NOVELTY - The light detection and ranging (Lidar) system comprises: a mode-locked fiber laser that generates an electromagnetic radiation comprising an optical frequency comb with a frequency comb spacing of a pulsed repetition rate of the mode-locked fiber laser; an acousto-optic modulator comprising an input and an output, where the electromagnetic radiation is coupled to the input of the acousto-optic modulator, and the acousto-optic modulator (310) is configured to down select the pulse repetition rate of the electromagnetic radiation; and a power amplifier (315) comprising an input and an output.    USE - The differential absorption Lidar system is useful for simultaneously measuring multiple gas components such as carbon dioxide or methane, in the atmosphere and its concentration variations over distance, where the Lidar is free optical frequency comb based differential absorption Lidar system.    ADVANTAGE - The Lidar system achieves accurate control of eight lasers and corresponding data/signal processing in an economic manner, provides long distance measurement with high sensitivity and accuracy, and is portable and robust to environmental changes.    DETAILED DESCRIPTION - The light detection and ranging (Lidar) system comprises: a mode-locked fiber laser that generates an electromagnetic radiation comprising an optical frequency comb with a frequency comb spacing of a pulsed repetition rate of the mode-locked fiber laser; an acousto-optic modulator comprising an input and an output, where the electromagnetic radiation is coupled to the input of the acousto-optic modulator, and the acousto-optic modulator (310) is configured to down select the pulse repetition rate of the electromagnetic radiation; a power amplifier (315) comprising an input and an output, where the input of the power amplifier is coupled to the output of the acousto-optic modulator, and the power amplifier is configured to amplify the down selected pulse repetition rate electromagnetic radiation; a collimator comprising an input and an output, where the input of the collimator is coupled to the output of the power amplifier, and the collimator is configured to collimate the amplified down selected pulse repetition rate electromagnetic radiation; an azimuth and elevation scanner (335) comprising an input and an output, where the input of the azimuth and elevation scanner is coupled to the output of the collimator, and the azimuth and elevation scanner is configured to steer the collimated amplified down selected pulse repetition rate electromagnetic radiation onto a target; a telescope comprising an input and an output, where the input of the telescope is configured to collect electromagnetic radiation backscattered from the target; a dense wavelength division multiplexing (DWDM) comprising an input and an output, where the input of the DWDM is coupled to the output of the telescope; a photon counting receiver (380) coupled to the output of the DWDM; a signal processing and control unit coupled to the photon counting receiver and configured to synchronize and gate the mode-locked fiber laser, the acousto-optic modulator and the photon counting receiver; and a computer (390) coupled to the signal processing and control unit, where the computer is configured to compare an on-resonant band intensity of the backscattered electromagnetic radiation with an off-resonant band intensity of the backscattered electromagnetic radiation. The pulse repetition rate of the electromagnetic radiation generated from the mode-locked fiber laser is 1-100 MHz. The pulse repetition rate of the down selected electromagnetic radiation is 1-100 kHz. The amplified down selected pulse repetition rate electromagnetic radiation comprises: a power of greater than 1 W; an energy of greater than 1 mu J; and a pulse width of 0.1 ps to 200 ns. The azimuth and elevation scanner comprises dual oscillating plane mirrors. The photon counting receiver comprises a single photon avalanche photodiode or a photomultiplier tube. The power amplifier comprises a doped fiber amplifier. The Lidar further comprises an OPO and/or an OPA that are configured to extend spectral wavelength ranges of the electromagnetic radiation. The amplified down selected pulse repetition rate electromagnetic radiation spans from near-infrared to mid-wave Infrared and/or longwave-infrared. An INDEPENDENT CLAIM is included for a method for operating a differential absorption Lidar.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an optical frequency comb based differential absorption Lidar system.    Acousto-optic modulator (310)    Power amplifier (315)    Azimuth and elevation scanner (335)    Photon counting receiver (380)    Computer. (390)