Three-frequency heterodyne detection

Finally, we observe that the use of a two-quantum photomixer in a three-frequency nonlinear heterodyne detection receiver would result in a reduction of the SNR by the factor corresponding to the absorption of 2... 

Three-Frequency Single-Photon Heterodyne Detection Using a Nonlinear Device... 

It is of interest to examine the operation of the three-frequency nonlinear heterodyne system in a variety of configurations [7.59] different from those assumed earlier. In this section, we consider the behavior of the system under the following conditions 1) at zero frequency (dc), 2) without a final bandpass filter, 3) with increased Doppler information, 4) as an optimum system with no uncertainty in Doppler shift, and 5) with a vth law nonlinear device other than square-law. We also consider the consequences of four-frequency nonlinear heterodyne detection this will be examined in greater detail in Section 7.4. 

As an example of three-frequency nonlinear heterodyne detection, we consider a CO2 laser radar operating at 10.6 (un in the infrared [7.14,61,62] (see Fig. 7.3). If we assume that we wish to acquire and track a 1-m-radius satellite with a... 

Whereas the previous section (7.3,8) was concerned with the calculation of system performance for the vacuum channel, we now turn to the error probabilities for three-frequency nonlinear heterodyne detection for the atmospheric channel. The behavior of the clear-air turbulent atmosphere as a lognormal channel for optical radiation has been well documented both theoretically and experimentally [7.76-78, 80-82], We therefore choose the amplitudes /4i and A 2 to be lognormally distributed, and the phases < j and (j>2 to be uniformly distributed over (0,27t). Since A ocAj and while... 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

Finally, we observe that the use of a two-quantum photomixer in a three-frequency nonlinear heterodyne detection receiver would result in a reduction of the SNR by the factor PJP2), corresponding to the absorption of 2 nonmonochromatic photons as discussed earlier. It therefore does not appear to be suitable for this application. The next section is devoted to a discussion of the three-frequency technique, but using a single-photon detector, in which case the nonlinearity is derived from a circuit element rather than from a multiphoton process and the (undesirable) reduction factor does not appear. 

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