FMCW LIDAR SYSTEM AND METHOD FOR SIMULTANEOUS RANGE AND VELOCITY MEASUREMENT
The invention relates to a system and a method for simultaneous range and velocity measurement in an FMCW LiDAR system. A first light source produces first light having a first frequency that varies according to a first chirp rate. A second light source produces second light having a second frequency that is constant or that varies according to a second chirp rate. A splitter separates the measuring light into reference light and output light, and a scanning unit directs the output light towards an object and receives input light that is obtained by reflection of the output light at the object. A detector detects a superposition of the reference light and the input light. A computing unit computes unambiguously the range and relative velocity by analyzing beat frequencies resulting from the superposition. Ambiguities due to Doppler frequency shifts are removed by performing a decision tree analysis.
1 . An FMCW LiDAR system for simultaneous range and velocity measurement, comprising:
a first light source configured to produce first light having a first frequency that varies the first frequency varies according to a chirp rate CR1 during a first portion of a measurement interval and according to a chirp rate CR2 during a second portion of the measurement interval,
a second light source configured to produce second light having a second frequency that is constant,
an optical combiner configured to combine the first light and the second light, thereby obtaining measuring light having at least two different frequency components during the measurement interval,
a splitter configured to separate the measuring light into reference light and output light,
a scanning unit configured to direct the output light towards an object along different directions and to receive input light that is obtained by reflection of the output light at the object,
a detector configured to detect a superposition of the reference light and the input light, and
a computing unit configured to compute unambiguously a range to the object and a relative velocity between the system and the object by analyzing beat frequencies resulting from the superposition detected by the detector,
the computing unit is configured to compute beat frequencies separately for each portion of the measurement interval, and to compute a range to the object and a relative velocity between the system and the object by analyzing the beat frequencies measured during both portions of the measurement interval,
wherein
the computing unit is further configured to remove ambiguities due to Doppler frequency shifts by performing a decision tree analysis.
2 . The system of claim 1 , wherein the computing unit is configured, when performing the decision tree analysis, to determine, separately for each of the first and second portion of the measurement interval, how many beat frequencies have been measured, and whether there is a beat frequency that occurs in both portions of the measurement interval.
3 . The system of claim 2 , wherein the chirp rate CR1 is a positive chirp rate so that the frequency increases during the first portion of the measurement interval, and wherein the chirp rate CR2 is a negative chirp rate so that the frequency decreases during the second portion of the measurement interval.
4 . The system of claim 1 , comprising an optical circulator connecting the splitter, the scanning unit and the detector so that the output light is directed towards the scanning unit and the input light is directed towards the detector.
5 . A method for simultaneous range and velocity measurement in an FMCW LiDAR system, comprising the following steps:
a) producing first light having a first frequency that varies according to a chirp rate CR1 during a first portion of a measurement interval and according to a chirp rate CR2 during a second portion of the measurement interval;
b) producing second light having a second frequency that is constant;
c) combining the first light and the second light, thereby obtaining measuring light having at least two different frequency components during the measurement interval;
d) separating the measuring light into reference light and output light;
e) directing the output light towards an object along different directions and receiving input light that is obtained by reflection of the output light at the object;
f) detecting a superposition of the reference light and the input light;
g) computing beat frequencies separately for each portion of the measurement interval; and
h) computing a range to the object and a relative velocity between the system and the object by analyzing the beat frequencies computed for each portion of the measurement interval;
wherein step h) includes the step of removing ambiguities due to Doppler frequency shifts by performing a decision tree analysis.
6 . The method of claim 5 , wherein, when performing the decision tree analysis, it is determined, separately for each of the first and second portion of the measurement interval, how many beat frequencies have been measured, and whether there is a beat frequency that occurs in both portions of the measurement interval.
7 . The method of claim 5 , wherein the chirp rate CR1 is a positive chirp rate so that the frequency increases during the first portion of the measurement interval, and wherein the chirp rate CR2 is a negative chirp rate so that the frequency decreases during the second portion of the measurement interval.