HYBRID PULSED/COHERENT LIDAR SYSTEM
In one embodiment, a lidar system includes a light source configured to emit (i) local-oscillator light and (ii) pulses of light, where each emitted pulse of light is coherent with a corresponding temporal portion of the local-oscillator light. The lidar system also includes a receiver configured to detect the local-oscillator light and a received pulse of light, the received pulse of light including a portion of one of the emitted pulses of light scattered by a target located a distance from the lidar system. The receiver includes a detector configured to produce a photocurrent signal corresponding to the local-oscillator light and the received pulse of light. The photocurrent signal includes a sum of a first term, a second term, and a third term.
1 . A lidar system comprising:
a light source configured to emit (i) local-oscillator light and (ii) pulses of light, wherein each emitted pulse of light is coherent with a corresponding temporal portion of the local-oscillator light;
a receiver configured to detect the local-oscillator light and a received pulse of light, the received pulse of light comprising a portion of one of the emitted pulses of light scattered by a target located a distance from the lidar system, wherein the receiver comprises:
a detector configured to produce a photocurrent signal corresponding to the local-oscillator light and the received pulse of light, the photocurrent signal comprising a sum of a first term, a second term, and a third term, wherein (i) the first term corresponds to an optical property of the received pulse of light, (ii) the second term corresponds to a coherent mixing of the local-oscillator light and the received pulse of light, and (iii) the third term corresponds to an optical property of the local-oscillator light; and
a pulse-detection circuit configured to determine a time-of-arrival for the received pulse of light based on the first term and the second term; and
a processor configured to determine the distance from the lidar system to the target based on the time-of-arrival for the received pulse of light.
2 . The lidar system of claim 1 , wherein the photocurrent signal is proportional to |ε Rx (t)+ε LO (t)| 2 , wherein:
ε Rx (t) represents an electric field of the received pulse of light; and
ε LO (t) represents an electric field of the local-oscillator light.
3 . The lidar system of claim 2 , wherein:
the first term corresponds to an optical power of the received pulse of light and is represented by |ε Rx (t)| 2 ;
the second term, which corresponds to the coherent mixing of the local-oscillator light and the received pulse of light, is represented by 2·|ε Rx (t)|·|ε LO (t)|·cos[Δω(t)·t+Δϕ(t)], wherein:
Δωo(t) represents a frequency difference between the electric field of the received pulse of light and the electric field of the local-oscillator light; and
Δϕ(t) represents a phase difference between the electric field of the received pulse of light and the electric field of the local-oscillator light; and
the third term corresponds to an optical power of the local-oscillator light and is represented by |ε LO (t))| 2 .
4 . The lidar system of claim 1 , wherein the second term is a coherent-mixing term that is proportional to a product of (i) an amplitude of an electric field of the received pulse of light and (ii) an amplitude of an electric field of the local-oscillator light.
5 . The lidar system of claim 4 , wherein the coherent-mixing term of the photocurrent signal is proportional to E Rx (t)·E LO (t)·cos[(ω Rx −ω LO )t+ϕ Rx (t)−ϕ LO (t)], wherein:
E Rx (t) represents the amplitude of the electric field of the received pulse of light;
E LO (t) represents the amplitude of the electric field of the local-oscillator light;
ω Rx represents a frequency of the electric field of the received pulse of light;
ω LO represents a frequency of the electric field of the local-oscillator light;
ϕ Rx (t) represents a phase of the electric field of the received pulse of light; and
ϕ LO (t) represents a phase of the electric field of the local-oscillator light.
6 . The lidar system of claim 1 , wherein, when the first term is greater than the second term, the receiver is configured to act as a pulsed-lidar receiver, wherein the pulse-detection circuit determines the time-of-arrival for the received pulse of light primarily based on the first term.
7 . The lidar system of claim 6 , wherein the first term being greater than the second term is associated with the distance to the target being less than a threshold distance or a reflectivity of the target being greater than a threshold reflectivity.
8 . The lidar system of claim 1 , wherein, when the second term is greater than the first term, the receiver is configured to act as a coherent-lidar receiver, wherein the pulse-detection circuit determines the time-of-arrival for the received pulse of light primarily based on the second term.
9 . The lidar system of claim 8 , wherein the second term being greater than the first term is associated with the distance to the target being greater than a threshold distance or a reflectivity of the target being less than a threshold reflectivity.
10 . The lidar system of claim 1 , wherein:
the optical property of the received pulse of light is an optical power, optical intensity, optical energy, or electric field of the received pulse of light; and
the optical property of the local-oscillator light is an optical power, optical intensity, optical energy, or electric field of the local-oscillator light.
11 . The lidar system of claim 1 , wherein the local-oscillator light and the received pulse of light are coherently mixed together at the receiver to produce the photocurrent signal.
12 . The lidar system of claim 1 , further comprising an optical combiner configured to:
combine the local-oscillator light and the received pulse of light to produce a combined beam comprising at least a portion of the local-oscillator light and at least a portion of the received pulse of light; and
direct the combined beam to the detector.
13 . The lidar system of claim 1 , wherein the detector comprises a first input side and a second input side located opposite the first input side, wherein the received pulse of light is incident on the first input side of the detector, and the local-oscillator light is incident on the second input side of the detector.
14 . The lidar system of claim 1 , further comprising an optical polarization element configured to alter a polarization of the emitted pulses of light, the local-oscillator light, or the received pulse of light to allow the local-oscillator light and the received pulse of light to be coherently mixed.
15 . The lidar system of claim 14 , wherein the optical polarization element comprises (i) a quarter-wave plate configured to convert the polarization of the local-oscillator light to circularly polarized light or (ii) a depolarizer configured to depolarize the polarization of the local-oscillator light.
16 . The lidar system of claim 1 , wherein:
the light source comprises:
a seed laser diode configured to produce a seed optical signal and the local-oscillator light; and
a semiconductor optical amplifier (SOA) configured to amplify temporal portions of the seed optical signal to produce the emitted pulses of light, wherein each amplified temporal portion of the seed optical signal corresponds to one of the emitted pulses of light; and
the lidar system further comprises a photonic integrated circuit (PIC) comprising an optical combiner and one or more optical waveguides, wherein:
each of the seed laser diode, the SOA, and the detector is attached to, connected to, or integrated with the PIC;
the optical waveguides are configured to (i) convey the local-oscillator light to the optical combiner, (ii) convey the received pulse of light to the optical combiner, and (iii) convey a combined beam from the combiner to the detector; and
the optical combiner is configured to combine the local-oscillator light and the received pulse of light to produce the combined beam, the combined beam comprising at least a portion of the local-oscillator light and at least a portion of the received pulse of light.
17 . The lidar system of claim 16 , further comprising an input lens attached to, connected to, or integrated with the PIC, wherein the input lens is configured to focus the received pulse of light into one of the optical waveguides of the PIC.
18 . The lidar system of claim 1 , wherein:
the light source is further configured to impart a spectral signature of one or more different spectral signatures to each of the emitted pulses of light; and
the receiver further comprises a frequency-detection circuit configured to determine, based on the second term of the photocurrent signal, a spectral signature of the received pulse of light.
19 . The lidar system of claim 1 , wherein the light source comprises:
a seed laser diode configured to produce a seed optical signal and the local-oscillator light; and
a semiconductor optical amplifier (SOA) configured to amplify temporal portions of the seed optical signal to produce the emitted pulses of light, wherein each amplified temporal portion of the seed optical signal corresponds to one of the emitted pulses of light.
20 . The lidar system of claim 19 , wherein each emitted pulse of light being coherent with the corresponding temporal portion of the local-oscillator light corresponds to the temporal portion of the seed light that is amplified being coherent with the corresponding temporal portion of the local-oscillator light.
21 . The lidar system of claim 19 , wherein the seed laser diode comprises a front face from which the seed optical signal is produced and a back face from which the local-oscillator light is produced.
22 . The lidar system of claim 19 , wherein the light source further comprises an optical splitter disposed between the seed laser diode and the SOA, wherein the optical splitter is configured to split off a portion of the seed optical signal to produce the local-oscillator light.
23 . The light source of claim 19 , wherein the SOA comprises a tapered optical waveguide extending from an input end of the SOA to an output end of the SOA, wherein a width of the tapered optical waveguide increases from the input end to the output end.
24 . The lidar system of claim 19 , wherein the light source further comprises an electronic driver configured to:
supply a substantially constant electrical current to the seed laser diode so that the seed optical signal comprises light having a substantially constant optical power; and
supply pulses of electrical current to the SOA, wherein each pulse of current causes the SOA to amplify one of the temporal portions of the seed optical signal to produce one of the emitted pulses of light.
25 . The lidar system of claim 19 , wherein the light source is configured as a three-terminal device, wherein (i) the light source comprises a common anode, wherein an anode of the seed laser diode is electrically connected to an anode of the SOA or (ii) the light source comprises a common cathode, wherein a cathode of the seed laser diode is electrically connected to a cathode of the SOA.
26 . The lidar system of claim 19 , wherein the light source is configured as a four-terminal device, wherein:
the seed laser diode comprises a seed laser anode and a seed laser cathode;
the SOA comprises a SOA anode and a SOA cathode;
the seed laser anode and the SOA anode are electrically isolated from one another; and
the seed laser cathode and the SOA cathode are electrically isolated from one another.
27 . The lidar system of claim 1 , wherein the light source comprises:
a seed laser diode configured to produce a seed optical signal and the local-oscillator light;
a semiconductor optical amplifier (SOA) configured to amplify temporal portions of the seed optical signal to produce initial pulses of light; and
a fiber-optic amplifier configured to receive the initial pulses of light from the SOA and further amplify the initial pulses of light to produce the emitted pulses of light, wherein each amplified temporal portion of the seed optical signal corresponds to one of the emitted pulses of light.
28 . The lidar system of claim 1 , wherein the emitted pulses of light have optical characteristics comprising:
a wavelength between 900 nanometers and 2000 nanometers;
a pulse energy between 0.01 μJ and 100 μJ;
a pulse repetition frequency between 80 kHz and 10 MHz; and
a pulse duration between 1 ns and 100 ns.
29 . The lidar system of claim 1 , wherein:
the receiver further comprises an electronic amplifier configured to amplify the photocurrent signal to produce a voltage signal that corresponds to the photocurrent signal; and
the pulse-detection circuit comprises one or more comparators coupled to one or more respective time-to-digital converters (TDCs), wherein:
each comparator is configured to provide an electrical-edge signal to a corresponding TDC when the voltage signal rises above or falls below a particular threshold voltage; and
the corresponding TDC is configured to produce a time value corresponding to a time when the electrical-edge signal was received, wherein the time-of-arrival for the received pulse of light is determined based on one or more time values produced by one or more of the TDCs.
30 . The lidar system of claim 1 , wherein:
the time-of-arrival for the received pulse of light corresponds to a round-trip time (ΔT) for the portion of the one of the emitted pulses of light to travel to the target and back to the lidar system; and
the distance (D) to the target is determined from an expression D=c·ΔT/2, wherein c is a speed of light.
31 . The lidar system of claim 1 , further comprising a scanner configured to scan the emitted pulses of light across a field of regard of the lidar system.
32 . The lidar system of claim 31 , wherein the scanner comprises:
a polygon mirror configured to scan the emitted pulses of light along a first direction within the field of regard; and
a scan mirror configured to scan the emitted pulses of light along a second direction within the field of regard, the second direction different from the first direction.
33 . The lidar system of claim 31 , wherein scanning the emitted pulses of light comprises scanning a field of view of the light source and a field of view of the receiver across the field of regard of the lidar system, wherein the light-source field of view and the receiver field of view are scanned synchronously with respect to one another, wherein a scanning speed of the light-source field of view and a scanning speed of the receiver field of view are approximately equal.