201207457 六、發明說明： 【發明所屬之技術領域】 本發明有關光通訊系統之領域，尤其有關致能經由多 模態波導通訊之裝置。 【先前技術】 經由多模態波導通訊中的一個實驗係關於「 Fundamentals and Challenges of Optical Multiple-Input M u 11 i p 1 e - O u t p u t M u 11 i m o d e F i b e r L i nk s」，由 A· Tarighat 等人’ IEEE通訊雜誌，2007年5月。然而，傳輸距離仍然 非常有限。將多模態波導應用至長程及/或高容量傳輸上 仍需要發展。 【發明內容】 依據一個實施例，本發明提供一種光信號接收器，包 含： 輸入，欲連接至多模態波導而用來接收以數位資料所 調變之輸入光信號’該輸入光信號包含複數個空間模態； 光源，用以產生本地振盪器信號； 複數個同調光偵測器，該等同調光偵測器之各者包含 :同調混合器，用以產生該本地振盪器信號與來自該輸入 之待偵測光信號之間的干涉信號；及光電偵測器，用以產 生例如對應於該干涉信號之同相分量及正交分量的偵測信 號； -5- 201207457 一或更多個數位處理模組，用以處理該等偵測信號以 便找出由該輸入光信號所載送之該數位資料；及 模態解多工模組，配置在該輸入與該等同調光偵測器 之間，該模態解多工模組包含複數個分別與該等同調光偵 測器關聯之模態選擇濾波器； 該等模態選擇濾波器之各者能夠將待偵測光信號提供 給關聯的同調光偵測器的同調混合器，該待偵測光信號實 質對應於該輸入光信號之個別空間模態。 在其他有利的實施例中，此種光信號接收器可展現下 列特性之一或更多者： -設置分束器以分配該輸入光信號至該等模態選擇濾 波器。 -該等模態選擇濾波器之一者或各者包含：單模態波 導’其連接至關聯的同調混合器；及光裝置，其能 夠耦合該單模態波導之基本空間模態與該輸入光信 號之實質一個已挑選空間模態。 -該接收器可包含單模態波導，其連接該光源至該等 同調光偵測器之一者或各者的同調混合器。因此， 該同調混合器內之信號的重疊可實質在基本空間模 態內進行（其確保同調重疊是有效的），產生具有 滿意振幅的干涉信號。此外，以基本模態的信號操 作的同調混合器較爲容易建構，這在成本及可靠性 方面構成了優點。 -該分束器可爲多模態。在另一實施例中，該分束器 -6 - 201207457 及該等模態選擇濾波器可用一體式組件共同進行該 輸入光信號的分割及個別模態的選擇之形式加以建 構。 -該等模態選擇濾波器選擇該輸入光信號的個別模態 ，該等個別模態包含基本模態及頂模態。 -該等模態選擇濾波器選擇該輸入光信號的個別模態 ，該等個別模態包含多個頂模態。 -該等頂模態屬於由LP02、LP1 1、LP21、及LP03所組 成之群組。 -該輸入光信號產生自複數個重疊模態分量經由該多 模態波導的傳播，該等模態分量之各者在該多模態 波導遠離該接收器的一端內已經以該數位資料之子 集加以調變。 -該數位處理模組實施矩陣計算，該矩陣計算欲反轉 該多模態波導內的傳播期間在該等個別空間模態之 間所發生的耦合。 -該波導可爲薄弱的多模態，例如在所檢視之波長具 有少於10個模態。 在一個實施例中，本發明也揭示一種光通訊系統，其 包含：前述接收器；多模態波導，連接至該接收器之輸入 ;及光發送器，連接至該多模態波導遠離該接收器的一端 ，該光發送器係能夠在該多模態波導內發送複數個模態分 量之重疊，該等模態分量之各者係以該數位資料之子集加 以調變。 201207457 基於本發明的一個槪念在於，在光通訊系統內使用展 現相對高效之橫剖面的光纖或其他波導可能減少影響已傳 輸信號的非線性效應，這可促進光信號之功率位準的增加 而延長傳輸距離。本發明的一些方面得自下列觀察結果： 此種光纖可能使得該傳輸爲多模態，且因此造成符號間的 干涉，其必須在接收器端上加以補償以再發現該資料。本 發明的其他方面建立在下列槪念：經由多模態波導將已接 收信號分離成複數個多模態分量，且以一種反轉該等傳播 效應之方式處理對應於這些個別多模態分量的偵測信號。 本發明的另外方面建立在下列槪念：建構本地振盪器信號 與多模態信號之分量間的模態自適應（mode adaptation) ，以減少這些分量的有效同調偵測。本發明的另外方面建 立在下列槪念：使用在給定波長的空間模態多工，以獲得 等同於複數個空間模態之容量總和的總傳輸容量。 【實施方式】 參照第1圖，以略圖方式描繪光通訊系統。此系統包 含光發送裝置10、光接收裝置30、及能夠將光信號自光發 送裝置10傳導至光接收裝置3 0的傳輸線20。傳輸線20包含 多模態光纖2 1。該傳輸線也可包含此處未顯示之其他單模 態或多模態光元件，諸如光放大器、色散補償裝置、連接 器、增刪多工器、透明切換裝置、不同類型的光纖、及其 他。此處並非必然要較詳細地描述此種常用於光通訊網路 中的元件。 • 8 - 201207457 多模態光纖21爲一種光纖，其結構在用於通訊之波長 下致能多個適當橫向模態的傳播。適當橫向模態爲在正交 於傳播方向之平面中的電磁場空間分佈（其在傳播過程中 約略維持相同），受縱向相位因數及縱向衰減因數影響。 波導的適當橫向模態（爲簡潔之目的將稱爲空間模態）傳 統上由符號LP (線性極化）接著二個整數加以標示。在此 格式中，第一個數字代表沿著以光纖之縱軸爲中心的圓周 的電磁場相位變化，測量單位爲2π ;且第二個數字代表沿 著光纖之半徑的電磁場相位變化，測量單位爲π。 多模態光纖2 1可支援介於二與超過一百種的模態，視 其設計而定。優先地，多模態光纖2 1爲薄弱的多模態，空 間模態的數量不超過10或甚至3。此種光纖尤其可展現相 對寬的有效橫剖面，尤其大於300平方微米的橫剖面，諸 如400或5 00平方微米。此處所檢視的有效橫剖面可爲基於 模態的有效橫剖面或等效的有效橫剖面，其有關針對多個 空間模態之傳播的已注入總光功率相對於整體非線性效應 的容限（tolerance)。 光發送裝置10產生以資料調變的一或更多個光信號， 且將該或該等信號注入傳輸線20內，該傳輸線將該或彼等 信號載送至接收裝置30。爲了說明之目的，將首先檢視單 色信號經由給定波長通道的傳輸。類似地，將首先考慮以 單極化方式所發射、傳播、及接收的信號。由於光纖21的 多模態結構，到達接收裝置3 0的光信號在其所使用的波長 下包含多個空間模態。這些空間模態在不同群組速度下傳 -9 - 201207457 播，這在傳播期間造成干涉出現，尤其是符號間的干涉。 此外，在傳播期間可能有模態間的耦合，尤其在多個光纖 接縫（fiber seam)內，使干涉加重。接收裝置30實施同 調光偵測及偵測信號的數位處理，該數位處理尤其用以補 償多模態色散及再發現已傳輸資料。 爲了這樣做，接收裝置3 0包含模態解多工器3 1，該模 態解多工器的一個輸入32連接至光纖21而用來接收已傳輸 的多模態信號。模態解多工器3 1將輸入信號分離成多個個 別模態信號，經由個別輸出3 3將該等模態信號引導至個別 同調偵測器34。在模態解多工器的輸出3 3，各模態信號實 質對應於輸入信號的個別空間模態。換句話說，在模態解 多工器3 1的輸出所獲得之模態信號的功率的超過50%來自 模態解多工器3 1內之輸入信號的給定空間模態。這些空間 模態嚴格來說只存在於多模態光纖2 1內。在多模態光纖外 側，對應於此種模態的電磁場分佈也由詞語「空間模態」 加以標示。 每一次，同調偵測器34諸如經由光纖或波導36接收來 自模態解多工器3 1的待偵測模態信號，且諸如經由光纖或 波導3 8接收來自本地振盪器3 7的本地振盪器信號。將本地 振盪器信號調整至與用來傳輸該資料的波長相同的波長。 優先地，光纖或波導36及3 8在此波長爲單模態，這讓設計 相對簡單的單模態同調混合器有可能被使用。 數位處理模組35自各同調偵測器34接收該同調偵測器 所偵測之模態信號之代表同相分量的電偵測信號I及代表 -10 - 201207457 正交分量的電偵測信號Q。處理模組3 5對這些信號取樣且 施加處理至一給定時刻所接收之所有偵測信號，以反轉傳 播效應且再發現最初已調變的資料。爲了這樣做，可適當 地修改同調偵測領域中常使用之自適應演算法。尤其，爲 了此目的可能使用一種已知用以補償極化模態色散（ dispersion )的自適應濾波器。可將濾波器的時間深度（ temporal depth )調整至被檢視之該等空間模態間存在的 最大時間偏移。 在第1圖中使用分束器40而分配來自單一本地振盪器 37的本地振盪器信號至各同調偵測器34。在一個變型中， 多個個別本地振盪器可連接至各個同調偵測器3 4。 參照第4圖，可用於第1圖之接收裝置30中的同調偵測 器60包含：常由半反射片（semi-re f_,lective blade)及聚光 透鏡之組合所形成的同調混合器6 1、光電偵測器62及信號 結合器63。同調混合器6 1自它的二個輸入64上所接收的光 信號來形成干涉信號。光電偵測器62將這些干涉信號轉換 成電信號。信號結合器63結合這些電信號以形成同相偵測 信號I及正交偵測信號Q。 參照第2圖，可用於第1圖之接收裝置30中的模態解多 工器70包含：輸入元件7 1，例如一段多模態光纖’用以接 收待解多工的多模態光信號78 ;多模態分束器72 ’用以分 配該多模態光信號至多個模態選擇濾波器73 ;及輸出元件 74，用以連接每一次由關聯的模態選擇濾波器73所選擇的 模態信號79。各模態選擇濾波器73選擇輸入信號的個別空 -11 - 201207457 間模態，即所描繪之實例中的模態LP02、LPl 1、及LP01 。換句話說，模態選擇濾波器73允許進入輸出元件74的是 :其能量的超過50% (且較佳超過66% )來自已指示之空 間模態的光信號。 可自透鏡及半透明片（semi-transparent blade)建構 多模態分束器72。 參照第3圖，在一較佳實施例中，模態選擇濾波器73 及輸出元件74可用模態轉換器80及單模態波導8 1之組合的 形式加以建構。模態轉換器80接收來自波導82的待濾波多 模態信號，且將該信號的給定空間模態（例如模態LP 02 ) 轉換成基本空間模態LP0 1，該模態轉換器將該信號傳輸至 單模態波導8 1。由於其結構，單模態波導8 1只允許基本空 間模態通過，這讓任何較高階空間分量有可能被消除。 此種模態轉換器80可依據文件US-A-6377726中的揭示 而加以建構，其具有透鏡83及84與相位遮罩85及86。轉換 至基本模態的模態分量取決於相位遮罩8 5及8 6的確切組態 。不同的模態轉換器可自此模型而加以建構，以選擇輸入 信號之不同的較高階模態，例如LP02、LP 1 1、LP2 1、及 LP03。爲了選擇輸入信號的基本模態LP01，可使用此裝置 或較簡單的裝置，例如聚光透鏡。 選 態. 模妻 的以 圖口 2 力 第式 當形 每的 器 換 時 構 轉波 態態 模模 的單 圖以 3 &WL : 以 η之 7 擇 器選 波ί e 擇 可的 , 測 中偵 況待 情 行 此進 在中 ο 器 器合 測混 偵調 調同 同的 至器 供收 提接 式在 形性 的效 態有 模的 本好 基常 的非 導用 -12- 201207457 模態信號與本地振盪器信號的重疊，因爲該待偵測的模態 信號與該本地振盪器信號間的模態一致。這是爲何較佳的 是在此狀況中經由單模態波導傳輸本地振盪器信號。產生 自此組態的另一優點爲可選擇（於同調偵測器34之各者中 )使用市場上低成本可取得的傳統同調混合器。 在一個實施例中，可用與待偵測空間模態之選擇功能 結合的方式來進行多模態光信號之分配功能。 視發送及接收裝置10及30的組態而定，可用數種方式 使用第1圖中所描繪之光通訊系統。在第一應用中，稱爲 SISO ( Single Input-Single Output，單輸入單輸出）或 SIMO ( Single Input Multiple Output，單輸入多輸出）， 將傳播的多模態本質視爲複製不同傳播路徑上的相同資料 信號。對於此種應用，發送裝置1 0被設計成將以該資料信 號調變的光信號注入傳輸線20的一端中，以此種方式使得 此信號被耦合至多模態光纖2 1的一或更多個空間模態。在 傳輸線2 0的另一端，數位處理模組3 5以待再發現之資料信 號的多個已時間偏移的複本的不同線性組合之方式，處理 各偵測信號。 在第二應用中，稱爲 ΜΙΜΟ (Multiple lnput Multiple Output ’多輸入多輸出），將傳播的多模態本質視爲傳輸 通道的解多工，藉此有可能藉由經多個空間流或多組個別 空間流傳輸多個個別資料流而增加通訊系統的容量。對於 此種應用，參照第1圖，發送裝置1 0可包含：多個信號調 變器1 1，彼等經組態成以個別資料流Dl、D2、...、Dk調 -13- 201207457 變光信號；及模態解多工器1 2，經組態成耦合各已調變光 信號S 1、S2、…、Sk與多模態光纖2 1的個別空間模態（或 一組此種模態）。爲了進行已調變信號與光纖2 1之特定空 間模態的此種選擇性耦合，模態解多工器1 2可包含類似於 參照第3圖所描述的模態轉換器，在此例子中將轉換方向 反轉。 在此應用中，在光纖20的該另一端，數位處理模組35 在已知傳播期間出現模態間的耦合的情況下，以待再發現 之資料信號的混合之方式來處理各偵測信號。只要足夠數 量的不同偵測信號被提供至數位處理模組3 5，這些模態間 耦合（intermodal coupling )可藉由數値方法加以反轉。 由於各同調偵測器34用來實質上擷取有關一個給定空間模 態的資訊，較佳設置與信號調變器1 1 —樣多的同調偵測器 3 4，且模態解多工器3 1選擇與最初傳輸的光信號S 1、S2、 …、Sk分別耦合的個別空間模態。 倘若是傳輸期間的模態多工，如果模態沒有在多模態 光纖中混合，也可能以逐模態方式處理已偵測信號，諸如 藉由設置用於各已偵測空間模態之分開的數位處理模組。 在以上實施例中，僅利用載波波長而已經描述經由多 模態傳輸線的光通信實施方式。模態多工讓波長通道的容 量增加有可能實現。爲了產生較大容量的通訊系統，上述 方法可與波長劃分多工（wavelength division multiplexing ) 技術及/或極化劃分多工 （polarization division m u 11 i p 1 e x i n g )技術結合。 -14- 201207457 爲了這樣做’可將波長結合器（例如波長劃分多工器 )設置在發送裝置10內，且可將波長分離器（例如波長劃 分解多工器）設置在接收裝置30內。在一個對應實施例中 ，可將以上關於波長所描述之發送裝置10及/或接收裝置 30的元件增加成爲與用以處理之波長通道一樣多。 類似地，爲了實施極化劃分多工，可將一或更多個極 化結合器設置於發送裝置10內，且可將一或更多個極化分 離器設置於接收裝置30內。 無論在傳輸期間有或沒有極化劃分多工，較佳的是建 構分集式極化（diverse-polarization )接收裝置，尤其能 夠補償模態極化色散。爲了這樣做，存在多個可能性。 分集式極化接收裝置的一個實施例在第1圖中加以槪 述。此處，入射光信號進入極化分離器元件25，在該極化 分離器元件的輸出將二個具有正交極化的多模態信號分離 。類似地，極化分離器元件26將本地振盪器信號分離成二 個正交極化的分量。第1圖中所描繪之方塊50因此代表極 化分量的同調偵測鏈。第二個相同的方塊5 0 (圖未示）必 須連接至極化分離器25及26的其他埠51，以偵測其他極化 分量。在此情況中，數値極化模組35可由方塊50兩者共享 ，以便同時處理對應於該二個極化分量的偵測信號。代替 分離器26，可針對各極化設置不同的本地振盪器。 第5圖中所描繪之分集式極化接收裝置的另一實施例 中，將光信號分離成二個正交極化的分量是在將空間模態 分離的下游處完成。至於其他，與第1圖中的元件相同或 -15- 201207457 類似的元件具有相同元件符號加上1 00。 —些所描繪的元件（尤其是控制單元及各模組）可利 用硬體及/或軟體組件而以各種形式（獨立或分散式）加 以建構。可使用的硬體組件爲應用特定積體電路、場效可 程式化閘極陣列、或微處理器。軟體組件可用諸如C、 C + +、Java、或VHDL的各種程式語言加以編寫。此清單並 不詳盡。 雖然本發明已連同多個具體實施例而加以描述，本質 上不以任何方式限於這些實施例，且包含所述手段的所有 技術性等效物以及彼等之組合，如果該等組合落在本發明 之範圍內。 使用動詞「包含」或「包括」及彼等之動詞變化並非 排除申請專利範圍中所陳述之元件或步驟以外的元件或步 驟。對元件或步驟使用不定冠詞「一」並非排除複數個此 種元件或步驟的存在。多個手段或模組可由單一硬體元件 加以描繪。 在申請專利範圍中，不應該將括號內的任何元件符號 證釋爲限制申請專利範圍8 【圖式簡單說明】 在檢視本發明之多個特定實施例的說明以後，將較佳 地理解本發明，且將更清晰地明瞭本發明之其他目的、細 節、特性及優點，這些實施例只藉由例示及非限制性實例 之方式加以敘述。在這些圖式中： -16- 201207457 第1圖爲依據一個實施例之光傳輸系統的功能方塊圖 ， 第2圖描繪模態解多工器的一個實施例，其尤其可用 於第1圖的系統中， 第3圖描繪模態轉換器的一個實施例，其尤其可用於 第2圖的解多工器中， 第4圖描繪同調接收器的一個實施例，其尤其可用於 第1圖的系統中，及 第5圖描繪光接收裝置的一個實施例，其尤其可用於 第1圖的系統中。 【主要元件符號說明】 10 :光發送裝置 1 1 :信號調變器 1 2、3 1、7 0、1 3 1 :模態解多工器 20 :傳輸線 2 1、1 2 1 :多模態光纖 25、26、125、126:極化分離器 30、130:光接收裝置 32、 64、 132:輸入 33 、 133 :輸出 34、60、134 :同調偵測器 3 5、1 3 5 :數位處理模組 36、 38、 82、 136、 138：波導 -17- 201207457 37、137:本地振盪器 40、1 40 :分束器 5 0 :方塊 51 :埠 6 1 :同調混合器 62 :光電偵測器 63 :信號結合器 7 1 :輸入元件 72 :多模態分束器 73 :模態選擇濾波器 74 :輸出元件 78 :多模態光信號 79 :模態信號 8 0 :模態轉換器 8 1 :單模態波導 8 3、8 4 :透鏡 8 5、8 6 :相位遮罩201207457 VI. Description of the Invention: [Technical Field] The present invention relates to the field of optical communication systems, and more particularly to devices capable of communicating via multi-modal waveguides. [Prior Art] An experiment in multi-modal waveguide communication is about "Fundamentals and Challenges of Optical Multiple-Input M u 11 ip 1 e - O utput M u 11 imode F iber L i nk s" by A· Tarighat Et al.' IEEE Communications Magazine, May 2007. However, the transmission distance is still very limited. The application of multi-modal waveguides to long-range and/or high-capacity transmissions still needs to evolve. SUMMARY OF THE INVENTION According to one embodiment, the present invention provides an optical signal receiver comprising: an input to be connected to a multi-modal waveguide for receiving an input optical signal modulated by digital data. The input optical signal includes a plurality of a spatial modality; a light source for generating a local oscillator signal; a plurality of coherent photodetectors, each of the equivalent dimming detectors comprising: a coherent mixer for generating the local oscillator signal and from the input An interference signal between the optical signals to be detected; and a photodetector for generating a detection signal corresponding to, for example, an in-phase component and a quadrature component of the interference signal; -5- 201207457 one or more digital processing a module for processing the detection signals to find the digital data carried by the input optical signal; and a modal demultiplexing module disposed between the input and the equivalent dimming detector The modal demultiplexing module includes a plurality of modal selection filters respectively associated with the equivalent dimming detector; each of the modal selection filters can provide an optical signal to be detected to Coherent cohomology photodetector associated mixer, the substantial optical signal to be detected corresponding to the individual spatial modes of the input optical signal. In other advantageous embodiments, such an optical signal receiver can exhibit one or more of the following characteristics: - A beam splitter is provided to distribute the input optical signal to the modal selection filters. One or each of the modal selection filters comprising: a single mode waveguide 'connected to an associated coherent mixer; and an optical device capable of coupling the fundamental spatial mode of the single mode waveguide with the input The essence of the optical signal is a selected spatial mode. - The receiver may comprise a single mode waveguide that connects the light source to a coherent mixer of one or each of the same dimming detectors. Thus, the overlap of the signals within the coherent mixer can be substantially performed within the fundamental spatial mode (which ensures that the coherent overlap is effective), producing an interference signal having a satisfactory amplitude. In addition, coherent mixers operating with fundamental modal signals are easier to construct, which provides advantages in terms of cost and reliability. - The beam splitter can be multimodal. In another embodiment, the beam splitter -6 - 201207457 and the modal selection filters can be constructed in the form of splitting of the input optical signal and selection of individual modalities together with the integrated component. The modal selection filters select individual modalities of the input optical signal, the individual modalities comprising a fundamental mode and a top mode. The modal selection filters select individual modalities of the input optical signal, the individual modalities comprising a plurality of top modes. - These top modes belong to the group consisting of LP02, LP1 1, LP21, and LP03. - the input optical signal is generated from propagation of a plurality of overlapping modal components via the multimodal waveguide, each of the modal components having a subset of the digital data in an end of the multimodal waveguide remote from the receiver Make changes. The digital processing module performs a matrix calculation that computes the coupling that occurs between the individual spatial modes during propagation of the multimodal waveguide. - The waveguide can be a weak multimode, e.g. having less than 10 modes at the wavelength being examined. In one embodiment, the present invention also discloses an optical communication system comprising: the aforementioned receiver; a multi-modal waveguide connected to the input of the receiver; and an optical transmitter connected to the multi-modal waveguide away from the receiving At one end of the device, the optical transmitter is capable of transmitting an overlap of a plurality of modal components within the multimodal waveguide, each of the modal components being modulated by a subset of the digital data. 201207457 A complication based on the present invention is that the use of optical fibers or other waveguides exhibiting a relatively efficient cross section within an optical communication system may reduce the non-linear effects affecting the transmitted signal, which may contribute to an increase in the power level of the optical signal. Increase the transmission distance. Some aspects of the invention result from the observation that such an optical fiber may cause the transmission to be multimodal, and thus cause intersymbol interference, which must be compensated at the receiver end to rediscover the data. Other aspects of the invention are based on the concept of separating a received signal into a plurality of multi-modal components via a multi-modal waveguide and processing the corresponding multi-modal components in a manner that reverses the propagation effects. Detect signals. Another aspect of the invention is based on the concept of constructing mode adaptation between the local oscillator signal and the components of the multimodal signal to reduce effective coherent detection of these components. A further aspect of the invention resides in the complication of using spatial modal multiplexing at a given wavelength to obtain a total transmission capacity equal to the sum of the capacities of a plurality of spatial modes. [Embodiment] Referring to Fig. 1, an optical communication system is depicted in a schematic manner. This system includes an optical transmitting device 10, a light receiving device 30, and a transmission line 20 capable of transmitting an optical signal from the optical transmitting device 10 to the optical receiving device 30. Transmission line 20 includes a multimode fiber 2 1 . The transmission line may also include other single mode or multimode optical components not shown herein, such as optical amplifiers, dispersion compensating devices, connectors, add-drop multiplexers, transparent switching devices, different types of optical fibers, and the like. It is not necessary here to describe such components commonly used in optical communication networks in more detail. • 8 - 201207457 Multimode fiber 21 is an optical fiber whose structure enables the propagation of multiple suitable transverse modes at the wavelengths used for communication. A suitable transverse mode is the spatial distribution of the electromagnetic field in a plane orthogonal to the direction of propagation, which approximately remains the same during propagation, and is affected by the longitudinal phase factor and the longitudinal attenuation factor. The appropriate transverse mode of the waveguide (referred to as the spatial mode for the sake of brevity) is conventionally indicated by the symbol LP (linear polarization) followed by two integers. In this format, the first number represents the phase change of the electromagnetic field along the circumference centered on the longitudinal axis of the fiber, measured in units of 2π; and the second number represents the phase change of the electromagnetic field along the radius of the fiber, measured in units of π. Multimode fiber 2 1 supports between two and more than one hundred modalities, depending on the design. Preferentially, the multimode fiber 21 is a weak multimode with no more than 10 or even 3 spatial modes. Such fibers may in particular exhibit a relatively wide effective cross section, especially a cross section of more than 300 square microns, such as 400 or 500 square microns. The effective cross-section viewed here can be a modal-based effective cross-section or an equivalent effective cross-section that relates to the tolerance of the injected total optical power relative to the overall non-linear effect for propagation of multiple spatial modes ( Tolerance). The optical transmitting device 10 generates one or more optical signals modulated with data and injects the signals into the transmission line 20, which carries the signals or signals to the receiving device 30. For purposes of illustration, the transmission of a single color signal through a given wavelength channel will be examined first. Similarly, signals transmitted, propagated, and received in a single polarization manner will be considered first. Due to the multi-modal structure of the optical fiber 21, the optical signal arriving at the receiving device 30 contains a plurality of spatial modes at the wavelengths it uses. These spatial modes are transmitted at different group speeds, which causes interference during propagation, especially between symbols. In addition, there may be modal couplings during propagation, especially within multiple fiber seams, which exacerbate interference. The receiving device 30 performs digital processing of the same dimming detection and detection signals, which is used to compensate for multimodal dispersion and rediscover the transmitted data. In order to do so, the receiving device 30 includes a modal demultiplexer 3 1, an input 32 of which is coupled to the optical fiber 21 for receiving the transmitted multimodal signal. The modal demultiplexer 3 1 separates the input signal into a plurality of individual modal signals, and directs the modal signals to the individual coherent detectors 34 via the individual outputs 33. At the output 3 3 of the modal demultiplexer, each modal signal substantially corresponds to an individual spatial mode of the input signal. In other words, more than 50% of the power of the modal signal obtained at the output of the modal demultiplexer 31 comes from a given spatial mode of the input signal within the modal demultiplexer 31. These spatial modes are strictly only present in the multimode fiber 2 1 . On the outside of the multimode fiber, the electromagnetic field distribution corresponding to this mode is also indicated by the word "space mode". Each time, the coherent detector 34 receives the modal signal to be detected from the modal demultiplexer 31, such as via fiber or waveguide 36, and receives local oscillations from the local oscillator 37, such as via fiber or waveguide 38. Signal. The local oscillator signal is adjusted to the same wavelength as the wavelength used to transmit the data. Preferentially, the fibers or waveguides 36 and 38 are single mode at this wavelength, which makes it possible to design a relatively simple single mode coherent mixer. The digital processing module 35 receives, from each of the coherent detectors 34, an electrical detection signal I representing an in-phase component of the modal signal detected by the coherent detector and an electrical detection signal Q representing a quadrature component of -10 - 201207457. The processing module 35 samples the signals and applies processing to all of the detected signals received at a given time to reverse the propagation effects and rediscover the originally modulated data. In order to do this, the adaptive algorithm commonly used in the field of coherent detection can be modified as appropriate. In particular, an adaptive filter known to compensate for polarization modal dispersion may be used for this purpose. The temporal depth of the filter can be adjusted to the maximum time offset that exists between the spatial modes being examined. The local oscillator signal from a single local oscillator 37 is distributed to each coherent detector 34 using beam splitter 40 in FIG. In one variation, a plurality of individual local oscillators can be coupled to each of the coherent detectors 34. Referring to FIG. 4, the coherent detector 60 that can be used in the receiving device 30 of FIG. 1 includes a coherent mixer 6 which is often formed by a combination of a semi-ref_, a lective blade and a collecting lens. 1. Photodetector 62 and signal combiner 63. The coherent mixer 6 1 forms an interference signal from the optical signals received on its two inputs 64. Photodetector 62 converts these interference signals into electrical signals. The signal combiner 63 combines these electrical signals to form an in-phase detection signal I and a quadrature detection signal Q. Referring to FIG. 2, the modal demultiplexer 70 usable in the receiving device 30 of FIG. 1 includes: an input element 7.1, such as a multi-mode optical fiber, for receiving a multi-modal optical signal to be multiplexed. 78; a multi-modal beam splitter 72' for distributing the multi-modal optical signal to the plurality of modal selection filters 73; and an output element 74 for connecting each time selected by the associated modal selection filter 73 Modal signal 79. Each modal selection filter 73 selects an individual null -11 - 201207457 modality of the input signal, namely the modalities LP02, LPl 1, and LP01 in the depicted example. In other words, the modal selection filter 73 allows access to the output element 74 to be: more than 50% (and preferably more than 66%) of its energy from the optical signal of the indicated spatial mode. A multi-modal beam splitter 72 can be constructed from a lens and a semi-transparent blade. Referring to Figure 3, in a preferred embodiment, modal selection filter 73 and output element 74 can be constructed in the form of a combination of modal converter 80 and single mode waveguide 81. The modal converter 80 receives the multimode signal to be filtered from the waveguide 82 and converts a given spatial mode of the signal (eg, modal LP 02 ) into a fundamental spatial mode LP0 1, which will The signal is transmitted to the single mode waveguide 81. Due to its structure, the single mode waveguide 81 allows only the basic spatial modes to pass, which makes it possible to eliminate any higher order spatial components. Such a modal converter 80 can be constructed in accordance with the teachings of the document US-A-6,377,726, which has lenses 83 and 84 and phase masks 85 and 86. The modal components converted to the fundamental mode depend on the exact configuration of the phase masks 8 5 and 8 6 . Different modal converters can be constructed from this model to select different higher order modes of the input signal, such as LP02, LP 1 1, LP2 1, and LP03. In order to select the basic mode LP01 of the input signal, this device or a simpler device such as a concentrating lens can be used. Selecting the state. Modeling the mouth of the wife's mouth 2 force the first type of the shape of each device change the structure of the wave state mode of the single picture to 3 & WL: select the wave of η 7 choose the wave ί e select, test In the middle of the situation, the situation is in the middle of the apparatus. The same is the same as the instrument. The mode is the same as the one. The shape is effective. The mode is effective. The good non-conducting -12- 201207457 The overlap of the state signal with the local oscillator signal is due to the modality of the modal signal to be detected and the local oscillator signal. This is why it is preferred to transmit the local oscillator signal via a single mode waveguide in this situation. Another advantage resulting from this configuration is that it is optional (in each of the coherent detectors 34) to use a conventional coherent mixer that is commercially available at low cost. In one embodiment, the multi-modal optical signal distribution function can be performed in a manner that is combined with the selection function of the spatial mode to be detected. Depending on the configuration of the transmitting and receiving devices 10 and 30, the optical communication system depicted in Figure 1 can be used in several ways. In the first application, called SISO (Single Input-Single Output) or SIMO (Single Input Multiple Output), the multimodal nature of propagation is considered to be replicated on different propagation paths. The same data signal. For such an application, the transmitting device 10 is designed to inject an optical signal modulated with the data signal into one end of the transmission line 20 in such a manner that the signal is coupled to one or more of the multimode optical fibers 21 Spatial mode. At the other end of the transmission line 20, the digital processing module 35 processes the detected signals in a manner of different linear combinations of a plurality of time-shifted replicas of the data signals to be rediscovered. In the second application, called Multiple Input Multiple Output (Multiple Input Multiple Output), the multimodal nature of the propagation is regarded as the demultiplexing of the transmission channel, whereby it is possible to Groups of individual spatial streams transmit multiple individual data streams to increase the capacity of the communication system. For such an application, referring to FIG. 1, the transmitting device 10 may include: a plurality of signal modulators 1 1, which are configured to be adjusted by individual data streams D1, D2, ..., Dk-13-201207457 a dimming signal; and a modal demultiplexer 12, configured to couple the individual spatial modes of the modulated optical signals S1, S2, ..., Sk to the multimode fiber 2 (or a set of Mode) In order to perform such selective coupling of the modulated signal to a particular spatial mode of the optical fiber 21, the modal demultiplexer 12 may comprise a modal converter similar to that described with reference to Figure 3, in this example Reverse the direction of the transition. In this application, at the other end of the optical fiber 20, the digital processing module 35 processes the detection signals in a manner of mixing the data signals to be rediscovered in the case where modal coupling occurs during the known propagation. . As long as a sufficient number of different detection signals are supplied to the digital processing module 35, these intermodal couplings can be inverted by the digital method. Since each of the coherent detectors 34 is used to substantially capture information about a given spatial modality, it is preferable to set a coherent detector 3 4 as many as the signal modulator 1 1 and the modal solution is multiplexed. The device 31 selects individual spatial modes that are respectively coupled to the initially transmitted optical signals S1, S2, ..., Sk. In the case of modal multiplexing during transmission, if the modality is not mixed in the multimode fiber, the detected signal may be processed modally, such as by setting the separation for each detected spatial mode. Digital processing module. In the above embodiments, the optical communication implementation via the multi-modal transmission line has been described using only the carrier wavelength. Modal multiplexing allows for an increase in the capacity of the wavelength channel. In order to produce a larger capacity communication system, the above method can be combined with wavelength division multiplexing technology and/or polarization division m u 11 i p 1 e x i n g technology. -14-201207457 In order to do so, a wavelength combiner (e.g., a wavelength division multiplexer) may be disposed in the transmitting device 10, and a wavelength separator (e.g., a wavelength division demultiplexer) may be disposed in the receiving device 30. In a corresponding embodiment, the components of the transmitting device 10 and/or the receiving device 30 described above with respect to wavelengths may be increased as much as the wavelength channel used for processing. Similarly, to perform polarization division multiplexing, one or more polarization combiners can be placed within the transmitting device 10, and one or more polarization splitters can be disposed within the receiving device 30. Whether or not there is polarization division multiplexing during transmission, it is preferred to construct a diversity-polarization receiving device, particularly to compensate for modal polarization dispersion. In order to do this, there are multiple possibilities. An embodiment of a diversity polarization receiving device is described in detail in FIG. Here, the incident optical signal enters a polarization separator element 25 where the output of the polarization separator element separates two multi-modal signals having orthogonal polarizations. Similarly, polarization separator element 26 separates the local oscillator signal into two orthogonally polarized components. The block 50 depicted in Figure 1 thus represents a coherent detection chain of polar components. The second identical block 50 (not shown) must be connected to the other turns 51 of polarization separators 25 and 26 to detect other polarization components. In this case, the digital polarization module 35 can be shared by both blocks 50 to simultaneously process the detection signals corresponding to the two polarization components. Instead of the splitter 26, a different local oscillator can be set for each polarization. In another embodiment of the diversity polarization receiving device depicted in Figure 5, separating the optical signal into two orthogonally polarized components is accomplished downstream of spatial mode separation. As for the others, elements similar to those in Figure 1 or similar to -15-201207457 have the same component symbol plus 100. The components (especially the control unit and the modules) can be constructed in various forms (independent or decentralized) using hardware and/or software components. The hardware components that can be used are application-specific integrated circuits, field-effect programmable gate arrays, or microprocessors. Software components can be written in a variety of programming languages such as C, C++, Java, or VHDL. This list is not exhaustive. Although the present invention has been described in connection with the specific embodiments, the invention is not limited to the embodiments in any way, and includes all technical equivalents of the means and combinations thereof, if such combinations fall Within the scope of the invention. The use of the verb "comprise" or "comprises" or "the" or "the" or "the" or "the" The use of the indefinite article "a" or "an" Multiple means or modules can be depicted by a single hardware component. In the scope of the patent application, any component symbol in parentheses should not be construed as limiting the scope of the application. 8 [Simplified Description of the Drawings] After reviewing the description of specific embodiments of the invention, the invention will be better understood. Other objects, details, features and advantages of the present invention will be apparent from the accompanying drawings. In these figures: -16- 201207457 Figure 1 is a functional block diagram of an optical transmission system in accordance with one embodiment, and Figure 2 depicts an embodiment of a modal demultiplexer, which is particularly useful in Figure 1 In the system, FIG. 3 depicts an embodiment of a modal converter that is particularly useful in the demultiplexer of FIG. 2, which depicts an embodiment of a coherent receiver, particularly useful in FIG. In the system, and Figure 5 depicts an embodiment of a light receiving device that is particularly useful in the system of Figure 1. [Main component symbol description] 10: Optical transmitting device 1 1 : Signal modulator 1 2, 3 1, 7 0, 1 3 1 : Modal demultiplexer 20: Transmission line 2 1 , 1 2 1 : Multimodal Optical fibers 25, 26, 125, 126: Polarization splitters 30, 130: Light receiving devices 32, 64, 132: Inputs 33, 133: Outputs 34, 60, 134: Coherent detectors 3 5, 1 3 5: Digital Processing Modules 36, 38, 82, 136, 138: Waveguide-17-201207457 37, 137: Local Oscillator 40, 1 40: Beamsplitter 50: Block 51: 埠6 1 : Coherent Mixer 62: Photodetection Detector 63: signal combiner 7 1 : input element 72 : multimode beam splitter 73 : modal selection filter 74 : output element 78 : multimode optical signal 79 : modal signal 8 0 : modal converter 8 1 : Single mode waveguide 8 3, 8 4 : Lens 8 5, 8 6 : Phase mask