Lidar remote sensing system

• US 6,147,747 A
• Filed: 08/14/1998
• Issued: 11/14/2000
• Est. Priority Date: 08/22/1997
• Status: Expired due to Fees

• Current Assignee: National Aeronautics and Space Administration (Government of the United States of America)
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Kavaya, Michael J., Amzajerdian, Farzin
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US09/134,704
• Time in Patent Office: 823 Days
• Field of Search: 356/141.1, 356/4.01, 356/5.01, 356/5.1
• US Class Current: 356/4.01
• CPC Class Codes: G01B 11/26 for measuring angles or tapers; for testing the alignment of axes
• CPC Class Codes: G01S 17/58 Velocity or trajectory determination systems; Sense-of-movement determination systems
• CPC Class Codes: G01S 7/4812 transmitted and received beams following a coaxial path
• CPC Class Codes: G01S 7/4817 relating to scanning

Abstract

A lidar remote sensing system wherein a laser signal is transmitted along an optical path through a telescope having a primary and secondary mirrors and a rotating prism at the telescope output. When the reflected signal from the target is received it is passed back through the system to a detector, where it is heterodyned with a signal from a local oscillator to detect Doppler frequency shifts in the returned signal. Since the prism is rotating, the prism will be at one position when the signal is transmitted and at another when the returned signal is received. This causes the reflected signal to be off the optical path, reducing the power of the returned signal. To correct this problem a de-rotator or prism is mounted for rotation, in synchronism with the rotating prism, about the optical path in a position to intersect the returned beam and refract it back onto the optical path to reduce the power loss in the returned signal.

Claims

1. A lidar remote sensing system having an optical path, comprising

a. a frame;

b. a rotating scanning element supported by the frame and positioned between the remainder of the system and a target for directing a transmitted beam from the optical path toward a target in a conical scan;

c. means for injecting a transmitted laser beam into said optical path to be directed to the scanning element;

d. a detector positioned on the optical path for detecting a reflected beam from the target; and

e. a rotating prism positioned on the optical path for refracting said reflected beam back onto the optical path, said scanning element and said prism being rotated in synchronism.

2. The apparatus of claim 1 wherein the scanning element is a prism.

3. The apparatus of claim 1 wherein the scanning element is a mirror.

4. A lidar remote sensing system, comprising

a. a frame;

b. a telescope mounted on the frame and having opposite ends with a primary mirror at one end and a secondary mirror at the other end, said secondary mirror being positioned at one end of an optical path;

c. a rotating scanning element positioned between the primary mirror and a target for directing a laser beam transmitted through the telescope toward the target in a conical scan;

d. means for injecting a transmitted laser beam into said optical path to be directed through the telescope and the scanning element toward the target;

e. a detector positioned at the other end of the optical path for detecting a reflected beam from the target; and

f. a rotating prism positioned on the optical path for refracting said reflected beam back onto said optical path, said scanning element and said prism being rotated in synchronism.

5. The system of claim 4 wherein the system includes means for injecting a heterodyning laser beam into said optical path to be directed onto the detector with the reflected beam for determining the Doppler frequency shift in said reflected beam.

6. The system of claim 5 wherein a quarter waveplate positioned on the optical path between the first beam splitter and the secondary mirror changes the polarization of the transmitted beam from linear to circular and the reflected beam from circular to linear.

7. A lidar remote sensing system, comprising

a. a support frame;

b. a telescope mounted on the frame, said telescope having opposite ends with a primary mirror at one end a secondary mirror at the other end, said secondary mirror being positioned at one end of an optical path, said telescope having a rotating scanning element positioned at the primary mirror and a target for directing a laser beam transmitted through the telescope toward the target in a conical scan;

c. a detector positioned at the other end of the optical path for detecting a reflected beam from the target;

d. a first beam splitter mounted on said optical path between the detector and the secondary mirror;

f. a laser transmitter mounted to direct a transmitted laser beam into the beam splitter and along said optical path;

said secondary mirror being positioned to reflect said transmitted beam traveling along the optical path onto the primary mirror and through the scanning element to the target and to reflect the reflected beam from the primary mirror back toward the detector;

g. a prism mounted on the optical path between the beam splitter and the detector, said prism being rotated in synchronism with the scanning element for refracting the reflected beam onto the optical path; and

h. a second beam splitter and a local oscillator for directing a heterodyning laser beam onto the optical path and into the detector with the reflected beam for detecting the Doppler shift in the return beam.

8. The system of claim 7 wherein said prism is made of a material transparent to laser radiation and having opposed flat surfaces at a wedge angle to each other.

9. The system of claim 8 wherein the prism is rotated about the optical path with one of said flat surfaces being at an acute angle to said optical path.

10. The system of claim 9 wherein said one flat surface is positioned at a 45°

angle to said optical path.

11. The system of claim 9 wherein the scanning element is a prism.

12. The system of claim 9 wherein the scanning element is a mirror.