JP3974471B2 - Optical transmission system - Google Patents

Description

【０００９】
このことを開示した文献として、
（３）K. S. Kim et al., "Temperature dependence of chromatic dispersion-sifted fibers: Experiment and analysis", J. Appl. Phys., 1993, 73, pp.2069-2074.
がある。
【００１０】
ここでΔＤ（ｐｓ／ｎｍ）は２次波長分散変化量、Ｚ（ｎｍ／ｄｅｇ）は零分散波長シフトの温度係数、Ｓ（ｐｓ／ｎｍ²／ｋｍ）は分散スロープ、Ｌ（ｋｍ）は光ファイバ長である。
【００１１】
この温度変化による２次分散変動量を補償するため適応等化型の分散補償器の研究開発が進められている。具体的にはＶＩＰＡ（Virtually Imaged Phased Array）やファイバグレイティング等が用いられている（上述の文献（３））。
【００１２】
ただし、これまで分散スロープは温度依存性を持たないとされてきた。そのため上記の適応等化型の分散補償器は全波長域で同じ分散変動量を補償するように設定されていた。
【００１３】
しかしながら、実際には光ファイバの分散スロープは温度依存性を有していることが最近わかった。図２は、従来から主に伝送路として使用されている１．３μｍ零分散光ファイバ（ＳＭＦ；Single Mode Fiber）と、最近開発された逆分散光ファイバ（ＲＤＦ；Reverse Dispersion Fiber）の分散スロープの温度依存性を測定した一例を示す図である。
【００１４】
これまで温度依存性を無視していた１．３μｍ零分散光ファイバの分散スロープの温度係数は１．７９×１０^-6（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）である。さらに逆分散光ファイバの分散スロープは、１．３μｍ零分散ファイバや分散シフトファイバに比べて１桁程度大きな温度依存性の分散スロープの温度係数１．４８×１０^-5（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）を持っている。
【００１５】
光信号の波長帯域をΔλ（ｎｍ）および伝送用光ファイバの分散スロープの温度係数をα_T（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）（１℃当たり変化する分散スロープの値）とすると、２次波長分散変化量ΔＤは次式で計算される。
【００１６】
【数４】

【００１７】
従って例えば伝送路を逆分散光ファイバと同様の値α_T＝１．４８×１０^-5（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）を持つとし、光ファイバ長Ｌを１０００（ｋｍ）、年間の温度変動ΔＴを５０（ｄｅｇ）、および波長帯域Δλを１００（ｎｍ）とするとΔＤは７５．０（ｐｓ／ｎｍ）となる。つまり、システムの運用初期に波長帯域Δλ１００ｎｍの全範囲において２次波長分散を０に設定したとしても、分散スロープの温度変化により最短波長のチャネルと最長波長のチャネルで７５．０（ｐｓ／ｎｍ）の２次波長分散の差が生じることになる。
【００１８】
従来の適応等化用分散補償回路を用いても、全波長帯域で同じ分散補償量を増加（または減少）するだけである（図３の波長分散特性３ｄから波長分散特性３ｅ）。この場合図３の波長分散特性３ｅに示したように、１チャネルのビットレート４０Ｇｂｉｔ／ｓのＷＤＭ伝送システム（許容２次波長分散が約４０ｐｓ／ｎｍ（図中符号３７の２Ｄ₀））においては、符号３６のΔＤ₃を上述のΔＤとするとΔＤ₃＞２Ｄ₀となってしまう場合があり、従来の適応等化用分散補償回路の適用は困難である。
【００１９】
【発明が解決しようとする課題】
以上のように、従来の光伝送系においては、実際には光ファイバの分散スロープは温度依存性を有している。このため、高速・広帯域のＷＤＭ伝送では、温度変化によってチャネル間で２次波長分散の影響が異なるという解決すべき課題が従来技術にはあった。
【００２０】
本発明は、このような課題に鑑みてなされたもので、その目的とするところは、分散スロープの温度変動の影響による伝送特性劣化がない伝送を実現することができる光伝送システムを提供することにある。
【００２１】
【課題を解決するための手段】
このような目的を達成するために、本発明の光伝送システムは、光信号を発生する送信系と、該光信号を伝送する伝送用光ファイバと、当該伝送された光信号を受信する受信系とを備えた光伝送システムにおいて、前記受信系が受信する伝送された光信号について、波長分散の分散補償量を調整するための波長分散補償回路をさらに備え、ビットエラーレートが所定値以下になる許容２次波長分散変動値を±ΔＤ₀、前記伝送用光ファイバが受ける温度変化をΔＴ、前記伝送用光ファイバの全長をＬ、前記光信号の波長帯域をΔλ、および前記伝送用光ファイバの１℃当たり変化する分散スロープの値である分散スロープ温度係数をα_Tとした場合、前記ΔＴに対して、式
【００２２】
【数５】

【００２３】
を満たすようにα_T、Ｌ、Δλが設定されており、前記波長分散補償回路が、−ΔＤ ₀ ≦前記波長帯域の全波長の２次波長分散≦＋ΔＤ ₀ となるように前記分散補償量を調整することを特徴とする。
【００２４】
以上の構成により、光伝送システムは分散スロープの温度変動の影響を受けない。
【００２５】
ここで、前記伝送用光ファイバは、複数の光ファイバの組み合わせからなり、該複数の光ファイバの各々は、それぞれ異なる分散スロープ温度係数（α_T1、α_T2、…α_Tn（ｎは２以上の正数）（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）；１℃当たり変化する分散スロープの値）および異なる光ファイバ長（Ｌ₁、Ｌ₂、…Ｌ_n（ｋｍ））を有し、前記ΔＴに対して、前記式(1)に替えて、式
【００２６】
【数６】

【００２７】
を満たすようにα_T1、Ｌ₁、α_T2、Ｌ₂…α_Tn、Ｌ_n、Δλを設定したことを特徴とする。
【００２８】
以上の構成により、異なる種類の光ファイバを用いた光伝送システムでも、分散スロープの温度変動の影響を受けない。
【００２９】
また、前記分散スロープ温度係数α_Tを設定するために、前記伝送用光ファイバの表面にコーティングを施したことを特徴とすることができる。
【００３０】
また、前記コーティングは、前記伝送用光ファイバとは温度変化に対する膨張係数が逆である材料によるコーティングであることを特徴とする。
【００３１】
また、前記分散スロープ温度係数（α_T1、α_T2、…α_Tn）を設定するために、前記複数の光ファイバの各々の表面にコーティングを施したことを特徴とすることができる。
【００３２】
また、前記コーティングは、前記複数の光ファイバの各々について、該光ファイバとは温度変化に対する膨張係数が逆である材料によるコーティングであることを特徴とする。
【００３３】
【発明の実施の形態】
以下、図面を参照して本発明の実施形態を詳細に説明する。なお、各図面において同様の機能を有する箇所には同一の符号を付し、説明の重複は省略する。
【００３４】
［実施形態１］
図１は、本実施形態１の光伝送システムの概略図であり、図中符号１１は送信系、１２−１〜１２−ｎは伝送用光ファイバ、１３−１、１３−２は光中継器、１４は波長分散補償回路、１５は受信系である。送信系１１から発生された光信号（符号１ａ）は伝送用光ファイバ１２−１〜ｎに送信され、ある距離ごとに光中継器１３−１、１３−２等で光増幅されながら波長分散補償回路１４および受信系１５に送られる。
【００３５】
伝送直後の光信号は、伝送用光ファイバ１２−１〜１２−ｎおよび光中継器１３−１、１３−２等の波長分散により波形が劣化する（符号１ｂ）。この光信号を直接受信すると隣接光パルス間の干渉により信号読み取りに誤りが生じる。また、光信号の速度が増加するほど１タイムスロット（１ビットの占める時間幅）が減少するため、波長分散Ｄ（ｐｓ／ｎｍ）による伝送特性への影響は増大する。従って、この伝送路（伝送用光ファイバと光中継系）の波長分散を補償する波長分散補償回路１４が必要となる。
【００３６】
ここで、ビットエラーレートが所定値以下になる許容２次波長分散変動値を±ΔＤ₀（ｐｓ／ｎｍ）、伝送用光ファイバが受ける温度変化をΔＴ（ｄｅｇ）、伝送用光ファイバの全長をＬ（ｋｍ）、光信号の波長帯域をΔλ（ｎｍ）および伝送用光ファイバの分散スロープの温度係数をα_T（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）（１℃当たり変化する分散スロープの値）とする。
【００３７】
本実施形態１の光伝送システムでは、式（４）に示した２次波長分散変化量ΔＤが、伝送用光ファイバが受ける温度変化ΔＴに対して、次式、
【００３８】
【数７】

【００３９】
を満足するようにα_T、Ｌ、Δλを設定する。
【００４０】
図１の図中符号１ｅは本実施形態１における受信系１５直前での波長分散特性である。ここでは例えば、１チャネルのビットレート４０Ｇｂｉｔ／ｓのＷＤＭ伝送システムであって、ビットエラーレートが１０^-9以下になる許容２次波長分散が約４０ｐｓ／ｎｍ（図中符号１７の２ΔＤ₀）とする。
【００４１】
波長分散特性１ｅに示したように、符号１６のΔＤ₁を式（５）を満たすΔＤとすると、分散スロープの温度変化により最短波長のチャネルと最長波長のチャネルで２次波長分散の差が生じたとしても、従来の適応等化用分散補償回路を用いて同じ分散補償量を増加（または減少）するだけで（図１の波長分散特性１ｄから波長分散特性１ｅ）、ΔＤ₁１６を伝送システムの許容２次波長分散量２ΔＤ₀１７以内に押さえることができる。
【００４２】
すなわち、本実施形態１の光伝送システムを用いることにより、分散スロープの温度変動の影響による伝送特性劣化がない伝送を実現することができる。
【００４３】
尚、本実施形態１の上述の手法は、伝送用光ファイバが１本の無中継の光伝送システムにも適用可能なことは明らかである。
【００４４】
［実施形態２］
図１は、本実施形態２の光伝送システムの概略図であり、図中符号１１は送信系、１２−１〜１２−ｎは伝送用光ファイバ、１３−１、１３−２は光中継器、１４は波長分散補償回路、１５は受信系である。送信系１１から発生された光信号（符号１ａ）は伝送用光ファイバ１２−１〜ｎに送信され、ある距離ごとに光中継器１３−１、１３−２等で光増幅されながら波長分散補償回路１４および受信系１５に送られる。
【００４５】
伝送直後の光信号は、伝送用光ファイバ１２−１〜１２−ｎおよび光中継器１３−１、１３−２等の波長分散により波形が劣化する（符号１ｂ）。この光信号を直接受信すると隣接光パルス間の干渉により信号読み取りに誤りが生じる。また、光信号の速度が増加するほど１タイムスロット（１ビットの占める時間幅）が減少するため、波長分散Ｄ（ｐｓ／ｎｍ）による伝送特性への影響は増大する。従って、この伝送路（伝送用光ファイバと光中継系）の波長分散を補償する波長分散補償回路１４が必要となる。
【００４６】
ここで、ビットエラーレートが所定値以下になる許容２次波長分散変動値を±ΔＤ₀（ｐｓ／ｎｍ）、伝送用光ファイバが受ける温度変化をΔＴ（ｄｅｇ）、光信号の波長帯域をΔλ（ｎｍ）とする。
【００４７】
本実施形態２において、図１の光伝送システムは、伝送用光ファイバ１２−１〜１２−ｎが各々異なる分散スロープ温度係数（α_T1、α_T2、…α_Tn（ｎは２以上の正数）（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）；１℃当たり変化する分散スロープの値）および異なる光ファイバ長（Ｌ₁、Ｌ₂、…Ｌ_n（ｋｍ））を有するような、複数の種類の伝送用光ファイバの組み合わせからなる。
【００４８】
本実施形態２の光伝送システムでは、伝送用光ファイバ１２−１〜１２−ｎの式（４）に示した各２次波長分散変化量ΔＤの総和ΔＤ´が、伝送用光ファイバが受ける温度変化ΔＴに対して、次式、
【００４９】
【数８】

【００５０】
を満足するようにα_T1、Ｌ₁、α_T2、Ｌ₂…α_Tn、Ｌ_n、Δλを設定する。
【００５１】
図１の図中符号１ｅは本実施形態１における受信系１５直前での波長分散特性である。ここでは例えば、１チャネルのビットレート４０Ｇｂｉｔ／ｓのＷＤＭ伝送システムであって、ビットエラーレートが１０^-9以下になる許容２次波長分散が約４０ｐｓ／ｎｍ（図中符号１７の２Ｄ₀）とする。
【００５２】
波長分散特性１ｅに示したように、符号１６のΔＤ₁を式（６）を満たすΔＤ´とすると、分散スロープの温度変化により最短波長のチャネルと最長波長のチャネルで２次波長分散の差が生じたとしても、従来の適応等化用分散補償回路を用いて同じ分散補償量を増加（または減少）するだけで（図１の波長分散特性１ｄから波長分散特性１ｅ）、ΔＤ₁１６を伝送システムの許容２次波長分散量２Ｄ₀１７以内に押さえることができる。
【００５３】
すなわち、本実施形態２の光伝送システムを用いることにより、異なる種類の光ファイバを用いた光伝送システムでも、分散スロープの温度変動の影響による伝送特性劣化がない伝送を実現することができる。
【００５４】
［実施形態３］
図４は、実施形態１、２で使用可能な伝送用光ファイバ４０の構造を示す図である。ここで４１はコア、４２はクラッド、４３は液晶ポリマーである。図４に示したようにコア、クラッドから成る光ファイバの表面には液晶ポリマーコーティングが施されている。
【００５５】
光ファイバ（石英）とは逆方向の膨張係数を有する液晶ポリマー４３で光ファイバ４０表面のコーティングを行い、温度変化による物理的な伸縮を抑制することにより、信号の伝搬時間の安定化が実現できる。ここで、光ファイバ心線の単位長さ（１ｋｍ）、単位温度変化（１ｄｅｇ）あたりの伝搬時間差係数Ｋ（ｐｓ／ｋｍ／ｄｅｇ）は次式で与えられる。
【００５６】
【数９】

【００５７】
但し、Ｎは光ファイバの群屈折率、ｃは真空中の光速、Ａは屈折率の温度変化率、Ｂは歪みによる屈折率変化率、Ｅは光ファイバ心線の等化線膨張係数、Ｅ_glassは光ファイバ（ガラス）の線膨張係数である。このＫの波長による２回微分ｄ²Ｋ／ｄλ²は、分散スロープの温度係数α_T（ｐｓ／ｎｍ²／ｋｍ／ｄｅｇ）と等価である。
【００５８】
光伝送システムにおける光信号の波長帯域をΔλ（ｎｍ）、および液晶ポリマー４３を施した伝送用光ファイバ４０の伝搬時間差係数と長さをそれぞれＫ（ｐｓ／ｋｍ／ｄｅｇ）、Ｌ（ｋｍ）、温度変化をΔＴ（ｄｅｇ）とすると、２次波長分散変化量ΔＤ（ｐｓ／ｎｍ）は次式で表される。
【００５９】
【数１０】

【００６０】
したがって、伝送システムの許容２次分散の範囲が−ΔＤ₀（ｐｓ／ｎｍ、但しΔＤ₀＞０）以上ΔＤ₀以下である場合、次式を満たすような伝搬時間差係数Ｋが得られるように液晶ポリマーコーティング４３を施した光ファイバ４０を用いることによって、分散スロープの温度依存性の影響を抑制することが可能となる。
【００６１】
【数１１】

【００６２】
【発明の効果】
以上、説明したように、本発明によれば、光伝送システムは、伝送路の分散スロープの温度変動の影響を受けない。
【００６５】
このため、光伝送システムにおいて温度変動の影響による伝送特性劣化がなくなる。
【図面の簡単な説明】
【図１】本発明の実施形態１、２の光伝送システムの概略図である。
【図２】従来の分散スロープの温度依存性を測定した一例を示す図である。
【図３】従来の光伝送システムの概略図である。
【図４】実施形態１、２で使用可能な伝送用光ファイバの構造を示す図である。
【符号の説明】
１１ 送信系
１２−１〜１２−ｎ 伝送用光ファイバ
１３−１、１３−２ 光中継器
１４ 波長分散補償回路
１５ 受信系
１６ ２次波長分散変化量
１７ 許容２次波長分散
光信号 １ａ、１ｂ、１ｃ
波長分散特性 １ｄ、１ｅ
伝送用光ファイバ ４０
４１ コア
４２ クラッド
４３ 液晶ポリマー[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmission system, and more particularly, an optical transmission system, an optical fiber for transmission, and an optical fiber that reduce the influence of wavelength dispersion on a transmission line in high-speed time-division multiplexing (TDM) optical communication. About.
[0002]
[Prior art]
Currently, research and development of high-speed and large-capacity optical communication that will be responsible for future multimedia is underway. One of the important issues in realizing such optical communication is compensation for chromatic dispersion in the transmission path.
[0003]
FIG. 3 is a schematic diagram of a conventional optical transmission system, in which reference numeral 31 is a transmission system, 32-1 to n are optical fibers for transmission, 33-1 and 33-2 are optical repeaters, and 34 is chromatic dispersion compensation. A circuit 35 is a receiving system. The optical signal (symbol 3a) generated from the transmission system 31 is transmitted to the transmission optical fibers 32-1 to 32-n, and is chromatic dispersion compensated while being optically amplified by the optical repeaters 33-1 and 33-2 at every certain distance. It is sent to the circuit 34 and the receiving system 35.
[0004]
The waveform of the optical signal immediately after transmission deteriorates due to wavelength dispersion of the transmission optical fibers 32-1 to 32-n and the optical repeaters 33-1 and 33-2 (reference numeral 3b). When this optical signal is directly received, an error occurs in signal reading due to interference between adjacent optical pulses. Further, as the speed of the optical signal increases, one time slot (time width occupied by one bit) decreases, so that the influence of the chromatic dispersion D (ps / nm) on the transmission characteristics increases. Therefore, a chromatic dispersion compensation circuit 34 for compensating the chromatic dispersion of this transmission line (transmission optical fiber and optical repeater) is required.
[0005]
Up to now, as the chromatic dispersion compensation circuit 34, an optical fiber or an optical fiber grating having a dispersion value equal in absolute value with a sign opposite to that of the transmission line has been used. As a document disclosing such technology,
(1) Optical fiber: AM Vengsarkar et al, "Fandamental mode dispersion-conpensating fibers: disign considerations and experiments", OFC'94, Thk2,1994
(2) Fiber grating; KO Hill et al., "A periodic in-fiber Bragg gratings for optical fiber dispersion compensation", OFC'94 PD2, 1994
There is.
[0006]
It is also necessary to compensate for the dispersion slope S (ps / nm ² / km). This dispersion slope S means the amount of change in wavelength dispersion (dD / dλ) with wavelength. In WDM (wavelength division multiplexing) transmission that handles multi-channel optical signals in a wide wavelength band, the influence of chromatic dispersion due to the channel differs due to this dispersion slope, resulting in a difference in transmission characteristics. Therefore, in the conventional broadband WDM transmission, the transmission characteristics of all channels are made uniform using an optical fiber or the like that compensates for the dispersion slope.
[0007]
A further problem is fluctuation due to temperature of chromatic dispersion of the optical fiber used in the transmission line. Conventionally, in the case of a 1.3 μm zero dispersion fiber or dispersion shifted fiber that has been mainly used, the second-order chromatic dispersion changes as shown in the following equation by shifting the zero dispersion wavelength due to the temperature change ΔT (deg). It has been known.
[0008]
[Equation 3]

[0009]
As a document disclosing this,
(3) KS Kim et al., “Temperature dependence of chromatic dispersion-sifted fibers: Experiment and analysis”, J. Appl. Phys., 1993, 73, pp.2069-2074.
There is.
[0010]
Where ΔD (ps / nm) is the amount of change in secondary chromatic dispersion, Z (nm / deg) is the temperature coefficient of zero dispersion wavelength shift, S (ps / nm ² / km) is the dispersion slope, and L (km) is the light. The fiber length.
[0011]
Research and development of an adaptive equalization type dispersion compensator is underway in order to compensate for the amount of secondary dispersion fluctuation caused by this temperature change. Specifically, VIPA (Virtually Imaged Phased Array), fiber grating, etc. are used (the above-mentioned literature (3)).
[0012]
However, it has been said that the dispersion slope has no temperature dependence so far. For this reason, the above adaptive equalization type dispersion compensator has been set so as to compensate for the same dispersion fluctuation amount in all wavelength regions.
[0013]
However, it has recently been found that the dispersion slope of an optical fiber is temperature dependent. FIG. 2 shows the dispersion slopes of a 1.3 μm zero-dispersion optical fiber (SMF: Single Mode Fiber) that has been used mainly as a transmission line and a recently developed reverse dispersion fiber (RDF). It is a figure which shows an example which measured temperature dependence.
[0014]
The temperature coefficient of the dispersion slope of the 1.3 μm zero-dispersion optical fiber, which has so far ignored temperature dependence, is 1.79 × 10 ⁻⁶ (ps / nm ² / km / deg). Furthermore, the dispersion slope of the reverse dispersion optical fiber has a temperature coefficient of 1.48 × 10 ⁻⁵ (ps / nm ² / km), which is a temperature-dependent dispersion slope that is about an order of magnitude larger than that of a 1.3 μm zero dispersion fiber or dispersion shifted fiber. / Deg).
[0015]
When the wavelength band of the optical signal is Δλ (nm) and the temperature coefficient of the dispersion slope of the transmission optical fiber is α _T (ps / nm ² / km / deg) (dispersion slope value changing per 1 ° C.), the second order The chromatic dispersion change ΔD is calculated by the following equation.
[0016]
[Expression 4]

[0017]
Therefore, for example, assuming that the transmission line has the same value α _T = 1.48 × 10 ⁻⁵ (ps / nm ² / km / deg) as the reverse dispersion optical fiber, the optical fiber length L is 1000 (km), and the annual temperature When the variation ΔT is 50 (deg) and the wavelength band Δλ is 100 (nm), ΔD is 75.0 (ps / nm). That is, even if the secondary chromatic dispersion is set to 0 in the entire range of the wavelength band Δλ100 nm in the initial operation of the system, 75.0 (ps / nm) for the shortest wavelength channel and the longest wavelength channel due to the temperature change of the dispersion slope. That is, the difference in the second-order chromatic dispersion occurs.
[0018]
Even if a conventional adaptive equalization dispersion compensation circuit is used, the same dispersion compensation amount is only increased (or decreased) in all wavelength bands (from chromatic dispersion characteristic 3d to chromatic dispersion characteristic 3e in FIG. 3). In this case, as shown in the chromatic dispersion characteristic 3e of FIG. 3, in a WDM transmission system with a single channel bit rate of 40 Gbit / s (allowable secondary chromatic dispersion is about 40 ps / nm (2D _{0 of reference} numeral 37 in the figure)). If ΔD ₃ of reference numeral 36 is ΔD described above, ΔD ₃ > 2D ₀ may occur, and it is difficult to apply a conventional adaptive equalization dispersion compensation circuit.
[0019]
[Problems to be solved by the invention]
As described above, in the conventional optical transmission system, the dispersion slope of the optical fiber is actually temperature dependent. For this reason, in the high-speed / broadband WDM transmission, there is a problem to be solved in that the influence of secondary chromatic dispersion differs between channels due to temperature changes.
[0020]
The present invention has been made in view of such a problem, and an object of the present invention is to provide an optical transmission system capable of realizing transmission without degradation of transmission characteristics due to the influence of temperature fluctuation of the dispersion slope. It is in.
[0021]
[Means for Solving the Problems]
In order to achieve such an object, an optical transmission system of the present invention includes a transmission system that generates an optical signal, a transmission optical fiber that transmits the optical signal, and a reception system that receives the transmitted optical signal. And a chromatic dispersion compensation circuit for adjusting a dispersion compensation amount of chromatic dispersion for the transmitted optical signal received by the receiving system, and the bit error rate becomes a predetermined value or less. Allowable secondary chromatic dispersion fluctuation value is ± ΔD ₀ , temperature change that the transmission optical fiber receives is ΔT, the total length of the transmission optical fiber is L, the wavelength band of the optical signal is Δλ, and the transmission optical fiber When the dispersion slope temperature coefficient, which is the value of the dispersion slope that changes per 1 ° C., is α _T , the equation
[Equation 5]

[0023]
The satisfying manner alpha _T, L, [Delta] [lambda] is set, the chromatic dispersion compensation circuit, the dispersion compensation amount so that secondary chromatic dispersion ≦ + [Delta] D ₀ of all wavelengths -ΔD ₀ ≦ said wavelength band It is characterized by adjusting .
[0024]
With the above configuration, the optical transmission system is not affected by the temperature fluctuation of the dispersion slope.
[0025]
Here, the transmission optical fiber is a combination of a plurality of optical fibers, and each of the plurality of optical fibers has a different dispersion slope temperature coefficient (α _T1 , α _T2 ,... Α _Tn (n is 2 or more). Positive number) (ps / nm ² / km / deg); dispersion slope value varying per 1 ° C.) and different optical fiber lengths (L ₁ , L ₂ ,... L _n (km)), On the other hand, instead of the formula (1), the formula
[Formula 6]

[0027]
Α _T1 , L ₁ , α _T2 , L ₂ ... Α _Tn , L _n , Δλ are set so as to satisfy
[0028]
With the above configuration, even an optical transmission system using different types of optical fibers is not affected by the temperature fluctuation of the dispersion slope.
[0029]
In addition, in order to set the dispersion slope temperature coefficient α _T , the surface of the transmission optical fiber may be coated.
[0030]
Further, the coating is a coating made of a material having an expansion coefficient opposite to the temperature change of the transmission optical fiber.
[0031]
In addition, in order to set the dispersion slope temperature coefficient (α _T1 , α _T2 ,... Α _Tn ), the surface of each of the plurality of optical fibers may be coated.
[0032]
Further, the coating is characterized in that each of the plurality of optical fibers is a coating made of a material having an expansion coefficient opposite to that of the optical fiber.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, the same code | symbol is attached | subjected to the location which has the same function in each drawing, and duplication of description is abbreviate | omitted.
[0034]
[Embodiment 1]
FIG. 1 is a schematic diagram of an optical transmission system according to the first embodiment. In the figure, reference numeral 11 is a transmission system, 12-1 to 12-n are optical fibers for transmission, and 13-1 and 13-2 are optical repeaters. , 14 is a chromatic dispersion compensation circuit, and 15 is a receiving system. The optical signal (symbol 1a) generated from the transmission system 11 is transmitted to the transmission optical fibers 12-1 to 12-n, and is optically amplified by the optical repeaters 13-1, 13-2, etc. at certain distances while being chromatic dispersion compensated. It is sent to the circuit 14 and the receiving system 15.
[0035]
The waveform of the optical signal immediately after transmission deteriorates due to wavelength dispersion of the transmission optical fibers 12-1 to 12-n and the optical repeaters 13-1 and 13-2 (reference numeral 1b). When this optical signal is directly received, an error occurs in signal reading due to interference between adjacent optical pulses. Further, as the speed of the optical signal increases, one time slot (time width occupied by one bit) decreases, so that the influence of the chromatic dispersion D (ps / nm) on the transmission characteristics increases. Therefore, a chromatic dispersion compensation circuit 14 for compensating the chromatic dispersion of this transmission line (transmission optical fiber and optical repeater) is required.
[0036]
Here, the allowable secondary chromatic dispersion fluctuation value at which the bit error rate becomes a predetermined value or less is ± ΔD ₀ (ps / nm), the temperature change experienced by the transmission optical fiber is ΔT (deg), and the total length of the transmission optical fiber is L (km), Δλ (nm) of the optical signal wavelength band, and α _T (ps / nm ² / km / deg) of the dispersion slope temperature coefficient of the transmission optical fiber (dispersion slope value changing per 1 ° C.) And
[0037]
In the optical transmission system according to the first embodiment, the secondary chromatic dispersion change amount ΔD shown in the equation (4) is expressed as
[0038]
[Expression 7]

[0039]
Α _T , L, and Δλ are set so as to satisfy
[0040]
In FIG. 1, reference numeral 1e denotes a chromatic dispersion characteristic immediately before the receiving system 15 in the first embodiment. Here, for example, in a WDM transmission system with a single channel bit rate of 40 Gbit / s, the allowable secondary chromatic dispersion at which the bit error rate is 10 ⁻⁹ or less is about 40 ps / nm (2 Δ D _{0 of reference} numeral 17 in the figure). ).
[0041]
As shown in the chromatic dispersion characteristic 1e, when ΔD ₁ of reference numeral 16 is ΔD satisfying the equation (5), a difference in secondary chromatic dispersion occurs between the shortest wavelength channel and the longest wavelength channel due to the temperature change of the dispersion slope. Even if the same dispersion compensation amount is increased (or decreased) using the conventional adaptive equalization dispersion compensation circuit (from chromatic dispersion characteristic 1d to chromatic dispersion characteristic 1e in FIG. 1), ΔD ₁ 16 is transmitted to the transmission system. The allowable secondary chromatic dispersion amount of 2 ΔD ₀ can be kept within 17.
[0042]
That is, by using the optical transmission system according to the first embodiment, it is possible to realize transmission without deterioration of transmission characteristics due to the influence of temperature fluctuation of the dispersion slope.
[0043]
It is obvious that the above-described method of the first embodiment can be applied to an optical transmission system having one transmission optical fiber and no repeater.
[0044]
[Embodiment 2]
FIG. 1 is a schematic diagram of an optical transmission system according to the second embodiment. In the figure, reference numeral 11 is a transmission system, 12-1 to 12-n are optical fibers for transmission, and 13-1 and 13-2 are optical repeaters. , 14 is a chromatic dispersion compensation circuit, and 15 is a receiving system. The optical signal (symbol 1a) generated from the transmission system 11 is transmitted to the transmission optical fibers 12-1 to 12-n, and is optically amplified by the optical repeaters 13-1, 13-2, etc. at certain distances while being chromatic dispersion compensated. It is sent to the circuit 14 and the receiving system 15.
[0045]
The waveform of the optical signal immediately after transmission deteriorates due to wavelength dispersion of the transmission optical fibers 12-1 to 12-n and the optical repeaters 13-1 and 13-2 (reference numeral 1b). When this optical signal is directly received, an error occurs in signal reading due to interference between adjacent optical pulses. Further, as the speed of the optical signal increases, one time slot (time width occupied by one bit) decreases, so that the influence of the chromatic dispersion D (ps / nm) on the transmission characteristics increases. Therefore, a chromatic dispersion compensation circuit 14 for compensating the chromatic dispersion of this transmission line (transmission optical fiber and optical repeater) is required.
[0046]
Here, the allowable secondary chromatic dispersion fluctuation value at which the bit error rate becomes a predetermined value or less is ± ΔD ₀ (ps / nm), the temperature change experienced by the transmission optical fiber is ΔT (deg), and the wavelength band of the optical signal is Δλ. (Nm).
[0047]
In this embodiment 2, the optical transmission system of FIG. 1, the transmission optical fiber 12-1 to 12-n are each different dispersion slope temperature coefficient _{(α T1, α T2, ...} α Tn (n is 2 or more positive ) (Ps / nm ² / km / deg); dispersion slope values varying per degree Celsius) and different optical fiber lengths (L ₁ , L ₂ ,... L _n (km)) It consists of a combination of transmission optical fibers.
[0048]
In the optical transmission system according to the second embodiment, the total ΔD ′ of each secondary chromatic dispersion change amount ΔD shown in Expression (4) of the transmission optical fibers 12-1 to 12-n is the temperature that the transmission optical fiber receives. For the change ΔT,
[0049]
[Equation 8]

[0050]
Α _T1 , L ₁ , α _T2 , L ₂ ... Α _Tn , L _n , Δλ are set so as to satisfy
[0051]
In FIG. 1, reference numeral 1e denotes a chromatic dispersion characteristic immediately before the receiving system 15 in the first embodiment. Here, for example, in a WDM transmission system with a bit rate of 40 Gbit / s for one channel, the allowable secondary chromatic dispersion at which the bit error rate is 10 ⁻⁹ or less is about 40 ps / nm (2D _{0 of reference} numeral 17 in the figure). To do.
[0052]
As shown in the wavelength dispersion characteristic 1e, when the ΔD' satisfying the expression (6) [Delta] D ₁ of the code 16, the difference of the secondary chromatic dispersion in the channel of the channel and the longest wavelength of the shortest wavelength by a temperature change in the dispersion slope Even if it occurs, the same dispersion compensation amount is simply increased (or decreased) using the conventional adaptive equalization dispersion compensation circuit (from chromatic dispersion characteristic 1d to chromatic dispersion characteristic 1e in FIG. 1), and ΔD ₁ 16 is transmitted. The allowable secondary chromatic dispersion amount of the system can be kept within 2D ₀ 17.
[0053]
That is, by using the optical transmission system according to the second embodiment , transmission without deterioration of transmission characteristics due to the influence of the temperature variation of the dispersion slope can be realized even in an optical transmission system using different types of optical fibers.
[0054]
[Embodiment 3]
FIG. 4 is a diagram illustrating a structure of a transmission optical fiber 40 that can be used in the first and second embodiments. Here, 41 is a core, 42 is a cladding, and 43 is a liquid crystal polymer. As shown in FIG. 4, a liquid crystal polymer coating is applied to the surface of the optical fiber composed of the core and the clad.
[0055]
By coating the surface of the optical fiber 40 with a liquid crystal polymer 43 having a coefficient of expansion opposite to that of the optical fiber (quartz) and suppressing physical expansion and contraction due to temperature changes, the signal propagation time can be stabilized. . Here, the propagation length difference coefficient K (ps / km / deg) per unit length (1 km) and unit temperature change (1 deg) of the optical fiber core wire is given by the following equation.
[0056]
[Equation 9]

[0057]
Where N is the group refractive index of the optical fiber, c is the speed of light in vacuum, A is the temperature change rate of the refractive index, B is the refractive index change rate due to strain, E is the equalized linear expansion coefficient of the optical fiber core, E _glass is the linear expansion coefficient of the optical fiber (glass). This double differential d ² K / dλ ² with respect to the wavelength of K is equivalent to the temperature coefficient α _T (ps / nm ² / km / deg) of the dispersion slope.
[0058]
The wavelength band of the optical signal in the optical transmission system is Δλ (nm), and the propagation time difference coefficient and the length of the transmission optical fiber 40 to which the liquid crystal polymer 43 is applied are K (ps / km / deg), L (km), If the temperature change is ΔT (deg), the secondary chromatic dispersion change amount ΔD (ps / nm) is expressed by the following equation.
[0059]
[Expression 10]

[0060]
Accordingly, when the allowable second-order dispersion range of the transmission system is −ΔD ₀ (ps / nm, where ΔD ₀ > 0) or more and ΔD ₀ or less, the liquid crystal is set so that a propagation time difference coefficient K satisfying the following equation is obtained. By using the optical fiber 40 provided with the polymer coating 43, it is possible to suppress the influence of the temperature dependence of the dispersion slope.
[0061]
[Expression 11]

[0062]
【The invention's effect】
As described above, according to the present invention , the optical transmission system is not affected by the temperature fluctuation of the dispersion slope of the transmission path.
[0065]
For this reason, transmission characteristic deterioration due to the influence of temperature fluctuation is eliminated in the optical transmission system.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an optical transmission system according to first and second embodiments of the present invention.
FIG. 2 is a diagram showing an example of measuring the temperature dependence of a conventional dispersion slope.
FIG. 3 is a schematic diagram of a conventional optical transmission system.
FIG. 4 is a diagram showing a structure of a transmission optical fiber that can be used in the first and second embodiments.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 Transmission system 12-1 to 12-n Transmission optical fiber 13-1, 13-2 Optical repeater 14 Wavelength dispersion compensation circuit 15 Reception system 16 Secondary chromatic dispersion change amount 17 Allowable secondary chromatic dispersion optical signal 1a, 1b 1c
Chromatic dispersion characteristics 1d, 1e
Transmission optical fiber 40
41 Core 42 Clad 43 Liquid crystal polymer

Claims (8)

In an optical transmission system including a transmission system that generates an optical signal, a transmission optical fiber that transmits the optical signal, and a reception system that receives the transmitted optical signal,
A chromatic dispersion compensation circuit for adjusting a dispersion compensation amount of chromatic dispersion for the transmitted optical signal received by the receiving system;
The allowable secondary chromatic dispersion fluctuation value at which the bit error rate is below a predetermined value is ± ΔD ₀ , the temperature change that the transmission optical fiber is subjected to is ΔT, the total length of the transmission optical fiber is L, and the wavelength band of the optical signal is When Δλ and the dispersion slope temperature coefficient, which is the value of the dispersion slope that changes per 1 ° C. of the optical fiber for transmission, is α _T ,
For ΔT, the equation

The satisfying manner alpha _T, L, [Delta] [lambda] is set,
The chromatic dispersion compensation circuit is configured such that −ΔD ₀ ≦ second-order chromatic dispersion of all wavelengths in the wavelength band ≦ + ΔD _0.
The dispersion compensation amount is adjusted so that: The optical transmission system according to claim 1,
The transmission optical fiber is a combination of a plurality of optical fibers,
Each of the plurality of optical fibers, dispersion slope temperature coefficient alpha _T1 is a value of dispersion slope which varies per different _{1 ℃, α T2, ... α} Tn and different optical fiber lengths _{L 1, L 2, ... L} n ( n is a positive number of 2 or more),
For ΔT, instead of the equation (1), the equation

Α _T1 , L ₁ , α _T2 , L ₂ ... Α _Tn , L _n , Δλ are set so as to satisfy the above. The optical transmission system according to claim 1,
Wherein in order to set the dispersion slope temperature coefficient alpha _T, an optical transmission system, characterized in that a coated on the surface of the transmission optical fiber. The optical transmission system according to claim 3.
The optical transmission system according to claim 1, wherein the coating is a coating made of a material having an expansion coefficient opposite to the temperature change of the transmission optical fiber. The optical transmission system according to claim 2,
In order to set the dispersion slope temperature coefficient (α _T1 , α _T2 ,... Α _Tn ), a coating is applied to the surface of each of the plurality of optical fibers. The optical transmission system according to claim 5,
The optical transmission system according to claim 1, wherein the coating is a coating made of a material having an expansion coefficient opposite to the temperature change of each of the plurality of optical fibers.   An optical fiber for transmission used in the optical transmission system according to claim 3 or 4.   An optical fiber used in the optical transmission system according to claim 5 or 6.

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