch 1002 between the downstream optical signal transmitter / continuous light source 109 and the optical coupler 110. As the polarization optical switch 1002, a polarization optical switch shown in Reference Document 1 is used. The configuration of other parts is the same as that shown in FIG.
[0050]
Reference 1: R.A. C. Alfness and L.L. L. Buhl, “Waveguide electro-optical polarization transformer”, Applied Physic Letters, vol. 38, pp. 665, 1981 or M.M. A. Santoro and C.M. D. Poole, “Polarization scrambling using a short piece of high-birefringence optical fibre and a multifrequency diol fow.” 12, no. 2, pp. 288-293, 1994
[0051]
The downstream optical signal transmitter / continuous light source 109 on the station K side can transmit the continuous light Pc and the downstream optical signal Pd with the extinction ratio lowered to about 3 to 5 dB in a time division manner. The downstream optical signal receiver / reflective optical modulator 112 on the user terminal U side can perform time-division reception of downstream optical signal Pd, modulation of continuous light Pc, and downstream optical signal Pd. The normal upstream optical signal Pu1 and the backup upstream optical signal Pu2 generated in this manner can be transmitted in a time-sharing manner.
[0052]
The downstream optical signal receiver / reflective optical modulator 112 uses a reflective semiconductor amplifier, applies a reverse bias during reception to monitor the received light current, and injects a positive bias modulation current during transmission. Furthermore, by optimizing the positive bias modulation current applied to the downstream optical signal receiver / reflective optical modulator 112, the downstream optical signal receiver / reflective optical modulator 112 can also function as an optical limiter amplifier. It is possible to have it. That is, the downstream optical signal receiver / reflection optical modulator 112 can function as a downstream optical signal receiver / reflection optical modulator / optical limiter amplifier.
[0053]
The polarization optical switch 1002 outputs the polarization direction when the continuous light Pc output from the downstream optical signal transmitter / continuous light source 109 in a time division manner passes, and outputs from the downstream optical signal transmitter / continuous light source 109 in a time division manner. The polarization direction when the transmitted downstream optical signal Pd passes is switched. For this reason, the polarization directions of the continuous light Pc and the downstream optical signal Pd that have passed through the polarization optical switch 1002 are shifted by 30 to 40 degrees.
[0054]
As described above, since the polarization directions of the continuous light Pc transmitted from the station K and the downstream optical signal Pd are shifted by 30 to 40 degrees, one of the upstream optical signals is similar to the first embodiment shown in FIG. Even if the polarization direction of (for example, the preliminary upstream optical signal Pu2) completely coincides with the polarization direction of the downstream continuous light Pc or the downstream optical signal Pd and the waveform is disturbed, the other of the upstream optical signals (for example, the regular upstream optical signal) The polarization direction of Pu1) deviates from the polarization direction of the downstream continuous light Pc or the downstream optical signal Pd. As a result, it is possible to prevent waveform degradation due to Rayleigh scattering.
[0055]
[Third Embodiment]
FIG. 3 shows a reflection-type polarized wave optical communication system according to the third embodiment of the present invention. In the third embodiment, a polarization controller 1003 is further provided in the first embodiment shown in FIG. The polarization controller 1003 is provided between the polarization maintaining optical coupler 1001 and the optical coupler 103. The continuous light Pc and the downstream optical signal Pd are transmitted to the outside of the station after passing through the polarization controller 1003. In addition, the polarization controller 1003 detects the polarization of the continuous light Pc and the downstream optical signal Pd. The wave state (polarization direction) can be controlled.
[0056]
In the present embodiment, on the user terminal U side, the same signal is assigned to the normal upstream optical signal Pu1 that modulates the continuous optical portion and the backup upstream optical signal Pu2 that modulates the downstream optical signal portion. Then, the upstream optical signal receiver 108 on the station K side receives these two signals (regular upstream optical signal Pu1 and backup upstream optical signal Pu2) and compares them. Here, if the backup upstream optical signal Pu2 and the regular upstream optical signal Pu1 match, it can be determined that the signal is correct. If the two do not match, the polarization controller 1003 controls the polarization state (polarization direction) of the continuous light Pc and the downstream optical signal Pd until the two signal states match. By doing so, it is possible to transmit and receive high-quality optical signals without errors.
[0057]
[Fourth Embodiment]
FIG. 4 shows a reflection polarization type optical communication system according to the fourth embodiment of the present invention. In the fourth embodiment, a polarization controller 1004 is further provided in the second embodiment shown in FIG. The polarization controller 1004 is provided between the polarization optical switch 1002 and the optical coupler 110. The continuous light Pc and the downstream optical signal Pd are transmitted to the outside of the station after passing through the polarization controller 1004. In addition, the polarization controller 1004 detects the polarization of the continuous light Pc and the downstream optical signal Pd. The wave state (polarization direction) can be controlled.
[0058]
In the present embodiment, on the user terminal U side, the same signal is assigned to the normal upstream optical signal Pu1 that modulates the continuous optical portion and the backup upstream optical signal Pu2 that modulates the downstream optical signal portion. Then, the upstream optical signal receiver 108 on the station K side receives these two signals (regular upstream optical signal Pu1 and backup upstream optical signal Pu2) and compares them. Here, if the backup upstream optical signal Pu2 and the regular upstream optical signal Pu1 match, it can be determined that the signal is correct. If the two do not match, the polarization controller 1004 controls the polarization state (polarization direction) of the continuous light Pc and the downstream optical signal Pd until the two signal states match. By doing so, it is possible to transmit and receive high-quality optical signals without errors.
[0059]
[Modification]
In the embodiment shown in FIGS. 3 and 4, the polarization controllers 1003 and 1004 are provided. However, instead of the polarization controllers 1003 and 1004, an optical delay line is provided, and the normal upstream optical signal Pu1 and the spare upstream light are provided. Until the reception results of the signal Pu2 match, the phase of the downstream optical signal may be controlled by the optical delay line to control the polarization state (polarization direction) of the continuous light Pc and the downstream optical signal Pd. .
[0060]
Further, instead of providing the polarization controllers 1003 and 1004, an electrical delay circuit is provided at the electrical signal input portion of the downstream optical signal transmitter 102 in FIG. 3 or the downstream optical signal transmitter / continuous light source 109 in FIG. A similar effect can be obtained by controlling the amount.
[0061]
For reference, a control circuit for controlling the polarization controller 1003 or the polarization controller 1004 will be described with reference to FIG. When this control circuit is used, a preamble pattern signal (10 to 16-bit fixed pattern signal) is given before the regular upstream optical signal Pu1 and the backup upstream optical signal Pu2. Note that the preamble pattern signal applied to the regular upstream optical signal Pu1 and the preamble pattern signal applied to the backup upstream optical signal Pu2 are the same pattern signal. In this example, it is assumed that the same signal is assigned to the regular upstream optical signal Pu1 and the backup upstream optical signal Pu2.
[0062]
As shown in FIG. 5, the regular upstream optical signal Pu1 and the backup upstream optical signal Pu2 branched by the optical coupler 103 (or 110) are transmitted through the optical fiber 501, and are provided in the upstream optical signal receiver 108 (or 113). The received light is received by the received light receiver 502.
The light receiver 502 outputs a regular upstream signal Eu1 (including an electrical signal corresponding to a preamble pattern signal) obtained by converting the regular upstream optical signal Pu1 into an electrical signal, and a preliminary upstream signal obtained by converting the spare upstream optical signal Pu2 into an electrical signal. A signal Eu2 (including an electrical signal corresponding to the preamble pattern signal) is output.
[0063]
The preamble detection circuit 503 detects a preamble pattern signal that is an electrical signal. The high speed switch 504 performs switching each time the preamble detection circuit 503 detects a preamble pattern signal. For this reason, one of the regular upstream signal Eu1 and the backup upstream signal Eu2 is output to the line L1, and the other signal is output to the line L2.
[0064]
The preamble detection circuit 505 detects a preamble pattern signal attached to the upstream signal passing through the line L1, and the preamble detection circuit 506 detects a preamble pattern attached to the upstream signal passing through the line L2. The coincidence comparison circuit 507 performs an AND operation on the detection results of the preamble detection circuits 505 and 506, and the power monitor 508 detects an average output power of the AND operation result. The phase circuit adjustment circuit 509 adjusts the phase state of the phase circuit 510 interposed in the line L1 so that the average output power becomes maximum. As a result of such phase adjustment, the coincidence comparison circuit 511 receives the normal upstream signal Eu1 and the spare upstream signal Eu2 whose phase states are identical.
[0065]
The coincidence comparison circuit 511 performs an AND operation on the normal upstream signal Eu1 and the spare upstream signal Eu2 whose phase states coincide with each other, and the power monitor 512 detects the average output power of the AND operation result. The polarization controller adjustment circuit 513 adjusts the polarization state (polarization direction) of the polarization controller 1003 (or 1004) so that the average output power is maximized.
[0066]
Thus, by adjusting the polarization direction control for the continuous light Pc and the downstream optical signal Pd by the polarization controller 1003 (1004), it is possible to transmit and receive a good optical signal without error.
[0067]
Further, the reception result (one signal of the normal uplink signal Eu1 and the backup signal Eu2 having the same phase state and signal information) is output from the line L1 side.
[0068]
【The invention's effect】
As described above, in the reflection-type polarization-use optical communication system of the present invention and the station and user terminal used therefor, the downstream optical signal whose extinction ratio is reduced by reducing the extinction ratio of the downstream optical signal is used as the second electrical signal. The spare upstream optical signal is generated by modulating the optical upstream and the spare upstream optical signal is sent back from the user terminal to the station. For this reason, a normal upstream optical signal obtained by modulating continuous light with the first electrical signal, and a backup upstream optical signal obtained by modulating the downstream optical signal having a reduced extinction ratio with the second electrical signal are designated as upstream optical signals. Therefore, an upstream optical signal can be efficiently transmitted.
In addition, since the polarization directions of the continuous light and the downstream optical signal are different, one waveform of the upstream optical signal (regular upstream optical signal and backup upstream optical signal) is caused by Rayleigh scattering, which is interference between the upstream and downstream optical signals. Even if the signal quality deteriorates, the waveform of the other upstream optical signal can be prevented from being deteriorated, so that a high-quality optical signal can be transmitted and received.
[0069]
Further, the signal state of the first electrical signal and the second electrical signal are made the same, and the normal upstream optical signal and the backup upstream optical signal are compared in the user terminal, thereby improving the code error rate of the upstream optical signal. Can measure.
Further, by controlling the polarization directions of the continuous light and the downstream optical signal until the signal contents of the regular upstream optical signal and the backup upstream optical signal match, it becomes possible to transmit and receive a higher quality optical signal.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a reflection polarization type optical communication system according to a first embodiment of the present invention.
FIG. 2 is a configuration diagram showing a reflection polarization type optical communication system according to a second embodiment of the present invention.
FIG. 3 is a block diagram showing a reflection polarization type optical communication system according to a third embodiment of the present invention.
FIG. 4 is a configuration diagram showing a reflection polarization type optical communication system according to a fourth embodiment of the present invention.
FIG. 5 is a circuit diagram showing an example of a control circuit that controls a polarization controller;
FIG. 6 is a block diagram showing a reflection type optical communication system recently developed by the inventor of the present application.
FIG. 7 is a characteristic diagram showing a signal waveform of an optical signal in a reflection type optical communication system.
FIG. 8 is a block diagram showing another example of a reflective optical communication system recently developed by the inventors of the present application.
FIG. 9 is a signal waveform diagram when there is no waveform deterioration due to Rayleigh scattering in a reflective optical communication system.
FIG. 10 is a signal waveform diagram when waveform deterioration occurs due to Rayleigh scattering in a reflective optical communication system.
[Explanation of sign]
DESCRIPTION OF SYMBOLS 101 Continuous light source 102 Downstream optical signal transmitter 103 Optical coupler 104 Optical fiber 105 Optical coupler 106 Reflective optical modulator 107 Downstream optical signal receiver 108 Upstream optical signal receiver 109 Downstream optical signal transmitter and continuous light source 110 Optical coupler 111 Light Fiber 112 Downstream optical signal receiver / reflection type optical modulator 113 Upstream optical signal receiver 1001 Polarization maintaining optical coupler 1002 Polarization optical switch 1003, 1004 Polarization controller K Station U User terminal Pc Continuous light Pd Downstream optical signal Pu Upstream light Signal Pu1 Regular upstream optical signal Pu2 Backup upstream optical signal Eu1 Regular upstream signal Eu2 Backup upstream signal

Claims

Station and user terminal are connected by optical fiber,
The station transmits continuous light and downstream optical signals to the user terminal in a time division manner, and receives upstream optical signals transmitted from the user terminal,
In the reflective optical communication system, the user terminal receives a downstream optical signal transmitted from the station, and sends back an upstream optical signal generated by modulating the received continuous light to the station.
The station transmits the continuous light and the downstream optical signal to the user terminal with different polarization directions,
In the user terminal, not only the regular upstream optical signal generated by modulating the received continuous light with the first electric signal is sent back to the station, but also the extinction ratio of the received downstream optical signal is decreased to reduce the extinction ratio. A reflection-type polarized light optical communication system, wherein a spare upstream optical signal generated by modulating a reduced downstream optical signal with a second electrical signal is sent back to the station.

In claim 1,
The signal contents of the first electric signal and the second electric signal used in the user terminal are the same,
The reflection-type polarization-based optical communication system, wherein the station determines that the signal content is correct when the received regular upstream optical signal and the backup upstream optical signal match.

In any one of Claim 1 or Claim 2,
The reflection-type polarized wave optical communication system, wherein the station transmits a downstream optical signal having an extinction ratio of 3 to 5 dB.

A station used in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
This station includes a continuous light source that transmits continuous light, a downstream optical signal transmitter that transmits downstream optical signals, and an upstream optical signal receiver that receives normal upstream optical signals and standby upstream optical signals that are upstream optical signals. Have
In addition, the polarization-polarized optical communication system station is characterized in that the polarization directions of the continuous light transmitted from the continuous light source and the downstream optical signal transmitted from the downstream optical signal transmitter are different.

A station used in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
The station includes a continuous light source that transmits continuous light, a downstream optical signal transmitter that transmits downstream optical signals, an upstream optical signal receiver that receives normal upstream optical signals and backup upstream optical signals that are upstream optical signals, A polarization controller,
Moreover, the polarization direction of the continuous light transmitted from the continuous light source and the downstream optical signal transmitted from the downstream optical signal transmitter are different,
Furthermore, the polarization controller controls the polarization direction of the continuous light transmitted from the continuous light source and the downstream optical signal transmitted from the downstream optical signal transmitter, and then transmits the polarization outside the station. A station for reflection-type polarized optical communication systems.

In claim 5,
A reflection type wherein the polarization controller controls the polarization directions of the continuous light and the downstream optical signal until the signal contents of the regular upstream optical signal and the backup upstream optical signal received by the upstream optical signal receiver match. Station for polarization-based optical communication systems.

A station used in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
This station includes a downstream optical signal transmitter and continuous light source that transmits downstream optical signals and continuous light in a time division manner, an upstream optical signal receiver that receives normal upstream optical signals and standby upstream optical signals that are upstream optical signals, and A polarization optical switch,
The polarization optical switch switches the polarization direction so that the downstream light signal transmitted from the downstream optical signal transmitter / continuous light source and the polarization direction of the continuous light are different from each other. Bureau.

A station used in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
This station includes a downstream optical signal transmitter and continuous light source that transmits downstream optical signals and continuous light in a time division manner, an upstream optical signal receiver that receives normal upstream optical signals and standby upstream optical signals that are upstream optical signals, and A polarization optical switch and a polarization controller,
The polarization optical switch switches the polarization direction so that the downstream light signal transmitted from the downstream optical signal transmitter and continuous light source is different from the polarization direction of the continuous light,
Further, the polarization controller controls the polarization direction of the continuous light and the downstream optical signal transmitted from the polarization optical switch, and then transmits the polarization light outside the station. Bureau.

In claim 8,
A reflection type wherein the polarization controller controls the polarization directions of the continuous light and the downstream optical signal until the signal contents of the regular upstream optical signal and the backup upstream optical signal received by the upstream optical signal receiver match. Station for polarization-based optical communication systems.

A user terminal for use in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
The user terminal includes a downstream optical signal receiver that receives a downstream optical signal, an optical limiter amplifier that reduces an extinction ratio of the received downstream optical signal, and a regular upstream by modulating the received continuous light with a first electrical signal. An optical signal is generated and sent back to the station, and a downstream optical signal whose extinction ratio is reduced by passing through the optical limiter amplifier is modulated by a second electrical signal to generate a preliminary upstream optical signal and sent back to the station. A user terminal of a reflection polarization type optical communication system, comprising: a reflection type optical modulator.

A user terminal for use in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
The user terminal receives the downstream optical signal, modulates the received continuous light with the first electrical signal, generates a normal upstream optical signal, sends it back to the station, and transmits the received downstream optical signal to the second electrical signal. A user terminal of a reflection-type polarization optical communication system, comprising a downstream optical signal receiver / reflective optical modulator that modulates a signal to generate a preliminary upstream optical signal and sends it back to the station.

A user terminal for use in the reflection-type polarized light optical communication system according to any one of claims 1 to 3,
The user terminal receives the downstream optical signal, reduces the extinction ratio of the received downstream optical signal, further modulates the received continuous light with the first electric signal to generate a normal upstream optical signal, and And a downstream optical signal receiver / reflective optical modulator / optical limiter amplifier, which generates a backup upstream optical signal by modulating the downstream optical signal having a reduced extinction ratio with a second electrical signal and returns it to the station. A user terminal of a reflection polarization type optical communication system.