1. A coherent optical detection system receiving an input optical signal containing digital information, the input optical signal having signal phase noise and additive noise, the coherent optical detection system comprising: a local oscillator laser, the local oscillator later having local oscillator phase noise; and a digital signal processor, the digital signal processor having a parallel architecture; wherein: the digital signal processor applies a nonlinear function to values of the complex electric field of the input optical signal to produce nonlinear function output values; and the digital signal processor applies a filter to values derived from the nonlinear function output values, said filter being close to a Wiener filter appropriate to the additive noise, signal phase noise and local oscillator phase noise; whereby said digital signal processor estimates the phase of the input optical signal compared to light from the local oscillator.