Slope-Baseline-Suppressed and Wavelength-Precise Scanning Off-Axis Integrated Cavity Output Spectroscopy with Active Locking for ¹³ CO ₂ / ¹² CO ₂ Isotope Measurement

Conventional off-axis integrated cavity output spectroscopy (OA-ICOS) measurements suffer from a tilted baseline in the <i>Y</i>-axis direction and nonlinear wavelength variations across the <i>X</i>-axis, both of which degrade measurement accuracy. Here, we present an improved OA-ICOS variant, featuring ramp-baseline immunity and wavelength-precise scanning capabilities, realized through active locking technology. A custom-designed wavelength meter with simple construction and rapid response was designed to enable precise wavelength measurement and feedback locking. Simultaneously, laser power amplification combined with feedback control ensures wavelength-independent uniform optical power. This approach delivers three significant advantages. First, the amplified probe light mitigates the throughput constraint in off-axis configurations and partially eases the demands on the photodetector for weak signal detection and amplification. Second, the flat spectral baseline reduces estimation errors associated with nonlinear ramp backgrounds while simultaneously simplifying signal processing and improving measurement accuracy. Finally, active wavelength locking delivers accurate wavelength acquisition, enabling precise spectral characterization. Using an off-axis configured optical cavity, as well as a near-infrared diode laser, the feasibility and practical efficacy of this active stabilization approach is experimentally validated by recording the spectra of ¹²CO₂ and ¹³CO₂ isotopologues in the laboratory and in the field.