Zero-background tunable diode laser absorption spectroscopies using balanced interferometers

Tunable diode laser spectroscopy (TDLS) is one of the most successful techniques for trace-gas environmental monitoring. By modulating the light source at high frequency for suppressing the noise, frequency modulation or wavelength modulation TDLS can measure optical absorption in atomic or molecular samples with high resolution and sensitivity. Apart from these absorptive techniques, there exists a group of zero-background spectroscopic methods, e.g. laser-induced fluorescence spectroscopy, photo-acoustic spectroscopy, and polarization spectroscopy. Here a spectroscopic signal rises from a zero or low background with little noise and these methods therefore can achieve good signal-to-noise ratio (SNR) even without any modulation on the light source.