1. A method of detecting a received signal in an optical fiber communication system using Viterbi algorithm methodology, the method comprising: amplifying the received signal by an optical amplifier; converting the received signal into an electrical signal by a photon detector; decoding the electrical signal by a processor using a Viterbi algorithm comprising the following expression of Viterbi branch metrics: λ ⁡ ( a k , σ k ) ≅ 2 ⁢ Z k ⁢ S R ⁡ ( a k , σ k ) - S R ⁡ ( a k , σ k ) - N 0 2 ⁢ ( v - 1 2 ) ⁢ ln ⁡ [ S R ⁡ ( a k , σ k ) ] , wherein: σ k is the state of the receiver at the k th bit interval; S R (a k , π k ) is the substantially noise-free received sample at the k th bit interval; v is the number of degrees of freedom; a k is a possible value of the bit transmitted at the k th bit interval; Z k is the sample received at k th bit interval; and N 0 is the power spectral density.