Optical frequency comb-based coherent LIDAR
A coherent laser radar that uses two coherent femtosecond fiber lasers to perform absolute ranging at long distance. One coherent femtosecond fiber lasers acts as a source and the other as a local oscillator for heterodyne detection of the return signal from a cooperative target. The system simultaneously returns a time-of-flight range measurement for coarse ranging and an interferometric range measurement for fine ranging which is insensitive to spurious reflections that can cause systematic errors. The range is measured with at least 3 μm precision in 200 μs and 5 nm precision in 60 ms over a 1.5 m ambiguity range. This ambiguity range can be extended to 30 km through reversal of signal and LO source roles.
1. A method of comb-based coherent Light Detection and Ranging (LIDAR) comprising:
generating a signal comb that transmits a signal pulse train reflected from a reference and a target to generate a target signal from the target and a reference signal from the reference;
generating a local oscillator (LO) comb that transmits a LO pulse train at a slightly different repetition rate than the signal pulse train with a coherent optical carrier;
detecting the reflected signal pulse train through linear optical sampling against the LO pulse train in which consecutive samples of an overlap between a signal pulse from the target signal and the references signal, and a LO pulse from the LO pulse train yields a high-resolution measurement of the target signal and reference signal;
Fourier transforming the target signal to extract the optical phase vs. frequency of the target signal;
Fourier transforming the reference signal to extract the optical phase vs. frequency of the reference signal;
determining a frequency dependant optical phase difference between the optical phase of the target signal and the optical phase of the reference signal;
fitting the optical phase difference to a straight line;
obtaining a time-of-flight distance measurement from a slope of the straight light and the known group velocity of light at the frequency of the coherent optical carrier;
obtaining an interferometric distance measurement between the target and the reference from an intercept of the straight line and the wavelength of the coherent optical carrier.
2. A method as recited in claim 1 , wherein the time-of-flight measurement is repeated and averaged until a desired uncertainty of the time-of-flight distance measurement is achieved.
3. A method as recited in claim 1 , wherein the time-of-flight measurement is repeated until an uncertainty of the time-of-flight distance measurement is below half the wavelength of optical carrier.
4. A method as recited in claim 3 , wherein the time-of-flight measurement is repeated to remove an ambiguity of the interferometric distance measurement.
5. A method as recited in claim 1 , wherein generating the signal comb that transmits the signal pulse train reflected from the reference and the target with a first mode locked laser.
6. A method as recited in claim 5 , wherein generating the local oscillator (LO) comb that transmits the LO pulse train with a second mode locked laser.
7. A method as recited in claim 6 , wherein the coherent optical carrier is generated by phase locking the signal comb and the LO comb with a common frequency reference that is a continuous wave laser.
8. A Light Detection and Ranging (LIDAR) comprising:
a first mode locked laser which generates a signal comb that transmits a signal pulse train reflected from a reference and a target subject to a distance measurement relative the reference;
a second mode locked laser that generates a local oscillator (LO) comb;
a first continuous wave laser and a second continuous wave laser stabilized to an optical reference cavity, said first continuous wave laser phase locked at a first tooth of the signal comb and the LO comb, said second continuous wave laser phase locked at a second tooth of the signal comb and the LO comb different than the first tooth to relate a distance between the reference and the target to a known distance of said optical reference cavity;
a beamsplitter including first and second output ports, said beamsplitter being arranged to receive and combine said signal comb, or said LO comb with said first continuous wave laser and said second continuous wave laser and direct a first output from said first output port to said target; and
wherein said beamsplitter is arranged to direct a second output from said second output port to a 2-channel, optical filter with each channel centered on one of said continuous wave lasers.
9. A LIDAR as recited in claim 8 , wherein the signal comb and the LO comb are phase locked with said first continuous wave laser such that their optical carriers are mutually coherent and phase locked with said second continuous wave laser such that their repetition rates have a slight, known difference.
10. A LIDAR as recited in claim 8 , further comprising two detectors that are each operable to detect a beat signal between a respective one of said combs and a respective one of said continuous wave lasers.
11. A LIDAR as recited in claim 10 , further comprising phaselock electronics arranged to receive said second output from said detectors and generate an error signal for feedback to said frequency combs.
12. A LIDAR as recited in claim 8 , wherein said beamsplitter is a polarizing beamsplitter.
13. A LIDAR as recited in claim 12 , wherein said beamsplitter is a polarization insensitive beamsplitter of any ratio with regard, respectively, to said first output and said second output.