Dual frequency FMCW lidar and method
A lidar including a laser having a first frequency-modulated laser radiation and a second frequency-modulated laser radiation, a first waveguide coupled to the laser, wherein the first frequency-modulated laser radiation and the second frequency-modulated laser radiation are transmitted by the laser into the first waveguide, a second waveguide, a filter coupled between the first waveguide and the second waveguide, wherein the filter is configured to couple and pass the first frequency-modulated laser radiation through the filter to the second waveguide, and is configured to not couple or pass the second frequency-modulated laser radiation through the filter to the second waveguide, and a photodetector coupled to the second waveguide.
1. A lidar comprising:
a laser having a first frequency-modulated laser radiation and a second frequency-modulated laser radiation;
a first waveguide coupled to the laser, wherein the first frequency-modulated laser radiation and the second frequency-modul-ated laser radiation are transmitted by the laser into the first waveguide;
a second waveguide;
a filter coupled between the first waveguide and the second waveguide, wherein the filter is configured to couple and pass the first frequency-modulated laser radiation through the filter to the second waveguide, and is configured to not couple or pass the second frequency-modulated laser radiation through the filter to the second waveguide; and
a photodetector coupled to the second waveguide;
wherein the first frequency-modulated laser radiation comprises a linearly chirped ramp modulation v 1m ;
wherein the second frequency-modulated laser radiation comprises a linearly chirped ramp modulation v 2m ;
wherein a waveform received by the second waveguide comprises a scattered radiation of v 2m from an object shifted by a Doppler shift f D caused by a velocity of the object so that the received waveform is ν 2m +f IF ±f D ; and
wherein the photodetector mixes the linearly chirped ramp modulation v 1m with the received waveform ν 2m +f IF ±f D to produce a beat frequency Δν+f IF ±f D , where Δν=ν 1m −ν 2m ;
where
f
IF
=
B
Δ
t
T
,
Δ t= 2 L/c,
L is a distance to the object,
c is a speed of the light,
B is a bandwidth or range of frequency modulation,
T is a period of frequency sweeping; and
L
=
cTf
IF
2
B
.
2. The lidar of claim 1 further comprising:
a first collimator coupled to the first waveguide; and
a second collimator coupled to the second waveguide.
3. The lidar of claim 1 wherein:
the laser has at least two longitudinal modes.
4. The lidar of claim 1 wherein:
the laser comprises a semiconductor laser, a laser diode, a quantum cascade laser, or an optical fiber laser.
5. The lidar of claim 1 :
wherein Δν=ν 1m −ν 2m is equal to c/(2nL c ), where c is a speed of light, n is a refractive index of a cavity material of the laser, and L c is a cavity length of the laser.
6. The lidar of claim 5 :
wherein an optical length of the cavity nL c is greater than 5 mm, so that Δν=ν 1m −ν 2m is equal to or less than 30 GHz.
7. The lidar of claim 1 wherein:
the filter comprises an optical microresonator, a microring resonator, a plurality of mutually coupled ring resonators, or a plurality of coupling microresonators.
8. The lidar of claim 1 :
wherein the waveform received by the second waveguide comprises a scattered radiation from an object shifted by a Doppler shift caused by a velocity of the object; and
wherein the photodetector mixes the first frequency-modulated laser radiation with a waveform received by the second waveguide to produce a beat frequency.
9. A method of frequency-modulated continuous-wave (FMCW) detection of range and velocity of an object comprising:
transmission of a first frequency-modulated laser radiation from a laser and a second frequency-modulated laser radiation from the laser through a first waveguide;
redirection by a filter of the first frequency-modulated laser radiation into a second waveguide;
outcoupling the second frequency-modulated laser radiation from the first waveguide for exposure of an object;
coupling a received waveform of scattered radiation from the object into the second waveguide; and
mixing the received scattered radiation with the first frequency-modulated laser radiation;
wherein the first frequency-modulated laser radiation comprises a linearly chirped ramp modulation v 1m ;
wherein the second frequency-modulated laser radiation comprises a linearly chirped ramp modulation v 2m ;
wherein a waveform received by the second waveguide comprises a scattered radiation of v 2m from an object shifted by a Doppler shift f D caused by a velocity of the object so that the received waveform is ν 2m +f IF ±f D ; and
wherein the photodetector mixes the linearly chirped ramp modulation v 1m with the received waveform ν 2m +f IF ±f D to produce a beat frequency Δν+f IF ±f D , where Δν=ν 1m −ν 2m ;
where
f
IF
=
B
Δ
t
T
,
Δ t= 2 L/c,
L is a distance to the object,
c is a speed of the light,
B is a bandwidth or range of frequency modulation,
T is a period of frequency sweeping; and
L
=
cTf
IF
2
B
.
10. The method of claim 9 :
wherein Δν=ν 1m −ν 2m is equal to c/(2nL c ), where c is a speed of light, n is a refractive index of a cavity material of the laser, and L c is a cavity length of the laser.
11. The method of claim 10 :
wherein an optical length of the cavity nL c is greater than 5 mm, so that Δν=ν 1m −ν 2m is equal to or less than 30 GHz.
12. The method of claim 9 further comprising:
determining the velocity of the object by making periodic measurements of f IF ±f D .
13. The method of claim 9 further comprising:
spectral measurement of a beat frequency from the mixed received scattered radiation and the first frequency-modulated laser radiation; and
derivation of range and velocity of the object.
14. The method of claim 9 wherein:
the laser comprises a semiconductor laser, a laser diode, a quantum cascade laser, or an optical fiber laser.
15. The method of claim 9 wherein:
the filter comprises an optical microresonator, a microring resonator, a plurality of mutually coupled ring resonators, or a plurality of coupling microresonators.
16. The method of claim 9 :
wherein the received scattered radiation comprises a Doppler shift caused by a velocity of the object; and
wherein a photodetector mixes the received scattered radiation with the first frequency-modulated laser radiation to produce a beat frequency.
17. A lidar comprising:
a laser having a first frequency-modulated laser radiation and a second frequency-modulated laser radiation;
a first waveguide coupled to the laser, wherein the first frequency-modulated laser radiation and the second frequency-modulated laser radiation are transmitted by the laser into the first waveguide;
a second waveguide;
a filter coupled between the first waveguide and the second waveguide, wherein the filter is configured to couple and pass the first frequency-modulated laser radiation through the filter to the second waveguide, and is configured to not couple or pass the second frequency-modulated laser radiation through the filter to the second waveguide;
a third waveguide; and
a photodetector coupled to the second waveguide and to the third waveguide;
wherein the first frequency-modulated laser radiation comprises a linearly chirped ramp modulation v 1m ;
wherein the second frequency-modulated laser radiation comprises a linearly chirped ramp modulation v 2m ;
wherein a waveform received by the third waveguide comprises a scattered radiation of v 2m from an object shifted by a Doppler shift f D caused by a velocity of the object so that the received waveform is ν 2m +f IF ±f D ; and
wherein the photodetector mixes the linearly chirped ramp modulation v 1m with the received waveform on the third waveguide ν 2m +f IF +f D to produce a beat frequency Δν+f IF ±f D , where Δν=ν 1m −ν 2m ;
where
f
IF
=
B
Δ
t
T
,
Δ t= 2 L/c,
L is a distance to the object,
c is a speed of the light,
B is a bandwidth or range of frequency modulation,
T is a period of frequency sweeping; and
L
=
cTf
IF
2
B
.
18. The lidar of claim 17 further comprising:
a first collimator coupled to the first waveguide; and
a second collimator coupled to the third waveguide.
19. The lidar of claim 17 wherein:
the laser has at least two longitudinal modes.
20. The lidar of claim 17 wherein:
the laser comprises a semiconductor laser, a laser diode, a quantum cascade laser, or an optical fiber laser.
21. The lidar of claim 17 :
wherein Δν=ν 1m −ν 2m is equal to c/(2nL c ), where c is a speed of light, n is a refractive index of a cavity material of the laser, and L c is a cavity length of the laser.
22. The lidar of claim 21 :
wherein an optical length of the cavity nL c is greater than 5 mm, so that Δν=ν 1m −ν 2m is equal to or less than 30 GHz.
23. The lidar of claim 17 wherein:
the filter comprises an optical microresonator, a microring resonator, a plurality of mutually coupled ring resonators, or a plurality of coupling microresonators.
24. The lidar of claim 17 :
wherein the waveform received by the third waveguide comprises a scattered radiation from an object shifted by a Doppler shift caused by a velocity of the object; and
wherein the photodetector mixes the first frequency-modulated laser radiation with a waveform received by the second waveguide to produce a beat frequency.