A series of experiments are carried out by current modulating a tunable diode laser, and slowly ramping the wavelength to scan weak absorption lines in gases at pressures ranging from 2 to 60 Torr. A lock-in amplifier detects the second harmonic (2f) of the modulation frequency, and the experimental 2f signals are compared with theory. Detailed measurements are made on Lorentzian, Voigt, and Gaussian line profiles, over a wide range of modulation amplitudes. Excellent agreement between experiment and calculation is obtained in all cases. This quantitative understanding enables one to derive true lineshapes and linewidths of very weak absorption lines from measurements of 2f lineshapes only. Results are applicable to trace gas detection using tunable diode lasers, and to other areas of spectroscopy and magnetic resonance where harmonic detection techniques are routinely employed to monitor weak signals.
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