Use of a Fabry–Perot/monochromator system for enhancement of the spectral line-to-noise power ratio from line and continuum sources

A Fabry–Perot (FP) interferometer, used in conjunction with a conventional double-slit monochromator (DSM), is shown to yield a spectral line-to-(photon) noise power ratio (L/N) that is greater than the ratio for the DSM alone. The effects of the double-slit, étalon, and fringe-limiting-aperture functions upon the transmitted powers of the spectral line and continuum are derived. Measurements of the power transmitted through the DSM + FP and the DSM systems are made with a source that consists of a high-pressure D2 continuum superimposed upon a narrow Ne i 3520 Å line from a hollow-cathode discharge. When the spacer thickness l, slit widths s, and radiation source are varied, the measured and calculated values of several power ratios compare favorably. An L/N ratio is defined that is proportional to PL/(PL + 2PC)1/2, where PL and PC are the peaks of the transmitted line and continuum powers, respectively. Curves are presented that show the dependences of PL, PC, and the L/N ratio upon l and the plate reflectance R in the DSM + FP system. Typically, for slit widths equal to 0.028 cm, a plate-defect finesse (ND) equal to 21, absorptance (Â) equal to 0.02, and for continuum–radiation noise alone, the L/N-ratio curves achieve maxima at l = 0.02 cm and at R = 0.84. Under these conditions, the L/N ratio of the DSM + FP system is larger than that of the DSM alone by a factor of approximately 2.1. With improved plate properties, e.g., ND = 100 and  = 0.004, the foregoing factor becomes 4.4.