Stacked modulation formats enabling highest-sensitivity optical free-space links

A new modulation scheme with a sensitivity of 2.3 photons per bit at a bit-error ratio (BER) of 10-3 is discussed theoretically and demonstrated experimentally. We achieve a limiting sensitivity of 2.3 photons per bit (3.7 dB photons per bit) by stacking the modulation formats 64PPM, 4FSK and polarization-switched (PS) QPSK. This modulation stack encodes 11 bit per symbol (PPM: 6 bit, FSK: 2 bit, PS-PQSK: 3 bit). We also replaced 4FSK by 2ODFM (2-channel multiplex) for comparison. With 64PPM-2OFDM-PS-QPSK a total of 12 bit are encoded (PPM: 6 bit, 2 OFDM channels with PS-QPSK: 2 x 3 bit). Both modulation stacks show a similar limiting sensitivity and are probably the highest sensitivities so far reported for a BER of 10-3, Our theoretical considerations are supported by simulations and experiments.