Use of lasers in modulation spectroscopy

A theoretical analysis is made of the sensitivity of modulation spectroscopy using single-mode solid-state and gas lasers, and various types of photodetectors. The analysis is based on calculations of the signal-to-noise ratio allowing for laser and photodetector noise. It is shown that the sensitivity of modulation spectroscopy is essentially governed by the photodetector noise. For a photodiode the smallest detectable relative fluctuations of the absorption are ~2×10^–8 whilst those of the refractive index and birefringence are ~4×10^–12. It is demonstrated that intracavity modulation spectroscopy using a solid-state laser can raise the sensitivity by a factor of 10–50 for a photodiode and by a factor of 10³–10⁴ for a photomultiplier, as compared with the extracavity technique. For gas lasers there is no improvement using photodiodes but an approximately sixtyfold improvement with photomultipliers.