Scanning the Cavity

To measure the spectral line area it is necessary to sweep the source frequency across the absorption line of the gas contained in the cavity. The equivalent path length Zequiv in the cavity is given by 

This would be 32 m at 150 GHz in a cavity of Q = 10 that would have a 

One consequence of the high Q attained in these structures is that they become sharply tuned the system described above would show a FWHM of 1.5 MHz, comparable with the Doppler width of spectral lines in this region. Thus spectral lines viewed in a cavity may appear as an increased loss that lowers the Q at high pressures whereas at lower pressures their profile becomes distorted because the incident power density varies markedly with offset from the cavity resonant frequency. 

Quantum Electronics, John Wiley and Sons, London, 1975. 

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The main signal detection system was a commercial EG G phase-coherent detector that consisted of a broadband preamplifier of adjustable 30-60 dB gain that transmitted both 1 kHz and 2 kHz signals from the MMW detector, but was selectively tuned to 2 kHz. The outputs from this were split, with one half input to a home-made 1 kHz phase detector. This generated the piezoelectric actuator control signal that scanned the cavity. The other half went to the EG G precision low-noise 2 kHz phase sensitive amplifier that passed the spectral to the computer for processing and display (Figure 2.1). 

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