Tunable Light Source for Frequency-Modulated Continuous Wave (FMCW) LiDAR Devices
A tunable light source for a frequency-modulated continuous wave (FMCW) LiDAR device includes an external cavity laser including a laser gain medium having a light output facet emitting laser light, a reflector and an external cavity waveguide extending between the light output facet and the reflector. A variable phase shifter is arranged in the external cavity waveguide. An optical splitter splits off a portion of the laser light and has an output port. An optical phase-locked loop is optically coupled to the optical splitter and electrically coupled to the laser gain medium. The optical phase-locked loop includes a first arm and a second arm, a double stage single-sideband Weaver mixer including an optical stage and an electrical stage, a phase detector and a loop filter that are electrically connected in series. The first arm and the second arm have different optical path lengths.
1 . A tunable light source for frequency-modulated continuous wave (FMCW) LiDAR devices, comprising:
an external cavity laser that comprises:
a laser gain medium having a light output facet configured to emit laser light;
a reflector; and
an external cavity waveguide extending between the light output facet and the reflector;
a variable phase shifter arranged in the external cavity waveguide;
an optical splitter configured to split off a portion of the laser light; and
an optical phase-locked loop that is optically coupled to the optical splitter and electrically coupled to the laser gain medium, wherein the optical phase-locked loop comprises a first arm, a second arm, a double stage single-sideband Weaver mixer comprising an optical stage and an electrical stage, a phase detector and a loop filter that are electrically connected in series, wherein the first arm and the second arm have different optical path lengths.
2 . The light source of claim 1 , wherein the optical stage of the double stage single-sideband Weaver mixer comprises a quadrature optical mixer, which is connected to the first arm and the second arm and is configured to produce in-phase and quadrature shifted radio beat frequencies from optical signals guided in the first arm and the second arm.
3 . The light source of claim 2 , wherein the quadrature optical mixer comprises:
a quadrature hybrid coupler comprising:
a first input connected to the first arm;
a second input connected to the second arm; and
four exits that combine light from the first arm and light from the second arm with different phase delays; and
two first photodetectors and two second photodetectors, wherein:
each photodetector has an electrical output and is connected to one of the exits of the quadrature hybrid coupler;
the electrical outputs of the first photodetectors are combined to produce the in-phase radio beat frequency; and
the electrical outputs of the second photodetectors are combined to produce the quadrature shifted radio beat frequency.
4 . The light source of claim 2 , wherein the electrical stage of the double stage single-sideband Weaver mixer:
comprises a local oscillator that is configured to produce a first oscillator signal and a second oscillator signal that is phase shifted relative to the first oscillator signal;
a first mixer that is configured to mix the first oscillator signal with the in-phase radio beat frequency so that a first lower side band signal is obtained; and
a second mixer that is configured to mix the second oscillator signal with the quadrature shifted beat frequency so that a second lower side band signal shifted in quadrature is obtained.
5 . The light source of claim 2 , comprising:
a waveform generator;
a laser driver connected to the loop filter and the waveform generator; wherein the laser driver is configured to produce a laser injection current that is applied to the laser gain medium; and
a phase shifter driver that is connected to the waveform generator and configured to produce a control signal that is applied to the variable phase shifter.
6 . The light source of claim 2 , comprising a further optical splitter configured to split off a portion of the laser light, said portion forming a signal that is emitted by the FMCW LiDAR device.
7 . The light source of claim 6 , comprising an optical isolator arranged in a light path downstream of the further optical splitter.
8 . A frequency-modulated continuous wave (FMCW) LiDAR device for measuring a range to an external object, comprising the tunable light source of claim 1 , a detector configured to detect a superposition of a portion of the laser light that was reflected at the external object with a portion of the laser light that was not reflected at the external object, and a calculation unit configured to determine the range to the object based on the superposition detected by the detector.