Patent ID: 8265491

Claim:
A method for mitigating distortion in coherent single-ended photo-detection comprising receiving an optical signal carried on an optical transmission medium and coherently detecting the received optical signal to produce a digitized signal; estimating a time-dependent random variable introducing distortion to the coherently detected signal; subtracting the distortion from the digitized signal to produce a distortion mitigated output signal; wherein signal power received by photo-detectors in the coherent detection is represented by: P I ⁡ ( t ) = P S ⁡ ( t ) + P L ⁡ ( t ) + 2 ⁢ P S ⁡ ( t ) ⁢ P L ⁡ ( t ) ⁢ cos ⁡ ( θ ⁡ ( t ) ) P Q ⁡ ( t ) = P S ⁡ ( t ) + P L ⁡ ( t ) + 2 ⁢ P S ⁡ ( t ) ⁢ P L ⁡ ( t ) ⁢ sin ⁡ ( θ ⁡ ( t ) ) wherein P S (t) denotes the signal power and P L (t) denotes local oscillator power of a reference optical signal, and θ(t) represents a relative phase between the received optical signal and the reference optical signal, and further, wherein a digitized signal with AC coupling is represented by: I I ⁡ ( n ) ≅ I ~ S ⁡ ( n ) + 2 ⁢ I S ⁡ ( n ) ⁢ I L ⁡ ( n ) ⁢ cos ⁡ ( θ ⁡ ( t n ) ) I Q ⁡ ( n ) ≅ I ~ S ⁡ ( n ) + 2 ⁢ I S ⁡ ( n ) ⁢ I L ⁡ ( n ) ⁢ sin ⁡ ( θ ⁡ ( t n ) ) and I S (n) denotes the coherently detected signal and I L (n) denotes a local oscillator signal, and Ĩ S (n) is the time-dependent random variable introducing distortion; wherein a first-order approximation is represented by: I I ⁡ ( n ) ≈ 2 ⁢ I S ⁡ ( n ) ⁢ I L ⁡ ( n ) ⁢ cos ⁡ ( θ ⁡ ( t n ) ) I Q ⁡ ( n ) ≈ 2 ⁢ I S ⁡ ( n ) ⁢ I L ⁡ ( n ) ⁢ sin ⁡ ( θ ⁡ ( t n ) ) and ⁢ : I I 2 ⁡ ( n ) + I Q 2 ⁡ ( n ) ≈ 4 ⁢ I ~ S ⁡ ( n ) ⁢ I _ L ⁡ ( n ) + 4 ⁢ I _ S ⁢ I _ L I I 2 + I Q 2 _ = 4 ⁢ I _ S ⁢ I _ L and Ī S (n) and Ī L (n) denote a DC component of I S (n) and I L (n) respectively, and I I 2 +I q 2 denotes a DC component of I I 2 (n)=I q 2 (n), and, the first-order approximation of Ĩ S (n) is represented by: I ~ S ( 1 ) ⁡ ( n ) = I I 2 ⁡ ( n ) + I Q 2 ⁡ ( n ) - I I 2 + I Q 2 _ 4 ⁢ I _ L and, a known local-oscillator-to-signal power ratio (LOSPR), defined as P L / P S , is approximated as Ī L /Ī S such that Ī L is represented by: I _ L ≈ I I 2 + I Q 2 _ · LOSPR 2 .