The receiver structure described here improves upon the performance of optical communications links, achieving high sensitivity and high data rates even with extremely weak optical signals.
The proposed quantum receiver in this innovation partitions each binary signal interval into two unequal segments: a short “pre-measurement” segment in the beginning of the symbol interval used to make an initial guess with better probability than 50/50 guessing, and a much longer segment used to make the highsensitivity signal detection via field-cancellation and photon-counting detection. It was found that by assigning as little as 10% of the total signal energy to the pre-measurement segment, the initial 50/50 guess can be improved to about 70/30, using the best available measurements such as classical coherent or “optimized Kennedy” detection.
However, 70% detection probability (or, equivalently, 30% error probability) is not good enough for communications, even with the most powerful codes, which require error probabilities in the 0.01–0.1 range to achieve the desired coded performance. Due to the requirement to maintain a constantenvelope local laser field, the recently reported “optimized Kennedy” measurement was selected for making this initial guess. The outcome of this first measurement is used to decide which signal the receiver should try to null. Hence, the local field envelope and phase are adjusted to nearly cancel the more likely signal, and photon counting is used for the rest of the interval to confirm this initial decision. If the wrong signal is selected initially, then the local laser adds instead of subtracting a constant matched laser field to the received signal, yielding a higher probability error; that is, higher probability of erroneously pre-selecting the “other” binary signal. Optimum partitioning of the signal interval is critical, and must be carried out for each new value of Ks. This concept can be extended directly to more than two intervals, by partitioning the first interval itself into two segments, optimized for the smaller initial energy, to further improve the “pre-measurement” upon which the final high-sensitivity measurement strategy is based.
The performance gain of the partitioned- interval quantum receiver over the well-known Kennedy receiver detection strategy is shown in Figure 2, along with the gains of the classical coherent receiver and the optimized Kennedy receiver. It is noted that the coherent receiver peaks at an average received photon-count of Ks = 0.095 attaining a maximum gain of 1.272 over the Kennedy receiver, whereas the optimized Kennedy receiver peaks at Ks = 0.165, with a maximum gain of 1.381, after which both gains decrease as the average signal energy increases: the optimized Kennedy receiver approaches 1 at high signal energies, reverting back to the conventional Kennedy receiver, whereas the coherent receiver continues towards zero. However, the partitioned-interval receiver described here attains higher gains, and tends to maintain these gains near their maximum value even with increasing signal energy.
The only receiver structure known to achieve the quantum limit theoretically on binary signal detection (the curves labeled “Helstrom bound” in Figures 1 and 2) is known as the Dolinar receiver. This approach applies a rapidly timevarying local laser field to the signal during each bit-interval, but such time-varying fields are difficult to generate in practice at high data rates. In addition, the phase and sign of the local laser fields must be switched instantaneously with the detection of each new photon for best performance, placing significant burdens on the processing speed of the receiver and on the response of the local laser. The proposed solution overcomes these problems by employing constant local laser intensities that can be precomputed based on estimates of signal-strength, while attaining nearly the same bit-error rate as the more complex quantum-optimum receiver. The solution proposed here will therefore enable high-sensitivity deep-space optical communications at data rates up to gigabits/second as required for future deep-space optical communications.
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