Enhanced transmission capacity for laser communication at the single-photon level using the multi-channel frequency coding scheme

The statistical properties of a radiation sources are commonly characterized by second-order-correlation or Mandel parameter. Our research found that the single photons modulation spectrum provides us another optional way which is more sensitive to the high frequency information contained in the photon sequence. In this paper, we present direct laser communication by using a multi-channel frequency coding scheme based on the single photons modulation spectrum in which the multi-frequency modulation makes the transmission capacity efficiently enhanced. The modulation frequencies could be operated in a wide band without frequency aliasing due to the inherent randomness of photons arrival time of weak coherent light. The error rate less than 10-5 has been achieved experimentally when the mean signal photon count is 80 kcps. The modulated coherent light field shows nonlinear effects of single photons modulation spectrum. The studies of statistical properties of the single photons modulation spectrum, including the dependence of mean noise photon count, integration time, channel spacing and the number of frequency component, helped us to optimize the error rate and transmission capacity.