Method and system for time separated quadrature detection of doppler effects in optical range measurements
Doppler correction of broadband LIDAR includes mixing, during a first time interval, a returned optical signal with an in-phase version of the transmitted signal to produce a first mixed optical signal that is detected during the first time interval to produce a first electrical signal. During a non-overlapping second time interval the returned optical signal is mixed with a quadrature version of the transmitted signal to produce a second mixed optical signal that is detected during the second time interval to produce a second electrical signal. A complex digital signal uses one of the digitized electrical signals as a real part and a different one as the imaginary part. A signed Doppler frequency shift of the returned optical signal is determined based, at least in part, on a Fourier transform of the complex digital signal. A device is operated based on the Doppler frequency shift.
1. A method comprising:
modulating an optical signal from a laser to produce a broadband optical signal; transmitting the broadband optical signal;
receiving a returned optical signal in response to transmitting the broadband optical signal;
mixing during a first time interval the returned optical signal with a first reference optical signal based on an in-phase version of the broadband optical signal to produce a first mixed optical signal;
detecting a real part of the first mixed optical signal at a first optical detector during the first time interval to produce a first electrical signal;
mixing during a non-overlapping second time interval the returned optical signal with a second reference optical signal based on a quadrature version of the broadband optical signal to produce a second mixed optical signal;
detecting a real part of the second mixed optical signal at a second optical detector during the second time interval to produce a second electrical signal;
producing on a processor a complex digital signal by using a digitized sample of one of the first electrical signal and the second electrical signal as a real part of the complex digital signal and a digitized sample of a different one of the first electrical signal and the second electrical signal as an imaginary part of the complex digital signal;
determining on a processor a signed Doppler frequency shift of the returned optical signal based, at least in part, on a Fourier transform of the complex digital signal; and
operating a device based on the signed Doppler frequency shift.
2. The method as recited in claim 1 , wherein the first optical detector is also the second optical detector.
3. The method as recited in claim 1 , wherein:
both mixing steps are performed using one 2×2 multimode interference structure that outputs a positive optical component and a negative optical component; and
both detecting steps are performed using one balanced optical detector that outputs an electrical signal that indicates a difference between the positive optical component and the negative optical component.
4. The method as recited in claim 1 , wherein the broadband optical signal is a phase-encoded optical signal.
5. The method as recited in claim 1 , wherein the broadband optical signal is an up and down chirped optical signal.
6. The method as recited in claim 1 , wherein the first time interval and the second time interval alternate an equal number of times during a processing time interval.
7. The method as recited in claim 1 , wherein the first time interval and the second time interval occur an equal number of times randomly during a processing time interval.
8. The method a recited in claim 1 , wherein operating the device based on the Doppler frequency shift further comprises presenting on a display device an image that indicates a Doppler corrected position of any object at a plurality of spots illuminated by the transmitted broadband optical signal.
9. The method a recited in claim 1 , wherein operating the device based on the Doppler frequency shift further comprises communicating, to the device, data that identifies at least one object based on a point cloud of Doppler corrected positions at a plurality of spots illuminated by the transmitted broadband optical signal.
10. The method a recited in claim 1 , wherein operating the device based on the Doppler frequency shift further comprises presenting on a display device an image that indicates a size of the Doppler frequency shift at a plurality of spots illuminated by the transmitted broadband optical signal, whereby moving objects are distinguished from stationary objects and absent objects.
11. The method a recited in claim 1 , wherein operating the device based on the Doppler frequency shift further comprises moving a vehicle to avoid a collision with an object, wherein a closing speed between the vehicle and the object is determined based on a size of the Doppler frequency shift at a plurality of spots illuminated by the transmitted broadband optical signal.
12. The method a recited in claim 1 , wherein operating the device based on the Doppler frequency shift further comprises identifying an object based on a point cloud of Doppler corrected positions at a plurality of spots illuminated by the transmitted broadband optical signal.
13. The method a recited in claim 1 , wherein producing the complex digital signal further comprises producing the complex digital signal by subtracting from an output of the first optical detector an average output from the first optical detector over a plurality of returned optical signals received in response to transmitting a corresponding plurality of the broadband optical signal.