Sensitive Microwave Spectrometer Using MW Double Resonance Modulation and Two Confocal Open Resonators

One of the major sensitivity limitations common to most microwave (MW) cavity spectrometers is due to the difficulty of introducing an effective modulation into a large cavity without degrading the MW characteristics. The described MW/MW double resonance spectrometer operating with two open resonators and a travelling wave amplifier appears to be a promising solution. The means used to obtain a good microwave isolation between pump and observing circuits, which must be better than when a klystron pumping source is used, are discussed. The electrical field vectors of pump and observing waves are perpendicular and the frequency of the modulated source is locked to the corresponding cavity one. This yields flexibility and stability over a wide frequency range. The influence of pump and observing electromagnetic fields amplitude upon signal amplitude has been widely studied: our design increases sensitivity by a factor of 20 over a conventional Stark modulation spectrometer and allows observing a double resonance signal at pressures as high as 500 mT.