System and Method for Measuring a Range to an Object
An FMCW LiDAR system for measuring a range to an object has a light source that produces measuring light having a varying frequency. An optical circuitry emits the measuring light and receives measuring light that was reflected from the object. A detector has a quadrature optical mixer that receives the measuring light and reference light, which was not reflected from the object. The detector produces an in-phase radio beat frequency and a quadrature shifted radio beat frequency based on a superposition of the measuring light and the reference light. A phase-locked loop produces, based on the in-phase radio beat frequency and the quadrature shifted radio beat frequency received from the quadrature mixer, an electrical signal representing an absolute value and a sign of a synthesized beat frequency. A computing unit computes a range to the object based on the electrical signal.
1 . A frequency-modulated continuous wave (FMCW) LiDAR system for measuring a range to an object, comprising:
a light source configured to produce light having a varying frequency,
an optical circuitry configured to emit the light produced by the light source and to receive measuring light that was reflected from the object,
a detector comprising:
a quadrature optical mixer configured to:
receive the measuring light and reference light, which was not reflected from the object, and
produce an in-phase radio beat frequency and a quadrature shifted radio beat frequency based on a superposition of the measuring light and the reference light, and
a phase-locked loop configured to produce, based on the in-phase radio beat frequency and the quadrature shifted radio beat frequency received from the quadrature optical mixer, an electrical signal representing an absolute value and a sign of a synthesized beat frequency, and
a computing unit configured to compute a range to the object based on the electrical signal.
2 . The FMCW LiDAR system of claim 1 , wherein the quadrature optical mixer comprises:
a quadrature hybrid coupler comprising a first input configured to receive the measuring light, a second input configured to receive the reference light, and four exits combining the measuring light and the reference light with different phase delays;
a first pair of photodetectors connected to two of the four exits of the quadrature hybrid coupler and configured to produce the in-phase radio beat frequency; and
a second pair of photodetectors connected to the other two of the four exits of the quadrature hybrid coupler and configured to produce the quadrature shifted radio beat frequency.
3 . The FMCW LiDAR system of claim 1 , wherein the phase-locked loop comprises:
an oscillator,
a first electrical mixer configured to mix the in-phase radio beat frequency with an in-phase oscillator signal received from the oscillator, and
a second electrical mixer configured to mix the quadrature shifted radio beat frequency with a quadrature shifted oscillator signal received from the oscillator.
4 . The FMCW LiDAR system of claim 3 , wherein the oscillator is fed by a driving signal produced by a lag-type or lag-lead type loop filter.
5 . The FMCW LiDAR system of claim 3 , wherein the oscillator is a voltage-controlled oscillator.
6 . The FMCW LiDAR system of claim 3 , comprising a quadrature splitter that is connected to the oscillator and comprises a first exit port connected to the first electrical mixer and a second exit port connected to the second electrical mixer.
7 . A method for measuring a range to an object using a frequency-modulated continuous wave (FMCW) LiDAR system, comprising the following steps:
a) producing light having a varying frequency;
b) emitting the light and receiving measuring light that was reflected from the object;
c) producing an in-phase radio beat frequency and a quadrature shifted radio beat frequency from a superposition of the measuring light and reference light;
d) producing, in a phase-locked loop and based on the in-phase radio beat frequency and the quadrature shifted radio beat frequency, an electrical signal representing an absolute value and a sign of a synthesized beat frequency; and
e) computing a range to the object based on the electrical signal.
8 . The method of claim 7 , wherein the in-phase radio beat frequency is mixed with an in-phase oscillator signal received from an oscillator, and the quadrature shifted radio beat frequency is mixed with a quadrature shifted oscillator signal received from the oscillator.
9 . The method of claim 8 , wherein the oscillator is fed by a driving signal produced by a lag-type or lag-lead type loop filter.