Multispecies Heterodyne Phase Sensitive Dispersion Spectroscopy over 80 nm Using a MEMS-VCSEL

Vertical cavity surface emitting lasers fabricated with movable microelectromechanical mirrors can offer continuous single-mode wavelength tuning up to 100 nm with high efficiency and in a very small package. Wide tunability makes these diode lasers ideally suited for multispecies and high-density gas analysis and the first demonstrations of direct absorption spectroscopy and wavelength modulation spectroscopy have already been published. The performance of these systems could nevertheless be enhanced by the use of the new molecular dispersion spectroscopic methods, as heterodyne phase sensitive dispersion spectroscopy. This technique bases its operation on the detection of the profile of the refractive index of the sample under analysis, in contrast to traditional architectures based on the measurement of optical absorption, and this provides noticeable advantages. First, the method is normalization-free, and therefore, the characteristic issue of the nonmonotonic intensity profile during wavelength tuning of tunable vertical cavity surface emitting lasers is directly overcome. In addition, dispersion spectroscopy also provides an intrinsic linearity with concentration, high suitability for calibration-free operation, and an extended dynamic range that are very desirable features to have in an optical gas analyzer. In this Letter we present the first multispecies spectrometer based on a widely tunable vertical cavity surface emitting laser and heterodyne phase sensitive dispersion spectroscopy that is capable of operating in a tuning range of more of 80 nm for the simultaneous detection of several species.