Effects of amplitude and frequency modulation with diode lasers, using higher harmonic detection

Wavelength modulation spectroscopy with diode lasers has been used by many researchers to perform sensitive measurements in gaseous media. However, there is one distinct complication that often arises from the use of diode lasers which is that frequency modulation, performed by modulation of the injected current, is accompanied by simultaneous amplitude modulation. In this paper, we treat the effects of simultaneous amplitude and frequency modulation and obtain a complete expression (accounting also for the phase difference between the two types of modulation), for the signal in a modulation spectroscopic experiment in which phase sensitive detection is performed at any harmonic of the modulation frequency.