Frequency-stepwise pulse train with switchable frequency spacing for CO₂ spectroscopy

We report a frequency stepwise pulse train (FSPT) generation system based on an amplified frequency shifting loop (AFSL) with switchable frequency spacing. The frequency spacing switching was achieved with a composite module with two acousto-optic modulators, providing different frequency shifts per round trip. In this way, one single frequency laser pulse can be extended to an equidistant pulse train in the time domain and the generated pulses possess frequency stepwise behavior with nonuniform spacing in the optical frequency domain. In a FSPT generation prototype, 52 pulses with different frequency spacing of 800 MHz and 200 MHz, covering the two absorption wings and the peak of the R16 line for CO₂, respectively, were obtained with the specially designed AFSL. Both static and dynamic CO₂ spectroscopy were carried out to verify the performance of the FSPT with switchable frequency spacing. Good agreement was obtained between the retrieved spectral transmittance and theoretical curves calculated from the HITRAN database. It is believed that such a FSPT generation method with switchable frequency spacing has paved a promising path towards practical spectroscopy, especially for varying atmosphere such as laser occultation.