Longitudinally resolved temperature measurement in diode-pumped alkali lasers via dual-wavelength CH4 absorption spectroscopy with one-step fitting.

Temperature rise in the alkali-vapor gain medium critically limits the output performance of diode-pumped alkali lasers (DPALs) and amplifiers (DPAAs). Here we report the first longitudinal spatially resolved temperature measurements in operating rubidium DPAL and DPAA systems, achieved via a cross-beam near-infrared tunable diode laser absorption spectroscopy sensor with 1 mm spatial resolution. A dual-wavelength one-step fitting algorithm jointly constrains temperature retrieval from methane lines at 1650.9 nm and 1653.7 nm, enforcing a shared temperature parameter to enhance fitting robustness. The static temperature measurement standard deviation is 0.97 K, and the dynamic measurement standard deviation under pump loading is 1.90 K. Retrieved thermal profiles, parameterized by an asymmetric double-Gaussian model, are qualitatively consistent with the 780-nm pump fluorescence intensity distributions. These spatially resolved heat source terms furnish quantitative experimental constraints for validating existing DPAL thermal models. The dual-wavelength one-step fitting algorithm is readily extendable to other absorption spectroscopy thermometry applications.