Laser distance measuring device

• US 3,901,597 A
• Filed: 09/13/1973
• Issued: 08/26/1975
• Est. Priority Date: 09/13/1973
• Status: Expired due to Term

• Current Assignee: Loral Aerospace Corp. (MHR Holdings LLC (New York))
• Original Assignee: Matthew B. White
• Inventors: White, Matthew B.
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Buczinski, S. C.
• Application Number:
  US05/396,802
• Time in Patent Office: 712 Days
• Field of Search: 356/4,5,167,123,109,120 331/94.5A,94.5C 250/201
• US Class Current: 356/4.05
• CPC Class Codes: G01S 17/08 for measuring distance only indirect measurement G01S17/46; active triangulation systems G01S17/48

Abstract

When a diffusely reflecting surface, such as the kind ordinarily encountered on most physical objects, is located in the focal saddle of a lens, the combination produces reflection that has spatial coherence characteristics similar to those obtained from a specular surface. Such a combination can be employed in conjunction with a conventional mirror to provide a low-Q FabryPerot resonant cavity. A high gain laser medium can produce oscillation with such a low-Q resonator. When the diffuse surface is outside the lens focal saddle the cavity Q is too low and laser action will not occur. As the focal saddle is advanced toward the surface, the cavity Q will increase and the onset of laser oscillation will signal contact between the focal saddle and the surface. This contact position is repeatable to high precision and is therefore a useful distance measurement. In one alternative embodiment the active laser medium is not contained within the low-Q cavity but instead the low-Q cavity is optically coupled to a conventional laser and its optical length modulated at a convenient frequency. If the laser output is examined for modulation components at the cavity modulation frequency, contact between the focal saddle and the diffuse surface can be established by their presence. In a second alternative embodiment the low-Q cavity includes a quarter-wave plate and is optically coupled to a dual polarization laser having greater than critical adjacent mode coupling. Such a laser will oscillate in only one mode at a time but two orthogonal polarization states are permitted. When the low-Q cavity is operative, that is, when the diffuse surface is inside the lens focal saddle, moving the surface relative to the lens will cause the laser polarization state to flip for each quarter wave length of motion. Thus if the laser polarization state is monitored, entry of the surface into the focal saddle can be sensed by the onset of polarization flipping and motion inside the focal saddle can be established by counting the number of flips.

Claims

1. An optical system for use in measuring the distance between a part of said system and an object having a non-specular reflecting surface, said system comprising:

a laser, a focusing lens associated with said laser and positioned to direct laser optical energy into a focal saddle, said lens and said surface comprising a specular reflector for the physical condition where said surface is present inside said focal saddle, a specular mirror positioned with respect to said lens so that it forms in combination with said lens and said surface a low-Q resonant cavity when said surface is present in said focal saddle, means operatively associated with said laser for indicating when said surface is present in said focal saddle, and means associated with and responsive to the position of said lens for determining said distance measurement.

2. The system of claim 1 wherein said low-Q resonant cavity encloses a laser active medium having sufficient gain to produce oscillation and said indicating means includes a photo detector responsive to light generated by said laser for providing an electrical output only when said surface of said object is in said focal saddle and said laser is oscillating.

3. The system of claim 1 wherein said low-Q resonant cavity is optically coupled to an oscillating laser and contains a phase modulator driven at a predetermined frequency, and said laser has optically coupled thereto an optical detector responsive to modulation products having said frequency so that the presence in the output of said detector of said frequency indicates that said surface of said object is present in said focal saddle.

4. The system of claim 1 wherein said specular mirror comprises one resonator element in a dual polarization laser, said low-Q resonant cavity is optically coupled by virtue of said specular mirror to said dual polarization laser, said dual polarization laser having sufficient adjacent mode coupling to suppress one mode and containing a quarter wave retardation plate oriented at about 45 degrees with respect to permitted laser modes, and said dual polarization laser has coupled thereto a photodetector preferentially responsive to only one mode so that the presence of said surface of said object and the motion of said object in said focal saddle is indicated by mode changing.

5. A laser distance measuring device for use with an object having diffuse reflection characteristics, said device comprising:

a laser active medium characterized in having high gain, a lens associated with said active medium for focusing light from said medium into a focal saddle region, a mirror located on the side of said active medium away from said lens and positioned to act in conjunction with said lens as a low-Q optical resonator when the surface of said object is in said focal saddle region of said lens, means for moving said lens along the axis of said resonator relative to said surface, and means associated with said laser for indicating when said laser is oscillating.

6. A laser distance measuring device for use with an object having diffuse reflection characteristics, said device comprising:

an oscillating laser having an optical resonant cavity with at least one partly transmissive mirror, a lens associated with said laser for focusing light emitted from said mirror into a focal saddle region, said lens forming in combination with a surface of said object a specular reflector and said specular reflector forming in combination with said mirror a low-Q optical resonant cavity when said surface is in the focal saddle of said lens, means for moving said lens axially with respect to said laser and relative to said object, a phase modulator located between said lens and said mirror and driven at a predetermined modulating frequency, and means for anAlyzing the energy in said laser for a modulation component at said frequency, the presence of which denotes the location of the surface of said object inside said focal saddle region.

7. A laser distance measuring device for use with objects having diffuse reflection characteristics, said device comprising:

an oscillating dual-polarization laser having an optical resonator with at least one partly transmissive mirror and sufficient adjacent mode coupling to limit its oscillation to a single polarization at a time, a lens associated with said laser for focusing light from said mirror into a focal saddle region, means for moving said lens along the optical axis of said laser through said focal saddle and relative to said object, and means for detecting changes in the state of polarization of said laser.

8. The device of claim 7 wherein said resonator is translated to change its tuning and to produce polarization changes at a rate related to the speed of said translation.

9. The device of claim 8 further including a phase modulator located between said mirror and said lens and driven at a frequency that is substantially greater than said rate.