Amplified frequency double-shifting loop enabled frequency-stepped pulse train for direct time domain CO2 measurement

We have developed an amplified frequency double-shifting loop with a bidirectionally operated acousto-optic modulator as the in-loop frequency shifter to generate a frequency-stepped pulse train (FSPT) with frequency spacing twice as much as its operation frequency. Based on this configuration, a FSPT with 51 equidistant optical frequencies and 800 MHz frequency spacing around 1572 nm was generated, covering the whole R16 absorption peak of C O 2 for direct spectral measurement in the time domain. The retrieved transmission curve from a C O 2 gas cell with concentration of 4.02% is in good agreement with the calculated Voigt lineshape from the HITRAN database. The maximal relative errors between the measured and calculated results are within 0.5% for each optical frequency, indicating its great potential for practical C O 2 monitoring.