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The theory is proposed which deals with physical grounds of a new type detection process. This means to use the following experimental conditions. A crystal platelet, e.g., CdS, is excited by optical or electron beam pumping at near or above visible laser generation threshold. If a surface of the platelet will be illuminated by two IR rays with different frequencies and angles of incidence, then the running wave of beatings between these rays will arise. It is assumed that the excitation is capable to create the sufficient concentration of spherical excitons having absorption lines with frequencies that correspond to incident IR radiation ones. Thus, the running wave of absorption and, consequently, the net laser gain change, moving with velocity v determined by difference of frequencies divided by wave vectors difference module for two IR (signal and reference) rays, appears. Here we have taken into consideration the possibility for spherical excitons at impurity atoms of the crystal to participate in IR-induced absorption and amplification of laser (visible) radiation due to indirect process. The amplitude and (or) frequency of the laser generation can be changed if initially the group velocity vg of the laser radiation equals to v. In the case, when the external probe laser beam is used, instead of intrinsic laser generation one, for read-out of the incident IR-radiation information, the output visible radiation will be only changed in the amplitude. As it follows from above, modulated in frequency coherent incident IR radiation will lead to amplitude modulated signal in visible range. The results show that this way open the possibilities to design one-mode fast operating excitonic FM and AM receiver with highest sensitivity that meet all the demands for free space laser communication and LIDARs in spectral interval near 3 mkm wavelength.
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