CHANNEL BALANCING OF A WAVELENGTH DIVISION MULTIPLEXED OPTICAL SIGNAL

Abstract: The invention relates to methods and arrangements for channel balancing of a wavelenght division multiplexed optical signal. Channel balancing according to the invention is performed by using a resonator that provides a selection region in which a selected channel has a substantially increased power density relative to channels out of resonance. The selected channel is attenuated a desired amount, i.e. a desired amount of poer is removed therefrom, by adjusting the prperties of the selection region. In a preferred embodiment, attenuation is achieved by adjusting the selection region such that destructive interference is obtained for the selected channel in a fibre carrying the multiplexed optical signal.

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LQ Φ ϋ φ ϋ 3 μ- rt SD rt Ω ø LQ Z ^ φ μ- SD 3 3 μ- Ω 0 Φ 0 Φ ø SD TJ LQ 0 0 SD φ Φ 03 rt Φ 03 Ω 0 rt tr Φ rt ø μ- 0 SD 03 0 TJ 0 μ- " 0 μ- rt ø rt 3 TJ ii Ω tr 03 • 0 i μ- Φ . μ- SD rt rt Ω ø rt ^ ø rt 0 ø SD s; 0 CQ ø- Φ tr $. φ ii 03 ri SD φ 0 Φ ø 0 tr tr tr 03 tr 03 <! μ- tr Ω LQ rt 03 Φ 0 SD SD Φ SD 0 Φ rt μ- rt 0 TJ SD SD LQ Ω Ω 0 . 0 Φ SD ii μ- Φ ti μ- Φ Ω SD Φ μ- μ- H μ- ø Ω <1 rt CQ 0 TJ rt ii 03 tr ø- ø ø Φ LQ Φ ø ø SD μ- 1 i ø ø LQ ≤ Pi tr 3 φ φ 0 rt Φ rt rt € SD SD μ- rt 03 ø LQ 0 03 3 LQ rt SD Φ Q ø ϋ tr SD SD Φ " Hi 0 ϋ μ- μ- μ- μ • 0 ti 0 03 Φ Φ μ- SD Φ SD μ- ø Hi LQ SD φ Φ Ω ø rt φ 0 rt 0 0 03 S . i SD 0 SD μ- 1 0 " μ- Φ 0 03 Ω 0 rt 0 ii tr ϋ ø tr 0 μ 0 <! ι-3 M π Ω φ tr Hi φ • ø φ ø ϋ ø μ- μ- μ μ- tr 03 φ SD Φ Φ 0 LQ μ- Φ SD tr rt SD Hi Ω Ω 1 SD LQ ø LQ Φ μ- ø CQ P. rt TJ 0 0 tr ø 0 μ- Φ % Φ 03 Φ rt Td SD rt 3 0 1 0 LQ rt μ- SD rt 03 ø μ- - tr Φ <! Hi φ rt 03 <! tr ii 03 φ Ω μ- 0 μ- 0 μ- ø 0 SD tr tr Q 0 ■ ; SD Φ ti μ- H Φ Ω μ- μ- Φ μ- φ Si rt 0 ii rt ϋ 0 rt Ω μ- Φ Φ Pi μ- Φ : > ø Si Hi ø 03 0 0 SD LQ SD ø 03 T μ- 0 0 rt 0 03 φ i fi ~. ϋ μ- Ω μ- 0 rt 0 3 0 ϋ φ rt rt SD 0 ø Φ μ- tr μ- μ- ø 3 μ, Φ SD Φ rt <l μ μ- 3 φ ii ø Φ TJ μ- μ 0 LQ ϋ X 03 ■ <! ϋ TJ ø rt μ- TJ Φ Hi Ω rt Ω rt TJ tr 0 μ- 3 o φ TJ SD Φ 03 i Ω SD Φ SD φ " Φ rt φ 03 0 rt rt SD tr tr 0 μ- Pi 03 μ- 0 SD ii 0 ii 0 03 rt SD 0 LQ 3 TJ SD LQ φ SD X tr μ. 0 μ- Φ rt SD Φ Z ø μ- μ- ø TJ TJ 0 LQ SD rt < μ- rt μ- TJ 0 α μ- X rt o Hi ø 0 0 μ- ø Φ ø 0 CQ TJ μ TJ <! Φ μ< ø- μ- O 0 Pi tr O 03 l_I- o φ 03 •« ø 0 SD SD ø ø <: ø ii ii 03 LQ ø ϋ Φ SD μ- 3 Φ Pi Hi 0 φ Φ ø Φ 03 0 0 Si Φ rt μ ø ø SD Φ Φ Φ SD rt 0 Hi ii ϋ Φ O Φ φ ii - μ- 03 0 3 rt ^ ϋ rt 03 -> μ- rt 3 tr <! Φ φ Φ 0 tr TJ P. td μ- TJ Ω tr rt μ- μ- φ Hi SD φ Φ μ- rt 0 Φ ϋ tr 0 Φ μ- μ 0 03 rt c_ι. ϋ μ ø Ω SD rt μ- 03 ø Ω 0 i Ω Ω CQ ø 0 tr 0 < Φ P. ii rt 03 φ 0 μ- SD μ- " 0 tr φ ø rt rt > < ø- tr μ- LQ SD SD φ o rt 3 φ Φ O Ω r ø LQ rt 0 SD Φ CQ Hi rt μ- tr Pi SD SD 0 0 03 0 TJ 03 μ- Φ 03 Si S rt SD rt LQ - 03 0 φ rt φ tr 0 φ SD μ- 0 ø 0 03 rt Ω . rt rt TJ rt tr ø" Ω Φ Ω Φ μ- 0 SD Ω 0 rt Φ ø Ω ii 0 ø SD tr SD φ μ- Φ ø * SD Φ Φ Φ Ω μ- 3 0 LQ LQ φ 03 tr ø" 03 rt Φ φ Φ μ- Φ Φ Ω TJ φ O 3 ø r 0 03 03 0 ii 0 tr μ- SD SD Φ μ- o μ SD ø Ω " rt ϋ ii 03 TJ 03 SD 03 X i 0 * ii 0 SD T rt 03 ø TJ ø TJ Φ 3 SD rt • tr 0 i rt φ - 0 rt Hi i SD Φ 1 rt ^ ø TJ TJ rt rt LQ φ o Φ 0 tr 0 SD o ϋ Pi tr Φ μ- 03 Ω μ- Φ μ 0 μ- μ- •<! TJ 1 1 tr Hi Hi μ- Φ SD ø 1 1 1 tr 0 1 ø 0 rt 1 1 0 I I LQ φ 1 ø μ ω ω to to μ> μ> in o in o (Jl o in SD μ- 03 £ 3 Φ < Ω i 03 SD 0 SD Ω Ω ø 03 TJ TJ Ω rt SD SD Φ ø Ω $, 0 μ- TJ 03 SD 0 rt rt 03 Φ tr Φ 03 Φ Φ TJ 3 0 μ. tr tr φ Φ Φ ii SD tr rt SD φ SD tr SD SD <i 0 rt ø TJ μ- TJ μ- rt rt 0 0 03 0 TJ μ- Φ SD SD 0 0 φ 03 Φ φ Ω <! 0 SD <! rt φ s; SD φ o ι-> SD SD Ω tr SD rt Pi μ- 0 03 ø ø 03 i 03 ϋ Φ Pi tr Φ rt ø φ μ- Φ ϋ o ti ø Φ Ω tr 0 tr μ- μ- ri rt μ- Φ LQ ø ø — ' μ- fu φ 03 T Ω ø LQ 0 H rt TJ SD SD tr SD ϋ H 0 ø Φ SD Hi μ- φ φ ø rt Hi 0 tr i Ω π > ; Φ ø 0 rt rt 3 03 μ- rt 3 0 μ- 0 iQ P ø μ- μ- <! CQ Φ μ- $ SD Φ tr 0 " μ- 1 μ- Hi rt μ- μ- μ- rt Φ Φ φ Hi 03 Φ φ 03 φ 03 H ø rt Φ ø 03 SD Hi SD rt ϋ μ- μ- Φ 0 Ω <! rt <! - rt 0 0 tr ø ø < 0 ti ø φ tr rt Hi Φ tr i ø μ- ø ø 0 03 μ- CQ " <! SD φ tr 0 Φ ^ tr μ 0 μ- 0 CQ SD ø 03 Φ Hi X Φ tr 0 ϋ Φ Φ ii ø 0 ø φ ø rt Φ μ, 0 i 0 φ ø rt φ μ- 03 X SD 0 tr Φ μ -> φ Φ 0 Φ Si TJ LQ SD Φ Φ • <! $D Φ Pi Ω LQ ii ø μ- TJ μ μ- ϋ TJ ϋ Φ 03 Pi Φ Φ SD TJ ii 03 Hi tr n tr SD tr φ Φ ø φ tr tr Φ 0 i Φ < *• 03 ϋ CQ TJ μ- ^ 0 μ- Ω s; $ ■ ! tr 03 SD 03 rt φ μ- 03 3 Ω μ- £. μ- 03 TJ ø 0 03 Φ <! 3 3 tr μ- SD rt ø μ- ø tr 0 SD SD SD μ- ø TJ tr μ- ø μ- rt rt iQ tr rt μ- rt 0 Hi μ- μ- SD μ- SD SD ϋ rt ti μ- iQ SD ϋ 0 < <J TJ ø ø ø Φ Φ ø TJ Ω 03 tr Φ tr μ- 03 ti μ- 3 μ- ϋ ø rt <! Φ μ- ø φ rt Φ 0 Φ Φ ii ø IQ Φ Ω LQ ø 0 03 Φ α Φ Ω φ Ω ø φ 0 0 tr Φ SD Pi 0 Ω ϋ 0 Φ - Φ LQ 0 Φ ft rt ø μ- ii tr rt Pi SD φ ϋ Hi 3 φ μ- 0 0 rt ø tr Ω 0 Φ <J H φ Φ 0 rt tr • tr . ø φ Ω Ω ø rt rt 0 μ- SD rt μ- 0 μ- tr 03 μ- tr Si ø ti ≥i TJ 3 Φ <! 0 <! SD μ- ø μ- tr H 0 μ- ø Φ P- LQ Pi 03 i > SD rt μ- 3 ■ ! SD 0 0 0 SD 03 03 en SD Ω H3 rt LQ ø SD SD Φ rt 03 Pi μ- ft μ- φ SD ø" μ- tr μ- 0 μ- ϋ CQ μ- 03 ø " tr tr ø- rt LQ ø rt - SD tr ϋ 03 Φ ø tr ø T < SD Φ φ rt 3 φ ø 03 03 Φ Φ SD 0 LQ Φ Ω ø Φ Φ tr ø rt Φ SD rt rt LQ LQ SD Φ 0 Pi φ Φ 3 Φ ii 3 φ ø 03 tr Φ Ω Ω μ- Φ Φ Ω Ω ø μ- O Ω μ, Ω TJ P. SD CQ Hi μ- 0 φ SD 0" SD S! Ω ø 0 O SD ϋ TJ ϋ 0 03 tr SD SD φ μ- μ <! 0 φ μ- 03 μ- O Ω μ- TJ Ω SD CQ Φ D tr Ω 03 ø 3 ϋ Φ Hi SD rt SD rt ø ø Φ SD rt 03 rt tr 03 03 Φ μ- CQ 0 0 μ 0 0 Φ 03 3 SD SD TJ Φ 03 μ- ii ϋ ø M μ- iQ LQ CO 0 ii μ- <! 03 μ- 0 ---, o ø 0 μ- ø ø rt ϋ Hi 0 ø φ Ξ $. ø ø φ 0 rt Φ ø o SD φ 0 μ- Φ φ iQ " TJ SD Hi Φ rt ϋ 0 ø i μ- μ- 0 0 03 03 SD SD O in Φ 03 ø tr 0 ø Φ SD Hi Ω ø ø ø Si ti μ- Si tr φ TJ Φ Φ 0 03 SD μ- 0 < < rt 0 Hi rt CQ 0 Ω SD rt μ- SD Φ ft 03 Φ Si rt 03 0 μ- " 0 P. ø Φ Φ ø 0 SD Ω • ■ <; SD 0 3 03 μ. rt 03 \-> SD 1 ii φ tr μ- ø > rt Φ SD " LQ ii S SD Ω tr SD μ- i TJ ^ 0 tr I Φ SD ^ 1 Φ rt Ω <! Ω rt SD LQ rt φ 0 Φ μ- rt Ω SD 0 TJ Ω Si 03 i SD φ rt 03 iP 0 O tr SD SD 0 P. μ- 03 , rt 0 ø μ- • rt 0 0 ø s! <! SD 0 μ- μ- rt 0 ø X 0 0 ø 03 Hi 03 SD L_l. i φ SD tr μ. LQ ϋ ø 4 li ti ø a φ ri 0 ø Hi Φ Ω TJ 1 SD 3 0 Φ σ ø ø SD 0 μ- 1 Φ < φ rt μ- s! μ- Si Φ 3 ø rt rt iQ μ- 3 φ rt φ 0 tr μ- μ. rt φ ø φ 03 03 03 φ Φ ø" ø tr ø rt μ- rt μ- H φ i tr Ω O Φ μ- 03 Φ μ- Si rt tr O Ω μ, n LQ Φ tr ø 03 0 μ- Ω 0 rt ϋ -- ^ 0 rt μ- CQ 03 Φ SD φ Ω iQ Φ 3 φ ø rt Φ Φ 0 μ- ø 03 SD LQ TJ 0 0 0 03 03 tr 03 Φ 03 03 SD μ- X Ω iQ 0 ø 03 Φ SD Φ 0 03 ø ø 03 SD rt Ω rt ø ^-^ 0 ^ Φ i Φ 0 rt 0 SD Ω 0 tr ø SD μ- 0 TJ 0 Si LQ O 03 rt rt SD μ- rt SD μ- SD Ω SD • Ω Ω μ- ø 3 0 SD SD ϋ rt ti rt rt ø Φ Φ ϋ P> μ- 0 0 Ω TJ ii 0 0 tr Hi μ- tr 0 TJ ii i Φ 0 TJ tr SD lP μ ø li 03 SD ri 0 * Q Hi 0 ø SD 0 CQ i • SD tr ϋ 0 <! "< 0 -- ^ rt 0 <! Ω Φ 0 rt tr " 0 3 CQ Φ SD 0 rt rt ^ φ μ- 0 tr SD Hi μ- μ. 03 0 φ SD rt 03 TJ H tr Φ <! φ 0 μ- 0 03 ri rt μ- tr 0 03 •» ø μi Ω Φ 03 < Si Φ rt - — * 0 ø 0 rt Φ Φ 03 μ- rt ø Φ tr O SD Hi rt IQ ø μ- SD rt φ 0 μ- ^> rt CQ ii 0 μ- 03 SD μ- ϋ tr φ TJ i SD Φ tr Φ ~. rt Φ Ω ø Φ SD t ϋ ø" O Φ SD Ω 0 μ- rt IQ Φ 0 ø SD 3 π μ- Ω 3 0 rt 0 LQ id 0 0 Φ SD 0 H Φ ^ $ rt SD 0 ø rt ø 03 Pi rt TJ SD Ω 03 μ- φ tr 0 0 rt ø Φ TJ 0 Hi Hi μ. 03 0 SD < Φ SD rt - μ< 1 3 tr ^ • SD SD tr " tr Ω SD SD • TJ ϋ Ω O Hi ø ϋ 0 Φ 0 SD Hi tr μ- TJ ^ 03 φ Ω ø Φ Si 03 φ SD 03 0 Φ Φ Hi SD Φ SD Ω 03 ft ø ø •> 0 Φ μ- Hi rt tr 1 Hi 0 μ ø 03 H3 S ϋ ϋ rt Ω tr μ- μ- rt SD r ø μ- Φ Φ Φ 0 Ω 3 03 Hi 0 tr Φ 0 SD rt rt tr 03 LQ rt μ- rt tr μ- 1 rt φ 03 3 μ 0 φ Φ 0 Ω 0 μ 3 Φ I μ- SD 1 0 0 Φ ø tr ϋ 03 Φ 3 03 μ μ- 03 μ- ø LQ 03 0 ϋ 1 Φ 1 1 1 1 tr UJ to to DO μ> in o in O in O in rt Hi $ Ω 0 SD <! rt Hi ii Si Φ rt 0 H3 O SD Ω 03 TJ Ω TJ : > SD SD 0 SD rt SD ϋ LQ μ- 0 ø 0 μ- 0 tr 0 TJ Φ ø μ- SD ^ 03 μ- TJ tr ϋ M tr 0 μ μ- ii μ- 0 0 SD ϋ φ rt o μ- ø μ- SD SD ti SD Φ TJ ø 0 H" 0 ø 03 o rt 0 ii SD tr 0 Hi μ- rt SD ii ii rt 3 o LQ rt 0 SD rt i ø μ. rt Φ rt LQ SD Φ 0 μ- 03 tr SD ø 03 <! μ- 03 tr SD • SD 3 Φ μ- ø tr rt • Φ 03 ø Hi 0 μ- SD Φ Φ 3 0 Ω " 0 ø rt μ- Ω Φ μ- rt • 0 CO 0 ø rt ϋ ii " > Φ μ- X 0 tr i Si μ- H ft Ω SD LQ Φ (D Pi 03 ø rt CQ μi LQ H ø SD 0 0 Ω H Φ CQ μ- ø 03 Ω 0 μ- SD H tr SD φ μ> 0 Φ Φ μ- tr φ 0 0 SD ø Hi φ 4 tr 03 •» tr - rt 3 Φ μ- SD 0 •<: ϋ i 03 rt 03 fu SD 03 ø ø Φ 3 Hi rt rt rt 03 SD Φ μ- * ; Φ SD μ- rt ø 0 SD 03 μ- < SD ø iQ Φ SD μ- μ- tr TJ 0 i rt μ- ii rt 03 μ- tr TJ 0 tr μ- TJ 0 rt 0 SD SD φ ii μ- SD SD 0 0 S 0 ^ Φ tr 03 rt Φ ø SD Ω rt 0 ■ : Φ LQ ii CQ 0 ø 1 μ> Φ rt rt ri rt ø Φ φ 0 0 SD 0 Ω Ω Φ Φ ϋ Φ 0 rt rt rt SD μ- rt ø 0 rt rt 03 φ 03 O tr ϋ rt <! SD ϋ μ- rt CQ TJ μ- 0 • J ø £ 03 Ω tr 0 SD SD <! μ- SD ^ Φ ø 0 O μ. φ SD Hi tr Φ 3 TJ Φ 0 0 TJ rt Φ 03 SD SD SD Ω SD Φ 0 rt 1 μ- 0 ft O iQ 0 TJ 0 0 Φ Φ TJ SD <i rt tr ø i μ. ii s; rt rt < li SD μ. 0 rt ϋ LQ rt Hi μ- φ rt SD rt Hi TJ LQ ii ii Φ SD μ- 03 φ 0 SD SD O SD μ- TJ SD tr φ Φ ϋ I SD μ- μ- Φ ø Ω tr ft μ- 0 o φ 0 0 μ- Φ ø 03 rt Hi ø 3 o < Φ SD Φ Hi SD ϋ μ- CQ ø rt ø ii Ω rt 0 Ω μ 3 3 Ω SD IQ 03 φ ϋ rt 0 o μ- 03 TJ CQ Φ Φ tr • μ- 03 μ- ø LQ ø" ø φ H μ- Ω SD Φ Φ tr 03 ft ϋ μ- O SD ϋ H- 1 3 0 03 03 TJ 0 0 TJ 03 φ TJ Φ SD 03 φ 0 tr SD ϋ 0 SD TJ rt ø- T TJ 0 0 Φ μ- CO μ- φ SD LQ rt H3 Φ Ω ϋ SD rt O SD 0 SD rt rt rt ø φ • Φ rt 0 0 1 03 Φ 03 μ- tr ø 3 ø rt tr tr Φ Ω ϋ Φ TJ Φ 0 03 ø tr CQ rt 03 ø Ω i 0 : > - ii 0 03 CQ ø LQ Φ ii tr Φ ii rt μ- tr ϋ μ, μ SD Φ ii ø SD μ- Φ TJ Ω Φ rt Φ Φ tr rt T μ- LQ Φ rt SD Ω CQ - 3 ^ 0 φ 0 03 0 (i Φ Φ rt LQ 0 Φ 0 "• 03 rt μ. SD 03 03 SD O O tr 3 O TJ ~ μ- 3 iQ TJ SD Ω LQ 0 ø Ω 3 φ ø" rt 0 rt tr ii ø rt rt 03 SD μ- o 0 ϋ rt ø 0 μ- μ- Φ μ- Φ TJ μ- μ- SD SD rt TJ tr rt Φ ϋ rt ø SD tr 0 <! rt ø rt Φ 0 * SD 03 ø 0 ii ϋ • 0 0 3 03 rt li ϋ SD tr 3 Φ Φ SD 0 μ- Φ O Φ Ω Φ ■ <: Φ fi 03 ii Φ rt μ- SD 0 rt :> 0 μ- • μ- ø μ- Φ μ- ϋ ø 0 0 rt 03 X Hi 03 0 Φ μ- ø ø μ- 0 T Φ rt ii 0 3 03 SD LQ Φ 03 0 rt μ- tr SD Φ tr 0 φ 0 SD 0 Ω TJ ø LQ rt : > Φ TJ ø ii g, tr Φ Hi LQ Φ ø TJ tr 03 μ- SD SD Φ 3 rt Hi SD 03 ø Hi ϋ rt 0 ϋ LQ - tr 03 rt μ tr μ- 03 tr 0 03 Ω ii rt 0 rt rt s; 03 TJ ø" HI TJ Φ LQ <! SD ø Φ SD Φ μ- rt i Φ ø 0 Φ 03 Hi μ- φ 0 ^ μ- SD μ- Φ Φ Φ rt Hi SD tr μ- TJ CO μ- rt Ω μ O Ω tr Φ ϋ 0 μ- SD ø 03 tr SD • 0 <! ^ SD φ Ω rt tr μ- 0 ø φ ø rt tr Φ Hi SD Φ 0 Φ rt SD SD ø rt LQ φ Φ 0 0 Φ -• Hi rt rt 0 rt Φ <! φ 0 φ SD 1 ii 03 0 * 0 ii LQ tr 03 0 rt μ- ϋ μ- 03 tr rt SD φ μ. rt rt 0 ø 0 rt 3 μ- SD Φ rt ϋ Φ TJ 0 rt ø rt μ- φ ø Φ rt Φ SD td Φ LQ μ- ø 03 φ ø ø ø tr 03 O rt SD Φ Φ l-h 0 Ω rt 03 0 Ω LQ ø" rt 0 > μ. Φ ø rt - μ- ϋ SD Φ Φ Φ μ- ϋ ^ 03 0 rt 0 0 Ω μ- ø tr Φ LQ μ. μ- Φ Φ Hi Hi 03 03 μ- 0 3 rt LQ Φ Φ TJ 0 LQ TJ ft SD ø 1 SD ø TJ rt Φ 0 i μ- Φ rt 0 SD 0 rt < 03 μ- Ω 03 ø - μ- rt Φ SD rt ii <! fi t ø ø ii tr SD r ø SD ø Ω Hi rt ø tr φ 0 03 tr φ SD ø φ μ- SD 3 <i μ- 0 s! φ μ- ø SD 0 03 μ. Ω rt Φ 0 SD rt SD ø 0 0 TJ SD rt Ω Ω rt Φ φ Ω 3 D ø CQ Φ rt < Ω φ 3 SD SD TJ Φ Ω μ- 0 rt Φ TJ rt rt Hi Φ ø Φ tr ii rt SD ø SD 3 rt Ω μ- μ- tr α ø" 0 0 rt ø tr 0 ϋ μ- 3 Ω ø Ω tr Φ φ Φ rt Φ SD μ- ii 0 ϋ SD Φ 03 0 μ- Φ LQ tr Φ ø 0 Ω 0 : > Φ μ- Φ rt ^ Si Φ 0 0 0 0 μ- 0 Φ ø TJ i 3 tr ii ø Si φ SD < SD 0 SD SD Hi μ- SD SD 03 SD Hi LQ 03 SD TJ 0 3 SD Pi ø 0 Φ μ- φ SD LQ 3 TJ rt μ- 0 0 μ- O SD rt rt 03 μ- 03 ø rt μ- rt μ- μ- Φ € 03 • 3 0 rt μ μ- ϋ TJ Φ 03 Ω s: ø Ω rt μ- μ- LQ rt μ tr iQ ^ μ- TJ ø rt 03 μ- φ rt 03 rt μ- SD ϋ ø Φ 0 φ tr ϋ μ- rt Φ 0 CQ ø rt ø 03 Ω μ SD tr ø 03 μ- SD rt H <! φ ø 03 ø 03 Φ O φ rt SD rt μ μ- ø ø 0 SD O tr Ω SD Φ SD 0 rt o rt • ø 1 Φ 0 μ- ■ <; 0 Φ Φ Φ <! ø tr rt ø- Φ 0 ø SD 0 Φ tr μ <! μ- 0 tr 03 ϋ ø " rt ϋ ø SD 0 SD μ- CQ SD rt μ- LQ CQ SD rt Φ ϋ SD μ- ø LQ 0 SD > Φ ii 0 0 0 Φ tr rt 0 TJ ii ϋ rt rt φ ø μ- rt 0 ø Hi 03 Pi iQ tr 03 μ- Ω ø Φ Ω ii Hi rt ø SD rt ϋ rt 1 φ tr ø 0 ø O ø" SD 03 Ω ø SD 0 μ- φ Si Ω 03 03 ø Φ tr LQ ø 0 μ- μ- 03 SD ø rt ø TJ H SD 0 TJ 0 Φ 0 μ LQ 03 μ- rt TJ O μ- 0 ii ϋ Φ rt ø Ω μ. SD ø" O 03 ϋ φ 3 ϋ 1 ø tr ii ii ii rt 1 <! fi tr 1 SD Φ I Φ 0 μ- 1 1 03 SD Φ φ φ i ^ Φ 03 SD 0 1 1 of the filter can be flattened or broadened by allowing multiple filters to act on the same wavelength channel. In this way, a super dynamic capability can be achieved, further increasing the versatility of the channel balanc- ing according to the present invention. Preferably, in order to achieve this super dynamic capability, a set of sub-resonators are associated with a common channel within the WDM signal . In one embodiment, properties of the selection re- gion are adjusted in order to change the phase of light in the selection region. By directing part of the power of the selected channel into the selection region, and leaving part of the channel in the optical signal, a phase change of the light in the selection region can cause destructive interference of the channel when it is fed back into the multiplexed signal . Consequently, the selected channel can be removed from the multiplexed optical signal by destructive interference. In fact, the destructive interference can be arbitrarily tuned by properly adjusting the phase of the singled-out portion, thereby allowing attenuation of the channel to any chosen degree. When attenuation is effected by means of destructive interference as described above, light is instead forced out of the selection region by making it impossi- ble for the light to continue to propagate in the multiplexed optical signal. Thus, according to one embodiment of the present invention, attenuation of one channel is achieved by dividing the power of said channel in two portions, and then adjusting the phase of one of said portions so that when the two portions are brought back together, destructive interference occurs in the propagation direction of the optical signal. More particularly, an arrangement for attenuation by destructive interference comprises a waveguiding structure, preferably an optical fibre, into which the wavelength division multiplexed optical signal is directed. In said fibre, there is provided a deflector arranged to deflect light into an external resonator, said deflector preferably comprising a blazed, or tilted, phase grating. The external resonator can be tuneable in order to allow tuning of the wavelength to which said resonator is resonant. The deflector is weak, in that it only deflects a small fraction of light impinging thereupon. However, a strong field will quickly build up inside the resonator at the wavelength interval (or intervals) to which the resonator is resonant and thereby es- tablish a selection region in which the power density of the resonant wavelength is considerably higher than of other wavelengths. Thus, at equilibrium, higher power will be reflected back and forth between the mirrors inside the external resonator, and the power deflected into the resonator from the fibre will be equal to the power deflected out from the resonator back into the optical fibre. By adjusting the phase of the light inside the external resonator, destructive interference between light from the resonator and light remaining in the fibre can be obtained in a selected propagation direction. Hence, by tuning the external resonator, it is possible to prevent light of the resonant wavelength from propagating in a selected direction in the fibre. Likewise, it is possible to achieve fractional destructive interference, and thereby prevent only a fraction of the resonant wavelength from propagating in a selected direction. In this way, tuneable attenuation of the channel power is attained. The features of this embodiment will be fully understood when the detailed description below is read. In another embodiment, similar to the one described above, the external resonator is constantly tuned (or fixed) to provide constructive interference. According to this embodiment, attenuation is achieved by introducing a loss in the external resonator. Consequently, when the two portions of the channel are brought back together, the resulting power is lower than the power before divid- ω . to to μ> M in o in o in o in CQ TJ tr 03 0 tr ι-3 rt ii o rt TJ rt P. Ω 03 tr rt μ- 03 Si 0 TJ rt T tr £ μ- Hi μ- rt μ- μ- ϋ μ- Φ ø Φ tr μ- Φ o tr ø" μ- 0 μ- Φ ø ø rt μ- Φ 0 0 0 ii φ H> rt ø φ 0 03 --~- Φ μ- Φ <! 0 rt μ- ii LQ H ø 03 03 μ- ø 0 rt < <", 03 μ- 0 0 ø tr Φ LQ ! tr <l LQ ϋ tr Φ P. ø 0 < \ ø μ- μ- ø Φ ø φ Ω μ- 03 ø Φ TJ € Φ rt μ. φ ø Φ 0 φ Ω 0 0 o Ω LQ Ω 03 Φ 0 LQ • ø Ω 0 ii ø LQ ø φ Φ tr Φ ø tr rt 3 ii rt Hi 0 0 Ω 0 00 0 0 μ- 0 0 rt rt ii μ- rt O Ω tr ii Φ Φ Φ 03 rt 0 tr SD μ- φ Hi 03 ø 0 Ω 1 ø ii 0 03 03 i TJ ø σ Φ rt 0 tr rt 0 0 ø • μ- Ω 0 φ TJ ø rt Si rt CO ø Ω Ω to TJ 03 ii ø μ- Φ μ - μ 3 Φ ϋ tr 0 Φ Φ μ- LQ 0 Ω rt TJ SD tr tr rt Φ 0 μ> Φ 03 Φ 0 CQ Φ Φ φ μ- si ø φ rt Φ rt ft ϋ H3 0 0 Ω μ. ø SD Ω Φ tr Ω tr li 03 0 μ- 03 3 ! 0 Hi ø . rt ø μ. tr £ 0 03 tr tr Φ tr - ii in tr 0 rt ø tr Φ 0 Φ TJ Pi — Pi ω TJ ø 0 ø Φ CO tr 0 φ Φ rt Φ 03 Φ Pi o 0 SD SD 0 ø i μ- ~ μ- rt 0 tr 0 ø ϋ rt en tr Φ Φ rt o tr TJ rt μ- ø Ω ■ <: rt 0 TJ Hi 0 0 ø ø Φ ø rt φ LQ ø ø 0 rt 0 ii 3 μ- 0 Φ Hi rt Φ 0 03 tr ø TJ ø tr ø" 0 rt μ- tr tr <! LQ $ LQ P Pi μ- rt tr 0 Ω tr < Φ tr 0 0 £ rt 0 Φ Φ LQ Φ Φ D Φ φ μ- tr φ rr μ- tr μ- TJ μ rt " ø tr Φ Φ 03 o rt rt 0 rr 0 % 03 μ. rt Hi Ω ii ø Pi rt μ- 0 tr ø φ tr rt Φ rt Ω • 0 HI TJ SD Φ o μ- φ 0 Ω TJ ^ iQ φ 3 ^ φ . 0 P. ϋ tr SD tr φ μ- φ Φ 0 μ- TJ 0 μ- 0 φ rt φ Hi φ tr Ω X 0 φ Φ 0 μ- •» Hi ø 03 tr TJ X 0 td ø ii H3 0 3 Pi μ- μ- ø rt rt 0 Hi ø Ω $ ø φ rt ø ii φ <! 03 Pi 03 ii rt 0 tr rt φ ø rt o Φ tr Φ μ- Φ • : rt Ω rt φ Hi 01 CQ Si ø ø" μ- μ- Φ Pi φ 0 μ- 0 ø Φ TJ Φ 0 ø Ω t. 0 X μ 03 0 μ, Φ τ tr 03 ii rt μ- 0 μ- Φ 03 Ω Ω CO P. 3 Hi μ- LQ ϋ 0 ii rt φ 0 rt Φ 0 Hi 0 ø rt ø μ- μ- tr tr μ- 3 tr 03 0 tr 0 0 φ Hi Pi LQ ø LQ μ- 03 Pi o Φ 03 μ- O 0 rt Ω φ rt Si Φ ii Pi TJ ^ φ TJ tr rt μ- ø TJ Hi • Φ ø ø ø Hi Hi μ Pi ϋ ii ø rt ø <! 0" Φ Φ μ> Φ μ- ii ø rt ϋ μ iQ LQ ι-> rt rt ϋ 0 μ- μ- ø Ω Φ 0 Φ φ φ 0 03 • 3 Φ μ- φ Φ μ. o TJ ø μ- o > φ ϋ ii μ- φ ø Hi ø tr rt 0 0 rt rt LQ μ> O 0 03 Φ μ- ø Ω CQ Φ 0 Φ Ω Ω 3 0 ø ø ii 0 ii rt rt ii ør 0 s; tr μ- Φ rt ø rt 03 μ- en Ω 0 rt rt Φ tr Pi Φ ø Ω rt rt rt Φ 0 ø tr rt 0 Φ φ 0 φ 0 03 tr ø ^ ø Φ rt Φ μ- ϋ 0 μ, μ- ø ^ s . 0 0 μ- μ- 03 ø Φ ø" 03 li tr ø 0 Φ rt 0 *< Ω ø 0 rt 0 0 rt O td 0 0 ^ ø ø 0 P. μ. Φ rt 03 Φ Φ Hi Ω ø μ> μ- H- ID Φ Φ rt Ω I 0 P-. 0 μ- TJ ii s! 03 Φ rt 0 LQ LQ 0 μ- 4 Φ 03 μ- 0 TJ 0 0 rt SD Ω 0 μ- φ tr μ. 0 μ- ø rt ø ø μ- li <! ø ti ii rt TJ Φ 0 rt tr ø ϋ rt Φ 0 tr rt Hi 0 Φ Φ Ω ii 0 Ω μ- LQ <! LQ Ω 0 Φ μ- μ- rt Φ Φ " μ φ 0 0 μ. TJ 0 0 3 μ- Φ 0 rt ø - μ- Φ <! 0 Ω iQ φ ø tr 03 ø O Ω CQ 0 Φ Φ rt Ω ø 0 Φ ϋ tr 3 03 Φ Si 0 tr Pi 0 LQ Φ ii ø ø 0 Φ i rt 0 0 03 X Φ rt rt φ 03 μ- μ- 0 μ- rt μ. Φ Hi LQ μ- ø TJ LQ φ ø ø 0 ø rt μ- 0 Φ rt ϋ Φ 0 Pi • ø Pi Φ Pi rt TJ Ω tr Φ Ω o 0 rt 0 i ø ø tr LQ tr μ- 0 ø rt ø φ ø Pi ii iQ φ 0 μ- tr 0 0 t φ LQ TJ 0 ø μ- ii o ø LQ ø Φ rt Ω φ 03 ø rt tr rt ii ø - tr i-3 Pi Ω 3 03 03 0 ϋ μ- 0 0 Hi 0 0 0 rt rt rt φ ø" tr 0 φ ø rt Ω *<: ø 0 Φ rt 03 03 0 0 s; 03 Φ 0 μ- ø rt 0 μ- ø" CQ μ- 3 TJ 0 ii μ- ø μ- ø- ø Φ Hi tr 0 ø tr μ- 0 Ω ø Φ μ- X μ ø - Φ TJ ø rt 0 0 ii 0 ϋ Hi φ 0 <! ø 0 0 ^ 03 rt μ- tr μ- φ tr ø 0 Pi LQ 03 tr Hi 3 0 TJ <! o μ- X <! Φ 0 Pi Pi Ω rt Φ 03 03 SD μ- LQ 3 Φ ^ rt μ- Φ φ 0 rt Φ φ rt 03 rt Φ ϋ 0 L_l. 0 tr 01 03 0 Φ . ø 03 i TJ X tr tr tr ø μ- " rt ø Pi LQ μ- tr Φ 0 Φ ^ Φ rt 0 ø Φ 0 Hi φ rt ø φ 0 μ- Φ ^ LQ 03 φ TJ tr ø Pi ø i Φ TJ ϋ rt 03 * tr 03 TJ μ- rt ft O Φ 03 0 μ- Φ 3 Ω 5 Ω o Φ rt μ- td μ- φ ϋ ø μ- 0 Φ rt μ Pi 3 rt 1 o 0 0 03 LQ T tr μ- μ. P rt 03 Hi ii Pi Pi £. Φ ø 0 ø ø ø li μ- μ- Φ 0 tr 0 ø 0 03 tr μ- tr SD ø Φ ^ — ^ . φ μ- μ- :> O Hi SD μ- SD Hi ø SD rt Φ ø ø 03 ii φ P. 0 rt rt *. 03 rt rt Φ 0 rt Hi α 03 <i rt ø Hi Φ CQ ø tr <! Φ rt <l tr LQ CQ o φ O μ- Ω μ- tr rt ^ 3 - rt Φ Φ en Ω φ μ- <! μ LQ LQ ^ φ ϋ ii ii 0 ø rt * ø 03 T TJ 3 tr φ rt ø rt μ- rt ri μ μ. 0 Φ TJ φ LQ 0 Φ Φ ii ii t rt Hi 0 0 rt μ φ μ- μ- ϋ tr 0 0 0 Ω 0 Φ ø Φ μ- Ω 0 0 0 LQ 03 0 0 tr 03 • μ- Φ TJ tr 0 0 ø μ- Φ ø φ 03 Φ rt Hi ϋ 0 ø ø TJ tr O φ $ rt li rt ii μ- O ■ <; ø ϋ Φ Ω rt ø Ω 03 rt <! Φ Hi μ- o μ> ø Ω TJ φ 0 0 0 Φ ø t ii ϋ ■ <: ø tr Ω 0 rt rt ø φ • φ Φ ø ø Φ 1 0 ø φ Si 1 Hi LQ TJ rt ø ø μ- ø 0 0 < 0 0 0 0 0 ø 03 1 rt 0 μ- Ω 0 ø Ω rt ø φ ø μ- μ ii rt ø 1 CQ 03 μ- tr t ø 1 IQ LQ 0 0 Pi TJ 03 ø LQ 1 1 ii Φ ω 10 to to H μ> in o ( l o in o in Φ < ø 0 Φ 03 Ω φ 0 rt Ω 0 0 T μ- ø IQ 0 Hi μ- iQ tr rt μ> Hi O μ 0 3 rt tr Φ CD Φ φ Ω rt μ- 0 ø ri Φ LQ μ- 0 0 conjunction with the drawings, in which figure 1 schematically shows an arrangement according to the present invention comprising external resonators; figure 2 schematically shows an arrangement according to the present invention having a super dynamic capability; figure 3 schematically shows a magnified picture of an arrangement according to the present invention; figure 4 schematically shows an arrangement according to the present invention comprising external resonators in which controllable absorbers are provided; figure 5 schematically shows an arrangement according to the present invention comprising controllable ab- sorbers and having a super dynamic capability; figure 6 schematically shows how a super dynamic filtering capability is achieved; figure 7 schematically shows an input signal, a filter function and a filtered output signal; figure 8 schematically shows, in block form, a setup for channel balancing of an optical signal propagating in a fibre; figure 9 schematically shows, in block form, a setup for channel balancing, including a pre-amplifier and a power-amplifier for amplifying the optical signal; figure 10 schematically shows an arrangement according to the present invention comprising two fibres provided within external resonators; figure 11 schematically shows an arrangement accord- ing to the present invention comprising two fibres with internal resonators; and figure 12 schematically shows two embodiments of an arrangement according to the present invention comprising a single fibre with internal resonators. Throughout the drawings, like parts are designated by like references. ω ω o DO μ > μ> in o in O in o in Ω tr Ω rt μ- Ω α μ- 0 0 tr ø 0 Φ t <! μ φ ii rt Ω μ- ii Hi Pi ø ø ø Φ tr μ- μ- μ- LQ d ø Φ CQ ø ø μ- rt CO 0 iQ φ rt ø LQ rt ii Hi P. 0 ø ø tr Φ rt μ- ii μ. ø 3 0 μ. i μ- φ 0 0 M CO φ DO ø g, Pi • rt rt 03 • TJ μ> Φ tr Ω ø Φ H Φ TJ μ. μl rt 03 Pi 0 tr μ> μ- tr tr LQ Hi Φ TJ φ TJ Φ Hi o φ ϋ Hi rt μ- £, • Ω 0 Φ φ μ- Ω tr 03 ø μ. 03 μ o μ- ϋ μ μ, Φ μ 0 ii φ 0 ii 0 ø Φ Ω ■ <! ø rt P. 0 ø μ 1 Ω μ- CQ Hi tr ø Φ μ- Φ 0 rt 0 iQ 3 ø 3 TJ rt tr ø Φ <! t ϋ 0 0 ø 0 ø Φ 0 Φ ϋ rt Φ 0 rt 0 "1 , , rt rt μ- DO μ- 03 μ- 3 ϋ ffi i^, 03 tr rt tr 0 φ ii μ- 0 tr 0 ø ø Φ 03 Ω φ s; rt Pi O ø ø μ- ø 0 ø μ- 03 0 Φ ii ø Ω ø μ- Hi 3 ii Φ μ- < 03 03 ^ ø Ω ø φ Ω 0 0 ø rt iQ ø rt tr 0 Pi LQ Φ rt 0 φ μ- Φ Pi ø μ- 3 0 3 Pi i 0 tr ø μ. φ rt ϋ ø Φ rt li ø rt 0 ø μ- ø rt 0 0 ϋ Ω 0 03 ø LQ tr Φ ø LQ Pi 0 φ 0 LQ φ μ- 3 03 ø 3 ii O Φ φ ii ^ Φ Φ TJ 0 Pi Pi ø Ω rt rt Φ 03 rt rt μ- 0 Pi tr Ω ø 0 ø ø o ø s; Ω 03 1 ω 10 to to μ 1 μ > in σ in o in o in IO to to DO μ> in o in O in O in tr μ 0 α 0 3 Ω μ rt IQ rt Hi Φ 03 μ Ω Ω ro n rt 0 Pi Ω 0 ø Φ TJ si TJ ø ø φ tr μ- 0 0 X rt 0 tr Φ 0 μ- 0 tr TJ Φ ø Ω Ω rt σ rt i 03 Φ 0 rt μ μ μ- 0 CQ μ- ø Φ rt 03 0 tr ø rt μ- 3 μ- rt μ- 0 0 ø Φ 0 03 μ < 0 tr 3 03 μ- rt μ- ø φ Ω Ω ø ø 0 3 03 ø ^ μ Ω TJ Φ rt rt μ- Φ ii Ω μ μ Φ Ω Pi ø 03 0 μl ø LQ 0 ø K 0 rt H -J Φ 0 ø id CD ø ø Φ <! μ- H H- tr Φ rt H £ Φ tr 0 μ- rt - p. rt ri 0 0 0 Hi " Ω rt Φ ø μ- LQ Φ 0 03 0 rt tr 0 Pi < tr Hi ti ¬ φ μ rt ø i LQ ø Ω 0 Ω ø 0 μ μ- ro Ω Φ 0 φ μ- 00 LQ Φ to ro 0 0 TJ μ- μ rt H- 0 0 ø CQ Ω 03 TJ K Φ 03 Ω μ CQ ri 0 P. rt o ø < ø CQ μ- 0 μ H 3 μ tr H 1 ro μ- tr φ μ- 0 03 μ- 0 rt rt Φ 0 03 Ω 3 μ ^ en Φ μ Φ 03 ø rt rt < ø CD Hi μ- tr rt Φ Pi o rt 0 μ- 03 μ ^ Ω rt 0 μ- ø ø μ X Ω rt < μ- 0 tr TJ tr H 03 ø ø 0 φ μ- φ 0 tr Pi 0 Φ 0 ti ¬ rt Ω 0 Φ ø φ Φ ø μ- LQ 0 03 03 i μ Φ 0 0 μ tr LQ ro r tr 3 rt φ rt rt rt μ rt TJ Ω Φ 0 μ Pi μ rt Hi Φ 3 μ- 0 tr μ- 0 Φ tr tr Hi μ- 0 μ 0 3 ø 0 μ- 0 ro Φ Φ O φ in 03 ø rt φ ø IQ μ Φ 0 0 ø μ 0 rt Φ 0 0 ø TJ 03 X LQ Hi μ μ ø ro ø tr Pi rt Hi rt μ rt TJ μ- 0 ■ <: rt i rt TJ rt ii 0 CO Φ 0 0 ^ ro Φ Ω Φ tr Φ 0 3 Φ to 0 0 rt 0 μ- H- 0 Φ Φ ø Pi μ 03 0 0 X μ J 0 Φ μ CQ ø ø μ <J Ω ø μ Φ Ω Φ D ^ 3 μ 0 φ rt 0 Pi 3 Hi 0 CQ 03 0 03 μ- ø p. 0 • • ! i-3 Φ μ rt ^ μ- 0 μ μ- Hi ø μ- Φ μ ti ¬ tr Pi " μ- 0 tr 03 tr tr 03 Φ ø Ω Ω P. 0 ro 0 4 ø ø TJ μ- Ω rt μ ro rt 03 rt Φ ø μ- Tj μ- 3 Pi μ- $ Φ ø ø CO CQ 0 03 ø 0 0 μ μ- TJ μ- TJ ø tr LQ μ ø ø ø μ rt μ- μ 03 0 0 ø ro LQ μ TJ 0 rt Φ Ω tr 0 rt rt rt iQ Ω Φ 03 3 Pi IQ - tr 0 ø ø Ω rt 3 Φ tr H3 μ- 0 Φ ^ ø tr 03 Φ φ tr tr ti ¬ tr Φ p. Φ 0 μ- Φ tr ø Hi 0 ø 0 0 Pi 0 Φ rt Hi μ- X μ- Φ X μ rt Hi ro μ- 0 0 ø ø ø tr Φ 0 ro tr φ ø ø rt tr Ω 0 Hi ^ tr μ- μ- ø Hi Ω ø Pi • ø ø 03 0 0 μ- ii ø μ- Pi Q 03 Φ Φ 0 Φ tr ø 0 μ- tr rt ι_l. ro rt 0 tr rt rt ro rt rt ii μ- 1 rt μ TJ TJ £, LQ 0 μ- 0 td 0 CQ rt rt tr Φ μ ro 0 £. ø 0 ø tr μ- Φ 0 rt tr 0 ø 0 CQ ø CQ ii tr ø Pi Φ X LQ Pi TJ ø CO ø Ω rt Φ ø rt μ- μ- μ ø 0 rt Ω 03 $ 0 μ- μ- μ 0 ■ : rt <! 0 tr 0 rt io Ω Ω φ Φ ø tr 0 Φ ti ¬ ø Hi 3 rt μ- Φ 3 μ- μ μ φ Hi 0 tr > Ω tr μ TJ H> Φ Hi 0 TJ μ- 0 Ω Φ μ φ Φ μ t, μ- > 03 ø 0 ro μ ro φ Hi 03 ø Φ Φ < DO 03 3 tr tr 0 . 0 φ Hi 0 0 μ Φ μ -> 0 J 03 φ • 0 Φ Φ i 03 μ- 03 <J ø Ω tr φ rt μ- ro LQ 0 0 i ø μ Φ μ- s; 03 tr 03 rt μ- μ- 03 tr ^ 0 tr rt ø rt 03 rt μ ø 0 μ- φ LQ α Φ 0 tr ø Pi rt Φ tr CO 0 μ- tr rt 0 tr ø ø μ> 0 3 TJ Φ LQ φ TJ Ω 0 μ ti ¬ μ LQ μ- rt * rt rt rt rt • ø μ ro rt 0 tr P. H li 0 ro ø ø Ω 0 0 Φ Φ tr 03 0 Hi tr Φ Φ 03 μ ø l_l- Φ Φ 0 0 ø tr 0 μ 0 μ s- Φ ø μ- < Hi 0 X Φ rt ø 0 <! CD CQ Si rt 0 ø 0 Hi 0 φ LQ μ- φ rt ti ¬ 1 ø 03 φ 0 tr O • ø rt tr ti ¬ ø <J μ- Ω i ø ø Ω - ro 0 φ ^ Φ rt 0 Hi ø tr *< ro rt φ tr tr ti ¬ ø Φ rt μ 0 Ω μ- 03 0 rt 0 rt S! Φ Φ μ μ- μ 0 μ- ro P. φ tr ø rt rt tr 0 rt tr μ tr tr μ Ω μ- < Φ φ ø 03 μ P. * 0 μ- Ω LQ 0 O Φ Ω Φ Φ ro tr ø Φ 0 ø 03 o μ- Hi 0 0 0 Hi μ ro ø μ- 0 0 CQ CQ io Φ P. μ ø 03 Ω μ ø Ω 0 rt Pi Ω 03 Hi ø rt μ- rt μ- 0 0 tv tr Pi 3 μ- . LQ rt ^ tr 1 tr ø ø 1 03 3 μ φ tr φ Φ μ- rt μ μ- rT ro ø 1 φ i Φ 1 μ 0 1 1 ø Φ 1 iQ tirety with respect to the fibre 3. In this way, more than one resonator can be made resonant to almost the same wavelength. Consequently, one channel within the multiplexed optical signal can be filtered (attenuated) by more than one selection region. This feature gives the arrangement according to the invention great versatility and provides a "super dynamic" capability, i.e. the capability of shaping the filter function. This super dynamic capability will be further described with reference to figure 2. The variable attenuation by destructive interference will now be described in greater detail with reference to figure 3. Figure 3 is a schematic magnified picture of one external resonator 6 and its associated deflector 9. Also shown in the figure is the back-reflecting grating 5 , now shown as a plane mirror in the waveguiding structure 3. The blazed grating, constituting the deflector 9, is shown as a single domain between the resonator mirrors. The above simplifications are for the purposes of graphical clarity only. Light (the optical signal) is sent into the arrangement from the left in the figure, as indicated by the reference numeral 31. When impinging upon the deflector 9, some of the light is deflected towards the upper reso- nator mirror (as indicated by 32) , and some light is transmitted towards the back-reflector (as indicated by 33) . Light deflected towards the upper resonator mirror 32 is reflected back towards the deflector and, again, some light is reflected back to the left (as indicated by 34) in the figure (counter-propagating to the incoming light) and some light is transmitted towards the lower resonator mirror (as indicated by 35) . Light initially transmitted towards the back-reflector 33 will, at the back-reflector 5, be reflected back towards the deflector 9. At the deflector, some of this light is reflected towards the lower resonator mirror (as indicated by 37) , and some of the light is transmitted back to the left (as indicated by 36) in the figure (counter-propagating to the incoming light) . Now, in the lower part of the external resonator 6a, two beams are superimposed; namely light from the back- reflector 37 and light from the upper resonator mirror 35. On the other hand, two beams are also superimposed in the waveguiding structure to the left of the deflector. For clarity only, the beams are shown with a small offset. By a parallel displacement of the external resonator 6 with respect to the waveguiding structure 3, or by changing the refractive index in at least some portion of the external resonator, destructive interference between the two superimposed beams can be obtained either in the lower part of the external resonator (beams 35 and 37) , or in the waveguiding structure to the left of the deflector (beams 34 and 36) . Consequently, if destructive interference is obtained in the waveguiding structure to the left of the deflector, all light is forced into the external resonator 6. The channel at issue is thus blocked from returning to the circulator 2 (not shown in figure 3) . On the other hand, if destructive interference is obtained in the lower part of the external resonator 6a, light is actually prevented from entering the external resonator 6 at all, and hence all light is trans- mitted back towards the circulator 2. The channel at issue is thus unaffected and transmitted back to the circulator 2 in its entirety. Figure 2 shows schematically an arrangement according to the present invention having a super dynamic capa- bility. Each of the external resonators 6, as compared to the arrangement shown in figure 1, is divided into six separate external sub-resonators. It is to be understood that any number of sub-resonators is conceivable, the shown embodiment of six sub-resonators being an example only. In this embodiment, a wavelength division multiplexed optical signal coming into the optical circulator 2 from the input fibre 1 is directed into the intermedi- io to to DO H 1 μ> in o in O in o in 0 0 rt Ω TJ μ CO 0 Φ 03 rt Φ rt rt rt Ω 0 μ Hi 03 0 3 to Hi Ω S3 Ω Hi CO rt μ- 0 0 Hi tr O 0 φ ø SD SD tr Φ X μ- Φ O O tr Hi φ μ 0 ø φ μ- ø μ- tr μ- o O . φ ti¬ Φ 0 μ- CQ tr Ω O μ rt iQ μ μ μ 0 03 o tr H" Pi ø tr rt μ- ø μ φ ro Φ *< rr μ 0 1 tr S 0 Φ tr 3 CO 03 0 li O 3 1 μ- 0 μ tr μ ^ rt rt • CQ X μ ii ø li ii ø 0 H rt Φ 0 Φ 0 li li ø Pi Φ CQ μ- T Hi tr tr Hi rt Φ 0 0 ø Φ φ Φ μ. 0 CQ O φ 03 ø ø Φ Φ tiμ- ø Φ H 0 Φ Φ Ω O μ- Φ H CD Hi ø CO 03 X μ- tr ø Hl μ- 0 O rt ø CQ 03 ro tr io LQ Pi 0 μ tr Hi tr μ rt 0 rt O O tiø Φ μ μ ø TJ 0 0 O •<; 0 o Φ - ø 0 rt 0 O 0 μ ø ø ø CQ ø 0 ro φ O rt Φ μ 0 μ 0 0 Hi ø ω rt tr TJ TJ ø ft Φ SD μ- ø tr 0 s- ø 0 μ Hi CQ 03 μ 3 Φ μ φ μ- rt 0 0 0 μ- Pi rt TJ μ tr Φ rt rt ø tr 03 0 tr ø rt rt ø μ- ø 0 Φ μ- rt O tr rt rt rt tr μ- tr μ 0 Φ μ μ- μ- Φ Φ IO rt Φ < O O 0 iQ TJ 0 CO rt Hi 3 Ω tr μ Φ tr O o μ μ Φ μ- Φ IQ Ω Ω -> • "i μ- 3 Φ μ μ ø φ 0 O tr μ- TJ 0 μ- 03 μ- Φ μ μ Φ Φ 03 Pi IQ μ- rt ø ø 0 φ 3 S3 μ- μ- CO μ μ rt ø ro tr O 0 ø ø μ CO 03 Ω to Φ ø μ tr TJ H CQ TJ 0 ø μ φ - μ- μ φ 0 ø μ 03 TJ μ- LQ . io rt μ- ø Hi IQ rt 0 0 rt 0 0 O < μ ø 03 φ Si μ rt μ- Φ Φ rt 03 Φ en Φ LQ 0 μ- μ Φ ø 03 Ω μ- 0 μ φ 0 0 LQ φ 03 to μ^ O ø . Pi φ tr 0 X H - Pi ø rt 0 0 μ CQ μ- μ- 0 Ω rt SD rt μ μ rt 0 Pi O 0 CTl μ rt Pi Φ Ω 03 ti0 rt 0 φ rt 3 3 LQ μ ø ø tr rt 0 Φ P. 03 tr Ω μ- 0 μ- 0 Φ μi 0 0 φ ro Φ tr μ- 3 Pi μ- Φ μ- 0 Ω Ω Φ 0 0 0 Φ - tr <J ø 03 3 μ- en μ tr 0 rt 0 0 Pi μ Hi ø Φ ø ø 0 fi CQ ø 0 μ- μ rt LQ μ μ- rt μ- ø 3 Φ O TJ TJ ø 0 Ω μ rt s- ø s- iQ μ- 03 ø 03 μ- CO rt 03 td tr Pi O TJ Ω rt Φ rt rt 0 ø Ω tr Φ 0 μ- μ- 0 ø μ- 0 LQ μ- ø rt tr rt 0 0 0 Φ μ μ O ø Pi Pi tr rt μ- Φ 0 rt TJ < in rt 0 μ- rt iQ rt μ- 0 0 μ- Ω <! Pi • O Ω 0 μ- Φ Φ tr Ω fi 0 μ- rt 0 φ Φ 0 03 O μ- ø Φ 03 O P. tr μ- <! ø rt Pi 0 Hi Φ ø > < μ Hi CD tr μ 03 ø μ ø Hi 0 ø μ- > μ- T tr rt Ω rt φ ø Φ μ 0 φ μ Hi Pi 3 Ω 0 03 03 LQ ø 03 P. 0 Φ rt Φ Φ tr μ- O rt 03 S3 ø Φ rt ø φ μ- o μ- DO TJ Φ 0 i Φ 0 0 rt Φ tr O Ω μ- ø 03 O 0 03 tr μ 0 rt Φ LQ Hi tr S3 μ φ ø & ■ , , 0 Φ 0 0 tr tr μ- O 3 . μ- μ- Hi rt μ rt μ- ø 0 μ- μ ø 0 rt μ 0 Φ Hi μ μ μ- μ 1 TJ μ Ω s; 1 1 rt φ ø Pi μ- O TJ Φ CQ O TJ ø tr Ω ø rt μ tr φ Φ Ω μ Hi Φ pi -J I- 1 CQ tr 3 μ CO ø 0 tr μ- rt Φ tr μ Φ μ ø μ 0 rt 1 tr ø TJ ø S3 Ω rt rt 0 0 rt φ cr rt μ- Φ rt ■ <: rt Φ Hi μ 03 Pi 3 ø μ- O μ LQ ø μ- μ rt to tr Φ 3 μ 0 φ . ø CD ø μ 03 O TJ μ- ^ μ Φ Φ 0 ø μ φ Ω φ rt rt rt 0 μ- Φ P- 3 rt 0 0 0 μ 0 S3 O rt 3 Φ 0 td Pi • 03 rt 03 0 Pi O tr ø ø rt Φ rt Hi rt i tr μ- 0 ø ø ø 0 tr O Φ Ω CO μ μ μ- 03 O 3 Hi LQ IQ rt 0 μ- tr P. Φ μ- 0 1 ø μ 0 μ Φ Φ Hi Pi rt Φ 0 ø i 0 O ti- ø T ø μ- rt μ- Φ tr TJ ø Φ 0 tr rt μ LQ CD rt TJ 0 fi μ- μ- LQ rt tr Ω ø tr ø μ tr tr ø . Φ Φ rt rt ø μ SD tr Φ 0 tr \-> 03 rt ø ø 0 LQ Φ Φ μ- rt ro rt μ- TJ μ Φ 03 μ μ- O μ- s; ø φ 0 03 0 tr μ 0 μ- ø ø tr tiμ- rt μ 0 O 03 φ rt H 03 φ Ω ø tr ti ¬ Ω rt O Hi Φ 03 ( tr tr Φ ro LQ O μ IQ Hi Φ rt rt Φ μ Φ ø O tr tr Φ ø rt rt φ ro ω 0 ø μ- μ- Φ *< O 1 tr 0 μ μ- X Φ tr X 03 μ TJ φ 0 ø μ- tr Φ ø - 3 S" ø rt ø CO < μ 0 Hi rt rt ø 0 tr rt P. 0 TJ μ- 0 • rt 03 rt 03 Φ μ 3 tr rt tr LQ 0 O Φ Φ μ μ- tr LQ rt μ Φ rt μ 03 0 μ- μ- - μ- Ω 3 μ- μ- 03 0 Φ 0 φ P. ø Hi > 03 li tr Hi φ Φ μ- φ μ S3 Φ μ- > Ω ø ø TJ 0 0 Φ rt ø Φ 0 ø μ μ- φ p. O 0 μ μ Pi 0 ø μ- 0 CO 0 TJ rt rt ø tiμ- " μ 3 0 P. P. TJ TJ ø Hi μ- Φ 0 ø Φ 0 μ IQ CO 0 rt 03 O ^ ro Hi 0 3 rt μ- Ω μ- 0 Ω SD μ- tr P. μ- μ- ø SD SD IQ 3 Φ rt tr TJ μ- rt 0 rt μ < <: ø J SD o <! μ tr ø 0 μ- ø rt Pi Ω rt φ io CQ O μ- tr !- ■ 0 μ o tr Ω 3 rt μ- 0 ø rt 3 μ- ø 0 TJ Si 03 < Φ 03 μ- tr O 3 • Φ TJ 0 *< μ 0 ø i 0 H! φ μ- Φ tr ø Pi μ- ft Φ Φ Pi rt ø μ- φ H- Pi < μ φ ø Φ Pi Φ ø μ μ- μ Pi Φ Φ Ω 03 ø Φ ø 0 < Pi O μ- Φ 03 0 Ω Ω Φ rt P. CD 0 03 Hι μ- φ μ- Ω μ- 0 Hi Hi ø Φ μ μ- 0 0 tr S3 Pi X rt " rt Hi tr Φ 0 Ω 0 Hi rt X ø 0 ø 0 rt μ- μ- tr M Ω ø P- ø 0 *< ø ø ti ¬ μ- μ- Φ Hi 0 0 μ- tr Φ •<: rt rt rt T ø μ- tr tr 0 tr ø ø ro ø Φ <! Φ 0 3 rt 1 μ Φ Φ ø Φ rt μ- O μ μ Ω 0 Φ φ I ^ 0 μ- 1 H 0 μ X Φ Ω μ- φ 1 3 • : φ 0 μ μ rt μ- LQ 0 rt *< 1 1 rt ø Ω O 0 Hi 1 O 1 tr 03 1 ^ 1 1 ø rt μ rt ^ io io t to * in o in o in o in 03 Φ ø Hi 03 td 0 rt H ci ¬ Pi tr tr 03 0 03 0 μ- rt ø rt rt rt 03 TJ 03 Ω μ- ø Hi TJ iP 0 s- μ- μ 0 0 ø 03 tr DO ts" • <; Φ μ ø μ 0 Φ ø 0 Pi μ- O r μ- 0 0 rt ø 0 ø 0 μ- 3 0 μ μ μ rt μ- 0 O ro TJ -> CJ- < μ 0 cQ ro tr TJ CO 3 φ CO Φ 3 rt TJ tr 0 φ tr tr 03 O rt 0 03 ø rt Φ • Hi TJ 0 φ CO ti ¬ ø Pi 1 ø μ- Ω ro 0 Φ tr H 0 ø LQ Φ Φ O μ 3 ø P. μ- to μ 0 μ 03 0 μ tr 0 Ω ti ¬ p. rt fi μ- μ- ii LQ 0 li ø Ω 03 td μ- T Φ ø . μ3 Ω 0 rt > *< ø ro rr ro μ- S3 > ro μ- 1 ø Ω 0 φ μ- ω LQ ø 03 4 φ Ω Φ i tr rt Hi ø LQ <J φ 03 ■ ø ø . 0 03 ø Ω Pi 0 CQ μ- ti ¬ 3 ø CO μ- o μ Φ td Φ > μ- Pi Pi tr 0 μ- 0 rt ø ^ 0 rt rt ro φ LQ μ> Ω ø Φ 3 S3 φ Ω P. Hi K ø Pi μ- 0 s! ø ø rt Si TJ tr tr tr tr ^ 0 to O 03 μ- ø μ 0 Pi μ- 0 03 03 Φ rt rt ø μ- 0 Φ 0 0 0 ro rt 1 Φ • rt 0 μ TJ O rt 3 0 μ- O tr Ω rt Ω rt Hi tr 0 Ω 0 tr μ 0 μ- rt rt rt ø μ- μ- 0 ø μ ti ¬ 0 μ- TJ X ø 0 Φ ro tr 0 0 Hi -> μ- LQ p Ω Φ Φ 0 μ ø ø H μ tr tr μ ø 0 tr ø ro ø Ω Φ ø CO 03 3 μ O Φ h- 1 tr tr 03 TJ μ tr tr O φ Φ LQ μ- 3 3 μ μ- μ P. Pi rt μ- 03 3 03 μ Ω μ- μ- Φ O rt ø 0 rt μ- *< 3 Hi H μ- O ø Ω TJ μ- Φ Pi μ ø rt 0 O rt rt μ- ø 0 Hi ø μ- ø < tr 03 μ Φ μ- ø μ- Ω Pi Hi LQ ø O Hl CQ μ- 0 ti ¬ tr Ω 0 0 ro O rt rt rt Hi ø Ω CQ φ 0 0 0 Φ ø CQ 0 rt - μ- 0 TJ 03 Hi μ- Hi ti ¬ ro ro tr μ 0 μ O tr Φ rt ø φ . rt Φ tr μ CO ø 0 S3 K Hi 0 rt ø μ- Φ μ Hi ro μ Ω μ Ω S3 Ω ro Ω O 3 3 Φ 0 03 μ ø μ- Ω tr tr rt μ φ Φ P 3 Ω 0 tr 0 tr μ- O rt μ rt μ φ td μ- Φ ø Φ rt ø φ rt ro μ- μ- φ Pi μ φ o ø 0 0 μ- rt ø tr 03 03 CO ø 0 rt 0 ø 0 μ- 0 i Pi μ- -> 0 ø φ μ- Pi 0 ø LQ Ω rt tr ro S3 μ- rt S3 Pi 0 i rt ø 1 0 03 μ- ø rt ø 0 ø μ- TJ 0 ø ro tr Si tr 0 ,~- iQ φ μ- rt O 03 Hi rt 0 0 rt rt rt 0 μ ro fi 0 μ- Pi μ- ro Hi rt ø 3 < ø 3 rt 0 μ μ- Ω μ- tr μ. tr K 0 Hi (T Hi ti ¬ μ- μ ø 03 μ- tr ø ø O ø ø φ Ω Φ Φ CQ CD P- o LQ Φ tr Hi [ Hi μ- Φ μ μ- ro ø 0 0 ø 01 Φ ti ¬ 0 • μ tY ø 0 ti ¬ Φ 3 ø Φ O μ O ø φ CQ iQ ø S3 Pi ro TJ rt ro rt rt CD rt ø ro 0 TJ μ 0 μ ro Hi rt ti ¬ ø 03 ø Hi LQ μ- t_j. μ μ TJ μ 4 tr tr ø Si φ rt μ φ μ Hi 03 tr ro ø 0 μ μ- tr φ rt ø Φ μ- 0 3 φ 0 rt Φ μ- CD μ- rt φ tr μ- μ μ- 0 0 rt Φ μ rt 0 Φ tr φ 3 tr Ω μ- Hi tr 03 ro 03 rt ø 0 03 μ- rt Pi Φ 03 ^ 0 H 1 ø tr 03 03 μ- 0 μ rt ro μ- ro 03 Φ 0 03 fi μ 0 ^ Φ E ∞ TJ 3 0 O φ - ø ti ¬ Ω 0 Φ φ O 0 en ro S3 ø ø ø μ rt μ- μ- φ tr TJ 03 μ 3 Φ CO 0 0 Φ 03 iQ ro tr rt Hi 0 rt - φ rt rt TJ μ Φ tr 0 03 1 ø Φ Φ ø o rt 0 X 0 Φ μ 0 O 0 03 O Ω Φ -. rt μ φ P. ti ¬ o μ μ Hi ø 0 3 μ- tr titi ¬ Ω 0 3 μ- ø μ μ μ- μ ε- φ 0 ø tr rt 0 P. Φ ro ø Φ rt Φ φ K Hi φ 03 φ ro O Φ ø ø CD μ X rt ø ø ø rt ro 0 <J μ ø 03 μ- X μ S3 er μ ø φ ø cQ Φ 0 rt tr <! Ω rt tr μ- rt φ rt Hi ø 0 Ω ti ¬ μ Ω rt tr TJ TJ Φ ø rt 3 ti ¬ μ- ø Hi tr ro Φ tr tr tr ro s; ro p. tr CQ O μ- rt 0 ø ro φ ø tr 0 μ μ 0 μ t μ- 0 LQ μ- ro μ ro Φ α ø ro 0 0 tr ø μ Pi CQ Φ rt O 0 rt 0 0 ro 03 Ω Pi Φ Φ Φ 03 Ω 3 Ω i rt ø ø μ Ω rt 0 ø Φ <! TJ 0 P. X 0 μ- 3 ti ¬ 0 tr ø 0 0 Φ tr tr ø Ω ro tr O μ 0 tr 03 Hi 03 μ- Φ 0 μ H 1 μ- tr rt 0 <! Φ ro ø *<; LQ tr rt X μ- 0 tr rt rt tr • 0 μ *< Φ μ- P. μ φ Φ 0 3 μ- tr μ- ø μ ro rt [ μ- ti ¬ μ μ 0 K tr O μ- ø 03 Ω rt 0 3 Φ 0 03 tr Φ tr φ tr Pi rt 03 Ω tr μ ro TJ rt Φ μ φ ø tr μ < tr rt TJ Pi S3 tr 0 ø μ- μ Φ ø 0 0 φ μ- μ ro α tr 0 CO <i Φ ø 0 ro μ- μ- 0 0 ø φ rt rt φ ø CO TJ ø 0 0 CO 03 ø i s! ro CD φ μ ø CD μ 03 μ Φ tr <J Φ 0 03 Hi Hi tr μ ϋ CO ø 0 Φ 0 α μ TJ 0 03 Φ ø 0 rt φ μ • <! Φ ft 0 μ o O O rt Pi ø P. LQ 3 TJ Φ 0 Φ ø ø 03 rt 0 tr Φ 0 μ 0 K S3 < 03 μ- ø μ 0 Φ ø 0 CO 0 Φ φ rt CD Φ tr μ CD H> ro rt ø μ- 0 rt 0 rt φ μ ø O 0 μ μ Ω 03 μ 0 — rt CO o tr SD 0 Φ 03 0 ø 03 ti ¬ P. 3 ø Pi O Hi Φ rt TJ ø 0 tr 0 φ TJ • O 0 03 Hi 0 μ- LQ 3 μ rt 0 03 ro ( μ- φ CD rt L_l. μ Hi 03 μ- rt Pi -> P< TJ φ μ Hi ø rt 03 en tr Φ TJ 0 ø tr ø 03 μ- 0 Φ 0 ø μ- μ- tr Φ Pi μ o 03 rt tr Φ • Φ Pi φ μ- ro Φ Ω ø 03 μ- Ω ø LQ Ω Ω rt ø 0 μ Φ μ- 0 μ- tr rt ø . μ μ P. Pi Hi rt 4 ω rt 03 1 0 ø 0 K ø <! 1 3 0 ø rt φ tr r <! H 03 rt μ μ- tr 0 ^. Φ 1 0 3 μ- Si 1 CD μ CO Φ 1 Φ 0 O rt Φ CQ Φ ø Pi 1 0 μ- ø Φ CD 03 O 1 I 1 Hi Ω LQ 1 I ω CO to to μ» in o in o in O in Hi Ω rt Pi rt O 0 Φ tr 3 Pi Hi Φ 03 μ μ 0 tiTJ rt S3 μ- H3 0 0 rt μ- μ tr φ O Hi μ ø 0 μ- μ- μ- ø 0 0 0 M ro μ O ø ø tr 0 rt ø 0 K Φ Pi μ μ Ω <! μ <! iQ Ω tr ø M μ- ø 0 < i ø M rt tv CD CO μ- 01 rt 0 tr Φ μ μ- ø tr 1 LQ μ- T Pi Ω μ Φ μ- 03 Φ Φ 03 rt 0 Ω • tr ø 0 P. μ μ φ rt φ ro rt ro M Ω μ 0 CQ ø tr ø Φ CQ 03 Hi μ ø Φ Ω Φ 3 Pi H μ- Pi Φ ø Φ rt ø M 03 rt T Φ 0 μ- 03 ø tr CQ Φ rt 4 Ω 0 ø tirt 03 ø TJ tr 0 Φ ø Ω 3 tr X M in 0 O ø O tr rt φ ø Ω LQ ro tr 0 tiM ro Ω μ Pi μ 0 φ 1 Φ Pi ø ø rt Hi Φ O M Φ rt P. ro ø ro 0 ro tr rt 0 0 μ Pi Φ rt ø 0 0 tr 0 P. Ω 0 M Φ ø tr rt rt Φ Hi ø ! Φ rt Ω Φ 0 tr μ 0 rt ø ro rt Φ rt M μ tr Φ μ 03 03 μ- ø M M O O X μ 0 μ TJ tr P. tr O rt 03 03 μ O μ- 0 0 ø Φ rt μ 3 tiμ <i 0 TJ Φ μ- 0 1 μ r • Ω 0 -— - o μ- o M 03 ø tr μ- ø tr 03 TJ ro 0 ro ø M < < 0 0 ø ø o μ < M 0 1 ø tr 0 φ μ μ ø LQ μ- CO μ- ro i μ- 0 m 3 tr Φ 0 O rt 3 LQ μ- 0 0 0 μ- ø LQ CQ Φ Ω μ- 03 μ- 03 rt rt 0 tr TJ Φ μ tr ø 0 μ- M LQ Ω ø 03 0 Φ 0 3 0 CQ μ- 3 tr tr rt Φ μ μ CQ M M μ μ rt μ- tr tr Pi μ- M 3 TJ Φ (ϊ ø O tr CD 0 φ Φ μ- μ- 0 Φ K μ tr rt ø Φ ro ø μ- ø ø Φ ø tr 0 rt co Hi rt μ- O Φ K μ- CO Ω iQ μ ø μ rt o M rt 03 o Φ 0 μ φ 0 μ- 0 rt 03 μ rt φ 0 Φ rt 0 3 ø 0 rt X 0 CD μ M tr ø CO 03 tr rt 3 Φ 03 p. CO 03 03 ø CQ O μ tr tiμ- ø μ- 03 0 μ . 0 K μ- TJ X 0 03 K tr rt M Φ Hi tr Φ ro ø CO ø Φ rt 0 Φ Φ tr CQ O μ rt ø tr ø 0 μ rt ϋ Φ μ μ 3 ø 0 K μ Si CO Sj 1 0 Hi μ- Φ 0 0 0 S3 rt φ μ- Hi P. ø φ μ- Ω tr O tr μ Pi Ω 03 μ rt s; 3 0 tr ø TJ rt μ- tr 0 ø rt ø tr μ- Φ tr ø Φ φ 1—1. φ 03 μ- ø O ø μ- μ- LQ M O LQ 0 M Ω rt μ 03 μ *< ø ø 03 0 μ 0 ø ø μ Ω μ- 03 rt ro 0 rt ø tr ø μ CD CO ø O CQ rt tr LQ M CO μ- ø tr X Φ μ tr Pi μ ø ø 0 ø 0 rt rt ø rt ø 1 ø Hi μ- ro M φ Φ • » Φ 03 Ω ■ : tr Ω μ- Pi tr ø ø μ- μ ø μ ø μ- Hi 03 tr Pi μ- 03 0 Φ 1 tr ø c_ι. rt Φ rt tr ø Φ φ CO Hi M μ- •<; 3 . Ω μ- O tr O μ μ- 03 0 O O M M CD Ω CD μ- rt LQ CQ 0 μ- tr Hi ø 1 0 Hi Φ φ μ- CD 03 μ K K O O O 03 iQ φ ø 0 tr TJ ø Ω 0 ø μ Hi 03 < Pi rt ø 0 03 ø ø ø ø ø μ μ Hi 0 rt 0 0 ø ø rt Φ Ω 0 φ φ μ- tr 3 TJ 0 rt ø tr μ μ- CD M μ- ti0 ø M O 03 Φ O ø P. ø Hi 1 O O rt μ rt 1 Φ ø μ- Ω ro M Φ M μ O 0 0 0 rt LQ μ- 3 μ- 0 03 O 0 O μ cQ Ω . ø fi M ø Ω rt rt tr tr X μ- rt 0 03 μ M μ φ to μ- M 0 M ø tr μ O ■ ! Φ rt φ μ rt μ- 03 M 03 03 Ω Ω ro ffi 0 TJ μ- rt rt 0 μ tr Pi μ μ- μ- tr — 0 O ø ø ø o 03 Ω 0 0 iQ tr 0 Ω M • TJ Φ φ 0 03 ø M tr ø ø tr TJ 0 rt S3 μ. tr rt S3 tr Φ μ φ M μ X S3 μ LQ φ μ- M μ ø K ø φ LQ 0 - φ rt M 0 TJ 0 rt rt ø 03 TJ 03 Φ μ rt tr tr μ- <! 0 ø μ Ω M tr μ <! Φ μ < μ rt rt 0 O 0 μ- ro ø φ 0 ø H- μ- r M Φ μ- μ ø Φ 0 Φ tr O 03 0 ø μ 03 M Pi μ M ro 0 μ- ø 0 ft φ Hi Pi ø 0 M μ Hi Φ tr tr iQ 03 μ- μ- Hi D M 03 ø tr Φ μ- ø 03 Φ Φ Φ ø O CQ Φ • ø rt 0 ro rt rt ø ro M μ ø M 3 0 μ 03 M S3 0 fi iQ μ- μ Pi μ- tr Hi tr tr tr Φ ø μ- ø iQ μ- IQ Hi μ ø M ø μ ro rt ø 0 μ ro 0 φ M iQ tr μ rt rt μ- φ tr O tr μ- ø ø 03 tr rt 0 M ø M ø Φ rt tr X P. 1 S3 μ- Φ ø - ro 3 3 Ω < φ μ- Φ μ- K 03 ø φ ø μ rt ø TJ μ 0 O 0 Φ X eo 1 Pi O 0 fi rt μ- O μ M 3 03 ø rt 03 Pi tiø Ω H O ø Hi 1 0 ø ro rt M tr Φ ro ø ø Φ tr 1 o 1 Ω X K ro μ O 1 Φ μ tr 1 1 ti ¬ J io DO CO " in o in o in O in 0 μ- 0 S3 tr rt S3 ø μ TJ Φ CQ ø H Hi 0 0 0 P. μ- 0 CO > rt ø rt TJ Hi Hi 0 CD μ ø μ- 0 μ ø 3 tr 0 tr μ- TJ Φ μ ø tr rt 0 0 μ ø TJ μ- 03 ø Ω μ tr μ CD μ- μ- Hi M φ 0 M Hi μ LQ 0 Φ 03 Φ ø TJ iQ 0 Ω o ti ¬ μ Φ M rt <! i μ μ μ- Pi 0 μ M iQ μ- 03 ø M 0 0 ø P. M μ- <! tr 0 ro rt μ- μ- TJ μ- 0 03 Φ ø μ- ti ¬ ø rt LQ 0 rt 0 ft ø rt 0 TJ Ω 0 μ- 0 μ- M ø tr Ω ] TJ Ω 03 φ ιπ tr ø CQ 03 3 ro μ tr tr 0 CQ tr LQ tr rt φ 0 Φ S3 Φ Pi 0 P φ — ø i ø φ tr • 0 ø μ- μ T CD CQ Ω μ- 3 TJ μ Φ ro rt 0 rt rt ro μ3 Φ 0 03 ø 03 P. Φ Hi ø 0 S3 M 0 Hi μ- Pi CD 0 μ μ- 0 CQ • M 0 O μ tr 3 - Ω 03 rt μ- CO tr rt μ- ø μ] φ ø 0 LQ 3 T φ 03 CO Hi K rt 0 TJ φ Φ CD tr φ μ 0 rr 03 rt 0 μ- M ø tr μ Hi μ ø 0 φ 0 Si 03 μ- μ- μ. μ- 0 ti ¬ μ ø μ- μ- P. φ μ 0 ø tr <! 0 M ro Φ 03 μ- 3 3 μ tr ø tr μ- ø 03 tr M O S3 ro ro 0 rt Q rt Ω Φ rt Φ φ ø 0 M tr ø Φ Φ 0 rt μ- TJ 0 LQ φ ti ¬ < ø S3 Pi 03 ø tr rt Pi Φ tr μ 03 0 μ M Pi <! 03 M μ ø μ- ro Φ < 0 ro 9 < 1 μ> 0 μ- 0 tr μ- TJ ø Φ φ Ω rt ø rt tr TJ 0 CD rt • ! ro μ- 0 0 ø c μ μ Φ < ø 0 o M 03 ø μ ø μ Ω ø 03 tr tr 0 μ 0 rt 03 • μ- tr rt Hi Ω Pi O M M Φ 3 rt • ø ø 0 LQ 0 tr TJ 0 Φ Pi 0 M μ- TJ > 03 0 ø K μ- 0 TJ μ- rt Ω 0 Φ M 0 rt - — . Ω Φ ø μ <! 0 φ ø rt SD Ω μ- H3 M tr ø < M μ M Ω ø 0 TJ ø Φ μ μ- ø n Ω 0 M LQ 0 μ- Ω Pi Ω ø 03 rt Φ ø ø ø tr CD TJ 0 Pi μ- μ- φ <! 0 ø M μ TJ LQ 0 Φ 0 LQ 0 0 0 tr 3 Pi tr (_l. rt Φ 0 tr fi Ω M Pi ro X M s; ø 03 φ μ ti ¬ μ- rt LQ <! Φ ø μ TJ tr φ 0 0 μ- M 3 Φ μ- μ- ro 0 CD 0 CQ ---. 0 ro Φ CD ø tr rt rt ro 0 CQ μ M 0 rt 0 P. ø 03 < 03 Φ Ω ø CO Ω Pi Si μ μ μ 03 -- ^ M Pi Hi 03 LQ tr tr 0 ø Φ Φ Φ 03 tr Hi ø ti ¬ φ i ø iQ μ- 0 μ- ø CD ro rt ro tr rt μ- Hi ^ μ- 0 rt 03 iP CQ i Φ Hi Φ ro TJ Φ CQ LQ ti ¬ Hι o M Ω tr Hi 0 tr rt rt Φ ø Ω ø μ- 0 TJ tr Hi 0 M Pi 03 0 0 03 φ ø ø ro Hi TJ μ- tr CO μ- ro TJ φ K 0 Ω 0 rt rt M 0 Hi Φ 0 μ- Φ rt μ- μ • Φ S3 μ- Ω ø Pi ro rt rt Φ Ω 03 μ φ rt CD tr Φ 0 0 ø Φ 0 0 ø Φ μ CQ CO M μ φ μ Ω M μ μ- ■ <; 3 tr μ Hi O rt Φ μ CD ø 0 rt Pi rt ø 0 ø Φ H 0 Φ φ μ Φ ø 0 μ- ro Ω 0 ro 03 ro μ- Hi tr O μ- μ <! Φ Φ μ- rt M μ- 0 ø μ 0 0 Ω 03 Pi M μ μ- ø ø 0 0 rt 3 0 Pi M ro Hi ø φ ø 03 ø K ø 0 03 Φ Ω rt rt 0 M M Si 03 rt M Hi μ- 0 TJ Hi rt Hi Pi tr M rt μ CQ rt rt 3 tr 0 tr μ- ø Φ μ μ- rt Ω rt ro rt M φ μ- μ μ- μ- ■ <! O φ 03 4 φ ^ ^ r rt μ- -J rt 0 0 ø <! φ Ω ø M tr Ω 03 Ω ø μ- μ 0 0 μ M μ- ø Ω O 03 ø o rt rt φ Φ 03 . Ω tr rt 0 0 φ 03 tr CQ ø ro tr μ- tr M Ω rt ti ¬ iQ 0 TJ Hi ø 0 3 0 tr 0 0 03 tr Φ Ω M 0 0 ø 0 CQ ø M ø Pi 0 rt Ω ro tr rt ti ¬ μ ø 3 Hi Φ rt ø μ- S3 0 μ TJ μ Φ M ø rr ø 0 ø ø ø K M *< 03 Φ ø μ rt tr ro 0 Φ CQ ø ø rt μ ø 0 tr ø μ Φ - rt ø o Ω TJ ø ø ø M ø 0 μ 03 μ- Pi < ø Φ rt μ 03 0 tr 0 rt ø Φ ø s; 0 LQ μ- 03 Φ tr TJ ro M Φ tr • <: tr Φ <! O 03 μ- Ω M 0 μ- 0 TJ Φ ø Φ ø Φ 0 TJ μ- 03 μ- μ- M rt Φ ro M M 0 3 ro Pi φ μ- Hi rt Pi tr Φ μ eo TJ rt 03 P. s; M μ Φ 0 LQ 03 CD tr P. μ 03 μ- 01 S3 CQ μ- μ- — ø Ω 0 μ- Ω CD ro μ- 0 3 μ- rt i μ 0 Φ ø ro 3 CQ tr Ω ø 0 rt ø ø li ti ¬ μ- i ii Ω 0 μ- tr ø tr 03 03 ø rt 0 μ- 0 tr s; Ω 0 ^Q μ Ω rt tr 03 rt LQ Φ ro 03 ø rt 03 K i Φ ø ^ 0 rt M tr μ TJ ø 03 0 TJ μ- tr S3 Ω ø tr ro φ tr CQ Si μ- P. M TJ 03 0 CQ rt td ø 0 φ μ ft rt < μ rt SD ø ø 03 tr 0 μ- Φ ø o rt ø K 0 μ- 0 s- 0 rt rt ø tr CD rt ro O Φ 0 t ø TJ 0 μ CD Hi Hi 0 Ω ø 0 CQ rt 0 TJ μ- μ Φ Ω M rt 03 03 Pi TJ μ- ø μ- 0 TJ 0 tr μ- 0 M 03 0 tr ø Hi 0 μ- 0 Pi -— , rt ø 0 0 tr μ- μ ø φ Φ ø μ- i 0 ø ro ø tr Φ 0 Φ S3 LQ 0 Φ ø rt 0 Φ μ- 3 LQ Hi φ ø μ- Si ø ø 03 φ M M ø ø ø μ- LQ M μ- μ Pi ø Hi 03 ø μ Hi TJ 0 ø P. CQ μ- 0 μ- Ω Ω μ- CQ ø CO tr 0 M φ rt LQ Hi 0 0 Hi M Φ rt K μ rt Φ μ ø ø Ω 0 tr μ- μ- rt 0 Ω 3 rt φ φ LQ Ω ø μ Hi ti ¬ tr μ- μ- Pi ø ø ro Pi μ ro rt μ M μ er μ ø M ø μ- μ- M Pi rt Ω ti ¬ μ- μ 0 ro ro S3 0 LQ rt ■ : Φ μ- μ- μ- TJ μ- M 0 ro Ω φ μ φ ø rt 03 0 Φ tr tr ro tj μ 3 μ Si ø M 0 K 0 P; M rt Hi ø 0 03 • ø tr Φ Pi M CO Φ CO 0 Ω rt 0 Pi < ro < 0 μ 0 O tr td Hi 03 tr en TJ 0 0 M μ- Pi μ- rt tr • ø Φ ø 0 ø μ- Φ 0 ro ø 3 • μ ro *< φ ( μ- m Φ • 0 CD Hi o K LQ 0 D μ- 0 ø Hi ø LQ Hi rt ø M Ω 03 μ μ- ro μ tr rt tr tr 0 0 μ- ø 0 rt Td Φ 0 rt Φ Φ μ- ro Ω m P. 03 ro Φ μ- ro μ μ- tr rt ii 0 ø μ- 0 0 M μ μ- 3 Hi CQ ø 0 ø to tr 0 ø 0 Φ CO M Φ tr μ M CQ μ 03 o Φ μ- μ- ø iQ LQ Pi 0 rt 1 rt ø 03 μ- Φ Φ ^ 0 ø iQ LQ rt ro 03 O o rt ø 1 rt 1 1 tr 1 - 1 subject to channel balancing, in which the strength of all channels has been equalised, as indicated in the lowermost diagram of figure 7. By using other levels of attenuation for each chan- nel, it is straightforward to achieve any power profile of the balanced signal . Figure 8 illustrates how channel balancing according to the present invention is implemented at an optical communications link. An attenuator arrangement 10 accord- ing to any one of the embodiments of the present invention is connected to an optical fibre 40 capable of carrying a wavelength division multiplexed optical signal. A controller 41 is operatively connected to the attenuating arrangement 10 in order to control the channel balancing. Downstream and/or upstream in the fibre 40 from the attenuating arrangement 10 there is provided a spectrum analyser 42. The spectrum analyser 42 is arranged to analyse the power spectrum of the optical signal propagating in the optical fibre 40. The measured power spectrum is sent to the controller 41 and at least one channel within the optical signal having higher than a desired power level is identified and selected for attenuation. The controller 41 adjusts the attenuating arrangement 10 in order to provide the desired attenuation of the at least one selected channel. Either of the spectrum analysers 42 can be omitted. If spectrum analysis is performed only upstream from the attenuating arrangement, the optical signal is analysed prior to being subjected to attenuation, and the desired channel balancing is performed at the attenuating arrangement in accordance with the measured power spectrum forwarded to the controller. If, on the other hand, spectrum analysis is performed only downstream from the attenuating arrangement, the optical signal is analysed af- ter being subjected to attenuation, and the measured power spectrum is fed back to the controller in order to provide the required channel balancing. IO J to DO H H en o in o (Jl o (Jl Ω Ω 0 en TJ 0 rt ø 0 3 Ω i- Ω Ω μ- 3 03 0 rt Ω 0 ø TJ μ- TJ H rt 03 Pi Hi rt Pi TJ O CD in ø 0 μ- 0 ø tr Φ o 0 ø ø K CQ μ- 0 μ ø CD > φ rt tr rt Φ μ tr 0 0 0 μ- tr Ω S3 TJ O Ω P. K 0 μ μ 0 LQ ti ¬ CO < 0 fi ø μ CD μ 0 μ 03 0 ro S3 S3 rt <! μ- tr φ ii ø Φ ø i fi rt . ro ti ¬ 0 Φ 0 Φ M Hi • Hi μ- rt ro μ- 3 ø ro μ 0 Φ> iQ ii ro μ ø tr ø Φ μ- μ Si ro M Φ ii K 0 0 03 0 ri 3 03 i 0 tr en tr Ω 0 T rt rt Φ ro P. ø 0 Td 3 • ; Td Ω 03 μ Hi ii rt 3 φ rt ro rt K M M 0 ø 0 Hi μ- ø M LQ M 0 0 CD 0 rt rt μ- 3 μ 3 rt tr Pi tr 0 μ 03 0 M φ Ω Φ ø 3 3 LQ μ- *< TJ 03 rt M μ μ ■ <; Ω μ μ- O 03 ro 0 φ 0 ro 0 ro 03 φ TJ S3 CD 0 ø Hi T TJ rt ø ø Φ ii 0 rt CD 0 tr 0 rt 0 Pi 3 Pi Hi 0 0 0 ro CD μ rt ø 0 μ- M M tr μ rt φ μ- 0 μ Φ 0 0 0 tr 0 3 μ- tr fi rt 0 μ 3 Ω < 0 ø iQ μ- μ- Φ φ TJ Hi ø ø X TJ Pi 0 0 Φ CD μ- 03 rt μ Φ ro rt ti ¬ rt ro rt CD ^ tr LQ ø Hi Hi O ø φ Ω ø φ. 0 ro μ- μ Φ μ . ø 3 tr ro 03 tr ro rt 3 0 μ μ H Ω 0 tr μ μ- μ- 0 ID S3 ø μ rt tr μ> 3 μ 03 Pi Ω 3 μ- TJ ro ro 3 ø μ- ro 0 ro Φ Hi ø *> 0 ! 0 Φ Φ μ CD 0 μ tr μ- ø TJ 0 TJ ti ¬ 0 3 φ < 0 ø μ 0 ø ø 3 0 φ TJ rt μ μ μ 03 μ rt Φ ro φ CD μ- M rt M ro μ μ- μ ro TJ ti ¬ P. ro 0 0 0 rt rt μ- rt 0 μ- 0 tr tr fi <! S3 ø φ 0 0 0 Si Φ 03 Hi μ M ro ro i rt rt 0 tr 0 rt μ 0 ε; 0 ^ ^ ø 0 TJ Φ 0 ro ^ μ • <: φ 0 rt 0 μ ti ¬ μ- rt 0 Φ Hi 0 Φ 0 en in CQ S3 μ μ rt TJ CD 0 TJ rt μ tr Ω Hi 03 Ω ro 0 μ- Hi μ- M TJ μ Φ 03 0 0 rt 0 Ω μ μ- 0 03 ro 0 rt rt 3 ro Φ μ rt tr ø ø 0 0 ft 03 μ μ- P. ø 3 Hi μ- μ- tr Pi rt CQ Ω μ tr tr Φ M 0 0 0 0 CQ rt rt tr rt Φ M 03 0 TJ Φ 03 μ- ø 0 Ω 0 ro ro 03 μ μ- μ- Φ φ i ro S3 3 o ø 0 tr ti ¬ ro 03 tr Pi Φ S3 CD ø Ω M 03 Ω ø ø 3 μ 3 μ- ro TJ ø CD tr Pi ø rt ø ø Φ ro 0 φ Φ <! 0 ø rt rt tr φ rt tr M μ- ro ø 03 ro 4 TJ Ω ø Ω CD φ rt ro μ- 0 μ 0 0 3 Hi φ Hi CD μ Φ . ø μ- Hi • rt ■ : ti ¬ ø ro ø 0 Φ M ~. μ tr rt M ø i μ TJ 0 TJ φ LQ μ- M ti ¬ rt Φ Hi 3 ø ro 03 o 0 ro μ rt ø Φ 0 Φ Φ tr iQ ø 0 0 rt rt 0 ø ro μ 0 0 0 rt < μ- μ tr Pi 0 rt fi 03 μ- 0 tr rt ø S3 rt μ- μ- μ- Φ 0 0 φ 3 μ rt ø LQ 3 rt TJ φ ro M Φ> μ ro rt rt 0 rt 0 ø tr iQ φ μ- Ω 3 0 Hi 0 μ- μ- ø μ Ω rt LO 0 Hi tr rt M μ ro Φ μ ø ø φ TJ 3 Hi Ω Ω rt 0 ø 03 0 0 Pi 0 Ω 3 M 0 rt μ- Φ ti ¬ rt 0 μ 3 LQ M 03 U 0 LQ rt μ μ 4 ø M μ- 0 < φ ø . o 0 M μ 0 03 ro tr •> ø ø 3 Φ 03 DO Hi ø tr ro Hi 0 0 M 3 ø CQ Φ μ- 03 rt ø 0 Φ 0 0 ro Ω ø ro ø TJ ø 03 TJ μ- φ Hi μ 3 ø M ø 0 ø Pi μ- μ μ- Pi μ Hi rt TJ Ω 0 μ- CQ 0 Cϊ φ μ μ. μ rt 0 Φ 0 ø K *< μ- 0 0 μ- CD μ 0 μ- rt Φ tr μ rt 0 03 ø CD 03 Ω μ LQ φ tr - Ω μ 3 rt Φ Hi rt M ø ro M ø 0 0 > Φ ro ro ø rt TJ LQ 3 0 μ- rt ø 0 Hi φ tr μ tr M 0 r 0 1 03 Ω fi μ ø Pi Hi ø μ- ro CD μ 03 μ ø 0 φ tr 0 φ rt Φ 0 ro μ 03 rt rr tr 0 0 μ- rt tr Ω ro M ø Ω S3 ø ro 0 LQ M μ 0 0 ø Pi tr tr 3 0 0 0 0 rt Hi li ø TJ μ- 0 tr μ LQ rt ø rt 3 ø 3 CD μ rt ø Φ 0 0 Φ TJ 3 1 0 ø 0 Pi LQ ø 03 Ω 0 μ Ω μ 03 ro 03 3 tr Φ TJ Φ μ- rt M X μ TJ Φ 0 rt ø rt ro rt ? ø ro tr ø 0 μ- ø Φ TJ CD 0 Pi M μ- M ø 0 ti ¬ 0 ti ¬ Φ 0 0 ro μ tr μ ø TJ ø fi 0 μ 3 0 03 rt P. φ 0 rt μ- rt 03 rt ro ø ro Hi S3 0 μ- μ- M ro ro ø > μ 0 ro ø μ Ω μ ft tr rt 0 ti ¬ 0 Ω ø Φ φ rt 03 rt Ω ( P. Φ> Φ ø M rt ø 0 ø rt 0 TJ rt Hi • 0 μ- μ- μ ro 0 μ- i μ μ tr ro tr 0 Ω 0 Hi Pi tr Φ ø ø ø 03 ro 0 0 0 Φ 03 m i 0 ø ø ro μ Φ 3 0 ø tr 03 Pi φ tr 0 M LQ ø ø ro μ μ H TJ ø μ tr φ 0 rt LQ Pi Φ Pi μ μ ø M Hi 0 0 ø ro μ Hi 0 rt Φ M M Pi Hi CD 3 0 03 0 03 ø Φ μ 0 0 - fi μ- 0 0 μ- ø M μ < 0 Φ M tr μ- 3 M 0 CD φ rt ø M rt μ 03 μ- rt ø iQ ø M <! μ- tr Φ 0 03 0 φ CO ■ <! rt μ rt Pi rt μ M Hi 03 0 rt 0 μ- .3 rt rt ø μ- tr Ω ø ro 0 CD Ω M Pi ø TJ M 0 μ- O 3 ø tr CD K μ 0 3 0 μ μ> ø TJ 0 μ 03 0 Φ μ- μ M S3 μ- Φ K Hi Ω ro 0 rt 03 TJ ro tr 0 μ- ø 03 0 TJ o Ω M μ- rt ø Φ M 03 0 μ- Ω ø μ Ω Pi M TJ 03 o 3 Φ 3 Ω M ø M r ø tr ø 0 TJ LQ tr SD M o μ ø rt Ω μ> Ω O tr μ- CO 0 μ- Ω o ø 3 Ω 0 0 Hi μ rt 0 0 ø TJ S3 LQ μ μ- o 0 <! Φ Pi 03 rt Ω ø rt rr 0 l-h tr • ø fi Φ tr rt rt Φ 0 tr M rt ø 03 Φ ø 0 ø - 0 μ- Ω 0 tr μ 0 ø tr tr 1 μ- μ- ø μ- ø μ- μ- rt 1 Ω Φ 0 0 Ω 0 μ μ 3 LQ Pi tr S3 Φ CO P. Φ Φ φ φ Pi ø rt φ 0 μ- tr ø 1 CD Ω 0 Pi μ- tr CD 0 0 ø ro μ < iQ ø < ø rt ro Ω tr 1 ø μ- ro Pi 1 3 03 φ ø φ 03 UJ 10 DO to H 1 in o in o in O in LQ 03 P- μ- 0 μ- 0 3 Hi ø Φ Hi CD . Φ - i H Φ μ- tr 0 ø Φ rt CO s. φ S3 tr 0 μ- μ <! Ω Φ Φ tr 03 M 0 φ rt 0 0 tr ø LQ Φ 0 rt Ω tr μ Φ 03 φ 03 CQ 0 0 Pi rt 0 φ 0 Hi 0 μ- μ- rt ø Hi Φ Hi DO s; Φ 0 ø ii < Φ Φ ø 0 M 0 rt Φ 0 & TJ Q Φ 0 rt μ 03 tr μ- 0 rt tr Hi μ- 0 0 03 CQ 0 Φ 03 0 M Φ ø 03 Ω M 0 rt 0 r μ- 0 CO 0 LQ rt 0 ø rt 0 μ tr 1 φ φ 1 UJ ω to CO H in o in o in o in rt μ- 0 ø TJ ø μ TJ 0 03 Φ 03 0 μ Φ 0 CO 0 0 M M K 0 0 rt D ^ TJ 0 M 0 μ μ- 03 0 rt μ- 0 ø 3 0 μ- ø rt ø μ- ø μ- ø LQ DO M O in O in 0 Ω 3 S3 03 0 Pi 03 Ω ø μ TJ TJ 03 M 2! 03 TJ cf H3 Hi 0 0 μ- O μ Φ 0 0 μ 0 μ o 0 0 D 0 μ tr tr 0 Si M 0 Φ 03 tr M φ rt Φ S3 rt μ 3 Cf Φ 0 Φ Cf Ω μ- M LQ Ω L-J. M Φ CD φ μ- μ- μ- CO rt ii tr Φ Hl tr ø μ Φ K μ M Φ μ 0 03 0 0 Φ φ Φ μ- μ- μ cf 0 μ- m Ω Φ K 0 0 ø TJ 0 ø rt Hi < Ω Φ • rt tr tr rt 0 Ω rt Ω rt rf rt tr 0 μ- 0 0 0 Φ Φ 0 0 ø 0 3 μ- μ- Si Φ μ ø tr rt M o μ- fi rt cf 0 μ- 0 ø 0 Ω μ- μ- Φ < M μ- μ- 0 0 Φ 0 1 Pi 0 rf μ- 0 0 μ ø Ω - φ φ 0 03 rt 0 3 TJ μ- < μ ø M Φ <! tr ø ø φ cf Φ tr tr 03 o ø μ- Φ O rt Ω Ω φ 0 μ- rf S3 tr ø 0 0 ø M Φ ø ø M ø 0 03 cf ø ø rt μ- μ- 0 cf Φ P. < Pi μ- 0 fi cf μ- 3 μ- μ- rt ø 0 rt Hi tr Φ Φ μ- Pi μ Ω μ- -- ^ ø TJ LQ 0 μ- Φ μ- ø tr SD Ω P. 3 μ- μ- 0 0 0 μ- O ø ø 0 M 03 μ- ft rt 0 ø φ TJ 03 ø ø ø μ ø 0 ø 03 ø μ- ø tr ø rt 0 ø 0 0 rt Ω CQ Φ M • 03 TJ eo Φ 0 rt 0 P. rt S3 cf 0 Φ μ- Ω 0 μ- TJ μ rt μ CO φ μ- μ 03 tr Hi cf μ- H 03 φ φ P. tr Φ ø 0 M CO μ 0 cf rt 0 μ- 03 03 rt Ω Hi φ Φ 0 μ μ- Ω 0 ø tr TJ ø tr TJ μ- φ Hi Cj 1 K 3 tr Hi Ω Φ Φ ø μ 0 03 μ- Φ Hi μ μ- 03 0 tr μ- φ 0 3 en μ φ φ 0 φ 03 μ μ- μ ø Ω i Φ ef 3 rf 0 ø rt 0 Hi M TJ Hi Ω Φ φ 0 μ- μ ø 0 tr cf μ- μ tr o 0 TJ 0 fi Pi TJ 3 0 rt (f Φ μ ø 0 CO μ tr μ- 03 μ μ μ TJ Φ Φ 0 • <; tr μ- 0 cf ø φ 3 0 ø Φ 0 φ 3 0 μ- ø Hi Φ Ω TJ M 0 03 Λ φ M 0 rt Hi Φ s; ø 0 rt TJ ø μ • μ- TJ ø fi 0 S3 Cf Φ Φ Φ w 0 ^n Hi CO Pi Φ 03 M Φ ø 0 φ ø tr P. μ μ P. rt μ- μ Ω M CD μ- μ ø 0 Ω μ- μ- μ 0 tr rt 03 Hi 03 0 ■ : Φ ø rt rt ø μ- Ω ø μ- Φ ø Ω μ- Φ ΪT tr Hi 0 TJ ø LQ M ø tr Cf ø fi 0 ø φ 0 φ ø Φ 03 rt — " Hi K tr LQ 0 rt rf 0 0 S3 μ tr μ- 0 a 0 S3 ft 03 TJ μ- ø φ 03 0 0 μ- ø tr rt tr ø Hi 0 μ 0 TJ ø ø t Hi s; 0 Hi μ rt tr ø 0 rt 0 0 3 Φ <! ø rf μ- ø < Cf φ tr Ω ø μ S3 μ 0 M Φ ø Φ 0 M rt Φ Φ φ Ω Ω ø Φ o μ p. ø fi φ M tr μ- ø μ . — . 0 0 Φ 03 φ rt 0 Ω Φ cf φ Φ ø cf S3 rt 3 μ M TJ o X tr Ω ø Pi μ- P. t Pi μ- tr tr S3 TJ fi Φ μ ø o Ω CQ 0 φ 0 TJ rt 0 0 μ- 0 0 μ- 03 Ω cf 3 LQ 0 Φ Ω ø CQ Pi μ- μ tr ø <! Ω 0 ø Φ rt tr TJ 0 μ CO M Ω μ- ø 0 Φ μ- tr Ω φ iQ M μ- 0 M 0 Si SD μ 3 cf cf ø O 0 Φ <! LQ Φ SD μ- Φ μ- m μ- φ rt Φ Pi tr iQ μ- 3 tf rt Ω Φ 0 M 0 CO 3 tr tr ø tr Ω μ 0 co TJ 03 0 Cf ø LQ 03 CQ Φ φ rt Φ rt ø (f tr 0 μ- TJ 0 ft TJ Φ • Pi 03 μ- S3 φ rt ø 03 cf <! 0 M tr 0 rt 0 03 ~. 0 0 μ. 0 φ Φ φ tr φ M 0 M 0 Cf Ω μ 0 0 μ ø τ ø TJ Φ 0 TJ rt 0 μ- 0 ø 0 rt LQ Φ μ- rt 0 μ 0 μ cf ø T μ 0 CD 1 03 1 μ- 1 rt μ- tr M Φ φ Si 1 tr 1 Φ Φ

Claims: CLAIMS 1. A method for channel selective power control of a wavelength division multiplexed optical signal, the method including the steps of : selecting at least one channel within said optical signal having higher than a desired power level; establishing a resonance to the selected channel, the resonance providing a selection region where said se- lected channel has a substantially increased power density relative to channels out of resonance; and attenuating said selected channel a desired amount by adjusting the properties of said selection region. 2. A method as set forth in claim 1, in which the step of selecting at least one channel having higher than a desired power level is performed by means of spectrum analysis of the wavelength division multiplexed optical signal . 3. A method as set forth in claim 1, in which the step of establishing a resonance comprises the steps of providing an external resonator, which is defined by a first and a second mirror, said first and said second mirror being provided outside and on opposite sides of a waveguiding structure, preferably an optical fibre, carrying the optical signal; and deflecting light between the waveguiding structure and the external resonator, said deflecting being ef- fected by a deflector provided in said waveguiding structure . 4. A method as set forth in claim 1, in which the step of attenuating is performed by introducing a loss in the selection region. 5. A method as set forth in claim 4, in which the step of attenuating is performed by introducing an absorbing element in the selection region. 6. A method as set forth in claim 3, in which the step of attenuating is performed by introducing an absorbing element inside the external resonator. 7. A method as set forth in claim 4, in which the step • of attenuating is performed by making the selection region leaky, light thereby being caused to leak out of the same . 8. A method as set forth in claim 3, in which the step of attenuating is performed by changing the phase of the selected channel in the selection region relative to the phase of the selected channel in the waveguiding structure, thereby causing destructive interference on the selected channel . 9. A method as set forth in claim 8, in which the phase of the selected channel is changed by making a parallel displacement of the first and the second mirror with respect to the waveguiding structure . 10. A method as set forth in claim 8, in which the phase of the selected channel is changed by altering the refractive index in at least some portion of the external resonator, thereby altering the optical path length in the resonator. 11. A method as set forth in claim 9 or 10, further comprising the step of altering the separation between the first and the second mirror. 12. A method as set forth in claim 1, further comprising the steps of deflecting the selected channel from a first waveguiding structure carrying the optical signal into an external selection region; and coupling the selected channel from the selection re- gion into a second waveguiding structure; the step of attenuating the selected channel being performed by absorbing light in said selection region. 13. A method as set forth in claim 12, in which the step of establishing a resonance comprises the step of providing an external resonator enclosing both the first and the second waveguiding structures, said external resonator being defined by a first and a second mirror arranged outside and on opposite sides of the first and the second waveguiding structures. 14. A method as set forth in claim 12, in which the step of establishing a resonance comprises the step of providing at least one Bragg grating in each of the first and the second waveguiding structures. 15. A method as set forth in claim 14, in which at least one of the Bragg gratings is a chirped grating. 16. A method as set forth in any one of the claims 12 to 15, in which the step of deflecting the selected channel from the first waveguiding structure is performed by means of a first blazed phase grating in the first waveguiding structure, and the step of coupling the se- lected channel into the second waveguiding structure is performed by means of a second blazed grating in the second waveguiding structure . 17. A method as set forth in claim 1, in which the reso- nance to the selected channel is established by arranging one or several Bragg gratings inside a waveguiding struc- ture, preferably an optical fibre, carrying the optical signal . 18. A method as set forth in claim 17, in which the resonance is established by arranging a chirped Bragg grating in the waveguiding structure, said grating being resonant to different wavelength channels at different portions along the same. 19. A method as set forth in claim 18, in which the selection region is comprised within the resonance, and attenuation is provided by introducing a loss in said selection region. 20. A method as set forth in claim 19, in which the selection region is made leaky by bending a selected portion of the waveguiding structure, light of predominantly the selected channel thereby being caused to leak out from the selection region. 21. A method as set forth in claim 19, in which the selection region is made leaky by moving a light guiding probe close enough to the waveguiding structure to allow evanescent coupling of light from the waveguiding struc- ture into said probe. 22. A method as set forth in claim 3, in which the deflector is provided within an internal resonator in the waveguiding structure, thereby enhancing the spectral se- lectivity of the channel balancing. 23. A method as set forth in claim 3, further comprising the step of tuning the external resonator to the wavelength of the selected channel . 24. A method as set forth in claim 23, in which the step of tuning the resonance is performed by adjusting the separation between the first and the second mirror. 25. A method as set forth in claim 23, in which the step of tuning the resonance is performed by tilting the external resonator with respect to the waveguiding structure . 26. A method as set forth in claim 23, in which the step of tuning the resonance is performed by adjusting the separation between the first and the second mirror; and tilting the external resonator with respect to the waveguiding structure. 27. A method for controlled removal of power from a selected channel within a wavelength division multiplexed optical signal propagating in a waveguiding structure, preferably an optical fibre, comprising the steps of: establishing a selection region in which the power density of said selected channel is increased relative to other channels within said optical signal; and removing a controlled amount of power from said se- lected channel by controlling the properties of said selection region. 28. A method as set forth in claim 27, in which a channel is selected by performing spectrum analysis of the wavelength division multiplexed optical signal, and selecting at least one channel having higher than a desired power level . 29. A method as set forth in claim 27, in which the step of establishing a selection region comprises the step of providing a resonance to the selected channel , in which resonance the power density of the selected channel is increased relative to other channels. 30. A method as set forth in claim 29, in which the se- lection region is established outside the resonance. 31. A method as set forth in claim 29, in which the selection region is established within the resonance. 32. A method as set forth in claim 27, in which the step of removing a controlled amount of power from the selected channel is performed by introducing a variable loss in the selection region. 33. A method as set forth in claim 32, in which the variable loss is introduced by absorbing a controlled amount of light in the selection region. 34. A method as set forth in claim 27, in which a con- trolled amount of power is removed by adjusting the properties of the selection region in such a way that destructive interference is achieved, which prevents light from propagating in the waveguiding structure. 35. An arrangement for channel selective power control of a wavelength division multiplexed optical signal propagating in a waveguiding structure, preferably an optical fibre, the arrangement comprising a spectrum analyser arranged to analyse the power spectrum of said optical signal and to identify and select at least one channel within said optical signal having higher than a desired power level; an attenuator arranged to attenuate a selected channel within said optical signal; and a resonator arranged to provide a selection region where the selected channel has a substantially increased power density relative to channels out of resonance, the attenuator further being arranged to attenuate said selected channel by changing the properties of said selection region. 36. An arrangement as set forth in claim 35, comprising a plurality of attenuators and a plurality of resonators, said attenuators and said resonators being arranged to attenuate a plurality of wavelength channels within a wavelength division multiplexed optical signal. 37. An arrangement as set forth in claim 35, further comprising a controller, said controller being arranged to receive, from the spectrum analyser, information identifying the at least one channel having higher than a de- sired power level, and to control the attenuator to provide a desired level of attenuation to said at least one channel . 38. An arrangement as set forth in claim 37, wherein the spectrum analyser is operatively connected to the optical fibre upstream from the attenuator. 39. An arrangement as set forth in claim 37, wherein the spectrum analyser is operatively connected to the optical fibre downstream from the attenuator. 40. An arrangement as set forth in claim 38, wherein a second spectrum analyser is operatively connected to the optical fibre downstream from the attenuator, said second spectrum analyser also being operatively connected to the controller. 41. An arrangement as set forth in claim 35, wherein the resonator is an internal resonator arranged in the waveguiding structure, said internal resonator comprising a chirped Bragg grating. 42. An arrangement as set forth in claim 35, wherein the resonator is an external resonator arranged outside the waveguiding structure, said external resonator being defined by two mirrors arranged outside and on opposite sides of the waveguiding structure, said external resonator being coupled to said waveguiding structure by a deflector. 43. An arrangement as set forth in claim 42, wherein the attenuator is arranged to introduce a loss in the selection region. 44. An arrangement as set forth in claim 42, wherein the attenuator is arranged to change the properties of the selection region in such a way that the phase of light being coupled back into the waveguiding structure from the external resonator is out of phase with light of the resonant wavelength propagating in the waveguiding structure, thereby causing attenuation by destructive inter- ference. 45. An arrangement as set forth in claim 42, wherein a plurality of external resonators is coupled to a common channel of the WDM signal, said plurality of external resonators thereby constituting a set of sub-resonators associated with said channel . 46. An arrangement as set forth in claim 42, wherein the external resonator is adjustable in such way that the wavelength to which the external resonator is resonant can be tuned. 47. An optical device, comprising a waveguide, preferably an optical fibre, capable of carrying an optical signal having a plurality of wavelength channels; a resonator operatively connected to said waveguide, the resonator being resonant to at least one wavelength interval within said plurality of wavelength channels, said resonator establishing a region where the resonant wavelength interval has a substantially increased power density relative to wavelength intervals out of resonance; and a controller arranged to adjust said resonator such that a controlled amount of power is removed from the resonant wavelength. 48. A device as set forth in claim 47, wherein the resonator is controllable such that the wavelength interval to which the resonator is resonant can be tuned, thereby allowing removal of power from different wavelength in- tervals at different instants. 49. A device as set forth in claim 47, wherein the resonator is an external resonator arranged outside the waveguide, said external resonator being defined by two mirrors arranged outside and on opposite sides of the waveguide, said external resonator being coupled to said waveguide by a deflector. 50. A device as set forth in claim 49, wherein the de- flector comprises a blazed phase grating provided in a core of the waveguide . 51. A device as set forth in claim 49, wherein the controller is operative to change the phase of light of the resonant wavelength in the resonator relative to the phase of light of the same wavelength in the waveguide, thereby causing destructive interference on said wavelength. 52. A device as set forth in claim 51, wherein the controller is operative to change the phase of light in the resonator by causing a parallel displacement of the external resonator with respect to the waveguide . 53. A device as set forth in claim 51, wherein the con- troller is operative to change the phase of light in the resonator by causing a change in the refractive index in at least some portion of the external resonator. 54. A device as set forth in claim 49, wherein the con- troller is operative to provide absorption in the external resonator. 55. A device as set forth in claim 54, further comprising a controllable liquid crystal provided inside the ex- ternal resonator, the controller being operative to provide absorption by changing the transmittance of said liquid crystal. 56. A device as set forth in claim 47, wherein the reso- nator is an internal resonator arranged inside the waveguide, said internal resonator comprising at least one Bragg grating. 57. A device as set forth in claim 56, wherein the reso- nator comprises a chirped Bragg grating. 58. A device as set forth in claim 56 or 57, wherein the waveguide comprises a bend that makes the resonator leaky. 59. A device as set forth in claim 56 or 57, wherein a light guiding probe is arranged within evanescent contact with the waveguide, light thereby leaking out from the resonator to said probe .

Inventors: ASSEH ADEL, BERGMAN MIKAEL, SAHLGREN BENGT, SANDGREN SIMON, STUBBE RAOUL, ASSEH, ADEL, BERGMAN, MIKAEL, SAHLGREN, BENGT, SANDGREN, SIMON, STUBBE, RAOUL
Applicants: ASSEH ADEL, BERGMAN MIKAEL, PROXIMION FIBER OPTICS AB, SAHLGREN BENGT, SANDGREN SIMON, STUBBE RAOUL, ASSEH, ADEL, BERGMAN, MIKAEL, SAHLGREN, BENGT, SANDGREN, SIMON, STUBBE, RAOUL
Application year: 2001
Main classification: H04J14
IPC codes: H04B10, H04J14