Profiles of Lamb dip and their first derivative curves on 43μm fluorescence from a dilute CO
2 gas cell in a CO
2 laser cavity were observed. H
P-P, the difference of intensity between the peaks and Δν
P-P, the frequency width between the peaks of the derivative curve depended on the modulation amplitude, the pressure of CO
2 gas in the cell and the laser power. The Δν
P-P, max, which corresponds to the modulation amplitude for the maximum H
P-P, is smaller than the theoretical value. Rather narrower back ground of the fluorescence partly explains the reason. The pressure dependence of the H
P-P and the Δν
P-P was explained by assuming the Lorentzian profile for the dip. The power decrease causes the relative displacement of the dip from the center of the fluorescense and this seems one of the reasons for the decrease of the H
P-P and invariance of the Δν
P-P in the weak power.
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