Frequency-locked single-frequency 2 micron fiber laser is proposed to be used for airborne/spaceborne coherent lidar measurements, i.e., Active Sensing of CO2 Emissions over Nights, Days, and Seasons. The laser is based on our proprietary fiber technology and extensive experience in fiber laser development, which features a compact, highly stable, frequency-stabilized light source. Advanced frequency-locking schemes for both center-line frequency-locking and offset-frequency locking is developed in the laser source to address the bandwidth issue associated with airborne and space-borne coherent lidar measurements. Important key concepts in the proposed laser have been successfully demonstrated in the Phase I effort. This Phase II program will focus on the development of prototype units of two frequency-locked lasers at a specific wavelength of NASA interest. The prototype units will be delivered to NASA Langley Research Center at the end of this Phase II program for evaluation test.