Feedback stabilized tandem Fabry-Perot interferometer

A new system for measuring the isotopic ratio of uranium, in which two plane-type Fabry-Perot interferometers (tandem FP) are connected in series. The parallelism between the two FPs is achieved automatically by a feedback control mechanism based on laser interference fringe monitoring. The structure of the tandem FP, feedback control system, automatic parallelism adjustment mechanism and wavelength synchronization mechanism are described in detail. For experiments, a hollow cathode discharge tube of a pulse discharge type is employed. Measurements are made to determine the effects of pulse width on the 238U peak height of 502.7 nm line, recorder traces of 235U and 238U lines, half width for 238U component of the 502.7 nm line, SN ratio, reproducibility of the 235U/238U peak height ratio and 235U/238U intensity ratio. Considerations are made on the spectral line width, contrast, transmission factor, and stability of automatic parallelism control and wavelength synchronization. Results obtained indicates that a single-type interferometer would serve adequately for measuring the 235U/238U ratio if the automatic parallelism control developed here is used. The ultimate object of the tandem system is to make measurement of 236U. Satisfactory results have not obtained as yet, but most likely the present system would make it possible if a light source of a higher intensity and advanced photometric techniques are developed. (Nogami, K.)