Patent ID: 7969647

Claim:
An apparatus for flattening a gain profile G(λ) of a hybrid amplifier comprising an erbium doped fiber amplifier (EDFA) and a Raman amplifier (RA) for amplifying a stream of optical signals, the apparatus comprising: a detection device arranged to receive a tapped portion of the stream of optical signals in the form of N+1 sub-portions and to provide N+1 output signals P 0 . . . P N in dependence upon said N+1 sub-portions, wherein N is an integer positive number, the detection device comprising: N spectral filters having respective transmission functions F 1 (λ) . . . F N (λ); and N+1 photodetectors for producing the N+1 output signals P 0 . . . P N , in response to a light impinging thereon, wherein the first sub-portion of the tapped portion of the stream of optical signals is coupled to the first photodetector for producing the signal P 0 , and each one of remaining N of said sub-portions of the tapped portion of the stream of optical signals is coupled to one of the N spectral filters coupled to one of the remaining N photodetectors for producing the signals P 1 . . . P N ; a controller arranged to receive said signals P 0 . . . P N from the detection device and suitably programmed to provide M control signals x 1 . . . x M in dependence upon said signals P 0 . . . P N , wherein M is an integer positive number and x m =f m (C k , P 0 . . . P N ), wherein f m is a pre-determined function and C k are pre-determined constants, for each m=1 . . . M; and M spectral actuators S 1 . . . S M arranged to receive said control signals x 1 . . . x M , respectively, and modify the gain profile G(λ) by a value ΔG(λ) according to the equation Δ ⁢ ⁢ G ⁡ ( λ ) = ∑ m = 1 ⁢ ⁢ … ⁢ ⁢ M ⁢ A m ⁡ ( λ ) · x m , wherein A m (λ) is a fraction of said modification caused by the m th actuator S m upon receiving a unitary control signal by said actuator; wherein the functions f 1 . . . f M and F 1 . . . F N are selected so as to increase flatness of the gain profile G, wherein the set of functions f m is defined as f m = C m , 0 + ∑ n = 1 ⁢ ⁢ … ⁢ ⁢ N ⁢ C m , n ⁢ P n P 0 , where C m,n are M·N pre-determined constants.