Frequency modulation of a single-frequency laser with a linear triangular waveform is of great interest to many laser remote sensing and interferometry applications. Linear frequency modulation of a continuous-wave (CW) laser beam can make target distance measurements with great precision. If the frequency modulation is perfectly linear with time, such a laser radar (lidar) system will have higher resolution and accuracy compared with pulsed lasers.
The motivation for developing this technique was its application in a coherent Doppler lidar system that could enable precision safe landing on the Moon and Mars by providing accurate measurements of the vehicle altitude and velocity. The multifunctional coherent Doppler lidar is capable of providing high-resolution altitude, attitude angles, and ground velocity, and measuring atmospheric wind vector velocity. It can operate in CW or quasi-CW modes.
A low-power laser is used as a seed or master laser source. The seed laser can be either a fiber laser with volume grating or a semiconductor laser with an external cavity Bragg grating to generate a very narrow linewidth spectrum. The output of the seed laser is frequencymodulated by an electro-optical nonlinear device. Part of the laser beam is split for use as the local oscillator for optical heterodyne detection. The remaining part of the seed laser output is amplified by a single mode fiber amplifier to increase its power to several watts. The fiber amplifier output is divided into three or more parts, each expanded and transmitted by a lens toward different directions. The reflected laser radiation is collected by the same lenses and focused into optical fibers. Each individual returned signal passes through a transmit/receive switch that directs it to a detector where it is mixed with the local oscillator beam.
In the linear frequency modulation waveform of a laser beam, the modulation waveform has a triangular shape. The transmitted waveform is delayed by the light round-trip time, upon reflection from the target. When mixing the delayed return waveform with the transmitted waveform at the detector, an interference signal will be generated whose frequency is equal to the difference between the transmit and receive frequencies. This frequency is directly proportional to the target range. When the target or lidar platform is not stationary during the beam round-trip time, the signal frequency will be shifted due to the Doppler effect. Therefore, by measuring the frequency during “up chirp” and “down chirp” periods of the laser waveform, both the target range and velocity can be determined.