Laser radar with remote local oscillator

Patent metadata

Field	Value
Publication number	US-10180496-B2
Application number	US-201313841115-A
Country	US
Kind code	B2
Filing date	Mar 15, 2013
Priority date	Nov 21, 2012
Publication date	Jan 15, 2019
Grant date	Jan 15, 2019

Abstract

Laser radar systems include a pentaprism configured to scan a measurement beam with respect to a target surface. A focusing optical assembly includes a corner cube that is used to adjust measurement beam focus. Target distance is estimated based on heterodyne frequencies between a return beam and a local oscillator beam. The local oscillator beam is configured to propagate to and from the focusing optical assembly before mixing with the return beam. In some examples, heterodyne frequencies are calibrated with respect to target distance using a Fabry-Perot interferometer having mirrors fixed to a lithium aluminosilicate glass-ceramic tube.

First claim

We claim:

1. A measurement apparatus, comprising:
a measurement beam source coupled to provide a probe beam and a reference beam from a measurement beam;
a focus adjustment optical system having a movable reflector that is movable so as to focus the probe beam at a target surface;
at least one photodetector configured to receive a portion of the probe beam returned by the target surface to the focus adjustment optical system and a portion of the reference beam from the focus adjustment optical system; and
a signal processor configured to determine a target distance estimate based on a signal from the probe beam and the reference beam,
wherein the focus adjustment optical system includes a probe beam reflector and a reference beam reflector, the probe beam reflector situated to receive the probe beam from the movable reflector and direct the probe beam back to the movable reflector, and the reference beam reflector situated to receive the reference beam from the movable reflector and direct the reference beam back to the movable reflector so as to deliver the reference beam from the reference beam reflector to the at least one photodetector.

2. The measurement apparatus of claim 1 , wherein the focus adjustment optical assembly is situated so that the portion of probe beam returned by the focus adjustment system from the target surface and the reference beam have a common number of traverses of the movable reflector of the focus adjustment optical system as received at the at least one photodetector.

3. The measurement apparatus of claim 1 , wherein the movable reflector is a movable retroreflector.

4. The measurement apparatus of claim 3 , wherein the reference beam is a collimated beam in the focus adjustment system.

5. The measurement apparatus of claim 3 , wherein the focus adjustment optical system includes a reference beam retroreflector, wherein the reference beam retroreflector is situated to receive the reference beam from the movable retroreflector and direct a displaced reference beam to the reference beam reflector through the movable retroreflector, and the reference beam reflector is configured to direct the displaced reference beam back to the reference beam retroreflector.

6. The measurement apparatus of claim 5 , wherein the measurement beam source includes an optical fiber configured to provide the probe beam and the reference beam, and the focus adjustment optical system is configured to deliver the portion of the probe beam returned by the target surface and the reference beam from the reference beam retroreflector to the optical fiber.

7. The measurement apparatus of claim 3 , wherein the measurement beam source includes an optical fiber configured to provide the probe beam and the reference beam, and the focus adjustment optical system is configured to deliver the portion of the probe beam returned by the target surface and the reference beam to the optical fiber.

8. The measurement apparatus of claim 7 , further comprising an optical system that includes a lens situated to receive the probe beam and focus the probe beam at a target distance.

9. The measurement apparatus of claim 8 , wherein the lens is situated to direct the portion of the probe beam returned by the target surface into the optical fiber.

10. The measurement apparatus of claim 9 , wherein the measurement beam is a swept frequency beam and the photodetector is configured to produce a signal at a difference frequency that is associated with a target distance.

11. The measurement apparatus of claim 10 , further comprising a compound rotational stage that includes an azimuthal rotational stage and an elevational rotational stage secured to the azimuthal rotational stage, wherein the optical system is secured to the elevational stage so that the probe beam is directed to the target surface based on an elevational angle and an azimuthal angle.

12. The measurement apparatus of claim 11 , wherein the signal processor is configured to determine a target distance estimate based on the difference frequency.

13. The measurement apparatus of claim 10 , further comprising a compound rotational stage that includes an azimuthal rotational stage and an elevational rotational stage secured to the azimuthal rotational stage, wherein the optical system is fixed so as not to move with either the azimuthal rotational stage or the elevational rotational stage, and further comprising a mirror coupled to the azimuthal rotational stage and the elevational rotational stage so as to direct the probe beam.

14. The measurement apparatus of claim 10 , wherein the signal processor is configured to determine a target distance estimate based on the difference frequency.

15. The measurement apparatus of claim 1 , wherein the movable reflector is an air corner cube or an air roof prism.

16. The measurement apparatus of claim 1 , further comprising a beam divider configured to provide the probe beam and the reference beam from the measurement beam.

17. The measurement apparatus of claim 1 , further comprising a beam divider that includes a beam splitter situated to receive the measurement beam and transmit one of the probe beam or the reference beam.

18. The measurement apparatus of claim 17 , wherein at least one of the probe beam or the reference beam is directed by the beam splitter to the focus adjustment optical system.

19. The measurement apparatus of claim 18 , wherein the beam splitter is a polarizing beam splitter (PBS), and further comprising a wave plate situated so as the reference beam is coupled from the PBS to the movable reflector in a first state of polarization (SOP) and from the movable reflector to the PBS in a second SOP that is orthogonal to the first SOP.

20. The measurement apparatus of claim 19 , wherein the first and second SOPs are linear SOPs, and the at least one wave plate is configured to provide a ¼wave retardation.

21. The measurement apparatus of claim 1 , further comprising a beam divider that includes at least one optical surface configured to select a first portion of a measurement beam cross section as the probe beam and a second portion of the measurement beam cross section as the reference beam.

22. The measurement apparatus of claim 21 , wherein the at least one optical surface is a refractive surface having a first curvature in a surface area corresponding to the probe beam portion of the measurement beam cross section and a second curvature in a surface area corresponding to the reference beam portion of the measurement beam cross section.

23. The measurement apparatus of claim 21 , wherein the at least one optical surface includes a first reflective surface area situated so as to reflect either the probe beam portion of the measurement beam or the reference beam portion of the measurement beam.

24. The measurement apparatus of claim 23 , wherein the first reflective surface area is situated to provide a first beam divergence for the probe beam and a second beam divergence for the reference beam, wherein the first beam divergence and the second beam divergence are different.

25. The measurement apparatus of claim 21 , wherein the beam divider includes a first optical surface and a second optical surface configured to select the first portion of the measurement beam cross section as the probe beam and the second portion of the measurement beam cross section as the reference beam.

26. The measurement apparatus of clai

Assignees

• Nikon Corp
• Nikon Metrology Nv

Inventors

• Smith Daniel G
• Goodwin Eric Peter
• Slotwinski Anthony R
• Rezk Mina A
• Cooper Alexander
• Hedges Thomas M

Classifications

• G01S7/4817

  relating to scanning · CPC title

• G01S7/4917

  superposing optical signals in a photodetector, e.g. optical heterodyne detection · CPC title

• G01S17/42 Primary

  Simultaneous measurement of distance and other co-ordinates (indirect measurement G01S17/46) · CPC title

• G01S7/4812

  transmitted and received beams following a coaxial path · CPC title

• G01S7/4818

  using optical fibres · CPC title