Stanford EE259 I Gaussian beams, beam scanning techniques I 2023 I Lecture 16

Stanford EE259 I Gaussian beams, beam scanning techniques I 2023 I Lecture 16

Introduction to Lidar

  • Lidar uses laser light to illuminate the environment and measure the time it takes for the light to return to the sensor.
  • Components of lidar include the laser source, optics, and photo detector.
  • Different types of laser sources used in lidar: edge-emitting lasers and vertical cavity surface emitting lasers.
  • Two techniques for illuminating the scene with laser light: flashlight lidar and scanning lidar.
  • Flashlight lidar emits a broad beam to illuminate the entire scene at once.
  • Scanning lidar uses a narrow beam and scans it across the scene.
  • Gaussian beams and the paraxial wave approximation are used to describe the propagation of laser beams.

Angle of Divergence

  • The angle of divergence (Theta D) determines how quickly a beam diverges.
  • Theta D is almost equal to Lambda over Pi Omega zero, where Omega zero represents the waist radius of the beam at the focus point.
  • The angle of divergence is inversely proportional to the waist radius.
  • A smaller focus point results in quicker divergence.

Fiber Coupling

  • Red laser coupled to a fiber, limiting the beam waist to the diameter of the fiber's core (10 microns).
  • Laser beam tightly focuses at the tip of the fiber but quickly diverges afterward.
  • Quick divergence leads to larger spot sizes when objects are farther away.

The Importance of Optics

  • Optics, particularly lenses, are necessary for collimating beams that diverge quickly.
  • Lenses can transform the beam waist diameter to a much larger size, reducing divergence.
  • Lens collimates the beam coming out of the fiber, increasing beam diameter from 10 microns to approximately 7 millimeters.
  • Collimated beam does not diverge as quickly as the original beam, allowing for better visibility even at a distance.

Application-specific Optics

  • Optics can be designed according to specific application requirements.
  • Lens selection based on desired beam diameter at a certain distance.
  • Optics, such as lenses, are essential for controlling the divergence angle of a beam.
  • Presenter acknowledges that the lens used in the demonstration was not designed for a red laser, but still serves to illustrate the purpose of optics in controlling divergence.

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