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EP-3942325-B1 - LIDAR SYSTEM WITH A MODE FIELD EXPANDER

EP3942325B1EP 3942325 B1EP3942325 B1EP 3942325B1EP-3942325-B1

Inventors

  • BEHZADI, Behsan
  • GAGNE, Keith
  • AVCI, Oguzhan
  • KOCAOGLU, Omer P.
  • OZA, Neal N.
  • REZK, MINA

Dates

Publication Date
20260506
Application Date
20200302

Claims (15)

  1. A frequency modulated continuous-wave, FMCW, light detection and ranging, LIDAR, apparatus, comprising: an optical source (202) to emit a frequency modulated optical beam (218) towards a target (220); a single mode optical fiber operatively coupled to the optical source; a larger mode area optical fiber operatively coupled in series with the single mode optical fiber; and a mode field expander (210) operatively coupled to the optical source (202) to: expand a mode area of the optical beam (218) from a first mode area of the single mode optical fiber to a second mode area corresponding to the larger mode area optical fiber, during transmission towards the target (220) through the mode field expander (210), to an optical lens (212), and to a scanner (216); and reduce the mode area of the optical beam (218) to the first mode area, upon receipt from the scanner (216), to the optical lens (212), through the mode field expander (210), and to a beam combiner (224), the beam combiner to generate a combined signal comprising a local oscillator signal (228) and a target signal (222) associated with the optical beam (218).
  2. The FMCW LIDAR apparatus of claim 1, further comprising: a first beam separator (204) operatively coupled between the optical source (202) and the mode field expander (210), the first beam separator (204) to separate a first portion of the optical beam (218) in a first direction towards the target (220) and a second portion of the optical beam (218) in a second direction as the local oscillator signal (228).
  3. The FMCW LIDAR apparatus of claim 1, further comprising: a photodetector (226) operatively coupled to the beam combiner (224) to receive the combined signal from the beam combiner (224).
  4. The FMCW LIDAR apparatus of claim 1, wherein the mode field expander (210) is operatively coupled between at least one optical device and the beam combiner (224), wherein the mode field expander (210) is configured to receive the target signal (222) from the at least one optical device, expand the mode area of the target signal (222) and provide the target signal (222) to the beam combiner (224).
  5. The FMCW LIDAR apparatus of claim 1, further comprising: a polarization beam splitter operatively coupled between the optical source (202) and the mode field expander (210), the polarization beam splitter to pass a first polarization state of light through the polarization beam splitter in a first direction and reflect a second polarization state of light in a second direction different than the first direction.
  6. The FMCW LIDAR apparatus of claim 1, further comprising: a polarization wave plate to transform a polarization of the optical beam (218).
  7. The FMCW LIDAR apparatus of claim 6, wherein the polarization wave plate further comprises a reflector or a coating to return a second portion of the optical beam (218) as a local oscillator signal (228).
  8. The FMCW LIDAR apparatus of claim 1, further comprising: a second optical source (202) to emit a second optical beam (218) towards the target (220), wherein a first wavelength of the optical beam (218) is different than a second wavelength of the second optical beam (218).
  9. The FMCW LIDAR apparatus of claim 1, further comprising: an optical amplifier (206) operatively coupled between the optical source (202) and the mode field expander (210), the optical amplifier (206) to amplify the optical beam (218).
  10. A method comprising: generating, by an optical source (202) of a frequency modulated continuous-wave, FMCW, light detection and ranging, LIDAR, system, an optical beam (218) towards a target (220); converting, by a mode field expander (210) operatively coupled to the optical source (202) by a single mode optical fiber, a mode area associated with the optical beam to expand the mode area from the single mode optical fiber to a larger mode optical fiber, operatively coupled in series with the single mode optical fiber, during transmission towards the target through the mode field expander, to an optical lens, and to a scanner, and to reduce the mode area of the optical beam to the mode area of the single mode optical fiber, upon receipt from the scanner, to the optical lens, through the mode field expander, and to a beam combiner; and generating by the beam combiner, a combined signal comprising the optical beam and a local oscillator signal.
  11. The method of claim 10, further comprising: receiving, by a first beam separator (204) operatively coupled between the optical source (202) and the mode field expander (210), the optical beam (218); and separating, by the first beam separator (204), a first portion of the optical beam (218) in a first direction towards the target (220) and a second portion of the optical beam (218) in a second direction as the local oscillator signal (228).
  12. The method of claim 10, further comprising: receiving, by a photodetector (226), the combined signal from the beam combiner (224).
  13. The method of claim 10, further comprising: generating, by a second optical source (202) of the FMCW LIDAR system (100, 200, 300, 400), a second optical beam (218) towards the target (220), wherein the second optical beam (218) has a different wavelength than the optical beam (218); and converting, by the mode field expander (210), a second mode area associated with the second optical beam (218).
  14. The method of claim 10, wherein the mode field expander (210) comprises an adiabatic mode expander.
  15. The method of claim 10, further comprising: amplifying, by an optical amplifier (206) operatively coupled between the optical source (202) and the mode field expander (210), the optical beam (218) generated by the optical source (202).

Description

RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application No. 16/359,217 (publication number US 2020/300980 A1) filed on 20 March 2019. TECHNICAL FIELD The present disclosure relates generally to light detection and ranging (LIDAR) that provides range (and for some types a simultaneous measurement of velocity) across two dimensions. BACKGROUND Fast-scanning mirrors are the primary components used to illuminate a scene in most conventional LIDAR systems. One mirror typically scans quickly along the X direction (azimuth), while another mirror scans slowly along the Y direction (elevation). Light emission and detection from target reflections are done coaxially, typically via a single mode fiber. The collected light has a measured delay or an altered frequency signature that is used to extract range, and potentially velocity, information. A 3D point cloud can be established when the point-wise detected range information is combined with angular position feedback from the scanning mirrors. To achieve higher frame rates, the mirror's angular velocity is increased, especially that of the scanner in faster scan direction (X scanner in our case). When using the mirrors with a high angular velocity and single-mode fiber-based detection, the target signal from distant objects is severely degraded. Signal degradation is mainly due to the difference in angular position of the scanner mirror from the launch time of the optical signal (pulsed or frequency swept) to the collection time of the same signal from a distant scattering target. This slight angular change causes a walk-off of the target signal at the fiber tip decreasing the coupling efficiency, which manifests itself as a weaker signal detection. Such degradation becomes more severe as the fiber diameter decreases, e.g. a single mode fiber with ~10 µm diameter, or as the mirror's angular velocity increases. US 4 846 571 A describes a laser for generating a beam of continuous wave (CW) electromagnetic energy having a nominal frequency and amplitude. US 2014/209798 A1 concerns devices and methods for multimode light detection. EP 1019773 A1 concerns a displaced aperture beamsplitter for a laser transmitter/receiver opto-mechanical system. US 2019/025431 A1 concerns a precisely controlled chirped diode laser and coherent lidar system. US 2003/043363 A1 concerns a combined laser obstacle awareness (LOAS) and light detection and ranging (LIDAR) system. SUMMARY The present disclosure includes, without limitation, the following example implementations. The present invention is defined by the appended independent claims, to which reference should now be made. Specific embodiments are defined in the dependent claims. These and other features, aspects, and advantages of the present disclosure will be apparent from a reading of the following detailed description together with the accompanying figures, which are briefly described below. BRIEF DESCRIPTION OF THE FIGURE(S) Embodiments and implementations of the present disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various aspects and implementations of the disclosure, which, however, should not be taken to limit the disclosure to the specific embodiments or implementations, but are for explanation and understanding only. Figure 1 illustrates a LIDAR system according to example implementations of the present disclosure.Figure 2 illustrates aspects of a LIDAR system in accordance with embodiments of the present disclosure.Figure 3 illustrates aspects of a LIDAR system in accordance with other embodiments of the present disclosure.Figure 4 illustrates aspects of a LIDAR system in accordance with some embodiments of the present disclosure.Figure 5A is an illustration of an example of a mode field expander expanding a mode area of an outgoing optical beam in accordance with embodiments of the disclosure.Figure 5B is an illustration of an example of a mode field expander reducing a mode area of an incoming target signal in accordance with embodiments of the disclosure.Figure 6 depicts a flow diagram of a method for utilizing a mode field expander to convert the mode area of incoming and outgoing light of a LIDAR system in accordance with implementations of the present disclosure. DETAILED DESCRIPTION Example implementations of the present disclosure are directed to an improved scanning LIDAR system. Example implementations of the present disclosure are based on a type of LIDAR that uses frequency modulation (FM) and coherent detection to overcome the shortcomings of traditional LIDAR systems and the limitations of prior FM LIDAR systems. Historically, FM LIDAR systems suffer from significant losses in the beam's return path; thus, such systems, which are often quite bulky, require a higher average beam output power to measure distances comparable to time-of-flight (TOF) LIDAR systems. However, the range is l