JP3875597B2 - Optical bandpass filter using a long-period grating. - Google Patents

Description

上式で、ΔＤは、ＢＢ−ＬＰＧによって結合されている２つのモード間の分散の相違であり、Ｌは格子の長さ、λ_ｒｅｓは（最大結合が生じる）共振波長、ｃは真空における光速、Ａは格子の最大結合強度によって決定される一定値である。したがって、この概念を使用して、様々な広帯域スペクトルを獲得することが可能である。通常のＢＢ−ＬＰＧの波長の範囲は、４０から１００ｎｍの範囲とすることが可能である。
【００１６】
ファイバ・セクション１３は、ファイバ１１に類似の数モードのファイバであるが、１１２μｍのＯＤにドローされている。このファイバのＮＢ−ＬＰＧのスペクトルを、図３の曲線３２によって示す。このスペクトルは、１２０μｍの格子周期と１×１０^−４の指数摂動を有する、５．７ｃｍの長さの均一のＬＰＧに対するものである。３ｄＢの帯域幅は、７ｎｍであり、１５５５ｎｍの共振波長において、９９．６％（２４ｄＢ）のモード変換を提供する。ＬＰＧスペクトルは、ファイバの分散特性、または、格子の物理パラメータを修正することによって、帯域幅についてテイラーすることが可能である。
ＮＢ−ＬＰＧの帯域幅Δλは、次式によって与えられる。
【数２】

上式で、Δｎ_ｇは、ＮＢ−ＬＰＧによって結合されている２つのモード間のグループ指数における相違であり、残りの項は、以前に確定されているものである。帯域幅上での制御の他に、格子の周期をチャープすること、または、格子の指数摂動をアポダイズすることにより、複数のスペクトルの形状（矩形、ガウス型など）をもたらすことができる。
【００１７】
図１の要素を参照すると、装置の機能は以下の通りである。
本質的に、ＢＢ−ＬＰＧ１２を通過する通信帯域全体のすべての光は、ＬＰ_０２モードに変換される。ＬＰ_０２モードの光が、ファイバ・セクション１１をファイバ・セクション１３にスプライスするスプライス１５を横切る際、ファイバ１１のＬＰ_０２モードの光は、ファイバ１３のＬＰ_０２モードに断熱的に変換される。次いで、ＮＢ−ＬＰＧ１４は、スペクトルの望ましい狭い部分を選択し、それを再びＬＰ_０１モードに変換する。これにより、図４の曲線３５によって示した特性を有する、帯域通過フィルタが得られる。
【００１８】
図１に示した装置は、ＬＰＧを有する帯域消去フィルタとは対照的に、帯域選択を構築するための一般的なプラットフォームを提供する。第１モード・コンバータＢＢ−ＬＰＧ１２は、ＮＢ−ＬＰＧ１４に対する空間的に修正された入力を提供する装置としてのみ役立ち、帯域通過フィルタのスペクトル特性を確定しない。フィルタのスペクトルは、ＮＢ−ＬＰＧ１４の反転スペクトルによって確定される。さらに、唯一のＮＢ−ＬＰＧ１４の代わりに、いくつかのＮＢ−ＬＰＧを直列に追加することが可能である。代替として、ＮＢ−ＬＰＧは、複数の狭帯域共振を有するように設計することが可能である。これにより、１つまたは複数のＮＢ−ＬＰＧ１４のスペクトル特性を変化させることによって、帯域通過構成におけるスペクトルの形状の予想が可能になる。
【００１９】
直前に記述した装置は、コアブロッキング要素を使用しないという点で、ＬＰＧを使用する従来のフィルタに対して、利点を有する。これにより、その構成を有する装置に固有の損失が回避される。この区別を確定するために、ＢＢ−ＬＰＧとＮＢ−ＬＰＧの間の送信経路は、断熱的に機能する。さらに、ＬＰＧを結合する送信経路は、アクティブな減衰要素、すなわち、減衰光の意図した計画的な機能を有する要素を含まない。最小限の光の減衰に対して設計されている従来のスプライスは、この文脈では、アクティブな減衰要素ではないことを意図していない。
【００２０】
図１のスプライス要素１５は、ファイバ１１のＨＯＭを（ＮＢ−ＬＰＧを横切った後）ファイバ１３のＨＯＭに断熱的に変形する。これは、２つのＨＯＭが効率的に結合することを保証するために、スプライスに沿って熱プロファイルを誘導することによって、および／または、一方のファイバを他方に対して先細にすることによって、達成することが可能である。図１に示した実施形態では、２つのファイバ１１と１３は、それらを接合する物理スプライスを有するように示されている。すなわち、ファイバ１１と１３は別々のファイバである。物理スプライス１５と、そのスプライスにおける付随する損失は、２つの異なる送信特性を有する単一のファイバを作成することによって、回避することが可能である。例えば、ファイバの直径は、ファイバの全長に沿って、長手方向に変化するように作成することが可能である。すなわち、第１の直径を有する第１セクションと、第２の直径を有する第２セクションである。代替として、クラッディングは、単一ファイバの１つの部分から他の部分へ、選択的に変更することが可能である。したがって、スプライス１５の機能的な動作は、重要な態様であり、一特性を有するＨＯＴから、異なる特性を有する対応するＨＯＭへの変更を実施するための手段として確定することが可能である。
【００２１】
均一なＬＰＧは、分散型フィルタではないので、ここで説明した帯域通過フィルタは、分散型ではない。
【００２２】
また、本発明のフィルタは、ＮＢ−ＬＰＧの位相整合曲線（図２の曲線２３）にシフトを誘導することによって、同調することが可能である。代替として、ファイバのクラッディングをドープすること、または、電気光学材料または非線形光学材料でファイバの外部クラッディングをコーティングすることにより、帯域通過フィルタの共振波長の電気的または光学的制御が可能になる。
【００２３】
本発明の帯域通過フィルタは、様々なシステムにおいて使用することが可能である。例えば、様々なレーザ装置は、空洞内要素として、本発明の帯域通過フィルタを使用して、同調することができる。１つの高反射器と１つの弱反射器（出力鏡）の２つの狭帯域反射器によって画定される従来のレーザ空洞では、２つの狭帯域反射器は、空洞に本発明の帯域通過フィルタを有する２つの広帯域反射器（１つが高で１つが弱）によって置換される。レージング波長は、フィルタを同調することによって、調節することが可能である。複数のレージング波長は、複数の共振を有する帯域通過フィルタにおける１つまたは複数のＮＢ−ＬＰＧを有することによって、生成することができる。
【００２４】
レーザのこの一般化した形態の例を図５に示す。この図では、利得ファイバを３８、高反射器を３９、弱反射器を４１、入射信号を４３、レーザ・ポンプを４４、信号とポンプを組み合わせるためのＷＤＭを４５に示す。全体を４６で示し、ＢＢ−ＬＰＧ４７、ＮＢ−ＬＰＧ４８、およびスプライス４９を備える本発明の帯域通過フィルタは、図示したように、レーザ空洞の内部に配置される。
【００２５】
レーザの特有の形態、カスケード・ラマン・ファイバ・レーザは、本発明のフィルタと共に使用するために、特に適合される。これらの装置は、狭帯域の高反射器と出力カプラ／弱反射器を備えるブラッグ格子の複数のセットを使用することによって、複数の波長において動作するように作成することができる。ブラッグ格子のこれらの対は、レーザ空洞を画定し、狭帯域格子の共振波長は、レーザ波長を確定する。
【００２６】
この装置の実施形態を図６に示す。ラマン共振器５１は、ファイバ５２と格子５３、５４を備える。従来の装置では、共振器は、狭帯域格子によって、両側面上で境界を接している。この実施形態では、共振器は、広帯域高反射器４８と弱反射器４９の格子によって、境界が定められる。この場合、共振波長は、本発明の帯域通過フィルタ５６によって制御される。帯域通過フィルタ５６は、以前に記述したように、ＢＢ−ＬＰＧ５７、ＮＢ−ＬＰＧ５８、およびスプライス５９を備える。同調は、帯域通過フィルタのＮＢ−ＬＰＧを同調することによって達成される。ＬＰＧ（および、したがって、ＬＰＧの帯域通過フィルタ）は、以前に記述した方法によって、広範に同調可能なので、ラマン増幅光通信システムにおいて有用である掃引波長源は、容易に実現可能である。これらのシステムに関するより詳細と、これらのシステムにおける掃引波長の実装については、参照によって、本明細書に組み込まれている、２００２年３月１５日に出願された、「Ｗｉｄｅｂａｎｄ Ｒａｍａｎ Ａｍｐｌｉｆｉｅｒ」という名称の、同時係属中の出願第１０／０９８，２００号を参照されたい。より一般的には、いくつかのレージング波長にわたって、装置の同時同調可能動作を提供するために、複数のＮＢ−ＬＰＧ５８を直列に追加することが可能である。
【００２７】
また、本発明の帯域通過フィルタは、自己位相変調（ＳＰＭ）に基づく光再生成装置との使用に、よく適合されている。これらの装置は、比較的簡単であり、通常、帯域通過フィルタに関連付けられた非線形ファイバを備える（米国特許第６，１４１，１２９号参照）。信号は、非線形ファイバ・セクション内に入射され、それにより、信号にＳＰＭを誘導する。その効果は、選択した強度値を超える信号の部分のスペクトルを広げることである。次いで、帯域通過フィルタは、広くなったスペクトルの所定の部分を選択する。理想的には、この応用のための帯域通過フィルタは、スペクトルの時間的な変化を説明するように、同調可能であるべきである。また、フィルタは、分散のないことが望ましい。これらの是非望ましいものは、本発明の帯域通過フィルタによって満足される。
【００２８】
本発明の帯域通過フィルタを使用する光再生成装置の概略図を図７に示す。非線形ファイバ・セクションを６２、ＢＢ−ＬＰＧ６７、ＮＢ−ＬＰＧ６８、およびスプライス６９を備える帯域通過フィルタの全体を６６に示す。
【００２９】
光受信器のための前置増幅器は、受信した信号から自然発生雑音を除去するための雑音フィルタを使用する。この雑音は、送信経路に沿って、増幅器から発信される。雑音は、通常、信号の波長と、信号の実質的な側波帯とを含むスペクトル範囲全体にわたる。この背景雑音は、受信器への信号の信号対雑音比の質を下げる。帯域通過フィルタは、信号の波長外の雑音を選択的に減衰させ、したがって、光の信号対雑音比を向上させることができるが、帯域通過フィルタに分散がないとき、特に効果的である。この特性は、本発明の帯域通過フィルタによって提供される。
【００３０】
直前に記述した光前置増幅器システムを図８に示す。この図では、光増幅器７２には、ＢＢ−ＬＰＧ７７、ＮＢ−ＬＰＧ７８、およびスプライス７９を備える、全体を７６で示した帯域通過フィルタが続く。次いで、フィルタリングした信号は、感光ダイオード８１によって図に表した受信器に導入される。
【００３１】
本発明について、本発明の帯域通過フィルタを実装するために光ファイバを使用して、上記で説明した。光集積回路（ＯＩＣ）装置の平面導波路など、導波路の他の形態を使用して、同様の装置を構築することが可能である。
【００３２】
本明細書に記述した長周期の格子は、様々な技術によって形成することが可能である。一般的な手法は、ＵＶ光を使用して、Ｇｅをドープしたファイバに格子を書き込むものである。しかし、他の方法を使用することも可能である。例えば、マイクロベンド誘導ＬＰＧが適切である。これらは、音響光学格子、アークスプライサ誘導周期マイクロベンドで、または、必要な格子の周期性を有する波形ブロック間においてファイバをプレスすることによって、実現することができる。
【００３３】
当業者なら、本発明の様々な追加の修正を思いつくであろう。当技術分野が進展してきた原理とその等化物に基本的に依存する本明細書の特有の技術からの逸脱は、すべて、記述および主張されている本発明の範囲内において、適切に考慮されている。
【図面の簡単な説明】
【図１】本発明の帯域通過フィルタの概略図である。
【図２】異なる光ファイバにおける２つのＬＰＧの関係を示す格子周期対波長のプロットである。
【図３】ＢＢ−ＬＰＧとＮＢ−ＬＰＧの効果的なモード変換特性を示す、強度対波長のプロットである。
【図４】図１のフィルタに対する、すなわち組み合わされたＬＰＧを有する、図３のプロットと類似のプロットである。
【図５】レーザにおいて、本発明の光学フィルタの使用を示す概略図である。
【図６】ラマン・レーザにおいて、本発明の光学フィルタの使用を示す概略図である。
【図７】光信号再生成装置において、本発明の光学フィルタの使用を示す概略図である。
【図８】光受信器のための光前置増幅器において、本発明の光学フィルタを使用するシステムの概略図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical filter, and more particularly to a low-loss optical bandpass filter.
[0002]
[Prior art]
An optical bandpass filter transmits light over a predetermined band of wavelengths while removing all other wavelengths by absorption, radiation or scattering. Such filters are useful in laser cavities or optical communication systems. For example, it can be used to constrain the wavelength of operation of the laser when placed inside or outside the laser cavity. In an optical communication system, it can be used at the input of an optical receiver to separate unwanted light, such as spontaneous emission noise outside the signal wavelength band. D. M.M. Shamon, J.A. M.M. H. Elmirghani, R.A. A. See Cryan's “Characterization of optically received receivers with fiber fibre Bragg-grading optical fibers”, IEEE Colloquium on Optical 96. An optical regeneration device based on self-phase modulation needs to extract a predetermined wavelength from a wide spectrum of light.
[0003]
Several devices have been proposed and demonstrated to provide the functionality of bandpass filtering. The fiber Bragg grating can be used in reflection mode with a circulator, or transmission mode, to select a narrow wavelength band. Xu, M .; G. Alavie, A .; T.A. Maaskant, R .; Ohn, M .; M.M. "Tunable fiber bandpass filter based on a linearly chirped fiber Bragg grating for wavelength demultiplexing", Electron Lett. , 32, pp. 1918-1919 (1996). Operation in the reflective mode requires adding a circulator to the transmission line. This increases the cost, loss and complexity of the device. Another drawback of Bragg gratings is that when used for transmission or reflection, such filters can be highly dispersive, which causes distortions in the pulse shape. Lenz, G.M. Eggleton, B .; J. et al. Giles, C .; R. Madsen, C .; K. Sluster, R .; E. "Dispersive properties of optical filters for WDM systems", IEEE Journal of Quantum Electronics, 34, pp. 1390-1402 (1998). Alternatively, thin film dielectric filters can also be used as bandpass filters (see US Pat. No. 5,615,289), but in addition to their dispersive nature (similar to Bragg gratings), free space devices Is subject to the additional drawback of being. Thus, for use in such a fiber optic system, light must be coupled into the fiber pigtail. This increases loss, cost, and complexity.
[0004]
An alternative technique for creating a bandpass filter uses two identical long period fiber gratings (LPGs) spliced in series in a fiber optic transmission line with a core block between them (US Pat. No. 6,151,151). No. 427). The first long-period grating converts the narrow wavelength band of the core mode light into the cladding mode, and the second identical grating couples the cladding mode light back into the core mode. The core block between the two LPGs attenuates or scatters any light that has not been converted to the cladding mode. There is an associated disadvantage with this device-(1) the core block attenuates all modes of light simultaneously, and therefore the device is inherently lossy. Therefore, lower order cladding modes present even greater losses, so only higher order cladding modes can be used; (2) the core block is a discrete discontinuity in the fiber This results in undesirable mode coupling and complex one-conventional transport interference; (3) such a filter works properly only when both LPGs have the same spectrum, Tuning requires that both LPGs be tuned simultaneously by the same amount. Bandpass filters using acousto-optically generated or microbend-guided LPG have the additional disadvantage that the device is inherently sensitive to polarization.
[0005]
[Problems to be solved by the invention]
Therefore, there is a need for a bandpass filter that is an inline fiber device, has low loss, is insensitive to polarization, is tunable, and is easy to implement.
[0006]
[Means for Solving the Problems]
In accordance with the present invention, LPG is used in a new configuration to provide in-line bandpass filtering of optical signals in optical systems. In the preferred case, the optical system is a fiber optic system. In the basic filter embodiment of the present invention, two dissimilar LPGs are attached to an optical waveguide cascaded in series with a transmission line. In the first waveguide of the filter, the signal light is first transmitted in the fundamental mode or the LP ₀₁ mode. The first LPG converts the signal light into a higher order mode (HOM) of the first optical waveguide over a wide wavelength range. This conversion is accomplished using a broadband LPG that provides strong mode conversion over a wide spectrum. S. Ramachandran, M.M. F. Yan, L. C. Cowsar, A.M. C. Carra, P.M. Wisk, R.A. G. Huff, and D.D. Peckham's "Large bandwidth, high efficient mode coupling using long-period gratings in dispersal tailed fibers", Optics Letters, vol. 27, pp. 698-700 (2002) and US Pat. No. 6,084,996. Both are hereby incorporated by reference. In the following description, the broadband LPG is referred to as BB-LPG. The BB-LPG operates by exciting the core guided HOM or the cladding guided HOM of the first optical waveguide. Next, the mode conversion signal having the mode LP _{m, n} is coupled to the second optical waveguide having transmission characteristics different from the transmission characteristics of the first optical waveguide. The second waveguide is strongly coupled to the LP _{m, n} mode because the second LPG, which can be a conventional LPG, is the LP _{m, n} mode and LP ₀₁ mode of the second optical waveguide. This is because a strong narrow band is provided. The second LPG is referred to herein as a narrow band LPG (NB-LPG).
[0007]
The light incident on the dual LPG filter first encounters BB-LPG, where over 99% of the signal is converted to LP _{m, n} mode over a wide wavelength range. When the converted signal encounters the NB-LPG, the signal on the selected narrowband of the wavelength accepted by the NB-LPG is converted back to the LP ₀₁ mode of the second waveguide. The selected narrow band of the LP ₀₁ mode propagates efficiently over the remainder of the transmission path of the optical waveguide, i.e. the remainder of the second waveguide.
[0008]
_{LP m,} and _n modes excited in _{BB-LPG, NB-LPG LP} m , which was returned in _the mode distribution profile of the _n as long as the order of the mode (m and n) are the same, have to be aligned Absent. BB-LPG and NB-LPG can have separate waveguide sections or can be formed in the same length of waveguide. The waveguide is preferably an optical fiber. If the LPG comprises separate optical fibers spliced together, the signals are adiabatically coupled at the splice because the mode shape is the same.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, the configuration shown shows the general form of the bandpass filter of the present invention. The filter comprises a first fiber section 11 having a BB-LPG 12, a second fiber section 13 having an NB-LPG 14, and a splice 15 connecting the two fiber sections. The splice 15 adiabatically converts the HOM of the fiber 11 to the HOM of the fiber 13. BB-LPG 12 has a selected grating period for wideband mode conversion, and NB-LPG 14 has a different grating period selected for narrowband, ie bandpass mode conversion. The selection of the grating period is shown in FIG.
[0010]
Referring to FIG. 2, the phase matching relationship is shown for LPG to bind (for example) LP mode ₀₁ mode to the _{LP 02} mode. Curve 21 represents the fiber phase matching relationship resulting in BB-LPG at about 1470 nm. A unique feature of the curve 21 is the turning point (TAP) of the phase matching relationship. In TAP, the vertical line 22 is a tangent to the phase matching curve 21. When the grating period has a value corresponding to the tangent line 22, a wide range of wavelengths almost satisfies the phase matching relationship. At wavelengths in this range, BB-LPG occurs because the wavelength resonates or is close to resonance in the vicinity of the TAP. In this example, BB-LPG with a period of 115 microns (line 22) yields BB-LPG at about 1470 nm.
[0011]
Curve 23 corresponds to the phase matching relationship of LPG in the second fiber. From curve 23 it can be seen that LPG gives NB-LPG for any wavelength longer than about 1300 nm. That is, for wavelengths longer than 1300 nm, the vertical line corresponding to the selected grating period never touches the phase matching curve 23. Therefore, there is no wide range of wavelengths that substantially satisfies the phase matching relationship. For example, line 24 intersects the phase matching relationship only at one wavelength.
[0012]
In this example, curve 23 represents a fiber having the same exponential profile as represented by curve 21, but the fiber associated with curve 23 has a diameter that is 80% of the original diameter of the fiber associated with curve 21. Drawn. Thus, by reducing the fiber dimensions, the fiber TAP is shifted. Thus, a fiber that yields BB-LPG when drawn to one diameter yields NB-LPG when drawn to a different diameter. Reducing the size is one of several ways to shift or adjust the fiber TAP. The same purpose can be achieved by inducing a constant index change in the fiber or by etching the outer diameter of the cladding. The former technique results in a TAP shift in both the cladding mode as well as the core induction mode, while the latter is useful when the HOM used for the bandpass filter is in the cladding mode.
[0013]
To show the characteristics of the fiber sections 11 and 13 in the filter of FIG. 1, the experimentally acquired spectrum is shown in FIG. The phase matching curve 21 of FIG. 2 for the fiber 11 (outer diameter OD = 121 μm) has a TAP at 1540 nm. An LPG written in the corresponding grating period (112.5 μm) converts the incident LP ₀₁ mode into the LP ₀₂ mode over the entire C band. The length of this lattice is 1 cm and the exponential perturbation is 5 × 10 ⁻³ . This indicates that over 99% (> 20 dB) of the light is converted over the spectral range between 1527 nm and 1571 nm.
[0014]
More generally, the wavelength range of BB-LPG can be controlled by appropriately designing fibers with different dispersion characteristics for the fundamental mode and the HOM.
[0015]
The bandwidth Δλ of BB-LPG is given by the following equation.
[Expression 1]

Where ΔD is the dispersion difference between the two modes coupled by BB-LPG, L is the length of the grating, λ _res is the resonant wavelength (where maximum coupling occurs), and c is the speed of light in vacuum. , A is a constant value determined by the maximum coupling strength of the lattice. Therefore, using this concept, it is possible to acquire various broadband spectra. The wavelength range of normal BB-LPG can be in the range of 40 to 100 nm.
[0016]
Fiber section 13 is a few mode fiber similar to fiber 11 but drawn to an OD of 112 μm. The NB-LPG spectrum of this fiber is shown by curve 32 in FIG. This spectrum is for a 5.7 cm long uniform LPG with 120 μm grating period and 1 × 10 ⁻⁴ exponential perturbation. The 3 dB bandwidth is 7 nm, which provides 99.6% (24 dB) mode conversion at a resonant wavelength of 1555 nm. The LPG spectrum can be tailored for bandwidth by modifying the dispersion characteristics of the fiber or the physical parameters of the grating.
The bandwidth Δλ of NB-LPG is given by the following equation.
[Expression 2]

Where _Δng is the difference in group index between the two modes coupled by NB-LPG and the remaining terms are those previously determined. In addition to control over bandwidth, multiple spectral shapes (rectangular, Gaussian, etc.) can be produced by chirping the period of the grating or apodizing the exponential perturbation of the grating.
[0017]
Referring to the elements of FIG. 1, the function of the device is as follows.
Essentially all the light in the entire communication band that passes through the BB-LPG 12 is converted to the LP ₀₂ mode. As LP ₀₂ mode light traverses the splice 15 splicing the fiber section 11 to the fiber section 13, the LP ₀₂ mode light of the fiber 11 is adiabatically converted to the LP ₀₂ mode of the fiber 13. The NB-LPG 14 then selects the desired narrow portion of the spectrum and converts it back to the LP ₀₁ mode. As a result, a bandpass filter having the characteristics shown by the curve 35 in FIG. 4 is obtained.
[0018]
The apparatus shown in FIG. 1 provides a general platform for constructing band selections as opposed to band elimination filters with LPG. The first mode converter BB-LPG 12 serves only as a device that provides a spatially modified input to the NB-LPG 14 and does not determine the spectral characteristics of the bandpass filter. The spectrum of the filter is determined by the inverted spectrum of NB-LPG14. Furthermore, instead of a single NB-LPG 14, it is possible to add several NB-LPGs in series. Alternatively, NB-LPG can be designed to have multiple narrowband resonances. This makes it possible to predict the shape of the spectrum in a bandpass configuration by changing the spectral characteristics of one or more NB-LPGs 14.
[0019]
The apparatus just described has an advantage over conventional filters using LPG in that no core blocking element is used. This avoids losses inherent in the device having that configuration. In order to establish this distinction, the transmission path between BB-LPG and NB-LPG functions adiabatically. Furthermore, the transmission path coupling the LPG does not include active attenuating elements, i.e. elements having the intended planned function of attenuating light. Conventional splices designed for minimal light attenuation are not intended to be active attenuation elements in this context.
[0020]
The splice element 15 of FIG. 1 adiabatically transforms the HOM of the fiber 11 (after traversing the NB-LPG) into the HOM of the fiber 13. This is accomplished by inducing a thermal profile along the splice and / or by tapering one fiber relative to the other to ensure that the two HOMs are effectively combined. Is possible. In the embodiment shown in FIG. 1, the two fibers 11 and 13 are shown as having a physical splice joining them. That is, the fibers 11 and 13 are separate fibers. The physical splice 15 and the accompanying loss in that splice can be avoided by creating a single fiber with two different transmission characteristics. For example, the fiber diameter can be made to vary longitudinally along the length of the fiber. That is, a first section having a first diameter and a second section having a second diameter. Alternatively, the cladding can be selectively changed from one part of a single fiber to another. Therefore, the functional operation of the splice 15 is an important aspect and can be established as a means for implementing a change from a HOT having one characteristic to a corresponding HOM having a different characteristic.
[0021]
Since uniform LPG is not a distributed filter, the bandpass filter described here is not distributed.
[0022]
The filter of the present invention can also be tuned by inducing a shift in the NB-LPG phase matching curve (curve 23 in FIG. 2). Alternatively, doping the fiber cladding, or coating the outer cladding of the fiber with an electro-optic material or a non-linear optical material, allows electrical or optical control of the resonant wavelength of the bandpass filter. .
[0023]
The bandpass filter of the present invention can be used in various systems. For example, various laser devices can be tuned using the bandpass filter of the present invention as an intracavity element. In a conventional laser cavity defined by two narrowband reflectors, one high reflector and one weak reflector (output mirror), the two narrowband reflectors have the bandpass filter of the present invention in the cavity. Replaced by two broadband reflectors (one high and one weak). The lasing wavelength can be adjusted by tuning the filter. Multiple lasing wavelengths can be generated by having one or more NB-LPGs in a bandpass filter with multiple resonances.
[0024]
An example of this generalized form of laser is shown in FIG. In this figure, the gain fiber is 38, the high reflector is 39, the weak reflector is 41, the incident signal is 43, the laser pump is 44, and the WDM for combining the signal and pump is shown at 45. The bandpass filter of the present invention, indicated generally at 46 and comprising BB-LPG 47, NB-LPG 48, and splice 49, is placed inside the laser cavity as shown.
[0025]
A unique form of laser, a cascaded Raman fiber laser, is particularly adapted for use with the filter of the present invention. These devices can be made to operate at multiple wavelengths by using multiple sets of Bragg gratings with narrowband high reflectors and output coupler / weak reflectors. These pairs of Bragg gratings define the laser cavity, and the resonant wavelength of the narrow band grating determines the laser wavelength.
[0026]
An embodiment of this device is shown in FIG. The Raman resonator 51 includes a fiber 52 and gratings 53 and 54. In conventional devices, the resonator is bounded on both sides by a narrow band grating. In this embodiment, the resonator is bounded by a grating of broadband high reflector 48 and weak reflector 49. In this case, the resonance wavelength is controlled by the band pass filter 56 of the present invention. The band pass filter 56 includes a BB-LPG 57, an NB-LPG 58, and a splice 59 as previously described. Tuning is achieved by tuning the NB-LPG of the bandpass filter. Since the LPG (and hence the LPG bandpass filter) can be widely tuned by the methods described previously, a swept wavelength source useful in a Raman-amplified optical communication system is easily feasible. For more details on these systems and the implementation of the swept wavelength in these systems, named “Wideband Raman Amplifier”, filed Mar. 15, 2002, which is incorporated herein by reference. See copending application Ser. No. 10 / 098,200. More generally, multiple NB-LPGs 58 can be added in series to provide simultaneous tunable operation of the device over several lasing wavelengths.
[0027]
The bandpass filter of the present invention is also well adapted for use with an optical regeneration device based on self-phase modulation (SPM). These devices are relatively simple and typically comprise a nonlinear fiber associated with a bandpass filter (see US Pat. No. 6,141,129). The signal is incident into the nonlinear fiber section, thereby inducing SPM in the signal. The effect is to broaden the spectrum of the portion of the signal that exceeds the selected intensity value. The bandpass filter then selects a predetermined portion of the broadened spectrum. Ideally, the bandpass filter for this application should be tunable so as to account for temporal changes in the spectrum. Further, it is desirable that the filter has no dispersion. These highly desirable ones are satisfied by the bandpass filter of the present invention.
[0028]
FIG. 7 shows a schematic diagram of an optical regeneration device using the bandpass filter of the present invention. An overall bandpass filter comprising a nonlinear fiber section 62, BB-LPG 67, NB-LPG 68, and splice 69 is shown at 66.
[0029]
Preamplifiers for optical receivers use a noise filter to remove naturally occurring noise from the received signal. This noise is emitted from the amplifier along the transmission path. Noise typically spans the entire spectral range including the wavelength of the signal and the substantial sideband of the signal. This background noise degrades the signal-to-noise ratio of the signal to the receiver. A bandpass filter can selectively attenuate noise outside the wavelength of the signal and thus improve the signal-to-noise ratio of light, but is particularly effective when the bandpass filter has no dispersion. This property is provided by the bandpass filter of the present invention.
[0030]
The optical preamplifier system just described is shown in FIG. In this figure, the optical amplifier 72 is followed by a bandpass filter, indicated generally at 76, comprising BB-LPG 77, NB-LPG 78, and a splice 79. The filtered signal is then introduced by the photosensitive diode 81 into the receiver represented in the figure.
[0031]
The present invention has been described above using an optical fiber to implement the bandpass filter of the present invention. Similar devices can be constructed using other forms of waveguides, such as planar waveguides in optical integrated circuit (OIC) devices.
[0032]
The long-period grating described herein can be formed by various techniques. A common approach is to write a grating in a Ge-doped fiber using UV light. However, other methods can be used. For example, microbend derived LPG is appropriate. These can be realized with acousto-optic gratings, arc splicer induction periodic microbends, or by pressing the fiber between corrugated blocks with the required grating periodicity.
[0033]
Those skilled in the art will envision various additional modifications of the invention. All deviations from the specific technology of the present specification, which are fundamentally dependent on the principles developed by the art and their equivalents, are all properly considered within the scope of the invention as described and claimed. Yes.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a bandpass filter of the present invention.
FIG. 2 is a plot of grating period versus wavelength showing the relationship between two LPGs in different optical fibers.
FIG. 3 is a plot of intensity versus wavelength showing the effective mode conversion characteristics of BB-LPG and NB-LPG.
4 is a plot similar to the plot of FIG. 3 for the filter of FIG. 1, ie with combined LPG.
FIG. 5 is a schematic diagram illustrating the use of the optical filter of the present invention in a laser.
FIG. 6 is a schematic diagram illustrating the use of the optical filter of the present invention in a Raman laser.
FIG. 7 is a schematic diagram showing the use of the optical filter of the present invention in an optical signal regeneration device.
FIG. 8 is a schematic diagram of a system that uses the optical filter of the present invention in an optical preamplifier for an optical receiver.

Claims (6)

a. An optical waveguide having a first portion and a second portion connected in series, wherein the first portion has a light transmission characteristic different from that of the second portion;
b. All of the incident signal light propagating in mode LP _01, the first long-period fiber grating for converting signal light propagating in mode LP _{0, n,}
c. An optical coupler configured to transmit signal light propagating in mode LP _{0, n} in the first portion to light in LP _{r, s} mode in the second portion;
d. A second long-period fiber grating that converts a selected band Δλ ₂ of the signal light propagating in mode LP _{r, s} in the second portion into light propagating in mode LP ₀₁ , and
The first long-period fiber grating is a refractive index grating having a grating period that satisfies a phase matching relationship over the entire wavelength range of the incident signal light ,
The second long-period fiber grating is a refractive index grating having a grating period that satisfies a phase matching relationship over only the selected band Δλ ₂ .
These refractive index gratings are formed by periodic refractive index changes written in an optical waveguide by UV light.   The optical filter according to claim 1, wherein each of the first portion of the optical waveguide and the second portion of the optical waveguide is made of an optical fiber.   The optical filter of claim 1, wherein the optical coupler comprises a fiber splice.   A device in which the optical filter of claim 1 is disposed in a resonant cavity as part of a laser. A method for filtering an optical signal having a band Δλ ₁ comprising:
a. Transmitting an optical signal having a band Δλ ₁ composed of an LP ₀₁ mode through a first optical waveguide;
b. Transmitting the LP ₀₁ signal mode through a long-period fiber grating in the first optical waveguide, wherein the long-period fiber grating has a wide passband so that all of the LP ₀₁ signal modes are transmitted. into a higher order signal mode _{LP 0n,} it consists of a step,
The long-period fiber grating in the first optical waveguide is a refractive index grating generated by a periodic refractive index change written in the optical waveguide by UV light, and satisfies the phase matching relationship over the Δλ ₁ Having a grating period , the method further comprises:
c. Transmitting the higher order mode LP _0n through a second optical waveguide having a transmission characteristic different from that of the first optical waveguide, whereby the higher order signal mode in the optical signal is transmitted. LP _0n propagates as higher order signal modes LP _{r, s} in the second optical waveguide;
d. Wherein the second optical waveguide via a long-period fiber grating transmits a signal mode LP _{r, s} of the higher order, the higher order signal mode LP _r, selected portion [Delta] [lambda] ₂ signal mode of the _s of And converting to LP ₀₁ ,
Δλ ₁ is sufficiently larger than Δλ _{2, and the} long-period fiber grating in the second optical waveguide is a refractive index grating generated by a periodic refractive index change written in the optical waveguide by UV light, and the Δλ A method for filtering an optical signal having a band Δλ ₁ having a grating period satisfying a phase matching relationship over only _two .   A device in which the optical filter of claim 1 is connected in series to a cascaded Raman resonator having a Raman resonator cavity.

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