FMCW LiDAR methods and apparatuses including examples having feedback loops
Methods and apparatuses are described for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR). Examples are provided where high-closed-loop bandwidth, active feedback applied to laser frequency chirps may provide increases in the free-running laser coherence length for long-range FMCW distance measurements. Examples are provided that use an asymmetric sideband generator within an active feedback loop for higher closed-loop bandwidth. Examples of using a single shared reference interferometer within multiple active feedback loops that may be used for increasing the coherence length of multiple chirped lasers are described. Example calibrators are also described.
1. A method comprising:
applying an actuator signal to a laser source to provide a laser beam having a frequency and a free-running coherence length; and
controlling the actuator signal using a feedback loop to control a chirp of the frequency of the laser beam; and
wherein the feedback loop has a closed-loop bandwidth selected to cause the laser beam to be having an actual coherence length, wherein the actual coherence length is longer than the free-running coherence length, and wherein the closed-loop bandwidth is greater than a free-running laser linewidth of the laser beam divided by 10.
2. The method of claim 1 , wherein controlling the actuator signal comprises controlling the actuator signal to chirp the frequency of the laser beam linearly.
3. The method of claim 1 , wherein the feedback loop comprises splitting the laser beam into at least two optical paths and generating an interference signal based on the at least two optical paths.
4. The method of claim 1 wherein the actual coherence length is greater than a coherence length determined by a Schawlow Townes linewidth limit of the laser source.
5. A method comprising:
applying an actuator signal to a laser source to provide a laser beam having a frequency and a free-running coherence length; and
controlling the actuator signal using a feedback loop to control a chirp of the frequency of the laser beam; and
wherein the feedback loop has a closed-loop bandwidth selected to cause the laser beam to be having an actual coherence length, wherein the actual coherence length is longer than the free-running coherence length, wherein the feedback loop comprises splitting the laser beam into at least two optical paths and generating an interference signal based on the at least two optical paths, wherein controlling the actuator signal comprises controlling the actuator signal to chirp the frequency at a chirp rate, and wherein the closed-loop bandwidth is greater than one tenth of a product of the chirp rate and a difference between transit times of the at least two optical paths.