Lidar system with pulsed and frequency-modulated light

Abstract

In one embodiment, a lidar system includes a light source configured to emit an output optical signal and a local-oscillator optical signal. The output optical signal includes (i) pulses of light and (ii) frequency-modulated (FM) output-light signals, where each pair of consecutive pulses of light is separated in time by one or more of the FM output-light signals. The local-oscillator optical signal includes FM local-oscillator light signals corresponding to the FM output-light signals. The lidar system also includes a receiver configured to detect the local-oscillator optical signal and an input optical signal. The input optical signal includes (i) a received pulse of light that includes a portion of one of the emitted pulses of light scattered by a target located a distance from the lidar system and (ii) a received FM light signal that includes a portion of one of the FM output-light signals scattered by the target.

What is claimed is: 
     
         1 . A lidar system comprising:
 a light source configured to emit an output optical signal and a local-oscillator optical signal, wherein:
 the output optical signal comprises (i) pulses of light and (ii) frequency-modulated (FM) output-light signals, wherein each pair of consecutive pulses of light is separated in time by one or more of the FM output-light signals; and 
 the local-oscillator optical signal comprises FM local-oscillator light signals corresponding to the FM output-light signals; 
   a receiver configured to detect the local-oscillator optical signal and an input optical signal, the input optical signal comprising:
 a 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; and 
 a received FM light signal comprising a portion of one of the FM output-light signals scattered by the target, wherein the received FM light signal and the local-oscillator optical signal are coherently mixed together at the receiver; and 
   a processor configured to:
 determine a coarse distance to the target based on a round-trip time for the portion of the emitted pulse of light to travel from the lidar system to the target and back to the lidar system; and 
 determine a precise distance to the target based on (i) the coarse distance to the target and (ii) a frequency of a beat signal resulting from the coherent mixing of the received FM light signal and the local-oscillator optical signal. 
   
     
     
         2 . The lidar system of  claim 1 , wherein each FM local-oscillator light signal is coherent with at least one of the FM output-light signals. 
     
     
         3 . The lidar system of  claim 1 , wherein each FM local-oscillator light signal has a frequency modulation that matches a frequency modulation of at least one of the FM output-light signals. 
     
     
         4 . The lidar system of  claim 1 , wherein each FM output-light signal and each FM local-oscillator light signal comprises an optical frequency that increases or decreases with time. 
     
     
         5 . The lidar system of  claim 4 , wherein the optical frequency increases or decreases linearly with time. 
     
     
         6 . The lidar system of  claim 1 , wherein the coarse distance (D coarse ) is determined from an expression D coarse =c·ΔT/2, wherein c is a speed of light, and ΔT is the round-trip time for the portion of the emitted pulse of light to travel from the lidar system to the target and back to the lidar system. 
     
     
         7 . The lidar system of  claim 1 , wherein:
 each FM output-light signal and each FM local-oscillator light signal has a duration T and a frequency-modulation range B;   the duration T is related to a frequency-modulation (FM) distance d FM  by an expression d FM =c·T/2, wherein c is a speed of light;   the frequency (ΔF) of the beat signal is related to an offset distance d offset  by an expression d offset =d FM ·(ΔF/B);   the coarse distance (D coarse ) is associated with an integer number N of the FM distances based on an expression N=INT(D coarse /d FM ), wherein INT(D coarse /d FM ) is an integer portion of (D coarse /d FM ); and   the precise distance D precise  to the target is determined from an expression D precise =N·d FM +d offset .   
     
     
         8 . The lidar system of  claim 1 , wherein the precise distance has a higher accuracy than the coarse distance. 
     
     
         9 . The lidar system of  claim 1 , wherein:
 the received FM light signal is received after the received pulse of light; and   the processor is configured to determine the precise distance to the target after determining the coarse distance to the target.   
     
     
         10 . The lidar system of  claim 1 , wherein the FM output-light signals and the FM local-oscillator light signals each comprise optical frequencies that alternately increase and decrease with time. 
     
     
         11 . The lidar system of  claim 1 , wherein:
 the received FM light signal is a first received FM light signal, and the portion of the FM output-light signal is a portion of a first one of the FM output-light signals;   the frequency of the beat signal is a first frequency of a first beat signal;   the input optical signal further comprises a second received FM light signal comprising a portion of a second one of the FM output-light signals scattered by the target, wherein the second received FM light signal and the local-oscillator optical signal are coherently mixed together at the receiver; and   the processor is further configured to determine a speed of the target based on (i) the first frequency of the first beat signal and (ii) a second frequency of a second beat signal resulting from the coherent mixing of the second received FM light signal and the local-oscillator optical signal.   
     
     
         12 . The lidar system of  claim 11 , wherein:
 the first one of the FM output-light signals comprises an optical frequency that increases with time; and   the second one of the FM output-light signals comprises an optical frequency that decreases with time.   
     
     
         13 . The lidar system of  claim 11 , wherein the speed of the target (V r ) is a radial speed of the target relative to the lidar system and is determined from an expression V r =ΔF D λ/2, wherein:
 λ is a wavelength of the FM output-light signals; 
 ΔF D  is a Doppler shift of an optical frequency of each of the first and second received FM light signals, wherein the Doppler shift is determined from an expression ΔF D =(ΔF 2 −ΔF 1 )/2, wherein ΔF 1  is the first frequency of the first beat signal and ΔF 2  is the second frequency of the second beat signal. 
 
     
     
         14 . The lidar system of  claim 1 , wherein:
 a wavelength of the FM output-light signals is between 900 nanometers (nm) and 1600 nm; and   the portion of the FM output-light signal scattered by the target experiences a Doppler shift in optical frequency between [1.2 MHz/(m/s)]×V r  and [2.3 MHz/(m/s)]×V r , wherein V r  is a radial speed of the target relative to the lidar system in units of meters/second.   
     
     
         15 . The lidar system of  claim 1 , wherein the light source comprises:
 a seed laser diode configured to produce (i) seed light comprising the FM output-light signals and (ii) the FM local-oscillator light signals; and   a semiconductor optical amplifier (SOA) configured to amplify temporal portions of the seed light to produce the emitted pulses of light, wherein each amplified temporal portion of the seed light corresponds to a pulse of light of the emitted pulses of light.   
     
     
         16 . The lidar system of  claim 15 , wherein the light source further comprises a fiber-optic amplifier configured to further amplify the amplified seed light to produce the emitted pulses of light. 
     
     
         17 . The lidar system of  claim 15 , wherein the light source further comprises an electronic driver configured to supply an electrical current to the seed laser diode comprising a modulated electrical current, wherein the modulated electrical current causes the seed laser diode to produce the FM output-light signals and the FM local-oscillator light signals. 
     
     
         18 . The lidar system of  claim 17 , wherein the electrical current supplied to the seed laser diode further comprises a substantially constant electrical current. 
     
     
         19 . The lidar system of  claim 17 , wherein the modulated electrical current comprises a time-varying electrical current having a sawtooth-wave shape or a triangle-wave shape. 
     
     
         20 . The lidar system of  claim 17 , wherein the modulated electrical current comprises modulated seed-current portions, each modulated seed-current portion having an electrical-current magnitude that increases or decreases with time. 
     
     
         21 . The lidar system of  claim 20 , wherein each modulated seed-current portion causes the seed laser diode to produce one of the FM output-light signals and one of the FM local-oscillator signals. 
     
     
         22 . The lidar system of  claim 20 , wherein each modulated seed-current portion that has an increasing electrical-current magnitude causes the seed laser diode to produce (i) one FM output-light signal having a decrease in optical frequency with time and (ii) one FM local-oscillator light signal having a corresponding decrease in optical frequency with time. 
     
     
         23 . The lidar system of  claim 17 , wherein the modulated electrical current comprises electrical-current portions, each electrical-current portion configured to produce a corresponding temporal portion of the seed light that is amplified by the SOA. 
     
     
         24 . The lidar system of  claim 23 , wherein:
 consecutive electrical-current portions have different electrical-current amplitudes; and   the different electrical-current amplitudes are configured to impart different spectral signatures to corresponding consecutive pulses of light emitted by the light source.   
     
     
         25 . The lidar system of  claim 15 , wherein the light source further comprises an electronic driver configured to supply pulses of electrical current to the SOA, wherein each pulse of electrical current results in the SOA amplifying one of the temporal portions of the seed light to produce one of the emitted pulses of light. 
     
     
         26 . The lidar system of  claim 25 , wherein consecutive pulses of electrical current supplied to the SOA have different electrical-current characteristics; and
 the different electrical-current characteristics are configured to impart different spectral signatures to corresponding consecutive pulses of light emitted by the light source.   
     
     
         27 . The lidar system of  claim 25 , wherein the electronic driver is further configured to supply a substantially constant electrical current to the SOA, wherein the constant electrical current is configured so that the SOA transmits or amplifies the FM output-light signals produced by the seed laser diode. 
     
     
         28 . The lidar system of  claim 1 , wherein the coherent mixing of the received FM light signal and the local-oscillator optical signal comprises a coherent mixing of the received FM light signal and one of the FM local-oscillator light signals. 
     
     
         29 . The lidar system of  claim 1 , wherein:
 the frequency of the beat signal resulting from the coherent mixing of the received FM light signal and the local-oscillator optical signal comprises one or more frequencies of one or more beat signals; and   the receiver is further configured to determine the frequency of each of the one or more beat signals.   
     
     
         30 . The lidar system of  claim 1 , wherein the receiver comprises a frequency-detection circuit configured to determine the frequency of the beat signal resulting from the coherent mixing of the received FM light signal and the local-oscillator optical signal. 
     
     
         31 . The lidar system of  claim 30 , wherein the frequency-detection circuit comprises a plurality of electronic filters, each electronic filter associated with a particular frequency component. 
     
     
         32 . The lidar system of  claim 30 , wherein the frequency-detection circuit comprises:
 a derivative circuit configured to produce a derivative signal corresponding to a derivative of the beat 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 receiver comprises:
 one or more detectors configured to produce one or more respective photocurrent signals corresponding to the input optical signal, each photocurrent signal comprising (i) a pulse of photocurrent corresponding to the received pulse of light and (ii) a beat-signal photocurrent corresponding to the coherent mixing of the received FM light signal and the local-oscillator optical signal; and   one or more electronic amplifiers configured to amplify one or more of the photocurrent signals to produce one or more voltage signals corresponding to the photocurrent signals.   
     
     
         34 . The lidar system of  claim 33 , wherein:
 the receiver further comprises a pulse-detection circuit configured to determine, based on the one or more voltage signals, a time-of-arrival for the received pulse of light; and   the processor is further configured to determine the round-trip time for the portion of the emitted pulse of light to travel from the lidar system to the target and back to the lidar system based on the time-of-arrival for the received pulse of light.   
     
     
         35 . The lidar system of  claim 1 , wherein the received pulse of light and the local-oscillator optical signal are coherently mixed together at the receiver. 
     
     
         36 . The lidar system of  claim 35 , wherein:
 the receiver comprises a frequency-detection circuit configured to determine a frequency associated with the coherent mixing of the received pulse of light and the local-oscillator optical signal; and   the processor is further configured to determine, based on the frequency associated with the coherent mixing matching a spectral signature of the one of the emitted pulses of light, that the received pulse of light is associated with the one of the emitted pulses of light.   
     
     
         37 . The lidar system of  claim 35 , wherein:
 the receiver comprises a frequency-detection circuit configured to determine a frequency associated with the coherent mixing of the received pulse of light and the local-oscillator optical signal; and   the processor is further configured to determine, based on the frequency associated with the coherent mixing matching a spectral signature of the emitted pulses of light, that the received pulse of light is a valid received pulse of light that is associated with a pulse of light of the emitted pulses of light.   
     
     
         38 . The lidar system of  claim 1 , wherein the light source comprises a direct-emitter laser diode configured to emit the output optical signal and the local-oscillator optical signal. 
     
     
         39 . The lidar system of  claim 1 , further comprising a scanner configured to scan the output optical signal across a field of regard of the lidar system. 
     
     
         40 . A method comprising:
 emitting, by a light source of a lidar system, an output optical signal and a local-oscillator optical signal, wherein:
 the output optical signal comprises (i) pulses of light and (ii) frequency-modulated (FM) output-light signals, wherein each pair of consecutive pulses of light is separated in time by one or more of the FM output-light signals; and 
 the local-oscillator optical signal comprises FM local-oscillator light signals corresponding to the FM output-light signals; 
   detecting, by a receiver of the lidar system, the local-oscillator optical signal and an input optical signal, the input optical signal comprising:
 a 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; and 
 a received FM light signal comprising a portion of one of the FM output-light signals scattered by the target, wherein the received FM light signal and the local-oscillator optical signal are coherently mixed together at the receiver; and 
   determining, by a processor of the lidar system, a coarse distance to the target based on a round-trip time for the portion of the emitted pulse of light to travel from the lidar system to the target and back to the lidar system; and   determining, by the processor, a precise distance to the target based on (i) the coarse distance to the target and (ii) a frequency of a beat signal resulting from the coherent mixing of the received FM light signal and the local-oscillator optical signal.   
     
     
         41 . A lidar system comprising:
 a light source configured to emit an output optical signal and a local-oscillator optical signal, wherein:
 the output optical signal comprises (i) pulses of light and (ii) frequency-modulated (FM) output-light signals, wherein each pair of consecutive pulses of light is separated in time by one or more of the FM output-light signals; and 
 the local-oscillator optical signal comprises FM local-oscillator light signals corresponding to the FM output-light signals; 
   a receiver configured to detect the local-oscillator optical signal and an input optical signal, the input optical signal comprising:
 a 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; 
 a first received FM light signal comprising a portion of a first one of the FM output-light signals scattered by the target, wherein the first received FM light signal and the local-oscillator optical signal are coherently mixed together at the receiver; and 
 a second received FM light signal comprising a portion of a second one of the FM output-light signals scattered by the target, wherein the second received FM light signal and the local-oscillator optical signal are coherently mixed together at the receiver; and 
   a processor configured to:
 determine the distance to the target based on a round-trip time for the portion of the emitted pulse of light to travel from the lidar system to the target and back to the lidar system; and 
 determine a speed of the target based on (i) a first frequency of a first beat signal resulting from the coherent mixing of the first received FM light signal and the local-oscillator optical signal and (ii) a second frequency of a second beat signal resulting from the coherent mixing of the second received FM light signal and the local-oscillator optical signal.