Theoretical Considerations of the Three-Level Maser Detector for the Far-Infrared

In the case when the pumping transition in a three-level system of a molecule does not exactly coincide with the output frequency of a maser oscillator, four different types of operation at off-resonance of the three-level maser detector are discussed. At first the use of pressure broadening is considered; secondly a method which observes the double-quantum transition and thirdly the other method which measures a change in population caused by the double-quantum pumping are considered, and finally the level shift due to the far-infrared input is considered. It is found that the third method of double-quantum pumping in which three waves are involved is the most sensitive. The minimum detectable power of order 10-6W/cm2 with the resolving time of about 10-6s is expected in detecting the maser output at the far-infrared which differs from the resonant frequency of the detector by as large as 10 GHz.