Abstract
We propose a new scheme of trace gas detection by the use of a Fabry–Perot cavity (FPC) and the wavelength modulation technique. Optical propagation length is enhanced using a 25 cm FPC with a finesse of ∼100. By applying the same modulation to both the laser and cavity frequency, we observe the first derivative signal of acetylene absorption at 1.5 µm. It is found that a sample-and-hold circuit serves well for restoring the signal level, which otherwise is degraded because of the pulsation of the detected laser intensity due to a feedback loop required for scanning the laser and cavity frequency simultaneously. Compared with the signal intensity observed with a 20 cm Brewster cell, this experiment yields the improvement of signal intensity by a factor of 81.2 with an acetylene pressure of 2.6 Pa.
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