Abstract
Polarization-modulated optical signals can be demodulated via direct detection of Stokes vectors. In this article, we develop a quantum theory of noise in the Stokes vector receiver. Examining the conditions that enable simultaneous measurement of three elements of the Stokes vector, we demonstrate that an optical pre-amplifier in front of the Stokes vector receiver can reduce the power penalty associated with simultaneous measurements, down to the quantum limit of 3 dB. Then, the 3-D Gaussian noise model for the shot noise and optical pre-amplifier noise is discussed. Subsequently, based on the noise analyses, we derive bit error rate formulae for the cubic-lattice polarization modulation format and evaluate the sensitivity of the Stokes vector receiver.
Highlights
A STOKES vector receiver with low-complexity digital signal processing (DSP) circuits can track random fluctuations in the state of polarization (SOP) of the optical signal, even if it is based on direct detection [1], [2]
The following crucial problems regarding its performance remain unsolved [8], [9]: 1) the shot noise is the most fundamental quantum noise generated in direct detection receivers, it has not been demonstrated how the shot noise distributes on the 3D Stokes vector
Such a cubic-lattice modulation format is the 3D extension of the 2D quadrature amplitude modulation (QAM) format, which is most commonly used in coherent optical communication systems
Summary
A STOKES vector receiver with low-complexity digital signal processing (DSP) circuits can track random fluctuations in the state of polarization (SOP) of the optical signal, even if it is based on direct detection [1], [2] Such a receiver facilitates the employment of the multi-level SOP-modulation format, in which constellations are designed in the 3-dimensional (3D) Stokes space. 4) The theoretical sensitivity limit of the Stokes vector receiver for the most basic cubic-lattice SOP modulation format has not yet been demonstrated.
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