Hybrid pulsed/coherent lidar system with spectral signatures
In one embodiment, a lidar system includes a light source configured to emit (i) local-oscillator light and (ii) pulses of light. Additionally, the light source is configured to impart a spectral signature of one or more different spectral signatures to each of the emitted pulses of light, where the emitted pulses of light include an emitted pulse of light having a particular spectral signature of the one or more different spectral signatures. 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 the emitted pulse 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 receiver also includes a pulse-detection circuit and a frequency-detection circuit.
1 . A lidar system comprising:
a light source configured to:
emit (i) local-oscillator light and (ii) pulses of light; and
impart a spectral signature of one or more different spectral signatures to each of the emitted pulses of light, wherein the emitted pulses of light comprise an emitted pulse of light having a particular spectral signature of the one or more different spectral signatures;
a receiver configured to detect the local-oscillator light and a received pulse of light, the received pulse of light comprising a portion of the emitted pulse 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;
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 frequency-detection circuit configured to determine, based on the second term of the photocurrent signal, a spectral signature of the received pulse of light; and
a processor configured to determine whether the spectral signature of the received pulse of light matches the particular spectral signature of the emitted pulse of light.
2 . The lidar system of claim 1 , wherein, in response to determining that the spectral signature of the received pulse of light matches the particular spectral signature of the emitted pulse of light, the processor is further configured to determine that the received pulse of light is associated with the emitted pulse of light, wherein the received pulse of light includes a portion of light from the emitted pulse of light.
3 . The lidar system of claim 1 , wherein, in response to determining that the spectral signature of the received pulse of light matches the particular spectral signature of the emitted pulse of light, the processor is further configured to determine the distance from the lidar system to the target based on a time difference ΔT between (i) the time-of-arrival for the received pulse of light and (ii) a time-of-emission for the emitted pulse of light, wherein the distance (D) to the target is determined from an expression D=c·ΔT/2 wherein c is a speed of light.
4 . The lidar system of claim 1 , wherein the spectral signature of the received pulse of light matching the particular spectral signature of the emitted pulse of light corresponds to a spectral-signature score being greater than a particular threshold value, the spectral-signature score representing an amount of correlation between the spectral signature of the received pulse of light and the particular spectral signature.
5 . The lidar system of claim 1 , wherein:
the receiver is further configured to detect the local-oscillator light and a second received pulse of light having a second spectral signature that is different from the particular spectral signature of the emitted pulse of light; and
the processor is further configured to determine that the second received pulse of light is not associated with the emitted pulse of light based on the second spectral signature of the second received pulse of light not matching the particular spectral signature of the emitted pulse of light.
6 . The lidar system of claim 5 , wherein the second spectral signature of the second received pulse of light not matching the particular spectral signature of the emitted pulse of light corresponds to a spectral-signature score being less than a particular threshold value, the spectral-signature score representing an amount of correlation between the second spectral signature and the particular spectral signature.
7 . The lidar system of claim 1 , wherein:
the receiver is further configured to detect the local-oscillator light and a second received pulse of light having a second spectral signature that is different from the particular spectral signature of the emitted pulse of light; and
the processor is further configured to determine that the second received pulse of light is associated with another one of the emitted pulses of light based on the second spectral signature of the second received pulse of light matching a spectral signature of the another one of the emitted pulses of light.
8 . The lidar system of claim 1 , wherein:
the receiver is further configured to detect the local-oscillator light and a second received pulse of light having a second spectral signature that is different from each of the one or more spectral signatures of the emitted pulses of light; and
the processor is further configured to determine that the second received pulse of light is not associated with any of the emitted pulses of light based on the second spectral signature of the second received pulse of light not matching any of the one or more spectral signatures.
9 . The lidar system of claim 8 , wherein, in response to determining that the second received pulse of light is not associated with any of the emitted pulses of light, the processor is further configured to discard or ignore the second received pulse of light.
10 . The lidar system of claim 1 , wherein determining whether the spectral signature of the received pulse of light matches the particular spectral signature of the emitted pulse of light comprises determining a spectral-signature score that represents an amount of correlation between the spectral signature of the received pulse of light and the particular spectral signature.
11 . The lidar system of claim 10 , wherein determining whether the spectral signature of the received pulse of light matches the particular spectral signature of the emitted pulse of light further comprises:
comparing the spectral signature of the received pulse of light to the particular spectral signature of the emitted pulse of light to determine the spectral-signature score; and
determining that the spectral signature of the received pulse of light matches the particular spectral signature when the spectral-signature score exceeds a particular threshold value.
12 . The lidar system of claim 1 , wherein:
the emitted pulses of light comprise n emitted pulses of light, each having a different spectral signature, wherein n is an integer greater than or equal to 2, and the emitted pulse of light having the particular spectral signature is one of the n emitted pulses of light; and
the processor is further configured to:
store spectral-signature information associated with each of the n emitted pulses of light;
compare the spectral-signature information of each of the n emitted pulses of light to the spectral signature of the received pulse of light to determine n spectral-signature scores, each spectral-signature score representing an amount of correlation between the spectral-signature information of one of the n emitted pulses of light and the spectral signature of the received pulse of light; and
determine that the spectral signature of the received pulse of light matches the particular spectral signature of the emitted pulse of light based on the spectral-signature score for the emitted pulse of light having a highest value of the n spectral-signature scores.
13 . The lidar system of claim 1 , wherein the spectral signature of the received pulse of light matching the particular spectral signature corresponds to the spectral signature of the received pulse of light including at least a particular minimum amount of frequency components associated with the particular spectral signature.
14 . The lidar system of claim 13 , wherein the spectral signature of the received pulse of light matching the particular spectral signature further corresponds to the spectral signature of the received pulse of light including less than a particular maximum amount of frequency components not associated with the particular spectral signature.
15 . The lidar system of claim 1 , wherein each spectral signature imparted to each of the emitted pulses of light comprises a broadened optical spectrum, wherein a spectral linewidth of each emitted pulse of light is greater than a spectral linewidth of the local-oscillator light.
16 . The lidar system of claim 1 , wherein each spectral signature imparted to each of the emitted pulses of light comprises a shifted optical spectrum, wherein (i) an optical frequency of each emitted pulse of light and (ii) an optical frequency of the local-oscillator light are offset by a particular frequency difference.
17 . The lidar system of claim 16 , wherein:
the received pulse of light is offset by a frequency difference of Δf with respect to the local-oscillator light; and
the second term of the photocurrent signal corresponding to the coherent mixing of the local-oscillator light and the received pulse of light comprises periodic temporal pulsations separated by a time interval of 1/Δf.
18 . The lidar system of claim 16 , wherein the particular frequency difference is greater than 1/Δτ, wherein Δτ is a duration of the emitted pulses of light.
19 . The lidar system of claim 16 , wherein the particular frequency difference is between 10 MHz and 50 GHz.
20 . 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 the emitted pulses of light; and
an electronic driver configured to:
supply an electrical current to the seed laser diode to produce the seed optical signal; 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 a corresponding pulse of light of the emitted pulses of light.
21 . The lidar system of claim 20 , wherein one or more characteristics of each spectral signature imparted to each emitted pulse of light depend on one or more of (i) the electrical current supplied to the seed laser diode and (ii) an amplitude, duration, rise time, fall time, or shape of the corresponding pulse of electrical current supplied to the SOA.
22 . The lidar system of claim 20 , wherein each pulse of current further causes the SOA to impart one of the spectral signatures to the amplified temporal portion of the seed optical signal so that the corresponding pulse of light includes the one of the spectral signatures.
23 . The lidar system of claim 20 , wherein:
a particular pulse of electrical current supplied to the SOA by the electronic driver causes the SOA to produce the emitted pulse of light having the particular spectral signature;
the particular spectral signature imparted to the emitted pulse of light comprises a broadened optical spectrum, wherein a spectral linewidth of the emitted pulse of light is greater than a spectral linewidth of the local-oscillator light; and
the broadened optical spectrum of the emitted pulse of light depends on one or more of an amplitude, duration, rise time, fall time, and shape of the particular pulse of electrical current.
24 . The lidar system of claim 20 , wherein:
the particular spectral signature imparted to the emitted pulse of light comprises a shifted optical spectrum, wherein (i) an optical frequency of the emitted pulse of light and (ii) an optical frequency of the local-oscillator light are offset by a frequency difference of Δf, and
the electronic driver is further configured to change the electrical current supplied to the seed laser diode by a particular amount to cause the frequency difference of Δf between the optical frequency of the emitted pulse of light and the optical frequency of the local-oscillator light.
25 . The lidar system of claim 24 , wherein the frequency difference of Δf results from a corresponding change of Δi in the electrical current supplied to the seed laser diode, wherein:
the electrical current supplied to the seed laser diode is in when the seed laser diode produces a temporal portion of the seed optical signal that is amplified to produce the emitted pulse of light; and
after the temporal portion of the seed optical signal is produced, the electrical current supplied to the seed laser diode is changed to io, wherein Δi=i 0 −i 1 .
26 . The lidar system of claim 1 , wherein the light source imparts the spectral signatures to the emitted pulses of light in a deterministic manner, wherein each emitted pulse of light includes a predetermined spectral signature of the one or more spectral signatures.
27 . The lidar system of claim 1 , wherein the light source imparts the spectral signatures to the emitted pulses of light in a pseudo-random manner, wherein each emitted pulse of light includes a non-predetermined spectral signature of the one or more spectral signatures.
28 . The lidar system of claim 1 , wherein:
the receiver is further configured to detect the local-oscillator light and another portion of the emitted pulse of light, wherein the another portion is produced prior to the emitted pulse of light exiting the lidar system; and
the frequency-detection circuit is further configured to determine, based on another second term corresponding to a coherent mixing of the local-oscillator light and the another portion of the emitted pulse of light, the particular spectral signature of the emitted pulse of light.
29 . The lidar system of claim 1 , wherein the second term of the photocurrent signal includes temporal pulsations corresponding to the spectral signature of the received pulse of light.
30 . The lidar system of claim 29 , wherein a frequency of the temporal pulsations is approximately equal to a frequency difference between an optical frequency of the received pulse of light and an optical frequency of the local-oscillator light.
31 . The lidar system of claim 1 , wherein:
the frequency-detection circuit comprises one or more electronic band-pass filters, each band-pass filter having a particular pass-band center frequency; and
the frequency-detection circuit is configured to determine the spectral signature of the received pulse of light based on determining an amplitude of each of one or more frequency components of the second term of the photocurrent signal.
32 . The lidar system of claim 1 , wherein the frequency-detection circuit comprises:
a derivative circuit configured to produce a derivative signal corresponding to a derivative of the photocurrent signal; and
a zero-crossing circuit configured to determine two or more zero crossings of the derivative signal.
33 . The lidar system of claim 1 , wherein the frequency-detection circuit comprises a matched filter.
34 . The lidar system of claim 1 , wherein the processor is further configured to determine, based on a frequency difference between (i) the particular spectral signature of the emitted pulse of light and (ii) the spectral signature of the received pulse of light, a speed of the target relative to the lidar system.
35 . The lidar system of claim 34 , wherein:
the frequency difference between the spectral signatures of the received pulse of light and the emitted pulse of light comprises a frequency difference of ΔF; and
the speed of the target (Sr) relative to the lidar system is a radial speed of the target relative to the lidar system and is determined from an expression S r =ΔFλ/2, wherein λ is a wavelength of the emitted pulse of light.
36 . The lidar system of claim 1 , wherein each emitted pulse of light is coherent with a corresponding temporal portion of the local-oscillator light.
37 . 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.
38 . The lidar system of claim 1 , wherein the photocurrent signal is proportional to |ε Rx (t)+ε LO (t)| 2 , wherein:
Σ Rx (t) represents an electric field of the received pulse of light; and
Σ LO (t) represents an electric field of the local-oscillator light.
39 . The lidar system of claim 38 , 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)+Δϕ(t)], wherein:
Δω(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 |.