Wavelength modulation of semiconductor lasers for absorption spectroscopy employing harmonic detection

Abstract

Wavelength modulation spectroscopy using semiconductor lasers is a sensitive tool for identifying species and measuring concentrations, velocities, and temperature in gaseous media. One advantage of modulation spectroscopy is that detection is carried out at frequencies far from the base-band noise, improving the signal-to-noise ratio. An additional benefit of performing wavelength modulation spectroscopy is that it allows for detection at harmonics of the modulation frequency. When a sinusoidal modulation is imposed, the harmonic detection signals resemble the frequency derivatives of the absorption profile, where the derivative order corresponds to the harmonic detection order. For this reason, the higher harmonic signals can be exploited to obtain more sensitive measurements of spectroscopic parameters, such as lineshape profile, absorption cross-section, temperature, and concentration. Wavelength modulation of semiconductor lasers, such as Fabry-Perot, vertical cavity, and external-cavity lasers, is discussed along with the advantages and disadvantages associated with each laser source. Applications of wavelength modulation spectroscopy are explored and it is shown that the higher harmonic detection signals exhibit structure and magnitude variations that are extremely useful in many spectroscopic applications.