Frequency stabilization of distributed-feedback laser diodes at 1572 nm for lidar measurements of atmospheric carbon dioxide

We demonstrate a wavelength-locked laser source that rapidly steps through six wavelengths distributed across a $1572.335\mathrm{nm}$ carbon dioxide (${\mathrm{CO}}_2$) absorption line to allow precise measurements of atmospheric ${\mathrm{CO}}_2$ absorption. A distributed-feedback laser diode (DFB-LD) was frequency-locked to the ${\mathrm{CO}}_2$ line center by using a frequency modulation technique, limiting its peak-to-peak frequency drift to $0.3\mathrm{MHz}$ at $0.8\mathrm{s}$ averaging time over 72 hours. Four online DFB-LDs were then offset locked to this laser using phase-locked loops, retaining virtually the same absolute frequency stability. These online and two offline DFB-LDs were subsequently amplitude switched and combined. This produced a precise wavelength-stepped laser pulse train, to be amplified for ${\mathrm{CO}}_2$ measurements.