Multiple quantum heterodyne detection

From the foregoing, it is clear that multiple-quantum heterodyne detection is somewhat more complex than the analogous single-quantum process. In particular, the average detector response and the SNR are found to depend on the higher-order correlation functions of the radiation field and on the LO irradiance. From a physical point of view, it has been possible to associate various terms in the detected current with specific kinds of photon absorptions. Calculations of the SNR and MDP for a number of cases have been carried out. The results of a preliminary two-quantum photomixing experiment are in agreement with the theory. 

After briefly considering the relevant results pertinent to multiple-quantum direct detection (Sec. 7.2.1), we derive the combination device response for the general multiple-quantum photomixing process (including the important two-quantum case) in Section 7.12. In Section 7.2.3, we obtain the SNR for a receiver using a multiphoton optical heterodyne device, and compare it with the SNR for conventional optical heterodyne detection. The results of a two-photon experiment are presented in Section 7.2.4, while a suggested setup for future experiments, as well as the applicability of the scheme in general, is reserved for Section 7.2.5. 

Since multiple-quantum optical direct detection has also been studied in great detail, it seems natural to investigate the behavior of such a detector in the presence of more than one frequency [7.16]. In this section, we obtain the response and the signal-to-noise power ratio (SNR) for a multiple-quantum absorption heterodyne receiver, with particular attention devoted to the simplest case, i.e., the mixing of two waves in a double-quantum infrared or optical device. 

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